Crystalline-amorphous features crystallite

Two additional features of a semi-crystalline network should be considered one is the presence of some completely amorphous chains, and the other is the displacement of crosslinks by growing crystallites. As a network crystallizes the crystallites upset the balance of forces about the crosslinks, which are then forced to new positions of equilibrium. These problems have recently been attacked (7) by consigning the chains to their most probable positions. 

In the 1970s a model for semi-crystalline polymers was presented by Struik (1978) it is reproduced here as Fig. 2.13. The main feature of this model is that the crystalline regions disturb the amorphous phase and reduce its segmental mobility. This reduction is at its maximum in the immediate vicinity of the crystallites at large distances from the crystallites will the properties of the amorphous phase become equal to those of the bulk amorphous material. This model is similar to that of filled rubbers in which the carbon black particles restrict the mobility of parts of the rubbery phase (Smith, 1966). 

An important and general feature of CPs is that they are not well-ordered materials an amorphous phase is always present, sometimes dominant and the crystalline phase is always made of small, imperfectly ordered crystallites. These problems are seldom reviewed and are discussed at some length in Chapter 12, Section II. 

It should be emphasized that all of the crystalline polymers discussed here are not totally crystalline but partly crystalline and partly amorphous. Thus one expects to see behavior characteristic of the crystalline regions superposed upon behavior characteristic of the amorphous regions. This superposition is not necessarily a linear one but may rather be a complex coupling of the response of each region. Furthermore, the degree of crystallinity as well as the crystallite size is not a unique feature of any polymer system. Both of these properties are determined by prior thermal history. 

Cokes, chars, and activated carbons are frequently termed amorphous carbon. X-ray studies have shown that many so-called amorphous substances have crystalline characteristics, even though they may not show certain features, such as crystal angles and faces, usually associated with the crystalline state. An amorphous-looking powder may be composed of crystals of submicroscopic dimensions, so-called crystallites. This is true of chars and cokes. 

The above-listed features of the formation of X-ray amorphous and crystalline products are observed in the decompositions of many other compounds. Haul and Schoning [41] applied an X-ray method (line-width technique) to study the structure of the decomposition products of dolomite as a function of temperature. The experiments were carried out both in vacuum and in a CO2 environment. In vacuum, decomposition proceeded to the CaO and MgO oxides, and in a CO2 environment, to CaCOs and MgO. This difference is reasonable. It originates from the calcium carbonate being more stable than the magnesium carbonate (the enthalpies of formation of CaCOs and MgCOg at 800 K are —1,154 and —1,045 kJ mol respectively). The size of the crystallites formed in vacuum [41] increased monotonically from 6nm for MgO and 13 nm for CaO to 120-140 nm with the temperature increasing within the range 700-1,000° C. (Oversaturation decreases in these conditions from 10 to 10 for MgO, and from 10 to 10 for CaO.)... 

PSt and PMMA formed by electron beam irradiation have no effect on the crystallinity of cellulose insofar as the blackness in the X-ray diffraction patterns of both WPC (irradiation) systems taken at room temperature are concerned. However, the high temperature feature of the E,p pattern of the PSt-WPC (irradiation) system indicates the enhancement of band (c) at ca. 190°C and the lowering of bands (a) and (b) (Figure 9). Therefore, it seems likely that grafted PSt somewhat perturbs the crystallites and increases the amorphous portion in wood by thermal shear at high temperatures. 

In one extreme, termed the regularly folded-adjacent-re-entry stmcture, the molecular chains appear to be accordion-like, making precise hairpin turns in order to yield the optimum level of possible crystallinity. However, equally consistent with the gross morphological features is the other model illustrated. Here, there is a distinct, disordered, amorphous overlayer. This schematic representation has popularly been termed the switchboard model. Both of these interfacial stmctures, and those in between, are consistent with the electron micrographs. The reason for introducing these concepts here is that a lamellar-type crystallite is also the universal mode of crystallization of a homopolymer from the pure melt. 

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