Starch percolation

As time elapses, the microbes access the starch percolation pathways and invade deeper into the material. The microbes increase in number by scission and growth and are largely resident on the invasion front. Unaccessed areas of starch are left behind as islands which are not connected to the percolation pathway. The vacated starch sites are occupied by either microbes or water. Some of the microbes die and can be seen near the invasion front. Since dead microbes are used as food by the other microbes and reside on the connected pathways, relatively few dead microbes exist in this phase. This phase accounts for the majority of the degradation of the blend. 

Wool and Cole (6) described a simulation model based on percolation theory for predicting accessibility of starch in LDPE to microbial attack and acid hydrolysis. This model predicted a percolation threshold at 30% (v/v) starch irrespective of component geometry, but the predicted values are not in accordance with results of enzymatic or microbial attack on these materials (Cole, M.A., unpublished data). Since a model that incorporates component geometry provides a better fit to experimental data than a geometry-independent model does, development of advanced models should be based on material geometry and composition, rather than on composition alone. 

Porosity characteristics also influence the degradation rate of blends containing intact starch grains. Amylase removal of starch from these films was not highly correlated with starch content, since films whose starch content was above possible "percolation thresholds" (6) were degraded at very different rates when starch content was not very different (Table I). 

FIGURE 20.5 Cumulative particle size distribution pt= 0.62 for different ratios of the binary mixture lactose (L)/corn starch (MS). The granule diameter is critically linked to the concentration ratio (percolation effect ). (From Leuenberger, H., Usteri, M., Imanidis, G, and Winzap38ll,. Chem. Farrn 128, 54-61... 

Peanasky, J.S. Percolation Effects in Degradable Polyethylene/Starch Blends,... 

Starch nanocrystals were used to reinforce a non-vulcanised NR matrix. The NR was not vulcanised to enhance biodegradability of the total biocomposite. Non-linear dynamic mechanical experiments demonstrated a strong reinforcement by starch nanocrystals, with the presence of Mullins and Payne effects. The Payne effect was able to be predicted using a filler-filler model (Kraus model) and a matrix-filler model (Maier and Goritz model). The Maier and Goritz model showed that adsorption-desorption of NR onto the starch surface contributed the non-linear viscoelasticity. The Kraus model confirmed presence of a percolation network. ... 

A. Dufresne and J-Y. Cavaille, Clustering and percolation effects in microcrystalline starch-reinforced thermoplastic. J. Polym. Sci. Part B-Polym. Phys. 36,2211-2224 (1998). 

Peanasky, Long and Wool applied percolation theory to analyse the degradation of starch in polymer-starch blends [11]. They found that in 3rf, the equilibrium accessible fraction of starch AJj) ), was determined by... 

Figure 7.2 Microbial percolation into starch blends (P = 0 31, r = 0.41) at different stages (continued overleaf).

Dynamic degradation of a polymer-starch blend is defined as the time-dependent accessibility of starch by micro-organisms A t) [13]. In this chapter, the percolation theory is extended to investigate the time dependence of starch removal in polymer-starch blends as a function of starch concentration p, the invasion mechanism, enzyme diffusion, microbial population, and starch size distribution. In this case, the time dependence of the accessibility on the fractal pathways representing the connected starch network with emphasis on both invasion and diffusion controlled mechanisms is explored. 

As the degradation nears completion, the microbes eventually find less food, die and decrease in number. Thus, during the course of the invasion the microbes increase in number to a maximum and then decrease. It will be seen later that this observation is in agreement with experimental results. When the invasion is complete, the accessed starch fraction is determined from percolation (equation 7.3) with / = 0.61 and = 0.5928 as 66%, while the unaccessed fraction remains as clusters embedded in the polymer matrix. 

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