Molecular Weight Block Copolymers Research Articles

Poly(styrene- b-methyl methacrylate) block copolymers as compatibilizing agents in blends of poly(styrene- co-acrylonitrile) and poly(2,6-dimethyl-1,4-phenylene ether): 2. Influence of concentration and molecular weight of symmetric block copolymers

The influence of the molecular weight of the symmetric block copolymer poly(styrene- b-methyl methacrylate) (P(S- b-MMA)) in blends with high-molecular-weight poly(styrene- co-acrylonitrile) (PSAN) and poly(2,6-dimethyl-1,4-phenylene ether) (PPE) is investigated by dynamic mechanical analysis and transmission electron microscopy. Total molecular weights of the block copolymers vary from 16 up to 275 kg mol −1. Independent of molecular weight, all block copolymers locate to the interface with strong dispersing efficiency. The different block copolymers also showed approximately the same emulsifying efficiency. The degree of segmental mixing of the blocks with the respective phases is evaluated from the glass transition behaviour. A qualitative model is developed to relate the observed glass transition behaviour to the segmental distribution. In blends with large block copolymers, polystyrene blocks and PPE are rather uniformly mixed. The degree of mechanical coupling of the phases increases with the block copolymer molecular weight. The favourable enthalpic interaction between the blocks and the blend components is a major factor determining the phase behaviour. In contrast to this, the molecular weight of PPE showed little influence on blend behaviour.

Analysis of block copolymers by high-performance liquid chromatography under critical conditions

The complete chromatographic characterization of block copolymers (molecular weight versus chemical composition) is an important problem in polymer chemistry. The experimental validity of the concept of so-called “chromatographic invisibility”, predicted theoretically by Gurbunov and Skvortsov on the basis of the phenomenon of critical conditions known in liquid chromatography, was examined. The theoretical approach predicts the possibility of one component of an A-B block copolyner being eluted under gel permeation chromatographic conditions, whereas the size of the alternate “invisible” component exerts no effect on the overall elution profile of the block copolymer. This applies only when special thermodynamic conditions i.e., eluent composition and temperature, are fulfilled, where the distribution coefficient K is unity, regardless of molecular weight. Block copolymers of poly(styrene-methyl methacrylate) and poly(styrene- tert.-butyl methacrylate) wre used as examples with binary and ternary mixtures of acetonitrile-dichloromethane, methanol-chloroform, tetrahydrofuran-dichloromethane and tetrahydrofuran-dicloromethane-hexane as eluents for chromatography under critical conditions on wide-pore silica gel in narrow-bore columns.

Compatibilizing effect of block copolymers in heterogeneous polystyrene/poly(methyl methacrylate) blends

The morphology of heterogeneous polystyrene/poly(methyl methacrylate) (PS/PMMA) blends has been modified by the addition of diblock copolymers of PS and PMMA and investigated as a function of block copolymer composition, molecular weight and concentration by optical and electron microscopies. A sharp decrease in dispersed phase dimension is observed with the addition of a few per cent of block copolymers having equal segmental mass (50/50 PS/PMMA), followed by a levelling off as the copolymer content is increased above the critical micelle concentration. For concentrations below the critical value, the particle size reduction is linear with copolymer volume fraction and agrees well with the predictions of Noolandi and Hong. The compatibilizing efficiency of the copolymer is strongly dependent on its composition and molecular weight. The effects of processing conditions, such as casting solvents and compression moulding, on the phase morphology have also been studied. Finally, the conformation of the copolymer at the interface is discussed with the help of models: calculations based on the experimental results indicate that the compatibilizer is neither located entirely at the interface, nor penetrates completely into the corresponding homopolymer phases.

Emulsifying effects of block copolymers in incompatible polymer blends

AbstractWe discuss some theoretical aspects of interfacial and emulsifying activity of block copolymers A‐B in incompatible blends of homopolymers A and B. We predict the reduction in interfacial tension due to equilibrium adsorption of copolymers at the interface. It is shown that for an appropriate choice of block copolymer composition and molecular weight a very low interfacial tension can be achieved. This suggests a possible existence of a thermodynamically controlled stable droplet phase in which minority homopolymer particles protected by an interfacial film are suspended in a majority homopolymer matrix. The size distribution of the droplets depends on the rigidity and spontaneous radius of curvature of the interfacial film which, as we show, can be controlled by molecular parameters of copolymers.

An explicit expression for the weight‐average molecular weight of block copolymers

An explicit expression for the weight‐average molecular weight of block copolymers

Determination of molecular-weight distribution and average molecular weights of block copolymers by gel-permeation chromatography

The problem of preparation of a block copolymer of precise molecular-weight distribution (MWD) and with heterogeneous composition on the basis of gel-permeation chromatography (GPC) data has been investigated. It has been shown that in MWD calculations the distribution ƒ( p) of the composition p in individual GPC fractions should be taken into account. The type of the ƒ( p) functions can be simultaneously established by an independent method, such as use of adsorption-column or thin-layer chromatography sensitive to the composition of the copolymer. It has also been shown that the actual ƒ( p) may be replaced by a corresponding piecewise distribution, of simple form, without decrease in the precision of calculation of the MWD and average molecular weights of most known block copolymers.

Synthesis and characterization of styrene- b-dimethylsiloxane diblock and styrene- b-isoprene- b-dimethylsiloxane triblock copolymers

Poly(styrene- b-dimethylsiloxane) (PS-PDMS) diblock and poly(styrene- b-isoprene- b-dimethylsiloxane) (PS-PI-PDMS) triblock copolymers have been synthesized by living anionic sequential addition polymerization in tetrahydrofuran as solvent with n-BuLi as initiator. These copolymers were characterized by gel permeation chromatography (GPC), membrane osmometry, viscometry, nuclear magnetic resonance (NMR) spectroscopy, small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The universal calibration curve of [η] M w vs GPC peak elution volume in methyl ethyl ketone for polystyrene (PS) and poly(dimethylsiloxane) (PDMS) standards was not unique suggesting that this method cannot be used for the calculation of true molecular weights of these diblock copolymers. Number-average molecular weights ( M n ) of the diblock copolymers were computed from their uncorrected GPC chromatograms using log M vs elution volume calibration curves of PS and PDMS in combination with the following equation: log M block = W PS log M PS + W PDMS log M PDMS where W PS and W PDMS are the weight fractions of PS and PDMS in the block copolymer ( W PS + W PDMS = 1). For M n < 4 × 10 4 , the calculated values were close to those measured experimentally. Micelles composed of PS-PDMS diblock copolymer formed in heptane were studied using SAXS. Radii of gyration of these micelles varied as the 1 3 power of the block copolymer molecular weight. SAXS measurement of the toluene cast films of the PS-PI-PDMS triblock copolymer yielded lamellar thickness of 54 nm. This value was larger than that (18.5—35 nm) obtained with TEM.

Block copolymer molecular weight by GPC

Molecular weights of narrow distribution styrene–butadiene block copolymers were determined from combined GPC–intrinsic viscosity data using the universal calibration principle and from GPC data alone using the assumption of weighted average of log molecular weights suggested by Runyon et al. The results indicate that, experimentally for block copolymers in good GPC solvents, the simpler second method is more precise despite the objection on theoretical grounds raised by Ho-Duc and Prud'homme. The calibration curves of polystyrene and polybutadiene in THF were found to be parallel, and the ratio for the two molecular weights at equal elution volume was found to be 1.75, differing from the 2.0 value reported earlier.

Gel permeation chromatography for determining molecular weight distribution and composition distribution based on the molecular weight of block copolymers

The method proposed for determination of the molecular weight distribution (MWD) of block copolymers involves the use of GPC data along with continuous composition determinations during block copolymer fractionation processes. Apart from the normal procedure of gel chromatogram transformation into MWD the proposed method comprises a further correction of the gel chromatogram to allow for the composition curve, for non-uniform detector sensitivity to block copolymer components and, finally, for the principle of3additivity of intrinsic viscosity rates for block copolymer components. Satisfactory results were obtained with block copolymers of PSPMMAPS, PSPAN, PSPAN, PSPB and PIP-α-MSPi in toluene, DMF and THF. Good agreement was found between average characteristics of the block copolymers calculated in line with GPC data and determined from independent measurements.

An Attempt to Adapt Superacid Chemistry for “Living” Carbenium Ion Polymerizations

Abstract The discovery that carbocations can be stabilized in super acid media, e.g., SbF5-SO2, etc., raises the possibility of “living” carbenium ion polymerization. Polymerization experiments with isobutylene and styrene carried out at high acid concentrations and in the virtual absence of nucleophile, i.e., under conditions conducive for living polymerization, failed to indicate a linear conversion vs molecular weight relationship and/or block copolymer formation. Additional model experiments with 2,4,4-trimethyl-1-pentene substantiate our conclusions that “living” carbocation polymerizations are unlikely to be produced by superacid chemistry.

Preparation of Block Copolymer of ε-Caprolactone with Living Polystyrene

Abstract ε-Caprolactone was polymerized by using living polystyrene prepared by means of an anionic polymerization technique. A gross polymer was elution-fractionated into four fractions which were methanol-soluble low-molecular-weight polyester, high-molecular-weight polyester, a small amount of polystyrene and polystyrene-polyester block copolymer. Both yields and molecular weights of block copolymers were observed to be higher in benzene than in tetrahydrofuran. Formation of block copolymers was concluded from the results of an elementary- and infrared analyses and turbidometry. Polymerization mechanism of ε-caprolactone is discussed.