Block Length Ratio Research Articles

On the thread-like morphology of LC/I block copolymers in nematic solvents

AB and ABA block copolymers are synthesized by direct living anionic polymerization where the A-block is a liquid crystalline side chain polymer (LC-block) and the B-block is an isotropic polymer (I-block). Similar to amphiphilic block copolymers with a “hydrophilic” water soluble segment and a “hydrophobic” water insoluble part, the LC/I block copolymers consist of the “nematophilic” LC-block that is soluble in a nematic solvent and the “nematophobic” segment that is insoluble in nematic solvents. The behaviour of these LC/I block copolymers and corresponding homopolymers is analyzed in dilute nematic solutions of low molar mass liquid crystals. The effects of varying molecular weight, chemical constitution of blocks and block length ratio are investigated. Copolymers with a high fraction of the I-block are insoluble in the nematic solvent as indicated by phase separation. Copolymers with a high fraction of the LC-block are completely soluble. If the weight fraction of the LC-block is in the range of 0.46 to 0.85, thermoreversible thread-like aggregates are formed in the nematic solvent. These thread-like aggregates have a uniform diameter of 2–3 μm and their long axis perfectly follows the director field of the nematic matrix. The formation of these structures is discussed in terms of copolymer concentration, temperature and influence of aligning surfaces. From simple geometrical packing arguments it can be assumed that these thread-like aggregates consist of bilayered vesicles. In contrast to the spherical shape of amphiphilic block copolymer vesicles in aqueous solution, the rod-like shape of vesicles in nematic solution is caused by the minimization of free elastic energy density of the nematic matrix.

On the thread-like morphology of LC/I block copolymers in nematic solvents

AB and ABA block copolymers are synthesized by direct living anionic polymerization where the A-block is a liquid crystalline side chain polymer (LC-block) and the B-block is an isotropic polymer (I-block). Similar to amphiphilic block copolymers with a “hydrophilic” water soluble segment and a “hydrophobic” water insoluble part, the LC/I block copolymers consist of the “nematophilic” LC-block that is soluble in a nematic solvent and the “nematophobic” segment that is insoluble in nematic solvents. The behaviour of these LC/I block copolymers and corresponding homopolymers is analyzed in dilute nematic solutions of low molar mass liquid crystals. The effects of varying molecular weight, chemical constitution of blocks and block length ratio are investigated. Copolymers with a high fraction of the I-block are insoluble in the nematic solvent as indicated by phase separation. Copolymers with a high fraction of the LC-block are completely soluble. If the weight fraction of the LC-block is in the range of 0.46 to 0.85, thermoreversible thread-like aggregates are formed in the nematic solvent. These thread-like aggregates have a uniform diameter of 2–3 μm and their long axis perfectly follows the director field of the nematic matrix. The formation of these structures is discussed in terms of copolymer concentration, temperature and influence of aligning surfaces. From simple geometrical packing arguments it can be assumed that these thread-like aggregates consist of bilayered vesicles. In contrast to the spherical shape of amphiphilic block copolymer vesicles in aqueous solution, the rod-like shape of vesicles in nematic solution is caused by the minimization of free elastic energy density of the nematic matrix.

On the thread-like morphology of LC/I block copolymers in nematic solvents

AB and ABA block copolymers are synthesized by direct living anionic polymerization where the A-block is a liquid crystalline side chain polymer (LC-block) and the B-block is an isotropic polymer (I-block). Similar to amphiphilic block copolymers with a “hydrophilic” water soluble segment and a “hydrophobic” water insoluble part, the LC/I block copolymers consist of the “nematophilic” LC-block that is soluble in a nematic solvent and the “nematophobic” segment that is insoluble in nematic solvents. The behaviour of these LC/I block copolymers and corresponding homopolymers is analyzed in dilute nematic solutions of low molar mass liquid crystals. The effects of varying molecular weight, chemical constitution of blocks and block length ratio are investigated. Copolymers with a high fraction of the I-block are insoluble in the nematic solvent as indicated by phase separation. Copolymers with a high fraction of the LC-block are completely soluble. If the weight fraction of the LC-block is in the range of 0.46 to 0.85, thermoreversible thread-like aggregates are formed in the nematic solvent. These thread-like aggregates have a uniform diameter of 2–3 μm and their long axis perfectly follows the director field of the nematic matrix. The formation of these structures is discussed in terms of copolymer concentration, temperature and influence of aligning surfaces. From simple geometrical packing arguments it can be assumed that these thread-like aggregates consist of bilayered vesicles. In contrast to the spherical shape of amphiphilic block copolymer vesicles in aqueous solution, the rod-like shape of vesicles in nematic solution is caused by the minimization of free elastic energy density of the nematic matrix.

Stabilization of aqueous α‐Al2O3 suspensions with block copolymers

AbstractWater‐soluble diblock copolymers (DBCPs) with a polyelectrolyte block are accessible by sequential anionic polymerization of tert‐butylmethacrylate (TBMA) and ethylene oxide (EO), followed by polymer analogous conversion of the PTBMA into poly(methyacrylic acid) (PMAA) blocks. These materials are highly efficient dispersants for oxide ceramic powders in aqueous media. A series of block copolymer samples with Mn ranging from 1300 to 38 900, and (EO:MAA) block length ratios from 0.5 to 11.7 were prepared with polydispersities close to 1.2. The influence of overall molecular weight and block length ratio, pH and ionic strength on the stability of aqueous α‐Al2O3 suspensions was investigated by sedimentation and adsorption experiments, surface plasmon resonance (SPR) and electrokinetic measurements. The copolymers are capable of stabilizing alumina suspensions by a combination of specific adsorption of one block on the particle surface and a shielding effect provided by the nonadsorbing block. In addition, the adsorption of the negatively charged PMAA block on the oppositely charged alumina surface reverses the electrophoretic potential of the oxide particles, a process which is strongly pH dependent. With respect to the powder dispersing efficiency, an optimum was found when the DBCP consisted of a short PMAA anchoring and an approximately tenfold longer PEO stabilizer block with an overall molecular weight of about 5000. Alumina suspensions with the DBCP added were highly dispersed and rather stable against salt addition in a much wider pH window than block‐copolymer‐free suspensions.

Novel Poly(vinyl ether) Block Copolymers: Adsorption from Aqueous Solutions on α-Fe2O3(Hematite) and the Mechanism of Colloidal Stabilization

The adsorption of diblock copolymers containing different block length ratios of polyvinylmethyl ether (PVME) and poly (vinyloxy-4-butyric acid) (PVOBA) from aqueous solutions onto α-Fe2O3(hematite) was compared with the adsorption of the PVOBA and PVME homopolymers. The effects on the colloidal stabilization in relation to the chemical composition of the polymers were evaluated. The pH was between 7.5 and 9 in all experiments. The PVME homopolymer hardly adsorbs on α-Fe2O3, while an adsorbed amount of 0.31 mg/m2was found for PVOBA, which behaves as a polyelectrolyte at the pH values used. The adsorbed amount of the block copolymers increases with increasing PVME chain length up to 0.95 mg/m2. The high affinity character in the adsorption isotherm is lost above a certain length of the PVME chain in the block copolymer. The adsorbed layer thickness increases with the PVME chain length, while at the same time the chain density on the surface decreases. Adsorption of PVOBA causes a high absolute value of the zeta potential (|ζ|) of the α-Fe2O3particles. The value of |ζ| drops with an increasing length of the PVME chain. The stability of α-Fe2O3dispersions stabilized with block copolymers with a short PVME chain is mainly due to electrical double-layer repulsion. Full steric stabilization is obtained with sufficiently long PVME chains in the block copolymer, making these dispersions insensitive to the salt concentration.

Structures and properties of block copolymers in solution

AbstractStructures and properties of block copolymers in solution depend on the total block length, the block length ratio, the block sequence and the solvent quality. The supramolecular structures formed by core/shell or flower‐like micelles in the concentrated solution region are examined by dynamic light scattering to ascertain some of the conditions which can enhance the formation of open associated structures using a triblock copolymer in a selective solvent for the middle block.

Synthesis and characterization of amphiphilic diblock copolymers of methyl tri(ethylene glycol) vinyl ether and isobutyl vinyl ether

Water-soluble diblock copolymers of methyl tri(ethylene glycol) vinyl ether (hydrophilic block) and isobutyl vinyl ether (hydrophobic block) of different molecular weights and composition were synthesized by living cationic polymerization. The molecular weight and comonomer composition of these copolymers were determined by GPC and 1H NMR spectroscopy, respectively. Aqueous solutions of the copolymers were characterized in terms of their micellar behavior using dynamic light scattering, aqueous GPC, and dye solubilization. All the copolymers formed aggregates with the exception of a diblock copolymer with only two hydrophobic monomer units. The micellar hydrodynamic size scaled with the 0.61 power of the number of hydrophobic units, in good agreement with a theoretical exponent of 0.73. An increase in the length of the hydrophobic block at constant hydrophilic block length or an increase in the overall polymer size at constant block length ratio both resulted in lower critical micelle concentrations (cmcs). The cloud points of 1% w/w aqueous solutions of the polymers were determined by turbidimetry. An increase in the length of the hydrophobic block at constant hydrophilic block length caused a decrease in the cloud points of the copolymers. However, an increase in the overall polymer size at constant block length ratio led to an increase in the cloud point. © 1996 John Wiley & Sons, Inc.

Architecture-Induced Phase Immiscibility in a Diblock/Multiblock Copolymer Blend

Ordered diblock copolymer blends have recently become the subject of tremendous research interest since they can be used to elucidate the intramicrodomain segregation of blocks differing in length, as well as to identify the molecular and blend parameters yielding phase immiscibility. In this work, we explore the influence of molecular architecture on block copolymer blend miscibility by examining an equimolar mixture of two symmetric styrene (S)/isoprene (I) block copolymers, one an SI diblock and the other an (SI)4 octablock. Their molecular weights are identical, so that the ratio of block lengths is 4:1 SI:(SI)4. While this ratio is expected to yield a single phase in diblock copolymer blends, transmission electron microscopy reveals here that the diblock/multiblock blend is macrophase-separated due to the linear multiblock architecture and midblock conformations of the (SI)4 copolymer. Electron tomography (3D imaging) permits direct visualization of connected SI and (SI)4 microdomains at the SI/(SI)4...

Effects of Molecular Weight and Block Length Ratio on the Rheological Behavior of Low Molecular Weight Polystyrene-block-polyisoprene Copolymers in the Disordered State

Effects of molecular weight (M) and block length ratio (Φ) on the rheological behavior of low molecular weight polystyrene-block-polyisoprene (SI diblock) copolymers in the disordered state were investigated. Here Φ is defined by N PS /(N PS + N PI ), where N PS and N PI are polymerization indices for PS and PI blocks, respectively. For the study, a series of SI diblock copolymers were synthesized via anionic polymerization, and their dynamic storage and loss moduli (G' and G) were measured as functions of angular frequency at various temperatures. The disordered state of each block copolymer was ascertained by observing that plots of log G' versus log G were independent of temperature and had a slope of 2 in the terminal region. Using log G' versus log G plots for the block copolymers in the disordered state, we were able to investigate the effects of M and Φ on their linear viscoelastic properties. We found that the melt elasticity, as judged from the values of G' in the log G' versus log G plots, increased with increasing M but decreased with increasing Φ. Reduced plots for dynamic storage modulus and dynamic viscosity were prepared with reference temperature, which was chosen by two different ways : (1) by choosing the temperature at an equal distance from the glass temperature of component polystyrene in each SI diblock copolymer and (2) by choosing the temperature at an equal distance from the order-disorder transition temperature of each SI diblock copolymer. We point out an urgent need for developing a comprehensive molecular theory predicting the rheological behavior of block copolymers.

Rheological behaviour of binary mixtures of polystyrene- block-polyisoprene copolymers in the disordered state

The rheological behaviour of binary mixtures of polystyrene- block-polyisoprene (SI diblock) copolymers in the disordered state was investigated. For this study, two SI diblock copolymers having different molecular weights and block length ratios were synthesized via anionic polymerization, and then binary mixtures were prepared to measure their linear viscoelastic properties (namely, dynamic storage and loss moduli, G′ and G″, as functions of angular frequency ω) at various temperatures. The disordered state of the block copolymers and their mixtures was determined by observing a critical temperature, often referred to as the order-disorder transition temperature ( T ODT ), at and above which logarithmic plots of log G′ versus log G″ become independent of temperature and have a slope of 2 in the terminal region. We found that log G′ versus log G″ plots for the binary mixtures lie between those of the constituent components, and at a fixed value of G″ the value of G′ increases monotonically with increasing blend composition. The composition dependence of the linear viscoelastic properties of the binary mixtures was investigated by preparing reduced plots using two different methods of determining reference temperature T 0: (i) at an equal distance from the glass transition temperature of the polystyrene block ( T g, PS + 55° C ) in the respective block copolymer and also (ii) at an equal distance from the T ODT of the respective block copolymer ( T 0 = T ODT + 10° C). We found that the two different methods of yielded very different composition dependence of the linear viscoelastic properties of the binary mixtures of SI diblock copolymers.

Rheological behaviour of binary mixtures of polystyrene-block-polyisoprene copolymers in the disordered state

The rheological behaviour of binary mixtures of polystyrene- block -polyisoprene (SI diblock) copolymers in the disordered state was investigated. For this study, two SI diblock copolymers having different molecular weights and block length ratios were synthesized via anionic polymerization, and then binary mixtures were prepared to measure their linear viscoelastic properties (namely, dynamic storage and loss moduli, G ′ and G ″, as functions of angular frequency ω) at various temperatures. The disordered state of the block copolymers and their mixtures was determined by observing a critical temperature, often referred to as the order-disorder transition temperature ( T ODT ), at and above which logarithmic plots of log G ′ versus log G ″ become independent of temperature and have a slope of 2 in the terminal region. We found that log G ′ versus log G ″ plots for the binary mixtures lie between those of the constituent components, and at a fixed value of G ″ the value of G ′ increases monotonically with increasing blend composition. The composition dependence of the linear viscoelastic properties of the binary mixtures was investigated by preparing reduced plots using two different methods of determining reference temperature T 0 : (i) at an equal distance from the glass transition temperature of the polystyrene block ( T g , PS + 55° C ) in the respective block copolymer and also (ii) at an equal distance from the T ODT of the respective block copolymer ( T 0 = T ODT + 10° C ). We found that the two different methods of yielded very different composition dependence of the linear viscoelastic properties of the binary mixtures of SI diblock copolymers.

Interaction of acrylic diblock copolymers with aluminium oxide surfaces

Amphiphilic diblock copolymers with methacrylic acid blocks (A) for anchoring to hydrophilic surfaces and with either methyl methacrylate or n-butyl methacrylate blocks (B) as hydrophobic stabilizers are highly efficient dispersants for oxide and nitride ceramic powders in organic media. In a series of copolymers with number-average molecular weight M n ranging from 4000 to 135 000, the block length ratios (A/B) have been varied systematically from 0.07 to 2, which correspond to values for the asymmetry ratio β as defined by Marques and Joanny (Macromolecules, 22 (1989) 1454) between 4.93 and 0.66. The influence of these parameters as well as that of the solvent quality on the interaction of the copolymers with α-Al 2O 3 have been studied by adsorption experiments and surface plasmon resonance. Owing to the strong interaction between the A blocks with the alumina surface, the diblock copolymers are adsorbed spontaneously and irreversibly. Generally, the adsorbed mass increases with the molecular weight of the adsorbed polymer; however, some differences between samples with asymmetric block lengths (β > 1 ) and with symmetric or moderately asymmetric block lengths (β ⩽ 1 ) have been observed. The adsorbate layer thickness L of samples with β > 1 as well as with β < /1 increases with the degree of polymerization of the B blocks. In all cases, L can be scaled with the radius of gyration of undisturbed coils formed by the B blocks in relatively good solvents. Thus, the structure of the adsorbed diblock copolymers appears to be “mushroom-like” rather than “brush-like”. With increasing “quality” of solvent, stabilizer coil expansion and stretching of the macromolecules perpendicular to the surface have been observed.

Adsorption of charged block copolymers with two adsorbing blocks

We have investigated the adsorption of diblock copolymers consisting of uncharged water-soluble dihydroxypropyl methacrylate (HMA) and positively charged dimethylaminoethyl methacrylate (AMA) on TiO2 and SiO2. Adsorbed amounts of the block copolymers and of the homopolyelectrolyte AMA were determined by means of reflectometry, as a function of pH, ionic strength and block length ratio. These results are compared with theoretical calculations using the self-consistent-field (SCF) theory of Scheutjens and Fleer extended for charged block copolymers.We find that the adsorption behaviour of charged block copolymers is essentially different from that of neutral block copolymers, because the electrostatic interactions, including an induced interaction, play a key role. This induced interaction influences the dissociation of the surface groups, thereby reducing the build-up of the electrostatic potential on the surface. When the composition of the block copolymer is varied a maximum in the amount adsorbed is observed, which is similar to that found for adsorption of polyelectrolytes with varying segment charge. Also, depending on the pH, charged block copolymers can be adsorbed with both blocks in contact with the surface, whereas neutral block copolymers usually have one anchoring block.Qualitative agreement is found between the experimental and theoretical results.

The interaction of acrylic diblock‐copolymers with aluminum oxide surfaces and their application for ceramic powder processing

AbstractAmphiphilic diblock copolymers were prepared by anionic copolymerization of t.‐butylmethacrylate (TBMA) with methylmethacrylate (MMA) and n‐butylmethacrylate (NBMA) respectively, followed by polymer analogous conversion of the TBMA‐into methacrylic acid‐ (MAA) blocks. These materials are highly efficient dispersants for oxide and nitride ceramic powders. A series of samples with Mn ranging from 4.000 to 135.000, and with block length ratios from 0.5 to 14 (MMA/MAA) with polydispersities close to 1.0 were obtained.The influence of total molecular weight and block length distribution of the copolymers and quality of the solvent system on their interaction with α‐Al2O3 were studied by sedimentation, adsorption experiments and surface plasmon resonance (SPR). The acrylic diblock copolymers are adsorbed irreversibly at the alumina surface. The conformation of the adsorbed macromolecules depends on the “quality” of the solvent. With respect to powder dispersing efficiency, an optimum is found when the polymers consist of short anchoring and stabilizing blocks.

Rheological behaviour of block copolymers in the disordered state: effects of molecular weight and block length ratio

The effects of molecular weight and block length ratio on the rheological behaviour of polystyrene- block-polyisoprene- block-polystyrene (SIS) copolymers in the disordered state were investigated. For the study, SIS triblock copolymers were synthesized via anionic polymerization, and linear dynamic viscoelastic properties (i.e. dynamic storage and loss moduli, G′ and G″) of the SIS block copolymers were measured at various temperatures as functions of angular frequency (ω), using a cone-and-plate rheometer. The disordered state of the samples was determined by the threshold temperature at which the plot of log G′ versus log G″ becomes virtually independent of temperature as the temperature was increased. It has been found that when the block length ratio is kept the same, the melt elasticity of SIS block copolymers in the disordered state becomes independent of molecular weight, while the viscosity increases with molecular weight, and the block length ratio greatly influences both melt viscosity and elasticity.

Rheological behavior of homogeneous poly(α-methylstyrene)-block-polystyrene copolymers

Oscillatory shear flow properties of α-methylstyrene)/styrene block copolymers having molecular weights ranging from 120000 to 250000 and different block length ratios prepared by anionic polymerization were measured using a Rheometrics mechanical spectrometer at temperatures ranging from 116 to 251°C. It is concluded that these polymers are homogeneous as determined by DSC and by logarithmic plots of dynamic storage modulus versus dynamic loss modulus. The Stockmayer-Kennedy molecular viscoelasticity theory which was developed on the basis of the Rouse theory was modified using the concept of the tube model

Block copolymer adsorption and stabilization of colloids

The self-consistent field model of Evers et al. for the adsorption of block copolymers from solution is briefly reviewed and illustrated with simple examples for monomer and dimer adsorption. Equations for the free energy of interaction between surfaces bearing adsorbed layers of block copolymers are given, both for full equilibrium and for restricted equilibrium. The adsorption and stabilization behaviour of diblock copolymers is compared with that of triblock copolymers with an adsorbing middle block. It is shown that diblock copolymers give thicker adsorbed layers than triblock copolymers with the same length of the soluble blocks. If the adsorption energy of the adsorbing block is high enough, the structure of the adsorption layer and the stabilizing action of diblock copolymers are about the same as for end-grafted chains with the same coverage and length of the stabilizing hairs. There exists an optimum block length ratio for diblock copolymers, for which the adsorbed amount and polymer-induced repulsion between colloidal particles are a maximum.

An X-ray photoelectron spectroscopy study of the surface of siloxane-containing block copolymers

The formation of the surface of polysulphone-polymethylsiloxane and polycarbonate-polydimethylsiloxane block copolymers is studied by X-ray photoelectron spectroscopy over a wide range of block length ratios. When the bulk content of the polydimethylsiloxane block is 0·7–15·0% its surface concentration is 60–100%. Surface modification of the polycarbonate and polysulphone homopolymer films by introduction of block copolymers is studied. The surface layer morphology is described for various block length ratios in the block copolymers.