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Segmented Poly(urethane-urea) Elastomers Based onPolycaprolactone: Structure and Properties

L. May-Hernandez,1 F. Hernandez-Sanchez,1 J. L. Gomez-Ribelles,2,3,4 R. Sabater-i Serra2

1Unidad de Materiales, Centro de Investigacion Cientfica de Yucatan (CICY), Calle 43, No. 130,Col. Chuburna de Hidalgo, C.P. 97200, Merida, Yucatan, Mexico2Centro de Biomateriales e Ingeniera Tisular. Universidad Politecnica de Valencia.Camino de Vera s/n 46022, Valencia, Spain3CIBER en Bioingeniera, Biomateriales y Nanomedicina, Valencia, Spain4Centro de Investigacion Prncipe Felipe, Avda. Autopista del Saler 16, E-46013, Valencia, Spain

Received 10 July 2009; accepted 1 June 2010DOI 10.1002/app.32929Published online 26 August 2010 in Wiley Online Library (wileyonlinelibrary.com).

ABSTRACT: A series of segmented poly(urethane-urea)polymers have been synthesized varying the hard segmentscontent, based on the combination of polycaprolactone

diol and aliphatic diisocyanate (Bis(4-isocyanatocyclohexyl)methane), using diamine (1,4-Butylenediamine) as the chainextender. The microstructure and properties of the materialhighly depend on the hard segments content (from 14 to40%). These PUUs with hard segment content above 23%have elastomeric behaviors that allow high recoverable

deformation. The chemical structure and hydrogen bondinginteractions were studied using FTIR and atomic forcemicroscopy, which revealed phase separation that was also

confirmed by DSC, dynamic-mechanical, and dielectricspectroscopy. VC 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119:20932104, 2011

Key words:poly(urethane-urea); block copolymers; elastomers;phase separation; polycaprolactone

INTRODUCTION

Segmented Polyurethane (PU) and Poly(urethaneurea) (PUU) have received much attention for thedevelopment of biodegradable polymers because oftheir great potential in the tailoring polymeric struc-

ture, and their role in obtaining mechanical proper-ties and biodegradation rates to cover a varietyof medical applications.1,2 PU and PUU can befound in vascular applications, such as pacemakerinsulation, heart valves, and intra-aortic balloons.Pellethane (Dow Chemical), Biomer (Ethicon) andTecothane (Thermedics) are examples of commer-cially available PU/PUU. They are nondegradablematerials containing a polyether soft segment and ahard segment derived from an aromatic diiso-cyanate. PU/PUU have shown promising results inapplications, such as ligament reconstruction,3,4

meniscus replacement,5,6 and bone regeneration.7

For example, the biodegradable PU Artelon (Artim-plant, Vastra Frolunda, Sweden), known for its longand controlled in vivo degradation time, has beenused in several medical applications. In vitro andin vivo studies have demonstrated its safety andexcellent biocompatibility.4

PUUs are blends of copolymers that present alter-nating hard and soft segment sequences. Hardsegments are blocks formed by the reaction of adiisocyanate with a low molecular weight diamine,whereas the soft segments are blocks of a highmolecular weight diol. Immiscibility between softand hard segments induces microphase separationor segregation, which is a factor that highly influen-ces the biocompatibility of this multiblock copoly-mer.2 By adding diamine chain extenders ratherthan diols, urea groups are included at the hard seg-ment, resulting in the formation of multiple hydro-gen bonds between proton donors (urethane NAHand urea NAH groups) and proton acceptors(urethane CO, urea CAO and CAOAC groups) inthe hard domain.8 These hard segment domainsplay an important role in the mechanical behavior ofthe material, acting as physical crosslinks and/or asfillers of the rubbery soft segment matrix that

increase the elastic modulus of the rubber, suchresults are higher than typical chemically crosslinkedelastomers. Another interesting characteristic of PU/PUU thermoplastic elastomers is the possibility of

being processed by conventional molding techniques.Polycaprolactone (PCL) is commonly used as soft

segment in the synthesis of biodegradable poly-urethanes.9 PCL is a semicrystalline polymer thatdegrades slower than both polylactide and polyglyco-lide and thus, it is preferably used for several longterm applications.10 It is degraded in vivo throughhydrolysis and enzymatically, and a part of the

Correspondence to: L. May-Hernandez ([email protected]).

Journalof AppliedPolymer Science,Vol. 119,20932104(2011)VC 2010 Wiley Periodicals, Inc.

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degradation occurs by intracellular mechanisms.11

Recent works have verified the cytocompatibility andnoncytotoxicity of PCL based PUU elastomers.1214

On the other hand, methyl diisocyanate (MDI),has been used for the synthesis of medical-grade

polyurethanes, despite the observation that theirdegradation products are suspected to be carcino-gens, namely methylene diamine (MDA).15 Never-theless, a solution to the potential carcinogenic effectof MDA has already been proposed that is the useof a hydrogenated version of MDI, bis(4-isocyanato-cyclohexyl)methane (HMDI). Aliphatic diisocya-nates, such as lysine-methyl-ester diisocyanate or1,4-butanediisocyanate are also used to reduce thetoxicity of the degradation products. An advantageto the use of these is the ease of crystallization of theisocyanate groups, which is a variable that allowsmodifying the mechanical properties of the finalpolymer and increasing the microphase separation.16

The majority of the PUUs produced commerciallyare based on polyesters, mainly because of theirmechanical properties (tensile and tear strength andabrasion resistance). However, they are susceptibleto hydrolytic degradation when implanted in the

body, even for short periods of time.2,17,18 Thus,when looking for more biostable materials, polyols

based on ether links have been used, although theyare still susceptible to degradation by scission of theurethane and CH2-O links.

19 The combination ofether and ester groups, include both hydrophilicand hydrophobic characteristics, respectively.20,21

If a diamine such as Putrescine,2226 which is agrowth factor essential for the cell division in mam-mals26 is added at the backbone of the PUUs, duringthe chain extension step, the mechanical propertiescan be improved as a result of a higher molecularweight. These, then allow to obtain a variety ofphase-separated morphologies.27

In this work, we analyze the structure of seg-mented PUU synthesized from an aliphatic diisocya-nate, HMDI, polycaprolactone-diol of high molecularweight, and 1-4 diaminobutane (putrescine) as thechain extender. It is expected that this system to be

biodegradable with noncytotoxic degradation prod-ucts, in according to the pointed out in the preced-

ing paragraphs. The obtained materials are similarin structure to those synthesized by Guan et al.13,14

except by the isocyanate (HMDI). The effect ofincreased hard segments content on the thermal,mechanical, and dielectric properties is analyzed.

EXPERIMENTAL

Materials

HMDI, Stannous octoate (SO), and Dimethylforma-mide anhidro (DMF) were purchased from Aldrich

and 1-4 diaminobutane and Isopropylic alcohol(IPA) were purchased from Fluka and used asreceived.

Polycaprolactone-diol (Mn 2000, purchased fromAldrich) was dried under vacuum at 55C for 24 h

to remove moisture. This polymer contains an ethyl-ene glycol unit in the center of each polymer chainsince its polymerization is initiated by diethyleneglycol.

Synthesis of PUUs

In this work, the hard segment fraction (wt % HS)was calculated from the weight ratio of diisocyanateand butanediamine to that of all the reactants,including diisocyanate, butanediamine, and polyol.In other words, the content of hard segments was

based on the content of urea-urethane groups,27 asshown in eq. (1):

wt%HS 100RMdi R 1Mda

MPCL RMdi R 1Mda; (1)

where the variable M refers to the molecular weightof the monomer, R is the mole ratio of isocyanate todiol (NCO/OH), and the subindexes PCL, di and darefer to polycaprolactone-diol, diisocyanate, anddiamine, respectively. In the case of polycaprolac-tone-diol the average molecular weight was includedin eq. (1).

To obtain the linear polymer (Fig. 1), a molar ratioNCO:OH and NH2 was maintained at 1 : 1. PUUswere synthesized by two-step polymerization (pre-polymer method) under an inert atmosphere ofhigh purity nitrogen, in a 250 mL three-neck round-

bottomed flask equipped with a stirrer and a thermo-meter. A 10% w/v solution of PCL in DMF wasmixed with the catalyst, stannous octoate (about 0.01wt %) at 40C for 30 min to obtain a homogenoussolution. In a first step an excess of diisocyanate wasadded to obtain an isocyanate-terminated prepolymerat several NCO/OH ratio (1.2, 2, 3, and 4), and thereaction was left for 3 h at 75C. The prepolymersolution was cooled to room temperature and chainextension step was performed adding slowly stoichio-

metric amounts of putrescine under vigorous mag-netic stirring for 180 min. The reaction was completed

by increasing the temperature to 75C for 180 min.The solution was cooled (25C) and the polymer

was precipitated in cool distilled water and washedin IPA at room temperature for 3 days, changing theIPA every 12 h, to remove unreacted monomer. Thesamples were then dried at 50C for 24 h underreduced pressure. A series of four PUUs with hardsegment contents ranging between 14 and 40% weresynthesized. The sample designation is included inTable I.

2094 MAY-HERNANDEZ ET AL.

Journal of Applied Polymer Science DOI 10.1002/app

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Synthesis of polyurea

The polymerization of polyurea was carried out atroom temperature under stirring. A solution ofHMDI in DMF (10% w/v) was placed in a three-neck reactor, and putrescine was added (equimolarratio 1 : 1 of NCO : NH2) dropwise under vigorous

stirring. The reaction took place immediately and awhite precipitate was formed.

After 30 min, the polymer was removed, washedrepeatedly with an ethanol/water mixture (75 : 25, v/v),and dried under vacuum for 24 h at 50C.

Film preparation

PUUs were dissolved (10% w/v) in DMF at 90C for2 h and subsequently poured onto Teflon plates. Thesolvent was allowed to evaporate at room tempera-ture for 24 h, and later the samples were dried at

50C for 48 h under low pressure. The resultingfilms were 400 lm thick. The urea films subjectedto infrared characterization were obtained by com-pression molding.

Measurements

Gel permeation chromatography (GPC) was usedto determine the average molecular weights andpolydispersity of the synthesized. A HP/Agilent1100 GPC (CA) instrument equipped with a refrac-tive index detector (RID A) was used. The mobilephase was THF at a flow rate of 1 mL/min. Calibra-tion was performed with polystyrene standards. Thepolymers were dissolved (1.5% w/v) in THF bystirring at 65C for 60 h and 20 lL was injected.

Phase images were obtained using an atomic forcemicroscope (AFM) Nanoscope IIIa system fromDigital Instruments (Santa Barbara, CA). Experiments

Figure 1 ATR-FTIR spectrum of PUUs samples, Polyurea and PCL as references.

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were performed in tapping mode under ambientconditions. Images were acquired with a 2.8 N/mforce constant tip. The tapping frequency wasaround 10% lower than the resonance frequency(75 KHz). The ratio between the setpoint and driveamplitude was set to 0.8.

Thermal properties were determined by differen-tial scanning calorimetry (DSC) from a MettlerToledo DSC-823e system (Zurich, Switzerland),

between 100 and 140C, the heating and coolingrate of the scans was 10C/min. Nitrogen was usedas the purge gas.

Attenuated total reflectance-Fourier transforminfrared (ATR-FTIR) spectra of PUUs were recordedwith a Nicolet Protege 460 (Wisconsin) between600 and 4000 cm1 with a resolution of 8 cm1,using film samples cast from 10 wt % solutions inDMF and dried for 24 h in a vacuum oven at 30 C.

Dynamic mechanical analysis (DMA) was performedusing a Perkin-Elmer DMA-7 (Norwalk, CT) at a fre-quency of 1 Hz. The temperature dependence of thestorage modulus and loss tangent were measured inthe temperature range between 100 and 100C at aheating rate of 5C/min. Samples were cut from thedried films with approximate dimensions 12 3 0.4 mm3.

Uniaxial stressstrain experiments were performedwith a Shimadzu AG-100 KN equipment (Kyoto,

Japan) at 25C at a cross-head speed of 125 mmmin1.Samples were cut from a sheet of PUUs film of

about 0.4 mm thickness. The specimen gauge lengthwas about 45 mm long and 7 mm wide.

Thermogravimetric analysis (TGA) was carriedout on PUUs samples using a Perkin- Elmer TGA-7analyzer (Norwalk, CT). Approximately 8 mg ofeach sample was heated from 50 to 600C in a nitro-gen atmosphere at the rate of 10C/min.

Dielectric relaxation spectroscopy (DRS) experi-ments were performed using a impedance analyzerAlpha-S (Hundsangen, Germany). The temperaturecontrol was assured by the Quatro cryosystem fromNovocontrol GmbH. The real and imaginary parts of

the complex dielectric permittivity were measuredisothermally from 101 to 106 Hz. The samples werekept between gold-plated stainless steel electrodesand the temperature was varied from 140 to 80Cat intervals of 5C.

RESULTS

A series of four poly(urethane-urea) polymers weresynthesized (Table I). The polyurethanes obtained inthe study were linear materials and soluble in polarsolvents, such as dioxane, THF, DMF, and DMSO.These solvents were chosen for freeze drying testswhose results will be published in a later article.The samples containing 14 and 23 wt % of hardsegments were soluble in dioxane, THF, and DMF at120C for 48 h. On the other hand, all samples weresoluble in DMSO at temperatures close at 90C for48 h, which is more polar than the other solvents.The solubility of the samples was not tested withapolar solvents. The molecular weights of PUUswere less than 3 104 Da. A polyurea sample wasalso synthesized as a reference for the interpretationof the properties of the PUUs. The polyurea wasdissolved in organic solvents such as acetic acid, at50C for 24 h.

FTIR analysis

Figure 2 shows the FTIR spectra, which are similar

in PUUs containing different contents of hard seg-ments. Spectra were similar to those obtained byGuan et al.14 It is worth noting the absence of the2260 cm1 band that is characteristic of theunreacted isocyanate groups. Stretching vibrations ofthe ester groups are assigned to the 1060, 1160, and1239 cm1 bands. The peak at 1536 cm1 is charac-teristic of the carbonyl stretching vibration, which isvery intense and clearly increase with the content ofhard segments. But also the amide II peak (a combi-nation peak of NAH bending and CAN stretchingvibration) at 1560 cm1 is studied. The urea carbonyl

TABLE IComposition and Some Characteristic Parameters of the Thermal Properties of the PUUs Samples

Samples

Molarcomposition

HMDI/PCL/BDA

Hardsegmentscontentsa

Molecularweight(Mw)

Tgb

(C)Tm

b

(C)TaDMAc

(C)

DCp (J/gC)

(1st/2ndheating

DSC scan) Characteristics

PCL2000 2 103 64 4651 0.22/0.26 Brittle, opaquePUU14 1.2/1/0.2 14.2 14% 2.5 104 48 46.47 31 0.30/0.38 Brittle, opaquePUU23 2/1/1 23.5 23% 2.6 104 53 38 0.55/0.62 Lightly Flexible, translucentPUU32 3/1/2 32.5 32% 2.8 104 57 40 0.43/0.50 Slightly elastic, clearPUU40 4.5/1/3.5 39.6 40% 57 44 0.38/0.44 Tough, rubbery, opaque

a Obtained from eq. (1).b Obtain by DSC (first heating).c Temperature of the maximum of the loss tangent in the main relaxation process measured by DMA (see text).



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(hydrogen bonded) peak at around 16401629 cm1

,which also increases with the content of hardsegments indicating the presence of orderly arrange-ment of urea groups.28 The characteristic bands ofester and urethane free carbonyls appear overlapped

between 1735 and 1710 cm1. The absorption peaksat 2854 and 2924 cm1 correspond to symmetric andasymmetric CH2 groups, respectively. Bands at 1458,1420, 1394, and 1365 cm1 correspond to variousmodes of CH2vibration.

The absorbance at 1100 cm1 was attributed to theether group at the center of the polycaprolactonesegment. The absorption band at 3330 cm1 is

caused by hydrogen-bonded NA

H (urea and urethane)stretching, which increase with the content of hardsegments. The absorption band in PUUs with lowercontents of HS appears at 3366 cm1 and is associatedto nonbonded free NAH (Fig. 3).

The peak intensities of the hydrogen-bonded urethanecarbonyl groups depend on the increase in hard seg-ments content and it is correlated with the decrease

observed in the peak intensity of the free carbonylband (nonbonded).

DSC analysis

The calorimetric results of PUU samples [Fig. 4(a)]show the effect of the hard segment content onto thecrystallization and glass transition behavior of thesoft segments. An endotherm at 120C was attrib-uted at the hard segments. The first heating scan ofthe sample containing 14% hard segments has amelting endotherm associated with the polycapro-lactone segments. The melting peak appears at atemperature slightly higher than in the polycapro-lactone-diol used in the synthesis process whosethermogram is also shown in Figure 4(a) forcomparison. Note that in the heating scan of PCL adouble peak appears due to the superposition of the

exothermal crystallization that take place during theheating scan, immediately after the first endotherm.In the PUU containing 23 or 32% HS a small endo-therm appears in the same temperature interval aswell, showing that some PCL crystallization is alsopossible in these samples. On the contrary, thePUU40 sample shows a broad endotherm similar tothe heating thermogram of pure polyurea alsoincluded in Figure 4(a).

On the other hand the thermograms show theglass transition of PCL blocks in the same tempera-ture region than in the PCL-diol but slightly shiftedtowards higher temperatures (Table I).

These heating scans are recorded with the samplesas were obtained from the synthesis process inwhich the last stage consisted in solvent casting at50C. Later a cooling scan at 10C/min showed thatonly the sample containing 14% HS was able tocrystallize, showing an exotherm shift around 10Ctowards lower temperatures with respect to purePCL-diol [Fig. 4(b)]. The rest of samples just show

Figure 2 Synthesis of segmented poly(urethane-urea).

Figure 3 Possible intermolecular interchain interactions in segmented poly(urethane-urea) elastomers.



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the glass transition in the temperature range around60C. The second heating scan shows only theglass transition of PCL blocks in all samples withthe exception of those containing 14% HS, whichmelt at a temperature close to that of the PCL-diol[Fig. 4(c)]. The enthalpy of melting for samplePUU14 is around 40% smaller than the pure PCLsample. The heat capacity increment and the glasstransition temperature are always higher in thesecond scan than in the first (Table I).

AFM analysis

AFM images of PUU samples (Fig. 5) show the pres-ence of a phase separation with phase domains inthe nanometer scale. The phase morphology stronglydepends on the sample composition. The structureobserved in the phase image in the case of thePUU14 sample [Fig. 5(b)] consists of large aggregatesaligned parallel to each other. This structurewas found only in the surface of sample PUU14,and is related to PCL crystallization, disappearingin sample PUU23 in which a homogeneous distribu-tion of small domains appears [Fig. 5(d)]. Thesedomains grow in the sample PUU32, which shows a

great dispersion of domain size, with most of theaggregates ranging between 20 and 50 nm in size.Some of them even reach 100 nm. The sample withthe highest hard segment content shows a veryuniform distribution of domains that seem topercolate [Fig. 5(h)]. The comparison of phase andheight images allows for ascribing the structureshown by phase measurements to differences in theviscoelastic properties of the soft and hard domains(or crystalline and amorphous domains in the caseof sample PUU14) and not to artifacts produced bythe surface topography.

DMA analysis

In the temperature interval of our dynamic-mechani-cal experiments the polycaprolactone shows only themain relaxation, associated with the co-operativerearrangements of the polymer chains in the amor-phous phase. A secondary relaxation appearing atmuch lower temperatures, which will be detected indielectric experiments.29,30 The dynamic mechanicalresults of PUU samples (Fig. 6) present this relaxa-tion process in the temperature interval between70 and 10C. The temperatures of the maximum

Figure 4 DSC thermograms of PUUs samples: (a) first heating at 10C/min, (b) first cooling at 10C/min, (c) secondheating at 10C/min.



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of the loss tangent peak, which we will call TaDMAhave been listed in Table I. This temperature is gen-erally situated around 15 above the calorimetricglass transition temperature, Tg. It is higher in thesample containing 14% HS than in the rest ofthe samples. The drop of the elastic modulus in the

relaxation process highly depends on the HS con-tent of the polymer [Fig. 6(a)]. At temperaturesabove the transition in the region of the elastomericplateau, the highest modulus corresponds to thePUU14 sample while that of PUU23 is the lowestone increasing from this HS content on, a feature

Figure 5 AFM height (left) and phase (right) images of 1 lm2 of the PUUs films surface: (a, b) 14% HS, (c, d) 23% HS,(e, f) 32% HS and (g, h) 40% HS.



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that as we will discuss below must be related to thecrystallinity and crosslinking density of the polymer.

TGA analysis

The TGA curves of PUUs show a two-step decompo-sition, which are related at the existence of twodifferent segments in the structure: hard and softsegments. The degree of phase separation plays arole in the decomposition of the polyurethanes as itis observed in the Figure 7. Apparently, a low con-

tent of HS improve the phase separation allowing atboth soft and hard segments to crystallize to delaythe degradation rate; higher content of HS induce apoor phase separation (phases mixed) that increasethe degradation rate of the urea and urethane

groups; however, mixture of phases allowed toimprove the thermal stability of the soft segments(secondary step) even after the dissociation ofhydrogen bonds interurethane.

Stressstrain measurements

Results of the elastic modulus and ultimate stressobtained by tensile testing in PUU samples at roomtemperature are shown in Figure 8. The sample with14% HS shows a behavior characteristic of ductilematerials. This sample presents the highest elasticmodulus, and the lower tensile strength and elonga-

tion at break. PUUs with the highest hard segmentcontents show a different stressstrain curve with a

behavior typical of the elastomeric materials. Themodulus of PUU23 decreases one order of magni-tude with respect to PUU14 and then increases with

Figure 6 DMA curves of PUUs: (a) storage modulus, (b)

tand.

Figure 7 TGA thermograms of synthesized PUUs.

Figure 8 Results of the stressstrain measurements showing the effect of increase the hard segments content: (a) typicalstressstrain curves of PUUs samples, (b) Dependence of the elastic modulus (n) and ultimate stress (*) on hard segmentcontents of the samples.



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increasing HS content, in good agreement with thevalues of the storage modulus measured by DMAin the elastomeric plateau region. The mechanicalproperties obtained in this work were similar to theobserved by Guan et al.13,14

On the other hand the ultimate stress and theelongation at break in PUU23 and PUU32 are alsoone order of magnitude higher than in PUU14 anddecrease in PUU40, which is stiffer material due tothe high HS content.

Dielectric relaxation spectroscopy analysis

The dielectric relaxation spectrum of PCL-diol showstwo relaxation processes, a secondary relaxation thatwill be called hereafter cDRS and the main dielectricrelaxation associated with cooperative conforma-tional rearrangements of the polymer chains in theamorphous phase, which will be called aDRS (Fig. 9).These relaxation processes were already described inprevious papers3032 both in PCL-diol and in highmolecular weight PCL. In the PUU samples, cDRSand aDRS also appear and should be related to thesoft PCL-diol segment. In addition, a third relaxa-tion, bDRS, with the characteristics of a secondaryrelaxation is shown clearly in the plots of Figures9(a,b). This relaxation is ascribed to the local motionsof the permanent dipoles of the urethane and ureagroups in the hard segment.

The bDRS can hardly be characterized, due to theoverlapping with aDRS since the intensity of thelatter is higher. The position of the local cDRS relaxa-tion in the temperature axis does not systematicallydepend on the hard segment content.

At low frequencies, bDRS relaxation appears attemperatures below the temperature interval of theaDRS relaxation. Thus the dependence of the latteron the fraction of hard segments in PUUs can beclearly observed [Fig. 10(a)]. At higher frequency, asshown in Figure 10(b), bDRS relaxation appears attemperatures immediately higher than aDRS and

both relaxations partially overlap. The strength ofthe main dielectric relaxation of the PCL segmentsin PUU14 is in the order of that of pure PCL-diol

but shifts towards higher temperatures around 15,as shown in Figure 10(a) at 1 Hz. The broad relaxa-tion at 10 kHz in this sample is the result of thesuperposition of bDRS and aDRS, which have similarintensities in this sample. The behavior of samplePUU23 is completely different the aDRS is muchhigher than in PUU14 or PCL-diol. For samplesPUU32 and PUU40, the height of the e peakdecreases with the hard segment content of thePUU.

The temperature dependence of the relaxationtimes smax obtained from the maxima of e isshown in an Arrhenius diagram [Fig. (11)]. The max-

imum of frequency (smax 1/2pfmax) related withthe secondary relaxation processes has been calcu-lated from isothermal curves. From these plots anapparent activation energy Ea 99 61 kJ mol

1 canbe calculated for the bDRS relaxation of PUU23,PUU32, and PUU40, while for PUU14 a slightlyhigher value was found (Ea 105 6 1 kJ mol

1). Inthe case ofcDRSrelaxation a small displacement towardsa higher temperature is shown compared to the PCLdiol. For PUU14 and PUU40, Ea 41 6 1 kJ mol

1

and for PUU23 and PUU32 a value 43 6 1 kJ mol1

was obtained. These results show a slight increase

Figure 9 Three-dimensional plots show the shift of the three relaxation processes with frequency and temperature for

PUU14 (a) and PUU40 (b) (note the change of scale in the later with respect to figures 6a).



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with respect to the Ea obtained for the PCL diol(37 6 1 kJ mol1).In the inset of Figure 11, the frequency position of

the maxima values of e corresponding to aDRSrelaxation, obtained from isochronal curves, is shown.In the case of the PCL diol and PUU14, aDRS showsthe characteristic curvature of the co-operative mainrelaxation process, while in the other samples, theArrhenius diagram is more linear. The main processaDRS in sample PUU14 is shifted to higher tempera-ture respect to PCL diol. Nevertheless in the remain-ing samples this process shows a displacement tolower temperatures.

DISCUSSION

A series of block copolymers were synthesized inthis work, one of which is polycaprolactone whilethe other is the urea hard segment. These structuresallow the modification of the mechanical behavior ofPCL, modulating its mechanical properties betweenthose corresponding to a stiff semicrystalline poly-mer and those of an elastomer with up to 750%deformation at break. Interestingly enough, solubil-ity with some specific solvents seems to indicate thatno chemical crosslinks appear due to the synthesis

conditions. Nevertheless hydrogen bonding provedby FTIR spectroscopy seems to be enough to ensurethe recovering of deformation.

The DSC results show that the ability of PCL tocrystallize is maintained in PUUs up to a certaincontent of hard segments. The cooling scan showsthat the kinetics of crystallization on cooling is modi-fied by the presence of the hard segments and themelting enthalpy shows that a significantly smallerfraction of PCL chains participate in the crystallinephase. This means that the presence of the hard seg-ments in this sample is a significant constraint for

PCL segment diffusion during crystal growth althoughit does not impede crystallization. On the otherhand, the temperature of the melting peak in PUU14crystallized from the melt is 47C, while in highmolecular weight PCL a value around 60C isobtained. This difference gives information about thesmaller crystal size of PCL segments in PUU. PUUswith higher content of hard segments show no traceof crystallization in the cooling or heating scans(Fig. 4). PCL segments in polymer networks of blockcopolymers of PCL (with molecular weight 2000 Da)and hydroxyethyl acrylate were able to crystallizewhen the HEA content was below 30% by weight.32

Van Bogart et al.

33

observed a similar behavior inHMDI-BDO-PCL samples with relatively low hardsegment content; at higher hard segment content thecrystallization of the soft segments is suppressed.

Figure 10 Temperature dependence of the dielectric loss, e at 1 Hz (a) and 10 kHz (b) for PCL diol (D), PUU14 (n),PUU23 (h), PUU32 (l), and PUU40 (*).

Figure 11 Temperature dependence of relaxation times(smax 1/2pfmax). PCL diol (D), PUU14 (n), PUU23 (h),PUU32 (l), and PUU40 (*). Solid lines represent theArrhenius fit for the secondary relaxations. The inset showsin more detail the region for the mainaDRS-relaxation.



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Bogdanov et al.9 observed a decrease in the crystalli-zation on poly(ester urethane) samples, mainly witha molecular weight less than 4000 and seems to van-ish with increasing length of the block.

The glass transition temperature of PUU14

(48

C), is higher than in the rest of the PUUs,while the increment of the heat capacity in thetransition is smaller (Table I), since a fraction of thePCL blocks pertains to the crystal phase and doesnot contribute to the glass transition process. Thisfeature is confirmed by dynamic mechanical anddielectric results that also show an increase of thetemperature of the a relaxation and a decrease of theintensity with respect to PUUs with higher contentof hard segments. The effect of the proximity of thehard domain can hinder the cooperative motions ofthe amorphous chains producing an increase ofthe glass transition temperature with respect to thefully amorphous polymer. Nevertheless, it must benoted that the glass transition of PCL-diol is 68C,and the crystalline fraction is higher than in PUU14.The great difference in Tg must be ascribed tosome degree of miscibility in the amorphous phase

between the hard and the soft segments. Thereare no conclusive results supporting phase separa-tion in the amorphous phase between hard and softsegments since the aggregates observed in AFMphase pictures should correspond to the crystallinedomains of PCL.

The behavior of samples with 23% or more weightfraction of hard segments is completely different tothat of PUU14. Samples cooled from the melt arecompletely amorphous. Phase separation betweenhard and soft segments is clearly observed in AFMpictures. Only the glass transition corresponding tothe soft segments is observed in DSC thermogramsin the same temperature range as PCL-diol. Thismeans that the PCL blocks into copolymer chainscan aggregate to form domains larger than thelength of cooperativity at the glass transition, whichis expected to be on the order of some tens of nano-meters. Both the glass transition temperature andthe temperature of the a relaxation measured usingDRS or DMA techniques decrease with increasingcontent of hard segments. This cannot be explained

by partial miscibility or interphase phenomena sincein both cases an increase of the glass transitionshould be expected in these polymers. A decrease ofthe glass transition temperature has been foundin some amorphous polymer confined in nano-metric three-dimensional domains3436 and one canspeculate that the dispersion of the PCL phase innanometric domains dispersed between the hardsegment blocks could produce the decrease of theirglass transition temperature. The increment of theheat capacity in the glass transition and the intensityof the arelaxation decrease as the crystalline fraction

of PCL decreases in PUUs as it should (Table I andFigs. 6, 8, and 9).

There is a significant difference from the first tothe second heating DSC scans. The last step in poly-mer synthesis is casting from a solution and thus the

PUU obtained have crystallized from solution. Inthe first heating scan a small endothermic peak isshown by the samples with hard segments contentequal or above 23% which means that crystallizationof PCL chains is still possible in these samples. Onthe other hand, the broad endotherm shown

between 40 and 120C by the polyurea sample syn-thesized as a representative of the behavior of thehard segments also appears in the first scan of thePUU40 sample, which means that crystallization ofhard blocks is also possible but only when is crystal-lized from solution, since this peak is completelyabsent in the second DSC heating scan.

There are no significant differences in the glasstransition temperature of the soft segment phasefrom the first to the second heating scans althoughthe value of the heat capacity increment at the glasstransition is lower in the first scan due to the highercrystallinity.

The dynamic-mechanical experiments show themain, aDMA, dynamic mechanical relaxation as a

broad loss tangent peak [Fig. 6(b)] whose composi-tion dependence has been already commented onabove. At higher temperatures the samples show a

broad elastomeric plateau covering the temperatureinterval between 30 and 40C. The value of theelastic modulus in this region is characterized by thereinforcing effect of the PCL crystals in the case ofPUU14, the sample that present the highest plateaumodulus, and that of the hard segment blocks in theother compositions. In the absence of PCL crystalli-zation, clearly the plateau modulus increases as thehard segment fraction does. Chain connectivity

between soft and hard segments makes it so thatwhen phase separations takes place, the stiff urea-urethane block domains acts as crosslinking pointsfor the rubbery PCL phase and are responsibleof the elastomeric behavior of the polymer. In addi-tion hard blocks act as a filler of the PCL phase.34

Above 60C, the PUU14 sample collapses due to

the melting of the crystalline phase, whereas in therest of compositions, the elastic modulus decreasessmoothly when temperature increases, showing nomore transitions in the experimental temperatureinterval.

Mechanical properties at large strains are a conse-quence of the copolymer structure. At room temper-ature, PUU14 shows the characteristic behavior of astiff semicrystalline polymer, with an elastic modu-lus around 1 MPa. These properties are desire forthe engineering of soft tissue mainly as elasticscaffolds.



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After yield and neck formation strain follows withconstant stress until break at moderate deformation.The rest of PUU samples show an elastomeric

behavior, deformation at break attains 750% insamples PUU23 and PUU32 decreasing for the high-

est hard segment content. The increase of the elasticmodulus with HS content above 23% agrees withthe behavior found with DMA measurements.

CONCLUSIONS

The PUUs synthesized in this work are promisingmaterials for biomedical applications in which highelastic deformations are required. The introduction ofthe Putrescine segments as chain extender allow formodifying the mechanical properties of PCL, which isa material well-known in biomedical applications, toproduce a deformable amorphous material. Thedegradation properties of the polymers are expectedto be quite different as well. PCL is a material thatdegrades hydrolytically at very low rates because ofits hydrophobicity, which hinders the access of watermolecules to the polymer chains and also because ofits crystallinity. Hard block domains impede on theone hand the crystallization of PCL chains but alsomay allow some hydrogen bonding that is expectedto improve water sorption of the material.

The mechanical behavior of the materials can bemodulated by varying the fraction of hard segmentsto yield elastomers that can deform up to 750%. Insamples containing more than 20% hard segments,they seem to play the role of crosslinking PCLamorphous blocks acting as reinforcing filler at thesame time. On the contrary hard segment contents

below 20% allow for PCL segments crystallizationand the mechanical behavior is completely differentat the other polymers of the series.

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