Glassy amorphous polymers behavior

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. 

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... 

It is well known that the mechanical behavior of glassy amorphous polymers is strongly influenced by hydrostatic pressure. A pronounced change is that polymers, which fracture in a brittle manner, can be made to yield by the application of hydrostatic pressure Additional experimental evidence for the role of a dilatational stress component in crazing in semicrystalline thermoplastics is obtainai by the tests in which hydrostatic pressure suppresses craze nucleation as a result, above a certain critical hydrostatic pressure the material can be plastically deformed. 

In a detailed discussion of secondary transitions in glassy, amorphous polymers, it is shown that the fi transition in polymethacrylates is caused by motion of the entire —COOR group (69). This same study notes that the Arrhenius plot lines for secondary transitions of various polymers all converge to a common frequency, about 10 Hz this is the same type of empirical behavior observed for the glass transition (56). 

Time and Temperature Effects The viscoelastic characteristics of stress-strain behavior of glassy amorphous polymers can be approached through considering that the stress at any time t is the sum of all the little stresses, 5g, each of which is the result of many incremental relaxing stresses each started at a progressively different time, f. Each 5o produces an incremental strain, 5e, and... 

Mechanism of water penetration in water-soluble cellulose derivatives Solvent transport in glassy amorphous polymers has been shown to undergo varying behavior according to the dominant process diffusion of the penetrant into the polymer or relaxation of the polymer chains [34-37]. A diffusional Deborah number, (DEB)q, has been proposed by Vrentas et al. [38, 39] as a means of characterizing penetrant transport ... 

Why do the tensile behaviors of rubbery amorphous, glassy amorphous and semicrystalline polymers differ as they do ... 

At sufficiently low temperatures a polymer will be a hard, brittle material with a modulus greater than lO N m (10 dyn/cm ). This is the glassy region. The tensile modulus is a function of the polymer temperature and is a useful guide to mechanical behavior. Figure I 1-8 shows a typical modulus-temperature curve for an amorphous polymer. 

Most crystalline polymers with metylenic groups in their structure and with a degree of crystallinity below 50% present a sub-glass relaxation whose intensity and location scarcely differ from those observed for the amorphous polymer in the glassy state. The temperature dependence of this relaxation follows Arrhenius behavior, and its activation energy is of the same order as that found for secondary processes in amorphous polymers. 

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