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Molecular investigation of the swelling behaviour of polymer model networks

Applicant Dr. Michael Lang
Subject Area Experimental and Theoretical Physics of Polymers
Term from 2017 to 2022
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 329888557
 
Final Report Year 2023

Final Report Abstract

With the present project, we analyzed “slide-ring” gels and networks with reversible junctions as model systems for variable netpoints, with the goal of a better understanding of these systems to develop theoretical concepts for their description. Polymers with a large number of slide-rings and an asymmetric attachment to the network develop a stongly non-linear deformation behavior, if the chains are stretched through the slide-rings. The limiting case of maximum asymmetry was called “tendomer”, and it were accessible through a modified version of the original synthesis. Tendomers exhibit a critical deformation behavior, so that after a threshold force, an initially stiff polymer turns into a very soft chain. For the swelling of tendomder networks, a complex parameter space arises from the non-linearity of the deformation curve and the different interactions between the 3 components of the system (polymer, slide rings, and solvent). Here, in particular, a suitable exchange of the solvent quality and the addition of charges lead to extreme changes of the degree of swelling. A uniaxial deformation leads to the formation of two populations of chains, remaining either below the threshold force or being strongly deformed. For intermediate degrees of deformation, this implies a clear reduction of the modulus. For reversible networks, we developed a new approach based upon balance equations, and we tested it for the case of linear reversible polymerization with cyclization. With this approach, it was also possible to describe the formation of reversible model networks of star polymers in a good approximation. These develop a universal critical concentration in the limit of large f. For small f, different corrections are necessary for even and odd f . Intramolecular reactions involving more than one star lead to correlations between pairs of stars, which are particularly relevant for co-polymerizations. With this model, precise predictions for the network structure and the elastic properties can be made. These were tested with a Monte Carlo method that was particularly designed for this task. Within the project, fundamental questions were answered regarding the impact of different quantities like the overlap of elastic chains or the mobility and accessibility of reactive groups on the gel point. Moreover, we developed several analysis methods that allow to quantify different corrections to the phantom modulus. It was shown that the cross-linking process leads to an expansion of the equilibrium conformations of network chains. Existing simulation methods for lattice polymers were further optimized, completed, and subsequently published.

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