It is shown that the morphologies and other characteristics of
crew-cut aggregates of
polystyrene-b-poly(acrylic acid) (PS-b-PAA)
diblock copolymers are related to the nature of the
initial
common solvent in which the micelle-like aggregates are prepared.
Polymer−solvent interactions
determine the dimensions of both the core and the corona of the
aggregates. Solubility parameters and
dielectric constants of the solvents can be used to estimate the
strength of the PS−solvent interaction
(which influences the solvent content in the core) and the strength of
PAA−solvent interaction (which
influences the repulsion among corona chains). The closer the
match between the solubility parameter
of the solvent and that of the core forming block, the higher the
solvent content of the core and the
higher the degree of stretching of the core chains. The lower the
polarity of the solvent, the weaker the
PAA−solvent interaction and the weaker the repulsive interactions
among the corona chains; this increases
the aggregation number and degree of stretching in the core. As
the degree of stretching of PS chains in
the cores increases and the repulsion among the corona decreases, the
morphology of the aggregates can
change progressively from spheres to cylinders, to vesicles, or to
large compound micelles. In
N,N-dimethylformamide, the core dimensions are
smaller and the corona dimensions are larger than in
tetrahydrofuran (THF) or dioxane, so spherical aggregates are favored.
In THF, the solvent content in
the core and the corona dimension are larger than in dioxane.
Since an increase of solvent content in
the core favors a morphological change away from spheres, but a larger
corona dimension favors spheres,
the final morphology is determined by a balance of these two factors.
Relationships between the nature
of common solvent and critical water content as well as degree of
micellization are studied in detail.
Finally, morphological changes for the same diblock copolymer in
different solvents are explored further.
Micelles formed from polycaprolactone-b-poly(ethylene oxide) (PCL-b-PEO) diblock copolymers were investigated as a novel drug delivery system. The affinity of the micelles for hydrophobic solubilizates was assayed by determining the partition coefficient for the lipophilic compound, pyrene, between the micelles and water; the partition coefficient was found to be on the order of 10(2). The Trypan blue and Alamar blue survival assays were used to assess the in vitro biocompatibility of the micelles with PC 12 cells, MCF-7 breast cancer cells, and primary cultures of human microglia, astrocytes, and cortical neurons. The micelles were then studied as a delivery vehicle for the neurotrophic agents FK506 and L-685,818 in PC 12 cell cultures. In both cases, the micelle-incorporated drugs, in the presence of nerve growth factor (5 ng/mL), were able to promote the degree of differentiation of the PC 12 rat pheochromocytoma cells.
Mesosize crystal-like aggregates with an internal structure of hexagonally packed hollow hoops (HHH) in a polystyrene matrix have been prepared in solution by self-assembly of asymmetric polystyrene-b-poly(acrylic acid) diblock copolymers. Most of the aggregates are cylindrical or in the shape of truncated cones. The external surface of the aggregates and the internal surface of the hollow hoops are lines with short poly(acrylic acid) chains. The hoop morphology is imposed because the end-capping energy of a rod on this size scale is more important than the curvature energy. A strong interdependence between the external shape and the internal structure in these mesosize particles is demonstrated. [S0031-9007 (97)04815-1] PACS numbers: 61.41. + e
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