Amorphous versus semicrystalline

The formation of a gas/polymer solution depends on gas absorption and diffusion into the polymer matrix, which can be affected by the nature of the polymer matrix (amorphous versus semicrystalline), gas type, saturation pressure, and temperature [44, 45]. The sorption behaviors of gas in polymers can be explained by Henry s law (gas solubility) and Pick s law (gas diffusivity), as shown in the following equations [46] ... 

Once again, there are no simple answers. Plastics are unique materials, and the categories used to describe and compare and contrast them are not always black and white. There are some high-level distinctions such as thermoplastics versus thermosets (described earlier), and amorphous versus semicrystalline. These distinctions are useful, and important, but by no means absolute. 

Figure 8.4 shows generic load versus elongation curves for rubbery amorphous, glassy amorphous, and semicrystalline polymers. In each case, the effect of extension on a dogbone specimen is shown at various points along the curve. 

Figure 15 illustrates the comparison of ten-second tensile modulus E(10 sec) versus temperature of the amorphous and semicrystalline Nafion-Na as observed by dry state and underwater stress relaxation studies. It is evident that the rate of stress relaxation is faster and the relaxation temperature is lower in amorphous Nafion, relative to those of semicrystalline Nafion. 

As displayed in Figures 14.4 to 14.9, k and a behave differently across the main transition region of the amorphous and semicrystalline polymers, Tg and Tm respectively. While Tg is embedded within the transition region, Tm separates melt from the supercooled liquid with its small quantity of dispersed crystals. Figure 14.9 shows k and a versus T dependencies for PA-6 and the two PNCs containing 2 and 5 wt% clay. For... 

Thermomechanical behaviour is most probably the most widely exploited property of engineering thermoplastics. Figure 3.1 shows the behaviour of two types of thermoplastic, one amorphous and the other semicrystalline, versus temperature. We can see several steps moving from low to high temperatures ... 

Figure 3.1. Examples of modulus variations versus temperature for an amorphous and a semicrystalline thermoplastic...

Figure 3.2 Specific volume (volume/unit mass) versus temperature curves for an amorphous and a semicrystalline polymer.

Draw a logB versus temperature plot for a linear, amorphous polymer and indicate the position and name the five regions of viscoelastic behavior. How is the curve changed if (a) the polymer is semicrystalline, (b) the polymer is cross-linked, and (c) the experiment is run faster ... 

For semicrystalline polymers, the curve of specific volume versus temperature follows an intermediate path between the ones for pure amorphous and pure crystalline polymers. 

Whether the polymer is totally amorphous or partially crystalline, the material will be glassy (hrittle) or ruhher-like (soft) depending on its temperature with respect to Tg. If an amorphous polymer is at a temperature helow Tg, it will be brittle and will show properties of a glassy material for example, it will fracture more easily. As the temperature of the sample increases and approaches Tg, it adopts a leathery behavior and its elastic modulus decreases. When the sample has reached several degrees above Tg, it shows a clear rubbery behavior and is easily deformable. If the temperature is increased even more, the polymer reaches liquid flow behavior. If the polymer is semicrystalline, it exhibits similar behavior, but when it reaches the melting temperature the crystals will break up, and the polymer will then reach the melted liquid state. This behavior is illustrated in Fig. 3.45 where the elastic modulus is plotted versus temperature. 

Finally, the left curves of Fig. 2.45 show that above about 260 K, melting of small, metastable crystals causes abnormal, nonlinear deviations in the heat capacity versus crystallinity plots. The measured data are indicated by the heavy lines in the figure. The thin lines indicate the continued additivity. The points for the amorphous polyethylene at the left ordinate represent the extrapolation of the measured heat capacities from the melt. All heat capacity contributions above the thin lines must thus be assigned to latent heats. Details of these apparent heat capacities yield information on the defect structure of semicrystalline polymers as is discussed in Chaps. 4-7. 

Fig. 22. Nomialized pull-off energy measured for polyethylene-polyethylene contact measured using the SFA. (a) F, versus rale of crack propagation for PE-PE contact. Change in the rate of separation does not seem to affect the measured pull-ofF force, (b) Normalized pull-off energy, Pn as a function of contact time for PE-PE contact. At shorter contact times, P does not significantly depend on contact lime. However, as the surfaces remain in contact for long times, the pull-off energy increases with lime. In semicrystalline PE, the crystalline domains act as physical crosslinks for the relatively mobile amorphous domains. These amorphous domains can interdiffuse across the interface and thereby increase the adhesion of the interface. This time dependence of the adhesion strength is different from viscoelastic behavior in the sense that it is independent of rate of crack propagation.

Hoffman-Weeks plots have also been drawn for several other amorphous/crys-taUine miscible blends, such as PVDF/PEMA (Eshuis et al. 1982), PEG/PMMA (Martuscelli 1984), PCL/SARAN (Zhang and Prud homme 1987), as well as for some miscible blends containing two semicrystalline components, PCL/PC (Jonza and Porter 1986) and PCL/Penton (Guo 1990). Table 3.10 represents equilibrium melting points derived from versus Tc plots for some of these systems. 

The symbol Lg derives from latent heat, ie, the recoverable heat in a reversible process. Thermolytic cleavage of primary chemical bonds in the polymer backbone to produce volatile fuel and char is obviously not a reversible process, but the symbol Lg will be used throughout to conform with the literature in the fire sciences. Table 2 illustrates the magnitude of these enthalpic terms for amorphous PMMA, polystyrene (PS), and semicrystalline polyethylene (PE). The stored heat Ahs was obtained by numerical integration of heat capacity versus temperature from ambient to the dissociation temperature as per equation 24. The dissociation... 

Fig. 2.6. Specific volume versus temperature for semicrystalline linear polyethylene showing the effect of heating a specimen from 20 C to above the melting point The specific volume of the specimen at 20 °C is v. The specific volume of the amorphous fraction is obtained by extrapolating the v-T curve for the liquid down to 20 °C. The specific volume of the crystalline fraction is obtained from the lattice constants of the unit cell (see Problem 2.1). The crystal fraction can be obtained using these quantities in eqn (2.2).

A special problem in heat of fusion determination is presented by linear, flexible macromolecules. They usually crystallize only partially, so that the heat of fusion measured is not the total heat of fusion and cannot be used directly for a discussion of the equilibrium entropy of fusion, for example. Similarly to the heat capacity treatment described in Fig. 5.17, one assumes that the partially crystallized samples can be described by a crystallinity, iv. . A definition of based on density is given in Fig. 5.17. Equation (1) of Fig. 5.25 shows how crystallinity can also be expressed in terms of the heat of fusion. The measured heat of fusion of the semicrystalline sample is A/7f, while Ai/f is the heat of fusion of the perfect crystal. As long as a two-phase model of semicrystalline polymers holds, the two definitions of Wf. give identical values. If perfect crystals are not available for comparison, calibration can in this case be obtained by measuring the heat of fusion of a sample of crystallinity known from some other measurement, such as density measurement. X-ray diffraction, or infrared absorption. In some cases it is possible to plot the change in heat capacity at the glass transition temperature, ACp(Tg), versus the measured heat of fusion A7/f. At the extrapolated value for ACp(Tg) = 0, A//f corresponds to the heat of fusion for the fully crystalline state, A//f. Special difficulties of this method arise from rigid amorphous fractions sometimes found in semicrystalline polymers. In this case the observed AC is lowered, as is discussed in Sect. 5.6. 

Numerous studies have been reported on the methods that enable one to obtain temperature independent log T) versus log OjY plots, usually referred to as reduced (or master) plots, where is called a temperature-dependent shift factor. We will show different ways of obtaining Uj. The availability of reduced plots for any given polymer will enable one to estimate the shear viscosity of the polymer at any desired shear rate and temperature. There are two different ways of obtaining reduced plots for shear viscosity, depending on whether a polymer is semicrystalline or amorphous. 

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