Secondary crystallites

Re-organization of amorphous chains in the interstitial regions between pre-existing crystallites leads to the formation of secondary crystallites. The new crystallites have smaller a , b , and "c dimensions than the primary crystallites. 

Figure 5 Poly(ethylene)oxide lamella crystals grown within thin layers of melt (the white bar correspon

TTie presence of crystallites is very important for product performance because crystallites serve as physical crosslinking points which reinforce material and improve its viscoelastic properties. It is therefore very important to notice that the mechanism peculiar to PVC allows for melting of crystallites and their formation on cooling. Melting of crystallites improves PVC processability (lowers melt viscosity and improves its flow), and recovery of crystallites on cooling improves performance characteristics of PVC. The gelation depends on the melting enthalpies of primary and secondary crystallites. 

Figure 2.23. Fusion enthalpy of primary and secondary crystallites vs. process temperature. [Data from Li, T. Qi, K., J. Appl. Polym. Sci., 63,13, 1747-1754,1997.]...

Figure 7.29 is the transmission electron microscopy images of the above samples. Figure 7.30 is the distribution of the secondary crystallite size. It can be seen from... 

First set of values refers to crystallization under quiescent conditions. CNTs nucleated iPP crystals which further nucleated secondary crystallites (mother-daughter lamellae). Second set of values show Avrami exponents for crystalhzation during a step shear of 20 s for 5 s. 

The glass temperature, T, you will remember, is the temperature at which the secondary bonds start to melt. Well below the polymer molecules pack tightly together, either in an amorphous tangle, or in poorly organised crystallites with amorphous... 

Nylon fibers are semicrystalline, that is, they consist of crystallites separated by amorphous regions. Hydrogen bonding is an important secondary valence interaction in nylon-6 and nylon-6,6. Individual chains in the microcrystalline regions of nylons are held together by hydrogen bonds. Nylons are resistant to aqueous alkali but deteriorate more readily on exposure to mineral acids. 

The Tg of P-plastomers changes as a function of ethylene content. The Tg decreases with increasing ethylene content, primarily due to an increase in chain flexibility and loss of pendant methyl residues due to incorporation of ethylene units in the backbone. It is well known that PP has a Tg of 0°C, and polyethylene a Tg< —65°C. The addition of ethylene to a propylene polymer would therefore be expected to decrease the Tg, as is observed here. A secondary effect would be the reduction in the level of crystallinity associated with increasing ethylene content, which is expected to reduce the constraints placed upon the amorphous regions in proximity to the crystallites. Thus, an increase in ethylene content will result in a lower T as well as an increase in magnitude and a decrease in breadth of the glass transition. 

Crystallite dimensions play a role in determining the rates of reactions and their control is of fimdamental importance not only for the catalytic activity, but also for the selectivity, since, with low rates of the desired reaction, the relative importance of secondary reactions may be greater. The effects of crystallite dimensions have been demonstrated for 1-butene epoxidation and for phenol hydroxylation, and they are significant for many reactions carried out with hquid phase reactants [17]. 

Describe the mechanism by which crystallites form, starting with nucleation and ending with secondary crystallization. 

The distribution of M, B and C products strongly depended on the crystallites size (Figure 2). With the 0.02 pm crystallites, monobranched isomers M were primary reaction products, while multibranched isomers B and cracking products C were secondary reaction products. M and B isomers are both primary reaction products with the 1-1.5 pm crystallites. Lastly, all the products (M, B and C) appear as apparent primary reaction products with the 10-15 pm crystallites. 

Micro-XRD confirms that secondary phases are generally aggregates of micro-or nano-scale crystallites, or in some cases amorphous or short-range ordered. Arsenic-mineral associations within... 

Figure 13. Secondary crystallization of small hexagonal crystallites of (NH )ZSM-5 on the original single crystal of the same (Reproduced with permission, from ref (26), Elsevier Sci. Publ. Co.).

From these considerations there evolved the concept of "primary valence chains" in cellulose, held together in bundles, or micelles (crystallites) by secondary forces, as propounded by Meyer and Mark (5). This view was then extended to encompass other high polymers as well. It should be noted however, that Freudenberg had already proposed a chain structure for cellulose, based on degradation experiments (6). If the micelles were to... 

Co catalysts, metal crystallite size and support effects, 39 242-246 Ru catalysts, metal crystallite size and support effects, 39 237-242 Thiele modulus effect, 39 275 reaction-transport models, 39 222-223 readsorption probability, 39 264-265 secondary chain growth, hydrogenation, and depolymerization reactions, 39 224—225... 

Site of the. acidic surface oxides. The question whether the acidic surface oxides are bound to the periphery of the carbon layei-s or to the basal planes of the crystallites could be resolved by oxidation of a graphitized carbon black (46). The particles of carbon black are, at first approximation, spherical. The graphite-like crystallites show such preferential orientation that their c axis are aligned in a radial direction (64, 65). A schematic representation of this secondary structure is given in Fig. 1. On recrystallization between 2000 and 3000°, many small... 

Systems such as the concentrated solution of the UHMWPE in paraffin oil (2-8% w/w) contain a three-dimensional molecular network in which the junction points are produced by secondary valence bonds which cause crystalline regions and by physical entanglements of different life times. Entanglements that are trapped between crystallites have, like the crystallites, essentially infinite life times. 

Further cellulosic structural variation is displayed by DP and crystallite size parameters. Aceiobacter and vascular plant primary wall celluloses are low in DP (2,000-6,000), while siphonocladalean and vascular plant secondary wall celluloses are relatively high in DP (> 10,000) (2). During cotton fiber development, the cellulose IV polymorph is produced during primary wall formation, while in secondary walls, cellulose I is observed... 

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