Helena Valentová

Molecular order and dynamic mechanical behavior of polyurethanes based on liquid crystalline diol

Abstract Synthesis of the liquid crystalline (LC) diol 6,6′-[ethylenebis(l,4-phenylene-oxy)]-dihexanol (I) is described. The structure of polyurethanes prepared from diol I and 4,4′-methylenedi(phenyl isocyanate) (MDI), 4,4′-methylenedi(cyclohexyl isocyanate) (HMDI), or 2(4)-methyl-l,3-phenylene diisocyanate (TDI) at 1:1 molar ratios of isocyanate and hydroxy groups is studied by dynamic mechanical spectroscopy, differential scanning calorimetry (DSC), polarizing microscopy, and x-ray scattering. The polymer prepared from HMDI and the diol (I/HMDI) shows, on cooling, thermal behavior typical of amorphous polymers. A frequency-temperature superposition could be applied to the mechanical data, and the horizontal shift factor satisfied the Williams-Landel-Ferry (WLF) equation. A more-complex thermal behavior was found for I/HMDI polymer during subsequent heating; above 70°C, the formation of an ordered structure takes place, and this structure melts at about 120°C. Complex thermal behavior is exhibited by I/...

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.

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.

Dynamic Mechanical Behavior of Ordered Off-Stoichiometric Polyurethane Systems at the Gel Point Threshold

Dynamic mechanical and thermal behavior of ordered off-stoichiometric polyurethane (PU) systems, before and after the gel point, based on the mesogenic diol 6,6′;-[ethylenebis(1,4-phenyleneoxy)]dihexan-1-ol (D),2(4)-methyl-1,3-phenylene diisocyanate (DI), and poly(oxypropylene)triol (T) were studied. Polymer samples were prepared at various initial molar ratios of the reactive groups, r = [OH]T/[NCO]DI/[OH]D, ranging from 1/10/9 to 5/10/9 (the ratio [NCO]DI/[OH]D = 10/9 was constant); the total mole ratio of hydroxy (OH) and isocyanate (NCO) groups, rOH = [OH]/[NCO] = ([OH]D)/[NCO]DI,changed from 1 to 1.4. Dynamic mechanical measurements during the curing reaction showed that the power law parameters that characterize the critical gel state (gel strength S and relaxation exponent n) are dependent on the initial composition (the ratio r OH). The gel-point critical ratio of reactive groups rc OH, found during curing in the ordered state of the diol (at low curing temperature), has revealed that the critical gel (CG) structure is determined by a contribution of strong physical interactions as well as chemical junctions and does not correspond to pure chemical gelation (CG structure formed at low temperature exhibits flow at elevated temperatures in the isotropic state). This fact suggests that formation of the mesophase enhances the connectivity of the molecular structure at the gel point. Dynamic mechanical behavior of fully cured chemical networks (r OH < r OH c ) and un-cross-linked (r OH > r OH c ) samples (and a CG sample) has also been investigated. Decreasing the rOH ratio (increasing concentration of chemical cross-links in the systems) inhibits conformational rearrangements required for ordering; at the same time, the intensity of the slow relaxation process in the rubbery region decreases.

Dynamic mechanical and thermal behavior of novel liquid-crystalline polybutadiene-diols with azobenzene groups in side chains

Liquid-crystalline (LC) polymers containing methoxy- or butoxy-substituted azobenzene in side chains have been prepared via radical addition of the in advance synthesized nematic thiols onto double bonds of poly(butadiene)diol [Polakova et al., Pol. Bull. 54, 315–326 (2010)]. In the present work, thermal behavior of these comblike polymers has been characterized by differential scanning calorimetry and polarizing optical microscopy. LC transitions have been also determined by rheological measurement. Time–temperature superposition of mechanical functions has been successfully applied to samples undergoing direct nematic/glassy state transition.

Thermal, dynamic mechanical and dielectric behavior of liquid-crystalline linear and crosslinked polyurethanes with mesogenic group in side chains

This paper describes DSC, dielectric and dynamic mechanical behavior of linear and crosslinked liquid crystalline (LC) polyurethanes based on LC diols with a mesogenic group in the side chain, diisocyanates of various flexibility and two triols. From our investigations it follows: a) Linear polymers prepared from diols with simple end side chain substituents (as hydrogen, nitro and nitril group) exhibit only amorphous behavior regardless of the structure of used diisocyanate; generally, the most pronounced LC behavior exhibited polymers prepared from a diol with phenyl substituent. b) Investigation of the curing reaction showed that rheological power-law parameters, which are characteristic of the structure at the gel point, are dependent on the initial ratio of the reactants (amount of LC diol in EANCs). c) Strong physical interactions between the mesogens support the cyclization in the course of crosslinking reaction. d) Introduction of chemical junctions (amount of triols) suppresses LC ordering in the networks.

Chemical Clusters in Polyurethane Networks. SAXS, Photoelastic and Dynamic Mechanical Behavior of Networks from Poly(Oxypropylene)Diol, Diisocyanate, and Trimethylolpropane Prepared One‐Stage and Two‐Stage Process

The effect of the initial ratio of components and of the one‐ or two‐stage process of formation of polyurethane networks prepared from poly(oxypropylene)diol (PD, M n=2100), 4,4′‐diisocyanatodiphenylmethane (MDI) and trimethylolpropane (TMP) on their extraction, swelling, small‐angle X‐ray spectroscopy (SAXS), photoelastic, and dynamic mechanical behavior was investigated. The networks were prepared at various ratios of hydroxy (OH) to isocyanate (NCO) groups, r HT=[OH]TMP/(0.5[NCO])=1–1.5, by a one‐ or two‐stage process (in the first step [OH]PD/[NCO]=1/2) up to the full conversion of NCO groups. Due to decreasing reactivity of OH groups on TMP a nonuniform distribution of hard (TMP‐MDI‐TMP) segments (chemical clusters) was developed in both network series. The SAXS experiments proved that one‐stage networks exhibit a more heterogeneous structure than two‐stage ones. As expected, the weight fraction of the gel, w g, and equilibrium modulus, G e, decrease with increasing deviation from stoichiometry; the G e and w g values of one‐stage networks at constant r HT are always higher than those of two‐stage networks. The frequency‐temperature (f‐T) superposition of both components of the complex compliance J* (=J′–iJ″) was performed and the horizontal shift factor, log a T, satisfied the WLF equation for all networks. From the dependence of superimposed storage, J′p, and loss, J″p compliances on reduced frequency f.a T, three contributions could be determined from the Cole‐Cole distribution; each process was characterized by the relaxation strength, ΔJ i, relaxation time, τi and distribution broadness, b i. The broadness b and strength ΔJ of the high and medium frequency process (i=1,2), which correspond to the glass‐rubber transition, are roughly independent of r HT and the preparation method. The τ3 and ΔJ 3 values of the low‐frequency process in networks prepared by the two‐stage process are always greater than those in networks prepared by the one‐stage process. Dedicated to Professor John L. Stanford on the occasion of his 60th birthday.