Effects of molecular structure of modifiers on the thermodynamics of phenolic blends: An entropic factor complementing PCAM

Different thermodynamics in phenolic blends with different polymeric modifi ers, i.e., phenoxy, poly(decamethylene adipate), poly(ethylene oxide), and poly(vinyl alcohol), calculated by the Painter and Coleman association mode l (PCAM) are examined. The thermodynamics is calculated based upon the equi librium constants derived experimentally from infrared spectroscopies of lo w molecular weight analogues with similar hydrogen-bonding formation. The d iscrepancies between the PCAM predictions and the experimentally observed T -g and free volume variation with blending composition are attributed to th e additional entropic effects introduced by the long repeated units of modi fiers. The structural characteristics and hydrogen-bonding heterogeneity as derived from solid-state NMR and IR spectra support the notion that the le ngth and size of the modifier repeated unit are responsible for such discre pancies. These observed nonidealities can be interpreted as competition bet ween inter- and intraassociations (Delta H-m favored), which depend on the entropy rise associated with the amount of increase of the breaking off of the self-association in phenolic and modifiers within blends. While PCAM is based on "true" miscibility, however, minor modification is required to be tter describe the thermodynamics for "real" blends where microdomain hetero geneity with size greater than that defined by thermodynamic criteria may b e present.