Local Structure and Relaxation Dynamics in the Brush of Polymer-Grafted Silica Nanoparticles

Advancing elastomer performance with dynamic bond networks in polymer-grafted single-chain nanoparticles: a molecular dynamics exploration

This research introduces a method to enhance the mechanical properties of elastomers by grafting polymer chains onto single-chain flexible nanoparticles (SCNPs) and incorporating dynamic functional groups. Drawing on developments in grafting polymers onto hard nanoparticle fillers, this method employs the distinct flexibility of SCNPs to diminish heterogeneity and enhance core size control. We use molecular dynamics (MD) simulations for a mesoscale analysis of structural properties, particularly the effects of dynamic functional group quantities and their distribution. The findings demonstrate that increased quantities of functional groups are correlated with enhanced mechanical strength and toughness, showing improved stress-strain responses and energy dissipation capabilities. Moreover, the uniformity in the distribution of these functional groups is crucial, promoting a more cohesive and stable dynamic bonding network. The insights gained from MD simulations not only advance our understanding of the microstructural control necessary for optimizing macroscopic properties, but also provide valuable guidance for the design and engineering of advanced polymer nanocomposites, thereby enhancing the material performance through strategic molecular design.

Confined Dynamics in Spherical Polymer Brushes

We investigate the dynamics of polymers grafted to spherical nanoparticles in solution using hybrid molecular dynamics simulations with a coarse-grained solvent modeled via the multiparticle collision dynamics algorithm. The mean-square displacements of monomers near the surface of the nanoparticle exhibit a plateau on intermediate time scales, indicating confined dynamics reminiscent of those reported in neutron spin-echo experiments. The confined dynamics vanish beyond a specific radial distance from the nanoparticle surface that depends on the polymer grafting density. We show that this dynamical confinement transition follows theoretical predictions for the critical distance associated with the structural transition from confined to semidilute brush regimes. These findings suggest the existence of a hitherto unreported dynamic length scale connected with theoretically predicted static fluctuations in spherical polymer brushes and provide new insights into recent experimental observations.

The Role of Interchain Force and/or Chain Entanglement in the Melt Strength and Ductility of PLA-Based Materials

As an eco-friendly material, PLA was a desirable alternative to polyethylene and polypropylene films due to its biodegradability. The preferable melt strength of PLA-based materials was a key factor in ensuring its processing using extrusion blow. This paper focuses on the influence of interchain force and/or chain entanglement on the melt strength and ductility of PLA-based materials in recent years. In addition, the preparation of PLA-based materials via physical blending or reactive processing was also summarized. The blending of PLA with a flexible heteropolymer, driven by the interchain force and/or chain entanglements, were characterized as a practicable method for toughening PLA-based materials. Also, the restructuring of PLA chains, by branching based on chain entanglement, was suitable for increasing chain entanglements in PLA matrix, yielding satisfactory melt strength and ductility. This review aims to elucidate the relationship between interchain forces and/or entanglement with the melt strength and ductility of PLA-based materials. An essential and systematic understanding of the tailoring melt strength and rheological properties of PLA by interchain forces and/or entanglement was apt to improve and perfect the processing technology of the extrusion blow, and consequently improve the tensile strength and toughness of PLA films.

Investigation of the Effect of Molecular Weight, Density, and Initiator Structure Size on the Repulsive Force between a PNIPAM Polymer Brush and Protein

This paper focuses on the effect of degree of polymerization (N), density ( $\sigma$ ), and pattern size ( $x$ ) on the interaction force between a periodically patterned Poly(N-isopropylacrylamide) (PNIPAM) brush and protein. The hydrophobic interaction, the Van der Waals attractive force, and the steric repulsive force were expressed in terms of $N$ , $\sigma$ , and $x$ . The osmotic constant (k1) and the entropic constant (k2) were determined from the fit of the steric repulsive force to an experimentally obtained force distance curve. The osmotic constant was 0.105, and the entropic constant was 0.255. Using these constants, the steric repulsive force was plotted as a function of the separation distance(s) between the substrate and the protein. The forces were determined at a separation distance equal to 0.3 nm, where L0 is the equilibrium thickness of the PNIPAM brush. At this separation distance, the value of the steric repulsive force was much higher than the value of the sum of the hydrophobic interaction and the Van der Waals attractive force for large degree of polymerization ( $N>100$ ) and density ( $\sigma >0.2$ chains/nm2). However, the repulsive force was comparable to the sum of the hydrophobic interaction and the Van der Waals attractive force for a small degree of polymerization ( $N<100$ ) and density ( $\sigma =0.2$ ). Furthermore, the steric repulsive force was plotted as a function of pattern size $x$ . The plot indicated that the steric repulsive force becomes nearly zero for all degrees of polymerization and density when the value of the initiator structure size was less than 200 nm. In addition to the steric repulsive force, the lateral extension of the chains in the periodically patterned PNIPAM brush was calculated by scaling low and compared with the experimental data taken from previously published literatures. The polymer brush structure was modelled as if the immediate bare substrate is so wide that even a stretched polymer segment cannot reach to the next polymer brush structure. In such models, the value of the lateral extension was equal to the thickness of the homogenous brush. It was independent of the pattern size. However, when the polymer brush structure was modelled as if there is another polymer brush structure at a distance half of the size of the period, the lateral extension was found to be dependent on the size of the initiator structure size due to chain bridging. This was witnessed by the patterning of polymer brushes using the interferometric patterning of PNIPAM brushes and an atomic force microscopy imaging of the polymer brush structures both in air and in water. The polymer brush structure resolution in water was much lower than the resolution in air, which indicates the lateral extension of the polymer chains in water. For such kind of periodic polymer brush structures, the gap between them was calculated, and it was found dependent on the degree of polymerization, density, and initiator structure size.

Local Structure of Polymer-Grafted Nanoparticle Melts

Polymer-grafted nanoparticle (GNP) membranes show unexpected gas transport enhancements relative to the neat polymer, with a maximum as a function of graft molecular weight (MW_g ≈ 100 kDa) for sufficiently high grafting densities. The structural origins of this behavior are unclear. Simulations suggest that polymer segments are stretched near the nanoparticle (NP) surface and form a dry layer, while more distal chain fragments are in their undeformed Gaussian states and interpenetrate with segments from neighboring NPs. This theoretical basis is derived by considering the behavior of two adjacent NPs; how this behavior is modified by multi-NP effects relevant to gas separation membranes is unexplored. Here, we measure and interpret SAXS data for poly(methyl acrylate)-grafted silica NPs and find that for very low MW_gs, contact between GNPs obeys the two-NP theory─namely that the NPs act like hard spheres, with radii that are linear combinations of the NP core sizes and the dry zone dimensions; thus, the interpenetration zones relax into the interstitial spaces. For chains with MW_g > 100 kDa, the interpenetration zones are in the contact regions between two NPs. These results suggest that for MW_gs below the transition, gas primarily moves through a series of dry zones with favorable transport, with the interpenetration zone with less favorable transport properties in parallel. For higher MW_gs, the dry and interpenetration zones are in series, resulting in a decrease in transport enhancement. The MW_g at the transport maximum then corresponds to the chain length with the largest, unfavorable stretching free energy.

Unusual High-Frequency Mechanical Properties of Polymer-Grafted Nanoparticle Melts

Brillouin light spectroscopy is used to measure the elastic moduli of spherical polymer-grafted nanoparticle (GNP) melts as a function of chain length at fixed grafting density (0.47  chains/nm^{2}) and nanoparticle radius (8 nm). While the moduli follow a rule of mixtures (Wood's law) for long chains, they display enhanced elasticity and anomalous dissipation for graft chains <100  kDa. GNP melts with long polymers at high σ have a dry zone near the GNP core, surrounded by a region where the grafts can interpenetrate with chain fragments from adjacent GNPs. We propose that the departures from Wood's law for short chains are due to the effectively larger silica volume fraction in the region where sound propagates-this is caused by the short, interpenetrated chain fragments being pushed out of the way. We thus conclude that transport mechanisms (of gas, ions, sound, thermal phonons) in GNP melts are radically different if interpenetrated chain segments can be "pushed out of the way" or not. This provides a facile new means for manipulating the properties of these materials.

Diffusion of polymer-grafted nanoparticles with dynamical fluctuations in unentangled polymer melts

The dynamics of polymer-grafted nanoparticles (PGNPs) in melts of unentangled linear chains were investigated by means of coarse-grained molecular dynamics simulations. The results demonstrated that the graft monomers closer to the particle surface relax more slowly than those farther away due to the constraint of the grafted surface and the confinement of the neighboring chains. Such heterogeneous relaxations of the surrounding environment would perturb the particle motion, making them fluctuating around their centers before they can diffuse through the melt. During such intermediate-time stage, the dynamics is subdiffusive while the distribution of particle displacements is Gaussian, which can be described by the popular fractional Brownian motion model. For the long-time Fickian diffusion, we found that the diffusivity D decreases with increasing grafting density Σ_g, grafted chain length N_g, and matrix chain length N_m. This is due to the fact that the diffusivity is controlled by the viscous drag of an effective core, consisting of the NP and the non-draining layer of graft segments, and that of the free-draining graft layer outside the "core". With increasing Σ_g, the PGNPs become harder with greater effective size and thinner free draining layer, resulting in a reduction in D. At extremely high Σ_g, the diffusivity can even be estimated by the diameter-renormalized Stokes-Einstein (SE) relation. With increasing N_g, both the effective core size and the thickness of the free-draining layer increase, leading to a reduction in diffusivity by D ∼ N-γg with 0.5 < γ < 1. Increasing N_m would lead to the enlargement of the effective core size but meanwhile result in the reduction of the free-draining layer thickness due to autophobic dewetting. The counteraction between these two opposite effects leads to only a slight reduction in the diffusivity, significantly different from the typical SE behavior where D ∼ N_m^-1. These findings bear significance in unraveling the fundamental physics of the anomalous dynamics of PGNPs in various polymers, including biological and synthetic.

Simulation of the Coronal Dynamics of Polymer-Grafted Nanoparticles

Polymer-grafted nanoparticles (PGNPs) are an important component of many advanced materials. The interplay between the nanoparticle surface curvature and spatial confinement by neighboring chains produces a complex set of structural and dynamical behaviors in the polymer corona surrounding the nanoparticle. For example, experiments have shown that the inner portion of the corona is more stretched and relaxes more slowly than the outer region. Here, we perform systematic core-modified dissipative particle dynamics (CM-DPD) simulations and analyze the relaxation dynamics using proper orthogonal decomposition (POD) of the monomer coordinates. We find that grafted chains relax more slowly than free chains and that the relaxation time of the grafted chains scales inversely with the confinement strength. For PGNPs in a polymer melt, the relaxation processes are always Rouse-like. However, we observe either Zimm-like or Rouse-like dynamics for PGNPs in solution depending on the confinement strength.

Effect of Dispersity on the Conformation of Spherical Polymer Brushes

We show that dispersity (D̵) markedly alters the conformation of spherical polymer brushes. The average lengths (l_b) of poly(tert-butyl acrylate) (PtBA) brushes of varying D̵ grafted to nanoparticles were measured using dynamic light scattering. In the semidilute polymer brush (SDPB) regime, the l_b of PtBA and polymers from earlier studies of various D̵ could be cleanly collapsed onto a master curve as a function of the scaling variable N_wσ^1/3, where N_w is the weight-average degree of polymerization and σ is the grafting density. In the concentrated polymer brush (CPB) regime, however, l_b collapsed onto a bifurcated curve as a function of the scaling variable N_wσ^1/2, indicating D̵ more strongly affects the average length of brushes with low N_wσ^1/2. We propose that the stretching of the stem near the particle surface due to interchain interactions in the CPB regime leads to greater l_b in broad dispersity brushes of low but not high N_wσ^1/2.

Structure and thermodynamics of grafted silica/polystyrene dilute nanocomposites investigated through self-consistent field theory

Polymer/matrix nanocomposites (PNCs) are materials with exceptional properties. They offer a plethora of promising applications in key industrial sectors. In most cases, it is preferable to disperse the nanoparticles (NPs) homogeneously across the matrix phase. However, under certain conditions NPs might lump together and lead to a composite material with undesirable properties. A common strategy to stabilize the NPs is to graft on their surface polymer chains of the same chemical constitution as the matrix chains. There are several unresolved issues concerning the optimal molar mass and areal density of grafted chains that would ensure best dispersion, given the nanoparticles and the polymer matrix. We propose a model for the prediction of key structural and thermodynamic properties of PNC and apply it to a single spherical silica (SiO2) nanoparticle or planar surface grafted with polystyrene chains embedded at low concentration in a matrix phase of the same chemical constitution. Our model is based on self-consistent field theory, formulated in terms of the Edwards diffusion equation. The properties of the PNC are explored across a broad parameter space, spanning the mushroom regime (low grafting densities, small NPs and chain lengths), the dense brush regime, and the crowding regime (large grafting densities, NP diameters, and chain lengths). We extract several key quantities regarding the distributions and the configurations of the polymer chains, such as the radial density profiles and their decomposition into contributions of adsorbed and free chains, the chains/area profiles, and the tendency of end segments to segregate at the interfaces. Based on our predictions concerning the brush thickness, we revisit the scaling behaviors proposed in the literature and we compare our findings with experiment, relevant simulations, and analytic models, such as Alexander's model for incompressible brushes.

Potential of Mean Force between Bare or Grafted Silica/Polystyrene Surfaces from Self-Consistent Field Theory

We investigate single and opposing silica plates, either bare of grafted, in contact with vacuum or melt phases, using self-consistent field theory. Solid-polymer and solid-solid nonbonded interactions are described by means of a Hamaker potential, in conjunction with a ramp potential. The cohesive nonbonded interactions are described by the Sanchez-Lacombe or the Helfand free energy densities. We first build our thermodynamic reference by examining single surfaces, either bare or grafted, under various wetting conditions in terms of the corresponding contact angles, the macroscopic wetting functions (i.e., the work of cohesion, adhesion, spreading and immersion), the interfacial free energies and brush thickness. Subsequently, we derive the potential of mean force (PMF) of two approaching bare plates with melt between them, each time varying the wetting conditions. We then determine the PMF between two grafted silica plates separated by a molten polystyrene film. Allowing the grafting density and the molecular weight of grafted chains to vary between the two plates, we test how asymmetries existing in a real system could affect steric stabilization induced by the grafted chains. Additionally, we derive the PMF between two grafted surfaces in vacuum and determine how the equilibrium distance between the two grafted plates is influenced by their grafting density and the molecular weight of grafted chains. Finally, we provide design rules for the steric stabilization of opposing grafted surfaces (or fine nanoparticles) by taking account of the grafting density, the chain length of the grafted and matrix chains, and the asymmetry among the opposing surfaces.

Structural and Dynamical Coupling in Solvent-Free Polymer Brushes Elucidated by Molecular Dynamics Simulations

We investigate the chain configuration and segmental dynamics in interacting solvent-free polymer brushes using molecular dynamics simulations. The brush systems are designed to mimic the interstitial space between a pair of neighboring polymer-grafted nanoparticles in solvent-free nanoparticle-organic hybrid materials. Each brush consists of uniformly grafted chains formed by a given number of monomer beads. In monodisperse systems, two opposing brushes have the same chain length and grafting density. In mixed conditions, we consider binary systems with two surfaces being separately grafted with polymers of distinct chain lengths at different grafting densities as well as bidisperse systems with polymers of two different lengths being tethered to the surfaces at a fixed grafting density. We demonstrate that the brush configuration and interpenetration are both governed by the need that monomer beads have to uniformly fill the space. For systems with longer chain lengths and/or higher grafting densities, the larger interwall separation yields more stretched brush conformations and reduced extents of interbrush mixing. As a result, the polymer configurational entropy is generally decreased and the segment-to-segment relaxation dynamics is slowed down accordingly. The grafting of chains at a high density not only makes the relaxation dynamics deviate from the standard Rouse prediction but also leads to distinct relaxation times for the free and tethered segments. The more slowly relaxing tethered segments play a more important role in determining the overall end-to-end fluctuations. Moreover, the two distinct relaxation processes are consistent with the two-stage decay in the Rouse mode fluctuation autocorrelation function. In the presence of brush bidispersity, the collaboration between polymers of different lengths is evidently observed in the brush profiles. The variations of the chain configuration for the two polymers are complementary, and the associated relaxation dynamics of the two species are significantly coupled.

Intermediate scattering functions of a rigid body monoclonal antibody protein in solution studied by dissipative particle dynamic simulation

In the past decade, there was increased research interest in studying internal motions of flexible proteins in solution using Neutron Spin Echo (NSE) as NSE can simultaneously probe the dynamics at the length and time scales comparable to protein domain motions. However, the collective intermediate scattering function (ISF) measured by NSE has the contributions from translational, rotational, and internal motions, which are rather complicated to be separated. Widely used NSE theories to interpret experimental data usually assume that the translational and rotational motions of a rigid particle are decoupled and independent to each other. To evaluate the accuracy of this approximation for monoclonal antibody (mAb) proteins in solution, dissipative particle dynamic computer simulation is used here to simulate a rigid-body mAb for up to about 200 ns. The total ISF together with the ISFs due to only the translational and rotational motions as well as their corresponding effective diffusion coefficients is calculated. The aforementioned approximation introduces appreciable errors to the calculated effective diffusion coefficients and the ISFs. For the effective diffusion coefficient, the error introduced by this approximation can be as large as about 10% even though the overall agreement is considered reasonable. Thus, we need to be cautious when interpreting the data with a small signal change. In addition, the accuracy of the calculated ISFs due to the finite computer simulation time is also discussed.

Impact of the Solvent Quality on the Local Dynamics of Soft and Swollen Polymer Nanoparticles Functionalized with Polymer Chains

Grafting polymer chains on the surface of nanoparticles (NPs) is a strategy used to control the interaction between the NPs and their environment. The fate of the resulting particles in a given environment is strongly influenced by the solvent-polymer interaction. The solvent quality affects the behavior, conformation, and dynamics of the grafted polymer chains. However, when this polymer grafting strategy is used to functionalized polymer particles, the influence of solvent quality becomes even more complex; when the grafted polymer chains and the polymer nanoparticles are tethered together, the effect of the solvent quality on the behavior and dynamics of the system depends on the solvent interaction with both polymer components. To explore the relationship between the solvent quality and the dynamics of polymer-functionalized soft polymer NPs, we designed a system based on cross-linked polystyrene (PS) NPs grafted with a canopy of poly(methyl acrylate) (PMA). PS and PMA, two immiscible polymers, can be selectively solvated by using binary mixtures of solvents. NMR spectroscopy was used to address the effect of those selective solvents on the local mobility of the PS-PMA core-canopy NPs and revealed an interplay between the local mobility of the core and the local mobility of the canopy. A selective reduction of the solvent quality for the PMA canopy resulted in the expected reduction of the local mobility of the PMA chains, but also in the slower dynamics of the PS core. Similarly, a selective reduction of the solvent quality for the PS core resulted in a slower dynamics for both the PS core and the PMA canopy.

Influence of the Architecture of Soft Polymer-Functionalized Polymer Nanoparticles on Their Dynamics in Suspension

The behavior of nanogels in suspension can be dramatically affected by the grafting of a canopy of end-tethered polymer chains. The architecture of the interfacial layer, defined by the grafting density and length of the polymer chains, is a crucial parameter in defining the conformation and influencing the dynamics of the grafted chains. However, the influence of this architecture when the core substrate is itself soft and mobile is complex; the dynamics of the core influences the dynamics of the tethered chains, and, conversely, the dynamics of the tethered chains can influence the dynamics of the core. Here, poly(styrene) (PS) particles were functionalized with poly(methyl acrylate) (PMA) chains and swollen in a common solvent. NMR relaxation reveals that the confinement influences the mobility of the grafted chain more prominently for densely grafted short chains. The correlation time associated with the relaxation of the PMA increased by more than 20% when the grafting density increased for short chains, but for less than 10% for long chains. This phenomenon is likely due to the steric hindrance created by the close proximity to the rigid core and of the neighboring chains. More interestingly, a thick layer of a densely grafted PMA canopy efficiently increases the local mobility of the PS cores, with a reduction of the correlation time of more than 30%. These results suggest an interplay between the dynamics of the core and the dynamics of the canopy.

Dynamics of Soft and Hairy Polymer Nanoparticles in a Suspension by NMR Relaxation

The design of surface-modified functional nanoparticles (NPs) is used to control the properties of the NPs and the NP/environment interactions. The efficient control of the final behavior of the NPs demands a comprehensive understanding of the resulting system. This is particularly challenging for systems with an architecture of the type polymer core-polymer canopy. In such systems, one of the key parameters influencing the behavior of the NPs is the local dynamics of the polymer canopy. However, because the grafting points of the canopy are experiencing their own local dynamics, predicting the final behavior of such systems is difficult. To get a deeper understanding of NPs made of a soft and swollen polymer core and a swollen polymer canopy, we prepared a library of hairy NPs made of a polystyrene (PS) core and a canopy of grafted poly(methyl acrylate) (PMA) chains. The softness of the PS core and the thickness of the PMA canopy were controlled, and the behavior and dynamics of the soft and hairy PS-PMA NPs in suspension were measured by 1H NMR relaxation and dynamic light scattering. It was observed that the rigid PS core slowed down the subsegmental dynamics of the PMA chains, while thick PMA canopies accelerated the relaxation of the PS core. The dynamics of the NPs in suspension was the result of the interplay between the PS core and the PMA canopy.

Polymer-functionalized polymer nanoparticles and their behaviour in suspensions

In concentrated suspensions of polymer-functionalized nanoparticles, the softness of the core nanoparticles has a crucial effect on the mechanical behaviour of the resulting colloidal gels.

Effect of Polymer Chemistry on Chain Conformations in Hairy Nanoparticle Assemblies

Matrix-free, polymer-grafted nanoparticles, called hairy nanoparticle assemblies (aHNPs), have proven advantageous over traditional nanocomposites, as good dispersion and structural order can be achieved. Recent studies have shown that conformational changes in the polymer structure can lead to significant enhancements in the mechanical properties of aHNPs. To quantify how polymer chemistry affects the chain conformations in aHNPs, here we present a comparative analysis based on coarse-grained molecular dynamics simulations. Specifically, we compare the chain conformations in an anisotropic cellulose nanoparticle grafted to four common polymers with distinct chemical groups, fragility, and segmental structures, that is, poly(methyl methacrylate) (PMMA), polystyrene (PS), polycarbonate (PC), and polybutadiene (PB). We observe that semiflexible glassy polymers such as PMMA and PS have a higher critical chain length (N_cr), the transition point where the polymer conformation changes from concentrated to semidilute brush regime. Flexible rubbery polymers (PB) can overcome the N_cr barrier at relatively lower molecular weights. We have used theoretical scaling laws based on Daoud-Cotton theory to uncover a direct correlation between empirical constants and physical parameters, such as persistence length and monomer excluded volume. Furthermore, we carried out a systematic study to understand the role of backbone rigidity and side-group size of polymer, and it revealed that the backbone rigidity significantly affects N_cr but the side-group size doesn't seem to have an appreciable effect on N_cr. We find that normalization of the monomer radial distribution curves using N_cr and other key molecular parameters collapses the curves for 110 distinct model aHNP systems studied. Our work paves the way for systematic quantification of these molecular design parameters to accelerate the design of polymer-grafted nanoparticle assemblies in combination with universal scaling relationships.

Accelerated Local Dynamics in Matrix-Free Polymer Grafted Nanoparticles

The tracer diffusion coefficient of six different permanent gases in polymer-grafted nanoparticle (GNP) membranes, i.e., neat GNP constructs with no solvent, show a maximum as a function of the grafted chain length at fixed grafting density. This trend is reproduced for two different NP sizes and three different polymer chemistries. We postulate that nonmonotonic changes in local, segmental friction as a function of graft chain length (at fixed grafting density) must underpin these effects, and use quasielastic neutron scattering to probe the self-motions of polymer chains at the relevant segmental scale (i.e., sampling local friction or viscosity). These data, when interpreted with a jump diffusion model, show that, in addition to the speeding-up in local chain dynamics, the elementary distance over which segments hop is strongly dependent on graft chain length. We therefore conclude that transport modifications in these GNP layers, which are underpinned by a structural transition from a concentrated brush to semidilute polymer brush, are a consequence of both spatial and temporal changes, both of which are likely driven by the lower polymer densities of the GNPs relative to the neat polymer.

Polymer dynamics under confinement

We review recent neutron scattering work and related results from simulation and complementary techniques focusing on the microscopic dynamics of polymers under confinement. Confinement is either realized in model porous materials or in polymer nanocomposites (PNC). The dynamics of such confined polymers is affected on the local segmental level, the level of entanglements as well as on global levels: (i) at the segmental level the interaction with the surface is of key importance. At locally repulsive surfaces compared to the bulk the segmental dynamics is not altered. Attractive surfaces slow down the segmental dynamics in their neighborhood but do not give rise to dead, glassy layers. (ii) Confinement generally has little effect on the inter-chain entanglements: both for weakly as well as for marginally confined polymers the reptation tube size is not changed. Only for strongly confined polymers disentanglement takes place. Similarly, in PNC at higher NP loading disentanglement phenomena are observed; in addition, at very high loading a transition from polymer caused topological constraints to purely geometrical constraints is observed. (iii) On the more global scale NSE experiments revealed important information on the nature of the interphase between adsorbed layer and bulk polymer. (iv) Polymer grafts at NP mutually confine each other, an effect that is most pronounced for one component NP. (v) Global diffusion of entangled polymers both in weakly and strongly attractive PNC is governed by the ratio of bottle-neck to chain size that characterizes the 'entropic barrier' for global diffusion.

Intensive study on structure transformation of muscovite single crystal under high-dose γ-ray irradiation and mechanism speculation

Intensive study on structure transformation of muscovite single crystal under high-dose γ-ray irradiation is essential for its use in irradiation detection and also beneficial for mechanism cognition on defect formation within a matrix of clay used in the disposal of high-level radioactive waste (HLRW). In this work, muscovite single crystal was irradiated with Co-60 γ ray in air at a dose rate of 54 Gy min-1 with doses of 0-1000 kGy. Then, structure transformation and mechanism were explored by Raman spectrum, Fourier-transform infrared spectrum, X-ray diffraction, thermogravimetric analysis, CA, scanning electron microscope and atomic force microscopy. The main results show that variations in the chemical/crystalline structure are dose-dependent. Low-dose irradiation sufficiently destroyed the structure, removing Si-OH, thus declining hydrophilicity. With dose increase up to 100 kGy, CA increased from 20° to 40°. Except for hydrophilicity variation, shrink occurred in the (004) lattice plane which later recovered; the variation range at 500 kGy irradiation was 0.5% close to 0.02 Å. The main mechanisms involved were framework break and H2O radiolysis. Framework break results in Si-OH removal and H2O radiolysis results in extra OH introduction. The extra introduced OH probably results in Si-OH bond regeneration, lattice plane shrink and recovered surface hydrophilicity. The importance of framework break and H2O radiolysis on structure transformation is dose-dependence. At low doses, framework break seems more important while at high doses H2O radiolysis is important. Generally, variations in the chemical structure and surface property are nonlinear and less at high doses. This indicates using the chemical structure or surface property variation to describe irradiation is correct at low doses but not at high doses. This finding is meaningful for realizing whether muscovite is suitable for detecting high-dose irradiation or not, and mechanism exploration is efficient for identifying the procedure for defect formation within the matrix of clay used in disposal HLRW in practice.

Strained Bottlebrushes in Super-Soft Physical Networks

ABA triblock copolymers composed of a poly(dimethylsiloxane) (PDMS) bottlebrush central block and linear poly(methyl methacrylate) (PMMA) terminal blocks self-assemble into a physical network of PDMS bottlebrush strands connected by PMMA spherical domains. A combination of small- and ultrasmall-angle X-ray scattering techniques was used to concurrently examine dimensions of PMMA spherical domains and PDMS bottlebrush strands both in the bulk and at the PMMA-PDMS interface. In agreement with scaling model predictions, the degrees of polymerization of the bottlebrush backbone (n_bb) and PMMA block (n_A) correlate with the measured PMMA domain size and area per molecule at the PMMA-PDMS interface as D_A ∝ (n_bbn_A)^1/3 and S ∝ n_A^2/3n_bb^-1/3, respectively. In the bulk, bottlebrush strands are extended due to steric repulsion between the side chains and unfavorable interactions between the different blocks. At the PMMA-PDMS interface with large curvature, packing constraints require additional bottlebrush backbone extension and alignment of side chains along the backbone in the direction perpendicular to the interface.

Nanoscale Particle Motion Reveals Polymer Mobility Gradient in Nanocomposites

Polymer mobility near nanoparticle surfaces has been extensively discussed; however, direct experimental observation in the nanocomposite melts has been a difficult task. Here, by taking advantage of large dynamical asymmetry between the miscible matrix and surface-bound polymers, we highlighted their interphases and studied the resulting effect on the nanoparticle relaxation using X-ray photon correlation spectroscopy. The local mobility gradient is signified by an unprecedented increase in the relaxation time at length scales on the order of polymer radius of gyration. The effect is accompanied by a transition from simple diffusive to subdiffusive behavior in accord with viscous and entangled dynamics of polymers in the matrix and in the interphase, respectively. Our results demonstrate that the nanoparticle-induced polymer mobility changes in the interphases of nanocomposite melts can be extracted from the length-scale-dependent slow particle motion.

Intensive evaluation of radiation stability of phlogopite single crystals under high doses of γ-ray irradiation

The evaluation of radiation stability of clay is important for the disposal of high-level radioactive waste (HLRW). In this study, phlogopite single crystals were irradiated by Co-60 γ-rays in air at a dose rate of 3.254 kGy h⁻¹ with doses up to 1000 kGy. Subsequently, the radiation stability and mechanism of radiation damage were explored by RS, FT-ATR, XRD, TGA, CA, and SEM techniques. In general, phlogopite single crystals show worthwhile radiation resistance toward their chemical structure but poor radiation stability toward their crystalline structure. Upon irradiation, their chemical structure changed slightly, while their crystalline structure varied obviously. For the 1000 kGy-irradiated sample, the interlayer space d of the (001) lattice plane increased by more than 1% with a value close to 0.13 Å, showing expansion. This could be mainly ascribed to H₂O radiolysis and framework breakage: the former seems more important. These variations had a considerable impact on surface hydrophilicity, while they had marginal impacts on thermal stability and morphology: the effect on surface hydrophilicity is dose-dependent. A lower dose of irradiation sufficiently reduced the hydrophilicity, while a higher dose recovered the hydrophilicity. For instance, the CA increased from 14° to 28° with dose increases from 0 kGy to 200 kGy and then decreased to approximately 20° as the dose continued to increase to 1000 kGy. In general, the crystalline structure is more sensitive toward γ-ray irradiation and phlogopites could be regarded as poorly radiation-resistant. In this procedure, H₂O radiolysis occupies a crucial role and seems to be the dominant factor. This finding is meaningful to evaluate the radiation stability of clay matrixes and to understand the microscopic property variations in clays used in practice when they are under irradiation.

Upon irradiation, the framework underwent breakage, H₂O underwent radiolysis, and the radiolysis products reacted with the framework, expanding the lattice plane.

Quantitative Measure of the Size Dispersity in Ultrasmall Fluorescent Organic-Inorganic Hybrid Core-Shell Silica Nanoparticles by Small-angle X-ray Scattering

Small-angle X-ray scattering (SAXS) was performed on dispersions of ultrasmall (d < 10 nm) fluorescent organic-inorganic hybrid core-shell silica nanoparticles synthesized in aqueous solutions (C' dots) by using an oscillating flow cell to overcome beam induced particle degradation. Form factor analysis and fitting was used to determine the size and size dispersity of the internal silica core containing covalently encapsulated fluorophores. The structure of the organic poly(ethylene glycol) (PEG) shell was modelled as a monodisperse corona containing concentrated and semi-dilute regimes of decaying density and as a simple polydisperse shell to determine the bounds of dispersity in the overall hybrid particle. C' dots containing single growth step silica cores have dispersities of 0.19-0.21; growth of additional silica shells onto the core produces a thin, dense silica layer, and increases the dispersity to 0.22-0.23. Comparison to FCS and DLS measures of size shows good agreement with SAXS measured and modelled sizes and size dispersities. Finally, comparison of a set of same sized and purified particles demonstrates that SAXS is sensitive to the skewness of the gel permeation chromatography elugrams of the original as-made materials. These and other insights provided by quantitative SAXS assessments may become useful for generation of robust nanoparticle design criteria necessary for their successful and safe use, for example in nanomedicine and oncology applications.

Polymers on nanoparticles: structure & dynamics

Grafting polymers to nanoparticle surfaces influences properties from the conformation of the polymer chains to the dispersion and assembly of nanoparticles within a polymeric material. Recently, a small body of work has begun to address the question of how grafting polymers to a nanoparticle surface impacts chain dynamics, and the resulting physical properties of a material. This Review discusses recent work that characterizes the structure and dynamics of polymers that are grafted to nanoparticles and opportunities for future research. Starting from the case of a single polymer chain attached to a nanoparticle core, this Review follows the structure of the chains as grafting density increases, and how this structure slows relaxation of polymer chains and affects macroscopic material properties.

Location of Imbibed Solvent in Polymer-Grafted Nanoparticle Membranes

Membranes made purely from nanoparticles (NPs) grafted with polymer chains show increased gas permeability relative to the analogous neat polymer films, with this effect apparently being tunable with systematic variations in polymer graft density and molecular weight. To explore the structural origins of these unusual transport results, we use small angle scattering (neutron, X-ray) on the dry nanocomposite film and to critically examine in situ the structural effects of absorbed solvent. The relatively low diffusion coefficients of typical solvents (∼10^-12 m²/s) restricts us to thin films (≈1 μm in thickness) if solute concentration profiles are to equilibrate on the 1 s time scale. The use of such thin films, however, renders them as weak scatterers. Inspired by our nearly two decades old previous work, we address these conflicting requirements through the use of a custom designed flow cell, where stacks of 10 individual ≈1 μm thick supported films are used, while ensuring that each film is individually exposed to solvent vapor. By using isotopically labeled solvents, we study the solvent distribution within the film and show surprisingly that the solvent homogeneously swells the polymer under all conditions that we examined. These results are not anticipated by current theories, but they suggest that, at least under some conditions, the free volume increases due to the grafting of chains to nanoparticles is apparently distributed isotropically in these materials.