Kinetics and dynamics of thermally-induced shape-memory behavior of crosslinked short-chain branched polyethylenes

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  • ucth

    analleany

    Received in revised form31 August 2009Accepted 21 September 2009Available online 26 September 2009

    Keywords:Shape-memory effectEthylene-1-octene copolymersPeroxidic crosslinking

    applications only biocompatible polymers come into consider-ation [14,916]. In many cases the SMP needs to be biodegrad-able as well [9,12,1720].

    Preconditions for proper observation of the thermally-inducedSM effect (SME) are the existence of a covalently crosslinked ora stable physically network, as well as a thermal transition (e.g.,

    material is necessary. Secondly, the crystalline phase formed aftercooling of the stretched sample must be able to x the strain andthe elastic/viscoelastic forces stored in the network. The ability ofSMP to x a temporary shape and to restore a permanent (original)shape are characterized by the strain xity ratio (Rf) and strainrecovery ratio (Rr), which are dened according to [1] as:

    Rf 3rem3pr

    and Rr 3pr 3rec;m3pr

    (1)* Corresponding author. Tel.: 49 0 3461 46 2590; fax: 49 0 3461 46 3891.

    Contents lists availab

    Polym

    ls

    Polymer 50 (2009) 54905498E-mail address: hans-joachim.radusch@iw.uni-halle.de (H.-J. Radusch).1. Introduction

    Shape-memory polymers (SMP) represent a promising class ofsmart polymers [16], which can be advantageously used invarious elds of application, e.g., as heat-shrinkable tubing andlms, particularly in electrical cables, packaging [7,8] and auto-motive industry [3,4]. SMP are also being developed for biomed-ical applications, e.g., such as stents and microtubing fortherapeutic actuators, and suture materials [914]. For such

    melting) at convenient temperatures Ttrans [1]. For covalentlycrosslinked semi-crystalline polymers, e.g., if crosslinking is real-ized by peroxide decomposition, a SM behavior with high perfor-mance can be achieved, if the following requirements are fullled.Firstly, the polymeric material must possess a thermally andmechanically resistant networkwith suitable high crosslink densityin order to produce sufciently large elastic and viscoelastic forcesunder load at programming temperature Tpr> Ttrans and preferablyminimal residual strain. Also, a sufcient elongation at break of the0032-3861/$ see front matter 2009 Elsevier Ltd.doi:10.1016/j.polymer.2009.09.062covalent network and the crystalline domains of a crosslinked polymer, i.e., crosslink density andcrystallinity, respectively, was performed using homogeneous ethylene-1-octenecopolymers (EOC) asmodel polymers. The EOCs have been crosslinked by 2,5-dimethyl-2,5-di(t-butylperoxy)hexane (DHBP)decomposition. Two EOCs with a degree of branching of 30 and 60 hexyl side chains per 1000C atomswith each four different crosslink densities were employed. The investigated EOCs differ signicantly incrystallinity, melting temperature (Tm) and crosslink density. The crosslinked EOC undergone theprogramming at a strain of 100% showed high strain xity ratio (Rf) and strain recovery ratio (Rr). The Rfand Rr values increase with increasing crystallinity and crosslink density, respectively, and decline onlyslightly in a subsequent SM cycle. The switching temperature (Tsw) is strictly related to Tm and decreaseswith increasing degree of branching as well as crosslink density in the temperature range of 10163 C.Tsw remains nearly unchanged when the programming temperature (Tpr) or the load during SM recoveryis varied. The kinetics of SM recovery as characterized by the temperature dependence of recovery rate iscontrolled by the melting behavior. The specic work generated by the programmed specimen duringthermally-induced recovery under constant load, gains with increasing crosslink density, and is proposedas dynamical characteristic of practical relevance. The opportunity of tailoring Tsw by variation of thedegree of branching and crosslink density makes such polymers attractive candidates for applicationsrequiring Tsw temperatures in the range from 60 to 100 C.

    2009 Elsevier Ltd. All rights reserved.Article history:Received 28 May 2009Systematic investigation of the relation between shape-memory (SM) behavior and characteristics of thea r t i c l e i n f o a b s t r a c tKinetics and dynamics of thermally-indcrosslinked short-chain branched polye

    Igor S. Kolesov a, Karl Kratz b, Andreas Lendlein b, HaMartin Luther University Halle-Wittenberg, Center of Engineering Sciences, D-06099 Hb Institute of Polymer Research, GKSS Research Center, Kant Str. 55, 14513 Teltow, Germ

    journal homepage: www.eAll rights reserved.ed shape-memory behavior ofylenes

    s-Joachim Radusch a,*

    (Saale), Germany

    le at ScienceDirect

    er

    evier .com/locate/polymer

  • merwhere 3pr is the programming strain, i.e. strain produced bystretching during programming, 3rem is the remaining strain aftercompleted programming, i.e. after cooling to the lowest tempera-ture of experiment, unloading and recovery of the stretched spec-imen at room temperature, and 3rec,m is the residual strainmeasured as thermally-induced recovery (shrinkage) at maximumtemperature of experiment. In the case of ideal SM behavior therelations 3rem 3pr and 3rec,m 0 are valid.

    If strain recovery is recorded at constant heating rate, the rela-tion between temperature and measuring time is linear. In thisconnection the kinetics of SM recovery can be described by meansof the recovery rate, i.e. the rst derivative of recovery strain (3rec(T)/dt) with respect to the time (d3rec(T)/dt). For characterization of theSM recovery kinetics the absolute peak value of recovery ratejd3recT=dtjmax has been employed. The switching temperature(Tsw), as a further important characteristic of SME, was estimated asthe temperature, which corresponds to the peak of recovery rate, incontrast to the other denitions of Tsw, as for example a temperaturecalculated by the equation 3rec(Tsw)$100% 0.5 Rr [21,22].

    Unfortunately, in many publications [222] on the dynamicalaspects of SME in polymers has not been reported. Therefore,a series of excellent works performed in both three-point bending[2325] as well as uniaxial tension and compression modes [26,27]should be especially emphasized. Both the thermally-inducedrecovery and the release of stored stress under various externalconstrains of SMP programmed at a strain of about 10% at differenttemperatures were investigated experimentally and theoreticallyanalyzed on the basis of a developed constitutive model [2327].An important nding is in particular the observation that the SMrecovery under load and the corresponding effective values of Rrdecrease considerably with increasing constraining stress srec andapproach to zero, if srec reaches the programming stress spr [24,25].However, it should be noted that rstly the last result wasdemonstrated only in tree-point bendingmode and secondly all thedetected results are obtained only for relatively low strain and onlyfor SMP with glass transition as switching mechanism [2327]. Inthe present work the dynamics of SM behavior is evaluated on thebasis of SM recovery under load by constant external opposed forceusing the relatively high programming strain of 100% and bymeansof short-term stress relaxation at Tpr.

    The recently published results about SME in ethylene copoly-mers [21,22] and in polycyclooctene [28] crosslinked by means ofdicumyl peroxide (DCP) demonstrate the potential of these types ofmaterial for technical applications in comparison to crosslinkedconventional polyethylenes (PEs). Moreover, the commerciallyavailable homogeneous ethylene-1-octene copolymers (EOC) aresuitable model polymers for the study of the mechanisms of ther-mally-induced SME in crosslinked semi-crystalline polymers due tothe possibility for adjusting the temperature range of melting aswell as the ability to crystallization and crosslinking by means ofthe variation of comonomer content [29]. As a matter of fact theinvestigated EOCs are short-chain branched polyethylenes(SCBPEs) with the random intrinsic homogeneous distribution ofshort-chain branches or in other words of tertiary carbon atomsalong the main chains. The crystallinity, melting and crystallizationtemperature of SCBPEs decrease with increasing degree ofbranching due to the shortening of crystallizable ethylenesequences between neighboring tertiary carbon atoms, as alreadyreported for EOC [29]. The experimental results also showed thathydrogen atoms on tertiary carbon atoms are much more easilyabstracted than hydrogen atoms attached to secondary andprimary carbon atoms [30,31]. These results are in agreement withtheoretical calculations [32]. According to [33] the crosslinkabilityof SCBPEs increases with increasing degree of branching in the

    I.S. Kolesov et al. / Polyrange between approximately 16 and 31 CH3/1000C. Tertiaryradicals can also initiate the undesired scission reaction appearingin SCBPEs [34]. However, the contribution of scission reactionprevails only at high degree of branching (31 CH3/1000C) andhigh peroxide concentration, e.g., at dicumyl peroxide 4 wt% [33].

    For the depicted reasons, the objectives of the present work arethe systematic investigation of the correlation between the SMbehavior of EOCs crosslinked by means of peroxide, on the onehand, and the characteristics of network and crystalline phase aswell as SM programming and measuring conditions, on the otherhand. The study focuses on the description of basic mechanisms ofthe thermally-induced SME in crosslinked semi-crystalline poly-mers and in particular, on the kinetics and dynamics of SMrecovery. From the point of view of application as SMP, theimportant advantage of crosslinked EOCs/SCBPEs compared tocrosslinked conventional PEs is the relatively low meltingtemperature, which even can be only 60 C depending on thedegree of branching and crosslink density. This allows to expectalso low Tsw values at which the thermally-induced SME can beactivated by simple procedures such as heating in hot water.

    2. Experimental

    2.1. Materials and processing

    The SCBPEs used in the present study are commercially availablehomogeneous EOCs (Dow Chemical) produced using metallocenecatalysts with approx. 30 and 60 hexyl branches per 1000C (EOC30and EOC60, respectively). The relevant parameters of these mate-rials are compiled in Table 1. As crosslinking agent 2,5-dimethyl-2,5-di(t-butylperoxy)hexane (DHBP) was employed. The use ofDHBP resulted in a higher efciency and a good processability ofthe chosen systems compared to DCP due to the availability of twoperoxy groups and the higher thermal stability (up to 145 C) aswell as uidity at room temperature. Pellets of the processedpolymers were impregnated with DHBP via mechanical mixing ina hermetic closed plastics ask and diffusion of the peroxide intothe polymer pellets for three days at room temperature. For samplepreparation the impregnated EOC30 and EOC60 pellets containingthe nominal DHBP concentration of 1, 2, 3 and 4 wt% were used.Subsequently, the DHBP containing polymers were homogenizedusing a single-screw mixing extruder (Brabender) at 130 C barreltemperature. The true DHBP content determined by thermogravi-metric analysis (TGA,MettlerToledo), as given in Table 2, was lowercompared to the desired values in all samples. Thereby, the differ-ences between true and desired values increase signicantly withincreasing crystallinity of the polymer.

    The processing experiments show that the applied crosslinkingmethod is suitable for PEs with the chosen melt-ow index MFI(see Table 1) measured by means of the melt index test apparatusMI 21,6 (Gottfert). DHBP as a low-molecular uid substance can actas a lubricant during processing, i.e., the effective MFI values ofDHBP-impregnated PEs are evidently higher and the viscosities arelower than that of pure materials. Thus, the DHBP can conduce tothe mixing process at 130 C. The extrudates were compressionmolded to lmswith a thickness of 0.5, 1 and 2 mm and crosslinkedat a temperature of 190 C in the press under load. The chosencrosslinking temperature and crosslinking duration were opti-mized on the basis of time dependency of torque measured usinga vulcameter Elastograph (Gottfert) at four different temperaturesof 170, 180, 190 and 200 C.

    2.2. Network characterization

    The crosslink density, which reects the density of network

    50 (2009) 54905498 5491chains, is connected with Mc and the material density r at

  • measurement temperature through the simple equation 2.3. Instrumentation

    Table 1Designations as well as physical and molecular parameters of the used EOCs.

    Designation Melt-ow index [dg/min] MeltingtemperatureTm [C]

    Density[kg/m3]

    Mass-averagemolecular mass Mw[kg/mol]

    Poly dispersityMw=Mn [-]190 C/2.16 kg 190 C/5.0 kg 130 C/5.0 kg

    EOC30 5.0 17.7 4.2 96 900 64 2.3EOC60 4.4 15.9 3.4 59 870 87 2.5

    I.S. Kolesov et al. / Polymer 50 (2009) 549054985492vc Mc=r. The average molecular mass of the links, i.e., thenetwork chains between two neighboring network nodes Mc ofEOC crosslinked using different DHBP amounts were calculatedby means of the FloryRehner equation [35] on the basis ofswelling measurements. The swelling measurements were per-formed at the samples stored in xylene at 110 C after at least 10 husing the gel of crosslinked EOC obtained after extraction of thesoluble fraction as described in [33,36]. The extraction experi-ments were carried out in a Soxhlet extractor in xylene at 110 Cfor at least 28 h using about 0.7 g samples partitioned into 65pieces. The nc and Mc values for both crosslinked EOCs were alsoevaluated from stress-strain diagrams ascertained at 120 C onthe basis of the MooneyRivlin equation [37,38]. The resultsdetermined for crosslinked EOCs using the both abovementionedmethods are in satisfactory agreement with each other (seeTable 2). The difference between the results of both used methodsis within the experimental error range of 1020%, which becomeswider with declining nc. Hence, in the text, tables and legendsbelow, the mean nc values calculated on the basis of both swellingand tensile tests are given. Interestingly, the same quantities ofDHBP cause a more close-meshed network in EOC30 compared toEOC60, i.e. the efciency of crosslinking agent in SCBPEs withhigh degree of branching is lower, presumably because of anincreasing contribution of the scission reaction initiated simul-taneously with the crosslinking [33].

    However, it has to be also taken into account that the rela-tively homogenous distribution of network nodes may beexpected only in highly branched and correspondingly lowcrystalline PEs whose very small crystallites, e.g., fringed micelles[29], are dispersed in the continuous and quasi homogenousamorphous phase. The peroxide distribution in medium and highcrystalline PEs homogenized in the extruder is denitely no morehomogeneous at molecular level after subsequent cooling of theextrudates due to reallocation of the peroxide caused by crys-tallization. By denition, the originated crystalline regions arefree from peroxide. In this connection by reheating until 190 Cthese nano-regions obviously will not be crosslinked, and retainthe ability to crystallization. Hence, the experimentally acquiredaverage values of crosslink density are denitely considerablylower than the true local crosslink density of the amorphousphase.

    The elongation at break values received from tensile tests pointto the possibility of using a standard programming strain 3pr of100% for the investigation of the SM behavior of used materials.Table 2Crosslink density nc and average molecular mass of link Mc of medium branched EOC3depending on DHBP content.

    EOC30

    DHBPcontent [wt%]

    nc [molm3] Mc [kg/mol]

    Tens. Test Swell. Test Tens. Test Swell. Test

    0.5 56 75 14.39 10.371.1 142 134 5.68 6.021.6 213 224 3.78 3.601.9 274 264 2.94 3.05Tensile tests in the temperature range of SM tests programmingat 120 C has been carried out using a testing machine Zwick 1425(Zwick) with heating chamber and a load cell 10 N at a cross headspeed of 5 mm/min after DIN EN ISO 527-1. As specimens, shoul-dered test bars with the cross-section area 2.0 2.0 mm2 wereused. The initial distance between the clamps was 21 mm.

    Thermal analysis of crosslinked EOC30 and EOC60 was per-formed with DSC-7 (PerkinElmer) at a heating rate of 20 K/min.The experimental data were converted into temperature depen-dencies of apparently heat capacity cp(T) as described earlier[39,40]. The measured cp(T) values in conjunction with theoreticalcp(T) values for crystalline and amorphous PE were utilized for thecalculation of an enthalpy-based crystallinity as proposed in [41].

    SM behavior investigations were carried out in tensile modeusing a mechanical spectrometer measuring head Mark III (Rheo-metric Scientic). The samples shaped as strips with a cross-sectionarea 2.01.0 mm2 were tested during the specic cyclic thermomechanical experiment at an initial clamps distance of 12 mm. Thesamples were programmed by a tensile strain of 3pr 100% duringa standard dwell period of 120 s at the Tpr, and cooled down to thelowest temperature Tlow of 10 or 0 C for EOC30 and EOC60,respectively, in the constant deformed state at an average coolingrate of 11 K/min. The values of programming stress spr weremeasured at Tpr for 120 s after loading of the sample. The stretchedspecimen was thermally equilibrated for 10 min at Tlow, unloadedand heated to 25 C. After a dwell time of 10 min at 25 C theremaining strain 3rem for estimation of Rf was determined, and thanthe unconstrained thermally-induced recovery of the completelyprogrammed sample in a heating run at a rate of 2 K/min and zerostress of 70 Pawasmeasured. In order to estimate the fatigue of SMpolymer the described cyclewas additionally repeated two times foreach specimen. The effect of load on the specic work, generated bythe programmed specimen during SM recovery was investigatedunder constraining stress srec of 0.1 and 0.2 MPa. Because of theunconventional nature of this kind of experiment, the experimentaldetails are discussed together with the results in Section 3.3.

    3. Results and discussion

    3.1. Melting behavior and crystallinity

    Fig.1 shows the enthalpy-based crystallinity versus temperatureobtained in rst heating runs for both uncrosslinked and cured0 and highly branched EOC60 estimated on the basis of tensile and swelling tests

    EOC60

    DHBPcontent [wt%]

    nc [molm3] Mc [kg/mol]

    Tens. Test Swell. Test Tens. Test Swell. Test

    0.9 38 63 21.21 11.541.9 105 126 7.68 6.402.7 151 149 5.34 5.433.6 192 193 4.20 4.17

  • EOC30 and EOC60. Both highly branched EOC60 and mediumbranched EOC30 exhibit the fragmentation of the cp melting peakwhich results in the discontinuity of crystallinity as function oftemperature. This phenomenon is caused by annealing at roomtemperature during storage of prepared samples. The secondcrystal population generated by annealing has a poorer quality andsubsequently lower thermal stability as it was shown for uncros-slinked EOCs especially with high branch degree [42].

    The consistent decrease of the nal melting temperature Tm and

    crystallinity for medium branched EOC30 is more distinct than thatof highly branched EOC60. The depicted reduction of Tmvalues withincreasing crosslink density reects obviously the reduction ofperfection and size of crystallites in particular the decrease oflamellae thickness.

    3.2. Effect of degree of branching and crosslink density,programming temperature and cyclic thermomechanical load onunconstrained SM behavior of EOCs

    Fig. 1. Enthalpy-based crystallinity versus temperature obtained for medium branched EOC30 (a) and highly branched EOC60 (b) depending on the crosslink density nc.

    I.S. Kolesov et al. / Polymer 50 (2009) 54905498 5493the crystallinity in the entire temperature range takes place withincreasing crosslink density for both EOC30 (from 96 to 88 C andfrom 35 to 29% at 25 C, respectively) and EOC60 (from 64 to 56 Cand from 14 to 13% at 25 C, respectively). The decrease ofFig. 2. Temperature dependencies of SM recovery strain (a, c) and recovery strain rate (b, d)and EOC60 (c, d) with four different crosslink density nc after programming at 100 and 80Fig. 2 depicts the inuence of crosslinking degree of mediumEOC30 (a, b) and highly branched EOC60 (c, d) on the conventionalcharacteristics and kinetics of thermally-induced SM recovery inmeasured in three repeating thermomechanical load-recovery cycles for EOC30 (a, b)C, respectively.

  • echa

    merTable 3Inuence of crosslink density nc, programming temperature Tpr and cyclic thermombranched EOC.

    EOC type nc [molm3] Tm [C] Tpr [C] Cycle #

    EOC30 65 93.1 100 1st2nd3rd

    140 91.0 100 1st2nd3rd

    120 1st140 1st

    2nd3rd

    220 89.0 100 1st2nd3rd

    270 88.2 100 1st2nd3rd

    EOC60 50 60.2 80 1st2nd3rd

    120 1st2nd3rd

    120 57.9 80 1st2nd3rd

    120 1st

    I.S. Kolesov et al. / Poly5494the three-cycles thermomechanical SM test. These experimentswere performed at the programming temperature Tpr of 100 C forEOC30 and 80 C for EOC60 which are near to the nally meltingtemperature of these SCBPE.

    In combination with Fig. 1, Fig. 2 demonstrates the strictcorrelations between the crystallinity and temperature depen-dence of the SM recovery strain 3rec(T) (Fig. 2a and c) and SMrecovery rate d3rec(T)/dt (Fig. 2 b and d) as well as between meltingTm and switching Tsw temperatures, respectively. The EOCs showa reduction of the Tsw value and SM recovery rate in the tempera-ture range of melting and especially at the switching temperaturejd3recT=dtjmax with decreasing melting temperature and crystal-linity, respectively, due to the increase of both the degree ofbranching as a major inuencing factor and the crosslink density nc(see Figs. 1 and 2 as well as Table 3). The shoulders in the d3rec(T)/dtcurves of EOC60 are caused by the existence of a thermodynami-cally less stable crystal population. The Rr values are approximately95% and 96% for EOC30 and EOC60, respectively.

    With increasing crosslink density nc of both EOCs a small increaseof Rr of about 1% is observed. At the same time, Rf of EOC60 decreasesconsiderably, whereas Rf of EOC30 increases only slightly withincreasing nc value (see Fig. 2 and Table 3). In consequence of theincrease of stresses occurred by programming (spr) with constantstrain of 100% and decrease of the ability of crystalline phase to xthese stresses due to the decrease of crystallinity and perfection ofcrystallites, Rf is generally expected to decreasewith an increase of nc.

    There is yet another difference in the SM behavior betweenEOC30 and EOC60. All highly branched EOC60 networks

    2nd3rd

    150 56.7 80 1st2nd3rd

    120 1st2nd3rd

    190 55.4 80 1st2nd3rdnical load on programming stress spr and SME characteristics of medium and highly

    spr [MPa] Rf [%] Rr [%] Tsw [C] jd3recT=dtjmax[103 s1]

    0.97 96.0 95.2 100.6 2.410.93 95.6 93.7 100.2 2.250.91 95.5 93.5 100.2 2.261.45 96.5 95.0 97.8 2.241.41 96.5 94.5 97.7 2.171.42 96.5 94.3 97.7 2.171.51 96.6 95.3 98.2 2.231.65 97.1 93.4 98.3 2.161.45 97.2 90.2 98.2 2.111.56 97.2 87.7 98.2 2.071.85 97.7 95.2 95.9 2.031.84 98.1 95.0 95.9 2.011.84 98.1 94.9 95.9 2.002.05 98.6 95.0 94.5 1.902.05 99.0 94.9 94.4 1.882.04 99.0 94.9 94.4 1.880.60 95.5 95.7 71.2 1.220.58 95.0 94.5 71.0 1.190.57 94.9 94.2 71.1 1.170.56 94.7 93.4 71.4 1.120.54 95.0 92.2 71.3 1.100.53 95.0 91.6 71.2 1.090.943 92.1 96.3 66.9 1.120.927 91.4 96.0 66.9 1.100.926 91.4 95.9 67.0 1.091.01 91.2 95.6 67.2 1.08

    50 (2009) 54905498demonstrate a continuous decrease of 3rec(T) with increasingtemperature. On the other hand, the medium branched EOC30networks with nc values of 70 and 130 molm

    3 exhibit an increaseof 3rec(T) values in the temperature range from 25 to 66 C and49 C, respectively. Since these temperatures are consistent withthe temperature of a-relaxation the described behavior can beexplained by the contribution of a-relaxation, which is explicitevident only for slowly cooled low and medium branched SCBPEs[41]. This conclusion agrees well with the 3rec(T) behavior, which isdemonstrated by high density PE for temperatures up to approxi-mately 100 C [43]. In highly branched EOC60 the temperature andintensity of a-relaxation is considerably lower and correspondinglythe contribution of a-relaxation to SM recovery is insignicant [40].

    Generally, the atter and wider the melting endotherm thesmaller and broader is the 3rec(T) peak of SMP (see Fig. 2). Thus, it canbe concluded that the decrease of crystallinity and crystal perfectionas well as the broadening of crystal size distribution, especially indirection of lower values, leads in any case to a deceleration of theSM recovery kinetics, even if the forces produced by programmingincrease as in the case of increasing crosslink density. This can beexplained by an increment in difference between the elastic andviscoelastic forces produced by programming and stored/xed inthe specimen by crystallization due to an increase of a part of poorerand smaller crystallites in the entire spectrum.

    For samples with a high degree of crosslinking the temperaturedependencies of 3rec(T) and d3rec(T)/dt obtained from the cyclicthermomechanical SM test at relatively low programming temper-atures are practically congruent for all cycles (see Figs. 2 and 3ad).

    1.00 91.1 94.2 67.1 1.060.99 90.9 94.2 67.2 1.051.15 88.0 96.6 64.7 1.051.14 88.0 96.4 64.7 1.051.14 88.1 96.4 64.7 1.051.32 87.0 96.0 64.6 1.021.31 87.0 95.8 64.5 1.011.31 86.4 95.8 64.5 1.001.36 86.6 96.6 62.6 1.011.34 86.6 96.6 62.6 1.011.34 86.3 96.6 62.6 1.01

  • merI.S. Kolesov et al. / PolyOnly the samples of EOC60 and EOC30 with lower degree of cross-linking of approximately 50 and 70 molm3 exhibit certain smalldifferences between the curves detected in rst, second or third cycle.In particular, such samples exhibit in the rst cycle some lower 3rec(T)at T> Tsw and correspondingly some higher Rr values as well asslightly higher values of jd3recT=dtjmax. In any case, the differencesbetween second and third cycle were generally negligible.

    The situation changes appreciably, if Tpr becomes 40 K higher(see Fig. 3). At Tpr 140 and 120 C for EOC30 and EOC60, respec-tively, the reduction of Rr value in repeating cycles of SM test, ascompared to 40 K lower Tpr, is evident. However, this behavioroccurs more explicitly for specimens with nc values of approxi-mately 50 to 140 molm3. For EOC networks with higher nc valuesthe inuence of Tpr on the change of SM characteristics in cyclicthermomechanical test is not so distinct. In other respects thecrosslinked EOCs show the similar SM behavior, as described abovefor 40 K lower Tpr. The optimal Tpr value must be chosen obviouslybetween melting and crystallization temperatures, but in this case,the specimen should be pre-heated at a temperature higher thanTmbefore programming.

    Fig. 3. Inuence of programming temperature Tpr on dependencies of SM recovery strain (a,highly branched EOC60 with different crosslink density nc (cf).50 (2009) 54905498 5495Table 3 demonstrates that the programming stress spr gainswithincreasing programming temperature Tpr and crosslink density ncfor both EOCs as expected according to the rubber elasticity theory.The medium branched EOC30 network produces markedly higherspr values compared to highly branched EOC60 with similar cross-link density nc (140 and 150 molm

    3, respectively) at the same Tpr.This difference can be explained by the abovementioned hypothesisdiscussed in Section 2.2, which assumes that the true local crosslinkdensity of the amorphous phase is considerably higher as theexperimentally obtained average values of crosslink density.

    3.3. Effect of load during SM recovery of programmed EOC networks

    The effect of load on SM recovery behavior of crosslinked EOC30and EOC60 is demonstrated in Fig. 4. As previously discussed forunconstrained SM recovery, the SM recovery curves under loadpossess no signicant change in the second and third SM cyclescompared to the rst cycle. The SME characteristics obtained in therst SM cycle are provided in Table 4. In this case, besides the Rf, Rr,Tsw and jd3recT=dtjmax values, it seems to be reasonable to use the

    c, e) and recovery strain rate (b, d, f) measured for medium branched EOC30 (a, b) and

  • merI.S. Kolesov et al. / Poly5496mechanical specic work Wsp, which is done by the elastic andviscoelastic forces stored in the volume unit of completely pro-grammed specimen. The Wsp will be proposed for the character-ization of SM recovery behavior under load with the external force,which opposes the clamp motion. At this point an importantremark is tomake that in presented experiments the nominal valueof constraining stress srec is related to the initial cross-section areaof the sample before stretching.

    Therefore, at programming strain of 100% used in the presentwork the true stress reduces in recovery run in the case of ideal SMbehavior (Rf Rr 100%) from 2srec to srec with increasingtemperature, due to the increasing initial cross-section area ofsample. In real case (Rf< 100% and Rr< 100%) the true stress struedecreases in the range between 2srec> strue> srec. By contrast, herethe force was kept constant. For this reason, the specic work canbe calculated as the product of maximum contraction of sample (athighest temperature of the experiment) and constant externalopposed force per unit volume of sample.

    In comparison to unconstrained recovery runs (srec 70 Pa) theSM recovery curves under load (here srec 0.1 or 0.2 MPa) are

    Fig. 4. Inuence of load during SM recovery on dependencies of SM recovery strain (a, c, e)EOC30 (a, b) and EOC60 (cf) with different cross-link density nc after programming by 1050 (2009) 54905498notably shifted upwards and evince distinctly lower Rr values aswell as higher apparent Rf values, which can exceed 100%. Similar tothe unconstrained recovery runs the temperature dependencies ofrecovery strain and recovery rate obtained under load srec by cyclicthermomechanical SM test are nearly congruent, especially forsamples with high crosslink density. The differences between rstand third cycle were negligible.

    For all investigated samples the peaks of SM recovery rateascertained under loadwere shifted slightly to higher temperaturesand evinced some higher jd3recT=dtjmax values compared tounconstrained recovery runs. The differences in switchingtemperatures and peak values of SM recovery rate betweenunconstrained recovery runs and recovery runs detected underload rise with increasing srec value. However, these changes arerelatively low (see Fig. 4 b, d and f as well as Table 4). Such a weakacceleration of the recovery kinetics could be connected withchanged melting behavior of the samples under load.

    With increasing crosslink density nc the programming stress sprand obviously the stored elastic and viscoelastic forces increasemarkedly and correspondingly the recovery becomes more

    and SM recovery strain rate (b, d, f) measured at different srec values (see legends) for0% strain.

  • Table 4Inuence of crosslink density nc and nominal value of constraining stress during SMrecovery srec on SME characteristics of medium and highly branched EOCs obtainedin rst SM cycle.

    EOCtype

    Tpr[C]

    nc[molm3]

    srec[Pa]

    Rf[%]

    Rr[%]

    Tsw[C]

    jd3recT=dtjmax[103 s1]

    EOC30 100 65 70 96.0 95.2 100.6 2.412 105 97.0 64.7 102.0 2.52

    140 70 96.5 95.0 97.8 2.242 105 99.3 81.4 99.0 2.36

    220 70 97.7 95.2 95.9 2.032 105 99.7 85.2 97.0 2.17

    270 70 98.6 95.0 94.5 1.902 105 100.3 85.4 95.5 2.04

    EOC60 80 50 70 95.5 95.7 71.2 1.221 105 100.9 73.0 72.5 1.29

    120 70 92.1 96.3 66.9 1.121 105 98.3 84.3 68.2 1.192 105 103.5 68.8 69.2 1.19

    150 70 88.0 96.6 64.7 1.051 105 93.0 87.4 65.5 1.122 105 98.0 76.1 66.5 1.14

    190 70 86.6 96.6 62.6 1.015

    I.S. Kolesov et al. / Polymercomplete for both EOCs that results in a certain rise of strainrecovery ratio Rr (see Fig. 4 a, c and e, Table 4 and Fig. 5). Fig. 5demonstrates that the spr value shows yet no linear dependence onnc, as expected according to the rubber elasticity theory. Regardlessthe rubber elasticity theory, the spr, Rr and Wsp values exhibita distinct tendency to the formation of a plateau with increasing nc.Thereby the Wsp value for highly branched EOC60 increases insig-nicantly. Allowing for the considerable increment of program-ming stress spr with increasing crosslink density nc it can beconcluded that there is only aweak qualitative correlation betweenthe increase of spr values and of specic work Wsp as well as ofstrain recovery ratio Rr. This observation points to the decrease ofability of crystalline phase to x the forces generated in thenetwork by programming with increasing crosslink density due tothe abovementioned decrease of crystallinity and perfection ofcrystallites that results in an increase of the difference betweenprogramming stress and stored elastic/viscoelastic stress.

    1 10 90.8 88.7 63.8 1.082 105 96.1 78.8 64.9 1.10Two other peculiarities of SM recovery under load should beemphasized. Firstly, theWsp value shows surprisingly a considerablerise with increasing external load, e.g., for EOC60 with

    Fig. 5. Inuence of cross-link density and degree of branching of EOCs on program-ming stress spr, strain recovery ratio and Rr specic work Wsp done by SM recoveryunder load srec of 0.2 MPa (black points and curves); the results obtained for EOC60 bysrec 0.1 MPa are given in gray.nc 150 molm3 the Wsp value changes from 82 to 131 kJm3 asa result of srec increase from 0.1 to 0.2 MPa. Secondly, the mediumbranched EOC30 produces a notably higher programming stress sprand correspondingly realizes a considerably larger specicmechanical work Wsp compared to highly branched EOC60 withnearly the same crosslink density nc, e.g., EOC30 withnc 140 molm3 and EOC60with nc 150 molm3 show the valuesspr 1.48 and 1.17 MPa as well as Wsp 165 and 137 kJm3,respectively. This means that the medium branched EOC30 incomparison to the highly branched EOC60 does not only producea larger programming stress due to the higher programming/crys-tallization temperature and local crosslink density, but EOC is alsoable to x the produced forces better as a consequence of highercrystallinity and crystal perfection.

    4. Conclusions

    Crystallinity, melting temperature and crosslinking density ofcrosslinked homogeneous ethylene-1-octene copolymers (EOCs)can be well controlled by the 1-octene content in the copolymer(degree of branching) and the amount of suitable peroxide, e.g.,DHBP. Thus, short-chain branched polyethylenes (SCBPEs) can beapplied as model shape memory (SM) polymers for the investiga-tion of principles and mechanisms of the thermally-induced SMeffect in covalently crosslinked semi-crystalline polymers.

    The inuence of crystallinity, melting temperature and crosslinkdensity of semi-crystalline networks as well as of programmingconditions, which include also the temperature-triggered changesof crystallinity on the kinetics and dynamics of the SM behavior ofSCBPEs, has been analyzed. The ndings of the investigation allowto make the following ascertainments, proposals and suggestions:

    - The resulting switching temperature for all investigatedcrosslinked SCBPEs decreases systematically with increasingboth the degree of branching and the crosslink density withinthe temperature range of 101 to 63 C and shows a rather strictdependence on the melting temperature.

    - In unconstrained SM recovery experiments the generatedSCBPE networks exhibit high values of strain recovery ratio ofabout 97%, which increases only slightly with increasingcrosslink density, in spite of simultaneous considerable rise ofprogramming stress. The highly branched and correspondinglylow crystalline SCBPE networks demonstrate a distinctdecrease of strain xity ratio with increasing crosslink densityand programming stress. In contrast, the strain xity ratio ofmedium crystalline SCBPE networks increase weakly withincreasing crosslink density and programming stress as a resultof the well developed crystalline structure and the markedlycontribution of a relaxation to the SM recovery.

    - The increase of branching degree and crosslink density resultsin the deceleration of the SM recovery kinetics, even though,the programming stress increase simultaneously. This is causedby the drop of ability of crystalline phase to x the elastic/viscoelastic forces generated in the network due to thedecrease of crystallinity, crystal size and perfection, and thebroadening of crystal size distribution.

    - Although, the elasticity modulus of covalent network andcorrespondingly the stress at constant strain increase withrising temperature, the stored stress, which will be xed in thenetwork by the crystalline structure, cannot exceed the loweststress generated by the stretched network at crystallizationtemperature. Furthermore, the considerable increase ofprogramming temperature causes a slight decrease of strainrecovery and strain xity ratios as well as of SM recovery rate,

    50 (2009) 54905498 5497which can be explained by the acceleration of thermo-

  • oxidative degradation. Therefore, the optimal programmingtemperature must obviously be as low as possible and locatedin the window between melting and crystallization tempera-tures. In this case, the short-term heating above the meltingtemperature is necessary before programming.

    - The mechanical specic work generated by the stored elastic/viscoelastic forces during SM recovery under load, which wascalculated as the product of nal contraction of sample andconstant external opposed force per unit volume of the sample,canbeused asdynamical SMcharacteristic of practical relevance.The investigation of constraining stress inuence on the SMrecovery behavior is very important for the estimation of appli-cation limits of given SM polymers. In comparison to uncon-strained recovery runs, the SM recovery under load leads todistinctly lower strain recovery ratio but higher apparent strainxity ratio, which can exceed 100%. The strain recovery ratio andmechanical specic work under load increase moderately withincreasing crosslink density, in spite of simultaneous consider-able increaseof programming stress, that is especially evident forhighly branched SCBPE. This points to distinct differencesbetween the generated and stored stresses and to the strong

    results can serve for the development of innovative SM poly-mers and applications such polymers as heat-shrinkable mol-

    References

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    I.S. Kolesov et al. / Polymer 50 (2009) 549054985498ded items with reduced energy input. The results of the workcontribute to a better understanding of the correlationsbetween crystallization/relaxation processes, which are behindthe SM effect, and the parameters of the SM behavior, neces-sary for process optimization.

    Acknowledgment

    The authors gratefully acknowledge the nancial support byDeutsche Forschungsgemeinschaft (DFG). Samples of DHBP peroxidewere kindly provided by Evonic Degussa GmbH (Germany).effectof crystallinityandperfectionof crystalline structureon theability of SM polymer to store the generated forces.

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    Kinetics and dynamics of thermally-induced shape-memory behavior of crosslinked short-chain branched polyethylenesIntroductionExperimentalMaterials and processingNetwork characterizationInstrumentation

    Results and discussionMelting behavior and crystallinityEffect of degree of branching and crosslink density, programming temperature and cyclic thermo-mechanical load on unconstrained SM behavior of EOCsEffect of load during SM recovery of programmed EOC networks

    ConclusionsAcknowledgmentReferences

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