TEMPERATURE AND pH SENSITIVITIES OF POLY(<I>N</I>,<I>N</I>-DIMETHYLAMINOETHYL METHACRYLATE) AND THEIR EFFECTS ON LATEX STABILITY

Amine-containing acrylate latex has been widely used in fabricating water-borne adhesives and coatings because of its good desirable performances.Such latex is also a fundamental component for making ambient self-curable latex compositions.During emulsion copolymerization amine-containing monomer can polymerize in water phase due to its high water solubility,and the polymer generated dissolves in water phase.This phenomenon is believed to have strong effect on the stability of latex colloids during polymerization stage and shelf stage.N,N-Dimethylaminoethyl methacrylate(DMAEMA) is a typical tertiary amino monomer employed in preparation of amine-containing latex.Its homopolymer(PDMAEMA) is water soluble and sensitive to temperature and pH.In the present study,phase transition behavior and colloid formation of PDMAEMA(M_w=8.56×10~4) in aqueous solution were investigated by dynamic light scattering at different temperatures,pHs and ionic strength.The effect of emulsifier on the thermo-induced phase transition of PDMAEMA was also studied.It was found that:(1) 1% PDMAEMA aqueous solution showed no phase transition on heating at pH8.2.The lower critical solution temperature(LCST) of the solution decreased sharply from 67℃ to 42.5℃ as pH increased from 8.2 to 9,and decreased slowly to 40℃ when pH was further increased to 11;(2) A higher ionic strength(NaCl concentration) in the solution resulted in a lower LCST;(3) The presence of emulsifier,sodium dodecyl sulfate at the concentration of 1.92%,in PDMAEMA aqueous solution(pH 8.8) could keep the polymer soluble even on heating at 70℃(no phase transition occurred).Moreover,the relationship between the formation of homo-PDMAEMA or DMAEMA-rich copolymer in aqueous phase during emulsion polymerization and the stability of the latex were also investigated.The results obtained suggest that the stability of DMAEMA-containing polymer latex can be improved by selecting a proper emulsifier,designing optimized formulation and polymerization conditions.