Isobaric thermal expansivities of polyethylenes with various crystallinities over the pressure range from 0.1 to 300 MPa and over the temperature range from 303 to 393 K

Abstract

A pressure‐controlled scanning calorimeter (PCSC) has been applied for measuring the isobaric volume thermal expansivities (αp) of crystalline polymers as a function of pressure up to 300 MPa at various temperatures. The measurements have been performed for several well‐defined polyethylenes with various degrees of crystallinity at 302.6, 333.0, 362.6, and 393.0 K. The results are reported as values of coefficients in a correlation equation, which facilitates the use of reported data over large ranges of temperature and pressure. The general pressure‐temperature behavior of αp for all polyethylenes under study is such that αp increases with temperature and decreases with pressure. The increase with temperature is smaller at high pressures and the isotherms of αp have a tendency to converge at high pressures; αp decreases linearly with the crystallinity of the polyethylene over the whole range of pressure and temperature under investigation. From the linear approximation of experimental data for polyethylenes with various crystallinities the estimated αp for both crystal and amorphous phases of polyethylenes have been determined as a function of pressure up to 300 MPa at 302.6, 333.0, and 362.5 K. The obtained results have been compared with available literature crystallographic data and with the values derived from the Pastine theoretical equation of state for both crystalline and amorphous phases. © 1996 John Wiley & Sons, Inc.

Using vibrational mode analysis for predicting the coefficient of thermal expansion of amorphous polymers

“…The structure was initially prepared using PolyPack with trans–gauche conformational statistics consistent with 350 K and then cooled to 50 K. Upon heating from 50 to 350 K, the structure showed thermal expansion. However, the volumetric CTE was only 15 × 10 −6 /K, which is at least 30 times smaller than the reported experimental values for amorphous fraction in PE (470 × 10 −6 /K at 160–200 K, 750 × 10 −6 /K at 200–250 K,41 and between 750 × 10 −6 /K42 and 800 × 10 −6 /K43 at room temperature).…”

Modification of molecular organization of polymers by gas sorption: Thermodynamic aspects and industrial applications

“…In this context, a study of different PEs of low, middle, and high densities (LDPE, MDPE, and HDPE, respectively) have been completed at several temperatures between 300 and 400 K and under different pressures up to 300 MPa. Extrapolation of these results allows us to obtain α p for a PE supposedly totally crystallized [31], remarkably and very satisfactorily, the extrapolated value at atmospheric pressure is in very good agreement with the value calculated using the Pastine equation of state, which equation has the particularity to have been designed from crystallographic data. The real advantage of using a differential detection between the two cells of a scanning transitiometer is the possibility to directly compare in identical conditions of temperature and pressure two polymer samples submitted to the same pressurizing gas.…”

Thermal expansion of polymers submitted to supercritical CO2 as a function of pressure

“…For comparison, cubic expansion coefficients of pure polymers under investigation in the present study have also been determined according to procedures similar to those described previously 17. In this study, the measurements were performed with the thermal differential mode [Table 1(a)], the reference vessel was not connected to the pressure line.…”

“…In the present study, pressure was used as inducing variable at constant temperature and only the thermal response was recorded, because the simultaneous recording of volume variations was not precise enough due to the high compressibility of the fluid phase over the investigated pressure range. This mode of measurement is named pressure‐controlled scanning calorimetry (PCSC), and both its detailed analysis16 and application for determination of cubic expansion coefficient of pure polymers17, 18 were described previously. The thermodynamic basis for the functioning of PCSC is the Maxwell relation: (∂ V /∂ T ) p = −(∂ S /∂ p ) T = −1/ T (∂ Q /∂ p ) T .…”