Conformations and Dynamics of Polymer Chains in Cis and Trans Polybutadiene/Silica Nanocomposites through Atomistic Simulations: From the Unentangled to the Entangled Regime

Abstract

The conformations and the dynamics of poly­(butadiene) (PB) chains, of various molecular weights, in PB/silica nanocomposites are studied through long-time atomistic molecular dynamics simulations at T = 413 K, well above T g. The effect of the stereochemistry of PB chains is addressed by simulation of cis-1,4-PB/silica and trans-1,4-PB/silica nanocomposites. The model systems contain 30 wt % (≈12 vol %) silica nanoparticles (NPs) of diameter ≈4 nm. The nanocomposites are characterized through analyzing (i) interfacial packing and the dimensions of the PB chains; (ii) statistics of the train, bridge, loop, and tail conformations of adsorbed chains and the coupling between segmental orientational dynamics and chain conformations; and (iii) the orientational and translational dynamics of the polymer chains and the desorption kinetics of chains and segments. The dimensions of PB chains, excluding a small fraction of chains that wrap around the NP, are not affected. The segmental and terminal dynamics of PB chains are slower in the nanocomposites than in the respective bulk melts. Moreover, the dynamics of PB chains in the nanocomposites is very heterogeneous, and a coupling between the dynamics and the conformation of PB chains is observed: the adsorbed segments (trains) and the chains that have a higher number of contacts to the NPs are more decelerated. Additionally, at long times, bridge segments exhibit a very slow orientational decorrelation. The self-diffusion coefficients, D, of PB chains in the nanocomposites are also reduced compared to the respective bulk systems. A clear crossover from the unentangled (Rouse-like) to the entangled (reptation-like) regime is observed based on the calculation of the segmental mean-square displacement and D as a function of the chain length. The effective tube diameter of entangled PB chains in the nanocomposites is estimated to be slightly smaller than in the pure melts. The deceleration of dynamics in the nanocomposites, in both Rouse and reptation-like regimes, is discussed in terms of a higher effective monomeric friction coefficient. Finally, the correlation times for the desorption of segments and chains are much larger than the segmental and end-to-end-vector correlation times, respectively.

Effect of SiO2 Particles on the Relaxation Dynamics of Epoxidized Natural Rubber (ENR) in the Melt State by Time-Resolved Mechanical Spectroscopy

“…Such morphological features are well known in the field of reinforced elastomers and are associated with the formation of both interfacial chemical bonds and interphase domains of higher cross-linking density. Recent computer modeling studies have shown how double bond interactions from a diene elastomer ( cis - and trans- polybutadiene) with silica nanoparticles are sufficient to bring about the formation of an interphase layer [ 38 ].…”

Coupling between Polymer Conformations and Dynamics Near Amorphous Silica Surfaces: A Direct Insight from Atomistic Simulations

“…It is important to note that the shift was observed for the nanocomposite systems where the radius of the nanoparticle was comparable to the Kuhn length of the polymer. The data presented in Figure 5 correspond to a system in which the radius of nanofillers is around two times bigger than the Kuhn length of the polymer matrix (see [ 54 ] for the estimation of the Kuhn length for the used PB model). The probability of finding trains formed by the whole chain, i.e., for which its length equals to the length of the polymer chain, is higher in the systems with shorter cPB30 chains than cPB100 due to the lower entropic penalty associated with the full-chain adsorption in the former case, as also reported in the literature for flat surfaces [ 35 ] and silica nanocomposites [ 54 ].…”

“…In the long trains, the enthalpic contribution (i.e., increased number of contacts with the surface) and the connectivity effect wins over the entropic one, and the dynamics becomes slower as the length of the train increases (notice the difference in the actual values of , e.g., for trains consisting of 9 and 3 segments in Figure 11 ). By contrast, the segments of the shortest trains (up to three monomers, i.e., up to the Kuhn length [ 54 ]) are even slower than the first (terminal) monomers of the longer trains. Due to the low probability of finding long trains (see Figure 5 ), we cannot obtain statistically relevant results for much longer trains than those presented in Figure 11 .…”

“…The polymer chains are modelled by a united atom model, meaning that the hydrogens are not simulated explicitly but are included together with the carbon atoms in one united atom, e.g., CH. The details of the force field used for cPB can be found elsewhere [ 54 ]. Both 30 and 100-mers in bulk were also simulated as reference systems.…”

“…In their study, Guseva et al investigated the effect of the film thickness and, thus, of the imposed confinement, reporting mainly the overall properties of the material averaged over the whole film [ 52 ]. Recently, conformational and dynamical properties of cis- and trans -polybutadiene/silica nanocomposites were analyzed by classifying the chains according to their adsorption state and their distance from the nanoparticle’s center of mass [ 54 ]. Due to the confinement effects induced by a high volume fraction of the filler as well as due to the curvature of the nanoparticles, a small fraction of PB chains also acted as bridges between the nanoparticles or was wrapped around the filler in addition to trains, loops and tails [ 54 ].…”