Study On The Structure And Properties Of Poly (N-isopropylacrylamide) Prepared By Inverse Emulsion Polymerization At Room Temperature

Poly（N-isopropylacrylamide）（PNIPAM）has always been the studying focus of temperature-responsive polymers because of its lower critical solution temperature（LCST）,which is close to the human body temperature.The Combination of branched polymers and temperature-responsive polymers endows PNIPAM with the special properties of branched polymers and offers more possibilities for their temperature-responsive behavior.However,the general synthesis of branched polymers needs to be carried out under strict condition while the product is still inclined to crosslink and to have low molecular weight.Here,the inverse emulsion polymerization was employed which suit for water-soluble monomers,such as NIPAM.A redox initiation system with a lower initiation temperature was used to avoid the precipitation of polymer during the whole reaction process.The inverse emulsion polymerization of branched PNIPAM at room temperature using a redox initiation system provides a simple and efficient method for the synthesis of high molecular weight temperature-responsive branched PNIPAM.Inverse emulsions are generally less stable than traditional emulsions,so a stable inverse emulsion system is the key to synthesize the polymers successfully.Benzene in the aromatic oil phase can interact with polar groups on the nonionic emulsifier,while the alkane oil phase basically does not affect other components.The stability of the two inverse emulsion systems under different conditions will be quite different.Sorbitan trioleate（Span 85）and polyoxyethylene sorbitan monooleate（Tween 80）were used as emulsifiers,and the liquid paraffin of alkanes and the anisole of aromatic were oil phases respectively.The effects of compound emulsifier HLB value,water-oil phase volume ratio,monomer concentration,temperature and electrolyte concentration on the stability of the two emulsions and the types of emulsions were compared by centrifugal coefficient method,electrical conductivity method and microscope observation method.Liquid paraffin was found more suitable for the oil phase of NIPAM inverse emulsion polymerization,and the formed inverse emulsion had good stability when the HLB of the compound emulsifier was 4.4,the temperature was 25℃,and the oil phase volume fraction was 80 vol%.In addition,using 25°C as the reaction temperature also satisfies the polymerization requirements.2-（dimethylamino）ethyl methacrylate（DMAEMA）containing tertiary amine and double bond was used as reducing agent monomer and form a redox initiation system with potassium persulfate（KPS）to initiate NIPAM through inverse emulsion polymerization at room temperature.A series of branched PNIPAM with different molecular weights and different branching degrees were obtained.Dimethylaminoethyl isobutyrate（DMAEi B）was designed and prepared as model reducing agent,a series of linear PNIPAM reference were obtained under the same conditions.The molecular weight(M_n.SEC,M_w.SEC and M_w.MALLS)and molecular weight distribution（（?））of the polymers were measured by gel permeation chromatography（GPC）,dynamic and static laser light scattering（DLS-SLS）respectively.The intrinsic viscosity（[η]）and branching factor（g’）were measured by ubbelohde viscometer.When the mole ratio of NIPAM,DMAEMA and KPS was 80:1:1,the monomer conversion was97.61%,the PNIPAM was found a high weight average molecular weight with M_w.MALLS=3757000 g/mol,and no crosslinking with 0.84 of g’.The degree of branching increased with the increase of the DMAEMA,but it did not change significantly when it exceeded KPS by 2 times.The increasing of emulsifier led to an increase in molecular weight,but the degree of branching was basically unchanged.The properties of both the linear and branched PNIPAM with high molecular weight prepared by the inverse emulsion polymerization carried out above were studied further in view of the important connection between the propperties of PNIPAM with its special structure.The microrheological propertie,low critical solution temperature（LCST）and temperature response behavior were measured by diffusing wave spectroscopy（DWS）,ultraviolet-visible spectrophotometer（UV-Vis）,dynamic light scattering（DLS）respectively.The solution viscosity and loss modulus of branched PNIPAM were found lower than those of the linear PNIPAM,which had lower molecular weight.The LSCT of branched PNIPAM（about 34.5℃）was higher than that of linear PNIPAM（about 33.7℃）,and the"hysteresis effect"was weaker.The temperature response process was further studied by dynamic light scattering comparing the changes of the particle size and the particle size distribution（PDI）of linear and branched PNIPAM with similar molecular weight under varying temperature.In the heating process,branched PNIPAM was found to have smaller particle size and larger volume of its polymer agglomerates.In the cooling process,the branched PNIPAM agglomerates deagglomeration faster.In this study,the branched PNIPAM synthesized with high molecular weight has lower solution viscosity and weaker"hysteresis effect"than linear PNIPAM,which means more advantages in the cyclic response process in the fields of biomedicine,sensor and etc.