Poly stress-induced crystal phase

Perhaps, the most dramatic example of the effects of drawing on the spectra of a polymer comes from the stress-induced crystal transition of poly(butylene terephtha-late) 292,29 298 299). The phase transition of PTMT has been investigated above and below the glass transition temperature by means of dynamic infrared measurements... 

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. 

Some polymorphic modifications can be converted into one another by a change in temperature. Phase transitions can also be 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 [41]. Stress-induced transitions are also observed in some crystalline polymers, such as PBT [42,43] and its block copolymers with poly(tetramethylene oxide) (PTMO) [44], poly(ethylene oxide) (PEO) [45,46], polyoxycyclobutane [47], nylon 6 [48], poly(vinylidene fluoride) [49,50], polypivalolactone [51], keratin [52,53], 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 substances with the former include PBT, PEO, and keratin. 

---