Shape Memory Behavior of a Polyethylene-Based Carboxylate

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Shape Memory Behavior of a Polyethylene-Based Carboxylate Ionomer R. Dolog and R. A. Weiss* Department of Polymer Engineering, University of Akron, 250 South Forge Street, Akron, Ohio 44325, United States ABSTRACT: Shape memory behavior of a partially zincneutralized, poly(ethylene-co-methacrylic acid) ionomer was investigated. The ionomer was a semicrystalline ionomer with a broad melting transition in the range 60−100 °C. Physical crosslinks in the ionomer due to an ionic nanodomain structure provided a “permanent” crosslinked network, while polyethylene crystallinity provided a temporary network. The broad melting transition allowed one to tune the dual-shape memory behavior by choosing a switching temperature, Tc, anywhere within the melting transition. Similarly, multiple-shape memory behavior was achieved by choosing two or more switching temperatures within the melting transition, though the effectiveness of fixing (F) depended on how much material was melted and recrystallized to support the specific temporary shape. Crosslinking improved the recovery efficiency (R), and the crosslinked ionomer exhibited nearly ideal shape memory behavior in the dual-shape memory cycle.

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INTRODUCTION Shape memory polymers (SMPs) are an emerging class of polymers that can change shape when exposed to an external stimulus such as heat, light, electric or magnetic fields, pH, or the presence of specific ions.1 Thermally activated SMPs have a permanent shape as a consequence of a crosslinked network, but they can be deformed and fixed into a temporary shape when heated above a critical switching temperature, Tc, of a second, reversible network. The temporary shape remains unchanged until the material is reheated above Tc, which in the absence of an external stress produces a recovery of the material to its original, permanent shape. A variety of thermal SMPs have been reported with a wide range of activation temperatures and applications such as medical devices, mechanical actuators, sensors, and self-deployable structures. A number of recent reviews have surveyed the progress in the development and understanding of SMPs and their applications.2−5 The original shape memory polymer was a heat-shrinkable film prepared from radiation crosslinked low density polyethylene (XLDPE)6 by Raychem. In XLDPE the chemical crosslinks provide a network that maintains a permanent shape, while the crystalline polymer provides the thermally reversible, physically crosslinked network that allows the material to be frozen into a temporary shape. That technology is still widely used commercially as shrinkable film and tubing. Partially neutralized poly(ethylene-co-r-methacrylic acid), which are commercially available ionomers, such as Surlyn from Dupont de Nemours Co. are similar to crosslinked LDPE, except that, in this case, the crosslinks arise from intermolecular associations of metal-neutralized methacrylic acid groups instead of covalent crosslinks. This paper describes the shape memory behavior of ionomers neutralized with zinc counterions. Two objectives were considered: (1) to ascertain whether neat PEMA ionomers produce viable SMPs and (2) whether multiple shape memory can be achieved with PEMA by exploiting the broad melting temperature range of the ionomer. © XXXX American Chemical Society

EXPERIMENTAL DETAILS

Materials. The PEMA ionomer, Surlyn 9520, was obtained from DuPont de Nemours Co. It had a melt index of 1.56 g/10 min measured at 190 °C and 2.16 kgf with a Dynisco Polymer Test D4003x melt indexer. Ionomer had a nominal methacrylic acid concentration of 10 wt % that was 70% neutralized to the zinc methacrylate.7 Films, ∼0.5 mm thick, were prepared by compression molding at 140 °C using a Carver Hydraulic Press, Model #3912. Although the PEMA films were highly crystalline, they were optically clear, which is due to the very small crystallite sizes that occur in this ionomer.8 For comparison to the neat ionomers, some samples were covalently crosslinked to assess the relative effectiveness of the physical crosslinks due to ionic group associations at providing shape memory behavior. The crosslinking was done by exposing compression molded films to electron beam (EB) radiation in air at NEO Beam Alliance Ltd. (Middlefield, OH) using radiation doses of 50, 100, and 200 kGy. Materials Characterization. Thermal transition temperatures were measured with a TA Instruments differential scanning calorimeter (DSC), Model Q200, by scanning between −50 and 150 °C using cooling and heating rates of 10 °C/min. The samples were crimped inside aluminum pans and blanketed with a dry nitrogen atmosphere. The temperature and enthalpy were calibrated with an indium sample. The melting point of the ionomer, Tm, was defined as the highest temperature peak observed in the DSC heating thermogram, which corresponded to the maximum rate of melting for the primary crystallites. The crystallinity of the components was calculated as the ratio of the measured heat of fusion (normalized by the mass fraction of the component of interest) and the heat of fusion of a perfect crystal of polyethylene, 278 J/g.9 Mechanical properties were measured with an Instron Universal Testing Machine, model 5567, using a tensile fixture. Samples were compression molded and cut into dogbone specimens with gauge length dimensions of 7.62 × 3.18 × 0.50 mm3. Pneumatic grips were Received: August 20, 2013 Revised: September 17, 2013

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used to hold the specimens, which were elongated at a rate of 5 mm/ min at room temperature. The energy to break was calculated as the area under the stress−strain curves. A TA Instruments Q800 dynamic mechanical analyzer (DMA) with a tensile film fixture was used do study the viscoelastic properties of the polymers and to measure the average molecular weight between crosslinks in the neat ionomer and in the ionomer samples that were EB-crosslinked. The samples were rectangular films (about 20 mm ×5 mm × 0.5 mm), and the DMA measurements were performed using a constant frequency of 1 Hz, a 0.01 N preload force, and strain amplitude of 20 μm. The temperature was equilibrated at −75 °C and then increased at 2 °C/min to 150 °C. The shape memory properties of the ionomer were evaluated using shape memory cycles (SMC) carried out with the DMA using a tensile film fixture and the same sample preparation procedure described above for dynamic mechanical measurements. The DMA’s controlled force mode was used to deform the sample with a fixed heating and stretching protocol. In most studies where a crystalline phase is used as the temporary network, Tc = Tm. However, since polymers have a broad melting transition, essentially composed of a large number of individual melting points that depend on the size of the crystallites, one can arbitrarily choose Tc (or multiple Tc’s) as any temperature within the melting region. That is discussed in this paper with regard to multiple shape memory behavior. With the exception of the experiments where Tc > Tm, a preload force of 0.001 N was used to prevent sagging of the sample. When Tc > Tm, a 0.0050 N preload force was used. The sample was heated at the rate 10 °C/min to a temperature above Tc, at which point it was stretched isothermally with a stresss of ∼1.2 MPa for Tc < Tm (i.e., in the range 60 to 80 °C), or ∼0.020 MPa for Tc > Tm. Those stresses were chosen so as to keep the strain below 100%. The sample was then cooled at a constant stress to fix the temporary shape, after which the applied stress was removed. Shape fixity (F), i.e., the effectiveness at fixing the temporary shape, was defined by eq 1,10 F=

εf (N ) × 100% εs

Figure 1. DSC cooling and heating thermograms (10 °C/min) between −75 to 150 °C for PEMA. The cooling thermogram was obtained immediately after completing the first heating scan, and the second heating scan immediately followed the cooling scan.

of the sample at room temperature, which is above the glass transition, and is only observed in the first heating thermogram in Figure 1. The PEMA also features microphase separation of nm-size ion-rich domains, often termed ionic clusters that provide multifunctional, physical crosslinks within the amorphous phase of the ionomer that persist to temperatures above the melting point.13 A schematic of the three phase microstructure of PEMA is shown in Figure 2. This includes an amorphous phase

(1)

where εf is the strain of the fixed temporary shape after removing the stress, εs is the strain after stretching the sample, i.e., before the stress was removed, and N is the cycle number (for experiments with only a single SMC, N = 1). The original, permanent shape of the sample was recovered by reheating the unconstrained sample to above Tc. The effectiveness of the shape recovery, R, was defined by eq 2,10

R=

εs − εr(N ) × 100% εs − εr(N − 1)

(2)

Figure 2. Schematic of the microstructure of PEMA (features are not drawn to scale).

where εr is the residual strain of the sample at the completion of the SMC (when N = 1, the initial strain of the sample at the beginning of the experiment εr(N − 1) is equal to zero). For an ideal shape memory material, F = R = 100%. The melt index of PEMA at 190 °C and 2.16 kgf was measured with a Dynisco Polymer Test D4003 melt indexer following ASTM Standard D-1238.11

composed primarily of polyethylene with perhaps some isolated or hydrogen bonded methacrylic acid groups, contact ion pairs and multiplets of ion pairs, a crystalline polyethylene phase, and an ionic nanophase (the clusters) that contains most of the ion pairs. The key features of the microstructure with respect to shape memory behavior is that the crystalline phase can provide a physical network to support a temporary shape, similar to the function of the crystalline regions in crosslinked polyethylene SMPs, and that the ionic clusters may be sufficiently robust so as to provide a “permanent” network and shape. In this situation, the term “permanent” is appropriate if the relaxation time of the network formed from the multifunctional ionic clusters is much greater than the time scale during which the network chains are under load. Generally, the ionic clusters in PEMA persist to relatively high temperatures (>150 °C), so they were expected to be strong enough to serve as a permanent network for an SMP.

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RESULTS AND DISCUSSION Shape Memory Behavior of the Ionomer (PEMA). The DSC thermograms in Figure 1 show that PEMA exhibits two melting endotherms that are due to primary and secondary crystallization of the ethylene sequences. The primary crystallites melt in the range 60−100 °C, with a maximum rate of melting at 96 °C. The broad melting region indicates a broad distribution of crystallite sizes, though all the crystallites are relatively small, which imparts good optical clarity to the sample.8 The lower temperature endotherm in Figure 1, between 50 and 55 °C, is due to secondary crystallization of thinner polyethylene crystals between the primary lamellar crystals.12 The secondary crystallization results from annealing B

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Tunable Shape Memory Behavior of PEMA. Xie15 recently showed that multiple shape memory behavior can be achieved by exploiting a very broad glass transition region for a crosslinked polymer. Shape memory behavior can be achieved by deforming the material at any temperature either above or within the transition region, which effectively sets the value of Tc. The sample can then be cooled to a lower temperature within the transition region to fix a temporary network, which in that case is composed of the fraction of the amorphous phase for which short-range segmental motion occurred for T ≤ Tc. Multishape memory was achieved by heating the sample to a temperature above or within the transition region and then stretching and fixing sequentially at multiple temperatures within the transition region. Xie used the ionic clusters of a perfluorosulfonate ionomer (Nafion) to provide a physical “permanent” network, which is similar to the strategy used in the work discussed herein, except that in Nafion the clusters contain associated sulfonate groups that have stronger interactions than the carboxylate groups used in the present study. Triple shape memory has also been reported for a graft copolymer system with two distinct melting points16 and a multiphase polymer blend with two or more semicrystalline polymers with distinct melting points.17 Those reports suggest that a broad melting transition in a polymer with a broad distribution of crystallite sizes may also be used to produce multiple thermal shape memory18 in a manner similar to Xie’s use of a broad glass transition region. In that case, one might use any temperature within the melting region as Tc to develop dual shape memory or multiple shape memory with many Tc’s using sequential deformation and fixing at a variety of temperatures within the melting region. Only partial melting is achieved by setting Tc within the melting region, but since the material is above Tg, it may be deformable. When the constrained sample is cooled, the partially melted material recrystallizes and fixes a temporary network that may be recovered by reheating to above Tc to remelt those crystals. Since theoretically there are infinite choices for Tc between the onset and completion of melting, one may consider the shape memory behavior of a semicrystalline polymer with a broad melting region as tunable. The maximum rate of melting (Tm) of PEMA determined by DSC analysis was at 96 °C, Figure 1. Figure 4 shows SMCs for PEMA deformed at four different temperatures (i.e., Tc) between 60 and 105 °C, and the values of F and R are summarized in Table 1. The SMCs shown are for the second SMC, since the first cycle included the effects of the sample preparation, as discussed earlier in this paper. The crystalline phase during stretching of the sample held isothermally within the melting region (Tc < Tm) was only partially melted. The fraction of melted crystals increased as Tc increased, and as a consequence, the modulus of the partially molten sample decreased with increasing Tc. Shape fixity exhibited a strong dependence on the choice of Tc within the melting region, which is a consequence of fewer unmelted material as Tc increased. F increased from 78% to 99% when Tc increased from 60 to 105 °C. Stretching the ionomer, which is essentially a crosslinked elastomer, at Tc generates a restoring stress in the network chains, i.e., the polyethylene chains between ionic clusters, as a consequence of rubber elasticity. When the deformed sample is then cooled, the restoring stress must be supported by the network formed by the recrystallized material that melted at Tc, if a temporary

A shape memory cycle (SMC) of PEMA is shown in Figure 3. The ionomer film was preloaded with a force 0.005 N to

Figure 3. Shape memory cycle for PEMA. “S” denotes the start of the cycle. The sample was heated along path 1 to 105 °C and then stretched to a constant force of 0.05 N, and the sample length was allowed to achieve equilibrium, path 2. The sample was cooled under load to 25 °C along path 3, and then the stress was removed along path 4 to fix the temporary shape. The sample was reheated to 105 °C along the recovery path 5 and finally cooled along path 6. “E” denotes end of the cycle.

maintain a taut sample and then heated at 10 °C/min from 25 to 105 °C, which was above the melting point of the ionomer. It was then stretched at a rate of 0.01 N/min to a constant applied force of 0.05 N (engineering stress, σ = 20 kPa) and maintained at that force for 5 min to allow the sample length to reach an equilibrium strain of 65%. The sample was then cooled at the rate 10 °C/min to 25 °C while under a load of 0.05 N, after which the force was decreased to 0.005 N (to prevent sag of the sample) to fix the temporary shape. The recovery step involved reheating the sample at 10 °C/min to 105 °C, with a 0.005 N load, and allowing it to relax for 20 min to the final strain. The PEMA exhibited a shape fixity of F = 99% and shape recovery of R = 72% for the initial SMC. For a second SMC using the same sample and the same SMC conditions, F = 99% and R improved to 88%. The improvement of shape recovery after an initial SMC is commonly observed for SMPs and has been attributed to the history of the sample such as processing and storage.1 As a consequence of residual stresses, the polymer microstructure may exhibit some reorganization upon heating in the first cycle. The source of the permanent set in these ionomers during the SMC, i.e., R < 100%, was not investigated in this study, but based on the results work with SEPDM networks previously studied,13 it is thought that the permanent deformation was due to creep recovery of the physical network formed by the ionic clusters (i.e., the “permanent” network for this system). Ionic networks formed by carboxylate salts are generally weaker than those due to sulfonate salts,14 so in comparison with the SEPDM ionomers, which exhibited permanent deformation of the ionic network upon holding it in the temporary shape for a long time,13 more creep relaxation of the ionic network would be expected in the PEMA. C

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Table 1. Shape Memory Behavior of PEMA for Various Values of Tca Tc (°C)

F (%)

R (%)

60 70 80 105

78 87 93 99

88 88 89 88

a

Calculated from the second shape memory cycle. See text for explanation.

the SMP. The value of R, however, was only ∼90% for each Tc, which indicates that there was about 10% permanent deformation of the samples as a result of the SMC. If the “permanent” network were truly permanent, as one expects for an ideal network, R should be 100%. The values of R below 100% indicate defects in the behavior of the ionic nanodomain network used as the permanent network for PEMA. The defect is most likely due to creep recovery of the network chains connected to the ionic domains, which can occur as a consequence of the physical nature of these crosslinks. The electrostatic forces that hold the ions in the ionic network are also opposed by the restoring force generated in network chains during the stretching step, and that when an imbalance occurs an ion pair can pull out of a nanodomain and hop to another nanodomain. That not only relaxes the stress but also produces flow of the network chain and a permanent set in the sample. This same mechanism was observed previously in a study of the shape memory behavior of compounds of SEPDM ionomers filled with fatty acid salts.14 Multiple Shape Memory Behavior of PEMA. Tripleshape memory behavior was achieved for the PEMA ionomers by extending the procedure described in the preceding section to include two different Tc’s and temporary shapes; see Figure 5. The sample was deformed and fixed into two different

Figure 4. Initial shape memory cycles for PEMA as a function of Tc: (a) 105 °C; (b) 80 °C; (c) 70 °C, and (d) 60 °C.

Figure 5. Triple-shape memory properties of PEMA (First cycle). The quantitative shape memory properties were: FS0→S1 = 77%; FS1→S2 = 67%; RS2→S1 = 88%; RS1→S0 = 33%; R*S1→S0 = 34%.

shape is to be achieved. If the restoring stress exceeds the strength of the temporary network formed by the remaining crystallinity, the sample length will recover until the two opposing stresses are equal. Thus, as Tc decreased, the ability of the recrystallized temporary network became less efficient at fixing the temporary shape. The shape recovery efficiency, R, was insensitive to the choice of Tc, which was due to the fact that the shape recovery properties depend on the integrity of the permanent network of

temporary shapes, 1 and 2, at 80 and 60 °C with applied stresses of 0.6 and 1.8 MPa, respectively. When the sample was reheated to 60 °C, the temporary shape “1” was restored, and when the sample was reheated to 80 °C the sample shape changed toward the original shape, “0”. The shape fixity and recovery for each shape change in Figure 5 were calculated using eqs 3 and 4,15 where i and j D

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represent the different shapes and S, f, and r denote the stretched, fixed, and recovered shape, respectively. εfj − εfi × 100% FSi → Sj = εSj − εfi (3) R Sj → Si =

εfj − εri εfj − εfi

Table 2. Fixity and Recovery Values for Six Consecutive Triple-Shape Memory Cycles of PEMAa

× 100% (4)

The shape fixity and recovery values for the triple shape memory cycle shown in Figure 5 are listed in the caption of the figure. The fixity values for the two temporary shapes, 77% and 67%, were relatively low, because for each fixing step the temporary shape was supported by only a fraction of the ionomer crystals. As explained in the previous section, if a fraction of material recrystallized at either Tc had insufficient strength to oppose the stress developed in the network strands, the fixed shape will partially relax. The second shape fixity, FS1→S2, was lower than first shape fixity, FS0→S1, because the fraction of polymer that recrystallized to form the temporary network was lower at the lower Tc and, thus, was less efficient at supporting the temporary shape. Shape fixity and shape recovery values also depend on the deformation strain, since the larger the strain, the larger the restoring stress generated in the network chains due to rubber elasticity. The strain of the recovered shape at Tc, εro, is affected by the thermal expansion of the polymer, since the cycle started at room temperature. When cooled to room temperature, the sample recovered further which provided a final recovered strain of ε*ro. Using that strain in eq 4 provided the actual shape recovery efficiency R*S1→S0. Both values are included in the caption to Figure 5. A quadruple shape memory effect is shown for the PEMA in Figure 6. Three temporary shapes were fixed at 90, 75, and 60 °C. As with the triple shape memory behavior, the fixity and recovery values decreased with decreasing Tc.

a

run no.

FS0→S1

FS1→S2

RS2→S1

RS1→S0

R*S1→S0

1 2 3 4 5 6

77 46 44 42 41 41

67 61 57 56 54 54

88 88 90 92 94 95

33 69 76 77 80 80

34 74 80 85 89 89

Tc1 = 80 °C and Tc2 = 60 °C; σ1 = 0.75 MPa and σ2 = 1.9 MPa.

respectively. The largest decrease occurred between the first and second SMCs, which, as explained earlier, is probably due to the prior molding thermal and stress histories. The decreasing behavior of F is a consequence of annealing of the polyethylene crystals during each fixing step. The broad melting point of the ionomer is due to a broad distribution of crystallite sizes, the smaller ones melting at the lower temperatures. During the stretching and fixing steps of each cycle, the polymer is essentially annealed at those temperatures, which tends to increase the average crystallite size and shifts the melting point distribution to higher temperatures. As a consequence less material is melted at a fixed value of Tc, which makes the temporary network less efficient at fixing the temporary shape. In contrast to the fixity results, the two shape recovery values, RS2→S1 and RS1→S0, increased for each consecutive cycle, which is largely a consequence of the decreasing fixity values; that is, a large part of the shape recovery was actually achieved during the fixing step when the shape relaxed due to insufficient strength to support the stress. A surprising observation with regard to the fixity results in Table 2 is that while FS0→S1 (Tc1 = 80 °C) was higher, as expected, than FS1→S2 (Tc2 = 60 °C) for the first SMC, for subsequent cycles it was lower. The results after the first cycle are inconsistent with the earlier conclusion that decreasing Tc decreases F. However, that disparity can be reconciled by considering that it is actually the distribution of crystal sizes and melting temperatures that controls how much material will melt at a specific Tc. The relationship between the crystallization rate and crystallization temperature for polymers is generally nonlinear due to the opposing influences of the thermodynamic potential and the chain diffusion rate. The thermodynamic driving force for crystallization increases with increasing supercooling, but the diffusion rate decreases as a consequence of an increasing viscosity. As a consequence the crystallization rate usually increases as supercooling increases to a point, but at some value of supercooling, crystallization becomes diffusion controlled and the rate decreases with further increases in supercooling. Thus, when the PEMA was annealed during the SMCs, it is not apparent how that process changes the distribution of crystal sizes. The DSC data in Figure 7 provide a measure of the crystal size distribution, though not the absolute sizes of the crystals. The dashed thermogram is for a PEMA sample quenched into its permanent shape from the melt and stored at room temperature. This sample shows a bimodal distribution of melting points as a consequence of primary and secondary crystallization of polyethylene sequences. The solid curve is the DSC heating thermogram of the sample after it was stretched and fixed into its temporary shape. It is clear that even during

Figure 6. Quadruple-shape memory properties of PEMA. FS0→S1 = 87%; FS1→S2 = 50%; FS2→S3 = 81% RS3→S2 = 73%; RS2→S1 = 31%; RS1→S0 = 10%; R*S1→S0 = 20%.

The reproducibility of the three-way shape memory behavior of PEMA was assessed by running six consecutive multiple SMCs using Tc1 = 80 °C and Tc2 = 60 °C with applied stresses of 0.75 and 1.9 MPa, respectively. The fixity and recovery efficiencies are summarized in Table 2. The fixity ratios FS0→S1 and FS1→S2 generally decreased with each cycle and appeared to be approaching asymptotic values of ∼40% and ∼55%, E

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one cannot completely rule out effects on the shape memory behavior due to changes in the polyethylene crystalline morphology as a result of melting/recrystallization and/or annealing during the reaction. Similarly, changes in the ionic domain structure, which still contributes to the overall crosslink density of the material, may have occurred. However, neither effect was evaluated in these experiments, and it was assumed that the major effects on the shape memory behavior originated with the introduction of the permanent chemical crosslinks. The average molecular weight between crosslinks (chemical or physical) was calculated from the general theory of rubber elasticity,19 eq 5 ρRT (5) G where G is the shear modulus, ρ is the mass density, R is the gas constant, and T is absolute temperature. The shear modulus above the melting point of the ionomer melting point was approximated as E′/3, where E′ was the dynamic tensile modulus measured by DMA. Below the melting point of the ionomer, the polyethylene crystallites contribute to the crosslink network, but above Tm, the network is due only to the chemical crosslinks and the physical crosslinks due to the ionic nanodomains. The effect of EB radiation dose on the Mc is shown in Figure 8. The value of Mc for zero radiation dose is Mc =

Figure 7. Comparison of DSC heating scans of PEMA at different stages of a shape memory cycle: original, permanent shape (dashed curve); sample after fixing temporary shape (solid curve).

the dual-shape SMC, the distribution of polyethylene crystallite sizes is changed. The reproducibility of the quadruple-shape memory cycle was studied in a similar manner, and the results are summarized in Table 3. The temperatures and stresses used for this study Table 3. Fixity and Recovery Values for Six Consecutive Quadruple-SMCs for PEMAa run no.

FS0→S1

FS1→S2

FS2→S3

RS3→S2

RS2→S1

RS1→S0

R*S1→S0

1 2 3 4 5

87 59 57 56 56

50 52 51 49 48

81 83 82 83 83

72 68 66 65 65

32 26 28 27 24

10 3 12 12 13

21 47 64 67 69

a

Tc1 = 90 °C, Tc2 = 75 °C, and Tc3 = 60 °C.

are identical to the ones shown on Figure 6. The fixity efficiencies FS0→S1, FS1→S2, and FS2→S3 decreased with each cycle, but appear to approach asymptotic values of ∼55%, ∼50%, and ∼80%, respectively. The fixity efficiencies tended to increase with decreasing Tc, similar to what was observed for the triple-shape memory cycle, and for the same reason discussed above. However, FS1→S2 was slightly less than that of FS0→S1, which may indicate that the conclusion of increasing F with decreasing Tc is an oversimplification or simply that there was not a significant difference in the results for Tc = 75 and 90 °C. Effect of Chemical Crosslinking on the Shape Memory Behavior of the PEMA. Previous research on ionomeric SMPs where both the “permanent” and temporary networks were derived from physical bonds showed that the fixity efficiency of the SMP decreased with time as a result of creep recovery of the sample shape during long periods above Tg.13 That study indicated that the source of the creep recovery was relaxation of the “permanent” network formed by the nanodomain crosslinks formed by microphase separation of associated ion pairs. The influence of covalent crosslinks on the shape memory behavior of the PEMA ionomers was assessed by EB crosslinking the ionomer. Although the EB crosslinking was done at room temperature, the sample temperature increased appreciably during the exothermic crosslinking reaction. Thus,

Figure 8. Effect of the EB radiation dose on Mc for crosslinked PEMA ionomers.

due to the ionic nanodomain, physical network. Mc decreased with increasing radiation dose, which is expected if the radiation did not significantly affect the physical crosslink structure. Figure 9 shows the DMA results for the uncrosslinked PEMA (0 KGy) and three crosslinked PEMA samples using 50, 100, and 200 kGy EB radiation. The arrows in Figure 9 indicate increasing EB radiation or, as seen in Figure 8, decreasing Mc. Below Tm, there was little difference between the properties of the four samples, which is due to the dominant nature of the crystallites that form a physical network in the polymer. As the material melts, however, E′ decreases several orders of magnitude. Note that the ionomer shows two melting transitions (see Figure 1) and the large losses associated with those transitions were greatly suppressed in the crosslinked samples. The most significant differences between the four samples show up after the crystallites have completely melted. In that case, the effects of the ionic and chemical networks dominate the modulus and the modulus increases with increasing EB exposure, which is consistent with a higher crosslink density. The loss, however, decreases with increasing EB exposure, which is also consistent with the more elastic F

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Figure 9. Dynamic storage modulus (E′) and tan δ for PEMA ionomers crosslinked with 0 to 200 kGy EB-radiation. The arrows indicate increasing radiation dose (i.e., decreasing Mc).

crosslinked material. The dramatic differences in the values of tan δ of the ionomer and the crosslinked ionomers above the melting transitions are due to thermoplastic nature of the neat ionomer. That is, at elevated temperature, the ionic network is sufficiently weakened such that viscous flow begins, though it is not yet obvious in the modulus data. The weak peak in tan δ at about 0 °C is the glass transition of the ionomer,20 which did not appear to be affected by the EB-crosslinking. The tensile properties at room temperature of PEMA and the crosslinked PEMA are summarized in Table 4. Incorporation of the chemical crosslinks had little effect on Young’s modulus of the ionomer, which as discussed above is influenced for the most part by the crystallinity of the sample. Similarly, the effect of crosslinking on the yield point (stress and elongation) was small. The ultimate properties, however, were sensitive to crosslinking. The stress at break, the elongation at break, and the energy to break of the crosslinked ionomers were greater than those for the uncrosslinked material. The effect of the crosslink density, however, of the crosslinked samples was not clear from the data in Table 4, and the experimental uncertainty of those properties was large, which may be due to an inhomogenous crosslinking distribution. Five consecutive SMCs were run for the uncrosslinked PEMA and PEMA crosslinked with a 200 kGy radiation dose using Tc = 105 °C. The second SMC for each are compared in Figure 10. The fixity values for both samples shown in Figure 10 were similar, 98−99%, while the recovery efficiency was significantly improved by the covalent crosslinking, 88% vs 99% for the uncrosslinked and crosslinked samples, respectively. The insensitivity of F to crosslinking indicates that any effect of crosslinking on the crystalline temporary network was insignificant. The improvement of the recovery by crosslinking, however, indicates that the ionic nanodomain network is much

Figure 10. Second of five consecutive dual shape memory cycles for (a) uncrosslinked and (b) crosslinked PEMA. Tc = 105 °C.

less efficient than covalent crosslinks at retaining the permanent shape of the ionomer. The F and R results of the five consecutive SMCs for both samples are shown in Table 5. Except for the first cycle of the Table 5. Effect of Crosslinking on Shape Memory Properties of PEMA for Five Consecutive Shape Memory Cycles (Tc = 105°C) PEMA uncrosslinked

PEMA crosslinked

R

F

R

F

72 88 89 89 91

99 99 99 99 99

98 98 99 99 99

87 98 98 98 98

uncrosslinked PEMA, the shape memory behavior for both samples was reproducible. The low value of R for the first SMC of each may be attributed to the thermal and stress history of the molding step and the radiation step. The data in Table 5 demonstrate that while the neat ionomer constitutes a reasonably well performing dual SMP, the crosslinked ionomer exhibited nearly ideal behavior.

Table 4. Summary of the Tensile Properties of the Uncrosslinked and Crosslinked PEMA at Room Temperaturea sample PEMA PEMA PEMA PEMA a

(0 kGy) (50 kGy) (100 kGy) (200 kGy)

E (MPa) 130 107 122 132

(3.15) (7.09) (4.69) (10.2)

σy (MPa) 14.0 14.9 13.7 14.0

(0.121) (0.748) (0.104) (0.139)

εy (%) 38.7 34.0 33.9 35.3

(3.87) (0.714) (0.110) (0.127)

σb (MPa) 24.9 32.2 35.6 38.1

εb (%)

(0.13) (4.42) (3.08) (2.42)

702 821 916 817

(31.5) (142) (58.6) (61.7)

energy to break (MJ/m3) 47.1 93.6 126 119

(2.20) (37.5) (23.9) (20.1)

Standard deviation is shown in the parentheses. G

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Table 6. Shape Fixation and Shape Recovery Efficiencies in Triple Shape Memory Cycle for Uncrosslinked and Crosslinked PEMA (200 kGy) for Five Consecutive Cycles uncrosslinked PEMA

crosslinked PEMA (200 kGy)

run no.

FS0→S1

FS1→S2

RS2→S1

RS1→S0

run no.

FS0→S1

FS1→S2

RS2→S1

RS1→S0

1 2 3 4 5

77 46 44 45 41

67 61 57 54 54

88 88 90 94 95

33 69 76 69 80

1 2 3 4 5

69 43 40 39 38

73 70 68 66 67

90 88 90 92 93

49 89 94 98 99

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The effect of crosslinking on the triple-shape memory properties of PEMA is shown in Table 6. The shape fixity efficiencies generally decreased in the crosslinked sample. That is probably a consequence of the higher crosslink density (lower Mc), which produces higher stresses in the network chains for a fixed strain. In addition higher crosslink density influences crystallization which also affects shape fixity. Crosslinking improved the shape recovery, which is consistent with the results for the dual-shape memory cycle and can be explained by strengthening of the permanent network.

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CONCLUSIONS The PEMA ionomer exhibits tunable shape memory properties and the capability to exhibit multiple shape memory behavior due to its broad melting transition. Microphase separation of ionic clusters that arises from ionic interactions of the metal salt groups in the ionomer provides physical crosslinks that can serve as a permanent network. A polyethylene crystalline phase serves as a temporary network, and a switching temperature can be chosen anywhere within the broad melting transition. Multiple shape memory can be achieved by choosing multiple switching temperatures, Tci, within the melting transition. However, if insufficient material is melted at Tci, the recrystallized network that fixes the temporary shape may have insufficient strength to maintain the fixed shape. As a result fixity efficiency may suffer. In addition, the annealing of the crystals that takes place during a melting/recrystallization fixing step changes the morphology of the crystallites and changes the melting point distribution toward higher melting crystals. That can complicate the reproducibility of the various recovery steps. Crosslinking the ionomer reinforces the permanent network and produces a nearly ideal shape memory polymer, i.e., R and F ≈ 100%. Crosslinking also improved the ultimate tensile properties of the ionomer, without significantly affecting the low strain properties.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant from the Polymer Program of the National Science Foundation (DMR- 1309853). REFERENCES

(1) Lendlein, A.; Kelch, S. Angew. Chem., Int. Ed. 2002, 41, 2034− 2057. (2) Xie, T. Polymer 2011, 52, 4985−5000. H

dx.doi.org/10.1021/ma401631j | Macromolecules XXXX, XXX, XXX−XXX