Again in contrast to solutions, all colloidal systems (dispersions) are always highly structured, i.e., the dispersed phase is not irregularly and statistically distributed, but self-organized to a rich variety of complex structures. These may range from soap-like layers, over micelles to bicontinous sponge structures (as have been shown for microemulsions [1] and also proposed to exist in PAni dispersions [44b]). These self-organized structures are due to the non-equilibrium character of colloidal systems (hence a minimum of entropy) [14]. Solutions, in contrast, are equilibrium systems and have reached a maximum of entropy.
The same considerations apply for side-chain modified ICPs (or their neutral conjugated analogues), like poly-octylthiophenes, poly-alkoxy-phenylene-vinylenes or for block copolymers like PAc-PIB or PAc-PS. Here, J. P. Aimé [2] and others [48] have shown with SANS, that “solutions” in fact exhibit a layered structure, where the PAc-blocks are arranged (and aggregated, not solvated!) in flat layers, like drawn in [3] (fig. 8).
We expect a comparable soap-like layer structure also for alkoxy-substituted poly-phenylene-vinylenes to result after deposition from “solution” (dispersion). These will lead to multilayer structures. It will be found, that this arrangement is responsible for the incredibly high gain and small gain width (< 50Å), Heeger et al [4]c. Also the fact, that - in contrast to organic laser dyes - no concentration dependent self-absorption effect was observed, could be found being based on this very specific structure resulting from the colloidal nature of the “solution” from which the polymers were deposited.
figure captions:
fig 8: structure model for a flat, soap-like layer structure of a side-chain
modified conjugated polymer (like poly-octyl-thiophene), where the conjugated
units are aggregated
[2] P. Garrin, J. Aimé, D. Reibel, C. Mathis, Synth. Met. 51, 37-44 (1992)
[3] B. Wessling, reprint from Mair/Roth, Elektrisch
leitende Kunststoffe (1989)
cf: also [16 a]
[4] a) D. Halliday, P. Burn, D. Bradley, R. Friend, O. Gelsen, A. Holmes, A. Kraft, J. Martens, K. Pichler, Adv. Mater. 5, 41 (1993)
b) C. Zhang, D. Braun, A. Heeger, Appl. Phys. 73, 5177 (1993)
c) F. Hide, : Diaz-Garcia, B. Schwartz, M. Andersson, Q. Pei, A. Heeger, Science 273 (5283), 1833 (27. Sept. 1996)
A. Heeger, Bayreuth Polymer Symposium, april 7th, 1997 (Proceedings)
