Nucleating agents epitaxy

Keywords nudeation, primary nudeation, nudeation rate, heterogeneous nudeation, supercooling, homogeneous nudeation, aimeal-ing, self-nudeation, nucleating agents, epitaxy, nudeation in blends, nudeation in copolymers, spherulite, lamellae, p-crystal form, secondary nudeation, nudeation rate. 

As shown by Fig. 27c, NaCl appears to serve as a nucleating agent as well as an epitaxial substrate. Double striations are nucleated at the surface, growing out perpendicular, as well as being oriented on the surface parallel to the faces. In this case there even seems to be some climbing of the molecules up and over the NaCl surface. 

Lotz, B. Wittman, J.C. Epitaxy of helical polyolefins polymer blends and polymer-nucleating agent systems. Makromol. Chem. 1984,185, 2043. 

Substrates which are suitable as host materials for epitaxial growth of synthetic polymers, include inorganic materials (mainly alkali hahdes), organic materials (aromatic hydrocarbons, aromatic carboxylic acids, their salts, hydroquinone, and pigments). Some of these are used as the common nucleating agents. 

The role of epitaxy of nucleating agent in nucleation mechanism of polymers was studied to formulate the nucleation rate, I, as a function of the concentration of nucleation agent, Cna> the lateral size of a nucleation agent crystal, Th following relationship was considered ... 

Epitaxy of nucleating agent controls the nucleation according to the results of this research. ... 

Because of epitaxy matching, carbon nanotubes are good nucleating agent for linear polyethylene. The presence of single-walled carbon nanotube, SWCNT, accelerates the crystallization kinetics of polyethylene not only in the qiriescent conditions but also at the relatively low shear rates. For the low shear (<50/s), the shish-kebab structure that is formed due to the chain orientation is enhanced by the presence of SWCNTs. ... 

Although a number of nucleating agents are used for simple polymers, such as polyethylene (PE), and for iPP and sPP, the real nature of the interactions has been worked out only in the last ten years or so. These interactions are essentially of epitaxial character, i.e. do not involve chemical reactions. For polymers with a linear envelope (such as PE), the epitaxial relationship involves, as a rule, the chains lying flat on the substrate (with the helix axis parallel to the substrate). The major dimensional match involves the interchain distance in the contact plane the latter may differ for different substrates. As a consequence, the polymer lamellae stand edge on on the substrate [1]. 

Keywords epitaxy, isotactic polypropylene, syndiotactic polypropylene, crystal structure, crystal polymorphism, helical hand, nucleating agents, AFM, a-phase, 3-phase, lamellae. 

Wittmann JC, Lotz B. Epitaxial crystallization of polyethylene on organic substrates, A reappraisal of the mode of action of selected nucleating agents. J Polym Sci B 1981 19 1837-1851. 

Epitaxial crystallization of PE may be considered as a rather academic concern. It is indeed so for most linear PEs produced for example by Ziegler-Natta catalysis, which crystallize fast, as they have high nucleation and growth rates. However, modem PEs, such as the linear low density ones (LLDPE), have much reduced growth rates. It is therefore of technological interest to develop nucleating agents that are efficient for this class of polymers. 

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