Surface Crystallization and Melting

Another point is important The envisaged process can only be realized if the continuously renewed equilibration is kinetically possible. A shift of the interface requires a rearrangement of the chains in the crystallites and this can only be accomplished if chains are perfect, i.e. do not include co-units or branches which cannot enter the crystallites, and in particular, if chains possess sufficient mobility to facilitate a longitudinal transport. For linear polyethylene both conditions are fulfilled, the necessary longitudinal mobility being provided by the action of the a-process . Direct evidence comes from a nuclear magnetic resonance (NMR) experiment. 

A second observation in Fig. 4.35 is also noteworthy. We see non-vanishing values of p (uji,uj2) also away from the peaks and in particular between the peak c and the off-diagonal peaks ac and ca . This is just the range where contributions of the transition zones are expected, when units are passing through during a change from the crystallite into the amorphous phase. One 

Surface crystallization and melting being the exception, the insertion mode is the rule and is indeed mainly responsible for the generally observed secondary crystallization. As it does not require a mobile crystalline phase, it can always occur. 

This chapter would not be complete without a warning. It is natural that a textbook based on a lecture course has a personal touch, expressed in the selection of the topics and the weights attibuted to the various arguments and concepts in the literature. For this chapter this is even more applicable than for the others. Readers who already have some knowledge in the field 

Barham Crystallization and Morphology of Semicrystalline Polymers in R.W. Cahn, P. Haasen, E.J. Kramer, E.L. Thomas (Eds.) Materials Science and Technology Vol.12 Structure and Properties of Polymers VCH Publishers, 1993 E.W. Fischer Investigation of the Crystallization Process of Polymers by Means of Neutron Scattering in L.A. Kleintjens, P.J. Lemstra (Eds.) Integration of Fundamental Polymer Science and Technology Elsevier, 1886 U.W. Gedde Polymer Physics, Chapman Hall, 1995. 

The existence of this process demonstrates that the structure of the amorphous intercrystalline layers in a semicrystalline polymer is different from a polymer melt. The reason can be easily seen All the chain sequences are fixed with their ends in the crystallites and, furthermore, the concentration of entanglements is enhanced. As a consequence, the mean chemical potential of the units is higher than in a melt and varies with the layer thickness. The direction of change is obvious. The numbers of entanglements and points of chain entry into the crystallites are constant. The motional restrictions thus become diminished if the layer thickness increases, which implies a decrease in the chemical potential. Under such conditions each change in temperature leads to a new local equilibrium between crystallites and amorphous regions, via a surface crystallization or melting process. 

The experiment demonstrates that a motional mechanism is active, which produces the exchange. It appears reasonable to assume that a longitudinal 

The dynamic heat capacity of the sample, C (w), expresses the ratio between the amplitudes of the heat flow and the heating rate 

The normally used dynamic specific heat c(tu) as given by the heat capacity per unit mass is in general a complex, frequency-dependent quantity of the 

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TMDSC) and heat wave spectroscopy (HWS) are particularly suited to study a reversible process like surface crystallization and melting. They extract heat flows associated with reversible structure changes and separate them from latent heats of fusion and crystallization. Experiments probe the reaction of a sample onto an imposed oscillating temperature... 

The results above have the following applications (i) estimation of diffusive crystal dissolution distance for given crystal and melt compositions, temperature, pressure, and duration if diffusivities are known and surface concentrations can be estimated and (ii) determination of diffusivity (EBDC) and interface-melt concentrations. Those diffusivities and interface concentrations can be applied to estimate crystal dissolution rates in nature. 

Kobayashi (143) presented the first computer simulations that considered the determination of the crystal radius as part of the analysis but avoided the capillary problem by considering a flat melt-ambient surface, which is consistent with <)>o = 99°. Calculations were performed for a fixed crystal radius, and then the growth rate was adjusted to balance the heat flux into the crystal. Crowley (148) was the first to present numerical calculations of a conduction-dominated heat-transfer model for the simultaneous determination of the temperature fields in crystal and melt and of the shapes of the melt-crystal and melt-ambient surfaces for an idealized system with a melt pool so large that no interactions with the crucible are considered. She used a time-dependent formulation of the thermal-capillary model and computed the shape of an evolving crystal from a short initial configuration. 

In a series of papers, Derby and Brown (144, 149-152) developed a detailed TCM that included the calculation of the temperature field in the melt, crystal, and crucible the location of the melt-crystal and melt-ambient surfaces and the crystal shape. The analysis is based on a finite-ele-ment-Newton method, which has been described in detail (152). The heat-transfer model included conduction in each of the phases and an idealized model for radiation from the crystal, melt, and crucible surfaces without a systematic calculation of view factors and difiuse-gray radiative exchange (153). 

The degree of emulsification in such materials can also be estimated by the measurement of ultrasound velocity in conjunction with attenuation [4]. It is possible to determine factors such as the degree of creaming (or settling ) of a sample, i.e. the movement of solid particles/fat droplets to the surface (or to the base) [5], Such information gives details, for example, of the long-term stability of fruit juices and the stability of emulsions such as mayonnaise. The combination of velocity and attenuation measurements shows promise as a method for the analysis of edible fats and oils [6], and for the determination of the extent of crystallization and melting in dispersed emulsion droplets [7]. 

Suspension melt crystallization crystals and melt same temperature, design on degree of supersaturation, separation of crystals from melt depends on density difference in countercurrent operation. Scraped surface crystallizer. Section 4.6. The suspension methods have slower rates of crystal growth compared with the solid layer processes. 

Differential scanning calorimetry (DSC) is a suitable technique to monitor temperature-depen-dent enthalpic changes in a system related to phase transition. It is well known that PDMS polymer shows a glass transition temperature at about -120°C, and also phase transitions of crystallization and melting. Solid state H-NMR [4] revealed a marked shift in its Tg, to about 0°C, when a PDMS polymer is adsorbed strongly on a silica surface. In this study we investigated the use of DSC to monitor the mobility of the PDMS chains on hydrophilic and silylated silica surfaces. 

Differences between surface and volume stages of the crystallization and melting of glasses were revealed by means of DSA. The results obtained for the systems Pb0-Si02 and Ge-Se-Te are presented in [27,28]. 

Androsch R, Wunderlich B (2001) Reversible Crystallization and Melting at the Lateral Surface of Isotactic Polypropylene Crystals. Macromolecules 34 5950-5960. 

The interface between the lamellar crystals and the non-crystalline, interlamellar region was studied using the technique of the Gibbs dividing surface. In so doing, one is able isolate the effects of the interface alone, irrespective of thickness of the lamellae and, to some degree, of the interlamellar domain. Therefore, the properties attached to the sharp interface can be used in a three-component model with arbitrary composition, which accounts for the interface contribution explicitly, in addition to the crystal and melt bulk contributions. 

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