Linear thermoviscoelasticity and characterization of slightly crystalline EPDM networks

Three EPDM network families with various degrees of chemical crosslinking and slightly crystalline below about 350 K have been characterized by thermoviscoelastic and equilibrium swelling measurements. Between 300 and 450 K the thermoviscoelasticity of these elastomers, whether cured or not, is complex. Important differences from previous results on noncrystalline EPDM networks are their dependence on waiting time and temperature history below 350 K, and an equilibrium modulus for the uncured elastomers. These effects increase with rising ethylene content of the EPDM and are related to the crystallinity in the networks. A previously proposed modified time-temperature (t-T) superposition scheme is also applicable to these data, and the resulting frequency shift functions a_T are of the usual WLF type. The vertical shift functions for the relaxational components b_T are different from those for the equilibrium moduli. Experimental data demonstrate that the absolute values of the relaxational components, i.e., G″ (ω) and H(τ), are not changed by small variations in crystallinity. In addition, larger variations in crystallinity do not affect their frequency dependence, only their absolute values. On the contrary, the equilibrium modulus, and therefore all characteristics containing this as a component, is extremely sensitive to small variations in crystallinity, and hence in temperature. The linear viscoelastic characteristics of these slightly crystalline networks and their deviations from thermoviscoelastically simple behavior are discussed as functions of the ethylene content, temperature, and degree of crosslinking of the elastomer. The data provide evidence for three contributing factors, namely chemical crosslinks, crystallinity, and a more disordered type of crystallinity termed microparacrystallinity.     

A critical analysis of the effect of crosslinking on the linear viscoelastic behavior of styrene–butadiene rubber and other elastomers

The linear viscoelastic behavior in dynamic shear and tensile creep at temperatures from −30 to 70 °C is measured for an styrene–butadiene rubber (SBR) elastomer cured with dicumyl peroxide to crosslinking densities between 0 and 23.5 × 10⁻⁵ mol/cm³. The G′, G″, and tan δ isotherms are analyzed by time–temperature superposition (TTS), where the tan δ master curves are consistent with those of Mancke and Ferry. However, to achieve the TTS in the lightly crosslinked SBR systems, an anomalous vertical shift is required in the narrow temperature region from 10 to 30 °C. The vertical shift factor in this temperature region is not the standard from rubber elasticity. No anomalous behavior is detected in the equilibrium modulus, which is a linear function of temperature in accordance with the classical theory of rubber elasticity. In contrast to SBR, standard vertical shifts are required to effect TTS for uncrosslinked polybutadiene and an ethylene propylene diene monomer elastomer. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Viscoelastic properties of nanostructured natural rubber/polystyrene interpenetrating polymer networks

The effects of the blend ratio and initiating system on the viscoelastic properties of nanostructured natural rubber/polystyrene‐based interpenetrating polymer networks (IPNs) were investigated in the temperature range of ?80 to 150 °C. The studies were carried out at different frequencies (100, 50, 10, 1, and 0.1 Hz), and their effects on the damping and storage and loss moduli were analyzed. In all cases, tan δ and the storage and loss moduli showed two distinct transitions corresponding to natural rubber and polystyrene phases, which indicated that the system was not miscible on the molecular level. However, a slight inward shift was observed in the IPNs, with respect to the glass‐transition temperatures (T_g's) of the virgin polymers, showing a certain degree of miscibility or intermixing between the two phases. When the frequency increased from 0.1 to 100 Hz, the T_g values showed a positive shift in all cases. In a comparison of the three initiating systems (dicumyl peroxide, benzoyl peroxide, and azobisisobutyronitrile), the dicumyl peroxide system showed the highest modulus. The morphology of the IPNs was analyzed with transmission electron microscopy. The micrographs indicated that the system was nanostructured. An attempt was made to relate the viscoelastic behavior to the morphology of the IPNs. Various models, such as the series, parallel, Halpin–Tsai, Kerner, Coran, Takayanagi, and Davies models, were used to model the viscoelastic data. The area under the linear loss modulus curve was larger than that obtained by group contribution analysis; this showed that the damping was influenced by the phase morphology, dual‐phase continuity, and crosslinking of the phases. Finally, the homogeneity of the system was further evaluated with Cole–Cole analysis. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1680–1696, 2003     

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).     

Time–temperature superposition of time-dependent birefringence for low-density polyethylene

The time-dependent birefringence has been measured simultaneously with the stress relaxation on quenched and annealed low-density polyethylene at various temperatures from 10 to 70°C. The strain-optical coefficient increases generally with increasing time, and approaches the equilibrium value, which depends upon the temperature. When the strain-optical coefficient at a fixed time is plotted against temperature, it first increases and then decreases after passing through a maximum at T_max with increasing temperature. The higher the degree of crystallinity, the higher are the equilibrium values of the strain-optical coefficient and T_max. The curves for strain-optical coefficient versus time and relaxation modulus versus time below T_max can be superposed well by a horizontal shift along the abscissa. The optical shift factor obeys the original WLF equation, while the mechanical shift factor is much larger than the optical one. The molecular mechanisms corresponding to this dispersion of the strain-optical coefficient and viscoelastic α_c absorption peak near T_max are discussed.     

Viscoelastic properties of crosslinked solutions of poly(β-hydroxyethyl methacrylate) in diethylene glycol. II. Dependence of creep compliance on concentration in the rubbery zone

Creep compliance data, J(t), at 35°C for poly(β-hydroxyethyl monomethacrylate), crosslinked by ethylene glycol dimethacrylate in a range of concentration C from 0.0855 to 2.053 × 10^?4 mole/cm³ and swollen to various degrees in diluents, were examined for time-concentration superposition. From the dependence of time scale shift factors on v₂, the volume fraction of polymer, free volume parameters were calculated for two samples with C = 0.0855 × 10^?4 and 0.136 × 10^?4 mole/cm³, swollen in the range of v₂ from 0.134 to 0.591. Special attention was given to the magnitude of the shift factor on the log J(t) axis and its dependence on concentration, which was found to depend substantially on the crosslinking and the swelling degrees of the samples. This shift was approximately log v₂ for lightly crosslinked samples, swollen to a small degree, measured in the neighborhood of the main transition. For higher degrees of crosslinking and/or swelling, the shift was much less and for the most highly crosslinked networks swollen to equilibrium it was even negative. The correction appears to be very sensitive to the strain of the effective chains and to the location on the time scale with respect to the transition and rubberlike zones of viscoelasic behavior. It was found that the parameters of the WLF equation calculated in our previous study from the time-temperature superposition of the creep curves in the rubber-glass transition are valid also for the rubberlike region.     

Effect of the reference chain dimension on the viscoelastic and stress–strain behavior of poly(n-butyl methacrylate) networks

The linear viscoelastic and stress-strain behavior of poly(n-butyl methacrylate) networks at a content of crosslinking agent (ethyleneglycol dimethacrylate) of c? 0–1 × 10^?4 mole/cm³ was investigated in the main transition and rubberlike region in the temperature interval from 20 to 150°C. The dependence of the unperturbed chain dimensions on temperature was determined from thermoelastic measurements in the rubberlike region; this dependence was unaffected by the content of crosslinking agent. Application of time–temperature superposition to the linear viscoelastic behavior did not give a continuous superimposed curve in the proximity of the rubberlike region; superposition within the whole time region required introducing the change of the unperturbed chain dimensions with temperature. This correction was sufficient for a sample with a higher content of the crosslinking agent. However, for loose networks (c< 0.1 × 10^?4 mole/cm³) it was insufficient, because of another relaxation mechanism in the region of high temperatures. It was found that the intensity and temperature dependence of this relaxation mechanism, which is probably due to a change of the number of entanglements with temperature, are connected with the magnitude and the temperature dependence of the C₂ constant of the Mooney-Rivlin equation.     

Effect of the crosslinking density and the method of sample preparation on the observed microstructure of macroporous and conventional poly(N,N-dimethylacrylamide) hydrogels

SEM micrographs of macroporous and conventional poly(N,N-dimethylacrylamide) hydrogels were obtained for specimens synthesized in different conditions and prepared for microscopy by different methods (freeze drying of different solvents and critical point drying). The crosslinking density of both types of samples was determined through T _g measurements. Open structures (honeycomb-like, fibrillar networks) were more frequently observed in specimens prepared by freeze drying of benzene, which was attributed to its large pressure and temperature at the triple point. In spite of the different structure in the millimeter scale, there is no significant difference in the mesh size of fibrillar networks observed for macroporous and conventional samples, and in both cases it decreases with increasing crosslinking density. Other effects of the crosslinking density are that only incomplete honeycomb-like structures were formed in low-crosslinking samples and that collapsed structures were developed by phase separation throughout polymerization in highly crosslinked samples. Fibrillar networks of 1-μm mesh size were observed for the uncrosslinked polymer.     

Mechanical properties of composite heterogeneous gels

Composites with a matrix of poly(2-hydroxyethyl methacrylate) (PHEMA) and 10% by volume of various crosslinked PHEMA polymer fillers (prepared by copolymerization with 0.1, 0.4, 1.0, and 20.0% by weight of ethylenedimethacrylate) of particle size about 1 μm were prepared. Some polymer matrixes were prepared from soluble branched PHEMA (Hydron S), and others by copolymerization, in the presence of the filler with 0.4 and 1.0% of ethylenedimethacrylate as a crosslinking agent. In the case of the uncrosslinked matrix, a linear polymer–crosslinked polymer system, resulted; in the case of the crosslinked matrix, a composite heterogeneous network was formed (in the latter case, the particles of the filler were swollen with monomer during the crosslinking polymerization). Stress–strain, equilibrium, and ultimate characteristics were measured at 3, 10, 25, 40, 60, and 80°C on samples swollen to equilibrium in water (T_g ≈ ?50°C) and at 80, 110, and 140°C on dry samples (T_g ≈ 100°C). Depending on experimental conditions, above all on the distance from the main transition region and on whether the polymer is dry or swollen, it was found that the measured hydrophilic composite systems behaves as a filled system (with the polymer filler acting mostly as solid particles, irrespective of the crosslink density) or as a system with crosslink density fluctuations (where both networks, the matrix and the filler, contribute roughly additively to the properties of the system), or finally as defect heterogeneous systems (where the properties depend primarily on the character of the polymer–filler interface).     

Quantitative nanoscale measurements of the thermomechanical properties of poly-ether-ether-ketone (PEEK)

Understanding the internal structure and organization of semicrystalline polymers, especially at the nanoscale, has many challenges for researchers to date. In this article, we demonstrate a quantitative method for investigating the local viscoelastic properties (i.e., storage and loss moduli, as well as loss tangent) of semicrystalline polyether-ether ketone (PEEK) through the combination of contact resonance atomic force microscopy (CR-AFM) and in situ local heating with a thermal probe. Furthermore, the local viscoelastic properties of the crystalline and amorphous phases were decoupled by performing thermal CR-AFM array mapping near the glass transition temperature of PEEK (T_g, 143 °C). A distinct bimodal distribution of tip-sample interaction was observed for PEEK near its T_g, providing a means to estimate the T_g and the degree of crystallinity of PEEK.     

Rheo-optical studies of high polymers. XVIII. Significance of the vertical shift in the time-temperature superposition of rheo-optical and viscoelastic properties

To determine the true reason for the increase in birefringence and the decrease in relaxation modulus for high-density polyethylene with rising temperature, changes in crystalline structure as well as in thermal, viscoelastic, and rheo-optical properties with temperature were measured, by several techniques, including DSC, DLI, infrared dichroism, x-ray diffraction, and NMR. The values for degree of crystallinity obtained from the DSC fusion curve, density, and infrared absorbances coincide very well and show almost no divergence till about 80°C. The optical vertical shift factor pT can be related to the ratio of the orientation function for the crystal c axis at an arbitrary temperature to that at the references temperature, f_ε/f_ε0. The mechanical vertical shift factor b_T, on the other hand, is associated with the temperature dependence of the mobile fraction, as determined by NMR measurements, but not with variations in degree of crystallinity.     

Formation,structure, and elasticity of loosely crosslinked epoxy-amine networks. II. Mechanical and optical properties

The mechanical and optical behavior in the dry and swollen states of loosely crosslinked epoxy networks prepared from the diglycidyl ether of bisphenol A, phenylglycidyl ether, and 4,4′-diaminodiphenylmethane was investigated, and the weight fraction of sol in the networks was determined. The crosslinking density was controlled by an excess of diamine and by the fraction of monoepoxide. The reaction proceeded to almost full conversion of epoxy groups. With increasing content of monoepoxide or with increasing excess of diamine, the main transition region is shifted to lower temperatures. The dependence of the viscoelastic modulus on temperature and the optical behavior indicate that the networks are homogeneous. In all cases, the sol fraction is adequately described by the theory of branching processes (cf. Part I). The equilibrium modulus related to the dry state is the same irrespective of whether it is obtained by measurements in the dry or swollen state. The mechanical behavior in the rubbery state can be described by the theory of phantom networks with fully suppressed fluctuations of crosslinking (front factor A = 1) or by the theory of phantom networks with fully released fluctuations of crosslinks (front factor) A = f_e?2/f_e] and contribution of trapped entanglements of the Langley-Graessley type (cf. Part I). In the analysis of the equilibrium behavior, it is advantegeous to use the plot of superimposed dependences of G_e on the gel fraction, which considerably reduces the effect of experimental inaccuracy in determination of composition and degree of conversion.     

The photoelastic behaviour of dry and swollen networks of poly (N,N-diethylacrylamide) and of its copolymer with N-tert.butylacrylamide

The viscoelastic and equilibrium behaviour of dry and water-swollen networks of poly(N,N-diethylacrylamide) and of its copolymer with N-tert.butylacrylamide with a small amount of methylenebis-acrylamide as the cross-linking agent was investigated. Superimposed curves of the mechanical and optical functions of dry networks have a form typical of amorphous polymers with side-chains: the mechanical and optical shift factors obtained by using the time-temperature superposition exhibit the same temperature dependence. Increasing temperature pronouncedly reduces the degree of swelling of gels in water but in the range 275–325 K the gels are in the rubberlike state, because the rise in temperature compensates for deswelling. The equilibrium mechanical behaviour corresponds to that predicted for Gaussian networks, while the optical behaviour is more complicated. The observed fast increase in the negative stress-optical coefficients (C_e) with swelling and the network density may be qualitatively explained by assuming correlations between the side chains. Cross-links and swelling interfere with these correlations, while the introduction of the tert. butyl group into the side-chain stabilizes them, owing to the steric hindrances to the motion of ethyl groups of diethylacrylamide units.     

Rheology of polystyrene/poly(vinyl methyl ether)blends near the phase transition

Mixtures are expected to show anomalous behavior in their viscoelastic properties close to a critical point. In this study, the reheological behavior of blends of polystyrene and poly (vinyl methyl ether) below, close to, and above the phase separation temperature T_s was investigated. Rheological measurements were carried out at three different compositions in the melt. Below and far from T_s, a satisfactory superposition of the storage and loss moduli G' and G″ was observed at all temperatures and frequencies. Close to T_s deviations were observed for G' at low frequencies (the so-called terminal zone). Above T_s G″ values was still observed over the whole range of frequencies and temperatures. The deviations observed for G' near T_s can be interpreted as due to the presence of significant concentration fluctuations. Plots of log (G'/G″²) as a function of temperature were shown to be sensitive to this anomalous behavior.     

Radiation effects on SBR–EPDM blends: A correlation with blend morphology

The effect of γ radiation on the morphological and physical properties of Styrene–butadiene rubber (SBR) and Ethylene–propylene–diene monomer (EPDM) blends has been investigated. An attempt has been made to establish a correlation between various parameters like Gordon–Taylor parameter (k), hydrodynamic interaction parameter (Δ[η]_mix), chemical shift factor (b), Charlesby–Pinner parameter (p₀/q₀) and polymer–polymer interaction parameter (χ). The results showed a close dependence of mechanical and physical properties of irradiated blends on these parameters. The probability of spur overlap has been found to increase with the increase in EPDM content in the blends, which in turn results in significant improvement in the mechanical properties of the irradiated SBR–EPDM blends with higher EPDM fraction. The efficiency of four multifunctional acrylates as crosslinking aid for the radiation‐induced vulcanization of SBR–EPDM blend was also studied. The results established lower efficiency of methacrylates over acrylates in the process and indicated that among the crosslinking agents studied trimethylolpropane triacrylate is the most efficient one. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1676–1689, 2006     

Bulk and shear rheology of a symmetric three‐arm star polystyrene

The bulk and shear rheological properties of a symmetric three‐arm star polystyrene were measured using a self‐built pressurizable dilatometer and a commercial rheometer, respectively. The bulk properties investigated include the pressure–volume–temperature behavior, the pressure‐dependent glass transition temperature (T_g), and the viscoelastic bulk modulus and Poisson's ratio. Comparison with data for a linear polystyrene indicates that the star behaves similarly but with slightly higher T_gs at elevated pressures and slightly higher limiting bulk moduli in glass and rubbery states. The Poisson's ratio shows a minimum at short times similar to what is observed for the linear chain. The horizontal shift factors above T_g obtained from reducing the bulk and shear viscoelastic responses are found to have similar temperature dependence when plotted using T ? T_g scaling; in addition, the shift factors also exhibit a similar temperature dependence to linear polystyrene. The retardation spectra for the bulk and shear responses are compared and show that the long time molecular mechanisms available to the shear response are unavailable to the bulk. At short times, the two spectra have similar slopes, but the short‐time retardation spectrum for the shear response is significantly higher than that for the bulk, a finding that is, as yet, unexplained. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

A relationship between network chain concentration and time dependence of rupture for SBR vulcanizates

The failure properties of crosslinked elastomers are influenced by variables such as the test rate, test temperature, chemical nature of the elastomers, degree of crosslinking, etc. For an elastomer with a fixed degree of crosslinkingthe time dependence of both the stress- and strain-at-break, when determined as a function of the test rate and test speed, can be correlated using a time–temperature shift such as the WLF a_T shift factor. When a similar study is carried out with a sample having a different degree of crosslinking, the time dependence of both the stress- and strain-at-break is shifted along the time scale. It is demonstrated here that the time dependence of failure obtained as a function of the degree of crosslinking can be superposed on a master curve by the use of a time shift factor whose value depends directly on the degree of crosslinking.     

Pressure–volume–temperature behavior of two polycyanurate networks

The pressure–volume–temperature (PVT) behavior was studied for two polycyanurate networks having different crosslink densities using a pressurizable dilatometer. The samples were studied at temperatures ranging from 60 to 180 °C and at pressures up to 170 MPa to yield PVT data in both rubbery and glassy states. The Tait equation is found to well describe the isobaric temperature scan and isothermal pressure scan data. The thermal expansion coefficients, instantaneous bulk moduli, and thermal pressure coefficients are extracted from the data and their dependence on crosslink density is examined. The time‐dependent viscoelastic bulk modulus (K(t)) is also calculated in the vicinity of the α‐relaxation from previously published pressure relaxation experimental data, and the strength and shape of the dispersion are found to be independent of crosslink density. The limiting bulk moduli depend strongly on temperature with those of the more loosely crosslinked sample being lower at a given temperature and pressure, although at T_g(P), the limiting moduli of the more loosely crosslinked sample are slightly higher than those of the more highly crosslinked sample. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Thermoplastic elastomers by hydrogen bonding 4. Influence of hydrogen bonding on the temperature dependence of the viscoelastic properties

The temperature dependence of the viscoelastic properties of thermoreversible polybutadiene networks based on hydrogen bond linkages is analyzed from the logarithmic shift factors loga _T . For binary hydrogen bond complexes thermorheologically simple behavior is observed. The temperature dependence of loga _T is described by the Williams-Landel-Ferry (WLF) equation. The thermoreversible linkages cause an increase in the apparent activation enthalpy of flow which is related to the number of complexing sites in the polymer. Thermorheologically complex behavior is observed in a system with more complex association.     

Viscoelastic properties of crosslinked solutions of poly(2-hydroxymethyl methacrylate) in diethylene glycol. VI. Comparison with poly(butyl methacrylate) and poly(butyl acrylate) networks crosslinked to very low degrees

On the basis of our own experimental and some literature data, the contributions of slow relaxation mechanisms to the shear modulus, (G_eN — G_e), and the parameter C₂ of the Mooney-Rivlin equation have been examined for lightly crosslinked poly(butyl methacrylate), poly(butyl acrylate), poly(2-hydroxyethyl methacrylate), and some rubber networks. For the rubbers, increasing degree of crosslinking caused a decrease in G_eN — G_e and an increase in C₂; for the other networks, both G_eN — G_e and C₂ diminished with increasing crosslinking. The effectiveness of the crosslinking polymerization, and also the absolute values of the physical crosslinking degree, decreased in the order of poly(2-hydroxyethyl methacrylate), poly(butyl methacrylate), and poly(butyl acrylate). The values of the equilibrium compliances J of the networks studied, obtained by various methods, have also been compared, and good agreement has been found.