Information on the noncrystalline phase of nascent iPP given by slow calorimetry

Abstract

Nascent isotactic polypropylene (iPP) has a lower crystallinity and a different X-ray pattern than recrystallized iPP. Heat flows during the melting of nascent highly stereoregular iPP samples are recorded with a Setaram C80 calorimeter over a wide T range (30–280 °C). The rate of heating–cooling is 1–3 K/h, i.e., much lower than with conventional DSC. Melting is performed after annealing at 30 < Tannealing < 140 °C and with or without substrate. The main peak, that found by DSC, is associated with melting of monoclinic crystals and gives an enthalpy ΔHDSC. Two other peaks, usually above and below the main peak, are observed. These are associated with a slower process of disordering a physical network, which was produced in the sample during polymerization. The sum of ΔHDSC and ΔHnetwork equals ΔHtotal. When melting is complete ΔHtotal is equal to ΔH0, the heat of fusion of perfect iPP crystals. This work presents new information on: (i) the noncrystalline phase of nascent iPP and the heat content of a semicrystalline polymer; (ii) the modifications of the melting process due to strain development, brought about by expansion in the material during the temperature ramp when a physical network is present; and (iii) the effect of a substrate on the polymer melting process. Keywords: nascent iPP, slow calorimetry, strain-melting, change of enthalpy, substrates.