Thermodynamic analysis of the phase separation in solutions of monodisperse polystyrene in an epoxy/amine solvent undergoing a linear stepwise polymerisation.

ILEANA A. ZUCCHI; MARÍA J. GALANTE; JULIO BORRAJO; ROBERTO JJ WILLIAMS

Viña del Mar - Chile

Congreso; II Simposio Binacional de Polímeros Argentino-Chileno (Archipol II).; 2003

Universidad de Chile

The thermodynamic analysis of the polymerization-induced phase separation in a modified thermoset is usually performed using the Flory-Huggins (F-H) model. The mathematical modeling of the reaction-induced phase separation finds a major difficulty when conversion approaches gelation. A good test of the applicabilility of the F-H equation results when gelation can be excluded by selecting a model system consisting of a monodisperse linear polymer dissolved in a bifuntional monomer/co-monomer solvent. The system composed of stoichiometric amounts of diglycidyl ether of bisphenol A (DGEBA) and benzylamine (BA), is particularly useful because the stepwise polymerization follows an ideal path. A monodisperse polystyrene (PS) was chosen, because the location of the CPC in a temperature vs. composition diagram, for solutions in DGEBA or in DGEBA/BA, can be shifted by varying its molar mass. Solutions of two monodisperse polystyrenes (PS, Mn=83,000 or 217,000) in DGEBA, exhibited an upper-critical-solution-temperature (UCST) behaviour. Cloud–point curves (CPC) were fitted with the F-H model using an interaction parameter depending on both temperature and concentration, c = (a+b/T)/(1-c.f2), where f2 represents the volume fraction of PS. CPC’s curves were shifted to lower temperatures (higher miscibility) when PS’s  were dissolved in stoichiometric amounts of DGEBA and BA, in conditions where no reaction took place. The resulting curves were fitted using a higher value of constant b but keeping the same a and c values. Cloud-point times in the course of the polymerizations carried out at 70ºC y 80ºC, were determined for solutions containing 2.5 to 15 %wt PS (Mn=83,000). The fitting of the cloud-point conversions with the F-H model required the use of a parameter b decreasing with conversion and a slight change in the value of c. Using the resulting functionality of the interaction parameter, cloud-point conversions were predicted for the other PS (Mn=217,000), and found to be in excellent agreement with the experimental results.