Crystaf

When crystaf structure is involved, it gives rise to special features in the reactions and makes their mechanisms more complex. Therefore, at first we consider... 

The metal surface is polycrystalline and has a rather complex profile. Because of different crystallite orientations at the surface, different crystaf faces are exposed, such as smooth fow-index faces and stepped high-index faces. Surface texture where a particufar kind of face is predominant can devefop in individual cases. Microcracks and various lattice defects (dislocations, etc.) will also emerge at the surface. 

Rytter et al. reported polymerizations with the dual precatalyst system 14/15 in presence of MAO [30]. Under ethylene-hexene copolymerization conditions, 14/MAO produced a polymer with 0.7 mol% hexene, while the 15/MAO gave a copolymer with ca. 5 mol% hexene. In the mixed catalyst system, the activity and comonomer incorporation were approximate averages of what would be expected for the two catalysts. Using crystallization analysis fractionation (CRYSTAF) and differential scanning calorimetry (DSC) analysis, it was concluded in a later paper by Rytter that the material was a blend containing no block copolymer [31],... 

K. Dokko, M. Nishizawa, M. Mohamedi, M. Umeda, L. Uchida, J. Akimoto, Y. Takahashi, Y. Gotoh, S. Mizuta, Electrochem. and Sohd State Lett Electrochemical studies of Li-ion extraction and insertion of LiMn204 single crystaf ... 

Soares JBP, Abbott RF, Kim JD (2000) Environmental stress cracking resistance of polyethylene the use of CRYSTAF and SEC to establish structure-property relationships. J Polym Sci Part B Polym Phys 38(10) 1267-1275... 

In addition to the above, the intermolecular co-monomer distribution, that is the extent of the co-monomer s incorporation in different molecular mass fractions of the polymer, is an important aspect. When feasible, this is evaluated by fractionating the polymer and measuring the co-monomer content in each fraction, by solubility (TREF),511 crystallizability (CRYSTAF),512,513 or by GPC-IR.514 Whereas ZN catalysts tend to produce LLDPE co-polymers of broad co-monomer distribution (with more co-monomer incorporated in the lower molecular mass fractions), single-center catalysts always have a nearly perfect intermolecular co-monomer distribution, and this aspect is taken as implicit in most co-polymerization studies with the latter systems. 

Oxo-A4-androsten-3p,6p-diol-3-acetate 2.8 A solution of 17-oxo-A5-androsten-3p-ol acetate 1 (1,03 g, 31 mmol) and mercury(ll) trifluoro acetate (3,1 g, 72 mmol) in dichloromethane (100 mL) were stirred for 24 h at 20°C. Part of the solvent (66mL) was evaporated in vacuum and the reaction mixture was filtered over glass fiber filter paper. The filtrate was washed with 5% Na2C03 aqueous solution, water and again filtered. After evaporation of the solvent the residue (720 mg) was dissolved in MeOH. After crystafization there was obtained a first crop of 411 mg of 2 (40%), mp 148-150°C [a]D2S= +25° (CHCI3). 

Results and Discussion The differences between the actuaf and theoreti-caffy cafcufated mofar enthalpies are seen to be insignificant. The largest discrepancy, 9-12 kJ moP (about 4%), is observed for ZnS and ZnSe. This is comparable with the scatter of the experimental data given for the same compounds (CdS and CdSe) in different papers. Therefore, there are no grounds to attribute the observed discrepancies to deviation of the product composition from the equifibrium composition. This conclusion is, on the whole, consistent with the general features of the effect of the crystaf structure on the decomposition products (see Chapter 9). 

Fig. 3 (a) Various tacticity configurations in PP. (b) Crystallization temperatures in solution (CRYSTAF) of the three tacticity forms of PP... 

In experimental practice, a straight-line correlation between temperature and comonomer composition has been obtained by various authors with TREF [61,62], DSC [63], and CRYSTAF [64]. These correlations are practically independent of molar mass. 

Crystallization analysis fractionation (CRYSTAF) was developed by Monrabal [99] in 1991 as a process to speed up the analysis of the CCD, which at that time lasted around 1 week per sample with the TREF technique. CRYSTAF shares with TREF the same principle of separation according to crystallizability. In CRYSTAF, the samples are not crystallized in a column but in a stirred vessel with no support, and only a temperature cycle (crystallization) is required [64], thus speeding up the analysis process and simplifying the hardware requirements. 

Fig. 21 CRYSTAF sampling process in a vessel with an internal filter T temperature,...

In CRYSTAF, the analytical process is followed by monitoring the polymer solution concentration during crystallization by temperature reduction. Aliquots of the solution are filtered (through an internal filter inside the vessel) and analyzed by a concentration detector at different temperatures, as shown in Fig. 21. The whole process is similar to a classical stepwise fractionation by precipitation with the exception that, in this new approach, no attention is paid to the polymer being precipitated but to the one that remains in solution. 

Fig. 22 CRYSTAF cumulative data points circles) and the first derivative CCD curve...

Fig. 24 Simultaneous CRYSTAF analysis of five LLDPE resins of different density (cumulative curves). 40 mg/40 mL in TCB crystallization rate 0.2°C/min...

The same solvents, same IR detector and similar calculation parameters to those presented in Sect. 4.1.2 for TREF are applicable for CRYSTAF analysis. The calibration of temperature to comonomer content can be performed by using narrow composition standards (metallocene-type resins) of the same comonomer type, with similar results to TRFF as discussed in Sect. 4.1.2. Octene and hexene copolymers follow the same calibration curve [92]. 

Reviews of the CRYSTAF technique and applications have been presented [80, 83, 84]. Mathematical modeling of CRYSTAF crystallization kinetics has also been investigated [100]. 

Both techniques share the same principles of fractionation on the basis of crystallizability. TRFF is carried out in a packed column and demands two full temperature cycles, crystallization and elution (dissolution), to obtain the analysis of the composition distribution. In CRYSTAF, the analysis is performed in a single step, the crystallization cycle, which results in faster analysis time and simple hardware requirements. 

TRFF has the advantage that a continuous elution signal is obtained and molar mass detectors can be easily added to obtain composition molar mass interdependence an autosampler can also be added for multiple sample analysis. CRYSTAF takes advantage of discontinuous sampling to analyze a set of samples simultaneously. 

Both techniques provide similar results the comparison of TREF and CRYSTAF has aheady been discussed [84] and the most significant difference is the temperature shift due to the undercooling, as analytical conditions are far from equilibrium CRYSTAF data are obtained during the crystallization whereas TREF data are obtained in the melting-dissolution cycle. Both techniques, however, can be calibrated and the results expressed in branches/lOOOC will be similar for PE copolymers. 

Fig. 25 TREF and CRYSTAF analysis of PE-PP combinations. TREE separates iPP + PE better and CRYSTAF separates EP + PE better...

Crystallization elution fractionation (CEF) is a new separation technique developed by Moiuabal [102] for the analysis of the CCD that combines the separation power of CRYSTAF and TREF. The CEF technique is based on a new and patented separation principle, referred to as dynamic crystallization (DC) [87], that separates fractions inside a column according to crystallizabUity while a small flow of solvent passes through the column. The separation by DC occurs during the crystallization step. CEF combines the separation power of DC in the crystallization step with the separation during dissolution of the TREF technique. 

Other components of lower crystallinity, still in solution, move along the column until they reach their own crystallization temperature. At the end of the crystallization cycle, the three composition families shown in Fig. 26b are physically separated inside the column according to crystallizability this process is referred to as dynamic crystallization and can separate components in a similar fashion as CRYSTAF although all polymer molecules still remain inside the column in three different locations. 

Once the DC separation step has been completed, it is easy to realize the possibility to combine it with a final elution cycle as in TREF to obtain a new extended separation as shown in Fig. 26b by the improved separation of the three components at the exit of the column in CEF analysis as compared to the TREF approach. It is quite interesting that the separation power of CRYSTAF and TREF are combined in CEF when both systems are based on the same crystallizability principles. To obtain the maximum benefit of the DC process, the colunm volume must be large enough and the flow rate in the crystallization (FC) has to be adapted to the crystallization rate (CR), crystallization temperature range (ATc) of the components to be separated, and colunm volume (V) as described by (8) ... 

Bosch JV, Ortiri A, Momabal B (1998) Development of a highly stable multiple wavelength IR detector fon on-line GPC, CRYSTAF and TREF analysis. In Proceedings International GPC Symposium, Arizona, OctobCT 1998, 633-640... 

Monrabal B (2004) Microstructure Characterization of Polyolefins. TREF and CRYSTAF. In Proceedings of the 17th International symposium on polymer analysis and characterization (ISPAC), Heidelbergh, 6-9 June 2004... 

Bec Becker, G., Braun, D. Kunststoff Handbuch 4, Polystyrol. Carl Hanser Verlag, Munich, 1996. OOSoa Soares, J. B. P., Abbott, R. F., Kim, J. D. Environmental stress cracking resistance of polyethylene The use of CRYSTAF and SEC to establish structure-property relationships. J. Polym. Sci. Part B-Polym. Phys. 38 (20(X)) 1267-1275. 

A% of D2014)take due to Ii9and bindng mapped onto Crystaf lucture ... 

Important data on physical architecture of crystallable polymers can be obtained from temperature rising elution fractionation, TREF and crystallization fractionation, CRYSTAF. 

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