Department of Materials Engineering, Indian Institute of Science, Bangalore-560012, India.
J Phys Chem B. 2013 Jul 18;117(28):8589-602. doi: 10.1021/jp4033723. Epub 2013 Jul 8.
Crystallization-induced phase separation and segmental relaxations in poly(vinylidene fluoride)/poly(methyl methacrylate) (PVDF/PMMA) blends was systematically investigated by melt-rheology and broadband dielectric spectroscopy in the presence of multiwall carbon nanotubes (MWNTs). Different functionalized MWNTs (amine, -NH2; acid, -COOH) were incorporated in the blends by melt-mixing above the melting temperature of PVDF, where the blends are miscible, and the crystallization induced phase separation was probed in situ by shear rheology. Interestingly, only -NH2 functionalized MWNTs (a-MWNTs) aided in the formation of β-phase (trans-trans) crystals in PVDF, whereas both the neat blends and the blends with -COOH functionalized MWNTs (c-MWNTs) showed only α-phase (trans-gauche-trans-gauche') crystals as inferred from wide-angle X-ray diffraction (WXRD) and Fourier transform infrared (FTIR). Furthermore, blends with only a-MWNTs facilitated in heterogeneous nucleation in the blends manifesting in an increase in the calorimetric crystallization temperature and hence, augmented the rheologically determined crystallization induced phase separation temperature. The dielectric relaxations associated with the crystalline phase of PVDF (αc) was completely absent in the blends with a-MWNTs in contrast to neat blends and the blends with c-MWNTs in the dielectric loss spectra. The relaxations in the blends investigated here appeared to follow Havriliak-Negami (HN) empirical equations, and, more interestingly, the dynamic heterogeneity in the system could be mapped by an extra relaxation at higher frequency at the crystallization-induced phase separation temperature. The mean relaxation time (τHN) was evaluated and observed to be delayed in the presence of MWNTs in the blends, more prominently in the case of blends with a-MWNTs. The latter also showed a significant increase in the dielectric relaxation strength (Δε). Electron microscopy and selective etching was used to confirm the localization of MWNTs in the amorphous phases of the interspherulitic regions as observed from scanning electron microscopy (SEM). The evolved crystalline morphology, during crystallization-induced phase separation, was observed to have a strong influence on the charge transport processes in the blends. These observations were further supported by the specific interactions (like dipole induced dipole interaction) between a-MWNTs and PVDF, as inferred from FTIR, and the differences in the crystalline morphology as observed from WXRD and polarized optical microscopy (POM).
通过熔融流变学和宽带介电谱研究了聚偏二氟乙烯/聚甲基丙烯酸甲酯（PVDF/PMMA）共混物在多壁碳纳米管（MWNTs）存在下的结晶诱导相分离和链段松弛。通过在高于 PVDF 熔点的温度下熔融混合，将不同官能化的 MWNTs（胺，-NH2；酸，-COOH）掺入共混物中，其中共混物是混溶的，并通过剪切流变原位探测结晶诱导相分离。有趣的是，只有 -NH2 官能化的 MWNTs（a-MWNTs）有助于 PVDF 中β相（反式-反式）晶体的形成，而纯共混物和 -COOH 官能化的 MWNTs（c-MWNTs）的共混物仅显示α相（反式- gauche-trans-gauche'）晶体，如广角 X 射线衍射（WXRD）和傅里叶变换红外（FTIR）所推断。此外，仅含有 a-MWNTs 的共混物有利于在共混物中进行异相成核，表现为比热结晶温度升高，从而提高了流变法确定的结晶诱导相分离温度。与纯共混物和共混物中含有 c-MWNTs 的共混物相比，与 PVDF 结晶相（αc）相关的介电弛豫在含有 a-MWNTs 的共混物中完全不存在。在介电损耗谱中。这里研究的共混物中的弛豫似乎遵循 Havriliak-Negami（HN）经验方程，更有趣的是，在结晶诱导相分离温度下，可以通过在更高频率处的额外弛豫来映射系统中的动态不均匀性。评估了平均弛豫时间（τHN），并观察到在共混物中存在 MWNTs 时会延迟，在含有 a-MWNTs 的共混物中更为明显。后者还显示出介电弛豫强度（Δε）的显著增加。电子显微镜和选择性蚀刻用于通过扫描电子显微镜（SEM）确认 MWNTs 在球晶间区无定形相中的定位。在结晶诱导相分离过程中观察到的演化结晶形态对共混物中的电荷输运过程有很强的影响。这些观察结果进一步得到了 FTIR 推断的 a-MWNTs 与 PVDF 之间的特殊相互作用（如偶极诱导偶极相互作用）以及 WXRD 和偏振光显微镜（POM）观察到的结晶形态差异的支持。
