Solution properties and characterization of polyisoprenes at a lower critical solution temperature (LCST)

Abstract

This paper reports two investigations on polyisoprene. One consists of obtaining the molecular weight distribution (MWD) of cis-polyisoprene (PIP) standards by a new method using the turbidity at a lower critical solution temperature (LCST). Advantage is taken for this analysis of the fact that the MW dependence of the critical temperature is greater for systems with a narrow solubility gap, i.e. for systems whose LCST and upper critical solution temperature (UCST) are not too far apart. Typically, in n-pentane, a narrow MW standard phase-separates over 25 K. When the solution is heated by temperature increments of 2 K, 15 fractions can be identified, without physical separation. The relative amount of each phase is analysed by turbidity and the average MW of the phases by the temperature at which they phase separate. The MWs recalculated with two constants for the standards are in excellent agreement with gel permeation chromatography (g.p.c.) data. Possible effects leading to a higher polydispersity found by the LCST are discussed. Application to rubber analysis is feasible. The second investigation consists of reporting and analysing the temperatures of phase separation of medium MWD cis- and trans-PIP samples in a series of linear and branched alkanes. The occurrence of correlations of molecular orientations (CMO) in the melt, similar to those found from thermodynamic analysis of PE solutions, was considered. Trans-PIP has a LCST about 25 K lower than cis-PIP in alkanes. Linear solvents are better solvents than branched ones. The low LCSTs (100°C in n-pentane) support CMO or order in polyisoprene melt or concentrated solutions.