Polyethylene terephthalate relaxation transitions

Recently Moore and Petrie (5) have demonstrated that control of sample thermal history can result in transition from ductile to brittle behavior for polyethylene terephthalate. This transition in behavior was related to volume relaxation of the glassy state. 

Orientation of typical random coil polymer films, such as polypropylene and polyethylene terephthalate, involves stretching the film at a temperature above the glass transition temperature (Tg) and below the melt temperature (Tm) [5]. This allows the polymer chains sufficient mobility to be aligned in the strain directions, but not so much mobility that they will relax to a random state. With LCP films, the development of orientation is quite different. 

Amorphous polyethylene terephthalate is a glassy polymer. During exposure to temperatures below glass transition temperature, molecular rearrangements facilitate equilibrium in the material. This process changes the free volume and causes a reduction in relaxation enthalpy. 

Fig. 20 Apparent activation energy of glass transition (a-relaxation) vs activation energy of p-relaxation dependence obtained at 10-2-1 Hz obtained by DSC or DMA techniques for polymers (1) polyisoprene (2) poly(dimethylsiloxane) (3) polyethylene (4) polypropylene (5) polyamide-6 (6) poly(ethylene terephthalate) (7) poly(vinyl acetate) (8) poly(vinyl chloride) (9) poly(vinyl alcohol) (10) poly(methyl methacrylate) (11) boron oxide glass (12) polyacrylonitrile (13) polycarbonate (14) poly(cyclohexyl methacrylate) (15) poly(a-methylstyrene) (16) polystyrene (17) polyarylate (18) polyimide I (19) polyimide II (20) polyimide III (21) poly(m-phenylene isoph-thalamide)...

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