Structure effect of phosphite on the chain extension in PLA

PLA is synthesized from natural resource and can degrade easily, so it is a reasonable substituent of petroleum base plastic. Phosphites can increase the stability of PLA through the chain extension with PLA. In this paper, the molecular weights, complex viscosities and storage modulus of virgin PLA and PLA stabilized by different phosphites were characterized by gel permeation chromatography and rheometer. The results show that when there is a bigger substituent around the P-O bound in the phosphite molecule, the contact between the PLA and phosphite chain extender is inhibited and the chain extension becomes weak. In point of the three phosphite chain extenders of bis-2,2′-methyl-4,6-di-tert-butylphenyl phosphite (M46TBPP), tris(2,4-di-tert-butylphenyl) phosphite (Irgafos 168) and triphenylphosphite (TPP), the hindrance order around P-O bound is M46TBPP > Irgafos168 > TPP, and the hindrance of TPP is the smallest. So TPP takes the most effective chain extension in PLA. Furthermore, the by-product phenol formed due to chain extension of TPP has the weakest hydrogen donating ability, which makes PLA degrade weakest. So the average molecular weight values, complex viscosities and storage modulus of PLA stabilized by TPP are the biggest. In addition, the product, which was formed due to the chain extension of PLA and TPP, has some plasticization, so it makes PLA stabilized by TPP move free, crystallize easy. At last, compared to virgin PLA, PLA-TPP represents better mechanical properties.