Styrene-butadiene Diblock Research Articles

Preliminary investigation of bioadvantaged polymers as sustainable alternatives to petroleum-derived polymers for asphalt modification

Due to recent volatility in the petroleum market, the possibility of using sustainable alternative materials as substitutes has gained great interest and viability. The purpose of this research is fourfold: to demonstrate how bioadvantaged polymers are produced using non-food soybean oil and showcase their economic and environmental value; to evaluate how they perform rheologically in asphalt against commonly used petroleum-derived polymers; to optimize the formulation of the bioadvantaged polymer for warm climate regions pavement applications based on the grading results, and to verify the modification effects of optimized bioadvantaged polymer by running performance grade tests. The study on economic and environmental implications demonstrated that poly(styrene-acrylated epoxidized soybean oil) (PS-PAESO) is more cost-effective, environmentally friendly, and safer to produce than styrene-butadiene (SB) polymer. The commercially produced petroleum-derived SB diblock polymer was used for comparison, while the laboratory produced bioadvantaged polymers were targeted at 1.25 MDa molecular weight of PS-PAESO diblock copolymers that consist of PAESO with various polystyrene (PS) molecular weights and contents. The effectiveness and rheological performance of the polymer modified asphalt binders were evaluated through binder investigations. Rheology test results indicated that the bioadvantaged polymers improved the stiffness, elasticity, and rutting resistance of the neat asphalt binder. Best-fit prediction models were developed through response surface modeling to optimize the PS-PAESO formulation in terms of PS content and molecular weight and the models were verified to be highly accurate based on the grading results. It was found that lower polystyrene content in PS-PAESO polymer could be beneficial in the improvement of critical high temperatures.

How Temperature and Loading Rate Affect the Yield Behavior in Polymer-modified Asphalt Systems

This paper reports on how temperature and loading rate affect the low-temperature yield properties of two asphalts (California Valley and Bow River sources), straight and modified with 5% by weight of four commonly used commercial polymers (styrene–butadiene diblock, triblock and radial and ethylene–vinyl acetate copolymers). The Bow River asphalt was also tested with the addition of mineral filler. Both the source of the bitumen as well as the nature of the additive and the presence of filler were found to have a significant influence on how the yield stress changes with temperature and loading time. Only one homogeneous, California Valley binder modified with SBS triblock copolymer, provided a typical Eyring plot with a constant activation energy of 42 kJ/mol and activation volume of 3.7 nm3. Other systems showed atypical behavior in that the activation parameters changed rapidly with both temperature and rate of loading.

Low-Temperature Binder Specification Development: Thermal Stress Restrained Specimen Testing of Asphalt Binders and Mixtures

Results of low-temperature testing of Bow River asphalt binders and concrete mixtures, unmodified and modified with three commercially used polymers [diblock styrene-butadiene (SB), radial styrene-butadiene-styrene (SBS), and ethylene-vinyl acetate], are discussed. Thermal stress-restrained specimen tests, at a cooling rate of 10°C/h, were used to evaluate the performance of binders and mixtures. Binders were restrained as direct tension specimens with and without a sharp starter crack. Notching of the samples resulted in lower failure loads for all systems. But three of the four binders failed at warmer temperatures. The fourth binder, an exceptionally tough radial SBS-modified system, remained unaffected in this regard. Changes in failure temperatures ranged from 0°C (radial SBS) to +12°C (diblock SB). A comparison of binder results with those obtained for the mixture yields additional insights. It appears that there are differences between binder and mixture failure modes. The higher stress levels encountered in the mixture give rise to energy dissipating mechanisms that do not occur at the lower stress levels in the binder test. It is unlikely that a specification based on binder failure stress or strain will predict performance in polymer-modified systems. Three-point bend tests on notched asphalt binder or mastic beams may provide more realistic conditions for measuring materials properties. A compression test on the binder or mastic may be used to determine additional important parameters, such as Young’s modulus and yield stress in compression, which along with the failure properties, will likely produce an improved fracture mechanics-based low-temperature failure criterion.

Strain energy effects on the ordering process in diblock styrene-butadiene copolymer

The technique of time-resolved small-angle x-ray scattering is used to monitor the disorder-order transformation occurring in asymmetric, diblock styrene-butadiene. A rapid thermal quench is applied to drive the system from its initial high-temperature disordered state to a low-temperature ordered structure, a body-centered cubic lattice of styrene spheres characterized by a series of concentric Debye rings in the two-dimensional scattering profile. At relatively late times in the microdomain ordering process, the uniform Debye rings were seen to rapidly develop nonuniformities indicating the existence of preferred orientation with a fiber texture, the 110] axis of the bcc structure defining the fiber axis. These results are interpreted in the context of a developing transformation strain energy during the ordering process and shown to be consistent with accompanying changes in lattice spacing as well as earlier observations of apparent fluctuations in the ordering process.

Finite impurity size effects on heterogeneous nucleation occurring in diblock copolymers

The role played by heterogeneous nucleation in phase transformations occurring in asymmetric, diblock styrene-butadiene is examined using the experimental technique of time-resolved small-angle x-ray scattering. Transient nucleation effects are clearly identifiable through comparison of the early time transformation kinetics seen with samples containing varying impurity sizes. These results are interpreted in the context of a simple model of transient nucleation rate incorporated into classical theories of isothermal transformation kinetics.

The influence of styrene–butadiene diblock copolymer on styrene–butadiene–styrene triblock copolymer viscoelastic properties and product performance

AbstractThe effects of the styrene–butadiene (SB) diblock copolymer on the viscoelastic properties of styrene–butadiene–styrene (SBS) triblock copolymers were examined in both in the the neat state and within specific product applications. The addition of the SB diblock copolymer into a pure SBS triblock copolymer resulted in a significant decrease in the plateau storage modulus and a quantitative linear rise in tan delta. In a pure triblock, in which all endblocks are anchored in polystyrene domains, all entanglements are physically trapped. The SB diblock embodies untrapped polybutadiene endblocks that are able to relax stress by chain reptation through the rubbery polybutadiene matrix. The SB diblock copolymer quantitatively lowered the microphase separation temperature (MST) of the SBS triblock copolymer. These changes in linear viscoelastic behavior manifest themselves into a reduction in the efficiency and performance of the SBS triblock copolymer in asphalt pavement binders and hot‐melt adhesive blends. Specifically, the SB diblock diminished the complex shear modulus and elasticity of a polymer‐modified asphalt, which translated into lower predicted rutting specification values. The increase in diblock content altered the viscoelastic response of the hot‐melt adhesive blend, translating into a reduction in the shear holding power and shear adhesion failure temperature. The lack of network participation, coupled with the relaxation of the polybutadiene endblocks, accounts for the lower strength and greater temperature susceptibility of the diblock‐containing systems. © 1995 John Wiley & Sons, Inc.

Nucleation and growth kinetics in diblock styrene-butadiene

Abstract Time-resolved small-angle x-ray scattering has been used to study the phase transformation kinetics of a thermally quenched, asymmetric, diblock copolymer. The roles played by homogeneous and heterogeneous nucleation in the microdomain ordering process are considered through comparison of results from pure samples and samples doped with impurities. This crystallization process is unique in that it involves constituent particles (microdomains) that are hundreds of angstroms in size ordering onto a lattice with spacings on a similar length scale. Models derived from classical nucleation and growth theories are used to analyze results from pure and doped samples. Clear experimental evidence of a heterogeneous nucleation process occurring in the doped samples suggests that impurity effects cannot be ignored in these materials. In addition, it is proposed that the late-time coexistence of crystalline and amorphous microdomain structures is due to strain energy effects induced by the volume constraints...

Morphology of polyethylene blends: 3. Blend films of polyethylene with styrene-butadiene or styrene-isoprene copolymers obtained by extrusion-blowing

The morphologies of blends of polyethylene (PE) with a range of copolymers with different basic structure (diblock, triblock, starblock or statistical) have been compared in order to assess the relative importance and interrelation of rheological and molecular parameters. The development of well defined dispersed phases in extruded-blown PE films results from two successive steps. The first is the dispersion step, dominated by shear force fields and shear viscosity in the extruder barrel and die. The second step involves drawing of the melt and freezing of the structures. If the dispersed phase is characterized by a high value of the shear viscosity ratio η d η m and hence by a low deformability, elongation can be performed in neither step. This was observed for diblock styrene-butadiene (SB) in a low viscosity PE matrix, for SBS r2 and for StatSB which all give spheres or ellipsoids. The dimension of the spheres is governed by the viscosity of the matrix and by the value of η d η m according to the Wu equation. If the dispersed phase is highly deformable ( η d η m < 1 ), it is elongated in one or two dimensions at the exit of the die. For a Newtonian melt, these structures break during the elongation time preceding freezing of the structures if the breaking time given by Tomotika's theory is shorter than the elongation time. Broken filaments have indeed been observed for SBS at 240°C and also for styrene-isoprene-styrene (SIS) at 270°C. For a non-Newtonian dispersed phase, stabilization of the structures occurs by different mechanisms involving the elastic contribution of the elongational viscosity. It is related to entanglements and increases with molecular weight. Yielding or tension thickening behaviour can be involved. This is illustrated by the formation of co-continuous elongated structures when the high-molecular-weight diblock SB and triblock SIS are blended with PE, respectively at 240 and 220°C. Two starblock copolymers, SBS r1 and SBS r3, although non-Newtonian and characterized by a rather low value of η d η m , do not give continuous structures. This is attributed to the low molecular weight of the branches which does not allow the formation of a sufficient number of entanglements.

Recent morphological surprises in diblock copolymer systems

Diblock copolymers form ordered interfacial surfaces that range from lamellae, to cylinders, to the ordered bicontinuous double diamond (OBDD) structure, and spheres on a cubic lattice as the size of one component is increased. Flexible control of these materials presents an opportunity for structural materials, adhesives and selective separations. Mixtures of diblock copolymers with homopolymers exhibit similar structures due to changes in the interfacial curvature that accompany "swelling" the like-component of the diblock with homopolymer. Ordering behavior in systems of this type depends on the elastic properties and molecular weights (MW's) of the polymers, annealing temperature and annealing time. We have examined the changes in morphology of a styrene-butadiene (SB) diblock copolymer mixed with homopolymer polystyrene (hS). Sample made by casting from toluene solution, annealing at 130°C for 2-10 days, staining B-rich regions with OsO4, and microtoming thin-sections at -90°C. The SB MW was 49 800 g/mol with an S-block MW of 25 500 g/mol, and 24 400 g/mol for the B-block. Two hS components were used, having MW's of 26 000 g/mol (h26) and 19 300 g/mol (h19). We refer to the samples by the hS molecular weight and total polystyrene volume fraction, e.g. h26/0.67.

Time-resolved small-angle x-ray-scattering study of ordering kinetics in diblock styrene-butadiene.

A detailed study of the kinetics of phase transformations of the diblock copolymer, styrene-butadiene, is reported. The technique of in situ time-resolved small-angle x-ray scattering with the use of synchrotron radiation has been used to study the first-order phase transitions of microphase separation and microdomain ordering. These transitions occur following a rapid, thermal quench from the homogeneous, disordered state to temperatures below the transition point. The isothermal ordering process is discussed in the context of classical theories of nucleation and growth. Anomalous temporal oscillations in the ordered-volume fraction are observed following quenches to temperatures just below the ordering transition. These results are reported and qualitatively discussed.

Geometrical constraints on microstructural development in block copolymer ultrathin films

The fact that microstructures form in microphase-separated block copolymers, endowing the materials with unique thermo-mechanical properties, is well-established. However, no thermodynamic theories directly address the problem of microstructural development in ultrathin films, which might be useful as adhesives or resists. To predict the microstructural dimensions in such films, a model based on unit-cell geometries and material/volume balances is developed. Predictions of the ratio of the characteristic length of the domain core to domain repeat distance are obtained for poly (styrene-butadiene) diblock (SB) copolymers at 298 K. The finite interphase region is included in the volume balances with the parameterf, the volume fraction of interphase material, obtained from a modified version of the Leary-Henderson-Williams thermodynamic theory and shown here not to be a strong function of composition. An approach, implementingf, for accurately estimating the critical molecular weight of microphase separation in the strong-segregation limit (Mc), as a function of bulk composition for monodisperse SB copolymers at 298 K, is also presented.