Direct Measurement of the Enthalpy of Mixing in Miscible Blends of Poly(<scp>dl</scp>-lactide) with Poly(vinylphenol)

Abstract

Blends of poly(dl-lactide) (PDLLA) with poly(vinylphenol) (PVPh), obtained by solvent-casting and solution/precipitation, have been studied by DSC and FTIR. DSC results obtained in the first heating scan suggest that as-prepared blends are phase separated in nearly pure components. In addition, the initial FTIR spectra of the as-precipitated blends show no sign of specific interactions, which is also consistent with the phase separation. In solution/precipitation blends, the first calorimetric scan shows an exothermic peak attributed to the enthalpy of mixing. To our knowledge, this is the first polymer blend system for which this thermodynamic parameter has been possible to directly measure. The exothermic peak occurs at temperatures just below the Tg of PVPh. The consecutive DSC scans show a single Tg for the whole composition range, indicating complete miscibility. The interaction energy density (B) has been calculated and shows a strong dependence on composition. The decrease in B (in terms of absolute value) as the content of PVPh decreases is related to the higher energy consumed to break its strong autoassociation. Phase separation into the neat polymers for the as-prepared blends is attributed to accessibility issues between interacting groups, including steric shielding, group spacing, and chain stiffness, aggravated by blending temperatures below the corresponding Tgs. At high temperatures thermal motion increases chain mobility, allowing the development of an intimately mixed single-phase blend. On the other hand, phase separation is observed in solution-cast blends with high PVPh content, certainly due to the Δχ effect, resulting in macroscopic domains that prevent obtaining a homogeneous blend at high temperatures in the absence of a shearing mechanism. Interaction development during heating has been followed by FTIR. Both the hydroxyl and carbonyl stretching regions indicate hydrogen bonding between OH groups of PVPh and CO groups of PDLLA, suggesting weaker interactions than in other PVPh/polyester systems.