Costas Vlahos

Asymmetric Star Polymers: Synthesis and Properties

The synthesis and the properties, both in bulk and in solution, of asymmetric star polymers are reviewed. Asymmetry is introduced when arms of different molecular weight, chemical nature or topology are incorporated into the same molecule. The phase separation, aggregation phenomena, dilute solution properties etc. are examined from a theoretical and experimental point of view. Recent applications of these materials show their importance in modern technologies.

Brownian Dynamics Simulations on the Self-Assembly Behavior of AB Hybrid Dendritic—Star Copolymers

The micellization behavior of hybrid dendritic-star copolymers with solvophilic dendritic units is studied by means of Brownian dynamics simulations. The critical micelle concentration and the micelle size and shape are examined for different solvophobic/solvophilic ratios r as a function of the number of the dendritic and linear arms. Hybrid dendritic-star copolymers with one dendritic and up to three solvophobic linear branches form spherical micelles with preferential aggregation number. Those with two dendritic arms and three solvophobic branches form micelles with wide aggregation numbers only for small values of r. For hybrid dendritic-star copolymers with three dendritic arms and two or three solvophobic linear arms, micelles with wide aggregation numbers are also formed but for slightly higher values of r. Our results for the aggregation number are compared with existing results of other architectures obtained at the same temperature, and an inequality for the aggregation number is proposed.

Dendritic brushes under good solvent conditions: a simulation study.

The structural properties of polymer brushes, formed by dendron polymers up to the third generation, were studied by means of Brownian dynamics simulations for the macroscopic state of good solvent. The distributions of polymer units, of the free ends, of the dendrons centers of mass, and of the units of every dendritic generation and the radii of gyration necessary for the understanding of the internal stratification of brushes were calculated. Previous self-consistent field theory numerical simulations of first-generation dendritic brushes suggested that at high grafting densities two kinds of populations are evident, one of short dendrons having weakly extended spacers and another with tall dendrons having strongly stretched spacers. These Brownian dynamics calculations provided a more complicated picture of dendritic brushes, revealing different populations of short, tall, and in some cases intermediate height dendrons, depending on the dendron generation and spacer length. The scaling dependence of the height and the span of the dendritic brush on the grafting density and other parameters were found to be in good agreement with existing theoretical results for good solvents.

Effects of the excluded volume interactions on the conformational properties of star polymers

Abstract The effects of the excluded volume interactions on the conformational properties of star polymers have been studied. First order calculations at the critical dimensionality d = 4 yield the critical exponents of the average quantities up to first order in e = 4-d. We thus find the partition function, the probability of the end of a branch to reach the central core and the probability of contact of the ends of two branches. The size of the macromolecule, expressed by the mean square radius of gyration 〈s2〉star is studied in the region where the interactions between the polymeric units repel one another and in the region where the units attract one another. The results are compared with the results of previous works and with experiments.

Dendritic brushes under theta and poor solvent conditions

The effects of solvent quality on the internal stratification of polymer brushes formed by dendron polymers up to third generation were studied by means of molecular dynamics simulations with Langevin thermostat. The distributions of polymer units, of the free ends, the radii of gyration, and the back folding probabilities of the dendritic spacers were studied at the macroscopic states of theta and poor solvent. For high grafting densities we observed a small decrease in the height of the brush as the solvent quality decreases. The internal stratification in theta solvent was similar to the one we found in good solvent, with two and in some cases three kinds of populations containing short dendrons with weakly extended spacers, intermediate-height dendrons, and tall dendrons with highly stretched spacers. The differences increase as the grafting density decreases and single dendron populations were evident in theta and poor solvent. In poor solvent at low grafting densities, solvent micelles, polymeric pinned lamellae, spherical and single chain collapsed micelles were observed. The scaling dependence of the height of the dendritic brush at high density brushes for both solvents was found to be in agreement with existing analytical results.

Conformational properties of regular comb polymers

Regular comb polymers with excluded volume interactions are studied and compared with regular star and linear polymers. First-order calculations in the excluded volume parameter, at the critical dimensionality d=4, yield the characteristic exponents of the macroscopic properties to order epsilon =4-d. The number of total configurations and the number of configurations with the backbone forming a ring are found. The evaluation of the mean end-to-end square distances of the backbone and the branches give an insight to the spatial distribution of the macromolecule.

On the miscibility of chemically identical linear homopolymers of different size

Abstract The effects of asymmetry due to the chain-size differences on the miscibility of linear polymer blends composed from the same monomers are studied by means of an analytical theory. The partition function of the blend and the resulting free energy are obtained through the summation of the series of the one-loop diagrams at any dimensionality d . From the stability conditions, no macrophase separation is found at d =3 for any size disparity or volume fractions in agreement with other recent theoretical calculations. At d =2, the effective repulsions between the two different species are much larger leading to macrophase separation. This result strongly indicates that a way to observe de-mixing of chemically identical homopolymer, is to increase the two-dimensional character of the chains like in the cases of strong adsorption or ultra-thin polymeric films.

Comparison of the stability of blends of chemically identical and different homopolymers in the bulk and in a film

We present a criterion of the stability of a polymeric blend of two species in terms of the three different interaction parameters between the same or different monomeric units. Long chain correlations among different chains are taken into account by including the contribution from all configurations having one loop. The special cases of chemically identical polymers of different size where all three interaction parameters become the same can be deduced and compared with that of different polymers. The stability limits of the system is analyzed in the three dimensional (3D) and in the two dimensional (2D) space in order to describe the behavior of a blend in the bulk and in a thin film where the 2D character of the chains is increased. Going from the 3D to the 2D case while the stability of a system of chemically identical homopolymers of different size decreases that of two different homopolymers increases but may decrease too.

Complexation of Polyelectrolyte Micelles with Oppositely Charged Linear Chains

The formation of interpolyelectrolyte complexes (IPECs) from linear AB diblock copolymer precursor micelles and oppositely charged linear homopolymers is studied by means of molecular dynamics simulations. All beads of the linear polyelectrolyte (C) are charged with elementary quenched charge +1e, whereas in the diblock copolymer only the solvophilic (A) type beads have quenched charge -1e. For the same Bjerrum length, the ratio of positive to negative charges, Z+/-, of the mixture and the relative length of charged moieties r determine the size of IPECs. We found a nonmonotonic variation of the size of the IPECs with Z+/-. For small Z+/- values, the IPECs retain the size of the precursor micelle, whereas at larger Z+/- values the IPECs decrease in size due to the contraction of the corona and then increase as the aggregation number of the micelle increases. The minimum size of the IPECs is obtained at lower Z+/- values when the length of the hydrophilic block of the linear diblock copolymer decreases. The aforementioned findings are in agreement with experimental results. At a smaller Bjerrum length, we obtain the same trends but at even smaller Z+/- values. The linear homopolymer charged units are distributed throughout the corona.

Monte Carlo simulation of star/linear and star/star blends with chemically identical monomers

The effects of chain size and architectural asymmetry on the miscibility of blends with chemically identical monomers, differing only in their molecular weight and architecture, are studied via Monte Carlo simulation by using the bond fluctuation model. Namely, we consider blends composed of linear/linear, star/linear and star/star chains. We found that linear/linear blends are more miscible than the corresponding star/star mixtures. In star/linear blends, the increase in the volume fraction of the star chains increases the miscibility. For both star/linear and star/star blends, the miscibility decreases with the increase in star functionality. When we increase the molecular weight of linear chains of star/linear mixtures the miscibility decreases. Our findings are compared with recent analytical and experimental results.