Thermodynamic properties of polyolefin solutions at high temperature: 1. Lower critical solubility temperatures of polyethylene,polypropylene and ethylene-propylene copolymers in hydrocarbon solvents

Lower critical solubility temperatures (LCST) of linear polyethylene (PE), isotactic polypropylene (PP) and of five random ethylene-propylene (EP) copolymers of different composition have been measured in (i) five linear alkanes (n-C₅ to n-C₉); (ii) sixteen branched alkanes and (iii) four cycloalkanes. The effect of correlations of molecular orientations (CMO) on the LCST was investigated. The main results of this work are the following: (1) the LCST for PE are much lower than those for PP although the expansion coefficients of the two polymers are similar. Calculations using the van der Waals model for a liquid would predict them 10° to 20° apart while the experimental difference can reach 90°. (2) In PE solutions, the linear alkanes are much better solvents than those which are branched. This constitutes a rare example in non-polar solutions in which the magnitude of the equation of state term is not sufficient to predict or even to compare the LCST. The importance on x of polymer-segment and solvent shapes even above the boiling point of the solvent is to be noted. (3) The LCST of the five copolymer samples are almost a linear function of their composition over all the composition range. (4) These results can be understood if the existence of CMO between the (CH₂Ch₂) sequences is assumed in the pure PE melt and in the copolymers but not between the CH(CH₃)CH₂ sequences. CMO in solution between the polymeric chains and the linear alkanes make the linear alkanes better solvents than the branched ones. From LCST data in n-C₇ and its isomers, the temperature to which CMO in PE disappear can be estimated to be above 170°C, a value which is consistent with those found for long linear alkanes. (5) Branched volatile alkanes such as 2,2-dimethylpentane appear to be a good choice for dosage of the ethylene content of an EP copolymer because of the large interval of LCST between PP and PE in such solvents. LCST measurements could become a sensitive and routine analytical tool for polymer and copolymer characterization for some polymers in well-chosen solvents.