Copolymers In Dilute Solution Research Articles

Synthesis and Aqueous Solution Properties of Novel Thermoresponsive Graft Copolymers Based on a Carboxymethylcellulose Backbone

The synthesis of thermoresponsive graft copolymers based on a carboxymethylcellulose (CMC) backbone is reported. Thermal responsive properties are introduced by grafting the CMC sample with amino-terminated poly(N-isopropylacrylamide) (PNIPAM) side chains of a relatively low molecular weight. Turbidity measurements in dilute copolymer solutions showed that, due to the hydrophilic CMC backbone, macroscopic phase separation by increasing temperature above the lower critical solution temperature (LCST) of PNIPAM is not allowed for pH ≥ 3. Pyrene fluorescence probing studies in aqueous solutions revealed the formation of hydrophobic microdomains above the LCST of PNIPAM. In semidilute solution these microdomains interconnect the polymer chains, leading to the thermally induced formation of a physical network. The macroscopic result is the observation in semidilute solutions of a pronounced thermally induced viscosity enhancement. This thermothickening phenomenon is almost irrespective of pH, and it remains very important even at pH values as low as 3.

PHOTODEGRADATION OF St-MMA COPOLYMERS: CORRELATION OF DEGRADATION, STYRENE CONTENT, AND PHOTOPHYSICS

A series of styrene(St)-methyl methacrylate(MMA) copolymers with compositions ranging from 0 to 100% St were synthesized and exposed to UV light for various periods of time. The changes in yellowing (yellowness index—YI) and absorbance of exposed films were evaluated as a function of photolysis time and St content. Both the changes in YI and absorbance at 400 nm were found to increase dramatically for polymer films with styrene (St) content greater than about 50%. A direct correlation between increases in the YI and absorbance at 400 nm with mean styrene sequence length was found with a maximum change being obtained for copolymers with mean sequence length of approximately 5 St units or greater. A small absorbance shoulder at 270–290 nm was observed for copolymers with St content less than 30% after relatively short irradiation times (e. g. 2 hours). Excimer formation of copolymers in dilute solution was found to increase with an increase in St content and St sequence length; in films, a plateau in the relative excimer emission was attained with a St content of 60%, corresponding to a mean St sequence length of 2.

Self-Assembled Nanostructure of a Novel Coil−Rod Diblock Copolymer in Dilute Solution

A novel narrowly distributed rod−coil diblock copolymer, poly(styrene-block-(2,5-bis[4-methoxyphenyl]oxycarbonyl)styrene) (PS-b-PMPCS), was synthesized by 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) mediated living free radical polymerization. The rodlike PMPCS block is a mesogen-jacked liquid crystalline polymer and is soluble in p-xylene at temperatures higher than 100 °C. When a PS-b-PMPCS dilute solution is cooled, the copolymer chains can self-assemble into a core−shell nanostructure. The temperature-induced self-assembly was studied. We showed, for the first time, that instead of a neutron scattering, a combination of static and dynamic laser light scattering results could also lead to the core radius (Rc) and the shell thickness (ΔR). Using this novel method, we found that when more chains were assembled into the nanostructure, Rc remained as a constant, close to the contour length of the PMPCS block, but the shell became thicker, indicating that the attraction between the insoluble rigid rodlike PMPCS blocks led to their insertion into the core, while the repulsion between the soluble coillike PS blocks forced them to stretch in the shell.

Micellization and relaxation kinetics of diblock copolymers in dilute solution based on A–W theory: I. Description of a model for core–corona type micelles

On the basis of the Aniansson–Wall (A–W) theory, a calculation method for the time evolution of association-number distribution during micelle formation of diblock copolymer in solution is presented. The rate constant for the elemental process of a chain expulsion from a micelle is evaluated as a function of the association number by application of Halperin's treatment based on the Kramers rate theory. Numerical calculations are carried out for both cases of micellization from unimer state and micellar relaxation under a temperature jump from one micellar state to another. In the micelle relaxation, the micelle size changes stepwise with two steps, clearly showing the characteristic feature of the A–W mechanism, where there exist two processes, the fast process undergoing by consuming/releasing free unimers and the slow process accompanied with almost no change of unimer concentration. On the contrary, in the micellization from unimer state, the very fast process is observed, where the free chains get together quickly to form temporal micelles, and is followed by an ordinary micellar relaxation.

Association of ethylene vinyl acetate copolymer in dilute solutions. V. Stability and pour point depression to wax solutions

The stability of two kinds of aggregation states of ethylene vinyl acetate (EVA) copolymer in dilute solution of 1,2-dichloroethane has been investigated by viscometry. It was found that good storage-time, shearing, and dilution resistance were displayed for both states of EVA molecule with intra- and interchain association. And for the pour point depression of wax solution doped with EVA, the EVA in intrachain aggregation state was better than the EVA in interchain aggregation state. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 836–841, 2000

New chiral photochromic menthone-containing homopolymers and copolymers - synthesis, phase behaviour and photo-optical properties

A new approach for the preparation of photosensitive materials for coloured data recording and storage is advanced. This approach involves the synthesis of copolymers containing nematogenic and combined (-)-arylidene-p-menthan-3-one chiral photochromic fragments in one monomer unit. The conditions for the formation of an LC phase in the chiral photochromic homopolymers as a function of the structure of the side groups have been identified; for a new series of copolymers, the effect of their composition on the phase behaviour and photooptical properties is considered. Planarly oriented films of the copolymers show selective light reflection in the UV, visible, and near IR spectral regions. The photochemical behaviour of the homopolymers and copolymers in dilute solution and as films has been studied. In these systems, under UV radiation, the E-Z isomerization of the (-)-arylidene-p-menthan-3-one fragment is the dominating process. The kinetic features of the photoprocess are revealed, and the effective quantum yields calculated. The isomerization process leads to dramatic changes in the anisometry of the chiral side groups, and the helical twisting power decreases. Therefore, as a result of UV radiation of films of the copolymers, the selective light reflection peak is shifted to a longer wavelength spectral region. This means that such polymeric films can be considered as promising materials for colour data recording and as storage media.

The physics of block copolymers

1. Introduction 2. Melt Phase Behaviour of Block Copolymers 3. Block Copolymers in Dilute Solution 4. Block Copolymers in Semidilute and Concentrated Solutions 5. Solid State Structure of Block Copolymers 6. Blends Containing Block Copolymers Appendix: The Self Consistent Field Theory

Vesicles Formed from a Poly(ethylene oxide)−Poly(propylene oxide)−Poly(ethylene oxide) Triblock Copolymer in Dilute Aqueous Solution

Micelle formation of block copolymers in selective solvents (organic solvent or water) has been studied for many years.1-3 The morphology of polymeric micelles has been the subject of more recent investigations both theoretically and experimentally.4-8 Diblock copolymers with a long soluble block compared to the nonsoluble block may self-assemble into micelles of a starlike structure while copolymers with a short soluble block often form so-called crew-cut micelles.3,9 One class of water-soluble block copolymers that has attracted great interest in the literature is the triblock copolymers of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO), often abbreviated as PEOPPO-PEO or EOa-POb-EOa. These copolymers exist in a wide range of different compositions and display a rich phase behavior in water depending on their relative block lengths.10,11 It is well established that the more hydrophilic copolymers associate into micelles, composed of a PPO core and a PEO corona, with a spherical or rodlike shape depending on temperature.7,12-14 The present work focuses on the investigation of one of the most hydrophobic PEO-PPO-PEO copolymers (L121) in dilute water solutions. New interesting structures found in the L121/water system are presented and visualized by the cryogenic transmission electron microscopy (cryo-TEM) technique. For the first time, to our knowledge, unilamellar vesicles are shown to exist in a triblock copolymer system of this type. Vesicles of branched PEO-PPO block copolymers in water have only been predicted as a metastable state in earlier model calculations.15 There are only a few reports in the literature on vesicle formation of diblock and multiblock copolymers in dilute solutions of selective solvents.16-21 Vesicles were one among several different crew-cut aggregates found in polystyrene-poly(acrylic acid) (PS-PAA) and PS-PEO diblock copolymer aqueous systems.9 These vesicular aggregates were prepared by a dialysis method. Both copolymers form so-called glassy vesicles, which are preserved in water since the water-insoluble PS block is in a glassy state at ambient temperatures. However, a recent study on a polybutadiene-PAA (PBD-PAA) copolymer/water system has concluded that the vesicular morphology is not controlled by the glass temperature of the nonsoluble block.22 Other publications have reported the existence of multilamellar block copolymer vesicles in a homopolymer-rich matrix as a selective solvent and in a concentrated ternary block copolymer system exposed to a shear flow.23-27 This communication presents a different and new preparation procedure of block copolymer vesicles than described in earlier work. The method uses an extrusion method often utilized for the preparation of single-tailed and double-tailed surfactant (or lipid) vesicles, during which the dispersed bilayers close into a spherical structure with a water interior.28 The formation of unilamellar surfactant vesicles normally requires the input of energy by extrusion, ultrasonication, or dialysis, although it has been suggested that they may form spontaneously in some mixed cationic and anionic surfactant systems.29 The PEO-PPO-PEO triblock copolymer, denoted Pluronic L121, was a gift from BASF corporation and used without further purification. It has a nominal molecular weight of 4400 g/mol, and the average composition is EO5PO68EO5, which corresponds to 10 wt % of PEO. L121 copolymers do not form micelles. At low concentrations and temperatures, unimers (individual copolymer chains) coexist with larger unimer aggregates (an L1 phase). The hydrodynamic radii (RH) of the unimers and the aggregates, obtained using dynamic light scattering (DLS) at 10 °C, are 1.9 and 44 nm, respectively. Figure 1 displays one part of the binary phase diagram of L121 in water. Because of the short hydrophilic PEO chains, and therefore the small headgroup area, the copolymers can easily pack into a lamellar phase (LR) which appears at higher copolymer concentrations (>55 wt %). The lamellar phase boundaries, as preliminary determined by small-angle X-ray scattering (SAXS), are also indicated in the figure. At low concentrations and at ambient temperatures, the system phase separates, and the phase separation curve determined by visual inspection is shown in Figure 1. The two-phase region is an equilibrium between the lamellar phase and the L1 phase. The L121 block copolymer vesicles were prepared at 25 °C from dilute block copolymer solutions in this L1/LR two-phase regime by repeated extrusion of the * To whom correspondence should be addressed. E-mail address: Karin.Schillen@fkem1.lu.se. Figure 1. Schematic phase diagram of L121 in water. The vesicles were prepared in the two-phase region (2φ) at 25 °C from aqueous L121 solutions with concentrations of 0.2, 0.4, 0.6, and 1.2 wt %. 6885 Macromolecules 1999, 32, 6885-6888

Thermodynamic vs Kinetic Aspects in the Formation and Morphological Transitions of Crew-Cut Aggregates Produced by Self-Assembly of Polystyrene-b-poly(acrylic acid) Block Copolymers in Dilute Solution

Thermodynamic vs kinetic aspects of the formation of crew-cut aggregates of various structures prepared from polystyrene-b-poly(acrylic acid) diblock copolymers in DMF/water mixtures are explored. In particular, the reversibility of the transitions between the crew-cut aggregates of various morphologies is studied by “jumps” in the polymer or ion concentration at various water contents. The aggregates are prepared by two different methods: micellization is induced either by the addition of water to polymer/DMF solutions or by the direct dissolution of the polymer in DMF/water mixtures. It is shown that as the polymer concentration increases, the morphology of the aggregates changes from spheres to rodlike micelles, to interconnected rods, and then to bilayers. As the water content increases, the boundaries for the formation of the different morphologies move toward lower polymer concentrations. The thermodynamic vs kinetic control of the morphology depends largely on the water content and the method of a...

Conformational structure of amphiphilic copolymers in dilute solution

We present the equations of the extended Gaussian self-consistent method for study of equilibrium and kinetics of conformational transitions of arbitrary copolymer solutions. As applications of the method we consider the effect of the chain topology on the kinetics of homopolymer folding, sequence dependence of the phase diagram and folding kinetics of a single copolymer chain, and stability of mesoglobules in dilute polymer solutions.

Dielectric relaxation of dialkyl fumarates monomers homopolymers and copolymers in dilute solutions

Abstract The dielectric absorption of some disubstituted ethylenes (dialkyl fumarates) namely dimethyl fumarate (DMF), diethyl fumarate (DEF), diisopropyl fumarate (DIPF), dicyclohexyl fumarate (DCHF), dicyclohexyl maleate (DCHM), dinonyl fumarate (DNF), didodecyl fumarate (DDF) and dicetyl fumarate(DCF) has been measured in the microwave region 0.2–18 GHz. The measurements were carried out at 20°C. The solvent used in all cases was CCl 4 and the concentration does not exceed 0.003 mole fraction. The data obtained for the specific absorption Aϵ/x were analyzed using Debye equation into one or even more than one relaxation time. The results were interpreted in terms of dipole reorientation by molecular and intermolecular rotation. Dielectric measurements were also carried for some dialcyl fumarate polymers and copolymers in CCl 4 solutions at 30 °C. The dielectric data revealed the existence of three distinct absorption regions related to the main chain and its related motions.

Lattice Monte Carlo investigations on copolymer systems, 1. Diblock copolymers

Symmetric diblock copolymers in dilute solution were examined by means of Monte Carlo simulations on a cubic lattice with respect to chain- and block dimensions, shape, local structure and number of contacts. The solvent was either a common good one, a common θ-solvent or a selective one for the two blocks. In all cases, repulsive interactions are operative between the blocks. In addition, the underlying homopolymers (athermal and θ) were divided into two parts (and treated as a block copolymer) for comparison. Chain-length was varied from 40 to 1280 segments leading to the expected values for the critical exponent 2v ≈ 1.2 for good solvent quality and 2v ≈ 1.0 for θ-solvent. Copolymers in a selective solvent scale with an intermediate exponent, 2v ≈ 1.13. The deviation of the mean squared dimensions of the copolymers from the sum of those of two homopolymers of the same length and for the same solvent quality as the blocks is largest for block copolymers in a common θ-solvent (where it exceeds 20%), while the blocks themselves have mostly the same dimensions as their underlying homopolymers of equal length. The shape of the copolymers, expressed by the parameter δ (asphericity) becomes more rod-like with increasing chain-length if there are (compact) θ-blocks in the molecule which are subject to mutual repulsive interaction. In these cases, θ exceeds the value of the homopolymers in the limit of infinite chain-length. The number of contacts per segment approaches a limiting value with increasing chain-length which is ≈0.20 for athermal chains and athermal blocks. For θ-chains and θ-blocks, a limiting value is not yet reached within the range of chainlengths investigated. The number of contacts per segment between two different blocks quickly tends to zero with increasing chain-length.

Conformations of (AB)n star copolymers in dilute solution

Molecular simulation of amphiphilic (AB)n star diblock copolymers in dilute solution shows that the properties of the polymer are significantly affected by which of the ends of the diblock arm is attached to the center of the star. In dilute aqueous solution, stars having the solvophilic end of the diblock attached to the center of the star can undergo a dramatic conformational change in which the outer solvophobic blocks aggregate into one or more compact solvophobic globules. This aggregation transition is accompanied by a significant change in the size of the polymer as measured by the radius of gyration.

Lattice Monte Carlo investigations on copolymer systems, 3. Alternating and random copolymers

Alternating and random copolymers in dilute solution are investigated by means of Monte Carlo simulations on a cubic lattice. Each molecule consists of an equal number of A and B segments, either randomly distributed along the chain or forming an alternating sequence. The energy parameters chosen represent selective solvent conditions (the solvent is a good one for monomers of type A and a θ-solvent for B; between A and B repulsive interactions are operative). Comparison with di- and triblock copolymers of equal overall composition reveals that the behaviour of random or alternating copolymers (subject to the same selective solvent) is quite different. Their properties rather resemble those of homopolymers in a solvent of intermediate quality. The absolute chain dimensions (e. g. the mean square radius of gyration, , and the mean square end-to-end distance, > of random and alternating copolymers as well as their scaling exponents are significantly larger than those of block copolymers. The ratio between and as well as the shape of the polymer (expressed by the asphericity δ) are similar to those of athermal polymers indicating that there is no pronounced selectivity of the solvent. In contrast to block copolymers, these parameters exhibit no significant chain-length dependence. The number of the various types of polymer-polymer contacts (A-A, B-B and A-B) is almost independent of the type of contact at least for the solvent conditions investigated. This is in contrast to block copolymers where A-B contacts are widely suppressed and where the number of B-B contacts is approximately twice as high as that of A-A contacts.

Lattice Monte Carlo investigations on copolymer systems, 1. Diblock copolymers

Symmetric diblock copolymers in dilute solution were examined by means of Monte Carlo simulations on a cubic lattice with respect to chain- and block dimensions, shape, local structure and number of contacts. The solvent was either a common good one, a common θ-solvent or a selective one for the two blocks. In all cases, repulsive interactions are operative between the blocks. In addition, the underlying homopolymers (athermal and θ) were divided into two parts (and treated as a block copolymer) for comparison. Chain-length was varied from 40 to 1280 segments leading to the expected values for the critical exponent 2v ≈ 1.2 for good solvent quality and 2v ≈ 1.0 for θ-solvent. Copolymers in a selective solvent scale with an intermediate exponent, 2v ≈ 1.13. The deviation of the mean squared dimensions of the copolymers from the sum of those of two homopolymers of the same length and for the same solvent quality as the blocks is largest for block copolymers in a common θ-solvent (where it exceeds 20%), while the blocks themselves have mostly the same dimensions as their underlying homopolymers of equal length. The shape of the copolymers, expressed by the parameter δ (asphericity) becomes more rod-like with increasing chain-length if there are (compact) θ-blocks in the molecule which are subject to mutual repulsive interaction. In these cases, θ exceeds the value of the homopolymers in the limit of infinite chain-length. The number of contacts per segment approaches a limiting value with increasing chain-length which is ≈0.20 for athermal chains and athermal blocks. For θ-chains and θ-blocks, a limiting value is not yet reached within the range of chainlengths investigated. The number of contacts per segment between two different blocks quickly tends to zero with increasing chain-length.

Dynamics of Diblock Copolymers in Dilute Solutions

We consider the dynamics of freely translating and rotating diblock (A-B), Gaussian copolymers, in dilute solutions. Using the multiple scattering technique, we have computed the diffusion and the friction coefficients D_AB and Zeta_AB, and the change Eta_AB in the viscosity of the solution as functions of x = N_A/N and t = l_B/l_A, where N_A, N are the number of segments of the A block and of the whole copolymer, respectively, and l_A, l_B are the Kuhn lengths of the A and B blocks. Specific regimes that maximize the efficiency of separation of copolymers with distinct "t" values, have been identified.

Micellization of Model Graft Copolymers in Dilute Solution

The micellization behaviors of graft copolymers consisting of polystyrene, PSt, backbones and poly(tert-butyl methacrylate), PtBuMA, or poly(methacrylic acid), PMAA, branches were studied in dilute solutions by low angle laser light scattering, LALLS, viscometry, and dynamic light scattering, DLS. tert-Amyl alcohol, a selective solvent for the PtBuMA branches, and tetrahydrofuran, THF, a selective solvent for the PSt backbones, were used to promote the formation of micelles from the PSt-g-PtBuMA and PSt-g-PMAA graft copolymers, respectively. Low aggregation numbers were obtained in all cases due to the specific architectural characteristics of the samples. Aggregation numbers were dependent on the type of the component (backbone or branches) that was insoluble in the selective solvent. The formation of micelles from the PSt-g-PtBuMA graft copolymers in tert-amyl alcohol is evidenced by the appearance of large and rather loose aggregates at a certain concentration range. This kind of behavior was not obser...

Photophysics of carbazole-containing systems. 1. Dilute solution behavior of poly(N-vinyl carbazole) and N-vinyl carbazole/methyl acrylate copolymers

Steady-state and time-resolved fluorescence techniques have been used to study the photophysical behaviors of poly(N-vinyl carbazole), PNVCz and a series of N-vinyl carbazole-methyl acrylate (NVCz-co-MA) copolymers in dilute solution as a function of both NVCz composition and temperature. A kinetic scheme, intended to describe intramolecular excimer formation across the entire NVCz composition range, is proposed. In low aromatic content copolymers, two monomer species (unquenched and quenched monomer) and two excimer species (the sandwich-like excimer and a higher energy excimer) exist. The contribution from monomer emission to the overall fluorescence decreases with increasing NVCz content through increased excimer formation: this is likely to be consequent upon (1) an increase in the number of excimer forming sites, and (2) increasing efficiency of energy transfer from the excited monomers to the excimer forming sites. In the homopolymer, PNVCz, the only emission that can be observed on a nanosecond timescale is excimeric. This fluorescence appears to originate from three excimer species (the sandwich-like excimer, and two higher energy forms). For the homopolymer, the current observations are consistent with the model proposed by Vandendriessche and De Schryver [Polym. Photochem. 7, 153 (1986)]. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 963–978, 1997

Ion-Induced Morphological Changes in “Crew-Cut” Aggregates of Amphiphilic Block Copolymers

The addition of ions in micromolar (CaCl2 or HCl) or millimolar (NaCl) concentrations can change the morphology of "crew-cut" aggregates of amphiphilic block copolymers in dilute solutions. In addition to spherical, rodlike, and univesicular or lamellar aggregates, an unusual large compound vesicle morphology can be obtained from a single block copolymer. Some features of the spontaneously formed large compound vesicles may make them especially useful as vehicles for delivering drugs and as models of biological cells. Gelation of a dilute spherical micelle solution can also be induced by ions as the result of the formation of a cross-linked "pearl necklace" morphology.

Commentary: Reflections on “Copolymers in dilute solution. I. Preliminary results for styrene-methyl methacrylate,” by W. H. Stockmayer, L. D. Moore, Jr., M. Fixman, and B. N. Epstein,J. Polym. Sci., XVI, 517 (1955)

Commentary: Reflections on “Copolymers in dilute solution. I. Preliminary results for styrene-methyl methacrylate,” by W. H. Stockmayer, L. D. Moore, Jr., M. Fixman, and B. N. Epstein,J. Polym. Sci., XVI, 517 (1955)