Dielectric studies of cellulose and its derivatives: 1. Acetylation of cellulose

Dielectric conductivity (dc) and water absorption measurements are reported on a series of cellulose linter samples acetylated to varying degrees. The data illustrates the effects of the original structure of the cellulose linters both on the ease of acetylation and also on the subsequent dynamic characteristics of the polymer. It is only in the most forcing conditions that the primary fibrillar structure is destroyed. A change in properties is observed to occur at 40% acetylation of the hydroxyl groups, this coincides with the concentration of accessible groups as determined by deuterium exchange. A correlation is observed between the absorption and electrical observations on these systems.     

Dielectric studies of cellulose and its derivatives: 3. Glycerol in cellulose acetate

Dielectric studies are reported on cellulose acetate doped with glycerol. The addition of glycerol initially leads to the generation of a strongly interacting layer manifest by an increase in the amplitude of the side chain motion of the cellulose acetate and also the appearance of a higher temperature relaxation. Further addition of glycerol allows development of a multilayer situation which has relaxation properties similar to that of pure glycerol and is essentially independent of the cellulose acetate matrix. The high temperature conductivity of the samples investigated is critically dependent on the concentration of the dopant, increasing markedly for concentrations above 2–3%. This change in behaviour appears to correlate with the fibrillar surface area and is associated with the generation of proton conduction in the system. The general characteristics of this behaviour are similar to those previously reported¹ for the addition of water to cellulose derivatives.     

Dielectric studies of cellulose and its derivatives: 2. Effects of pressure and temperature on relaxation behaviour

Dielectric and water absorption measurements are reported for cellulose acetate, a plasticized cellulose acetate and ethyl cellulose as a function of pressing temperature and pressure. The observed effects are explained on the basis that the work done in producing the discs used in these measurements destroys residual fibre structure and leads to an increase in the dipolar activity of the sample. The observation of gelatinization is discussed in terms of the effects on the side chain mobility and the densification of the polymer matrix. The effects observed in the case of dibutylphthalate are in agreement with data from other measurements on another cellulose derivative and indicate strong interactions between the plasticizer and the polymer. Water absorption data also reflects the changes which occur in the morphology with changes in the pressing conditions.     

Dielectric relaxation studies of heterogeneously substituted cellulose acetate and its plasticization by dibutyl phthalate

Dielectric relaxation measurements are reported on plasticized commercial and heterogeneously substituted cellulose acetate. Differences in the observed behaviour are attributed to the retention of a degree of crystallinity and microscopic structure in the original substituted cellulose samples. A comparison of dynamic mechanical and dielectric relaxation data further emphasizes the influence of the nature of the original morphology on the micro dynamics.     

Dielectric and infrared study of some cellulose derivatives

Measurements of the dielectric constant of cotton cellulose, mercerized cellulose acetate, methyl cellulose, and carboxymethyl cellulose have been carried out in the temperature range of 0–70°C at different frequencies. The results showed that the values of the dielectric constant, with all cellulose derivatives at a given frequency, are greater than that of cotton cellulose and lower than that of mercerized cellulose. The variation of the dielectric constant with temperature showed a transition at 30–40°C with all the samples studied. These results, together with those from infrared spectra and specific volume measurements, indicated that the dielectric behavior depends greatly on the nature of the side group, the degree of hydrogen bonding between the different chains, and the micropores present in the fiber.     

Dielectric relaxation studies in some Polymer-PZT composites

Composites of PZT with different polymeric compositions viz. Vinyl Acetate-Vinyl Chloride (VA-VC) co-polymer, Amide cured Epoxy (ACE), Nitrile Rubber-Poly Vnyl Chloride (NR-PVC) blend and piezoelectric poly Vinyledene Fluoride (PVDF) were prepared and the dielectric properties were studied. New dielectric relaxation peaks were observed in the VA-VC and PVDF containing composites owing to the interaction of the molecular motion of the polymer chains with PZT.     

Rheological studies of cellulose derivatives solutions

The rheological behavior of solutions of cellulose derivatives, acetate, and hydroxypropyl was studied as a function of different parameters; molecular weight, concentration,…, at T = 25°C in the vicinity of the liquid crystal phase transition. A variation of the strain rate sensitivity parameter with concentration is found. A degradation of the solution in trifluoroacetic acid is demonstrated.     

Dielectric analysis of cellulose and its vinylic copolymers

The real and imaginary parts of the complex dielectric permittivity, ε′ and ε″, for some vinylic copolymers of cellulose [prepared with vinyl acetate (VA) and methyl acrylate (MA) and Ce (IV) ions as initiator ] and for cellulose were measured over a frequency band of 0.1–10² kHz and a temperature range from ?40 to 100°C. In vinylic copolymers of cellulose, we observed one dielectric relaxation attributed to the α-relaxation of the vinylic side chain grafted on cellulose. In cellulose dielectric spectra, this relaxation did not appear, but we detected one relaxation that may correspond to the β-relaxation. For these vinylic copolymers of cellulose, the ε″ against ε′ plot gives a skewed are that closely resembles that of the Davidson–Cole model, with a broader distribution for high frequencies that shows the overlap of several relaxations in the process considered. Some differences observed between the vinylic copolymers of cellulose may be due to the composition and the length of the vinylic side chains and to the frequency of grafting on the cellulose.     

Dielectric and dynamic mechanical relaxation studies of styrene-acrylonitrile copolymers

Dynamic mechanical measurements (～1 Hz) and dielectric measurements (10^?4 to 10⁵ Hz) are presented for styrene-acrylonitrile copolymers containing 25 and 32.5% acrylonitrile. Two dielectric (α, β) and one mechanical (α) processes were observed and their mechanisms are discussed. It is shown that the Montrose-Litovitz fluctuation theory and Phillips-Barlow-Lamb defect-diffusion theory give reasonable representations of the dielectric α relaxation process.     

Nuclear magnetic proton relaxation studies of oxidized coals

Coals of NCB rank 301 a (coking), 502 (caking) and 802 (very weakly caking) are oxidized in air at 373 K or 383 K for up to 42 days. Spin-lattice and spin-spin relaxation times, T₁ and T₂ respectively, of oxidized coals are measured using a Bruker SXP 4–100 and FT spectrometer. Free radical concentrations in the coals are obtained using a JES PE e.s.r. spectrometer. Infrared spectra of oxidized coals are obtained and optical textures of cokes from fresh and oxidized coals are assessed by optical microscopy. For two coking coals, decreasing values of T₁, and increasing concentration of free radicals occurred with oxidation at 383 K to 16 and 28 days. Thereupon values of T₁, increased and free radical concentrations decreased with further progressive oxidation. At the point of inflexion in properties, resultant cokes from the coals ceased to shown any anisotropy in their optical textures and became isotropic resembling cokes from low-rank coals. For the caking coals, T₁ increased at all stages of oxidation to 42 days with decreasing concentrations of free radicals. Two values of T₂ were found in each coal corresponding to a rigid and mobile component ((T₂)_r < (T₂)_m). The rigid component (T₂)_r was not affected by oxidation but values of (T₂)_m decreased with increasing duration of oxidation. It is considered that coking and caking coals exhibit different effects of oxidation with perhaps phenols and quinones in caking coals acting as inhibitors to the growth of stable free radicals. Oxidized coking coal may behave like fresh caking coal.     

Liquid-crystalline state of cellulose and its derivatives

Conclusions Literature material on the liquid-crystalline state of solutions and melts of cellulose and its derivatives has been analyzed.Translated from Khimicheskie Volokna, No. 3, pp. 8–12, May–June, 1983.     

Structure and properties of ethylene—methacrylic acid copolymers and their sodium salts: Dielectric and proton magnetic relaxation studies

Dielectric and proton magnetic relaxation data have been obtained for an ethylene-methacrylic acid copolymer (containing about 4 mole% methacrylic acid units) and its 53% ionised sodium salt. The degrees of crystallinity and percentage ionisation of the samples investigated were estimated by infra-red methods. The dielectric results were obtained principally in the frequency range 100 Hz to 10 kHz and at temperatures ranging from 80° to – 120°. A few results are also reported at frequencies down to 0·1 Hz and up to 100 MHz. For the acid copolymer, two dielectric loss regions are observed and these are correlated with the reported mechanical β′- and γa-processes respectively. The partly ionised copolymer exhibits three dielectric relaxation regions which correlate with the mechanical α-, β- and γa-relaxations respectively. In addition, a dielectric peak appears at about —40° in the presence of absorbed water, a result similar to that found in the polyamides. The proton magnetic relaxation results were obtained by pulse methods which yielded the spin-lattice relaxation times T₁ (at 30 MHz) and T_1p (at kilohertz frequencies) as a function of temperature from ?180° to 100°. Two components were generally observed for T_1p. For the acid copolymer the β′- and γ-processes have been observed from these results, as well as a lower-temperature (δ) process which has not been detected by the mechanical or dielectric methods. For the sodium salt the γ- and δ-processes are also found, in addition to a high-temperature process in the region of the merged α- and β-processes. The present data are consistent with previous assignments for the β′-, α-, β- and γ-processes. The ‘water’ relaxation appears to involve some rotation of water molecules, or of ionic segments to which water molecules are attached, in the proposed ionic domains. The δ-process is ascribed to the rotations of methyl groups present in the methacrylic acid units.     

Dielectric relaxation studies of low thermal expansion polymer composites

The results of temperature‐dependent dielectric and rheological measurements are reported on polymer‐ceramic composite films, poly(methyl methacrylate) (PMMA) : lead titanate (PbTiO₃). Analyses of relaxational processes of the PMMA host matrix have been investigated using temperature‐dependent dielectric and rheological measurements. It is found that the α‐relaxation is more significantly affected by the addition of filler in comparison to β‐relaxation. The composite films are found to have much lower dielectric constants in comparison to the pure ceramic material. Suitable models have been used to explain the observed dielectric constant of the composite films. Using rheological measurements, occurrence of reinforcement in these composite films due to the addition of ceramic filler has also been observed and the results are discussed in the article. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Rubber-modified epoxy resins: 2. Dielectric and ultrasonic relaxation studies

Dielectric and ultrasonic relaxation measurements are reported on a series of rubber-modified epoxy resins over a temperature range from 190 to 423K. The resins were prepared by the reaction of the diglycidyl ether of bisphenol A with a chain-extender formed from carboxyl-terminated acrylonitrile-butadiene copolymer and cured with either triethylene tetramine or diethylene glycol bis-propyl amine. At low temperature, a dielectric relaxation was detected which was also active ultrasonically and can be associated with molecular motion of the acrylonitrile-butadiene chain extender. Changes in the concentration of the rubbery phase and type of curing agent used have marked effects on both the activation energy for molecular relaxation and in the breadth of the relaxation curves. Extensive studies of the dielectric properties at higher temperatures failed to reveal a resolvable relaxation associated with either of the higher temperature transitions observed using equilibrium measurements. Electrical conductivity, permittivity and acoustic velocity measurements performed as a function of temperature, however, indicate the existence of a change in slope at approximately the value of the glass transition of the epoxy phase. A further change in slope at approximately the temperature of the highest temperature transition was detected. The observed relaxation properties are compatible with the resins being phase-separated at low rubber concentrations and changing to a semicontinuous interpenetrating network structure at higher rubber concentrations.     

Positron annihilation and X-ray diffraction studies of cellulose and some derivatives

Positron lifetime measurements are reported over a temperature range from 290 to 380K on cellulose and certain of its derivatives. The lifetime spectra were resolved into three components each attributed to a different mode of positron annihilation in the polymer. The longest lifetime component is associated with the decay of ortho-positronium and is a function of the average cavity distribution in the sample. It is observed that significant differences exist in the lifetimes of positrons in cellulose and its derivatives which can be interpreted as evidence for changes in the free volume distribution in these materials. Comparison of positron decay in samples of cellulose acetate prepared at both room temperature and elevated temperature indicate that the phenomena of gelation in this material is accompanied by a reduction of free volume. The correlation of such observations with the dynamic properties of these polymers is discussed.     

Structure and relaxation in cellulose hydrogels

Cellulose‐based hydrogels show great potential for a wide range of applications. However, the structure of these hydrated gels is not fully understood. The impact of moisture on the structure and stability of cellulose based hydrogels is reported in this article. Analytical data based on GPC, NMR, and rheology are discussed. It was found that moisture‐induced gelation greatly reduces the crystallinity of the hydrogels, and the release of water from the hydrogels leads to permanent structural changes in the network structure due to the reformation of hydrogen bonding. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42071.     

Dielectric relaxation of polyorganophosphazenes

Dielectric relaxation spectra of poly(bis-trifluoroethoxy phosphazene) (PBFP), poly(bis-n-propoxy phosphazene) (PBPP), and poly(bis-n-propylamino phosphazene) (PBPAP) were measured in wide temperature and frequency regions, and the -relaxation caused by micro-Brownian motion of segments in the rubbery state and the -relaxation due to the local motions of the polymer in the glassy region were observed. The dielectric spectra of PBFP at temperatures higher than the rubbery region exhibited a mesophase transition at 358 K. From conformation analysis according to the two-site model, it has been elucidated that the mechanism for -relaxation of these samples is hindered internal rotation of the side groups.     

Dielectric relaxation in epoxy-polyglycol elastomers

Dielectric relaxation data is reported on a series of elastomers formed between bisphenol A and either polyethylene glycol or polypropylene glycol. The observed dispersion may be ascribed to relaxation of the “soft” block and is found to vary with both change in the glycol block length and with the ratio of bisphenol A to polyglycol. Elastomers prepared with a poly-glycol component of mixed composition allowed the relaxation frequency at a particular temperature to be shifted over a frequency range of almost three decades. Activation parameters obtained from the temperature dependence of the mean relaxation frequency are used in evaluation of the factors influencing the molecular motion of the “soft” block.