Preparation of mixed esters of starch or use of an external plasticizer: two different ways to change the properties of starch acetate films

In view of the brittleness of acetylated starch-based materials, we investigated the possibility of obtaining internal plasticization of such products by preparing mixed esters of starch with both acetate groups and fatty-acid ester chains. We prepared such compounds and determined their compositions by use of quantitative solid-state NMR. The method detailed in this work is not usually used in the area of modified polysaccharides. The presence of fatty-acid esters is shown to decrease the glass transition temperature. External plasticization of starch acetate by triacetin is also studied. A wide range of polymer-plasticizer compositions was investigated, and the variation of glass transition temperature is found to follow a Couchman law. The mechanical behaviour of mixed esters is also investigated. The introduction of fatty-acid esters is shown to depress the glass transition temperature and the alpha relaxation associated with the glass transition, corresponding to a plasticizing effect. The introduction of these substituents is shown to suppress the beta relaxation, corresponding to a local antiplasticizing effect. However, the strength modulus of mixed esters remains almost unchanged compared to that of starch acetate, and the materials remain brittle in spite of internal plasticization. This is due to a decrease of glass transition temperature, which is not sufficient to produce a soft material. In contrast, external plasticization resulted in an increased flexibility, when sufficient amounts of plasticizer were used. (C) 1998 Elsevier Science Limited. All rights reserved.