Polymer-related Parameters Determining Biodegradation

While a number of aliphatic components which alter the biodegradation behaviour of aromatic polyesters have been tested, the aromatic component predominantly used was terephthalic acid. The materials commercially available on the market also contain this aromatic dicarboxylic acid. 

The degradation behaviour of aliphatic-aromatic copolyesters generally depends on the monomer composition, as well as on the structure of the polymer chains at a given composition. 

This phenomenon is attributed to the melting point of the material - a correlation already demonstrated by Tokiwa and co-workers for different aliphatic polyesters [79]. In aliphatic-aromatic copolyesters, the melting behaviour is mainly determined by the length of the aromatic sequences in the polymer chains, which depends both on the composition and structure [86, 87]. For many aliphatic polyesters, a correlation of the degradability with the melting point was observed [2]. Marten [86] interpreted it as a decrease in the mobility of the polyester chains at lower temperatures in this situation the polymer chains are highly fixed in the polymer crystals and cannot adjust easily into the active sites of the extracellular enzymes. A random insertion of some aromatic monomers in aliphatic polymer chains disturbs the formation of crystals. 

While the reduced enzymatic susceptibility of the aromatic ester bonds is caused by the interaction of the long chains in the polymer, the chain length itself has no direct influence on the biodegradability above a minimum molar mass. Tests with copolyesters from terephthalic acid, adipic acid and 1,4-butanediol, which were chain extended with hexamethylene-diisocyanate, did not result in a decrease in the biological degradation rate (in a compost simulation test), although the molar mass of the prepolyesters of about 48,000 D was increased to 232,000 D by the chain extension [85]. 

Polyester initial molecular weight (g/mol) melting point 

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