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Computer Simulations of Polymer Gels: Structure, Dynamics, and Deformation
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry.
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
Keyword [en]
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: urn:nbn:se:uu:diva-332575ISBN: 978-91-513-0144-0 (print)OAI: oai:DiVA.org:uu-332575DiVA, id: diva2:1153466
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
List of papers
1. Construction of a closed polymer network for computer simulations
Open this publication in new window or tab >>Construction of a closed polymer network for computer simulations
2014 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 141, no 15, p. 154113-Article in journal (Refereed) Published
Abstract [en]

Computer simulations are an important tool for linking the behaviour of polymer materials to the properties of the constituent polymer chains. In simulations, one normally uses periodic boundary conditions to mimic a macroscopic system. For a cross-linked polymer network, this will impose restrictions on the motion of the polymer chains at the borders of the simulation cell. We present a new method for constructing a three-dimensional closed network without periodic boundaries by embedding the system onto the surface of a sphere in four dimensions. This method can also be used to construct finite-sized gel particles for simulating the swelling of particles in a surrounding solvent. The method is described in algorithmic detail to allow the incorporation of the method into different types of simulation programs. We also present the results of Brownian dynamics simulations, analyzing the end-to-end distribution, radial distribution function, and the pore size distribution for different volume fractions and for chains with varying stiffness.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-239581 (URN)10.1063/1.4897447 (DOI)000344346000015 ()25338887 (PubMedID)
Available from: 2014-12-30 Created: 2014-12-29 Last updated: 2017-12-05Bibliographically approved
2. Tracer diffusion in a polymer gel: simulations of static and dynamic 3D networks using spherical boundary conditions
Open this publication in new window or tab >>Tracer diffusion in a polymer gel: simulations of static and dynamic 3D networks using spherical boundary conditions
2016 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 28, no 47, article id 475101Article in journal (Refereed) Published
Abstract [en]

We have investigated an alternative to the standard periodic boundary conditions for simulating the diffusion of tracer particles in a polymer gel by performing Brownian dynamics simulations using spherical boundary conditions. The gel network is constructed by randomly distributing tetravalent cross-linking nodes and connecting nearest pairs. The final gel structure is characterised by the radial distribution functions, chain lengths and end-to-end distances, and the pore size distribution. We have looked at the diffusion of tracer particles with a wide range of sizes, diffusing in both static and dynamic networks of two different volume fractions. It is quantitatively shown that the dynamical effect of the network becomes more important in facilitating the diffusional transport for larger particle sizes, and that one obtains a finite diffusion also for particle sizes well above the maximum in the pore size distribution.

Keyword
tracer diffusion, Brownian dynamics simulation, polymer network, polymer gel, spherical boundary conditions, 3-sphere, hypersphere
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-307510 (URN)10.1088/0953-8984/28/47/475101 (DOI)000385445100001 ()27662260 (PubMedID)
Available from: 2016-11-21 Created: 2016-11-17 Last updated: 2017-11-29Bibliographically approved
3. Collapse Dynamics of Core-Shell Nanogels
Open this publication in new window or tab >>Collapse Dynamics of Core-Shell Nanogels
2016 (English)In: Macromolecules, ISSN 0024-9297, E-ISSN 1520-5835, Vol. 49, no 15, p. 5740-5749Article in journal (Refereed) Published
Abstract [en]

Stimuli-responsive core shell nanogels display collapse properties which are determined by both the core and the shell. We examine the equilibrium properties and the detailed structural changes during a collapse transition of polymer core shell nanoparticles using Brownian dynamics simulations. Gel particles with randomly distributed cross-linking nodes are created. The influence of the cross-linking degree, core/shell mass ratio, and the strength of the interparticle attractive interaction on the collapse behavior is investigated. Both collapsed core and collapsed shell structures are considered and compared with collapsed homopolymer networks. The transition time was found to be reduced with increasing cross-linking degree and inversely related to the depth of the Lennard-Jones potential. Similar to the kinetics of single chain collapse, there is an initial formation of clusters, in this case near cross-linking nodes, and a subsequent coarsening to form a compact globule. Where the nanogels were collapsed into a compact core, the deformation was found to be essentially symmetric, with a significantly slower relaxation time for the shell units compared to the core collapse transition time. Reducing the core size, the shell units were less affected by the presence of a collapsing core. For the system with a collapsing shell, our simulations reveal in some cases an inversion, with the shell compressing and eventually squeezing out the core units. This effect was more pronounced with decreasing cross-linking degree and core/shell mass ratio.

National Category
Polymer Chemistry
Identifiers
urn:nbn:se:uu:diva-303278 (URN)10.1021/acs.macromol.6b01206 (DOI)000381320300043 ()
Available from: 2016-09-16 Created: 2016-09-15 Last updated: 2017-11-21Bibliographically approved
4. Deformation Behavior and Failure of Bimodal Networks
Open this publication in new window or tab >>Deformation Behavior and Failure of Bimodal Networks
2017 (English)In: Macromolecules, ISSN 0024-9297, E-ISSN 1520-5835, Vol. 50, no 19, p. 7628-7635Article in journal (Refereed) Published
Abstract [en]

Using computer simulations, we have investigated the deformation and stress-strain behavior of a series of ideal gels without any defects, with a bimodal molecular weight distribution, subject to tensile strains. These networks were prepared with a spatially homogeneous distribution of short and long chains, where all chains are elastically active, without needing to consider possible effects of chain aggregation or entanglements on the physical properties. For all fractions of short chains, the first chains to rupture were the short chains that were initially oriented along the strain axis. The average orientation of the short chains slightly increased with decreasing fraction of short chains. This could be explained by the detailed structure of the network at different compositions. Analysis of the stress-strain relation for the short and long chains showed that the stress was not uniformly shared. Instead, the short chains are more strongly deformed whereas the long chains only make a negligible contribution at smaller strains. The mechanical properties of the bimodal networks at lower fractions of short chains also deviated from the behavior of equivalent unimodal networks with the corresponding average chain length, showing that bimodality alone is sufficient to increase both the maximum extensibility and toughness.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-332557 (URN)10.1021/acs.macromol.7b01653 (DOI)000412965900023 ()
Available from: 2017-10-30 Created: 2017-10-30 Last updated: 2018-01-09Bibliographically approved
5. Deformation behaviour of homogeneous and heterogeneous bimodal networks
Open this publication in new window or tab >>Deformation behaviour of homogeneous and heterogeneous bimodal networks
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.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-332560 (URN)
Available from: 2017-10-30 Created: 2017-10-30 Last updated: 2017-10-30

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