Synthesis And Characterization Of Novel Temperature-sensitive Poly(N-isopropy-lacrylamide) Based Microgels

The temperature sensitive microgels based on poly(N-isopropylacrylamide) (PNIPAM)have been paid much attention for their potential application in the field of drug controlled release,biomaterial separation and photonic crystal.The aim of many researches is mainly focus on the preparation of microgels with smaller hydrodynamic diameter(D_H),good monodispersity,high stability and clean surface.The research interest of this dissertation is concentrated on the preparation and characterization of novel temperature-sensitive microgels based on PNIPAM by adopting different cross-linkers,comonomers and polymerization processes.The structure,morphology and phase transition behavior were studied in detail by means of FTIR,NMR,SEM, TEM,DLS,UV/Vis and DSC.Furthermore,the microgel particles were severed as additives to prepare the PNIPAM hydrogels with fast response rate and better mechanical properties.The main results are as follows:1.A series of temperature-sensitive poly(NIPAM-co-tBA)microgels were prepared by surfactant-free emulsion polymerization(SFEP)with N,N'-methylene-bis-acrylamide(MBA)as chemical cross-linker,and tert-butyl acrylate(tBA)and NIPAM as functional monomers.The results indicated that the microgels thus prepared had good monodispersity,surface-clean,and colloidal stability.Their volume phase transition temperature(VPTT)was found to be around 33℃.As the molar ratio of MBA/NIPAM was increased from 5.65%to 22.58%,the sizes of microgel particles decreased initially and then increased,and the VPTT range became broader.As molar ratio tBA/NIPAM was increased from 12.90%to 38.71%,the colloidal particles tended to be arranged orderly,while the VPTT decreased from 33℃to 18℃,and the deswelling ratio decreased from 23 to 12,and the DH ranged from 800nm to 200nm.2.Adopting hectorite as a physical cross-linker and tBA as a strong hydrophobic monomer,poly(NIPAM/Clay/tBA)microgels were prepared by SFEP method.The results showed that exfoliated hectorite platelets could act as an effective multifunctional cross-linking agent through specific hectorite-polymer chain interaction,including hydrogen bod,ionic bond or coordinate bond,and the crosslinks uniformly distributed,having high crosslinking efficiency.The appearance of poly(NIPAM/clay)microgel dispersions after centrifugation presented light blue,and the D_H of the microgels was in a range from 150nm to 360nm.Volume phase transition occurred in a narrow temperature range.As the weight ratio of hectorite/NIPAM increased from 7%to 28%,the VPTT of poly(NIPAM/clay) microgel still remained around 32℃but the temperature sensitivity became weaker, and the DH increased after initial decrease while the deswelling ratio was on the contrary.In addition,with increasing tBA content,the monodispersity poly(NIPAM/tBA/clay)microgels was improved,but the temperature sensitivity became weaker,the D_H was initially decreased and then increased while the VPTT of the microgels was downwards.3.Semi-batch SFEP method was adopted to prepare PNIPAM and poly(NIPAM-co-AAc)microgels,which had low hydrodynamic diameter and high transparency,and narrow phase transition temperature but poor monodispersity.The particle size could be controlled by tuning feed rate.The DH values of poly(NIPAM-co-AAc)microgel dispersions thus prepared ranged from 60nm to 230nm,and decreased with increasing AAc content.The carboxyl groups of which mainly distributed in the interior of colloidal particles.As a comparision,the microgels of poly(NIPAM-co-AAc)prepared by traditional batch method,the carboxyl groups of which mainly distributed in the outer,the D_H values ranged from 350nm to 1700nm,which increased with increasing AAc content whereas decreased with raising temperature,and changed from high to low values with increasing pH values. prepare temperature sensitive PNIPAM microgels,which remained a VPTT around 32℃.The D_H of microgel particles thus prepared was ranged from 180nm to 308nm,and decreased with increasing temperature.The monodispersity of the colloidal particles was improved,which arranged in order but tended to aggregate each other.5.Two kinds of microgels based on PNIPAM and poly(NIPAM-co-AAc) prepared by SB-SFEP method were used as additives to prepare chemical cross-linked poly(NIPAM/MBA)and physical cross-linked poly(NIPAM/Clay)hydrogels. Compared to a normal PNIAPM hydrogel.The microgel-impregnated PNIPAM hydrogels exhibit fast response rate and good mechanical properties whereas the VPTTs of which still remained around 33℃.The introduction of the microgel particles increased the pore sizes of poly(NIPAM/MBA)hydrogels with reducing pore numbers.The efficiency of improving responsive properties by incorporating poly(NIPAM-co-AAc)microgel is much better than that of pure PN1PAM microgels. The smaller the DH and more ionic groups containg in the microgels,the higher in the swelling rates.In addition,the hydrogels thus prepared exhibited good swelling-deswelling reversibility under the stimulus of temperature and improved tensile strength of the poly(NIPAM/clay)hydrogels.