Tuning thermal stability soft nanoparticle membranes through entropic and enthalpic interactions with substrates.

Enthalpic interactions at the interface between nanoparticles and matrix polymers is known to influence various properties of the resultant polymer nanocomposites (PNC). For athermal PNCs, consisting of grafted nanoparticles embedded in chemically identical polymers, the role and extent of the interface layer (IL) interactions in determining the properties of the nanocomposites is not very clear. Here, we demonstrate the influence of the interfacial layer dynamics on the fragility and dynamical heterogeneity (DH) of athermal and glassy PNCs. The IL properties are altered by changing the grafted to matrix polymer size ratio, f , which in turn changes the extent of matrix chain penetration into the grafted layer, λ. The fragility of PNCs is found to increase monotonically with increasing entropic compatibility, characterised by increasing λ. Contrary to observations in most polymers and glass formers, we observe an anti-correlation between the dependence on IL dynamics of fragility and DH, quantified by the experimentally estimated Kohlrausch-Watts-Williams parameter and the non-Gaussian parameter obtained from simulations.

Viscosity and fragility are key parameters determining the processability and thermo-mechanical stability of glassy polymers and polymer nanocomposites (PNCs).

Understanding the fundamentals of nanoparticle (NP) penetration into soft matter systems is indispensable for numerous applications ranging from targeted nanoparticle-based drug delivery to generating hybrid polymer nanocomposite materials. Hence, it is crucial to identify the parameters which control the extent of NP penetration. Here we study the penetration of polystyrene-grafted Au nanoparticles (PGNPs) into an entropically/enthalpically coupled soft polymer film. The system consists of two layers: ultrathin monolayer of ordered grains of PGNPs on top of a bulk polymer film.To study enthalpic effects on nanoparticle penetration, PGNP monolayer was coupled to two different polymers, polystyrene (PS) and poly(tert-butyl acrylate) (PtBA). When the temperature of the system is increased toward the glass transition temperature of underlying films, the width and extent of penetration of the PGNP layer depends on the Flory−Huggins parameter between the graft chain of the PGNPs and the underlying matrix polymer. In athermal cases (PGNP/PS) (χ = 0), the initially compact monolayer undergoes structural disordering and individual PGNPs penetrate into PS films to form a broad layer. However, in the second case (PGNP/PtBA) (χ ≈ 0.26), unfavorable enthalpic interactions results in PGNPs penetrating together as a monolayer into PtBA leading to the formation of a narrow layer of PGNP. The extent of PGNP penetration is improved upon increasing the entropic and enthalpic compatibility between PGNPs and underlying bulk layer. The experimental findings are corroborated by molecular dynamics simulation studies, where the time evolution of PGNP penetration into a bottom polymer layer is found to be similar to that in experiments.

We investigate the thermal gelation of egg white proteins at different temperatures with varying salt concentrations using X-ray photon correlation spectroscopy in the geometry of ultra-small angle X-ray scattering. Temperature-dependent structural investigation suggests a faster network formation with increasing temperature and the gel adopts a more compact network which is inconsistent with the conventional understanding of thermal aggregation. The resulting gel network shows a fractal dimension δ, ranging from 1.5 to 2.2. The value of δ displays a nonmonotonic behavior with increasing amount of salt. The corresponding dynamics in the q range of 0.002 to 0.1 nm-1 is observable after major change of the gel structure. The extracted relaxation time exhibits a two-step power law growth in dynamics as a function of waiting time. In the first regime, the dynamics is associated with structural growth, whereas the second regime is associated with the aging of the gel which is directly linked with its compactness as quantified by the fractal dimension. The gel dynamics is characterized by a compressed exponential relaxation with a ballistic-type of motion. The addition of salt gradually makes the early-stage dynamics faster. Both gelation kinetics and microscopic dynamics show that the activation energy barrier in the system systematically decreases with increasing salt concentration.