聚(甲基丙烯酸N,N-二甲氨基乙酯)的温度和pH敏感性及其对乳液稳定性的影响

甲基丙烯酸N,N-二甲氨基乙酯的均聚物(PDMAEMA)在水中的溶解性具有温敏性,即低温溶解、高温不溶,而且其低临界溶液温度(LCST)与pH密切相关.本文重点考察了PDMAEMA水溶液在不同温度、pH值、溶液离子强度时的相转变特性,并研究了水溶液中乳化剂对PDMAEMA的疏水相互作用和增溶稳定作用.将PDMAEMA的温敏相转变行为同有关乳液的稳定性相关联,揭示了改善乳液稳定性的内在机制.

TEMPERATURE AND pH SENSITIVITIES OF POLY(N,N-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 pH<8.2.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.