Conformational Behavior of Guest Chains in Uniaxially Stretched Poly(diethylsiloxane) Elastomers:  ²H NMR and SANS

Abstract

2 H nuclear magnetic resonance (NMR) and small angle neutron scattering (SANS) data are reported for deuterated guest chains of polydiethysiloxane (PDES) in end-linked PDES networks as a function of the molecular weight of guest chains relative to that of the network elastic chains. We exploit the ability of PDES networks to form a strain-induced mesophase to demonstrate the tendency of longer guest chains to phase separate or partition selectively to the amorphous phase and the tendency of smaller guest chains to remain distributed between the mesophase and the amorphous phase. The segmental orientation of the guest chains measured via 2 H NMR peak splitting can be interpreted in terms of an enthalpic orientational coupling of the chain segments in the amorphous state. SANS results show that the radius of gyration of guest chains in the unstretched networks and in the networks stretched below the mesomorphic transition remains essentially unchanged from that in the melt state. The scattering intensity from SANS patterns for networks with guest chains of any size has a peak in the direction parallel to the stretch direction that reflects the domain spacing of the lamellae in the mesophase.

Stretch-Induced Crystallization and Phase Transitions of Poly(dimethylsiloxane) at Low Temperatures: An in Situ Synchrotron Radiation Wide-Angle X-ray Scattering Study

“…The mesophase of PDMS has been discussed by Ohlberg and Tosaka, , which is a short-range ordered structure and differentiated from the oriented amorphous component. Similar phase structures and mutual transformations were also found in the study of the crystallization behavior of PDES. − ,− …”

“…At low temperature (lower than room temperature but higher than T g ), PDMS can form three phases, including mesophase, α form, and β form crystals . The mesophase of PDMS may have a structure close to columnar phase with parallel helices, similar to the liquid crystal phase of poly­(diethylsiloxane) (PDES). − The α form crystal of PDMS has a monoclinic lattice with unit cell parameters of a = 1.439 nm, b (fiber axis) = 0.840 nm, c = 0.854 nm, and β = 55.6°, in which chains adopt a 2/1 helical conformation, while the β form is a centered tetragonal crystalline structure with unit cell parameters of a = b = 0.83 nm and c (fiber axis) = 1.20 nm with a 4/1 helical conformation. , At quiescent condition, the onset temperature of crystallization ( T onset ) of PDMS during cooling is reported in a wide range from −50 down to −90 °C, which is influenced by different constraints, such as molecular weight (MW), , solvents, cross-links, , and nanofiller. − Increasing molecular weight or cross-links within a certain range can increase T onset , and the presence of solvent and nanofiller results in the decrease and increase of T onset , respectively. Note that adding nanosilica filler to PDMS can not only promote crystallization but also form a double-network structure − to sustain deformation to a large strain, which ensures the occurrence of SIC.…”

“…With WAXS measurements on the prestretch PDMS, Tosaka et al observed different phases under different temperatures. At room temperature, the mesomorphic phase − was obtained under large strain, while at −100 °C PDMS crystallized into β and α forms under small and large stains, respectively. According to the results, they speculated that these phases may transform from one to another in strain–temperature space.…”

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…”

Probing the cross-effect of strains in non-linear elasticity of nearly regular polymer networks by pure shear deformation

“…2 H NMR is sensitive to chain segment anisotropy through the orientation-dependent quadropolar interaction. 52,53 Several experimental and theoretical studies showed that the polymeric systems with finite nematic interaction exhibited larger stress-optical coefficients than those calculated from the density fluctuation theory. [66][67][68][69][70] Using a theta solvent for the constituent polymer for gel specimens enables us to exclude the excluded volume effect.…”