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Polymer 53 (2012) 5423e5435

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Polymer

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Studies of 4,40-diphenylmethane diisocyanate (MDI)/1,4-butanediol (BDO) basedTPUs by in situ and moving-window two-dimensional correlation infraredspectroscopy: Understanding of multiple DSC endotherms from intermolecularinteractions and motions level

Cong Li a,*, Jiwen Liu b, Junjun Li c, Fei Shen d, Qishan Huang a, Hongxing Xu a

aNational Polyurethanes Engineering Research Center, Yantai Wanhua Polyurethanes Company, Shandong Province 264000, People’s Republic of ChinabKey Laboratory of Rubber-plastics, Ministry of Education, Qingdao University of Science and Technology, Qingdao City 266042, People’s Republic of ChinacGM R&D Center, Chemical Sciences and Materials Systems Lab, Rab 3-149, 30500 Mound Rd, Warren, MI 48090, United Statesd Polymer Processing Institute GITC Bldg, STE 3901, 218 Central Ave. Newark, NJ 07031, USA

a r t i c l e i n f o

Article history:Received 8 April 2012Received in revised form8 September 2012Accepted 14 September 2012Available online 20 September 2012

Keywords:Thermoplastic polyurethanesEndotherms originTwo-dimensional correlation analysis

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

0032-3861/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.polymer.2012.09.030

a b s t r a c t

In situ infrared spectroscopy and moving-window two-dimensional (MW2D) correlation spectroscopywere employed to study the multiple endotherms observed in differential scanning calorimetry(DSC) curves for linear thermoplastic polyurethanes (TPUs). A 4,40- diphenylmethane diisocyanate (MDI)/1,4-butanediol (BDO) polyester based TPU with a hard segment content of 45 wt % was used in this study.Temperature-dependent infrared absorbance of H-bonded NeH stretching peak before and after 170 �Cannealing both showed three stages, in which the absorbance slopes showed different variation trendswith temperature increase. Correspondingly, two boundary temperatures existed between these stages.The lower boundary temperatures coincided with the onset of endotherms above 100 �C in the corre-sponding DSC curves. While the higher boundary temperatures were located around 200 �C and almostindependent of the annealing effect. These distinct absorbance slopes reflected the interaction changesbetween NeH and C]O groups with temperature increase and revealed the endotherms origin from theintermolecular interactions level. MW2D autocorrelation spectra exhibited the enthalpy relaxation ofamorphous hard segments, which was consistent with the endotherms below 100 �C observed by DSC.MW2D correlation spectra also revealed the specific orders of temperature response during hydrogenbonds dissociation, and indicated free NeH group might play a key role as the starting point oftemperature response during the dissociation. The micro-crystalline structures of hard segments andmorphologies of these samples before and after annealing were characterized by Wide-angle X-rayDiffraction (WAXD) and Atomic Force Microscopy (AFM) respectively. Finally, a schematic evolutioncontaining factors influencing the endotherms behaviors was presented.

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1. Introduction

Thermoplastic polyurethanes (TPUs) are linear segmentedcopolymers, composed of micro-phase structure deriving from thethermodynamic incompatibility between the soft and hardsegments. This microstructure endows these materials witha variety of properties and can be applied in many fields. Therefore,the microstructure of TPUs has been extensively investigated bymany techniques [1e8].

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Differential scanning calorimetry (DSC) is one of the mostcommon methods used to characterize the micro-phase behavior.The origin of the apparent multiple melting endotherms observedvia DSC for many commercial aromatic MDI/BDO based TPUs is animportant and intriguing feature, which is not fully made certainuntil now [9e13]. For this kind of TPUs usually three endothermsare observed. Thefirst endotherm is usually observed at around 20e30 �C above the annealing temperature and is designated as T1. Thisendotherm is considered to be from the enthalpy relaxation of theamorphous hard segment [15e17]. The endotherm detected athigher temperature between 120 and 200 �C is generally ascribed tothe melting of ordered hard segment structure [14e17] and micro-phasemixing of the soft and hard segment [18,19], designated as T2.

Fig. 1. In situ IR spectra measured at room temperature (～25 �C) for (a) TPU-RT(before annealing) and (b) TPU-170C (after annealing).

Table 1Main band assignments of TPUs in the regions of 4000e500 cm�1 according to insitu IR spectrum [33,48e50].

Wavenumber (cm�1) Assignment Vibration type

3441 Free NeH Stretching3335 H-bonded NeH Stretching2960 eCH2e Asymmetry stretching2874 eCH2e Symmetry stretching1733 Free C]O Stretching1702 H-bonded C]O Stretching1597 Benzene ring Framework vibration1533 NeH Bending1416 eCH2e Bending1224 CeO Stretching771 Characteristic absorption of

benzene substitution

C. Li et al. / Polymer 53 (2012) 5423e54355424

The last endotherm observed above 200 �C is designated as T3,which is typically attributed to the melting of the hard segmentmicro-crystalline region [14e17].

The nature of endotherms mentioned above was mainly studiedby DSC and SAXS in the last decades [14e21]. In order to illustratethe origin of these multiple endotherms effectively, some modelthermoplastic polyurethanes were synthesized [15,21,22]. As weknow, the multiple endotherms observed by DSC are directlycorrelated to the microscopic structural transformation, which, inturn, originate from the temperature-dependent molecular inter-actions and chain motions. This inspires us that the origin ofmultiple endotherms can be revealed from the molecular interac-tions and chain motions level instead of the conventionalmorphology level characterized mainly by DSC and SAXS.

In situ infrared spectroscopy has been proved to be a usefulmethod to detect the temperature-dependent intermolecularinteractions and transitions of TPUs [23,24]. However, the inter-molecular interactions relationship and some subtile changes in thesystem cannot effectively be obtained from the in situ infraredspectroscopy. Fortunately, a powerful 2D correlation analysistechnique provides a possibility for further understanding themolecular chain motions behavior. The 2D correlation techniquehas high resolution to the complex band intensity variation and candistinguish the relative rates of change of the spectral intensities[25e33].

In the present study, we employ in situ as well as moving-window two-dimensional (MW2D) infrared correlation spectros-copy to study the origin of multiple melting endotherms of TPUs. Acommon MDI/BDO polyester based TPU with a hard segmentcontent of 45 wt % was used in this study. The temperature-dependent infrared spectra of the sample before and after 170 �Cannealing was investigated, combined with a DSCmeasurement forcomparison. The results obtained by in situ and MW2D e FTIR inthis study cast some light on the origin of multiple endothermsfrom the intermolecular interactions andmotions level. In addition,the specific temperature response orders during hydrogen bondsdissociation for TPUs were revealed by MW2D e FTIR. The corre-sponding micro-crystalline structure and morphology of thesamples before and after annealing were also characterized byWAXD and AFM respectively for the convenience of comparison.

2. Experimental

2.1. Materials

Diphenylmethane diisocyanate (MDI)-MDI100 was obtainedfrom Yantai Wanhua Polyurethanes CO., LTD and butanediol (BDO)was obtained from BASF. Polyester polyol CMA-44 was kindlyprovided by Yantai Huada Chemical Industry Co., Ltd, correspond-ing to the number average molecule weight (Mn) of 2000 g/mol.MDI was stored in a freezer to minimize dimmer formation. Itapproached 50 �C under dry conditions before use. All othermaterials (BDO, POLYOL) were dried at 100 �C in a heated vacuumoven with a pressure of 0.001 MPa for 10 h before use.

2.2. Synthesis

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 and BDO was stirred in a 5 Lmetal bucket for 3 min at 300 rpm. After mixing was completed,MDI was added and stirred at 300 rpm until the viscosity andtemperature started to increase rapidly. The resulting mass was

then transferred to a plate mold and cured for 10 h at 100 �C. Thenthe cured materials were ground to pieces by a pulverizer. Thesepieces were dried at 100 �C for 4 h and injection molded to a 2 mmthick sheet for later use. The mold was kept at a temperature of20 �C and the injection speedwas set to 25 cm s�1 with amaximuminjection pressure of 1600 bar. The injection unit temperatureranging from nozzle to feeder was set 205 �C, 200 �C, 195 �C and190 �C, respectively.

2.3. Characterization methods

2.3.1. In Situ FTIR spectroscopyTPU sheet was initially melted at 240 �C and then casted on one

side of a KBR disk (2.1mm thick) to form a thin layer no thicker than

C. Li et al. / Polymer 53 (2012) 5423e5435 5425

10 mm. A conventional solvent casting method was not applied heredue to the fact that an opaque film would be formed by the solventinduced crystallization effect. TPU film disk samplewas then placedto a vacuum oven with a pressure of 0.001 MPa at room tempera-ture for 24 h. This sample was designated as TPU-RT. Part of theTPU-RT was annealed at 170 �C for 90 min. The annealed samplewas designated as TPU-170C. The TPU film disk samples were thenplaced into an Instec HCS302 variable temperature cell. Thetemperature-dependent absorbance IR spectra were measuredwith Vertex-70 IR spectrometer, which was equipped witha deuterated triglycine sulfate (DTGS) detector. The spectral reso-lution was 4 cm�1 and the number of scans of each spectrum was16. The TPU film samples were protected by dried high-puritynitrogen gas during measurement. A total of IR spectra werecollected from 30 �C to 230 �C at 2.5 �C increments. Simultaneously,the temperature increasing rate was 5 �C/min.

Fig. 2. Temperature-dependent IR spectra of the NeH stretching region for MDI

2.3.2. MW2D correlation analysisBefore performing the 2D correlation analysis, the intensity

of the each IR spectrum was normalized by dividing itsintensity mean value. In this study, all MW2D correlation spectrawere processed and calculated by 2Dshige (Shigeaki Morita,Kwansei-Gakuin University, 2004e2005). During the calculation,the window size was 2m þ 1 ¼ 11. The 5% autocorrelationintensity of MW2D correlation spectra was regarded as noise andwas cut off.

2.3.3. Differential scanning calorimetry (DSC)The thermal behavior was determined using DSC-823e (MET-

TLER Instrument). DSC thermograms were obtained at tempera-tures ranging from �80 to 250 �C with a heating rate of 5 �C/minunder a nitrogen atmosphere. The sample was obtained by peelingoff the film from the surface of the corresponding film KBR disk

/BDO based TPUs in the range 3500e3200 cm�1 for TPU-RT and TPU-170C.

C. Li et al. / Polymer 53 (2012) 5423e54355426

sample mentioned at the FTIR Spectroscopy section. Each sampleweighed 8 mg was cooled at �80 �C for 2 min, then measurementswere carried out up to 250 �C. The temperatures and transitionenthalpy were calibrated with In and Zn standards with a heatingrate of 5 �C/min.

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

the reflection mode using a Philipsgoniometer. Nickel-filtered CuKa radiation (wavelength ¼ 0.1542 nm) was produced by an XRG-3000 generator at an operating voltage of 40 kV and a current of20 mA. The scattering intensity was monitored on a strip-chartrecorder as a function of the scattering angle (2q) between 5 and35� using a goniometer arm speed of 1�/min.

Fig. 3. The normalized intensity variations of H-bonded NeH stretch

2.3.5. Atomic Force Microscopy (AFM)The surfaces of TPU film disk samples were characterized by

AFM. Measurements were carried out by a NanoScope IIIa set-up(made by Digital Instruments), in the tapping mode using phaseimaging. Tapping mode was operated utilizing cantilever vibrationair amplitudes in the range 0.5e2 V. The level of the force applied tothe surface was adjusted by the ratio of the set point amplitude tothe free vibration amplitude in air. The set point amplitude, whichwas used for feedback control, was adjusted to 70e80% of the freeamplitude. A contrast in phase can be due to the result ofdifferences in mechanical properties of the components near thesurface of the scanned material. Soft segments are anticipated togive dark contrast in the phase imaging, while hard segmentsappear as bright areas, provided that phase angle differences due

ing peak as a function of temperature for TPU-RT and TPU-170C.

C. Li et al. / Polymer 53 (2012) 5423e5435 5427

to adhesion and topography contrasts do not have significanteffects.

Fig. 4. DSC thermograms for (a) TPU-RT and (b) TPU-170C.

3. MW2D correlation spectroscopy theory

The basic principles of 2D correlation spectroscopy were firstproposed by Noda to provide insights into the properties of poly-mer systems [34,35]. The general experimental approach in 2Dcorrelation spectroscopy involves the application of an externalperturbation that can selectively excite various chemical compo-nents of a given system. The excitation and subsequent relaxationprocesses, which are manifested in the changes in peak intensities,shifts in spectral band frequencies and variations in peak shapes,are monitored by a given spectroscopic probe. The dynamic spectraare then transformed into 2D data. However, this method loses allinformation along the dynamic coordinate. Thomas and Richardsonintroduced a moving-window two-dimensional correlation spec-troscopy method in 2000 [36]. This method, which uses the datesubdivision technique, is an extension of generalized two-dimensional correlation spectroscopy [37e39], proposed by Nodain 1993 [40]. MW2D can directly observe spectral correlationvariation along both spectral variables (e.g., wavenumber) andperturbation variables (e.g., temperature) axes [41e44]. Accord-ingly, the transition point can be determined from the correlationintensity along the perturbation variables direction.

The theory of MW2D correlation spectroscopy is describedbriefly as below. A detailed description on this theory can bereferred to Tao’s and Noda’s work [29,41].

The formal definition of the generalized 2D correlation spec-trum can be expressed as

Fðv1; v2Þ þ iJðv1; v2Þ ¼ 1pðTmax � TminÞ

ZN

0

Y1w

ðuÞ,Yw

2*ðuÞdu

(1)

where F(v1,v2) is the synchronous correlation spectra and J(v1,v2)is the asynchronous correlation spectra. More detailed interpreta-tion can be found in the paper by Noda [40,45].

The MW2D method is essentially the division of a spectralintensity data matrix into a series of submatrices. Correspondingly,the synchronous correlation spectra F(v1,v2) and asynchronouscorrelation spectra J(v1,v2) can be converted to the sub-spectra Fjand Jj, which are expressed as

Fj ¼ 12m

�Aw

Jðv; tÞ�T

��Aw

Jðv; tÞ�

(2)

Jj ¼12m

�Aw

Jðv; tÞ�T

�H ��Aw

Jðv; tÞ�

(3)

where AwJðv; tÞ is the mean-centered jth submatrix and H is the

Hilbert-Noda [46] transformation. More detailed description can befound in the papers by Tao and Noda [29,41].

For the MW2D autocorrelation spectrum, each row (UAJ(v,tj)) ofMW2D autocorrelation spectrum matrix is obtained by extractingfrom a diagonal line of Fj matrix. Finally, the MW2D autocorrela-tion spectrum matrix can be obtained by sliding window positionfrom j¼1þm to Mem (Considering the number of rows for spectralintensity data matrix and submatrix areM andm, respectively). Forthe MW2D correlation spectrum based on a slice spectrum, eachrow (UFj(v,tj)) of synchronous MW2D correlation spectrum matrixis obtained from a fixed row of Fj(v,tj) matrix, where the ½A

wJðv; tÞ�T

in Eq. 2 is replaced by a fixed row ½AwJðv2; tjÞ�T in A

wJðv; tÞ matrix.

Sliding window position from j¼1þm to Mem, UF(v,t) matrix isobtained. Based on the same principle, asynchronous MW2Dcorrelation spectrum (Uj(v,t)) matrix can also be obtained.

MW2D correlation analysis is especially powerful in studies oftransitions of polymers in response to applied perturbations, andhas been successfully applied in many cases. Vasilis G hassuccessfully applied MW2D spectroscopy for the study of elasticand viscous orientation behavior of a complex side chain liquidcrystalline segmented polyurethane [47]. Slobodan has also appliedMW2D spectroscopy to investigate the precise supramolecular self-assembly nature of poly[di(butyl)vinyl terephthalate] (PDBVT) [30].

4. Results and discussion

4.1. In situ FTIR analysis

In situ IR spectra of TPU-RT and TPU-170C measured at roomtemperature are presented in Fig.1a and b respectively.Major bandshave been assigned as listed in Table 1. Among these assignments,two principal bands corresponding to the free NeH stretching(3441 cm�1) and the H-bonded NeH stretching (3335 cm�1)between NeH and C]O groups were studied [49]. Spectra of theseNeH stretching regions, displayed on the absorbance scale and

C. Li et al. / Polymer 53 (2012) 5423e54355428

recorded as a function of increasing temperature, are shown inFig. 2. It’s clearly shown that with temperature increase, the H-bonded NeH stretching peak declined, while the absorbance peakof free NeH stretching raised up. This phenomenon is attributed tothe dissociation of the hydrogen bonds between NeH and C]Ogroups and has been discussed bymany researchers [48e52]. In thispaper, our interest is focused on the intensity variation as a functionof temperature for H-bonded NeH stretching peak (around 3335e3350 cm�1). For the convenience of comparison, the intensitieshave been normalized and the corresponding temperature-dependent variations are shown in Fig. 3 for TPU-RT and TPU-170C, respectively. It can be found there were three obviousstages in which the slope showed different variation trends. At thefirst stage, the H-bonded NeH peak dropped gradually with theincrease of temperature. While at the second stage, the peakintensities dropped remarkably. Finally, at the third stage, the slopedeclined relatively slowly again. The boundary points betweenthese stageswere defined as P1, P2 from the lower temperature side.Comparing these boundary points, we can find some useful infor-mation. For TPU-RT, P1 was located around 139 �C compared toa higher location around 180 �C of TPU-170C. However, the P2

Fig. 5. (a) MW2D autocorrelation spectra of TPU-RT calculated from the temperature-depelevels represent the projection of MW2D autocorrelation intensity. (b) For the convenience

locations were both located around 200 �C for TPU-RT and TPU-170C. Combined with the corresponding DSC results, we can findthe correlation between them.

Fig. 4 shows the DSC curves of TPU-RT and TPU-170C, respec-tively. Above 100 �C, the endotherm began at 138 �C for TPU-RT and176 �C for TPU-170C.When comparing these temperatures with theP1 temperatures shown in Fig. 3, they were very close to each other.As the previous investigations indicated, the endotherms appearingbetween 120 and 200 �C were assigned to the melting of orderedhard segment structure and the mixing of the soft and hardsegment [14e19]. Therefore, above 138 �C, for TPU-RT ordered hardsegment began to melt and consequently the dissociation ofhydrogen bonds between NeH and C]O groups became vigorous.This explains a remarkable slope decline at the second stage forTPU-RT shown in Fig. 3. Similar to TPU-RT, the same mechanismoccurred in TPU-170C sample with a P1 temperature of 180 �C.

With temperature increase, the slope changed again around200 �C as shown in Fig. 3, which indicates another transitionwithinthe samples. It should be noticed that the changing temperature is200 �C. Previous research works proved that for MDI based TPUsthe endotherm appearing above 200 �Cwas assigned to themelting

ndent IR spectra of TPU-RT (45e230 �C) in the region 3600e3100 cm�1. The contourof comparison, the corresponding DSC curve of TPU-RT is listed in lower part.

C. Li et al. / Polymer 53 (2012) 5423e5435 5429

of hard segment micro-crystalline region [14e17]. As shown inFig. 4, a broad endotherm region appeared below 200 �C and endedabove 200 �C. It was hard to distinguish the boundary temperatureat which the melting origin changed from ordered hard segmentstructure to micro-crystalline region. However, the boundarytemperature can be obviously observed in Fig. 3 by in situ IR, wherea distinct slope change occurred around 200 �C. It can be found theslope declined slowly again above 200 �C. On the other hand, above200 �C, the decline of H-bonded NeH stretching absorbance isknown to be related to the melting of hard segment micro-crystalline region. Therefore, the relatively slower decline of slopein stage 3 indicated that the hydrogen bonds were more stable inmicro-crystalline region and dissociation become harder. Thoughthe annealing effect improved a regular arrangement of the hardsegment and leaded to a higher P1 temperature for TPU-170C, theP2 temperatures were almost the same for both samples. This resultindicated that P2 temperature was determined by the microdomainstructures, almost independent of the annealing effect.

It’s also worth mentioning that the endothermic peak below100 �C. For TPU-RT, a weak endotherm appeared between 40 and90 �C (Fig. 4a). After annealing at 170 �C for 90 min, endothermcould hardly be found below 100 �C (Fig. 4b). The endothermappearing in this temperature range was usually attributed to theenthalpy relaxation of the amorphous hard segment [14e17]. After

Fig. 6. (a) MW2D autocorrelation spectra of TPU-170C calculated from the temperature-depelevels represent the projection of MW2D autocorrelation intensity. (b) For the convenience

a 170 �C annealing, amorphous hard segment rearranged to anordered structure, which restricted the mobility of hard segmentaround its glass transition and the corresponding enthalpy relax-ation as well as glass transition could not occur. However, thisrestriction could not freeze the dissociation of hydrogen bondsbetween NeH and C]O groups. It could be seen from Fig. 3 thata continuous decrease of H-bonded NeH absorbance at stage 1,implying the dissociation of hydrogen bonds between NeH and C]O groups. This phenomenon also indicates that the dissociation ofhydrogen bonds make no contribution to the endotherms in theDSC. A similar result was found by Seymour, R. W [15].

The discussion above investigated the efficiency of in situ FTIRin studying the multiple endotherms origin of MDI based TPUs.In spite of the dissociation of hydrogen bonds between NeHand C]O groups had no contribution to the endotherms, thetemperature-dependent interaction change detected by in situFTIR can reveal the endotherms origin from the intermolecularinteractions level.

Compared with in situ FTIR, MW2D infrared correlation spec-troscopy has a high resolution to the band intensity variation andcan distinguish the relative rates of change of the spectral inten-sities. For a further comprehension of the intermolecular interac-tions and molecular motions in the samples, MW2D infraredcorrelation spectroscopy was applied in this study.

ndent IR spectra of TPU-170C (45e230 �C) in the region 3600e3100 cm�1. The contourof comparison, the corresponding DSC curve of TPU-170C is listed in lower part.

C. Li et al. / Polymer 53 (2012) 5423e54355430

4.2. MW2D correlation analysis

The absorbance intensity changes are characteristics of thespecificity or magnitude of the H-bonded NeH stretching, whichcan reflect the states of hydrogen bonds formed in different kinds ofhard segment domains. Figs. 5a and 6a show theMW2D correlationspectrum based on autocorrelation calculations in the NeHstretching region (3100e3600 cm�1) for TPU-RT and TPU-170C,respectively. Figs. 5b and 6b are the corresponding DSC curves. Asshown in Fig. 5a, around 3335 cm�1 positive correlation intensitypeak was observed at 170 �C. As mentioned above, 3335 cm�1 wasassigned to the H-bonded NeH stretching. It indicated thata remarkable dissociation of hydrogen bonds between NeH andC]O groups occurred at 170 �C for TPU-RT. A similar peak appearedfor TPU-170C, however, the correlation intensity peak was located

Fig. 7. Temperature-dependent IR spectra of the C]O stretching region for MD

at a higher temperature around 190 �C. This meant a more orderedhard segment structure was formed in TPU-170C sample due to theannealing effect at 170 �C, which leaded to a higher temperature forthe remarkable dissociation of hydrogen bonds. In addition, nocorrelation intensity peak was observed above 200 �C, whichmeant the dissociation of hydrogen bonds above 200 �C was not asremarkable as it did below 200 �C.

In addition, what should be paid attention is the long tail of thecorrelation intensity peak around 3335 cm�1, which extends from112 �C to 45 �C as shown in Fig. 5a. Associated with the corre-sponding DSC curve, we can speculate the origin of the tail. It can beseen from Fig. 5b that an endotherm appeared below 100 �C, whichis assigned to the enthalpy relaxation of the amorphous hardsegment in TPU-RT sample. In this process, the slow relaxation ofthe hard segment chains from its equilibrium conformation leaded

I/BDO based TPUs in the range 1800e1650 cm�1 for TPU-RT and TPU-170C.

C. Li et al. / Polymer 53 (2012) 5423e5435 5431

to a relative change rate of the H-bonded NeH spectral intensity.Correspondingly, aMW2D correlation intensity tail shown in Fig. 5aappeared ranging from 45 �C to 112 �C.

On the contrary, this correlation intensity tail was not observedin Fig. 6a. Associated with the corresponding DSC curve shown inFig. 6b, endotherm below 100 �C could hardly be found. Therefore,for TPU-170C sample, a relatively ordered hard segment structuredue to the annealing at 170 �C restricted the mobility of molecularchain below 120 �C. The difference between molecular chainmotions states below 120 �C can be detected by the MW2D

Fig. 8. MW2D correlation spectra based on a slice spectrum calculated from the temperaturein the top rectangle is a corresponding static IR spectrum. The contour levels represent the prbonded C¼O vibration of the carbonyl group. (a) Synchronous MW2D correlation spectra;

correlation infrared spectra compared with the in situ infraredspectra, which lacks the distinguish ability.

During the hydrogen bonds dissociation process, the specifictemperature response order between NeH and C]O groups is alsoan interesting aspect to study. Except for enhancing spectral reso-lution, 2D correlation spectroscopy can also discern the specificorder taking place under external perturbation. Therefore, theMW2D correlation spectrum, based on a slice spectrum in the NeHstretching region (3600e3100 cm�1), was calculated. Fig. 7 showsthe IR spectra of carbonyl groups as a function of temperature for

-dependent IR spectra of TPU-RT (45e230 �C) in the region 3600e3100 cm�1. The curveojection of MW2D correlation intensity. The slice point, vb ¼ 1702 cm�1, assigned to the(b) Asynchronous MW2D correlation spectra.

C. Li et al. / Polymer 53 (2012) 5423e54355432

TPU-RT and TPU-170C, respectively. It can be found there were twopeaks in this band region, and the peak located at lower wave-number around 1702 cm�1 corresponded to the bonded carbonylgroup [53,54]. The wavenumber 1702 cm�1 was thus used as theslice point in the MW2D correlation spectrum calculation.

Fig. 8 shows the synchronous MW2D correlation spectrum andasynchronous MW2D correlation spectrum for TPU-RT. Around205 �C, Uj (3335 cm�1, 205 �C)< 0, UF (3335 cm�1, 205 �C) > 0 and

Fig. 9. MW2D correlation spectra based on a slice spectrum calculated from the temperatucurve in the top rectangle is a corresponding static IR spectrum. The contour levels representto the bonded C¼O vibration of the carbonyl group. (a) Synchronous MW2D correlation sp

Uj (3441 cm�1, 205 �C) > 0, UF (3441 cm�1, 205 �C) > 0. Accordingto Noda’s rule [40], the intensity changes at band 3335 cm�1

occurred after the changes at 1702 cm�1, while the intensitychanges at bands 3441 cm�1 occurred before the changes at1702 cm�1. This indicates that the specific temperature responseorders in the hydrogen bonds dissociation process are as follows(/means earlier than): 3441 cm�1 / 1702 cm�1 / 3335 cm�1,that is vf (NeH) / vb (C]O) / vb(NeH). It revealed that the free

re-dependent IR spectra of TPU-170C (45e230 �C) in the region 3600e3100 cm�1. Thethe projection of MW2D correlation intensity. The slice point, vb ¼ 1702 cm�1, assignedectra; (b) Asynchronous MW2D correlation spectra.

Fig. 10. X-ray diffractometer scans of TPU-RT and TPU-170C.

C. Li et al. / Polymer 53 (2012) 5423e5435 5433

NeH group firstly responded to the temperature increase, and thenthe bonded carbonyl group, finally the bonded NeH group. Basedon this analysis result, it is speculated that the free NeH groupmight play a key role as the starting point of temperature responsein hydrogen bonds dissociation process. Based on the same prin-ciple, similar specific orders in the hydrogen bonds dissociationprocess were obtained in Fig. 9. Compared with TPU-RT, it can befound that the asynchronous correlation intensity peak of TPU-170C appeared around at 215 �C, 10 �C higher than that of TPU-RT. For TPU-170C, a more distinct micro-phase separation withlarger regular hard segment domains existed due to the annealingeffect. This means the content of free NeH group is lower in TPU-170C sample compared with TPU-RT sample. This speculation canalso be testified by the ratio of free NeH absorbance height to thebonded NeH absorbance height around 200 �C (Af-TPU-RT (NeH)/Ab-

TPU-RT (NeH) ¼ 0.51 > Af-TPU-170C (NeH)/Ab-TPU-170C (NeH) ¼ 0.40).As mentioned above, the free NeH group might play a key role asthe staring point of temperature response during hydrogen bondsdissociation. Because of the relatively low content of free NeHgroup in TPU-170C, the action as starting point of temperatureresponse probably need a higher temperature relative to TPU-RT,which leaded to the asynchronous correlation intensity peakappeared at a higher temperature.

Compared with the dissociation of hydrogen bonds occurringabove 200 �C, the dissociation was more prominent below 200 �Cand had been discussed in in situ FTIR section. However, the

Fig. 11. AFM topographical image (a, vertical scale 0e5

asynchronous correlation intensity peak was not observed below200 �C. This indicates that there are no specific orders of temper-ature response in the hydrogen bonds dissociation process. Theprobable reason is that the intersegmental mixing between hardand soft segment weaken the bonded strength between NeH andcarbonyl group, which is stronger in micro-crystalline domains dueto a regular lattice arrangement, and causes a faster temperatureresponse relative to the behavior above 200 �C.

4.3. Micro-crystalline structure and morphology characterization

In order to have a certain understanding of the micro-crystallineand morphology before and after annealing, WAXD and AFM wereused respectively. X-ray diffractions of the samples are shown inFig. 10. The crystalline structure of the hard segment based on MDI/BDO has been studied by ROBERT, M [55] and a remarkablediffraction peak around 2q ¼ 20� is a characteristic of MDI/BDOhard segment reflection. The fairly wide diffraction peak indicatesa lack of high organized crystal structure in these samples, even asTPU-170Cwhich had been annealed at 170 �C for 90min. Accordingto Scherrer method [56], a narrower half-breadth of diffractionpeak indicates a larger average crystallite size. Compared with TPU-RT, a narrower half-breadth of diffraction peak for TPU-170C wasfound in Fig. 10, meaning that a relatively larger crystallite sizeformed after a 170 �C annealing.

The topographical and phase images for TPU-RT and TPU-170Care shown in Figs. 11 and 12 respectively. The peak to valleyheight variation for topographical image is on the order of 5 nm,meaning a flat topography and less topographical contrast, whichhas little effect on the phase contrast. Light and dark areas shown inphase images are correlated with different phase responses. Weattribute the light areas to the hard domains and dark areas to thesoft domains. It can be found hard domains and soft domains arerandomly mixed and multi-phase separation exists. No regularhard domain can be found even for TPU-170C (Fig. 12b), which isconsistent with the fairly broad diffraction peak observed in Fig. 10.In addition, compared with Fig. 11b, a larger scale light areas cor-responding to hard domains exist in Fig. 12b.

So far we have a comprehensive recognization of the multipleendotherms origins from the point of view of intermolecularinteractions to aggregate structures. The schematic evolution withtemperature increase based on the analysis above is presentedin Fig. 13.

nm) and phase image (b, scale 0e20�) for TPU-RT.

Fig. 12. AFM topographical image (a, vertical scale 0e5 nm) and phase image (b, scale 0e20�) for TPU-170C.

Fig. 13. Schematic evolution for TPU-RT and TPU-170C with temperature increase: Yellow rectangle represents hard segment; Blue curve represents soft segment; Red circle in themorphology sketch represents enthalpy relaxation of amorphous hard segments; Brown dotted circle area in the morphology sketch represents ordered hard segments; Greendotted circle area in the morphology sketch represents hard segment micro-crystalline region; Red dashed arrows in the hard segment chemical structure represent the specificorders of temperature response between NeH and C]O groups in hydrogen bonds dissociation process. (For interpretation of the references to color in this figure legend, the readeris referred to the web version of this article.)

C. Li et al. / Polymer 53 (2012) 5423e54355434

5. Conclusions

In this paper, in situ FTIR and MW2D correlation spectroscopywere employed to investigate the multiple DSC endotherms ofMDI/BDO based TPUs before and after 170 �C annealing. In situ FTIRrevealed three stages of variation in the absorbance with temper-ature increase. Boundary temperatures between these stages werediscussed in detail. P1 temperatures were close to the onsettemperatures of endotherms observed in the corresponding DSCcurves, indicating a start of melting of the ordered hard segmentstructures. P2 temperatures were located around 200 �C and almostindependent of the annealing effect, indicating a start of melting ofhard segment micro-crystalline region. Compared with in situ FTIR,MW2D correlation spectroscopy has greater power to discern the

endotherms origins. A long tail observed in autocorrelation spec-trum around 3335 cm�1 for TPU-RT indicated the enthalpy relax-ation of the amorphous hard segment, which was consistent withthe endotherms observed below 100 �C for TPU-RT in DSC curve. Inaddition, the specific temperature response order during hydrogenbonds dissociation was revealed by MW2D correlation spectros-copy. It was found that the free NeH group firstly responded to thetemperature increasing, then the bonded carbonyl group, andfinally the bonded NeH group. Free NeH group might play a keyrole as the staring point of temperature response during hydrogenbonds dissociation. WAXD and AFM data verified larger size hardsegment domains formed after 170 �C annealing and a sketch waspresented to describe the morphological transitions with temper-ature increase. The endotherms origins were revealed from the

C. Li et al. / Polymer 53 (2012) 5423e5435 5435

intermolecular interactions and motions level by in situ FTIR andMW2D correlation spectroscopy.

Acknowledgments

The authors would like to thank Dr. Chong Sun for FTIR exper-iment assistance, Dr. Shaolei Wang for help with the AFM andWAXD experiments at Analyzing & Testing Center of East ChinaUniversity of Science and Technology, and Professor JianmingZhang for helpful technical discussions.

References

[1] Miller JA, Lin SB, Hwang KKS, Wu KS, Gibson PE, Cooper SL. Macromolecules1985;18:32e44.

[2] Mclean RS, Sauer BB. Macromolecules 1997;30:8314e7.[3] Abouzahr S, Wilkes GL. Polymer 1982;23:1077e86.[4] Blackwell J, Lee CD. J Polym Sci Polym Phys Ed 1983;21:2169e80.[5] Tocha E, Janik H, Debowski M, Vancso GJ. J Macro Sci Part B 2002;41:1291e304.[6] Yamasaki S, Nishiguchi D, Kojio K, Furukawa M. Polymer 2007;48:4793e803.[7] MishraAK,ChattopadhyayS,RajamohananPR,NandoGB.Polymer2011;52:1071e83.[8] Li C, Han JJ, Huang QS, Xu HX, Li XH. Polymer 2012;53:1138e47.[9] Saiani A, Novak A, Rodier L, Eeckhaut G, Leenslag JW, Higgins JS. Macromol-

ecules 2007;40:7252e62.[10] Wilkes GL, Bagrodia S, Humphries W, Wildnauer RJ. Polym Sci Part C: Polym

Lett 1975;13:321e7.[11] Ryan AJ, Macosko CW, Bras W. Macromolecules 1992;25:6277e83.[12] Chang YJP, Wilkes GL. J Polym Sci Part B: Polym Phys 1975;13:455e76.[13] Koberstein JT, Galambos AF. Macromolecules 1992;25:5618e24.[14] Chen TK, Shieh TS, Chui JY. Macromolecules 1998;31:1312e20.[15] Seymour RW, Cooper SL. Macromolecules 1973;6:48e53.[16] Hesketh TP, VanBogart JWC, Cooper SL. Polym Eng Sci 1980;20:190.[17] VanBogart JWC, Bluemke DA, Cooper SL. Polymer 1981;22:1428.[18] Leung LM, Koberstein JT. Macromolecules 1986;19:706.[19] Koberstein JT, Russell TP. Macromolecules 1986;19:714e20.[20] LiYJ, Liu J,YangHC,MaDZ,ChuBJ. J PolymSciPartB:PolymPhys1993;31:853e67.[21] Bonart R. J Macro Sci Part B 1968;B2(1):115e38.[22] Saiani A, Daunch WA, Verbeke H, Leenslag JW, Higgins JS. Macromolecules

2001;34:9059e68.[23] Zimmermann B, Vrsaljko D. Polym Test 2010;29:849e56.[24] Suplee H, Wang YK, Hsu SL. Macromolecules 1987;20:2089.

[25] Boyer RF. J Appl Polym Sci 1986;32:4075.[26] Donth E, Beiner M, Reissig S, Korus J, Garwe F, Vieweg S, et al. Macromolecules

1996;29:6589.[27] Ngai KL, Plazek DJ, Bero CA. Macromolecules 1993;26:1065.[28] Plazek DJ, Chay IC, Ngai KL, Roland CM. Macromolecules 1995;28:6432.[29] Zhou T, Zhang AM, Zhao CS, Liang HW, Wu ZY, Xia JK. Macromolecules 2007;

40:9009e17.[30] Sun ST, Tang H, Wu PY, Wan XH. Phys Chem Chem Phys 2009;11:9861e70.[31] Shen Y, Wu PY. J Phys Chem B 2003;107:4224e6.[32] Wang JP, Chen JX, Hochstrasser RM. J Phys Chem B 2006;110:7545e55.[33] Wang XA, Huang GS, Wu JR, Nie YJ, He XJ. J Phys Chem B 2011;115:

1775e9.[34] Noda IJ. Am Phys Soc 1986;31:520.[35] Noda IJ. Am Chem Soc 1989;111:811.[36] Thomas M, Richardson HH. Vibr Spectrosc 2000;24:137e46.[37] Noda I, Dowrey AE, Marcott C, Story GM, Ozaki Y. Appl Spectrosc 2000;54:

236e48.[38] Noda I, Ozaki Y. Two-dimensional correlation spectroscopy applications in

vibrational and optical spectroscopy. Chichester, U.K: John Wiley &Sons;2004.

[39] Wu YQ, Meersman F, Ozaki Y. Macromolecules 2006;39:1182e8.[40] Noda I. Appl Spectrosc 1993;47:1329e36.[41] Morita S, Shinzawa H, Tsenkova R, Noda I, Ozaki Y. J Mol Struct 2006;799:

111e20.[42] Shinzawa H, Morita S, Noda I, Ozaki Y. J Mol Struct 2006;799:28e33.[43] Morita S, Shinzawa H, Noda I, Ozaki Y. J Mol Struct 2006;799:16e22.[44] Sasic S, Katsumoto Y, Sato N, Ozaki Y. Anal Chem 2003;75:4010e8.[45] Noda I. Vibr Spectrosc 2004;36:143e65.[46] Noda I. Appl Spectrosc 2000;54:994e9.[47] Gregoriou VG, Rodman SE, Nair BR, Hammond PT. J Phys Chem B 2002;106:

11108e13.[48] Wang FC, Feve M, Lam TM, Pascault JP. J Polym Sci Part B: Polym Phys 1994;

32:1305e13.[49] Wang FC, Feve M, Lam TM, Pascault JP. J Polym Sci Part B: Polym Phys 1994;

32:1315e20.[50] Coleman MM, Lee KH, Skrovanek DJ, Painter PC. Macromolecules 1986;19:

2149e57.[51] Xiu YY, Zhang ZP, Wang DN, Ying SK. Polymer 1992;33:1335e8.[52] Coleman MM, Skrovanek DJ, Hu JB, Painter PC. Macromolecules 1988;21:

58e65.[53] Zha LS, Wu MY, Yang JJ. J Appl Polym Sci 1999;73:2895e902.[54] Yilgör E, Burgaz E, Yurtsever E, Yilgör I. Polymer 2000;41:849e57.[55] Robert M, Thomas EL. J Macro Sci Part B 1983;22:509e28.[56] Kasai N, Kakudo M. X-ray diffraction by macromolecules. Japan: Springer;

2005.