End-Linked Poly(dimethylsiloxane) Elastomer Structure:  ²H-NMR Transverse Dephasing Data Compared to Predictions of Statistical and Thermodynamic Models

Abstract

2H-NMR measurements of deuterated poly(dimethylsiloxane) [PDMS(d)] elastomers with systematically varied elastic and swelling properties are presented. 2H-NMR transverse dephasing data are compared to the predictions of a model describing elastomer deuterium transverse dephasing data as a linear superposition of contributions from elastic and pendant chains. Structural parameters obtained from the model for 2H-NMR transverse dephasing are compared to structural parameters inferred from swelling measurements using statistical and thermodynamic models. As predicted by the model for transverse dephasing data, dephasing time constants for elastic chains are directly proportional to number-average molecular weights of elastic chains obtained from swelling measurements.

End-Linked Poly(dimethylsiloxane) Elastomers: 2H-Nuclear Magnetic Resonance Investigations of Compression-Induced Segment Anisotropy

“…We postulated that the difference is because entangled portions of pendant chains may not reorient isotropically on the NMR time scale and thus may contribute to the elastic component of the 2 H-NMR dephasing data. 17 Thus, we regard f el values obtained from the Macosko-Miller model as lower bounds for the weight fractions of segments which reorient anisotropically on the NMR time scale due to constraints at chain ends.…”

“…These networks are well characterized; mechanical and swelling properties, light scattering from swollen samples of similar networks, and 2 H-NMR of unconstrained samples have been reported previously. [14][15][16][17] Background 2 H-NMR is sensitive to elastomer segment anisotropy through the orientation-dependent quadrupolar interaction. In PDMS(d) the NMR frequency shift caused by the quadrupolar interaction, ν Q , reflects the timeaveraged orientation of a C-2 H bond:…”

“…Specifically, 2 H-NMR spectra of elastomers have been described as the sum of two distinct components contributed by segments of pendant and elastic chains. 12,17,18 Segments of both pendant and elastic chains move rapidly relative to the NMR time scale, as evidenced by the substantiallynarrowed 2 H-NMR spectrum of the elastomer, compared with the spectrum of the rigid solid. Although segments of undeformed elastomers move rapidly, they do not move isotropically.…”

Effects of structural imperfections on the dynamic mechanical response of main‐chain smectic elastomers

“…38 Even the optimal networks have w sol values substantially higher than the accepted range for ''model'' networks, which can have w sol < 1.0 mass %. 39,41,44,45 Thus, although the ''optimal'' samples are presumed to have a lower concentration of architectural defects than the ''imperfect'' networks, they cannot be fairly described as ''model'' networks. The high soluble fractions may be attributed to intramolecular cyclization reactions, encouraged by the presence of solvent during crosslinking and by the flexibility of the oligosiloxane connectors.…”

Recent NMR Studies of Thermotropic Liquid Crystals

“…The two characteristic T 2 times, ascribed to a faster and a slower dynamic motion with different correlation times τ c were determined in a wide temperature range within the I (PN) phase [330]. The fast motion, corresponding to a τ c in the range of 10 À8 -10 À9 s, is compatible with the internal reorientation of the deuterated phenyl ring in the crosslinkers [331], while the slower motion, which is characterized by a τ c in the range of 10 À7 -10 À8 s, can be ascribed to the OPF of the long axis of the deuterated moiety around the average orientation. One particular aspect of using quadrupolar splittings to monitor S(T) is the ability to relate this microscopic property to the thermomechanical deformation e(T), a macroscopic strain property of LSCE as demonstrated by Domenici et al [321].…”