Session R54: Confined Polymer Glasses III: Elasticity, Nanoparticles, and Brushes

Enhanced Thermal Stability of Polymers under Extreme Nanoconfinement

It has been shown that the properties of polymers in weakly-interacting polymer nanocomposites (PNCs), such as Polystyrene(PS) and SiO₂ nanoparticles (NP), can deviate from bulk due to segmental and chain confinement. Capillary Rise Infiltration (CaRI) of polymers into densely-packed NP films can produce PNCs with ultra-high NP loadings. Depending on the degree of confinement, the glass transition temperatures (T_g) of PS/SiO₂ CaRI films can increase up to ~57 K. In this study, unentangled PS/SiO₂ CaRI films were used to study the thermal degradation of polymers under extreme nanoconfinement. The degree of confinement was controlled by using NPs with different diameters, thus adjusting the effective pore diameter by 3~30 nm. We show that the thermal stability of PS is significantly increased in smaller pores under both thermo-oxidative and pyrolytic conditions. The characteristic degradation time during isothermal degradation at high temperatures is proportional to the viscosity of PS at lower temperatures, indicating an entropic origin of the improved stability. The details of the kinetics of the degradation process through surface or bulk is also explored.   

Unexpected strengthening effect in brush particle-based hybrid materials with intermediate grafting density

Surface functionalization of nanoparticles with polymers has emerged as a versatile platform to enable hybrid building blocks that can be assembled into functional nanocomposites with controlled microstructure and enhanced mechanical properties. However, the dense functionalization by conventional method presents a barrier to the utilization of this concept due to the limited inorganic content. This contribution will present a systematic analysis of the role of brush architecture on the elastic properties of brush particle films. Material systems with continuous variation of the grafting density as well as bimodal graft architectures were synthesized using modified atom transfer radical polymerization and films were characterized using indentation and thermal analysis. The results reveal that chain interdigitation plays a dominant role for the modulus and strength of the films. In particular, the results demonstrate an ‘optimum degree of functionalization’ to maximize cohesive interactions as a consequence of the enhanced dispersion interactions in interdigitated brush particle assemblies. The results are used to postulate design guidelines for the synthesis of brush particles capable of forming hybrid materials with enhanced performance.   

Dramatic Modification of Nanoparticle Surface Mobility in Polymer Colloids with a Core-Shell Structure

Advances in polymer nanoparticle synthesis and assembly techniques have enabled new applications, from drug delivery carriers to novel coatings. However, the thermal transition of polymer dynamics, especially at the particle surfaces, must be understood in order to realize their potential. Using Brillouin light scattering as a direct probe of the particle surface mobility via polymer nanoparticle vibrations, recent experiments confirm the correlation between the glass transition behavior and surface dynamics of polymer nanoparticles. This raises a challenge of how the glass transition dynamics of polymer nanoparticles can be engineered. To address this need, we modify the surface condition of polystyrene nanoparticles with different shell architecture layers. We demonstrate that a shell layer composed of two polyelectrolytes, as thin as a single polymer chain, is able to eliminate the effect of enhanced particle surface mobility. Other shell structures, such as a single polyelectrolyte layer, enables tailoring the glass transition temperature, softening behavior, and elasticity of the nanoparticles.   

Materials by Design for Stiff and Tough Nanoparticle Assemblies with Polymeric Hairs

In this work, a computational methodology for predicting the mechanical behavior of assembled hairy nanoparticle systems (aHNPs) using coarse-grained molecular dynamics simulations coupled with Gaussian process metamodeling will be presented (Hansoge et al. ACS NANO,2018). The coarse-graining approach for the polymeric hairs involves systematic parameter development based on the energy renormalization approach, which allows us to describe the behavior of the glassy nanocomposites with greater accuracy at larger length-scales. Simulations reveal that for cellulose nanocrystal-PMMA aHNPs, the Pareto frontier, which marks the optimal trade-offs between modulus and toughness within the design parameter space, can be reached when the weight percentage of nanoparticles is around 60% and the grafted chains exceed the critical length scale governing transition into the semidilute brush regime. A theoretical model with computationally determined parameters based on the Daoud−Cotton model adequately explains the dependence of the critical length scale on graft density and nanoparticle size. The computational results agree well with recent experiments and suggest that high stiffness and high toughness could be achieved by carefully tuning the molecular design parameters, most notably by keeping a relatively low grafting density while having high graft length and nanoparticle volume fraction.   

Diminishing Interfacial Effects with Decreasing Nanoparticle Size in Polymer-Nanoparticle Composites

Using molecular simulations on model polymer nanocomposites at fixed
filler loading we show that interfacial polymer dynamics are affected
less with decreasing nanoparticle (NP) size. However, the glass
transition temperature $T_g$ changes substantially more for extremely
small NP. The reason for this apparent contradiction is that the mean
NP spacing decreases with decreasing particle size. Thus, all polymers
are effectively interfacial for sufficiently small NP, resulting in
relatively large $T_g$ shifts. For larger NP, interfacial relaxations
are substantially slower than the matrix for favorable NP-polymer
interactions. The minority ``bound'' polymer dynamically decouples
from the polymer matrix, and we only find small changes in $T_g$
relative to that of the bulk polymer for large NP. These results are used
to organize a large body of relevant experimental data, and we propose an apparent
universal dependence on the ratio of the face-to-face distance between the NPs and
the chain radius of gyration.   

Disentangling the Role of Chain Conformation on the Mechanics of Particle Brush Materials

The modification of particle surfaces with polymeric ligands is widely used to enhance the interactions between particles, to allow their integration in polymeric matrices or for the processing of particulate materials through techniques such as molding or extrusion. A critical question that is of both fundamental and applied relevance pertains to the role of polymer architecture in controlling the interactions and resulting mechanical properties of polymer-tethered particles. Because of its ability to resolve the mechanical characteristics on submicrometer scale without sample destruction, Brillouin light scattering (BLS) has become an important analytical tool to elucidate the connection between structure, dynamics and macroscopic response in hybrid material systems. In this contribution a combined approach of experimental BLS analysis and coarse-grained simulations is presented to identify the respective contribution of grafting density and degree of polymerization on the macroscopic mechanical properties of films from polystyrene tethered silica colloids. The bulk modulus is maximized in intermediate to low grafting density systems, where the hard silica cores are partially exposed due to conformational fluctuations of the grafted polymers.   

Interfacial entropic interactions tunes fragility and dynamic heterogeneity of confined polymer glasses with embedded nanoparticles.

The nature of nanoparticle-polymer interfacial layer is known to play a vital role in determining various physical properies of polymer glasses. The influence of interfacial interactions on viscosity, fragility and dynamical heterogeneity of athermal melts are explored in this study¹. Dynamical parameters are studied using X-ray photon correlation spectroscopy and results are corroborated with molecular dynamics simulations. Influence of various microscopic parameters such as interfacial layer diffusivity, matrix chain penetration depth etc on fragility of polymer glasses are demonstrated combining experiment and simulation results. Thin film-substrate interface adsorbed layer effects on dynamical properties of confined glasses are also investigated. Reduction (enhancement) in viscosity is observed for melts with low (high) entropic compatibility parameter, f (molecular weight ratio of graft to matrix chains). Fragility is found to increase with increasing f. Anticorrelation between fragility and dynamical heterogeneity is found in these systems, which are contrary to various earlier studies on glass froming polymers.

1. Begam, Nafisa, et al., Soft Matter, 2018, DOI: 10.1039/C8SM01729H.   

Interactions of Ligand-Coated Nanoparticles at a Liquid Surface

The pair interaction potential of ligand-coated silica nanoparticles (NPs) at a liquid surface were determined by scanning electron microscopy, exploiting the nonvolatility of ionic liquids to stabilize the liquid specimens against microscope vacuum. Even at near contact, individual, two-dimensionally well-dispersed NPs were resolved. The potential of mean force, reduced to the pair interaction potential for dilute NPs, was extracted with good accuracy from the radial distribution function as both NP diameter and poly(ethylene glycol) ligand length varied. While NP polydispersity broadened the core repulsion, the pair potential well-approximated a hard sphere interaction. For short (5 kDa) ligands, a weak (<kT) long-range attraction was discerned, and for ligands of identical length, the potentials overlapped for NPs of different diameter; the attraction is suggested to arise from ligand-induced menisci. To probe the factors underlying the potential, NP surface-binding energies were measured by interfacial tensiometry, and NP contact angles were assessed by new atomic force microscopy and transmission electron microscopy methods.   

Switching mixed polymer brushes surfaces through external stimulation

Binary and mixed brushes are comprised of two (or more), chemically distinct polymers randomly grafted to a surface in close proximity. This approach is considered a potent means to manufacture responsive surfaces with tunable physical properties. We highlight recent work in using orthogonal polymerization techniques to tailor such mixed surfaces, comprised chemically disparate polymers covalently grafted to a silica surface. A combination of X-ray photoelectron spectroscopy, synchrotron hard (GISAXS) and soft resonant X-ray techniques (RSoXS), as well as Infrared Atomic Force Microscopy (AFM-IR) was used to provide evidence for responsiveness towards external stimuli. The choice of selective solvents for either of the two species resulted in reversible microphase segregation and can provide a direct pathway towards switchable surface properties.
  

The TUFF Method: Stretching Free-Standing Ultra-Thin Glassy Polymer Films

TUTTUT (The Uniaxial Tensile Tester for Ultra-Thin films) is a recently developed method for quantifying the complete uniaxial stress-strain relationship for ultra-thin polymer films. The key to TUTTUT is the use of liquids to help support ultra-thin, often fragile, films. Although this method has provided new insights into mechanical properties and deformation mechanisms for dimensionally-confined polymer systems, for some polymers the presence of a liquid may influence the ultra-thin properties. Here, we introduce a new method, TUFF (Tensile tester for Ultra-thin Freestanding Films) to directly measure the uniaxial stress-strain response of freestanding polymer thin films. Using polystyrene thin films, with thickness 20nm-100nm, we observe and quantify large strain deformation mechanisms, as well as yield stress and elastic moduli, and compare these results to TUTTUT measurements. These results provide new fundamental insights into how the surface interactions can alter polymer behavior in thin confined polymer films.   

Unveiling the Elasticity of Confined Multilayer Hybrid Materials

The superior elasticity and processing easiness of polymers render them promising as an interfacial filler material. With the progress in hybrid materials fabrication and manufacturing precision, the thickness of interfacial polymer layers has gone down to the nanometer range that can feature strong confinement. A critical question of scientific and applied importance is how the confined polymer layers affect the effective mechanical properties of the hybrid material. In this contribution, we utilize Brillouin light scattering experiments and a continuum-mechanics based model to examine the anisotropic elastic properties of confined hybrid materials composed of alternating polyvinylpyrrolidone and hectorite nanolayers. The clear resolution of the direction-dependent quasi-longitudinal, quasi-transverse, and pure-transverse phonon modes from the Brillouin scattering experiments leads to the full elastic properties of these highly anisotropic samples. We find that as the polymer layer thickness increases from about 1 to 3 nm, the material’s Young’s moduli in the directions parallel and perpendicular to the layers drop by nearly 50%, implying the crucial role of polymer confinement on the sample’s mechanical properties.   

Tensile elastic modulus of free-standing single-layer and bilayer polymer films

Studies have shown that the elastic modulus, E, of polymer films may vary with the film thickness, h, when h is tens of nanometers. For polystyrene (PS) supported by a polydimethylsiloxane (PDMS) substrate, E had been found to decrease with decreasing h below ~60 nm. For free-standing PS films, however, E was found to be constant with h for h < ~80 nm. In this experiment, we conducted uniaxial tensile tests on free-standing PS films (with a molecular weight, MW, of 1,049 kg/mol and h = 40 to 150 nm) and bilayer films of PS-PDMS (with various MW from 151 to 1,049 kg/mol and h = 12 to 150 nm). The measured elastic modulus of the films agreed with published value of bulk PS, showing insignificant dependences on h or MW.   

Surface and Interfacial Tension of Graft Polymer Melts

Understanding surface properties of polymer melts is crucial for designing new polymeric coatings, adhesives, and composites. Here, we study the effect of molecular architecture on surface and interfacial tension of graft and linear polymer melts by molecular dynamics simulations. In particular, we elucidate the effect of the degree of polymerization of the side chains n_sc and their grafting density 1/n_g on the surface tension of the graft polymer/vacuum interface, and the interfacial tension of the interface between graft and linear polymer melts. For the case of the graft polymer/vacuum interface, our simulations confirm that the surface tension is a linear function of the fraction of the backbone ends f_be and side-chain ends f_se. This dependence of the surface tension highlights the entropic origin of the surface tension corrections associated with the redistribution of the grafting points and ends at the interface. However, the interfacial tension between graft and linear polymer melts does not show any significant dependence on the molecular structure of the graft polymers, thus pointing out the dominance of the enthalpic contribution to the interfacial tension.