Self-nucleation temperature

Fig. 18 Peak crystallization temperature as a function of self-nucleation temperature for hydrogenated and nonhydrogenated S27B15C58 triblock copolymers. (Reprinted with permission from [97]. Copyright 1998 American Chemical Society)...

For 11U4,13 thermal cycles were applied starting from the ideal self-nucleation temperature, that is, 124 C. Therefore, 12 sharp meeting peaks are appreciated in Figure 5.8b corresponding to those Tj temperatures that produced annealing and thermal fractions. 

The SSA technique is now being used rather frequently for all the applications quoted above. Unfortunately, in many recent references, the authors do not perform a previous self-nucleation study to determine the ideal self-nucleation temperature. Therefore, they start their SSA protocol at an arbitrary value that will have an important impact on the fractionation profile. Any attempt to quantify the data and calculate SCB or MSL distributions will not be correct unless the first value employed to perform SSA is the ideal self-nucleation temperature. 

DSC isothermal scans were also recorded in order to study the crystallization kinetics of PPDX after self-nucleating the sample at 117 °C. This self-nucleation temperature was determined by analyzing the self-nucleation domains and choosing an intermediate temperature within Domain II (more on the self-nucleation protocol can be found in Ref [52,60,91]). 

The ideal self-nucleation temperature is probably the best possible choice, because the use of the ideal self-nucleation temperature will provide the polymer with an extremely high nucleation density (typically on the order of 10 nuclei/cm ), which can produce a full completion of the nucleation step before any isothermal crystallization is performed. This novel treatment could be used to study the relative contributions of nucleation and growth in semicrystalline homopolymers and semicrystalline components within diblock copolymers or polyblends. 

Figure 1 shows the DSC cooling scan of iPP in the bulk after self-nucleation at a self-seeding temperature Ts of 162 °C (in domain II). The self-nucleation process provides a dramatic increase in the number of nuclei, such that bulk iPP now crystallizes at 136.2 °C after the self-nucleation process this means with an increase of 28 °C in its peak crystallization temperature. In order to produce an equivalent self-nucleation of the iPP component in the 80/20 PS/iPP blend a Ts of 161 °C had to be employed. After the treatment at Ts, the cooling from Ts shows clearly in Fig. 1 that almost every iPP droplet can now crystallize at much higher temperatures, i.e., at 134.5 °C. Even though the fractionated crystallization has disappeared after self-nucleation, it should also be noted that the crystallization temperature in the blend case is nearly 2 °C lower than when the iPP is in the bulk this indicates that when the polymer is in droplets the process of self-nucleation is slightly more difficult than when it is in the bulk. In the case of block copolymers when the crystallization is confined in nanoscopic spheres or cylinders it will be shown that self-nucleation is so difficult that domain II disappears. 

Chen et al. [92] also performed self-nucleation experiments by DSC in PB-fr-PEO diblock copolymers and PB/PB-b-PEO blends. The cooling scans presented in their work showed that a classical self-nucleation behavior was obtained for PEO homopolymer and for the PB/PB-b-PEO blend where the weight fraction of PEO was 0.64 and the morphology was lamellar in the melt. For PB/PB-fr-PEO blends with cylinder or sphere morphology, the crystallization temperature remained nearly constant for several self-seeding temperatures evaluated. This observation indicates that domain II or the self-nucleation domain was not observable for these systems, as expected in view of the general trend outlined earlier. 

Bergen and Borisy (1980) used the axoneme-hased approach to explore the commitment to treadmilling, and they also found that the efficiency was quite low. An 5 value of 0.07 0.04 was obtained, corresponding to the net addition of 1.6 0.8 tubulin protomers/second. Interestingly, their estimates of the dissociation constants for the two ends were 2.2 0.6 and 3.2 0.6 iiM for the assembly and disassembly ends, respectively. We calculate that this corresponds to only about 0.2 kcal/mol. In a more recent investigation from the same laboratory (Cote and Borisy, 1981), porcine tubulin was found to exhibit an s value of about 0.26, and the rate of the tubulin flux was about 28 protomers/ second. The authors of the latter study suggested that the discrepancy might be accounted for in terms of the need to use MAP-depleted tubulin with the axoneme system to prevent self-nucleation, or in terms of the temperature differences in the two studies. 

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