Melt crystallization temperature profiles

Fig. 16. Schematic of a typical temperature profile in a crystallization experiment and the resulting evolution of the fraction of crystalline polymer and dynamic moduli with time. The preheating temperature Tp is above the melting temperature Tm...

A sample of the polymer to be studied and an inert reference material are heated and cooled in an inert environment (nitrogen) according to a defined schedule of temperatures (scanning or isothermal). The heat-flow measurements allow the determination of the temperature profile of the polymer, including melting, crystallization and glass transition temperatures, heat (enthalpy) of fusion and crystallization. DSC can also evaluate thermal stability, heat capacity, specific heat, crosslinking and reaction kinetics. 

Rg.7 X-ray diffraction profiles of two MBBE-6 samples prepared by the melt-crystallization and solution-precipitation processes. The two diagrams became nearly identical for anneahng at a temperature 10 degrees below the melting point (379.8 K)... 

The CCD is the second most important microstructural distribution in polyolefins. Differently from the MWD, the CCD carmot be determined directly only the distribution of crystallization temperatures (CTD) in solution can be measured and one can try to relate this distribution to the CCD using a calibration curve. Two techniques are commonly used to determine the CTD or CCD of polyolefins TREF and Crystaf. Both operate based on the same principle chains with more defects (more comonomer molecules or stereo-and/or regioirregularities) have lower crystallization temperatures than chains with fewer defects. Figure 2.11 compares the TREF and Crystaf profiles of an ethylene/1-butene copolymer made with a heterogeneous Ziegler-Natta catalyst. Notice that they have very similar shapes the Crystaf curve is shifted toward lower temperatures because it is measured as the polymer chains crystallize, while the TREF curve is determined as the polymer chains dissolve (melt) and are eluted from the TREF column, as explained in the next few paragraphs. 

The conditions under which the different profiles of dominant lamellae occur are as follows (59). At crystallization temperatures > 127°C, melt-crystallized lamellae of linear pol5uner have 201 fold surfaces, the same basal surface typical of n-alkanes, allowing folds more surface area in a subcell with a comparatively small cross-sectional area per fold stem. Their profile viewed down b, the growth direction, has lamellar normals inclined at 35° to c, indicating 201 surfaces. For higher mass materials, lamellae are planar, but for molecular masses of 30,000, 201 inclinations alternate in ridged lamellae (Fig. 3a). At immediately lower temperatures, the corresponding profile of dominant lamellae differs it is then... 

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