Rubber‐elastic mesophase siloxane systems

Abstract

Thermotropic poly(diethylsi1oxane) is a new type of the mesophase elastomers. Mechanical measurements in extension and contraction and stretching calorimetric measurements of heat and energy effects resulting from extension and contraction were combained with thermo-mechanical measurements under constant loads and opto-mechanical measurements to obtain molecular understanding of the deformational behaviour of these mesophase rubber-like materials in a wide temperature range with particular emphasis on the temperature range of the mesophase stability as a function of extension. The stress-strain curves of semi-mesomorphic films resemble the strain-strain beahaviour of crystalline polymers with a very low degree of crystallinity. During stretching of amorphous films the strain-induced mesophase occurs. The influence of stress on the temperature range of the stability of the strain-induced mesophase is very strong due to a very small heat of isotropization. The cyclic deformations of the mesophase elastomers are accompanied by the necking-renecking phenomena resulting from the strain-induced mesophase formation on stretching followed by the immediate isotropization during contraction. Some potential practical applications of the mesophase elastomers are considered.Paper presented at the meeting of the GDCh Fachgruppe 99Makromolekulare Chemie" on "Kautschukelastische Polymersystemel' in

Influence of strain rate and temperature on necking transition in a polydomain smectic main chain elastomer

“…However, cold drawing is rarely observed, if ever, in noncrystallizable elastomers above their respective glass transition temperatures. Previous reports of necking in elastomers have been limited to smectic polydomain MCLCE and mesomorphic poly(diethylsiloxane) (PDES) elastomers 30–35. Like semicrystalline polymers, smectic MCLCE are also thought to contain high concentrations of chain‐folded conformers (hairpins),5, 12 suggesting that the necking transition might involve loss of chain folding, a view supported by our recent X‐ray diffraction studies 12…”

“…Previous reports of necking in elastomers have been limited to smectic polydomain MCLCE and mesomorphic poly(diethylsiloxane) (PDES) elastomers. [30][31][32][33][34][35] Like semicrystalline polymers, smectic MCLCE are also thought to contain high concentrations of chain-folded conformers (hairpins), 5,12 suggesting that the necking transition might involve loss of chain folding, a view supported by our recent X-ray diffraction studies. 12 The molecular basis for neck formation and cold drawing in MCLCE is interesting not only from a fundamental standpoint, but also in a practical sense, because mechanical elongation is the preferred means of achieving globally oriented LCE, including ''monodomain'' samples.…”

Effects of Molecular Structure on Segment Orientation in Siloxane Elastomers. 1. NMR Measurements from Compressed Samples

“…Alignment of the chain backbones decreases the internal energy of the network, a result that has been verified by calorimetric measurements. 21,22,[39][40][41][42] For the mesophase transition to be spontaneous, this favorable decrease of internal energy must offset the entropic penalty for alignment. The entropy change associated with mesophase formation 33 is small (<1 kJ/mol), lending credibility to these ideas.…”

“…In uniaxial extension, initially amorphous PDES elastomers can spontaneously change to an aligned mesomorphic state under appropriate conditions of temperature and molecular weight between cross-links. 19,21,22 A stable mesophase can also be observed in PDES melts of molar mass larger than 28 kg/mol. 23 Because we have not observed this transition in compressed PDES networks, the mesophase itself will not play a large role in the current paper.…”

Effects of Molecular Structure on Segment Orientation in Poly(diethylsiloxane) Elastomers. 2. NMR Measurements from Uniaxially Stretched Samples

“…The proposed energetic interactions are consistent with the decrease of internal energy in PDES elastomers that accompanies mesophase formation during stretching. 1,2,14,[29][30][31] The proportionality between ∆ν and (λ 2λ -1 ) for conventional elastomers was derived 23 assuming Gaussian chain statistics and affine deformation. Given the tendency of PDES chains to align spontaneously, neither of these assumptions may be strictly correct, even at low deformations in the amorphous state.…”