This
work deals with effects of polymer molecular weight, W
m, below the entanglement threshold, W
m,e, on molecular dynamics of polydimethylsiloxane
(PDMS) adsorbed onto silica particles, employing differential scanning
calorimetry (DSC) and two dielectric techniques: broadband dielectric
spectroscopy (BDS) and thermally stimulated depolarization currents
(TSDC). The rigid amorphous polymer fraction at interfaces, RAFint, was found suppressed for larger W
m by all techniques in qualitative agreement with each other.
Results on RAFint were supported by evaluating, for the
first time, the coverage of hydroxyls at the surfaces of nanoparticles
by polymer chains (S relaxation). The mobility of
interfacial polymer (αint relaxation) was followed
by BDS and TSDC, showing suppression of dynamics and cooperativity
with decreasing W
m. We suggest that interfacial
polymer fraction and dynamics are dominated by the concentration of
polymer–particle contact points, the latter increasing for
smaller W
m
due to more
free chain ends, as expected below W
m,e. Furthermore, adopting models that describe multiple conformations
for polymers adsorbed on solid surfaces, we explain our results in
terms of promotion of tail/loop-like conformations in the particle–polymer
interfacial layer for shorter/longer polymer chains, respectively.
The model was further checked by employing surface modification of
initial silica, which resulted in smoothening of nanoparticle surface
and led to further suppression of RAFint and interfacial
polymer dynamics.
A new multi-dimensional quasi-discrete model is suggested and tested for the analysis of heating and evaporation of Diesel fuel droplets. As in the original quasi-discrete model suggested earlier, the components of Diesel fuel with close thermodynamic and transport properties are grouped together to form quasi-components. In contrast to the original quasi-discrete model, the new model takes into account the contribution of not only alkanes, but also various other groups of hydrocarbons in Diesel fuels; quasi-components are formed within individual groups. Also, in contrast to the original quasidiscrete model, the contributions of individual components are not approximated by the distribution function of carbon numbers. The formation of quasi-components is based on taking into account the contributions of individual components without any approximations. Groups contributing small * Corresponding author.Tel. +44 (0)
Abstract:In this review, the importance of water in hydrogel (HG) properties and structure is analyzed. A variety of methods such as 1 H NMR (nuclear magnetic resonance), DSC (differential scanning calorimetry), XRD (X-ray powder diffraction), dielectric relaxation spectroscopy, thermally stimulated depolarization current, quasi-elastic neutron scattering, rheometry, diffusion, adsorption, infrared spectroscopy are used to study water in HG. The state of HG water is rather non-uniform. According to thermodynamic features of water in HG, some of it is non-freezing and strongly bound, another fraction is freezing and weakly bound, and the third fraction is non-bound, free water freezing at 0 • C. According to structural features of water in HG, it can be divided into two fractions with strongly associated and weakly associated waters. The properties of the water in HG depend also on the amounts and types of solutes, pH, salinity, structural features of HG functionalities.
The extensive oxygen-group functionality of single-layer graphene oxide proffers useful anchor sites for chemical functionalization in the controlled formation of graphene architecture and composites. However, the physicochemical environment of graphene oxide and its single-atom thickness facilitate its ability to undergo conformational changes due to responses to its environment, whether pH, salinity, or temperature. Here, we report experimental and molecular simulations confirming the conformational changes of single-layer graphene oxide sheets from the wet or dry state. MD, PM6, and ab initio simulations of dry SLG and dry and wetted SLGO and electron microscopy imaging show marked differences in the properties of the materials that can explain variations in previously observed results for the pH dependent behavior of SLGO and electrical conductivity of chemically modified graphene-polymer composites. Understanding the physicochemical responses of graphene and graphene oxide architecture and performing selected chemistry will ultimately facilitate greater tunability of their performance.
The influence of solution pH (in the range 3–9) on the adsorption of polyacrylic acid (PAA) on the mixed silica–alumina surface (SA-3: SiO2 97 %–Al2O3 3 % and SA-96: SiO2 4 %–Al2O3 96 %) was investigated. The following methods were applied in experiments: spectrophotometry, viscosimetry, potentiometric titration, and microelectrophoresis, which enable determination of adsorbed amount of the polymer, thickness of its adsorption layers, surface charge density, and zeta potential of solid particles in the presence and absence of PAA, respectively. The obtained results indicate that rise of solution pH causes the decrease of PAA adsorption and the increase of its adsorption layer thickness on surfaces of both solids. Moreover, significantly higher adsorption of polyacrylic acid was obtained on the SA-96 surface. This is a result of more favorable electrostatic interactions occurring between the adsorbing polymer chains and the SA-96 surface and formation of a greater number of adsorbate-adsorbent connections through hydrogen bridges.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.