An approach to high performance zero VOC waterborne coatings, polymer interdiffusion vs. crosslinking in carboxylic acid-carbodiimide latex films

grantor: University of TorontoThis thesis examines the competition between polymer interdiffusion and crosslinking reaction in latex films. These latex films were prepared from a blend of two similar soft latexes based on poly(2-ethylhexyl methacrylate) (PEHMA). Polymers of the first latex contain a small amount of carbodiimide (-N=C=N-) groups plus 1 mol% 9-anthryl methacrylate (An-MA, fluorescent acceptor), and polymers of the second latex contain a small amount of carboxylic acid (-COOH) groups plus 1 mol% 9-phenanthylmethyl methacrylate (PheMMA; fluorescent donor). The -N=C=N- group reacts with the -COOH group to form an N-acyl urea, and the Phe donor, when excited, can transfer its energy to the An acceptor. In the initially formed film, these latex particles are adjacent to each other, but the reactive groups and the fluorescent molecules are still confined within their own cells. Polymer interdiffusion will bring the reactive groups into close proximity for them to react, and will bring the fluorescent chromophores close enough for energy transfer (ET) to occur. We use this energy transfer to follow polymer interdiffusion, and at the same time use Fourier transform infrared (FTIR) to monitor the extent of the chemical reaction. We find that polymer interdiffusion is necessary for the reaction to proceed, but the reaction retards the rate of polymer diffusion. Once the reaction occurs, long branches and high molecular weight polymers are formed. The diffusion rate of the branched polymers is much slower than that of the original linear polymers. Because the reaction is coupled to diffusion, the reaction rate also drops. We also found that polymers containing -COOH groups and polymers containing -N=C=N- groups have limited miscibility, but the crosslinking reaction promotes mixing between the two polymers. We also find that rapid polymer interdiffusion at the early stages is important for films to develop to 100% of gel formation. Factors that promote formation of branched polymers at the inter-particle interface, e.g. a decrease in annealing temperature, or an increase in the reactive comonomers, or an increase in the reactivity of the reaction, lead to a decrease in polymer interdiffusion. Here films with less than 100% gel content are obtained. We also find that these films exhibit the maximum toughness at early stages of annealing. The toughness decreases as the extent of both crosslinking and polymer diffusion increases. The modulus increases, while the extension-at-break decreases as the extent of both crosslinking and polymer diffusion increases.Ph.D