Biodegradation dynamics of polymer–starch composites

The dynamics of starch biodegradation in polyethylene–starch (PE–S) composites was investigated by aerobic biodegradation methods and computer simulations, with the starch fraction p above and below the percolation threshold pc. Two models for starch degradation were considered: (i) microbial invasion through the composite and (ii) macromolecular (enzyme) diffusion which results in the back-diffusion of small molecules to the surface for further assimilation by microorganisms. The microbial-invasion model was based on scanning electron microscopy (SEM) studies of PE–S composites that contained a 1–15-micron distribution of starch particles. Following exposure to soil test conditions, micrographs of thin films clearly showed the colonization of microorganisms within channels of the matrix that were initially occupied by starch. The enzymatic diffusion was based on hydrolytic experiments of PE–S composites. Following exposure of a composite to a hydrolytic test condition, small molecules were produced. The starch accessed by microbes and enzymes was computed by simulating degradation of a monodisperse and polydisperse (starch grains of 1–10-micron diameter) composite. Aerobic degradation studies in a biometer indicate that the starch accessibility. A follows a power-law dependence with time A ∼ tn, where the exponent n depends on the fractal dimension of the accessed starch clusters and pathways and approaches unity when p > pc. Microbial invasion simulations indicate that the average power-law exponent near pc is approximately 0.5 and approaches 1.0 at p > pc, whereas the enzymatic diffusion simulations indicate that the average power-law exponent near pc is about 0.25 and approaches 0.5 at p > pc. The observed exponent for the aerobic degradation study suggests that for composites with a starch fraction less than and greater than pc the starch is predominantly accessed by microbial invasion. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1643–1657, 2000