Preparation And Properties Of Amphiphilic Copolymer Hydrogels Reinforced By Crystallization Of Side Chains

Hydrogels,which are composed of hydrophilic group and absorb a large amount of water without dissolving,are novel type polymer materials.Hydrogels are divided into physicial hydrogels and chemical hydrogels according to the different crosslinking methods.Physical hydrogels are generally crosslinked by non-covalent bonds,which often have good self-healing and environmental responsiveness properties.However,most of the physical hydrogels have poor mechanical properties because of the weaker non-covalent interactions.Traditional chemical hydrogels are crosslinked by chemical bonds and generally have good mechanical properties.But there are few crosslinking sites,low crosslinking number density in hydrogel network because of the less chemical crosslinking agents.These defects limit the practical application of hydrogels.Crystallization has been a characteristic nature of polymer segments bearing high structural symmetry and tacticity.Because the polymer chains assemble in a more ordered and compact manner in the crystalline phase than those in the amorphous phase,crystallization of hydrophobic chains is effective to improve the number density and strength of crosslinking junctions in the hydrophobic interaction-induced physical hydrogels.In the first chapter,2-hydroxyethyl methacrylate(HEMA)terminated poly(L-lactic acid)and poly(D-lactic acid)(PLLA and PDLA)macromonomers were firstly synthesized by ring-opening polymerization with the presence of HEMA initiator.Subsequently,the copolymers poly(acrylic acid)-g-poly(L-lactic acid)(PAA-g-PLLA)and PAA-g-PDLA were copolymerized with acrylic acid(AA)and aforementioned macromonomers by free radical copolymerization.Hydrogels were prepared by simply casting and reswelling the films of copolymers.The PLLA and PDLA side chains of these amphiphilic copolymers segregate in water to form stereocomplexed or crystallized hydrophobic associations,serving as physical crosslinking junctions for hydrogels.The crystallinity and content of stereocomplex(SC)crystallization in the hydrogels can be well tailored by varying the mixing ratio of the PAA-g-PLLA/PAA-g-PDLA enantiomeric pair;this enables good control over the crosslinking density,microstructure,and mechanical properties of the obtained hydrogels.The mechanical properties of hydrogels are also tuned by varying the copolymer composition.Tensile strength and Young's modulus of the hydrogels increase,while their water content and elongation at break decrease,as the homocrystallization(HC)and SC content increase.As demonstrated by wide angle X-ray diffraction(WAXD)and small angel X-ray scattering(SAXS)analyses,the average distance between physical crosslinking junctions decreases and the number density of physical crosslinkers increases with an increase in HC and SC content.In addition,the hydrogels exhibit stereocomplexation dependent shape memory behaviors;the shape can be memorized by forming strong complexation between carboxylic acid groups and Fe3+ ions and relaxed by reduction of Fe3+ ions.In the second chapter,a series of poly(N-isopropylacrylamide-co-stearyl acrylate)(P(NIPAM-co-SA))copolymers with different compositions were synthesized by the free radical polymerization with the presence of 2,2'-azobis(isobutyronitrile)initiator and N,N'-methylene bisacrylamide crosslinking agent.The long hydrophobic alkyl chains of SA segregate in water to form hydrophobic associations,serving as the physical crosslinking junctions and energy dissipation network of hydrogels.The swelling ratio,transmittance,LCST and mechanical properties of hydrogels are tuned by varying the content of SA.Tensile strength and Young's modulus of the hydrogels increase,while elongation at break firstly increases and then decreases,as the SA content increases.The water content and LCST decrease with the increase of SA content.As demonstrated by the differential scanning calorimeter,the crystallization and melting temperatures of dry gels gradually increase with the increase of SA content.As indicated by WAXD and SAXS results,the long alkyl side chains align tail-to-tail perpendicular to the main chain of hydrogels and the thickness of tail-to-tail crystallization phase becomes thinner with the increase of SA content.The thickness of SA crystallization phase increases and water diffuses into SA melted phase with the temperature rising.According to the decrease of LCST of hydrogels in salt solution,the shape memory effect controlled by SA crystallization was designed.