Crystalline Formation

According to Hosemann-Bonart s model8), an oriented polymeric material consists of plate-like more or less curved folded lamellae extended mostly in the direction normal to that of the sample orientation so that the chain orientation in these crystalline formations coincides with the stretching direction. These lamellae are connected with each other by some amount of tie chains, but most chains emerge from the crystal bend and return to the same crystal-forming folds. If this model adequately describes the structure of oriented systems, the mechanical properties in the longitudinal direction are expected to be mainly determined by the number and properties of tie chains in the amorphous regions that are the weak spots of the oriented system (as compared to the crystallite)9). 

Use of systemic corticosteroids is discouraged in patients with OA. However, in a subset of patients with an inflammatory component or knee effusion involving one or two joints, intraarticular corticosteroids can be useful as monotherapy or as an adjunct to analgesics. The affected joint can be aspirated and subsequently injected with a corticosteroid. The aspirate should be examined for the presence of crystalline formation and infection. A single joint should not be injected more than three to five times per year. 

Table I demonstrates that most liquid crystalline polymers lacking a spacer are formed from a flexible polyacrylate backbone. In contrast, the methyl substituent in polymethacrylate backbones both reduce main chain mobility and imposes additional steric barriers to mesophase formation. Therefore, successful liquid crystalline formation of polymethacrylates has been achieved only...

Specifically, the improved solidification (cementation) technology involves the use of (a) a special dry powder admixture for the generation of a nonsoluble crystalline formation deep within the pores and capillary tracts of the concrete—a crystalline structure that permanently seals the concrete against the penetration or movement of water and other hazardous liquids from any direction (b) special nonmetal reinforced bars for enhancing the concrete block s tensile and compressive strengths and (c) a unique chemical crystallization treatment for the waterproofing and protection of the concrete block s surface. 

Dobutamine hydrochloride generally crystallizes in a random manner usually from an oil (1). This results in a nondescript crystalline formation. In only few cases does the drug exhibit any crystalline habit of interest. Upon careful and patient crystallization small thin plates and/ or small needles are formed. 

To facilitate and accelerate folding and crystallization of polymer chains, internal plasticizers are often added to PET to serve as crystallation promoters. Such additives are usually based on poly(ether ester)s. These plasticizers are liquids that are typically added at levels of 2-4 wt%. They reduce cycle time in injection moulding operations by increasing the rate of crystalline formation. They also plasticize the resin and act as processing aids by virtue of their lubricating action in the melt. On a molecular level, these plasticizers reduce the intermolecular... 

Noninteger values for n are not uncommon. As noted before, depending on the particular conditions several crystalline formations are possible and found for the same polymer. Sperling has collected a number of Avrami values for some common values given in the literature. The range of values for PE is 2.6-4.0 for poly(decamethylene terephthalate), 2.7. 0 for PP, 2.8-4.1 for poly(ethylene oxide), 2.0-4.0 and for iPP, 2.0-4.0. 

The third type of cross-linking involves formation of crystalline portions within an amorphous grouping (Figure 2.21). For vinyl polymers, the temperature that allows segmental chain mobility to occur is well below room temperature, but the temperature to disrupt crystalline formations is typically well above room temperature. Within mixtures of crystalline and amorphous structures below the T, the crystalline portions act to tie in or connect the surrounding areas acting as cross-links. 

In some situations, crystalline formation can occur because of addition of physical stress such as the stretching of a rubber band. As the rubber band is stretched, the amorphous random chains become aligned, forming small areas that are crystalline and oppose further stretching. Thus, rubber bands contain chemical cross-links and form crystalline domains on extension (Figure 2.22). 

Alkalies. — On heating 1 gm. of molybdic anhydride in a test tube, the particles adhering to the heated portion begin to fuse at a red heat, while at the upper, cooler portion of the tube a crystalline formation of sublimed anhydride may be observed. If the molybdic anhydride is contaminated with salts of the alkalies, the whole melts to a dark-colored mass. In such case the melting-point is low. 

We should also mention an early work by Slonimsky and Askadsky 74 who were apparently the first to observe structural changes taking place in extension under condition s of constant force. Three characteristic sections (see Fig. 20) were identified on the curves of strain versus tension time at F = const. These sections correspond to polymer flow in the amorphous state, the process of molecular ordering and crystallization, and, finally, to polymer flow in the crystalline state. The presence of crystalline formations on the latter section was detected with the help of X-ray-structural and electron-microscopic investigation of extended samples. As the tensile stress was lifted, the sample amorphised again and contracted. The occurrence of a drastic increase in strain on the second section was accounted for 74) by exhaustion of the longevity of supramolecular structures. 

The change from a crystalline into a liquid crystalline state can be brought about by changes in, for example, temperature or pressure. Furthermore, some molecules may be induced to form liquid crystals by the addition of a solvent such as water. This behavior is in reality a liquid crystalline formation in a two component system and is called solvent-induced liquid crystal formation or lyotropic mesomorphism (Small, 1986, p. 49). 

The formation of crystalline precipitates from starch sols by treatment with alcohol has been repeatedly mentioned in the literature. Alsberg describes birefringent crystal clusters obtained in this manner from autoclaved starch sols. WiegeP has recently reported crystalline formations obtained by treating starch pastes with various aliphatic alcohols, dioxane or ethylene chloride. However, the significance of these crystalline formations as a definite starch component has not been generally recognized, and little attempt has been made to isolate and purify this material. 

Sufficiently long heating at suitable temperatures brings about crystallization in glass, either in the form of isolated crystalline formations or as a layer growing from the surface. The final stable state is a totally transformed polycrystalline system. Exceptions to this rule are represented by B2O3 among oxides and by feldspar K2O. ... 

Bearing in mind that spontaneous formation of chemical and crystalline formations is accompanied by a decrease of the free energy of the system, which can be much smaller than its initial value (v i), especially when the newly produced compound is more stable than the mixture of initial components. The inverse case of p i is seldom encountered (nitrogen oxide or N2 + O2 mixture) or when (,i>2 = V l (HI or H2 + I2 mixture). As a rule, for porous materials phase transformations are accompanied by reduction of their specific surface area. 

The very sharp crystalline formation of the C. acidi-urici ferredoxin relative to that from C. pasteurianum has been correlated with their different amino acid compositions, in particular with the extra proline at site 16 (294). It is hence interesting that the kinetics of heat-inactivation at 70 °C indicates a higher stability for the former. Similarly, the aerobic reaction of the C. acidi-urici and C. tartarivorum ferredoxins with o-phenanthroline is faster for the most thermolabile of the two. This was reflected clearly in the temperature dependence of the reaction (285). Along these lines, Gillard et al. (295) have reported that iron is removed from native Peptostreptococcus elsdenii and from C. pasteurianum ferredoxins by o-phenanthroline while this ferrous chelator was found inactive on the C. acidi-urici protein. The magnitude of these differences needs further substantiation since other authors have claimed that o-phenanthroline can sequester iron from the C. acidi-urici protein as well (296). 

Extensive work on the formation of LSX from finely powdered metakaolin resulted in the discovery that essentially pure LSX could be prepared in a one-step synthesis without aging using a temperature of 50°C or slightly higher. Crystallization curves for LSX produced by circulating the alkaline mixture continually over the metakaolin at 50°C are shown in figure 3. In both the powdered and porous pellets, A zeolite did not form if the bases were circulated and the temperature maintained at 50°C. At this temperature, classical crystallization behavior was observed. After an induction period, the LSX crystallinity formation exhibits a smooth S-shaped curve. 

Typical condensation polymers, such as polyester and nylon, often exhibit these properties. If the fiber is to be ironed, its Tg should be above 200 °C if it is to be drawn from the melt, its Tg should be below 300 °C. Branching and cross-linking are undesirable because they disrupt crystalline formation even though a small amount of cross-linking may increase some physical properties if effected after the material is suitably drawn and processed. 

S-layer glycoproteins are usually assembled into a two-dimensional crystalline format on the cell envelope. Therefore it allows bacterial cell surfaces to be covered with the carbohydrate. This is comparable to the... 

If the fiber is to be ironed, its Tg should be above 200°C. Branching and crosslinking are nndesirable since they inhibit crystalline formation. Even so, some crossfinkmg may be present to maintain a given orientation, snch as desired in permanent press clothing. While most fibers are made from condensation polymers, new treatments allow some fibers to be made from olefinic materials snch as polypropylene (Table 3). 

The neutral pure ice is a crystalline formation. Khodzhaev et ah (48) have established that under long y-irradiation stabilization of the electrons in ice will be observed. Figure 7 shows the absorption spectrum of ice subjected to y-irradiation at 77°K. Band with Amax = 280 n.m. (e 450M 1 cm.-1) is because of the radical OH e tr absorbs the light in the visible part of the spectrum (Amax is about 620 n.m.). The yield of e tr in this system is very low (—10-3 electrons/100 e.v.). The concentration of the trapped electrons is very low therefore, these electrons are not displayed in the EPR spectrum. It should be stressed that the value of Amax of the trapped electron practically coincides with the value of Amax for e tT in the hypothetic glassy ice. 

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