Glassy liquid-crystalline polymers

Fig. 29. Observed and calculated 2H NMR spectra for the mesogenic groups of a) the nematic (m = 2), b) the smectic (m = 6) liquid crystalline polymer in the glassy state, showing the line shape changes due to the freezing of the jump motion of the labelled phenyl ring. The exchange frequency corresponds to the centre of the distribution of correlation times. Note that the order parameters are different, S = 0.65 in the frozen nematic, and S = 0.85 in the frozen smectic system, respectively...

With these properties a wide field of application is revealed As the l.c. side chain polymers can be orientated in the l.c. state by an electric or magnetic field, it is possible to store any information obtained in the l.c. state by cooling the liquid crystalline polymer down to the glassy state. Obvious applications are e.g. optical filters or reflectors, prepared for linearly or circularly polarized light by cholesteric polymers. Furthermore the glassy polymers can serve as anisotropic matrices for dissolved molecules. 

The expansivity of the rubbery or liquid polymer is always larger than that of the glassy or crystalline polymer. 

Finally, there are complex fluids that are intermediate between solid and liquid in more than one of the ways listed above. Liquid crystalline polymers (LCPs) are both viscoelastic and liquid crystalline. Ordered block copolymers are viscoelastic and anisotropic. Glassy polymers possess long viscoelastic time scales both because they are glassy and because they are polymeric. Filled polymer melts possess the properties of both polymer melts and suspensions. 

There are a large number of publications on photoinduced anisotropy in supramolecular-ordered azobenzene-containing systems, such as liquid crystals (Kreuzer et al., 2000 Szabados et al., 1998), liquid crystalline polymers (Rosen-hauer et al., 2001 Han et al., 2000), LbL (Geue et al., 1997 Stumpe et al., 1996), self-assembled monolayers (Sekkat et al., 1995), and adsorbed azobenezene molecules (West et al., 2001). But mass transport resulting in SRG was only reported till recently in glassy systems. 

The influence of chain packing (Le. free volume) on solubility, diffusivity and permeability in liquid crystalline polymers can be studied by comparing properties of LCPs in the disordered, isotropic state with those in the ordered, liquid crystalline state. HIQ-40 is a random, glassy, thermotropic, nematogenic terpolymer synthesized from 40 mole percent p-hydroxybenzoic acid and 30 mole percent each of isophthalic acid and hydroquinone. The chemical structures of the constituent monomers for fflQ-40are ... 

Table I. Oxygen permeability in glassy barrier polymers and liquid crystalline polymers at 35 C...

In the liquid crystalline phase, the molecules lose their positional order, but their mesogenic units still retain their orientational order. When the liquid crystalline polymers solidify, polymer glasses are formed in which the liquid crystalline structure may become frozen in. The glassy state may have the orientational order of a nematic phase or a smectic phase depending on the liquid crystalline phase present before the glass transition. This gives the material properties not normally found in amorphous glassy... 

At lower temperatures, a number of liquid-crystalline transitions may occur which again can be recorded by DSC/DTA as exothermic first-order transitions (Fig. 10.22). Hot-stage microscopy and X-ray diffraction are used to determine the nature of these transitions. At lower temperatures, solid crystals may be formed. The latter are revealed by DSC/DTA as an exothermic first-order transition, by TMA as an increase in sample stiffness and by X-ray diffraction as sharp Bragg reflections. Some liquid-crystalline polymers, e.g. copolyesters, are supercooled to a glassy state without crystallizing (Fig. 10.22). 

Fleischmann EK, Zentel R (2013) Liquid-crystalline ordering as a concept in materials science from semiconductors to stimuli-responsive devices. Angew Chem Int Ed 52 8810-8827 Garcia-Amoros J, Velasco D (2014) Understanding the fast thermal isomerisation of azophenols in glassy and liquid-crystalline polymers. Phys Chem Chem Phys 16 3108-3114 Garcia-Amoros J, Szymezyk A, Velasco D (2009) Nematic-to-isotropic photo-induced phase transition in azobenzene-doped low-molar liquid crystals. Phys Chem Chem Phys 11 4244-4250... 

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