Polymorphic transition strain-induced

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). 

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. 

The study of the strain-induced polymorphic transitions by microhardness measurement offers the opportunity to gain additional information on the deformation behaviour of more complex polymer systems such as polymer blends. Since polymer blends are usually multicomponent and multiphase systems the question arises of how the independent components and phases react under the external load. The polymorphic transition will reflect the behaviour of the crystalline phases provided strain-induced polymorphic transition is possible. 

Comparison of the curves presented in Fig. 6.9 for homo-PBT (a), the blend (b) and the copolymer of PBT with PEG (c) allows one to draw the following conclusions the initial drop in H at around e = 5% (Fig. 6.8) originates from the strain-induced polymorphic transition in the crystallites comprising only homo-PBT segments. The second change in H at around s = 25-30% is related to the strain-induced polymorphic transition in the PBT crystallites comprising PBT segments from its multiblock copolymer PEE. 

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... 

Thus, the observation of two sharp well defined, and clearly separated on the deformation scale, strain-induced polymorphic transitions convincingly demonstrates that the two populations of PBT crystallites of differing origin, undergo the mechanical loading not simultaneously but in two steps first those comprising homo-PBT (at s = 2-3%), followed by crystallites belonging to PEE (at s = 25%). 

In summarizing the results from the last three sections, one can conclude that the systematic variation of microhardness under strain performed on (a) homo-PBT (Section 6.2.1), (b) its multiblock copolymer PEE (Section 6.2.2) and (c) on blends of both of these (this section) is characterized by the ability of these systems to undergo a strain-induced polymorphic transition. The ability to accurately follow the strain-induced polymorphic transition even in complex systems such as polymer blends allows one also to draw conclusions about such basic phenomena as cocrystallization. In the present study of a PBT/PEE blend two distinct well separated (with respect to the deformation range) strain-induced polymorphic transitions arising from the two species of PBT crystallites are observed. From this observation it is concluded that (i) homo-PBT and the PBT segments from the PEE copolymer crystallize separately, i.e. no cocrystallization takes place, and (ii) the two types of crystallites are not subjected to the external load simultaneously but in a sequential manner. 

From the microhardness behaviour during the strain-induced polymorphic transition of PBT, differences were found for the three above described systems. The common characteristic feature among PBT (Section 6.2.1), its copolymer PEE (Section 6.2.2) and the PBT/PEE blend (Section 6.2.3) is the relatively sharp (within 2-4% of deformation) stepwise decrease in H (typically by 20-30% of starting H value). This drop appears at different deformation intervals for PBT the sharp decrease occurs between 5-8% (Fig. 6.2), for PEE it appears between 25-30% (Fig. 6.5), while for the blends one observes two sharp decreases with increasing strain (Fig. 6.8). The first 20% decrease in the starting H value coincides with the deformation interval... 

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 results shown in Fig. 6.11 not only support the reversibility of the strain-induced polymorphic transition (Boyle Overton, 1974) but also allow one to speak about reversibility of microhardness. This is feasible in cases in which, as a result of some treatment, it is possible to regenerate the starting structure of the polymer. Reversibility of the microhardness further emphasizes that this mechanical property depends primarily on the structure of material. 

In summary, it can be concluded that the microhardness technique is sensitive enough to detect strain-induced polymorphic transitions in polymers. The results in this chapter reveal that in materials characterized by a high and reversible deformation ability it is possible to observe reversible microhardness provided the strain-induced structural changes are reversible too. 

It is to be noted that is intimately related to the packing of the chains in the crystals (4). Since the crystal hardness reflects the response of the inter-molecular forces holding the chains within the lattice, it has been shown that the microhardness technique permits to distinguish between polymorphic modifications of the same polymer (20,21). Indeed, the study of the transition from the a to the form in iPP confirmed that changes in H were directly related to the different crystal hardness values of each phase (20). More recently, the microhardness technique has been successfully applied to follow the reversible strain-induced poljmiorphic a p transition occurring on PBT (21). 

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]. 

In conclusion, the presented results show that microindentation hardness allows one to follow the strain-induced polymorphic transition in PBT in real time [73]. The observed rather abrupt change in the H values (within 2-4% external relative deformation) makes the method competitive, with respect to sensitivity, with other commonly used techniques, such as WAXS, IR, or Raman spectroscopy, and others [42,43,54-58,61]. What is more, the same sensitivity of the technique has been observed when applied to strain-induced polymorphic transitions in multiblock copolymers of PBT. 

Strain-induced two-stage polymorphic transition in blends of PBT and PEE as revealed by microhardness measurements in real time. The results of the polymorphic transition obtained in real time on a blend of PBT and a PEE thermoplastic elastomer, the latter being a copolymer of PBT and PEG, are considered here. It should be mentioned that these blends are well characterized by DSC, SAXS, DMTA, and static mechanical measurements [28,78-81]. 

From the microhardness data presented in Figure 11, one could suggest that, in the present case, one deals with the third case for the following reasons. If PBT crystallites resulted from complete cocrystallization, one would expect a single strain-induced polymorphic transition in the entire deformation range. In case of partial cocrystallization, one would observe a more or less continuous polymorphic transition between the deformation ranges typical of neat PBT and of PEE. This interpretation was adopted in [30] and [69], based exclusively on the assumption of a continuous polymorphic transition, which actually is not observed (Figure 11). 

Fakirov S and Balta Galleja F J (2002) Strain-induced polymorphic transition in poly (butylene terephthalate), its copolymers and blends, in Handbook of Thermoplastic Polyesters (Ed. Fakirov S) Wiley-VCH, Weinheim, Ch. 21, pp. 927-964. 

If we take a value of Ha — 30 MPa (Martmez-Salazar et al, 1988) for the amorphous phase of i-PP and solve eqs. (4.15) and (4.16), values of // = 143 MPa and He = 119 MPa are obtained. The former value of 143 MPa fits well with the ab initio calculation for the a phase (Balta Calleja et al, 1988). In conclusion, the determination of microhardness is shown to be a technique capable of detecting polymorphic changes in polymers. Further examples of polymorphic crystal-crystal transitions induced by external field (stress or strain) are given in Chapter 6. 

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