Phase transition stress-induced

Uniaxial stress-induced phase transition in gels [29]. The changes in diameter and length of cylindrical N-isopropylacrylamide gels were measured under uniaxial stress along the axis. The transition temperature and discontinuity were increased with increasing stress, which indicates the possibility of the... 

Phase transition in gels can be affected by applying uniaxial stress. With increasing stress in the region below 1 x 104 N m 2 at gelation, the effects of uniaxial stress was qualitatively described by the mean field theory. The present results clearly indicate the possibility of a uniaxial stress-induced phase transition of gels. 

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. 

Small molecular mass liquid crystals do not respond to extension and shear stress. Liquid crystalline polymers may exhibit a high elastic state at some temperature due to the entanglements. However, the liquid crystalline network itself is an elastomer, showing rubber elasticity. In the presence of external stress, liquid crystalline networks deform remarkably and then relax back after the release of stress. The elasticity of liquid crystalline networks is more complicated than the conventional network, such as the stress induced phase transition, the discontinuous stress-strain relationship and the non-linear stress optical effect, etc. 

Stress-Induced Phase Transitions in Metallocene-Made Isotactic Polypropylene... 

C. De Rosa and F. Auriemma Stress-Induced Phase Transitions in Metallocene-Made Isotactic Polypropylene, Lect. Notes Phys. 714, 345—371 (2007)... 

Stress-induced Phase Transitions in Unoriented Fims... 

The samples show a complex polymorphism during tensile deformation. The relationships between the different mechanical behavior and the stress-induced phase transitions are discussed in terms of a general view, outlining the concept that stress-induced phase transitions during plastic deformation of... 

Relationship Between Molecular Configuration and Stress-Induced Phase Transitions... 

In the third step, starting from deformations close to the critical strain at which stress-induced phase transitions start occurring (e 200 %, Figs. 11.6 and 11.7), the... 

De Rosa C, Auriemma F (2007) Stress-induced phase transitions in metallocene-made isotactic polypropylene. Lect Notes Phys 714 345-371... 

VS1 Vshivkov, S.A., Rusinova, E.V., Dubchak, V.N., and Zarubin, G.B., Stress-induced phase transitions in polydimethylsiloxane-methyl ethyl ketone system (Russ.), Vysokomol. Soedin., Ser. A, 38, 844, 1996. 

Liquid crystalline elastomers are produced by the introduction of cross linking into liquid crystal polymer systems. This cross linking results in materials with a number of unusual properties, for example, stress-induced phase transitions and spontaneous ekmgatioo of samples in the liquid crystelline phase. When they contain chiral units. Sr elastomers are formed. For such materials piezoelectricity was predicted by Brand 103]. The helical structure of those systems can be untwisted by applicatioo of a mechanical stress, generating an electrical signal. This possibility provi the basis for the developnient of materials with piezoelectric properties. Such materials are of comaderablc interest, since the basis for their piezoelec c properties is rather different from that in the best-known piezoelectric polymer, poly(vinylidenefluoride) (PVDF). There exists rather more scope for the modification of their properties for example, the nature of the chiral unit may be varied to alter the helkal superstructure, or differences in cross link density can change the mechanical properties of the sample. 

There has been considerable interest in the mechanisms of the a — /3 transition, and this has been modeled for static and dynamic measurements [58-60], X-ray diffraction patterns and infrared (IR) and Raman spectra show specific changes through this a—>/3 transition [42-44,61-63], The stress and strain dependence of the molar fraction of the / -form, X, increases drastically above the critical stress / the fraction is almost linearly proportional to the strain the transition is reversible the stress-strain curve has a plateau of the critical stress /, i.e., the curve is divided into three regions the elastic deformation of the a-phase (0-4% strain), the a — /3 transition (plateau region 4-12% strain), and the elastic deformation of the /3-phase (> 12% strain). These experimental results indicate that this stress-induced phase transition is a thermodynamic first-order transition. 

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