Polyethylene Glycol-Modified Poly(Styrene-co-Ethylene/Butylene-co-Styrene)/Carbon Nanotubes Composite for Humidity Sensing

Polymeric composites of the linear triblock copolymer poly(styrene-co-ethylene/butylene-co-styrene) grafted with maleic anhydride units (SEBS-MA) or MA modified by hydrophilic polyethylene glycol (PEG) and containing various amounts of multiwall carbon nanotubes (MWCNTs) as conducting filler—were prepared by solvent casting. The MWCNT surface was modified by a non-covalent approach with a pyrene-based surfactant to achieve a homogeneous dispersion of the conducting filler within the polymeric matrix. The dispersion of the unmodified and surfactant-modified MWCNTs within the elastomeric SEBS-MA and SEBS-MA-PEG matrices was characterized by studying the morphology by TEM and SAXS. Dynamical mechanical analysis was used to evaluate the interaction between the MWCNTs and copolymer matrix. The electrical conductivity of the prepared composites was measured by dielectric relaxation spectroscopy, and the percolation threshold was calculated. The prepared elastomeric composites were characterized and studied as humidity sensor. Our results demonstrated that at MWCNTs concentration slightly above the percolation threshold could result in large signal changes. In our system, good results were obtained for MWCNT loading of 2 wt% and an ~0.1 mm thin composite film. The thickness of the tested elastomeric composites and the source current appear to be very important factors that influence the sensing performance

Glass transition and molecular dynamics in poly(dimethylsiloxane)/silica nanocomposites.

Abstract The molecular dynamics of a series of poly(dimethylsiloxane) networks filled with silica nanoparticles synthesized in situ was investigated using thermally stimulated depolarization currents, broadband dielectric relaxation spectroscopy and differential scanning calorimetry. The techniques used cover together a broad frequency range (10 K3 -10 9 Hz), thus allowing to gain a more complete understanding of the effects of the nanoparticles on the chain dynamics. In addition to the a relaxation associated with the glass transition of the polymer matrix, we observe in dielectric measurements a slower a relaxation which is assigned to polymer chains close to the polymer/filler interface whose mobility is restricted due to interactions with the filler surface. The thickness of the interfacial layer is estimated to be about 2.1-2.4 nm. Differential scanning calorimetry shows a change in the shape of the glass transition step, as well as a decrease in both the degree of crystallinity and the crystallization rate by the addition of silica.

Effects of Hydration/Dehydration on Interfacial Polymer Fraction and Dynamics in Nanocomposites Based on Metal–Oxides and Physically Adsorbed Polymer

Effects of hydration on the interfacial dynamics of polydimethylsiloxane (PDMS) adsorbed on silica and titania nanoparticles in the form of aggregates of different specific surface area (<i>S</i>_BET, 25–342 m²/g) were studied employing equilibrium water sorption/desorption isotherms (ESI/EDI) and broad-band dielectric spectroscopy (BDS). ESI revealed a systematic increase of the water content, <i>h</i>, of initial oxides (varying up to 60 wt %) with increasing <i>S</i>_BET, whereas changes in <i>h</i> in nanocomposites (PNCs) are very weak (0.1–0.3 wt %) and practically independent from <i>S</i>_BET. BDS allowed for the detection of an increase in the interfacial polymer fraction with hydration, accompanied by a significant enhancement of dynamics and cooperativity of the corresponding segmental relaxation in the interfacial layer (α_int relaxation), changes being reversible as recorded by hydration and subsequent dehydration of the samples. At the same time, bulk PDMS dynamics in PNCs (α relaxation) was not significantly affected. Effects of hydration on the characteristics of α_int in PNCs were found to be qualitatively similar here with changes imposed by thermal annealing (resulting in increasing crystallinity) and surface modifications (resulting in changes in <i>S</i>_BET) recorded recently in similar PNCs. Hydration effects were additionally followed via monitoring the dielectric relaxation process attributed to hydrated hydroxyls at the surfaces of the oxides (<i>S</i> relaxation) in the PNCs. The results may suggest that with increasing <i>h</i> the concentration of contact points between the solid surface (oxides) and the polymer chains increases, while the polymer chain conformations at the interface change toward an increase in looplike interfacial polymer segments

Dynamic glass transition of the rigid amorphous fraction in polyurethane-urea/SiO2 nanocomposites

We report molecular dynamics in the rigid amorphous fraction (RAF) of the polymer bound at the interfaces with nanoparticles in polymer nanocomposites and calculate the glass transition temperature, Tg, for this bound layer of polymer. We follow the '3-phase-model' for semicrystalline polymers where the polymer matrix consists of the crystalline fraction (CF), the mobile amorphous fraction (MAF) and the RAF. While the amorphous polymer bound by crystallites is completely rigid, neither contributing to the glass transition, nor displaying molecular dynamics, the amorphous polymer bound at the interfaces with filler displays decelerated dynamics, as compared to the bulk polymer. Reports in the literature suggest a discrepancy between Tg values obtained by Differential Scanning Calorimetry (DSC) and by Dielectric Relaxation Spectroscopy (DRS). As a plausible explanation we suggest that DRS results in Tg values taking into account the bound polymer, whereas DSC does not. For this investigation we use semicrystalline polyurethane-urea/SiO2 nanocomposites and employ, next to DSC and DRS, SEM, SAXS and WAXS for morphological characterization. It is our intention to use DRS as a tool for investigating the RAF

Effect of soft segment molecular weight on the glass transition, crystallinity, molecular mobility and segmental dynamics of poly(ethylene oxide) based poly(urethane-urea) copolymers

The effect of poly(ethylene oxide) (PEO) soft segment molecular weight (Mn = 2000, 4600 and 8000 g mol-1) on the glass transition, crystallinity, molecular mobility and segmental dynamics of a series of aliphatic polyurethaneurea copolymers (PUU) with a constant hard segment content of 30% by weight was investigated using differential scanning calorimetry and dielectric relaxation spectroscopy. The soft segment (PEO) glass transition temperature increased with increasing molecular weight. Furthermore, five different relaxations were observed in dielectric analyses of all copolymers. These included local glassy state motions (γ) and (β), segmental motion of the soft phase (α), conductivity relaxation, and interfacial Maxwell-Wagner-Sillars (MWS) polarization. Local relaxations follow Arrhenius behavior and their time scale is not affected by the soft segment molecular weight. α-Relaxation follows Vogel-Tammann-Fulcher (VTF) behavior and is slower for the copolymer based on PEO-4600. Conductivity relaxation and the interfacial MWS polarization also follow VTF behavior and have quite similar slopes since both are related to the same phenomena. Unexpectedly, the interfacial MWS polarization is not affected by the soft segment molecular weight. Although this result suggests that the soft segment molecular weight does not affect the microphase separation in these copolymers, we believe that no safe conclusions can be extracted for this system due to the high complexity and the presence of many phases with different conductivity. However, significant differences were observed in the conductivity relaxation, which is much faster for the copolymer based on PEO-2000, due to its lower crystallinity when compared with others

Nanostructure and molecular dynamics in rodlike polyimide/flexible-chain polyimide molecular composites

International audienc

Structure-property relationships in polyimide molecular composites

International audienc