Polymers isothermal crystallization

Thermodynamic Properties. The thermodynamic melting point for pure crystalline isotactic polypropylene obtained by the extrapolation of melting data for isothermally crystallized polymer is 185°C (35). Under normal thermal analysis conditions, commercial homopolymers have melting points in the range of 160—165°C. The heat of fusion of isotactic polypropylene has been reported as 88 J/g (21 cal/g) (36). The value of 165 18 J/g has been reported for a 100% crystalline sample (37). Heats of crystallization have been determined to be in the range of 87—92 J/g (38). 

Other crystallization parameters have been determined for some of the polymers. The dependence of the melting temperature on the crystallization temperature for the orthorhombic form of POX (T = 323K) and both monoclinic (T = 348K) and orthorhombic (T = 329K) modifications of PDMOX has been determined (284). The enthalpy of fusion, Aff, for the same polymers has been determined by the polymer diluent method and by calorimetry at different levels of crystallinity (284). for POX was found to be 150.9 J/g (36.1 cal/g) for the dimethyl derivative, it ranged from 85.6 to 107.0 J/g (20.5—25.6 cal/g). Numerous crystal stmcture studies have been made (285—292). Isothermal crystallization rates of POX from the melt have been determined from 19 to —50 C (293,294). Similar studies have been made for PDMOX from 22 to 44°C (295,296). 

The isothermally crystallized monomers were polymerized with 2 4 Mrad Y-ittadiation. The polymer film was removed from the substrate after C, Pt coating by dissolving the salt. A JEOL JEM 100B electron microscope was used to examine the samples. 

The overall isothermal crystallization kinetics of polymers can be described by the Avrami equation [88-90] ... 

Jabarin, S. A., Crystallization kinetics of polyethylene terephthalate. I. Isothermal crystallization from the melt,. /. Appl. Polym. Sci., 34, 85-96 (1987). 

Verhoyen, 0., Dupret, F. and Legras, R., Isothermal and non-isothermal crystallization kinetics of polyethylene terephthalate mathematical modeling and experimental measurement, Polym. Eng. Sci., 38, 1592-1610 (1998). 

F. J. Medellin-Rodriguez, C. Burguer, B. S. Flsiao, B. Chu, R. Vaia, S. Phillips, Time-resolved shear behavior of end tethered nylon 6-clay nanocomposites followed by non-isothermal crystallization, Polymer, vol. 42, pp. 9015-2023, 2001. 

L. Zhang, T. Tao, C. Li, Formation of polymer/carbon nanotubes nano-hybrid shish-kebab via non-isothermal crystallization, Polymer, vol. 50, pp. 3835-3840, 2009. 

Fig. 5.38 Normalized intensities obtained from SAXS data for the isothermal crystallization of a PCL-PB diblock (Mw = 12.5 kg mol 45% PCL) following a quench from the homogeneous melt to 26.5 °C (Tom is close to Tm for this polymer) (Nojima et al. 1992a) ( ) melt peak (o) crystal peak. The peak position was found not to change on crystallization.

Modelling non-isothermal crystallization is the next important step in a quantitative description of reactive processing. This is particularly important, because crystallization determines the properties of the end product. Therefore, the development the spatial distribution of crystallinity, a, and temperature, T, with time throughout the volume of the reactive medium must be calculated. It is also noteworthy that crystallization and polymerization processes may occur simultaneously. This happens when polymerization proceeds at temperatures below the melting point of the newly formed polymer. A typical example of this phenomenon is anionic-activated polymerization of e-caprolactam, which takes place below the melting temperature of polycaproamide. 

K. Nakamura, K. Katayama, and T. Amano, Some Aspects of Non-isothermal Crystallization of Polymers. Part II. Consideration of Isokinetic Conditions, J. Appl. Polym. Sci., 17, 1031-1041 (1982). 

The stereosequence length also has a marked effect on the isothermal crystallization kinetics of the propylene oxide polymers. These studies and analysis of results on crystallization kinetics will be described in detail in another communication. Here we summarize briefly the main conclusions of the effect of stereosequence length on the isothermal crystallization rates. 

Figure 20.2 sPS/PPE blends (75 25 wt%) at different isothermal crystallization temperatures. (a) DSC thermograms (b) WAXD patterns. Reprinted from Polymer, vol. 39, Hong B. K., Jo W. H., Lee S. C., Kim J., Correlation between melting behavior and polymorphism of sPS and its blends with PPE , p. 1793, Copyright 1998, with permission from Elsevier Science... 

Fig. 15.6 Raman spectra recorded along the diameter of an iPP spherulite during isothermal crystallization from the melt at Tj, = 130°C. The variation in Raman bands at 809 cm and 841 cm indicating the amorphous and crystalline content of iPP are matched -with their positions on the polymer (Reprinted from [38])...

Tkeda, T., Horiuchi, S., Karanjit, D. B., Kurihara, S., and Tazuke, S. Photochcmically induced isothermal phase transition in polymer liquid crystals with mesogenic phenyl K nzoate side chains. 1. Calorimetric studies and order parameters. Macromolecules 23. 16 (1990). 

---