The structural analysis of a series of amorphous statistical copolymers of n-butyl acrylate (A) and dimethyl(3-methacrylamidopropyl)(3-sulfopropyl)ammonium betaine (B) was performed through differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), and solid-state NMR spectroscopy ( broad line and 13C high resolution). In copolymers of low and moderate B content [0.04 < Fb (molar fraction) < 0.35] strong dipolar interactions between the zwitterionic units (µ =* 25 D) result in a typical biphasic structure. DSC clearly shows two well-separated glass transitions, allowing a quantitative description of the copolymer morphology in terms of a soft matrix (-46 < Tgs (°C) < -20) containing only very low amounts of B units (FBS < 0.07) and hard domains (30 < TgH (°C) < 120) containing rather high amounts of A units (0.45 < Fah < 0.73). Solid-state NMR analysis confirms the fair purity of the mobile phase and the chemical heterogeneity of the rigid phase, which may be quantitatively correlated with the hard domains identified in DSC. The presence of a single SAXS peak and the variations of the corresponding Bragg spacings d (4-7 nm) according to a power law of the type d <* "°•27 ( = volume fraction of B units) suggest a random distribution of the dipolar scattering entities within the apolar matrix. A low fraction of the zwitterions still remain strongly associated in rigid aggregates stable at very high temperatures such as TgH + 100 °C. With respect to phase separation, the zwitterionic copolymers may appear as a very versatile alternative to the corresponding quaternary ammonium cationic ionomers.
The morphology of two statistical copolymers of n-butyl acrylate (A) and dimethyl(3methacrylamidopropyl) (3-sulfopropyl)ammonium betaine (B) of B molar fraction 0.04 and 0.12 (copolymers AB-4.0 and AB-Í2) were analyzed through solid-state NMR spectroscopy (13C CP/DD/MAS) over a broad temperature range (160-360 K). In spite of the complexity of molecular motions and carbon line overlapping, the measurement of selective hydrogen rotating frame spin-lattice relaxation time Tu through 13C detection clearly shows that spin diffusion actually occurs over rigid microdomains of about 1 and 2.2 nm for the unclustered AB-4 and clustered AB-12 copolymers, respectively. These minimum dimensions, corresponding to the shortest path for magnetization relaxation, are likely characteristic of the zwitterionic multiplets which are the primary dipolar aggregates in these heterogeneous materials and are compatible with literature models relying on antiparallel alignment of the dipoles in close-packed structures.
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