Polydimethylsiloxane networks

Figure 8. SANS measurements of R /Fg° and R /Rg° versus A of tetrafunctional polydimethylsiloxane networks. Mw and Mn are weight and number average molecular weights before crosslinking, cp is the polymer fraction in solution before cross-linking. Data from Ref. 20.

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. 

The bare vulcanized silicone pol)mier, a polydimethylsiloxane network, shows only a low mechanical strength. Adding 30 wt. % of fumed silica by mixing and dispersing increases the elongation and stress at break by a factor larger than 10 (Fig. 8). 

Calculate the variation in entropy corresponding to a polydimethylsiloxane network of mass 6.89 g, with an average molecular weight between crosslinks Me = 8.3 X 10, subjected to a reversible uniaxial extension at 25°C until the length is double its initial length. 

The goal of this work was to show how the fluorescence probe with the general structure I (Scheme 1) behaves in swollen polydimethylsiloxane networks and how the solvation can be examined of linear polydimethylsiloxanes in the presence of covalent bonded probes. This molecule consists of a donor part, e.g., a dialkylamino group, and an acceptor, which is usually an ester group. The following points are important to discuss for a better understanding of the fluorescence behavior... 

Fig. 3. Fluorescence spectra of dimethylaminobenzoic acid esters in swollen polydimethylsiloxane networks as a function of the crosslinker concentration corresponding [-Si-0H] [-Si-0-C2H5 ratios are drawn in the figure, the intensity is plotted in a.u.

The surface of inert fillers such as powdered quartz, diatomaceous earth, calcium silicates, calcium carbonate or iron oxides, just to mention the most frequently used, does not show major chemical or physical interaction with the polydimethylsiloxane network. Thus, this kind of filler... 

A point worth noting here is that several of the molecular models that will be described in the subsequent sections are Neo-Hookean in form. Normally, dry rubbers do not exhibit Neo-Hookean behavior. As for the Mooney-Rivlin form of strain energy density function, rubbers may follow such behavior in extension, yet they do not behave as Mooney-Rivlin materials in compression. In Fig. 29.2, we depict typical experimental data for a polydimethylsiloxane network [39] and compare the response to Mooney-Rivlin and Neo-Hookean behaviors. The horizontal lines represent the affine and the phantom limits (see Network Models in Section 29.2.2). The straight line in the range A <1 shows the fit of the Mooney-Rivlin equation to the experimental data points. 

Rossi, N. A. A. Duplock, E. J. Meegan, J. Roberts, D. R. T. Murphy, J. J. Patel, M. Holder, S. J., Synthesis and Characterization of Pyrene-Labelled Polydimethylsiloxane Networks Towards the In Situ Detection of Strain in Silicone Elastomers. J. Mater. Chem. 2009,19, 7674-7686. 

Vera-Graziano, R. Hernandez-Sanchez, F. Cauich-Rodriguez, J. V., Study of Crosslinking Density in Polydimethylsiloxane Networks by DSC. J. Appl. Polym. Sci. 1995, 55,1317-1327. 

Falcao, A. N. Pedersen, J. S. Mortensen, K. Boue, F., Polydimethylsiloxane Networks at Equilibrium Swelling Extracted and Nonextracted Networks. Macromolecules 1996, 29, 809-818. 

Clement, F. Bokobza, L. Monnerie, L., On the Mullins Effect in Silica-Filled Polydimethylsiloxane Networks. Rubber Chem. Technol. 2001, 74, 847-870. 

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

Sharaf, M. A. Mark, J. E. Hosani, Z. Y. A., Regular Bimodal Polydimethylsiloxane Networks. Elastomeric Properties of the Tetrafunctional Networks. 

Jiang, C.-Y. Mark, J. E., The Effects of Various Catalysts in the In-Situ Precipitation of Reinforcing Silica in Polydimethylsiloxane Networks. Makromol. Chem. 1984,185, 2609-2617. 

Clement F, Bokobza L, Monnerie L (2005) Investigation of the Payne effect and its temperature dependence on silica-filled polydimethylsiloxane networks. Part I Experimental results. Rubber Chem Technol 78 211-231... 

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... 

Sharaf MA, Mark JE, Hosani ZYA. Regular bimodal polydimethylsiloxane networks. Elastomeric properties of the tetrafunctional networks. Eur Polym J 1993 29 809-17. 

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

These siloxanes are easily hydrolyzed by ambient humidity and water bound to the substrates to generate transient silanols 77 and 78. Subsequent dehydration initially provides a precursor 79 of crosslinked polydimethylsiloxane network, which is formed by further condensation of the silanol groups. With many variants, this process constitutes the base of the RTV silicones that are mainly used as protection shields in the glob top encapsulation process. Low-stress conductive adhesives have been prepared by the condensation reaction of ethylphenylsila-nediol, l,4-phenylenebis(dimethylsilanol), and trimethylsilyl-terminated poly-diethylsiloxane. When loaded with carbon particles and cured at 180°C, this composition exhibits a volume resistivity of 1.2 X 10 flcm and a thermal conductivity of 2.1 W m K. ... 

Figure 10 Formation of linear and crosslinked polydimethylsiloxane networks. Room temperature vulcanization (RTV) is performed by reacting methoxysilyl-terminated polymer 75 and methyltrimethoxysilane 76. Hydrolysis produces transient silanols 77 and 78 whose dehydration yields silicone precursor 79. Two-part systems are based on the platinum-catalyzed addition reaction between vinyl-terminated polysiloxane 80 and polydimethylsiloxane carrying either only terminal 81 or multiple silane (Si-H) pendent groups. Silicone main chains of the resulting polymer 82 are linked through silicon-...

Mark JE, Pan J. Reinforcement of polydimethylsiloxane networks by in-situ precipitation of sihca A new method for preparation of filled elastomers. Makromol Chem Rapid Commun 1982 3 681-685. 

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... 

---