Preparation And Properties Of Novel Poly (n-isopropylacrylamide) (pnipa) Based Copolymer And Interpenetrating Network (ipn) Hydrogels

In recent years,considerable research attention has been focused on the intelligent hydrogels due to their wide applications in the biotechnology and biomedical area,including drug delivery system, enzyme immobilization and biomaterials separation and purification,etc. Such intelligent polymers hydrogels exhibit swelling changes in response to the external stimuli,such as temperature,pH value,or electric field. Poly(N-isopropylacrylamide)(PNIPA)has been one of the most studied thermosensitive polymers which exhibits a volume phase transition temperature(VPTT)near 32℃.Conventional PNIPA hydrogels prepared using organic cross-linkers had some serious disadvantages,such as poor mechanical properties and low swelling/deswelling rates.Therefore, several routes,includeing copolymerization,blending,nanocomposite and interpenetrating polymer network(IPN)technology have been adopting to improve the mechanical properties and the responsive rate for PNIPA hydrogels.In this thesis,from the molecular design point of view,a series of novel P(NIPA-co-HEMA)/Clay nanocomposite hydrogels and PAA/ PNIPA/Clay IPN nanocomposite hydrogels were prepared with free-radical copolymerization and IPN route based on the thermosensitive N-isopropylacrylamide(NIPA)with hydrophilicity of the monomers, 2-hydroxyethyl methacrylate(HEMA)or acrylic acid(AA),using inorganic clay as a cross-linker.The structure and morphology,the pH and temperature responsive behaviors,the swelling and deswelling behaviors,and the mechanical properties of these novel hydrogels were investigated in detail.In addition,the synthesis mechanism of the hydrogels was preliminarily discovered.The main results obtained are as following;1A new kind of temperature-responsive P(NIPA-co-HEMA)/ Clay nanocomposite hydrogel was successfully prepared by in situ free-radical copolymerization based N-isopropylacrylamide with hydrophilic 2-hydroxyethyl methacrylate(HEMA),using inorganic clay as a cross-linker in place of the conventional organic cross-linker.It was found that the silicate layers were substantially exfoliated and homogeneously dispersed in the hydrogel,which cross-linked the polymer chains to form network structure.2P(NIPA-co-HEMA)/Clay nanocomposite hydrogel has good temperature-sensitivity and tensile properties,which are related to the contents of clay and HEMA.The experimental result indicated that the comprehensive properties of the hydrogels are more desirable when clay amount in the mass of NIPA monomer is 20%.The swelling ratios of PNIPA / Clay hydrogel were greatly enhanced by the addition of hydrophilic HEMA,meanwhile,the volume phase transition temperature(VPTT)of the hydrogels are also enhanced,but the VPTT of which was closed to that PNIPA / Clay hydrogel at lower content of HEMA.The elongation at break of the P(NIPA-co-HEMA)/Clay nanocomposite hydrogels was more than 1000%and its tensile strength was increased with the addition of HEMA.As the content of clay increased,the tensile strength of the hydrogels with same amount of the monomers also increased,while the elongation at break decreased.3The swelling and deswelling kinetics of P(NIPA-co-HEMA)/Clay nanocomposite hydrogels were studied.The result indicated that at 25℃(below the VPTT)and pH=7.4 the swelling ratio increased with increasing HEMA content in the hydrogel.The transport mechanism of P(NIPA-co-HEMA)/Clay hydrogel is Fickian diffusion at acid(pH 2.2) and weak alkaline(pH 7.4)conditions.The P(NIPA-co-HEMA)/Clay hydrogel has more rapid deswelling ratio,which was remarkably affected by clay amount used.The more amount of clay,the slower the deswelling rate.Furthermore,with addition of hydrophilic monomer(HEMA),the hydrophilic / hydrophobic balance in PNIPA / Clay hydrogel was changed,as a result,the swelling and deswelling rates were significantly improved.4A new kind of pH/temperature-responsive PAA/PNIPA/Clay nanocomposite hydrogel was successfully prepared by interpenetrating polymer network(IPN)method.The experimental results showed that the hydrogels this prepared exhibited pH- and temperature-sensitivity,with a VPTT around 32℃,which is independent of the amount of PAA in the hydrogel.Below this temperature,the hydrogels precede high swelling ratio while contract as the temperature is increased above the VPTT.PH value of the medium has great influence on swelling behavior of PNIPA / PAA /Clay.The swelling ratio of hydrogel is lower at acid condition, while it increases gradually at weak alkaline condition.These behaviors are mainly due to the ionization and protonation of the ion groups on the PAA chains which give raise to the changes of the swelling ratios of the hydrogels.5A preliminary study on the formation mechanism of the nanocomposite hydrogels showed that well separation and dispersion of the clay in the solution,as well as the interaction between clay layers and macromolecular chains is the key factors of forming nano-composite hydrogels.The result from DLLS analysis proved that particle size of the clay in different monomer solution either HEMA or NIPA monomer, only has little change,which was still uniformly distributed.The TEM observation for the P(NIPA-co-HEMA)/Clay nanocomposites hydrogel also showed that the clay layers were uniformly dispersed in the hydrogel and formed network with the P(NIPA-co-HEMA)matrix.However,in the PNIPA/PAA/Clay IPN hydrogels,the clay tended to be partly aggregated due to the addition of AA,which caused the hydrogels uneven, as a result,the performance of the hydrogels was deteriorated.