Abstract
Gelation of poly(ethylene terephthalate) by heating at 263°–300°C was investigated. Under nitrogen flow, crosslinks were scarcely formed. However in air, degradation and crosslinking were common, and these were accelerated by purging gaseous and sublimable degradation products out of the system with a stream of air. The main component of the sublimate was terephthalic acid. Infusible and insoluble gel was treated with methanol at 260°C, and then the methanolysis products were separated into two parts. The methanol‐insoluble part exhibited a polyene structure with ester groups, and the methanol‐soluble part contained dimethyl terephthalate, ethylene glycol, and some 1,2,4‐butanetriol. To clarify the relation between the crosslinking and the formation of vinyl ester groups, the degradation of vinyl methyl terephthalate was studied. Thermoxidative degradation of linear polyesters other than poly(ethylene terephthalate) was also studied. Poly(ethylene isophthalate) and poly(ethylene sebacate) were easily gelated. However, poly(trimethylene terephthalate) and poly(neopentyl terephthalate) were scarcely gelated. The primary reaction leading to crosslinking is assumed as follows. At first, the random scission of polyester chain may take place forming carboxylic acids, vinyl esters, aldehydes, etc. After accumulation of vinyl esters to some extent, vinyl polymerization of the esters takes place and network structures are formed.