Dielectric relaxation time of polymers

The temperature dependence of the dielectric relaxation time of amorphous polymers can be described quite satisfactorily by an expression derived from the theory of the relaxation time for local conformational transitions in a polymer chain. This theory was recently developed using the Kramers theory of the rate constant together with the free-volume theory.     

Dielectric relaxation in styrene-related polymers at low temperatures

The complex dielectric constants ?^* = ?′ ? j?″ of each of several members of a system of copolymers of 4-chlorostyrene and 4-methylstyrene have been measured from 1.6°K to 300°K and from 0.1 kHz to 20 kHz. The principal experimental findings are: the strength of the relaxation process which occurs near 50°K at 1 kHz varies linearly with changing copolymer composition; both the apparent activation energy (H = 2.7 ± 0.7 kcal/mole) and the shape of the relaxation curve are independent of the composition variable and of the temperature (or frequency) within the ranges studied; and the ratio of the relaxation strength of poly-4-methylstyrene to that of poly-4-chlorostyrene in the 50°K process is about 25 times the corresponding ratio for the primary relaxation process that occurs in the neighborhood of the glass-transition temperature. These findings suggest that in the 50°K process the phenyl groups relax independently of one another; that the apparent activation energy and the shape of the relaxation spectrum are determined primarily by the nature of the intrachain forces; and that the strength of the relaxation process depends primarily on effects of intermolecular forces that are governed by the molar “free volume” of the copolymers.     

Dielectric relaxation and intramolecular mobility in LC polymers

Intramolecular motion and mechanisms of local mobility were considered in linear polyesters, Including thermotropic systems, on the base of experimental results of dielectric absorption research. In some cases the role of molecular motion in the formation of the mesomorphic structure is established.     

Structure investigations of solid organosilicon polymers by high resolution solid state Si NMR

The high resolution ²⁹Si NMR spectra of five solid silicon polymers of different structure have been studied and the ²⁹Si chemical shifts of characteristic structure units determined. ²⁹Si---¹H cross-polarization in combination with high speed magic angle sample spinning and high power proton decoupling was used to achieve high resolution in the solid state spectra. Comparison of the latter with the results obtained in the liquid state clearly indicates that no special solid state effects on ²⁹Si chemical shifts arise and the relations between δ(Si) and the molecular structure, well known from investigations of liquids, can be used for interpretation of the solid state spectra. It is shown that high resolution solid state ²⁹Si NMR spectroscopy offers detailed information about the structural units of the siloxane resin framework, and this opens up new possibilities for structural determinations of solid organosilicon polymers.     

Dielectric relaxation and effect of plasticisers on siloxane polymers

Studies of dielectric relaxation of polydimethyl siloxane have shown that there is anomalous behaviour near its melting point (≈ ?50°), probably due to the presence of “Rotator Phases”. A detailed study of polyphenyl methyl siloxane, with various polar and non-polar diluents, has been carried out. The most interesting aspect of this study is the systematic broadening of the loss curves with increasing concentration of diluents while retaining conformity with the empirical decay function π(t) = exp ? (t/τ_o)^β. The broadening of loss curves was explained in terms of a distribution of local concentration of the diluents. The range of concentration (ΔC) was calculated for various diluents. It was found that ΔC was larger for the polar diluents than for the non-polar diluents. This result shows that polar plasticiser molecules have changed considerably the environment of the segmental motion of the basic polymer. There is also a linear relationship between the loss factor maximum (?^″_m) with the number of carbon atoms in the plasticiser molecule.     

Dielectric relaxation studies of some linear crosslinked and branched polymers

Dielectric loss measurements are reported for polystyrene, crosslinked polystyrene, polyacrylamide, branched polyacrylamide, and poly(methyl methacrylate) at 1 and 10 kHz f over the temperature range ?85 to +100°C. Crosslinking and branching have a pronounced effect on the dielectric relaxation spectra of polymers. The methods of preparation of these polymers and their viscosity molecular weight data are also reported.     

Structure investigations of solid organosilicon polymers by high resolution solid state 29Si NMR

The high resolution ²⁹Si NMR spectra of five solid silicon polymers of different structure have been studied and the ²⁹Si chemical shifts of characteristic structure units determined. ²⁹Si¹H cross-polarization in combination with high speed magic angle sample spinning and high power proton decoupling was used to achieve high resolution in the solid state spectra. Comparison of the latter with the results obtained in the liquid state clearly indicates that no special solid state effects on ²⁹Si chemical shifts arise and the relations between δ(Si) and the molecular structure, well known from investigations of liquids, can be used for interpretation of the solid state spectra. It is shown that high resolution solid state ²⁹Si NMR spectroscopy offers detailed information about the structural units of the siloxane resin framework, and this opens up new possibilities for structural determinations of solid organosilicon polymers.     

Theory of the local mode relaxation in the glassy state of polymers

The local modes refer to vibrational motions of the main chain in the glassy state which are thermally excited with a relatively large amplitude and are consequently strongly damped into relaxational motions by the intermolecular viscous force. This paper describes a theory of strengths of the dynamic mechanical and dielectric local mode relaxations, in the latter of which the correlation of dipole arrangement along the main chain is considered. The results are compared with the observed strength and its dependence on temperature and pressure, in particular for the dielectric β relaxation of poly(vinyl chloride). Satisfactory agreement is obtained between theory and experiment.     

Spin-lattice relaxation of radicals in the solid state. Part 3

Some causes of spin-lattice relaxation in the solid state are considered. It is found that anisotropic hyperfine interaction can make an appreciable contribution to the spin-lattice relaxation time, which becomes dependent on the orientation of the radical relative to the external field and on the number of HS components. There is no appreciable contribution from frequency modulation of the vibrations (of the radical as a whole or purely intramolecular) caused by the lattice vibrations.     

Chemically resolved solid state NMR images of polymers

Polymers are generally spatially heterogeneous, either in terms of morphology or as blends of various components. Solid state NMR imaging provides a means of characterizing both the chemical/morphological composition and its spatial variation. Here we discuss multiple-pulse line-narrowing approaches to acquiring high resolution NMR images and how these sequences can be modified so as to be sensitive to the chemical composition of a sample.     

Length scales in heterogeneous polymers from solid state NMR

New methods for probing structure and dynamics of heterogeneous polymers by multidimensional nuclear magnetic resonance (NMR) are described. On the molecular level high resolution multiple quantum spectroscopy of solids probes connectivities of different functional groups on length scales below 1 nm. On a mesoscopic scale NMR spin diffusion techniques probe phase separation and interfacial effects in polymer blends and block copolymers. On a macroscopic scale NMR imaging techniques allow to spatially resolve differences in order and mobility in the necking region or in shearbands of deformed polymers. The techniques are illustrated with amorphous polymers, elastomers, and core-shell systems.     

Approaches to conjugated polymers via solid state polymerization

Topochemical solid state polymerization is a method of obtaining fully crystalline macromolecules. Herein, approaches to conjugated polymers via new solid state polymerizations and the properties of materials, especially polydiacetylenes(PDA), obtained via solid state polymerization are discussed. Our attempts to polymerize cyanoalkynes and to define the thermal reaction product of a dicyanodiazepine are presented. Certain PDA exhibit absorption, likely charge-transfer, at energies below the exciton. Nanocrystals of PDA-DCH react with liquid bromine to give the mixed polyacetylene PDA-DCHBr₆ whose visible spectrum exhibits structure not observed in spectra of bulk crystals of PDA-DCHBr₆. The thermochromic PDA of the dialkylurethanes of 5,7-dodecadiyn-1,12-diol were studied by variable temperature solid state ¹³C nmr. The upfield shift of the acetylene carbons as the temperature is raised above that of the thermochromic phase transition indicates the role of mechanical strains in achieving thermochromism.     

Energetics of stress relaxation in glassy polymers at high pressures

Kinetics of relaxation of the shear stress in glassy poly(methyl methacrylate) were studied in isobaric, isothermal and isochoric conditions. In isochoric conditions the relaxation rate depends only on temperature and polymer volume is the main parameter for the rate of stress relaxation. Isobaric and isochoric activation energies and activation volume of the stress relaxation were determined. The results are shown to satisfy the thermodynamic relation of activation parameters of the relaxation process. The size of the area in which the elementary stress relaxation process occurs in glassy poly(methyl methacrylate) was evaluated.     

Dielectric relaxation analysis of cellulose oligomers and polymers in dependency on their chain length

For all cellulose‐like oligo‐ and polyglucans, beginning with the dimer cellobiose, a broad relaxation process at low temperatures is observed using the dielectric relaxation spectroscopy method. This relaxation has its molecular origin in orientational motions of the sugar rings via the glucosidic linkages. We investigated the dynamics of this main chain motion for β(1‐4) oligoglucans with 2, 3, 4, or 5 anhydroglucose units (AGUs), as well as for β(1‐4) polyglucans having a degree of polymerization molecular weight averages (DP_w) of 23, 37, 50, and 140 up to 3000. As a result we found that the activation energy (E_a) of the segmental chain motion has the lowest value (32 ± 1 kJ/mol) for cellobiose, followed by passing through a maximum for a DP between 7 and 15 with E_a = 51 ± 1 kJ/mol. Subsequently, the activation energy is decreased at a value around 44.8 ± 1.2 kJ/mol for chains containing more than 100 AGUs. Obviously, from a distinctly molecular dimension (DP_w ～ 100) the mean number of AGUs that take part in the local chain motions and cross‐correlation between the motions of neighboring AGUs are nearly the same and the chain length has no influence on the segmental motion. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2491–2500, 2001     

Dielectric relaxation in two liquid crystalline polyacrylates under high hydrostatic pressure

For two polyacrylates with p-cyano-phenyl benzoate side groups and different spacer lengths the dielectric relaxation was measured in the nematic and isotropic phase at pressures up to 5000 bar. The relaxation of the cyano groups was observed as a separate relaxation process; it seems to be closely correlated with the glass relaxation. The pressure dependence of the glass temperature and of the clearing temperature could be derived from the experiments.