Department of Materials Engineering, Indian Institute of Science, Bangalore-560012, India.
J Phys Chem B. 2013 Jul 18;117(28):8633-46. doi: 10.1021/jp404610w. Epub 2013 Jul 2.
The effects of multiwalled carbon nanotubes (MWNTs) on the concentration fluctuations, interfacial driven elasticity, phase morphology, and local segmental dynamics of chains for near-critical compositions of polystyrene/poly(vinyl methyl ether) (PS/PVME) blends were systematically investigated using dynamic shear rheology and dielectric spectroscopy. The contribution of the correlation length (ξ) of the concentration fluctuations to the evolving stresses was monitored in situ to probe the different stages of demixing in the blends. The classical upturn in the dynamic moduli was taken as the rheological demixing temperature (T(rheo)), which was also observed to be in close agreement with those obtained using concentration fluctuation variance, <(δϕ)(2)>, versus temperature curves. Further, Fredrickson and Larson's approach involving the mean-field approximation and the double-reptation self-concentration (DRSC) model was employed to evaluate the spinodal decomposition temperature (T(s)). Interestingly, the values of both T(rheo) and T(s) shifted upward in the blends in the presence of MWNTs, manifesting in molecular-level miscibility. These phenomenal changes were further observed to be a function of the concentration of MWNTs. The evolution of morphology as a function of temperature was studied using polarized optical microscopy (POM). It was observed that PVME, which evolved as an interconnected network during the early stages of demixing, coarsened into a matrix-droplet morphology in the late stages. The preferential wetting of PVME onto MWNTs as a result of physicochemical interactions retained the interconnected network of PVME for longer time scales, as supported by POM and atomic force microscopy (AFM) images. Microscopic heterogeneity in macroscopically miscible systems was studied by dielectric relaxation spectroscopy. The slowing of segmental relaxations in PVME was observed in the presence of both "frozen" PS and MWNTs interestingly at temperatures much below the calorimetric glass transition temperature (T(g)). This phenomenon was observed to be local rather than global and was addressed by monitoring the evolution of the relaxation spectra near and above the demixing temperature.
采用动态剪切流变学和介电谱系统地研究了多壁碳纳米管（MWNTs）对聚苯乙烯/聚（甲基乙烯基醚）（PS/PVME）共混物近临界组成的浓度涨落、界面驱动弹性、相形态和链局部段动力学的影响。通过原位监测浓度涨落相关长度（ξ）对演变应力的贡献，来探测共混物中不同的分相阶段。动态模量的经典上升被视为流变分相温度（T(rheo)），也与使用浓度涨落方差（<(δϕ)(2)>）与温度曲线获得的结果非常吻合。此外，采用弗雷德里克森和拉尔森（Fredrickson and Larson）的方法，涉及平均场近似和双重蠕动自浓缩（DRSC）模型，来评估旋节分解温度（T(s)）。有趣的是，在 MWNTs 存在下，共混物中的 T(rheo)和 T(s)都向上移动，表现出分子水平的混溶性。这些显著变化进一步被观察到是 MWNTs 浓度的函数。通过偏光显微镜（POM）研究了形态随温度的演变。观察到在分相的早期阶段，PVME 演变成相互连接的网络，在后期阶段粗化为基质-液滴形态。由于物理化学相互作用，PVME 优先润湿 MWNTs，这使得 PVME 的相互连接网络在更长的时间尺度上得以保留，这得到了 POM 和原子力显微镜（AFM）图像的支持。介电弛豫光谱研究了宏观混溶性体系中的微观不均匀性。有趣的是，在“冻结”PS 和 MWNTs 的存在下，观察到 PVME 中段松弛的减缓，温度远低于量热玻璃化转变温度（T(g)）。这种现象被观察到是局部的而不是全局的，并通过监测分相温度附近和以上的弛豫谱的演变来解决。
