Nucleation of spherulites

Polarized light micrographs show details of the spherulitic structure in molded nylon (Fig. 5.42A) which is similar to PBT [201]. The nonspherulitic skin, a transition zone and a spherulitic core region are observed. This is as expected, as the quench rate declines away from the surface, e.g. in the transition zone, and thus there is some nucleation of spherulites. A view of the transition zone (Fig. 5.42B) shows round spherulites in a fine textured matrix. The... 

It has to be emphasized that the classic Avrami and Evans equations, and consequently the Nakamura approach, were derived by assuming random positions of nuclei in a material therefore, they do not apply strictly when there is a correlation between positions of nucleation sites. Such nucleation of spherulites is accounted for in the model developed originally for fiber-reinforced polymers [53], described in Chapter 13. 

We can nucleate crystallization from the melt by incorporating finely ground inorganic crystalline compounds such as silica. Nucleation of injection molded nylons has three primary effects it raises the crystallization temperature, increases the crystallization rate, and reduces the average spherulite size. The net effect on morphology is increased crystallinity. This translates into improved abrasion resistance and hardness, at the expense of lower impact resistance and reduced elongation at break,... 

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

The hme evoluhon of the formahon of the him during evaporation is shown in Fig. 5.8 as recorded with a CCD camera assembled to an optical microscope with crossed polarizers. The 2D spherulites are labelled by order of formahon. The in-plane spherulitic crystallization is completed 300 s after the observahon of the hrst spherulite, 1 in Fig. 5.8(a). From Fig. 5.8 we observe that the nucleation centres do not appear simultaneously and that the density of spherulites is rather low ( 1.6 mm ). This low density permits a detailed analysis of the time evoluhon of... 

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. 

Crystallisation of polymers depends on the possibilities of nucleation and growth. The structural regularity of the polymer has a profound influence on both. Interesting correlations were found for estimating the rate of spherulitic crystallisation. Besides this normal mode of bulk crystallisation, other modes are frequently observed induced crystallisation by pressure and stress and extended chain crystallisation. The latter mode occurs under special conditions for flexible chain polymers, but is the normal mode for rigid chain polymers. All modes of crystallisation are correlated with the structure of the polymer chain and with the two main transition temperatures, Tg and Tm. 

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-... 

When a polymer crystallizes from the melt without disturbance, it normally forms spherical structures that are called spherulites [1,2]. The dimensions of spherulites range from micrometers to millimeters, depending on the structure of the polymer chain and the crystallization conditions, such as cooling rate, crystallization temperature, and the content of the nucleating agent. The structure of spherulites is similar regardless of their size they are aggregates of crystallites [1-6]. 

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