Polymorphic transitions in crystalline polymers

When polymers crystallize from the melt or solution, the crystalline regions may exhibit various types of polymorphic modification, depending on the cooling rate, evaporation rate of solvent, temperature and other conditions. These crystal modifications differ in their molecular and crystal structures as well as in their physical properties. Many types of crystalline modifications have been reported (Tashiro Tadokoro, 1987). (See also Chapter 4.) 

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. 

Two crystalline phases of PBT can appear under tension and relaxation the so called and a forms (Hall Pass, 1976) (see Fig. 6.1). The main difference in the molecular structure between the a and the forms lies in the methylene conformation  

X-ray diffraction patterns and infrared Raman spectra show specific changes through this a transition (Tashiro Tadokoro, 1987 Jakeways et al, 1976 Ward Wilding, 1977). The stress and strain dependence of the molar fraction of the form, Xp, can be evaluated by quantitative analysis of the infrared spectra. The characteristic behaviour of this phase transition observed in a uniaxially oriented sample is as follows Xp increases drastically above the critical stress / Xp is almost linearly proportional to the strain the transition is reversible and the 

It has been also pointed out that in block copolymers of PBT and PTMO, the transition is smeared over a wide range of stresses due to the effect of the PTMO soft segments (Tashiro et ai, 1986). 

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