“complete” system monitoring of polymer matrix composites

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“Complete” system monitoring of polymer matrix composites

F.J. Johnsona, W.M. Crossa, D.A. Boylesb, J.J. Kellara,*

a Department of Materials and Metallurgical Engineering, South Dakota School of Mines and Technology, Rapid City, SD 57701-3995, USAb Department of Chemistry and Chemical Engineering, South Dakota School of Mines and Technology, Rapid City, SD 57701-3995, USA

Received 20 July 1999; received in revised form 6 February 2000; accepted 23 February 2000

Abstract

In situ, real time analysis of the chemical interactions occurring within the interphase of fiberglass fiber-reinforced polymer matrix

composites (PMCs) has been accomplished through utilization of fiber optic sensors. The fiber size, as well as the composition of the

thin cladding, closely approximates industrial fiberglass fibers, which allows the fibers to function simultaneously as model fiber-reinforce-

ments and evanescent wave sensors. Using this sensor technology, the current research has shown that reactions occurring between the fiber

surface, adsorbed silane coupling agent, and polymer can be monitored using FT-NIR spectroscopy. Two PMC systems have been chosen to

demonstrate the ability to monitor complex interphase chemistry in situ and in real time. 2000 Elsevier Science Ltd. All rights reserved.

Keyword : Interphase

1. Introduction

The region adjacent fibrous reinforcements in polymer

matrix composites (PMCs), referred to as the interphase,

is thought to have properties unlike those of either the

matrix or the reinforcement [1]. Therefore, a need existsfor an improved understanding of the chemical interactions

and mechanical properties of this region. With regard to the

former, molecular-level monitoring of PMC interfacial

reactions involving fibers, adsorbed species (such as silane

coupling agents), and a polymer matrix (monomer and

curing agent), has recently been accomplished in our labora-

tories [2]. Previously, such analyses were considered nearly

impossible for a number of reasons. First, few analytical

methods are available that allow in situ measurements in

such a relatively small, highly localized region. Evidencing

the small scales involved, recent atomic force microscopy

measurements have shown that for a carbon fiber-reinforced

polyphenylene sulfide system, the interphase region is onthe order of 20–80 nm [3]. Similarly, recent nano-indenta-

tion and atomic force microscopy (phase imaging) results

from our laboratories (see Fig. 1) suggest the existence of a

modulus gradient throughout the interphase region adjacent

glass fibers in an epoxy matrix; this gradient is less than 1-

m in thickness [4]. Second, given the relatively low

concentration of the adsorbed coupling agent (usually sub-

monolayer to several monolayers of adsorbed material) in

relation to the monomer and curing agent concentrations,

discriminating between distinct signals from each constitu-

ent is difficult. For this reason, most analytical methods do

not have adequate sensitivity for the detection of coupling

agent reactivity concurrently with monomer/curing agent

reactivity.Infrared evanescent wave spectroscopy, alternately called

attenuated total reflectance spectroscopy, has proven

capable of monitoring various aspects of interphase chem-

istry. For example, numerous researchers have employed

this method to monitor monomer and/or curing agent reac-

tivity, utilizing a high refractive index crystal or fiber to

allow propagation of the evanescent wave into the polymer

phase [5–10]. Use of a high refractive index substrate is

necessary because the evanescent wave sampling method

requires the refractive index of the sample (in this case

the polymer) be less than that of the material in which

internal reflection occurs (crystal or fiber). For example,

Connell et al. [10] used fused silica glass fibers as evanes-cent wave sensors to monitor the organofunctional reactiv-

ity of an amino-silane with a partially fluorinated epoxy

monomer. The fluorination of the epoxy lowered the mono-

mer refractive index, allowing the use of the relatively low

refractive index fused quartz fiber as an evanescent wave

sensor [10].

Such an approach works well for analysis of monomer

and/or curing agent reactivity. However, monitoring of

adsorbed coupling agent reactivity concurrently with

monomer/curing agent reactivity via evanescent wave

analysis is problematic. Specifically, the high refractive

Composites: Part A 31 (2000) 959–968

1359-835X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved.

PII: S1359-835X(00) 00044-0

www.elsevier.com/locate/compositesa

* Corresponding author. Tel.: 1-605-394-2341; fax: 1-605-394-

3369.



index materials commonly employed (high lead content

oxides, sapphire, chalcogenide or selenide glasses) do not

mimic the substrate chemistry of oxide glasses used indust-rially. The composition of the fiber is relevant because

previous work indicates that substrate effects can signifi-

cantly influence composite properties [11,12]. For example,

previous work suggests that quartz exhibits much different

bonding characteristics when exposed to a silane coupling

agent than either glass or alumina [11]. Furthermore,

because sapphire, chalcogenide or selenide fibers (tradi-

tional high refractive index fibers used for evanescent

wave analysis) are not used as reinforcements for polymer

matrix composites, their utilization, from a substrate-speci-

fic surface chemistry perspective, can be considered irrele-

vant. Substrate-specific interactions are important whenconsidering the mechanisms associated with coupling

agents; i.e. silane coupling agents can be represented by

the general structure X3SiRY, where X is a readily hydro-

lyzable functional group, and Y an organofunctional group

[1,13]. These compounds, when applied as surface treat-

ments to the reinforcement, have been widely recognized

to improve overall strength and modulus, as well as hygro-

thermal stability of glass fiber-reinforced polymer matrix

composite.

With regard to silane coupling agent chemistry, the

earliest molecular rationale for coupling agent performance

was predicated on several seemingly logical assumptions,

including:

1. a chemical reaction of the organofunctional group, Y,

with reactive organic functionality of the resin (e.g. reac-

tion of the amino moiety of aminofunctional silanes with

the oxirane ring of epoxy resins);

2. condensation/oligerimerization between adjacent adsorbed

silanol groups; and

3. the chemisorption of the coupling agent (through conden-

sation of silanol groups with surface hydroxyl groups) to

the glass surface.

Numerous studies with organofunctional silanes have

demonstrated that assumptions (2) and (3) are correct

[12–15]. However, direct in situ evidence of organofunc-

tional reactivity, as specified in (1) above, has been elusivedue to the sampling problems delineated previously.

Assumption (1) serves as the basis for the chemical bonding

theory, as described by Plueddemann [13] and Ishida [16].

Plueddemann [13] suggests that the X groups of the silane

are hydrolyzed in water, after which the –OH groups react

with the hydrated glass surface, forming a chemisorbed

layer of silane via Si–O–Si bonds. Coupled with the reac-

tion in (1), the silane can be said to have formed a chemical

bridge between the matrix and the reinforcement. In addi-

tion, Ishida [16] describes the existence of physically

adsorbed (physisorbed) silane molecules in the outermost

layers of the silane interphase. These molecules are heldin place through weak van der Waals forces, as well as

through hydrogen bonding to adjacent –OH groups on the

silanols [16]. An analysis of the chemical reactions occur-

ring at the interphase is necessary to determine the efficacy

of these theories. Such an analysis should ultimately lead to

improved understanding and/or modification of the beha-

viors and properties of glass fiber-reinforced PMCs.

Ideally, to monitor the interphase chemistry of an oxide

fiberglass PMC via evanescent wave spectroscopy, an inter-

nal reflection element (IRE) possessing surface chemical

properties closely approximating those of fiberglass should

be employed. However, most silicon-based oxide glasses

have refractive indices in the range 1.45–1.51 [17], whichare lower than those of most polymers; therefore, evanes-

cent wave sensing of polymer chemistry through such glass

IREs cannot occur. Thus, monitoring “complete” interphase

chemistry (coupling agent/monomer/curing agent) by utili-

zation of common oxide glasses and evanescent wave spec-

troscopy would seem to be impossible.

In fact, due to sensitivity problems associated with

adsorbed silane coupling agent monolayers, investigations

of their reactivity have been limited. A noteworthy excep-

tion is the work of Nishio et al. [14], which involved the

chemisorption and oligimerization of the silane headgroup

F.J. Johnson et al. / Composites: Part A 31 (2000) 959–968 960

Fig. 1. Image of a DJ fiber with nano-indentations in the interphase region.



to a soda-lime glass microscope slide. The slide served as

both substrate and internal reflection element; however,

only glass/silane reactivity was monitored (items 2 and 3

given earlier), i.e. no monitoring of polymer interphasechemistry was reported. Presumably, interphase analysis

(with the polymer present) was not performed in this

study, due to the aforementioned refractive index problem.

Another example of interphase analysis is the work of

Johnson et al. [2], who used thin-clad optical fibers to

perform a qualitative analysis of an adsorbed silane

coupling agent/epoxy-amine polymer system. The fibers,

obtained from Dolan-Jenner, Inc. (DJ), consisted of 25-

m diameter flint-glass core with an 1-m thick soda-

lime cladding. Thus, the diameter of the DJ fibers was

close to that of industrial fiberglass fibers. In addition,

these fibers were advantageous as internal reflection

elements because the high refractive index flint glass coren 162 allowed evanescent wave sensing, while the

soda-lime cladding (which serves as the substrate) was

thin enough to allow extension of the evanescent wave

into the polymer phase [18]. Furthermore, the composition

of the cladding was 74% SiO2 (with other oxides), which is

similar to industrial E-glass fibers commonly used in fiber-

glass reinforced PMCs. The fiber sensors were used in both

a silane/epoxy/amine system and silane/epoxy/aniline

system. This work illustrated the utility of the DJ fibers to

allow simultaneous monitoring of the silane and epoxy/

hardener reactivity, a feat of which most analytical methods

have not proven capable. However, in each epoxy/hardener

system examined, the primary NIR amino band of the hard-

ener either masked or overlapped that of the adsorbed silane

[2].Therefore, research efforts herein were directed toward

development of a system in which the relevant NIR amino

bands of the adsorbed silane and the hardener, as well as a

band attributable to the epoxy, could be monitored simulta-

neously without any overlap or masking of the individual

bands. In addition, a polyester polymer/silane system was

examined, to illustrate the ability of the fiber sensors to

monitor multiple PMC systems in a complete manner.

2. Experimental

The DJ fibers were tested as evanescent wave sensorsthrough a number of experiments, which were performed

on bundles of approximately 250–350 fibers. Each experi-

ment was monitored in the near-infrared (NIR) using a Bio-

rad Digilab FTS-40A Fourier transform infrared (FT-IR)

spectrometer. Complete experimental details can be found

elsewhere [2].

Initially, optimum fiber bundle thickness and length were

established to maximize sensitivity. Bundles of DJ fibers

from 10 to 30 in. in length and 100–450 fibers (per

bundle) were examined. A back-ground single-beam spec-

trum (1024 scans) was collected, after which the bundles


Fig. 2. General chemical structures of complete composite system components.



were placed in aqueous 5 wt% -aminopropyltrimethoxysi-

lane (-APS) solution for 1 h, dried at 75C for 1 h, and

immersed in Epon 828 (Shell Chemical Co.). The variables

examined in the optimization process included length of the

transparent region (i.e. the wavelength region over which

the NIR signal is not absorbed by the fibers) and noise,

especially near the wavelength region where the fiber

bundles are no longer transparent.

Previous X-ray photoelectron spectroscopic (XPS)analyses indicated significant contamination on the surface

of the as-received fibers [19]. Therefore, prior to complete

composite monitoring, the fiber bundles were first cleaned

for approximately 10–15 min in NoChromix, a sulfuric

acid-based cleaning solution, to remove contaminants. The

bundles were then rinsed several times with distilled water.

The clean fibers were dried for 1 h at 75C, then cooled to

room temperature over approximately 1 h.

The curing agent chosen for the Epon 828 epoxy system

was methyl-5-norbornene-2,3-dicarboxylic anhydride (NMA,

Aldrich Chemical Company, Inc.), with 0.5% imidazole

initiator (Avocado Research Chemicals, Ltd.). The secondpolymer matrix was a polyester monomer/styrene mixture

with a methyl ethyl ketone peroxide (MEKP) initiator

(Dynatron/Bondo corp.). The coupling agent used in both

systems was -APS; these systems are both of industrial

relevance [20]. The formulae of the constituents of these

systems are shown in Fig. 2. The concentration of -APS

in aqueous solution treatment of the DJ fibers was either 1 or

2 wt%.

Once the fiber optimization process (number/length) was

completed, clean fiber bundles were used to collect spectra

of the individual neat constituents on the fibers. A back-

ground spectrum of each clean bundle was collected (reso-

lution 16 cm1, 1024 scans). The spectra were collected by

ratioing 512 co-added scans against the background spec-

trum. The fibers were immersed in the desired constituent

and a spectrum collected at room temperature. A spectrum

of powdered imidazole was obtained through diffuse reflec-

tance FT-IR (DRIFT) spectroscopy. The remaining bundles

were treated with silane and then placed in either the epoxy

or the polyester system and subjected to cure monitoring.

The fiber bundles used with the epoxy system were

immersed in an aqueous 1-wt% -APS solution at room

temperature for 1 h, dried at approximately 115C for 1 h,

and cooled to 93C. A spectrum of the adsorbed silane on

the dried fibers was then collected. The treated fibers were

placed in a 100:90 Epon 828/NMA mixture, with 0.5%

imidazole added as an initiator. The mixture was contained

in an insulated aluminum curing cell, and maintained at

93C using a Harrick Scientific temperature controller.

After 1 h, the temperature was increased over a period of

approximately 10 min to 115C. The spectra were then

collected every 20 min for 2 h. After approximately 17 h,

a final spectrum of the cured composite was collected at

115C.

The fiber bundles used with the polyester system were

treated with an aqueous 2-wt% -APS solution for 1 h, dried

at approximately 115C for 1 h, and cooled to room

temperature over 1–2 h. A spectrum of the adsorbed silane

on the dried fibers was collected, after which the bundles

were placed in 3 oz of a polyester monomer/styrene

mixture with 2 ml MEKP initiator added, and cured at

room temperature. The spectra were collected every

10 min over the first two hours; the polymer was cured for

approximately 17 h, after which a final spectrum wascollected.

3. Results and discussion

With respect to transparent region versus fiber bundle

length, lengths approximating 30 in. were found to provide

a shorter region of transparency than those of shorter

lengths, as the IR light was lost through the length of the

fibers. Of particular importance is the loss of signal around

the lower energy end of the spectrum; i.e. signal loss

occurred prior to 4600 cm

1

. Since the desired epoxyband for cure monitoring, given in Table 1, is at

4530 cm1, lack of a signal in this region makes monitor-

ing of the band nearly impossible. Conversely, those

bundles with lengths approximating 10 in. provided suffi-

cient signal for monitoring of all desired bands. When

comparing signal intensity versus fiber bundle length,

longer bundles were found to have lower signal intensities

than shorter bundles. However, the high signal intensity

around 4530 cm1 in 10-in. bundles resulted in large absor-

bance values of the epoxy band; this high absorbance often

tended to dwarf the relevant silane band occurring at

4930 cm1. Based on these observations, the optimum

fiber bundle length was determined to be 24 in.Sensing region length was not found to have a relation-

ship to bundle thickness. However, the signal-to-noise ratio

tended to increase with thickness. Specifically, bundles of

100 fibers or less had low signal intensities, so that moni-

toring changes associated with bands related to curing reac-

tions was difficult. Bundles containing 450 fibers had

high signal intensities, but once again the resulting absor-

bance of the relevant epoxy band tended to dwarf that of the

relevant -APS band. Therefore, bundles of 300–350 fibers

were chosen as the optimum quantity.

Table 1 lists the relevant bands for each constituent in the


Table 1

Relevant NIR bands for PMC cure monitoring

Constitutent Relevant bands

-Aminopropyltrimethoxysilane (-APS) 4930 cm1

Epon 828 4530 cm1

Methyl-5-norbornene-2,3-dicarboxylic

anhydride (NMA)

4830 cm1

Imidazole None present

Polyester monomer/styrene mixture 4493, 4721, 6137 cm1

Methyl ethyl ketone peroxide (MEKP) 4783 cm1



PMC systems analyzed. A spectrum of adsorbed -APS

(1 wt% solution) on the DJ fibers is shown in Fig. 3. The

band at 4930 cm1 can be attributed to the –NH2 bend-

ing–stretching combination of the -APS [14]. Individual

spectra of Epon 828, NMA, imidazole, polyester resin, and

MEKP initiator are shown in Figs. 4–8.

In Fig. 4 (Epon 828), the band at 4530 cm1 can be

attributed to the –CH2 bending–stretching combination of

the epoxide ring; this band should decrease with curing of

the polymer [21–23]. The band at 4830 cm1 in Fig. 5

(NMA) is used for identification of the presence of the

curing agent. In Fig. 7, the bands at 4493 and 4721 cm 1


Fig. 3. 1 wt% gamma-APS absorbed on DJ fibers.

Fig. 4. Spectrum of Epon 828.



are assigned to HCyCH bending–stretching combination of

the polyester monomer, and the band at 6137 cm1 is the

first overtone of the C–H stretching of the yCH2 of the

styrene [24]. Because the reaction of styrene with the poly-

ester results in the elimination of the double bonds [25],

these bands were used to monitor the reactivity of the

mixture. The band at 4783 cm1 in Fig. 8 is used to monitor

the reactivity of the MEKP initiator.

Figs. 9 and 10 are spectra of the complete epoxy system at

93C (initial) and 115C (final), respectively. In Fig. 9, the

bands at 4930 cm1 (-APS), 4830 cm1 (NMA), and

4530 cm1 (Epon 828) are clearly visible; i.e. none of the


Fig. 5. Spectrum of methyl-5-norbornene-2,3-dicarboxylic anhydride.

Fig. 6. DRIFT spectrum of imidazole initiator.



bands are overlapping or masking the others. The bands at

4530 and 4830 cm1 are seen to decrease considerably after

curing. This decrease is assumed to be due to the reaction of

the epoxy and curing agent. The band at 4930 cm1 does not

observably decrease, suggesting little reaction has occurred

between the -APS and the polymer constituents. Regarding

the spectrum of the imidazole initiator in Fig. 6, no bands

are visible in the regions where the aforementioned consti-

tuent bands are present; therefore, the initiator should not

interfere with monitoring of the epoxy PMC system.


Fig. 7. NIR spectrum of polyester monomer/styrene mixture.

Fig. 8. NIR spectrum of MEKP initiator.



Figs. 11 and 12 are spectra of the complete polyester

system before and after curing, respectively. Again, Fig.

11 clearly shows the amino band of the -APS at

4930 cm1. In addition, the bands at 4493, 4721, and

6137 cm1, all of which can be attributed to the polyester

monomer mixture, are also present. The band at 4783 cm1,

which identifies the presence of the MEKP initiator, is

small, due to the small quantity used. In Fig. 12, this

band, as well as the relevant polyester bands, has decreased

significantly. As with the epoxy system, the relevant -APS

band is still present, suggesting a lack of silane organofunc-

tional reactivity with the polymer.


Fig. 9. Spectrum of complete epoxy system before curing.

Fig. 10. Spectrum of complete epoxy system after curing.



As discussed previously, in situ analysis of a complete

composite system has hitherto not been reported. The resul-

tant ability to differentiate the individual components is

significant because the reactivity of these constituents can

now be monitored using NIR evanescent wave spectro-

scopy, both in situ and in real time. Ultimately, using this

approach, a more complete understanding of the chemical

reactivity at the interphase can therefore be established, and,

more specifically, the degree of organofunctional silane

reactivity determined. Furthermore, this approach will


Fig. 11. Spectrum of complete polyester system before curing.

Fig. 12. Spectrum of complete polyester system after curing.



allow the role of silane treatments on bulk polymer curing to

be ascertained.

4. Conclusions

Previous work in our laboratories has resulted in the

development of a system that utilizes a thin-clad opticalfiber bundle as an FT-NIR evanescent wave sensor. The

system approximates those of industrial fiberglass-rein-

forced composites. Study of the reactivity of coupling

agent, polymeric monomer, and hardener requires that

each constituent have an NIR reactive band separate from

the reactive bands of the other system components. There-

fore, a system was established whereby each constituent was

discernable. Specifically, the hardener, methyl-5-norbor-

nene-2,3-dicarboxylic anhydride, was chosen to combine

with the bisphenol-A epoxy and aminosilane coupling

agent. Subsequent FT-NIR evanescent wave examination

showed each constituent was separate and discernable

from the others. In addition, another complete polymer

system consisting of an aminosilane coupling agent, polye-

ster monomer mixture, and MEKP initiator, was monitored,

to illustrate the ability of the fiber sensors to be used in a

variety of PMC systems.

The significance of these results rests in the ability to

monitor the reactivity of complete composite systems,

both in situ and in real time. This system is the first to not

only approximate that of a real composite (fiberglass-rein-

forced polymer), but to allow for analysis of the reactivity of

each constituent during the curing process; this is an impor-

tant first step in the development of an understanding of the

chemical reactivity within the interphase region, which isimportant industrially for engineered modification of rein-

forcements which maximize the desired properties at the

interphase within a composite system. Future work will

entail system monitoring at aqueous silane treatments of

less than 1 wt% (such as 0.5 wt%), which would more

closely approximate concentrations employed industrially.

In addition, an investigation of organofunctional reactivity

will be conducted to determine potential silane reactivity.

Acknowledgements

Support for this work was provided by the NationalScience Foundation (NSF) under Grants # CMS-9453467

and CMS-9900383. The authors would also like to thank Mr

Travis Downing for providing the Hysitron image.

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