Generally, an increase in molecular weight results in a decline
of polymer degradability by microorganisms. In contrast, monomers,
dimers, and oligomers of a polymer's repeating units are
much easily degraded and mineralized. High molecular weights
result in a sharp decrease in solubility making them unfavorable
for microbial attack because bacteria require the substrate to be
assimilated through the cellular membrane and then further
degraded by cellular enzymes. At least two categories of
enzymes are actively involved in biological degradation of
polymers: extracellular and intracellular depolymerases (Doi
1990; Gu et al. 2000). In biodegradable mulch films, the crosslinked
structures limit the segmental mobility of the plastic
molecules as well as the accessibility of the water and microbes
to the polymer chain. Consequently, under such conditions, the
biodegradation process is inhibited after the film is plowed into
the soil (Kijchavengkul et al. 2008a).