Dynamics in entangled polymer melts: The theory of dynamics of entangled polymer melts has been primarily based on the tube model established by de Gennes, Doi, and Edwards. While experiments support the tube concept in monodisperse long linear polymers, additional relaxation mechanisms, such as contour length fluctuations (CLF) and constraint release (CR) effects, need to be incorporated into the model to provide quantitative predictions. I employ large-scale fine-grained molecular dynamics (MD) simulations, in combination with coarse-grained 1D Rouse-like simulation algorithms, to investigate the constraint release effects in entangled binary blends of linear polymers, in which the rapid motion of the shorter chains releases the constraints on the longer ones. Comparison between the simulation results with theoretical predictions and available experimental data indicates the importance of considering a broad distribution of the disentanglement rates and the effect of chain stiffness or packing length in any quantitative predictions of constraint release effects.
Solutions of surfactant micelles: Aqueous solutions of surfactant micelles and mixtures of micelles with nonionic water-soluble polymers are widely used in commercial and industrial products, such as detergents, paints, adhesives, cosmetics, pharmaceuticals, foodstuffs and oil recovery fluids. Despite the numerous applications and studies of these solutions, the microscopic mechanisms of the salt-induced micellar shape transition, the micelle-micelle and micelle-polymer interactions remain poorly understood. We thus perform extensive atomistic molecular dynamics simulations to study these mechanisms for various surfactant solutions, including aqueous solutions of thread-like cetyl-trimethyl ammonium chloride (CTAC) micelles and mixtures of spherical sodium dodecyl sulfate (SDS) micelles with poly(ethylene oxide) polymers.
Rheological properties of branched polymers: Understanding the relation between the long chain branching (LCB) structures of commercial polymers and their rheological properties is perhaps the most important and practical problem remaining in polymer melt rheology. The degree of LCB has strong effects on the rheological, and therefore processing properties, of these polymers. The hierarchical model proposed by Larson is one of the most promising theoretical models in resolving this relationship. Based on this model, we are making collaborative efforts to develop a numerical program which would enable the accurate prediction of linear rheological properties for all kinds of branch polymers.
 
Selected Publications
[1] Z. Wang and R.G. Larson, Macromolecules 41:4945-4960, 2008 “Constraint release in entangled binary blends of linear polymers: A molecular dynamics study.”
[2] B. Shang, Z. Wang, and R.G. Larson, J. Phys. Chem. 112:2888-2900, 2008 “Molecular dynamics simulation of interactions between a sodium dodecyl sulfate micelle and a poly(ethylene oxide) polymer.”
[3] Z. Wang and M. Rubinstein, Macromolecules 39:5897-5912, 2006 "Regimes of conformational transitions of a diblock polyampholyte."
[4] Z. Wang, C. Holm, and H.W. Müller, Phys. Rev. E 66:021405, 2002 "Molecular dynamics study on the equilibrium magnetization properties and structure of ferrofluids."
[5] Z. Wang and C. Holm, J. Chem. Phys. 115:6351-6359, 2001 "Estimate of the cutoff errors in the Ewald summation for dipolar systems."
[6] Z. Wang, H. Fang, Z. Lin, and L. Zhou, Phys. Rev. E 61:6837-6844, 2000 "Simulation of field-induced structural formation and transition in electromagnetorheological suspensions.''
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