Crystallinity orientation effects, glassy polymer

In order to deal with the four non-crystalline forms in a unified way, we define a network chain in a crosslinked system, as the section of network between neighbouring crosslinks (Fig. 3.6). The shape of both a network chain in a rubber, and a molecule in a polymer melt, can be changed dramatically by stress, and both can respond elastically. However, when the polymer is cooled below Tg, the elastic strains are limited to a few per cent (unless a glassy polymer yields), so the molecular shape is effectively fixed. If the melt or rubber was under stress when cooled, the molecular shape in the glass is non-equilibrium. This molecular orientation may be deliberate, as in biaxially stretched polymethylmethacrylate used in aircraft windows, or a by-product of processing, as the oriented skin on a polystyrene injection moulding. Details are discussed in Chapter 5. 

Oriontation-Induced Effects. Orientation and combined heat and orientation processing affect the transport properties of glassy polymers. Especially when crystallites are present, the effects can become surprisingly large. As noted for rubbery semicrystalline materials, the obvious improvements in barrier properties associated with organization of lamellar crystalline domains with their platelets perpendicular to the direction of penetrant flow can produce significant... 

Orientation of the polymer may also influence the permeation properties. However, the overall effect is highly dependent upon crystallinity. For example, deformation of elastomers-has little effect on permeability until crystallization effects occur. " Orientation of amorphous polymers can result in a reduction in permeability of around 10-15%, whereas in crystalline polymers, e.g, poly(ethylene terephthalate), reductions of over 50% have been observed. At high degrees of orientation, time-dependent effects on permeability occur in both glassy and semi-crystalline polymers. These effects have been related to the relaxation recovery of strain-induced areas of free volume generated during orientation. ... 

The chain stiffness inflnences the height of the glass-rnbber transition temperatnre (and of the melting point), bnt not the stiffness of the polymer below Tg (in the glassy state). Extremely stiff chains show the effect of the formation of LCP s (liqnid-crystalline polymers), by which very high stiffness is reached, bnt only in the direction of the orientation. 

Mixtures of clay platelets and polymer chains compose a colloidal system. Thus in the melt state, the propensity for the clay to be stably dispersed at the level of individual disks (an exfoliated clay dispersion) is dictated by clay, polymer, stabilizer, and compatibilizer potential interactions and the entropic effects of orientational disorder and confinement. An isometric dimension of clay platelets also has implications for stability because liquid crystalline phases may form. In addition, the very high melt viscosity of polypropylene and the colloidal size of clay imply slow particulate dynamics, thus equilibrium structures may be attained only very gradually. Agglomerated and networked clay structures may also lead to nonequilibrium behavior such as trapped states, aging, and glassy dynamics. 

Again, depending on the temperature, many changes from one state to another may occur per second. In the glassy or solid state few will take place, while in the liquid state many rotations will occur. Further, which state is preferred will depend upon whether the molecule is in a crystalline close packed state or in the more loosely packed amorphous state. As a result, it is clear that many factors tend to determine the conformations of a polymer molecule. Effects of orientation and temperature will be discussed in later Sections and Chapters. 

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