Apparent hydrodynamic thickness of densely grafted polymer layers in a theta solvent

We study the drainage of a near-theta solvent through densely grafted polymer layers and compare to recent notions that these layers display little permeability to solvent flow at surface separations less than a “hydrodynamic thickness.” The solvent is trans-decalin (a near-theta solvent at the experimental temperature of 24°C). The polymer is polystyrene (PS) end-attached to two opposed mica surfaces via the selective adsorption of the polyvinylpyridine (PVP) block of a PS-PVP diblock copolymer. The experimental probe was a surface forces apparatus modified to apply small-amplitude oscillatory displacements in the normal direction. Out-of-phase responses reflected viscous flow of solvent alone—the PS chains did not appear to contribute to dissipation over the oscillation frequencies studied. The value of the hydrodynamic thickness (R_H) was less than the coil thickness (L_o) measured independently from the onset of surface–surface interactions in the force-distance profile, implying significant penetration of the velocity field into the polymer layer. As the surface–surface separation was reduced from 3L_o to 0.3L_o, the apparent hydrodynamic thickness (R) decreased monotonically to values R ≪ R_H. Physically, this indicates that the “slip plane” moved progressively closer to the solid surfaces with decreasing surface–surface separation. This was accompanied by augmentation of the effective viscosity by a factor of up to approximately 5, indicating somewhat diminished permeability of solvent through the overlapping polymer layers. Similar results hold for the flow through surface-anchored polymers in a good solvent. It is interesting to note the strong stretching of densely end-grafted polymers in a theta solvent. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2961–2968, 1997     

Analysis of glass transition and relaxation processes of low molecular weight polystyrene-b-polyisoprene diblock copolymers

The objective of this study is to analyze the glass transition temperature and relaxation processes of low molecular weight polystyrene-block-polyisoprene diblock copolymers with different compositions, synthesized via anionic polymerization. Thermal properties were investigated by differential scanning calorimetry and dynamic-mechanical thermal analysis, while the morphologies at room temperature were investigated by transmission electron microscopy and small-angle X-ray scattering. The χN values indicate that the diblock copolymers lie near the weak segregation regime. Three different experimental techniques were applied to determine the dynamic properties, i.e., linear viscoelastic shear oscillations, creep recovery experiments, and dielectric spectroscopy. The rheological experiments were performed above the order–disorder transition temperature where the diblock copolymers behave like a Maxwell fluid. Our results indicate that the presence of the polyisoprene segments strongly influences the monomeric friction coefficient and the tendency to form entanglements above the order–disorder temperature. Consequently, the zero-shear rate viscosity of a diblock copolymer is much lower than the zero-shear rate viscosity of the neat polystyrene block (the polystyrene precursor of the polymerization procedure). Dielectric spectroscopy enables the analysis of relaxation processes below the glass transition of the polystyrene microphase. Frequency sweeps indicate the dynamic glass transition of the polyisoprene blocks, which are partly mixed with the polystyrene blocks, which are always the majority component in the block copolymers of this study.     

Synthesis and characterization of poly[styrene‐b‐methyl(3,3,3‐trifluoropropyl)siloxane] diblock copolymers via anionic polymerization

A series of narrow molecular weight distribution (MWD) polystyrene‐b‐poly[methyl(3,3,3‐trifluoropropyl)siloxane] (PS‐b‐PMTFPS) diblock copolymers were synthesized by the sequential anionic polymerization of styrene and trans‐1,3,5‐trimethyl‐1,3,5‐tris(3′,3′,3′‐trifluoropropyl)cyclotrisiloxane in tetrahydrofuran (THF) with n‐butyllithium as the initiator. The diblock copolymers had narrow MWDs ranging from 1.06 to 1.20 and number‐average molecular weights ranging from 8.2 × 10³ to 37.1 × 10³. To investigate the properties of the copolymers, diblock copolymers with different weight fractions of poly[methyl(3,3,3‐trifluoropropyl)siloxane] (15.4–78.8 wt %) were prepared. The compositions of the diblock copolymers were calculated from the characteristic proton integrals of ¹H NMR spectra. For the anionic ring‐opening polymerization (ROP) of 1,3,5‐trimethyl‐1,3,5‐tris(3′,3′,3′‐trifluoropropyl)cyclotrisiloxane (F₃) initiated by polystyryllithium, high monomer concentrations could give high polymer yields and good control of MWDs when THF was used as the polymerization solvent. It was speculated that good control of the block copolymerization under the condition of high monomer concentrations was due to the slowdown of the anionic ROP rate of F₃ and the steric hindrance of the polystyrene precursors. There was enough time to terminate the ROP of F₃ when the polymer yield was high, and good control of block copolymerization could be achieved thereafter. The thermal properties (differential scanning calorimetry and thermogravimetric analysis) were also investigated for the PS‐b‐PMTFPS diblock copolymers. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4431–4438, 2005     

Dynamics in disordered block copolymers and miscible blends composed of poly(vinyl ethylene) and polyisoprene

We investigated the segmental and terminal relaxation dynamics of a well‐characterized disordered diblock copolymer, poly(isoprene‐b‐vinyl ethylene) (PI‐PVE), and miscible blends of polyisoprene (PI)/poly(vinyl ethylene) (PVE), using dielectric and viscoelastic spectroscopies. Generally, the concentration fluctuation (CF) amplitude of a disordered diblock copolymer is smaller than that of the miscible blend, especially in a length scale longer than the size of the whole block chain. To test whether the difference in the CF amplitudes causes the difference in the segmental relaxation spectra, we compared the shape of the dielectric loss curves between PI‐PVE and PI/PVE with the same composition (PI/PVE ratio = 17:83). However, no appreciable difference was observed, indicating that the CF amplitudes in PI‐PVE and PI/PVE are not so different in the length scale of the segmental motions. We also examined the effect of distinct friction coefficients of the PI and PVE chains on the terminal relaxation dynamics by comparisons of the viscoelastic and dielectric normal mode relaxations in PI‐PVE. The former probes the whole chain motion and the latter probes motions of the PI block. Shift factors (a_T) for the viscoelastic and dielectric relaxations were compared. The dielectric normal mode a_T was found to have weaker temperature dependence than the viscoelastic a_T, which indicates that the friction for the PI block chain is lower than the average friction for the PI‐PVE chain. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4084–4094, 2004     

Normal and shear forces between a polyelectrolyte brush and a solid surface

The diblock copolymer poly(methyl methacrylate)‐b‐poly(sodium sulfonated glycidyl methacrylate) (PMMA‐b‐PSGMA) was end‐attached by its hydrophobic block (PMMA) onto mica hydrophobized by a stearic trimethylammonium iodide (STAI) layer, to form a polyelectrolyte brush immersed in water. With a surface force balance (SFB), we extended earlier measurements between two such brush layers for the case of normal and shear forces at different shear rates, surface separation, and compressions between one mica surface coated with STAI or a STAI‐diblock layer against a bare mica surface. After coating one of the surfaces with STAI, a long range attraction that results in a jump into an adhesive flat contact between the hydrophobic and hydrophilic surfaces was observed. A very different behavior was seen after forming the polyelectrolyte brush on the STAI‐coated surface. The long range attraction was replaced by repulsion, accompanied by very low friction during shear (ca. three orders of magnitude lower than with adsorbed polyelectrolytes). On further compression, a weak attraction to the adhesive contact was observed. From the final surface–surface contact separation, we deduce that most of the polyelectrolyte diblock brush layer was squeezed out from the gap, leaving the STAI layer and a small amount of the polymer attached to the surface. Stick‐sliding behavior was seen while applying shear, suggesting a dissipation mechanism caused by the trapped polyelectrolyte. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 193–204, 2005     

Interactions between surfactant-coated surfaces in hydrocarbon liquids containing functionalized polymer dispersant

The forces and viscosity between calcium benzene sulfonate surfactant-coated mica surfaces in various hydrocarbon liquids containing a polyamine-functionalized hydrocarbon polymer (M _W≈8000) have been measured using the surface forces apparatus technique. The polymer is found to adsorb to the substrate surfaces by displacing the surfactant layer, and to produce forces that are monotonically repulsive. The forces have a maximum range of 50–100 nm (>3R _H), indicating that tails play a particularly important role in the interaction of this relatively low molecular weight polymer. The forces become steeply repulsive below about 10 nm (∼0.6R _H), at which point a “hard-wall” repulsion comes in that can sustain pressures greater than 100 atm. Thin-film viscosity measurements indicate that the far-field positions of the slipping planes Δ_H depend on the shear rate, showing that significant shear thinning/thickening effects occur within the outermost tail regions of the adsorbed layers during shear. The position of the slipping plane, or hydrodynamic layer thickness Δ_H, varies from 0.6R _H to 2R _H away from each surface (mica and surfactant-coated mica surfaces). Beyond the hydrodynamic layer the far-field fluid viscosity is the same as that of the bulk polymer solution. At separations below D = 2Δ_H the viscosity increases as each polymer layer is compressed. The static forces exhibited various time- and history-dependent effects, which further indicate that a number of different relaxation/equilibration processes are operating simultaneously in this complex multicomponent system. The results reveal that the interactions of tails of functionally adsorbed polymers play a more important role than previously thought. This is especially true in this study where the adsorbed polymers are of low molecular weight and where the tails may represent the largest fraction of interacting segments. Received: 22 September 1998 Accepted: 11 January 1999     

Viscoelasticity of nanobubble‐inflated ultrathin polymer films: Justification by the coupling model

The nanobubble inflation method is the only experimental technique that can measure the viscoelastic creep compliance of unsupported ultrathin films of polymers over the glass–rubber transition zone as well as the dependence of the glass transition temperature (T_g) on film thickness. Sizeable reduction of T_g was observed in polystyrene (PS) and bisphenol A polycarbonate by the shift of the creep compliance to shorter times. The dependence of T_g on film thickness is consistent with the published data of free‐standing PS ultrathin films. However, accompanying the shift of the compliance to shorter times, a decrease in the rubbery plateau compliance is observed. The decrease becomes more dramatic in thinner films and at lower temperatures. This anomalous viscoelastic behavior was also observed in poly(vinyl acetate) and poly (n‐butyl methacrylate), but with large variation in the change of either the T_g or the plateau compliance. By now, well established in bulk polymers is the presence of three different viscoelastic mechanisms in the glass–rubber transition zone, namely, the Rouse modes, the sub‐Rouse modes, and the segmental α‐relaxation. Based on the thermorheological complexity of the three mechanisms, the viscoelastic anomaly observed in ultrathin polymer films and its dependence on chemical structure are explained in the framework of the Coupling Model. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Improving mechanical properties and chemical resistance of ink‐jet printer color filter by using diblock polymeric dispersants

The diblock copolymers of polystyrene and poly(tert‐butyl acrylate) (PSt‐b‐PtBA) with various molecular weights and hydrophobic/hydrophilic (styrene/acrylic acid) chain length were prepared by atom transfer radical polymerization (ATRP). Selective hydrolysis of the diblock copolymers (PSt‐b‐PtBA) resulted in amphiphilic block copolymers of polystyrene and poly(acrylic acid) (PSt‐b‐PAA). The amphiphilic block copolymers of PSt‐b‐PAA with average molecular weight (M_n) <7500 were proved to be critical in dispersing the pigments of UV curable ink‐jet inks for manufacturing the color filter. Incorporating DB2 diblock copolymer dispersants with styrene/acrylic acid ratio at 1.5 allowed more UV curable compositions in the red and blue inks without deteriorating pigment dispersing stability and jetting properties of the ink‐jet inks. The ink drops can be precisely ejected into the tiny color area. Better properties of the cured red stripe such as nanoindentation hardness and chemical resistance were found. The competing absorption of UV light by the blue pigment hindered the through cure of monomers near the interface between glass substrate and the blue stripe. This leads to lower hardness and poor chemical resistance of the UV cured blue stripe. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3337–3353, 2005     

Poly(styrene-b-2-vinylpyridine-1-oxide) and poly(dimethylsiloxane-b-2 vinylpyridine-1-oxide) diblock copolymers. 1. Preparation and characterisation

Sequential anionic polymerisation routes have been used to prepare AB diblock copolymers, where A is either polystyrene or polydimethylsiloxane, and B is poly(2-vinylpyridine-1-oxide). The latter block, which is water-soluble, was obtained from the oxidation of poly(2-vinylpyridine) using peroxyacetic acid (giving 100% yield).The resultant diblock copolymers were characterised by gel-permeation chromatography, proton nuclear magnetic resonance and gravimetric microanalysis to give relative block lengths and polydispersity indices. For both types of block copolymersM _w /M _n values <1.25 could be readily obtained under carefully controlled conditions.     

Friction and normal interaction forces between irreversibly attached weakly charged polymer brushes

Polyelectrolyte brushes were built on mica by anchoring polystyrene-poly(acrylic acid) (PS-b-PAA) diblock copolymers at a controlled surface density in a polystyrene monolayer covalently attached to OH-activated mica surfaces. Compared to physisorbed polymer brushes, these irreversibly attached charged brushes allow the polymer grafting density to remain constant upon changes in environmental conditions (e.g., pH, salt concentration, compression, and shear). The normal interaction and friction forces as a function of surface separation distance and at different concentrations of added salt (NaCl) were investigated using a surface forces apparatus. The interaction force profiles were completely reversible both on loading and receding and were purely repulsive. For a constant polymer grafting density, the influence of the polyelectrolyte charges and the Debye screening effect on the overall interaction forces was investigated. The experimental interaction force profiles agree very well with scaling models developed for neutral and charged polymer brushes. The variation of the friction force between two PAA brushes in motion with respect to each other as a function of surface separation distance appeared to be similar to that observed with neutral brushes. This similarity suggests that the increase in friction is associated with an increase in mutual interpenetration upon compression as observed with neutral polymers. The effect of the PAA charges and added ions was more significant on the repulsive normal forces than on the friction forces. The reversible characteristics of the normal force profiles and friction measurements confirmed the strong attachment of the PAA brushes to the mica substrate. High friction coefficients (ca 0.3) were measured at relatively high pressures (40 atm) with no surface damage or polymer removal.     

Effect of domain morphology of butadiene—styrene block copolymers on viscoelastic properties and in tension shear

Dynamic viscoelastic properties of S—B—S block copolymers were measured in the tensile and shear deformation modes. Between the glass transitions of the polybutadiene and polystyrene domains the ratio of storage moduli E'/G' in tension and in shear for the same polymer varied from 3 to more than 30, depending on sample preparation. For films cast from good solvents this ratio was near 3; large ratios resulted from deposition from poor polybutadiene solvents or from compression molding. Above the polystyrene glass transition, E'/G' approached 3 for all samples. The effect is ascribed to various degrees of polystyrene domain connectivity. Electron micrographs confirm this interpretation. For morphologies of high polystyrene domain connectivity, the loss tangent in tension is heavily weighted by mechanical losses in the polystyrene phase; the loss tangent in shear is affected only moderately by differences in domain morphology.     

Phosphine-Functionalized Syndiotactic Polystyrenes: Synthesis and Application to Immobilization of Transition Metal Nanoparticle Catalysts

The precise control of monomer sequence and stereochemistry in copolymerization is of much interest and importance for the synthesis of high performance materials, but studies toward this goal have met with only limited success to date. The coordination polymerization of diphenylphosphinostyrene (p-StPPh₂ and o-StPPh₂) and its copolymerization with styrene (St) by (C₅Me₄SiMe₃)Sc(CH₂C₆H₄NMe₂-o)₂ have been achieved for the first time to afford a new series of phosphine functionalized syndiotactic polystyrene. By the design of the polymer structure, the copolymer of o-StPPh₂ and St (poly(o-StPPh₂-alt-St)-b-sPS) containing o-StPPh₂ and St atactic alternating copolymer block and syndiotactic polystyrene block (sPS) showed excellent thermal stability and chemical resistance. The simple combination of the triphenylphosphine and syndiotactic polystyrene realized the stable immobilization of metal nanoparticles to afford highly robust metal@poly(o-StPPh₂-alt-St)-b-sPS nanocatalysts at high temperature and various atmospheres. The Cu@poly(o-StPPh₂-alt-St)-b-sPS catalyst can serve as a highly efficient heterogeneous catalyst for the synthesis of quinoline derivatives by acceptorless dehydrogenative coupling of o-aminobenzylalcohol with ketones.     

Normal and frictional forces between surfaces bearing polyelectrolyte brushes

Normal and shear forces were measured as a function of surface separation, D, between hydrophobized mica surfaces bearing layers of a hydrophobic-polyelectrolytic diblock copolymer, poly(methyl methacrylate)- block-poly(sodium sulfonated glycidyl methacrylate) copolymer (PMMA- b-PSGMA). The copolymers were attached to each hydrophobized surface by their hydrophobic PMMA moieties with the nonadsorbing polyelectrolytic PSGMA tails extending into the aqueous medium to form a polyelectrolyte brush. Following overnight incubation in 10 (-4) w/v aqueous solution of the copolymer, the strong hydrophobic attraction between the hydrophobized mica surfaces across water was replaced by strongly repulsive normal forces between them. These were attributed to the osmotic repulsion arising from the confined counterions at long-range, together with steric repulsion between the compressed brush layers at shorter range. The corresponding shear forces on sliding the surfaces were extremely low and below our detection limit (+/-20-30 nN), even when compressed down to a volume fraction close to unity. On further compression, very weak shear forces (130 +/- 30 nN) were measured due to the increase in the effective viscous drag experienced by the compressed, sliding layers. At separations corresponding to pressures of a few atmospheres, the shearing motion led to abrupt removal of most of the chains out of the gap, and the surfaces jumped into adhesive contact. The extremely low frictional forces between the charged brushes (prior to their removal) is attributed to the exceptional resistance to mutual interpenetration displayed by the compressed, counterion-swollen brushes, together with the fluidity of the hydration layers surrounding the charged, rubbing polymer segments.     

Phase separation induced by shear quenching in polymer blends with a diblock copolymer

The effects of adding A–B diblock copolymer to a polymer blend (A/B) on phase‐separation kinetics and morphology have been investigated in a fixed shallow‐quench condition (ΔT = 1.5 °C) by in situ time‐resolved light scattering and phase‐contrast optical microscopy. A shear‐quench technique was used in this study instead of a conventional temperature‐quench method. Mixtures of nearly monodisperse low relative‐molecular masses of polybutadiene (M_w = 2.8 kg/mol), polystyrene (M_w = 2.6 kg/mol), and a near‐symmetric butadiene–styrene diblock copolymer (M_w = 6.3 kg/mol) as an interfacial modifier were studied. We observed that the addition of the diblock copolymer could either retard or accelerate the phase‐separation kinetics depending on the concentration of the diblock copolymer in the homopolymer blends. In contrast to the conventional temperature quench, we observed complex phase‐separation kinetics in the intermediate and late stages of phase separation by the shear‐quench technique. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 819–830, 2001     

Mesoscale simulations of the behavior of charged polymer brushes under normal compression and lateral shear forces

Dissipative particle dynamics (DPD) was used to investigate the behavior of two opposing end-grafted charged polymer brushes in aqueous media under normal compression and lateral shear. The effect of polymer molecular weight, degree of ionization, grafting density, ionic strength, and compression on the polymer conformation and the resulting shear force between the opposing polymer layers were investigated. The simulations were carried out for the poly(tert-butyl methacrylate)-block-poly(sodium sulfonate glycidyl methacrylate) copolymer, referred as PtBMA-b-PGMAS, end-attached to a hydrophobic surface for comparison with previous experimental data. Mutual interpenetration of the opposing end-grafted chains upon compression is negligible for highly charged polymer brushes for compression ratios ranging from 2.5 to 0.25. Under electrostatic screening effects or for weakly charged polymer brushes, a significant mutual interpenetration was measured. The variation of interpenetration thickness with separation distance, grafting density, and polymer size follows the same scaling law as the one observed for two opposing grafted neutral brushes in good solvent. However, compression between two opposing charged brushes results in less interpenetration relative to neutral brushes when considering equivalent grafting density and molecular weight. The friction coefficient between two opposing polymer-coated surfaces sliding past each other is shown to be directly correlated with the interpenetration thickness and more specifically to the number of polymer segments within the interpenetration layer.     

Bulk and shear rheology of a symmetric three‐arm star polystyrene

The bulk and shear rheological properties of a symmetric three‐arm star polystyrene were measured using a self‐built pressurizable dilatometer and a commercial rheometer, respectively. The bulk properties investigated include the pressure–volume–temperature behavior, the pressure‐dependent glass transition temperature (T_g), and the viscoelastic bulk modulus and Poisson's ratio. Comparison with data for a linear polystyrene indicates that the star behaves similarly but with slightly higher T_gs at elevated pressures and slightly higher limiting bulk moduli in glass and rubbery states. The Poisson's ratio shows a minimum at short times similar to what is observed for the linear chain. The horizontal shift factors above T_g obtained from reducing the bulk and shear viscoelastic responses are found to have similar temperature dependence when plotted using T ? T_g scaling; in addition, the shift factors also exhibit a similar temperature dependence to linear polystyrene. The retardation spectra for the bulk and shear responses are compared and show that the long time molecular mechanisms available to the shear response are unavailable to the bulk. At short times, the two spectra have similar slopes, but the short‐time retardation spectrum for the shear response is significantly higher than that for the bulk, a finding that is, as yet, unexplained. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

RELAXATION IN DOMAINS:STRUCTURE AND SEGMENTAL DYNAMICS OF LC/I DIBLOCK COPOLYMERS

A series of block copolymers consisting of an isotropic (polystyrene) block and a side-chain liquid crystallineblock (LC) have been studied using small-angle X-ray scattering and dielectric spectroscopy. The triblock copolymer (PS-LC-PS) displays an order-to-order transition (OOT) together with the isotropic/nematic transition of the LC phase. The seriesof diblock copolymers show no clear OOT but the phase diagram differs from that of non-LC block copolymers. Thesegmental dynamics as measured with dielectric spectroscopy is dominated by the α and δ relaxation of the LC block. Bothdisplay a WLF like temperature dependence. The relaxation times are influenced by the constraints of the nanoscale domains.They are decreased for the LC confined in the domain as compared to the LC in the continuous matrix.     

Matched ring diblock and linear triblock copolymers in dilute solution

A unique diblock copolymer ring and its linear triblock copolymer precursor composed of polystyrene and polydimethylsiloxane have been characterized by static and dynamic light scattering in dilute solution. The measurements were carried out with cyclohexane as the solvent over a temperature range of 12–35°C. Cyclohexane has the useful property that it is nearly isorefractive with the PDMS so that the PDMS block segments are invisible to the light-scattering technique and it is a theta solvent for polystyrene at 34.5°C. The block polymers in this work contain 35.1 wt % of styrene as determined by proton NMR. In the linear triblock polymer, the polystyrene is the center block with PDMS blocks on each side. Static light scattering measurements give 4.31 × 10⁴ for the average molecular weight of the whole polymer. Light scattering also shows that the apparent theta temperature for the linear triblock is shifted by 15°C to a value of 20°C at which point the second virial coefficient drops sharply and phase separation begins to induce aggregation. The diblock ring, however, shows a strongly positive second virial coefficient and no aggregation even at 12°C which is the limit of these experiments. The diffusion coefficients of cyclic diblock (D_c) and linear triblock copolymer (D₁) are measured by dynamic light scattering. The ratio of diffusion coefficients of cyclic and linear copolymers at 14.9°C and 30°C are D_c/D_l = 1.13 and 1.107 respectively. These compare well with prediction of 1.18 for this ratio from consideration of the hydrodynamics of matched linear and cyclic polymer chains. Dynamic light scattering quantitatively confirms that the linear copolymer experiences a solvent quality change near 20°C but the cyclic polymer remains in good solvent over the entire experimental temperature range. © 1993 John Wiley & Sons, Inc.     

The influence of hydrogen bonding on the preparation and mechanical properties of PS-diblock copolymer blends

In this article, the preparation of nanosized core-shell particles to induce ductility in polystyrene (PS) is described. FTIR spectroscopy, solid-state NMR spectroscopy, and DSC were used to examine the extent of miscibility of PS and poly(butylacrylate)-b-polyolefin diblock copolymers in a blend in which PS was chemically modified by copolymerization with 0.5–5 mol % of p-(hexafluoro-2-hydroxy isopropyl) styrene (HFS). Hydrogen bonding between the hydroxyl-groups and the carbonyl-groups of polybutylacrylate enhanced the miscibility and lead to randomly distributed polyolefin particles surrounded by a homogeneous PBA/PS matrix. Morphological parameters such as the size of the dispersed phase or extent of interpenetration between the components are controllable simply by changing the amount of interacting groups in the blend. The mechanical properties of the prepared blends were also studied. The intrinsic deformation behavior was investigated by compression tests, whereas the microscopic mode of deformation was studied by time-resolved small-angle X-ray scattering. It was shown that the macroscopic strain at break depends to a large extent on the diblock copolymer content and the degree of demixing between the rubber shell and PS matrix. Brittle behavior was observed for PS blends that contain more than 3 mol % HFS and show complete miscibility between the PS matrix and acrylate shell. For the blends showing partial miscibility, the compression tests demonstrated a pronounced decrease in strain softening with increasing diblock copolymer concentration. Furthermore, it was illustrated that dependent on the degree of demixing the microscopic deformation mode changes from crazing to cavitation induced shear yielding. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2137–2160, 2004     

A lutetium allyl complex that bears a pyridyl-functionalized cyclopentadienyl ligand: dual catalysis on highly syndiospecific and cis-1,4-selective (co)polymerizations of styrene and butadiene

A novel linked‐half‐sandwich lutetium–bis(allyl) complex [(C₅Me₄? C₅H₄N)Lu(η³‐C₃H₅)₂] ( 1 ) attached by a pyridyl‐functionalized cyclopentadienyl ligand was synthesized and fully characterized. Complex 1 in combination with [Ph₃C][B(C₆F₅)₄] exhibited unprecedented dual catalysis with outstanding activities in highly syndiotactic (rrrr>99 %) styrene polymerization and distinguished cis‐1,4‐selective (99 %) butadiene polymerization, respectively. Strikingly, this catalyst system exhibited remarkable activity (396 kg copolymer (mol_Lu h)^?1) for the copolymerization of butadiene and styrene. Irrespective of whether the monomers were fed in concurrent mode or sequential addition of butadiene followed by styrene, diblock copolymers were obtained exclusively, which was confirmed by a kinetics investigation of monomer conversion of copolymerization with time. In the copolymers, the styrene incorporation rate varied from 4.7 to 85.4 mol %, whereas the polybutadiene (PBD) block was highly cis‐1,4‐regulated (95 %) and the polystyrene segment remained purely syndiotactic (rrrr>99 %). Correspondingly, the copolymers exhibited glass transition temperatures (T_g) around ?107 °C and melting points (T_m) around 268 °C; typical values for diblock microstructures. Such copolymers cannot be accessed by any other methods known to date. X‐ray powder diffraction analysis of these diblock copolymers showed that the crystallizable syndiotactic polystyrene (syn‐PS) block was in the toluene δ clathrate form. The AFM micrographs of diblock copolymer showed a remarkable phase‐separation morphology of the cis‐1,4‐PBD block and syn‐PS block. This represents the first example of a lutetium‐based catalyst showing both high activity and selectivity for the (co)polymerization of styrene and butadiene.