Composites bulk crystallization rates

Since most applications of PLA-based materials are in the sohd state, flie detailed knowledge of the composition, bulk structure, and conformation of these materials is crucial. For example, it is weU known that poly(L-lactide) (PLLA) obtained from LL-lactide is usually molded at 100-120 C in industrial melt processing because of the higher crystallization rate. It has been reported that when it is crystallized within this temperature region a mixture of crystals, a- and P-forms, is formed [2]. Although infrared and Raman spectroscopy is a very promising analytical technique, as will be... 

The BA is relatively well crystallized, which tends to lower hydrolysis when considering a long-term perspective. Dissolution rates are generally lowered if the crystalline fraction is higher and the growth of less resistant phases like meli-lites can be reduced. This is mainly the case for bulk compositions with a higher Si/Ca ratio. 

It is this large and continuous variability in bulk composition coupled with the fact that crystal structures may be different for the same anhydrous or hydrous bulk composition which makes zeolite identification so difficult (see Breck, 1970, for example and Deer, et al., Vol. 4, 1962). The factors determining which species of zeolite will crystallize are undoubtedly complex, involving such variables as the chemical activity of dissolved ionic species, crystal growth rate and ease of nucleation however, certain patterns of mineral paragensis can be discerned through a survey of the literature. 

The most important factor in zeolite synthesis in the laboratory, or factory, is the rate of crystallization. Composition and concentration of the liquid solution acting on the solids is important to the process as is the absolute necessity of maximum disorder of the Si-O-Al bonds in the initial solids reacted (Zhdanov, 1970). It is thus evident that not only bulk chemical (equilibrium) factors are important in the initial crystallization of zeolites but also the. relative free energies of the reactants. It is apparent that zeolite equilibria are essentially aqueous i.e., that silicate equilibrium or approach to it is attained through reaction with solutions, and thus the solubilities of the solids present are of primary importance. If materials are slow to enter into solution they are essentially bypassed in the rapid crystallization sequence (Schwochow and Heinze, 1970 Aiello, et al , 1970). In most studies the zeolites precipitated from solution appear to respond to the laws concerning chemical activity of solutions (Zhdanov, 1970). 

Connection between Transport Processes and Solid Microstructure. The formation of cellular and dendritic patterns in the microstructure of binary crystals grown by directional solidification results from interactions of the temperature and concentration fields with the shape of the melt-crystal interface. Tiller et al. (21) first described the mechanism for constitutional supercooling or the microscale instability of a planar melt-crystal interface toward the formation of cells and dendrites. They described a simple system with a constant-temperature gradient G (in Kelvins per centimeter) and a melt that moves only to account for the solidification rate Vg. If the bulk composition of solute is c0 and the solidification is at steady state, then the exponential diffusion layer forms in front of the interface. The elevated concentration (assuming k < 1) in this layer corresponds to the melt that solidifies at a lower temperature, which is given by the phase diagram (Figure 5) as... 

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