Copolymerization of styrene and butadiene in emulsion. III. Crosslinking studies by partial conversion properties

Crosslinking in the styrene–butadiene emulsion copolymerization (21 parts styrene:79 parts butadiene) prior to gelation has been studied by means of partial conversion number-average molecular weights at 5, 15, and 25°C. It is shown that the macromolecular population begins to diminish at progressively lower conversions as the reaction temperature is increased. This is attributed to the relative increase in crosslinking over propagation as the reaction temperature is increased and also to the decrease in mercaptan regulating index with increasing temperature.     

Effect of temperature on the ozone cracking of butyl rubbers

The rates of growth of single ozone cracks have been measured for vulcanizates of two butyl rubbers over the temperature range of 20–160°C. Over most of this range the rates are quantitatively related to the segmental mobility of the polymer and depend upon temperature in accord with the appropriate from of the WLF relation. The rates are also proportional to the concentration of ozone. It is therefore concluded that diffusion of ozone into the polymer before reaction is the rate-controlling step. This is contrasted with the behavior of butadiene–styrene copolymers, for which the rates of crack growth are also quantitatively related to the segmental mobility, but the rates are somewhat larger at equivalent mobilities and the dependence upon ozone concentration is smaller. The difference is attributed to different penetration distances before reaction in polymers containing low and high densities of reactive sites.     

Grafting of styrene onto low molecular weight polymers and copolymers of butadiene

The grafting of styrene onto low molecular weight polybutadienes and butadiene–styrene co-polymers was studied. A mathematical method was used for the design of experiments and for the determination of the optimum grafting conditions with respect to the conversion of styrene and the efficiency of grafting. The reaction parameters were temperature (65–105°C), time (2–10 hr), concentration of the initiator, polymer to monomer ratio (10/90–90/10) and dilution by solvent (toluene). The optimum grafting conditions were chosen under which 50–60 wt-% of styrene was grafted onto backbone polymer at a high conversion of the monomer. It was found that the reactions producing graft copolymer prevailed over the styrene homopolymerization when the temperatures employed were lower (65–85°C), and the reaction time (8–10 hr), backbone polymer/monomer ratio, and the dilution by solvent were higher. The efficiency, density, and degree of grafting were found to increase with the increase in the molecular weight of the backbone polymer. The efficiencies and densities of grafting onto low molecular weight polybutedienes were higher than those of grafting onto low molecular weight butadiene–styrene copolymers. Grafting efficiencies and grafting densities were in the ranges 37.8–61.6 wt % and 0.06–0.26, respectively, in the studied range of number-average molecular weights (M?_n = 2400–6000).     

Experimental branching results for diene polymers by use of gel-permeation chromatography

Branching analyses in styrene–butadiene rubbers and polybutadiene rubbers have revealed large differences in branching between rubbers polymerized in different ways. The functionalities of several star-branched solution-polymerized styrene–butadiene rubbers were calculated and compared to their expected structures. Emulsion-polymerized polybutadiene rubber and a series of solution-polymerized polybutadienes made with different catalysts had different degrees of random branching, and evidence is presented indicating that the different available catalyst systems provide some latitude in making rubbers of different branching contents. Random branching analyses on a series of emulsion-polymerized styrene–butadiene rubbers revealed the dependency of branching on molecular weight and molecular weight distribution. The influence of polymerization temperature on the branching of emulsion-polymerized styrene–butadiene rubber was also studied.     

Peroxide-Initiated crosslinking of styrene-grafted polymers and copolymers of butadiene

Low molecular weight polymers and copolymers of butadiene were grafted with styrene. The graft products were then crosslinked by using dicumyl peroxide as initiator. The optimum peroxide concentration was established (5 phr). Infrared analysis showed that the reactivity of 1,2-vinyl and that of 1,4-trans double bonds in styrene-grafted polybutadiene is similar. Crosslinking of the graft product seems to involve a radical-chain polymerization of double bonds in the polymer. The reaction rate is proportional to the square root of peroxide concentration and to the concentration of polymer double bonds. Activation energy, reaction heat, reaction order, and crosslinking efficiency were also determined from DSC measurements. No relation was found between the activation energy of crosslinking and the molecular weight of backbone polymer or density of grafting. Crosslinking efficiency was to 25–50 crosslinks per molecule of decomposed peroxide. The crosslinking efficiency for grafted butadiene–styrene copolymers is somewhat lower than that for grafted polybutadienes. From thermogravimetric measurements it was found that the crosslinked grafted polymers show lower resistance to thermal degradation than ungrafted polymers.     

Detailed kinetic modeling of 1,3‐butadiene oxidation at high temperatures

The high‐temperature kinetics of 1,3‐butadiene oxidation was examined with detailed kinetic modeling. To facilitate model validation, flow reactor experiments were carried out for 1,3‐butadiene pyrolysis and oxidation over the temperature range 1035–1185 K and at atmospheric pressure, extending similar experiments found in the literature to a wider range of equivalence ratio and temperature. The kinetic model was compiled on the basis of an extensive review of literature data and thermochemical considerations. The model was critically validated against a range of experimental data. It is shown that the kinetic model compiled in this study is capable of closely predicting a wide range of high‐temperature oxidation and combustion responses. Based on this model, three separate pathways were identified for 1,3‐butadiene oxidation, with the chemically activated reaction of H^· and 1,3‐butadiene to produce ethylene and the vinyl radical being the most important channel over all experimental conditions. The remaining uncertainty in the butadiene chemistry is also discussed. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 589–614, 2000     

Dynamic mechanical and dielectric properties of epoxidized SBS triblock copolymer

The morphological and dynamic properties of epoxidized styrene–butadiene–styrene block copolymers were studied and compared with their parent styrene–butadiene–styrene block copolymer (SBS). Two peaks were observed in the mechanical loss (tan δ) curve which can be attributed to segmental motion of epoxidized polybutadiene (EPPB) and polystyrene. Analysis by DSC thermograms also showed the linear increase of glass transition temperature for EPPB domain with the epoxy group content. Phase separated structures of epoxidized SBS as observed by TEM suggests a considerable degree of mixing occurred between phases after 80 mol % of the double bonds in SBS were epoxidized. The interfacial region displays a third peak and causes much steeper drop in modulus at higher temperature than T_g of EPPB. Parallel dielectric relaxation measurements were also made in the frequency range of 30 Hz–1 KHz as a function of temperature. In each dielectric constant (?′) curve, there is a maximum near the T_g of EPPB determined from the dielectric loss tangent curve. The shift in T_g of EPPB versus epoxy group content was consistent with that measured by the thermal and dynamic mechanic analysis. These findings indicated an 8°C shift in glass transition temperature as the epoxy group content in EPPB increased 10%.     

Styrene‐butadiene rubber synthesized by anionic polymerization

In this paper we report on the copolymerization of styrene with butadiene in cyclohexane as solvent. Some experiments were also done using toluene as a solvent in order to check the influence of this solvent on the polydispersity index of the copolymers obtained, n‐ Butyllithium (BuLi) was used as initiator, while methyl‐tert‐butyl ether (MTBE) was used as an active center modifier. Reaction parameters such as polymerization temperature, the [MTBE]/[BuLi] molar ratio and the nature of solvent were studied in relation to their effects on the copolymerization rate, the randomness of the polymer and the microstructure of the butadiene units incorporated.     

The influence of the arrangement of styrene in methyl methacrylate/butadiene/styrene on the properties of PMMA/SAN/MBS blends

A series of methyl methacrylate‐butadiene‐styrene (MBS) core–shell impact modifiers were prepared by grafting styrene (St) and methyl methacrylate (MMA) onto polybutadiene (PB) or styrene‐butadiene rubber (SBR) seed latex in emulsion polymerization. All the MBS modifiers were designed to have the same total chemical composition, and Bd/St/MMA equaled 39/31/30, which was a prerequisite for producing transparent blends with poly(MMA)/styrene‐acrylonitrile (PMMA/SAN) matrix copolymers. Under this composition, different ways of arrangement for styrene in MBS led to the different structure of MBS modifier. The concentration of PB or SBR rubber of MBS in PMMA/SAN/MBS blends was kept at a constant value of 15 wt.%. The effects of arrangement of St in MBS on the mechanical and optical properties of PMMA/SAN/MBS blends were investigated. The results indicated that Izod impact strength of PMMMA/SAN/MBS blend with the amount of St grafted on core in MBS was higher than that of blend with the amount of St copolymerized with Bd in core of MBS, while the transparency of blend is opposite. From transmission electron microscopy, it was found that the arrangement of St in MBS influenced the dispersion of blend, which led to different toughness. Copyright © 2013 John Wiley & Sons, Ltd.     

Evidence for and against concentration gradients in polymerizing latex particles

A single-charge emulsion polymerization involving a monomer which is a good solvent for its polymer is considered. It is shown to be unlikely that within the polymerizing latex particles there are concentration gradients large enough measurably to affect the kinetics of the reaction. The average displacement of monomers due to Brownian motion within the latex particles and in the absence of concentration gradients is calculated. This diffusive mean free path, corresponding to an interval involving less than 1% change in conversion, is shown to be much longer than the radius of the latex particle. Consequently, loci where monomer concentration is perturbed by conversion to polymer are immediately swamped by unreacted monomer. Also, direct experimental evidence exists showing that the monomer concentration in latex particles is about the same when nonpolymerizing latex particles are saturated or during polymerization in the presence of monomer excess. The thermodynamics of saturation swelling preclude the possibility of the existence of large concentration gradients. The arguments that have been advocated in the literature for core–skin separation within polymerizing latex particles were based on conversion data which were thought to be linear with time, while a reexamination indicates that they were not. The observed core–skin separation obtained when butadiene or tritiated styrene were copolymerized with styrene in the presence of a polystyrene homopolymer seed latex has questionable relevance to single-charge homopolymerization. There are reasons to doubt that the distribution of co-monomers within latex particles can be frozen by their conversion to polymers in a two-stage emulsion polymerization.     

Dynamic mechanical properties of two copolymers of styrene and n-hexyl methacrylate

The storage (J′) and loss (J″) shear compliances have been measured for two random copolymers of styrene and n-hexyl methacrylate with styrene contents of 18% and 30% (by weight) in the frequency range 45–4400 Hz and the temperature range 31–107°C. The data at different temperatures were combined by the method of reduced variables, and the WLF coefficients were calculated from the temperature shift factors by the method of Pierson and Kovacs. The data were compared with earlier data for the two homopolymers. The thermal expansion coefficient of the fractional free volume, and the free volume at the glass transition temperature, varied monotonically with composition, but the fractional free volume at a reference temperature of 100°C appeared to pass through a maximum as a function of concentration. Comparison of isothermal plots of J′ at 100°C, plots of the relaxation spectrum at 100°C, the monomer friction coefficient and its temperature dependence, and isochronal plots of the storage shear moduls at 100 radians/see all show that the properties of poly(n-hexyl methacrylate) are very slightly affected by incorporation of 18% styrene and only moderately affected by 30% styrene. By contrast, comparison of styrene–butadiene rubber with 1,4-polybutadiene shows a very large effect of incorporation of 23.5% styrene. These differences may be associated with local packing relations of the comonomer residues and suggest that copolymer properties cannot be readily predicted from those of the component homopolymers.     

Generic crack patterns in rubber‐modified polymers under biaxial stress states

The damage mechanisms in three different systems, namely, acrylonitrile‐butadiene‐styrene, methacrylate‐butadiene‐styrene modified poly(vinyl chloride), and styrene‐butadiene‐styrene have been investigated. The damage was analyzed over a range of biaxial stress states with confocal microscopy and scanning electron microscopy. The macroscopic yield followed a linear behavior for all the systems in an octahedral shear stress versus mean stress plot, whereas popular models for this class of materials predicted a nonlinear response. Over a certain range of biaxial stress states, a damage pattern generic to all the systems was observed. The damage pattern consisted of an array of cracks propagating perpendicular to the direction of the maximum tensile principal stress and arranged itself in a more or less periodic fashion. There was also self‐similarity in the patterns at various length scales. Similar patterns have also occurred in several other polymeric systems. The interaction in the ensemble of cracks created seems to lead to stress reduction at the crack tips, thereby limiting the crack sizes. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2248–2256, 2003     

Anionic synthesis of polystyrene and polybutadiene heteroarm star polymers

The use of living linking reactions of poly(styryl)lithium with 1,3-bis(1-phenylvinyl)benzene followed by crossover reactions with styrene or butadiene monomers has been used to prepare four-armed heteroarm, star-branched polymers. Bimodal molecular weight distributions have been observed for crossover reactions with both styrene and butadiene. Addition of THF ([THF]/[Li]=14–32) for crossover to styrene and lithium sec-butoxide for crossover to butadiene produces monomodal molecular weight distributions. Symmetrical, four-armed star polystyrenes have been synthesized; properties have been compared with a corresponding polymer prepared via a silicon tetrachloride linking reaction. Heteroarm, star-branched polymers with two polystyrene arms and two polybutadiene arms with high 1,4-microstructure have been prepared.     

The research for low-temperature rheological properties and structural characteristics of high-viscosity modified asphalt

High-viscosity modified (HVM) asphalt was prepared by the addition of styrene–butadiene–styrene, furfural exact oil (FEO, plasticizer), sulphur (crosslinker). The low-temperature rheological properties of HVM asphalt were investigated by using bending beam rheometer, and different analysis ways including Fourier transform infrared (FTIR) analysis, thermal analysis, ¹hydrogen nuclear magnetic resonance (¹NMR) analysis, elemental analysis, optical microscopy were used to investigate the structural characteristics of modified asphalts and FEO. Rheological tests demonstrated the effect of each modifier on low-temperature rheological performances of asphalt and displayed the structural characteristics of each binder to some extent. FTIR analysis indicated the effect of ageing and modifier on the distribution of functional groups of modified asphalt before and after ageing. Morphology observation showed the distribution of polymer in asphalt with different modifications or ageing. The thermal analysis showed the effect of each modifier on thermal behaviour of asphalt before and after ageing and confirmed the result of FTIR analysis and morphology observation further. Besides, the constituents of base asphalt and plasticizer were also investigated and compared further by adopting elemental analysis, and ¹HNMR and FTIR tests.     

钴系催化剂催化丁二烯—苯乙烯共聚动力学研究

合成高顺-1,4-丁苯胶的催化剂主要是镍、钴、钛系3类。钴系催化剂基本上是三元体系,也有加入添加剂(含N,O,S化合物)的多元体系。该体系的特点是,共聚物顺1,4含量高(～98％),分子量大。但苯乙烯的共聚活性低,有均聚苯乙烯和凝胶形成。本文以Co(nap)_2-Al_2Et_3Cl_3进行了丁苯共聚的动力学研究。     

Epoxidation of polybutadiene and styrene–butadiene triblock copolymers with monoperoxyphthalic acid: Kinetic and conformation study

The rates of epoxidation of polybutadiene homopolymers and styrene–butadiene triblock copolymers with monoperoxyphthalic acid were measured at four temperatures in homogeneous solution. Dioxane and mixed solvents with chloroform were used as reaction media. Activation energy values were also calculated for polymers different in microstructure and styrene content. Viscometry in a modified viscometer was performed and combined with kinetic measurement to monitor the conformational change during epoxidation. Cis-content in polybutadiene and styrene content in SBS exhibit only slight effect on epoxidation in dioxane. The addition of chloroform promotes the reaction rate remarkably and enlarges the difference between polymers. Explanations were given including the solvent effect on reduced viscosity, which was used to correlate the conformational change of polymer chains. NMR and GPC analysis confirmed the absence of ring opening and degradation during epoxidation up to 54% epoxy group content.     

Effect of mixing conditions on the morphology and properties of polystyrene/polyethylene blends compatibilized with styrene–butadiene block copolymers

The effect of mixing conditions on the morphology, molten‐state viscoelastic properties, and tensile impact strength of polystyrene/polyethylene (80/20) blends compatibilized with styrene–butadiene block copolymers containing various numbers and lengths of blocks was studied. Under all mixing conditions, an admixture of a styrene–butadiene block copolymer led to a finer phase structure and to an increase in the dynamic viscosity, storage modulus, and tensile impact strength. The effects were stronger for S–B diblock with a short styrene block than for S–B–S–B–S pentablock with long styrene blocks (where S represents styrene and B represents butadiene). For all blends mixed longer than 2 min, the mixing time had only a small effect on their morphology and properties. Surprisingly, the localization of S–B diblock copolymers was strongly dependent on the rate of mixing. The mixing rate had a nonnegligible effect on the viscoelastic properties of the compatibilized blends. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 609–622, 2003     

The effect of pressure on the mechanical properties of polymers. IV. Measurements in torsion

Stress relaxation was studied in torsion under superposed hydrostatic pressure using a newly constructed device. Two materials, a styrene–butadiene rubber and a butadiene rubber, were studied in the range from ?60 to 30°C at pressures up to 500 MPa. The time–temperature–pressure superposition of the data can be described by the FMT form of the free-volume theory which uses input from relaxation experiments at high pressure. The applicability of the Havlí?ek–Ilavský–Hrouz form of the Adam–Gibbs theory was also examined. This theory, which does not require information from high-pressure relaxation experiments, offers less flexibility than the FMT theory and is less successful in predicting superposition. The data are also examined in light of an adaptation of the Simha–Somcynsky equation of state to the free-volume theory. Prediction of the effect of pressure by the theory requires a single adjustable parameter. It was possible to calculate the free-volume parameters of several polymers without use of information from high-pressure experiments.     

Formation and Conversion of Surface Compounds upon Interaction of Ethanol with Cu/CeO<Subscript>2</Subscript> Catalyst According to in situ IR Spectroscopy

In the conditions of ethanol conversion on the surface of a 5%Cu/CeO₂ catalyst, the method of in situ IR spectroscopy reveals ethoxy groups, acetate and formiate complexes, and consolidation products. Acetaldehyde, acetone, croton aldehyde, butadiene, hydrogen, CO, and CO₂ are observed in the reaction products. As the temperature of the experiment increases, the concentration of acetaldehyde passes through a maximum at T = 250°C. This product is formed due to the interaction of ethoxy and hydroxyl surface groups. The concentration of acetone, croton aldehyde, and butadiene also passes through a maximum in the 350–400°C range. These products are associated with the decomposition of the consolidation products. The concentration of hydrogen, CO and CO₂ steadily increases with temperature and only these reaction products are left at T > 400°C. A mechanism of hydrogen formation based on the conversion of the highest temperature formiate surface complex is discussed.     

Filler‐polymer interactions of styrene and butadiene units in silica‐filled styrene–butadiene rubber compounds

Styrene–butadiene rubber (SBR) is a copolymer of styrene and butadiene, and the butadiene unit is composed of cis‐1,4‐, trans‐1,4‐, and 1,2‐components. Filler‐polymer interactions of each component of SBR in silica‐filled SBR compounds were examined by microstructure analysis of the bound and unbound rubbers. The composition ratio of butadiene and styrene units (butadiene/styrene) of the bound rubber was higher than that of the compounded rubber. Of the butadiene units, the 1,2‐component of the bound rubber was more abundant than the cis‐1,4‐ and trans‐1,4‐components. The filler‐polymer interaction of the butadiene unit with silica was stronger than that of the styrene unit, and the interaction of the 1,2‐component was stronger as compared with the others. The butadiene–styrene ratio of the bound rubber of the compounds containing the silane coupling agent was lower than for the compounds without the silane. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 577–584, 2004