Side-chain polymer

In side-chain polymers, where the chromophore is covalently bonded to a polymer main chain, the concentration of transport molecules can be increased as compared to MDPs without causing crystallization. Because the underlying physical mechanism of charge carrier transport (nearest neighbour hopping between weakly coupled, localized states) is the same for both MDPs and side-chain polymers, the transport properties are qualitatively similar. 

This finding offers the opportunity to tailor the thermal and mechanical properties of the photoconductor without seriously affecting the transport properties. This can be achieved either by variation of the spacer length between the chromophore and the backbone or by variation of the backbone itself A typical example for this group is PVK [12, 32, 34, 35]. Here, the transition from dispersive to non-dispersive transport could be observed [34]. Due to the fact, that the TOP curves have been measured over 10 decades in time, it was possible by means of a numerical inverse Laplace transform [34] to calculate from the measured photocurrent the trapping rate distribution, which is a measure for the density of localized states. 

Without doubt, this is by far the most studied family of Cso-polymers, also called on-chain or charm-bracelet, with a wide range of potential applications (Chapter 2). 

The first example belonging to this category was reported by the group of Wudl in 1992 [47]. Among the recent literature, we emphasize the outstanding performances showed by two recent examples of side-chain C,so-polymer-based organic solar cells reported by Sariciftd (2.0% efficiency) [48] and Fr chet (2.8% efficiency) [49]. 

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Fig. 17. Schematic stmcture of an LC side-chain polymer with embedded dyes (134).

Fig. 18. Single steps of a write/read/erase cycle for an LC side-chain polymer (134). (—), mesogenic group (0), dyestuff. transition temperature...

Besides the classical photothermoplasts, LC side-chain polymers with distinct phase changes also are well suited for holographic purposes, and biopolymers from genetically engineered bacteriorhodopsine (BR) have been discussed as a holographic material. 

Fig. 15. Molecular stmcture and phases of a typical polysdoxane side-chain polymer Hquid crystal.

Liquid crystal polymers are also used in electrooptic displays. Side-chain polymers are quite suitable for this purpose, but usually involve much larger elastic and viscous constants, which slow the response of the device (33). The chiral smectic C phase is perhaps best suited for a polymer field effect device. The abiHty to attach dichroic or fluorescent dyes as a proportion of the side groups opens the door to appHcations not easily achieved with low molecular weight Hquid crystals. Polymers with smectic phases have also been used to create laser writable devices (30). The laser can address areas a few micrometers wide, changing a clear state to a strong scattering state or vice versa. Future uses of Hquid crystal polymers may include data storage devices. Polymers with nonlinear optical properties may also become important for device appHcations. 

In 1950, Dahlquist et al. [82] reported the use of polyvinyl A -alkyl carbamates as PSA release materials. Since then, many other types of alkyl side chain polymers have been patented for use as release coatings, including copolymers based on higher alkyl acrylates or methaci ylates [83-86], polyvinyl esters of higher aliphatic fatty acids [87], higher alkyl vinyl esters or ethers and a maleic... 

Finkelmann, H. and Rehage,G. Liquid Crystal Side-Chain Polymers. Vol. 60/61, pp. 99 bis 172. 

Optical and electro-optical behavior of side-chain liquid crystalline polymers are described 350-351>. The effect of flexible siloxane spacers on the phase properties and electric field effects were determined. Rheological properties of siloxane containing liquid crystalline side-chain polymers were studied as a function of shear rate and temperature 352). The effect of cooling rate on the alignment of a siloxane based side-chain liquid crystalline copolymer was investigated 353). It was shown that the dielectric relaxation behavior of the polymers varied in a systematic manner with the rate at which the material was cooled from its isotropic phase. 

Finkelmann, H. and Rehage, G. Liquid Crystal Side-Chain Polymers. Vol. 60/61, pp. 99-172. Fischer, H. Freie Radikale wahrend der Polymerisation, nachgewiesen und identifiziert durch Elektronenspinresonanz. Vol. 5. pp. 463-530. 

The steric frustrations have also been detected in LC polymers [66-68]. For example, the smectic A phase with a local two-dimensional lattice was found by Endres et al. [67] for combined main chain/side chain polymers containing no terminal dipoles, but with repeating units of laterally branched mesogens. A frustrated bilayer smectic phase was observed by Watanabe et al. [68] in main-chain polymers with two odd numbered spacers sufficiently differing in their length (Fig. 7). 

Fig. 17a-c. Sketches of the molecular arrangements for the smectic structure with alternating layer-to-layer tilt a conventional and chevron smectic C layering in low molecular mass mesogens b ferroelectric hilayer chevron structures for achiral side-chain polymers c antiferroelectric hilayer chevron structures for achiral side-chain polymers. Arrows indicate the macroscopic polarization in the direction of the molecular tilt... 

In 2003, the van Hest group produced elastin-based side-chain polymers [123]. This research was motivated by the demonstration of the occurrence of an inverse temperature transition in a single repeat of VPGVG [124]. A methacrylate-functionalized VPGVG was synthesized and used as a monomer to perform atom transfer radical polymerization (ATRP) to produce homopolymers (Fig. 16b) or... 

Fig. 16 Various type of elastin-based side-chain polymers (a) ABA block copolymer produced via ATRP, (b) homopolymer produced via ATRP, (c) homopolymer produced via RAFT polymerization...

Inspired by the elastin-based side-chain polymers, Lemieux et al. prepared elastin-based stimulus-responsive gold nanoparticles. To this end, they capped gold particles with a layer of a single repeat of thiol-functionalized VPGVG peptides (Fig. 17a). These nanoparticles showed LCST behavior, which was modulated by varying the pH of the solution [131]. 

Ayres L, Vos MRJ, Adams P et al (2003) Elastin-based side-chain polymers synthesized by atrp. Macromolecules 36 5967-5973... 

M. Rahahn, A.-D. Schliiter, G. Wegner, and W.J. Feast, Soluble poly(p-phcnylcne)s. 1. Extension of the Yamamoto synthesis to dibromobenzenes substituted with flexible side chains, Polymer, 30 1054-1059, 1989. 

S. Dailey, W.J. Feast, R.J. Peace, I.C. Sage, S. Till, and E.L. Wood, Synthesis and device characterization of side-chain polymer electron transport materials for organic semiconducting applications, J. Mater. Chem., 11 2238-2243, 2001. 

C Wang, H Fei, Y Qiu, Y Yang, Z Wei, Y Tian, Y Chen, and Y Zhao, Photoinduced birefringence and reversible optical storage in liquid-crystalline azobenzene side-chain polymers, Appl. Phys. Lett., 74 19-21, 1999. 

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