TRIBOLOGY, UV DEGRADATION, AND STRUCTURE-PROPERTY-PROCESSING RELATIONSHIPS OF DETONATION NANODIAMOND-POLYETHYLENE

NANOCOMPOSITES

by

JOHN TIPTON

DERRICK DEAN, COMMITTEE CHAIR S. AARON CATLEDGE

NITIN CHOPRA ROBIN D. FOLEY

GREGG M. JANOWSKI

A DISSERTATION

Submitted to the graduate faculty of The University of Alabama at Birmingham, in partial fulfillment of the requirements for the degree of

Doctor of Philosophy

BIRMINGHAM, ALABAMA

2012

Copyright by JOHN TIPTON

2012

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STRUCTURE, PROPERTY, PROCESSING RELATIONSHIPS, TRIBOLOGICAL AND UV WEATHERING BEHAVIOR OF NANODIAMOND-HDPE/UHMWPE

POLYETHYLENE NANOCOMPOSITES

JOHN TIPTON

MATERIALS SCIENCE AND ENGINEERING

ABSTRACT

Nanoscale reinforcements offer the possibility of coupling the already proven

high strength to weight properties of polymer matrix composites with additional

multifunctional properties such as electrical conductivity, thermal conductivity, unique

optics, UV/IR radiation absorption, and enhanced wear resistance. This work presents

materials based on detonation nanodiamonds dispersed in two types of polyethylene. The

work begins with an understanding of nucleation phenomena. It was discovered through

isothermal kinetics using differential scanning calorimetry that nanodiamonds act as

nucleating agents during polyethylene crystallization. A processing technique to disperse

nanodiamonds into very viscous ultra-high molecular weight polyethylene was developed

and analyzed. These composites were further studied using dynamic mechanical analysis

which showed increases in both stiffness and energy absorbing modes over unfilled

UHMWPE. Exposure to UV degradation caused a failure of the polymer microstructure

which was found to be caused by residual tensile stresses between the polymer particles

formed during processing. These high stress regions were more prone to photo oxidation

even though the nanodiamond particles were shown to decrease surface oxidation.

Additionally, the tribological properties of UHMWPE/nanodiamond composites were

investigated. Ultra-high molecular weight polyethylene is an already proven ultra tough

and wear resistant polymer that is used in many high performance thermoplastic

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applications such as bearings, surfaces (skids/wheels), ropes/nets, and orthopedic

implants. This work showed that UHMWPE loaded with 5.0wt% nanodiamonds might

be a candidate to replace the currently used crosslinked polyethylene material used in

orthopedic implants.

Keywords: Ultrahigh molecular weight polyethylene wear, polyethylene kinetics, polyethylene UV degradation, detonation nanodiamond composites

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DEDICATION

To Jack T, Courtney, Lynn and Jack G.

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ACKNOWLEDGEMENTS

There can never be a finer mentor than Dr. Derrick Dean. His academic and

teaching philosophy is one of compassion, freedom, outreach, scientific

fellowship, and data perfection. Thank you Dr. Dean.

My family, Jack and Courtney, have been amazing through this process.

I have truly enjoyed the friends and colleagues made throughout my career at

UAB. Thanks to the committee. Thanks to the polymer lab group members from

past and present. Thanks to the Department of Materials Science and

Engineering; especially Cynthia Barham and Robin Mize.

Mentors like Vinoy Thomas and Gregg Janowski have offered much valuable

guidance along the way.

My friend Brendan for his help in editing; Thanks PT.

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TABLE OF CONTENTS Page

ABSTRACT ....................................................................................................................... iii

DEDICATION .................................................................................................................... v

ACKNOWLEDGEMENTS ............................................................................................... vi

LIST OF TABLES ............................................................................................................. ix

LIST OF FIGURES ............................................................................................................ x

1. INTRODUCTION ....................................................................................................... 1

2. LITERATURE REVIEW ............................................................................................ 3

2.1 Polymer Matrix Nanocomposites .................................................................... 3 2.2 Detonation Nanodiamond Particles ................................................................. 5 2.3 Detonation Nanodiamonds in Polymer Composites ........................................ 6

3. SPECIFIC AIMS ....................................................................................................... 10

3.1 Crystallization Kinetics of High-density Polyethylene / Detonation Nanodiamond Nanocomposites ..................................................................... 10

3.2 Fabrication and Morphology of Detonation Nanodiamond/Ultra-high Molecular Weigh Polyethylene Nanocomposites .......................................... 10

3.3 Thermomechanical Properties and Ultraviolet Degradation of Detonation Nanodiamond/Ultra-high Molecular Weight Polyethylene Nanocomposites 10

3.4 Tribological Behavior of Ultra-high Molecular Weight Polyethylene/Detonation Nanodiamond Nanocomposites on Cobalt-Chromium Alloys .......................................................................................... 11

4. MATERIALS AND EXPERIMENTAL METHODS ............................................... 12

4.1 Fourier-Transform Infrared Spectroscopy ..................................................... 12 4.2 Microscopy .................................................................................................... 12 4.3 Thermal Analysis ........................................................................................... 12 4.4 Nanodiamonds ............................................................................................... 12 4.5 Ultra-high molecular weight polyethylene .................................................... 13 4.6 Ultra-high molecular weight polyethylene/Detonation Nanodiamond

Composites .................................................................................................... 13

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4.7 Crystallization Kinetics of High-density Polyethylene / Detonation Nanodiamond Nanocomposites ..................................................................... 13

4.8 Thermomechanical Properties and Ultraviolet Degradation of Detonation Nanodiamond/Ultra-high Molecular Weight Polyethylene Nanocomposites ....................................................................... 14

4.9 Tribological Behavior of Ultra-high Molecular Weight Polyethylene/Detonation Nanodiamond Nanocomposites on Cobalt-Chromium Alloys .......................................................................................... 15

5. CRYSTALLIZATION KINETICS OF HIGH-DENSITY POLYETHYLENE / DETONATION NANODIAMOND NANOCOMPOSITES .................................... 18

6. FABRICATION AND MORPHOLOGY OF DETONATION NANODIAMOND/ULTRA-HIGH MOLECULAR WEIGHT POLYETHYLENE COMPOSITES .......................................................................................................... 42

7. THERMOMECHANICAL PROPERTIES AND ULTRA VIOLET DEGRADATION OF DETONATION NANODIAMOND/ULTRA-HIGH MOLCEULAR WEIGHT POLYETHYLENE NANOCOMPOSITES .................... 52

8. TRIBOLOGICAL BEHAVIOR OF ULTRA-HIGH MOLECULAR WEIGHT POLYETHYLENE/DETONATION NANODIAMOND NANOCOMPOSITES ON COBALT-CHROMINUM ALLOYS ................................................................. 73

9. CONCLUSIONS ..................................................................................................... 101

10. REFERENCES ........................................................................................................ 104

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LIST OF TABLES

Table Page

CRYSTALLIZATION KINETICS OF HIGH-DENSITY POLYETHYLENE / DETONATION NANODIAMOND NANOCOMPOSITES

1. Data extracted from DSC nonisothermal thermograms of HDPE/ND nanocomposites. * 286.2J/g used for 100% PE crystalline sample[25]. ................... 25

2. Nucleation constants obtained from Hoffman analysis for regimes II and III. ......... 37

THERMOMECHANICAL PROPERTIES AND ULTRA VIOLET DEGRADATION OF DETONATION NANODIAMOND/ULTRA-HIGH MOLCEULAR WEIGHT

POLYETHYLENE NANOCOMPOSITES

1. Tabulated heat of fusion and melting temperature data obtained from DSC scans. . 58 2. Tabulated crystallization exotherms and peak crystallization temperature data

obtained from DSC. ................................................................................................... 61

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LIST OF FIGURES

Figure Page

INTRODUCTION

1. Schematic illustration geometry of starting rod material and transformation into the hemisphere after treatment. .......................................................................... 17

CRYSTALLIZATION KINETICS OF HIGH-DENSITY POLYETHYLENE / DETONATION NANODIAMOND NANOCOMPOSITES

1. Nonisothermal DSC thermogram. Exothermic peaks are up. .................................. 25 2. DSC baseline shape changes observed in highly undercooled polyethylene

due to rapid crystallization. Data shown is a representative set of isothermal scans at varying crystallization temperatures obtained for the samples tested. ........ 27

3. Representative sample sigmoidal baseline correction for peak integration. ............. 28 4. Avrami constants as a function of isothermal crystallization temperature. .............. 30 5. Enthalpy evolved during isothermal crystallization as a function of isothermal

crystallization temperature. ....................................................................................... 31 6. Representative rolling integral curves used to obtain quasi-growth rates.

Dashed lines indicate time at 50% crystallization. .................................................... 35 7. Apparent activation energy as calculated from multiple isothermal

crystallization runs. ................................................................................................... 33 8. Hoffman plot, right box represents regime II, left box represents regime III. .......... 36 9. Nucleation constants as a function nanodiamond loading obtained from linear

regression of Hoffman plots at varying isothermal crystallization temperatures. ............................................................................................................. 37

10. Growth rate vs. crystallization temperature. ............................................................. 38

FABRICATION AND MORPHOLOGY OF DETONATION NANODIAMOND/ULTRA-HIGH MOLECULAR WEIGHT

POLYETHYLENE COMPOSITES

1. TEM micrographs at 120kV. As received nanodiamonds (A). Acid treated nanodiamonds (B). .................................................................................................... 47

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2. Infrared spectroscopy of oxidized (red), and as received (black) detonation nanodiamonds. The carbonyl stretching peak (1778cm-1) is evidence that carboxyl groups are present after acid treatment. ...................................................... 47

3. Scanning electron micrographs of UHMWPE powder. The arrows indicate nanodiamond clusters (5-15nms) indicating adequate dispersion along the particle surface. ......................................................................................................... 48

4. TEM micrographs at 120kV of 0.1wt% ND/UHMWPE composites: There was some preference for nanodiamond aggregation in the direction of microtomy marks (A). 20-50nm nanodiamond clusters were observed throughout the sample. ............................................................................... 49

5. 11kX TEM micrograph illustrating well dispersed individual nanodiamond particles away from the knife marks. ....................................................................... 49

THERMOMECHANICAL PROPERTIES AND ULTRA VIOLET DEGRADATION OF DETONATION NANODIAMOND/ULTRA-HIGH MOLCEULAR WEIGHT

POLYETHYLENE NANOCOMPOSITES

1. As processed differential scanning calorimetry thermogram of ND/UHMWPE composites as a function of loading. Crystallinity of the composite samples is lower than the unfilled sample. .............................................................................. 57

2. Storage modulus curves obtained from dynamic mechanical analysis (A), Loss modulus curves obtained from dynamic mechanical analysis. Vertical dashed lines represent relaxation regions (B). ...................................................................... 58

3. Crystallization exotherm obtained from differential scanning calorimetry; nucleating effects from the nanodiamonds are evident. ............................................ 60

4. This schematic illustrates a possible mechanism for chain folding initiation on the surface of the faceted nanodiamond particle. ................................................. 61

5. Scanning electron micrograph of unexposed neat UHMWPE (A), and 5.0wt% nanocomposite (B). Optical micrograph of the unexposed 0.1wt% nanocomposite sample (C). ...................................................................................... 62

6. SEM micrographs of neat UHMWPE (A) and 5.0wt% UHMWPE (B) after 400 hours of UV exposure. Particle de-coalescence is more pronounced in the nanocomposite sample. ................................................................ 62

7. Optical micrographs after 400 hours of ultraviolet exposure. (Neat-A, 0.1wt%-B, 1.0wt% C, 5.0wt% D) ............................................................................................... 63

8. De-coalescence of UHMWPE powder occurring in the 0.1wt% composite sample after 412 hours of UV exposure (A). Surface cracking and decoaslscence of UHMPWE powder in the neat sample after 400 hours of UV exposure (B). ........... 64

9. Intensity of carbonyl stretch (1720cm-1) as a function of exposure time. A decrease in the intensity corresponds to less photo oxidation. .............................. 65

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10. Optical micrographs of samples after 400 hours of UV exposure, neat (A), 0.1wt% (B), 1.0wt% (C), 5.0wt% (D). UHMWPE particle de- coalescence is more pronounced in the composite samples. ........................................................ 66

11. Schematic of sintering observed in UHMWPE processing. Red arrows indicate residual tensile stresses present from elastic recovering during processing. ............ 67

12. De-coalescence observed after UV exposure. Red arrows indicate residual tensile stresses present from elastic recovering during processing. .......................... 67

13. Plots of crystallinity calculated using the Zerbi relationships a function of UV exposure. Crystallinity values were calculated using both the 1464cm-1/1474cm-1 (left) and 720cm-1/730cm-1 (right) peak sets. ............................ 69

14. Nanoindentation results of Neat (black) and 5.0wt% (red) samples. Hardness vs. displacement (left) and Modulus vs. displacement (right); inset graphs represent %change between 400 hour exposed samples to controls. ... 70

TRIBOLOGICAL BEHAVIOR OF ULTRA-HIGH MOLECULAR WEIGHT POLYETHYLENE/DETONATION NANODIAMOND NANOCOMPOSITES ON COBALT-CHROMINUM ALLOYS

1. Neat GUR 4150B powder preprocessing (left). Transition of composite blanks into pins for POD testing (right). ................................................................... 78

2. Schematic of the pin on disk wear pins and PMMA holders containing CoCr disks (left). Pins loaded into the OrthoPOD instrument for tribological testing (right). ... 79

3. Polished and de-burred stock Stellite 21 rods cut to length before heat treatment (A). Hemispheres formed after heat treatment process (B). ............................................. 81

4. This schematic illustrates the CoCrMo hemisphere (top) and UHMWPE plate (bottom) fixed to aluminum stubs. These stubs were places in the AR2000 rheometer in the respective fixture locations. ........................................................... 82

5. Wear track diameter was controlled by the location of the hemisphere (left), and the distance from the center of the hemisphere to the center of the stub was used at the moment arm length, r. The force FT was obtained using the torque value, , and the moment arm length, r. ................................................... 83

6. Etched CoCrMo hemisphere with dendritic microstructure. ..................................... 84 7. Etched microstructure of homogenized hemispheres. ............................................... 84 8. Post wear stereo (upper) and Nomarski filter optical microscopy (lower)

micrographs. Neat (A), Cross-linked (B), 0.1wt% ND (C) after 2 million cycles. .. 86 9. Post wear stereo and Nomarski filter optical microscopy (lower) micrographs.

1.0wt% ND (D), 5.0wt% ND (E) after 2million cycles. ........................................... 86 10. Post wear CoCr disks surface through an optical microscope. Neat (A),

cross-linked (B), 0.1wt% ND (C) after 2 million cycles. .......................................... 87 11. Post wear CoCr disks surface through an optical microscope. 1.0wt% ND (D),

5.0wt% ND (E) after 2 million cycles. ...................................................................... 87

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12. Coefficient of friction (, z-axis) plotted over wear surface (y axis) and also as a function of cycle number (x-axis). Neat (left), Cross-linked (right). ............... 88

13. Coefficient of friction (, z-axis) plotted over wear surface (y axis) and also as a function of cycle number (x-axis). 0.1wt% ND (left), 1.0wt% ND (right). ...... 89

14. Coefficient of friction (, z-axis) plotted over wear surface (y axis) and also as a function of cycle number (x-axis). 5.0wt% ND. ............................................... 89

15. Center position coefficients of friction extracted from 3-dimensional wear surface data obtained from OrthoPOD instrument. ................................................... 90

16. Coefficient of friction () vs. wear distance obtained from microtribology experiments using the AR2000 rheometer. ............................................................... 92

17. Post wear stereomicroscopy at 1.6X after 6km. Neat (left), 0.1wt% ND (left-middle), 1.0wt% ND (right-middle), 5.0wt% ND (right). ........................................ 93

18. Post wear stereomicroscopy of CoCrMo disks at 1.6X after 6km. Neat (left), 0.1wt% ND (left-middle), 1.0wt% ND (right-middle), 5.0wt% ND (right). ............ 94

19. Sub-500m2 wear debris area distribution. ............................................................... 94 20. Average particle area (left axis) and average circularity parameters (right axis) of

wear debris as a function of nanodiamond loading. .................................................. 94 21. Circularity parameter distribution of wear debris as a function of nanodiamond

loading. ...................................................................................................................... 95

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1. INTRODUCTION

The area of nanocomposites has seen a tremendous amount of interest and

research since the discovery of carbon nanotubes[1] in 1991. The extremely high

modulus, strength, and intriguing electronic properties of these structures offer potential

to be a spectacular reinforcement material in polymeric matrices. Other nano particles

such as montmorillonite layered silicates, clays, and alumina have also been the subject

of many works in the area of polymer matrix nanocomposites. The outcomes in general

have been disappointing in terms of mechanical properties compared to the theoretical

potential. Most of the disappointing properties have been attributed to poor interfacial

interactions between the nanoparticles/polymer matrix and macroscale aggregation of the

particles. Many authors report chemical, physical, mechanical, and electrical methods to

modify nanoparticle surfaces and increase phase miscibility for their respective polymer

application. However, these attempts often leave the nanoparticle devoid of the original

structure of interest and therefore produce disappointing mechanical or electrical

properties. Perhaps the most successful thrust in nano materials has been in electronics

applications, specifically materials based on carbon nanotubes.

Recent works focused on multiscale, multifunctional composites are perhaps most

promising for full exploitation of nano material properties. In these cases mechanical

reinforcement is left up to proven materials, such as carbon and glass fibers, and the nano

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scale reinforcements are expected to provide value-added properties such as electrical

and thermal conductivity, UV protection, wear enhancements, and inter-fiber bridging.

Ultrahigh molecular weight polyethylene is currently used in many applications for

wear surfaces such as wheels, gears, and joint replacement implants. Although many

reports describe various ways to enhance the lifetime of implant systems by altering the

opposing (metallic/ceramic) wear surface, surprisingly very little literature exists on

enhancing the polymeric portion, especially through the use of nanocomposites. With an

aging population, demand for longer lasting, comfortable implant systems will continue

to increase. There is also potential for the use of light weight, high performance

composites in applications where energy is concerned also. To meet the ever increasing

demand for higher performance polymeric systems, new materials must be developed that

consider cost, availability, and performance.

Diamonds are well known for their extreme wear resistance; however use in most

applications is cost prohibitive. Nanodiamonds formed from detonation can be obtained

with controlled dimensions at low cost, high yield, and offer similar properties of macro

scale diamond. Incorporation of these particles into a polyethylene matrix presents a

platform for a multifunctional material with enhanced wear characteristics along with

enhanced strength, toughness, and environmental stability.

Presented in this document are sets of experiments to test and validate the use of

detonation nanodiamond particles as reinforcements in polyethylene matrices. The

fundamental effects from nanodiamonds on the crystallization of polyethylene are

investigated to gain insight into microstructure formation. Thermomechanical property

3

enhancements in the composites are analyzed using dynamic mechanical analysis and

differential scanning calorimetry. Since literature suggests that the nanodiamonds are

candidates for use in UV absorbing applications, composite samples are exposed to UV

radiation to test this property. Finally, the composite tribology is studied using several

techniques. These studies in totality present a practical, low cost, and high performance

multifunctional material.

2. LITERATURE REVIEW

2.1 Polymer Matrix Nanocomposites

Since the polymer of choice will be ultra-high molecular weight polyethylene

(UHMWPE), some important papers will be discussed in the next section. Extensive

work in the area of polyethylene morphology, structure, and property relationships has

been conducted since the 1940s [2-8].

Recently, thermoplastic resins such as polyethylene and polypropylene have been

used as a matrix material in composites filled with nanoscale reinforcements [9-11]. The

majority of fillers have been nano-carbon materials such as carbon nanotubes, carbon

nanofibers and nanographene. In particular, these carbon materials tend to interact well

with polyolefins due the similar nonpolar nature of their surfaces[12]. Due to the

extremely high surface-area-to-volume ratio of materials at the nanoscale and the

extraordinary properties of the reinforcements themselves, these composites offer new

classes of high performance engineering materials. Polymeric matrix composites filled

with nano-scale reinforcements have been shown to increase glass transition

4

temperatures, increase thermal stability, enhance wear resistance, and resist weathering

effects of UV radiation and moisture [13-19].

UHMWPE has been a very important polymer to industry and medicine due to its

chemical resistance, biocompatibility, extreme toughness and excellent wear properties.

It has been used in orthopedic implants, bearings, and even in bullet resistant vests.

However, due to the extremely high melt viscosity, it has been difficult to incorporate

nanoscale reinforcements into the resin by conventional means [20-22]. Therefore, only

a limited body of knowledge exists relating to UHMWPE composites. Wang et al[23]

showed that relatively low loadings of ball milled, micron-scale titanium particles in

UHMWPE did not show improvements in wear against a steel ball. Although at higher

loadings significant wear reduction occurred. Typically, with nanoscale reinforcements,

property enhancements can be realized at lower loadings due to the vast surface area and

hence high interface.

Weak interfacial bonding due to poor nano reinforcement/polymer affinity has

been a major factor in the disappointing mechanical properties exhibited by the final

composite. It was shown in papers by Li et al[24] and Uehara et al[25] that carbon

nanotubes acted as nucleation sites during the solution crystallization of UHMWPE,

therefore integrating the nano structure into the crystalline backbone of the polymer. Li

et al[24] achieved a new hybrid shish-kabob structure where the carbon nanotube

replaced the well known polyethylene fibril, or shish, and obtained growth of the lamella

(kabobs) on the surface of the CNT. The same experiment was done with nylon 6, 6, and

the hybrid shish kabobs were shown to still be intact even after melt processing of the

matrix which gives evidence of an interphase with unique properties. These types of

5

structures serve a dual purpose including maximizing dispersion by decreasing the

packing efficiency of the nano structures and also the formation of the lamellar crystals

provide affinity for the matrix polymer. In the case of a zero dimensional nano particles,

such as a nanodiamond (ND), replacing the fibrous polyethylene shish would not be

possible; however, the presence of the diamond facets might provide stable nucleation

sites for lamellae growth. Although these authors did not study thermo mechanical

behavior, it can be assumed that the direct mechanical interface formed in Lis study

would promote maximum thermomechanical property enhancements.

Recently Hill et al[26] published a paper on the wear of UHMWPE against

deposited nanodiamond coatings on various metallic substrates simulating an orthopedic

implant environment. These authors suggested that the wear rate of UHMWPE on their

diamond coatings decreased compared to the non-coated surfaces. They also suggested

that more work be done on the polymer to enhance wear properties. The potential of

enhanced UV and wear properties of nanodiamonds as reinforcements at low loadings

present a new opportunity for an even more useful UHMWPE material.

2.2 Detonation Nanodiamond Particles

Individual nanodiamond particles were subjects of many studies in the late 1980s

particularly in the study of explosive materials and chemical species of detonation

reactions. Although nanodiamonds were discovered previous to Greiners[27] 1988

Nature paper, this study was of great importance because all other studies synthesized

particles using a carbon source, where Greiner only used the explosive itself as the

6

carbon source. This work provided a new method for synthesis of nanodiamonds and

some interesting data for the study of detonation reaction products.

Unlike many heavily researched nano materials, detonation nanodiamond

particles are very cheap, some costing as low as $2.50/g. Also due to the formation

process, the particle size distribution is very uniform (5nm) so batch to batch variation

is minimized as long as the same technique is used. The first practical use of

nanodiamond particles has been in lubricating applications such as motor oils and as a

polishing medium[28]. Shenderova et al[29] describes many potential applications of

nanodiamonds in an extensive paper which pools much of their combined work. These

applications include wear resistant coatings, medical applications (including drug

delivery[30]), structural composite materials, UV protection, and functional

nanostructures including cold cathodes. However, very few of these applications have

been studied. Due to the unique faceted structure and varying surface energy on each

facet, many interesting drug delivery vehicles have been studied and predicted [30-33].

Although not the direct topic of this work, many functionalization methods, similar to the

reactions used on carbon nanotube structures, have been studied in detail [28, 33-37].

Nanodiamonds with many different functional groups can now be purchased

commercially.

2.3 Detonation Nanodiamonds in Polymer Composites

Lee and Lim[38] first studied the tribological behavior of PTFE/nanodiamond

composites. This study found that for their particular system, percolation occurred at 2

wt% of nanodiamond. Decreases in both the width of the wear track and friction

7

coefficients were also noted at this loading. The authors report that the necessary loading

of nanodiamond is much lower than previous reports using ZrO2 and Si3N4 and attribute

this to the higher surface area of the detonation nanodiamond particles. The composites

in this work were prepared using suspensions of PTFE and nanodiamond powder which

were then sprayed onto aluminum sheets. The sheets were then heated to allow

sintering of the PTFE powder, therefore dispersion and interface interactions were

probably not optimum. SEM micrographs showed extensive agglomeration of

nanodiamond particles over 2 wt% suggesting that more could be done during the

processing procedure, including functionalization, or completely dissolving the PTFE

while mixing with a nanodiamond suspension.

A study based on polyimides by Zhang et al[37] studied the effects of polyimide

grafted nanodiamond loading on the hardness of these films. The authors present a

method of polyimide grafting on NDs starting from a carboxylated structure to an

acylchloride and then to a polyimide monomer attachment. TEM images showed well

dispersed 100nm aggregates through the polyimide matrix with their grafted structures,

and 200-250nm aggregates with unmodified diamonds. Hardness was tested using a

micro hardness tester, and a 26% enhancement was seen with a 5wt% ND loading over

neat polyimide. The grafted NDs showed a 30% increase in hardness with just 1wt%

loadings over the neat polyimide. Clearly this paper lends additional evidence to the

potential for use of nanodiamonds in wear enhanced polymer composites at low loadings.

Shenderova [39] also studied detonation nanodiamonds as reinforcements in

polydi-methylsiloxane (PDMS) and polymethylmethacrylate (PMMA) matrices. They

tested thermal conductivity in the PDMS/ND composites as a function of ND loading and

8

found a 15% increase with only 2wt% ND. The data shows that no percolation was

reached with the highest concentration of NDs (2wt %). Clearly there is more potential

for increasing the thermal conductivity in those samples. Nominal increases in thermal

stability were shown in both the PMMA and PDMS composites as a function of

increasing ND concentration. Perhaps this property would show a higher increase with

higher loadings of NDs; however the apparently low percolation threshold might prevent

dramatic increases in thermal stability. The authors note difficulties in achieving good

dispersion by mechanical mixing. In the case of the PDMS composites, an intermediate

solvent (isopropyl alcohol) was used to first sonicate the NDs before adding to the PDMS

polymer. The additional solvent was removed using vacuum before curing.

dAlmeida [40] reported an aliphatic amine/epoxy system loaded with 10-30 wt%

detonation nanodiamonds. The relaxation behavior using dynamic mechanical analysis

(DMA) and tensile strength was measured as a function of ND loading and curing agent

concentration. Decreases in the tensile strength with increased ND loading were

recorded. The authors attributed the decreases to poor interfacial interaction between the

resin system and the ND particle. No high resolution images of the fracture morphology

were shown so there is no way to draw any conclusions about the dispersion of the NDs

in the matrix. The ND particles may have inhibited the curing reaction, resulting in a

more plasticized composite. The relaxation data in this paper showed enhancements in

both the storage modulus glass transition temperature with nanodiamond loading. Since

Shenderova et al[39] showed thermal conductivity enhancements, it would be worthwhile

to study these epoxy systems due to the wide range of applications for which they

currently are utilized.

9

Most recently Behler et al[41] studied electrospun polyacrylonitrile (PAN) and a

vegetable oil based polyamide 11 (PA11) nanodiamond composite. The mats were heat

treated after the electrospinning process to produce 3m thick, transparent films with

various ND loadings. Nanoindentation, UV-vis spectroscopy, and TEM microscopy

were performed on the finished samples. For the PA11 films, at 20wt% a 4x increase in

Youngs Modulus and 2x increase in hardness were observed. The highest concentration

of NDs achieved in the PAN system was 60 wt%. The authors indicated that the TEM

images suggest that these structures are essentially a ND fiber with a polymer binder.

The PA11 system was only able to hold a maximum of 40 wt% of nanodiamonds. Still

these materials might have potential application after removing the polymer by heating

which would leave a diamond coating on a substrate. UV-Vis spectroscopy showed

strong absorbance at very low loadings of NDs in the PAN system, indicating that only

small amounts of NDs were beneficial in increasing UV protection of a part. The authors

report to have also successfully electrospun nanodiamond composites using both

polycaprolactone (PCL) and polyethylene oxide (PEO) polymers [41].

Clearly, the available literature available on polymer/nanodiamond composites is

at an early state. The discussed studies give a broad view of what is potentially possible

with ND reinforcements.

10

3. SPECIFIC AIMS

3.1 Crystallization Kinetics of High-density Polyethylene / Detonation Nanodiamond

Nanocomposites

Since the final morphology and crystallinity of nanocomposites ultimately

determines the application, gaining understanding of how the matrix material and

reinforcement interact at the atomic-microscale bridge is important. This aim attempts to

understand how the uniquely faceted nanodiamond particles formed from detonation

processes effect polyethylene crystallization in order to describe a new nanocomposite

material.

3.2 Fabrication and Morphology of Detonation Nanodiamond/Ultra-high Molecular

Weigh Polyethylene Nanocomposites

UHMWPE is a difficult polymer to process due to the extensive chain

entanglement networks which ultimately cause an extremely high melt viscosity. Current

methods to disperse fillers, and specifically nanoscale fillers, are difficult and impractical.

This aim is an attempt to develop a practical processing technique for adding fillers to

raw UHMWPE powder that can easily be scaled to commercial production.

3.3 Thermomechanical Properties and Ultraviolet Degradation of Detonation

Nanodiamond/Ultra-high Molecular Weight Polyethylene Nanocomposites

The thermomechanical behavior of the UHMWPE/nanodiamond composites is

investigated in this aim using differential scanning calorimetry and dynamic mechanical

11

analysis. These properties are useful when defining temperature use ranges, and also

energy absorption characteristics and stiffness. The effect of the nanodiamonds on these

properties is discussed and important relationships between the stiffness and energy

absorption illustrated. Also, the literature identifies potential use for nanodiamond

particles in ultraviolet absorption applications due to its wide band-gap. A complete UV

weathering experiment is performed to assess these claims.

3.4 Tribological Behavior of Ultra-high Molecular Weight Polyethylene/Detonation

Nanodiamond Nanocomposites on Cobalt-Chromium Alloys

Since nanodiamonds were identified as candidates for enhancing tribological

properties in composite materials and UHMWPE is widely used for bearing surfaces and

skids, the wear behavior of the composites is tested. Tribological testing is a very

difficult area of study due to the dynamic nature of abrasion in real world applications. A

modified technique using a ball on disk was developed using a controlled-stress

rheometer. The wear debris, especially in biomedical applications, is a critical

component of wear and is often times the initiator of part failure. Debris generated from

this microtribology technique is analyzed and conclusions drawn on how nanodiamond

particles effect the formation and final morphology of the debris.

12

4. MATERIALS AND EXPERIMENTAL METHODS

4.1 Fourier-Transform Infrared Spectroscopy

A Nicolet 4700 (Thermo Electron Corporation, USA) FT-IR spectrophotometer

was used for infra-red spectra analysis. Unless otherwise noted, samples were scanned

using the attenuated total reflectance mode (ATR) with a diamond window.

4.2 Microscopy

Scanning electron microscopy (SEM) was conducted using a Phillips SEM 515 and

a QuantaTM 650 FEG (FEI Company). SEM samples were coated with gold/palladium

before analysis. Transmission electron microscopy (TEM) was performed on a Tecnai 12

(FEI Company). Samples for TEM were cast into epoxy and microtomed and then

placed onto copper grids coated with holey carbon film.

4.3 Thermal Analysis

All differential scanning calorimetry thermograms were obtained using a Q100 (TA

Instruments) modulated DSC. A TA Instruments AR2000 Rheometer with solid sample

clamps was used for dynamic mechanical analysis experiments.

4.4 Nanodiamonds

Nanodiamond powder (98% purity, Stock #1321JGY, CAS #7782-40-3, Lot

#1321-011208) was purchased from Nanostructured & Amorphous Materials

Incorporated. This powder was used throughout the embodied work.

13

4.5 Ultra-high molecular weight polyethylene

GUR 4150B ultrahigh molecular weight powder was kindly provided by the Ticona

Corporation. The powder was used as received. Pre-processed and machined pins were

obtained from Arcom, Bio-met which conformed to ASTM F732 and were gamma

irradiated in Argon at 25-40 kGy. Neat powder morphology was analyzed using a

scanning electron microscopy.

4.6 Ultra-high molecular weight polyethylene/Detonation Nanodiamond Composites

Zhang[22] and co-workers processing method with single-walled nanotubes in

UHMWPE was revisited and modified. The method developed utilized a high-speed

(10,000rpm) bladed mixer. The polyethylene powder and nanodiamonds were placed

into the mixer together and mixed for 3 minutes. The very high shear generated from the

mixer aerosolized the nanodiamonds which then adsorbed to the polyethylene powder.

The final UHMWPE powder morphology was similar to the powder in Zhangs

work[22]. This composite mix was then compression molded by heating to 190C for 20

minutes. The samples were allowed to cool adiabatically under a fume hood.

4.7 Crystallization Kinetics of High-density Polyethylene / Detonation Nanodiamond

Nanocomposites

Loadings of 0.1, 1.0, and 5.0wt% nanodiamonds were compounded with neat HDPE

in a HAAKE PolyLab torque rheometer at 8rpm. The samples were mixed for

approximately 10 minutes at 180C. The mixing time was determined by identifying a

minimum in the energy-versus-time curve provided by the instrument. A control sample

14

of neat HDPE was also mixed under the same conditions to account for any molecular

weight changes during the shearing process. The nanocomposite samples ranged from

light green to dark green in color as a function of increasing nanodiamond content.

A heat/cool/heat program was performed at 10C/min heating and 5C/min

cooling rates on the DSC with a temperature range from 25C to 180C. For isothermal

analysis samples were heated to 145C and held for 5 minutes to remove thermal history

before ramping quickly to the respective undercooling temperatures. Ranges of

undercooling temperatures were selected based on the locations of the crystallization

exotherms obtained from the nonisothermal DSC runs. Chosen temperature ranges

include: 102C-114C (increments of 2C) and 115C-117C (increments of 1C).

4.8 Thermomechanical Properties and Ultraviolet Degradation of Detonation

Nanodiamond/Ultra-high Molecular Weight Polyethylene Nanocomposites

Composite samples containing loadings of 0.1wt%, 1.0wt%, 5.0wt% were

fabricated and cut into rectangular (17x1.5x5mm) bars for dynamic mechanical analysis.

Due to the very low relaxation temperatures exhibited by polyethylene, the system was

cooled with a liquid nitrogen cooling accessory. A 5.0C/minute temperature ramp was

performed in controlled strain mode at 0.1% strain with 1Hz deformation frequency.

Differential scanning calorimetry was performed with a heating rate of 10C/min and a

cooling rate of 5C/min.

Samples were compression molded using the described processing techniques

above into 1.5mm thick plates for UV exposure experiments. Loadings of 0.1wt%,

1.0wt%, and 5.0wt% detonation nanodiamonds were selected for continuity with other

15

tests. Samples were cut, kept as control samples and stored for later microscopy and

infrared spectroscopy. The samples were suspended in an environmental chamber with

circular arrangement approximately 64mm from the ultraviolet source bulbs. The

chamber contained 16 ultraviolet bulbs that produced an intensity of 21000W/cm3 at

253.7nm. Small samples were taken and stored for analysis at 21, 54, 176, 412 hours.

4.9 Tribological Behavior of Ultra-high Molecular Weight Polyethylene/Detonation

Nanodiamond Nanocomposites on Cobalt-Chromium Alloys

Pin-On-Disk Wear Testing

For pin-on-disk tests conducted in the OrthoPOD instrument, cobalt chromium rods

were obtained from Biomet which conformed to the ASTM F1537 standard. The rods

were machined into disks and polished progressively to 0.1 m diamond grit. During the

microtribology experiments, cobalt-chromium hemispheres were manufactured using

stock Stellite 21 rods obtained from Deloro Inc.

Composite blanks were machined for fitment into the OrthoPOD (AMTI, Inc) with a base

diameter of 9.5mm and then a smaller 4.7mm diameter step for the wear surface. The

wear surface was lightly sanded with 1000 grit Al2O3 paper to remove machining marks

and then ultra-sonicated in isopropyl alcohol for 60 minutes to remove any residual

sanding media that might have been entrapped. One control sample was analyzed using

backscattered electron microscopy to confirm no residual Al2O3 was present.

The ASTM F732 standard was used as a guideline for testing parameters, however

no lubricant was used. A load of 60N was used during tested which gave a contact stress

of about 1.6MPa on each CoCr disk. To maximize translation distance on the disk and to

16

avoid extreme polymer chain alignment through unidirectional reciprocation, an arc

pattern was selected as the sliding action. The pins travelled 24.6mm per cycle at 1.5Hz

for a total of 2,000,000 cycles (49.2km).

Microtribology

A TA Instruments AR2000 rheometer was adapted for use as a mini scale tribometer.

Since the instrument contained a very sensitive normal force control mechanism (0.005N,

TA Instruments) and torque transducer (0.01Nm), it was hypothesized that this

instrument would be suitable for monitoring the coefficient of friction accurately over

time with small samples.

Hemispheres were formed from CoCrMo (Stellite 21) stock welding rod alloy

provided by The Deloro Stellite Group (Figure 1). The rods were polished to a mirror

finish to remove any flux or scale on the surfaces. Small sections were cut to roughly

6mm long studs. Any sharp edges were deburred using sand paper so that the studs

would stand up properly. The target microstructure and properties are outlined were

ASTM F75, a common CoCr cast alloy used in biomedical implants. The Stellite 21

contained 27.0% Cr and 5.0% Mo, which matched the requirements for the F75 alloy.

Studs were placed vertically on an Al2O3 plate and placed into an OxyGon Industries

(Epsom, NH) vacuum furnace for melting. Once evacuated to 10-6 torr, the samples

were heated to slightly above their melting point (1400C) and held for 30 seconds. This

allowed for off-gassing which helped to minimized surface porosity. Then the samples

were cooled at a rate of 150C/min to room temperature. The solidified samples retained

a hemispherical shape where diameter, although not quantified for this publication, was

17

an obvious function of rod size and mass. Taller rods produced hemispheres with larger

diameter and lower angles of curvature.

Figure 1: Schematic illustration geometry of starting rod material and transformation into the hemisphere after treatment.

The nanocomposite plates and CoCrMo hemispheres were fixed to stubs and

placed into the rheometer fixtures. Wear track diameter was controlled by the positioning

of the hemisphere on the stub and was maintained at 6mm. A steady-state shear

program was implemented with a linear velocity that was held constant at 8.4m/min. A

contact stress of 19.5MPa was applied to each sample in order for adequate formation of

wear debris. Data sampling occurred at intervals of 30 minutes at which time the normal

force, torque, and displacement was recorded by the instrument. Tests were conducted

for a total of 6km of wear travel.

18

5. CRYSTALLIZATION KINETICS OF HIGH-DENSITY POLYETHYLENE /

DETONATION NANODIAMOND NANOCOMPOSITES

by

JOHN TIPTON, W. JUD DUNLAP, THOMAS J LUCAS, ANDREW UEHLIN, DERRICK DEAN

19

ABSTRACT

Crystallization kinetics of high-density polyethylene (HDPE) and detonation

nanodiamond nanocomposites were investigated. Compounding in a Hakke torque

Rheometer was performed using nanodiamond loadings of 0.1wt%, 1.0wt% and 5.0wt%.

Isothermal and nonisothermal differential scanning calorimetry (DSC) was used to collect

crystallization exotherms at temperatures from 102C to 118C. The data was analyzed

using the well known Avrami and Hoffman models. Results indicated that the

nanodiamonds acted as nucleating agents at higher crystallization temperatures. Hoffman

analysis suggested this behavior in the regime II mode. At lower crystallization

temperatures there was little effect on the crystallization rates from the nanodiamonds

due to the high driving force of self (homogenous) nucleation of the polymer itself. This

region was identified as the regime III mode in Hoffman analysis. No obvious trend was

observed between crystallization rate and nanodiamond loading.

20

INTRODUCTION

Polyethylene is a globally important thermoplastic resin which holds a continually

growing consumption amount of 30 million tons. Heavily industrialized regions account

for almost 44% of this total consumption[1]. This material is easily processed by a wide

array of techniques and offers dynamic properties that are suitable to an almost limitless

range of applications. Polyethylene products occur in every sector of human civilization

including common plastic bags, lids, toys, tools, gasoline containers, and trash cans to

high performance gel-spun fibers used in bullet proof vests. The low melt viscosity

allows for easy formulation of composite materials reinforced with fibers, particles, and

biodegradable materials such as starches and biomass. Continuing to develop low cost,

high performance materials from polyethylene is of high importance. By understanding

the fundamental microstructure formation of these composite materials, it is possible to

continually develop materials based on polyethylene.

A relatively new resurgence of literature exists on nanodiamonds formed from

detonation processes. These highly uniform diamonds (d=4.5-5nm) possess the

extremely high thermal conductivity and hardness of macroscale diamond crystals while

maintaining intriguing nanoscale dimensions and low cost/high yield production

capabilities[2-6]. Use of these nanodiamonds as fillers in composite materials is still a

relatively new idea, but so far studies have shown benefits on properties such as wear

resistance, strength, toughness, stiffness, and enhanced gas barrier properties [3, 5, 7-9].

Semi-crystalline polymers, like polyethylene, fold into structures with long range

order. From the melt, these structures organize into features known as spherulites.

Material stiffness, strength, and use temperatures are mostly impacted by the overall

21

crystallinity of the final structure. Chain segments that do not crystallize usually exist in

interlamellar regions of the spherulites. These amorphous regions provide toughness and

ductility to the polymer by absorbing energy during deformation. The amorphous

regions are highly entangled and can require high degrees of deformation before these

entanglements can be separated.

Traditionally, by slightly stirring dilute solutions of polyethylene in xylene or

dichlorobenzene at approximately 80C, single crystal structures of polyethylene have

been grown mimicking shish-kabobs[10, 11]. A most interesting study by Li and

coworkers in shows these well known row-nucleated polyethylene structures manipulated

in a dilute solution environment[12]. The authors present a carbon nanotube as the

primary fibril portion of the structure with single crystal PE lamellae growing from the

surfaces for the kabob portions. The ability to control materials at this scale demonstrate

the possibility to create practical structures with the potential for extraordinary macro

scale properties and the importance of continuing to study fundamental crystallization

processes.

The model of polymer crystal nucleation and growth presented by Hoffman[13]

provides a description that is widely accepted in the polymer field[14]. This model

presents specific regimes based on the number and proximity of nucleation sites during

crystallization. Growth is either limited by the kinetic factor (regime III) of chain

reptation rate, or the number of nucleation sites (regime II); where the reptation rate is

how the polymer chains are reeled in to the crystal[15]. By using the model provided

by Hoffman, it is simple to connect crystallization kinetics data to physical mechanisms

that occur during crystallization.

22

The Avrami kinetics model is also a common analysis practice in the field of

polymer crystallization. Unlike Hoffman, Avrami does not provide corrections for any

chain reptation rate, or specific polymer diffusion limitations. There is more variability

in the literature regarding treatment of kinetics data from this model, especially relating

to physical interpretation [14, 16-19]. However, data studies using this model are very

prolific in the literature and provides some degree of comparison between other studies

conducted.

The analyses presented in this work describe the effects of nanodiamond fillers on

polyethylene crystallization. Nucleating effects from other fillers are well known in

polymeric matrix composites and have been shown in many reports published on

nanocomposite materials [20-24]. Here, detonation nanodiamonds are compounded with

polyethylene melts to fabricate nanocomposites. Both isothermal and non-isothermal

crystallization kinetics are studied using differential scanning calorimetry (DSC). DSC

measures transitions of polymeric materials by monitoring heat evolved or absorbed and

changes in heat capacity over time or temperature.

MATERIALS

HDPE

High density polyethylene was obtained from Solvay Polymers. Stock #: 60670,

Lot: C010722K02.

23

Nanodiamonds

Nanodiamond powder (98% purity, Stock #1321JGY, CAS #7782-40-3, Lot

#1321-011208) was purchased from Nanostructured & Amorphous Materials

Incorporated. This powder was used without further purification or treatment.

EXPERIMENTAL

Processing

Loadings of 0.1, 1.0, and 5.0wt% of nanodiamonds were compounded with neat

HDPE in a HAAKE PolyLab torque rheometer at 8rpm. The samples were mixed for

approximately 10 minutes at 180C. The mixing time was determined by identifying a

minimum in the energy-versus-time curve provided by the instrument. A control sample

of neat HDPE was also mixed under the same conditions to account for any molecular

weight changes during the shearing process. The nanocomposite samples ranged from

light green to a dark green color with nanodiamond content.

Nonisothermal DSC

Processed samples were analyzed in a Q100 (TA Instruments) differential scanning

calorimeter. A heat/cool/heat program was performed at 10C/min heating and 5C/min

cooling rates. The temperature range was from 25C to 180C.

Isothermal DSC

For isothermal analysis, samples were heated to 145C and held for 5 minutes to

remove thermal history before ramping quickly to the respective undercooling

temperatures. Ranges of undercooling temperatures were selected based on the locations

of the crystallization exotherms obtained from the nonisothermal DSC runs. Chosen

24

temperature ranges were: 102C-114C (increments of 2C) and 115C-117C (increments

of 1C).

RESULTS AND DISCUSSION

Nonisothermal Crystallization and Melting Analysis

The nanocomposite samples exhibited increased crystallization temperatures (Tc-max)

with respect to the neat HDPE (Figure 1, Table 1), which was indicative of the

nanodiamonds acting as nucleating agents. In a neat polymer melt, crystallization can

only occur when a region of low mobility occurs in which other chains can begin to fold.

The presence of the nanodiamond provided stable sites for chain folding, substituting the

typical regions of undercooled melt so crystallization occurred at higher temperatures.

As filler loading increased, no significant change in crystallization temperature was

observed. This was probably due to the saturation of nucleation sites at low loadings and

hence crystallization moved into a regime III mode. In this mode crystallization was

controlled by reptation rate.

Melting temperatures (Tm-max) of the nanocomposite samples were higher than in

the neat samples. This was subtle evidence that the nanoparticles might be integrating

into the crystal. Higher melting temperatures represent the extra needed energy to

destroy the interphase created between the polyethylene chains and the nanodiamond

surface. Interestingly, this melting temperature seemed to be independent of

nanodiamond loading and degree of crystallinity, furthering the idea of a unique

interphase region in this system.

25

Figure 1: Nonisothermal DSC thermogram. Exothermic peaks are up.

Table 1: Data extracted from DSC nonisothermal thermograms of HDPE/ND nanocomposites. * 286.2J/g used for 100% PE crystalline sample[25].

Sample Tm-max [C] Hm [J/g] Tc-max [C] Hc [J/g] Xc [%]* Neat HDPE 127.9 167.1 113.0 185.9 65.0

0.1 wt% ND 129.1 177.7 116.5 186.6 65.2

1.0 wt% ND 129.2 245.0 117.1 248.4 86.8

5.0 wt% ND 129.0 161.8 117.0 153.1 53.5

In the 1.0wt% nanocomposite sample, a high degree of crystallinity was obtained

(86.8%) and suggests a well optimized and fully percolated network for crystallization of

the polyethylene chains. However, in the 5.0wt % sample, crystallinity was shown to

decrease substantially (53.5%) when compared with both neat and other nanocomposite

samples. Several competing factors probably contributed to this result. Although the

-6

-4

-2

0

2

4

Heat

Fl

ow (W

/g)

100 105 110 115 120 125 130 135 140Temperature (C)

Neat HDPE 0.1 wt% ND 1.0 wt% ND 5.0 wt% ND

Exo Up Universal V3.9A TA Instruments

Tc-max

Tm-max

26

higher concentration of particles provided more potential nucleation sites in the 5.0wt%

sample, the melt viscosity of the 5.0wt% composite is much higher. Melt viscosity is

known to increase with fillers at low shear rates [16, 26-28]. Under low-shear conditions

fillers act to decrease chain mobility through various adsorptions and reversible bonding

mechanisms to the particle surface, and hence increase viscosity[16]. This increase in

viscosity impedes chains from efficient folding, resulting in lower crystallinity.

Additionally, the self-agglomeration of the nanodiamonds increases as a function of

concentration which probably decreases the nucleation efficiency of the nanodiamond

sites by increasing the proximity between sites[29]. Conversely, at 1.0wt%, this system

has adequate chain mobility along with sufficient heterogeneous nucleation sites to

facilitate maximum crystallinity.

A potential benefit in the composites that showed little change in overall crystallinity

might be the retention of impact strength. Since an enhancement in crystallinity is related

to embrittlement, applications that currently use HDPE where toughness is needed might

benefit from nanodiamonds fillers. The nanodiamond would provide a multifunctional

reinforcement without sacrificing toughness of the final part [30]. The multifunctional

properties might be achievable at low loadings with minimal modification to processing

techniques and final thermomechanical properties.

Avrami Kinetics

The crystallization rate of high density polyethylene is extremely high compared

to other polymers due to its simple and linear structure. During the experiments,

crystallization took place during the temperature equilibration process of the differential

scanning calorimeter[31], which led to an alteration in the shape of the final scans (Figure

27

2). To correct for the discontinuity in the baselines, integrations were performed using a

sigmoidal curve fit (Figure 3). Previous authors have addressed in more detail the

eccentricities with baseline behavior and describe ways to minimize the effects during the

experimental design and execution[17].

0.27min

-2

0

2

4

6

8

10

Hea

t Flo

w (W

/g)

0.0 0.5 1.0 1.5 2.0Time (min)Exo Up

Figure 2: DSC baseline shape changes observed in highly undercooled polyethylene due to rapid crystallization. Data shown is a representative set of isothermal scans at varying crystallization temperatures obtained for the samples tested.

28

Figure 3: Representative sample sigmoidal baseline correction for peak integration.

Avrami constants were calculated using the Sestak-Berggren (SB) model [18, 31,

32]. This model contains an additional order parameter that allows better fitting

compared to the single exponent of the Avrami model. The SB exponents (m, n) and rate

constant k were obtained using TA Instruments Thermal Specialty Library v2.2 which

uses a curve fitting algorithm similar to the steps described below. The term represents

the fraction converted (crystallinity); here it was considered the rolling integral area

fraction in the isothermal region. By evaluating the general rate equation ( 1 ) differential

at 0=dtd

, the Avrami ( 2 ) and SB equation ( 3 ) can be equated resulting in an

expression for the Avrami constant, nAvrami ( 4 ).

-2

0

2

4

6

8

10

Hea

t Flo

w (W

/g)

0.0 0.5 1.0 1.5 2.0Time (min)Exo Up

29

( ) fKdtd RT

Ea

= exp ( 1 )

( ) ( ) ( )[ ] = nnf 111ln1 ( 2 )

( ) ( )nmf = 1 ( 3 )

( ) ( )[ ]mnnn SBSBAvrami ++= lnln11

( 4 )

Avrami analysis yielded only slightly lower constants in the nanocomposite

samples (nAvrami1.4-1.6) than those obtained from the unfilled HDPE (nAvrami2.0-1.5)

(Figure 4). Typical Avrami constants for HDPE vary significantly in the literature from

nAvrami=1 through nAvrami=3; see for example Trujillo et al[24], and Martinez-Salazar et

al[16]. In general, higher Avrami constants are thought to correspond to less restrained

growth. An Avrami constant of 2 represents disk-like growth and adequately describes a

spherulitic structure seen in semi-crystalline polymers. Since the Avrami constants

changed only slightly in the nanocomposite samples over the neat (min 1.5, max2.0),

the growth geometry was hypothesized to remain relatively constant. However, it was

likely that the nanodiamonds introduced more active nuclei in the nanocomposite

samples and spherulite impingement occurred more quickly. This also meant that the

spherulite density was much higher in the composite samples based increased composite

crystallinity exhibited in the previous non-isothermal data.

30

At very low crystallization temperatures there was a convergence of Avrami

constants indicating similar homogenous crystallization mechanisms between the

composites and neat polyethylene. At lower crystallization temperatures the nucleating

effects of the nanodiamonds were probably diminished due to the strong driving force of

self nucleation of the HDPE. At higher crystallization temperatures, divergence of the

Avrami constants between loaded and neat samples occurred, indicating more

heterogeneous nucleation due to the nanodiamond reinforcements. Haggenmueller and

co-workers describe a similar deviation of Avrami constants in high density

polyethylene/carbon nanotubes composites[21].

There was no obvious evidence in the data that showed Avrami constant

dependence on the weight fraction of nanodiamonds. It might have been the case that the

critical weight fraction needed for the observed change was well below the lowest tested

values (wt %< 0.1)[33].

1.4

1.5

1.6

1.7

1.8

1.9

2

2.1

100 105 110 115 120

Avra

mi C

on

sta

nt

Temperature (C)

Neat Mixed0.1 wt% ND1.0wt% ND5.0wt% ND

Figure 4: Avrami constants as a function of isothermal crystallization temperature.

31

The 5.0wt% sample decreased overall crystallinity compared to the lower

loadings of nanodiamonds (Figure 5). As mentioned in the previous section, the melt

viscosity in polymers increases as filler is added. A higher viscosity limits reptation rate

and hampers the mobility needed to adequately fold into crystals.

Figure 5: Enthalpy evolved during isothermal crystallization as a function of isothermal crystallization temperature.

The isothermal enthalpy data suggested similar overall crystallinity values

between the 0.1wt% and 1.0wt% samples. This was unlike the non-isothermal data

(Figure 1, Table 1) where the 1.0wt% sample exhibited a very high degree of

crystallinity. This result was probably from incomplete crystallization since the

isothermal scans were cooled very quickly to their respect temperatures. Crystallinity

was shown to increase at lower crystallization temperatures where chains would become

immobile enough to rest in a crystal. At the higher temperatures, mobility of the polymer

chains was too high for complete crystallization to occur.

0

10

20

30

40

50

60

020406080

100120140160180

102 105 108 111 114 117 120

% Cr

ysta

llin

ity

Enth

alpy

(J/

g)

Temperature (C)

Neat Mixed0.1wt% ND1.0wt% ND5.0wt% ND

32

The final crystallinity in the nanocomposite samples was much higher than neat

HDPE in the isothermal scans. This suggested that spherulite density was much higher in

the nanocomposite samples[33]. The lower crystallinity observed in the lower

undercooled samples was probably due to regime II crystallization[15] where self

nucleation was relatively low. Nanodiamond particles acted as nucleating agents which

facilitated more complete crystallization whereas the unfilled polymer is dependent on

homogenous nucleation.

The nucleating effects of the nanodiamonds were further analyzed using basic

activation energy analysis. By plotting the natural logarithm of the rate constants (K)

obtained from the isothermal runs versus the reciprocal absolute temperature (T), a linear

plot was obtained. The slope of this line corresponded to the activation energy (Ea). Then

by plotting these activation energies versus their corresponding nanodiamond loading a

threefold decrease in value was noted (Figure 6). There was a vast decrease in the

activation energies between the neat HDPE and nanocomposite samples. It is important

to address the fact that this activation energy is probably not the real activation energy of

crystallization because of the simple Avrami model. This activation energy represents a

composite of the activation energy in each realm of crystallization, both homogenous and

heterogeneous [14]. Still this value does directly correspond to the rate at which the

system will crystallize and might be useful in some normalized form to compare kinetics

relationships between different systems.

33

Without detailed studies of physical morphology we hesitated to make any

significant conclusions on crystal/spherulite geometry using the Avrami constants. It was

however clear that the nanodiamonds provided heterogeneous nucleation sites that caused

faster crystallization at higher crystallization temperatures.

Hoffman Kinetics Analysis

Hoffman analysis is often used to link kinetic data to physical mechanisms during

polymer crystallization. Different regimes relating to the proximity and number of

nucleation sites as well as the idea of kinetically controlled growth are outlined in

Hoffman's papers [13, 34, 35]. One of the kinetic parameters discussed in this theory

relates to the rate at which a chain reels in, or folds and grows onto the substrate and is

known as reptation rate. The first regime is considered near equilibrium crystallization

where growth is limited by the number of nucleation sites and not reptational diffusion.

The third regime is a case in which numerous nucleation sites are available on the

substrate and growth is limited mostly by reptational diffusion (temperature), this regime

30

50

70

90

110

130

150

0 1 2 3 4 5 6

Act

ivat

ion

En

ergy

(kJ

/mo

l)

Nanodiamond Loading (wt%)Figure 6: Apparent activation energy as calculated from multiple isothermal crystallization runs.

34

being the fastest regime of crystallization. The second regime is an intermediate case

between the two extremes [13, 15].

In the Hoffman equation ( 5 ), 2/1

1t

is the reciprocal of the time required for 50%

of the total crystallization to finish. In the original definition by Hoffman, this value is

defined as the growth rate G. However it is known that the reciprocal half crystallization

times (t1/2) can be directly substituted for this growth rate [36]. The values *U and gK are

defined as the activation energy of reputational diffusion, and nucleation constant,

respectively; for polyethylene molJU /284,6* , KTT g 30= , the correction factor

( )TTTf m += 0/2 , undercooling TTT m += 0 , and the equilibrium melting point CTm = 45.141 [14, 25, 36].

( )

=

fTT

KTTR

Utt

gexpexp11

*

02/12/1

( 5 )

The quasi growth rates (2/1

1t

) were obtained using a rolling integral graph generated by

TA Universal Analysis software (Figure 7). Due to the baseline issues stated in the

previous section, it was important to determine the point at which time zero actually

occurred and the remaining time values shifted accordingly. In polymers and

temperatures where crystallization occurs quickly, choosing an inaccurate zero time can

quickly lead to erroneous values of the nucleation constant.

The Hoffman plots showed similar behavior to the Avrami plots in the previous

section. However a better defined transition was observed in this case which enabled

more accurate selection of regime II and regime III locales for linear regression. The

35

nucleation constant gK was calculated for each regime using data points in the

representative boxes in Figure 8. For analysis the results were compiled in Table 2 and

then plotted as a function of nanodiamond loading in Figure 9. Not surprisingly, there

was no evidence of regime I because of the rapid self nucleation of polyethylene at the

crystallization temperatures used.

0 20 40 60 80 1000

25

50

75

100

Rela

tive

Crys

talli

nity

(%

)

Time (s)

108C 102C

Figure 7: Representative rolling integral curves used to obtain quasi-growth rates. Dashed lines indicate time at 50% crystallization.

36

Figure 8: Hoffman plot, right box represents regime II, left box represents regime III.

The Hoffman plot also displayed higher resolution in regime III than the Avrami

analysis. The data however showed no obvious relationship between the nucleation

constants and growth rate (Figure 10). This could be due to the strong self nucleating

behavior experienced in regime III and the low interaction of the nanoparticles in

nucleation. This information might be useful when searching for the presence of unique

polymer/nanodiamond structures formed from the melt. These structures would likely be

found in the regime II area since more participation between the nanodiamonds and

polyethylene was present.

-0.5

0

0.5

1

1.5

2

6.5E-05 7.5E-05 8.5E-05 9.5E-05 1.1E-04 1.2E-04

1/TcTf

ln(1

/t1/2

)+U

*/R

[Tc-

(Tg-

30)]

Neat Mixed0.1wt% ND1.0wt% ND 5.0wt% ND

37

Table 2: Nucleation constants obtained from Hoffman analysis for regimes II and III. Sample IIgK IIIgK

0 wt% DND 6.85x104 2.87 x104

0.1 wt% DND 3.64 x104 3.33 x104

1.0 wt% DND 4.37 x104 3.52 x104

5.0 wt% DND 4.25 x104 2.26 x104

Figure 9: Nucleation constants as a function nanodiamond loading obtained from linear regression of Hoffman plots at varying isothermal crystallization temperatures.

0

1

2

3

4

5

6

7

8

0 1 2 3 4 5 6

Nucl

eatio

n Co

nst

ant (K

g*10

3 )

Loading (wt%)

Regime II

Regime III

38

Figure 10: Growth rate vs. crystallization temperature.

-1.9

-1.8

-1.7

-1.6

-1.5

-1.4

-1.3

-1.2

-1.1

-1

374 376 378 380 382 384 386 388 390 392T (K)

log(

G)

Neat Mixed0.1wt% ND1.0wt% ND 5.0wt% ND

39

CONCLUSIONS

Avrami analysis suggested lower activation energy for the composite samples without

a dependence on loading. There was probably saturation of nucleation sites at the lowest

loading (0.1wt%) used here. The Hoffman model provided a similar conclusion but only

in regime II where nucleation was primarily driven by the nanodiamond particles. In

regime III where self nucleation dominated, the nanodiamond particles showed minimal

influence on the crystallization mechanism. Further morphological studies, including

single crystal growth, conducted in the second regime of crystallization might provide

evidence of interesting structures formed between the nanodiamond particles and the

polyethylene crystals.

40

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42

6. FABRICATION AND MORPHOLOGY OF DETONATION

NANODIAMOND/ULTRA-HIGH MOLECULAR WEIGHT POLYETHYLENE

COMPOSITES

by

JOHN TIPTON AND DERRICK DEAN

43

ABSTRACT

Composites based on ultra-high molecular weight polyethylene (UHMWPE) are

still novel due to the extremely high melt viscosity and difficult processing of this

polymer. Methods to disperse nanofillers into this material need to be developed that are

practical and scalable to commercial production before widespread use of high-

performance nanocomposites can be realized. In this work, nanocomposites of

UHMWPE and detonation nanodiamonds were fabricated using a mechanical mixing

method without the use of solvent. The mixing produced UHMWPE powder with

nanodiamond coatings that could be compression molded. Morphology and dispersion of

the composites was investigated using transmission electron microscopy and scanning

electron microscopy. Some regions showed aggregations of nanodiamonds with

diameters of up to 200nm. It was also shown that there were regions in which

nanodiamonds appeared to be well dispersed and un-aggregated.

44

INTRODUCTION

Ultrahigh molecular weight polyethylene (UHMWPE) is widely used in many

applications due to its superior wear resistance, strength, toughness, and chemical

resistance. This polymer is unique from its other polyethylene relatives due to extremely

long chains of repeating ethylene units with molecular weights over one-million. These

chains form vast extended entangled networks in amorphous interlamellar regions which

provide remarkable toughness and strain to failure over other classes of polyethylene[1].

However, the same mechanisms that provide excellent properties make processing

difficult. High melt viscosity rules out traditional melt processing techniques such as

injection molding, extrusion, and etc. Typically raw UHMWPE is in powder form and so

the final microstructure of the bulk part is more similar to that of a sintered ceramic or

metal containing grain-boundary like interfaces from particle coalescence.

Nanocomposite materials based on a UHMWPE matrix are still novel due to the

difficulty in adequately dispersing reinforcements in such a viscous material. Because of

the very large surface area to volume ratio, and hence high surface energy, nanoscale

materials exhibit a tendency towards forming bundles/aggregates. Nanoparticles are

shown across the literature to be difficult to disperse into any medium, including

thermoplastic and thermoset polymers [2-8]. In order for adequate interface formation in

a composite, good dispersion is critical to avoid slippage and bundle deformation

between the nanoparticles[9, 10]. The additional effects from the extreme viscosity of

UHMWPE make dispersion even more difficult in this particular system[6, 11]. More

processing methods need to be developed to adequately disperse reinforcements into

UHMWPE powders.

45

Zhang reports a solvent based approach to physically adsorb nanoparticles to the

surface of UHMWPE powder[6]. These authors pre-coated the UHMWPE powder with

single walled nanotubes by first suspending the nanotubes in a solvent and then mixing

the suspension with UHMWPE powder. When the solvent evaporated, the nanotubes

remained on the outside of the powder simply from Van-der Waals interactions. They

showed that when these particles were compression molded, the high pressure and shear

further dispersed the nanotubes into the UHMWPE matrix.

Here, a mechanical mixing method was tested to create neat UHMWPE powder

coated with detonation nanodiamonds. This composite powder is compression molded

into samples and the resulting morphology and dispersion is investigated.

MATERIALS

Ultrahigh molecular weight polyethylene

GUR 4150B ultrahigh molecular weight powder was provided by the Ticona

Corporation. The powder was used as received.

Detonation Nanodiamonds

Nanodiamond powder (98% purity, Stock #1321JGY, CAS #7782-40-3, Lot

#1321-011208) was purchased from Nanostructured & Amorphous Materials

Incorporated.

EXPERIMENTAL

Nanodiamond Morphology

The nanodiamonds were placed into a 9:1 sulfuric to nitric acid solution for 3 days at

70C to remove amorphous carbon. The slurry was then filtered through a PTFE

46

membrane after wetting the membrane with methanol to increase hydrophilicity. Infra-

red spectroscopy (FTIR) was used on neat and modified samples pressed into potassium

bromide pellets to identify the presence of a carbonyl functional group. The diamonds

were sonicated in acetone for 30 minutes and then dropped onto a holey carbon micro

grid and imaged in an FEI Tecnai 12 transmission electron microscope (TEM).

Nanocomposite Processing

Neat UHMWPE powder was mixed with 0.1,1.0, and 5.0wt% nanodiamonds and

mixed for 3 minutes in a high speed (>10,000rpm) bladed mixer. The powder was then

compression molded at 190C and left to cool at an ambient rate. The composites were

cast into epoxy resin and sectioned for transmission electron microscope.

RESULTS AND DISCUSSION

Nanodiamond Morphology

The as-received nanodiamonds were cloudy in appearance when viewed under TEM

(Figure 1A). It was assumed that this was due to amorphous carbon and other impurities

on the surface. The diamonds were oxidized with an acid treatment and the resulting