Polysiloxanes Compressibility

There is also interest in synthesizing polysiloxanes in environmentally friendly compressed carbon dioxide207... 

Polysiloxanes, [-O-Si(RR )-], are linear resins that can be branched or crosslinked into elastomers. They have high compressibility, permeability to gases, low T and viscosity, exceptional weather-ability, low surface tension coefficient and are relatively expensive. Siloxane polymers or copolymers have been incorporated into engineering or specialty resins to improve processability, toughness, HDT, solvent and weather resistance. 

The example chosen here to illustrate this type of composite involves a polymeric phase that exhibits rubberlike elasticity. This application is of considerable practical importance since elastomers, particularly those which cannot undergo strain-induced crystallization, are generally compounded with a reinforcing filler. The two most important examples are the addition of carbon black to natural rubber and to some synthetic elastomers and silica to polysiloxane elastomers. The advantages obtained include improved abrasion resistance, tear strength, and tensile strength. Disadvantages include increases in hysteresis (and thus heat buUd-up) and compression set (permanent deformation). 

Folk, S. L. Desimone, J. M., Polysiloxanes in Compressed Carbon Dioxide. 

In the case of polysiloxane elastomers, segmental orientation has been of interest for both elongation - and compression. ... 

Locally oblate lyotropic elastomers with lamellar phase structure (L -phase) can be oriented by uniaxial compression, as outlined above for thermotropic smectic-A elastomers. Fischer et al. synthesized crosslinked polysiloxane elastomers carrying non-ionic amphiphilic side-groups attached with their hydrophobic end to the polymer backbone. They were able to compress elastomer samples between Teflon half-cylinders to about half of their original thickness. The orientation of the phase structure - except for some unoriented domains - was demonstrated by means of H-NMR spectroscopy on the directly deuterated samples as well as by X-ray scattering. The preferred orientation of the director, and hence the amphiphilic side chains, was found to be parallel to the axis of compression with the amphiphilic bilayers aligned perpendicularly [98, 99]. 

In acid catalyzed hydrolysis, protons from the dissociation of acidic species (e.g., carboxylic acids from residual catalyst) are able to protonate the relatively polar Si-O bond. Protonation makes the Si-O bond labile to nucleophiUc attack from water, the result of which is hydrolytic scission of the polysiloxane chain to produce two silanol functional free chain ends. The initial scission of the polysiloxane chain forms reactive silanol chain ends which are capable of subsequent recombination though condensation resulting in structural re-arrangement. Such processes contribute to time-dependant chemical stress relaxation and compression set that is often observed in commercial silicone elastomers (see Figure 13.5)... 

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