Non-crystalline forms

A non-crystalline polymer can be transformed reversibly into another form by a temperature change (Fig. 3.5). Alternatively an irreversible crosslinking reaction can convert a linear polymer into a network polymer. This reaction can only occur when polymer chains can change shape, i.e. the polymer is 

In order to deal with the four non-crystalline forms in a unified way, we define a network chain in a crosslinked system, as the section of network between neighbouring crosslinks (Fig. 3.6). The shape of both a network chain in a rubber, and a molecule in a polymer melt, can be changed dramatically by stress, and both can respond elastically. However, when the polymer is cooled below Tg, the elastic strains are limited to a few per cent (unless a glassy polymer yields), so the molecular shape is effectively fixed. If the melt or rubber was under stress when cooled, the molecular shape in the glass is non-equilibrium. This molecular orientation may be deliberate, as in biaxially stretched polymethylmethacrylate used in aircraft windows, or a by-product of processing, as the oriented skin on a polystyrene injection moulding. Details are discussed in Chapter 5. 

One way to specify molecular size is by the length of the end-to-end vector r (Fig. 3.6) that runs from the start to the end of an isolated chain, or between the crosslinks at the end of a network chain. The molecule is assumed to be unstressed, so its equilibrium shape has end-to-end vector tq. The average length of to will be calculated for a polymer with a C—C single bond backbone of a particular molecular weight. The average is over a large number of similar chains, and over a period of time. 

The model used is a walk on a diamond lattice, in which trans, gauche and gauche conformations have equal probabilities of 1/3. To obtain a simple result, the self-avoiding condition is relaxed, so the walks can intersect with themselves. The end-to-end vector r is the sum of the step vectors h. The mean square length of the walk is calculated from r, the scalar product of to with itself. The terms in the expansion are grouped according to the distance between the pairs of steps 

Using the Monte Carlo method, for walks that can intercept, confirms this result. The root mean square length of the 100 step walks in Fig. 3.7 is 14.135. The theoretical distribution in the figure is derived in Section 3.4. It is the product of a 4irr term (the surface area of a sphere of radius r on which the chain end lies) and the Gaussian distribution of Eq. (3.13). 

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Fig. 21.17. X-ray diffraction pattern of the cream, showing that SnC>2 is present (filled circles). The crystalline fatty acids (open circles) probably formed by deterioration of the animal fat component during burial. Starch is not evident and so must be in a non-crystalline form, consistent with mode of preparation. (Reprinted/redrawn from Nature, 432, 35-36, Copyright 2004, Nature Publishing Group, with permission.)...

Amorphous carbon is a general term that covers non-crystalline forms of carbon such as coal, coke, charcoal, carbon black (soot), activated carbon, vitreous carbon, glassy carbon, carbon fiber, carbon nanotubes, and carbon onions, which are important materials and widely used in industry. The arrangements of the carbon atoms in amorphous carbon are different from those in diamond, graphite, and fullerenes, but the bond types of carbon atoms are the same as in these three crystalline allotropes. Most forms of amorphous carbon consist of graphite scraps in irregularly packing. 

If a polymer is incapable of crystallizing or if it has retained non-crystalline form for kinetic reasons, it still remains in the liquid crystalline state in the entire region of temperatures and concentrations below curve 1 and not only between curves 1 and 2. The point Vj T " corresponds to the limiting concentration and limiting temperature for the existence of liquid crystalline state of the polymer. 

P-Cocaine 2P-carbomethoxy-3-p-benzoxytropane, methyl [lR-(cjro,exo)]-3-(benzoyloxy)-2-methyl-8-azabicyclo[3.2.1]octane-2-carboxylate [50-36-2] M 303.4, m 98°, b 187-188°/0.1mm, [a]p -15.8° (c 4, CHCI3), pK 8.39. Crystallises from EtOH and sublimes below 90° in a vacuum in a non-crystalline form. 

Crystallinity. Non-crystalline forms will dissolve more rapidly than crystalline forms the vast majority of biogenic silica in soils is non-crystalline. 

These observations led Saltman et al. to propose that there is an equilibrium between ferritin and the non-crystalline form and apoferritin subunits (Eq. 2) ... 

The improved polymerization process described in this patent operates at supercritical propylene conditions. The crystalline piolymer that is formed (and that is dissolved in the propylene) is separated from the non-crystalline form by pressure reduction. A separation between die two forms is effected because the crystalline polymer precipitates while the non-crystalline polymer remains dissolved. The noncrystalline jxilymer is separated from... 

Physicai Description Transparent to gray, odorless powder. [Note Amorphous silica is the non-crystalline form of Si02. 

The polymorphism of calcium carbonate is still not completely understood despite its very long history and the appearance of many published studies. In addition to calcite, aragonite and vaterite, non-crystalline forms of CaCOs of biolo cal origin exist. An account of the relationships between these solid phases has appeared recently [39] together with a summary of thermodynamic and kinetic data for the transformations of the metastable polymorphs aragonite and vaterite to the stable calcite. These authors describe the preparation of non-crystalline calcium carbonate and report preliminary values of the transition temperature and enthalpy change for its crystallization to calcite. The DSC method, siq)ported by TG and PXRD, was used in this study. 

Note Crystalline form given in brackets. The overall properties will depend on the bonding mechanism. Rapid cooling will favour the non-crystalline form since the time is insufficient to permit the formation of a regular crystalline structure. 

Very low FOg content indicates small quantities of non-crystalline form. Free iron, extracted with dithionite (Fej), corresponds to 58.1 and 17.9 gkg ... 

Another remarkable result of the interference between weak and em interactions is that the refractive index of a substance can be different for right and left circularly polarized light, even when the material is in a non-crystalline form. We shall consider the passage of a plane polarized... 

Elemental sulftn- may occur in a number of crystalline and non-crystalline forms, both natural and synthetic in origin. Of these the most important are the amorphous and orthorhombic (Sg) forms. Sulftn is commonly associated with volcanic deposits, particularly during the late stages of vulcanicity around hot springs and pools where it is associated with travertine (. v. a form of limestone), siliceous sinter and borates. It is also found in association with gypsum in sedimentary rocks and particularly with black shales where it is a product of an anaerobic environment and found associated with pyrite (qq.v.). It is also found associated with aerially weathered pyrite (and other similar sulfides) where the rate of oxidation is increased due to the presence of thiobacteria. In these cases it is associated with a range of iron sulfate compounds as well as sulfuric acid. 

Isotactic and syndiotactic polymers can crystallize, while atactic polymers cannot. Polymers other than polypropylene that have tacticity include polystyrene, polyfvinyl chloride), and poly(methyl methacrylate). Thus there are crystalline and non-crystalline forms of these polymers. By use of a copolymer, the number of side groups, and thus the crystallinity can be precisely controlled. An important example is the copolymerization of ethylene and a higher alpha-olefin, such as butene or octane, to make linear low-density polyethylene (LLDPE), in which the crystallinity is governed by the fraction of comonomer incorporated into the chain. Tacticity can affect important physical properties such as the intrinsic viscosity and thus must be taken into accoimt in characterization methods such as gel permeation chromatography. 

The gradual addition of water to an acetone solution of lutein causes marked changes in the structure of the absorption spectrum (Fig.4). In the first step only a short-wavelength absorbing form with one maximum is formed. The LWAF appears in the second step. It may then be concluded that the crystalline lutein and presumably some other xanthophylls are accompanied by non-crystalline forms of the pigments. 

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