Study On Shape Memory Polymers Based On Hydrogen Bonding Interaction

Stimulus-responsive materials can undergo conformational or phase changes in response to environmental signals. The materials represent one of the most exciting emerging areas of science due to their promising applications in such fields as actuator systems, tissue engineering and programmable delivery systems. Although some progress has been made in recent years, the design and engineering of a synthetic material with an ability to respond to stimuli in a controllable and predictable fashion remains a challenge.Among these responsive materials, shape memory polymers (SMPs) have attracted increasing attention for their potential medical applications. SMPs can change from their permanent shape into a temporary shape, and recover to their permanent shape upon application’of an appropriate stimulus such as temperature, light, water, magnetic field or chemicals. Thermo-induced SMPs are the most widely studied, but their biomedical applications are sometimes limited because the switching temperature should be in the range from room temperature to body temperature.In the second chapter, we attempt to incorporate liquid crystals compound into shape memory polyurethanes (SMPUs) to form a new composites to obtain novel shape memory properties. We firstly synthesized a new polyurethane-co-poly(ε-caprolactone) copolymer with carboxyl groups by using 2,2- dimethylol propionic acid (DMPA) as the chain extender, instead of the tradition butanediol reported in the literature.We choose the isonicotinic acid cholesteryl ester (INCh) as the mesogen, for which have the typical liquid crystal nematic phase. INCh was mixed with the SMPUs to fabricate the composites SMPUs-INCh-n. n stands for the molar ratio of INCh to the real carboxyl groups content in SMPUs. The results showed that the H-bonding interaction in the composite is helpful to improve the mechanical properties. The introducing of INChs promotes the bonded polyurethane (PU) segments to change into glassy state by restraining the movement of polymer chains, which is responsible for triple-shape memory property. To the best of our knowledge, it is the first time to synthesize supermolecular composites with triple-shape memory function based on the H-bonding interaction between PU and mesogen. This study has great potential for providing an effective and simple way to design and develop triple-shape memory polymers. These composites are biocompatibility with non-toxic, have great potential for applications in biomedical field.In the third chapter, we synthesize a highly pH-sensitive polymer by introducing pyridine rings into the backbone of polyurethane. Polyurethane has great potential for medical applications due to its excellent shape memory and biocompatibility. Pyridine is a Lewis base, and the N of the pyridine ring can readily combine with H+ to form an NH+ in acidic conditions, with H+ removed under basic conditions. In this system, the polymer will exhibit high pH sensitivity through the association and dissociation of the hydrogen bond interactions between the H of urethane and the N of the pyridine ring via the deprotonation and protonation of the pyridine ring. The H+ sensitive part of pyridine can be used as a switch to shift the polymer shape, while the hydrogen bonded urethane segments insensitive to H+ can be used as the fixed domain to hold the original shape. Unlike other systems with thermally sensitive functions, this shape memory function is independent of temperature and only dependent on the pH value of the environment. This pH sensitivity can be exploited for drug delivery to reversibly switch drug release on and off. Additionally, unlike other drug delivery systems such as polymer-drug conjugate nanoparticles that irreversibly de-assemble to release drug via the cleavage of pH-sensitive linkages in the polymeric backbone in the acidic microenvironment, this system can release drug on demand while maintaining a stable structure due to the reversible hydrogen bonding interactions.In the fourth chapter, the PVA-HDI-i are chemical cross-linked poly(vinyl alcohol) (PVA) by 1,6-diisocyanatohexane(HDI), and PVA-UPy-i are physical cross-linked PVA by 2(6-isocyanatohexylaminocarbonylamino)-6-methyl-4[1H]pyrimidinone(UPy) groups with quadruple-hydrogen bonding. The polymers PVA-HDI-5%, PVA-HDI-10%, PVA-UPy-10% and PVA-UPy-20% demonstrate thermo-induced and moisture-induced shape memory effects. The chemical cross-linked net points formed by HDI and physical cross-linked netpoint formed by UPy dimmer maintain the original shape, while the amorphous PVA segments as the switching to control the shape memory effects. PVA segments solidify to fix the temporary shape as cooling down to the glass transition temperature (Tg).As heating above the glass transition temperature, PVA segments melt to release the internal stress, inducing the shape recovery. Both the thermally and moisture induced shape memory effects are affected by Tg of PVA. Tg decreased lower the room temperature when cross-linked PVA immersing in water, resulting the shape recovery.