Design and characterization of thermally-induced shape memory polymers

Shape memory polymers (SMPs) are smart materials that can recover the permanent shape from a temporary shape when exposed to an external stimulus. They have drawn much attention due to their unique properties. In comparison to dual SMPs, complex SMPs with triple, multiple and two-way shape memories are more attractive due to their versatile properties. Triple and multiple SMPs could memorize three and more shapes, while two-way SMPs may automatically switch forth and back between two distinct shapes. The main purpose of this thesis is to design simple complex SMPs for their applications especially in biomedicine. Two biodegradable complex SMP systems based on bio-compounds or synthetic monomers have been synthesized. For multiple SMPs, we synthesized a series of random copolymers with pendent cholic acid groups by the use of a simple free radical polymerization method. Such copolymers have a glass transition temperature (Tg) range tunable by varying the monomers ratios, allowing the dual and triple shape memory effects (SMEs). The fixity and recovery ratios of dual and triple shape memory may be further improved by the incorporation of cinnamate groups into copolymers to enable a photo-cross-linking of the terpolymer. The cross-linked polymers show much improved recovery ratios for both dual and triple shape memory and even exhibit quadruple SMEs. The degree of cross-linking also affects the shape memory properties. The best shape memory behaviors were obtained with a 2.2 mol% cross-linking of the total monomers in the terpolymer.  Dual, triple and multiple SMPs are usually one-way SMPs, and their shape transformations are irreversible. Two-way reversible SMPs (2W-SMPs) may switch between two different shapes automatically when they are exposed to two reverse external stimuli. However, the actuation temperature (TA) of 2W-SMP is at a fixed value as it is determined by the melting temperature (Tm) of the actuator segment of the polymer network. In this study, a series of random copolymers containing ε-caprolactone (CL) and ω-pentadecalactone (PDL) were synthesized through ring opening polymerization catalyzed by Candida antarctica lipase B (CALB). The polymers segments made of these co-monomers are co-crystallizable and the Tms of the copolymers may be changed by adjusting the molar ratio of the co-monomers. Upon light irradiation which induces thiol-ene cross-linking, the polymer network showed 2W-SMEs under both stress-free and stress conditions with the absolute shape change up to 13.2%. Bending-unbending and coiling-uncoiling reversible shape motions were observed for the polymer network. The TA of the 2W-SMP under stress-free conditions may be tuned by simply selecting one or two different prepolymers as segments in the polymer network. The absolute strain change of 2W-SMPs increased under both stress-free and stress conditions, but the relative strain change reduced with increasing tensile stress under stress conditions. The evolution of the microstructure of 2W-SMPs under stress-free conditions was also elaborated. The 2W-SMPs are often actuated thermally, but indirect heating is desirable for certain applications because of its convenience. Thus, a series of light-actuated 2W-SMP composites were prepared via the incorporation of tiny amounts of polydopamine (PDA) nanospheres into polymer networks made of CL and PDL. PDA nanospheres have a strong photothermal effect which may convert energy of light into heat. The temperature change of the sample is larger with higher light intensity and higher content of PDA nanospheres. Such polymer composites exhibited excellent light-responsive 2W-SMEs under stress-free conditions with a reversible angle change of 45° when the light was turned on and off. The sample with a higher content of PDA nanospheres or under a stronger light led to a faster angle opening. A micro-robot is also designed and made of the 2W-SMP composite, which may walk on a home-made track with right triangle sawteeth when the light is turned on and off.

Keyphrases: Polyester co-cristallisables, Polydopamine, Shape memory polymers, Cholic acid, Co-crystallizable polyesters, Polydopamine