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as the temperature decreases, thermally sensitive spherical poly (n-isopropylacrylamide) (pnipam) microgels with a lower critical solution temperature (lcst～32 ℃) swell in a dispersion, leading to a possible volume-concentration induced sol-gel transition if the microgel concentration is sufficiently high. in such a formed gel, polymer chains inside each microgel were chemically cross-linked, but individual microgels were physically close-packed into a three-dimensional network. such hybrid gels can be used as model systems for studying the sol-gel transition, which avoids several problems, such as chain entanglements, phase separation, and vitrification, normally occurring in a gelling process. the sol-gel transition of such hybrid gels was studied by the change of viscoelastic modulus g¢ and g¢¢. as expected, the sol-gel transition depends on the polymer concentration, frequency, and shear stress. the gelation point could be roughly estimated by the method of winter and chambon. our results showed that the temperature dependence of g″ and tan σ(=g″/g′) had a minimum, which corresponded to the sol-gel transition, providing a better way to determine the sol-gel transition temperature.

two-dimensional simple shear flow of a self-avoiding macromolecular chain is simulated by a lattice monte carlo (mc) method with a pseudo-potential describing the flow field. the simulated velocity profile satisfies the requirements of simple shear flow unless the shear rate is unreasonably high. some diffusion problems for a free-draining bead-spring chain with excluded volume interaction are then investigated at low and relatively high shear rates. three diffusion coefficients are defined and examined in this paper: the conventional self-diffusivity in zero field, dself, the apparent self-diffusivity in flow field, dapp, and the flow diffusivity in simulation, dflow reflecting actually the transport coefficient. it is found that these three diffusivities for a flexible chain are different from each other. what is more important is that self-diffusion exhibits a high anisotropy in the flow field. the apparent self-diffusion along the flow direction is enhanced to a large extent. it is increased monotonically with the increase of shear time or shear strain, whereas the chain configuration can achieve a stationary anisotropic distribution following an interesting overshoot of the coil shape and size. besides a single self-avoiding chain, an isolated brownian bead and a group of self-avoiding beads with a quasi-gaussian spatial distribution are also simulated. according to the comparison, the effects of the connectivity of the chain on the diffusion behavior are revealed. some scaling relations of dapp versus t are consistent with the theoretical analyses in the pertinent literature.

simulations of phase separation under oscillatory shear flow have been performed based on the time-dependent ginzburg-landau (tdgl) equation. to calculate the stress tensor, the expression proposed by kawasaki was used. the results of the simulations have been confronted directly with experimental results on a lcst blend of prmsan/pmma to evaluate the potential of the simulations. the effect of quench depth, shear amplitude, and shear frequency on the morphology development as well as on the corresponding rheological properties has been investigated. the results show that the characteristic rheological behavior of phase-separating systems can be attributed to the interfacial relaxation, which is changing during the process of phase separation. the strength of the concentration fluctuations and the interfacial volume fraction are key factors determining the contribution of interfacial relaxation to the global rheological behavior of the blend. in the low frequency range, the oscillatory shear cannot affect the critical point, but it can accelerate the coagulation and growth of the blend morphology. the simulations qualitatively agree with the experimental findings.

the kinetics and chain length distributions occurring in living free-radical polymerizations are simulated using a hybrid monte carlo algorithm. the new algorithm is much faster than the conventional one because the activation/deactivation exchange reactions, which are cpu intensive, are treated by a biased-sampling method with an analytical expression for the exchange equilibrium, while the reactions of chain propagation, irreversible chain termination, etc. are treated by exact stochastic monte carlo simulation. two models of living radical polymerizations, i.e., the polymerization initiated by alkoxyamines and the nitroxide radical, 2,2,6,6-tetramethyl-1-piperidinyloxy, mediated radical polymerization, are simulated to study the effects of experimental variables, such as the concentration ratio of stable free radicals to initiators, initiation rate constants, etc., on the kinetics and molecular weight distributions. a comparison between simulated and analytical results in the literature is made. taking thermal initiation into consideration, the algorithm reproduces the experimental results very well. therefore, its feasibility and usefulness in studying living free-radical polymerization are demonstrated.