TY - BOOK

T1 - Reversible Cross-links in Polymer Chains The influence of sacrificial bonds on the mechanical behavior of polymeric system investigated using Monte Carlo simulations

AU - Nabavi, Seyedsoran

N1 - no embargo

PY - 2014

Y1 - 2014

N2 - Natural materials are a constant source of inspiration for material scientists in manufacturing materials with new and desired mechanical properties. However, this requires a thorough understanding of the structure and the mechanisms that make biological materials achieve their outstanding mechanical properties. One strategy to improve the mechanical performance of natural materials is sacrificial bonding that can be found in bone, wood, and in some softer biological materials like silk, mussel byssus threads and whelk egg capsules. Sacrificial bonds (SBs) are weaker than the covalent bonds that hold the structure together. Thus, upon loading SBs break before the covalent bonds rupture. The rupture of SBs reveals hidden length providing a very efficient energy dissipation significantly toughening the structure. Furthermore, SBs can form and open reversibly. Thus, they can reform after release of the load providing molecular repair and self-healing. In this thesis Monte Caro simulations are used to examine the role of SBs on the mechanical properties of single polymeric chains and chain bundles. The polymers are modeled as a string of hard spheres that are covalently connected to their two neighbors. Additionally some of the beads are defined as \textquotedbl{}sticky\textquotedbl{}. These so called \textquotedbl{}sticky sites\textquotedbl{} are allowed to form a SB. The SBs are assumed to be a factor of $4$ weaker than the covalent bonds. The influence of SB topology and thermal backbone fluctuations on the mechanical behavior of the chains is investigated by computationally mimicking tensile loading tests. It is shown that the topology of the bonds determines the position and spacing of the force peaks due to SBs in the load-displacement curves. The height of these peaks (i.e. the effective strength of SBs) is intimately tied to the magnitude of thermal fluctuations in the chain that are dependent on the effective chain length. This large influence of thermal fluctuations is surprising, because the lowest energy in the system is still a factor $50$ larger than $k_{B}T$. Furthermore, the effect of different density and arrangements of SBs on the work to fracture and dissipating energy is investigated in a (computational) cyclic loading test. The results show that increasing the density of SBs increases the work to fracture as well as the dissipation of energy. The arrangement of SBs has a strong influence on the work to fracture as well as on the strength and apparent stiffness of the single polymeric chain. The second part of this thesis investigates the role of reversible cross-links on the mechanical properties of a chain-bundle system. The biggest topological difference between a single chain and a chain bundle is the possibility of the cross-links to connect two different chains (i.e. forming an interchain cross-link), while for a single chain naturally only intrachain cross-links can be formed. This bears some surprising consequences, like that only two interchain cross-links (each having the strength of a quarter of a covalent bond) are necessary to provide backbone rupture of the chain. Load-displacement curves are simulated to investigate the influence of grafting density and cross-link density on the mechanical response of the system. Special emphasis is put on the interplay of inter- and intrachain cross-links. It is shown that the possibility of backbone failure reduces the strength of the bundle but increases the work to elongate the molecule. The results show that the most important factor influencing the ratio of intra- to interchain cross-links is the grafting density (i.e. the distance of the different chains). These results bear important implications for the understanding of natural systems and for the generation of strong and ductile biomimetic polymers.

AB - Natural materials are a constant source of inspiration for material scientists in manufacturing materials with new and desired mechanical properties. However, this requires a thorough understanding of the structure and the mechanisms that make biological materials achieve their outstanding mechanical properties. One strategy to improve the mechanical performance of natural materials is sacrificial bonding that can be found in bone, wood, and in some softer biological materials like silk, mussel byssus threads and whelk egg capsules. Sacrificial bonds (SBs) are weaker than the covalent bonds that hold the structure together. Thus, upon loading SBs break before the covalent bonds rupture. The rupture of SBs reveals hidden length providing a very efficient energy dissipation significantly toughening the structure. Furthermore, SBs can form and open reversibly. Thus, they can reform after release of the load providing molecular repair and self-healing. In this thesis Monte Caro simulations are used to examine the role of SBs on the mechanical properties of single polymeric chains and chain bundles. The polymers are modeled as a string of hard spheres that are covalently connected to their two neighbors. Additionally some of the beads are defined as \textquotedbl{}sticky\textquotedbl{}. These so called \textquotedbl{}sticky sites\textquotedbl{} are allowed to form a SB. The SBs are assumed to be a factor of $4$ weaker than the covalent bonds. The influence of SB topology and thermal backbone fluctuations on the mechanical behavior of the chains is investigated by computationally mimicking tensile loading tests. It is shown that the topology of the bonds determines the position and spacing of the force peaks due to SBs in the load-displacement curves. The height of these peaks (i.e. the effective strength of SBs) is intimately tied to the magnitude of thermal fluctuations in the chain that are dependent on the effective chain length. This large influence of thermal fluctuations is surprising, because the lowest energy in the system is still a factor $50$ larger than $k_{B}T$. Furthermore, the effect of different density and arrangements of SBs on the work to fracture and dissipating energy is investigated in a (computational) cyclic loading test. The results show that increasing the density of SBs increases the work to fracture as well as the dissipation of energy. The arrangement of SBs has a strong influence on the work to fracture as well as on the strength and apparent stiffness of the single polymeric chain. The second part of this thesis investigates the role of reversible cross-links on the mechanical properties of a chain-bundle system. The biggest topological difference between a single chain and a chain bundle is the possibility of the cross-links to connect two different chains (i.e. forming an interchain cross-link), while for a single chain naturally only intrachain cross-links can be formed. This bears some surprising consequences, like that only two interchain cross-links (each having the strength of a quarter of a covalent bond) are necessary to provide backbone rupture of the chain. Load-displacement curves are simulated to investigate the influence of grafting density and cross-link density on the mechanical response of the system. Special emphasis is put on the interplay of inter- and intrachain cross-links. It is shown that the possibility of backbone failure reduces the strength of the bundle but increases the work to elongate the molecule. The results show that the most important factor influencing the ratio of intra- to interchain cross-links is the grafting density (i.e. the distance of the different chains). These results bear important implications for the understanding of natural systems and for the generation of strong and ductile biomimetic polymers.

KW - Monte Carlo simulation Mechanische Eigenschaften Reversible Querverbindungen

KW - Monte Carlo simulation Mechanical properties sacrificial bonds Polymer chains

M3 - Doctoral Thesis

ER -