Demixing and colloidal crystallization in the mixture of charge stabilized colloidal poly(methyl methacrylate) particles and soluble poly(ethylene oxide) were investigated by means of synchrotron small-angle X-ray scattering (SAXS) technique. Phase diagram of the mixture was obtained based on visual inspection and SAXS results. The phase behavior is determined as a function of the concentration of the polymer as well as the volume fraction of the colloidal particles. The system shows a one phase region when the concentration of the polymer is low, whereas a two-phase region is present when the concentration of the polymer is larger than a critical concentration at certain volume fraction of the colloids. Interestingly, a face centered cubic colloidal crystalline structure was formed under certain conditions, which has been rarely observed in experiments of colloid-polymer mixtures with competing interactions.
Molecular dynamics simulations are useful tools to unveil molecular mechanisms of polymer phase separation,self-assembly,adsorption,and so on.Due to large molecular size and slow relaxation of the polymer chains,a great amount of issues related to large-distance chain displacement cannot be tackled easily with conventional molecular dynamic simulations.Systematic coarse-graining and enhanced sampling methods are two types of improvements that can boost spatiotemporal scales in polymer simulations.We present two typical ways to obtain the coarse-graining potential either by fitting to correct liquid structures or by fitting to available thermodynamic properties of polymer systems.The newly proposed anisotropic coarse-grained particle model can be used to describe aggregation and assembly of polymeric building blocks from disk-like micelles to Janus particles.We also present a stochastic polymerization model combined with coarse-grained simulations to investigate the problems strongly influenced by the coupling of polymerization and excluded volume effects.Finally,a facile implementation of integrated tempering sampling method is illustrated to be very efficient on bypassing local energy minima and having access to true equilibrium polymer structures.
LI YanChunWANG YongLeiLI ZhanWeiLIU HongLü ZhongYuan
It has been found recently that an ultrasmall nanoparticle whose size is smaller than the thickness of a cell membrane has unique roles in biomedical applications including the development of next generation of drugs or advanced nanoscale cargo carriers.However,the effect of physical properties of an ultrasmall nanoparticle on its adhesion to a bilayer membrane,which is a key step for Nano-Bio interaction as well as the biomedical applications,is still largely unknown.By using molecular dynamics,we find that both size and shape of an ultrasmall nanoparticle strongly affect its adhesion states on a bilayer membrane(e.g.,adhesion,separation or entwined by polymer chains).Interestingly,our simulations show that with decreasing particle size,the effect of particle shape becomes even more evident for the adhesion behavior.It is indicated that the competition between nanoparticle-polymer binding and polymer chain deformation,both of which are influenced by particle size and shape,determines the final adhesion states of an ultrasmall nanoparticle.Our results are helpful for the full understanding of interaction mechanism between nanoparticles and cell membranes and the practical applications of such ultrasmall nanoparticles.
