Copolymer segments, block interactions between

Here we turn to a consideration of the influence of two other pairwise interactions in the diblock copolymer, namely the interactions between the segments in the two blocks (which is assumed to be repulsive), and the interaction of the soluble block with the solvent (which is assumed to be attractive). The importance of the variation in these two terms, and the connection with the symmetry of the diblock copolymer, are deduced from the simulations. 

PIPAAm-PBMA block copolymers form a micellar structures by selfassociation of the hydrophobic PBMA segments in water, a good solvent for PlPAAm chains below the LCST but a nonsolvent for the PBMA chains. This amphiphilic system produces stable and monodispersed micelles from polymer/A-ethylacetamide (good solvent for the both polymer blocks) solutions dialyzed against water. Hydrophobic dmgs can be physically incorporated into the iimer micelle cores with PBMA chains by hydrophobic interactions between the hydrophobic segments and dmgs. 

In addition to the hydrophobic interaction mentioned above to encapsulate guest molecules, other types of nonspecific interactions have also been explored to enhance binding. For example, block copolymer micelles based on PEO as hydrophilic segments and poly(/3-benzyl L-aspartate) as hydrophobic blocks have used to encapsulate doxombicin. The encapsulation efficiency of doxombicin, an aromatic anticancer drug molecule, has been found to be significantly higher. This observation has been attributed to the tt-tt interaction between the anthracycUne moiety of doxorubicin and the benzyl group of poly(/3-benzyl L-aspartate) (Cammas-Marion et al. 1999). 

There have been a number of computer simulations of block copolymers by Binder and co-workers (Fried and Binder 1991a,ft), and this work was reviewed in Binder (1994). Although computer simulations are limited due to the restriction on short chain lengths that can be studied, finite size effects and equilibration problems at low temperatures, the advantages are that the models are perfectly well characterized and ideal (monodisperse, etc.) and microscopic details of the system can be computed (Binder 1994). In the simulations by Binder and co-workers, diblocks were modelled as self- and mutually-avoiding chains on a simple cubic lattice, with chain lengths N = 14 to 60 for/ = 1.A purely repulsive pairwise interaction between A and B segments on adjacent sites was assumed. A finite volume fraction of vacancies was included to speed the thermal equilibration process (Binder 1994). 

Specific Interactions between Block Copolymer Segments... 

These examples emphasize that not only the macromolecular architecture plays an important role in the determination of the aggregate morphology, but that also interactions between block copolymer segments can strongly influence the final structure of the assemblies formed by these types of amphiphiles as seen for the smaller molecular amphi-philes. 

The marginally compatible polyblend is analogous to the poor solvent case in that presence of the discrete dispersed phase results in less interaction between phases and, therefore, lower melt viscosities. Styrene-butadiene block copolymers have lower melt viscosities than random copolymers of the same composition and yield solutions of lower viscosity at the same concentration. This is because of the incompatibility but inseparability of the segments of the chain. 

Figure 4 Interaction between metal-complex anticancer drugs, cisplatin (CDDP), and the core-forming segments of the block copolymers PEG-P(Asp) and PEG-P(Glu). The methane group in the side chain of PEG-P(Glu) was reported to play an important role in significantly increasing the stabihty of the micelles compared to PEG-P(Asp).

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