Stereocomplexed and Homochiral Polyurethane Elastomers with

Letter Cite This: ACS Macro Lett. 2018, 7, 233−238

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Stereocomplexed and Homochiral Polyurethane Elastomers with Tunable Crystallizability and Multishape Memory Effects Jian Zhou, Heqing Cao, Ruoxing Chang, Guorong Shan, Yongzhong Bao, and Pengju Pan* State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, P. R. China S Supporting Information *

ABSTRACT: Design of the polymer networks with tunable mechanical properties and multishape memory effects (multiSMEs) is highly desired in the engineering applications. Herein, we report on the stereocomplexed and homochiral polyurethane (PU) elastomers with tunable multi-SMEs by cross-linking the triblock prepolymers bearing the poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) enantiomeric segments. The homochiral PU is nearly amorphous, yet the stereocomplexed PU becomes highly crystalline due to the stereocomplexation of enantiomeric segments. Moreover, the two distinct thermal (glass, melting) transitions of PLLA (or PDLA) segments in PUs are integrated to realize the thermally induced triple- and quadrupleSMEs. Control over the enantiomeric segmental ratios allows the feasible manipulation of crystallizability, mechanical and thermal properties, and multi-SMEs of PUs.

S

polymer crystals, the less-ordered crystalline mesophases can also be formed in polymer crystallization because of the restricted chain mobility.26−29 Mesophase has a much lower Tm than the normal crystals due to the disordered structure.27,28 The Tm of the mesophase is near the Tg of the amorphous phase; this allows for the combination of Tg and Tm and enables the multi-SMEs. Herein, we report on a novel approach to achieve the tunable multi-SMEs based on the Tg and Tm transitions of the same polymer segment in the polyurethane (PU) network. The PUs were cross-linked from the homochiral or enantiomerically mixed prepolymers, that is, poly(L-lactic acid)-b-poly(ethyleneco-butylene)-b-poly(L-lactic acid) (PLLA-PEB-PLLA) and poly( D-lactic acid)-b-PEB-b-poly(D -lactic acid) (PDLA-PEBPDLA). PLLA (or PDLA) can crystallize in the mesophase in the PUs, which has a Tm above yet near the Tg of the amorphous phase. Multi-SMEs were realized by the combination of Tg and Tm of PLLA (or PDLA) segments in the PUs. In addition, the cross-linked polymer networks usually have inferior crystallizability due to the strong confinement effects. We successfully improve the crystallizability of PUs through the stereocomplexation of PLLA/PDLA enantiomeric segments; this leads to the enhancement in thermal and mechanical properties of PUs. Dihydroxyl-terminated PLLA-PEB-PLLA and PDLA-PEBPDLA triblock prepolymers (PEB midblock: Mn = 3.6 kg/mol,

hape memory polymers (SMPs), as smart materials, are able to fix the temporarily programmed shapes and recover the permanent shapes under external stimuli.1−5 Recently, the multi-SMPs (triple- or quadruple-SMPs), which are able to memorize two or three temporary shapes and sequentially recover to the permanent shapes, have been realized.6−9 One strategy for developing the multi-SMPs relies on a single, broad thermal phase transition such as the broad glass or melting transition.7,8,10−15 Because the breadths of glass or melting transition of common polymers are usually narrow (typically 80 °C), and it is difficult to combine these two separate switches into a shape memory cycle. Compared to the typically ordered © XXXX American Chemical Society

Received: December 22, 2017 Accepted: January 18, 2018

233

DOI: 10.1021/acsmacrolett.7b00995 ACS Macro Lett. 2018, 7, 233−238

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ACS Macro Letters Mw/Mn = 1.11; PLLA, PDLA end block: Mn of each block = 1.5 kg/mol) were synthesized by the ring-opening polymerization of L- or D-lactide with the dihydroxyl-terminated PEB as macroinitiator (Figure 1A).30 Chemical structures of prepol-

Figure 1. Design of HC- and SC-PU networks and photographs of their film samples. Green dots: permanent cross-linking points; black lines: PEB segments; red lines: PLLA segments; blue lines: PDLA segments.

Figure 2. Characterization of crystalline structure and thermal behavior of the HC-PU and SC-PUs with various PLLA/PDLA segmental ratios. (A) WAXD pattern. (B) DSC heating curve. The asterisks indicate the melting peak of the mesophase. Tg,PEB, Tg of PEB segments; Tg,PLA, Tg of PLLA and PDLA segments; Tm,SC, Tm of PLLA/PDLA SCs. (C) Temperature-variable FTIR spectra of HC-PU collected upon heating at 10 °C/min. (D) Temperature-dependent changes of 918 cm−1 band intensity of HC-PU.

ymers were confirmed by gel permeation chromatography and NMR (Figures S1 and S2 and Table S1). The PU networks were prepared by cross-linking the L- and D-prepolymers with poly(hexamethylene diisocyanate) (PHDI) (Figure 1A). The use of triblock copolymers (rather than PLLA, PDLA, or PEB homopolymers) as prepolymers can avoid the macroscopic phase separation between PLLA (or PDLA) and PEB components in cross-linking and also ensure the alternating distribution of PLLA (or PDLA) and PEB blocks in the networks. For simplicity, the mass fraction of D-prepolymer in the prepolymers is denoted as FD, which equals to 0−0.5 and 1.0. The homochiral PU (HC-PU) made from the enantiopure L- or D-prepolymer is flexible and transparent (Figures 1 and S3). However, the stereocomplexed PUs (SC-PUs) made from the enantiomeric mixture of L- and D-prepolymers become rigid and turbid as the FD increases from 0 to 0.5, due to the increased crystallinity (as elaborated below). Although the PLLA and PDLA segments have strong crystallizability in the homochiral prepolymers,30 their crystallizabilities are significantly depressed in the HC-PU network because of the cross-linking effects. HC-PU (FD = 0 or 1) exhibits a weak diffraction at 2θ = 13.5° (characteristic of the α homocrystals of PLLA or PDLA)31 in the wide-angle X-ray diffraction (WAXD) profiles and an extremely small melting endotherm at ∼152 °C in the differential scanning calorimetry (DSC) heating curves (Figure 2A, B), indicating the very low crystallizability and near amorphous structure of HC-PUs. Because of the stronger driving force, stereocomplexation is feasible to improve and tune the crystallizability of non- or lesscrystallizable polymers or networks.32,33 As shown in Figure 2A,B, the PUs made from L-/D-prepolymer mixture show characteristic WAXD diffractions at 2θ = 9.7°, 16.8°, and 19.4° and a melting peak at ∼193 °C, characteristic of the stereocomplexes (SCs).31,34 The diffraction intensity and melting enthalpy of SC-PUs gradually increase as the FD increases from 0 to 0.5, suggesting that the crystallinity of PU

networks can be well controlled by the L-/D-prepolymer feed ratio. As shown in Figure 2B, the DSC heating runs of all samples exhibit two distinct glass transitions at around −50 and 50 °C, representing the Tg’s of PEB and PLLA (or PDLA) segments, respectively. Notably, the HC-PU and SC-PUs (FD ≤ 0.3) exhibit broad endotherms (indicated by asterisk) above the Tg of the PLLA (or PDLA) amorphous phase; this post-Tg endotherm can be more clearly observed in the enlarged DSC curves in Figure S4. The post-Tg endotherm of HC-PU is in the range 55−100 °C, and its peak top locates at ∼72 °C. This resembles the melting endotherm of the PLLA mesophase formed in the stretched PLLA27,35,36 and PLLA/poly(ethylene glycol) (PEG) block copolymer.28 However, the post-Tg endotherm becomes much broader (in the range 60−135 °C), and the peak top shifts to 115−120 °C for the SC-PUs (FD ≤ 0.3). The post-Tg endotherms of HC-PU and SC-PU (FD = 0.2) have the melting enthalpies of 5.7 and 7.0 J/g, respectively. The presence of the post-Tg endotherm suggests that the other structural phase is formed in the PU networks. In order to explain the physical origins of the post-Tg endotherm, the temperature-variable WAXD and FTIR measurements were conducted in the heating processes of HC- and SC-PUs; these results are shown in Figures S5−S7. As seen in Figures S5 and S6, the intensities of HC (200/110) diffraction of both HC- and SC-PUs vary little (or slightly increase) upon heating from 30 to 135 °C and decrease remarkably with further heating to 175 °C. This demonstrates that the HCs melt at 135−175 °C in the PU networks, and the post-Tg endotherm at 60−135 °C does not stem from the melts of HCs. In the case of SC-PUs, its SC diffraction intensity changes little (or slightly increases) with heating from 30 to 234

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ACS Macro Letters 160 °C and decreases drastically with further heating to 220 °C, because of the melt of SCs (Figures S5B and S6C). As shown in Figure 2C, the HC-PU exhibits an absorption peak at 918 cm−1, similar to the characteristic bands of the PLLA mesophase formed in the stretched PLLAs (918 cm−1)27,35 and PLLA-PEG-PLLA block copolymer (915 cm−1).28 However, this absorption band disappears as the HC-PU is heated from 50 to 85 °C, agreeing with the temperature range of the post-Tg endotherm (Figure 2D). Although the weak WAXD diffraction (2θ = 13.5°) of homocrystals is still present in HC-PU at >85 °C (Figure S5A), we do not observe the characteristic absorption of α homocrystals at ∼921 cm−1 upon heating to >85 °C.28 The FTIR absorption at 930−900 cm−1 is severely covered by the dominant absorption of SCs at 908 cm−1; therefore, we do not see the FTIR absorption of the mesophase at 918 cm−1 in SCPUs. Because the post-Tg endotherm is strongly correlated with the PLLA/PDLA segmental ratio, the post-Tg endotherm should associate with the structural transition of PLLA/PDLA segments but not the PEB segments or urethane linkages. Therefore, we conclude that the post-Tg endotherm does not originate from the melts or the other structural transitions of HCs or SCs but arises from the melt of the PLLA (or PDLA) mesophase. Because the post-Tg endotherm is only present in the HCPU and SC-PUs having much different PLLA/PDLA segmental contents, the mesophase would be formed from the homochiral PLLA or PDLA. The mesophase is an intermediate state between the orderly packed crystalline and amorphous states. The polymer chains in the mesophase have less ordered packing than those in the polymer crystals but more ordered packing than those in the amorphous phase. Thus, the mesophase formed in PU networks exhibits much lower Tm, a characteristic FTIR band similar to (but different frequency from) the homocrystals, while it does not show the distinct Xray diffraction.27,28 Because of the restricted mobility of PLLA (or PDLA) segments, most of the PLLA (or PDLA) chains cannot form the regular homocrystals but just pack into the mesophase in the PU networks. Mesophase formation in PUs is reversible, and the mesophase can be regenerated after thermal annealing or stretching. As shown in Figures S8 and S9, the melting endotherm and characteristic IR absorption of the mesophase are detected for the thermally treated (90 °C 3 min + 55 °C 6 h) and stretched (90 °C, strain = 200%) HC-PUs, as well as the thermally treated SC-PU (130 °C 3 min + 90 °C 4 h, 130 °C 3 min + 60 °C 4 h). The annealing and stretching temperatures are the same as the shape deformation, fixing, and recovery temperatures used in the following multishape memory cycles. As indicated by the temperature-variable FTIR results of thermally treated and stretched HC-PUs (Figure S10), the intensity of the characteristic FTIR band of the mesophase decreases significantly upon heating from 50 to 80 °C, agreeing with the melting range of mesophase. Due to the effects of crystallinity, the mechanical and thermomechanical properties of PUs depend strongly on the PLLA/PDLA segmental ratio. Figure 3A,B shows the tensile stress−strain curves, Young’s modulus (E), and elongation-atbreak (εmax) of HC- and SC-PUs. E increases, and εmax decreases with increasing FD from 0 to 0.5 because of the increase in crystallinity. HC-PU is flexible, having a large εmax of 537% and a small E of 47.5 MPa. However, the racemic SC-PU

Figure 3. Mechanical and thermomechanical characterizations of HCand SC-PUs. (A) Stress−strain curves. (B) Young’s modulus and elongation-at-break. (C) Storage modulus curves.

(FD = 0.5) is rigid, having a small εmax of 202% and a large E of 58.6 MPa. Figure 3C shows the typical dynamic mechanical analysis (DMA) curves of HC- and SC-PUs. The storage moduli (E′) of PUs decrease obviously at −50 ∼ −20, 30−60, and >150 °C, attributable to the glass transitions of PEB and PLLA (or PDLA) and the softening of PUs caused by the melt of PLLA/ PDLA crystals, respectively. Stereocomplexation of PLLA and PDLA significantly improves the thermal resistance and E′ of PUs. At >50 °C, E′ of PUs gradually increases as FD increases from 0 to 0.5. HC-PU softens completely with heating to 160 °C; this temperature enhances to ∼190 °C for the racemic SCPU (FD = 0.5). Notably, DMA curves of PUs lack the rubbery plateau above the Tm of SCs; this is possibly caused by the depolymerization and thermal decomposition of PUs. The thermogravimetric analysis indicates that the PUs start to decompose at above ∼180 °C (Figure S11). The chemical cross-links and SC phase act as the stationary phase,37 while the amorphous PLLA/PDLA and mesophase work as the reversible phase for shape recovery in the PUs. Accordingly, both HC- and SC-PUs are found to exhibit good dual-SMEs when deformed and recovered at 90 (for HC-PU) or 130 °C (for SC-PU), with the shape fixity (Rf) and shape recovery (Rr) above 85% in both PUs (Figure S12). The calculation methods of Rf and Rr are described in the Supporting Information. On the other hand, the existence of broad thermal transitions of PLLA (or PDLA) segments (Tg of amorphous phase, Tm of mesophase) in 50−130 °C endows PUs the thermally induced multi-SMEs. Figure 4A illustrates the thermally induced triple-SMEs of HC-PU at 90 + 55 + 20 °C. The rectangle specimen (original shape, S1) was first curled at 90 °C and cooled to 55 °C, yielding an “O” shape (temporary shape, S2). It was then stretched and folded into a “V” shape (temporary shape, S3), which was subsequently cooled to 20 °C for shape fixing. After reheating to 55 °C, the sample recovered to the “O” shape (recovered shape, S2re). Further heating to 90 °C resulted in recovering to the original shape (recovered shape, S1re). 235

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Figure 4. Triple-shape memory characterizations of HC-PU. (A) Visual demonstration. S1: original shape; S2, S3: temporary shapes; S1re, S2re: recovered shapes. (B) Quantitative characterization (see more details of Rf’s and Rr’s in Table S2).

Figure 5. Quadruple-shape memory characterizations of SC-PU (FD = 0.2). (A) Visual demonstration. S1: original shape; S2, S3, S4: temporary shapes; S1re, S2re, S3re: recovered shapes. (B) Quantitative characterization (see more details of Rf’s and Rr’s in Table S2).

Quantitatively, the triple-shape memory cycle of HC-PU is illustrated in Figure 4B. Besides, the shape recovery of HC-PU is also observed in the DMA analysis under continuous heating mode (Figure S13), which is a crucial criterion of multiSMEs.38 For the HC-PU, the SMEs between S2 (S2re)/S3 and S1 (S1re)/S2 (S2re) rely on the glass transition of the PLLA amorphous phase and the melting transition of the PLLA mesophase, respectively. Because the post-Tg endotherm of SC-PUs (FD≤ 0.3) locates at the higher temperature than that of HC-PU, SC-PU has a much broader transition at 50−130 °C; this enables the fixing and recovery of more temporary shapes. SC-PU (FD≤ 0.3) has the quadruple-SMEs. Figure 5A illustrates the quadruple-SMEs of SC-PU (FD = 0.2) at 130 + 90 + 60 + 20 °C. SC-PU can be subsequently programed and recovered between the rectangle (S1, S1re), “V” (S2, S2re), “N” (S3, S3re), and “M” (S4) shapes. It is obvious that the quadruple-shape memory of SC-PU is good, though the shape recovery at 90 °C is not so perfect. It is notable that HC-PU is incapable of the quadruple-SMEs under the same conditions (Figure S14). Quantitative measurement of DMA also reflects the quadruple-SMEs of SC-PU, although the Rf (S1 → S2), Rf (S2 → S3), Rr (S3 → S2), and Rr (S2 → S1) are not so high (which may due to the relatively low fraction of the mesophase) (Figure 5B, Table S2). In the SCPU, the SMEs between S3 (S3re)/S4 rely on the glass transition of the PLLA (or PDLA) amorphous phase, while those between S1 (S1re)/S2 (S2re) and S2 (S2re)/S3 (S3re) are due to the melting transition of the mesophase. Although the glass and melting transitions of PLLA (PDLA) segments have the distinct mechanisms in the molecular level, they can be integrated in one material to fulfill the multi-SMEs. On the other hand, both HC- and SC-PUs possess the temperature memory effects,39,40 as demonstrated in Figures S15−S17. When the deformation temperature (Td) was within the melting range of the mesophase, PUs exhibited the fastest strain recovery at the temperature around Td. In summary, we have attained the highly crystallizable PU networks through the stereocomplexation of enantiomeric

segments. In particular, control over the segmental ratio of enantiomers allows the feasible manipulation of the crystallinity, mechanical, thermal, and shape memory properties of PU networks. We also revealed that two distinct thermal (Tg, Tm) transitions of the same polymer segment can be integrated in the PUs to realize the triple- and quadruple-SMEs. This would be a simple, yet effective approach to prepare the functional multi-SMPs that could enable a variety of potential applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00995. Experimental details and characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-571-87951334. ORCID

Guorong Shan: 0000-0001-5676-6310 Pengju Pan: 0000-0001-6924-5485 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support of the National Natural Science Foundation of China (21422406); National Natural Science Foundation of Zhejiang Province, China (LR16E030003); and State Key Laboratory of Chemical 236

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