J. Nedbal

Effect of compression pressure on mechanical and electrical properties of polyaniline pellets

While conductivity and other electrical properties are key parameters in the design of polymer electronics, equally important mechanical properties of conducting polymers have rarely been reported. The influence of preparation conditions of polyaniline pellets on mechanical and electrical properties was therefore studied. Conductivity of polyaniline is commonly measured using pellets prepared by the compression of powder. It is shown that a pressure of at least 300 MPa is needed to obtain a reliable value of conductivity. At lower pressures, the samples have lower apparent conductivity, density, Young modulus, and hardness. Above the compression limit of 300 MPa, these parameters become constant, except for the density. The same behavior was observed both for conducting polyaniline hydrochloride and for the non-conducting polyaniline base. The puzzling observation that density of the pellets decreased as the compression pressure increased is discussed considering the relaxation processes.

Formation, structure, thermal and dynamic mechanical behaviour of ordered polyurethane networks based on mesogenic diol

Abstract Dynamic mechanical and thermal behaviour of ordered linear and crosslinked polyurethane systems based on a mesogenic diol, 4,4′-bis(11-hydroxyundecyloxy)biphenyl (D), 2(4)-methyl-1,3-phenylene diisocyanate (DI) and poly(oxypropylene)triol (T) was investigated during network formation and in fully cured samples. The networks were prepared at various stoichiometric initial molar ratios of the reactive groups, [OH]T/[NCO]DI/[OH]D ranging from 1/1/0 to 1/12/11; for comparison, linear sample with the ratio 0/1/1 was also investigated. From our measurements it followed: (a) The power-law parameters, which are characteristic of the critical structure at the gel point (the gel strength S and relaxation exponent n), are dependent on the initial ratio of the reactive groups. With increasing content of mesogenic diol in network chain (increasing length of elastically active network chain (EANC)) both, the gel strength S and relaxation exponent n decrease. (b) Introduction of chemical junctions reduces flexibility of the EANCs in fully cured networks and inhibits conformational rearrangements required for ordering. Increasing the content of mesogenic diol in EANCs increases the concentration of permanent physical interactions (trapped entanglements) in the systems and the intensity of the slow relaxation process in the rubbery region.

Gelation and dynamic mechanical behavior of liquid crystalline networks

Abstract The dynamic mechanical behavior of polyurethane networks based on liquid crystalline (LC) diol, 6,6′-[ethylenebis(1,4-phenyleneoxy)]dihexanol (D), 2(4)-methyl-1,3-phenylene diisocyanate (DI), and poly(oxypropylene)triol (T) at the stoichiometric molar ratio of isocyanate (NCO) and hydroxy (OH) groups was studied. Samples were prepared at various initial molar ratios of the reactive groups [OH]T/[NCO]DI/[OH]D from 1/1/0 to 1/40/39. The gelation studies during the curing reaction in the LC and isotropic states showed that the critical gel structure at the sougel transition exhibits a power-law mechanical behavior; the relaxation exponent in the LC state is always higher than that in the isotropic state. From viscoelastic results, it follows that formation of the LC mesophase enhances the connectivity of the molecular structure at the gel point. Introduction of chemical cross-links in fully cured networks reduces the flexibility of the elastically active network chains (EANCs) and inhibits conformat...

Dynamic Mechanical and Dielectric Properties of Ethylene Vinyl Acetate/Carbon Nanotube Composites

A comparative study of the dielectric and mechanical properties of ethylene vinyl acetate copolymer (EVA) filled with various concentrations of pristine and modified carbon nanotubes is reported. The surface of the carbon nanotubes was modified with 4-(2-(cholesteryloxycarboxy)ethyl) phenyl to improve the interaction of the filler with the block copolymer matrix. The improved interaction and the better dispersion of the modified carbon nanotubes (mMWCNTs) were demonstrated by a detailed study of the EVA molecular mobility through dynamic mechanical analysis and broadband dielectric relaxation spectroscopy. The storage modulus of the nanocomposite with 6 wt.% of mMWCNTs at −50°C was enhanced by 103%, whereas for the nanocomposite with the same amount of unmodified filler, the storage modulus was only enhanced by 76% compared to the pure elastomeric matrix. This difference is more pronounced in the rubbery region in which the storage moduli were increased by 117% and 48% for the composite with the modified and unmodified fillers, respectively. The morphologies of the nanocomposites were studied with scanning and transmission electron microscopies to demonstrate the dispersion of the mMWCNTs within the EVA matrix.

The relaxation and equilibrium behaviour of model polyurethane networks

SummaryThe mechanical, dielectric and rheo-optical behaviour of model networks prepared from OH-terminated poly(oxypropylene)triols and 4,4′-diphenylmethane diisocyanate with various molar ratios of hydroxyl to isocyanate groups, rH=0.95 to 1.7, was investigated. The observed independence of the stress-optical coefficient of rH indicates the homogeneity of networks. Networks prepared with rH≈1 exhibit a narrow transition zone indicating a narrow chain distribution. With increasing rh, the transition becomes distinctly broader; dielectric measurements are well correlated with the mechanical ones. The equilibrium modulus markedly decreases with increasing rH.

Dielectric behaviour of swollen poly(acrylamide) gels in the collapse region

SummaryThe dielectrical behaviour (in the frequency range f=20 Hz — 200 kHz) of two networks — polyacrylamide (PAAm) network and a network obtained by the copolymerization of acrylamide with 3 mol.% sodium methacrylate (MNa) in a mixed solvent deionized water — acetone (acetone concentration a=0–80 vol.%) at T=298 K and pH ∼ 7 was investigated. High values of complex permitivity ɛx(=ɛ′+iɛ″) were found for both networks. For the PAAm network the dependence of both components of ɛx on a is continuous. On the other hand, for the ionized network the jumpwise decrease in swelling at the phase transition is accompanied by a jumpwise increase in the values of both components of ɛx at all frequencies; the critical concentration of acetone a at the collapse is frequency independent. The high values of components of ɛx and their dependences on a observed with both networks can be explained qualitatively by the polarization of the space charge in the measured samples. While with the PAAm network the charge is given by the dissociation of water, in the case of the P(AAm-MNa) network ions arising by the dissociation of MNa also contribute to this charge. The collapse in the ionized network is accompanied by a jumpwise increase in the effective charge concentration in the gel. The fact that the extent of the jumpwise change of components of ɛx in the collapse is frequency dependent suggests that the dependence of ɛx on a is also affected by the mobility of charge carriers.

Dynamic mechanical and water sorption behaviour of ordered polyurethanes

Abstract Ordered polyurethanes were made from two diisocyanates (DI), mesogenic diol (D) and a polybutadiene diol (B) with stoichiometric ratios of reactive hydroxy and isocyanate groups. Two-(D/DI) and three-component ((D+B)/DI, D/B=1/1 by weight) polymers were prepared and their DSC, dynamic mechanical and water sorption behaviour was investigated. In two-component polymers nematic and smectic mesophases were found. Three-component systems exhibit two-phase structure with soft polybutadiene and hard polyurethane (formed from D and DI) phase; in the polyurethane phase, mesophases are formed.

Swelling and photoelastic behaviour of ionized hydrogels of poly(acrylic acid)

SummaryThe photoelastic and swelling behaviour of poly(acrylic acid) gels swollen in 1 M NaCl aqueous solution was studied as a function of crosslinking degree (1–5 wt.% ethylene glycol dimethacrylate) and degree of ionization αg = 0 − 1. The degree of swelling passes through a minimum at αg ≐ − 0.15 regardless of the degree of crosslinking, probably due to the formation of hydrogen bonds between ionized and nonionized carboxyl groups. In the same neutralization region, the equilibrium modulus reaches a maximum; on the other hand, the modulus related to the dry state is independent of α for all crosslinker concentrations. Thus the mechanical behaviour of poly(acrylic acid) gels is in accord with the Gaussian theory and no contribution of permanent physical interactions to the modulus is observed. The optical behaviour is more complex — the sign of the stress-optical coefficient Ce changes three times with increasing α due to the orientation of the side groups. The analysis of the optical data shows that the Ce value is controlled by the degree of swelling and by the ionization.

Dielectric and mechanical behaviour of polyurethane networks from poly(oxypropylene)diols or-triols and diisocyanate

SummaryThermally stimulated depolarization currents and dielectric and dynamic mechanical behaviour of polyurethane networks prepared from poly(oxypropylene)diols or-triols and 4,4-diphenylmethane diisocyanate were studied. While the frequency and temperature positions of mechanical and dielectric functions are predominantly determined by the concentration of polar urethane groups in the network chain, the equilibrium modulus correlates with network chain length. The effect of the chain polarity on the shape of loss permittivity vs. frequency curves is more pronounced in the low frequency region than in the high frequency one and is practically the same for stoichiometric and off-stoichiometric networks measured earlier.

High-Temperature dispersion zones in cross-linked natural rubber

Abstract The complex dielectric constant of natural rubber samples cross-linked by sulfur and dicumyl peroxide has been measured over a frequency range from 1.5 to 3.0 × 105 cps by the bridge method and from 3.0 to 1.0 × 104 cps by the relaxation method. Two dispersion peaks were detected which lay in the measured frequency range at temperatures above the main dispersion zone. Temperature and cross-link density-dependent shift of these peaks were studied and compared with the results obtained by Ferry et al. from complex compliance measurements performed on analogous systems.