Structure-Property Relationships in Polytetramethylene Terephthalate/Polytetramethylene Isophthalate Copolyesters Part I : Glass-Transition Temperatures

The glass-transition temperatures (T g ) of a series of polytetramethylene terephthalate/polytetramethylene isophthalate, copolyesters have been measured by a variety of methods including differential thermal analysis, dilatometry, and broad- line nuclear magnetic resonance. The values measured for T g are discussed in relation to differences in copolymer com position, sample crystallinity and the method used in the measurement of T g . An increase in the isophthalate content of the copolyester has been found to decrease T g , whereas annealing and increasing the test frequency have both been shown to raise Tg. Values calculated for the apparent activation energy of the glass-transition (Δ ET g ) for the series of copoly esters are also reported. In addition, the influence of water on the T g of polyesters has been investigated.