Styrene-isoprene Diblock Copolymer 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.

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

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.

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.

Dielectric normal mode relaxation of tethered polyisoprene chains in styrene-isoprene block copolymers

Abstract

High-Pressure Effects on the Disordered Phase of Block Copolymer Melts

We have measured the effect of high pressure on the scattering from the disordered state of a styrene-isoprene diblock copolymer containing 48 wt % styrene (M w ∼ 16 500). Small-angle X-ray scattering measurements performed at the Cornell High Energy Synchrotron Source (CHESS) show an increase in the aspect ratio of the scattering maximum and a decrease in the angle at which the maximum occurs with increasing pressure. Within the context of the mean-field theory of block copolymer phase behavior originally developed by Leibler, 1 this behavior is due to an increase in both the radius of gyration of the molecule (R g ) and the degree of segregation of the system (χN). Examination of the data using the theory of Tang and Freed 2 reveals an identical quantitative dependence of R g and χN on pressure without indicating a role for the free volume fraction used by these authors to treat compressibility effects. We therefore propose that the observed behavior arises primarily from shifts in the structural characteristics of the blocks, rather than from a reduction in free volume. From comparison of the pressure-induced changes in the structural characteristics of the system with the corresponding temperature-induced ones, we predict the pressure dependence of the order-disorder transition temperature in this material, provided that the phase state of this polymer is determined solely by the structural characteristics examined in this experiment.

Temperature and Molecular Weight Effect of Styrene–Isoprene Diblock Copolymers on the Interfacial Tension between Polystyrene and Polyisoprene

The effect of styrene–isoprene diblock copolymers on the interfacial tension between polystyrene (PS) and polyisoprene (PI) has been investigated by direct measurement of the interfacial tension using a pendant drop interfacial tension apparatus. The interfacial tension is nearly independent of the temperature when the diblock copolymers are added, although the interfacial tension of PS/PI without copolymers decreases with increasing the temperature. A decrease in interfacial tension is observed with the addition of a small amount of copolymer, followed by a leveling off when the copolymer concentration exceeds a critical micelle concentration. The critical micelle concentration increases with temperature. The effect of the molecular weight of block copolymers on the interfacial tension is not significant.

NMR Investigation of the Order-Disorder and Rheological Transitions in Styrene-Isoprene Diblock Copolymers

13 C NMR spectroscopy was used to investigate the physical basis of the rheological thermal transition (T rheo ) in symmetric poly(styrene-b-isoprene) (SI) diblock copolymers. Among the studied samples, those with M n = (3-32)×10 3 , which all exhibited two glass transition features (T g,1 and T g,2 ) in their DSC curves, had T rheo values ranging from -10°C to 205°C. The NMR line width data related to the styrene units in the bulk materials show that a strong segregation behavior persists over a wide range of temperature above T rheo

A Dielectric Study on Dynamics of Tethered Ciiains: Styrene-Isoprene Diblock Copolymers in Bulk and in Selective Solvent

The dynamics of cis-polyisoprene (I) blocks tethered on glassy polystyrene (PS) domains in microphase separated styrene-isoprene (SI) diblock copolymers in bulk, in blends with PS-polybutadiene (SB) diblock copolymer, and also in solution of n-tetradecane (C14) that is a nonsolvent to S blocks, is studied

Investigation on micelle forming behavior of poly(styrene-isoprene) diblock copolymers in selective solvents via size exclusion chromatography

Size Exclusion Chromatography(SEC) was used to investigate the micelle forming behavior of Poly(styreneisoprene) diblock copolymers, with same composition and various molecular weight in different selective solvents at different temperatures. The aggregation number and aggregation energy of micelle can be determined. The effects of selective solvent, temperature and molecular weight of PS-PI on micelle forming behavior have been discussed.