Polymers in External DC Fields

Physical Chemistry, University of Saarland, 66123 Saarbriicken, Germany 

General considerations and a simple Hiickel-like model 

Theoretical chemistry and physics are partly concerned with the development of new theoretical and computational methods for studying special properties and/or materials and partly concerned with the application of these methods to questions of current interest. Thereby, a central issue is that of exploring new materials or properties that so far has been out of reach with theoretical methods. 

Slightly more than 10 years ago one of the present authors together with colleagues in Uppsala, including Osvaldo Goscinski, used one of the simplest possible conjugated polymers, trawi -polyacetylene, (CH) as 

ADVANCES IN QUANTUM CHEMISTRY, VOLUME 47 ISSN 0065-3276 DOI 10.1016/S0065-3276(04)47021-X 

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We finally mention that one further reason for studying infinite, periodic polymers in external DC fields is the findings [20-22] that currently applied approximate density functionals (like the one we are using) may be inadequate when calculating responses to external DC fields. Thus, studies like the ones of this contribution may provide further insight into the failures of the functionals. On the other hand, we stress that our basic method is, in principle, not dependent on these problems and can be modified easily according to new proposals for approximate density functionals. 

Spingborg M, Dong Y (2004) Conjugated polymers in external dc fields. In Sabin JR, Brandas E (eds) Advances in quantum chemistry, vol 47. Elsevier Academic Press Inc., p 269... 

We next considered the dissipative system of BCPs in electric field, since virtually all polymer show some level of residual conductivi when put under an external potential. Here we are interested in the part of conductivity which is due to mobile dissociated ions. These ions are predicted to cause a phase-transition from bcc phase of spheres into hexagonal phase of cylinders in static DC field. The bcc to hex transition has just been experimentally carried-out in the group of T. Russell in Amherst (21), The surprisingly low fields used, 6-72 V/pm, cannot be explained by dielectric contrast alone, and agree with our theory and confirm our predictions. This kind of deformation should be applicable in many more ordered structures such as some of the triblock copolymer phases. 

As seen in the preceding section, the counterions play a crucial role in the mobility of the polyelectrolyte molecules. Even in the absence of an external electric field, the counterions exert an induced electric field in the immediate environment of a charged segment which in turn significantly modifies the collective diffusion coefficient of the polymer. This additional contribution is absent for uncharged polymers, where the cooperative diffusion coefficient Dc is given by the Stokes-Einstein law in dilute solutions. 

Polymer films with exceptionally large second-order non-linearities can be produced by the corona poling (Fig. 6) process.An efficient NLO chrornophore molecule is either doped in a host polymer matrix or covalently linked as pendant groups on a polymer backbone. The polymer is then cast as a film, usually by spin-coating techniques. Subsequently, the film is subjected to a strong external dc electric field and heated to the glass transition temperatures of the polymer. At this point, the chromophores are aligned parallel by the field and locked into position when the film is cooled to room temperature with the external field on. 

Figure 9.11 Angular dependence of die resonance fields Bo of the 4 dicarbene structures DC9, DCjo, DCn, and DC in perdeuterated TS-diacetylene crystals. The crystal is rotated so that the external magnetic field Bq is in the plane of the polymer backbone. The b-axis is the direction of the polymer chain, y and z are the principal axes of the fine structure tensor. The curves are fitted to the experimental points by computer calculations. A and B indicate the two magnetically equivalent directions of the molecular orientation within the monoclinic imit cell. Dots experimental values lines calculated by (Eqs. 4-7) [32, 42].

The k is also related to the coefficients measured under different conditions. For instance, the elastic compliance and (measured under constant field and charge, respectively) are related to each other as = (l - kl )sf. Thus, under different external electric boundary conditions, a polymer with a large k will see a large difference in the elastic compliance. This can be used to tune the elastic modulus of the polymeric material by varying the electric conditions. For example, the elastic compliance of the electrostrictive material decreases with the DC bias. Similarly, E33 = (1 — which means the... 

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