Effect of Polymer Composition on Polymer Diffusion in Poly(butyl acrylate-co-methyl methacrylate) Latex Films

We describe polymer diffusion measurements in poly(butyl acrylate-co-methyl methacrylate) [P(BA−MMA)] copolymer latex films by fluorescence resonance energy transfer (FRET). Four sets of copolymers were prepared from various weight ratios of butyl acrylate and methyl methacrylate by semicontinuous emulsion polymerization. Their glass transition temperatures range from 4 to 28 °C. Latex particles were labeled with phenanthrene (Phe) as the donor dye and with 4-(N,N-dimethylamino)benzophenone (NBen) as the acceptor dye. Latex films were cast from a 1:1 mixture of Phe- and NBen-labeled latex samples. Polymer diffusion was monitored as a function of annealing temperature, and apparent diffusion coefficients (D app) were calculated from the energy transfer data using a simple diffusion model. These values increased with annealing temperature and decreased with T g. Rheology measurements recorded the response of the dynamic moduli (G‘, G‘ ‘) with respect to oscillatory shear frequency (ω) over a range of temperature close to that of the diffusion experiments. The temperature dependence of polymer dynamics extracted by the rheology experiments is in good agreement with the temperature dependence of D app. Increasing the BA copolymer content leads to an apparent increase in long-chain branching, which is reflected in both the time dependence of D app and in the dynamic moduli measurements. A greater degree of branching leads to a broader distribution of polymer diffusion coefficients and a stronger time dependence of D app.