Isothermal spherulite

McLaren, J. V. A kinetic study of the isothermal spherulitic crystallization of polyhexamethylen adipamide. Polymer 4, 175—189 (1963). 

For homopolymers, the temperature dependence of the isothermal spherulite growth rate, G, is described by Eq 3.1 [Turnbull and Fischer, 1949] ... 

Isothermal spherulitic growth rates of PDS and one of its slower-to-crystaUize copolymers (PDS-co-Glycolide,92/8) as a function of crystallization temperature. HSOM images of PDS and its copolymer captured after isothermal crystallization at 60°C after 15 and 30 mliv respectively, are also included. 

The influence of PMMA content on the kinetic and thermodynamic parameters controlling the isothermal spherulitic growth and the overall crystallization rate of PEG from the molten blends has been analyzed on the basis of the modified Tumbul 1-Fisher equation ... 

G Isothermal spherulite growth rate G Theoretical spherulite growth rate... 

Figure 7.1 Polarized light micrographs of isothermal spherulitic crystallization in iPP thin film at 136°C (a-d) consecutive stages of crystallization.

The first attempts to describe the conversion degree of melt into growing domains concerned the radial growth from nuclei distributed randomly in a material [1-6]. In spite of different reasoning, they led to the equation, best known as the Avrami-Evans equation, which is widely used to analyze the isothermal spherulitic crystallization, which is the crystallization at constant temperature. 

In References [6,114,115] shear-induced crystallization of iPP-based nanocomposites with o-MMT was examined. In Reference [114] early stages of flow-induced isothermal spherulitic crystallization of intercalated iPP/PP-g-MA/o-MMT nanocomposites with 2 wt% of the clay were studied after application of shear flow for a short time of up to 30 seconds. The flow induced enhancement of crystallization was stronger in the nanocomposite than in neat iPP. However, acceleration of crystallization in iPP/PP-g-MA blend was also observed. 

Figure 11 Left Spherulites of a Ziegler-Natta isotactic poly(propylene) with Mw = 271,500 g/mol and mmmm — 0.95, isothermally crystallized at 148°C. Right Banded spherulites of a linear polyethylene with Mw = 53,600 g/mol slowly cooled from the melt.

Figure 17 Isothermal melting of Ziegler-Natta isotactic poly(propylene). (a) Spherulites with mixed birefringence at Tc = 148°C. The top middle figure displays the melting for the same thermal history, (b) Subsequent to crystallization, the temperature was raised to 171°C spherulites acquire negative birefringence, (c), (d) and (e) Isothermal melting at 171°C for 80, 200 and 300 min, respectively. Reproduced with permission from W.T. Huang, Dissertation, Florida State University, 2005. (See Color Plate Section at the end of this book.)...

The growth of the nuclei then occurs in one, two or three dimensions creating rods (fibrils), discs or spheres (spherulites). The development of crystallinity (Vc) under isothermal conditions with time ( ) is generally analysed according to... 

To confirm the shape of the spherulites described by the Avrami exponent, polarized optical micrographs of the isothermal crystallized melt blends were taken, and are shown in Figure 20.26 [44],... 

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. 

The isotherms obtained in dilatometric measurements of the crystallization rate could be fitted with an Avrami (3) type equation only by assuming the existence of a secondary crystallization process much slower than the rate of spherulitic growth observed microscopically, and by taking into account the experimentally determined form of the nucleation rate. The nucleation rate was found to be a first-order process. Assuming that the secondary crystalliza-... 

Fio. 64.—Adsorption isotherm of alpha amylase to spherulites at 25°C. (Reprinted with permission from V. M. Lelong, P. Colonna, and S. G. Ring, Biotechnol. Bioenerg., 38 (1991) 127-134.)... 

Applying the Avrami model to the analysis of the isothermal crystallization of interesterified and noninteresterified 20%SSS/80%000 at 30°C, 40 C and 50 C, many differences can be observed (Table 17.3). At 30°C and 40°C growth would be described as rodlike with instantaneous nucleation for both interesterified and noninteresterified samples. Also, for the noninteresterified system at 50°C spherulitic growth with instantaneous nucleation takes place. The half-time of nucleation... 

Wu and Woo [26] compared the isothermal kinetics of sPS/aPS or sPS/PPE melt crystallized blends (T x = 320°C, tmax = 5 min, Tcj = 238-252°C) with those of neat sPS. Crystallization enthalpies, measured by DSC and fitted to the Avrami equation, provided the kinetic rate constant k and the exponent n. The n value found in pure sPS (2.8) points to a homogeneous nucleation and a three-dimensional pattern of the spherulite growth. In sPS/aPS (75 25 wt%) n is similar (2.7), but it decreases with increase in sPS content, whereas in sPS/PPE n is much lower (2.2) and independent of composition. As the shape of spherul-ites does not change with composition, the decrease in n suggests that the addition of aPS or PPE to sPS makes the nucleation mechanism of the latter more heterogeneous. 

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])...

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