Amorphous material/polymers/regions

Melt transition temperature, Tm (Section 31.5) The temperature at which crystalline regions of a polymer melt to give an amorphous material. 

Secondary crystallization occurs most readily in polymers that have been quench-cooled. Quenched samples have low degrees of crystallinity and thus have relatively large volumes of amorphous material. A pre-requisite for secondary crystallization is that the amorphous regions must be in the rubbery amorphous state. Increased temperature accelerates the rate of secondary crystallization. The new volumes of crystallinity that form during secondary crystallization are generally quite small, amounting to less than 10% of the crystalline volume created during primary crystallization. 

Interestingly, the amorphous regions within the spherulite confer some flexibility onto the material while crystalline platelets give the material strength, just as in the case with largely amorphous materials. This theme of amorphous flexibility and crystalline strength (and brittleness) is a central idea in polymer structure-property relationships. 

Noncrystalline or amorphous materials produce patterns with only a few diffuse maxima, which may be either broad rings or arcs if the amorphous regions are partially oriented [3]. Synthetic polymers, which are branched or cross-linked, are usually amorphous, as are linear polymers with bulky side groups, which are not spaced in a stereoregular manner along the backbone [3]. 

Determination of the proportions of crystalline and amorphous material in partially crystalline polymers. Knowledge of the unit cell dimensions in high polymer crystals leads to a knowledge of the density of the crystalline regions. If the density of amorphous regions is also known, either by measurement of the density of an entirely amorphous specimen (if this can be obtained) or by extrapolation of the liquid density/temperature curve, it is possible to calculate, from the measured density of any partially crystalline specimen, the proportions of crystalline and amorphous material. Since the physical properties of polymer specimens are profoundly influenced by the degree of crystallinity, X-ray determinations of crystallinity are much used in such studies (see Bunn, 1957). 

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). 

Crystallisation of polymers tends to decrease the volume of amorphous material available for the diffusion crystalline regions obstruct the movement of the molecules and increase the average length of the paths they have to travel. 

Polymer glasses are widely used in the optics industry as bulk and micro optical components. The most commonly used materials are polyacrylates, polycarbonates, and polystyrenes. Low optical losses are found with these materials due to their amorphous nature. Crystalline regions in a material will cause scattering, thus reducing the optical quality. 

Transmission electron spectroscopy (TEM) studies of the co-deposited thin films revealed a continuos polymer phase with a largely inter-dispersed SiOj regions on a 5-50 nm scale, confirming the nanocomposite morphology. As mentioned in section 3.1.3, parylene thin films obtained are typically highly crystalline. In contrast, SiO is an amorphous material. From X-ray diffraction analyses, it was observed that by increasing the relative amount of polymer in the nanocomposite, the crystallinity was... 

Post-failure studies of the fracture surface morphology of bulk semi-crystalline polymers are more difficult than those of amorphous materials due to the more complex multiphase structure associated with semi-crystalline materials However, it was shown that some fractographic details point to the formation of a stress whitened region ahead of a notch prior to final fracture of the material. In particular, stress-whitened regions were easily visible in semi-crystalline polymers such as LDPE and HDPE The resulting, macroscopically apparently brittle fracture... 

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