Shape Memory Polymeric Metasurfaces
3.2. Shape memory polymer synthesis
3.2.1. Introduction
3.2.4.1. Synthesis and chemical structure of SMPs
154 3.2.4. Results and discussion
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vibration, C-O vibration of acrylate, and C-O vibration of the crosslinker, respectively [201,202]. The CH2 peak of S-MMA at 2870 cm-1 is lower than that of the others, which suggests that the others have much larger amount of the CH2 moiety than S-MMA in the chemical structure. This result can be supported from the monomer peaks at 2960 and 2870 cm-1 (Fig. 3.2.4(b)). The sharp carbonyl peak of S-MMA at 1720 cm-1 means that the amount of the acrylate moiety decreases with increasing the amount of the CH2 moiety in the SMPs. The decreasing area of the ester peaks of acrylates at 1140 cm-1 in S-BMA and S-BA can be explained by the reduced fraction of the ester moiety with an increase in the amount of the CH2 moiety. The ester peaks at 1100 cm-1 originate from the crosslinker peak as shown in Fig. 3.2.4(b). It is verified by the ATR-FTIR analysis that the SMPs were synthesized successfully as designed.
The XPS analysis was carried out to further elucidate the chemical composition and the ratio of methyl and butyl moieties in the SMPs. The XPS data of C1s spectra are presented in Fig. 3.2.6. All XPS spectra reveal that the two main components are C and O.
A high resolution scan of C1s spectra was performed for more detailed information of the SMPs. The deconvolution of the C1s scan for the five components (C1, C2, C3, C4, and C5) was conducted. The C1 (284.5 eV) peak was associated with a single carbon atom and hydrogen atoms. S-BMA has the highest ratio of C1 peak because of its bulky structure of butyl moiety. On the other hand, S-BA shows the second highest ratio of C1 peak. The C2 (285.30 eV) peak corresponds to the carbon associated with other CO2-
moiety. The C3 (286.1 – 286.2 eV) peak is allocated to the hydrocarbons bound to one oxygen. The C4 (286.6 – 287.4 eV) peak is related to the carbon atom bound to two oxygen atoms. The C5 (288.6 – 289.3 eV) peak corresponds to the carbon atom connected with one carbonyl oxygen and other non-carbonyl oxygen atoms. The chemical structures shown in Fig. 3.2.5 were confirmed by the peaks from C1 to C5 (Table. 3.2.3). It was found that the measured results were consistent with the results in the literature [203–205].
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Figure 3.2.3. Chemical reaction scheme for the SMPs synthesis. (a) Reaction initiated by UV light irradiation. (b) Generation of pre-polymer chains by free radical electrons. (c) Crosslinking reaction of the pre-polymer chains and the crosslinker. (d) Suggested chemical structure of SMPs. (e) Schematic illustration of shape memory copolymers with monomers (“A” and “B”) and a crosslinker (“C”).
Table 3.2.2. Polymeric chain composition of the SMP samples.
R1 R2 R1’ R2’
S-MMA Methyl Methyl Methyl Methyl
S-BMA Butyl Butyl Methyl Methyl
S-BA Butyl Butyl Hydrogen Hydrogen
S-MMA/BMA Methyl Butyl Methyl Methyl
S-BA/BMA Butyl Butyl Hydrogen Methyl
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Figure 3.2.4. ATR-FTIR spectra of (a) the basic chemicals and (b) the synthesized SMPs.
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Table 3.2.3. XPS peak information of the SMP samples.
S-MMA S-BMA S-BA S-MMA/BMA S-BA/BMA
# Name Position (At%)
Position (At%)
Position (At%)
Position (At%)
Position (At%) C1 C-C, C-
H
284.50 (37.39)
284.50 (45.97)
284.50 (40.11)
284.50 (35.36)
284.50 (39.52) C2 CH2-
CO2-
285.30 (18.74)
285.30 (11.46)
285.30 (18.81)
285.30 (19.75)
285.30 (16.60)
C3 C-O 286.24
(23.52)
286.17 (20.79)
286.06 (12.37)
286.10 (27.71)
286.10 (16.20) C4 O-C-O 286.95
(16.50)
287.36 (16.38)
286.64 (24.98)
286.90 (7.67)
286.97 (20.16) C5 O=C-O 288.93
(3.85)
289.34 (5.40)
289.01 (3.73)
288.61 (9.51)
288.71 (7.51)
Figure 3.2.5. Chemical formula of the SMPs; (a) S-MMA, (b) S-BMA, (c) S-BA, (d) S- MMA/BMA, and (e) S-BA/BMA.
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Figure 3.2.6. XPS spectra of the SMPs: (a) S-MMA, (b) S-BMA, (c) S-BA, (d) S- MMA/BMA, and (e) S-BA/BMA.
160 3.2.4.2 Thermomechanical properties of SMPs
Thermomechanical features of all samples were investigated by the DMTA tests (Fig. 3.2.7).
At temperature above the transition temperature of SMPs, the micro-Brownian motion of polymeric chains induces rubbery behavior of polymeric chains. However, this motion is frozen at temperature below the transition temperature, and the materials become glassy.
Storage moduli of the SMPs are presented as a function of temperature in Fig. 3.2.7(a). It was verified from the comparison between S-MMA and S-BMA that the storage modulus is highly dependent on the size of side moiety of monomers, such as methyl and butyl groups. S-MMA is transformed from a glassy state to a rubbery state at a relatively higher temperature than that of S-BMA, which is explained by the longer side chain of S-BMA and the resulting higher chain flexibility. Because the methyl group on α-carbon position of the acrylate group has a bulky structure, this feature does not let the polymer have larger flexibility [206]. Therefore, S-BA has the transition temperature lower than that of S-BMA.
It is found that the transition temperatures of the SMCPAs are determined by a simple rule of mixture based on the blended two monomers [207].
Tangent delta curves were evaluated by calculating the ratio of the loss modulus to the storage modulus (Fig. 3.2.7(b) and Table 3.2.4). Generally, a transition temperature region of amorphous SMPs is relatively broader than that of semi-crystalline SMPs, around 30℃.
Glass transition temperature ( ) is defined as the temperature at the center of the transition region, which is the same as temperature at a peak value of a tangent delta graph.
The permanent shape can be recovered by the micro Brownian motion at a temperature slightly higher than transition onset temperature ( ). Figure 3.2.7(c) shows the variation of , , and rubbery modulus ( ). It was verified that the synthesized SMPs shows higher and values in the order of S-MMA, S-MMA/BMA, S- BMA, S-BA/BMA, and S-BA as chemically designed. is an elastic modulus in the rubbery state after sufficient heating over . Generally, there is a large difference between the rubbery and glassy moduli of SMPs in a constrained condition [70]. All the samples showed a big drop in modulus larger than 3000 MPa in the cooling step, which is sufficient enough for achieving a high shape recovery ratio.
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Table 3.2.4. Thermomechanical properties of the fabricated SMPs.
(MPa) (℃) (℃)
S-MMA 60.7 68.8 52.6
S-BMA 52.0 24.1 0.3
S-BA 39.2 3.9 -14.7
S-MMA/BMA 53.6 48.3 26.1
S-BA/BMA 45.8 12.4 -5
Figure 3.2.7. DMTA results of the SMPs. (a) Storage modulus as a function of temperature.
(b) Tangent delta as a function of temperature. (c) Transition temperature, transition onset temperature, and rubbery modulus of the SMPs.
162 3.2.4.3 Shape recovery behavior of SMPs
Time-dependent shape recovery behavior of the SMPs was investigated by carrying out the transient shape recovery test. Three discrete recovery temperatures were selected for the test as 80, 40, and 0℃ . Figure 3.2.8(a) shows the transient shape recovery of S- MMA/BMA measured at 40℃ in the aqueous condition. The shape recovery ratios were characterized as a function of time by measuring the angle change of the sample upon heating and plotted in Figs. 3.2.8(b-d). It was identified from Fig. 3.2.8(b) that all samples can recover the original shape when the applied thermal energy is large enough to release stored strain energy. The S-MMA and S-MMA/BMA samples recovered the original shape perfectly within 7 seconds and 3 seconds, respectively. However, shorter time was enough for the shape recovery of the other samples.
S-MMA did not display any shape recovery behavior at 40℃ due to its high (68.8℃), however, S-MMA/BMA could recover from the deformed geometry as shown in Fig. 3.2.8(c). It was shown by the comparison between the recovery ratios at 40℃ and 80℃ that S-BMA and S-BA/BMA have shorter recovery time with respect to temperature.
The samples can be classified into two groups from the shape recovery test at 40℃: the first group which can recover to the original shape around body temperature and the other group which cannot do that. The shape recovery capability at body temperature is a desirable function for biomedical applications. On the other hand, S-BMA and S-BA cannot be employed to the biomedical applications due to their seriously low mechanical property at this temperature. Consequently, S-MMA/BMA is the best candidate for biomedical applications among all samples.
S-MMA and S-MMA/BMA did not show shape recovery behavior at 0℃ (Fig.
3.2.8(d)). The shape recovery time of S-BMA and S-BA/BMA at 0℃ was estimated seven times and three times higher than that at 40℃. Since S-BA/BMA has sufficiently high shape recovery ratio and high mechanical properties with short recovery time, it can be the best smart material for aerospace applications.
For the quantitative analysis of shape recovery performance, cyclic stress-strain- temperature (SST) curves for the two SMCPAs (S-MMA/BMA and S-BA/BMA) were obtained by using the 3-point bending machine. As shown in Figs. 3.2.9(a-b), the S- MMA/BMA sample was heated to 40℃ and then bended with a deflection of 2
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mm ( = 3%) . The deformed shape was frozen at 0℃ during the cooling step by generating glassy regions in the sample. Since the stored strain energy of rubbery phase was very small, the deformed shape was maintained even after the unloading step. When temperature was elevated to 40℃, the original shape was recovered with a recovery ratio over 90%. S-BA/BMA was also characterized experimentally by using the similar procedure. Heating temperature was changed from 40℃ to 0℃, and cooling temperature was altered from 0℃ to -40℃. The results are shown in Figs. 3.2.9(a) and (c). In this case, the original shape was fully recovered at 0℃.
The shape memory–recovery behavior can be explained by the two-fold molecular mechanism. The first mechanism is the release of stored elastic energy due to reduced stiffness of the SMCPAs in the heating process. The second mechanism is based on the driving force exerted by the crosslinking points to recover the original permanent shape.
Both S-MMA/BMA and S-BA/BMA were almost completely recovered to their original shapes within 20 minutes. The time required for full shape recovery was much longer than that required in aqueous condition (see Fig. 3.2.8), since heating the samples takes longer time in a convection heating chamber than in water.
To model the shape memory behavior of S-MMA/BMA and S-BA/BMA theoretically, numerical analysis was carried out using ABAQUS/CAE with the subroutines UMAT and SDVINI. When SMPs are deformed, the strain energy is accumulated in the bended region.
Thereafter, it is stored during glass transition and the stiffness is increased in the cooling process. The following heating process causes the shape recovery through transition from glassy phase to rubbery phase.
A constitutive model for glassy shape memory polymers was proposed by Khanolkar et al. and employed in this study [198]. The simulated SST curves are shown in Figs.
3.2.9(a-c). The numerical simulation results are in good agreement with the experimental data. The measured and simulated curves have different shapes during the cooling step. It was caused by the assumption that phase transition is linearly proportional to the glassy state fraction ( ). The change of α was simply defined as = ( − )⁄ − , where is the current temperature, is the temperature for fully glassy state and is the temperature for fully rubbery state.
Figure 3.2.10 shows the von Mises stress contours formed in the specimens as a result
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of the numerical simulation. The stress applied in the loading step fades out in the next cooling step since the newly generated glassy region is in a stress-free state. After unloading, the stress in the specimen disappears, but the sample cannot recover its original shape in contrast to general elastic materials. The deformed sample is recovered upon heating. From the numerical structural analysis, the underlying mechanism of the shape memory behavior was understood and the position where the strain energy accumulates were verified theoretically.
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Figure 3.2.8. Transient shape recovery tests. (a) Snap shots of the shape recovery process of S-MMA/BMA taken at 40℃ and shape recovery ratios measured at (b) 80℃, (c) 40℃, and (d) 0℃.
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Figure 3.2.9. Cyclic shape memory tests of the SMCPAs. (a) The measured and simulated SST curves of S-MMA/BMA and S-BA/BMA. 2D plot of the SST curves for (b) S- MMA/BMA and (c) S-BA/BMA.
Figure 3.2.10. Numerical simulation results of S-MMA/BMA.
167 3.2.4.4 Optical property of SMPs
Figure 3.2.11 shows the light transmittance of the SMPs measured by using the UV-vis spectrophotometer. Since the synthesized SMPs have a very low level of crystallinity due to the bulky moiety of acrylate chemicals, the transmittance of the samples was measured to be about 90% in the wavelength range from 430 nm to 800 nm. In the wavelength range from 380 to 430 nm, the samples revealed a decay of transmittance as shown in the inset figure. It was presumed that the decay was induced by the immiscibility of monomers (S- MMA/BMA and S-BA/BMA) and the bulkier moiety of monomers (S-MMA, S-BMA, and S-BA) [208]. The butyl moiety of the acrylate group of S-BMA and S-BA could cause the scattering of light in the lower wavelength range. And the reaction inhibitor and unreacted photo initiator remained in the samples could also provoke the light absorption in this wavelength range.
Figure 3.2.11. Light transmittance of the SMPs with respect to wavelength.
168 3.2.5. Conclusions
In this section, optically transparent shape memory copolymers were synthesized and fabricated by using a tri-copolymerization method for thermomechanical modulation. The chemical formula of the SMPs was suggested based on the sequential reaction scheme and confirmed by the spectroscopic experiments. The characteristics of SMPs were identified by XPS, FTIR, DMTA, shape recovery tests, and the optical tests. The numerical simulation results coincided with the experimental results and confirmed the underlying nature of shape recovery mechanism. All the samples were optically transparent due to their low crystallinities. S-MMA/BMA and S-BA/BMA provided useful thermomechanical properties compared with the other SMP samples. Consequently, these copolymer materials showed great potential as smart materials for medical and aerospace applications. This design strategy can be applied to other specific circumstances for accomplishment of optimized shape recovery performance.
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