Cooperativity and Structural Relaxations in PVDF/PMMA Blends in the Presence of MWNTs: An Assessment through SAXS and Dielectric Spectroscopy

Abstract

Intermolecular cooperativity and structural relaxations in PVDF/PMMA blends were studied in this work with respect to different surface modified (amine, ∼NH2; carboxyl acid, ∼COOH and pristine) multiwalled nanotubes (MWNTs) at 1 wt % near blend’s Tg and in the vicinity of demixing using dielectric spectroscopy, SAXS, DSC, and WAXD. Intermolecular cooperativity at Tg and configurational entropy was addressed in the framework of cooperative rearranging region (CRR) at Tg. Because of specific interactions between PVDF and NH2-MWNTs, the local composition fluctuates at its average value resulting in a broad Tg. The scale of cooperativity (ξCRR) and the number of segments in the cooperative volume (NCRR) is comparatively smaller in the blends with NH2-MWNTs. This clearly suggests that the number of segments cooperatively relaxing is reduced in the blends due to specific interactions leading to more heterogeneity. The configurational entropy at Tg, as derived from Vogel-Fulcher and Adam–Gibbs analysis, was reduced in the blends in presence of MWNTs manifesting in entropic penalty of the chains. The crystallite size and the amorphous miscibility was evaluated using SAXS and was observed to be strongly contingent on the surface functional groups on MWNTs. Three distinct relaxations—αc due to relaxations in the crystalline phase of PVDF, αm indicating the amorphous miscibility in PVDF/PMMA blends, and αβ concerning the segmental dynamics of PMMA—were observed in the blends in the temperature range Tg < T < Tc. The dynamics as well as the nature of relaxations were observed to be dependent the surface functionality on the MWNTs. The dielectric permittivity was also enhanced in presence of MWNTs, especially with NH2-MWNTs, with minimal losses. The influence of the MWNTs on the spherulite size and crystalline morphology of the blends was also confirmed by POM and SEM.