The mechanical properties and microstructure of soft materials
Polymer gels, cross-linked polymer membranes and soft materials in general, are an important area of great technological interest. The description of their microscopic structure poses many challenges and fundamental questions in polymer science. Hydrogels, in particular, possess great potential in, for example, tissue engineering applications. Their use is, however, often restricted due to their weak mechanical performance. The synthesis of soft materials, such as cross-linked polymer gels normally introduces inhomogeneities that affect the physical properties of the corresponding material, such as giving rise to low deformability and low mechanical strength. To fulfil the demands of application, it is of interest to be able to correlate the mechanical properties with the network structure and to be able to improve the mechanical properties. Inhomogeneities may be in the form of structural defects (e.g., loops and dangling ends), and a spatially inhomogeneous cross-linking density, with local domains of high cross-linking density loosely interconnected. An advantage of using computers simulations is that it enables us to separate these two types of inhomogeneities by working with networks with a precise structure. By introducing heterogeneity in a controlled way, it is possible to understand the effect of structural imperfections on the mechanical performance. There have only been limited studies using molecular simulations to investigate the mechanical properties of crosslinked soft polymer networks, not least due the difficulty in obtaining a closed and well defined network. To investigate more general types of networks, we previously developed a novel algorithm to create a closed network with an unbiased distribution of crosslinking nodes, which has been used to investigate the diffusion of small molecules, relevant for e.g. drug delivery applications. We have recently extended this to purposely inhomogeneous networks and made preliminary tests looking at the small scale elongation of spherical nanoparticles, obtaining very interesting and promising first results. In the proposed project, we would perform a systematic study of the influence of spatial inhomogeneities on different length scales on the deformational behaviour and mechanical properties of crosslinked gel networks, as a model for soft materials in general, using boundary conditions with experimental set-ups in mind. Our systems range from perfectly homogeneous networks to fully random and bimodal networks with local highly cross-linked regions of various sizes, mimicing available experimental data for the structure of relevant soft gel materials. The mechanical properties will mainly be investigated by analysing the stress-strain and the stress-relxation curves. An important property of such materials, both with respect to the mechanical characteristics and the structure, is the pore size distribution. We will also develop new methods for analysing the internal structure, including the fractal dimension (specifically comparing with experimental methods), as well as a cross-sectional pore size distribution. The need for cpu-time would be in the range 35 000+ cpu hours per month, and the disk space needed to store the resulting trajectories during the analysis stage would be 800Gb-1Tb during the present part of the project.