End-linked polydimethylsiloxane

Mark, J. E. Rahalkar, R. R. Sullivan, J. L., Model Networks of End-Linked Polydimethylsiloxane Chains. 111. Effect of the Functionality of the Cross-Links. J. Chem. Phys. 1979, 70,1794-1797. 

Oikawa, H., Dynamic Light Scattering Study on a Bimodal End-Linked Polydimethylsiloxane Network Structure. Polymer 1992, 33,1116-1119. 

In their studies, dynamic mechanical experiments in small amplitude oscillatory were carried out for the end-linking polydimethylsiloxane and polyurethane. The extent of reaction, p, of the end-linking is the variable to dominate sol-gel transition. The gel point of the polydimethylsiloxane network appears at p=pc- A simple power law was found to govern the viscoelastic behaviour of the critical gel, described by a complex modulus... 

Llorente, M. A. Mark, J. E. (1979). Model Networks of end-Linked Polydimethylsiloxane chains. IV Elastomeric Properties of the Tetrafunctional Networks Prepared at Different Degrees of Dilution. Journal of Chemical Physics, Vol. 71, No. 2, pp. 682-689, ISSN 0021-9606... 

Mark JE, Andrady AL. Model networks of end-linked polydimethylsiloxane chains. X. Bimodal networks prepared in two-stage reactions designed to give high spatial heterogeneity. Rubber Chem Technol 1981 54 366-73. 

Llorente MA, Andrady AL, Mark JE. Model networks of end-linked polydimethylsiloxane chains. XIII. The effects of junction fimctional-ity on the elastic properties of the bimodal networks. Colloid Polym Sci 1981 259 1056-61. 

Zhang ZM, Mark JE. Model networks of end-linked polydimethylsiloxane chains. XIV. 

Oikawa H. Dynamic light scattering study on a bimodal end-linked polydimethylsiloxane network structure. Polymer 1992 33 1116-9. 

Figure 18(a) presents normalized proton DQ buildup functions of end-linked polydimethylsiloxane model networks prepared by mixing and subsequent cross-linking of very short and rather long chains. Such bimodal networks are known to consist of clusters of highly cross-linked regions embedded in a long-chain elastomer matrix. The buildup curves as well as the cross-link density distributions (i.e., the distributions of D s) derived from these show clear bimodal... 

There are now a number of techniques which may be used to prepare elastomeric networks of known structure Q-8). Two particularly useful and convenient ones involve the multi-functional end-linking of hydroxyl-terminated (4-16) or vinyl-terminated polydimethylsiloxane (PDMS) chains (3,17-21), and the cross-linking of PDMS chains through vinyl side groups present in known amounts and in known locations along the chains (4,18,22-25). A typical reaction of this type is... 

Networks with tri- and tetra-functional cross-links produced by end-linking of short strands give moduli which are more in accord with the new theory if quantitative reaction can be assumed (3...13) However, the data on polydimethylsiloxane networks, may equally well be analyzed in terms of modulus contributions from chemical cross-links and chain entangling, both, if imperfect reaction is taken into account (J 4). Absence of a modulus contribution from chain entangling has therefore not been demonstrated by end-linked networks. 

Such model networks may be prepared by very specific chemical reactions in which the network cross-links are introduced in a carefully controlled manner. For example, polydimethylsiloxane (PDMS) networks of this type have been synthesized by end-linking hydroxyl-terminated chains by means of silicates, and vinyl-terminated or vinyl-substituted chains with silanes containing active hydrogen atoms ... 

The role played by the trapped entanglements has been recently investigated on polydimethylsiloxane networks swollen with heptane The samples were prepared by end-linking at volume fractions (pc ranging from 0.5 to 1. The results of shear modulus... 

Recent work has focused on a variety of thermoplastic elastomers and modified thermoplastic polyimides based on the aminopropyl end functionality present in suitably equilibrated polydimethylsiloxanes. Characteristic of these are the urea linked materials described in references 22-25. The chemistry is summarized in Scheme 7. A characteristic stress-strain curve and dynamic mechanical behavior for the urea linked systems in provided in Figures 3 and 4. It was of interest to note that the ultimate properties of the soluble, processible, urea linked copolymers were equivalent to some of the best silica reinforced, chemically crosslinked, silicone rubber... 

Silicones Silicone rubbers and oils are usually based on polydimethylsiloxane (Si(CH3)20) . They all give spectra readily identified through the positive ions at miz 28, 47, 147, 107, 221, and 281. The relative intensities of these peaks vary as a function of molecular weight, end groups, and cross-link density in the rubbers. 

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