Polystyrene-polyisoprene Diblock Copolymer Research Articles

Frustrated phases under three-dimensional confinement simulated by a set of coupled Cahn-Hilliard equations.

We numerically study a set of coupled Cahn-Hilliard equations as a means to find morphologies of diblock copolymers in three-dimensional spherical confinement. This approach allows us to find a variety of energy minimizers including rings, tennis balls, Janus balls and multipods among several others. Phase diagrams of confined morphologies are presented. We modify the size of the interface between microphases to control the number of holes in multipod morphologies. Comparison to experimental observation by transmission electron microtomography of multipods in polystyrene-polyisoprene diblock copolymers is also presented.

Effect of Macromolecular Architecture on the Morphology of Polystyrene–Polyisoprene Block Copolymers

The impact of block connectivity on the morphologies of four block copolymers of varying architecture containing polystyrene (PS) and polyisoprene (PI) has been studied. The volume fraction of PS and molecular weight are held constant while varying the architecture from a linear PS–PI diblock copolymer to three different miktoarm star architectures: PS2PI, PSPI2, and PS2PI2. Morphologies of the PS2PI and PSPI2 miktoarm stars are different from those observed for the linear copolymer and dependent on the connectivity of the copolymer blocks. The change in morphology with connectivity indicates that combining two chains at a junction point leads to chain crowding, where subsequent excluded volume effects drive the change in morphology for each sample. The PS2PI2 miktoarm star exhibits the same morphology as the linear diblock but with a reduction in the size of the domains. The extent of the decrease in domain size indicates that chain stretching impacts the formation of this morphology. Experimentally obse...

The effect of osmium staining on lamellar spacing in thin polystyrene-polyisoprene diblock copolymer films

Thin films of lamellar-structured polystyrene-block-polyisoprene were vapour stained with osmium tetroxide and compared using transmission imaging in an environmental scanning electron microscope to unstained films of the same polymer. Staining was found to swell the affected phase by a factor of 1.15 in-plane, compressing the unstained phase to a factor of 0.71 of its original size as it did so. Additionally, images exhibiting contrast between phases in the unstained sample were successfully taken with conventional transmission electron microscopy, showing that staining is not necessary for imaging such samples.

Liquid chromatography at the critical condition for polyisoprene using a single solvent.

Liquid chromatography at the chromatographic critical condition has drawn much attention as an attractive characterization method of block copolymers since it has been proposed that a part of a polymer chain becomes "chromatographically invisible" at this condition, which would permit the characterization of individual blocks. A critical condition for a polymer species has been commonly established by use of mixed-solvent systems. It is not easy, however, to reproduce the critical condition since the retention of polymers depends very sensitively on the solvent composition and purity. Furthermore, the preferential sorption of a component in a mixed solvent may cause an additional problem. Therefore, the use of a single solvent is highly desirable to improve the reproducibility as well as the repeatability. In this study, a single-solvent critical condition for polyisoprene was established with 1,4-dioxane and C18 bonded silica as the mobile and stationary phases, respectively. At this condition, the "chromatographic invisibility" of polystyrene-polyisoprene diblock copolymers was critically examined and it was found that a rigorous chromatographic invisibility was not achieved and the retention of the block copolymers was affected by the length of the blocks under the critical condition. Some other chromatographic applications using the single-solvent system are also reported.

Self-Assembly of Ionically End-Capped Diblock Copolymers

The self-assembly of ionically end-capped, symmetric polystyrene-polyisoprene diblock copolymers (PS-b-PI) has been studied. Structural data obtained from small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) were correlated with the aggregation behavior of charged chain ends as evidenced by a spin probe using electron paramagnetic resonance (EPR) spectroscopy. The resulting mesomorphic structures were shown to be determined by the chain end topology, i.e., the site where the ionic chain end has been introduced chemically: For omega-functionalized diblock copolymers (monofunctional species) microphase separation is significantly stabilized due to the presence of ionic aggregates within the respective phase separated homopolymer domains. In contrast, for salt-free alpha,omega-macrozwitterionic diblock copolymers a marked perturbation of the block copolymer superstructure was found. In this case, the formation of a network of mixed ionic aggregates creates an additional microdomain interface by joining the chemically distinct blocks at their chain ends. The alteration of the degree of microphase separation as observed for the different functionalities can be attributed to conformational changes of the copolymer chain. Chain end association in the present system is reminiscent of certain covalently joined star and graft copolymers.

Self-diffusion of a polystyrene-polyisoprene block copolymer

Forced Rayleigh scattering (FRS) has been used to measure the self-diffusion coefficient, D, of a lamellar-forming polystyrene-polyisoprene diblock copolymer (M PS = 1.0 x 10 4 , M PI = 1.3 x 10 4 ) as a function of temperature. The measurements traverse the order-disorder transition (ODT), which occurs at 160°C. There is no obvious change in either D or the temperature dependence of D at the ODT, in agreement with measurements on several other systems. Electron microscopy confirms that the sample in the ordered state is quenched, with no long-range orientation of lamellae, and a typical grain size well below 1 μm. In contrast to previous measurements on a similar styrene-isoprene diblock, these FRS signals are well-described by single exponential decays ; this may be largely attributed to differences in average grain size. The temperature dependence of D is modeled with several empirical expressions, based on the known monomeric friction factors for pure polystyrene and pure polyisoprene, but without quantitative success. These results underscore the need for a greater understanding of the composition and temperature dependences of local friction in polymer mixtures.

Competitive Adsorption at Homopolymer Interfaces from a Binary Mixture of Diblock Copolymers

Interfacial adsorption from a binary mixture of short and long polystyrene-polyisoprene diblock copolymers (PS-PI or its deuterated analogue dPS-PI) to interfaces of a PS(M = 3 x 10 6 ) homopolymer was studied using nuclear reaction analysis. Short symmetric diblocks dPS(M = 10 4 )-PI(M = 10 4 ) (designated dS), or a similar protonated analogue (hS), and a highly asymmetric, long diblock dPS(M = 10 6 )-PI(M = 10 4 ) (L) were used in the binary mixtures. We found that the shorter diblocks adsorb preferentially at the interfaces : the isotopic contrast between dS and hS in the mixtures with L enables us to extract detailed information on the surface segregation of the short and long species. Our data are analyzed in terms of a modified Flory-type mean field model due to de Gennes and Leibler. Interfacial segregation from the binary diblock mixture is well predicted by this model with no adjustable parameters, using the adsorption characteristics determined earlier for systems with a single diblock component.

Density Discontinuity at the Microphase Separation Transition of a Symmetric Diblock Copolymer

The temperature dependence of the density near the microphase separation temperature for a symmetric polystyrene-polyisoprene diblock copolymer is investigated by dilatometry and reflection ellipsometry.

Compressibility Effects in a Solvated Polystyrene-Polyisoprene Diblock Copolymer

Results from small angle neutron scattering experiments on a pressurized polystyrene-polyisoprene diblock copolymer dissolved in dioctyl phtalate are reported

Nucleation and Anisotropic Growth of Lamellar Microdomains in Block Copolymers

Experimental evidence is presented showing a slow nucleation process followed by an incubation time and an anisotropic growth of lamellar microdomains in a nearly symmetric polystyrene-polyisoprene diblock copolymer quenched to the ordered state very close to T ODT

Viscoplastic flow and shear thickening in concentrated diblock copolymer solutions

Concentrated (typically 6%) solutions of a polystyrene-polyisoprene diblock copolymer in low viscosity paraffinic solvents form a micelle system by precipitating the polystyrene blocks, whereas the polyisoprene blocks are in solution. Besides viscoplastic behavior without thixotropy, this system exhibits a pronounced shear thickening in steady-state shear flow. The micelles are stable up to shear rates of more than 105 s−1. The properties of the solutions, especially the shear-thickening behavior, depend on the thermal history of the samples as well as on the solvent properties and are sensitive to flow field disturbances occurring in rotational viscometer devices with a profiled surface structure as commonly used to avoid wall slip in dispersed materials. The shear thickening is found to be related to the formation of a long-range ordered structure which also gives rise to the yield point. This long-range order enables aggregate flow with less energy dissipation at low shear rates. Shear-induced break-up of the aggregates appears as a shear-thickening transition which is observed in different types of flow fields.

Dynamic Light Scattering from Block Copolymer Solutions under the Zero Average Contrast Condition

Dynamic light scattering measurements on a symmetric polystyrene-polyisoprene diblock copolymer, M w =3.4×10 5 , dissolved in neutral good solvents are reported. The solvents comprise mixtures of toluene and α-chloronaphthalene, and the solvent compositions necessary to index-match the entire copolymer, the polystyrene block, and the polyisoprene block have been determined. The first of these, the zero average contrast (ZAC) condition, is particularly advantageous as it eliminates the cooperative diffusion mode, thus enhancing the relative amplitude of the elusive internal mode. However, the ZAC condition is sensitive to small changes in either solvent composition or temperature. In dilute and semidilute ZAC solutions, the correlation functions consistently show two modes. The slower mode is diffusive, with a diffusivity in close agreement with that measured by pulsed-field-gradient NMR; it is thus interpreted as the heterogeneity mode, which is visible due to chain-to-chain variations in composition. The faster mode is the internal mode, with a decay rate that is independent of scattering angle and decreasing with concentration; the concentration dependence is that observed for the viscoelastic longest relaxation time. For polymer concentrations within a factor of 2 of the order-disorder transition, the total scattered intensity increases markedly, which is attributed to the appearance of large-amplitude concentration fluctuations. It is also shown that in dilute solution, the heterogeneity and cooperative modes have comparable decay rates, which can complicate the interpretation of the diffusional second virial coefficient, k d , for copolymers

Dynamic Light Scattering from Dilute, Semidilute, and Concentrated Block Copolymer Solutions

The dynamic light scattering (DLS) properties of four symmetric polystyrene-polyisoprene diblock copolymers have been examined in the neutral good solvent toluene. Concentrations (c) ranged from the dilute, through the semidilute, and into the concentrated regimes. Pulsed-field-gradient NMR has been used to determine the translational diffusivities (D s ) of the same copolymers, at selected concentrations. The results are in substantial agreement with models developed by Benmouna and coworkers and by Semenov. Three modes are predicted : cooperative diffusion (Dc), corresponding to relaxation of fluctuations in polymer concentration ; an internal mode (Γ I ), reflecting relative motion of the two blocks within one molecule ; a heterogeneity mode (D H ), due to chain-to-chain fluctuations in composition and which relaxes by translational diffusion. In dilute solutions, a single, diffusive mode is seen and is attributed to a superposition of Dc and D H . In semidilute and concentrated solutions, Dc and D H are cleanly resolved. Dc shows the expected scaling with c and molecular weight (M), i.e., Dc∼ c 0.7 M 0 . Comparison with the NMR results confirms that the values of the diffusion coefficients D H and D s are very similar ; this result extends the previous work of Balsara and co-workers, who first noted the appearance of a block copolymer DLS mode resembling translational diffusion. Selected measurements in two other good solvents, THF and chloroform, establish that Dc and D H are independent of solvent refractive index (n s ), as expected. The relative amplitudes of the cooperative and heterogeneity modes also scale with c, M, and n s in agreement with theory. The predicted internal mode is generally difficult to resolve but is seen for the highest molecular weight sample examined. The concentration dependences of DH and D s are insensitive to the order-disorder transition, in agreement with previous measurements in solutions and melts. Preliminary measurements of D H with a novel Fourier transform heterodyne DLS apparatus were also in good agreement with the other techniques. Finally, the results are compared with previous DLS measurements of symmetric diblock copolymers in solution. It is proposed that the apparent discrepancies among the previous studies can be resolved, by appropriate assignment of the heterogeneity mode.

Shear effects on solvated block copolymer lamellae: Polystyrene-polyisoprene in dioctyl phthalate.

Small-angle neutron scattering experiments were performed on a polystyrene-polyisoprene diblock copolymer in concentrated dioctyl phthalate solution under Couette shear. Lamellar orientations were monitored below and above the order-disorder transition temperature with the neutron beam incident either radially or tangentially on the cylindrical shear cell. It was found that lamellae are oriented mostly parallel to the plane of the shear cell walls. Oscillatory shear helps improve the lamellar alignment. Above the quiescent order-disorder transition, the shear-induced alignment (observed as an anisotropy of the scattering ring) follows a master curve for all temperatures.

Birefringence and diffraction of light in ordered block copolymer materials

The scattering of light from ordered block copolymer materials, consisting of irregularlyshaped birefringent grains with randomly oriented optic axes, is analyzed theoretically and experimentally. It is shown that independent estimates of the characteristic grain sizes can be obtained from the far-field angular spread of the diffracted light and from the total orthogonally polarized scattered power. Experimental measurements on ordered polystyrene-polyisoprene diblock copolymer solutions in toluene support the predictions of the theory. Grain sizes estimated from diffraction and total power measurements are in agreement. Such measurements should aid in the physical characterization of ordered block copolymer materials, as well as other random anisotropic media

Effects of chain pull-out on adhesion of elastomers

A thin layer of polystyrene-polyisoprene diblock copolymer was placed at the interface between a polystyrene-coated substrate and a lens-shaped block of cross-linked polyisoprene. The interface toughness was found to vary with the crack growth rate and the areal density of copolymer present at the interface. At high crack growth rates (0.05 mm/h) the toughness was increased by about a factor of 10 by the presence of 0.1 chain/nm 2 at the interface. At lower rates the toughness varied linearly with crack velocity and showed a threshold toughness, as the velocity tended to zero, that increased with the areal density of copolymer

Small-angle X-ray scattering from block copolymers in disordered state: 2. Effect of molecular weight distribution

Spatial segmental density fluctuations occurring in a polystyrene-polyisoprene diblock copolymer in bulk and in disordered state were investigated by small-angle X-ray scattering. The observed wavelength of the dominant mode of the fluctuations was found to be larger than that predicted by the random phase approximation by a factor up to 50%. This discrepancy was found to be essentially removed by taking into account the distributions of molecular weight and composition of the block copolymer.