Surface tension of polystyrene blends: Theory and experiment

Surface tension of linear–linear and star/linear polystyrene blends were measured using a modified Wilhelmy method. Our results show that for both polystyrene blend systems, the surface tension‐composition profile is convex, indicating a strong surface excess of the component with lower surface energy. Star/linear blends display more convex surface tension profiles than their linear–linear counterparts, indicative of stronger surface segregation of the branched‐component relative to linear chains. As a first step toward understanding the physical origin of enhanced‐surface segregation of star polymers, self‐consistent field (SCF) lattice simulations (both incompressible and compressible models) and Cahn‐Hilliard theory were used to predict surface tension‐composition profiles. Results from the lattice simulations indicate that the highly convex surface tension profiles observed in the star/linear blend systems are only possible if an architecture‐dependent, Flory interaction parameter (χ = 0.004) is assumed. This conclusion is inconsistent with results from bulk differential scanning calorimetry (DSC) measurements, which indicate sharp glass transitions in both the star/linear and linear/linear homopolymer blends and a simple linear relationship between the bulk glass transition temperature and blend composition. To implement the Cahn‐Hilliard theory, pressure‐volume‐temperature (PVT) data for each of the pure components in the blends were first measured and the data used as input for the theory. Consistent with the experimental data, Cahn‐Hilliard theory predicts a larger surface excess of star molecules in linear hosts over a wide composition range. Significantly, this result is obtained assuming a nearly neutral interaction parameter between the linear and star components, indicating that the surface enrichment of the stars is not a consequence of complex phase behavior in the bulk. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1666–1685, 2009     

Surface segregation of highly branched polymer additives in linear hosts

Entropy‐driven segregation of various branched and hyperbranched polymeric additives in chemically similar linear polymer hosts is studied using self‐consistent (SCF) mean‐field lattice simulations. The simulations account for the effect of molecular architecture on local configurational entropy in the blends, but ignores the effect of architecture on local density and blend compressibility. Star, dendrimer, and comb‐like additives are all found to be enriched at the surface of chemically identical linear host polymers. The magnitude of their surface excess increases with increased number of chain ends and decreases with increased segmental crowding near the branch point. Provided the number of arms and molecular weight of the branched additives are maintained constant, we find that the simplest branched architecture, the symmetric star, exhibits the strongest preference for the surface of binary polymer blends. We show that a single variable, here termed the “entropic driving force density,” controls the relative surface affinities of branched additives possessing a wide range of architectures. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1788–1801, 2008     

Effective χ and surface segregation in blends of star and linear polystyrene

The existence in blends of linear and star branched polystyrenes (PS) of a bulk thermodynamic interaction due to differences in macromolecular architecture has been demonstrated with small angle neutron scattering measurements. It has also been demonstrated that a regularly star‐branched polystyrene material segregates preferentially to the surface of a blend of star and linear molecules.     

Entropically driven phase separation of highly branched/linear polyolefin blends

Small‐angle neutron scattering (SANS) was used to examine the melt phase behavior of a heavily branched comb PEE polymer blended separately with two linear PEE copolymers. In this case, PEE refers to poly(ethylene‐r‐ethylethylene) with 10% ethylene units; therefore, the molecular architecture was the only difference between the two components of the blends. The molecular weights of the two linear random copolymers were 60 and 220 kg/mol, respectively. The comb polymer contained an average of 54 long branches, with a molecular weight of 13.7 kg/mol, attached to a backbone with a molecular weight of 10 kg/mol. Three different volume compositions (25/75, 50/50, and 75/25) were investigated for both types of blends. SANS results indicate that all the blends containing the lower molecular weight linear polymer formed single‐phase mixtures, whereas all the blends containing the high molecular weight linear polymer phase‐separated. These results are discussed in the context of current theories for polymer blend miscibility. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2965–2975, 2000     

Influence of long‐chain branching on the miscibility of poly(ethylene‐r‐ethylethylene) blends with different microstructures

The melt miscibility of two series of poly(ethylene‐r‐ethylethylene) (PE_Exx) polymers with different percentages (xx) of ethylethylene (EE) repeat units was examined with small‐angle neutron scattering (SANS). The first series consisted of comb/linear blends in which the first component is a heavily branched comb polymer (B90) containing 90% EE and an average of 62 long branches with a weight‐average molecular weight (M_W) of 5.5 kg/mol attached to a backbone with M_W = 10.0 kg/mol. The comb polymer was blended with six linear PE_Exx copolymers, all of which had M_W ≈ 60 kg/mol and EE percentages ranging from 55 to 90%; they were denoted L55 to L90, with the number referring to the percentage of EE repeat units. The second series consisted of linear/linear blends; the first component, with M_W = 220 kg/mol and 90% EE, was denoted L90A, and the second components were the same series of linear polymers, with M_W ≈ 60 kg/mol and various EE compositions. The concentrations investigated were 50/50 w/w, except for the blend of branched B90 and linear L90 (both components were 90% EE), for which 25/75 and 75/25 concentrations were also examined. The SANS results indicated that for the comb/linear blends, only the dB90/L90 blends were miscible, whereas the other five blends phase‐separated; for the linear/linear blends, dL90A/L83 and dL90A/L78 were miscible, whereas the other three blends were immiscible. These results indicate that long‐chain branching significantly narrowed the miscibility window of these polyolefin blends. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 466–477, 2002; DOI 10.1002/polb.10102     

Precise synthesis of well‐defined dendrimer‐like star‐branched polymers by iterative methodology based on living anionic polymerization

Dendrimer‐like star‐branched polymers recently developed as a new class of hyperbranched polymers, which resemble well‐known dendrimers in branched architecture, but comprise polymer chains between junctions, are reviewed in this highlight article. In particular, we focus on the precise synthesis of various dendrimer‐like star‐branched polymers and block copolymers by the recently developed methodology based on iterative divergent approach using living anionic polymers and 1,1‐bis(3‐tert‐butyldimethylsilyloxymethylphenyl)ethylene. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6659–6687, 2006     

Thermodynamic interaction parameter of star–star polybutadiene blends

The value of the thermodynamic interaction parameter, χ_eff, for star–star polybutadiene blends was determined with small‐angle neutron scattering. Blends in which the stars have the same number of arms and blends in which the stars have different numbers of arms are investigated. For star–star isotopic blends with components having the same number of arms, the presence of the junction point of the star leads to a value of χ_eff that is larger than that for an analogous linear–linear isotopic blend. However, changes in the value of χ_eff resulting from small dissimilarities in the number of arms of the two components in the isotopic star–star blends were too small to resolve. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 247–257, 2003     

Improved phase separation in polymer solar cells by solvent blending

The effect of solvent blending on the performance of an anthracene‐containing poly(p‐phenylene‐ethynylene)‐alt‐poly(p‐phenylene‐vinylene) backbone‐based donor polymer with asymmetrically substituted branched 2‐ethylhexyloxy and methyloxy side‐chains in bulk heterojunction solar cells is reported. This copolymer yields relatively high open‐circuit voltages with fullerene‐based electron acceptors. We systematically studied the thin‐film blend morphology and solar cell performance as a function of solvent composition (chlorobenzene to chloroform ratio) and polymer to [6,6]‐phenyl C61‐butyric acid methylester (PCBM) ratio. We combined photophysical investigations with atomic force microscopy and grazing incidence wide‐angle X‐ray scattering to elucidate the solid‐state morphology in thin films. In the investigated polymer system, the blend morphology becomes independent of the supporting solvent for high PCBM concentrations. Deposition from solvent blends rather than from pure chlorobenzene facilitates the beneficial phase separation between polymer and PCBM, leading to improved charge transport properties (short‐circuit currents) at lower PCBM concentrations. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013, 51, 868–874     

A novel method for the pore size control of the battery separator using the phase instability of the ternary mixtures

The battery separator plays a key role in determining the capacity of the battery. Since separator performance mainly depends on the pore size of membrane, development of a technique for the fabrication of the membrane having controlled pore size is essential in producing a highly functional battery separator. In this study, microporous membranes having the desired pore size were produced via thermally‐induced phase separation (TIPS) process. Control of the phase boundaries of polymer‐diluent blends is the main concern in manipulating pore size in TIPS process, because pore size mainly depends on the temperature gap between phase separation temperature of the blend and the crystallization temperature of polymer. Microporous membranes having controlled pore size were produced from polyethylene (PE)/dioctyl phthalate (DOP) blends, PE/isoparaffin blends, and polymer/diluent‐mixture ternary blends, that is, PE/(DOP/isoparaffin) blends. PE/DOP binary blends and PE/(DOP/isoparaffin) ternary blends exhibited typical upper critical solution temperature (UCST) type phase behavior, while PE formed a homogeneous mixture with isoparaffin above the crystallization temperature of PE. When the mixing ratio of polymer and diluent‐mixture was fixed, the phase separation temperature of PE/diluent‐mixture blend first increased with increasing DOP content in the diluent‐mixture, went through a maximum centered at about 80 wt % DOP and then decreased. Furthermore, the phase separation temperatures of the PE/diluent‐mixture blends were always higher than that of the PE/DOP blend when diluent‐mixture contained more than or equal to 20 wt % of DOP. Average pore size of microporous membrane prepared from PE/DOP blend and that prepared from PE/isoparaffin blend were 0.17 and 0.07 μm, respectively. However, average pore size of microporous membrane prepared from ternary blends was varied from 0.07 to 0.5 μm by controlling diluent mixing ratio. To understand the phase behavior of ternary blend, phase instability of the ternary mixture was also explored. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2025–2034, 2006     

Shear damping function measurements for branched polymers

We present results for step‐strain experiments and the resulting damping functions of polyethylene blends of different structures, including solutions of linear, star and comb polymers. Remarkably, an entangled melt of combs exhibits a damping function close to that for entangled linear chains. Diluting the combs with faster‐relaxing material leads to a more nearly constant damping function. We find similar behavior for blends of commercial low density polyethylene LDPE. Our results suggest a simple picture: on timescales relevant to typical damping‐function experiments, the rheologically active portions of our PE combs as well as commercial LDPE are essentially chain backbones. When strongly entangled, these exhibit the Doi‐Edwards damping function; when diluted, the damping function tends toward the result for unentangled chains described by the Rouse model – namely, no damping. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3117–3136, 2007     

Upper critical solution temperature type phase behavior of poly(styrene‐co‐acrylonitrile) blends with poly(styrene‐co‐n‐phenyl maleimide) and their interaction energies

To enhance the heat resistance of poly(styrene‐co‐acrylonitrile‐co‐butadiene), ABS, miscibility of poly(styrene‐co‐acrylonitrile), SAN, with poly(styrene‐co‐n‐phenyl maleimide), SNPMI, having a higher glass transition temperature than SAN was explored. SAN/SNPMI blends casted from solvent were immiscible regardless of copolymer compositions. However, SNPMI copolymer forms homogeneous mixtures with SAN copolymer within specific ranges of copolymer composition upon heating caused by upper critical solution temperature, UCST, type phase behavior. Since immiscibility of solvent casting samples can be driven by solvent effects even though SAN/SNPMI blends are miscible, UCST‐type phase behavior was confirmed by exploring phase reversibility. When copolymer composition of SNPMI was fixed, the phase homogenization temperature of SAN/SNPMI blends was increased as AN content in SAN copolymer increased. To understand the observed phase behavior of SAN/SNPMI blend, interaction energies of blends were calculated from the UCST‐type phase boundaries by using the lattice‐fluid theory combined with a binary interaction model. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1131–1139, 2008     

Mechanical properties of emulsion polymer blends

Polymer blend technology has been one of the most investigated areas in polymer science in the past 3 decades. The one area of polymer blends that has been virtually ignored involves simple emulsion blends, although several articles have recently appeared that address film formation and mechanical characteristics. In this study, we investigated the mechanical property behavior of emulsion blends composed of low/high‐glass‐transition‐temperature polymers (where low and high mean below and above the test temperature, respectively). The emulsions chosen for this study had similar particle sizes, and the mixtures were rheologically stable. Two conditions were chosen, a binary combination of polymers that were thermodynamically immiscible and another system that was thermodynamically miscible. The mechanical property results over the entire composition range were compared with the predictions of the equivalent box model (EBM) with the universal parameters predicted by percolation theory. An array of randomly mixed and equal‐size particles of differing moduli was expected to show excellent agreement with theory, and the emulsion blends provided an excellent experimental basis for testing the theory. For the immiscible blend, the EBM prediction for the modulus showed excellent agreement with experimental results. With tensile strength, the agreement between the modulus and theory was good if the yield strength for the higher glass‐transition‐temperature polymer was employed in comparison with the actual tensile strength. The phase inversion point (where both phases were equally continuous) was at a 0.50 volume fraction of each component (based on an analysis employing Kerner's equation), just as expected for a random mixture of equal‐size particles. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 1093–1106, 2001     

Miscibility of polysulfone blends with poly(1‐vinylpyrrolidone‐co‐styrene) copolymers and their interaction energies

The miscibility of polysulfone (PSf) with various hydrophilic copolymers was explored. Among these blends, PSf gave homogeneous mixtures with poly(1‐vinylpyrrolidone‐co‐styrene) [P(VP–S)] copolymers when these copolymers contained 68–88 wt % 1‐vinylpyrrolidone (VP). Miscible PSf blends with P(VP–S) copolymers underwent phase separation on heating caused by lower critical solution temperature (LCST)‐type phase behavior. The phase behavior depended on the copolymer composition. Changes in the VP content of P(VP–S) copolymers from 65 to 68 wt % shifted the phase behavior from immiscibility to miscibility and the LCST behavior. The phase‐separation temperatures of the miscible blends first increased gradually with the VP content, then went through a broad maximum centered at about 80 wt % VP, and finally decreased just before the limiting content of VP for miscibility with PSf. The interaction energies of binary pairs involved in PSf/P(VP–S) blends were evaluated from the phase‐separation temperatures of PSf/P(VP–S) blends with lattice‐fluid theory combined with a binary interaction model. The decrease in the contact angle between water and the membrane surface with increasing VP content in P(VP–S) copolymers indicated that the hydrophobic properties of PSf could be improved via blending with hydrophilic P(VP–S) copolymers. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1401–1411, 2003     

Phase behavior of tetramethylpolycarbonate blends with styrene‐based methacrylate copolymers and their interaction energies

The miscibility of tetramethylpolycarbonate (TMPC) blends with styrenic copolymers containing various methacrylates was examined, and the interaction energies between TMPC and methacrylate were evaluated from the phase‐separation temperatures of TMPC/copolymer blends with lattice‐fluid theory combined with a binary interaction model. TMPC formed miscible blends with styrenic copolymers containing less than a certain amount of methacrylate, and these miscible blends always exhibited lower critical solution temperature (LCST)‐type phase behavior. The phase‐separation temperatures of TMPC blends with copolymers such as poly(styrene‐co‐methyl methacrylate), poly(styrene‐co‐ethyl methacrylate), poly(styrene‐co‐n‐propyl methacrylate), and poly(styrene‐co‐phenyl methacrylate) increase with methacrylate content, go through a maximum, and decrease, whereas those of TMPC blends with poly(styrene‐co‐n‐butyl methacrylate) and poly(styrene‐co‐cyclohexyl methacrylate) always decrease. The calculated interaction energy for a copolymer–TMPC pair is negative and increases with the methacrylate content in the copolymer. This would seem to contradict the prediction of the binary interaction model, that systems with more favorable energetic interactions have higher LCSTs. A detailed inspection of lattice‐fluid theory was performed to explain such phase behavior. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1288–1297, 2002     

Surface migration of branched molecules: analysis of energetic and entropic factors

We have introduced energetic factors into the response theory developed by Wu and Fredrickson [Macromolecules 29, 7919 (1996)] to predict the enrichment of branched molecules due to architectural effects at surfaces. This development simultaneously increases the utility of the theory for guiding experimental investigations, and makes possible a rigorous assessment of theoretical predictions in careful studies of isotopically labeled linear/branched species binary blends at surfaces. For example, the introduction of energetic factors allows us to predict the existence of a crossover molecular weight, below which an energetically unfavorable species at a surface can be enriched entirely due to architecture. For binary blends of linear chains, the degree of polymerization (Kuhn) of the energetically unfavorable species at the crossover point is r(c) approximately =2U(e)/DeltaU(s). Here, U(e) is the attraction of chain ends towards the surface and DeltaU(s) is the difference in the interaction potential of main chain segments to the surface due to chemical differences and/or isotopic labeling. We also show that surface segregation of an additive in a host polymer due to architectural effects alone is significantly enhanced as the spinodal temperature of a branched/linear blend is approached. Detailed comparisons of the modified response theory with lattice simulations are used to evaluate the theory and to determine the limits of its applicability.     

Thermoplastic blend demixing by a high‐pressure,high‐temperature process

The analysis of a thermoplastic polymer blend requires a precise separation of the blend components, which is usually performed by selective solvent extraction. However, when the components are high‐molecular‐weight polymers, a complete separation is very difficult. The use of fluids in near critical and supercritical conditions becomes a promising alternative to reach a much more precise separation. In this work, a method to separate reactive and physical blends from high‐molecular‐weight commercial polymers is proposed. Polyethylene (PE)/polystyrene (PS) blends were separated into their components with n‐propane, n‐pentane, and n‐heptane at near critical and supercritical conditions. The selectivity of each solvent was experimentally studied over a wide range of temperatures for assessing the processing windows for the separation of pure components. The entire PE phase was solubilized by n‐pentane and n‐heptane at similar temperatures, whereas propane at supercritical conditions could not dissolve the fraction of high‐molecular‐weight PE. The influence of the blend morphology and composition on the efficiency of the polymer separation was studied. In reactive blends, the in situ copolymer formed was solubilized with the PE phase by chemical affinity. The method proposed for blend separation is easy, rapid, and selective and seems to be a promising tool for blend separation, particularly for reactive blends, for which the isolation of the copolymer is essential for characterization © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2361–2369, 2005     

Specific interactions and phase behavior of ternary poly(2‐vinylpyridine)/poly(N‐vinyl‐2‐pyrrolidone)/bisphenol A blends

Hydrogen‐bonding interactions between bisphenol A (BPA) and two proton‐accepting polymers, poly(2‐vinylpyridine) (P2VPy) and poly(N‐vinyl‐2‐pyrrolidone) (PVP), were examined by Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC). The Flory–Huggins interaction‐energy densities of BPA/P2VPy and BPA/PVP blends were determined by the melting point depression method. The interaction parameters for both BPA/P2VPy and BPA/PVP blend systems were negative, demonstrating the miscibility of BPA with P2VPy as well as PVP. The miscibility of ternary BPA/P2VPy/PVP blends was examined by DSC, optical observation, and solid‐state nuclear magnetic resonance spectroscopy. The experimental phase behavior of the ternary blend system agreed with the spinodal phase‐separation boundary calculated using the determined interaction‐energy densities. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1125–1134, 2002     

The effect of phase‐separated morphology on the rheological properties of polystyrene/poly(vinyl methyl ether) blend

The effect of phase‐separated morphology on the rheological properties of polystyrene/poly(vinyl methyl ether) (PS/PVME) blend was investigated by optical microscopy (OM), light scattering (LS) method, and rheology. The blend had a lower critical solution temperature (LCST) of 112°C obtained by turbidity experiment using LS at a heating rate of 1°C/h. Three different blend compositions (critical 30/70 PS/PVME by weight) and two off‐critical (50/50 and 10/90)) were prepared. The rheological properties of each composition were monitored with phase‐separation time after a temperature jump from a homogeneous state to the preset phase‐separation temperature. For the 30/70 and 50/50 blends, it was found that with phase‐separation time, the storage and loss moduli (G′ and G″) increased at shorter times due to the formation of co‐continuous structures resulting from spinodal decomposition. Under small oscillatory shearing, shear moduli gradually decreased with time at longer phase‐separation times due to the alignment of co‐continuous structures toward the flow direction, as verified by scanning electron microscopy. However, for the 10/90 PS/PVME blend, the rheological properties did not change with phase‐separation times. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 889–906, 1999     

Characterization of elastomeric blends by atomic force microscopy

The bulk mechanical properties of a blend of elastomers are found to depend on the micro and nano scale morphology of the phases of the materials in the blend. In this study, we examine the phase morphology of blends of incompatible elastomers using Atomic Force Microscopy (AFM). Specifically, nanoindentation and Tapping Mode AFM (TMAFM) imaging techniques are used as experimental tools for mapping the composition of unfilled elastomeric blends. Depending on the composition of the blend, either co‐continuous or discontinuous domain/matrix morphology is observed. To identify the different components in bromobutyl (BIIR)/natural rubber (NR) blends, nanoscale indentation measurements were made on the observed phase‐separated regions. Results from force mode AFM and mechanical measurements of bulk NR and BIIR are used to assist in the interpretation of the TMAFM results for the BIIR/NR blends. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 492–503, 2006     

Control of droplet size of polymer–diluent blends through thermally induced phase separation

The effects of process conditions and molecular structure of polymer and diluent on the droplet size of membranes formed by thermally induced phase separatiom (TIPS) process were examined. The observed upper critical solution temperature–type phase boundaries of nylon‐12 blended with poly(ethylene glycol) (PEG) and nylon 12 diluted with poly(ethylene glycol) dimethyl ether (PEGDE) and their interaction energy densities calculated using the Flory–Huggins theory suggest that the nylon‐12/PEGDE blends are less stable than the nylon‐12–PEG blends. Infrared spectra confirmed that the difference in phase stability might come from specific interactions of the hydroxyl terminal groups of PEG with the amide groups from nylon‐12, which are not be feasible in the nylon‐12–PEGDE blends. The phase stability of diluent PEG blended with various nylons that are different in the number of methyl groups in the repeat unit was ranked in the order of: nylon‐6–PEG blend < nylon‐12–PEG blend < nylon‐11–PEG blend. We also noted that the phase‐separated droplets grew by both coalescence and the Oswald ripening process after the onset of phase separation. As a result, the cubic exponent of average droplet radius (R³) plotted against time satisfied the linear relationship. As the blends became less stable, the droplet growth rate increased and larger equilibrium droplets formed at a constant quenching depth. The TIPS membranes with desired pore structure could be prepared by controlling the molecular structure of components as well as by varying processing conditions such as quenching depth and annealing time. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3042–3052, 2000