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

Abstract

2 H NMR spectral data are reported for two types of compressed, deuterium-labeled siloxane elastomers: 2 H-labeled poly(diethylsiloxane) (PDES) networks and 2 H-labeled poly(dimethylsiloxane) (PDMS) free chains dissolved at low concentration in PDES host networks. We find that compressed PDES networks have higher segment orientation under stress than PDMS networks, a result that follows partially from the larger segment size (persistence length) of PDES. The internal motions of PDES chain segments are found to be substantially less isotropic than in PDMS, which leads to larger 2 H NMR line widths. Chain segments in compressed PDES elastomers have enhanced orientation compared to predictions from excluded-volume calculations. The 2 H NMR spectral splitting for compressed PDES networks is not directly proportional to (λ 2λ -1 ), unlike the case of conventional elastomers. The behavior of the compressed PDES networks is tentatively attributed to an enthalpic orientational coupling between PDES segments.

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.…”

Segmental Anisotropy in Strained Elastomers Detected with a Portable NMR Scanner

“…Deuterium NMR line shape analysis (splitting of the line) reveals that in the presence of a uniaxial mechanical force, a molecular anisotropy of the chain segments is induced in a polymer network [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. The doublet structure of the 2 D NMR absorption line and its angular dependence described by a second-order Legendre polynomial P 2 ðcos YÞ, where Y is the angle between the uniaxial applied force F * and the static magnetic field directionB B 0 , reflect uniaxiality at a local scale.…”

Static uniaxial compression of polyisoprene–montmorillonite nanocomposites monitored by 1H spin–lattice relaxation time constants

“…NMR has recently been used not only to identify structural changes in the polymer, but also to determine how these changes influence the macroscopic mechanical properties. In situ and quasistatic experiments on a number of semicrystalline and elastomeric polymers have explored the changes in structure as a function of stress or strain 10–13. Little work has focused on the mechanical and structural effects of the addition of montmorillonite clay to polyisoprene.…”