UNIT 7 Morphology of Solid Polymers
The exact nature of polymer crystallinity has been the subject of considerable controversy. The fringed-micelle theory, developed in the 1930 s, considers polymers to consist of small-sized, ordered crystalline regions-termed crystallites-imbedded in an unordered, amorphous polymer matrix. Polymer molecules are considered to pass through several different crystalline regions with crystallites being formed when segments from different polymer chains are precisely aligned together and undergo crystallization. Each polymer chain can contribute ordered segments to several crystallites. The segments of the chain in between the crystallites make up the unordered amorphous matrix. This concept of polymer crystallinity is shown in Fig. 7.1
The folded-chain lamella theory arose in the late 1950’s when polymer single crystals in the form of thin platelets termed lamella were grown from polymer solutions. The diffraction patterns of these single crystals indicate that the polymer molecules fold back and forth on themselves like in an accordion in the process of crystallization (Fig 7.2).The theory of chain-folding applies generally to most polymers _not only for solution-grown single crystals, but also for polymers crystallized from the melt. Semi-crystalline polymers are considered by advocates of the folded-chain theory to be chain-folded crystals with varying amounts of defects. The crystallinity of polymers is pictured as being completely similar to that of low molecular weight compounds. The defects in the chain folded crystals may be imperfect folds, irregularities in packing chain entanglements, loose chain ends, dislocations, occluded impurities, or numerous other imperfections.
The fringed-micelle and folded-chain theories of polymer crystallinity are often considered to be mutually exclusive but they need not be so considered. It is usually practical to adopt a working model of polymer crystallinity which employs the features of both concepts. The folded chain theory is especially well suited for highly crystalline polymers where one can consider them to be one phase crystalline systems with defects. Polymers with medium to low crystallinity can often be advantageously treated by the fringed-micelle concept as two-phase systems composed of crystallites imbedded in uncrystallized, amorphous polymer. The structure of the crystallites in such polymers may be that of the folded-chain lamella. The extent and type of crystallinity in a polymer is experimentally determined by methods such as density, X-ray and electron diffraction infrared spectroscopy , and nuclear magnetic resonance. The results are often interpreted in simplified manner as a weight or volume percent crystallinity by comparison with measurements on completely crystalline and completely amorphous polymer samples.