Spherulitic growth

The size and number of sphemhtes in the polymer tends to affects the physical properties. Thus, the impact strength of polymer hhns or their flex life usually increases as the sphemlite size decreases. On the other hand, there does not appear to be any correlation between the yield stress and ultimate elongation and the size of the sphemhtes. 

When the melt or the solutions are stirred epitaxial crystallinity is usually observed. One crystalline growth occurs right on top of another. This arrangement is often caUed shish-kebab 

In order for the ordered phase to crystallize from an amorphous melt a nucleation barrier must be overcome. This barrier is a result of interfacial energy between the ordered phase and the melt that causes super cooling. Sirota [44] suggested that in order for the nucleation barrier of the stable phase to be sufficiently high to form out of the melt, another phase with a lower nucleation barrier and a free energy intermediate between that of the stable phase and the melt must form. This, he points out, is implied by Oswald s rule [45] and evidence presented by Keller [35] that crystallization in semicrystalline polymer systems is mediated by a transient metastable phase [47, 48]. 

It was reported recently, that polymeric can also form quasicrystals. Hayashida et al. [50] demonstrated that certain blends of polyisoprene, polystyrene, and poly(2-vinylpyridine) form starshaped copolymers that assemble into quaskrystals. By probing the samples with transmission electron microscopy and X-ray diffraction methods, they conclude that the films are composed of periodic patterns of triangles and squares that exhibit 12-fold symmetry. These are signs of quasicrystalline ordering. Such ordering differ from conventional crystals lack of periodic structures yet are well-ordered, as indicated by the sharp diffraction patterns they generate. Quasi-crystals also differ from ordinary crystals in another fundamental way. They exhibit rotational symmetries (often five or tenfold). There are still some basic questions about their stracture. 

The free energy change of a polymer from liquid to crystal is 

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It has been reported that the overall rate of crystallization of pure PHB is relatively low compared to that of common synthetic polymers, showing a maximum in the temperature range of 55-60°C [23]. The spherulite growth rate kinetics have been evaluated [59] in terms of the theory by Hoffmann et al. [63], At about 90 °C, the spherulite growth rate displayed a maximum, which is not excessively low compared to that of common synthetic polymers. Therefore it was stated that the low overall crystallization rate of PHB centers on the nuclea-tion process rather than the subsequent crystal growth. Indeed, it has been shown that PHB has an exceptionally low level of heterogeneous nuclei [18]. 

Figure 9.30 AFM images of polyethylene films formed on the planar CrO,/Si02/Si( 100) model catalyst. The small white stripes are lamellar crystals. These form the well-known spherulite superstructure upon crystallization from the melt. Depending on the layer thickness, spherulite growth stops at different stages of development (adapted from Thiine el at. [90]). 

Fig. 8 a Spherulitic growth rates for PPDX and the PPDX block within D7732C2310 diblock copolymer. Solid lines are fits to Lauritzen and Hoffman theory, b Lauritzen and Hoffman kinetics theory plot for PPDX (K = 17.2 x 104 K2) and the PPDX block within D7732C2310 diblock copolymer (K = 46 x 104 K2). (From [103]. Reproduced with permission of the Royal Society of Chemistry)... 

The rate of spherulitic growth is extremely temperature sensitive and seems to be independent of the nucleating agent. 

The very fast initial density increase due to nucleation and rapid spherulite growth as shown by the dotted lines, referred to as primary crystallization. 

The overall rate of crystallization is determined by both the rate of nuclei formation and by the crystal growth rate. The maximum crystal growth rate lies at temperatures of between 170 and 190 °C [71, 72], as does the overall crystallization rate [51, 61, 75], The former is measured using hot stage optical microscopy while the latter is quantified by the half-time of crystallization. Both are influenced by the rate of nucleation on the crystal surface and the rate of diffusion of polymer chains to this surface. It has been shown that the spherulite growth rate decreases with increasing molecular weight due to the decrease in the rate of diffusion of molecules to this surface [46, 50, 55, 71, 74],... 

The spouting bed temperature is generally in the range of 150-170 °C, which is close to the maximum spherulite growth rate, and therefore ensures quick completion of the primary crystallization. The material temperature at the outlet of the pulsed fluid bed is usually <180°C. 

Tant, M. R. and Culberson, W. T., Effect of molecular weight on spherulite growth rate of poly(ethylene terephthalate) via real-time small angle light scattering, Polym. Eng. Sci., 33, 1152-1156 (1993). 

Figure 5.7. Optical microscope image of a thin film (thickness 2 p.m) of a-p-NPNN grown on a glass substrate (1.6 x 1.0 mm, crossed polarizers). Reprinted from Journal of Crystal Growth, Vol. 209, J. Caro, J. Fraxedas and A. Figueras, Thickness-dependent spherulitic growth observed in thin films of the molecular organic radical p-nitrophenyl nitronyl nitroxide, 146-158, Copyright (2000), with permission from Elsevier.

The isothermal crystallization of PEO in a PEO-PMMA diblock was monitored by observation of the increase in radius of spherulites or the enthalpy of fusion as a function of time by Richardson etal. (1995). Comparative experiments were also made on blends of the two homopolymers. The block copolymer was observed to have a lower melting point and lower spherulitic growth rate compared to the blend with the same composition. The growth rates extracted from optical microscopy were interpreted in terms of the kinetic nucleation theory of Hoffman and co-workers (Hoffman and Miller 1989 Lauritzen and Hoffman 1960) (Section 5.3.3). The fold surface free energy obtained using this model (ere 2.5-3 kJ mol"1) was close to that obtained for PEO/PPO copolymers by Booth and co-workers (Ashman and Booth 1975 Ashman et al. 1975) using the Flory-Vrij theory. 

Spherulites. As a polymer melt solidifies, several folded chain lamellae spherulites form which are up to 0.1 mm in diameter. A typical example of a spherulitic structure is shown in Fig. 1.15. The spherulitic growth in a polypropylene melt is shown in Fig. 1.16. 

Hirai, N. Absolute rate theory for spherulitic growth of polymeric substances. J. Polymer Sci. 42, 213-222 (1960). 

Ohlberg, S. M., J. Roth, and R- A. V. Raff Relationship between impact strength and spherulite growth in linear polyethylene. J. Appl. Polymer Sci. 1. 114-120 (1959). 

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