On polymer-polymer miscibility and cellulose ester blends: A case study.

• CAB/CA blends cannot be characterized as miscible/immiscible. • CAB/CA blends and respective copolymers exhibit two thermal transitions. • New interpretations for the existence of two transitions in polymer blends. • The new term "heterogeneous mixtures of compatible polymers" is introduced. Differential Scanning Calorimetry (DSC) and micro-Raman Spectroscopy were used to study blends of cellulose acetate (CA) with cellulose acetate butyrate (CAB) prepared by solution blending. It is shown that these blends combine two characteristics: (a) values of enthalpy of mixing (ΔH mix) and Gibbs free energy of mixing (ΔG mix) close to zero, due to similar molecular interactions and (b) multiple, more or less equivalent, minimum values of ΔG mix leading to non-equilibrium structures, mainly due to the inherent microheterogeneity of pure polymers. New interpretations are provided for the observed thermal behavior (e.g. existence of multiple thermal transitions) of the blends. None of the terms miscible, partially miscible or immiscible is truly appropriate to characterize these blends. The thermal behavior of pure polymers, blends and chemically synthesized copolymers of similar degree of substitution is pretty much alike, thus, development of copolymers by physical blending of cellulose esters seems to be feasible. [ABSTRACT FROM AUTHOR]