Preliminary Study On Preparation Of Polyimide Porous Microspheres By Grafted Methoxyplyethylene Glycols

Polyimide(PI)porous microspheres,combining both unique physicochemical properties such as excellent thermal,strong mechanical properties,and low electrical properties of the PI and the advantages of porous structures such as large specific surface,huge volume of storage,low density and feasible functionalization,have attracted more attention to catalyst carriers,drug storage,delivery carrier and release,microcages and chromatographic carriers and so on.Also it has made some improvement on preparative technique and mechanism of porming pores.So it is important and necessary to study highly temperature PI porous microspheres.But the literature shows that the pore structure is not obvious and uniform,also the solid content is low when the linear oligomer is as porogen.To overcome the shortcomings of these methods mentioned above,this research,combining previous theory of non-aqueous emulsion system,produce chemical bond between linear oligomer and PI through grafting Methoxypolyethylene glycols(mPEG)side chains.The PI porous microspheres are formed after thermal decomposition of grafted side chains by thermal treatment.PI porous microspheres were obtained having higher thermal stability、fine sphere and good dispersivity.The main contents and conclusions of this paper are as follows:(1)the synthesis of graft monomerThe graft monomer was synthesized by esterification of 3,5-diaminobenzoic acid(3,5-DBA)with mPEG(350)or mPEG(750)respectively in the presence of catalyst.The structure of product was confirmed by FT-IR and 1H-NMR.The different ratio of reactant,the content of catalyst,water carrying agent and reaction time will affect the esterification rate.Through a lot of experiments,the range of all influencing factor were ensure.Finally the optimal condition through orthogonal experiment was obtained:the n(acid)/n(alcohol)is 1.3:1,the weight mass ratio of catalyst,water carrying agent and reaction time are 5%,125%(to mPEG)and 7h respectively.As the molecular weight of the mPEG increases from mPEG(350)to mPEG(750)the esterification rate is found to decrease.(2)Preparation of graft copolymerIn the non-aqueous emulsion system,the content of graft monomer from 20%to 100%reacted with different diamine and dihydrant respectively to complete polycondensation.Two-step imidization was subsequently performed to convert graft copolymer PAA-g-mPEG to PI-g-mPEG.The result product was characterized by FT-IF,SEM,TGA and DSC.When the content of graft monomer was 20%-40%,the morphology and monodispersity of graft copolymer were fine.The grafting ratio was determined by gravimetric method.As an increasing of concentration of graft monomer,the ratio of grafting was creasing and the thermostability was decresing.(3)Preliminary synthesis and discussion on PI porous microspheresThe relationship between temperature and time of thermal treatment and thermal decomposition grafted chain was confirmed by FT-IR.The PI porous microspheres with molecular-scale and nanoscale structure were obtained when the graft chain was completely pyrolysis,and had high thermal property.And with the increasing of grafting ratio,the specific surface area was increasing.On the basis,the mechanism of forming porous microspheres was provided.