Random lamellar structure

Very high molar mass polyethylene (M>2,000,000 g mol-1) crystallises without the, formation of a clear superstructure, sometimes referred to as the random lamellar structure [116]. The great many chain entanglements present in high molar mass polymers obstruct crystallisation and the crystals become small and their orientation less correlated with surrounding crystal lamellae. 

Co-precipitation is also applied in the synthesis of hydrotalcite-like precursors (HTlcs) for catalysts e.g. based on Cu-Zn-Al, or Co-Al catalysts. Synthetic HTlcs, with a general formula (OHjg] [A ] nHgO, are hydrated hydroxycarbonates of a random lamellar structure. These materials can be visualized as brucite-type octahedral layers, in which cations partially substitute for cations (x = The cations are located in the center of... 

Very high molar mass samples, typically with molar mass of 10 g mol or greater, form so-called random lamellar structures. The entanglement effect is so extensive that the crystalline mass becomes very low and regular lamellar stacking is absent in these samples. 

The low molar mass samples display axialitic (sheaf-like) morphology. A relatively high value of n is expected on the basis of the data presented in Table 8.3. The reduction in n for the fractions of intermediate molar mass is consonant with the spherulitic morphology observed in these samples. The crystallization of high molar mass polymers is strongly influenced by chain entanglements and the slow and incomplete crystallization leads to small, uncorrelated crystals, i.e, to a so-called random lamellar structure. A low value of n is expected for such a low-dimensional growth. 

Krigbaum has formulated a theory predicting the dimensions of the domains in the case of a lamellar structure. For AB copolymers53, each subchain behaves as a random flight with reflecting barriers. For BAB copolymers54, the possibility... 

The study of the raesophases by X-ray diffraction, electron microscopy, infrared spectroscopy and circular dichroism20-2S has shown that the structure is always lamellar and can be described as follows the lamellar structure consists of plane, parallel, and equidistant sheets of thickness d each sheet results from the superposition of two layers one of thickness dA formed by the polyvinyl chains in a more or less random coil conformation, the other with a thickness dB formed by the polypeptide chains in an a helix conformation, oriented perpendicular to the plane of the layers, arranged in a bidimensional hexagonal array, and generally folded. 

It is now generally accepted that the morphology of a polymer depends on the contributions of three different macro-conformations (a) the random coil or irregularly folded molecule as found in the glassy state, (b) the folded chain, as found in lamellar structures and (c) the extended chain. The fringed micelle (d) may be seen as mixture of (a), (b) and (c) (see Fig. 2.12) with paracrystallinity as an extreme. 

Kawai et al. have studied films of SI copolymers of different composition obtained by evaporation of about 5% toluene solution. Electron micrographs of sections perpendicular to the film surface have revealed four types of morphotogy heres of polyisoprene randomly distributed in a polystyrene matrix for a polystyrene content of 73 wt.-%, a rather disordered lamellar structure for polystyrene content of 49% and 43%, and heres of polystyrene randomly distributed in a polyisoprene matrix for polystyrene content of 18% The authors have also studied the effect of the nature of the solvent using one good solvent for polystyrene (methylethyl ketone)... 

Friberg et al. (73) have proposed a random structure of microemulsions with varying curvatures. Taupin and co-workers (74) have considered the presence of hard oil and water droplets with a relatively sharp transition between these, while Shinoda (75.761 bas proposed a lamellar structure with alternating water, amphiphilic... 

Figure 5.15 Model of structural evolution process in the isothermal crystallization of polyethylene chains. The random chain segments change to the locally regularized trans-form. The stacked lamellar structure is formed with the long period 800 A. The structure changes further to the stacked lamellar structure of 400 A period by inserting the new lamella into the original lamellae. It is noted that the radius of gyration is kept unchanged during this process. (From Reference 46 with permission from the Society of Polymer Science, Japan.)...

This solid exhibits an extended lamellar structure with randomly distributed aromatic and ahphatic regions, as well as a high amount of hydroxyl/carboxyl functional groups embedded in its layers. Hence, GO is endowed with sweUing, intercalation and ion exchange properties [25—30]. This layered sohd decomposes at relatively low temperature (<200°C), releasing CO2 and H2O. 

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