Kinetics of Helix Untwisting in Photosensitive Cholesteric Polymer Mixtures: Influence of Molecular Mass and Ordered Phase Formation Alexey Bobrovsky,* Natalia Boiko, and Valery Shibaev Faculty of Chemistry, Moscow State UniVersity, Leninskie gory, Moscow, 119992 Russia ReceiVed June 6, 2006; ReVised Manuscript ReceiVed June 7, 2006 ABSTRACT: Photooptical behavior of cholesteric mixtures based on two fractions of nematic polymer with different molar masses containing chiral-photosensitive dopant with CdC double was studied. UV irradiation of planarly oriented films of the mixtures leads to E-Z photoisomerization of dopant, helix untwisting and shift of selective light reflection peak to the longer wavelength spectral region. Kinetics of this process was studied at different temperatures. It was shown that a relative rate of helix untwisting for the mixture with the high molar mass fraction (DP ∼ 87) is 2 orders of magnitude less than for the mixture containing a low-molar-mass fraction (DP ∼ 22), whereas the activation energies of the helix untwisting for both mixtures have almost the same values. The formation of the ordered TDK* phase during annealing of the blend at 50 °C for 1 week completely prevents the helix untwisting. This fact is worthy of attention because it provides a possibility of fixing a helical structure even at temperatures above the glass transition; this advantage makes this system promising for optical data recording and storage. Introduction It is well-known that cholesteric mesophase is characterized by the helical supramolecular organization of mesogens which determines its unique optical properties. 1,2 One of these optical properties is the selective reflection of light with wavelength λ max depending of helix pitch (P) of the cholesteric structure according to the equation: where n j is the average refractive index of liquid crystal. The concrete value of the selective reflection wavelength depends on many internal and external factors. One of these factors is the structure and geometry of the chiral fragments responsible for the helical twisting of the whole system. The key parameter characterizing the ability of chiral groups for twisting cholesteric mesophase is so-called helical twisting power, expressed by the simple equation where X is the concentration of the chiral fragments. The first example of cholesteric low molar mass liquid crystal mixture with photovariable selective light reflection wavelengths was published in 1971 by Sackmann. 3 Since then, a number of new interesting approaches for photoregulation of selective light reflection wavelength were developed. 4-18 One of them is based on the introduction of combined chiral-photochromic fragments capable of photoisomerization into cholesteric or nematic matrix. Under the irradiation the helical twisting power of chiral- photochromic dopant decreases leading to reversible or irrevers- ible shift of selective light reflection wavelength. Synthesis of cholesteric polymers and preparation of the polymer-based mixtures allow one to create glass-forming materials with optical properties “frozen” in the glassy state for a prolonged time. 5,11-18 This advantage is very significant for the creation of new materials for optical data recording and storage. In this connection, it seems to be very important to study the kinetics of helix untwisting in such polymer materials in detail and evaluate an influence of different factors on this process. In particular, it would be essential to study an influence of molar mass of polymer on the rates and activation energies of helix rearrangement of irradiated polymer mixtures. It should be pointed out that up to now these processes have not been considered in the literature. In this work we prepared cholesteric mixtures based on two fractions of nematogenic polymer PAA with different molecular masses: We have selected two fractions of PAA which are distin- guished not only by degree of polymerization but also by phase behavior. 19 These fractions display the following phase transi- tions temperatures: Fraction I forms only the nematic phase, but for high molar mass fraction II an additional formation of ordered two- dimensional crystal TDK phase is observed. The structure of this phase was studied and discussed in detail elsewhere. 20 A local order of mesogenic groups of the mesophase is similar to the smectic K phase (tilted monoclinic cell) but layer ordering is completely absent. Formation of the ordered phase is kinetically controlled: this phase is formed after one month storage at room temperature, at higher temperature (45-50 °C) the process proceeds faster and takes only several days. * Corresponding author. E-mail: bbrvsky@yahoo.com. λ max ) n jP (1)  ) dP -1 /dX ) n j(dλ max -1 /dX) X)0 (2) fraction I, DP∼22: g 18 N 99 I fraction II, DP∼87: g 31 TDK 73 N 135 I 6367 Macromolecules 2006, 39, 6367-6370 10.1021/ma061258y CCC: $33.50 © 2006 American Chemical Society Published on Web 08/16/2006