Block copolymers critical properties

Recent developments in polymer chemistry have allowed for the synthesis of a remarkable range of well-defined block copolymers with a high degree of molecular, compositional, and structural homogeneity. These developments are mainly due to the improvement of known polymerization techniques and their combination. Parallel advancements in characterization methods have been critical for the identification of optimum conditions for the synthesis of such materials. The availability of these well-defined block copolymers will facilitate studies in many fields of polymer physics and will provide the opportunity to better explore structure-property relationships which are of fundamental importance for hi-tech applications, such as high temperature separation membranes, drug delivery systems, photonics, multifunctional sensors, nanoreactors, nanopatterning, memory devices etc. 

Formation of a complex between DNA and polycationic compounds appears to be the initial and quite possibly a critical parameter for nonviral gene delivery. Several synthetic vector systems, which are generally cationic in nature, including poly(lysines), cationic liposomes or various types of block copolymers and recently dendrimers, have been shown to self-assemble with plasmid DNA [13-15] [16]. Specific physicochemical properties manifested by these DNA complexes depend on the type of cationic agent used however, interesting patterns for such interactions are beginning to evolve [17, 18]. Under certain conditions, the interaction of DNA with polyvalent cations results in... 

Micelles are formed by association of molecules in a selective solvent above a critical micelle concentration (one). Since micelles are a thermodynamically stable system at equilibrium, it has been suggested (Chu and Zhou 1996) that association is a more appropriate term than aggregation, which usually refers to the non-equilibrium growth of colloidal particles into clusters. There are two possible models for the association of molecules into micelles (Elias 1972,1973 Tuzar and Kratochvil 1976). In the first, termed open association, there is a continuous distribution of micelles containing 1,2,3,..., n molecules, with an associated continuous series of equilibrium constants. However, the model of open association does not lead to a cmc. Since a cmc is observed for block copolymer micelles, the model of closed association is applicable. However, as pointed out by Elias (1973), the cmc does not correspond to a thermodynamic property of the system, it can simply be defined phenomenologically as the concentration at which a sufficient number of micelles is formed to be detected by a given method. Thermodynamically, closed association corresponds to an equilibrium between molecules (unimers), A, and micelles, Ap, containingp molecules ... 

Figure 47.2. Plui onic block copolymers witli vaiious numbers of hydi opliilic EO (n) and hydiophobic PO (m) units ai-e chai acterized by distinct hydi opliilic-lipopliilic balance (HLB). Due to tlieir ampliiphi-lic chai acter tliese copolymers display surfactant properties including ability to interact witli hydiophobic surfaces and biological membranes. hr aqueous solutions at concentiations above critical micelle concen nation (CMC) tliese copolymers self-assemble into micelles.

A H NMR relaxation study of di-block and tri-block copolymers of ethylene oxide and 1,2-butylene oxide aqueous solutions has shown a phase transition from a micelle to a gel in the relaxation time of the ethylene oxide block, consistent with gel formation by close packing of micelles. NOE shows that the blocks of ethylene oxide and 1,2-butylene oxide interpenetrate at the core-fringe boundary.307 Similar phenomena have been observed for the tri-block copolymer of ethylene oxide and propylene oxide. At the critical micellar temperature, a marked transition in the relaxation times of the hydrophobic propylene oxide block occurs, which is attributed to a change from well-solvated mobile chains below the critical micellar temperature to a more restricted concentrated micelle-core environment above this temperature. However, no transition in the properties of the hydrophilic block of ethylene oxide has been observed. NOE data indicate that in the micelles there is considerable interpenetration of the... 

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