Effect of crosslink density on some properties of electron beam-irradiated styrene–butadiene rubber

Crosslink densities of electron beam (EB)-irradiated styrene–butadiene rubber (SBR) samples were measured by using a novel magnetic resonance crosslink density spectrometer (MRCDS). With 1,1,1-trimethylolpropane triacrylate (TMPTA) loading increasing, the crosslink density of EB-irradiated SBR increases up to a certain level, and then decreases in the irradiation dose range 50–200 kGy. Tensile strength, elongation at break, thermal stability and pyrolysis products of the EB-irradiated SBR samples with different crosslink densities were also studied in this paper.     

Effect of high-temperature curing on the crosslink structures and dynamic mechanical properties of gum and N330-filled natural rubber vulcanizates

The effects of high-temperature curing and overcuring on the cure characteristics, crosslink structure, physical properties and dynamic mechanical properties (DMPs) of gum and carbon black (N330) filled natural rubber (NR) vulcanizates cured with conventional (CV), semi-efficient (SEV) and efficient (EV) cure systems, which have about the same total crosslink densities under a moderate curing temperature of 150°C, were investigated. The gum NR vulcanizates cured with CV, SEV and EV curing systems have about the same glass transition temperature (T_g) and tan δ values below the temperature of about 0°C, but showed some apparent differences in the tan δ values increasing in the order CVG′ and tan δ values above T_g higher than those of the gum NR vulcanizates.

High-temperature curing and overcuring cause decreases to various extents in the cure plateau torque, Shore A hardness, 300% modulus and tensile strength, and lead to apparent changes in the DMPs. Typically, there is an increase in T_g of all three kinds of gum and N330-filled NR vulcanizates because of changes in the total crosslink densities and crosslink types. The CV vulcanizates show the most significant change in cure characteristics, physical properties and DMPs since the highest content of polysulfidic crosslinks appears in the CV vulcanizate, causing the highest level of reversion and having a dominant effect on the properties.     

The relationship between crosslink system, network structure and material properties of carbon black reinforced EPDM

The crosslink density and sulfur-ranks of crosslinks formed during vulcanization of a carbon black reinforced ENB–EPDM compound are analyzed as a function of the selected curing system: Conventional, Semi-Efficient, Efficient and Nitrosamine-safe. Each vulcanization system results in a specific crosslink concentration and sulfur-rank distribution: mono-, di- and polysulfidic of nature. Tensile properties, tear strength and compression set of the vulcanized materials turn out to practically only depend on overall crosslink density, as resulting from the particular curing systems and vulcanization times. All trends in properties coincide when plotted as a function of the overall crosslink density. Surprisingly, the crosslink distribution: the ratios of mono- to di- and polysulfidic crosslinks, has only a minor effect on these properties. The differences in sulfur-rank as a function of the chosen vulcanization system turn out to be too small for EPDM to have a significant effect.     

Crosslink network evolution of BIIR/EPDM blends during peroxide vulcanization

Crosslink network evolution of brominated butyl rubber (BIIR)/ethylene–propylene–diene-monomer rubber (EPDM) blends during peroxide vulcanization is studied at a meso-scale level. In this work, EPDM is added as a co-agent to increase the crosslink density of BIIR vulcanization. With increasing EPDM content from 0 to 20 phr, the maximum torque of BIIR/EPDM compounds during vulcanization increases by 73%, reaching to 3.40 dNm. Vulcanization kinetic study shows that addition of EPDM favors to the crosslinking of BIIR compound. Meanwhile, the addition of 20 phr EPDM contributes to an increase in the crosslink density of BIIR/EPDM(80/20) vulcanizate, avoiding downward trend at post-cure period in comparison with BIIR only. Crosslink network evolution of BIIR/EPDM blends is divided into three periods during peroxide vulcanization at 150 °C. The role of EPDM in the crosslink network evolution is studied by proton nuclear magnetic resonance, and a “network patching” mechanism is proposed in which EPDM is implied to work as patch on damaged crosslink network resulted from the degradation nature of BIIR.     

Studies of conventional sulfur vulcanization of SBR rubber: Analysing the reaction products from thermal degradation of the accelerator by means of MCC-IMS technique

A combination of knowledge on curing process of rubber mixes with novel methods of chemical analysis, a new unconventional approach to analysis of rubber vulcanization is presented in this study. Six SBR samples containing various N-tert-butyl-2-benzothiazole sulfenamide (TBBS) accelerator: sulfur ratios (within) the range of conventional (CV) sulfur vulcanization system were studied using multi-capillary column ion mobility spectrometry (MCC-IMS) technique. For these analysis, calibration/dilution curves were established. Moreover, data from MCC-IMS were correlated with other parameters of the rubber vulcanizates – their crosslink density and structure as well as their tensile strength and modulus at elongation. For such comparison, one of the reaction products from thermal decomposition of TBBS, benzothiazole was selected. Furthermore, the concentration of benzothiazole released during the vulcanization process corresponded well with the crosslink density of the rubber vulcanizates studied. It was even possible to calculate the crosslink density from the concentration of benzothiazole determined by MCC-IMS, using Boltzmann fitting curve. The presented results could be an important contribution in understanding the mechanisms occurring during rubber vulcanization, demonstrating a new approach to testing and evaluation of the process.     

Diffusion in polymers dependence on crosslink density

Kinetics of N-methyl pyrrolidone evaporation from swollen photo-crosslinked polyacrylate was monitored thermogravimetrically at temperatures ranging from 323 to 398 K. Crosslink density dependence of evaporation kinetics was investigated in photo-crosslinked polyacrylates with crosslinked density ranging from ≈1.2 × 10² to ≈1.7 × 10⁴ mol m⁻³ and number of main chain atoms between crosslinks ranging from ≈70 atoms to ≈6 atoms, respectively. As was shown, evaporation kinetics was controlled by the solvent diffusion in polymer. Activation energies of evaporation (diffusion) were deduced from the rate measurements at different temperatures. Apparent activation energy of evaporation decreased from 48.7 to 31.1 kJ mol⁻¹ with crosslink density increase. Activation energy of pure N-methyl pyrrolidone evaporation was 50.6 kJ mol⁻¹. Decrease of the rate of solvent diffusion and unexpected decrease of diffusion activation energy with increase of crosslink density of swollen polymer matrix was explained by decrease in polymer chain segments mobility, as indicated by Eyring’s approach to diffusion in polymers.     

Photo-crosslinked acrylates degradation kinetics

A series of crosslinked polyurethane acrylate solids with glass transition temperatures ranging from –49 to +65 °C was prepared by photopolymerization of specially formulated solvent-free resins. The kinetics of thermooxidative and thermal (in N₂) degradation of these crosslinked acrylate networks at temperatures ranging from 100 to 400 °C was studied as a function of crosslink density using thermogravimetry. The polyacrylate network degradation rate decreased with the increase of crosslink density, while apparent activation energy of degradation increased. Polyacrylate thermal stability increase with crosslinking was explained by decreased rate of oxygen and volatile products diffusion and/or slowing of depolymerization due to increased radical recombination rate, and decreased chain segments mobility in systems with higher crosslink density.     

Effect of network heterogeneity on tensile and tear strengths of radiation,peroxide, efficient and conventional cured polyisoprene

At any given crosslink density, the ultimate tensile and tear strengths of vulcanizates depend on the curing system and decrease in the order: conventional (CV) > efficient (EV) > peroxide > radiation cured. Measurements on three network parameters were interpreted as indicating that crosslinks were not uniformly distributed throughout the networks and that network heterogeneity decreased in parallel with a decrease in physical properties. (1) Plots of the freezing point of the gel-solvent in a network, swollen to equilibrium, versus the volume fraction of rubber in the swollen gel, showed a straight line relationship for radiation cured networks and an increasingly large deviation from linearity on passing from radiation cured networks through peroxide and EV to CV cured networks. (2) The glass transition temperature of CV cured networks was higher than that of radiation cured networks of the same crosslink density. (3) Scanning electron micrographs of cryofracture surfaces showed a more uneven surface in CV than in radiation cured networks. The development of heterogeneity in the four networks studied is discussed in terms of the reactions that lead to crosslinking with the different curing systems. © 1994 John Wiley & Sons, Inc.     

Separation of organic mixtures by pervaporation using crosslinked rubber membranes

Natural rubber (NR) and poly(styrene-co-butadiene) rubber (SBR) has been crosslinked with sulfur and accelerator with three different doses of varied accelerator to sulfur ratios to obtain three crosslinked membranes from each of these two rubbers (NR-1, NR-2 and NR-3 and SBR-1, SBR-2 and SBR-3). These six rubber membranes were used for pervaporative separation of toluene–methanol mixture up to 10 wt% of toluene in feed. It has been found that with increase in accelerator to sulfur ratio from membrane-3 to membrane-1, the vulcanization system shifts from conventional to efficient system resulting in higher degree of crosslink density and permeation selectivity. All of these membranes showed reasonably good range of flux (45.26 gm/m² h for NR-3 to 12.0 gm/m² h for SBR-1) and separation factor (162 for SBR-1 to 35.12 for NR-3) for 0.55 wt% of toluene in feed. Among these membranes NR-1 and SBR-1 with highest crosslink density showed maximum separation factor for toluene along with good flux. It has also been found that for comparable crosslink density SBR membranes showed better separation factor than NR membranes.     

Characterization of radiation-induced aging in silica-reinforced polysiloxane composites

Changes in crosslink density and chemical structure of silica-reinforced silicone polymer composites due to aging in gamma radiation environments were examined in this study. Solvent swelling was utilized to determine the individual contributions of the matrix polymer and filler phase to the overall crosslink density of silica-reinforced silicone polymer composites. The results show how polymer–filler hydrogen bonding dominates the overall crosslink density of the material. Air-irradiated samples displayed decreased hydrogen bonding at the polymer–filler interface, while vacuum irradiation revealed the opposite effect. These results were supported by solid-state NMR experiments that correlated the motional dynamics of the polymer chains with crosslink density through T₂ relaxation time measurements. GC/MS analysis was used to identify degradation products formed as a result of irradiation and speculate upon likely degradation mechanisms.     

Microbial desulfurization of ground tire rubber by Thiobacillus ferrooxidans

This study focused on the microbial desulfurization of ground tire rubber (GTR) by Thiobacillus ferrooxidans, which selected from the soil of an iron mine had strong sulphur oxidizing capacity. GTR was desulfurizated in the modified Silverman medium during the cultivation of T. ferrooxidans for 30 days, and T. ferrooxidans was able to maintain a high biomass. The continuous increase of SO₄²⁻ in the medium indicated that the sulfur on the surface of GTR was oxidized. FTIR-ATR and XPS spectra revealed that a rupture of conjugated CC bonds and a reduction of sulfur content on the surface of GTR had occurred during desulfurization. The sol fraction of GTR increased from its original 4.69%-7.43%. Compared with GTR sheet, desulfurizated GTR (DGTR) sheets had much smoother surfaces, better physical properties, and higher swelling values. NR vulcanizates filled with DGTR (with 10-40 phr loading) had lower crosslink density and better mechanical properties than those filled with GTR at the same loading. The results determined by DMA suggested that NR vulcanizates filled with DGTR had a reduction of molecular chain friction resistance during glass transition and SEM photograph indicated a better interface coherence between DGTR and NR matrix.     

Filler-elastomer interactions: influence of silane coupling agent on crosslink density and thermal stability of silica/rubber composites

In this work, the crosslink density and thermal stability of the silica/rubber composites treated by silane coupling agents, i.e., gamma-aminopropyl triethoxysilane (APS), gamma-chloropropyl trimethoxysilane (CPS), and gamma-methacryloxypropyl trimethoxysilane (MPS), were investigated. The chemical structures of modified silicas were studied in term of solid-state 29Si NMR spectroscopy. The crosslink density of the composites was determined by swelling measurement. The development of organic functional groups on silica surfaces treated by coupling agents led to an increase in the crosslink density of the composites, resulting in increasing final thermal stability of the composites. The composites treated by MPS showed the superior crosslink density and thermal stability in these systems. The results could be explained by the fact that the organic functional groups of silica surfaces by silane surface treatments led to an increase of the adhesion at interfaces between silicas and the rubber matrix.     

Prevulcanized natural rubber latex/clay aerogel nanocomposites

Natural rubber latex (NR)/clay aerogel nanocomposites were produced via freeze-drying technique. The pristine clay (sodium montmorillonite) was introduced in 1-3 parts per hundred rubber (phr) in order to study the effect of clay in the NR matrix. The dispersion of the layered clay and the morphology of the nanocomposites were determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Cure characteristics, thermal stability, and the crosslink density of thermal and microwave-cured NR and its composites were investigated. XRD patterns indicated that both intercalated and exfoliated structures were observed at loadings of 1-3 phr clay. SEM studies revealed that the clay aerogel structure was formed at 3 phr clay loading. The increment in Shore A hardness of nanocomposites compared with pure NR signified excellent polymer/filler interaction and the reinforcing effect of the clay to rubber matrix. This was supported by an increase in maximum rheometric torque and crosslink density. The crosslink density of clay-filled NR vulcanizate was found to increase with the pristine clay content in both thermal and microwave curing methods. However, microwave-cured 2 and 3 phr-filled NR vulcanizates exhibited higher crosslink density than those which were thermal-cured under the same curing temperature. In addition, thermal stability studies showed that pristine clay accelerated the decomposition of NR by showing a slight decrease in onset and peak decomposition temperatures along with clay content.     

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     

Imidazolium ionic liquids as fracture toughening agents in DGEBA-TETA epoxy resin

Although epoxy resins are used in a broad variety of applications due to their good mechanical and thermal properties, their low fracture toughness is a limitation, exhibiting brittle behavior. This study explored the potential use of imidazolium ionic liquids (IL) as toughening agents for epoxy resin based on diglycidyl ether of bisphenol A (DGEBA) with triethylenetetramine (TETA) as curing agent. Fracture toughness was evaluated for DGEBA-TETA epoxy resins with eleven imidazolium IL and the best results were found for the IL with the chloride anion and the shortest N-alkyl side chain, C₄MImCl. The use of 1.0 phr of C₄MImCl lead to the reduction of the crosslink density of the post-cured resin, resulting in the increase of 25.5% in stress intensity factor and 8.2% in tensile strength with no significant loss in other mechanical properties.     

A novel approach to the determination of the crosslink density in rubber materials with the dipolar correlation effect in low magnetic fields

We present a novel NMR approach to the determination of crosslink densities in rubber materials. The method is based on the dipolar correlation effect (DCE) on the stimulated echo examined in a series of rubber samples and linear polyisoprene. The parameter evaluated from the echo attenuation curves is the mean‐squared dipolar fluctuation associated with anisotropic reorientations of macromolecular backbones. The contributions to the DCE of the constraints due to excluded volume effects and chemical crosslinks are estimated. A strong dependence of the mean‐squared dipolar fluctuation on the crosslink density of rubber combined with the simplicity of performing the measurements with inexpensive low‐field instruments suggests that the DCE is a useful tool for routine applications. The potential and problems of performing DCE measurements in low‐magnetic‐field conditions are discussed in detail. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2207–2216, 2001     

聚氨酯的后交联研究进展

通过交联可以将线型聚合物连接成三维网状结构,适度交联后聚氨酯的拉伸强度、模量、耐溶剂性、耐高温性等许多性能都能得到大幅提高,而交联方法、交联的种类及交联密度的大小都是重要的影响因素,因此对交联方法、交联密度的测定、交联密度与聚氨酯性能之间的关系研究对于聚氨酯的质量控制非常重要。本文首先详细介绍了聚氨酯后交联的常用方法,然后对聚氨酯交联密度的测定方法进行了分析比较,在此基础上进一步介绍了国内外关于聚氨酯交联密度与其性能的关系方面的研究进展。     

Oxygen density dominated gas sensing mechanism originated from CO adsorption on the hexagonal WO3 (001) surface

Background oxygen play important role in the detection of gases on metal oxide surfaces. In this work, a new mechanism dominated by oxygen density has been proposed based on density functional theory (DFT) calculation of CO adsorption on the oxygen pre-absorbed and oxygen deficient hexagonal WO₃ (h-WO₃) (001) surface. Taking clean WO-terminated h-WO₃ (001) surface as the datum, we can define the O- and WO-terminated h-WO₃ (001) surfaces to be situations with surface oxygen density (denoted as d_O) of 1 and 0, respectively. And the oxygen density will be positive (1 > d_O > 0) for oxygen absorbed surfaces and negative (0 > d_O > ?1) for oxygen vacancy presented surfaces. More importantly, environmental oxygen concentration can be reflected directly by surface oxygen density. A positive correlation between environmental oxygen concentration (surface oxygen density) and sensing ability (charge transfer number) can be constructed based on the data of CO sensing on h-WO₃ (001) surfaces (Zhao et al., 2013; Tian et al., 2014). And these ideas obtained for CO on h-WO₃ can also be generalized to other gases and materials. The new proposed oxygen density dominated gas sensing mechanism, combined the two existing models of surface absorbed oxygen and oxygen vacancy together by the use of one physical quantity of oxygen density, will simplify the understanding of the effect of environmental oxygen on gas sensing largely. And the new findings here will provide substantial chances for controllable sensing by surface tuning.     

Chain dynamics in partially cross-linked polyethylene by combined rheology and NMR-based molecular rheology

In recent years, ¹H double-quantum NMR (DQ NMR) was established as a suitable molecular-rheology technique to elucidate chain dynamics and to determine entanglement or crosslink densities in linear entangled polymer melts and permanent as well as transient networks. In this work, industrial grade high-density polyethylene, partially cross-linked via electron beam irradiation in the semicrystalline state, is probed in the melt state by low-field DQ NMR and shear rheology. The DQ NMR data is analyzed by two approaches, one assuming the presence of a permanent network and the other considering the potentially complex relaxation spectrum of the studied inhomogeneous systems. A correlation between the DQ NMR results and extent of cross-linking is found. By direct comparison of the rheological results and the NMR-based segmental orientation autocorrelation functions (OACF) via time–temperature superposition (TTS), qualitative consistency between the microscopic and macroscopic observables is established. In this way, the frequency range of shear rheology can be extended by about two decades into the 10 krad/s range. The NMR method is thus a valuable extension of the toolbox of characterization techniques, where gel content measurements by solvent extraction proved to be the least sensitive.     

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.