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Deformation behaviour of homogeneous and heterogeneous bimodal networks
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry. Uppsala University, Disciplinary Domain of Science and Technology, Mathematics and Computer Science, Department of Mathematics, Algebra and Geometry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry.
2017 (English)In: Macromolecules, ISSN 0024-9297, E-ISSN 1520-5835Article in journal (Refereed) Submitted
Abstract [en]

In this study, the effect of spatial heterogeneities on the deformation behaviour and ultimate properties of bimodal gels consisting of both short and long chains was investigated by simulating the uniaxial elongation of defect-free networks containing dense short-chain clusters and comparing with gels having a homogeneous distribution of chains. In both cases, the first chains to rupture were the ones already aligned along the strain axis prior to imposing a strain. The presence of clusters was generally not found to improve the ultimate stress or toughness; the short chains within the clusters were effectively shielded from deformation, even at large fractions of short chains. The heterogeneous network tended to be weaker than the corresponding homogeneous network at a given fraction of short chains, fracturing before any signicant deformation of clusters had taken place. The deformation behaviour was, however, found to be sensitive to the degree of heterogeneity and the number of inter-cluster connections. At large fractions of short chains, clustering offered an improvement in the ultimate strain compared to a homogeneous bimodal network and also an equivalent unimodal network with the corresponding number-average chain length, thus providing a small improvement in toughness.

Place, publisher, year, edition, pages
2017.
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:uu:diva-332560OAI: oai:DiVA.org:uu-332560DiVA, id: diva2:1153402
Available from: 2017-10-30 Created: 2017-10-30 Last updated: 2017-10-30
In thesis
1. Computer Simulations of Polymer Gels: Structure, Dynamics, and Deformation
Open this publication in new window or tab >>Computer Simulations of Polymer Gels: Structure, Dynamics, and Deformation
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis presents the results of computer simulation studies of the structure, dynamics, and deformation of cross-linked polymer gels. Obtaining a fundamental understanding of the interrelation between the detailed structure and the properties of polymer gels is a challenge and a key issue towards designing materials for specific purposes. A new off-lattice method for constructing a closed network is presented that is free from defects, such as looping chains and dangling ends. Using these model networks in Brownian dynamics simulations, I show results for the structure and dynamics of bulk gels and describe a novel approach using spherical boundary conditions as an alternative to the periodic boundary conditions commonly used in simulations. This algorithm was also applied for simulating the diffusion of tracer particles within a static and dynamic network, to illustrate the quantitative difference and importance of including network mobility for large particles, as dynamic chains facilitate the escape of particles that become entrapped.

I further investigate two technologically relevant properties of polymer gels: their stimuli-responsive behaviour and their mechanical properties. The collapse of core-shell nanogels was studied for a range of parameters, including the cross-linking degree and shell thickness. Two distinct regimes of gel collapse could be observed, with a rapid formation of small clusters followed by a coarsening stage. It is shown that in some cases, a collapsing shell may lead to an inversion of the core-shell particle which exposes the core polymer chains to the environment. This thesis also explores the deformation of bimodal gels consisting of both short and long chains, subject to uniaxial elongation, with the aim to understand the role of both network composition as well as structural heterogeneity on the mechanical response and the reinforcement mechanism of these materials. It is shown that a bimodal molecular weight distribution alone is sufficient to strongly alter the mechanical properties of networks compared to the corresponding unimodal networks with the same number-average chain length. Furthermore, it is shown that heterogeneities in the form of high-density short-chain clusters affect the mechanical properties relative to a homogeneous network, primarily by providing extensibility.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. p. 69
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1596
Keywords
computer simulations, Brownian dynamics, polymer gel, microgel, spherical boundary conditions, hypersphere, core-shell, deswelling, mechanical properties, uniaxial elongation
National Category
Physical Chemistry Polymer Chemistry
Identifiers
urn:nbn:se:uu:diva-332575 (URN)978-91-513-0144-0 (ISBN)
Public defence
2017-12-19, Polhemssalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2017-11-28 Created: 2017-10-30 Last updated: 2018-03-07

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