Elastomers having high functionality cross links viewed as bimodal networks. An alternative interpretation in terms of bimodal chain-length distributions (original) (raw)
Related papers
1980
Model networks were prepared by selective crosslinking through vinyl groups occurring as either chain ends or as side-groups on a poly(dimethy1siloxane) backbone. Iodometric titrations were used to determine the number of unreacted groups, thereby providing detailed information on the completeness of the reactions and the structure of the resulting networks. The end-linking reaction ofthe vinyl-terminated chains was generally found to be a t least 95% complete. In the case of very high junction functionality, however, the extent of reaction was significantly lower, presumably because of steric interferences in the vicinity of the junctions, as was concluded in a previous investigation. Lower extents of reaction were also found in the case of vinyl groups located along the chains, probably because such groups are constrained by two chain sequences instead of one. The equilibrium elastomeric properties of both types of networks were interpreted using the structural information thus obtained and were found to be in good agreement with previous experimental results. They are also in satisfactory agreement with theory, without introduction of the highly questionable assumption of large contributions from interchain entanglements.
Determination of the Crosslinking Density of a Silicone Elastomer
Journal of Chemical Education, 2019
A laboratory experiment for the determination of the crosslinking density of silicone elastomers is described on the basis of swelling experiments and mechanical tests. In the experiment, the macroscopic swelling and mechanical behaviors of the elastomers were discussed as a function of the cross-linking density, which can be easily controlled by varying the base-to-curing-agent ratio during elastomer synthesis. The crosslinking density and the average molecular weight between cross-links were calculated from the swelling degree at equilibrium through the Flory− Rehner theory and from a simple mechanical model. The results have shown a good convergence of the average molecular weight between crosslinks calculated from the two methods, respectively. On a more general note, the suggested experiments and the associated results offered numerous opportunities for discussions on macromolecular architectures, interactions between polymers and solvents, and mechanical properties of elastomeric materials.
Some dynamic mechanical properties of unimodal and bimodal networks of poly(dimethylsiloxane)
Polymer Bulletin, 1991
Elastomeric networks of polydimethylsiloxane prepared by end-linking chains having molecular weights in the range 18,500 to 220 g mo1-1 were studied from-128 to 50~ using a Rheovibron DDV Ill Viscoelastometer. In the case of the unimodal networks, the glass transition temperature Tg was generally insensitive to degree of cross-linking. The intensity of the tan 5 relaxation, however, increased by over an order of magnitude over the range of cross-link densities investigated. Bimodal networks prepared from mixtures of relatively long and very short PDMS chains also had values of Tg which were insensitive to degree of cross-linking. Finally, as expected, the intensities of the tan ~ peak for the bimodal networks could not be explained on the basis of simple additivity of contributions from the relatively long and the very short network chains.
Macromolecules, 1996
A recursive approach is used to evaluate the resulting molecular structure of a networkforming system composed of a mixture of bifunctional and monofunctional prepolymer chains reacting with a polyfunctional cross-linker (Af + B2 + B1). This system can be used to find formulations for model networks with pendant chains composed of linear molecules of known and uniform molecular weight. Several important molecular parameters, such as the weight fraction of pendant chains, number-and weight-average molecular weights, and polydispersity of the pendant chains in the network, are calculated as a function of the weight fraction of monofunctional chains (B1) added to the reaction mixture. In a naive analysis of the system Af + B2 + B1, it may be wrongly assumed that networks obtained from the reaction of these components are formed by elastically active chains comprising B2 molecules and pendant chains comprising B1 molecules. A careful analysis shows that this hypothetical situation would take place only under certain formulation conditions. Either a defect or an excess of cross-linker leads to networks with branched pendant structures. Calculations also show that, even for stoichiometrically balanced, completely reacted networks, there exists a limiting amount of monofunctional chains that can be added to the reaction mixture to obtain model networks with linear pendant chains.
Polymer Bulletin, 1993
Unimodal, bimodal, and trimodal networks were prepared by end linking functionally-terminated chains of poly(dimethylsiloxane). The resulting materials were characterized using a "thermoporometric" technique in which freezing points or melting points are determined for solvent absorbed into the network stuctures. The extent to which the normal melting point is suppressed depends on how much the solvent is constrained within the network pores. Several welldefined melting points were observed for some of the multimodal networks, which is consistent with their unusual distributions of network chain lengths..
The influence of cross-link coordination on the mechanical properties of polymers
2020
Reversible cross-linking is a powerful strategy employed by nature to specifically tailor the mechanical performance of load-bearing polymeric structures. These cross-links are found in a variety of biological systems such as bone, silk or mussel threads. They provide an efficient toughening mechanism due to hidden length unraveling and repeated rupture and reformation of cross-links during the course of deformation. In this dissertation, the main objective is to investigate the influence of cross-link coordination on the mechanical properties of polymeric structures. Three different structures are investigated: a single linear chain, fiber bundles and a stiff random network. The aim is to contrast the deformation behavior of polymers cross-linked with two-fold coordinated cross-links only (the "classical" system) with the behavior of system where three-fold coordination of cross-links is energetically most favorable. The inspiration for the current study stems from variou...
Origins of Material Failure in Siloxane Elastomers from First Principles
ChemPhysChem, 2009
Siloxanes are versatile elastomers with an exceptional chemical and physical stability that allows them to be used as adhesives, coatings, and sealants in applications ranging from biomedical to aerospace. Although these materials are exceptionally strong, they are limited by the ease of propagation of cracks through the elastomer when subjected to tensile stress. The current method of improving the material strength is to add silica filler particles, which hinder tearing in the bulk elastomer. However, the chemical mechanism that facilitates crack propagation and the way in which the filler particles hinder it have not been defined at the molecular level. Understanding these processes entails a full description of the electronic structure of a system during the process of bond rupture and the subsequent reactions between ruptured fragments to correctly determine the underlying chemistry.
Polymer, 2015
Crosslinked polymer formation commonly occurs when two or more multi-functional precursors react to form a three-dimensional network. The resulting networks may contain a significant number of topological imperfections such as loops or dangling ends when formed using crosslinkers with high functionality or when crosslinking at high temperatures. We employ molecular dynamics simulations to analyze these topological imperfections in coarse-grained networks generated from precursors consisting of 'chain extenders' composed of two beads (dimers) and a crosslinker of functionality f = 3 or 6 for a wide range of crosslinking temperatures and final conversions. It is shown that these imperfections result in networks in which the number of elastically active chains, the cycle rank and the number of elastically active junctions is smaller than predicted by the Miller-Macosko theory. Such defects must adversely affect the mechanical properties, resistance to solvent swelling and, possibly, the long-term protective properties of polymer networks.