STATISTICAL N-BUTYL ACRYLATE (SULFOPROPYL)AMMONIUM BETAINE COPOLYMERS .5. PLASTICIZATION STUDIES

The influence of additives of strongly different polarities such as dibutyl phthalate (DBP), ethylammonium nitrate (EAN), or water on the morphology of zwitterionomers of n-butyl acrylate (A) and 1,1-dimethyl-1-(3-methacrylamidopropyl)-1-(3-sulfopropyl)ammonium betaine (B) has been studied by differential scanning calorimetry (DSC) and small-angle X-ray scattering (SAXS). In the case of a clustered copolymer (F(B) (molar fraction) = 0.11), addition of DBP as a selective solvent of A units (W(DBP) (weight fraction) < 0.7) results in simultaneous plasticization of the soft matrix (T(g)S,p approximately 237-183 K) and of the hard zwitterionic domains (T(g)H,P approximately 330-260 K) without significant redistribution of A and B units: the DBP partition coefficient K, which measures the plasticizer fraction located in the matrix, is always in favor of a weakly preferential sorption of the additive by the matrix and slightly increases with the overall DBP content (K approximately 0.84-0.96). According to SAXS analysis (Bragg spacings corresponding to the ''ionic'' peak in the range 6-10 nm), the plasticization process differs only slightly from an isotropic homogeneous swelling of the system. Addition of EAN or water as a selective solvent of B units (number of polar additive molecules per zwitterionic unit <2.2) results in a fairly similar and nearly selective plasticization of the zwitterionic domains (T(g)S,p approximately 237-232 K, T(g)H,p approximately 330-250 K) with simultaneous reorganization of the B units: the rather strong increase of the Bragg spacings (d approximately 6-10 nm) suggests a reduction of the number of zwitterionic multiplets and an increase of their size and of their aggregation number.