Morphological Characterization of Blends of Metal-Sulfonated Poly(styrene) and a Methylated Poly(amide) by Solid State NMR

Various blends of atactic, low-MW (4000), metal-sulfonated poly(styrene) (MSPS) and a higher-MW (25 000) poly(amide) (PA) were studied by solid-state 13C and proton NMR techniques which include multiple-pulse irradiation, cross-polarization, and magic angle spinning. This study is an investigation of the morphology of these MSPS(n)/PA blends (n ) 100 mole fraction sulfonate ) 2.3, 7.0, or 11.9) as functions of blend composition and sulfonation level. Unsulfonated PS and PA are incompatible and phase separate. Decoration with sulfonate groups promotes mixing of the blend components owing to strong, polar metal-sulfonate/amide interactions. Metal ions used were divalent Zn (diamagnetic) and Cu (paramagnetic), the latter ions having a significant influence on the protons. The PA, N,N'-dimethylethylene sebacamide, was N-methylated to weaken interactions between PA chains, thereby promoting mixing. Pure PA is semicrystalline, and intimate mixing prevents PA crystallization. 13 C CPMAS spectra were used to assay PA crystallinity. The stability of the blend morphology in the presence of water was also studied since water is expected to modify or compete with the polar interactions of the blend. Many of the experiments performed relied, for their interpretation, on the phenomenon of proton spin diffusion. For ZnSPS(11.9)/PA blends, mixing was quite intimate and PA crystallinity was suppressed for PA mass fractions of 0.5 and lower. PA crystallinity first appeared with a PA mass fraction of 0.65; however, this crystallinity was not the result of large-scale phase separation of the PA from the MSPS. Rather, PA crystallinity develops in the mixed MSPS/PA phase in such a way that each PA crystallite is surrounded by a mixed MSPS/PA phase. Larger PA mass fractions gave higher PA crystallinities. When PA crystallinity is present, there is an average periodicity of about 20-25 nm; moreover, the noncrystalline regions surrounding each crystallite have nonuniform composition in the sense that there is a buffer zone adjacent to the PA crystallites which is mainly PA in composition. A few blends involving ZnSPS(7.0) and ZnSPS(2.3) were also studied. Only the 75/25 ZnSPS(7.0)/PA blend seemed well mixed and noncrystalline. Compositional heterogeneities on scales larger than 20 nm were seen in the remaining blends. Certain CuSPS ionomers and blends, analogous to the Zn-containing materials, were studied in an attempt to resolve some ambiguities present in the interpretation of the data taken for the Zn-containing materials. Zn and Cu ions show similar affinities for the amide moieties; hence, the morphology for analogous Zn- and Cu-containing blends is expected to be similar. Mainly proton longitudinal relaxation was measured because it is sensitive to the presence of paramagnetic Cu. Two matters were pursued for the Cu-containing materials: First, the uniformity of Cu distribution was probed in pure CuSPS(11.9). Our T1 H analysis gave a variation by a factor of 1.3 in averaged Cu concentration, where the averaging was done over dimensions of 14 nm. Second, given that the 75/25 CuSPS(2.3)/PA blend exhibits large-scale phase separation, where one phase contains nearly all of the PA plus a small fraction of the SPS, we addressed the question whether the level of decorations for the SPS chains in this mixed phase was significantly above the 2.3% average. Our analysis did not support such a claim within the assumption that the morphologies of the Zn- and Cu-containing blends are the same. Toward an evaluation of paramagnetic Cu ions as aids for the elucidation of morphology in organic systems, several qualitative characteristics of the proton and 13 C relaxation are noted for these Cu-containing materials. Also, an estimate of the electron relaxation time, T1 e , in CuSPS(11.9) is given as is the fraction (0.95) of observable protons using multiple-pulse techniques. T1 e is shown to vary strongly with overall Cu concentration in the pure ionomers and with the amount of water absorbed in the pure ionomer. T1 e also changes when Cu is bound to the amide moiety of PA. One cannot simply assume that