Polymers, glassy amorphous

In the glassy amorphous state polymers possess insufficient free volume to permit the cooperative motion of chain segments. Thermal motion is limited to classical modes of vibration involving an atom and its nearest neighbors. In this state, the polymer behaves in a glass-like fashion. When we flex or stretch glassy amorphous polymers beyond a few percent strain they crack or break in a britde fashion. 

Glassy amorphous polymers exhibit excellent dimensional stability and are frequently transparent. Everyday examples include atactic polystyrene, polycarbonate, and polymethylmethacrylate (Plexiglas ), which we encounter in such applications as bus shelters, motorcycle windshields, and compact disc cases. 

In Fig. 8.4 a), the glassy amorphous polymer extends only a few percent before it breaks abruptly. The extension in the sample up to the point of failure is largely reversible, that is, the material behaves elastically. Polystyrene and polycarbonate, which are used to make CD jewel cases, exhibit this type of behavior. 

The elongation at break of a sample is the strain at which at which it breaks. This value varies widely depending on polymer type and processing conditions. Glassy amorphous polymers typically exhibit low elongations at break because their chains cannot slide past one another. In rubbery amorphous polymers the situation is somewhat different. High molecular weight... 

Our principal concern is often the polymer s mechanical properties. For instance, the requirements of the handle of an electrician s screwdriver are very different from those of wire insulation. In the former application, we are free to choose stiff polymers of many types, including glassy amorphous polymers. In contrast, wire insulation must be flexible, which limits our choice to ductile polymers. 

Apparently all four curves intersect at the same point, namely at the temperature at which A and B have the same E-modulus. Each blend has the same modulus at this temperature. This situation may occur when B is a glassy amorphous polymer and A... 

TABLE 4.7 Molar volumes of glassy amorphous polymers at 25 °C... 

Dimensional stability is one of the most important properties of solid materials, but few materials are perfect in this respect. Creep is the time-dependent relative deformation under a constant force (tension, shear or compression). Hence, creep is a function of time and stress. For small stresses the strain is linear, which means that the strain increases linearly with the applied stress. For higher stresses creep becomes non-linear. In Fig. 13.44 typical creep behaviour of a glassy amorphous polymer is shown for low stresses creep seems to be linear. As long as creep is linear, time-dependence and stress-dependence are separable this is not possible at higher stresses. The two possibilities are expressed as (Haward, 1973)... 

FIG. 13.44 Typical tensile creep behaviour of a glassy amorphous polymer (a modified PMMA at 20 °C), where strain is plotted vs. log time, for various values of tensile stress. From bottom to top 10, 20, 30, 40, 50 and 60 MPa. From Haward (1973). Courtesy Chapmann Hall. 

For glassy amorphous polymers analogous expressions could be derived from the experimental data in the literature. The numerical values of the constants in the equations are somewhat lower, and also the accuracy is lower (this is probably due to the fact that the physical structure of the glassy state strongly depends on the processing of the polymer). 

The factors (298 Tg) and (Tg 298) are the "thermal distances" of Tg from room temperature for rubbers and glasses respectively. The influence of these "thermal distances" is probably connected with the fractional free volume of the polymer in rubbery amorphous polymers this f.f.v. increases with decreasing Tg, in glassy amorphous polymers the f.f.v. increases with increasing Tg (increasing formation of micro-voids), hence lowering of the activation energy. 

Glassy amorphous polymers Hydropol (hydrogenated p. butadiene) 15... 

TABLE 18.13 Dual mode sorption and mobility data of glassy amorphous polymers (values at 35 °C) Data from Sada et al. (1987), Chern et al. (1987) and Barbari, Koros and Paul (1988). 

With polymers, complications may potentially arise due to the material viscoelastic response. For glassy amorphous polymers tested far below their glass transition temperature, such viscoelastic effects were not found, however, to induce a significant departure from this theoretical prediction of the boundary between partial slip and gross slip conditions [56]. 

In conclusion, the deformation behavior of poly(hexamethylene sebacate), HMS, can be altered from ductile to brittle by variation of crystallization conditions without significant variation of percent crystallinity. Banded and nonbanded spherulitic morphology samples crystallized at 52°C and 60°C fail at a strain of 0.01 in./in. whereas ice-water-quenched HMS does not fail at a strain of 1.40 in./in. The change in deformation behavior is attributed primarily to an increased population of tie molecules and/or tie fibrils with decreasing crystallization temperature which is related to variation of lamellar and spherulitic dimensions. This ductile-brittle transformation is not caused by volume or enthalpy relaxation as reported for glassy amorphous polymers. Nor is a series of molecular weights, temperatures, strain rates, etc. required to observe this transition. Also, the quenched HMS is transformed from the normal creamy white opaque appearance of HMS to a translucent appearance after deformation. 

Whitney W (1964) Yielding behavior of glassy amorphous polymers. So.D. Thesis in Materials Science and Engineering, M.I.T., Cambridge, MA Wellinghoff ST, Baer E (1978) J. Appl. Polymer Sci., 22 2025 Kardomateas GA, Yannas IV (1985) Phil Mag. 52 39... 

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