Physical and microstructural effects of heat setting in polyester films

The effects of a controlled heat-setting treatment on the properties and mi crostructure of biaxially stretched poly(ethylene terephthalate) (PET) and poly(ethylene-2,6-naphthalene dicarboxylate) (PEN) films are described. Sub stantial changes in crystalline fraction, crystallite size, glass transitio n temperature and significant enhancement in dimensional stability are obse rved for both film types upon increase in heat-set temperature. A distinct melting peak in the vicinity of the heat-set temperature, observed in diffe rential scanning calorimetry thermograms for both materials, is shown to be an effective marker of the heat-setting process underlying the dual nature of the morphology of the heat-treated films. We also observe that the PET films undergo significant molecular realignment on heat setting, while the orientation of the PEN films is only weakly modified by the heat-set condit ions. A morphological transition is detected for both films at high heat-se t temperatures (T-HS*) near the onset of the primary melting range, marking a qualitative change in the physical response of the biaxially oriented fi lms; Measurable drop in planar orientation and glass transition temperature , and a sharp rise in crystallite size are noted for films heat-treated abo ve T-HS*. The morphological models of previous workers (Schultz et al. and Fischer and Fakirov), invoking the idea of fibrillar-to-lamellar transforma tion, are used to explain some of our observations.