Despite the wide use of polymer nanocomposites (PNCs)in various applications, our understanding of the microscopic parameterscontrolling their macroscopic properties remains limited. In this study, weexamine the dielectric strength of segmental dynamics, ΔεIL(T) in theinterfacial polymer layer surrounding the nanoparticles in PNCs. Thepresented analysis reveals a significant drop in ΔεIL(T) and its anomaloustemperature dependence in the polymer layer adsorbed to nanoparticles.The drop in ΔεIL(T) was observed in all samples regardless of whethersegmental relaxation time in the interfacial layer was slower or faster thanin the bulk polymer, excluding interpretation of the “dead” layer. Weascribe the observed decrease in the dielectric strength to the restrictedamplitude of segmental relaxation in the interfacial/adsorbed layer. Ourresults provide a new perspective on discussion of dynamics in theinterfacial layer in PNCs and thin polymer films, demonstrating that notonly segmental relaxation time but also its amplitude can be strongly affected by the interface.

The addition of hard fillers to a polymer matrix is a well-known process for achieving mechanical reinforcement. With a decrease in the size of the fillers, the contribution from polymer–particle nanometer-sized interfaces becomes significant, and these interfaces affect the mechanical performance of polymer nanocomposites (PNCs) beyond the limits established for conventional composites. However, the molecular mechanisms underlying the improvement in the mechanical performance of glassy PNCs remain unresolved, necessitating a deeper understanding of the structure–property relationships in these intrinsically heterogeneous systems. In this effort, by using Brillouin light scattering (BLS) and dynamic mechanical analysis (DMA), we demonstrated that adding shorter chains to a PNC prepared with high molecular weight polymers significantly improved the mechanical properties of the PNC in the glassy state. The strongest enhancement of mechanical properties occurred at an optimum concentration of short chains. This is in contrast to the behavior of the glass transition temperature of PNCs which shows a monotonic decrease with an increase in the concentration of shorter chains. Using experimental data and coarse-grained molecular dynamics (MD) simulations, we have identified the molecular mechanism leading to the observed nonmonotonic changes in mechanical reinforcement. This mechanism includes changes in the nanoscale organization at the interface combined with chain stretching amplified by the addition of the short chains. Overall, our approach paves a simple and cost-effective pathway to fabricating glassy PNCs with significantly improved mechanical properties that will fill various practical needs.

We analyze stress distributions in a two-dimensional bidisperse cemented granular packing for a broad range of the values of particle-size ratio, the volumes of large and small particles, and the amount of cementing matrix. In such textured porous materials, the stress concentration, which controls the fracture and fragmentation of the material under tensile loading or in grinding processes, reflects not only the porosity but also the contact network of the particle phase and the resulting stress chains. By means of peridynamic simulations under tensile loading, we show how both the texture and stress distribution depend on size ratio, volume ratio, and the amount of the cementing matrix. In particular, the volume fraction of the class of small particles plays a key role in homogenizing stresses across the system by reducing porosity. Interestingly, the texture controls not only the porosity but also the distribution of pores inside the system with its statistical variability, found to be strongly correlated with the homogeneity of stresses inside the large particles. The most homogeneous stress distribution occurs for the largest size ratio and largest volume fraction of small particles, corresponding to the lowest pore size dispersion and the cushioning effect of small particles and its similar role to the binding matrix for stress redistribution across the packing. At higher porosity, the tensile stresses above the mean stress fall off exponentially in all phases with an exponent that strongly depends on the texture. The exponential part broadens with decreasing matrix volume fraction and particle-size ratio. These correlations reveal the strong interplay between size polydispersity and the cohesive action of the binding matrix for stress distribution, which is significant for the behavior of textured materials in grinding operations.

We propose a method relying on structural measurements by small-angle scattering to quantitatively follow aggregation of nanoparticles (NPs) in concentrated colloidal assemblies or suspensions up to percolation, regardless of complex structure factors arising due to interactions. As experimental model system, the dispersion of silica NPs in a styrene-butadiene matrix has been analyzed by small-angle X-ray scattering and transmission electron microscopy (TEM), as a function of particle concentration. A reverse Monte Carlo analysis applied to the NP scattering compared favorably with TEM. By combining it with an aggregate recognition algorithm, series of representative real space structures and aggregation number distribution functions have been determined up to high concentrations, taking into account particle polydispersity. Our analysis demonstrates that the formation of large percolating aggregates on the scale of the simulation box (of linear dimension 1/qmin, here micron-sized) can be mapped onto the macroscopic percolation characterized by rheology. Our method is thus capable of determining aggregate structure in dense NP systems with strong – possibly unknown – interactions visible in scattering. It is hoped to be useful in many other colloidal systems, beyond the case of polymer nanocomposites exemplarily studied here.

Inspired by the path followed by Matthias Ballauff over the past 20 years, the development of thermosensitive core-shell microgel structures is reviewed. Different chemical approaches, from hard nanoparticle cores to double stimuli-responsive microgels have been devised and successfully implemented by many different groups. Some of the rich variety of these systems is presented, as well as some recent progress in structural analysis of such microstructures by small-angle scattering of neutrons or X-rays, including modeling approaches. In the last part, again following early work by the group of Matthias Ballauff, applications with particular emphasis on incorporation of catalytic nanoparticles inside core-shell structures – stabilizing the nanoparticles and granting external control over activity – will be discussed, as well as core-shell microgels at interfaces.

The radial density profile of deuterated poly(N,n-propyl acrylamide) shell monomers within core-shell microgels has been studied by small angle neutron scattering in order to shed light on the origin of their linear thermally-induced swelling. The poly(N-isopropyl methacrylamide) core monomers have been contrast-matched by the H2O/D2O solvent, and the intensity thus provides a direct measurement of the spatial distribution of the shell monomers. Straightforward modelling shows that their structure does not correspond to the expected picture of a well-defined external shell. A multi-shell model solved by a reverse Monte Carlo approach is then applied to extract the monomer density as a function of temperature and of the core crosslinking. It is found that most shell monomers fill the core at high temperatures approaching synthesis conditions of collapsed particles, forming only a dilute corona. As the core monomers tend to swell at lower temperatures, a skeleton of insoluble shell monomers hinders swelling, inducing the progressive linear thermoresponse.

Well-defined double hydrophilic block copolymers (DHBCs) of poly(ethylene glycol)-block-poly(N,N-diethyla-crylamide) (PEG-b-PDEAm) were synthesized via a reversible addition-fragmentation chain-transfer (RAFT) polymerization of N,N-diethylacrylamide (DEAm) using dithioester terminated PEG of different chain lengths as macro-chain transfer agents. Controlled molecular weight and narrow molecular weight distribution were achieved as proven by size exclusion chromatography (SEC). Cloud points (CP) were determined via turbidity measurements with ultraviolet-visible spectroscopy (UV-vis) and LCST by differential scanning micro-calori-metry (micro-DSC). They range from 32 to 40°C depending on the PDEAm length block and its concentration. Above the LCST, fluorescence spectroscopy and dynamic light scattering (DLS) demonstrated that the PEG-b-PDEAm copolymers self-organized into large aggregates. More interestingly, copolymers pre-aggregated at relatively high concentrations below the LCST.