Thermal crystallization transition

Thermal transitions can be studied by DSC. The crystallization transition is usually sharp with a good baseline. The melting transition is more complex and often not a single transition (Fig. 3.19)48 as it depends on the thermal history of the sample and the structural changes that can take place upon heating. In warming, solid-state transitions can take place in the unit cell, the lamellae can thicken, and secondary crystallization can also take place. The heats of crystallization and... 

Lin, S. and Koenig, J., The transitions and melting behaviour of thermally crystallized poly(ethylene terephthalate) and their correlations with FTIR and density measurements, J. Polym. Sci., Polym. Symp. Ed., 71, 121-135 (1984). 

At a given (low) temperature and pressure a crystalline phase of some substance is thermodynamically stable vis a vis the corresponding amorphous solid. Furthermore, because of its inherent metastability, the properties of the amorphous solid depend, to some extent, on the method by which it is prepared. Just as in the cases of other substances, H20(as) is prepared by deposition of vapor on a cold substrate. In general, the temperature of the substrate must be far below the ordinary freezing point and below any possible amorphous crystal transition point. In addition, conditions for deposition must be such that the heat of condensation is removed rapidly enough that local crystallization of the deposited material is prevented. Under practical conditions this means that, since the thermal conductivity of an amorphous solid is small at low temperature, the rate of deposition must be small. 

Some polymers undego other thermal transitions in addition to Ts and Tm. These include crystal-crystal transitions (i.e., transition from one crystalline form to another and crystalline-liquid crystal transitions. 

Small-angle X-ray diffraction was used to identify the time-averaged location of amiodarone in a synthetic lipid bilayer. The drug was located about 6 A from the center of the lipid bilayer (Figure 4.13) [125, 126]. A dielectric constant of k = 2, which is similar to that of the bilayer hydrocarbon region, was used to calculate the minimum energy conformation of amiodarone bound to the membrane. The studies were performed below the thermal phase transition and at relatively low hydration of lipid. The calculated conformation differed from that of the crystal structure of amiodarone. Even though the specific steric effects of the lipid acyl chains on the confor-... 

Some complexes show a strong interdependence between crystal structure and spin-transition features. In the series of compounds [Fe(Rtz)6](BF4)2 (Rtz = 1-alkyltetrazole) the spin crossover behavior varies with the substituent R and is strongly influenced by cooperative effects. For example, the propyl derivative shows a quantitative spin transition, which is accompanied by a first-order crystallographic phase transition in the methyl and ethyl derivatives the Fe11 complexes occupy two nonequivalent lattice sites, only one of which shows a thermal spin transition.29... 

The triplet dimer diradical DR2(Ti) finally will relax into thermal equilibrium (kT) with its singlet ground state DR2(So). As we have seen from the ESR spectra (see Fig, 10) the energy separation between the singlet and triplet diradical states is very low and thermally activated transitions occur even at low temperatures. Furthermore the ESR spectra have revealed an admixture of about 10% carbene character with the diradical intermediates. This carbene character may be important in determining the probability x of the side reactions (see Eq. (19)) for the DR -+ AC chain termination reaction. It surely is not, however, the only essential factor, otherwise there should be no difference in the optical and thermal termination reaction steps. Up to now a direct observation of the metastable triplet state Ti(M) has been possible only in two specific crystals where the polymerization reactions are very weak. 

Thermal analysis data (8) have been Interpreted in terms of a crystal transition at 785 C. Details of these studies are not available and it is not clear how reliable this interpretation is. We note, however, that the reported temperature (785 C) is essentially the same as the eutectic temperature of KBOg with KgB O (9, 1 ). We tentatively adopt a single Cp curve without a transition. 

Michaud has observed a B to a crystal transition, on heating anhydrous barium dihydroxide, at 250°C (7) and at 246 C (8) by both thermal and dilatometric techniques. In the absence of water vapor, the high temperature a form is maintained on cooling to room temperature (7 ). This "metastable" state has been confirmed by others (9, ) This present JANAF Table considers only the a-form. A transition temperature of 521 + 2 K (248 2 C) is adopted. 

Polymerisation liquid induced and pressure/shear forces induced a-crystallinity effects proved to be irreversible (DSC) or partly irreversible (XRD) after heating through their a/B-crystal transition. Additional DSC experiments were performed to investigate this aspect for the thermally induced a-crystallinity. 

A detailed thermal and X-ray analysis exists for a series of anhydrous Na soaps From these measurements it can be derived that the soaps with alkyl-chains of 8, 16 and 18 carbon atoms undergo a crystal to condis crystal transition at about 390 K... 

Fig. 5A Differential thermal analysis curve of a PPX sample polymaized from the cjclic dimer at 296 K and annealed below the transition to the isotropic state, at 625 K, fm- 12 h. The shoulder at about 475 K is an annealing peak (fusion of imperfect crystals). Transition ot-P, occurs at 500 K (Tj ). Transition P,-P2, at 570 K (Tdj). The endotherm for Tj (7(X) K) is not shown (see Ref.

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