engleski

Deuterium NMR study of segmental orientational order in some polymer systems

Material properties are mainly determined by its chemical structure, but the molecular architecture and packing density also play significant roles. Particularly important is the spatial orientation of molecules. Anisotropic systems, such as, for example, liquid crystals, have different properties in different directions because of the specific spatial arrangement of the molecules. Therefore, it is very important to know how the molecules are arranged in space and whether their dynamics is isotropic or there are some preferred directions around which the molecules or their segments precess. Among all the techniques that allow observation of the spatial arrangement of the molecules, deuterium magnetic resonance is the most sensitive and the most powerful. The analysis of 2H-NMR spectra depends on the way the quadrupolar Hamiltonian is averaged by molecular motions. In the presence of fast, anisotropic molecular motions the quadrupolar interaction is averaged to a non-zero value, which results in a doublet of Lorentzian lines characterized by a splitting Δν. On the base of measured Δν values, the factor S which is the mean orientational order parameter and describes the degree of motional anisotropy of the C−D bond with respect to the symmetry axis can be calculated. In the fast motion limit, the line width of each component of the doublet depends on the relaxation time T2 and is generally small or comparable to the value of the splitting. This leads to well resolved spectral lines. In a disordered system, the resulting spectrum is the superposition of such doublets, and the resulting line shape is thus directly related to the distribution of residual interactions in the system. In a macroscopically oriented system, based on measured splitting values and the known value of macroscopic angle, the mean degree of orientational order parameter (S) can be calculated. In order to apply deuterium NMR technique in the study of polymer systems, it is not necessary to have a deuterated polymer matrix. Various swelling agents like deuterated good solvents or deuterated matrix like chains of low molecular mass can be used as deuterium probes. The power of this technique in the study of anisotropic molecular motions will be demonstrated on various polymer systems: natural rubber, polybutadiene rubber and polydimethylsiloxane under uniaxial constraint, semicrystalline polydimethyl-siloxane and lamellar block copolymers of polystyrene-polybutadiene and polystyrene-polydimethylsiloxane. The physical background of the method and the ways of samples preparation will be systematically presented.

Deuterium NMR; segmental order; orientational dynamics

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