Isothermal recovery rates in shape memory ...

Isothermal recovery rates in shape memory polyurethanes Charly Azra, Christopher JG Plummer,1 Jan-Anders E Månson Laboratoire de Technologie des Composites et Polymères (LTC), Ecole Polytechnique Fédérale de Lausanne (EPFL), Station 12, CH-1015 Lausanne, Switzerland. E-mail: [email protected]

Abstract. The present work compares the time dependence of isothermal shape recovery in thermoset and thermoplastic shape memory polyurethanes (SMPUs) with comparable glass transition temperatures. In each case, tensile tests have been used to quantify the influence of various thermo-mechanical programming parameters (deformation temperature, recovery temperature and stress and storage times following the deformation step) on strain recovery under zero load (free recovery) and stress recovery under fixed strain (constrained recovery). It is shown that the duration of the recovery event may be tuned over several decades of time with an appropriate choice of programming parameters, but that there is a trade-off between the rate of shape recovery and the recoverable stress level. The results are discussed in terms of the thermal characteristics of the SMPUs in the corresponding temperature range as characterized by modulated differential scanning calorimetry and dynamic mechanical analysis, with emphasis on the role of the effective width of the glass transition temperature and the stability of the network that gives rise to the shape memory effect. Keywords: shape memory polymers, polyurethanes, actuators

1. Introduction On application of a stimulus such as heat [1], light [2] or immersion in water [3] or other solvents [4, 5], shape memory polymers (SMPs) are able to revert to a “primary” shape from a “secondary” shape, induced by significant mechanical deformation. Their introduction to the market in the early 1990’s by Mitsubishi Heavy Industries has led to considerable academic interest, as borne out by recent reviews, which focus on molecular design [1], theoretical concepts [6], comparison of existing materials [7], optimization [8], composites [9], modeling [10] and biomedical applications [11-13]. In thermally activated SMPs, the primary shape is defined during the post-polymerization "processing" step. The secondary shape results from a series of thermo-mechanical treatments generally referred to as "programming" [1]. The polymer is first deformed mechanically at a temperature Td in the vicinity of a thermal transition temperature, which is typically either a glass transition temperature, Tg, or a melting temperature, Tm. The secondary shape is fixed by cooling under mechanical constraint to a storage temperature, Ts, which is usually well below the transition temperature. Depending on the deformation mode, thermal stresses may develop during cooling, requiring unloading to a stress-free state. Recovery of the primary shape is then achieved by raising the temperature to the recovery temperature Tr, also in the vicinity of the transition temperature. The ability of SMPs to return to their primary shape is due to the presence of cross-links, which act as anchoring points for a network of flexible macromolecular chains, preventing slippage under large deformations. These cross-links may be either chemical (covalent bonds) or physical (entanglements or segregated rigid domains) in thermoset and thermoplastic SMPs respectively [1]. The transition from the rubbery state to the glassy or semi-crystalline state acts as a switching mechanism for the fixation and

1 Corresponding

author

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Isothermal recovery rates in shape memory polyurethanes

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subsequent recovery of the secondary shape. It also marks a transition from an essentially entropic response to applied deformations to a regime in which enthalpy changes dominate. Liu et al. [7] grouped SMPs into 4 classes, according to the fixation temperature (Tg or Tm) and type of cross-link (chemical or physical). Of the various materials investigated for their potential as SMPs, shape memory polyurethanes (SMPUs) are generally considered to possess the widest range of properties and processing characteristics, owing to their versatile chemistry. Indeed, SMPUs may belong to any of the 4 classes defined by Liu et al. [7], and there is consequently considerable scope for tuning their properties and shape memory performance. Moreover, SMPUs are ideally suited to biomedical applications owing to their proven biocompatibility [14] and/or biodegradability [15]. As with many polyurethane elastomers, SMPUs usually comprise two phases [16]. The so-called “soft” phase is made up of relatively long polymer chain segments and provides the switching mechanism for the secondary shape fixing. The “hard” phase forms micro or nano-domains dispersed in a matrix formed by the soft phase. These domains may be either amorphous or semicrystalline and act as physical cross-links. Their softening temperature (Tg or Tm) is chosen to be sufficiently high that they experience little or no deformation when the soft segments are in the rubbery state. The majority of publications on SMPs have focused on synthesis and characterization, tuning and optimization of the transition temperature and rubbery modulus, shape fixity and recovery ratios, recovery stress, biocompatibility and biodegradability. The time-dependent recovery of SMPs has received less attention, and only a few quantitative data are available for the recovery rates and duration of the recovery event in thermally activated SMPs [17-19], electrically activated SMPs [20] and solvent-activated SMPs [21]. This is presumably because most applications envisaged in the literature require highly reproducible shape fixity and shape recovery, well defined recovery temperatures, enhanced mechanical properties or high recovery stresses. However, many biomedical applications would also benefit from precise control of the shape recovery rate. For example, Sharp et al. [22] proposed the use of SMPs in self-deploying neuronal electrodes, for which limited recovery rates are important if excessive brain tissue trauma is to be avoided. Similarly, where SMPs are envisaged for microfluidics applications [23] such as low-cost, one-shot diagnostics, one or more fluids may need to be displaced at a reduced, controlled rate. Indeed, the relatively slow response of SMPs gives them a certain advantage over shape memory alloys in this respect. Gall et al. [17] found the shape recovery dynamics in SMPs under isothermal conditions to be strongly influenced by Td and Tr. They showed that the ratio of these temperatures to Tg controls the shape and extent of recovery as a function of time. For Td/Tg < 1 recovery is relatively rapid, whereas it is much slower for Td/Tg > 1. The recovery stress also passes through a maximum as a function of time when the material is programmed in the glassy state, whereas programming in the rubbery state generally leads to monotonic stress relaxation towards the level defined by the rubbery modulus. A number of models have been also been developed in which the time dependent behavior is treated explicitly for thermally activated SMPs [24, 25, 26, 27] and solvent-activated SMPs [21]. However, the corresponding experimental data are lacking, as are systematic studies of the correlation between programming and recovery parameters and the recovery profile as a function of time. The goal of the present work has been to investigate the time dependence of shape recovery in two types of SMPU and their dependence on the programming and recovery conditions. A tensile test apparatus with temperature control was then used to quantify the influence of various programming and recovery parameters on the shape recovery behavior of SMPUs over time. The results are discussed in light of the thermo-mechanical properties of the different SMPUs determined using classical characterization methods, and current models for the underlying physical mechanisms governing the transition between the deformed and undeformed states during programming and recovery, with the aim of providing pointers towards possible materials tailoring strategies for SMP applications requiring controlled recovery rates. 2. Materials and methods 2.1 Sample preparation The two SMPUs used for this study, a thermoset and a thermoplastic, were purchased from SMP Technologies (Japan). Both were nominally amorphous with similar Tg of about 55 °C. The thermoset SMPU, MP5510, was supplied as a two-part resin, and was processed following the manufacturer’s guidelines. The two components, A and B, were degassed at room temperature under vacuum (50 mbar) for 1 h. They were then thoroughly hand-mixed for 30 s in the ratio 40:60 by mass. The resulting reactive mixture had a pot life of about 5 min, so that it was necessary to degas and inject it into the mold immediately after mixing. To this end, the mixture was placed in a sealed chamber in which the air pressure


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was first reduced for degassing (50 mbar, 45 s) and subsequently increased (2 bars) in order to force the liquid from the chamber into a mold, which took about 3 min. The mixture was then cured under pressure for 10 min at room temperature and post-cured for 4 h at 70 °C, to give reproducible, void-free specimens in the form of 1 mm thick plaques. These were cut into 10 mm wide rectangular bars for the tensile shape memory tests. The thermoplastic SMPU, MM5520, was received in pellet form. The pellets were first dried at 50 °C under vacuum (50 mbar) for a minimum of 3 days. A hot press (Fontijne-Holland) with a steel mold was used to produce rectangular bars with the same dimensions as those obtained with the thermoset (10 mm wide, 1 mm thick). After molding with a clamping force of 10 kN for 5 min at 205 °C, the specimens were cooled under pressure to 50 °C over a period of 6 min using a hydraulic cooling circuit. All the specimens were stored at room temperature in a desiccator prior to testing. 2.2 Modulated differential scanning calorimetry (MDSC) The thermal response of the SMPUs was characterized using a TA Instruments Q100 DSC in the modulated mode, calibrated with sapphire standards. The parameters for the modulated heating ramp were: average ramp rate 5 K/min, period 70 s and amplitude of 1.7 K for the modulated signal. Samples cut from molded parts with a razor blade with a mass of about 5 mg were found to give reproducible results under these conditions. Three samples were used for each test and the tabulated transition temperatures are mean values. Tg was defined as the temperature corresponding to the half-height of the corresponding step-like transition in the reversing heat flow rate signal. 2.3 Dynamic mechanical analysis (DMA) DMA measurements were made using a TA Instrument Q800 DMA calibrated with steel standards. 1 x 5 x 10 mm3 rectangular samples were tested in a tensile mode in dry air at a heating rate of 2 K/min, a frequency of 1 Hz and a dynamic strain of 0.01 %.

Figure 1. Example of a typical programming sequence (Td = 60 °C, εm = 100 %, tr = 10 min, τr = 10 min), showing the various steps. 2.4 Tensile shape memory tests Rectangular SMPUs strips of 1 x 10 x 100 mm3 (ASTM standard D 882) were tested using a Universal Testing System (UTS, Walter+Bai AG, Switzerland) equipped with a 1 kN load cell and an environmental chamber (Noske-Kaeser, Germany), capable of raising and lowering the temperature under controlled conditions. A K-type thermocouple placed inside a dummy specimen close to the test specimen was used to provide a precise indication of the specimen temperature. The strain was determined from the cross-head displacement. Tests were performed with various Td and Tr and the test temperature adjusted using heating and cooling ramps of 10 °C/min. Fine tuning of the heating and cooling system allowed the temperature setpoint to be reached with a smooth, over-damped response, i.e. without overshoot. The time to reach a stable


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temperature within 0.5 K of the set-point was between 6 and 8 min for the range of set-points investigated. After equilibration, the temperature could be maintained indefinitely at within 0.5 K of the set-point. As illustrated by the example in figure 1, the complete programming sequence was as follows:      

Isotherm at Td for 10 min Deformation at Td to a strain εm at a strain rate of 100 %/min (step 1) εm maintained at Td for a time tr (step 2) Cooling to Ts (25 °C in all cases) while maintaining εm (step 3) Unloading to zero stress at Ts (step 4) Relaxation at Ts for a time τr (10 min unless mentioned otherwise) to give a fixed strain εu (step 5)

For all the experiments, εm = 100 %. The “stress relaxation time”, tr, is the time over which both Td and εm were maintained immediately after the deformation step. After unloading (step 4), significant stress build-up was observed if the specimen was constrained by the clamps over relatively long times. This was due to time-dependent thermal contraction of polymers at temperatures below Tg, attributed to "structural relaxation" or physical ageing, as described recently by Nguyen et al. [24], and which is a consequence of the non-equilibrium nature of the glassy state. Because this phenomenon has been reported to influence the recovery of SMPs [24], the influence of the storage time at Ts in the unloaded state, τr, was also varied. τr will be referred to as the "storage time" in what follows. The time interval between each programming and recovery sequence was fixed at exactly 1 min, which was sufficiently short for negligible stress build-up to occur in the clamped specimen. After programming, the recovery behavior was studied in the free and fully constrained states, corresponding to zero stress and fixed strain recovery respectively. Free recovery was initiated by first unloading during the 1 min post-programming period and then maintaining the stress at zero while heating to Tr. Fully constrained recovery was initiated by heating to Tr while maintaining the post-programming strain εu. The total recovery time, including the temperature equilibration at Tr and the isotherm, was 60 min in all cases. 3. Results and discussion 3.1 Basic thermo-mechanical response Figure 2 shows the overall MDSC heat flow rate for the different specimens as a function of temperature, along with the reversing and non-reversing components of the heat flow rate, which in principle allow one to discriminate between different types of thermal event. The reversing heat flow generally reflects the evolution of the heat capacity and is consequently sensitive to glass transitions, although it may also be influenced by reversible melting-recrystallization, whereas the non-reversing heat flow generally reflects enthalpic relaxation (subsequent to physical ageing), crystallization, irreversible melting, polymerization, cross-linking, oxidation and degradation. Thus the glass transition temperature of the soft phase, Tgs, could be identified unambiguously from the reversing heat flow rate, whereas it showed significant overlap with the corresponding enthalpic relaxation peak in the overall heat flow rate. The Tgs measured for MP5510 and MM5520 were 62 °C and 52 °C respectively, with an experimental error of 1 to 2 K, roughly consistent with the manufacturer's specifications and values given previously by Metzger et al. [28] and Baer et al. [29] (Tgs ≈ 55 °C for both materials). At higher temperatures, further transitions were apparent from the non-reversing heat flow. In MM5520, exothermic and endothermic peaks were detected at Tch = 110 °C and Tmh = 155 °C respectively, which were attributed to cold “crystallization” and “melting” of phase separated domains associated with the hard segments, that are assumed to act as physical crosslinks in this case. Depending on the strength of the interactions, the number of units and degree of symmetry in the hard segments, and the processing conditions, these rigid domains, which intermittent contact mode atomic force microscopy images have suggested to be of the order of 20 nm in diameter, may be either amorphous or show some degree of ordering. Given that under the present processing conditions, the specimens were quenched from the melt to room temperature over a few minutes, segregation and ordering of the rigid domains is assumed to have been at least partly suppressed, explaining the presence of the exothermic peak. In MP5510, a broad, relatively weak endothermic peak, again attributed to melting of domains associated with the hard segments, was visible in the non-reversing heat flow at Tmh = 129 °C. There was a


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suggestion of a further exothermic peak at higher temperatures in the non-reversing signal at around 180 °C, which was attributed to cross-linking and/or degradation reactions. The results are summarized in table 1.

Figure 2. MDSC heating scans at 5 K/min for (a) MP5510 and (b) MM5520. Table 1. Data for the thermal transitions obtained by MDSC.

Thermoset SMPU MP5510 Thermoplastic SMPU MM5520

Tgs reversing heat flow [°C]

Tch non-reversing heat flow [°C]

ΔHc non-reversing heat flow [J/g]

Tmh non-reversing heat flow [°C]

ΔHm non-reversing heat flow [J/g]

62

N.A.

N.A.

129

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52

110

-6

155

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The storage modulus, E', and loss factor, tan δ, are shown as a function of temperature in figure 3. The peaks in tan δ at Tδs were assumed to correspond to the thermal transitions Tgs observed by MDSC. The onset temperature for this transition, Tonset, was defined as the temperature corresponding to the intercept between the baseline and the leading edge of the peak in tan , and the breadth of the transition was defined as ΔT = 2(Tδs - Tonset) following Yackaki et al. [18]. The glassy and rubbery moduli, Eg' and Er', were determined from E' at Tonset and Tδs + 20 K respectively. The results are summarized in table 2. Table 2. Data for the thermal transitions obtained by DMA.

Thermoset SMPU MP5510 Thermoplastic SMPU MM5520

Tonset [°C]

Tδs [°C]

ΔT [°C]

Eg' [MPa]

Er' [MPa]

55.3

72.3

34.0

1193

7.65

48.9

57.7

17.6

1016

3.87

The observed increase in E' between 100 and 120 °C observed for MM5520 (figure 3(b)) could be ascribed to cold crystallization and a subsequent drop between 140 and 150 °C to melting of the domains associated with the hard segments, as implied by the MDSC measurements. In the case of the thermoset MP5510, a permanent network is assumed to be established by chemical cross-links located within the domains formed by the hard segments, which result from the use of polymeric methylene diphenyl diisocyanate (MDI) with a mean functionality greater than 2. Such "hybrid" cross-links, i.e. combined physical and chemical cross-links, have been referred to as "extended network junctions" [30], and their melting was reflected by a step-like decrease in E' between 120 and 140 °C in the DMA scans (figure 3(a)). The final increase in E' above 170 °C was attributed to temperature-induced cross-linking reactions, associated with the exotherm visible in the MDSC scans. Indeed, when the DMA heating cycle was repeated,


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the second step-like decrease in E' was no longer present, resulting in a single rubbery plateau with a plateau modulus intermediate between the two plateau moduli characteristic of the first heating curve. The increase in chemical cross-link density was therefore assumed to have prevented hard domain formation by decreasing the mobility and symmetry of the hard segments. Chemical cross-linking was assumed to be due to the presence of unreacted functional groups after the curing process at 70 °C. It was found to be possible to obtain a single plateau modulus by using higher curing temperatures, but these led to significant discoloration of the specimens and were therefore not used for systematic investigations.

Figure 3. DMA heating scans at 2 K/min for (a) MP5510 and (b) MM5520. 3.2 Tensile shape memory tests 3.2.1 Influence of the recovery temperature. The free recovery response of the SMPs for Td = 60 °C and tr = 0 is shown in figure 4 for various values of Tr. Approximately 90 % of the initial deformation was recovered within 30 min at 70 and 60 °C in MP5510 and MM5520 respectively, i.e. at Tg + 8 °C in both materials (cf. Table 1). On the other hand, the recovery rates decreased more gradually as Tr was decreased below Tg in MP5510 than in MM5520, with about 30 % of the initial deformation being recovered in MP5510 after 30 min for Tr = Tg - 7 °C (55 °C) as compared with only 5 % in MM5520 under the same conditions (45 °C).

Figure 4. Effect of different values of the recovery temperature, Tr, as indicated, on the free recovery of (a) MP5510 and (b) MM5520 programmed with Td = 60 °C and tr = 0. The constrained recovery response is shown in figure 5 for the same Td. In this case, the recovery stress was significantly higher for MP5510 than for MM5520 for a given Tr. For the highest values of Tr, a peak was observed in the recovery stress during the first few minutes of the tests, i.e. during the heating ramp of 10 K/min prior to stabilization of the temperature at Tr. This effect is known from the literature to occur during the transient recovery of samples programmed below or in the vicinity of Tδs, as in the present case


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[17]. It is also seen from figure 5 that the peak in recovery stress broadened with decreasing Tr and for the lowest values of Tr, the stress continued to increase over the whole of the recovery period. The decrease in the maximum stress with increasing Tr, which was particularly apparent in figure 5(a) for MP5510, reflects the decrease in maximum recovery stress observed by Liu et al. [31] when increasing the heating rate in transient experiments, attributed to increased stress relaxation at high temperatures.

Figure 5. Effect of different values of the recovery temperature, Tr, as indicated, on the constrained recovery of (a) MP5510 and (b) MM5520 programmed with Td = 60 °C and tr = 0. The effect of Tr on the free recovery may be interpreted in terms of a "temperature-delayed" recovery process [32], driven by entropic elasticity at the highest Tr, which were well above Tgs as measured by MDSC. Indeed, for the data corresponding to truly isothermal conditions (i.e. recovery times greater than about 10 min), approximate time-temperature superposition (TTS) was possible by shifting the recovery curves along the log(time) axis. Certainly, TTS may be shown to be justified for these materials in the low strain linear viscoelastic regime for temperatures around Tgs, and the principle, i.e. that relaxation times should decrease, and hence relaxation rates increase as the temperature increases, is expected to remain qualitatively valid, even at large deformations. Of particular interest for practical applications requiring low actuation rates and hence actuation temperatures below Tg is the observation that free recovery in this regime showed a weaker dependence on Tr - Tg for the thermoset SMPU, MP5510, than for the thermoplastic SMPU, MM5520. This may possibly be associated with the breadths of the corresponding relaxation peaks observed in the DMA temperature scans, a significantly broader transition being observed for MP5510 than for MM5520. Important materials parameters for the breadth of the recovery temperature window of SMPUs during continuous heating, include the chemical nature of the soft segments, cross-link density, soft segment length and the stiffness of the network junctions [30]. In the present study, the differences in recovery rate might therefore be attributed to differences in the polyol architecture, network structure and junction stiffness (physical vs. "hybrid" crosslinks) of the two SMPUs. The stress recovery behavior, and in particular the observation of stress peaks that are both well above the stresses associated with a purely entropic response (which may be inferred from the limiting behavior at long times and high Tr) and correlated with the deformation stresses at Td, provides a more direct indication of enthalpic contributions to recovery in the SMPUs. During deformation in the neighborhood of the glass transition, molecular processes associated with relatively long relaxation times are progressively frozen out on the time scale of the deformation as the deformation temperature is reduced, so that sub-chains on the scale of the entanglement length or effective chain length between crosslinks are no longer able to explore freely the conformational space fixed by the deformation of the network. Rather, the remaining local deformation induced conformational changes involve an enthalpic penalty associated with short-range intraand intermolecular interactions, reflected by large activation barriers to deformation (and hence large deformation stresses and limited strain recovery on unloading) and significant stored strain energy in the deformed state. It follows that the stress peaks observed during constrained recovery may be attributed to the release of this strain energy owing to the reactivation of the local molecular processes associated with deformation at Td, followed by relaxation at longer times, associated with the activation of the longer range


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molecular processes associated with the rubbery state, as described by Gall et al. [17] in accounting for the results of their non-isothermal experiments. Given that Td = 60 °C was slightly less than Tgs for MP5510 but about 8 K higher than Tgs for MM5520 (cf. table 1), it follows that the maximum programming stress was significantly higher in MP5510 (about 17 MPa) than in MM5520 (about 1.5 MPa). Therefore a greater proportion of the programming stress in MP5510 was presumably linked to enthalpic effects, accounting for the higher short term recovery stresses in this material. 3.2.2 Influence of the programming temperature. Figure 6 shows the free recovery of the SMPUs for various programming temperatures, Td. The recovery temperatures, Tr, were 60 °C and 50 °C for MP5510 and MM5520 respectively, i.e. close to the respective values of Tgs (cf. table 1), in order to provide for an objective comparison of the effect of Td on recovery in the two SMPUs. The range of Td was chosen to avoid necking, which imposed a lower limit of about 50 °C for both materials. The recovery rate was more sensitive to Td in MM5520 than in MP5510, but it appeared relatively weakly dependent on Td in the vicinity of Tgs in both materials. However, the final recoverable strain decreased strongly as Td increased above Tgs in MM5520. Such behavior has been reported previously for a range of physically cross-linked SMPUs [33, 34], and may be accounted for by the modification or destruction of the primary network at high deformation temperatures, at which the hard domains have relatively low mechanical stability, leading to a loss of memory of the primary shape. In MP5510, the final recovered strain also decreased somewhat with increasing Td, but was typically around 50 % and still increasing after 60 min for the highest Td, reflecting the presence of a permanent chemically crosslinked network in this case.

Figure 6. Effect of different values of the programming temperature, Td, on the free recovery of (a) MP5510 and (b) MM5520 at Tr = 60 and 50 °C respectively and tr = 0.

Figure 7. Effect of different values of the programming temperature, Td, on the constrained recovery of (a) MP5510 and (b) MM5520 at Tr = 60 and 50 °C respectively and tr = 0.


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Figure 7 shows the constrained recovery for various Td. A peak in the recovery stress was observed for both SMPUs at low Td and the time corresponding to the onset of significant stress recovery also decreased with decreasing Td. Moreover, the absolute values of the recovery stresses were comparable for Td close to Tgs, confirming the relatively large recovery stresses associated with MP5510 in the results for constrained recovery after programming at Td = 60 °C (figure 5) to be due to the higher Tgs of this material. At long times, however, the behavior of the two SMPUs differed significantly. In MP5510, the recovery stress tended to converge to a similar value under all programming conditions, whereas in MM5520, the level of the plateau in the recovery stress observed after long times decreased markedly with increasing Td. As with the free recovery, this behavior may be attributed to the loss of shape memory caused by alteration of the primary network of physical cross-links at high deformation temperatures.

Figure 8. Effect of different values of the stress relaxation time, tr, as indicated, on the free recovery of (a) MP5510 and (b) MM5520 (Td = 60 °C for both materials and Tr = 60 and 50 °C for MP5510 and MM5520 respectively).

Figure 9. Effect of different values of the stress relaxation time, tr, as indicated, on the constrained recovery of (a) MP5510 and (b) MM5520 (Td = 60 °C for both materials and Tr = 60 and 50 °C for MP5510 and MM5520 respectively). The influence of Td on shape recovery has previously been investigated for thermoset SMPs tested in flexion at a constant heating rate or during isothermal free recovery [17]. As in the present case, a reduction in Td is reported to lead to a "stress overshoot" in constrained recovery and an increased recovery rate. Fine control of the rate of deployment of SMPUs is therefore possible by adjusting Td, particularly in the case of devices that are passively activated at a specific Tr (body temperature, for example). However, while varying Td may be effective for tuning the recovery rate, it also influences the recovery stress profile and recovery onset time. Moreover, the range over which Td may be varied is limited on the one hand by the usual requirement of homogeneous deformation during programming and on the other hand by the stability of


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network responsible for the shape memory effect, as is particularly evident from the present results for MM5520. Tobushi et al [34] also reported substantial reductions in recoverable strain in MM6520, a thermoplastic SMPU similar to MM5520, but with Tgs = 65 °C, for Td > Tgs + 10 °C. Therefore, the higher sensitivity of the thermoplastic SMPU to Td is primarily attributed to the relatively low recoverable strains associated with the limited stability of the physical cross-links. That the effect was particularly pronounced in MM5520 in the present case is assumed to be linked to the injection conditions, which resulted in relatively little ordering in the domains corresponding to the rigid segments. Given the observation of an exothermic peak at 110 °C in the MDSC heating scans for MM5520 (figure 2(b)), it may be possible to improve the stability of the hard domains by annealing in this temperature range, i.e. below Tmh, and hence increase the effectiveness of the physical crosslinking at high programming temperatures. This will be investigated in future work. 3.2.3. Influence of the stress relaxation time. Figure 8 shows the influence on free recovery of tr (the time for which the specimen is maintained at Td once m has been applied), with Td = 60 °C for both materials and Tr = 60 and 50 °C for MP5510 and MM5520 respectively. In each case, the recovery rate decreased with increasing tr. However the effect of tr over the range investigated was generally small compared with that of Td and Tr. This follows from the representative programming sequence in figure 1, which indicates stress relaxation to be relatively rapid, so that even with tr set to zero, substantial stress relaxation occurred in the time it took for the cooling ramp to become effective (the slight increase in stress during the later stages of cooling was due to thermal contraction). In both MP5510 and MM5520, increasing tr had a similar effect to increasing Td, the onset of strain recovery shifting to longer times in MP5510, whereas in MM5520 there was significantly less long term strain recovery as tr increased, which may again be attributed to the instability of the hard domains in this case. Results for the influence of tr on constrained recovery are shown in figure 9. Again, increasing tr had a similar effect to increasing Td. Thus, there was a marked decrease in the limiting stress level in MM5520 after long recovery times as tr increased and its evolution was most marked for small tr, highlighting the importance of short-term enthalpic relaxation processes when deforming the materials close to Tgs. 3.2.4. Influence of the storage time. The influence of the storage time, τr (the time for which the specimen is maintained at the storage temperature, Ts, after shape fixation), on the free and constrained recovery of the SMPUs is shown in figures 10 and 11. As with the stress relaxation tests, Td = 60 °C for both materials and Tr = 60 °C and 50 °C for the thermoset and thermoplastic SMPU respectively, while tr was set to 0. Under these conditions, the effect of increasing τr beyond its default value of 10 minutes was limited to a slight delay in the free recovery of MM5520 (figure 10(b)) and an increase in the onset time for stress recovery (figure 11(b)). τr may be considered to be equivalent to the duration of physical ageing prior to recovery, and it is therefore significant that the ageing temperature Tg = 25 °C was closer to Tg in MM5520 than in MP5510, implying more rapid structural relaxation in the former case. Hence, while the effect of τr was relatively minor for the present choice of parameters, which were chosen to be representative of practical programming and recovery conditions, it would be of interest to extend the study to a wider range of Td and Tr, as well as much longer τr in order to simulate the effect of prolonged storage.


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Figure 10. Effect of different values of the storage time, r, as indicated, on the free recovery of (a) MP5510 and (b) MM5520 (tr = 0, Td = 60 °C for both materials and Tr = 60 and 50 °C for MP5510 and MM5520 respectively).