Effect of the GO Reduction Method on the Dielectric Properties, Electrical Conductivity and Crystalline Behavior of PEO/rGO Nanocomposites.

The effect of the reduction method to prepare reduced graphene oxide (rGO) on the melt linear viscoelastic properties, electrical conductivity, polymer matrix crystalline behavior and dielectric properties of PEO-rGO nanocomposites was investigated. Reduction was performed chemically with either sodium borohydride (NaBH4) or hydrazine monohydrate (N2H4·H2O) or both reduction agents consecutively as well as thermally at 1000 °C. The different reduction methods resulted in exfoliated rGO sheets with different types and amounts of remaining functional groups, as indicated by FT-IR, Raman, TGA and XRD characterization. Moreover, their electrical conductivity ranged between 10-4 and 10-1 S/cm, with the consecutive use of both chemical reduction agents being far superior. PEO nanocomposites with filler loadings of 0.5 wt %, 1 wt % and 2 wt % were prepared by solvent mixing. The rGO fillers affected the melt linear viscoelastic and crystalline behavior of the PEO matrix and resulted in nanocomposites with a substantially increased electrical conductivity. Despite the wide variability in filler conductivity, the effects on the polymer nanocomposite properties were less distinctive. A correlation was obtained between the reduction of the mobility of the polymer chains (evaluated by the glass transition temperature) and the dielectric strength of the interfacial polarisation originating from the effective entrapment of GO/rGO filler charges at the interface with the less conductive PEO. Thus, favorable interactions of the polar PEO with the filler led to reduced mobility of the PEO chains and thereby a more effective entrapment of the filler charges at the PEO interface.

Effect of Compression on the Molecular Arrangement of Itraconazole-Soluplus Solid Dispersions: Induction of Liquid Crystals or Exacerbation of Phase Separation?

Predensification and compression are unit operations imperative to the manufacture of tablets and capsules. Such stress-inducing steps can cause destabilization of solid dispersions which can alter their molecular arrangement and ultimately affect dissolution rate and bioavailability. In this study, itraconazole-Soluplus solid dispersions with 50% (w/w) drug loading prepared by hot-melt extrusion (HME) were investigated. Compression was performed at both pharmaceutically relevant and extreme compression pressures and dwell times. The starting materials, powder, and compressed solid dispersions were analyzed using modulated differential scanning calorimetry (MDSC), X-ray diffraction (XRD), small- and wide-angle X-ray scattering (SWAXS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), and broadband dielectric spectroscopy (BDS). MDSC analysis revealed that compression promotes phase separation of solid dispersions as indicated by an increase in glass transition width, occurrence of a peak in the nonreversing heat flow signal, and an increase in the net heat of fusion indicating crystallinity in the systems. SWAXS analysis ruled out the presence of mesophases. BDS measurements elucidated an increase in the Soluplus-rich regions of the solid dispersion upon compression. FTIR indicated changes in the spatiotemporal architecture of the solid dispersions mediated via disruption in hydrogen bonding and ultimately altered dynamics. These changes can have significant consequences on the final stability and performance of the solid dispersions.

The key role of polymer grafted nanoparticles in the phase miscibility of an LCST mixture.

Blends of bromo-terminated polystyrene (PS-Br) and poly(vinyl methylether) (PVME) exhibit lower critical solution temperatures. In this study, PS-Br was designed by atom transfer radical polymerization and was converted to thiol-capped polystyrene (PS-SH) by reacting with thiourea. The silver nanoparticles (nAg) were then decorated with covalently bound PS-SH macromolecules to improve the phase miscibility in the PS-Br-PVME blends. Thermally induced demixing in this model blend was followed in the presence of polystyrene immobilized silver nanoparticles (PS-g-nAg). The graft density of the PS macromolecules was estimated to be ca. 0.78 chains per nm(2). Although the matrix and the grafted molecular weights were similar, PS-g-nAg particles were expelled from the PS phase and were localized in the PVME phase of the blends. This was addressed with respect to intermediate graft density and favourable PS-PVME contacts from microscopic interactions point of view. Interestingly, blends with 0.5 wt% PS-g-nAg delayed the spinodal decomposition temperature in the blends by ca. 18 °C with respect to the control blends. The scale of cooperativity, as determined by differential scanning calorimetry, increased only marginally in the case of PS-g-nAg; however, it increased significantly in the presence of bare nAg particles.

An unusual demixing behavior in PS-PVME blends in the presence of nanoparticles.

The effect of silver nanoparticles (sNP) on the demixing and the evolution of morphology in off-critical blends of 90/10 and 10/90 (wt/wt) PS/PVME [polystyrene/poly(vinyl methyl ether)] was probed here using shear rheology and optical microscopy. The faster component (PVME) has a higher molecular weight (80 kDa) than the slower component (PS, 35 kDa), which makes this system quite interesting to study with respect to the evolving morphology, as the blends transit through the binodal and the spinodal envelopes. An unusual demixing behavior was observed in both PVME rich and PS rich blends. Temperature modulated differential scanning calorimetry measurements showed that the Tg value for the blends with sNP was slightly lower than that of the neat blends. A decreased volume of cooperativity at Tg suggests confined segmental dynamics in the presence of sNP. Although, the addition of sNP had no influence on the thermodynamic demixing temperature, it significantly altered the elasticity of the minor component during the transition of the blend from the homogeneous to the heterogeneous state. This is manifested from energetically driven localization of the sNP in the PVME phase during demixing. As a direct consequence of this, the formation of the microstructures upon demixing was observed to be delayed in the presence of sNP. Interestingly, in the intermediate quench depth, the higher viscoelastic phase evolved as an interconnected network, which subsequently coarsened into discrete droplets in the late stages for the 90/10 PS/PVME blends. Similar observations were made for 10/90 PS/PVME blends where threads of PVME appeared at deeper quench depths in the presence of sNP. The interconnected network formation of the minor phase (here PVME), which is also the faster component in the blend, was different from the usual demixing behavior.

Nanoparticle-driven intermolecular cooperativity and miscibility in polystyrene/poly(vinyl methyl ether) blends.

The effect of silver nanoparticles (nAg) in PS/PVME [polystyrene/poly(vinyl methyl ether)] blends was studied with respect to the evolution of morphology, demixing temperature, and segmental dynamics. In the early stage of demixing, PVME developed an interconnected network that coarsened in the late stage. The nAg induced miscibility in the blends as supported by shear rheological measurements. The physicochemical processes that drive phase separation in blends also led to migration of nAg to the PVME phase as supported by AFM. The segmental dynamics was greatly influenced by the presence of nAg due to the specific interaction of nAg with PVME. Slower dynamics and an increase in intermolecular cooperativity in the presence of nAg further supported the role of nAg in delaying the phase separation processes and augmenting the demixing temperature in the blends. Different theoretical models were assessed to gain insight into the dynamic heterogeneity in PS/PVME blends at different length scales.