Shape-memory polyurethanes minimally crosslinked with hydroxyl-terminated AB2-type hyperbranched polyurethanes

Shape-Memory Polyurethanes Minimally Crosslinkedwith Hydroxyl-Terminated AB2-TypeHyperbranched Polyurethanes

Chickiyan Sivakumar, A. Sultan Nasar

Department of Polymer Science, University of Madras, Guindy Campus, Chennai 600 025, India

Received 17 December 2009; accepted 21 June 2010DOI 10.1002/app.33084Published online 22 October 2010 in Wiley Online Library (wileyonlinelibrary.com).

ABSTRACT: A series of shape-memory polyurethanesbased on poly(e-caprolactone) diol were prepared withnovel hydroxyl-terminated hyperbranched polyurethanesas crosslinkers and were characterized by Fourier trans-form infrared spectroscopy, 1H-NMR, gel permeation chro-matography, differential scanning calorimetry, scanningelectron microscopy, wide-angle X-ray diffraction,dynamic mechanical analysis, tensile testing, and shape-memory testing. The molecular weight of the soluble poly-mers ranged from 5.1 � 104 to 29.0 � 104 g/mol. The dif-ferential scanning calorimetry and wide-angle X-raydiffraction data indicated that when the crystallinities ofthe crosslinked polymers were compared to that of linear

polyurethane, this parameter was improved when thecrosslinker was in low quantity. The storage modulusratios obtained from the dynamic mechanical analysesdata of the crosslinked polymers were also high comparedto that of the linear polyurethane. As a result, crosslinkedpolymers showed better shape-memory properties com-pared to the linear polyurethanes. Also, the that incorpora-tion of the hyperbranched polymer as a crosslinker intothe polyurethane chain improved the thermal and me-chanical properties of the polymer. VC 2010 Wiley Periodicals,Inc. J Appl Polym Sci 120: 725–734, 2011

Key words: branched; crosslinking; thermal properties

INTRODUCTION

Shape memory refers to the ability of certain materialsto remember their original shape even after rathersevere deformations. In recent years, shape-memorypolymers (SMPs), especially thermally stimulatedSMPs, have received increasing attention because oftheir wide availability and broad possible applica-tions in various areas of everyday life. Such applica-tions include medical devices for treating stroke1

and vascular and coronary diseases,2,3 orthodontics,4

catheters and smart stents,5 implants for minimallyinvasive surgery,6 information storage that can allowthermally reversible recording,7 and smart fabrics.8

One of the main approaches for the preparation ofSMPs is to polymerize two different monomers; thatprocess should result in a copolymer consisting of asoft-segment phase that is able to store the deformedtemporary shape and a hard-segment phase. In mostSMPs, the hard-segment phase is a physical networkthat is responsible for storing the permanent shapeand mechanical properties of the polymer. Many

chemically crosslinked SMPs have been reported.9–19

Among these reports, some have dealt with shape-memory polyurethanes.15–19 In polyurethane, the po-lar urethane groups, in which strong secondary mo-lecular forces are present, act as a physical network.In addition to this, chemical crosslinks can be intro-duced by the deliberate addition of crosslinker dur-ing polyurethane synthesis. A wide range of cross-linked SMPs were reviewed lightly by Behl andLendlein20 in their review.Hyperbranched polymers (HBPs) are a new class

of emerging materials. They have a large number ofterminal functional groups, as they grow in dendriticfashion; for this reason, these materials show un-usual properties compared to their linear analogues.Their applications span different technologicalareas.21 The crosslinking process is an importantapplication area for HBPs, provided the molecularweight of the polymer is substantially low and nar-rowly distributed. The reason to recommend low-molecular-weight HBPs as crosslinkers is that theyhave a lower number of terminal functional groups,which is essential to the control of gelation. Thenarrow molecular weight distribution is a usefulparameter for stoichiometric calculations. Recently,we reported the use of amine-terminated hyper-branched polyamides with weight-average molecularweight (Mw) values of 2337–13,063 as crosslinkers inthe preparation of polyurethane elastomers and

Correspondence to: A. Sultan Nasar ([email protected]).

Contract grant sponsor: Indian Space Research Organization.

Journal of Applied Polymer Science, Vol. 120, 725–734 (2011)VC 2010 Wiley Periodicals, Inc.

found that a very low amount (0.01 equiv vs 1.0equiv of the polyurethane prepolymer) of HBPs witha degree of polymerization (DP) approximatelyequal to 11 was sufficient for improving the tensilestrength to a great extent.22

In the light of the observation made in our previousstudy, we intended to extend the use of HBP as acrosslinker in the synthesis of shape-memory poly-urethanes based on poly(e-caprolactone) (PCL) diol,which is a soft-switching segment. Thus, in this arti-cle, we describe the synthesis of shape-memory poly-urethanes with novel hydroxyl-terminated hyper-branched polyurethanes (HBPUs) as crosslinkers, afeat not attempted yet. We believed that because thecrosslinker (i.e., HBPU) used here was in polymericform and was fully hard segmented, it wouldenhance the hard-segment/soft-segment microphaseseparation in the polyurethane. Because of thisenhancement in the phase separation, the crystalliz-ability of PCL chain and associated stimuli responsiveeffect should be improved. We also envisaged thatbecause of HBP as was used as a crosslinker, theresulting thermoresponsive polymer would showimproved mechanical properties at the temperature atwhich the temporary shape was fixed or the perma-nent shape was recovered (Ttrans); this is desirable forpolymer materials being considered for end uses.

EXPERIMENTAL

Materials

4,40-Methylene bis(phenylisocyanate) (MDI; Lancas-ter), 1,4-butanediol (BDO; Alfa Aesar, Heysham,Lancs, UK), and dibutyl tin dilaurate (DBTDL; Lan-

caster) were used as received. Poly(e-caprolactone)diol with a number-average molecular weight (Mn)of 4000 g/mol (PCL-4000) was obtained from DaicelChemical Industries, Ltd. (Japan) and was dried at100�C in vacuo for 6 h before use. Hydroxyl-termi-nated HBPUs (Mw’s ¼ 3500 and 4500) were preparedaccording to our previous report.23 Dimethylforma-mide (DMF; Merck, Mumbai, India) and methylethyl ketone (MEK; Merck) were purified accordingto standard procedures.

Synthesis of the HBPU-crosslinkedshape-memory polyurethanes

HBPU-crosslinked shape-memory polyurethaneswere prepared by a two-step method. In the firststep, 1.87 g (15 � 10�3 equiv) of MDI was placed ina reaction vessel. To this, we added 10 g (5 � 10�3

equiv) of PCL-4000 melt dropwise with stirringunder a nitrogen atmosphere; the dropping rate wassuch that it took 1 h for complete transfer. The reac-tion time was 8 h; the first 2 h were maintained at60�C, and the next 6 h were maintained at 75�C. Af-ter 8 h, the temperature was reduced to 40�C, andthe ANCO content was determined by the dibutylamine method;24 the results were close to theexpected theoretical value (observed value ¼ 3.65%;theoretical value ¼ 3.53%). On the basis of theANCO content, this amount of polyurethane prepol-ymer was found to be 0.01 equiv. In the second step,0.45 g (0.01 equiv) of BDO or a mixture of BDO andHBP (see Table I; the equivalents given were func-tionality based) and 0.04 g of DBTDL were dissolvedin 15 mL of 4 : 1 MEK/DMF mixed solvent, and this

TABLE IMolecular Weight Data for the Linear and Crosslinked Shape-Memory Polyurethanes

Polymercode

MDIPP/BDO/HBPU(10�2 equiv)

Hard-segmentcontent (wt %)a

Mw

(�104 g/mol)Polydispersity

index

LPUb 1/1/0 18.8 5.1 1.47CPU-1 1/0.99/0.01 19.0 5.5 1.59CPU-2 1/0.94/0.06 19.4 6.3 2.03CPU-3 1/0.90/0.10 19.8 8.7 2.50CPU-4 1/0.85/0.15 20.0 29.0 4.84CPU-5 1/0.83/0.17 20.5 – –CPU-6 1/0.80/0.20 20.8 – –CPU-7 1/0.85/0.15 19.8 5.9 1.62

The HBPU Mw values were 3500 g/mol for CPU-1 to CPU-6 and 4500 g/mol forCPU-7. MDIPP ¼ MDI based Polyurethane Prepolymer.

a The hard-segment content was calculated according to the feed weight ratio:

Hard-segment content ðwt%Þ ¼ ðWMDI þWBDO=HBPUÞ=ðWMDI þWBDO=HBPU þWPCL�4000Þ� � 100

where WMDI is the weight of MDI, WBDO/HBPU is the weight ratio of BDO and HBPU,and WPCL-4000 is the weight of PCL-4000.

b LPU was prepared from PCL diol, MDI, and BDO.

726 SIVAKUMAR AND SULTAN NASAR

Journal of Applied Polymer Science DOI 10.1002/app

solution was added to the reaction vessel with anadditional funnel for 15 min under strong agitation.The polymerization reaction was followed by thedisappearance of the absorption peak due to ANCOgroups in the IR region 2250–2270 cm�1. The timerequired for the completion of the reaction was 90min. After the completion of the reaction, the highviscous reaction mass was diluted with 10 mL of the4 : 1 MEK/DMF mixed solvent and was cast on aTeflon plate with a thickness of 5 mm. Initially, thefilm was dried at room temperature for 3 h and thenmaintained in a vacuum oven at 70�C for 6 h.

Characterization

Fourier transform infrared (FTIR) and 1H-NMRanalyses

The FTIR spectra of the polymers were recorded on aThermo Matson (Madison, USA) Satellite model FTIRspectrometer by a neat method. The 1H-NMR spec-trum of the HBPU-crosslinked polymer was recordedon a Bruker 300 MHz NMR instrument in hexadeuter-ated dimethyl sulfoxide with tetramethylsilane(Me4Si) as an internal standard.

Gel permeation chromatography (GPC) analyses

Mw, polydispersity (Mw/Mn), and molecular weightdistribution of the polymers were estimated with aPL-GPC 50 (Polymer Laboratory, UK) gel permeationchromatograph equipped with a refractive-index de-tector and 5-lL mixed columns. DMF was used as aneluent, and the instrument was calibrated with narrowmolecular weight polystyrene standards.

Thermal characterization

Thermal properties, such as the glass-transition tem-perature (Tg), crystalline melting temperature (Tm),and crystallization temperature (Tc) of the HBPU-crosslinked polyurethanes were measured with aNetzsch (Selb/Bavaria, Germany) DSC-200PC differ-ential scanning calorimeter: a first heating rate of10�C/min from �100 to 100�C, a cooling rate of 10�C/min from 100 to �100�C, and a second heating rate of10�C/min from �100 to 100�C were applied under anitrogen atmosphere. Thermogravimetric analysis(TGA) were carried out in a Zetzsch-STA 409C/CDthermal analyzer (Selb/Bavaria, Germany) from 30 to800�C at a heating rate of 10�C/min under a nitrogenatmosphere with a gas flow rate of 90 mL/min.

Wide-angle X-ray diffraction (WAXD) and scanningelectron microscopy (SEM) analyses

WAXD analyses were performed with a RigakuMiniflex desktop X-ray diffractometer with a Cusource of radiation (k ¼ 1.541 A) operating in trans-mission mode. The voltage and current used were

30 kV and 15 mA, respectively, and the exposuretime was 30 min. Diffraction spectra were scannedat 2y ¼ 10–45�. SEM measurements were carried outon a JEOL JSM 6362 instrument at 10 kV.

Dynamic mechanical analysis (DMA)

The sample films were prepared for DMA measure-ments through film casting. The film dimensionswere 0.4 mm (thickness) � 3.5 mm (width) � 25 mm(length). DMA measurements were carried out witha Netzsch (Selb/Bavaria, Germany) DMA 242 instru-ment under tensile mode at a fixed frequency of 1Hz, a maximum dynamic force of 4.0 N, and an os-cillation amplitude of 120 lm under a nitrogenatmosphere. The measurements were carried outfrom �100 to 150�C at a heating rate of 10�C/min.

Shape-memory behavior

The polymer samples were cut into rectangularstrips with dimension of 0.4 � 5 � 50 mm3. Theshape-memory behavior was examined by a bendingtest as follows. In a typical experiment, the specimenwas deformed to angle hi (hi ¼ initial deformationangle ¼ 90�) at 60�C and then cooled quickly to 0�Cto fix the temporary bent shape by crystallization.The deformed specimen was placed in an oven (fab-ricated with an additional glass door) and main-tained at testing temperature (50�C or 60�C or 70�C).Immediately, the changes in the angle (recovery oforiginal shape) were recorded through the glassdoor with a digital video camera (Canon Powershot)at a rate of 12 frames/s. We selected 25 frames fromthe video picture, and their deformed angle withrespect to time were calculated. With these data, theratio of recovery was defined as (hi � hf)/hi. (hf ¼final deformation angle ¼ 0� to 90�).

Mechanical properties

Tensile measurements were carried out on speci-mens with dimensions of 5 mm (width) � 40 mm(length) with a TKG-E-C-500N tensile testingmachine (Hi-Tech India Equipments, Ltd.). Thethickness of each specimen was measured (FineSpavy Associates & Engineers Pvt. Ltd., Miraj, India)and recorded as the average of four measurements.The grip separation rate was 100 mm/min. For eachpolymer, two tests were carried out, and the averagevalues are reported.

RESULTS AND DISCUSSION

Synthesis of the HBPU-crosslinkedshape-memory polyurethanes

Generally, polyurethanes are prepared with a poly-urethane prepolymer in combination with a short-chain glycol (chain extender) and/or a crosslinking

SHAPE-MEMORY POLYURETHANES 727


agent with a functionality of three or more. Isocya-nurates of different diisocyantes and polyols ofhigher functionality are used as crosslinking agents.In this study, the polyurethane prepolymer was pre-pared from MDI and PCL-4000. Here, PCL-4000was chosen as a switching segment, and its Tm was

60�C. The hydroxyl-terminated HBPUs (Mw ¼ 3500g/mol, dispersity ¼ 1.34) and polymer (Mw ¼ 4500g/mol, dispersity ¼ 1.54) were used as crosslinkers(Scheme 1). The approximate number of terminalhydroxyl groups (B) was calculated with the follow-ing equation:

Scheme 1 Synthesis of HBP-crosslinked shape-memory polyurethanes with hydroxyl-terminated HBPUs as thecrosslinkers.



Number of unreacted B¼ ðFunctionality� 1Þ�DPþ 1

In this equation, the term functionality equals 2because the monomer used for the preparation ofthe hydroxyl-terminated HBPU was AB2 type. Thecalculated functionalities were used for the stoi-chiometric calculation in terms of equivalents. Theamount of HBPU was varied from 0.01 to 0.2 equivrelative to 1.0 equiv of the polyurethane prepolymer.

Seven polyurethanes were prepared with variationin the amount of chain extender and crosslinker (Ta-ble I). The FTIR spectra of the crosslinked polyur-ethanes (CPUs) were identical, and they did notshow absorption for free ANCO groups at 2270cm�1, whereas the prepolymer showed a strongabsorption band in the 2270–2275-cm�1 range.Except CPU-6, all other polymers showed only onestrong absorption in the carbonyl region (1675–1720cm�1). This was because of the amount of cross-linker used, which was relatively low (up to 0.17equiv); hence, the carbonyl groups present in HBPUwere not distinguished. However, when the amountof the crosslinker was increased up to 0.2 equiv, thetwo different carbonyl groups in the polymer chain,one from the polyurethane prepolymer and anotherfrom HBPU, absorbed separately at 1720 and 1676cm�1, respectively (Fig. 1). The 1H-NMR spectrum(Fig. 2) of CPU-2 clearly showed three distinct sin-glets between 8.19 and 8.5 ppm due to the presenceof three different urethane ANH protons, one frompolyurethane prepolymer, one from HBPU, and the

third one formed during the chain extension reactionwith BDO. Thus, the IR and 1H-NMR spectral dataconfirmed the incorporation of HBPU into the poly-urethane chain.The hard-segment content and molecular weight

data are summarized in Table I. Because the cross-linker was fully hard segmented, the hard-segmentcontent of the polymers increased proportionallywith increasing amount of HBPU. The hard-segmentcontents of the polymers ranged from 18.8 to 20.8wt % and were substantially above the minimumvalue required to show a shape-memory effect.25 Asexpected, the use of fully hard-segmented HBPUenhanced the microphase separation in the poly-mers. Such morphological changes were clear in theSEM pictures. Figure 3(a) confirms the phase-sepa-rated morphology of linear polyurethane (LPU), butthe phases were uniformly distributed in oneanother, whereas Figure 3(b,c) reveals that the hardand soft phases were separated distinctly when theamount of crosslinker was increased. Except CPU-5and CPU-6, which were prepared with 0.17 and 0.2equiv of crosslinker, respectively, all of the otherpolymers were found to have adequate solubility inDMF to smoothly carry out GPC measurements, andthe GPC curves obtained are given in Figure 4.There was a very significant difference in the Mw

values of the polyurethanes prepared. Mw variedfrom 5.1 � 104 g/mol for polymers containing noHBPU (LPU) to 29.0 � 104 g/mol for CPU-4, forwhich the amount of HBPU used was 0.15 equiv.Such a significant increment in the molecular weightwas due to the crosslinker used, which itself was apolymer. Because the polymers prepared with cross-linker of more than 0.15 equiv were found to becompletely insoluble in DMF, the molecular weightdata for CPU-5 and CPU-6 are not given. The poly-dispersity values were found to increase withincreasing molecular weight.

Figure 1 FTIR spectra of (a) the polyurethane prepolymerand (b) an HBPU-crosslinked polyurethane (CPU-1).

Figure 2 1H-NMR spectrum (300 MHz) of an HBPU-crosslinked polyurethane (CPU-1).



Differential scanning calorimetry(DSC) and WAXD

Thermal properties, such as Tg, Tc, and Tm, and en-thalpy data for the LPU and CPUs were determinedwith DSC, and the values are presented in Table II.Both the first and second heating curves showed Tg

and Tm transitions. However, for some cases, the Tg

transition was very clear in the first heating curvescompared to the second heating curves. The soft-seg-ment crystallization was observed only in the cool-ing curves. Thus, the first heating and cooling curveswere considered for analysis, and typical examples

of DSC traces are given in Figure 5. The incorpora-tion of HBPU crosslinker up to 0.1 equiv was foundto increase Tg of the polymers by 44�C, and the poly-mers crosslinked with more than 0.1 equiv of cross-linker did not show Tg. For CPUs, the crystallizationof the soft segment was observed between 5 and8�C; this was comparable to LPU. The Tm values ofthe CPUs were between 55 and 60�C, and these val-ues were a little high compared to that of LPU,which underwent melting at 54�C.The crystallinities of the polymers were calculated

from the enthalpy data of the crystalline meltingpeak with a 140 J/g enthalpy value for the fusion of100% crystalline PCL,26 and the values are summar-ized in Table II along with the corresponding en-thalpy values. Within the series of CPU-1 to CPU-6,except for CPU-6, all of the CPUs showed high en-thalpy and crystallinity compared to LPU. This wasbecause crystallization occurred effectively in thesecases as a result of enhanced phase separationbetween the soft crystalline phase and the hardamorphous phase. Generally, crosslinking is knownto lower the overall crystallization of the soft seg-ment. However, the observation here was the

Figure 3 SEM micrographs of (a) an uncrosslinked poly-urethane (LPU), (b) a CPU with 0.1 equiv of HBPU as thecrosslinker (CPU-3), and (c) a CPU with 0.2 equiv ofHBPU as the crosslinker (CPU-6).

Figure 4 SEC traces of linear and HBPU-crosslinkedpolyurethanes.



reverse. The reason could have been that generallycrosslinkers used for polyurethanes are soft-seg-ment-type, low-molecular-weight polyols, but here,the crosslinker was a hydrogen-bonded HBPU,which was a hard segment. Compared to LPU, thelow enthalpy and associated crystallinity of CPU-6revealed that there was a threshold limit for theaddition of crosslinker; beyond that limit (0.2 equiv),the crosslinker’s own contribution to the amorphousphase determined the crystallinity of the polymer.

The molecular weight of the HBPU crosslinker had apronounced effect on the crystallinity of polymer,and this was observed in the crystallinity and en-thalpy data of CPU-4 and CPU-7. The patterns ofthe X-ray diffraction profiles of the crosslinked anduncrosslinked polymers were identical, and repre-sentative X-ray spectra are given in Figure 6. All ofthe profiles showed two prominent peaks at Braggangles of 22.5� (100) and 23.8� (101); these indicated

TABLE IIDSC Results for the Linear and Crosslinked Shape-Memory Polyurethanes

Polymercode Tg (

�C) Tc (�C) Tm (�C) Enthalpy (J/g)

Crystallinity (%)

DSCa WAXDb

LPU �46 11.0 54 62.5 36.6 33.5CPU-1 �10 7.0 57 69.8 40.3 36.2CPU-2 �10 8.0 58 67.0 38.5 35.2CPU-3 �2 6.0 57 67.4 38.6 34.5CPU-4 NO 7.0 57 63.0 37.0 34.0CPU-5 NO 5.0 60 64.5 36.6 33.5CPU-6 NO 7.0 55 61.3 34.6 32.7CPU-7 NO 8.0 58 70.9 40.5 39.0

NO ¼ not observed.a The crystallinity was calculated as follows:26

Crystallinity ¼ Enthalpy=140�wt % of PCL� 4000 in LPU or CPU

b The crystallinity was calculated as follows:

Crystallinity ¼ ðA100 þ A101Þ=ðA100 þ A101 þ AaÞ

where A100 and A101 are the areas under the PCL (100) and PCL (101) diffraction peaks,respectively, and Aa is the area under the amorphous halo.

Figure 5 (a) Heating and (b) cooling DSC traces of anHBPU-crosslinked shape-memory polyurethane (CPU-3).

Figure 6 WAXD profile of linear and HBPU-crosslinkedpolyurethanes.



that the hard- and soft-segment phases were suffi-ciently separated to result in no disturbance to thesoft-segment crystallization. The pattern also fea-tured a plateau from 15.5 to 21.5�; this suggested thepresence of an amorphous region. The d values of(100) and (101) were found to be almost constant forall of the polymers. In addition to the DSC method,the percentage crystallinity of the polymer was alsodetermined from X-ray profiles with a simple calcu-lation,27 and the values are given in Table II. Thevalues and the trend present in the values werecomparable to that determined from the DSC curves.The DSC and X-ray results showed that the crystal-linity of the CPUs was not affected by the incorpora-tion of HBPU crosslinker up to 0.2 equiv relative to1.0 equiv of prepolymer.

Dynamic mechanical properties andshape-memory effect

The tensile storage modulus (E0) ratios (E0g/E

0r,

where E0g is the modulus in the glassy region and

E0r is the modulus in the rubbery region), tan d val-

ues, and tan d peak temperatures of the LPU andCPUs were obtained from the DMA traces and arepresented in Table III. The E0 curves of the CPUs aregiven in Figure 7. In addition to a sharp Tg or Tm

transition, a large difference in the modulus aboveand below the transition temperature was also amost required property to render the materials capa-ble of shape-memory functions, and this property isclearly shown in Figure 7. Compared to LPU, all ofthe CPUs showed a high E0 ratio. These valuesincreased with increasing amount of crosslinkerused, and this result suggested that the shape recov-ery would be easily triggered at Ttrans. Compared toLPU, the CPUs showed a high tan d value, and thisvalue increased proportionally with the amount ofcrosslinker used. Because tan d value correspondedto the strain energy dissipated by viscous friction,we concluded that the HBPU-crosslinked polyur-ethanes were more likely viscous than elastic andpossessed good damping properties compared toLPU. For all of the polymers, the glass–rubber

TABLE IIIDMA Results for the Linear and Crosslinked

Shape-Memory Polyurethanes

Polymercode E0 ratio tanmax d

Temperatureat tanmax

d (�C)

LPU 6.4 0.06 40CPU-1 9.25 1.93 77CPU-2 – 1.84 94CPU-3 9.6 4.85 78CPU-4 13.6 3.10 84CPU-5 – 6.7 85CPU-6 13.75 – –CPU-7 12.5 – –

Figure 7 Curves of E0 versus the temperature for HBPU-crosslinked shape-memory polyurethanes.

Figure 8 Transition from the temporary shape (spiral) tothe permanent shape (strip) for LPU and an HBPU-cross-linked shape-memory polyurethane (CPU-1).The switchingtemperature of the polymers was 55�C. The recovery pro-cess took 10.0 and 16.0 s after heating to 60�C for thecrosslinked and linear polymers, respectively. [Color fig-ure can be viewed in the online issue, which is availableat wileyonlinelibrary.com.]



transition was found to start around �50�C and endaround 0�C, whereas the tanmax d temperature ofLPU was 40�C, which was close to the melting tem-perature of LPU. This value was significantly higherfor the CPUs. This observation indicated that thepolymers responded separately for the glass transi-tion and melting transitions.

A typical example of the shape-memory effect ofCPU is shown in Figure 8. Although the DSC curveshowed a peak for Tm of the LPU or CPU at above54�C, the thermograms clearly indicated that theactual melting started well below 50�C for all of thecases. Hence, the polymers were expected to show ashape-memory effect at 50�C and above. The quanti-tatively evaluated shape-recovery ratio for the linearand crosslinked polymers at 50, 60, and 70�C aresummarized in Table IV, and typical examples ofthe graphical representations are given in Figure 9.

On the basis of the E0 values determined from theDMA results, SEM micrographs, and the percentagecrystallinity calculated from the DSC and WAXDtechniques, we envisaged that all of the CPUs pre-pared should show shape-memory behavior andthat this effect should be better than that of the LPU.This anticipation is well reflected in Figures 8 and 9and the results given in Table IV. The CPU-6 tookless time compared to LPU for the transition fromtemporary shape to permanent shape. Such a per-formance was observed even in the third cycle ofdeformation. Within the series of CPU-1 to CPU-6,the shape-recovery rate was found to increase withincreasing amount of HBPU crosslinker used.

Thermal, mechanical, and solubility properties

TGA was performed for all of the polymers, and wefound that both linear and crosslinked polymersunderwent single-stage decompositions. A typicalexample of the TGA traces is given in Figure 10, andthe initial decomposition temperatures obtainedfrom the TGA traces are given in Table V. The initialdecomposition temperature revealed that the ther-mal stability of the CPUs improved significantlycompared to that of the LPUs. The mechanical pro-perties, including tensile strength and elongation ofpolymers, are also summarized in Table V. Gener-ally, the tensile properties are very sensitive to theamount of crosslinks present in the polymer. In thisstudy, the tensile strength of the polymers wasfound to increase gradually from LPU, which hadno added crosslinkers to CPU-6, which had the max-imum amount of added crosslinker.The LPU and CPUs prepared with up to 0.06

equiv of HBPU crosslinker were found to be highlysoluble in polar solvents, including tetrahydrofuranand MEK. The CPUs prepared with 0.10 and 0.15

TABLE IVShape-Recovery Data Obtained from Bending Tests of

the Linear and Crosslinked Shape-MemoryPolyurethanes

Polymercode

Time required for 100%shape recovery (s)

50�C 60�C 70�C

LPU 24.0 17.5 15.5CPU-1 18.0 12.0 8.0CPU-2 17.0 11.0 8.0CPU-3 16.0 10.0 8.0CPU-4 15.0 10.0 8.0CPU-5 14.0 9.0 7.0CPU-6 14.0 9.0 8.0CPU-7 16.0 11.0 9.0

Figure 9 Shape-memory behavior of linear and cross-linked shape-memory polyurethanes in the time seriesstudy.

Figure 10 TGA thermograms of LPU and an HBPU-crosslinked polyurethane (CPU-1).



equiv of crosslinker were found to be only partiallysoluble in DMF, dimethylacetamide, N-methylpyrro-lidone, dimethyl sulfoxide, tetrahydrofuran, andMEK. The polymer became insoluble in these sol-vents with increasing amount of crosslinker beyond0.15 equiv. Also, none of the CPUs were soluble inmethanol or ethyl acetate.

CONCLUSIONS

A series of shape-memory polyurethanes were pre-pared with PCL diol as a switching segment andnovel hydroxyl-terminated HBPUs as crosslinkers.The properties of the CPUs were compared withLPU. The microphase separation was more pro-nounced in the CPUs. The solubility of the polymerswas not affected when the crosslinker was used inan amount up to 0.06 equiv relative to 1.0 equiv ofthe polyurethane prepolymer, and at the same time,the crystallinity of the polymers was improved. TheE0 ratios of the polymers increased with increasingamount of crosslinker. The morphological feature,crystallinity, and storage modulus ratio cumulativelycontributed to improvement in the shape-memoryproperties of the CPUs over LPU. Obviously, theincorporation of crosslinker into the polymer chainimproved the tensile properties of the polymer.

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TABLE VThermal, Mechanical, and Solubility Properties of the Crosslinked Shape-Memory Polyurethanes

Polymercode

Initialdecomposition

temperature (�C)a

Mechanical properties Solubilitya

Tensilestrength(MPa)

Elongation(%)

DMF, DMAc,NMP, DMSO,THF, and MEK

MeOH andEtOAc

LPU 236 13.6 6 0.1 46 þ 6CPU-1 263 13.8 6 0.2 80 þ �CPU-2 275 13.8 6 0.2 83 þ �CPU-3 266 14.6 6 0.1 50 6 �CPU-4 252 14.9 6 0.2 70 6 �CPU-5 265 15.2 6 0.2 105 � �CPU-6 272 17.3 6 0.2 124 � �CPU-7 273 15.1 6 0.1 83 6 �

DMAc ¼ dimethylacetamide; DMSO ¼ dimethyl sulfoxide; EtOAc ¼ ethyl acetate; MeOH ¼ methanol; NMP ¼ N-meth-ylpyrrolidone; THF ¼ tetrahydrofuran.

a þ ¼ soluble; � ¼ insoluble; 6 ¼ partially soluble.



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Shape-memory polyurethanes minimally crosslinked with hydroxyl-terminated AB2-type hyperbranched polyurethanes