Non-equilibrium Effects of Polymer Dynamics under Nanometer Confinement: Effects of Architecture and Molar Mass

Capillary filling of star polymer melts in nanopores

The topology of a polymer profoundly influences its behavior. However, its effect on imbibition dynamics remains poorly understood. In the present work, capillary filling (during imbibition and following full imbibition) of star polymer melts was investigated by molecular dynamics simulations with a coarse-grained model. The reversal of imbibition dynamics observed for linear-chain systems was also present for star polymers. Star polymers with short arms penetrate slower than the prediction of the Lucas-Washburn equation, while systems with long arms penetrate faster. The radius of gyration increases during confined flow, indicating the orientation and disentanglement of arms. In addition, the higher the functionality of the star polymer, the more entanglement points are retained. Besides, a stiff region near the core segments of the stars is observed, which increases in size with functionality. The proportion of different configurations of the arms (e.g., loops, trains, tails) changes dramatically with the arm length and degree of confinement but is only influenced by the functionality when the arms are short. Following full imbibition, the different decay rates of the self-correlation function of the core-to-end vector illustrate that arms take a longer time to reach the equilibrium state as the functionality, arm length, and degree of confinement increase, in agreement with recent experimental findings. Furthermore, the star topology induces a stronger effect of adsorption and friction, which becomes more pronounced with increasing functionality.

Topology sorting: Separating linear/star polymer blend components by imbibition in nanopores

We report the imbibition and adsorption kinetics of a series of symmetric linear/star cis-1,4-polyisoprene blends within the long channels of self-ordered nanoporous anodic aluminum oxide (abbreviated: AAO). Using in situ nanodielectric spectroscopy, we followed the evolution of the longest chain modes in the blends with a judicious selection of molar masses for the constituent components. We demonstrated differences in the imbibition kinetics of linear and star components based on the relative viscosities (e.g., polymers with lower zero-shear viscosity penetrated first the nanopores). Following the complete imbibition of the pores, the adsorption time, τads, of each component was evaluated from the reduction in the dielectric strength of the respective chain modes. In the majority of blends, both components exhibited slower adsorption kinetics with respect to the homopolymers. The only exception was the case of entangled stars mixed with shorter linear chains, the latter acting as a diluent for the star component. This gives rise to what is known as topology sorting, e.g., the separation of linear/star blend components in the absence of solvent. Moreover, a simple relation (τads ∼ 10 × tpeak; tpeak is the time needed for the complete filling of pores) was found for linear polymers and stars. This suggested that the characteristic timescale of imbibition (tpeak) governs the adsorption process of polymers. It further implied the possibility of predicting the adsorption times of high molar mass polymers of various architectures by the shorter imbibition times.

New Insights from Nonequilibrium Kinetics Studies on Highly Polar S-Methoxy-PC Infiltrated into Pores

Herein, the annealing of highly polar (S)-(-)-4-methoxymethyl-1,3-dioxolan-2-one (S-methoxy-PC) within alumina and silica porous membranes characterized by different pore diameters was studied by means of dielectric spectroscopy. We found a significant slowing down of the structural dynamics of confined S-methoxy-PC with annealing time below and, surprisingly, also above the glass transition temperatures of the interfacial layer, T_{g,interfacial}. Furthermore, unexpectedly, a change in the slope of temperature dependencies of the characteristic time scale of this process τ_anneal, at T_{g,interfacial} for all confined samples, was reported. By modeling τ_anneal(T), we noted that the observed enormous variation of τ_anneal results from a decrease of the pore radius due to the vitrification of the interfacial molecules. This indicates that the enhanced dynamics of confined materials upon cooling is mainly controlled by the interfacial molecules.