Understanding the Differential Thermal Behaviour of an Oriented Polymeric Film, in Response to the Modulated Differential Scanning Calorimetry Variables, for Determination of the Degree of Crystallinity

摘要

The degree and the nature of crystallinity determine several key properties of an oriented polymeric system. Thermal analysis, although widely used for crystallinity determination, may have limited precision with oriented polymers, due to the differential nature and overlap of multiple thermal events (cold-crystallisation, chain-relaxation, etc). In this paper we have studied, how MDSC (Modulated Differential Scanning Calorimetry) variables manipulate the thermal behaviour of oriented materials, so that the degree and the nature of crystallisation can be well defined. MDSC curves suggested that the thermal events were significantly shaped by the amplitude (α) and the period (ρ) of thermal modulations. Anisotropic thermal conductivity of the oriented PLA film lead to generation of an error in the calculation of non-reversible signal, seen as an artefact in the crystallisation exotherm. Higher amplitude increased the sensitivity of the method. However, when the rise in the amplitude lead to a shift from a 'heat-only' to a 'heat-cool-heat' cycle, contribution from a poor baseline resulted in the low estimate of the crystallinity. For the 'heat-only' cycles, measured crystallinity decreased inversely with the heating rate and α, due to time dependent crystallisation and melting. Heat-cool-heat cycles lead to crystallisation of some part of the polymer in a more perfected crystal form, whose melting was visible as a non-reversible event. The observations suggested that the heat-only cycles with longer period and faster heating rates favour estimation of the crystallinity, whereas heat-cool-heat cycles with higher amplitude help in understanding pre-melting thermal events associated with polymer orientation. A clear understanding of such an effect is necessary to establish the suitability of MDSC in rapid estimation of crystallinity of the oriented polymers. Accuracy of the method was evaluated by studying the films oriented to different draw ratios and comparison to other established methods. However, this part is not included in this brief communication.