effect of the soft segment on the fatigue behavior of segmented polyurethanes

E�ect of the soft segment on the fatigue behavior ofsegmented polyurethanes

Guillermo Jimeneza, Shigeo Asaib,*, Atsushi Shishidob, Masao Sumitab

aPOLIUNA, Department of Chemistry, Universidad Nacional, Heredia, Costa RicabDepartment of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552,

Japan

Received 17 November 1997; received in revised form 1 September 1999; accepted 24 September 1999

Abstract

E�ect of the soft-segment structure and molecular weight on the microphase separation in segmentedpolyurethanes (SPU) was determined by means of di�erential scanning calorimetry, small-angle X-ray scattering,dielectric constant measurement, pulsed nuclear magnetic resonance, and thermoluminescence. Possible changes inthe structural properties of SPU after cyclic mechanical fatigue were monitored using the same techniques as

described above. Samples were divided in two series according to their soft-segment structure, i.e., one is linearpoly(tetramethylene glycol) (PTMG), and another one with a methyl group on the PTMG chain. Hard segmentconsisted of 4,4 '-diphenylmethane diisocyanate (MDI), and 1,4-butanediol (BD) as chain extender in both sets. It

was found that phase separation increased for both types of SPU as the soft-segment molecular weight increased.Samples with methyl group showed little soft-segment crystallization. Upon fatigue, samples with a methyl groupdemonstrated a better fatigue resistance. 7 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction

Segmented polyurethanes (SPU) are thermoplasticelastomeric (AB)n block copolymers that exhibit sev-

eral aspects of rubber-like elasticity but are not cova-lently crosslinked [1]. One block of the polymer chainconsists of a relatively long, ¯exible polyester or poly-

ether diol with typical molecular weight that rangefrom 1000 to 3000 [2]. These amorphous polyol blocksare usually termed soft segments since they impart

elastomeric character to the polymer. The secondblock of the copolymer is referred to as the hard seg-ment and is formed by the reaction of aromatic diiso-cyanates with low-weight diol or triol chain extenders.

Due to the polar nature of the urethane group in the

hard segments and their ability to form hydrogen

bonds, these hard segments are capable of intermolecu-

lar associations and possible domain segregation. The

thermally reversible network structure of these copoly-

mers provides for the elastomeric or apparent cross-

linked nature of these polymers.

SPU are biocompatible and do not show any unde-

sired reaction with biological ¯uids. Their apparent

thromboresistance is thought to reside in the ability to

preferentially adsorb serum albumin, which is widely

regarded as the ®rst step in surface passivation toward

circulating blood. Therefore, segmented polyurethanes

have been considered in various biomedical appli-

cations such as endotracheal tubes, aortic grafts and

vascular tubing, cardiac assist devices and heart by-

pass devices [3].

European Polymer Journal 36 (2000) 2039±2050

0014-3057/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved.

PII: S0014-3057(99 )00241-4

* Corresponding author. Fax: +81-3-5734-2876.

Several structural models have been proposed to

explain the microdomain structure of SPU [4,5]. Onewidely accepted has been deduced by Koberstein andStein [6], which considered that all until then models

were conceived to explain the wide-angle di�ractionpeaks observed in oriented heat-set polyurethanes.They assumed that a di�erent morphology was to be

expected in bulk elastomers, and adopted a lamellarmicrodomain morphology. Based on the hard-segment

structure, a model considering the amount of intrado-main hard-segment order could be ®gured out. In this``average'' view of a domain, the longer hard segments

are forced to fold back into the domain. The presenceof segregation might be expected to decrease the

amount of chain reentry into such domain.Fatigue behavior of SPU and segmented polyuretha-

neureas (SPUU), has received considerable interest as

concerning with arti®cial heart safety. As the numberof cyclic deformation applied to a diaphragm of bloodpump for arti®cial heart is 105 per a day, a long fati-

gue lifetime is demanded for diaphragm materials. Forinstance, Lawandy and Hepburn [7±9] studied the fati-

gue failure of MDI-based SPU, and determined thebest conditions for processing SPU by using DSC, andmechanical behavior investigation. Takahara et al.

[10,11] researched the phase structure of SPUU uponfatigue using a tensile fatigue tester which enabled con-

tinuous measurement of dynamic viscoelasticity duringfatigue process. They suggested that destruction of thehard segment domain and/or intermixing of the hard

and soft segments proceeded under cyclic deformation.Later, Shibayama et al. reported a series of papers[12±14] concerning with the structure, and properties

of fatigued SPUU using mainly IR dichroism exper-iments. They reported a destruction of the hard seg-

ment domains and the phase mixing between hard andsoft segment domains which proceeded with increasingfatigue time. IR dichroism showed that hard segments

oriented negatively, then positively with increasingelongation. The negative orientation resulted from thebehavior of the hard segment domain as a ®ller having

an anisotropic shape (domain orientation). Subsequentpositive orientation is explained by the hard segment

orientation coupled with fragmentation of the hardsegment domains (segment orientation). They notedthat in the fatigue process, proceeding of phase mixing

was dominant followed by phase demixing related torearrangement and reorientation of the hard segments

which would ®nally lead to fracture. Therefore, theyconcluded that the fatigue process consisted of threestages; (i) the domain orientation stage; (ii) the phase-

mixing stage, and (iii) the segment orientation stage.This fatigue mechanism was well interpreted by usinga spherulitic deformation model [15]. Also Liu et al.

[16] investigated the deformation process of SPUU byusing thermoluminescence (TL) and pulsed NMR. Fur-

thermore, they studied the fatigue mechanism of SPUby the same above mentionated techniques [17]. In

their ®rst paper, they reported that increasing the soft-segment molecular weight or decreasing the diisocya-nate content led to a better phase separation. They

observed in the elongation experiments, that orien-tation, disintegration, and phase mixing of the hardsegment domains occurred. In the latter paper, they

suggested the appearance of a fatigue-induced phase-mixed phase due to fatigue, which had a segmentalmobility higher than the interfaces around the hard

segment domains.More recently, Sakurai et al. [18] reported a hard-

segment microdomain dissociation due to fatigue inSPUU containing poly(dimethyl siloxane) (PDMS).

The better fatigue resistance gained in one sample con-taining PDMS was interpreted as due to the e�ect ofthe PDMS, that lower crystallizability of the soft seg-

ment, even if the soft segments were released from con-straint by hard-segment microdomains during fatigue.In the present work, we aim to determine the e�ect

of the chain length, and that of a methyl side group(asymmetry factor) on the soft segment, in MDI-basedSPU's, and its relationship to the microphase structure

and behavior upon mechanical fatigue.

2. Experimental

2.1. Materials

Six as-cast (only cast on glass plate without furtherannealing) SPU samples were kindly supplied byHodogaya Chemical Co. Ltd. Sample thicknesses oscil-

lated around 500 aÁ AÄ m. Table 1 shows the compo-sitions of SPU. Samples were divided into two seriesaccording their soft segment; SPU containing poly(te-

tramethyleneglycol) (PTMG) and SPU with poly(tetra-methylene/3-methyltetramethylene glycol) (MePTMG).Hereafter former SPU will be denominated simply by

SPU-X, and the latter ones will be called MeSPU-X,where X corresponds to the soft-segment molecularweight (SSMW). Molecular weights (Mn ) were 1000,2000 and 3000. Hard segment consisted of 4,4 '-diphe-nylmethane diisocyanate (MDI) with 1,4-butanediol(BD) as chain extender.

2.2. Fatigue test

A home-made fatigue tester was employed. The con-ditions for this experiment were chosen in order toexclude large plastic deformation, and elevation of

temperature in the sample during fatigue. Films of 80� 20 � 0.5 mm3 were attached between two clampsand fatigued at room temperature with 3% static de-

G. Jimenez et al. / European Polymer Journal 36 (2000) 2039±20502040

formation and 20% of cyclic extension. A frequency of3 Hz and 1.6� 106 cycles were imposed.

2.3. Di�erential Scanning Calorimetry (DSC)

DSC was carried out using a Shimadzu DSC-50.

Heating rate was 208C/min, and every sample wasscanned from ÿ100 to 2008C under gas nitrogen at¯ow of 30 ml/min. Samples were weighted up to 5.020.3 mg and were sealed into aluminium pans. Aluminapowder 10 mg was used as reference. A glass transitionwas de®ned as the temperature at the end-set of the

transition.

2.4. Thermoluminescence (TL)

A 20 � 22 mm2 ®lm sample attached to a coppersample holder was irradiated during 90 min at roomtemperature under X-ray generated by a D-3F Rigaku

copper tube operated at 35 kV and 20 mA. After that,the sample was ®xed into a cryostat and vacuum wasapplied. After 30 min, TL started being recorded, anda TL glow curve was obtained scanning up to 2008Cat a heating rate of 68C/min. More details about theTL system employed here are given elsewhere [19].

2.5. Dielectric constant

An Impedance Analyzer Hewlett±Packard 4192Aand a home-made isothermic cell were used. From

measured capacitance C and loss coe�cient D betweenboth surfaces of ®lms, it was calculated the real (e ')and imaginary (e0) parts of the dielectric function

according the following equations:

e 0 � Cd

Ae0�1�

e 00 � e 0D �2�where, d=®lm thickness, A=electrode area, e0=rela-tive permitivity in the free space. Dielectric constantwas calculated scanning up to 2008C at 20 kHz.

2.6. Pulsed NMR

Pulsed NMR analysis was performed with a NihonBruker PC-20 Spectrometer operated at 20 MHz in thediode mode. The spin±spin relaxation, T2 was

measured by a solid echo method at room tempera-ture. For each measurement, 1024 points were col-lected and averaged with a cumulative number of 100,

an attenuation of 35 and a retardation time of 3 s.Three components could be observed in the free induc-tion decay (FID) curve and corresponding T2 andphase fractions (%) were calculated by a least square

method (LSM).

2.7. Small-Angle X-ray Scattering (SAXS)

SAXS measurements were carried out using a RU-

200 Rigaku Co. instrument, working at 50 kV and 180mA with Ni-®ltered CuKa radiation (l=0.15418 nm)and slit collimation using 0.08 (1st slit), 0.06 (2nd slit),

0.1 (scattering slit), and 0.25 (receiving slit) mm widthslits. Scans were made between Bragg angles of 0.08±28 and SAXS intensity was registered every 0.0048. Allmeasurements were controlled with a Hewlett±Packard

712/60 computer and using a RINT 2000 Series soft-ware. Data were corrected for air scattering, absorp-tion and thickness due to the specimen. Fatigued

samples were scanned along the stretching direction.

3. Data analysis

3.1. Pulsed NMR

The spin±spin relaxation time T2 can be determined

from the FID of the echo signal. FID is usuallyexpressed as an exponential function,

M�t� �M�0� exp�ÿt=T2� �3�where M(0) is the initial FID amplitude.The rate at which a proton signal decays depends on

the mobility of the segments containing the protons.

Table 1

Compositions of the polyurethane elastomers based on MDI, BD, and two polyols

Designation Macroglycol NCO/OH+BDa Hard segment weight fraction

SPU-1000 PTMG-1000 1.0/0.59+0.36 0.36

SPU-2000 PTMG-2000 1.0/0.44+0.51 0.27

SPU-3000 PTMG-3000 1.0/0.36+0.59 0.25

MeSPU-1000 MePTMG-1000 1.0/0.59+0.36 0.37

MeSPU-2000 MePTMG-2000 1.0/0.44+0.51 0.27

MeSPU-3000 MePTMG-3000 1.0/0.36+0.59 0.24

a Data provided by Hodogaya Chemical Co. Ltd.

G. Jimenez et al. / European Polymer Journal 36 (2000) 2039±2050 2041

Assink [20,21] studied crosslinked polyurethanes byusing pulsed proton magnetic resonance, and deter-

mined that the FID of polyurethanes consisted of afast component from the glassy domains and a slowcomponent from the rubbery or soft domains. There-

fore, proton magnetic resonance can establish when apolyurethane has separated into domains and deter-mine the relative amount of material in each domain.

Liu et al. [19] could ®nd a good ®tting for FIDresolved in three components shown by the followingequation:

M�t� �M�s� exp�ÿ t=T2�s��m�s�

��M�i � exp

�ÿ t=T2�i ��m�i �

��M�h� exp

�ÿ t=T2�h��m�h�

��4�

where s, i, and h express for the soft, intermediate, and

hard phases, respectively. Weibull coe�cient, m, wasfound to be approximately 2, 1.5, and 2 for m(s ), m(i ),and m(h ) respectively. T2(s ), T2(i ), and T2(h ), and cor-

responding M(s ), M(i ), and M(h ) were obtained byLSM as the best ®t to the FID curve. T2(s ), T2(i ), andT2(h ) were found to have values of approximately 350,

140 and 40 ms, respectively. The fraction of each phasecan be calculated according to the next equations:

X�s� �M�s�=�M�s� �M�i � �M�h� �5a�

X�i � �M�i �=�M�s� �M�i � �M�h� �5b�

X�h� �M�h�=�M�s� �M�i � �M�h� �5c�

Fig. 1 shows a typical FID for SPU-1000 where themagnetization as a function of time, M(t ), is plotted

against time.

3.2. SAXS

SAXS technique provides a convenient mean forevaluating morphological details of materials with het-

erogeneities in the 1±100 nm size range [22]. The exper-iment involves measurement of the scattered intensityas a function of the angle measured with respect to the

direction of the incident X-ray beam. The direction ofthe scattered radiation is usually expressed in terms ofthe scattering vector, s, de®ned by,

jsj � 2 sin�2y=2�=l � 1=d �6�where l is the X-ray wavelength and 2y is the scatter-ing angle. If a relative maximum is observed in the

scattering vector, smax, the size (i.e., wavelength), d, ofthe periodic structure from which this interferencearises may be approximated through Bragg's law,

d1D � �s1D, max �ÿ1 �7�where s1D,max is the scattering vector corresponding toa maximum in a s vs s IÃ(s ) plot (Lorentz correction

for slit collimation). IÃ(s ) is the scattered intensity, andd1D is the one-dimensional interlamellar repeat distance(long period). Eq. (7) applies for one-dimensional scat-tering systems which are globally isotropic, i.e., ran-

domly oriented lamellar stacks, such we assumed forSPU materials.The scattering intensity arises due to local heteroge-

neities in the electron density of the material. For slitcollimation, and assuming in®nite length, the smearedintensity in the Porod region (tail region) for a sigmoi-

dal model may be expressed as,

~I�s� ��K 0

s3

��1ÿ 8p 2s 2s 2 �erfc�2pss�

� 4ssp1=2 exp� ÿ 4p 2s 2s 2 ��

�8�

where K is a constant. This equation can be simpli®edempirically to [23],

~I�s� ��K 0

s3

�exp

�ÿ 38�ss�1:81

��9�

A more precise way to determine the long period is

by using the correlation function analysis. The corre-lation function for a lamellar structure will have alocal maximum at a position r which corresponds to

the long period, d1D. The one-dimensional correlationfunction, l1D(r ), can be calculated by applying aninverse Fourier transform to the scattering relation for

Fig. 1. Typical FID and its resolution in three components by

means of LSM.


the experimental scattering pro®le as follows [24],

l1D�r� ��1

Q

��10

s ~I�s��J0�2pxs� ÿ 2pxsJ1�2pxs��ds

�10�where Jn is a Bessel function of order n and Q is the

so-called invariant. Fig. 2 depicts a typical one-dimen-sional correlation function.The integrations and transforms involved in calculat-

ing these structural parameters require thermal back-ground subtraction, and extrapolation to s = 0 byGuinier's law and extrapolation to 1 through the ap-plication of Porod's law,

lim s41�~I�s�� Kp

s3�11�

where Kp is the Porod constant. Intraphase mixing anddi�use microphase boundaries lead to deviations from

Porod's law [25,26] which may be analyzed to estimatetheir respective contributions to the overall mixing. Inthe case of collimation by in®nitely long slits, a modi-®ed Porod's law results in,

lim s41�~I�s��Kp

s3

�H 2�s� � ~{B �12�

where H(s ) is the Fourier transform of a functionwhich accounts for smoothing of the phase boundaries.The slit smeared intensity, IÄ(s ), contains a background

contribution, IÄB, arising from both thermal density andconcentration ¯uctuation, which lead to positive devi-ations from the Porod's law. The background intensity

can be calculated by the Bonart method [27], whichuses a plot of IÄ(s )s 3 vs s 3.

The presence of di�use phase boundaries damps outthe limiting Porod intensity by the factor H 2(s ), lead-ing to negative deviations of that law. When the inter-

phase concentration pro®le is sigmoidal in nature, thedamping function may be approximated by,

H 2�s� � exp�ÿ 38�ss�1:81

��13�

An equivalent thickness, E, for a linear interphase

gradient is given by E = 121/2s, where s is the half-width of the Gaussian smoothing function used to gen-erate the sigmoidal interphase pro®le. The value of s isobtained from the slope of a ln{[IÄ(s )ÿIÄB]s 3} vs s 1.81

plot.

4. Results and discussion

4.1. In¯uence of the soft segment on the structure ofsegmented polyurethanes

Table 2 shows the thermal information determinedfrom the DSC thermograms of SPU and MeSPUsamples as cast. Existence of multiple endotherms has

been documented in several studies of the thermalbehavior of segmented block copolymers [28±31]. Inthe case of SPU, we detected three transitions and inthe speci®c case of SPU-2000 and SPU-3000, an ad-

ditional fourth transition was detected. Correspond-ingly, we can make the next assignments for the SPUsamples:

Tg,s: it corresponds to the glass transition of thesoft segments;Tm,s: that corresponds to the melting of the crystal-

line soft-segment, seen in SPU-2000, and strongerin SPU-3000 by virtue of the greater soft segmentmolecular weight;

Tg,h: this results from the breakup of short-rangeorder induced by room temperature annealing, andit has been also called as the glass transition of thehard segments [22,28];

Tm,h: this is attributed to dissociation of long-rangeordering in the hard microphase.

For MeSPU samples, all the above transitions were

also detected, except the corresponding to Tm,s. Thismeans that crystallinity was diminished drastically inMeSPU if we compare, for instance, SPU-3000 and

MeSPU-3000. Chen et al. [32] by studying the e�ect ofa pendant 3-methyl side group on the soft segment ofa polyurethane, determined that the crystallinity of the

soft segments could be disrupted by the existence ofthe 3-methyl side group. Also Sakurai et al. [18], bycomparing the change provided by adding a dimethyl-

Fig. 2. Determination of the long period as calculated from

the one-dimensional correlation function.


siloxane component to the soft segment chain,

observed that the methyl-side groups prevented the

PTMG soft segments from crystallization.

It is well seen from Table 2 that as SSMW

increased, the Tg,s decreased in both kinds of materials.

This indicates that polyurethanes made with lower

MW polyols present more miscibility between the hard

and soft segments. The more hard segments are dis-

persed in the soft segments, the more restrictive the

soft segment. Therefore, the microphase separation will

increase as the SSMW increases. It is worthly to point

out that the increase of Tg from SSMW 1000 to 2000

was bigger than that of from 2000 to 3000 in both

SPU and MeSPU. On the other hand, there is a di�er-

ent behavior between both sets of samples as regarding

the change of the heat capacity at Tg,s, DCpg,s. For

MeSPU, there was almost no variation in DCpg,s, whilefor SPU samples, this quantity decreased as the

SSMW increased. This latter result can be explained

considering a higher degree in crystallinity in the soft

segment for SPU-2000 and SPU-3000. Therefore, frac-

tion of the amorphous phase of the soft segment is

constrained, and restricted for participing in the glass

transition process. From Table 2, we can observe that

in general as the SSMW increased, Tg,h and Tm,h

increased as well. This indicates that the improved

phase separation in the polyurethanes made from

SSMWe2000 contributes to the formation of a well-

de®ned hard segment domains. The presence of ``dis-

solved'' soft-segment units decreased as the SSMW

increased [33]. Again, the di�erence between samples

with SSMW 2000 and 3000 was smaller than that of

between 1000 and 2000 samples.

Fig. 3a and b show the TL glow curves of SPU and

MeSPU as cast, for various number-average molecular

weights of the PTMG, and MePTMG, respectively.

Each curve exhibits a main peak (marked I) at around

1758C, and a weak and broad peak at about 758C(marked II). According the DSC results, we may corre-

late the peak I to the dissociation of the long-range

order of the hard segments, and the peak II to the

glass transition of the hard phase. In both sets of

samples, as the SSMW increased, the intensity of the

main peak I increased. Their temperatures at the maxi-mum intensity on the curves, Tmax, remained almostconstant for all the cases. Accordingly, because theintensity of the main peak I is proportional to the elec-

tron traps in the hard segment domains, it can be

Table 2

E�ect of the soft segment on the thermal transitions of SPU and MeSPU ®lms as cast

Tg,s (8C) DCpg,s (J/g8C) Tm,s (8C) DHm,s (J/g) Tg,h (8C) Tm,h (8C)

SPU

1000 ÿ43.2 0.53 ± ± 76.3 147.6

2000 ÿ61.7 0.49 8.3 4.7 76.6 175.2

3000 ÿ62.1 0.28 13.9 24.0 79.7 176.4

MeSPU

1000 ÿ37.1 0.55 ± ± 75.5 123.0

2000 ÿ62.7 0.53 ± ± 81.0 163.2

3000 ÿ69.1 0.55 ± ± 87.0 180.0

Fig. 3. E�ect of SSMW on the TL glow curve of (a) SPU,

and (b) MeSPU.


deduced that an increase in the SSMW of PTMG andMePTMG led to an increment in the TL trap concen-

tration in the hard-segment domains. That implies ahigher electron density hr 2i, and therefore, a betterphase separation [16].

Fig. 4 shows the temperature dispersion of dielectricconstant at 20 kHz for all samples. Dielectric relax-ation processes observed in polymers have been inter-

preted in terms of motional modes of polar groups inlong chain molecules [34]. Few studies have been car-ried out in order to determine the dielectric properties

of segmented polyurethanes [35±37]. In our case, wedid not measure the dielectric process at Tg,s. We onlymeasured it from room temperature to 2008C. Thelarge increase of e ' can be seen at the temperature

range from 120 to 1908C. It is considered to be due tothe mobility of the dipole located in urethane carbonylcaused by the dissociation of the hydrogen bonding,

which is forming hard phase, during heating. In caseof SPU-1000 the major peak shifts to lower tempera-ture, i.e. around 1508C, compared to those of other

samples at around 1908C. This suggests that the hydro-gen bonding in hard phase of SPU-1000 is weakerthan those of other samples, and expects that SPU-

1000 has poor fatigue resistance, in other words,MeSPU-1000 is expected to have better fatigue resist-ance. As the SSMW increased, there was a generaltrend of a shift of this peak to higher temperatures for

both sets of samples. Also, we can see an unnegligibledi�erence in e ' value below 1008C between the ma-terials with SSMW = 1000 and those with SSMWe2000. We can consider two possible reasons for thedi�erence as below. One reason is that the weight frac-

tion of the hard segments which involve the polargroups in the samples with SSMW = 1000 is larger

than that in the SPU with SSMWe2000 as shown inTable 1. Another possible reason is that the SPU withSSMW = 1000 has poor microphase separation

because of the short PTMG soft segment, whichmeans that phase mixing between the soft and hardsegment in these samples is more pronounced com-

pared to the samples with SSMWe2000. As the result,the dipole located in the hard segments resolved in thesoft domains can easily rotate by the electrical ®eld

because of the poor restriction by hydrogen bonding.If the strong hydrogen bonding formed in hard

domain produces the good fatigue resistance, the SPUwhich has the soft segment with large molecular weight

and methyl side-group is expected to have better fati-gue resistance from the dielectric measurement.Fig. 5 shows the phase fractions (%) calculated by

LSM from FID's for SPU and MeSPU samples. Itwas found that each FID consisted of three com-ponents labeled slow, intermediate and fast com-

ponent. These components can be related to the soft,interphase, and hard phase respectively. If a polymerlattice were rigid, the nuclei would feel the full e�ect of

the spin±spin interactions, and T2 would be very short,(that is the case of the hard segments with a T2(h ) of40 ms). As motion is introduced into the lattice (as inthe soft segment matrix with longer T2(s ) of 350 ms),the interaction will ¯uctuate and begin to approach toan average value of zero [38]. Here, we are consideringa third component, T2(i ), as Liu et al. did [16,17].

Nevertheless, it is important to emphasize that theterm `phase' used in this work is de®ned based onrelaxation times, T2, obtained from NMR measure-

Fig. 5. Phase fractions, (%), of SPU (white dots) and MeSPU

(black dots) as cast.

Fig. 4. Dielectric spectra at 20 kHz of SPU and MeSPU

samples.


ments, which gives no structural information in a senseof spatial arrangement. However, these results can give

us an idea about the state of mixing in the materials.In general, T2 values remained almost constant forevery phase, when comparing SPU with MeSPU. Both

kind of materials experienced similar phase-fractionbehavior as the SSMW changed. At the lowest SSMW,the intermediate-phase fraction was the biggest one. As

SSMW increased to 2000, the intermediate-phase frac-tion decreased and the corresponding to soft phaseincreased. Besides, the hard-phase fraction decreased.

Liu et al. assumed that this reduction in the intermedi-ate phase was the result of a promoted phase separ-ation. This is consistent with data from DSC, TL anddielectric constant. A further increase of the SSMW up

to 3000 did not represent a continuation of the trendso far observed.On the other hand, comparing the behavior of SPU

and MeSPU's phase fractions from Fig. 5, it can beseen a lower soft-phase fraction in the samples withmethyl group for SSMWe2000, than for correspond-

ing samples without the methyl group. And, it can alsobe noted a slight increase in the intermediate and hardphase. This could be explained due to the restriction of

a methyl for motion, as it has been already said.From Fig. 6, it can be noted that for both groups of

samples the long period (L, determined from the one-dimensional correlation curves), increased as the

SSMW increased as well. This means that the scatter-ing maximum shifted towards to smaller angles withan increase of SSMW indicating that the average of

interdomain distance increased as well. This is inagreement with the results obtained by Sakurai et al.[18]. The interphase thickness (E ), remains practically

constant as the SSMW increased. It can be easily seenthat samples with methyl group had shorter long

periods and bigger interphase thicknesses than sampleswithout that pendant group. Based on the assumptionof a coiling or folding structure for the hard segments

[6,39], it can be said that the hard domains in SPU aremore ordered (L is larger) than in the case of MeSPU.This is because of the presence of the methyl group on

the soft segment in MeSPU, which could restrict itsmotion, reducing phase separation and therefore,restricting the hard segments from getting an ordered

structure by folding. This fact can be also re¯ected bythe boundary interphase, which was bigger in MeSPUthan in SPU. It was said earlier in this work, that astrong interaction between soft and hard segments

causes internal mixing in the phases, besides the mixingin the boundary zone [40], and a comparison of inter-phase thickness values had to be carried out between

similar materials with same history. Then, by lookingat Fig. 5, bigger E values in MeSPU could beexplained by saying that MeSPU samples may be more

phase mixed than SPU samples.It can be concluded that the phase separation

increased as the SSMW increased. The di�erence of

this separation was, in general, more notable changingSSMW from 1000 to 2000 than from 2000 to 3000.MeSPU samples showed no soft segment crystalliza-tion at all. MeSPU samples also showed more relative

phase mixing than that of corresponding SPU samples,due to the steric hindrance imposed by a methyl groupon the main chain of the soft segment.

4.2. Structural changes in segmented polyurethanes due

to cyclic mechanical fatigue

Table 3 shows the values of the DSC thermal tran-sitions for the samples after fatigue. It is important to

explain that as it can be seen in this table, the corre-sponding values for sample MeSPU-1000 are absentbecause the most of the time this specimen did not

endure upon all the fatigue period. This sample couldbe fatigued as just received from the supplier, but acertain time later, it was almost impossible to do that.

The Tg,s value increased after fatigue in SPU-1000and SPU-3000. This result would be in agreement witha phase mixing assumption due to fatigue. But, thiswas not the behavior of SPU-2000. A decrease of Tg,s

in SPU-2000 after fatigue was noted. For the case ofMeSPU, Tg,s for MeSPU-2000 did not change and Tg,s

for MeSPU-3000 decreased. It was observed as a gen-

eral trend, a decrease of Tm,h after fatigue except forSPU-1000.Comparatively speaking, the variation in all the

above mentioned thermal transitions after fatigue wasbigger for the SPU samples than for MeSPU ones. Infact, we can say from thermal analysis, that the

Fig. 6. Boundary interphase (E ), and long period (L ), of SPU

and MeSPU.


MeSPU-2000 sample experienced fewer changes than

the other ones.Fig. 7a and b shows the TL glow curves before and

after fatigue for SPU-1000 and MeSPU-1000, respect-

ively. It can be noted that more changes were induced

due to fatigue in the case of sample without Me group.

It was observed an increment of the intensity in thepeak denoted III, which Liu et al. [17] referred it as

ascribed to an intermediate phase. This increase wasmore pronounced for SPU-1000. Besides, the peak IIalso increased signi®cantly for SPU-1000. This suggests

that MeSPU-1000 was a more fatigue-resistant ma-terial than SPU-1000, from a TL analysis point of

view. Accordingly, TL analysis con®rmed the resultsobtained by DSC.

Fig. 8a and b show the temperature dispersion ofdielectric constant for SPU-1000 and MeSPU-1000 ascast and after fatigue, respectively.

The major peaks at around 1508C in Fig. 8a and ataround 1908C in Fig. 8b are due to the dissociation of

the hydrogen bonding as mentioned above in Fig. 4.The decrease of the major peak in its intensity and theincrease of the level of e ' value ranging from room

temperature to 1008C can be observed after fatigue forSPU-1000 in Fig. 8a. These results may be come from

the dissociation of the hydrogen bonding by cyclicextension and suggest that the hard-segment chainwith polar groups dissolved into the soft-segment

domains by the fatigue. On the other hand, only theshift of the major peak to lower temperature can be

seen in Fig. 8b for MeSPU-1000. The peak shift tolower temperature indicates the strength of hydrogenbonding decreased after fatigue, i.e., the hydrogen

bonding could be dissociated more easily at lower tem-perature. The fact that the e ' value did not increase at

around room temperature after cyclic extension indi-cates no apparent dissociation of the hard-segmentinto the soft-segment which produces the phase mixing

between the hard and the soft segments in MeSPU-1000. From these results of dielectric measurement

before and after fatigue it is concluded that theMeSPU-1000 has more fatigue resistance compared tothe SPU-1000 as expected.

Fig. 9a and b show the phase fraction values forSPU and MeSPU samples respectively before and after

fatigue as measured by solid NMR. Fig. 9a indicatesan increment of the intermediate phase after fatigue

Table 3

E�ect of the soft segment on the thermal transitions of SPU and MeSPU ®lms

Tg,s (8C) DCpg,s (J/g8C) Tm,s (8C) DHm,s (J/g) Tg,h (8C) Tm,h (8C)

SPU

1000 ÿ38.9 0.53 ± ± 75.4 159.0

2000 ÿ63.8 0.57 5.6 8.4 71.9 169.2

3000 ÿ59.3 0.41 12.7 28.0 ± 174.4

MeSPU

1000 ± ± ± ± ± ±

2000 ÿ62.1 0.56 ± ± 80.3 158.7

3000 ÿ71.9 0.58 ± ± 61.5 174.7

Fig. 7. E�ect of fatigue on the TL curve of (a) SPU-1000, and

(b) MeSPU-1000.


for the SPU samples. The increase of the intermediate

phase due to fatigue agrees with the results obtained

by Liu et al. [17], who used a soft segment based on

PTMG 2000. In contrast, from Fig. 9b, it is seen com-

paratively less change in the fractions after fatigue. In

this case, MeSPU-3000 underwent the lesser variation

in their three phases. In MeSPU-2000, the intermediate

phase remained constant, but changes in the fast and

slow components were observed. Therefore, from

NMR data we can conclude MeSPU samples were

more fatigue resistant, especially in the case of

MeSPU-3000.

Fig. 10a and b show the long period, L, and inter-

phase thickness, E, for SPU and MeSPU samples as

compared before and after fatigue. We can see that L

and E almost remained constant for MeSPU-2000

(Fig. 10b). A decrease in L for SPU-1000 and SPU-

2000 (Fig. 10a), can be interpreted as a fatigue-induceddisordering of the hard domains. For SPU-3000 and

MeSPU-3000, it was observed an opposite e�ect. Itseems, that after fatigue, hard domains in samples withSSMW equal to 3000 have obtained a more ordered

structure. Boundary interphase changed a little for allthe samples, except the MeSPU-3000. In this case, wecould suppose that if an ordered structure inside the

hard domains was obtained, the interphase thicknesscould decrease.Therefore, we can make some conclusions about the

e�ect of fatigue on the structure properties of segmen-ted polyurethanes. A trend can be deduced that a bet-ter fatigue resistance is found in the MeSPU samples,with the exception of MeSPU-1000. Better fatigue re-

sistance in MeSPU samples could be reached due tothe absence of crystallization of the soft segments, as itwas also proved by Sakurai et al. [18]. In the case of

Fig. 8. Fatigue e�ect on e ' of (a) SPU-1000, and (b) MeSPU-

1000.

Fig. 9. E�ect of fatigue on phase fractions in (a) SPU, and

(b) MeSPU. (White and black dots count for phase fractions

before and after fatigue respectively.)


MeSPU-1000, we believe that MeSPU-1000 could notendure all the fatigue period because of the presence of

a large phase mixing, compared with MeSPU-2000 andMeSPU-3000.

5. Conclusions

Structural properties of segmented polyurethanesupon mechanical fatigue were analyzed. E�ect of thesoft-segment molecular weight and a methyl side group

on the microphase separation was also studied.It was observed that the phase separation increased

as the soft-segment molecular weight increased as well.Di�erence of this separation was, in general, more pro-

nounced from 1000 to 2000 soft-segment molecularweight, than from 2000 to 3000. Samples with methylgroup showed no soft-segment crystallization. These

samples also showed more relative phase mixing thancorresponding samples without methyl group.

Upon fatigue, samples with methyl group demon-strated a better fatigue resistance, which could be re-

lated to the absence of crystallization of the softsegments. MeSPU-1000 could not endure fatigue morelonger, maybe due to a detected large phase mixing

compared with samples of the same type.

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effect of the soft segment on the fatigue behavior of segmented polyurethanes

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