Phase behaviour transition temperature

The phase behaviour of PIFs 177 and 178 resembles that of the corresponding PDAFs 89 and 90, but with higher phase-transition temperatures. Thus, the octyl polymer 177 shows two nematic phases with transition temperatures at 250 and 290 °C (reverse transitions at 270 and 140 °C), while the ethylhexyl polymer 178 has only a single nematic phase with a transition at 290 °C (220 °C for the reverse transition) [279]. 

Crystalline and amorphous phases glass transition temperatures, crystalline melting points levels of crystallinity viscoelastic behaviour... 

As mentioned above, PLA is a thermoplastic polymer with shape memory behaviour. This polymer can be used as co-monomer with the aim of improving its properties. For instance, copolymers from lactic acid with gly-coUc add (i.e., poly(lactic-co-glycolic add) (PLGA)) (Meng et al, 2009) or -caprolactone (to form poly(I lactide-co-e-caprolactone) (PCLA))(Lu dfl/.,2008) present better shape memory behaviour than the PLA homopolymer. Other copolymers like poly(ethylene oxide) (PEO) with poly(ethylene terephthalate) (PET) have been studied (Luo et /., 2000). In this copolymer PEO is the switching phase with transition temperature between 40°C and 50°C and PET is the fixity phase. 

The basic features of folding can be understood in tenns of two fundamental equilibrium temperatures that detennine tire phases of tire system [7]. At sufficiently high temperatures (JcT greater tlian all tire attractive interactions) tire shape of tire polypeptide chain can be described as a random coil and hence its behaviour is tire same as a self-avoiding walk. As tire temperature is lowered one expects a transition at7 = Tq to a compact phase. This transition is very much in tire spirit of tire collapse transition familiar in tire theory of homopolymers [10]. The number of compact... 

The presence of a second viscoelastic phase, the mesophase, obviously affects the behaviour of the composite, which exhibits a glass-transition temperature, different than that of the matrix material. 

The question arises as to how useful atomistic models may be in predicting the phase behaviour of real liquid crystal molecules. There is some evidence that atomistic models may be quite promising in this respect. For instance, in constant pressure simulations of CCH5 [25, 26] stable nematic and isotropic phases are seen at the right temperatures, even though the simulations of up to 700 ps are too short to observe spontaneous formation of the nematic phase from the isotropic liquid. However, at the present time one must conclude that atomistic models can only be expected to provide qualitative data about individual systems rather than quantitative predictions of phase transition temperatures. Such predictions must await simulations on larger systems, where the system size dependency has been eliminated, and where constant... 

Contrary to the phase separation curve, the sol/gel transition is very sensitive to the temperature more cations are required to get a gel phase when the temperature increases and thus the extension of the gel phase decreases [8]. The sol/gel transition as determined above is well reproducible but overestimates the real amount of cation at the transition. Gelation is a transition from liquid to solid during which the polymeric systems suffers dramatic modifications on their macroscopic viscoelastic behavior. The whole phenomenon can be thus followed by the evolution of the mechanical properties through dynamic experiments. The behaviour of the complex shear modulus G (o)) reflects the distribution of the relaxation time of the growing clusters. At the gel point the broad distribution of... 

Fig.9 Schematic of phase behaviour for dPS-fc-PnPMA with two different molecular weights (M2 >Mi). Tg and r

Besides these thermotropic phase transitions, a variety of pressure-induced phase transformations can be observed," and it has been demonstrated that temperature and pressure have non-congruent effects on the structural and phase behaviour of these systems. 

The fact that the order parameter vanishes above does not mean that Nature does not have an inkling of things to come well below (or above) T. Such indicators are indeed found in many instances in terms of the behaviour of certain vibrational modes. As early as 1940, Raman and Nedungadi discovered that the a-) transition of quartz was accompanied by a decrease in the frequency of a totally symmetric optic mode as the temperature approached the phase transition temperature from below. Historically, this is the first observation of a soft mode. Operationally, a soft mode is a collective excitation whose frequency decreases anomalously as the transition point is reached. In Fig. 4.4, we show the temperature dependence of the soft-mode frequency. While in a second-order transition the soft-mode frequency goes to zero at T, in a first-order transition the change of phase occurs before the mode frequency is able to go to zero. 

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