Nucleation kinetics, microscopy

Scanning probe microscopies are now able to study in situ the growth of metal clusters. These studies are performed sequentially after deposition. On metal/metal systems it has been possible to follow the nucleation kinetics and to derive the elementary energies like adsorption and diffusion energies (see the excellent review by Brune [68]). On oxide surfaces only recently such studies have been undertaken. STM can be only used on conducting samples, however it is possible to use as a support an ultrathin film of oxide grown on a metal. By this way it has been possible to study the nucleation of several... 

Some limitations of optical microscopy were apparent in applying [247—249] the technique to supplement kinetic investigations of the low temperature decomposition of ammonium perchlorate (AP), a particularly extensively studied solid phase rate process [59]. The porous residue is opaque. Scanning electron microscopy showed that decomposition was initiated at active sites scattered across surfaces and reaction resulted in the formation of square holes on m-faces and rhombic holes on c-faces. These sites of nucleation were identified [211] as points of intersection of line dislocations with an external boundary face and the kinetic implications of the observed mode of nucleation and growth have been discussed [211]. 

A rate enhancement effect due to secondary nucleation has been identified in the solution-mediated transformation of the 7-phase of (i)-glutamic acid to its / -phase [82]. In this study, the kinetics of the polymorphic transition were studied using optical microscopy combined with Fourier transform infrared, Raman, and ultraviolet absorption spectroscopies. The crystallization process of n-hexatriacontane was investigated using micro-IR methodology, where it was confirmed that single... 

The kinetics of transition from the liquid crystal to the fully ordered crystal of flexible, linear macromolecules was studied by Warner and Jaffe 38) on copolyesters of hydroxybenzoic acid, naphthalene dicarboxylic acid, isophthalic acid, and hydro-quinone. The analytical techniques were optical microscopy, calorimetry and wide angle X-ray diffraction. Despite the fact that massive structural rearrangements did not occur on crystallization, nucleation and growth followed the Avrami expression with an exponent of 2. The authors suggested a rod-like crystal growth. 

The isothermal crystallization of PEO in a PEO-PMMA diblock was monitored by observation of the increase in radius of spherulites or the enthalpy of fusion as a function of time by Richardson etal. (1995). Comparative experiments were also made on blends of the two homopolymers. The block copolymer was observed to have a lower melting point and lower spherulitic growth rate compared to the blend with the same composition. The growth rates extracted from optical microscopy were interpreted in terms of the kinetic nucleation theory of Hoffman and co-workers (Hoffman and Miller 1989 Lauritzen and Hoffman 1960) (Section 5.3.3). The fold surface free energy obtained using this model (ere 2.5-3 kJ mol"1) was close to that obtained for PEO/PPO copolymers by Booth and co-workers (Ashman and Booth 1975 Ashman et al. 1975) using the Flory-Vrij theory. 

Microscopy is the most appropriate technique for studying the kinetics of nucleation. The shapes, sizes, textures and distributions of nuclei can be determined and the kinetics of nucleation can be distinguished fi om the kinetics of growth. Details of the intranuclear material, which is often porous with small crystallites separated by fine channels that provide routes for escape of product gas, may be discemable. Changes in particle-size, topochemical relationships and the possibility of melting of the solid reactant can also be recognized. 

The reaction of Ni(OH)2 resembled [39] that of Fe(OH)2 in that the contracting area equation fitted the data and the value of was 95 kJ mol. The rate was appreciably decreased by water vapour. The textural changes that accompany water removal have been studied [41] by electron microscopy which identified rapid initial nucleation at crystallite edges to form a continuous interface. The dehydration is topotactic to yield particles of product which are pseudomorphs of the reactant. These textural changes are consistent with the earlier conclusions based on kinetic evidence. 

Support for the nucleation and growth model was provided by electron microscopy which showed that nucleation was not confined to surfaces, but occurred at lines of internal dislocation. Overlap, following growth, resulted in the formation of approximately cylindrical particles of product. The presence of gaseous hydrogen exerted [51] only a small influence on the kinetics of decomposition. 

For samples that have undergone small extents of reaction (low a), nuclei may be identified on crystal faces by the appearance of patches of characteristic texture (often with symmetrical shapes), each of which increases in size as a increases. This is a feature of nucleation and growth processes, and microscopy has enabled the kinetic rate laws for nucleation to be deduced (29). 

Crystalhzation studies in blends of iPP/POE reveal that the crystallization process of iPP is affected by the addition of POE and vice versa. It has been demonstrated how the POE promotes the nucleation and crystal growth processes of iPP, this effect being more appreciable at low POE concentration (< 10 wt% POE). Analysis of the crystallization kinetics of the iPP crystals isothermally crystallized at different temperatures in blends of iPP/POE is supported by the morphological observations (lamellae, dendritic, and eventually spherulitic texmres) through optical microscopy. 

---