Polymers epitaxial crystallization

Thin polymer films may also be investigated by TEM and high resolution images are obtained for e.g. thin films of liquid crystalline polymers [64]. Usually thin microtome cuts from bulk samples are investigated, but also epitaxial growth of polyoxymethylene on NaCl [152], chain folding of polyethylene crystals [153], epitaxial crystallization of polypropylene on polystyrene [154] or monomolecular polystyrene particles [155] are observed. The resolution is, however, in most cases not comparable to STM. 

Epitaxially grown thin films of vapor-deposited organic compounds and defect structures in these films have been studied by electron diffraction and high-resolution imaging with TEM [23]. Here, epitaxial crystallization of polymers is briefly described. 

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. 

Epitaxial crystallization utilizes some form of lattice (dimensional) and structural (surface topographies) matching of the deposit (usually the polymer) and the substrate crystal. This matching has been demonstrated in a number of ways in the field of polymer crystallization [18] and only the general rules are (briefly) recalled here. 

For polymers that can exist in different crystal structures based on different chain conformations, epitaxial crystallization can induce these various crystal structures (it can impose different chain conformations). The most... 

Epitaxial crystallization of helical polymers may involve three different features of the polymer chain or lattice. These are (a) the interchain distance (as for stretched out polymers), (b) the chain axis repeat distance, and (c) the interstrand distance - the distance between the exterior paths of two successive turns of the helix. The two former periodicities are normal and parallel to the chain axis direction, and are therefore not usually sensitive to the chirality of the helix (unless the substrate topography is asymmetric and favors a given helical hand). However, the interstrand distance is oblique to the helix axis (it is normal to the orientation of the outer chain path) and therefore has different, symmetric orientations relative to the helix axis for left-handed and right-handed helices (Fig. 2). In other words, epitaxies that involve the interstrand distances are discriminative with respect to helix chirality. This discrimination becomes visible if the crystal structure is based on whole layers of isochiral helices. Such a situation does indeed exist for isotactic poly(l-butene), Form I, that will be considered soon. 

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 double-striation morphology has tentatively been attributed to Pt nucleation on the fold edges of on-edge lamellae, similar to Au nucle-ation on the fold edges or interlamellar regions of, e.g., on-edge lamellae in drawn, annealed films of polyoxymethylene [37] and epitaxially crystallized samples of various other polymers [38]. AFM is currently being used to characterize further this feature. 

Wittmann JC, Lotz B (1990) Epitaxial crystallization of polymers on organic and polymeric substrates. Prog Polym Sci 15 909... 

Electron diffraction from these crystals indicated a high degree of crystallinity in both the trimer and polymer, and indexing of the patterns required the use of new unit cells different from the usual structures(Figure 4). X-ray diffrac-tometry was used to provide structural information about the third dimension in these epitaxially crystallized films. It should be noted that the characteristic spacing determined by X-ray diffrac-tometry is the b-axis, only if the unit cell is orthorombic (Table II). 

Finally, it should be emphasized that the tendency of polyimides containing aliphatic spacers to form stable layer structures of various dimensions and degrees of order can be utilized for an even wider variety of studies. A recent example is a study of the epitaxial growth of polyethylene on smectic crystallites of PEI [140]. Most likely smectic crystallites adopt a lamellar form with the large surface covered by loops of the aliphatic spacers. This hypothesis still need detailed studies and confirmation. Anyway, smectic crystalline polymers are interesting substrates for studies of epitaxial crystallizations. Furthermore, layer structures derived from long aliphatic spacers (alkanes or oligoethers) may play... 

Mesoscale crystalline morphology, crystallinity, and molecular orientation in these deposited thin films strongly depend on molecular properties [17,18], chemical nature of the solvent, and processing condition, resulting in very different field-effect mobilities [15,23,36]. Specifically, due to heterogeneous surface-induced (epitaxy) crystal growth as a nature of semicrystalline polymers, fine control of substrate properties and solvent evaporation rate tends to yield favorable molecular orientation of these polymers (i.e., edge-on structure with respect to dielectric substrates) in solution-deposited films [24,66]. 

Wittmann, J., Hodge, A., Lotz, B., 1983. Epitaxial crystallization of polymers onto benzoic acid ... 

D. A. Blackadder, Ten years of polymer single crystals, J. Macromol. Sci. (Rev.) Cl, 297 (1967). J. Willems, Oriented overgrowth (epitaxy) of macromolecular organic compounds, Experientia 23, 409(1967). 

R. Su, K. Wang, Q. Zhang, F. Chen, Q. Fu, N. Hu, E, Chen, Epitaxial crystallization and oriented structure of linear low-density polyethylene/isotactic polypropylene blends obtained via dynamic packing injection molding. Polym. Adv. Technol. 22, 225-231 (2011)... 

Stocker, W. et al. (1998). Epitaxial Crystallization and AFM Investigation of a Frustrated Polymer Structure Isotactic Poly(propylene), p Phase. Macromolecules, Vol.31, Issue 3, pp. 807-814. 

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. 

Cartier, L, Okihara, T, Ikada, Y, Tsuji, H., Puiggal, J., and Lotz, B. (2000) Epitaxial crystallization and crystalline polymorphism of polylactides. Polymer, 41, 8909-8919. 

In addition, to the 3-step growth, formation of other 3-dimentional morphologies is of interest in the polymer nucleation. The epitaxy is a growth of the guest ciystal on the surface of the host ciystal. The guest polymer chains have their chain axis parallel to the substrate surface. The crystalline lamellae in the epitaxial crystallization are built edge-on, i.e., they are normal to the surface of the host crystal. Films, fibers, and single crystals are used as substrates for epitaxial crystallization. Polymers deposition can be achieved in several... 

---