杨玉良，学者凯发k8国际首页主页-凯发网

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.

in this paper, the kinetics and morphology of viscoelastic microphase separation of diblock copolymers are investigated in detail. it is found that for a symmetric system (f=0.5), taking into account the bulk modulus differences between the two blocks, the classical randomly oriented lamellar morphology is not observed. instead, we find that droplets of the lower bulk modulus soft block disperse in a matrix of higher bulk modulus hard block. in the case of off-critical system (f=0.4), since an inherent composition asymmetry exists between the two blocks, the viscoelastic microphase separated morphology is also different from that normally observed, whether there is an apparent difference in the bulk moduli between two blocks. however, when a bulk modulus difference exists between the two blocks, the morphology is further altered. addition of random thermal noise to both the critical (f=0.5=and the off-critical system (f=0.4=weakens the suppression of concentration fluctuations, but no fundamental structural transformation is caused by the noise. since the abnormal morphology is observed on slow approach to the equilibrium state, we conclude that the viscoelastic effect may be largely responsible for the observed deviations between the experimentally measured and the mean-field theoretically calculated phase diagrams.

polystyrene clusters were prepared by using a trace amount of 4-methacryloyloxy-2,2,6,6-tetramethylpiperidine-1-oxy (mtempo) as a branching agent. such clusters can undergo a thermal decomposition into linear chains at temperatures higher than 100℃. the thermal decomposition was studied by a combination of static and dynamic laser light scattering (lls). the time dependence of the weight-average molar mass (mw), the root-mean-square z-average radius of gyration (〈rg2〉z1/2), and the average hydrodynamic radius (〈rh〉) was used to monitor the decomposition kinetics and cluster structure. it has been found that mw∝t-r, and the decomposition can be roughly divided into three stages; namely, from large clusters to smaller ones; from smaller clusters to less-branched ones; and finally to short linear chains. the scaling of 〈rg〉∝mw 0.33 (0.01 in the first stage indicates that these clusters are uniform in density, which is rare and much different from conventional polymer clusters whose density decreases from center to periphery. moreover, we observed, for the first time, that 〈rg〉/〈rh〉∝mw-0.20 ( 0.01, revealing that even for a uniform cluster swollen in a good solvent, its periphery is still more hydrodynamically draining.

microphases and triangle phase diagrams of abc star triblock copolymers are investigated on the basis of a real-sapce implementation of the self-consistent field theory (scft) for polymers. for the of numerical trability, the calculations are carried out in two dimensions (2d). nine stable microphases are uncovered, including hexagonal lattice, core-shell hexagonal lattice, lamellae, and lamellae with beads at teh interface as well as a variety of complex morphologies that are absent in liear abc triblocks, such as a "three-color" hexagonal honeycomb phase, knitting patern, otcgagon-octagon-tetragon phase, lamellar phase with altemating beads, and decagon-hexgon-tetragon phase. we have found that when the volume factions of the three species are comparable the star architecture of the polymer chain is a strong topological constraint taht regulates the geometry ofthe microphases formed. however, when at least one of the volume fractons of the three species is low, the influence of the star arc hitecture on the morphology is not significant. our calculations reasonably agree with previous theoretical and experimental results and can be used to guide the design of novel mocrosructures involving star triblock copolymers.

in this paper, computer simulation results of phase separation dynamics of ternary mixture (a, b, and c) coupled with an interfacial chemical reaction a b=c in two-dimension are presented. the effect of redection of interfacial free energy due to the presence of species c along the interface is taken into consideration in our study. in the case of fixed domain size, it is shown in simulations that for both reversible and irreversible reactions thegeneration of species c is not affected by the reaction rate constants, and it is a diffusion controlled process. also, the simulation reveals that for reversible chemical reaction in the case of fixed domain size. the equilibrium of concentrations is established. in the cases of coupling beween domain growth and reversible reaction kinetics, it is shown in simulations that the domain growth eventually freezes regardless of the magnitude of the reaction rate constants. the higher the reaction rate constants are, the slower the evolution to reach steady state is, the larger the steady state domain size is, and the smaller the average concentration of species c is.