Insoluble Block Research Articles

Critical micellization phenomena in block polyelectrolyte solutions

Critical micelle concentrations (cmc's) were measured for a range of block copolyelectrolytes based on styrene (the insoluble block) and sodium acrylate. The lengths of the styrene blocks ranged from 6 to 110, while those of the polyelectrolyte ranged from ca. 300 to ca. 1400. cmc results were interpolated for a constant polyelectrolyte block length of 1000 units. It was found that changing the insoluble block length from 6 to 110 lowered the cmc from 1.6×10 -5 to 5×10 -8 M. By contrast, changing the soluble blocks length from 300 to 1400 typically changed the cmc values by less than a factor of 2

Experiments on Association of Block Copolymers in Solution

Block copolymers in dilute and semidilute solutions in selective solvents self associate forming multimolecular spherical micelles, cores of which are formed by insoluble blocks and shells by soluble blocks. Experimental approaches are surveyed concerning structure and hydrodynamic properties of block copolymer micelles, thermodynamics of micelle formation, kinetics of micelle formation and dissociation, and dynamics of micellar equilibria. Some problems of present interest are briefly mentioned, like anomalous micellization or tentacled micelles

Segmental distribution functions for a micelle comprised of small symmetric diblock copolymers (short chain amphiphiles)

The micelle formed by short chain amphiphiles (small diblock copolymers) in a selective solvent has been simulated on a cubic lattice. The system contains twenty chains, each with ten beads of A and ten beads of B, at a volume fraction of 0.0376. These chains form a single micelle when the pairwise interaction of nonbonded beads in the insoluble block contribute an energy of −0.5 kT, all other interaction energies being zero. The micelle consists of an internal core that is a close packed region occupied almost exclusively by the insoluble blocks, and a corona, the outer portion of which consists primarily of solvent and the soluble blocks. However, the interface between these two regions is not sharp, as judged by distribution functions for the two types of beads, and for the junctions between the blocks. Letting 〈r 2j〉 denote the mean square radius of gyration of the junctions, the close packed core of insoluble beads extends from the center of mass no farther than ∼0.5 〈r 2j〉. The free ends of the insoluble block have a mean square radius of gyration of 0.88 〈r 2j〉, which places them, on average, farther from the center of mass than a randomly chosen bead in the insoluble block. The results are not compatible with models for a diblock copolymer micelle that assume a sharp interface, nor with models that assume the free ends of the insoluble block seek out the center of mass, or are randomly distributed throughout the core.

On the location of the free ends of the insoluble block in micelles formed by diblock copolymers

Nonradiative singlet energy transfer has been used to monitor the formation of micelles by diblock copolymers of poly(styrene) and poly(oxyethylene). The acceptor is placed at the junction between the two blocks in sample A1. The donor is placed either at the junction point between the blocks (D1) or free end of the block of poly(styrene) (D2). The experiment finds similar efficiencies of nonradiative singlet energy transfer in micelles formed by D1 and A1, and micelles formed by D2 and A1. This result implies that the free ends of the insoluble blocks do not seek out the center of mass of the micelle, but instead have a distribution throughout the micelle that is similar to the distribution of the junction points. Therefore the result confirms a crucial prediction from a recent simulation of the internal structure of the micelle formed by diblock copolymers in a selective solvent.

Micelles and networks formed by symmetric triblock copolymers in dilute solutions that are poor solvents for the terminal blocks

A simulation for symmetric triblock copolymers in dilute solution shows a rich variety of structures when the solvent produces a net attractive interaction between the terminal blocks. Depending on the strength of this interaction, the system can form transitory aggregates, large independent micelles, or branched structures in which bridging soluble internal blocks connect dense particles formed by the aggregation of insoluble terminal blocks. The formation of the branched structure with the ABA triblock copolymer can be seen under conditions where the corresponding AB diblock copolymer would provide steric stabilization for a polymer colloid.

Simulation of the steric stabilization of polymer colloids by diblock copolymers

The steric stabilization of polymer colloids by diblock copolymers is simulated on a 22×22×22 cubic lattice with periodic boundary conditions. In the systems described here, the lattice contains 20 chains, the volume fraction of polymer never exceeds 0.0376, and the number of beads in a chain never exceeds 20. The behavior is monitored by evaluation of the dimensionless ratios Mn/M0 and Mw/Mn, where M0 is the mass of an individual chain, and Mn and Mw are the number- and weight-average molecular weights of the particles in the system. By suitable variation in the pairwise interaction energies, one can cause the system to (1) establish a dynamic equilibrium, in which aggregates easily form and dissociate, (2) follow an irreversible path toward the formation of the largest possible aggregate, or (3) reach a long-lived metastable state in which the system prefers smaller aggregates than those that would exist at a true equilibrium. The metastable state arises from the steric stabilization of the dispersed state, as is shown by its characteristic response to the variation in the size of the soluble (stabilizing) block, at constant size of the insoluble block and constant interaction energies. The simulation also documents an interesting influence of the energy of the self-interaction of the insoluble block on the efficiency of the steric stabilization of the metastable state.

Dilute and semidilute solutions of ABA block copolymer in solvents selective for A or B blocks: 1. Small-angle X-ray scattering study

Solutions of a triblock copolymer, polystyrene- block-poly(hydrogenated butadiene)- block-polystyrene, in a selective solvent either for the aliphatic middle block (heptane) or for the polystyrene outer blocks (1,4-dioxane/25 vol% heptane) were studied by viscometry and small-angle X-ray scattering. In both systems, micelles were detected with uniform spherical cores formed by insoluble copolymer blocks. In semidilute solutions, in concentration regions above 0.01 g cm −3 in heptane and ∼0.1 g cm −3 in the mixed solvent, micelles were found to be space-correlated, the effect being accompanied by a steep increase in zero-shear viscosity. Aliphatic micellar cores in the mixed solvent are highly swollen due to the effect of selective sorption.

Micellar behaviour of styrene—isoprene block copolymers in selective solvents

The micellar phenomenon of block copolymers in selective solvents has been observed analogous to the classical surfactants. Spherical polymolecular micelles of block copolymers consisting of a core of insoluble block surrounded by the fringes of the soluble block have been analysed. The micellar behaviour of several styrene—isoprene two-block copolymers in two kinds of solvents, (i) selectively good solvent for polystyrene, e.g. dimethylformamide (DMF), dimethylacetamide (DMA), methyl ethyl ketone (MEK) and (ii) selectively good solvent for polyisoprene, e.g. n-alkanes (heptane, octane, decane and dodecane), has been examined by photon correlation spectroscopy (PCS), viscosity data and NMR. Polymolecular micelles with a narrow size distribution and having a size of several nanometers were detected from PCS data. The size and aggregation number of the micelles largely depended upon the molecular characteristics (total molecular weight and the block composition) of the copolymer. The formation of micelles in different solvents has been interpreted in terms of solubility parameters. The influence of temperature and the formation of mixed micelles have also been examined.

Colloidal Behaviour of Block Copolymers

Block copolymers .in selective solvent (good solvent for one of the sequences but precipitant for the other ) exhibit the aggregation of molecules through a closed association process in analogy to conventional surface active agents. Such an aggragation of molecules in to 'micelles' is owing to the presepce of two incompatible sequences which behave differently in selective solvent. The copolymer micelle is built up of the core of the insoluble block surrounded by the fringes of the soluble block, beingtof the order of several nanometer in size. In most cases such micelles are spherical in shape. The micellar parameters l critical micelle concentration, agregation number, micellar weight, shape and internal structure of micelle) largely depend upon the molecular characteristics of the micelle forming block.copolymer (molecular mass, relative composition of the blocks and structure), selectivity of the solvent and temperature.

Organized structures in block copolymers: stability domain and structural study

Abstract

Learned helplessness and reinforcement responsibility in children.

In an attempt to demonstrate the effects of low expectancy of reinforcement and low expectancy for control of reinforcement on performance in an achievement situation, 40 fifth-grade children (20 boys and 20 girls) were given successes (soluble block designs) by one adult (success experimenter) and failures (insoluble block designs) by another (failure experimenter) with trials from each being randomly interspersed. A number of children failed to complete problems administered by the failure experimenter when her problems became soluble, even though they had shortly before solved almost identical problems from the success experimenter and continued to perform well on the success experimenter's problems. The subjects who showed the largest performance decrements were those who took less personal responsibility for the outcomes of their actions and who, when they did accept responsibility, attributed success and failure to presence or absence of ability rather than to expenditure of effort. Those subjects who persisted in the face of prolonged failure placed more emphasis on the role of effort in determining the outcome of their behavior; moreover, males displayed this characteristic to a greater extent than did females. Implications of the results for strategies of behavior change are discussed.