Crystal polymers, amorphous

Lamellar morphology variables in semicrystalline polymers can be estimated from the correlation and interface distribution fiinctions using a two-phase model. The analysis of a correlation function by the two-phase model has been demonstrated in detail before [30,11] The thicknesses of the two constituent phases (crystal and amorphous) can be extracted by several approaches described by Strobl and Schneider [32]. For example, one approach is based on the following relationship ... 

Density, mechanical, and thermal properties are significantly affected by the degree of crystallinity. These properties can be used to experimentally estimate the percent crystallinity, although no measure is completely adequate (48). The crystalline density of PET can be calculated theoretically from the crystalline stmcture to be 1.455 g/cm. The density of amorphous PET is estimated to be 1.33 g/cm as determined experimentally using rapidly quenched polymer. Assuming the fiber is composed of only perfect crystals or amorphous material, the percent crystallinity can be estimated and correlated to other properties. 

Tensile Properties. Tensile properties of nylon-6 and nylon-6,6 yams shown in Table 1 are a function of polymer molecular weight, fiber spinning speed, quenching rate, and draw ratio. The degree of crystallinity and crystal and amorphous orientation obtained by modifying elements of the melt-spinning process have been related to the tenacity of nylon fiber (23,27). 

Polyisobutylene has a similar chemical backbone to butyl rubber, but does not contain double carbon-carbon bonds (only terminal unsaturation). Many of its characteristics are similar to butyl rubber (ageing and chemical resistance, low water absorption, low permeability). The polymers of the isobutylene family have very little tendency to crystallize. Their strength is reached by cross-linking instead of crystallization. The amorphous structure of these polymers is responsible for their flexibility, permanent tack and resistance to shock. Because the glass transition temperature is low (about —60°C), flexibility is maintained even at temperatures well below ambient temperature. 

The volume inside the semicrystalline polymers can be divided between the crystallized and amorphous parts of the polymer. The crystalline part usually forms a complicated network in the matrix of the amorphous polymer. A visualization of a single-polymer crystallite done [111] by the Atomic Force Microscopy (AFM) is shown in Fig. 9. The most common morphology observable in the semicrystalline polymer is that of a spherulitic microstructure [112], where the crystalline lamellae grows more or less radially from the central nucleus in all directions. The different crystal lamellae can nucleate separately... 

PTT is a rapidly crystallizing polymer. A melt-processed PTT tends to crystallize with a crystallinity of between about 15 and 30 wt%. It is therefore more difficult to dissolve in solvents commonly used for amorphous PET. Stronger solvents, such as hexafluoroisopropanol (HFIPA) or a 1 1 mixture of trifluo-roacetic acid and methylene chloride are typically used to dissolve PTT. However, HFIPA is a very expensive solvent for routine IV measurements, and methylene chloride is too volatile to maintain in a 1 1 mixture with trifluoroacetic acid at elevated temperatures or in prolonged storage. With care, a 60/40 mixture of phenol/tetrachloroethane can be used satisfactorily for IV measurement when it is heated to 110 °C to ensure complete dissolution of PTT [37],... 

A number of other characteristics are required in order to ensure a viable polymeric conductor. Chain orientation is needed to enhance the conducting properties of a polymeric material, especially the intermolecular conduction (i.e., conduction of current from one polymer molecule to another). This is a problem with many of the polymers that are amorphous and show poor orientation. For moderately crystalline or oriented polymers, there is the possibility of achieving the required orientation by mechanical stretching. Liquid crystal polymers would be especially advantageous for electrical conduction because of the high degree of chain orientation that can be achieved. A problem encountered with some doped polymers is a lack of stability. These materials are either oxidants or reductants relative to other compounds, especially water and oxygen. 

It is known that crystalloid lum an influence on the position of the glass transition temperature. For instance, amorphous poly (ethylene terephthalate) has a TfJ of 67°C. This increases to 81 C. for the crystallized polymer (11). This is explained by the fact that the rotation of the chain segments, which is made possible at the glass transition, is hindered tn the proence of the crystallites. 

Where a melt-crystallized polymer has been processed by drawing, rolling or other means to produce an aligned structure in which lamellae as well as polymer chains have discernible order, a pseudocrystalline unit cell is present. Provided that this unit cell contains elements of the crystals as well as the boundaries between crystals and that it is entirely typical of the material as a whole then it could be considered as a RVE within the meaning defined above. The lamella crystal itself sometimes considered as embedded in an amorphous matrix would not seem to be an acceptable RVE for reasons similar to those advanced against the Takayanagi model, namely that its modulus is dependent upon the surface tractions. The boundaries between lamella crystals in the matrix must be included in an acceptable RVE. 

Recent advances in polyolefin chemistry have led to the creation of polymer hybrids finking between different polymer segments, namely crystal polyolefins, amorphous polyolefens, and other polar polymers (Fig. 1) to create polyolefin materials with novel functionalities, including polarity, etc. 

The polymer surface, as in the bulk itself, may vary from partially polycrystalline to amorphous. Except for the polydiacetylenes, single crystal polymer surfaces are essentially unknown. 

In polymers crystallized from the melt, in most cases spherulitic structures are observed spherical agglomerates of crystals and amorphous regions, grown from a primary nucleus via successive secondary nucleation (Figure 4.18). The dimensions of the spherulites are commonly between 5 pm and 1 mm. When spherulites grow during the crystallization process, they touch each other and are separated by planes. In a microtome slice they show a very attractive coloured appearance in polarized light. 

Transparent Polymers. Amorphous thermoplastics, like poly (methyl methacrylate), polystyrene, SAN, PVC, or the cellulose esters are transparent and used for glazing, photographic film, blown bottles, or clear packaging containers. Only a few crystalline thermoplastics, like poly (4-methyl-l-pentane), where the crystalline and the amorphous phases have almost identical refractive indexes, or polycarbonate, which has smaller crystals than the wavelength of light, are also transparent. R. Kosfeld and co-workers analyzed the mobility of methyl groups in polycarbonate, poly (methyl methacrylate) and poly( -methyl styrene) by NMR spectroscopy. 

The water present in foods may act as a plasticizer. Plasticizers increase plasticity and flexibility of food polymers as a result of weakening of the intermolecular forces existing between molecules. Increasing water content decreases Tg. Roos and Karel (1991a) studied the plasticizing effect of water on thermal behavior and crystallization of amorphous food models. They found that dried foods containing sugars behave like amorphous materials, and that small amounts of water decrease Tg to room temperature with... 

Most polymers fall in the class of translucent resins. These include acetal, polyamide, polybutylene terephthalate (PBT), polyethylene, and polypropylene as examples. There are very few neat polymers that are truly opaque (this depends on thickness as well). Liquid crystal polymer (LCP) is an example of a typically opaque polymer. It is theorized that these semicrystalline and crystalline resins will scatter some portion of incident light due to spherulitic crystal structure and the amorphous-crystalline region interfaces themselves. 

While most conjugated polymers display partial crystallinity, see Section 1.2.7, examples are known of extended-chain single crystals and amorphous isotropic solids. The extent of regions in the polymer backbone with uninterrupted overlap of the 7i-electron wavefunctions will obviously be much greater in an extended chain than in one that is randomly folded. Since physical properties,... 

Because of chain folding, melt-crystallized polymers are not as strong as they could be. One can envisage that under a load a sample will at some point yield, with chains in the amorphous domains becoming oriented in the draw direction while the lamellar arms of the spherulite undergo shear and whole sections are pulled ont. This process is illustrated in Figure 8-65. 

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