Amorphous crystallization kinetics

Crystallization kinetics have been studied by differential thermal analysis (92,94,95). The heat of fusion of the crystalline phase is approximately 96 kj/kg (23 kcal/mol), and the activation energy for crystallization is 104 kj/mol (25 kcal/mol). The extent of crystallinity may be calculated from the density of amorphous polymer (d = 1.23), and the crystalline density (d = 1.35). Using this method, polymer prepared at —40° C melts at 73°C and is 38% crystalline. Polymer made at +40° C melts at 45°C and is about 12% crystalline. 

Since interactions at the molecular level between polymer components in the blends occur only in the amorphous phase, it is reasonable to assume that these effects are due to kinetic factors and, in particular, to the influence of a polymer component on the nucleation or crystallization kinetics of the other one. 

Many polymers solidify into a semi-crystalline morphology. Their crystallization process, driven by thermodynamic forces, is hindered due to entanglements of the macromolecules, and the crystallization kinetics is restricted by the polymer s molecular diffusion. Therefore, crystalline lamellae and amorphous regions coexist in semi-crystalline polymers. The formation of crystals during the crystallization process results in a decrease of molecular mobility, since the crystalline regions act as crosslinks which connect the molecules into a sample spanning network. 

The major results described could be partially anticipated from those previously reported for linear polyethylene (17) as well as those for cis polyisoprene. (] ) For the latter polymer, by taking advantage of its crystallization kinetic characteristics, it was possible to compare the relaxation parameters of the completely amorphous and partially crystalline polymer (31% crystallinity) at the same temperature, 0°C. This is a unique situation and allows for some unequivocal comparisons. It was definitively observed that for all the carbons of cis polyisoprene the T] s did not change with crystallization. 

Blends of polybutylene terephthalate and polyethylene terephthalate are believed to be compatible in the amorphous phase as judged from (a) the existence of a single glass-transition temperature intermediate between those of the pure components and (b) the observation that the crystallization kinetics of the blend may be understood on the basis of this intermediate Tg. While trans esterification occurs in the melt, it is possible to make Tg and crystallization kinetics measurements under conditions where it is not significant. When the melted blend crystallizes, crystals of each of the components form, as judged from x-ray diffraction, IR absorption, and DSC. There is no evidence for cocrystallization. There is a slight mutual melting point depression. 

Surana, R. Suryanarayanan, R. Quantitation of crystallinity in substantially amorphous pharmaceuticals and study of crystallization kinetics by X-ray powder diffractometry. Powder Diffr. 2000,15 (1), 2-6. 

See liquid, Newtonian glass amorphous solid liquid crystals kinetic theory. 

The crystallization kinetics of native P(3-hydroxyalkanoic acid) (PHA) granules isolated from the strains of A. eutrophus and P. oleovorans have been studied by measurements of the glass-transition temperature with a differential scanning calorimeter [36]. The comparison is made between PHA in vivo and the isolated polymer. It is demonstrated that the native granules do not contain a plasticizer and the amorphous state of in vivo PHA can be explained by straightforward crystallization kinetics. 

The blend composition, the crystallization conditions, the degree of miscibility and the mobility of both blend components, the nucleation activity of the amorphous component are important factors with respect to the crystallization kinetics (3.3.4). 

Table 3.22. Global crystallization kinetics of the crystaUizable matrix in some crystaUine/amorphous blend systems...

It should be noted that the crystallization kinetics is related to the size of the dispersed HOPE droplets and the nucleation density. An increase in the amount amorphous PS caused the HOPE phase to be dispersed into finer droplets that, as a result, exhibited a lower degree of crystallinity, X, when isothermally crystallized. Furthermore, a higher degree of undercooling was needed to reach the same X in blends where the HOPE... 

PP-g-SBH copolymers cocrystallize completely with bulk iPP, which increases PP crystaUinity degree part of the SBH component (SBH grafts of PP-g-SBH copolymers and bulk SBH) enters the mutual amorphous phase of the blends, leading to a decrease of the intensity of the SBH peak. That means each part of the compatibilizer is miscible with the corresponding bulk phase of the blend. The compatibilized iPP/LCP blends display improved crystallization kinetics, enhanced degree of crystallinity, and improved interphase adhesion (37,38). Consequently, an improvement of the mechanical characteristics should be expected for these blends. In fact, the investigation of the Vickers microhardness of uncompatibilized and... 

Recently, some other non- conventional ways have been used to prepare zirconia-mullite ceramics [12-17]. In our previous work, nano-zirconia/mullite composite ceramics were prepared by in-situ controlled crystallization from the Si-Al-Zr-0 amorphous bulk, the crystallization kinetics and the general view on the performance were discussed for the first time. In this article, the detail crystallization behavior, structure changes and their effect on mechanical properties for this new nano composite ceramics with high performance have been evaluated. 

XRD studies show that synthesized composites do not contain any crystal phase, just an amorphous phase. Optical absorption measurements prove that synthesized nanocomposites are containing Ti02 and Ti phases. For comparative analysis the pure Ti containing thin film was deposited onto the cold substrate (77 K) and onto the substrate at room temperature. The same result was obtained XRD analysis shows that the synthesized films only contain the amorphous phase. Kinetics of the electrical resistance increase with the air exposure of Ti/PPX nanocomposites (after synthesis under vacuum) is similar to that of the Al/PPX ones. For a metal content below the percolation threshold the metal particles became insulator within several seconds, whereas for the samples beyond the threshold the observed resistance increase is per cents within several hours. DTA analysis revealed that the heating of amorphous Ti02 nanoparticles up to a temperature of 480°C leads to a phase transformation to anatase, whereas heating up to 580°C results in the anatase transformation to the mtile structure. 

Crystallization. We measured the density of initially amorphous PET films following exposures to methylene chloride at unit activity, thereby obtaining crystallization kinetics. The density gradient technique was used (18) with carbon tetrachloride/n-heptane mixtures as the immersion media. To avoid spurious readings, efforts were made to remove residual solvent from the crystallized samples and to prevent the entrapment of air in surface cavities during the density... 

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