Epitaxial crystallization materials

Epitaxial crystallization of polymers has been investigated for a wide variety of substrates minerals (alkali halides, talc, mica, and so on), low molecular weight organic materials (condensed and linear aromatics, benzoic acid and many of its substituted variants and their salts or hemiacids, other organic molecules of different types), and other crystalline polymers. 

The second and third illustrations of epitaxial crystallization deal with syndiotactic polypropylene (sPP). Syndiotactic polymers are by design susceptible to forming either right-handed or left-handed helices, and are therefore suitable materials in the present context of helical hand selection. 

The improved sensitivity of 15N enriched nylon-12 samples allows the clear identification of a shoulder appearing at 122.8 ppm (see Fig. 6C ). Although the exact identity is unclear, this peak is assigned on the basis of spin-lattice relaxation data to material epitaxially crystallized on the surface of 7 crystallites or to an ordered interphase region. The spectra of quenched samples (Fig. 6B, 6B ) are very similar in shape. However, the broader base of natural abundance sample indicates that the residual monomer (lauryllactam) may be acting as a plasticizer and increasing the size of conformational mobility of the amorphous region. The residual monomer is probably responsible for the difference in the relative intensities between natural abundance and 15N enriched samples (Fig. 6A, 6A ) [50]. 

Values for both the hole and electron mobilities and carrier densities in various SiC polytypes are listed. Ionized and neutral impurity, acoustic phonon, piezoelectric and polar optical phonon scattering mechanisms are all found in SiC. In general, mobilities have increased and carrier concentrations decreased with time, reflecting the improvement in crystal quality whether bulk or epitaxially-grown material is considered. 

The latter form can be prepared at a high draw ratio and a high drawing temperature [28]. The 7-form is formed by epitaxial crystallization [29]. It has been observed that a blend with equivalent poly(L-lactide) PLLA and poly(D-lactide) PDLA contents gives stereo-complexation (racemic crystallite) of both polymers. This stereocomplex has higher mechanical properties than those of both PLAs, and a higher melting temperature of 230°C. The literature reports different density data [4] for PLA, with most values for the crystalline polymer around 1.29 compared with 1.25 for the amorphous material. 

The ability to control the stereochemical architecture enables a precise control over the size/shape of the PLA crystals, the degree of crystallinity, the rate of crystallization, and the thermomechanical properties of the material. PLA homopolymers crystallizes in three forms (a, p, and y), depending on the preparation conditions and the ratio of L and D enantiomers. The a-form (and related disorder a -form) is the most stable form with two antiparallel chains upon a twofold helix conformation distorted periodically from the regular s, while the P-form is a left-handed threefold helix and the y-form is obtained by epitaxial crystallization, containing two antiparallel s helices upon a threefold helix [34-36]. Interestingly, PLA is a clear, colorless thermoplastic when quenched from the melt and crystallizes slowly on cooling [16]. 

Epitaxial crystallizations are also observed in eutectic-forming systems of polymer and low molecular weight materials, e.g. poly(e-caprolactone) and trioxane, and also poly(propylene) and tetrachlorobenzene, in that the crystals, formed initially, act as a substrate for further growth, producing a highly ordered deposition of polymer crystals. 

Epitaxial Crystallization An epitaxial crystallization can occur when one polymer component crystallizes on the surface of the film of the uniaxially oriented other polymer component. The polymer chains grow into crystals with their chain axes parallel to the chain direction of the polymer substrate. We observe similar phenomena for nanocomposite material, in which... 

Epitaxial crystallization of materials has been known for a long time. The oriented crystallization of a given crystal (the deposit) on the surface of another crystal (the substrate) has been investigated in quite some detail by Royer in the late 1920s [1], His work dealt mostly with minerals, for which X-ray investigations had established either the unit-cell geometry and dimensions or even the detailed crystal structure. On the basis of this information and by analyzing the relative orientations of the substrate and deposit, Royer could estab-... 

Epitaxial crystallization is stUl the topic of numerous investigations in the field of materials science, especially in relation with the formation of nanostructured layers in devices. Epitaxial crystallization has comparatively... 

Epitaxial crystallization is also an ingredient in the build-up of composite materials. Two examples may be given at this stage, although many more probably exist. 

The rules governing epitaxial crystallization had been set up early on for low molecular weight materials. By and large, the same basic rules apply for polymers. However, the structural and conformational diversity of crystalline polymers, and their lamellar structure introduce new and subtle variables in the process. 

Thierry, A. Epitaxial crystallization (of linear polymers). In Polymeric Materials Encyclopedia, Vol. 3, Salamone, J.C., ed. CRC Press, Boca Raton, FL, 1996, pp. 2167-2179. 

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