Dipolar host polymers

In electric-field poling, a guest dipolar chromophore and a chemically and optically passive host polymer (the chromophore can also be covalently tethered to the polymer [74] or incorporated into the main chain of the polymer) is heated to above the glass transition temperature, Tg, of the mixture. A large electric... 

This chapter concentrates on the design of efficient dipolar NLO chromophores and the different approaches for their incorporation in non-centrosymmetric materials, including guest-host polymer systems, chromophore-functionalized polymers (side-chain and main-chain), cross-linked chromophore-macromolecule matrices, dendrimers, and intrinsically acentric self-assembled chromophoric superlattices. The different architectures will be compared together with the requirements (e.g., large EO coefficient, low optical absorption, high stability, and processability) for their incorporation into practical EO devices. First, a brief introduction to nonlinear optics is presented. 

Yuh and Pai argued that the role of the polymer was related to the activation energy. Borsenberger and Bassler explained their results on a model based on dipolar disorder. According to the model, a is determined by the dipole moment of both the dopant molecule and the polymer repeat unit. The effect of the polymer host is then related to the difference in dipole moments of the dopant molecule and the polymer repeat unit as well as the dopant concentration. Most recent studies have been described by dipolar disorder arguments. 

This is confirmed by comparing ID double-quantum filtered experiments with increasing recouphng times. Extension of this to 2D SQ/DQ correlation identifies the nature of the dipolar coupled pair, since the couphng partner of the directly detected spin can be identified in the indirect dimension, where the chemical shift is the sum of the shift of the two protons involved. This spectrum (Fig. 4b) exhibits intense and well-resolved cross peaks between CD and PDMS. The dipolar couphng constant between the guest and host must therefore be of the order of, or exceed, a few himdred Hz, proving that whatever fast motion the polymer chain performs, it must be a local one. 

Dumont and coworkers [136, 155, 156] have observed that shining doped (or functionalized) polymer thin hlms with noncentrosymmetric dipolar chromo-phores, induces a significant increase of electro-optic coefficient in the chromo-phore absorption band, corresponding to a better, polar orientation of chromo-phores. The measurements have been done by using the attenuated total reflection technique, and the optical field polarization was perpendicular to the applied low-frequency external electric field to the thin film (Fig. 33). A better stability of induced orientation was observed in the case of functionalized polymers than in guest-host system, as is usually the case with the static field poled polymers. The chromophores orient with dipolar moments perpendicular to the optical field (and parallel to the applied static (or low frequency) field. As will be discussed later, the chromophore orientation undergoes a trans-cis isomerization process (Fig. 34). 

Numerous simulations and experimental work showed that the width of the density of states crin Eq. (107) is strongly dependent on the polarity of the polymer host and the dipole moment of the dopant molecule. It was found that the total width o was comprised of a dipolar component oy, and an independent Van der Waals component Oydw [ 8] ... 

ESR data of the intrazeolite PANI show the presence of ca. 0.0025 Curie-type spins per aniline loaded (comparable to bulk spin densities if the lower polymer content in the zeolites is considered), with g-values (g=2.0034) similar to bulk polymers. ESR spectra of intrazeolite PPy confirm the low spin count expected for bipolaron formation and g-values (2.0027) characteristic of polypyrrole. Large linewidths in both cases (8-10 G) could indicate strong dipolar interactions with the zeolite host. 

The dipolar interactions between polar host CDs and included guests with permanent dipole moments are considered to affect at least the conformations and structures of soluble small-molecule guest/host CD-ICs.Our observations of the preference for inclusion of poly (e-caprolactone) (PCL) over poly(L-lactic acid) (PLLA) chains in both dissolved and crystalline suspended a-CD (a-CDcs), as well as the displacement of PLLA chains by PCL from PLLA-a-CD-ICs when suspended in PCL solution, indicate that dipole-dipole electrostatic interactions may not be critical to the formation of polymer-CD-ICs. If they were, then we might expect PLLA to be preferentially included in a-CD compared to PCL, because two PLLA repeat units and two ester group dipoles occupy each a-CD, while only single PCL... 

Because both aliphatic polyesters likely adopt nearly fully extended all-trans conformations when included in their a-CD-lCs/ the dipole moments in neighboring repeat units point in approximately opposite directions. This might cause partial cancellation of the net PLLA dipole moment in each a-CD, because two PLLA repeat units occupy each host a-CD, while only a single PCL ester group is included. Aside from this potential caveat, and because purely non-polar hydrocarbon polymers may be included in CDs, it is likely that dipolar electrostatic interactions do not play a major role in the nano-threading and subsequent formation of polymer-CD-lCs. 

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