Hard PDMS

Pa, would deform appreciably under the action of loads comparable to the pull-off force given by Eq. 16. It is for this reason that the JKR type measurements are usually done on soft elastic materials such as crosslinked PI rubber [45,46] or crosslinked PDMS [42-44,47-50]. However glassy polymers such as polystyrene (PS) and PMMA are relatively hard, with bulk moduli of the order of 10 Pa. It can be seen from Eq. 11 that a varies as Thus, increasing K a factor of... 

The role played by the various ingredients in the composition of sealant, and in particular on the durability of adhesion has been discussed recently [77]. Inert plasticizers, such as trimethylsilyl-endblocked-PDMS, are typically added to silicone sealant compositions in order to adjust the rheology of the uncured sealant. They result in a reduction of the modulus and hardness of the cured sealant. Differences in the durability of silicone sealants are found to be due to differences in their cure chemistry, and more specifically to the nature and... 

Plasticizer Trimethylsilyl-endblocked- PDMS Adjustment of mechanical properties such as hardness, viscoelasticity, rheology. 

Morphology of the anionically synthesized triblock copolymers of polyfp-methyl-styrene) and PDMS and their derivatives obtained by the selective chlorination of the hard segments were investigated by TEM 146). Samples with low PDMS content (12%) showed spherical domains of PDMS in a poly(p-methylstyrene) matrix. Samples with nearly equimolar composition showed a continuous lamellar morphology. In both cases the domain structure was very fine, indicating sharp interfaces. Domain sizes were estimated to be of the order of 50-300 A. 

Siloxane-urea copolymers were synthesized by the reaction of the aminopropyl terminated PDMS oligomers with MDI or HMDI with no chain extenders (Reaction Scheme XI and Tables 14 and 15). Therefore, in these copolymers the hard segments consist of the aminopropyl end groups on the siloxane oligomers and MDI or HMDI backbones as shown below. The soft segment is pure polydimethylsiloxane. 

Flocculation studies (6) indicated that the mechanism of steric stabilization operates for the PMMA dispersions. The stability of PMMA dispersions was examined further by redispersion of the particles in cyclohexane at 333 K. Above 307 K, cyclohexane is a good solvent for PS and PDMS, and if the PS-PDMS block copolymer was not firmly anchored, desorption of stabilizer by dissolution should occur at 333 K followed by flocculation of the PMMA dispersion. However, little change in dispersion stability was observed over a period of 60 h. Consequently, we may conclude that the PS blocks are firmly anchored within the hard PMMA matrix. However, the indication from neutron scattering of aggregates of PS(D) blocks in PMMA particles may be explained by the observation that two different polymers are often not very compatible on mixing (10) so that the PS(D) blocks are tending to... 

Such an example has been recently worked out, combining a very hard block i.e. polypivalolactone (PPVL), with a thermostable elastomer segment i.e. polydimethyl-siloxane (PDMS), using the reactions sequence described in scheme 3. (One should note that the starting PDMS has been obtained through a previously described procedure (14), avoiding any unstable Si-O-C bond in the final produc t). 

The results are shown in Fig. 9. A small amount of the filler strongly increases the energy contribution which is in full contradiction to the assumed increase in the concentration of active network chains caused by the filler. Curve 2 summarizes the results for filled PDMS rubber and for PDMS block and graft copolymers. It is seen that below 20% of the filler or hard phase, the energy contribution is practically independent of the amount of hard phase, but then a considerable increase of (AU/W)v>t is observed. Although in all these cases the energy contribution is... 

Fig. 9. Dependence of the energy contribution on the filler (filled rubbers) or hard phase (thermo elastoplastics) content. 1 — filled silicone rubber1221 Sil-51 (A), Sil-4600 ( ) multiblock copolymer polyarylate-PDMS (O) us) graft copolymer of PDMS and AN ( x) 128>. 2 — Butyl rubber with high abrasion furnace black125). 3 — Butyl rubber with medium thermal black 125). 4 — SBR-filled rubber 126). 5 — aerosil //j Si-filled silicon rubber138). 6 — EPR-filled rubber 129,130). 7 — plastisized PVC filled with aerosil131132). 8 — SBS block copolymers 134)...

Generally, it is claimed that PDMS/silica-type hybrids combine the flexibility and mechanical properties of the organic polymer chains with the hardness and stiffness of silica. In detail, the properties of the hybrids are governed by the chemistry, presence of side chains or pendant groups and length of polymeric chains. Especially important are the concentration and distribution of alkoxysilyl groups attached to the polymeric chain, as they directly affect the rigidity or flexibility of the system. 

It is also possible to obtain reinforcement of a PDMS elastomer by polymerizing a monomer such as styrene to yield hard glassy domains within the elastomer.5051 Low concentrations of styrene give low-molecular-weight polymer that acts more like... 

Figure 8.12 The hardness of a silica-PDMS composite as a function of the relative numbers of alkyl groups and silicon atoms.138 Reproduced by permission of Springer Verlag.

The hardness of such a composite, for example, can be varied by control of the molar ratio of alkyl R groups to Si atoms, as is illustrated for PDMS in Figure 8.12.138 Low values of R/Si yield a brittle ceramic, and high values give a relatively hard elastomer. The most interesting range, R/Si = 1, can yield a relatively tough ceramic of increased impact resistance. 

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