Small-Angle Neutron Scattering Study on Specific Polymerization Loci Induced by Copolymerization of Polymerizable Surfactant and Styrene during Miniemulsion Polymerization

We have investigated the origin of a specific polymerization loci that yielded copolymer particles with a bimodal gel permeation chromatographic profile in the early stage of miniemulsion polymerization of styrene (St) with the polymerizable surfactant N-n-dodecyl-N-2-methacryloyloxyethyl-N,N-dimethylammonium bromide (C(12)DMAEMA). The formation of particles by miniemulsion polymerization using C(12)DMAEMA was investigated as a function of the polymerization time (t(p)) and was compared with that using the nonpolymerizable surfactant cetyltrimethylammonium bromide (CTAB). St was miniemulsified with C(12)DMAEMA or CTAB and was polymerized with either a water-soluble initiator, 2,2'-azobis(2-amidinopropane) dihydrochloride (V50), or an Oil-soluble initiator, 2,2'-azobis(isobutyronitrile) (AIBN), at 60 degrees C in the presence of polystyrene (PSt) as a hydrophobe. Gel permeation chromatography (GPC) and elemental analysis indirectly predicted two different polymerization loci in the St/C(12)DMAEMA/V50 polymerization system. Time-resolved small-angle neutron scattering (SANS) was used to directly observe and examine the specific polymerization loci in the miniemulsion polymerization solutions. The droplets formed in the St/C(12)DMAEMA/V50/PSt at the initial stage of the polymerization were not fully stabilized by C(12)DMAEMA and poly(C(12)DMAEMA-ran-St) and were found to build up the large aggregates. The spherical droplets were stabilized later in the polymerization (t(p) > 20 min) by forming a homogeneous dispersion in the water phase. In contrast, the droplets stabilized by CTAB in St/CTAB/V50/PSt maintained their size and shape throughout the polymerization. As a result, in both polymerization systems of St/C(12)DMAEMA/V50/PSt and St/CTAB/V50/PSt, the particle size was found to depend strongly on the size of the droplets formed in the early stage of polymerization. Porod analysis of the power law scattering observed by SANS provided direct evidence for the specific polymerization loci, which appeared in the early stage of the polymerization system of St/C(12)DMAEMA/V50/PSt on the surface of the droplet, and was the origin of the bimodal peaks in the GPC chromatogram.