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Polymer 53 (2012) 1138e1147

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Polymer

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Microstructure development of thermoplastic polyurethanes under compression:The influence from first-order structure to aggregation structure and a structuraloptimization

Cong Li a,*, Jingjie Han b, Qishian Huang a, Hongxing Xu a, Jian Tao a, Xiuhong Li c

aNational Polyurethanes Engineering Research Center, Yantai Wanhua Polyurethanes Company, Shandong Province 264000, ChinabKey Laboratory of Pressure System and Safety of Ministry of Education, East China University of Science and Technology, Shanghai 200237, Chinac Shanghai Synchrotron Radiation Facility, Shanghai 200100, China

a r t i c l e i n f o

Article history:Received 8 September 2011Received in revised form6 January 2012Accepted 14 January 2012Available online 20 January 2012

Keywords:Thermoplastic polyurethanesMicrostructure developmentCompression

* Corresponding author.E-mail address: li.cong80@yahoo.com (C. Li).

0032-3861/$ e see front matter Crown Copyright �doi:10.1016/j.polymer.2012.01.019

a b s t r a c t

The microstructure development of 4,40-diphenylmethane diisocyanate (MDI) and 1,4-butanediol (BDO)based thermoplastic polyurethane (TPU) under compression was investigated. Influential factors on thepermanent compression deformation were discussed in detail. Two types of samples with the samechemical compositions while different aggregation structures were prepared by altering the stirringspeed during the synthesis process. The difference in the first-order structure of these samples wasstudied by 13C NMR. The aggregation structures and the corresponding development under compressionwere characterized by DSC, WAXD and SAXS, respectively. These observed results indicated that differ-ence of microstructures lead to different deformation behaviors, and the disruption of hard segmentdomains or reorganized structures by less ordered hard segments played significant roles in thepermanent deformation under compression. The viscoelastic behaviors of these samples were describedby DMA and simulated by Rouse model. The derived terminal viscosity and relaxation times were used toexplain the different permanent deformations of these samples. Finally, an optimized micro-crosslinkstructure was introduced in TPUs, and a better deformation resistance property was obtained.

Crown Copyright � 2012 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Thermoplastic polyurethanes (TPUs) are linear segmentedcopolymers, composed of micro-phase separated hard and softsegments. The hard segments are held together by hydrogen bonds,which form physical crosslinks. This microstructure enables thematerial to be processed like other thermoplastics. Moreover, theextensive diversification of possible chemical constituents endowsthese materials with a wide variety of mechanical properties. Withthe rapid development of industry, there is an increasing demandof seals in heavy-duty field. Traditional seals are usually made byrubber. However, due to the comprehensive performance, espe-cially the good wear resistance and proper processing conditions,rubber seals are substituted by TPU in more and more seal field.

As a seal part, compression set is an important criterion con-cerned by practical application. A low compression set means gooddeformation resistance for a material. When the polymer is

2012 Published by Elsevier Ltd. All

compressed and maintained at a fixed deformation size, a stressrelaxation occurs. Once the compression is released, a permanentdeformation is formed. As a vulcanized rubber is concerned,permanent deformation is usually caused by scission of thenetwork at chemical crosslinks or some chain points [1e4].However, the crosslinks in thermoplastic elastomer system areusually made up by associated hard blocks, which are normallystabilized by hydrogen bonding or van der Waals interaction forces[5,6]. Under an applied load, these physical crosslinks are relativelyeasy to be disrupted. So the deformation mechanism of thermo-plastic elastomer is more complex. In the last decades, there aremany studies on the deformation mechanisms of thermoplastics,such as SBS, SIS [7e10]. These materials possess a regularmorphology containing alternating hard and soft domains, whichhave been conformed by AFM, SAXS, TEM [7e12]. TPU possessesa more complex microstructure. The hard domains and softdomains are randomly mixed and multi-phase separation exists[13e15]. So the investigation of the microstructure developmentunder an applied load is complicated. Previous works on micro-structure development for thermoplastic polyurethanes havemostly focused on the extension behavior [16e19]. However,

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C. Li et al. / Polymer 53 (2012) 1138e1147 1139

comparatively little study was made on the compression defor-mation behavior. As mentioned above, lower compression setmeans a better deformation resistance during use. Thereforeresearch on influential factors of compression deformation of TPUhas an important practical meaning as well as theoretical meaning.

The purpose of this work is to investigate the microstructuredevelopment of TPUs under compression, and reveal the influentialfactors on permanent compression deformation. Two types ofsamples with the same chemical compositions while differentmorphologies were prepared by altering the stirring speed duringsynthesis. The compression set was tested by ASTM-D395 method[20] to compare the deformation resistance abilities. Difference inlength distribution of hard segments of these two samples wasanalyzed by 13C NMR. The change of endothermal response in hardsegment domains before and after compression was illustrated byDSC. WAXD based on Hosemann method [21] testified the crys-tallite size change and the distortions of crystallite domains aftercompression. Correspondingly, the development of aggregationstructure was revealed by SAXS. At last the viscoelastic behaviors ofthese samples were characterized by DMA. The master curves ob-tained with time-temperature superposition were simulated byRouse model [22]. The terminal viscosity and relaxation timesderived from Rouse model revealed the causes of differentpermanent deformations of these two samples from viscoelasticpoint of view. Based on this, an optimizedmicro-crosslink structurewas introduced in TPUs, which has higher terminal viscosity andlower permanent compression deformation.

2. Experimental

2.1. Materials

Diphenylmethane diisocyanate (MDI)-MDI100 was obtainedfrom Yantai Wanhua Polyurethanes CO., LTD and butanediol (BDO)was obtained from BASF. Trimethylol propane (TMP) was suppliedby Perstorp Corporation. Poly (caprolactone) based polyol (CMA-44) was kindly provided by Yantai Huada Chemical Industry Co.,Ltd, corresponding to the number-average molecule weight (Mn) of2000 g/mol. MDI was stored in a freezer to minimize dimmerformation. It approached 50 �C under dry conditions before use. Allother materials (BDO, TMP, POLYOL) were dried at 100 �C before usein a heated vacuum oven.

Fig. 1. Device for compression set test under constant deflection. Notes: 1 specimen, 2top steel plate, 3 spacer, 4 middle steel plate, 5 bottom steel plate, 6 bolt.

2.2. Sample preparation

The samples were prepared by a one step polymerization witha 45% HSC (hard segment content). Stoichiometry was followedwith a 2% molal excess equivalence of MDI to ensure completereaction (Considering the influence of water existing in polyol andchain extender). A mixture of polyol, chain extenders (BDO or BDOand TMP) and MDI was added at 100 �C and stirred until theviscosity and temperature started to increase rapidly. The resultingmass was then transferred to a plate mold and cured for 10 h at100 �C. Then the cured materials were grinded to pieces bya pulverizer. These pieces were injected to a slab with a thicknessabout 6 mm and cured for 10 h at 80 �C. Finally, the slab was cut toa 12 mm � 12 mm � (6e7 mm) cylindrical disk. These disks wereused as the standard specimens for the compression test. Duringthe mixing synthesis process, two distinct stirring speeds about2500 rpm and 300 rpmwere used to obtainmaterials with differentaggregation structures. Accordingly, test samples prepared by2500 rpm and 300 rpmwere designated toTPU-2500 and TPU-300,respectively. Based on the synthesis process of TPU-300, an addi-tional 1 phr TMP added system was designated to TPU-300-TMP.

2.3. Test methods

The specimen was compressed to a 4.5 mm thickness bya compression device in accordance with ASTM-D395-03B [20].The compression device consists of three flat steel plates betweenthe parallel faces of which the specimens were compressed asshown in Fig. 1. Steel spacers for the required percentage ofcompression were placed on each side of the specimens to controltheir thickness. Before testing, the original thickness of the spec-imen was measured to the nearest 0.02 mm. Then place the testspecimen between the plates of the compression device with thespacers on each side, tighten the bolts so that the plates weredrawn together uniformly until they were in contact with thespacers. The specimens were compressed at 25 �C and 70 �C for22 h, respectively. After the 22 h compression, the specimen wasremoved from compression device and rest at ambient conditionsfor 30 min. The final thickness was then measured and thepermanent compression deformation value was calculated as

C ¼ ½ðH0 � H1Þ=ðH0 � HnÞ� � 100 ð%Þ:Where H0 is the original height of the specimen, H1 is the finalheight of the specimen and Hn is the height of the spacer. So thespecimen after a compression at 25 �C for 22 h was designated asTPU-2500-25C (2500 means a 2500 rpm stirring speed during thesynthesis process) or TPU-300-25C (300 means a 300 rpm stirringspeed during the synthesis process). Similarly, the TPU-2500-70Cand TPU-300-70C were designated following the same principle.The specimens without compression were designated as TPU-2500and TPU-300, respectively.

2.4. Characterization methods

2.4.1. Nuclear magnetic resonance spectroscopy (NMR)13C NMR spectra were recorded on a Bruker AMX-500 spec-

trometer at 50 �C and a proton frequency of 500 MHz. TPU-300 andTPU-2500 (25 mg) were dissolved in 1 mL of dimethylformamide(DMF) and chemical shifts were internally referenced to tetrame-thylsilane (TMS). The measurement was carried out at 500 MHzwith 6000 scans, using the pulse and spectral widths were 4.3 ms(90�) and 18 kHz, respectively.

C. Li et al. / Polymer 53 (2012) 1138e11471140

2.4.2. Differential scanning calorimetry (DSC)The thermal behavior of the sample was determined using DSC

1 (METTLER Instrument). DSC thermograms were obtained attemperatures ranging from �80 to 250 �C with a heating rate of10 �C min�1 under a nitrogen atmosphere. Each sample weighed8 mg was cooled at �80 �C and after being held a �80 �C for 2 min,measurementswere carried out up to 250 �C. The temperatures andtransition enthalpy were calibrated with In and Zn standards witha heating rate of 10 K/min.

2.4.3. Dynamic mechanical analysis (DMA)Dynamic mechanical frequency sweep experiments were

conducted in parallel plate geometry with a shear modeby SDTA861e (METTLER Instrument). Cylindrical specimens(4 mm � 4 mm � 1 mm) were cut from TPU-300 and TPU-2500.The frequency was ranged from 0.01 Hz to 150 Hz during a seriesof set temperature ranged from 60 �C to 160 �C. A force no higherthan 5 N and a strain amplitude no larger than 10 um was set tocontrol the dynamic mechanical behavior, under a nitrogenatmosphere.

2.4.4. Wide-angle X-ray diffraction (WAXD)Wide-angle X-ray Diffraction (WAXD) profiles were collected in

the reflection mode using a Philipsgoniometer. Nickel-fiitered CuKa radiation (wavelength ¼ 0.1542 nm) was produced by an XRG-3000 generator at an operating voltage of 40 kV and acurrent of20 mA. The scattering intensity was monitored on a strip-chartrecorder as a function of the scattering angle (2q) between 5 and3 5� using a goniometer arm speed of 1�/min.

2.4.5. Small-angle X-ray scattering (SAXS)Small-angle x-ray scattering measurements were performed at

the laboratory center of Shanghai Synchrotron Radiation Facility(SSRF), Pudong New Area, China. The x-ray wavelength lwas 1.54 Åand a small-angle camera was used with a multiwire quadrantdetector. The samples were characterized at a sample to detectordistance of 1820 mm. Beam center and camera length wereroutinely calibrated using a silver behenate standard. The raw datawere corrected for background noise and transmission prior toazimuthal averaging. Scattering vector q in the work is defined as

q ¼ 4p sin q=l

where 2q is the scattering angle and l is the wavelength of 1.54 Å.Experimental data was processed by FIT2D _12_077_i686_WXPwritten by Dr. Andy Hammersley.

3. Results and discussion

The mechanical properties of segmented polyurethanes dependstrongly on the micro-morphology of the synthesized blockcopolymer which in turn depends upon the hard and soft blockcomposition, size, distribution and the processing details [23e31].The various factors decide the complex and variation of the TPUpreparation. In our recent work, we found different stirring speedsduring the polyurethane synthesis would result in quite differentmorphology, which directly leaded to the difference in themechanical properties, especially the permanent compression setvalue. In step polymerization, the reactivity of a functional groupwill be independent of the size of the molecule to which it isattached [32]. This means the hydroxyl group of the polyol reactswith the isocynate as readily as the diol hydroxyl group. On anotherhand, reactivity of a functional group is dependent on the collisionfrequency of the group [33]. A lower diffusion rate means that anytwo functional groups will undergo more total collisions before

diffusing apart. Therefore, compared with the stirring at a highspeed (2500 rpm), the polyol has higher probability reacting withthe isocynate group attached to a soft segment (NCO-POLYOL) atlower speed (300 rpm) during the polymerization. This meansa larger soft segment can be formed at a low stirring speed,accompanying the appearance of larger hard segments.

The distribution of hard segment has been investigated bymanymethods. One is based on the hydrolysis of the polymer andsubsequent analysis of the hydrolyzed products by size exclusionchromatography [34e36]. Recently, a comparatively convenientmethod by using 13C NMR was applied [37]. This method is basedon the different response of the signals arising from nonprotonatedaromatic carbons which appear to be sensitive to dyads sequencedistribution. Therefore, a 13C NMR method was used to depict thedifference of hard segments distribution of samples prepared bydifferent stirring speeds.

The spectra of TPU-300 and TPU-2500 are shown in Fig. 2. Thechemical shift range from 135 to 139 ppm should be paid attention.A splitting signal appears around this range, which arises from thenonprotonated aromatic carbons of MDI segments in the ipsoposition relative to urethane group and sensitive to the sequencedistribution at the level of dyads. The peak assignments of differentdyads (BB, BP, PP) are shown in Fig. 3 and an attached molecularstructure illustrates the corresponding sequence distribution. Themolar ratio of the dyads was estimated by integration of thesepeaks. Based on these, the number-average sequence lengths of �LB(BDO-MDI-BDO) and �LP (POL-MDI-POL) units, the probability ofsoft (hard) segment in hard (soft) matrix PHS (PSH), as while as thedegree of block sequence randomness B can be obtained. The cor-responding expressions were list below [37]:

PBB ¼ ABB=ðABBþABPþAPPÞ PBP ¼ ABP=ðABBþABPþAPPÞPPP ¼ APP=ðABBþABPþAPPÞ�LB ¼ ðPBB þ 1=2PBPÞ=1=2PBP �LP ¼ ðPPP þ 1=2PBPÞ=1=2PBP

PHS ¼ 1=2PBP=ðPBB þ 1=2PBPÞ PSH ¼ 1=2PBP=ðPPP þ 1=2PBPÞ

B ¼ PHSþPSH ¼ 1=2PBP=ðPBBþ1=2PBPÞþ1=2PBP=ðPPPþ1=2PBPÞ

Table 1 shows the results. Generally, number-average sequencelengths reflect the size of the hard or soft segments in statisticalsense. In addition, segmentmixing probability and randomness canbe used as a measure of the micro-phase separation degree. It canbe concluded from �LB and �LP that relatively longer hard segmentsand soft segments sequences had formed in TPU-300. The segmentmixing probability and randomness also clearly indicate that TPU-300 had a higher degree micro-phase separation. 13C NMR spectraare the reflection of polymer chemical structure, which successfullyrevealed the distinction between TPU-300 and TPU-2500 from thefirst-order structure. As is known, polymer aggregation structure isdirectly related to the first-order structure. The difference in hardsegments distribution as shown in 13C NMR spectra indicates thatthere were two distinct aggregation structures between TPU-300and TPU-2500.

DSC provides useful information to estimate the aggregationstructure of TPU by observing the endothermic peaks and glasstransition region. In the last decades, extensive research articlesfocus on the origin of melting endotherms in the TPU systems[38e44]. Though the exact nature of the multiple melting endo-therms is still discussed, one consensus is melting endothermsappearing at higher temperature means more ordered arrange-ment of hard segment domains. Figs. 4 and 5 show the DSC ther-mograms of TPU-2500 and TPU-300 respectively, together with the

Fig. 2. 500 MHz 13C NMR spectra of TPU-2500 (top) and TPU-300 (bottom).

C. Li et al. / Polymer 53 (2012) 1138e1147 1141

compressed counterparts at set temperatures. Table 2 shows theglass transition temperature (Tg) of the soft segments, heat capacity(DCp) at the glass transition region, melting enthalpy (DHi) andmelting point (Tmi) of the hard segment domains of the TPUs. DHi,Tmi (i ¼ 1, 2) were defined as DH1, Tm1, DH2, Tm2 from the lowertemperature side. Compared TPU-2500 with TPU-300, two distinctthermal responses were found. The glass transition temperature ofTPU-2500 shown in Fig. 4 was higher than that of TPU-300, whichmeans TPU-2500 had poorer micro-phase separation and a similar

Fig. 3. Peak assignments of nonprotonated carbons in

conclusion was derived from the 13C NMR analysis mentionedabove. In addition, after compression, the melting enthalpy value ofTPU-2500 decreased, especially TPU-2500-70C which had lowervalue than that of TPU-2500-25C. The decrease in melting enthalpyvalue means the hard segment domains were disrupted to rela-tively disordered structures or mixed in the soft segment domains.Compared with 25 �C, a higher temperature of 70 �C endowed themolecular chains with a more mobility, accelerating the disruptionof the ordered hard segment domains.

the aromatic region for TPU-2500 and TPU-300.

Fig. 5. DSC thermograms for TPU-300, TPU-300-25C, TPU-300-70C.

Table 1Calculated sequence distribution and randomness of BDO-POL-MDI polyurethaneswith different stirring speeds.

Sample Molar ratio of thedyads

Number-averagesequencelength

Segmentmixingprobability

Randomness

PBB PBP PPP �LB �LP PHS PSH B

TPU-2500 0.359 0.254 0.387 3.83 4.04 0.261 0.247 0.508TPU-300 0.383 0.206 0.411 4.72 4.99 0.212 0.201 0.413

C. Li et al. / Polymer 53 (2012) 1138e11471142

In Fig. 5, a higher melting endothermic peak above 200 �Cappeared. Seymour and Cooper [38,45,46] studied the thermalresponse of polyester MDI-BD polyurethanes using DSC, pointingout the hard segment associated endotherm observed at temper-atures of 120 �C�190 �C and above 200 �C were correlatedrespectively with long range and microcrystalline ordering of hardsegment domains. Therefore higher organized hard segmentdomains had formed in TPU-300. These hard segment domains hadlower dissolution ability within the soft micro-phase and a moremarked micro-phase separation appeared. After compression, anentirely different thermal response relative to the TPU-2500 wasshown in Fig. 5. It can be seen that a novel endothermic zoneappeared below 200 �C. The appearance of the endothermic zonebelow 200 �C was probably attributed to the reorganized structureof the less ordered hard segments, which aggregated togetherunder the compression pressure to form higher ordered domains.Higher temperature would accelerate the mobility of molecularchains, thus a more prominent endothermic peak appeared ata higher temperature around 170 �C for TPU-300-70C.

If correlated the permanent compression deformation values atset temperatures (25 �C and 70 �C) with the corresponding changevalues of melting enthalpy DH, an interesting relationship can befound. Table 3 shows the permanent compression deformationvalues and change values of melting enthalpy for TPU-2500-25C(70C) and TPU-300-25C (70C), respectively. It can be seen fromthe values that with the increase of jDHj, permanent compressiondeformation value became larger. Although a quantitative rela-tionship between them can not be obtained, a factor can be madecertain: permanent compression deformation was directly relatedto the melting enthalpy change. As mentioned above, the meltingenthalpy was associated with the hard segment domains. Underthe compression, hard segment domains were disrupted for TPU-

Fig. 4. DSC thermograms for TPU-2500, TPU-2500-25C, TPU-2500-70C.

2500 and reorganized for TPU-300. In both processes a perma-nent deformation occurred as while as a change of the corre-sponding melting enthalpy value. Therefore, the disruption orreorganization of hard segment domains plays a significant role inpermanent compression deformation. In addition, it can be seenfrom Fig. 5 that the melting peak location and enthalpy value athigher temperature above 200 �C were almost unchanged, whichimplied that well organized hard segment domains could be morestable under compression. However, this can not derive a conclu-sion that well organized hard segment domains guarantee lesspermanent deformation than relatively disordered hard segmentdomains under compression. In our recent work, it was found thathigh content of well organized hard segment domains weredestroyed under a compression test, following a decrease inmelting enthalpy above 200 �C and a large permanent deformation.In order to illustrate this phenomenon, the affine deformationassumption used to explain the elasticity of rubber network [47]was cited. Based on this assumption, the unit undergoes the sameratio as the corresponding deformation of the bulk sample. If thehard segment domain size in a unit is comparable to the unitdeformation degree, a notable deformation would probably beundertaken by hard segment domains. Otherwise, more deforma-tion would be undertaken by soft segment domains. Thus, a wellorganized stable hard segment domain can also be disruptedobviously under compression if its size is comparable to the unitdeformation degree. Based on this hypothesis, the difference inpermanent deformation between TPU-300 and TPU-2500 wastentatively explained. As far as TPU-300 was concerned, a propersize of higher organized hard segment domains was formed, whichwas far smaller than the unit deformation degree and had moredistinct separation with the soft segment domains compared toTPU-2500. As a result, most deformations were undertaken by softsegment domains under compression. As for TPU-2500, due to the

Table 2Thermal properties of TPU sheets.

Sample code Tg(�C) DCp(J/gK) DH1(J/g) DH2(J/g) Tm1(�C) Tm2(�C)

TPU-2500 �36.5 0.335 �19.72 e 173.4 e

TPU-2500-25C �37.8 0.370 �7.39 e 172.4 e

TPU-2500-70C �36.6 0.285 �5.02 e 172.9 e

TPU-300 �40.9 0.471 �2.40 e 215.4 e

TPU-300-25C �38.9 0.402 �2.25 �2.12 118.7 216.9TPU-300-70C �38.4 0.452 �4.02 �2.21 170.2 209.1

Fig. 7. X-ray diffractograms for TPU-300, TPU-300-25C, TPU-300-70C.

Table 3Permanent compression deformations and change values of melting enthalpy forTPU-2500 and TPU-300 at set temperatures.

Sample Permanentcompressiondeformationat 25 �C (%)

Permanentcompressiondeformationat 70 �C (%)

Change valueof meltingenthalpy for25 �C (J/g)

Change valueof meltingenthalpy for70 �C (J/g)

TPU-2500 26 55 �12.33 �14.70TPU-300 21 32 1.97 3.83

Note: The minus sign before change value of melting enthalpy means a decrease inmelting enthalpy after compression.

C. Li et al. / Polymer 53 (2012) 1138e1147 1143

poor micro-phase separation, the deformations were both under-taken by soft and hard segment domains. Although the size of hardsegment domains of TPU-2500 was smaller than that of TPU-300,lacking stability (the stability of hard segment domains wasrelated to the hydrogen bond density and strength, a furtherinvestigation will be done in the next work) leaded to a readilydisruption. Due to the elasticity of the soft segment domains, thepolyol soft chains would restore its original conformation afterremoving the compression, and little permanent deformation canbe formed in these domains. On the contrary, lack of elasticity ofhard segment domains leads to a permanent deformation. Conse-quently, TPU-300 has relatively lower permanent deformation inwhich most deformations were undertaken by soft segmentdomains.

As a matter of fact, the influential factors on permanent defor-mation of semi-crystalline polyurethanes are very complex.Primary chemical crosslinks, crystallites, hydrogen bonds, chainentanglements, and intermolecular chains flow are all concerned[48e50]. Therefore, what size and what types of components forthe hard segment domains were proper to realize a low permanentdeformation property? A further study will be carried out in thenext work, trying to specify the correlation between permanentcompression deformation and hard segment domains size as whileas their components.

X-ray diffractions of these samples are shown in Figs. 6 and 7.The crystalline structure of the hard segment based on MDI/BD hasbeen studied by ROBERT M [51], and a remarkable diffraction peakaround 2q ¼ 20� is a characteristic of MDI/BD hard segmentreflection. Compared the samples before and after compression, thediffraction intensity shows a regular change. In Fig. 6, the diffraction

Fig. 6. X-ray diffractograms for TPU-2500, TPU-2500-25C, TPU-2500-70C.

intensity decreased after compression, especially TPU-2500-70C,which showed the lowest diffraction peak. On the contrary, thediffraction intensity increased for TPU-300 after compression as itis shown in Fig. 7. As far as TPU-2500 is concerned, some hardsegment domains were disrupted under compression and thediffraction intensity associated with hard segment crystallite wasweakened. The decrease in diffraction intensity was consistent withthe thermal behavior observed by DSC. Similarly, the increase indiffraction intensity for TPU-300 was due to the reorganization ofthe less ordered hard segment domains and consistent with thethermal behavior observed in Fig. 5.

The fairly wide diffraction peak indicated a lack of high orga-nized crystal structure in these samples; therefore it is hard toobtain the accurate crystallite size. In this work, Hosemannmethod[21] was used to estimate the crystallite size and distortionparameters. This method was applied to an imperfect crystalstructure, i.e. paracrystalline, in which three-dimensional period-icity resembling that of an ideal crystal only persists over shortranges, whereas over long ranges there is a permanent disorderresembling that of the amorphous state. These are known as latticedistortions of the second kind [52], and the integral breadthb derived by Hosemann method was expressed as follows:

b ¼ 1=Lþ p2g2pm2=d0

Where b is the integral breadth corrected for distortions of thesecond kind, gp is a measure of the distortions of the second kind,mis the order of reflection, L is the weight-average crystallite width,and d0 is the distance between the first-order reflection planes.Based on the Bragg equation, the planes with calculated interplanardistances d ¼ 0.46, 0.41 nmwere used as the first and second orderreflection planes, respectively. So a plot of b against m2 is a straightline. L is obtained from the intercept and gp from the slope. Thecrystallite widths L and distortions gp deduced from these rela-tionships are listed in Tables 4 and 5. As it is shown in Table 4,crystallite widths of TPU-2500 were increased as a result of furthercompression. Combined with the thermal behaviors of DSC, itmight be that smaller hard segment domains were disrupted undercompression, while relatively larger hard segment domains wereremained. It is also seen from Table 4 that the increase in crystallitewidth was accompanied by an increase in distortion gp, whichprobably implied that the perfection of crystallites was weaken asa result of further compression. On the contrary, the variation of

Table 4Crystallite parameters of TPU-2500, TPU-2500-25C, TPU-2500-70C.

Crystallite parameters TPU-2500 TPU-2500-25C TPU-2500-70C

Crystallite Width L ðAoÞ 23.1 24.6 27.7Distortion gp (%) 4.2 4.9 6.6

Table 5Crystallite parameters of TPU-300, TPU-300-25C, TPU-300-70C.

Crystallite parameters TPU-300 TPU-300-25C TPU-300-70C

Crystallite Width L ðAoÞ 34.5 25.6 23.4Distortion gp (%) 7.6 5.0 2.9

Fig. 9. SAXS profiles for TPU-300, TPU-300-25C, TPU-300-70C.

C. Li et al. / Polymer 53 (2012) 1138e11471144

crystallite width of TPU-300 showed a different trend. Asmentioned above, a novel endothermic zone appeared below200 �C in TPU-300 after compression, corresponding to relativelysmaller reorganized hard segment domains, which leaded to theweight-average crystallite widths decreased. In addition, thedecrease in distortion gp of TPU-300 shown in Table 5 probablyimplied the perfection of crystallites was strengthened witha further compression.

In general, the morphology of thermoplastic polyurethanes ischaracterized by AFM with a tapping mode [15,53]. Standardelectron microscopy (TEM) procedures fail for segmented poly-urethanes because of low contrast even with staining. However, toensure the accuracy in AFM measurement, flat surface with theheight fluctuation no more than 5 nm is necessary. This is becausethe topography factor has a profound influence on the morphologyrealism of a sample. Based on this consideration, SAXS was used tocharacterize the aggregation structures of the samples, though thistechnique can not give a visualized morphology.

Fig. 8 shows the SAXS patterns of TPU-2500 system. Sincescattering peak height is roughly proportional to the electrondensities contrast between the phases, the descent of peak heightwith further compression indicated that some hard segmentdomains were disrupted by the compression and mixed within thesoft phases. The increase in distortion gp for TPU-2500 discussed inWAXD analysis meant the increase in disordering of hard segmentdomains, which was consistent with the declining behavior of thescattering peak observed in SAXS. Markedly different from TPU-2500, there was no evident scattering peak in TPU-300 system.

Fig. 8. SAXS profiles for TPU-2500, TPU-2500-25C, TPU-2500-70C.

This phenomenon indicated that there was no significant electrondensities contrast in TPU-300 system. As discussed in the DSCsection, higher organized hard segment domains were formed inTPU-300, so a more significant electron densities contrast betweenhard and soft phases should come into being, accompanying theappearance of an evident scattering peak. However, the observedSAXS patterns in Fig. 9 were contrary to the expectation. This wasprobably due to the low content of higher organized hard segmentdomains in TPU-300, which was supported by the low meltingenthalpy value above 200 �C observed in DSC thermograms.Therefore, except rare regions, the electron densities contrast inmost parts of TPU-300 were not distinct. With a further compres-sion, a sign of scattering peak could be found in TPU-300-70C. Thismeant some ordered hard segment domains were formed underthe compression, which was consistent with the thermal behaviorsobserved in Fig. 5.

With regard to segmented polyurethanes, the long period L(L ¼ 2P/q) obtained from the scattering maximum can be taken asthe distance between hard segment domains. Fig. 10 shows thevariation of L with the compression for TPU-2500s and TPU-300s.The distance between hard segment domains decreased with thecompression, indicating hard segment domains moved closer with

Fig. 10. Long period L as a function of different compression conditions for TPU-2500and TPU-300.

Fig. 11. 2D SAXS images for TPU-2500 and TPU-300 before and after compression.

C. Li et al. / Polymer 53 (2012) 1138e1147 1145

a further compression. In addition, the hard segment contentmaintained constant in this work so that an increase in long periodcorrelates directly to enhanced phase separation and increasedhard segment domains size. Consequently, the longer L value ofTPU-300-70C shown in Fig. 10 indicated a more complete phaseseparation and larger hard segment domains, being beneficial forlower compression set.

2D SAXS images before and after compression are shown inFig. 11. The isotropic SAXS images indicated the hard segmentdomains were still randomly oriented even though after thecompression.

The permanent deformation of segmented polyurethanes isrelevant to many factors, however, all of these can be attributed tothe viscoelastic behaviors under compression. The elasticityelement plays a role as restoring force, whereas the viscosityelement serves as restraining force and is responsible for thepermanent deformation. In this work, DMA was used to charac-terize the viscoelastic behaviors of TPU-2500 and TPU-300. Fig. 12shows the modulus vs frequency master curves using a referencetemperature of 70 �C and the corresponding data derived by Rousemodel [22]. In Fig. 12-a, a failure of superposition can be found atlower frequencies for TPU-2500. However, this phenomenon wasless prominent in TPU-300. Velanker S pointed out the most likelyexplanation for this superposition failure was the hard segment Tg:time-temperature superpositionwas not expected to apply close tothe hard segment Tg and the failure of superposition was roughlycoincident with the sample flow [54]. Compared with the thermalbehavior of TPU-300, it can be found an obvious melting endo-therm appeared at lower temperature for TPU-2500. As a result,TPU-2500 had more flowing possibility with the temperatureincrease, which leaded to the failure of superposition at lowerfrequencies. These master curves can be described quantitativelyusing Rouse model [22].

G’ ¼ 6h0sR2

XN u2p42 2 4

p

p¼1 1þ sRu p

Where u is the frequency, h0 and sR are the terminal viscosity andrelaxation times, respectively, used as fitting parameters. These fitcurves by Rouse model are shown as dash lines in Fig. 12, and thecorresponding terminal viscosities and relaxation times are listedin Table 6. The higher terminal viscosity of TPU-300 probablymeans a more difficulty in intermolecular chains flow comparedto TPU-2500. So the permanent deformation resulted by inter-molecular chains flow was lower in TPU-300. A shorter relaxationtime also means a higher elasticity of TPU-300 compared toTPU-2500. Due to some arbitrary in obtaining sR, the values h0and sR were for reference only. Nevertheless, we believe thetendency in viscoelasticity changing can be properly reflected byh0 and sR.

Based on the analysis above, it can be found the viscosityelement plays a significant role in permanent deformation.Restraining intermolecular chains flow so as to strengthen theviscosity is an effective method in improving the permanentcompression deformation. Therefore, a micro-crosslink structurewas introduced in TPU-300 by adding proper content of TMP,and a low compression set of 25% at 70 �C was realized. Fig. 12-cshows the modulus vs frequency master curve of TPU-300-TMP using a reference temperature of 70 �C, and the corre-sponding terminal viscosity and relaxation time obtained byRouse model were listed in Table 6. The longer relaxationtime compared to TPU-2500 and TPU-300 meant the morerestraint of molecular chains rotation caused by the citingof micro-crosslink structure. The higher terminal viscosity h0meant the more difficult in intermolecular chains flow anda lower permanent compression deformation property wasobtained.

Fig. 12. Dynamic mechanical frequency sweep data for TPU-2500, TPU-300, TPU-300-TMP and the corresponding fitted dash line by Rouse model.

Table 6Rouse parameters for TPU-2500, TPU-300, TPU-300-TMP.

Rouse parameters TPU-2500 TPU-300 TPU-300-TMP

h0 (PaS) 271 714 1343sR (S) 0.042 0.031 0.054

C. Li et al. / Polymer 53 (2012) 1138e11471146

4. Conclusions

The microstructure developments of two types of thermoplasticpolyurethanes under compression were investigated. These twotypes of samples had the same chemical compositions whiledifferent morphologies, and were prepared by altering stirringspeed during the synthesis process. The difference in the first-orderstructures was verified by 13C NMR, and longer hard segmentsequence length was formed in TPU-300. The aggregation structurebefore and after compression was characterized by DSC, WAXD,SAXS. Based on these characterizations, two distinct deformationmechanisms between these two types of samples were investi-gated. The hard segment domains of TPU-2500 were disruptedunder compression, whereas reorganized hard segment domainsappeared in TPU-300. It was concluded from these observations

that the disruption or reorganization of hard segment domainsplays a significant role in the permanent deformation undercompression. It seems that a proper hard segment domain size ishelpful in realizing low permanent deformation, and the corre-sponding size distribution need to be investigated in the furtherwork. Rouse model was used to describe the viscoelastic behaviorsof the samples and explain the different performances in perma-nent deformation. TPU-300 had a higher terminal viscositycompared to TPU-2500, which leaded to a lower compression set.Finally, an optimizedmicro-crosslink structurewas introduced, anda better compression deformation resistance property wasobtained.

Acknowledgments

The authors would like to thank Miss. Xiuhong Li for help withthe Synchrotron X-ray experiments at laboratory center ofShanghai Synchrotron Radiation Facility (SSRF), Dr. Qishian Huangfor helpful technical discussion, Mr. Hongxing Xu for experimentassistance. This work was also supported by the National NaturalScience Foundation of China (NSFC50903030).

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