Polydiethylsiloxane mesophase

From the above results, it is clear that the presence of classical mesogens is not necessary for linear chains like poly(di-n-alkylsilane)s or poly(di-n-alkoxyphos-phazene)s [77] to show a mesomorphic behaviour. This concept has also been applied to the case of poly(di-n-alkylsiloxane)s. For aUcyl chains containing between 2 and 6 carbon atoms, the materials were shown to form a columnar mesophase, in which the molecules are positionally and orienta-tionally ordered in a two-dimensional hexagonal lattice, but along the chain no long range correlations exists [78]. The most famous member of this series of polysiloxanes is polydiethylsiloxane which exhibits at room temperature a... 

Above r, PDMS is in the amorphous state, and there are no indications in the literature concerning the possible appearance of a mesophase in it. The first information on phase transitions in polydiethylsiloxane (PDES) and polydipropylsiloxane (PDFS) was published in [17], in which low-temperature solid-phase transitions were detected in these polymCTS, but the existence of a mesophase was not found. A mesophase which appears after melting (>270°K) and persists up to =3(X)°K was found in a more detailed study of PDES [18] above 300 K, the sample turned into an amorphous melt. Beatty et al. called the observed mesophase a viscocrystalline state and attempted to characterize it by several physical methods (calorimetry, NMR, dielectric relaxation, optical observations). However, neither the structure nor the physical properties of this mesomorphic state have been studied completely enough. The mesomorphic state in PDFS was first detected and studied in our research [19, 20]. There is no information on the following representatives of linear polysiloxanes in the literature at present. 

Linear polydiethylsiloxane (PDES) and its higher homologues form ordered mesophases in temperature intervals between the solid and the isotropic melt phases [159, 160]. This mesophase is to be juxtaposed to monomeric [161-166] or main-chain polymeric nematic liquid crystals [167]. The results to be described in the following demonstrate the sensitivity of NMR techniques to microstructural changes. 

Fig. 39. Fraction of the more mobile segments in the mesophase of polydiethylsiloxane (PDFS) as a function of the reduced temperature. The solid line represents Eq. 184...

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