Polymers, liquid crystalline glass transition temperature

The linkage of conventional low molar mass Lc s to a linear polymer main chain via a flexible spacer provides a method to realize systematically the liquid crystalline state in linear polymers. Above the glass transition temperature Tg the polymer main chain can be assumed to exhibit, at least in the nematic state, an almost free motion of the chain segments, causing a tendency towards a statistical chain conformation. Due to their mobility, the polymer main chains are able to diffuse past each other, which is a condition to obtain the liquid state. Therefore such polymers can be classified as liquids of high viscosity10O). 

U Wiesner, N. Reynolds, C. Boeffel, and H. W. Spiess, Photoinduced reorientation in liquid-crystalline polymers below the glass transition temperature studied by time-dependent infrared spectroscopy, Makromol. Chem., Rapid Commun. 12, 457-464 (1991). 

Initially, polyethers based chromophore 1 were synthesized to explore the chemistry of the dye and to develop discotic liquid crystalline materials. These materials were prepared by phase transfer catalyzed Williamson etherification (Scheme 1). The first prepared polyether was based on 1 and had a spacer length of four methylene units (n=4, x=0). This demonstrated the chemistry, but the material possessed limited solubility in common organic solvents. Consequently, longer spacers (n=10, x=0) were incorporated to prepare soluble homopolymers of 1. The iH-NMR spectrum confirmed the structure but the polymer displayed a glass transition temperature of 100 °C. Furthermore, the absence of a melting point and a WAXD crystalline structure indicated the materials were amorphous. 

Liquid crystalline solutions as such have not yet found any commercial uses, but highly orientated liquid crystal polymer films are used to store information. The liquid crystal melt is held between two conductive glass plates and the side chains are oriented by an electric field to produce a transparent film. The electric field is turned off and the information inscribed on to the film using a laser. The laser has the effect of heating selected areas of the film above the nematic-isotropic transition temperature. These areas thus become isotropic and scatter light when the film is viewed. Such images remain stable below the glass transition temperature of the polymer. 

Lastly, it was demonstrated with PPO substituted with a series of alkyl side-chains as we have here, that the glass transition temperature decreases with an increase in the side-chain length (28). At the same time, the Tg s of the more flexible side-chain liquid crystalline polymers investigated to date are always much higher than those of the corresponding polymers without the mesogenic side-chains (3). Therefore, it is quite likely that we may obtain side-chain liquid crystalline polymers of approximately the same Tg from PPO and PECH. 

When the glass transition temperature of the polymer sample is reached in the DSC experiment, the plot will show an incline. It is obvious that the heat capacity increases at T, and therefore DSC can monitor the of a polymer. Usually the middle of the incline is taken to be the Tg. Above Tg, the polymer chains are much more mobile and thus might move into a more ordered arrangement they may assume crystalline or liquid-crystalline order. When polymers self-or-ganize in that way, they give off heat which can be seen as an exothermal peak in... 

The benzobisazole family of rigid-rod polymers is soluble in acidic solvents such as PPA, methanesulfonic acid, chlorosulfonic acid, 100% sulfuric acid and Lewis acid salts such as antimony trichloride and bismuth trichloride. More recently, PBZT has been reported [22] to form liquid crystalline solutions in nitromethane containing aluminum trichloride or gallium trichloride. Since the glass transition temperature of these materials is above their decomposition temperature, they must be processed from solution. 

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