Styrene-isoprene Diblock Research Articles

The Static and Dynamic Structure Factor of a Diblock Copolymer Melt via Small-Angle X-ray Scattering and X-ray Photon Correlation Spectroscopy

We present a detailed X-ray scattering study of the static and dynamic behavior of a styrene–isoprene diblock copolymer melt with a styrene volume fraction of 0.3468. At 115 and 120 °C, we observe splitting of the principal Bragg peak, which we attribute to phase coexistence of hexagonal cylindrical and cubic double-gyroid structure. In the disordered phase, above 130 °C, we have characterized the dynamics of composition fluctuations via X-ray photon correlation spectroscopy. Near the peak of the static structure factor, these fluctuations show stretched-exponential relaxations, characterized by a stretching exponent of about 0.36 for a range of temperatures immediately above the MST. The corresponding characteristic relaxation times vary exponentially with temperature, changing by a factor of 2 for each 2 °C change in temperature. At low wavevectors, the measured relaxations are diffusive with relaxation times that change by a factor of 2 for each 8 °C change in temperature.

Anionic bulk polymerization to synthesize styrene–isoprene diblock and multiblock copolymers by reactive extrusion

ABSTRACTIn this study, styrene–isoprene diblock and multiblock copolymers were synthesized with n‐butyllithium as the initiator, in an intermeshing, corotating twin‐screw extruder. The diameter (D) of this extruder was 36 mm, and the ratio of length/diameter was 56. The weight content of polyisoprene in these copolymers was above 50% although in the past studies it had not been possible to accomplish levels higher than 30%. Gel permeation chromatography results of samples and their degraded products show that there is only one long block polystyrene in the diblock copolymer chains; while in the multiblock copolymer molecules, there is a long block and large numbers of small blocks. Dynamic mechanical analysis and transmission electron microscopy show that the two phases in the diblock copolymer are completely incompatible, leading to sharp phase separation. In the multiblock copolymer, the two phases are partly compatible without an obvious phase boundary. The successful syntheses of styrene/isoprene diblock and multiblock copolymers with high‐isoprene contents provide a novel method to synthesize polystyrene rubbers and styrene–diene–styrene thermoplastic elastomers. Traditionally, these products were mainly synthesized by solution polymerization. The present work in this article provides the possibility to synthesize them with very little or no solvent using bulk polymerization. This method fits the environmentally friendly trend to use low amounts of carbon while allowing commercial profitability. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 39429.

Hydrogenation of C=C bonds in styrene-isoprene-styrene block copolymers in the presence of monocyclopentadienyl titanium(IV) catalytic systems

Hydrogenation of C=C bonds in styrene-isoprene-styrene (SIS) block copolymers in the presence of CpTiCl2(OC6H4Y-p)/BuLi and CpTiCl2(OC6H4Y-p)/Et3Al catalytic systems (Cp = cyclopentadienyl, Bu = butyl, Et = ethyl; Y = Cl, H, OCH3) was investigated. All of the catalytic systems were active in these reactions. Their activity depended on the kind of the metal alkyl, the copolymer composition, reaction time, temperature and hydrogen pressure. It was established that access of catalytic species to the C=C bonds in the isoprene blocks is the most important factor in the hydrogenation efficiency. The smallest steric hindrances occur when phenoxy ligands in the catalytic systems are unsubstituted (Y = H) and styrene-isoprene diblocks (SI) are present in the SIS copolymer. Thus, the catalytic systems CpTiCl2(OC6H4Cl)/BuLi were the most active in KratonŽ D-1161 copolymers hydrogenation.

Hydrogenation of C=C bonds in styrene-butadiene-styrene block copolymers in the presence of monocyclopentadienyl titanium(IV) catalytic systems

Summary — Hydrogenation of C=C bonds in styrene-isoprene-styrene (SIS) block copolymers in the presence of CpTiCl 2 (OC 6 H 4 Y-p)/BuLi and CpTiCl 2 (OC 6 H 4 Y-p)/Et 3 Al catalytic systems (Cp = cyclopentadienyl, Bu = butyl, Et = ethyl; Y = Cl, H, OCH3) was investigated. All of the catalytic systems were active in these reactions. Their activity depended on the kind of the metal alkyl, the copolymer composition, reaction time, temperature and hydrogen pressure. It was established that access of catalytic species to the C=C bonds in the isoprene blocks is the most important factor in the hydrogenation efficiency. The smallest steric hindrances occur when phenoxy ligands in the catalytic systems are unsubstituted (Y = H) and styrene-isoprene diblocks (SI) are present in the SIS copolymer. Thus, the catalytic systems CpTiCl2(OC6H4Cl)/BuLi were the most active in Kraton ® D-1161 copolymers hydrogenation.

Interfacial change on morphological transitions in styrene–isoprene diblock copolymer

The specific interfacial area ( S/ V) and interfacial thickness in each microstructure of styrene–isoprene diblock copolymer were estimated by analyzing the deviations from Porod’s law. The thermally induced phase transitions proceeded from lamellae (L) to hexagonally ordered cylinder (HEX), via hexagonally perforated layer (HPL) and gyroid (G). The S/ V ratio increased stepwise at the order–order transition (OOT) from L to HEX, via HPL and G. The S/ V data can be utilized for OOT determination.

Controlling the colloidal behavior of styrene–isoprene diblock copolymers by selective end functionalization

ω-Lithium poly(styrene- b-isoprene) sulfonates were synthesized by anionic polymerization. The copolymers carrying the lithium sulfonate (SuLi) end group at either the soluble or the insoluble block form a variety of different microstructures in n-decane. In the case where the SuLi group is located at the PS insoluble end block, micellar aggregates with larger aggregation number, hydrodynamic radius and thermal stability are formed, compared to the non-functionalized diblock precursor case. When the polar group is located in the soluble PI block end, the solubility of the system depends on temperature. At high temperatures where end group association primarily exists, star-like aggregates are formed. At lower temperatures PS block association also takes place, resulting in a system of highly polydisperse and interconnected aggregates. The latter materials can be considered as ABC heterotelechelic associating macromolecules presenting new ways of controlling the colloidal behavior of block copolymer systems by selective end functionalization.

Collaborative studies of thermo-oxidative degradation of styrene–isoprene diblock copolymer

Mechanisms and dynamics of thermo-oxidative degradation of styrene- co-isoprene (SI) diblock at a temperature above the order–disorder transition temperature, T odt, have been investigated both experimentally and theoretically. The structural development of SI during degradation was monitored by optical microscopy, and the reduction of molecular weight and molecular weight distribution during the degradation was determined by gel permeation chromatography (GPC). The degradation process was modeled in the framework of a kinetic model based on the framework of the Flory principle. The theoretical predictions were compared with the GPC results. Fourier transformed infrared spectroscopy (FTIR) experiment was conducted in situ during the thermo-oxidative degradation of the SI copolymers, to gain further insight into the chemical modification mechanisms, the reduction of unsaturated double bonds and the formation of carbonyl and hydroxyl groups.

Interplay between Cubic and Hexagonal Phases in Block Copolymer Solutions

The phase behavior of a symmetric styrene-isoprene (SI) diblock copolymer in a styrene-selective solvent, diethylphthalate, was investigated by in situ small-angle X-ray scattering on isotropic and shear-oriented solutions and by rheology and birefringence. A remarkable new feature in this phase diagram is the coexistence of both body-centered cubic (bcc) and hexagonally close-packed (hcp) sphere phases, in a region between close-packed spheres (cps) and hexagonally packed cylinders (hex) over the concentration range phi approximately 0.33-0.45. By focusing on the transitions among these various ordered phases during heating and cooling cycles, we observed a strong hysteresis: supercooled cylinders persisted upon cooling. The stability of these supercooled cylinders is quite dependent on concentration, and for phi > or = 0.40, the supercooled cylinders do not revert to spheres even after quiescent annealing for 1 month. The spontaneous formation of spheres due to the dissociation of cylinders is kinetically hindered in this case, and the system is apparently not amenable to any pretransitional fluctuations of cylinders prior to the cylinder-to-sphere transition. This contrasts with the case of cylinders transforming to spheres upon heating in the melt. The application of large amplitude shear to the supercooled cylinders is effective in restoring the equilibrium sphere phases.

Rheological properties of hot melt pressure-sensitive adhesives based on styrene--isoprene copolymers. Part 1: A rheological model for [sis-si] formulations

The viscoelastic properties of hot melt pressure-sensitive adhesives (HMPSA) based on formulations of block copolymers and tackifying resins have been studied in detail, through the variation of the complex shear modulus, G*, as a function of frequency, y . In this first article, we analyze the individual behavior of the components of HMPSA blends: (1) the two copolymers, styrene-isoprene (SI) diblock copolymer and styrene-isoprene-styrene (SIS) triblock copolymer and (2) two tackifying resins. The viscoelastic behavior of the overall formulation is also presented. We have mainly studied the effects of (1) the molecular characteristics of the SI and SIS copolymers and (2) the composition of the blends (mainly the effect of SI content, S content in SIS and SI, resin content) on the viscoelastic properties. A theoretical approach based on concepts of molecular dynamics leads to a model which describes reasonably well the linear viscoelastic properties of individual components and their formulations. Our systematic study can be used to design new copolymer molecules which can mimic the rheological behavior and end-user properties of regular formulations at room temperature.

Mechanism of morphological transition from lamellar/perforated layer to gyroid phases

We investigated epitaxial relations of phase transitions between the lamellar (L), hexagonally perforated layers (HPL), and gyroid (G) morphologies in styrene-isoprene diblock copolymer (PSI) and polyisoprene (PI)/PSI blend using rheology and small angle X-ray scattering (SAXS) techniques. In HPL→G transitions, six spot patterns of G phase were observed in two-dimensitional SAXS pattern. On the other hand, in direct L→G transition without appearance of HPL phase, the polydomain patterns of G phase were observed. From present study, it was understood that direct L→G transition of blend may be suppressed by high-energy barrier of transition and mismatches in domain orientation between epitaxially related lattice planes.

Accounting for Auger yield energy loss for improved determination of molecular orientation using soft x-ray absorption spectroscopy

Partial (Auger) yield near edge x-ray absorption fine structure (NEXAFS) is a structural analytical technique that has been primarily used to measure the spatial orientation and chemical bonding of small molecules on solid (i.e., inorganic or semiconductor) surfaces. In this article we demonstrate that the building block (BB) scheme proposed by Outka and co-workers [Phys. Rev. Lett. 59, 1321 (1987)] for analyzing NEXAFS spectra can be applied to model the molecular orientation of larger molecules, provided one accounts properly for kinetic energy losses of the Auger electrons traversing through the sample and hence the attenuation in measured Auger yield. We test the applicability of the proposed “modified” BB (MBB) model by measuring the orientation of a self-assembled monolayer (SAM) of –O1.5Si–(CH2)2–(CF2)8F, SF–SAM (SiOx), deposited on top of SiOx-covered silicon wafer as a function of the entrance grid bias (EGB) of the channeltron photoelectron detector. Our measurements of the EGB-dependent electron escape depth reveal that a crude depth profiling within the top ≈5 nm of the sample is possible by increasing the negative EGB on the channeltron detector, at the highest bias thus selecting only the Auger electrons, which have suffered negligible energy loss. In addition, we discuss how the order parameter method introduced recently by Stöhr and Samant [J. Electron Spectrosc. Relat. Phenom. 98–99, 189 (1989)] can be used to determine the molecular orientation of large organic molecules on surfaces. We also show that by accounting for energy losses of the NEXAFS Auger electrons (attenuation of measured Auger yield), the corrected order parameter (COP) approach gives good estimates of the orientation of molecules. We present a comparison between the MBB and COP models using experimental data collected from NEXAFS experiments from semifluorinated (SF) mesogens, –(CH2)x(CF2)yF, which are attached to: (1) the isoprene backbone of polyisoprene or a styrene–isoprene diblock copolymer and (2) a SiOx-covered solid substrate. We show that on both surfaces, the SF groups are oriented and on average are tilted by an angle 〈τF-helix〉 from the sample normal. We show that at higher 〈τF-helix〉 the results from the COP approach agree almost quantitatively with those extracted using the MBB model.

The effect of block lengths on micelle structure in blends of styrene–isoprene diblock copolymer and poly(vinyl methyl ether)

The micellar structure of styrene–isoprene diblock copolymer and poly(vinyl methyl ether) blends was investigated by using small-angle X-ray scattering and transmission electron microscopy techniques. In order to determine the effect of styrene block length on the formation of micellar structure, three sets of diblock copolymers, with near-identical isoprene block molecular weights, but with different styrene block lengths were studied. With modeling based on the polydisperse Percus–Yevick hard sphere fluid model, the structural parameters characterizing the micelles were determined as a function of copolymer concentrations, temperature, and copolymer block lengths. The core radius was found to decrease on increasing the length of styrene block. The degree of swelling of the corona by PVME increased steadily with increasing the styrene block length.

Dynamic mechanical relaxation behavior of block copolymers at high temperatures

AbstractWe carried out dynamic mechanical measurements to investigate three different examples of block copolymers: styrene–isoprene diblock copolymers and styrene–butadiene–styrene and styrene–(styrene butadiene)–styrene triblock copolymers. Isochronal and isothermal measurements of the real and imaginary parts of the complex shear modulus were performed over wide ranges of temperature and frequency. The measurements showed the presence of an additional relaxation process appearing at temperatures higher than those of the glass relaxation of the polystyrene phase, which has been misinterpreted by some authors as an order–disorder transition. The frequency dependence revealed that this process was a relaxation process and did not belong to a first‐order transition. Moreover, the influence of crosslinking via dicumylperoxide was measured, and we constructed complete master curves to confirm the presence of two relaxation processes. The high‐temperature relaxation process was strongly suppressed by crosslinking. Therefore, it was possible to detect the glass relaxation process of the polystyrene phase in a precise manner. The results were compared with those of homopolymers. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2198–2206, 2001

Phase behavior and viscoelastic properties of entangled block copolymer gels

AbstractTriblock copolymers in midblock‐selective solvents can form physical gels. However, at low triblock contents (near the percolation threshold), the bridging of chains between micelles can lead to macrophase separation. Adding a styrene–isoprene diblock to a styrene–isoprene–styrene triblock copolymer in squalane can eliminate macrophase separation, yielding a wide range of stable, single‐phase gels with a disordered arrangement of micelles. The plateau modulus of these triblock gels scales with the 2.2 power of polymer content, indicating the importance of entanglements in dictating the modulus. Comparing gels made from the midblock‐saturated derivative of the same polymer [styrene‐(ethylene‐alt‐propylene)‐styrene] in squalane reveals that the modulus differences in the gels are a direct consequence of the difference in the entanglement molecular weight of the midblock homopolymer in bulk. Finally, the broad relaxation spectrum of these triblocks is well‐described by a recent theory for the dynamics of entangled star polymers, with the breadth of the relaxation spectrum dictated by the number of entanglements per midblock in the gel. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2183–2197, 2001

Micelle Formation in Mixtures of Styrene−Isoprene Diblock Copolymer and Poly(vinyl methyl ether)

The characteristics of micelle formed in blends of styrene-isoprene diblock copolymer (SI) and poly(vinyl methyl ether) (PVME) were investigated by using small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and light scattering techniques. The results of SAXS and TEM experiments indicate that the blends form spherical micelles with isoprene-rich cores and styrene-corona mixed with PVME in a matrix of PVME. As temperature increased, the blends underwent macrophase separation into a PVME-rich phase and a SI-rich phase which was also microphase-separated to ordered microdomains. Upon further heating, the ordered structure of the SI-rich region transforms to a disordered phase in a manner resembling the order-disorder transition in pure block copolymer melts.

Understanding and controlling the morphology of styrene–isoprene side-group liquid crystalline diblock copolymers

In side-chain liquid crystalline diblock copolymers, driving forces for ordering of the material may be provided by the chemical incompatibility between the blocks and by the liquid crystalline nature of one of the blocks. We study the microstructure and its development from initially isotropic solutions in side-group liquid crystalline copolymers based on styrene–isoprene diblocks. Hierarchical structure from the 5Å to the 500Å length scale is observed, the product of coherent block copolymer microphase separation and liquid crystalline mesophase formation. The presence of cylindrical microdomains of either the poly(isoprene-LC) or poly(styrene) block markedly increased the thermal stability of the LC. Confinement of the LC to cylindrical microdomains strongly inhibited defect formation within the mesophase after suitable orientation and thermal treatment, readily manifested by the significantly improved optical clarity of these samples versus LC matrix or LC lamellar samples. We consider the relative stabilities of parallel–transverse, perpendicular–parallel, parallel–parallel and transverse–perpendicular arrangements of the microdomain and mesophase structures in interpreting the development of structure in these materials under oscillatory shear.

Effect of dilution on a block copolymer in the complex phase window

The phase behavior of a styrene–isoprene (SI) diblock copolymer, with block molecular weights of 1.1 × 104 and 2.1 × 104 g/mol, respectively, is examined in the neutral solvent bis(2-ethylhexyl) phthalate (DOP) and the styrene-selective solvent di-n-butyl phthalate (DBP). DBP is a good solvent for PS, but is near a theta solvent for PI at approximately 90°C. Small-angle X-ray scattering (SAXS), rheology, and static birefringence are used to locate and identify order–order (OOT) and order–disorder transitions (ODT); all three techniques gave consistent results. The neat polymer adopts the gyroid (G) phase at low temperatures, with an OOT to hexagonally-packed cylinders (C) at 185°C, and the ODT at 238°C. Upon dilution with the neutral solvent DOP, the C window is diminished, until for a polymer concentration ϕ = 0.65, a direct G to disorder (D) ODT is observed. These results reflect increased stability of the disordered state, based on the different concentration scalings of the interaction parameter, χ, at the OOT and ODT. The OOT follows the dilution approximation, i.e., χOOT ∼ ϕ−1, but the ODT is found to follow a stronger concentration dependence, i.e., χODT ∼ ϕ−1.4, similar to the scaling of ϕ−1.6 found previously for lamellar SI diblocks in toluene and DOP. Addition of the selective solvent DBP produces dramatic changes in the phase behavior relative to DOP and the melt state; these include transitions to lamellar (L) and perforated layer (PL) structures. The observed phase sequences can be understood in terms of trajectories across the SI melt phase map (temperature vs. composition): addition of a neutral solvent or increasing temperature corresponds to a “vertical” trajectory, whereas adding a selective solvent amounts to a “horizontal” trajectory. When the solvent selectivity depends on temperature, as it does for the SI/DBP system, increasing temperature results in a diagonal trajectory. For both neutral and selective solvents the domain spacing, d*, scales with ϕ and χ as anticipated by self-consistent mean-field theory. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 3101–3113, 1998

Surfaces of Semi-Fluorinated Block Copolymers Studied Using Nexafs

ABSTRACTThe molecular orientation within a surface liquid crystalline layer made up of semifluorinated side-groups [-CO-(CH2)x−x-(CF2)yF] (SF groups) attached to the isoprene block of a styrene-isoprene diblock copolymer was determined by analyzing the partial electron yield Cedge NEXAFS signal. The results show that in contrast to the bulk, where the SF groups lie parallel to the diblock copolymer lamellae and thus parallel to the surface, the surface SF groups make an average angle with the surface normal of between 29 and 46° depending on x and y.

Fluctuation regime in the viscoelastic properties of block copolymer solutions

We report measurements of the dynamic shear moduli for solutions of three symmetric styrene isoprene diblock copolymers in the neutral solvent di-octyl phthalate. For each sample the concentration range was selected to extend from the disordered regime to the (lamellar) ordered state, with the emphasis placed on the intermediate regime, where large amplitude composition fluctuations are clearly evident. Temperatures from 0° to nearly 100°C were employed. The fluctuations were most clearly revealed when the data were plotted as phase angle versus reduced frequency, where the frequency axis was scaled by the longest relaxation time in the absence of fluctuations. The dynamic shear moduli and the apparent longest relaxation times, all normalized to their respective values in the absence of fluctuations, increased markedly as the order-disorder transition was approached. The rate of increase was consistent with recent theoretical predictions of Fredrickson, Larson, and Helfand, but the magnitude of the effect is larger than anticipated by theory. The onset of visible fluctuation effects depends on molecular weight in a manner that suggests that entanglements play an important role in determining the coupling of the fluctuations to the viscoelastic properties. This is consistent with previous measurements of translational diffusion, which demonstrated that entanglements severely diminish the mobility of lamellar-forming block copolymers along the interfaces between microdomains.

Miscibility in blends of PVME with styrene-based block copolymers

The phase behaviours in binary blends of styrene-isoprene diblock copolymer and poly(vinyl methyl ether) (PVME) were investigated by using small-angle X-ray and light scattering techniques. It was observed that the added homopolymer molecules were dissolved into the microdomains of the styrene blocks of the copolymer. The blends exhibited a lower critical solution temperature as seen in the blends of PVME with polystyrene. The experimental results were compared with that of the modified Meier's theory in a semiquantitative way.