Polyisoprene Matrix Research Articles

Small-angle X-ray scattering and rheological studies on the ordering process of cylindrical microdomains in a polystyrene- block-polyisoprene copolymer

Ordering of cylindrical microdomains in a low molecular weight polystyrene- block-polyisoprene (SI diblock) copolymer was investigated using small-angle X-ray scattering (SAXS) and rheology. The SI diblock copolymer was found to have (i) hexagonally packed cylindrical microdomains of polystyrene phase in a polyisoprene matrix in the ordered state, and (ii) an order—disorder transition temperature T ODT of ca. 90°C. To investigate the ordering process from the disordered state, a specimen was quenched rapidly from the disordered state to a temperature below its T ODT, and then the time evolution of the SAXS intensity and dynamic moduli ( G′ and G″) (i.e. the ordering process) was monitored. We have found that the ordering process in the block copolymer, when quenched to an ordered state close to its T ODT, takes place very slowly. Specifically, we found that when a specimen was quenched rapidly from 100°C (10°C above T ODT) to 80°C (10°C below T ODT), the ordering into the hexagonally packed cylinders occurs via a nucleation-growth process and the attainment of the equilibrium microdomain morphology took about 6 h when monitoring G′ and G″ and about 8 h when monitoring the SAXS intensity.

Preparation and morphologies of 4- and 12-armed styrene-isoprene star-shaped block copolymers

Styrene(S)-isoprene (I) star-shaped block copolymers of the (SI) 4 and (SI) 12 types were prepared by an anionic polymerization technique. Linear SI arm molecules were prepared with sec-butyllithium as an initiator in benzene, followed by a coupling reaction with the 4-functional linking agent 1,2-bis(methyl-dichlorosilyl)ethane and the 12-functional 1,1,2,2-tetrakis(2-(trichlorosilyl)ethyl)silane, respectively, under vacuum. The star-shaped block copolymers were isolated by precipitational fractionation from aspolymerized crude products using benzene-methanol mixtures. They were confirmed to have the designed molecular structures by careful characterization. Using a transmission electron microscope it was observed that the microphase-separated structures of solvent-cast films of the samples are polystyrene (PS) cylinders in a polyisoprene (PI) matrix, alternating lamellar structures and PI cylinders in a PS matrix for 4-armed star-shaped block copolymers when the values of the volume fraction of PS, ф s , are 0.23, 0.47 and 0.78, respectively, and lamellar structures for the 12-armed star-shaped block copolymers when the ф s values are ∼0.5. Small-angle X-ray scattering studies revealed that lamellar domain spacings of the star-shaped block copolymers are essentially the same as those of the arm molecules, that is, linear diblock copolymers, irrespective of the number of arms.

Ordering by flow near the disorder-order transition of a triblock copolymer styrene-isoprene-styrene

An oriented morphology has been generated by cooling a triblock copolymer styrene-isoprene- styrene (SIS) below ita order-disorder transition (ODT) temperature and annealing it there at constant quench depth AT while simultaneously shearing it. The polymer has an equilibrium microphase-separated morphology of hexagonally packed cylinders of polystyrene in a polyisoprene matrix. The evolving structure was investigated with rheology and small-angle X-ray scattering (SAXS). SAXS showed that mirophase separation occurred rapidly upon cooling below the ODT temperature while the growth of large-scale spatial order (-1-pm scale) needed long annealing times as shown with low-frequency dynamic mechanical measurements. The slow part of the structuring process gave sufficient time for flow alignment of microphase- separated domains during their growth phase. Near morphology was obtained through large- amplitude oscillatory shear at -25 K below ODT. The low-frequency linear viscoelastic properties of the single crystal structure were found to be affected by the domain alignment in the flow direction. The quench depth AT is an important parameter: at a small quench depth of AT 10 K, microphase separation was found to be suppressed by the oscillatory shear.

Thermoreversible morphology transition between spherical and cylindrical microdomains of block copolymers

Thermoreversible morphological transition (MT) of a poly(styrene-block-isoprene) diblock copolymer was studied by SAXS and TEM. The cylindrical and spherical microdomains of polystyrene (PS) which are embedded in the polyisoprene matrix were thermoreversibly observed at 150 and 200°C, respectively. As far as we know, this work may be the first to show that the cylindrical and spherical morphologies can reversibly change with temperature. Using experimentally determined interaction parameters χ, we compared SAXS and TEM results with the theory in the weak segregation limit presented by Leibler, which predicts the thermoreversible MT between spheres and cylinders

Thermoreversible Morphology Transition of Block Copolymers.

Thermoreversible morphological transition of a poly(styrene-block-isoprene) (SI) block copolymer was studied by small-angle X-ray scattering and transmission electron microscopy (TEM). Although block copolymers are theoretically predicted to exhibit a reversible morphological change between lamellae and disordered state via cylinders and spheres with temperature, this thermoreversibility in morphology transition has not been experimentally confirmed yet. In order to study this thermoreversible transition, the SI diblock copolymer may have to have a biased composition and sufficient mobility. To meet these requirements, we used the SI with the weight fraction of polystyrene (PS) 0.16 and with a relative low number-average molecular weight 8.2×104. Heat treatments of the specimen were carefully conducted. Moreover, the ultrathin sectioning was performed by cryo-ultramicrotomy at -85°C for TEM studies. The cylindrical and spherical microdomains of PS which are embedded in the polyisoprene matrix were thermoreversibly observed at 150 and 200°C, respectively.

Diffusion in microstructured block copolymer solutions

AbstractThis work explores the effect of heterogeneity of chemical composition on tracer diffusion, when the characteristic size of the heterogeneities approaches that of the diffusing molecule. A heterogeneous environment is created by the self‐assembly of diblock copolymers in solution. The system chosen for this study is polystyrene‐polyisoprene diblock copolymers in toluene, which is a common solvent for the two blocks. Above a certain critical concentration, these systems are known to microphase separate into swollen domains of polystyrene and polyisoprene. Diffusion of homopolystyrene through the microstructure is measured in this work. The characteristics of the microstructure are varied by studying block copolymers of different molecular weights and compositions. The tracer diffusion coefficients of the labelled polystyrenes are measured by forced Rayleigh scattering, while the microstructure of the matrix is inferred from small angle X‐ray scattering measurements.In this paper, we report results for the diffusion of polystyrenes (molecular weights 3.2 × 104 and 9.0 × 104) in microstructured solutions of three copolymers. Two copolymer samples forming lamellae of alternating polystyrene and polyisoprene microphases and one sample forming polystyrene cylinders embedded in a polyisoprene matrix have been examined. The data indicate that the tracer diffusion coefficient of 3.2 × 104 molecular weight polystyrene in lamellar and cylindrical solutions is comparable to that of the homopolymer in a homogeneous solution of the same concentration. In contrast, the diffusion of 9.0 × 104 molecular weight polystyrene is substantially slower in the structured solutions than in unstructured solutions of the same concentration. For example, the diffusion coefficient of 9.0 × 104 molecular weight polystyrene in a 47% block copolymer solution with cylindrical microstructure is lower than its value in a homogeneous solution of the same concentration by a factor of 30.

Dynamic IR Studies of Microdomain Interphases of Isotope-Labeled Block Copolymers

ABSTRACTBy probing the localized segmental motion of isotope-labeled block copolymers, the physical nature of the interphase region between microphaseseparated domains of block polymers was examined. Dynamic infrared linear dichroism (DIRLD) spectroscopy, which measures the reorientations of submolecular structures induced by a small-amplitude oscillatory strain, was combined with specific isotope-labeling using deuterium-substituted monomers. The latter technique enabled us to differentiate the dynamic responses of well-defined parts of block segments, e.g., near the segment junction, chain end, or middle of the block. The degree of segmental interactions near the interphase region of styrene-isoprene diblock copolymers were studied as a function of the segment location and temperature. The reorientational motion of the polystyrene segment, especially near the block junction, was monitored around the glass transition temperature of the polyisoprene matrix. From this result, the degree of segmental mixing in the interphase region which leads to local plasticization of the polystyrene segment was determined.

Polyisoprene matrix inhomogeneity studied by nitroxide spin probe motion

The spin probe ESR method was applied to study natural rubber and synthetic polyisoprenes with different content of cis-configuration over a wide temperature range. The ESR spectra of natural rubber near and above the glass transition indicate the existence of two distinctly different mobilities as a consequence of the spin probe distribution in sol and gel phase. The results show that the spin probe method can yield information about the inhomogeneity of the polyisoprene matrix and the character of gel phase.

The stress birefringence of vulcanizates of polyisoprene–polyethylene blends

AbstractAs a model for the vulcanizates of block copolymers, mixtures of polyisoprene and polyethylene vulcanized both with a peroxide and with sulfur were investigated by stress birefringence. It was found that the polyethylene dispersion showed a reinforcing effect only in the peroxide vulcanizates. On the other hand, the stress birefringence was decreased with increased polyethylene content except at high polyethylene content. In the latter case, the slope of the line in the birefringence–stress plot was almost equal to that for the pure polyisoprene vulcanizate at high stress levels. However, at lower stresses significant optical creep was observed, i.e., the stress increased without bire‐fringence. Such an optical creep as this exists also in styrene–butadiene block copolymer. These facts are interpreted by the assumption that polyethylene dispersion, when it is linked chemically with polyisoprene matrix, acts as a reinforcing agent by forming physical crosslinks similar to the hard domains in block copolymers. Such physical crosslinks can slip during elongation, resulting in the observed optical creep. These phenomena disappear at the elevated temperature.