Stimuli-responsive Physical Hydrogels Based On Acrylic Acid/Stearyl Acrylate Amphiphilic Copolymers

Hydrogel is a class of hydrophilic and water-swollen three dimensional polymer materials.Depending on the type of cross-linking,hydrogels can be divided into chemical and physical hydrogels.Physical hydrogels contain the reversible noncovalent interactions and are generally responsive to external stimuli;yet they usually have poor mechanical properties.To overcome this problem,many dynamic and noncovalent bonds such as hydrophobic interactions,hydrogen bonds and metal coordination bonds have been incorporated into the physical gels as the additional crosslinking points to develop the tough physical gels.The long alkyl chains can crystallize in water,which can be used to prepare the tough polymer hydrogels.2-Ureido-4[1H]-pyrimidone?UPy?is a well-studied supramolecular building block that forms self-complementary quadruple hydrogen bonding interaction in dimerization.Because the UPy hydrogen bonding interactions are very strong in hydrophobic environment,the tough hydrogel can be prepared via the cooperative effects of hydrogen bonding and hydrophobic interactions.Because of the dynamic nature of hydrophobic interactions and hydrogen bonds,these hydrogels would have good processability,shape-memory effect,and self-healing ability.In this thesis,a series of tough poly?stearyl acrylate-co-acrylic acid?[P?SA-co-AA?]physical hydrogels by simply casting and re-swelling the films of copolymers synthesized by the copolymerization of hydrophilic acrylic acid?AA?with a small fraction of hydrophobic stearyl acrylate?SA?.The long alkyl chains of SA units segregate in water to form hydrophobic associations,serving as the physical crosslinking junctions of gels.The gels are robust with tensile strength up to?2 MPa;the mechanical properties can be tuned by varying the composition of copolymers.As indicated by the differential scanning calorimeter?DSC?results,the formation of well-organized hydrophobic domains with increasing the molar ratio of SA.As showed by the wide-angle X-ray diffraction?WAXD?and small-angle X-ray scattering?SAXS?results,the long alkyl side chains align tail-to-tail perpendicular to the main chain of hydrogels.The gels are responsive to temperature,pH,and ethanol solvent,which mediate the stability of the hydrophobic associations and even lead to gel-to-sol transition.Based on this reversible transition,the gels show stimuli-mediated healing ability.Hydrogels with complex structures can be constructed by localized welding.In addition,the gels exhibit shape memory behavior.The temporary shape can be locked by forming strong coordination between carboxylic acid groups and relaxed by reduction of the Fe³⁺ ions in 10 wt%sodium citrate.To enhance the mechanical properties of hydrogels,the P?SA-co-AA-co-UPy?hydrogels were further prepared by one pot polymerization of 2-ureido-4[1H]-pyrimidone methacrylate?UPyMA?,SA and AA.UPy groups can form quadruple hydrogen bonds in dimerization,which act as the second physical crosslinking points in these gels.The water content of hydrogel was larger than 50%.As showed by the DSC results,there was no melting peak in the DSC heating curves when the molar ratio of SA was less than 2%.The hydrogen bonds between of UPy groups maintained the hydrophobic domains and enhanced the crystallizability of SA side chain.Due to the strong physical crosslinking stemming from the highly stable quadruple hydrogen bonds formed between UPy units,the mechanical properties of the hydrogels enhanced with an elongation at break up to 700%.As showed by the WAXD and SAXS results,the UPy hydrogen bonds promoted the long alkyl chains to form more stable crystalline domains.Furthermore,the reversible metal coordination bonds between Ca²⁺,Fe³⁺ and carboxylic acid groups imparted dual shape memory properties to the hydrogels.Moreover,the hydrogels showed temperature responsive shape memory in Ca²⁺ solution.