Stress-induced crystallinity

Chemically modified collagen has been demonstrated to be subject to stress induced crystallinity(4). Moreover, a differential activity to enzymatic degradation exists for amorphous and crystalline collagen(5)(6). 

An approach based on the traditional concepts of stress induced crystallinity and the differential enzyme activity on crystalline and amorphous collagen offers a relatively direct explanation for tissue growth and resorption. 

The peculiar behaviour of polyftetramethylene terephthalate) 4GT has received considerable attention with studies on multiple melting and crystallization behaviour, - the alpha crystal phase, the hysteresis of the stress-induced crystalline phase transition, and reversible crystal deformation and conformational changes. ... 

Crystalline Fraction (%) Thermally- Induced Crystallinity Stress-Induced Crystallinity... 

Brereton M G, Davies G R, Jakeways R, Smith T and Ward 1 M (1978) Hysteresis of the stress-induced crystalline phase transition in poly(butylene terephthalate). 

In (6), fa is the molecular orientation factor, K, and 0D are the maximum crystallization rate, maximum rate temperature, and crystallization half-width, respectively, and C is the stress-induced crystallinity coefficient (see [7]). 

This thermodynamic behaviour is consistent with stress-induced crystallisation of the rubber molecules on extension. Such crystallisation would account for the decrease in entropy, as the disorder of the randomly coiled molecules gave way to well-ordered crystalline regions within the specimen. X-Ray diffraction has confirmed that crystallisation does indeed take place, and that the crystallites formed have one axis in the direction of elongation of the rubber. Stressed natural rubbers do not crystallise completely, but instead consist of these crystallites embedded in a matrix of essentially amorphous rubber. Typical dimensions of crystallites in stressed rubber are of the order of 10 to 100 nm, and since the molecules of such materials are typically some 2000 nm in length, they must pass through several alternate crystalline and amorphous regions. 

The critical state of stress-induced crystallization at high spinning speeds is governed by the viscoelasticity of the polymer in combination with its crystallization behavior. Any kind of coarse particle obviously disturbs the structure and affects the resistance against deformation. The development of stress is controlled by the rheological properties of the polymer. Shimizu et al. [4] found that increasing the molecular weight drastically promotes the crystallinity under stress conditions. 

The Kirkendall effect alters the structure of the diffusion zone in crystalline materials. In many cases, the small supersaturation of vacancies on the side losing mass by fast diffusion causes the excess vacancies to precipitate out in the form of small voids, and the region becomes porous [11], Also, the plastic flow maintains a constant cross section in the diffusion zone because of compatibility stresses. These stresses induce dislocation multiplication and the formation of cellular dislocation structures in the diffusion zone. Similar dislocation structures are associated with high-temperature plastic deformation in the absence of diffusion [12-14]. 

Our wide-angle x-ray diffraction (WAXD) measurements have shown that stretching pure Epcar 847 induces considerable orientation and crystallinity—just as is the case with natural rubber. The blend of Epcar 847 with 50 phr LDPE also undergoes stress-induced crystallization. However, quantitative or qualitative differences in amounts of... 

In semi-crystalline polymers this rearrangement is so drastic that it may be called stress-induced crystallisation or recrystallisation. 

This peculiar superelastic behavior is owing to a stress-induced transformation from a high-strength crystalline phase to a very structurally similar, yet deformable, crystalline phase. On removing the stress, the deformed material transforms back to the initial... 

Some polymorphic modifications can be converted from one to another by a change in temperature. Phase transitions can be also induced by an external stress field. Phase transitions under tensile stress can be observed in natural rubber when it orients and crystallizes under tension and reverts to its original amorphous state by relaxation (Mandelkem, 1964). Stress-induced transitions are also observed in some crystalline polymers, e.g. PBT (Jakeways etal., 1975 Yokouchi etal., 1976) and its block copolymers with polyftetramethylene oxide) (PTMO) (Tashiro et al, 1986), PEO (Takahashi et al., 1973 Tashiro Tadokoro, 1978), polyoxacyclobutane (Takahashi et al., 1980), PA6 (Miyasaka Ishikawa, 1968), PVF2 (Lando et al, 1966 Hasegawa et al, 1972), polypivalolactone (Prud homme Marchessault, 1974), keratin (Astbury Woods, 1933 Hearle et al, 1971), and others. These stress-induced phase transitions are either reversible, i.e. the crystal structure reverts to the original structure on relaxation, or irreversible, i.e. the newly formed structure does not revert after relaxation. Examples of the former include PBT, PEO and keratin. 

The fact that the abrupt drop in H (Fig. 6.2) coincides with the well documented a-/3 transition in the deformation interval between 4 and 10-12% suggests that the observed H changes (Fig. 6.2) are exclusively related with the stress-induced polymorphic transition. Thus, the starting value oi H = 155 MPa should be typical for PBT containing the crystalline a phase and the lower value oi H = 118 MPa can be assigned to PBT comprising mostly the crystalline p phase. The observation that is obviously related to the fact that the a modification is distinguished by... 

One can conclude that the microindentation technique allows the strain-induced polymorphic transition in PBT to be followed. The observed rather abrupt variation in H (within 2-4% of external deformation) makes the method competitive with respect to sensitivity to other commonly used techniques such as WAXS, infrared spectroscopy, Raman spectroscopy, etc. (Tashiro Tadokoro, 1987). Furthermore, by applying the additivity law it is possible to calculate the microhardness of completely crystalline PBT, comprising crystallites of the /6-type, as = 122 MPa. This technique can also be used to examine the stress-induced polymorphic behaviour of PBT in copolymers and blends as will be demonstrated in the following sections. 

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