The sulfur reversion process in natural rubber in terms of crosslink density and crosslink density distribution

Unfilled natural rubber compounds composed of conventional (CV), semi-efficient (SEV), efficient (EV) and sulfur donor (SD) vulcanization systems were heat aged to promote sulfur reversion. Rheometry, hardness, strain-strain characteristics including Mooney-Rivlin analysis, equilibrium solvent swell and Double Quantum (DQ) Nuclear Magnetic Resonance (NMR) were used to monitor crosslink density changes. A loss of crosslink density was observed by rheometry, C₁, equilibrium swelling and by DQ NMR as a function of cure extent. No chain scission reactions were operating in the time/temperature conditions used. All crosslink distributions were unimodal and the network homogeneity followed the order of EV > SD > SEV > CV. The crosslink distribution narrowed during the curing process for the CV and SEV systems. Non-oxidative maturation reactions were advantageous in promoting a more random distribution of crosslinks in the polymer matrix.     

Effect of crosslink density on the pressure relaxation response of polycyanurate networks

The effect of crosslink density on the pressure-volume-temperature (PVT) behavior and on the pressure relaxation response for two polycyanurate networks is investigated using a custom-built pressurizable dilatometer. Isobaric cooling measurements were made to obtain the pressure-dependent glass transition temperature (T_g). The pressure relaxation studies were carried out as a function of time after volume jumps at temperatures in the vicinity of the pressure-dependent T_g, and the pressure relaxation curves obtained were shifted to construct master curves by time-temperature superposition. The reduced pressure relaxation curves are found to be identical in shape and placement, independent of crosslink density, when T_g is used as the reference temperature. The horizontal shift factors used to create the master curves are plotted as a function of the temperature departure from T_g (T − T_g), and they agree well with their counterparts obtained from the shear response. Moreover, the retardation spectra are derived from bulk compliance and compared to those from the shear. The results, similar to our previous work on polystyrene, indicate that at short times, the bulk and shear responses have similar underlying molecular mechanisms; however, the long-time mechanisms available to the shear response, which increase with decreasing crosslink density, are unavailable to the bulk response. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2477–2486, 2009     

Influence of crosslink density and chain flexibility on mechanical properties of model epoxy networks

Fully cured epoxy networks having different architectures were prepared by introducing either a monoamine acting as a chain extender or an aliphatic epoxy monomer. As expected an increase in the amount of the difunctional amine leads to a decrease in the crosslink density whereas the introduction of an aliphatic epoxy comonomer leads to more flexible chains between crosslinks. Values of the Young modulus and the dependence of the yield stress as a function of temperature were related to the occurrence and amplitude of secondary relaxations. The fracture toughness at room temperature was clearly associated with the glass transition temperature as a consequence of plastic deformation, rather than with chain scission.     

Fixed crosslink formation and viscoelasticity of polystyrene networks

The crosslinking formation in the 8/92 mol % random copolymer of ethynyl- styrene and styrene is clarified quantitatively from the IR spectrum analysis on the basis of the thermally homogeneous reaction of ethynylstyrenes between the chains. The viscoelastic properties of the crosslinked polystyrene, CL-P(ESt/St), are investigated based on the temperature and frequency dependencies of the dynamic Young modulus. The relaxation spectrum of CL-P(ESt/St) drops deeply like the Wedge type with a slope of −½ in the glass transition region, and became a plateau without a terminal zone in the rubbery elasticity region. The effects of the ethynyl crosslinked junctions for the micro-Brownian motions of the segments in the strand are discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3319–3327, 1999     

The effects of crosslink density and crosslink type on the tensile and tear strengths of NR,SBR and EPDM gum vulcanizates

The effects of crosslink density and crosslink type on the tensile and tear strengths of gum NR, SBR and EPDM vulcanizates have been studied. Over a wide range of crosslink densities, sulphur vulcanizates for these rubbers have higher strengths than the peroxide vulcanizates. These results show that crystallizability of the rubbers is not an important factor in producing separate curves in the strength vs crosslink density plots. Tear strengths appear to be more sensitive to crosslink structures than tensile strengths. A composite plot shows that tensile strengths are approximately proportional to tear strengths for all three rubbers.     

Birefringence of semicrystalline polymeric networks

A simple theoretical development is proposed to explain the variation of birefringence with degree of crystallization for crosslinked, stretched polymer networks. The model adopted for the calculations is one proposed by Flory in order to calculate the thermodynamic properties of such systems and hence is restricted to the special case of network orientation prior to the onset of crystallization. Our findings are briefly discussed and possible applications of the results are considered.     

Pyrolysis-mass spectrometry correlations of crosslink density in natural rubber vulcanizates

A set of twelve natural rubber vulcanizates was analyzed by Curie-point pyrolysis-mass spectrometry (Py-MS). Multivariate analysis successfully correlated the Py-MS data with either total or polysulfidic crosslink density measurements. (Polysulfidic means two or more sulfurs.) The principal correlation observed was an increase in the ratio of monomer to dimer pyrolyzate yield with an increase in crosslink density. The feasibility of using Py-MS for total crosslink density determination has thus been demonstrated. The Py-MS data, however, did not allow us to distinguish different types of crosslinks.     

Inorganic glasses as crosslinked polymers: The relationship between crosslink density and transformation temperature in phosphate glasses

Most substances that form stable glasses are polymers, and inorganic oxide glasses are no exception. Because most oxide glasses are highly crosslinked polymers it is not possible to measure properties in solution, and parameters such as molecular weight and molecular weight distribution which are used to characterize organic polymers are meaningless in the context of oxide glasses. Instead, the network connectivity or a measure of crosslink density must be used to characterize their structure. In silicate glasses, the network connectivity can be changed significantly only by altering the composition, but phosphate glasses of fixed composition can be prepared with variable hydroxyl contents and consequently different crosslink densities. In this paper it is shown that the relationship between the transformation temperature T(x) and crosslink density x of phosphate glasses is similar to that observed for organic polymers and can be described by an equation of the form where T(0) is the transformation temperature of the corresponding linear polymer and K is a constant which is approximately the same for three different phosphate glasses that have been studied.     

Effect of crosslink density on the properties of monolithic polymer matrices: Reversed-phase gas-chromatography study

Monolithic polymer matrices of different natures and different degrees of crosslinking have been synthesized in capillaries with an inner diameter of 100 μm. The properties of the monolithic matrices are characterized by reversed-phase gas chromatography. Solubility coefficient S, Flory-Huggins parameter gC₁₂^∞, and reduced Flory-Huggins parameter gC₁₂^∞ are evaluated. For all tested sorbates, the values of S depend on the degree of crosslinking of the polymer, which is characterized by parameter gC₁₂^∞. In the case of all monolithic polymer matrices under study, the logarithm of the solubility coefficient plotted as a function of the squared critical temperature of sorbate is described by a straight line, a circumstance that is likewise typical of linear polymers. Parameter D/d _f ², which characterizes the rate of diffusion of low-molecular-mass compounds in the monolithic matrix, is calculated. For both polar and nonpolar polymers, the dependence of D/d _f ² on the degree of crosslinking follows an extremum pattern.     

Effects of crosslink density on the behavior of VHB 4910 dielectric elastomer

Interpenetration of dielectric elastomers with fillers is an effective technique to improve requisite properties for voltage induced electromechanical performance. However, the requirement that the rigid frame has to maintain its pre-strained state, and time/temperature related phenomenon like stress relaxation/concentration, strain-induced crystallization and void propagation etc., limits this technology. To overcome these limitations, thermochemical and UV irradiation techniques of polymerization are used to modify the macromolecular structures of VHB 4910 elastomer with crosslink monomers. Both the methods are compared in terms of the modification in surface and molecular structure through optical microscope and infrared spectroscopy, respectively. Thermochemically modified samples are found flawless with regular surface reformation; crosslink density is evaluated and its influences on hysteresis, dielectric constant and material degradation at elevated temperature are characterized. Results from this study may contribute towards the complete understanding of space density for inclusion of filler, used for material development with user-defined properties.     

Effect of crosslink density on the creep behavior of natural rubber vulcanizates

Torsional creep measurements on four natural rubber vulcanizates, crosslinked to different degrees, were carried out in the temperature range from ?50 to 90°C. This investigation complements the studies on identical samples of the stress relaxation behavior by Chasset and Thirion and of the dynamic mechanical response by Ferry, Mancke, Maekawa, ōyanagi, and Dickie. The creep measurements reported are shown to be in agreement with the stress relaxation results. In addition to the usual temperature reduction, a superposed curve was obtained for the long time response using the apparent molecular weight between crosslinks, M_c, as a reduction variable. The variation in viscoelastic response with crosslink density is interpreted as a restrictive action of the chemical crosslinks on the transient entanglement network.     

Colouration efficiency measurements in electrochromic polymers: The importance of charge density

A poly(3,4-ethylenedioxythiopene) (PEDOT)/polyaniline (PANI) electrochromic device (ECD) was fabricated for the purpose of examining colouration efficiency (CE) as a function of charge density ingress/egress. CE is typically measured in cm²/C with the aim being to produce as large an optical density change as possible with the least amount of charge being consumed. Results indicate that CE is not constant but is highly dependent on charge density insertion and the switching voltage. At a switching voltage of 1.9 V the maximum CE_max was 1186 cm²/C, recorded at 60% of the full optical switch where as the CE_95% was 302 cm²/C at 95% of the full optical switch. Furthermore, CE_max varied depending on the switching voltage from a high of 2212 cm²/C at a switching voltage of 1.2 V, 1528 cm²/C at 1.6 V and down to 1186 cm²/C at 1.9 V. The results highlight the fact that the current practice of quoting CE as a single-valued number may not reveal enough detail about the performance of ECDs and polymers.     

Relation of elastic modulus to crosslink and entanglement concentrations in rubber networks

The theory of rubber elasticity relates the elastic modulus of unfilled amorphous rubber to the concentration of elastically effective strands. A theoretical relation between this concentration and the concentrations of potential entanglements, random tetrafunctional crosslinks, and chain ends was proposed recently. In the present work, the new relation was combined with the theory of rubber elasticity and verified experimentally. Polydimethylsiloxane samples were cured by ⁶⁰Co irradiation and were extensively extracted to determine gel fraction, which was used to calculate concentrations of crosslinking and scission due to irradiation. Equilibrium modulus values determined from creep tests were in excellent agreement with those calculated using the new relation if the average spacing between potential entanglements is 116 (CH₃)₂SiO units. Thus, in typical commercial silicone rubbers, the contribution to the modulus from trapped entanglements is greater than the direct contribution from crosslinks. The new relation allows the calculation of crosslink concentrations from modulus measurements on other unfilled rubbers once the potential entanglement spacing of the polymer is determined.     

Physical & thermal properties of model polysiloxane rubbers: impact of crosslink density and tin concentration

The influence of the tin octanoate catalyst on the physical and thermal properties of RTV 5370 polysiloxane rubbers has been studied. To assess the likely influence of crosslinking on a number of physical and thermal properties in polysiloxanes, “model siloxane networks” (representing networks of well defined composition/structure) have been formulated by the hydrosilylation of polysiloxane diols of known average molecular weight with tetraethoxysilane curing agent. It was found that linear swell and the crystallisation melting transitions of these systems were both significantly affected by changes in crosslink density. A selection of RTV5370 foamed rubbers with different tin concentrations were prepared in a similar manner to assess the influence of the tin catalyst. For these materials it was found that the area of the crystalline melting transition decreased with increasing tin concentration, an effect indicative of increased crosslinking. Samples with double the standard amount of tin (10% wt catalyst) show a shift in the crystallisation transition to higher temperature with no further effect beyond this concentration. Furthermore, the storage modulus (E') of the rubbers at room temperature appears to be independent of the tin catalyst concentration. Through correlation with the data from the model siloxanes, these results suggests that increasing the catalyst concentration appears to induce additional crosslinking interactions that are stable at low temperatures but not stable at room temperature.     

Continuous and gradual photo-activation methods: influence on degree of conversion and crosslink density of composite resins

Thermal properties and degree of conversion (DC%) of two composite resins (microhybrid and nanocomposite) and two photo-activation methods (continuous and gradual) displayed by the light-emitting diode (LED) light-curing units (LCUs) were investigated in this study. Differential scanning calorimetry (DSC) thermal analysis technique was used to investigate the glass transition temperature (T _g) and degradation temperature. The DC% was determined by Fourier transform infrared spectroscopy (FT-IR). The results showed that the microhybrid composite resin presented the highest T _g and degradation temperature values, i.e., the best thermal stability. Gradual photo-activation methods showed higher or similar T _g and degradation temperature values when compared to continuous method. The Elipar Freelight 2^TM LCU showed the lowest T _g values. With respect to the DC%, the photo-activation method did not influence the final conversion of composite resins. However, Elipar Freelight 2^TM LCU and microhybrid resin showed the lowest DC% values. Thus, the presented results suggest that gradual method photo-activation with LED LCUs provides adequate degree of conversion without promoting changes in the polymer chain of composite resins. However, the thermal properties and final conversion of composite resins can be influenced by the kind of composite resin and LCU.     

The importance of middle-range Hartree-Fock-type exchange for hybrid density functionals

Hybrid functionals are responsible for much of the utility of modern Kohn-Sham density functional theory. When rigorously applied to solid-state metallic and small band gap systems, however, the slow decay of their nonlocal Hartree-Fock-type exchange makes hybrids computationally challenging and introduces unphysical effects. This can be remedied by using a range-separated hybrid which only keeps short-range nonlocal exchange, as in the functional of Heyd et al. [J. Chem. Phys. 118, 8207 (2003)]. On the other hand, many molecular properties require full long-range nonlocal exchange, which can also be included by means of a range-separated hybrid such as the recently introduced LC-omegaPBE functional [O. A. Vydrov and G. E. Scuseria, J. Chem. Phys. 125, 234109 (2006)]. In this paper, we show that a three-range hybrid which mainly includes middle-range Hartree-Fock-type exchange and neglects long- and short-range Hartree-Fock-type exchange yields excellent accuracy for thermochemistry, barrier heights, and band gaps, emphasizing that the middle-range part of the 1/r potential seems crucial to accurately model these properties.     

The importance of electronic exchange-correlation in non-self-consistent frozen density approximation

The effects of different treatments for the exchange-correlation energy on the accuracy of non-self-consistent frozen density approximation (FDA) are discussed. Local spin density approximation (LSDA) and non-local spin density approximation (NLSDA) are employed, respectively. Corresponding results obtained by using full-self-consistent density functional theory (DFT) are also given for the purpose of comparison. Explicit calculations for hydrogen bonds, covalent bonds and ionic bonds indicate that, comparing with LSDA, NLSDA can improve the accuracy of FDA as well as that of DFT. This improvement attributed to the refinements in the treatment for the electronic exchange-correlation energy may help to extend the application of FDA.     

Solvent effects on the kinetics of gelation and the crosslink density of polysiloxane gels

A wide range of hydrocarbons were rapidly gelled by adding a polysiloxane copolymer in the presence of divinylbenzene and a platinum catalyst. The gel point was measured over a range of concentrations for hydrocarbons/solvents and organogels, using three separate methods: rheology, visual (tilt-tube) and FTIR. As the fraction of solvent was increased, the rate of reaction decreased, leading to an increase in the gelation time. The absolute value of the gel point depends upon the techniques used to measure it. For any particular system the gel point values always followed the order: rheology > visual > FTIR. The crosslink densities of the gel systems were determined using both swelling and thermomechanical analysis. The swelling measurements confirmed that the addition of large quantities of solvent markedly reduced the crosslink density of the obtained chemical gel networks, which helped in designing gels with suitable critical strength for effective field work. Also the effectiveness of gelation with the proposed gelling system for different hydrocarbons/solvents was evaluated, and discussed in relation to their dielectric properties.This paper is dedicated to Mike Owen on occasion of his winning the DeBruyn medal, the first silicon chemist to do so.     

The effect of natural rubber crosslink density on real time birefringence,true stress and true strain behavior

A newly developed real time spectral birefringence technique was implemented to follow the coupled relationships between birefringence, true stress and true strain behavior of varying crosslink density natural rubber vulcanizates at room temperature. It was shown that the stress optical law is valid even at early stages of crystallization during stretching and there exists a critical birefringence beyond which the metastable “near perfect” oriented and highly distorted crystalline regions form and upon retraction this crystalline order disappears at the same critical birefringence level. These crystalline regions exhibit nematic order with significant axial registry distortions at early stages of the formation. They are suggested to form at or near the juncture points of the chemical network formed by crosslinks, and physical network by chain entanglements where the orientability of the chains is the most efficient.