Dynamic shear compliance of polymer melts and networks in dependence on crosslinking density

Mechanical relaxation processes in polymer melts and networks are discussed. This is performed by decomposing master curves of the dynamic shear compliance into i) glass relaxation with its plateau complianceJ _eN ; ii) shearband process with its relaxation strengthJ _B , which is reciprocal to the total crosslink densityp _c ; and iii) flow relaxationJ _F and viscous flow (for uncrosslinked melts only). Plateau complianceJ _eN > is exponentially reduced only by effective crosslinks (p _c ^* p _c /30). This behavior is understood on the level of a meander superstructure, which includes shearbands. The observed saturation inJ _eN at higher dicumylperoxide (DCUP) crosslinking-which doesn't appear with radiation-can be explained by the lack of chemically induced effective crosslinks across the interfaces among meander cubes. This lack may be a consequence of DCUP molecules concentrating at the interfaces and thereby preventing the contact and radical recombination between chains at adjacent meander faces.Crosslink densitiesp _c (per monomer), determined from the reduction of shearband relaxation strength, vary linearly with the crosslinking agent and read: p_c2.4 · 10^–2 Dose/MGy andp _c 0.97 · 10^–2 DCUP/phr for radiation and DCUP crosslinking, respectively. This implies, e.g., that a dose of 0.4 MGy (40 Mrad) is equivalent to 1 part DCUP phr in a crosslinking polyisoprene. From activation-curve analysis it follows that3 r/d stays constant, and _s - _so (free energy of formation of a segment-dislocation) andQ _y -Q _yo (activation energy for segmental jumps) vary with the square ofP _c , as does the glass temperaturT _g -T _go from DSC measurements.     

Some structural and mechanical properties of bacterially produced poly-β-hydroxybutyrate-co-β-hydroxyvalerate

Some structural and mechanical properties of PHB homopolymer and copolymers containing 17 and 25–30% PHV have been investigated. X-ray scattering, optical microscopy, density measurements and differential scanning calorimetry were used to interpret the results of load-elongation curves and thermomechanical softening measurements.     

Network properties of mixtures of protonated and deuterated polyethylene

Mixtures of protonated and deuterated polyethylene were irradiated in the melt. The degree of crystallinity, the degree of crosslinking, as well as the enthalpyH and the melting pointT _M were determined. No significant differences in the degree of crosslinking between protonated and deuterated chains were found. The mass specific entropyS of the uncrosslinked samples remained constant and independent of the deuterium concentration. For the crosslinked samples, a netpoint entropy was postulated. A weaker Van der Waals interaction could explain the decrease in melting temperature by deuteration (for weakly crosslinked samples).     

Recovery of thermal and mechanical properties after thermal treatment in a polyester-based polyurethane

Solution casted films of segmented polyurethanes based on poly (ethylene adipate) glycol, 4,4diphenylmethanediisocyanate, and 1,4 butanediol were studied by thermal-mechanical methods including differential scanning calorimetry and mechanical hysteresis. Data demonstrate that, following thermal treatment at 70 °C for 15 min, these polymers show time-dependent thermal and mechanical properties. In fact, because 70 °C is a temperature greater than the melting point of soft segments (about 42 °C), the soft-segment crystals are melted and their crystallization is time dependent. The results are explained in terms of phase separation. In particular, the sample with better phase separation has a faster and larger recovery.     

Influence of the exchangeable cations on stability and rheological properties of montmorillonite suspensions

The effects of NaCl and CaCl₂ on the colloid stability and rheological properties of Na- and Ca-montmorillonite dispersions were studied. The distribution of cations between the surface and the bulk phase was determined. For both of monocationic montmorillonite, the critical coagulation concentration were 250 mmol NaCl/dm³ and 2 mmol CaCl₂/dm³. The changes in the Bingham yield stresses of Na- and Ca-montmorillonite dispersions as functions of the NaCl and CaCl₂ concentration could be explained in terms of the surface excess amount and equilibrium concentration of the cations, the second electroviscous effect and the formation of a gel structure.     

Investigations of the influence on the phase morphology of PP-EPDM-blends on their mechanical properties

This work covers the dependence of the mechanical properties of polymer blends on their composition and their phase morphology. Blends of EPDM-elastomers and polypropylene were prepared covering the whole concentration range. The phase morphology was varied strongly by employing different mixing techniques and its morphology was characterized by means of electron microscopy and light microscopy, as well as by x-ray scattering and calorimetry.Mechanical properties such as the complex shear modulus, the tensile modulus as well as the stress strain behavior were investigated as a function of the composition of the blends and their phase morphology. The experimental finding is that the complex modulus, the tensile modulus, the yield stress, and the ultimate stress are rather insensitive with respect to the phase morphology and vary continuosly with the composition. The elongation at break, on the other hand, as well as the impact strength were found to depend on the phase morphology and to vary discontinously with the composition. One conclusion to be drawn is that one is not always forced to control the phase morphology tightly during processing in order to obtain materials with sufficiently good mechanical properties. Rather, simple theoretical approaches, neglecting details of the phase morphology are frequently able to satisfactorily predict mechanical properties of multiphase blends.     

Temperature-dependence of mechanical and morphological properties of ultra-high molecular weight polyethylene cross-linked by electron beam irradiation

Cross-linking of ultra-high molecular weight polyethylene was performed with electron-beam irradiation in the range of radiation dose from 12 to 96 Mrad under nitrogen. Dry gel films and melt films were used as specimens. Two kinds of cross-linked specimens could be kept at 200°C for a prolonged time in an undeformed state and this tendency was independent of radiation dose. The elongation of the gel films hampered the heat-resistant effect and the drawn specimens were broken at temperatures lower than 175 °C. The elongation of the melt films could not be realized, because of a marked fixation of chains in the fiber network, even at a dose of 12 Mrad.     

Combinatorial calculations of topological defects in tetrafunctional polymer networks

The structure of real polymer networks admits many kinds of topological defects related to the way in which polymer chains are connected to crosslinks. The fractions of crosslink types have been derived from combinatorial considerations concerning the probabilities of various combinations of structural elements of crosslinks. 34 fractions of different crosslinks, possible in tetrafunctional systems, have been obtained as explicit functions of concentrations of seven structural elements — free-end chains, single-, double-, triple-, and quadruple-connected chains, loops, and unsaturated (void) functionalities. It was shown that different distributions (and different properties) result from end-to-end linking of precursor chains vs. vulcanization of long chains by bridging along their contours.     

Numerical analysis of complex dielectric mixture formulae

As a continuation of our earlier work (Ref. [9]) the complex (frequency dependent) dielectric behaviour of some mixture formulae are studied numerically. These include matrix-inclusion type formulae (as the Wagner-Sillars or the Bruggeman-Boyle equations), mean-field statistical mixture formulae (as the Böttcher-Hsu equation) and symmetrical integral formulae (as the Looyenga equation). The frequency dependent dielectric properties are first calculated for a model system at various particle shapes, field orientations and volume fractions. After this, the validity of these equations is checked on typical sets of experimental data. For low loss powders, the Böttcher and Looyenga equations are suggested; for emulsions, suspensions and filled polymers, the matrix-inclusion type formulae give acceptable results in most cases, while for metal-insulator composites mean-field statistical mixture formulae have to be used, as they are capable of describing the percolation phenomenon.     

Comment on the “dielectric properties of LDPE/Ny6 blends”

Data published by La Mantia et al. [1] on dielectric dispersion and loss in polyethylene/nylon 6 blends are analyzed in terms of dielectric mixture formulae. It is shown that an ohmic interfacial polarization process can not be responsible for the unexpected increase of and values observed in these blends at high temperatures. The observed phenomena are tentatively attributed to space charge processes at the electrodes or to other defects dipole mechanisms.     

Comparison of galactomannan crosslinking with organotitanates and borates

Water-soluble polymers crosslinked with metal ions form stable gels that are used in oil and gas production processes. We present an investigation of the chemistry of the binding between guar galactomannans and both borates and titanates using NMR and dynamic light scattering. The two monosaccharides comprising guar — methyl--D-galactopyranoside and methyl--D-mannopyranoside — have been studied as model compounds. High resolution¹³C NMR detected complexation of the sugars with borate but displayed no sign of complexation with either of the two titanates even at high titanate concentrations. Dynamic light scattering studies demonstrated the existence of colloidal titanium dioxide particles for both titanate crosslinkers. The loss of free titanium to the colloidal particles reduces the concentration of titanium crosslinking sites to below the detectable limits of NMR. The role of colloidal titanium dioxide colloids in these crosslinking reactions has not been reported previously. Data are presented following the growth of these particles with time as a function of pH and salt concentration. The observed particle growth kinetics explains the observed sensitivity of the guar gelation process to these variables.     

Broadband dielectric spectroscopy on a discotic liquid crystalline polymer

Broadband dielectric spectroscopy is employed for the first time to study the molecular dynamics in discotic liquid crystalline polymers. One dielectric relaxation process is found which is strongly broadened and asymmetric. It is assigned a local hindered rotation of the ester groups attaching the spacer to the discotic mesogen. This assignment is supported by NMR measurements on the identical substance.Dedicated to Prof. E. W. Fischer on the occasion of his 60th birthday.     

Effects of urea and a nonionic surfactant on the micellization and counterion binding properties of cetyltrimethyl ammonium bromide and sodium dodecyl sulfate

Micellization characteristics and counterion binding properties of cetyltrimethyl ammonium bromide (CTAB) in presence of urea and a nonionic surfactant polyoxyethylene sorbitan monolaurate (PSML), and of sodium dodecyl sulphate (SDS) in presence of urea as well as of several mixtures of CTAB with a bile salt, sodium cholate (NaC), and sodium chloride have been studied. Both urea and PSML have increased the critical micelle concentration (CMC) of the surfactants, the former being more effective than the latter. The analysis of the results supports the pseudophase micellar model to hold over the mass action model. Pure CTAB micelles bind more counterions (96 %) than pure SDS micelles (87 %), and the decreasing effect of urea on the binding is less in case of the former than the latter. A 41 mixture of CTAB and sodium cholate (NaC) can micellize and the micelles bind 87 % bromide ion, whereas 21 and 11 mixtures do not micellize. Micelles of 11 mixture of CTAB and NaCl can bind counter bromide ions to the extent of 92 %. The limiting concentrations of urea required to effect counterion binding by CTAB and SDS micelles are 0.15 mol dm^–3 and 0.25 mol dm^–3, respectively. Such effect is shown by PSML on CTAB at a ratio 0.281. The activation energy of conduction of SDS has increased in the presence of urea up to a concentration of 4 mol dm^–3, at higher concentrations the activation energy has decreased, the effect being more for surfactant concentration above CMC than below.     

On the observation of micellar structures by dielectric spectroscopy

Many cationic surfactants form, in the presence of certain organic acids, large supramolecular micellar structures in water. The dielectric response of one such system (cetyl trimethyl ammonium bromide-salicylic acid, CTMAS) has been studied as a function of frequency, concentration and temperature. The results are compared with dynamic mechanical data on the same system, which has been published in the literature.The dielectric response shows that the micelles form a rigid, open network structure, which does not impede ionic transport in the bulk liquid. However, the response also shows the presence of a frequency dispersive barrier capacitance. From the variation of the properties of this capacitance with CTMAS concentration and applied voltage over a range of frequencies, it is deduced that the barrier originates in an ordered micelle structure at each electrode.     

An interlayer model for the complex dielectric constant of composites

The complex dielectric constant of a composite with an interlayer was studied as a function of the volume fractions and the properties of the filler, the interlayer, and the matrix. The theoretical approach is analogous to the calculation of the shear modulus, the bulk modulus, and the termal expansivity of particulate filled polymers using the interlayer model (IM).An analytical expression describing the influence of an interlayer on the generalized dielectric constant of the composite as a function of the volume fraction and interlayer properties is derived.In the case of a composite with non-conductive constituents, the equations for static and oscillatory electric fields are similar. When conductive constituents are present, the complex dielectric constants have to be replaced by the generalized complex dielectric constants.For a composite of non-conductive materials, without interlayer, the obtained relation reduces to the classical Rayleigh equation. In the case of a composite with conductive constituents, also without interlayer, the complete solution of Wagner's theory is found. Special attention has been paid to the case of a water interlayer in a glass-bead filled non-conductive matrix material.     

Micelle aggregation numbers from colligative properties

An equilibrium model has been developed to derive micelle aggregation numbers n from colligative properties of surfactant solutions, with special regard to freezing point depression. The introduction of a mass action approach in the solute activity equation allows to get aggregation numbers close to those obtained from light scattering. The model can be applied with small formal changes to ionic and nonionic surfactants.     

Micellar properties of zwitterionic phosphobetaine amphiphiles in aqueous solution: Influence of the intercharge distance

Zwitterionic amphiphiles of the general formula H(CH₂)_y ⁺ N(CH₃)₂(CH₂)_n PO₂C₆H ₅ ^– , where the number of intercharge methylenesn is varied, were studied in dilute aqueous solution. Their critical micellar concentrations show a peculiar variation withn, first increasing asn varies from 1 to 4 and then slowly decreasing as methylenes are added up to 10. This behavior is interpreted as being the consequence of two opposite contributions. The first is the classical CMC lowering due to the increase of hydrophobic character with the total number of methylene groups in the surfactant molecule. The second contribution is the increase in the dipole moment of the zwitterionic headgroup withn, leading to stronger dipole-dipole repulsions between headgroups at the micellar surface. Experimental results suggest that the dipole moment does not increase linearly withn because of the polymethylene chain flexibility. This is supported by¹³C NMR relaxation experiments.     

Radiation-induced crosslinking: III. Effect on the crystalline and amorphous density fluctuations of polyethylene

Small-angle x-ray scattering (SAXS) was used to determine density fluctuation in radiation-induced crosslinked polyethylene of varying degrees of crystallinity. Density fluctuation FL decreases with increasing crystallinity, while it increases linearly with increasing radiation dose or degree of crosslinking. By means of extrapolation, density fluctuations in the crystalline and the amorphous phasesFL _c andFL _a were obtained. At a given dose,FL _a is greater thanFL _c . The increase inFL _a with radiation is found to be much greater than that ofFL _c compared with the initial values at 0 Mrad,FL _c showing only a negligible increase event at 312 Mrad. The present findings suggest that crosslinks are not introduced within the crystalline phase; they take place primarily in the noncrystalline phase, in agreement with the conclusions reached previously on the basis of changes in crystalline and amorphous densities in irradiated polyethylene.Dedicated to Prof. Dr. W. Pechhold on the occasion of his 60th birthday     

Morphology and mechanical properties of ultrahigh molecular weight polypropylene prepared by gelation/crystallization at various temperatures

Films of ultrahigh molecular weight (5.4×10⁶) polypropylene were produced by gelation/crystallization at various temperatures from dilute decalin solutions according to the method of Smith and Lemstra. The temperatures chosen were 20°, 30°, 50°, and 60°C. With increasing the temperature, the long period and crystallinity of the resultant gel film increased. By contrast, when the films were stretched up to 50 } 60 times, the increases in Young's modulus and crystallinity become more significant, as the temperature of the gelation/crystallization became lower. This interesting phenomenon is thought to be due to the dependence of the number of entanglements on the temperatures concerning gelation/crystallization and evaporation of solvent from the gel to form a film.     

The effect of segment length and concentration on dielectric properties of polypropyleneoxide-based polyurethanes

Three series of segmented polyurethanes based on MDI, variable chain extender, and polypropylene oxide of MW=1000, 2000, and 3000 were synthesized and their dielectric behavior examined.Dielectric relaxations in the segmented polyurethanes were investigated between –150°C and +150°C in the 100 Hz to 10 kHz range. In general, three transitions, designated as, , and were observed, and ascribed in accordance with calorimetric relaxations to glass transitions of the hard and soft segments, and Shatzki-type motions, respectively. The effect of structure variables such as soft segment size, type of chain extender (ethylene glycol, butane diol, and hexane diol) and soft segment concentration, as well as the effect of interaction of the phases on dielectric properties was discussed. It was found that a certain degree of phase mixing exists in all series, detected by the variation of theT _g of the soft segment with soft segment concentration, contrary to DSC results, which was ascribed to thermal treatment prior to the dielectric measurements. It appears that interfacial polarization becomes important only above the transition temperature.