Chang-Ji Qian

Study on the polymer diffusion in a media with periodically distributed nano-sized fillers

The effect of nano-sized fillers on the equilibrium and dynamical properties of a linear polymer is studied by using off-lattice Monte Carlo simulation. Fillers are arranged periodically in the system with period d and Lennard-Jones interaction between polymer and fillers is considered. Results show that the statistical dimension and dynamical diffusion of the polymer are dependent on the polymer-filler interaction strength ɛ(pf) and the relative size between R(G0) and d, here R(G0) is the radius of gyration of polymer in dilute solution. Normal diffusion of polymer is always observed in the regime 2R(G0) > d. And the diffusion coefficient D is scaled with chain length N as D ~ N(-α), where the exponent α increases with ɛ(pf). Whereas in the regime 2R(G0) < d ≪ Nl0 with l0 the mean bond length of polymer, normal diffusion is observed only at ɛ(pf) < 2, but the polymer will be adsorbed on the fillers and cannot diffuse at ɛ(pf) > 2. In addition, we find that there is a critical interaction strength ɛ*(pf) = 2 in our model system.

Simulation of a Flexible Polymer Confined between Two Parallel Surfaces

The conformational properties of a lattice self-avoiding polymer chain confined between two parallel impenetrable surfaces are simulated using Monte Carlo method. The statistical conformation size of polymer can be scaled by polymer chain length N and distance D between two surfaces. Chain is nearly free at large scaled distance Δ = D/Nνb with ν = 0.58 the scaling exponent and b = 1.42 mean bond length. Then it is squeezed along normal direction to surface at intermediate Δ, and at last it spreads out along surface at small Δ. For attractive surfaces, the chain is adsorbed at low temperature. For small D, we find an abnormal temperature dependence of chain size due to chain adsorbed to both surfaces simultaneously.

Simulation of conformational properties of end-grafted diblock copolymers

Conformational properties of end-grafted flexible diblock copolymers were studied by using Monte Carlo simulation. The copolymers, XAnBn and XBnAn, are grafted with the end X to a flat surface which attracts monomers A but repulses monomers B. Results show that the blocks A in XAnBn and XBnAn are adsorbed upon the surface at low temperature, and the adsorption of blocks A takes place roughly at the same temperature. However, the conformational size and instantaneous shape of the blocks A and B in XAnBn and XBnAn are different. The possible reasons were discussed and they were interpreted by different properties and grafting locations of the two blocks.

Critical adsorption of copolymer tethered on selective surfaces

Critical adsorption behaviors of flexible copolymer chains tethered to a flat homogeneous surface are studied by using Monte Carlo simulations. We have compared the critical adsorption temperature Tc, estimated by a finite-size scaling method, for different AB copolymer sequences with A the attractive monomer and B the inert monomer. We find that Tc increases with an increase in the fraction of monomers A, fA, in copolymers, and it increases with an increase in the length of block A for the same fA. In particular, Tc of copolymer (AnBn)r can be expressed as a function of the block length, n, and Tc of copolymer (AnB)r and (ABm)r can be expressed as a linear function of fA. Tc of random copolymer chains also can be expressed as a linear function of fA and it can be estimated by using weight-average of Tc of different diblocks in the random copolymer. However, the crossover exponent is roughly independent of AB sequence distributions either for block copolymers or for random copolymers.

Monte Carlo simulation of ramified aggregates on substrates with fixed impurities

Abstract A growth mechanism of ramified aggregates on substrates with fixed impurities is studied systematically. It is found in experiment that, on a melting glass surface with fixed impurities, deposited Au atoms first form a web-shaped film, then transfer to compact clusters, and at last form large ramified aggregates. Based on the experimental observations, we propose a restricted cluster–cluster aggregation (RCCA) model and simulate the growth mechanism of the ramified Au aggregates. In particular, the influence of the impurities on the fractal dimension and size of the final aggregates is investigated. The simulation results are in good agreement with the experimental findings.

SIMULATION FOR RAMIFIED AGGREGATES ON NONLATTICE SUBSTRATES WITH IMPURITIES

A growth mechanism of ramified aggregates on nonlattice substrates with fixed impurities is studied systematically. Based on the experimental observations of Au atomic aggregates on molten glass surfaces, an Improved Restricted Cluster-Cluster Aggregation (IRCCA) model is established. In this model, fixed impurities are distributed randomly on a nonlattice substrate, and all the aggregates with different sizes are allowed to diffuse and rotate. The influence of the liquid substrate and impurity on the structure of the ramified aggregates is studied systematically. The simulation results are in good agreement with the experimental findings.