Semicrystalline polymer physical properties

Semicrystalline polymers are those that consist of two or more sohd phases, in at least one of which molecular chain segments are organized into a regular three-dimensional array, and in one or more other phases chains are disordered. The nonciystalline phases form a continuous matrix in which the crystalline regions are embedded. Most polyolefins are semicrystalline their specific morphology is governed by molecular characteristics and preparation conditions. Polyethylene is no exception to this mle it is all but impossible to prepare a solid specimen of polyethylene that is not semicrystalline. All commercial polyethylene products are semicrystalline. The physical properties exhibited by polyethylene products are governed by the relative proportions of the crystalline and noncrystalline phases and their size, shape, orientation, connectivity, etc. with respect to one another. 

The present discussion of physical structure and properties is intended to serve merely as a basis for appraising the characteristics of various polymers here surveyed. The nature of the semicrystalline state in polymers and its influence on their physical properties will be dealt with in greater detail in a later chapter. 

Regiodefects are less readily incorporated into crystallites than defect-free chain sequences. In semicrystalline polymers, increasing levels of misinsertion result in reduced crystallinity. This can affect numerous physical properties, resulting in reduced modulus, lower heat distortion temperature, and decreased tensile strength. 

Experimental results are presented that show that high doses of electron radiation combined with thermal cycling can significantly change the mechanical and physical properties of graphite fiber-reinforced polymer-matrix composites. Polymeric materials examined have included 121 °C and 177°C cure epoxies, polyimide, amorphous thermoplastic, and semicrystalline thermoplastics. Composite panels fabricated and tested included four-ply unidirectional, four-ply [0,90, 90,0] and eight-ply quasi-isotropic [0/ 45/90]s. Test specimens with fiber orientations of [10] and [45] were cut from the unidirectional panels to determine shear properties. Mechanical and physical property tests were conducted at cold (-157°C), room (24°C) and elevated (121°C) temperatures. 

It is of course important to note that the overall rate of crystallization is not only determined by the growth rate of the spherulites, but also by the amount of nuclei being present in the system. This possibility is used as an effective method to influence the total crystallization rate of commercial polymeric materials in a controlled manner and to influence the size of spherulites and thus the physical properties of finished articles made from semicrystalline polymers. 

Above Tm, the viscosity of the melt has Arrhenius-type dependence, decreasing (exponentially) with increasing temperature. Therefore a sharp transition is observed in both mechanical and viscous properties of semicrystalline polymers at Tm, resulting in a physical situation that is closer to the classic melting interface of monomeric crystals where, on one side, there is a viscous liquid, and on the other side, an elastic solid. 

To a good approximation, Cps(T) and C fT) both increase linearly with T for most polymers. It is, therefore, possible to extrapolate Cp fT) into the temperature range (Ttemperature range (T>Tg for amorphous polymers and T>Tm for semicrystalline polymers) where the polymer is a "liquid". Such extrapolations are often performed, especially to estimate Cps and Cp1 at room temperature, namely Cps(298K) and Cp1(298K), for use as parameters in calculating other physical properties. The rate of increase of Cp T) with increasing T is smaller than that of Cps(T). 

The glass transition also plays a major role in determining both the physical properties and the processing characteristics of semicrystalline polymers. Amorphous portions "melt" or "soften" at Tg. Crystalline portions remain "solid" up to the melting temperature Tm. (In... 

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