Self-assembly/association

The major interest in the incorporation of POSS derivatives in polymers is how the structures affect the polymer to which they are attached or in which they are blended. Many studies reveal that POSS groups undergo self-assembly/association to form POSS-ilch domains that strongly affect polymer properties. For example, monofunctimial epoxy derivatives of POSS were incorporated into an epoxy matrix and it was shown that phase separation occurs in such a matrix, most likely because of the incompatibility of the isobutyl groups attached to the cage [69]. The resulting... 

Coacervation occurs in tropoelastin solutions and is a precursor event in the assembly of elastin nanofibrils [42]. This phenomenon is thought to be mainly due to the interaction between hydro-phobic domains of tropoelastin. In scanning electron microscopy (SEM) picmres, nanofibril stmc-tures are visible in coacervate solutions of elastin-based peptides [37,43]. Indeed, Wright et al. [44] describe the self-association characteristics of multidomain proteins containing near-identical peptide repeat motifs. They suggest that this form of self-assembly occurs via specific intermolecular association, based on the repetition of identical or near-identical amino acid sequences. This specificity is consistent with the principle that ordered molecular assembhes are usually more stable than disordered ones, and with the idea that native-like interactions may be generally more favorable than nonnative ones in protein aggregates. 

For instance, Pn-2 forms ribbons at concentrations >0.1 mM in water. At 0.6 mM, these ribbons self-assemble into fibrils and at higher concentrations into fibres. Conversely, glutamine-rich Pn-1 (Ac-QQRQQQQQEQQ-Am) forms ribbons at 1 mM and does not form fibrils, even at concentrations as high as 25 mM. Association of Pn-2 into ribbons is much more favourable due to it having hydrophobic aromatic residues arranged such that there is a large disparity between the hydrophobicity of each of its tape faces. 

Rajagopal K, Ozbas B, Pochan DJ et al (2006) Probing the importance of lateral hydrophobic association in self-assembling peptide hydrogelators. Eur Biophys J Biophys Lett 35 162-169... 

The value of EM for a cooperative self-assembled structure provides a measure of the monomer concentration at which trivial polymeric structures start to compete, and therefore EM represents the upper limit of the concentration range within which the cooperative structure is stable (Scheme 2). The lower limit of this range is called the critical self-assembly concentration (csac) and is determined by the stoichiometry of the assembly and the strength of the non-covalent binding interactions weaker interactions and larger numbers of components raise the csac and narrow the stability window of the assembly (8). Theoretical treatments of the thermodynamics of the self-assembly process have been reported by Hunter (8), Sanders (9), and Mandolini (10). The value of EM is lowered by enthalpic contributions associated with... 

The aniline-zinc porphyrin interaction has also been exploited to form dimers. Hunter (60) reported the dimerization of porphyrins functionalized at one meso position with ortho or meta aniline groups (47, 48, Fig. 15). Both compounds showed concentration-dependent H NMR spectra with large upfield shifts for the aniline protons. The dimerization constants are 160 and 1080 M-1 respectively for 47 and 48, and these values are an order of magnitude higher than the association constants of simple reference complexes (K — 10 and 130 M 1 respectively), which is indicative of cooperative self-assembly. The complexa-tion-induced changes in chemical shift were used to obtain three-dimensional structures of the dimers. 

Hunter (60) reported a self-assembled open polymer formed by a zinc porphyrin bearing one para-aniline substituent at the meso position. The ortho- and mela-analogs discussed above form closed dimers, but the geometry of the para-derivative precludes this, and polymerization is the only alternative (76, Fig. 31). Although the dilution experiments could be fitted to a non-cooperative polymerization model with a pairwise association constant (K = 190 M 1) practically identical to that found for simple aniline-zinc porphyrin complexes (K = 130 M 1), broadening of the 4H NMR spectrum at high concentrations is characteristic of oligomerization. 

---