Study On The Assembly Of Temperature-sensitive Microgels In Ice Crystal Template To Form Fibers

Temperature-sensitive fiber is a kind of smart fiber,which can respond to environmental temperature change and undergo volume phase transition, simultaneously going with the changes of both its diameter and its containing water percentage,hydrophilicity-hydrophobicity and refractive index.The response speed of temperature-sensitive fiber is obviously faster than temperature-sensitive block hydrogel,because of its smaller diameter and larger specific surface area. Temperature-sensitive fiber has potential applications in biomedical field,such as tissue engineering scaffold,protein separation material,enzyme immobilizing carrier, drug controlled release carrier,etc.In the thesis,poly(N-isopropylacrylamide) (PNIPAM)fibers,SiO₂/PNIPAM composite fibers and discontinuously hollow PNIPAM fibers were prepared by the assembly of temperature-sensitive PNIPAM microgels,SiO₂/PNIPAM core/shell composite microgels,hollow PNIPAM microgels in the ice-crystal template by means of unidirectional liquid nitrogen freezing method, respectively.The ice-templating method is greatly biocompatible,economical and environment-friendly,due to nonuse of any special spinning equipment,organic solvent,and additive,and no by-product,so the formed temperature-sensitive fiber will be more suitable to be used in biomedicine field.The main research works and results obtained are as follows:(1)Monodisperse temperature-sensitive PNIPAM microgels were firstly prepared by emulsion polymerization using N-isopropylacrylamide(NIPAM)as monomer, N,N'-methylene-bis-acrylamide(MBA)as crosslinker.The aqueous PNIPAM microgel dispersion of 1wt%concentration was frozen by refrigeratory and liquid nitrogen,respectively,and the results showed that the fibers were produced by assembly of PNIPAM microgels only in the ice crystal template formed by unidirectional liquid nitrogen freezing method.Field emission scanning electron microscopy(FE-SEM)images obtained under low accelerate voltage indicated that the fibers are composed of hexagonally packed PNIPAM microgels.The fiber diameter was lowered with the decrease of the particle size of PNIPAM microgels, and increased with the increase of the microgel dispersion concentration.(2) The colloidal silica particles synthesized by sol-gel process were modified by 3-(trimethoxysilyl)propyl methacrylate(MPS)to graft vinyl groups on their surfaces for radical polymerization.Then,the monodisperse SiO₂/PNIPAM core-shell composite microgels were prepared by seed-emulsion polymerization using modified colloidal silica particles as seeds,NIPAM as monomer,MBA as crosslinker.Finally, SiO₂/PNIPAM composite fibers were obtained by the assembly of the core-shell composite microgels in the ice crystal templates formed by unidirectional liquid nitrogen freezing method.FE-SEM images showed that the composite fiber surfaces were comprised of orderly packed core/shell microgels,which were more protuberant than those on the above-mentioned PNIPAM fiber surfaces.(3) Hollow PNIPAM microgels were produced by removing the silica cores of the above-mentioned SiO₂/PNIPAM core-shell composite microgels through chemical etching with hydrofluoric acid(HF).The characterization results of FTIR,TGA,TEM, SEM confirmed that the SiO₂ cores inside the core/shell composite microgels were successfully removed.The hollow PNIPAM microgels were assembled to produce the discontinuously hollow fibers in the ice crystal templates formed by unidirectional liquid nitrogen freezing method.Compared with the above-mentioned PNIPAM fibers and SiO₂/PNIPAM composite fibers,no microgel particles but many orderly arrayed dents were observed on the surface of the hollow PNIPAM fibers by FE-SEM.