Interdiffusion vs cross-linking rates in isobutoxyacrylamide-containing latex coatings

We describe the relative rates of polymer diffusion and cross-linking in a latex film containing 2 wt % (1.3 mol %) of N-isobutoxymethylacrylamide (IB MA) as a cross-linking agent. The latex base monomer is a 4:5 weight ratio copolymer of butyl acrylate and methyl methacrylate (BA-MMA) with a glass t ransition temperature of 12 degreesC. Polymer diffusion was monitored by di rect energy transfer (ET) in films prepared from latex particles labeled wi th phenanthrene as the donor and anthracene as the acceptor. In a model fil m without IBM-A, the quantum yield for ET (Phi (ET)) increased to its maxim um value of 0.62 in a few minutes at 120 degreesC, whereas at 80 degreesC t he films required hours of annealing to achieve full mixing. The temperatur e dependence of the diffusion rate indicated an effective activation energy of 43 kcal/mol. In contrast, a film prepared from latex of similar molecul ar weight containing 2 wt % IBMA formed gel rapidly at 80 degreesC in the p resence of 0.5 wt % toluenesulfonic acid. The system reached its maximum ge l content over 40 min. Diffusion was retarded over all annealing times, but after 20 min at 80 degreesC, when the gel content was 60%, polymer diffusi on ceased. If one uses a weaker acid, e.g., phosphoric acid instead of PTSA , the cross-linking rate is slower, whereas the polymer diffusion rate, pri or to extensive gel formation, is not very much affected.