Polymorphic transition stress-induced

Polymorphic transitions are known to be induced by an external field such as tensile stress, hydrostatic pressure or shearing stress, high electric field and others. Crystalline phase transitions under tensile stress are discussed in Chapter 6 in relation with the observed changes in microhardness. 

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. 

Stress-induced polymorphic transition in homopolymers, copolymers and blends... 

Effect of stress-induced polymorphic transition of PBT on microhardness... 

Thus, it seems to be of interest to examine the influence of stress-induced polymorphic changes on the microhardness. While in the case of f-PP two samples comprising the a or phase were characterized, here we wish to follow the microhardness behaviour during the a-j6 polymorphic transition caused by a mechanical field. For this purpose PBT has been selected as a suitable material because of its ability to undergo stress-induced polymorphic transition from the a (relaxed) to the P (strained) form. Bristles of commercial PBT with a diameter of about 1 mm were drawn at room temperature via neck formation (final diameter about 0.5 mm and draw ratio of 3.4) and thereafter annealed in vacuum at 200°C for 6 h with fixed ends (Fakirov etal., 1998). 

Although the stress-induced a-j6 polymorphic transition in PBT is well documented, comparative WAXS measurements of the samples were carried out in the same deformation range at which the H measurement are performed. In addition, the size of the coherently diffracting domains (crystal size) D/,ki in the (100) and (010) and (104) ( c axis direction) during stretching was calculated from the integral breadth of the equatorial reflections according to 1 /Dhki (Baltd Calleja Vonk, 1989). 

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... 

The strong increase in H obtained after completing the stress-induced polymorphic transition (from 118 to 150 MPa) can be explained by the additional chain... 

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. 

Let us now discuss the microhardness of PBT block copolymers during the stress-induced polymorphic transition for some block copolymers this transition is rather smeared (Tashiro et al, 1986). An additional reason for performing this work is the fact that the copolymers of PBT with PEO had not then been studied with reference to polymorphic transitions. The very detailed structural characterization of the copolymers with PEO as mentioned above was expected to shed more light on the nature of the microhardness behaviour. 

The most striking change in the H behaviour is the sharp decrease at e = 25-27%. Taking into account the similar study on homo-PBT (Fig. 6.2) where a similar change was observed (a drop in H by 20%) due to the well documented stress-induced polymorphic transition in PBT, one can assume that in the present case the same transition also takes place. 

This means that during the first H decrease (see Fig. 6.5), PEO is in a molten state. It is worth mentioning that for the same reason the observed stress-induced polymorphic transition cannot arise from PEO, though it is known that PEO is capable of undergoing such a transition (Takahashi etal, 1973 Tashiro Tadokoro, 1978). 

If we assume for He the value of 122 MPa which was described in the previous section on the stress-induced polymorphic transition in homo-PBT, then the calculations of H, before the stress-induced polymorphic transition (ff = 34 MPa) and after this transition (// = 24 MPa) would lead to negative values of H °f of —112 and -70 MPa, respectively, which are of course not physically acceptable. 

After following the microhardness behaviour during the stress-induced polymorphic transition of homo-PBT and its multiblock copolymers attention is now focused on the deformation behaviour of a blend of PBT and a PEE thermoplastic elastomer, the latter being a copolymer of PBT and PEO. This system is attractive not only because the two polymers have the same crystallizable component but also because the copolymer, being an elastomer, strongly affects the mechanical properties of the blend. It should be mentioned that these blends have been well characterized by differential scanning calorimetry, SAXS, dynamic mechanical thermal analysis and static mechanical measurements (Apostolov et al, 1994). 

The result shown in Fig. 6.8 that the two species of crystallites respond to the mechanical field in sequence - first the homo-PBT crystallites and later those arising from PEE, means that the homo-PBT crystals are probably dispersed within PEE in such way that they experience the mechanical field from the very beginning of loading. Moreover, one can assume that in the blend some internal stress and/or strain pre-exists since the strain-induced polymorphic transition starts even at lower... 

As mentioned above, PEE, is well characterized mainly by SAXS in the deformation regime under consideration (Fakirov et al, 1991, 1992, 1993, 1994 Stribeck et al., 1997). In addition to these morphological investigations a study of the strain-induced polymorphic transition in PEE using microhardness measurement will shed additional light on the stress- and strain-induced structural reorganization of this new class of polymeric materials. 

The occurrence of these two groups of values (below and above 5% residual deformation) can be explained by the strain-induced a p polymorphic transition in PBT. As stressed above, it is well known (Yokouchi et ai, 1976) that up to 5% deformation the a polymorphic modification characterized by higher microhardness // , dominates in the samples. Furthermore, for 12-15% deformation (for homo-PBT), the a p transition is essentially completed (see Fig. 6.11(a)) and the samples show predominantly the polymorphic modification, which has a lower microhardness < // . However, after removal of the load (a = 0) the samples contract (e.g. after a deformation of e = 5-10%, the plastic deformation is around 1% and after s = 15-20% the plastic deformation is around 3%). In all these cases the plastic... 

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... 

It was pointed out in Chapter 6 that the polymorphic transitions from one crystalline form to another can be induced by the application of an external stress on an axially oriented crystalline system. Anisotropic dimensional changes usually accompany the transformation. Typical examples are the classical a-p transition of the keratins,(21) and the crystal-crystal transition in poly(l,4-trans-butadienes).(22) The dimensional changes in these cases reflect the different axial or fiber repeat distances of the two polymorphs. The dimensional change would be expected to be... 

Apostolov A A, Boneva D, Balta Calleja F J, Krumova M and Fakirov S (1998) Microhardness under strain. III. Microhardness behavior during stress-induced polymorphic transition in blends of poly(butylene terephthalate) and its block copolymers, J Macromol Sci Phys B37 543-555. 

Strain-induced polymorphic transition in neat PBT and neat PEE as revealed by microhardness measurements in real time. Usually, the polymorphic modifications were characterized by microhardness, WAXS, or other techniques after the completion of the polymorphic transition, irrespective of the factor inducing the latter. In the present subsection, an attempt is made to characterize the respective polymorphic phases during the transition itself, i. e., the microhardness measurements have to be performed during stress-induced polymorphic transitions, rather than post mortem as, for example, in many other cases [68]. 

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