Session H38: Polymer Nanocomposites: Dynamics

Dynamics in Polymer Melts and Nanocomposites

Intense research has led to substantial progress in the field of polymer melts and nanocomposites, both regarding the fundamental understanding and the relationship to applications. From a fundamental point of view, knowing the microscopic single chain dynamics is important. It may even lead to optimized materials ranging from the classical car tire to battery or fuel cell applications. In polymer melts, different processes, such as diffusion, reptation, contour length fluctuations, etc. occur and determine the macroscopic results, e.g. obtained by rheology. In nanocomposites confinement effects and interactions of chains with surfaces play an important role. High resolution techniques, such as small-angle neutron scattering or neutron spin echo spectroscopy are suited to explore the structure and dynamics of chains. The presentation illuminates the fundamental relationship between the microscopic dynamics and the mesoscopic properties, exploiting different experimental techniques, such as dielectric spectroscopy, rheology, neutron scattering and neutron spin echo spectroscopy.   

Phase Stability and Dynamics of nanoparticles in Polymer Nanocomposites

In polymer nanocomposites, polymer grafted nanoparticles, where the tethered polymer chains are chemically identical to the host chains, have been reported to irreversibly aggregate if the length of host chains (P) become 5 or more times larger than the length tethered chains (N) due to the autophobic dewetting of the polymer brush. Utilizing Small Angle X-ray scattering as a tool, here we show that by choosing appropriate chemistry one can utilize the enthalpic attractions between the tethered chains and host chains to facilitate uniform nanoparticle dispersion in very large M$_{\mathrm{w}}$ hosts (P/N \textasciitilde 140). A generic phase diagram has also been proposed. Xray Photon Correlation Spectroscopy (XPCS) is employed as a sensitive probe of nanoparticle relaxation dynamics to investigate particle dynamics in these model PNCs. Remarkably, we find that for nanoparticle size $D, $slightly larger \quad than \quad the tube diameter of the host polymer ($a)$, particles undergo a transition from normal diffusion to hyperdiffusive relaxation dynamics,. In contrast, for unentangled hosts, diffusive particle relaxation are observed. Our experimental observations are rationalized by finding that nanoparticle motion in entangled melts only disturb sub-chain entangled segments of size comparable to the particle diameter.   

Nanoparticle effect on polymer chain dynamics and entanglement network

We investigated structure and dynamics of polymer nanocomposites through molecular modeling, by considering different molecular weights of polymers chains, and volume fractions of fillers. The dynamics of unentangled chains can be separated into two phases, a bulk polymer phase and a confined polymer phase between fillers. The dynamics of a confined polymer is slower than that of a bulk polymer, while still exhibiting high mobility. The amount of the bulk polymer phase is found to exponentially decay with increasing volume fraction of fillers. When highly entangled polymer chains are confined between fillers, their conformation and entanglement network are dramatically changed, in district with their unentangled counterparts. The entangled polymer chains are found to be significantly disentangled and flattened during increment of the volume fractions of spherical nonattractive fillers. A critical volume fraction is found to control the crossover from polymer chain entanglements to `nanoparticle entanglements', below which the polymer chain relaxation accelerates upon filling. These results provide a microscopic understanding of the dynamics of entangled polymer chains inside their composites, and offer an explanation for the unusual rheological properties of polymer composites.   

Thermally induced infiltration of polymer into nanoparticle packings

We present a novel approach in generating three-phase polymer nanocomposites via capillary rise infiltration (CaRI) of polymer into a dense nanoparticle packing, which we have previously utilized to generate dense nanocomposites with extremely high filler fraction. The CaRI process involves first generating a bilayer film of porous nanoparticle layer on a polymer layer, followed by annealing of the bilayer above the Tg of the polymer to induce polymer infiltration into the voids of the nanoparticle layer. By tuning the amount of polymer to be less than the void volume of the nanoparticle layer, we demonstrate that CaRI is capable of generating spatially homogeneous porous composite. We utilize spectroscopic ellipsometry to characterize and monitor the polystyrene infiltration process into the titania nanoparticle packing in-situ. The infiltration process occurs in two stages. Upon annealing, we observe that the polymer layer is depleted rapidly via capillary-induced infiltration to form a dense composite at the base of the nanoparticle packing. Eventually, the front of this composite layer propagates throughout the nanoparticle packing, just as the composite refractive index decreases, indicating the redistribution of polymer throughout the nanoparticle matrix.   

The effect of chain rigidity on the interfacial layer thickness and dynamics of polymer nanocomposites.

There are growing experimental evidences showing the existence of an interfacial layer that has a finite thickness with slowing down dynamics in polymer nanocomposites (PNCs). Moreover, it is believed that the interfacial layer plays a significant role on various macroscopic properties of PNCs. A thicker interfacial layer is found to have more pronounced effect on the macroscopic properties such as the mechanical enhancement. However, it is not clear what molecular parameter controls the interfacial layer thickness. Inspired by our recent computer simulations that showed the chain rigidity correlated well with the interfacial layer thickness,[1] we performed systematic experimental studies on different polymer nanocomposites by varying the chain stiffness. Combining small-angle X-ray scattering, broadband dielectric spectroscopy and temperature modulated differential scanning calorimetry, we find a good correlation between the polymer Kuhn length and the thickness of the interfacial layer, confirming the earlier computer simulations results. Our findings provide a direct guidance for the design of new PNCs with desired properties. [1] Carrillo, J.-M. Y. \textit{et al}; \textit{Macromolecules }\textbf{2015,} \textit{48}, (12), 4207-4219.   

Fragility-Controllable Polymer Grafted Nanoparticles.

20 years ago, the concept of `Fragility' has been suggested to categorize glass-forming liquids. Currently, we know there are two kinds of glass-forming liquids group. One is Arrhenius type liquids called as Strong glass (large Fragility). Another one is non-Arrhenius liquid called as Fragile glass (small Fragility). The physical meaning of Fragility is unknown yet, but people believe that to understand the physical meaning of Fragility leads to understand glass transition. Recently we found Polymer Grafted Nanoparticles (PGNPs) could behave like glass-forming liquids depending on their grafting density in MD simulations. Surprisingly, their Fragility can be controlled by grafting density and we can obtain both `Strong' and `Fragile' glass using this system.   

Activated Dynamics in Dense Model Nanocomposites

The nonlinear Langevin equation approach is applied to investigate the ensemble-averaged activated dynamics of small molecule liquids (or disconnected segments in a polymer melt) in dense nanocomposites under model isobaric conditions where the spherical nanoparticles are dynamically fixed. Fully thermalized and quenched-replica integral equation theory methods are employed to investigate the influence on matrix dynamics of the equilibrium and nonequilibrium nanocomposite structure, respectively. In equilibrium, the miscibility window can be narrow due to depletion and bridging attraction induced phase separation which limits the study of activated dynamics to regimes where the barriers are relatively low. In contrast, by using replica integral equation theory, macroscopic demixing is suppressed, and the addition of nanoparticles can induce much slower activated matrix dynamics which can be studied over a wide range of pure liquid alpha relaxation times, interfacial attraction strengths and ranges, particle sizes and loadings, and mixture microstructures. Numerical results for the mean activated relaxation time, transient localization length, matrix elasticity and kinetic vitrification in the nanocomposite will be presented.   

Effect of polymer-nanoparticle interactions on the capillary rise infiltration of polymers into nanoporous media

By wicking a polymer into a porous packing of nanoparticles, it is possible to generate polymer nanocomposites with extremely high filler fractions. Although capillary rise of simple fluids in porous media is fairly well understood based on the Lucas-Washburn model, there remain many unanswered questions related to the infiltration of high molecular weight polymer melts in nanoporous media. In this work, we probe the thermally induced infiltration of polymers into packings of nanoparticles using molecular dynamics (MD) simulations. In particular, we investigate the effect of polymer-nanoparticle interactions on the three phase contact angle of the polymer on the nanoparticle surface, and probe how the infiltration process is affected by changes in these interactions. We also study the effect of molecular weight on the capillary rise behavior of polymers in nanoparticle packings.   

Effects of Attractive Interactions on Nanoparticle Diffusion in Entangled Polymer Melts

Developing a complete picture for the mechanism of nanoparticle diffusion in model polymer nanocomposites remains a great challenge, especially experimentally. Using Rutherford backscattering spectroscopy, we have measured the translational diffusion coefficient of spherical nanoparticles (diameter $=$ 20 nm) infiltrated into poly(2-vinylpyridine) melts across a range of molecular weights (35-300 kg/mol). Our results reveal that the diffusion coefficient of nanoparticles in attractive nanocomposites is several times slower than what is predicted from the melt viscosity according to the Stokes-Einstein (SE) relation. This runs contrary to recent theoretical studies of non-attractive systems, where it is predicted that nanoparticle diffusion can be many orders of magnitude faster than SE predictions. Potential explanations for this unusual slowing of nanoparticle diffusion are discussed.   

Understanding the interfacial layer dynamics of polymer nanocomposites from broadband dielectric spectroscopy

Polymer nanocomposites show many advanced mechanical, thermal, optical, and transport properties mainly due to the vast interfacial area between the polymer matrix and nanoparticles. Recent studies show that there is an interfacial polymer layer with structure and dynamics that are different from the bulk polymer, and that contributes to the advanced macroscopic properties. It has been shown that broadband dielectric spectroscopy provides good method to study the interfacial dynamics in nanocomposites. However, current dielectric spectroscopy studies ignore the heterogeneous nature of polymer nanocomposites. Models based on a simple superposition of bulk polymer and interfacial layer spectra, or those that assume the interfacial layer is dynamically “dead” are inaccurate. In this talk, the prevailing methods in the literature will be compared with an accurate method accounting for the heterogeneity of the nanocomposites. Different nanocomposites with well-dispersed nanoparticles will be used as examples. The analysis clearly shows that the width and the amplitude of the relaxation peaks are affected by the data analysis. Thus accurate quantitative conclusions on properties and thickness of the interfacial layer can be achieved only using heterogeneous models.   

Molecular Dynamics Simulations of Silica-Filled Copolymers with Variable Sequence for Applications in Tire Treads

We simulate a simple nanocomposite relevant to tire tread compounds consisting of a single spherical nanoparticle surrounded by coarse-grained polymer chains. The polymers are composed of two different monomer types, which have different interaction strengths with the nanoparticle. The monomer sequence can be varied to model different copolymer configurations. We study the polymer end-to-end vector autocorrelation functions to obtain relaxation times of adsorbed and bulk polymer, showing how the interphase is affected by the polymer type and the monomer-nanoparticle interaction strengths. An understanding of the effect of copolymer sequence on the range of the polymer interphase and the magnitude of the effect on chain dynamics is critical to tire tread material design since the primary polymer component of modern tire tread is styrene-butadiene rubber (SBR) copolymer, which may be synthesized in primarily random or in various blocky copolymer configurations. Macromolecular adsorption to and desorption from filler surfaces has a significant effect on hysteresis, and in tire treads, hysteresis must be controlled to optimize the tradeoff between traction and rolling resistance. Superior tire tread materials must have high hysteresis under the operating conditions of traction while maintaining low hysteresis under the operating conditions of rolling resistance. An opportunity exists to control hysteresis through the use of SBR with specific monomer sequences.   

Distortion of chain conformation and reduced entanglement in polymer-graphene oxide nanocomposites

Graphene and related two-dimensional materials are excellent candidates as filler materials in polymer nanocomposites due to their extraordinary physical properties and high aspect ratio. To explore the mechanism by which the filler affects the bulk properties of these unique systems, and to build understanding from the macromolecular level upwards, we use a combination of small-angle neutron scattering (SANS) and oscillatory rheology. Where a good dispersion is achieved in poly(methyl methacrylate)-graphene oxide (PMMA-GO) nanocomposites, we observe a reduction in the polymer radius of gyration with increasing GO concentration that is consistent with the predicted behavior of polymer melt chains at a solid interface. We use concepts from thin-film polymer physics to formulate a scaling relation for the reduction in entanglements caused by the GO interfaces. Using these scaling arguments, we utilize SANS results to directly estimate the changes to the elastic plateau modulus of the network of entangled polymer chains, and find a correlation with the measured bulk rheology. We present a direct link between interfacial confinement effects and the bulk polymer nanocomposite properties, whilst demonstrating a model system for measuring thin film polymer physics in the bulk.   

Thin Film Deformation Behavior of Polystyrene Grafted Nanoparticle Assemblies

Assemblies of polymer-grafted ``hairy'' nanoparticles (HNPs) are of current interest for a wide array of mechanical, photonic and electrical applications. In contrast to nanoparticles dispersed in a free polymer matrix, the grafted polymer determines particle spacing and circumvents nanoparticle agglomeration. The extent to which these grafted polymers are entangled determines the robustness and strength of the HNP assembly. Here in, we investigate the correlations between grafted polymer conformation, entanglements and deformation mechanisms of thin film assemblies of polystyrene-grafted HNPs by controlling the HNP architecture (grafting density and molecular weight). HNPs with varied corona structures are synthesized with surface-initiated controlled/living radical polymerization. Thin films with controlled thickness are prepared by flow coating. Plastic deformation of thin films are examined using static (bright field, HAADF-STEM, tomography) and AFM techniques. Results show a decrease of void density in craze as grafted polymer length increases for semi-dilute polymer brushes. These correlations between HNP architecture and assembly deformation and failure modes refine the HNP design space for the synthesis and fabrication of assemblies with excellent mechanical properties.   

Tuning mechanical properties of polymer-grafted nanoparticle networks by using biomimetic catch bonds

Cross-linked networks of polymer-grafted nanoparticles (PGNs) constitute a class of composites with tunable mechanical properties that exhibit a self-healing behavior. A PGN network consists of nanoparticles that are decorated with end-grafted polymer chains. Reactive groups on the free ends of these grafted chains can form bonds with the chain ends on the nearby particles. We study these materials using a 3D computational model that encompasses the particle-particle interactions, the kinetics of bond formation and rupture, and the external forces applied to the network. In our model, a fraction of cross-links is formed through biomimetic “catch” bonds. In contrast to conventional “slip” bonds, catch bonds can effectively become stronger under a deformation. We show that by varying the fraction of these catch bonds in the network, the toughness, ductility, and tensile strength of the material could be tuned to desired levels.