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Brownian Dynamics Simulations of Associating Diblock Copolymers

Cass, M. J.; Heyes, D. M.; English, R. J.

Authors

M. J. Cass

D. M. Heyes

R. J. English



Abstract

A novel coarse-grained computational model for associating polymers is proposed that is based on a Gaussian “blob” representation of the polymer chains. The model allows a large number of model polymers to be simulated at moderate computational cost over a wide packing fraction range using the Brownian dynamics, BD, technique. The attraction of the hydrophobic part of the polymer to those on other molecules can lead to strong aggregation of the polymer molecules in real systems, and this is included in the model by an attractive potential felt by the Gaussian blobs to a common “nodal” point that represents the center of the micelle. Attention here is confined to model AB diblock copolymers in which the hydrophilic block, A, has a much higher mass than the hydrophobic moiety, B, which leads to relatively small aggregation numbers, Nagg, of ∼8. The aggregation number at low packing fractions is found to increase with packing fraction, as observed in experiments, with a functional form that closely follows a simple theory derived here that is based on entropy-derived mean-field terms for the free-energy change associated with the incorporation of the polymer molecule into the micelle. The computational model exhibits an extremely low critical micelle concentration (cmc), and micelles with Nagg ≈ 5 are observed at the lowest packing fractions, φ, simulated (∼10-4), which is consistent with experiment. The long-time self-diffusion coefficient of the polymers (and hence micelles) decreases logarithmically with packing fraction, and the viscosity increased with concentration according to the Huggins equation. The spherical blob coarse graining results in the simulable time scales being longer than the Rouse time of the chain, and hence for the nonassociating polymers the intrinsic viscosity is an input parameter in the model. The introduction of association leads to the partial inclusion of the intrinsic viscosity in the simulation and has an effect on the computed Huggins coefficient, kH, which is found to be ∼6 in those cases.

Citation

Cass, M. J., Heyes, D. M., & English, R. J. (2007). Brownian Dynamics Simulations of Associating Diblock Copolymers. Langmuir, 23(12), 6576-6587. https://doi.org/10.1021/la063210j

Journal Article Type Article
Online Publication Date May 12, 2007
Publication Date Jun 1, 2007
Deposit Date Oct 26, 2016
Journal Langmuir
Print ISSN 0743-7463
Electronic ISSN 1520-5827
Publisher American Chemical Society
Peer Reviewed Peer Reviewed
Volume 23
Issue 12
Pages 6576-6587
DOI https://doi.org/10.1021/la063210j
Keywords Polymer molecules, AB diblock copolymers,
Public URL http://researchrepository.napier.ac.uk/Output/315307