Surface morphology of latex film formed from poly(butyl methacrylate) latex in the presence of alkali-soluble resin

摘要

The film morphology of emulsifier-free poly(n-butyl methacrylate) (PBMA) latex in the presence of post-added alkali-soluble resin (ASR) was studied using atomic force microscopy (AFM). The film morphology was monitored, from the nascent stage after water was evaporated from the dispersion, as a function of annealing temperature and its duration. The AFM results suggest strongly the formation of a hard shell of ASR, namely poly(styrene/alpha-methylstyrene/acrylic acid) (SAA) over the soft PBMA particle. During annealing, the incompatibility of the two polymers leads to the migration and accumulation of SAA at the interstices and interfacial regions among the arrays of PBMA particles, and also at the film surface. The hard shell of SAA retards the diffusion of the PBMA molecules across the particle/particle interface and slows down the gradual coalescence of film formation. Annealing at high temperature ruptures the SAA shells and the PBMA particles interdiffuse at a faster rate than the SAA due to its lower Tg compared to that of SAA. The difference in the rates of fusion of the SAA and PBMA phases leads to the formation of indentations on individual particles at high annealing temperature and long annealing duration. The occurrence of this feature increases with the annealing temperature and its duration. These indentations are eventually transformed into `depressions' on the latex films when the indentations at the top of the individual particles sag inwords at sufficiently high annealing temperature and long annealing time. The surface enrichment of SAA is confirmed by the results of fainer transform infrared spectroscopy with attenuated internal reflectance of the film surface.