Isotropisation temperature

Fig. 33. Self-dewetting of a polymer smectic film into droplets was monitored by tapping mode SFM. The micrographs were measured at room temperature after annealing of a 10 nm thick film at 115°C during different time intervals a - 0 min.b - 15 min.c- 2h,d - 4 h. The annealing temperature was 20 °C above the bulk isotropisation temperature. The polymer consists of a polymethylmethacrylate backbone with perfluorooctyl side chains [323]...

Hydrogen bonding between pyridine units in a tetrafunctional compound (12) and benzoic acid units in difunctional compounds (13, Figure 6), resulted in the formation of reversible ladder-like polymers (14) or networks (15). These materials are liquid crystalline, and the large drop in material properties above the isotropisation temperature demonstrates that, also here, the interplay between association and liquid crystallinity is instrumental in the process of polymerization.63 DSC studies on these networks reveal a memory effect, resulting in a consistent decrease of crystallinity as the time the material is in the isotropic state increases.64... 

Nematic melts are also formed by PEIs based on imide diphenol alkylene diester (3). WAXD patterns and optical microscopy indicate a layered structure in the crystalline state melting to a nematic phase, once more with a strong odd-even effect of the isotropisation temperature [16]. 

Kricheldorf has reported the synthesis of lyotropic poly(amide-imide)s and poly(benzoxazole-amide)s. These were prepared by the polycondensation of N,N-bis(trimethylsilyl)-p-phenylenediamine or N,AT -bis(trimethylsilyl)-3,3 -dim-ethylbenzidine with the diacyl chloride of trimellitimide of p-aminobenzoic acid, or the imide formed from p-amino benzoic acid and terephthalic acid. Lyotropic behaviour was observed in cone, sulphuric acid solution [38]. A series of thermotropic poly(imide-amide)s was prepared based on trimellitimides formed from trimellitic anhydride and an a, -bis(4-aminophenoxy) alkane with carbon chain lengths 9-12. Melting points were in the range 250-300 °C. They formed smectic A phases and tended to degrade around the isotropisation temperatures (around 350 °C). Pendant methyl groups or occupied meta- groups tended to prevent mesophase formation [39]. Novel LC poly(imide-amides) have also been synthesised from new diamine spacers derived from linear diaminoalkanes and 4-nitrophthalic anhydride. A smectic and nematic phase were observed when 4,4 -biphenyl dicarboxylic acid was used as co-monomer [40]. 

Figure 4.42. Heat flow rate as a function of time in the transition temperature range of isotropisation of 4,4 -azoxyanisole on MTDSC (calorimeter with modulation control at the heater temperature, sinusoidal modulation) [54].

Geometry and time scale of structural relaxation of poly(n-alkylmethacry-lates) above the glass transition is studied by temperature dependent ID and 2D C NMR spectroscopy. The geometry of the isotropisation of the polymer backbone as deduced from detailed analysis of spectral line shapes is identified as random angular jumps. Analysis of echo decays confirms that at a given temperature this isotropisation, can adequately be described with a single correlation time. ... 

---