Hierarchical structural gradients in injection moulded poly(ethylene naphthalene-2,6-dicarboxylate) parts

Abstract

Structural development in injection moulded poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) was studied as a function of processing parameters including mould temperature, injection speed and holding time. This polymer exhibits a relatively low thermal crystallization rate and as a result quenches into wholly amorphous form when moulded into thick cavities at low mould temperatures. It, however, exhibits a three-layer structural gradient (an amorphous skin, a shear crystallized intermediate layer and an amorphous core) when moulded into thin cavities at mould temperatures up to about its cold crystallization temperature. Above this temperature, thermally activated crystallization starts playing a role particularly in the structural formation at the interior of the samples. At these temperatures the holding time in the mould becomes an important factor and as it gets longer the overall crystallinity in the sample increases. Thermal analysis results suggested that at mould temperatures up to its glass transition temperature the crystal structure of PEN consists of a mixture of ordinary α form and β that exhibits high melting temperature. Mechanical properties of injection moulded PEN were found to mostly depend on mould temperature and at high moulding temperatures on holding time.