Interchain aggregation

A left-handed double-heUcal stmcture has been proposed for geUan in the crystalline state, based on x-ray diffraction studies (227). The presence of acetyl groups presumably dismpts interchain aggregation, since these groups are postulated to be on the outside of the heUces. The role played by acetyl and glyceryl ester groups and their influence on the double-heUcal stmcture has been studied using computet models (232). 

A certain balance should be kept in distortion of the thiophene planarity as a way to prevent the formation of interchain aggregates. Introducing two substituents at positions 3 and 4 of PT allows a shift of the emission through the entire visible range and prevents interchain interactions (resulting in a smaller decrease of the quantum yield in the solid state compared to solution). Highly crowded disubstituted PTs 418 421 show very low luminescence efficiency already in solution (Table 2.5) due to substantial distortion of the backbone [107,498],... 

Short chain amphiphiles can be incorporated into the backbone of the polymer chains. The resulting graft macromolecules are able to form both intrachain and interchain aggregates. Polymeric surfactants assemble into a variety of intrachain micelles. These polymeric surfactants and/or amphiphilic polymacromonomers can also form mixed aggregates which incorporate free monomeric (macromonomer with a very small hydrophobic group) surfactants. 

Considering the competition between intrachain contraction and interchain association, we have to discuss an overlooked viscoelastic effect in the formation of stable mesoglobules in dilute solutions. Otherwise, it would be difficult to understand why copolymer chains with a high content of hydrophobic comonomers could form smaller interchain aggregates. In the micro-phase separation, copolymer chains in solutions contract and associate. The collision between contracted and associated chains would not be effective if the collision (or contact) time (rc) is much shorter than the time (re) needed to establish a permanent chain entanglement between two ap-... 

FIG. 11 Schematic illustration of the different types of complexes (a) with long polymers (interchain aggregates are formed essentially in the case of Coulombic complexation of polyelectrolyte and protein) (b) with short amphiphilic polymers (hydrophobic association). 

R.D. Schaffer, P.T. Snee, J.C. Johnson, L.F. Lee, K.R. Wilson, L.H. Haber, R.J. SaykaUy, T.Q. Nguyen, and B.J. Schwartz, Nanoscopic interchain aggregate domain formation in conjugated polymer films studied by third harmonic generation near-field scanning optical microscopy. J. Chem. Phys., 117, 6688 (2002). 

Comparative photophysical studies of 90 and 129 suggest that in dendro-nised polyfluorenes the bulkiness of the substituents hinders exciton migration due to increased chain separation [234]. Delayed PL studies [235] of 90 and 134 found that both polymers display a defect emission band at 530 nm, and a band at 480 nm previously attributed to interchain aggregates [205]. This is an interesting result as interchain aggregates had previously been assumed to arise from tt-stacking of the polymer chains. Calculations of... 

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