Monte Carlo simulation of molecular and structural properties of random copolymer thin films.

Molecular and structural properties of random copolymer thin films were studied by Monte Carlo simulation of coarse-grained copolymer model on the high coordination lattice. Random copolymer thin films with 50% comonomer fraction with varied interaction strength between comonomer units were studied. Intramolecular interaction was represented by Flory's rotational isomeric state (RIS) model of polyethylene (PE). The non-bonded interactions were treated by Lennard-Jones potential with different parameter sets for comonomer units. When the interaction strength of comonomer is increased, the density of thin films is higher in the inner region and significantly dropped near the free surface along the normal direction of the film. Higher bulk densities and narrower interfacial thicknesses are observed for random copolymer with stronger bead interaction. End beads were segregated at the free surface region and tend to orient perpendicularly to the surface. Copolymer size and shape were significantly changed as a function of bead interaction. For orientation of the whole chain, the largest molecular axis tends to orient along the film surface, but changed toward random orientation for weaker bead interaction. The intra-chain energies were decreased at the surface region while the non-bonded energies were increased. The stronger the comonomer bead attraction, the more change in energetics across the thin film.

Viscoelastic Relaxation of Polymerized Ionic Liquid and Lithium Salt Mixtures: Effect of Salt Concentration.

Polymerized ionic liquids (PILs) doped with lithium salts have recently attracted research interests as the polymer component in lithium-ion batteries because of their high ionic mobilities and lithium-ion transference numbers. To date, although the ion transport mechanism in lithium-doped PILs has been considerably studied, the role of lithium salts on the dynamics of PIL chains remains poorly understood. Herein, we examine the thermal and rheological behaviors of the mixture of poly(1-butyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide (PC4-TFSI)/lithium TFSI (LiTFSI) in order to clarify the effect of the addition of LiTFSI. We show that the glass transition temperature Tg and the entanglement density decrease with the increase in LiTFSI concentration wLiTFSI. These results indicate that LiTFSI acts as a plasticizer for PC4-TFSI. Comparison of the frequency dependence of the complex modulus under the iso-frictional condition reveals that the addition of LiTFSI does not modify the stress relaxation mechanism of PC4-TFSI, including its characteristic time scale. This suggests that the doped LiTFSI, component that can be carrier ions, is not so firmly bound to the polymer chain as it modifies the chain dynamics. In addition, a broadening of the loss modulus spectrum in the glass region occurs at high wLiTFSI. This change in the spectrum can be caused by the responses of free TFSI and/or coordination complexes of Li and TFSI. Our detailed rheological analysis can extract the information of the dynamical features for PIL/salt mixtures and may provide helpful knowledge for the control of mechanical properties and ion mobilities in PILs.