Siloxane polymers melts

J.P. Cohen-Addad, NMR approach to siloxane properties Melts, gels, silica mixtures. In S.J. Clarson and J.A. Semiyen (Eds.), Siloxane Polymers, Prentice Hall, Englewood Cliffs, 1993, p. 511. 

Examples of the synthesis of polysiloxane nanocomposites reported in the literature include Work by Ma et al (6) who modified montmorilIonite with short segments of PDMS and blended this into a polymer melt/solution to yield examples of fully exfoliated or intercalated PDMS/clay nanocomposites. Pan, Mark et al (7) synthesized well defined nano-fillers by reacting groups of four vinyl terminated POSS cages with a central siloxane core. These materials were subsequently chemically bonded into a PDMS network yielding a significant improvement in the mechanical properties of the polymer. 

To cause a polymer to deform or flow requires the application of a force. If a force is applied and then withdrawn quickly, the polymer molecules tend to revert to their previous undeformed configuration, a process called relaxation. In other words, the polymer melt exhibits a certain elastic quality. This elasticity comes about because the molecules were disturbed from what was a thermodynamically favorable arrangement. If, however, the force is applied gradually and consistently, the molecules begin to flow irreversibly. (Silly putty, a siloxane polymer, is ideal for demonstrating this effect. If dropped, it bounces but it can be shaped by the slow application of pressure.) Because of entanglement of the polymer chains and frictional effects, the flowing liquid will be very viscous. This combination of properties, namely elasticity and viscous flow, is why polymers are referred to as viscoelastic materials. 

Alternative processing-aid chemistries have the toughest arguments to make for their use since they have less history of use than standard lubricants. For example silicone-based processing aids, such as the Genioplast materials from Wacker Chernies are said to reduce to screw torque substantially and increase the melt-flow rate of both unfilled and filled PO compounds. These pelletized aids are composed of an ultra-high-molecular-weight siloxane polymer in a fumed-silica carrier that is compatible with thermoplastics. In trials... 

Cohen-Addad, J. P., NMR Approach to Siloxane Properties Melts, Gels, Silica Mixtures. In Siloxane Polymers, Clarson, S. J. Semiyen, J. A., Eds. Prentice Hall Englewood Cliffs, NJ, 1993 pp 511-566. 

Ex situ techniques can also be used. The effects of particle dispersion on the melt state have been documented using nanocomposites containing PS homopolymers or PS-grafted silica. PS has also been grafted onto the cross links in a siloxane polymer and onto filler particles used to reinforce an elastomer. ... 

The poly(diphenyl siloxane) chain is a rigid molecule because of the interaction of the phenyl substituents. Only a single melting temperature in the range of 247 °C to 260 °C has been reported for the homopolymer.(141,159,160) Other diaryl siloxane and aryl methyl siloxane polymers also have relatively high melting temperatures.(161)... 

This work deals with the statistical description of poly(dimethylsiloxane) chains adsorbed upon the surface of silica particles within silica-siloxane mixtures. In these systems, the incorporation of fumed silica into the PDMS polymer melt is obtained by mechanical mixing. Subsequently, the adsorption which occurs through the formation of hydrogen bonds between the oxygen atoms on the polymer chains and the silanol groups located on the silica s surface is a solvent free or melt adsorption process. Consequently, the law of adsorption observed from these systems is specific to such a process. 

Development of several new siloxane-imide copolymers for commercial applications have also been reported by Lee 181) and Berger58). Although no information was given in terms of the chemical compositions of these materials, most of these polymers were reported to be processable by solution or melt processing techniques, most probably due to their high siloxane contents. However, due to the presence of low (—20 to —120 °C) and high (>230 °C) temperature Tg s, it was clear that multiphase copolymers have been synthesized. Molecular weights and thermal stabilities, etc, were not reported. 

Synthesis of hydrolytically stable siloxane-urethanes by the melt reaction of organo-hydroxy terminated siloxane oligomers with various diisocyanates have been reported i97,i98) -yhg polymers obtained by this route are reported to be soluble in cresol and displayed rubber-like properties. However the molecular weights obtained were not very high. A later report56) described the use of hydroxybutyl terminated disiloxanes in the synthesis of poly(urethane-siloxanes). No data on the characterization of the copolymers have been given. However, from our independent kinetic and synthetic studies on the same system 199), unfortunately, it is clear that these types of materials do not result in well defined multiphase copolymers. The use of low molecular weight hydroxypropyl-terminated siloxanes in the synthesis of siloxane-urethane type structures has also been reported 198). 

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