Isothermal crystallization of polymers

DSC provides a rapid and sensitive evaluation of isothermal crystallization behavior. The sample of interest is annealed at a sufficiently high temperature for a period long enough to remove the previous thermal history. The sample is then rapidly cooled to the crystallization temperature, and the time period of exothermic crystallization is measured. 

Crystallization behavior is affected by the temperature of crystallization, the nucleation properties of additives (such as dyes), the relative molecular mass distribution and co-existing monomers, etc. It is important to investigate the crystallization behavior of the polymers, since the properties of polymeric products, such as appearance, dimensional stability and thermal stability, are influenced by crystallization processes. 

In the temperature range 118.0 121.5 C, the crystallization rate increases and the final 

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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). 

Abstract A number of theories for predicting non-isothermal crystallization of polymers have been proposed (1-5). Some are based on Avrami solution (2,3), others were derived independently (1,4,5). All are based on an "additivity" principle which states that the rate of crystallization at a time, t, depends on the extent of crystallization at time t, but not on the previous history (i.e., it is path independent). In this paper we present experimental results which show that, within a certain regime of cooling rates, the additivity principle applies to isotactic polystyrene. In a previous publication we have shown the same to be true for polyethylene (6). 

Di Lorenzo ML, Sdvestre C (1999) Non-isothermal crystallization of polymers. Prog Polym Sd 24 917-950... 

Isothermal crystallization of polymers is normally performed by cooling the sample at the fastest cooling rate possible from an (assumed) isotropic melt to a predetermined temperature where the crystallization process is monitored as a function of time. Coefficients similar to the reaction rate kinetic parameters have been developed to describe the isothermal crystallization of polymers based on Avrami s equation ... 

The kinetics of the isothermal crystallization of polymers have been thoroughly studied. However, practical processes such as extrusion, molding, injection and film production usually proceed under non-isothermal crystallization conditions. In order to obtain products with better properties, it is necessary to have quantitative evaluations of the non-isothermal crystallization process. A few methods have been developed to study the kinetics of non-isothermal crystallization by DSC for polymers. 

The lateral and fold surface free energies of the developing crystals during the isothermal crystallization of polymers, a and can be measured by DSC and/or polarizing optical microscopy according to the equation... 

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.

Figure 5.15 Model of structural evolution process in the isothermal crystallization of polyethylene chains. The random chain segments change to the locally regularized trans-form. The stacked lamellar structure is formed with the long period 800 A. The structure changes further to the stacked lamellar structure of 400 A period by inserting the new lamella into the original lamellae. It is noted that the radius of gyration is kept unchanged during this process. (From Reference 46 with permission from the Society of Polymer Science, Japan.)...

TAN 07] Tanaka T., Yabe T., Termachi S., et al, Mechanical properties and enzymatic degradation of poly[(R)-3-hydroxybutyrate] fibers stretched after isothermal crystallization neaxJg , Polymer Degradation and Stability, vol. 92, no. 6, 1016-1024, 2007. 

Alvarez, V. A., Kenny, J. M., and Vazquez, A., Isothermal crystallization of poly(vinyl alcohol-co-ethylene), J. Appl. Polymer Sci., 89, 1071-1077 (2003). 

In the isothermal crystallization of polypropylene montmorillonite (PP-MMT) nanocomposites, it was found that the crystallinity of the composites decreased with increasing montmorillonite content, indicating the dispersion of MMT layers in the polypropylene matrices. The nanoparticles confined the polypropylene chains and hindered the polymer crystallization. The spherulites of the PP-MMT nanocomposites were greatly decreased in size as MMT was introduced. On the other hand, the crystallization rate increased dramatically with the increasing of MMT content. The interfacial free-energy per unit area perpendicular... 

H. Shen, B. Xie, W. Yang, M. Yang, Non-isothermal crystallization of polyethylene blends with bimodal molecular weight distribution. Polym. Testing 32, 1385-1391 (2013)... 

Zang, Z. Kitano, T. Hatakeyama, T. (1995). Crystallization Behavior of Carbon Fiber Reinforced Polyamides 1. Dynamic and Isothermal Crystallization. International Polymer Processing, Vol.lO, No.2, pp. 165-171 ISSN 0930-777X... 

The crystallization of polymer in bulk as well as in solution is initiated by nucle-ation followed by growing of spheralites (Mandelkem, 2002). A common fundamental approach to study isothermal crystallization kinetics is the heuristic Avratni phase transition theory (Avrami, 1939 1940 1941). 

K " and n can be extracted from the intercept and the slope of Avrami plot, lg[-ln(l-.A0] versus lg(f-f ), respectively. The prime requirement of Avrami model is the ability of spherulites of a polymer to grow in a free space. Besides, Avrami equation is usually only valid at low degree of conversion, where impingement of polymer spherulites is yet to take place. The rate of crystallization of polymer can also be characterized by reciprocal half-time (/ 5). The use of Avrami model permits the understanding on the kinetics of isothermal crystallization as well as non-isothermal crystallizatioa However, in this chapter the discussion of the kinetics of crystallization is limited to isothermal conditions. 

Basic principle underlying the isothermal crystallization of PHAs will be emphasized in this chapter. Crystallization of polymer melts is often accompanied by a heat release in the system. This can be measured using differential scanning calorimeter (DSC). Following the isothermal crystallization of PHAs, the influence of thermal treatment of a semicrystalline polymer on the mechanical properties is also given. 

The overall rate of isothermal crystallization of PTT (semicrystalline polymer) can be monitored by thermal analysis through the evolution of heat of crystallization by DSC as depicted in Figure 10. The sample is isothermally crystallized at preselected crystallization temperature (T) until complete crystallization. Half time of crystallization for the polymer is estimated from the area of the exotherm at r = const, where it is the time taken for 50% of the crystallinity of the crystallizable component to develop. The rate of crystallization of PTT can be easily characterized by the experimentally determined reciprocal half time, (tg j) . ... 

Hu WB (2005) Molecular segregation in polymer melt crystallization simulation evidence and unified-scheme interpretation. Macromolecules 38 8712-8718 Hu WB, Cai T (2008) Regime transitions of polymer crystal growth rates molecular simulations and interpretation beyond Lauritzen-Hoffman model. Macromolecules 41 2049-2061 Jeziomy A (1971) Parameters characterizing the kinetics of the non-isothermal crystallization of poly(ethylene terephthalate) determined by DSC. Polymer 12 150-158 Johnson WA, Mehl RT (1939) Reaction kinetics in processes of nucleation and growth. Trans Am Inst Min Pet Eng 135 416-441... 

Ozawa T (1971) Kinetics of non-isothermal crystallization. Polymer 12 150-158 Permings AJ, van der Mark JMAA, Kiel AM (1970) Hydrodynamically induced crystallization of polymers from solution. III. Morphology. Kolloid Z Z Polym 237 336-358 Phillips PJ (1990) Polymer crystals. Rep Prog Phys 53 549-604... 

Andjelic, S., jamiolkowski, D., McDivitt, J., Fischer, J., Zhou, J., and Han, C.C. (2001) Time-resolved isothermal crystallization of absorbable PGA-co-PLA copolymer by synchrotron small-angle X-ray scattering and wide-angle X-ray diffraction. Polymer, 42, 8965-8973. 

Mucha and Krolikowski [39], studying the kinetics of isothermal crystallization of polypropylene, noticed that addition of a filler, e.g., wood flour, efficiently reduces the time of crystallization. This is desirable in the processing of composites as it reduces the injection-forming cycle and forms small spherulites improving the mechanical properties of composites. Transcrystalline structures are formed during polymer crystallization in the presence of lignocellulosic filler. 

Auliawan et al7 studied ternary polymer blends of PLLA, poly(methyl methacrylate) (PMMA) and poly(ethylene oxide) (PEO) as matrix for nanocomposites. Their conclusions were that nanoclays enhanced the non-isothermal crystallization of the blend, since this ternary polymer blend hardly crystallized when cooled from the melt. The addition of vermiculite at loadings larger than 1% by weight retarded the crystallization process, while montmorillonite enhanced the crystallization process to some extent before retarding it. 

E Devaux, B Chabert. Non-isothermal crystallization of glass fibre reinforced polypropylene. Polymer Communications 31 391-394, 1990. 

It is well known that the extent to which isothermal crystallization in polymers occurs can be profoundly influenced by molecular orientation. Recently Nakamura et al. (275) have performed a detailed theoretical analysis of the relationships between crystallization temperature, crystallinity, orientation and cooling conditions for nonisothermal processes. These investigators have also evaluated the crystallization rates under molecular orientation during melt spinning experiments and... 

Li Z M, Yang W, Li L-B, Xie B-H, Huang R and Yang M-B (2004) Morphology and non-isothermal crystallization of in situ microfibrillar PET/PP blend fabricated through slit-extrusion, hot-stretch quenching, J Polym Sci Part B Polym Phys 42 374-385. 

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