HIERARCHICAL STRUCTURAL GRADIENTS IN INJECTION-MOLDED POLY(ETHYLENE NAPHTHALENE-2,6-DICARBOXYLATE) PARTS

Structural development in injection moulded poly(ethylene naphthalene- 2,6-dicarboxylate) (PEN) was studied as a function of processing param eters including mould temperature, injection speed and holding time. T his polymer exhibits a relatively low thermal crystallization rate and as a result quenches into wholly amorphous form when moulded into thi ck cavities at low mould temperatures. It, however, exhibits a three-l ayer structural gradient (an amorphous skin, a shear crystallized inte rmediate layer and an amorphouts core) when moulded into thin cavities at mould temperatures up to about its cold crystallization temperatur e. Above this temperature, thermally activated crystallization starts playing a role particularly in the structural formation at the interio r of the samples. At these temperatures the holding time in the mould becomes an important factor and as it gets longer the overall crystall inity in the sample increases. Thermal analysis results suggested that at mould temperatures up to its glass transition temperature the crys tal structure of PEN consists of a mixture of ordinary alpha form and beta form that exhibits high melting temperature. Mechanical propertie s of injection moulded PEN were found to mostly depend on mould temper ature and at high moulding temperatures on holding time.