Insight into miscibility behaviour of cellulose ester blends with N-vinyl pyrrolidone copolymers in terms of viscometric interaction parameters

We previously offered miscibility maps for blend systems of cellulose esters (CEs) including cellulose acetate (CA), propionate (CP), and butyrate (CB) with vinyl copolymers containing an N-vinyl pyrrolidone (VP) unit, i.e., poly(N-vinyl pyrrolidone-[co]-vinyl acetate) [P(VP-[co]-VAc)] and poly(N-vinyl pyrrolidone-[co]-methyl methacrylate) [P(VP-[co]-MMA)]; the maps were constructed based on data of thermal analysis as a function of the degree of ester substitution (DS) of the CE component and the VP fraction in the copolymer component. The blend system using CP among the three CEs imparted the largest region of miscible pairings with the vinyl copolymers, and both of the maps for the CP/P(VP-[co]-VAc) and CP/P(VP-[co]-MMA) systems comprised a "miscibility window" associated with the respective copolymer compositions at high DSs of >2.65. The present work was made to interpret the expansion of the miscible markings for the CP/copolymer systems in comparison with the cases using CA and CB, in terms of a Krigbaum–Wall interaction parameter (μ) obtained by solution viscometry for selective polymer pairs involved in the respective CE/copolymer blends. The results of μ measurements were in good accordance with the earlier miscibility estimations. The assessment of very small negative μ values (i.e., extremely weak repulsion) for CP/PVAc and CP/PMMA combinations and that of considerably larger negative μ values for PVP/PVAc and PVP/PMMA combinations enabled us to give a rational explanation for the CP systems. The strongly repellent character of the two different monomer units constituting the copolymers permits accession of the CP component (DS > 2.65) to them, which would be responsible for the advent of the miscibility window. Further expansion of the window observed when cellulose acetate propionate (CAP) was adopted instead of CP as the CE component was also well explained on the basis of a μ data indicative of additional intramolecular repulsion in the CAP side.