Viscoelastic behaviour and time-temperature correspondence of HDPE with varying levels of process-induced orientation

Shear controlled orientation in injection moulding (SCORIM) is a non-conventional injection moulding technique that allows for the enhancement of the mechanical properties of semi-crystalline polymers. In this work the flexural mechanical feature, namely their time-dependence, of conventionally injection-moulded high-density polyethylene (HDPE) (PEc) and SCORIM-processed HDPE (PEs) are compared. The differences should be attributed to the appearance of highly oriented structures such as shish-kebab morphologies in the latter case. The quasi-static performance of HDPE shows a 59% increase in the flexural modulus following SCORIM application. Master curves of the frequency-dependence of the storage modulus and of the time-dependence of strain were constructed successfully in the region of the alpha -relaxation. The viscoelastic data are well described by the Cole--Cole model. The wide broadness of the distribution of relaxation time observed for both materials is ascribed to the molecular mobility of the amorphous phase involved in the relaxation process. Two approaches were used to investigate the curvature observed in the Arrhenius plots. In the first case, two separate processes were assumed to contribute to the alpha -relaxation, with activation energies of 150--170 and 180--210 kJ mol exp -1 , respectively (dynamic data). However, the two alpha -processes, alpha I and alpha II, often reported in the literature, did not appear explicitly in the original data, which showed a single peak in an E" vs T plot. Therefore, a unique process for the alpha -relaxation was also assumed and was well described taking into account changes in the reduced volume whose temperature-dependence obeys a general equation of state. This concept enabled a good agreement between creep and dynamic experiments. Copyright (c) 2001 Elsevier Science Ltd