The Crystalline State

Mendel kern [5] pointed out that essentially all properties of polymers are controlled by the molecular morphology. In contrast to the amorphous or liquid state, the crystalline state is relatively inelastic and rigid. In the crystalline state, the bonds adopt a set of successive preferred orientations, while in the liquid state the bond orientation is such that the chains adopt statistical conformations. In the crystalline state the properties of the polymers differ considerably. If a polymer molecule is coiled randomly, then it cannot fit readily into a geometrically arranged, regular crystalline lattice. So the molecules must change into a uniform shape to fit into a crystal pattern. In many cases they assume either a helix or a zigzag conformation. Such arrangements are more regular than in a random coil [29–40]. This can even be detected by spectral and thermodynamic studies. During crystallization, polymers with bulky substituents that are spaced close to each other on the polymeric chains tend to form helical conformations that remain in the crystalline phase. The arrangement allows close packing of the substituents without much distortion of the chain bonds. That is particularly true of many isotactic polymers that crystallize in helical conformations, taking on gauche and trans positions. For the gauche position steric hindrance always forces the rotation to be such as to place the substituents into a juxtaposition generating either a right-hand or a left-had helix. The helical conformations of isotactic vinyl polymers were illustrated by Gaylord and Mark [41] as shown in Fig. 2.9: When macromolecules possess a certain amount of symmetry, then there is a strong accompanying tendency to form ordered domains, or crystalline regions. Crystallinity, however, in polymers differs in nature from that of small molecules. When the small molecules crystallize, each crystal that forms is made up of molecules that totally participate in its makeup. But, when polymers crystallize from a melt, which means that certain elements of the polymeric system or segments of the polymeric chains have attained a form of a three-dimensional order. Complete crystallization, from the melt, however, is seldom if ever achieved The ordered conformations may be fully extended or may be in one of the helical forms as shown above. This resembles orderly arrangement of small molecules in crystals. The crystalline domains, however, are much smaller than the crystals of small molecules and possess many more imperfections.

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مواضيع ذات صلة

Rheology and Viscoelasticity of Polymeric Materials

Thermodynamics of Elasticity

Elasticity

The Glass Transition and the Glassy State

Induction Forces in Polymers

Effects of Dipole Interactions

Structure and Property Relationship in Organic Polymers

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الأسموزية والديلزة Osmosis and dialysis

الحجم والشكل

الوزن الجزيئي للبوليمرات

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