Session A70: Polymer Dynamics at the Nano-to Meso-Scale Revealed by X-ray and Neutron Spectroscopy I

Dynamics of nanoparticles in polyelectrolyte solutions

Recent theories indicate that nanoparticle dynamics in semidilute solutions of neutral, flexible polymers couple to the segmental relaxations of polymer chains. How nanoparticle dynamics couple to relaxations of charged, semiflexible polymers remains incompletely understood. Here, we probe the dynamics of polystyrene nanoparticles in solutions of a polyelectrolyte, sodium polystyrene sulfonate, using fluorescence microscopy. When the ionic strength of the solution is high, such that the polymer adopts a Gaussian conformation, the nanoparticle dynamics are coupled to the bulk solution viscosity across the dilute and semidilute regimes of polymer concentration. In near-salt-free solutions, in which the polymer adopts an extended conformation, the nanoparticle dynamics do not follow the Stokes-Einstein prediction based on the bulk viscosity across all polymer concentrations, and furthermore do not collapse onto a master curve as a function of normalized length scales. This result suggests that the coupling of nanoparticle and polymer segmental dynamics are altered by polymer conformation.   

Correcting the Generalized Stokes-Einstein Relation to Include Effects of Hydrodynamic Interactions from Periodic Images

In microrheology, the mean squared displacement (MSD) of micron-sized particles is measured to extract linear viscoelastic properties of entangled polymer melts using the generalized Stokes-Einstein (SE) relation. Probe rheology from molecular dynamics (MD) simulations has a significant computational cost due to the large simulation box sizes required to reduce the effect of the probe’s hydrodynamic interactions with its periodic images. Here, we use an analytical solution for Stokes flow around a body-centered cubic array of spheres to introduce a correction factor to the generalized SE relation. We show that this correction factor is a function of the ratio (R/L), where R and L are particle radius and simulation box length, and the fluid mass fraction. We then predict the dynamic moduli of an entangled melt of bead-spring polymer chains from the MSD of a probe particle at various R and L. Lattice Boltzmann simulations are also performed to calculate the transient drag force and velocity of a particle in a Newtonian fluid to account for effects of inertia at higher frequencies. This added correction factor should allow a reduction of at least one order of magnitude in MD simulation costs.   

Resolving structure-property-dynamics relationships in model polymer nanocomposite systems

Understanding the origins of mechanical enhancement in polymer-nanoparticle composites (PNC) is necessary to design materials with optimized mechanical properties. We use a combination of techniques to resolve the dynamics-structure-property relationships governing mechanical enhancement in a model PNC system of attractive SiO2 nanoparticles embedded in a glassy P2VP polymer matrix over a wide range of NP concentrations (1 – 25 vol.%). By tuning the interparticle distance between neighboring NPs, the formation of the proposed “structural bridge” and the resulting contribution to mechanical enhancement are resolved [1]. The nanoscale structure is investigated by SAXS and TEM while the micro/macro dynamics are resolved by x-ray photon correlation spectroscopy (XPCS) and shear rheology respectively. As concentration increases, the characteristic relaxation times increase dramatically. The dynamics transition to collective motion at the gel point, which correlates to the formation of a sample spanning network. Identification of “rubbery” and “glassy” bridges are discussed in the context of the observed dynamics. [1] Chen, Q.; et al. ACS Macro Lett. 2015, 4 (4), 398–402.   

Role of soft interactions in enhanced diffusivity of polymer-grafted nanoparticles in heterogeneous environments

The dynamic behavior of dilute solutions of polymers is well described by the colloidal model or the Zimm model. In denser systems, the colloidal description fails as interactions with the environment become important. Here, we investigate the behavior of polymer-grafted nanoparticles (PGNPs), where the grafting to the nanoparticle surface causes a high self-concentration of the polymer chains. When dispersed in semidilute polymer solutions, the grafted polymer chains compress and their dynamics are confined by neighboring chains. This change in grafted polymer dynamics arises from the soft repulsive interaction between PGNPs and surrounding polymer chains. This soft interaction profile also affects the dynamics on longer length and time scales. Whereas hard sphere diffusivity decouples from bulk predictions in polymer solutions when the particle becomes comparably sized to the polymer, the diffusivity of soft PGNPs decouples from solution viscoelasticity even when they are larger than the surrounding polymer. Furthermore, the PGNP dynamics strongly depend on the ratio of free to grafted polymer molecular weights. These findings indicate that soft interactions allow PGNPs to diffuse faster than hard spheres in heterogeneous materials.   

Dynamic Behavior of Polystyrene Soft Nanoparticles by Neutron Spin Echo

The inclusion of soft polystyrene nanoparticles improves the dynamic properties of bulk polystyrene, a behavior that is not yet fully understood. To provide insight into this phenomenon, the dynamics of the nanoparticles are evaluated using neutron spin echo (NSE) spectroscopy. These particles consist of a crosslinked polystyrene core with loosely connected loops and tails, which we term a fuzzy interface. NSE spectroscopy of dilute solutions of the nanoparticles (NPs) show the nanoparticle with the lowest crosslinking density and molecular weight exhibits the most Zimm-like dynamics. Varying the synthetic procedure to create nanoparticles with lower molecular weights and fuzzier surfaces results in more heterogeneous relaxation processes, indicating that the dynamics of the core differs significantly from that of the outer shell. Combined with our previous studies showing the relationship between the fuzziness of the nanoparticle and diffusion of bulk polystyrene, these results exemplify the importance of the NP molecular weight and the interaction of the fuzzy interface with linear chains in determining the dynamics of all polymer nanocomposite system.   

Universality in Microstructural Evolution of Deformed Polymer Melts as Revealed by Small-Angle Neutron Scattering and Molecular Dynamics Simulation

The viscoelastic properties of polymers are strongly influenced by the entanglement phenomenon. Historically, distinct theoretical models have been developed to understand the rheology of entangled and unentangled polymer melts. Using small-angle neutron scattering and nonequilibrium molecular dynamics simulations, we show that the microstructural evolutions of deformed entangled and unentangled polymer melts share a number of universal traits that cannot be comprehended within the classical theoretical framework. Our analysis suggests that interchain correlations play a fundamental role in nonlinear polymer melt rheology.   

Nanoparticle effect on multiscale polymer dynamics in nanocomposites: insights from neutron and x-ray spectroscopy

Dynamics of polymers in presence of nanoparticles has attracted much attention due to immediate consequences on the rheological behavior of polymer nanocomposites. Bulk techniques are usually insufficient to study the component dynamics mainly due to lack of space-time resolution required for intrinsically complex relaxation mechanisms. In our studies, we applied extensive neutron and x-ray scattering techniques, specifically backscattering, neutron spin echo, and x-ray photon correlation spectroscopy to gain microscopic insight on the role of nanoparticle size, shape, loading, dispersion state, polymer-particle interfaces and confinement on multiscale polymer dynamics as well as on slow nanoparticle relaxation in attractive nanocomposites [1, 2]. These interconnected and often competing effects result in the unusual rheological behavior, the origin of which has long been debated. By simultaneously accessing time scales from sub-nanosecond to hundreds of nanoseconds, and length scales from monomers to entanglement spacing, we directly measured the key dynamical parameters- Rouse rate and reptation tube size- in nanocomposites, and correlated them with the macroscopic mechanical relaxation and the nanoscale particle motion [3].

[1] Senses, E. et al. Physical review letters 2017, 118, 147801
[2] Senses, E. et al. ACS Nano 2018, 12, 10807-10816
[3] Senses, E. et al. Physical review letters 2017, 119, 237801   

Molecular Dynamics Simulations of a Polymer Star under Shear Flow

We performed large-scale coarse-grained molecular dynamics simulation of a polymer star in a melt of short polymer chains under shear flow in dilute star concentrations. The temperature of the system was maintained using a dissipative particle dynamics (DPD) thermostat to conserve momentum and model hydrodynamic interactions. Simulations at the quiescent state was used to determine the timescale for which the end-to-end vector of the star’s arm as it relaxes. The star has sixteen arms and its motion under shear in high Weissenberg numbers is characterized by a cycle of collapse and extension of the star’s arm, which is in line with expected tank-treading motion. We used the trajectories of these simulations to calculate quantities pertinent to neutron scattering experiments, such as small angle neutron scattering under high shear stress, or SANS in the rheo-SANS environment, and predict the resulting spectra from these experiments. Specifically, we calculate the star’s anisotropic single-molecule structure factor through the spherical harmonic expansion approach.   

Dynamics and Rheology of THF Swollen Ionic Polymer Melts: Molecular Dynamics Simulation Study

The macroscopic dynamics of ionic polymers are often constrained by the formation of ionic clusters. Traces of solvents however affect the structure, dynamics and viscoelastic response of these melts. Here, we probe the effects of THF on the dynamics of polystyrene sulfonate (PSS) melts by fully atomistic molecular dynamics simulations. We hypothesize that THF penetrates both hydrophilic and hydrophobic domains, affecting their interrelation. Melts swollen with THF as the sulfonation fraction is varied were studied. We find that with the addition of THF the average cluster size decreases. The number of THF molecules associated with ionic groups increases with increasing sulfonation fraction f. Shear measurements show that the addition of small amounts of THF decreases the viscosity dramatically. Correlations of the dynamics of swollen melts on different time and length scales as determined by MSD and S(q, t) with the rheology studies will be discussed.   

Influence of Chain Architecture on the Kinetics of Chain Exchange Between BCC-Ordered Copolymer Micelles: A Dynamical Self-Consistent Mean-Field Theory Study

Motivated by recent time-resolved small-angle neutron scattering results, we study the kinetics of symmetric ABA and BAB triblock copolymer exchange between BCC-ordered spherical micelles using dynamical self-consistent field theory simulations. We characterize the equilibrium properties of micelles in pure melts of AB, ABA, and BAB copolymers, and systematically examine the decay time of the fraction of core (A) blocks remaining in their original micelle, as well as the chain self-diffusion constant, as a function of segregation. Differences in the equilibrium micelle structure between these cases complicate comparison of the dynamical behaviour for the different chain architectures. This complication is overcome by simulating tracer triblock copolymers in a constant background of BCC-ordered diblock micelles, allowing for direct comparison of the chain diffusion for the different chain architectures, as a function of segregation.
  

Revealing Structures and Dynamics of Bound Chains in Filler-Reinforced Elastomers

Filler-reinforced elastomershave been of great interest to the broad materials community for at least the last three decades. The addition of nanofillers such as carbon black or silica to elastomers affects the overall rheological and mechanical properties mainly due to the creation of a few nanometer-thick bound polymer layer (BPL) on a filler surface at which the polymer is expected to have different structural and dynamical behavior from the bulk. However, the detailed descriptions of the BPL remain a challenge due to the lack of experimental techniques that directly probe the BPL. In this presentation, using simplified industrialcarbon black (CB) filled polybutadiene in conjunction with contrast-matched neutron scattering and spectroscopy techniques, we resolve the buried structure at the nanometer-scale and the dynamics at the picosecond-to-nanosecond time scales in the polymer matrix.   

The Effect of Rubbery/Glassy Block Copolymer Brushes on the Dynamic Behaviors of Silica Particles in Nanocomposite Elastomer

Matrix-free elastomer nanocomposites with high strength was prepared by rubbery/glassy block copolymer-grafted silica nanoparticles (BCP-g-SiNP). Investigation of dynamic behavior of the nanocomposits can help us to know the dynamic structure and properties. However, the dynamic behavior of SiNP in the BCP-g-SiNP has not been well-studied. In this study, X-ray photon correlation spectroscopy (XPCS) was utilized to analyze the dynamics of BCP-g-SiNP with inner poly(butyl acrylate) and outer poly(methyl methacrylate).
The XPCS measurement revealed that the relaxation rates of SiNP were proportional to q value, indicating that the behavior of SiNP in the block copolymer-grafted SiNP was non-Brownian and hyperdiffusion motion. At the same q value, the relaxation rates increased dramatically as the temperature exceeded T_g. This is due to the outer PMMA phase changed to the rubbery state, and the restriction of SiNP motion by PMMA decreased. Moreover, during elongation, the relaxation rates of SiNP in the direction perpendicular to elongation was faster than in the direction parallel to elongation. The extended polymer chains might restrict the motion of SiNP, leading the slower relaxation rate of SiNP in the direction parallel to elongation.   

The Role of Fast Relaxations in Cross-Linked Polymer Networks for Impact Mitigation

Composites for ballistic impact resistance integrate high strength reinforcements, such as high strength fibers or ceramic, with a polymeric binder that imparts ductility to the system. Together these components provide the strength plus ductility to realize tough ballistic composites. The binder is often a cross-linked polymer networks where the reactive liquid monomers can be impregnated into the composite structure and then be cross-linked into a rigid component. To this end, there is a significant interest in the molecular origins of toughness in cross-linked networks - why some systems lead to tough resins whereas others lead to a brittle response. Here we explore a series of model epoxy networks with moderate toughness to a series dicyclopentadiene networks which display incredible toughness deep into the glassy state. Quasielastic neutron scattering (QENS) is used to quantify molecular motions on the time scale of ps to ns in these networks. This reveals a strong correlation between these fast polymer relaxations and toughness. Collective many atom vibrations are important for toughness, but not enough. The many atom vibrations must lead to dissipative, many atom relaxations in order to dissipate energy and enhance toughness under ballistic impact conditions.