In Situ Rheodielectric, ex Situ 2D-SAXS, and Fourier Transform Rheology Investigations of the Shear-Induced Alignment of Poly(styrene-b-1,4-isoprene) Diblock Copolymer Melts

Linear and Nonlinear Rheology Combined with Dielectric Spectroscopy of Hybrid Polymer Nanocomposites for Semiconductive Applications

The linear and nonlinear oscillatory shear, extensional and combined rheology-dielectric spectroscopy of hybrid polymer nanocomposites for semiconductive applications were investigated in this study. The main focus was the influence of processing conditions on percolated poly(ethylene-butyl acrylate) (EBA) nanocomposite hybrids containing graphite nanoplatelets (GnP) and carbon black (CB). The rheological response of the samples was interpreted in terms of dispersion properties, filler distortion from processing, filler percolation, as well as the filler orientation and distribution dynamics inside the matrix. Evidence of the influence of dispersion properties was found in linear viscoelastic dynamic frequency sweeps, while the percolation of the nanocomposites was detected in nonlinearities developed in dynamic strain sweeps. Using extensional rheology, hybrid samples with better dispersion properties lead to a more pronounced strain hardening behavior, while samples with a higher volume percentage of fillers caused a drastic reduction in strain hardening. The rheo-dielectric time-dependent response showed that in the case of nanocomposites containing only GnP, the orientation dynamics leads to non-conductive samples. However, in the case of hybrids, the orientation of the GnP could be offset by the dispersing of the CB to bridge the nanoplatelets. The results were interpreted in the framework of a dual PE-BA model, where the fillers would be concentrated mainly in the BA regions. Furthermore, better dispersed hybrids obtained using mixing screws at the expense of filler distortion via extrusion processing history were emphasized through the rheo-dielectric tests.

Structure and Dynamics of Asymmetric Poly(styrene-b-1,4-isoprene) Diblock Copolymer under 1D and 2D Nanoconfinement

The impact of 1- and 2-dimensional (2D) confinement on the structure and dynamics of poly(styrene-b-1,4-isoprene) P(S-b-I) diblock copolymer is investigated by a combination of Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Grazing-Incidence Small-Angle X-ray Scattering (GISAXS), and Broadband Dielectric Spectroscopy (BDS). 1D confinement is achieved by spin coating the P(S-b-I) to form nanometric thin films on silicon substrates, while in the 2D confinement, the copolymer is infiltrated into cylindrical anodized aluminum oxide (AAO) nanopores. After dissolving the AAO matrix having mean pore diameter of 150 nm, the SEM images of the exposed P(S-b-I) show straight nanorods. For the thin films, GISAXS and AFM reveal hexagonally packed cylinders of PS in a PI matrix. Three dielectrically active relaxation modes assigned to the two segmental modes of the styrene and isoprene blocks and the normal mode of the latter are studied selectively by BDS. The dynamic glass transition, related to the segmental modes of the styrene and isoprene blocks, is independent of the dimensionality and the finite sizes (down to 18 nm) of confinement, but the normal mode is influenced by both factors with 2D geometrical constraints exerting greater impact. This reflects the considerable difference in the length scales on which the two kinds of fluctuations take place.