Thermophysical Behavior of Polycarbonate: Effect of Free Quenching above and below the Glass Transition Temperature

Abstract

Polycarbonate is a tough, amorphous and transparent high performance thermoplastic polymer. It is used in many fields of application due to its versatile thermophysical, mechanical and optical properties. However, one of its drawbacks is its relatively high thermal conductivity which prevents its application as an insulating material. An appropriate heat treatment can therefore be a useful route to improve the thermal insulating property. The objective of this work is to study the effect of heat treatment; namely the influence of the quenching temperature above and below the glass transition temperature (Tg) on the thermophysical properties of neat polycarbonate (PC). The effect of the quenching temperature above Tg was also studied for neat poly (methyl methacrylate) (PMMA). The effect of residual stresses (RS) generated by the free quenching process on the thermophysical properties of neat PC was investigated. The thermal conductivity (k) and thermal diffusivity (a) of neat PC were measured using a periodic measurement method (DICO), (DIffusivity and COnductivity), at room temperature. The DICO method developed in the CERTES laboratory (Center for Studies and Research in Thermal, Environment and Systems of Paris 12 University), allows simultaneous access to the conductivity and thermal diffusivity from which the specific heat (Cp) can then be deduced. This work showed that the quenching from a high temperature above Tg did not affect the thermal conductivity and thermal diffusivity of both PC and PMMA. However, quenching from a temperature below Tg (130 ° C) caused a decrease of both the thermal. In fact the thermal conductivity of PC annealed at 130 ° C which is 0.22 W. m-1 .K-1 decreased to 0.06 W. m-1 .K-1 and 0.14 W. m-1 .K-1 after quenching at 0° C and 40 ° C respectively. This means that quenching would therefore improve the insulating capacity of PC compared to the material which has undergone only annealing. Contrary to the thermal conductivity, the values of the specific heat capacity in this temperature range ( 0° C – 40° C) significantly increased as a result of quenching. They increase from 1118 J. kg-1. K-1 for the annealed sample to 1290 J. kg-1. K-1 for PC quenched at 0° C and increased to 2221 J. kg-1. K-1 for PC quenched at 40 ° C which corresponds to an increase by 98 %. It was also found that the values of thermal conductivity and specific heat were in good agreement with those reported in the literature for neat PC samples quenched below Tg.