Fundamental thermal properties of polyvinyl alcohol by fast scanning calorimetry

5 years ago

Fundamental thermal properties of polyvinyl alcohol by fast scanning calorimetry

Polyvinyl alcohol (PVA) is a synthetic, semi-crystalline polymer with a wide array of commercial uses ranging from textiles and packaging to medicine. Despite the fact that PVA is in common use, several important thermal properties have not been measured including: 1. temperature dependent liquid state specific heat capacity, cp ^Liquid(T); 2. specific heat capacity increment of amorphous PVA at the glass transition temperature, Δcp ^amor(Tg); and, 3. fraction of rigid amorphous phase in semi-crystalline PVA, ϕRA. Two rate-dependent effects have prevented these measurements: PVA thermally degrades at temperatures just in excess of 200 °C which is often within the onset of melting, and PVA crystallizes from the melt so rapidly that it is difficult to obtain fully amorphous polymer. To prevent degradation, and measure these fundamental thermal properties, we have used fast scanning calorimetry at rates ranging from 1000 K/s up to 600,000 K/s. The Mettler Flash DSC1 and a custom-built calorimeter were used to cover this range of heating and cooling rates. We determine the critical cooling rate, β c , needed to quench PVA into an amorphous glass as |β c | = 20,000 K/s. Using FSC in combination with conventional differential scanning calorimetry, we find cp ^Liquid(T) = ((0.0016 ± 0.0002)*T + (2.3 ± 0.2)) J/(gK). The specific heat capacity increment for fully amorphous PVA is Δcp ^amor(Tg) = (1.01 ± 0.05) J/(gK). For the semi-crystalline samples used in this study, PVA obeys a two phase model in which ϕRA ∼0. The approaches used in this work are applicable to any semicrystalline polymer or biopolymer which degrades upon heating, or crystallizes so rapidly from the melt that a fully amorphous material cannot be realized.

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DOI: S0032386118300041