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Synthesis, Properties, and Light-Induced Shape Memory Effect of Multiblock Polyesterurethanes Containing Biodegradable Segments and Pendant Cinnamamide Groups Linbo Wu,* Chunli Jin, and Xiangying Sun Department of Chemical and Biological Engineering, State Key Laboratory of Chemical Engineering, Zhejiang University, Hangzhou 310027, China Received October 12, 2010; Revised Manuscript Received November 12, 2010
Novel multiblock polyesterurethanes containing crystalline hard and amorphous soft segments and pendant cinnamamide moieties were designed and synthesized via a two-step polyaddition reaction using N,N-bis(2hydroxyethyl) cinnamamide (BHECA), biodegradable poly(L,L-lactide) (PLLA), and poly(ε-caprolactone) (PCL) diols as raw materials and hexamethylene diisocyanate (HDI) as coupling agent and characterized by 1H NMR, FTIR, UV, DSC, tensile and photomechanical tests, and so on. The copolymers behaved as typical thermoplastic elastomers and showed satisfactory thermal and mechanical properties. They also exhibited light-induced shape memory effect (LSME) at room temperature on exposure to light stimuli. The pendant cinnamamide groups work as photoresponsive molecular switches and provide the polymer with LSME via reversible [2 + 2] cycloaddition cross-linking. The strain fixity (Rf) increases with the content of BHECA and the strain recovery (Rr) increases with the content of PLLA. The Rf reaches 50% at a BHECA content of 20 wt % and the Rr reaches >95% at PLLA content of 50 wt %.
Introduction Shape memory polymers (SMPs) are a class of smart materials that can be deformed and fixed into a temporary shape and then recover their original shape on exposure to external stimuli. In general, thermal transitions like glass transition or melting of polymer upon direct or indirect heating are responsible for shape memory effect (SME). Thus, obtained SMPs are referred as thermally induced SMPs (TSMPs).1-4 TSMPs, including two-,5-11 triple-,12 and even quadruple-shaped,13 nonbiodegradable,5-7 and biodegradable,8-11 have been extensively reported in the last decades in terms of synthesis,5,6,8,12 thermomechanical properties,14,15 shape recovery characteristics,16 and applications.17-19 Owing to their large deformability, excellent strain fixity and recovery, and versatile shape configuration, they have been applied or are finding applications in many fields, including minimally invasive surgery,2,17 intelligent medical devices,18 and even self-deployable sun sails in spacecraft,19 and so on. Light-induced SMPs (LSMPs) have also been reported in recent years.20-24 These LSMPs include chemically cross-linked acrylic copolymers with grafted phororesponsive groups and acrylic interpenetrating networks loaded with four-arm star poly(ethylene glycol) containing photoresponsive terminal groups.20-22 The temporary shape is fixed owing to the formation of new photoresponsive cross-links via [2 + 2] cycloaddition reactions of the photoresponsive groups when exposed to >260 nm UV light. The photoresponsive cross-links can be reversibly cleaved by irradiation with 260 nm UV light, a [2 + 2] cycloaddition reaction will take place to form temporary chemical cross-links. After the external force is released, a temporary shape is thus fixed. When the sample is irradiated with 260 nm UV light to form temporary chemical crosslinks through a [2 + 2] cycloaddition reaction and then releasing the external force; (C) Final shape (recovered to original shape) after irradiation with 95%) similar to those exhibited by TSMPs. Although this may be affected by several factors such as molecular weight, content, and crystallinity of the hard PLLA segments, in general, higher PLLA content resulted in higher Rr (Figure 11, right). The shape memory properties remained constant in the second photomechanical cycle. They also remained unchanged (change less than 1-2%) when the wavelength of UV light for the first
Figure 12. In vitro hydrolytic degradation of four multiblock polyesterurethanes in PBS at 37 °C.
irradiation changed from 365 nm (No. 7-1) to 312 nm (No. 7-3), or when the measurement temperature was raised from 5 to 37 °C (Nos. 7-1, -4, -5). In Vitro Hydrolytic Degradation. In vivo biodegradation of a biomaterial is a complex process. It usually experiences extracellular chemically and/or enzymatic catalyzed hydrolytic degradation and then intracellular metabolism. So, in vitro hydrolysis in phosphate buffer saline (PBS) solution is often used to simulate the early biodegradation in body liquid environment. The presence of biodegradable segments in the multiblock polyesterurethanes may provide them with biodegradability. Figure 12 shows in vitro hydrolytic degradation of four multiblock polyesterurethane samples in PBS at 37 °C. The polymers lost 12-25% weight in 32 weeks, preliminarily indicating hydrolytic degradability. The slow degradation rates are attributed to the slow degradation of PCL and PLLA homopolymer segments. The homopolymer segments were used here just in order to simplify the synthesis. Clearly, copolymers such as poly(ε-caprolactone-co-glycolide) and poly(L,L-lactideco-glycolide) can be used instead to adjust the degradation rate as well as the mechanical properties, as previously reported.7,34-36 A forthcoming report will describe the biodegradation behavior of the multiblock copolymers. It has been reported that biodegradable polyesterurethanes often have good biocompatibility.35 In our polyesterurethanes, the PCL- and PLLA-based segments are all FDA-approved biocompatible polymers. It is expected that the cinnamamide moieties may also offer biocompatibility since BHECA and similar compounds are currently used in medicine as muscle relaxant, in fragrance and skincare, and so on. All these factors
Biodegradable Light-Induced Shape Memory Polymers
may provide the polyesterurethanes with good biocompatibility. Of course, the biocompatibility needs to be further assessed.
Conclusions Multiblock polyesterurethanes containing biodegradable segments and pendant cinnamamide groups were designed and synthesized via a two-step polyaddition reaction using PCL and PLLA diols as biodegradable segments, N,N-bis(2-hydroxyethyl) cinnamamide (BHECA) as photoresponsive monomer, and hexamethylene diisocyanate as coupling agent. The resultant thermoplastic elastomers showed a photoresponsive behavior at room temperature. The amorphous PCL segments and BHECA moieties compose the soft phase. The crystallized PLLA segments act as physical cross-links and constitute the hard phase. The pendant cinnamamide groups work as photoresponsive molecular switches and provide the polymer with LSME via reversible [2 + 2] cycloaddition cross-linking. The tensile strength, modulus, and elongation at break ranged from 10 to 20 MPa, from 20 to 230 MPa, and from 230 to 530%, respectively. The elongation increased with increasing content of soft PCL segments. The strain fixity, Rf, increased with the content of BHECA moieties and the strain recovery, Rr, increased with PLLA content. The Rf reached 50% at a BHECA content of 20 wt % and the Rr reached more than 95% at PLLA content of 50 wt %. The polyesterurethanes show hydrolytic degradability in aqueous medium. Details on biodegradability and biocompatibility will be assessed and reported later. Further modifications are also under way to adjust biodegradability and mechanical properties, as well as to improve light-induced shape memory properties. Acknowledgment. The authors thank the National Natural Science Foundation of China (20674067, 20406018, 20304012) and the National Basic Research Program of China (973 Program, 2011CB606004) for financial support.
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