MICROSTRUCTURAL CHARACTERIZATION OF METHYL METHACRYLATE AND n-BUTYL ACRYLATE GRADIENT LATEX PARTICLES BY NUCLEAR MAGNETIC RESONANCE

Methyl methacrylate (MMA) and n-butyl acrylate (n-BA) gradient latex particles with a controlled copolymer chain arrangement containing internal soft segments changing gradually to external hard ones (n-BA-centered) as well as internal hard and external soft particles (MMA-centered) were synthesized by gradient emulsion copolymerization, involving continuous addition of one monomer mixture into a stirred tank containing another mixture. Sodium dodecyl diphenyloxide disulfonate (DSB) was used as emulsifier and potassium persulfate (KPS) as initiator. The chemical composition and distribution of compositional triads of the copolymer formed in the resulting gradient latex particles were determined from H-1 NMR spectra by using the -OCH3 signal of MMA and -OCH2-signal of n-BA repeating units. The experimental results show that when the mole ratio of MMA in copolymer (F-M) is lower than 50%, the -OCH3 proton signal is sensitive to the sequences of MMA-centered gradient latex particles. Thus, for n-BA-centered gradient latex particles, compositional triads were distinguished from the -OCH3 proton signal, but for MMA-centered gradient latex particles, configurational-compositional triads were determined from the carbonyl signal in the C-13 NMR spectrum. The configurational effect observed for the carbonyl signal was introduced only by the MMA units and not by the n-BA units. The H-1 NMR results show that the dominant unit in the molecular chain gradually changes from n-BA to MMA with the reaction proceeding for the n-BA-centered gradient copolymer latex particles, but the trend of MMA-centered gradient latex particles is the opposite. The C-13 NMR results indicate that the dominant triad structure changes from MMM to BBB triad for MMA-centered copolymer latex particles.