Radial growth rate spherulites

The measured T]/2 values of many polypropylene samples of high isotacticity are shown in Fig. 14 as a function of T. It is seen that tj/2 increases very rapidly with Jc whereas at any given T, no statistically significant correlation with molecular mass is found. The independence of Ti/2 with molecular mass agrees with the fact that nucleation density and spherulite radial growth rate were found to have no identifiable dependence on molecular mass. 

Lim and coworkers [183] proposed the following modification of the Hoffman and Lauritzen Eq. (8) to measure spherulite radial growth rate, as a function of T and cooling rate ... 

For pure iPP and PB-1 homopolymers and their respective blends, the spherulite radius increases linearly with time t for all T, investigated. For all samples, the isothermal radial growth rate G was calculated at different as G = dR /dt. Generally, the G values decrease an increase in the values and with increase in the amount of noncrystallizable component in the blend. As shown in Fig. 6.1, where the relative G values of the iPP-based blends are reported for = 125°C, the depression of the G values was more pronounced for the blends prepared with HOCP as the second component. 

The spheruhte dimension, at constant T, increases with increasing concentration of noncrystallizable component. The spherulite radius R increases linearly with crystalhzation time for pure iPP and iPP/PB-l/HOCP blends for all investigated. For all samples, the isothermal radial growth rate, G = dR/dt, calculated at different Tc, is reported in Table 6.11. With the increase in the T, the G values appear to decrease for all investigated compositions. The blends prepared with the same fraction of iPP show G values that decrease with increasing of HOCP fraction at constant Tc value. 

Spherulite radial growth continues at a constant rate, even after other portions of the spherulite have impinged with its neighbors — indicating that lamellae within a given spherulite grow independently... 

FIGURE 11.1 Radial growth rate r of spherulites of isotactic polystyrene as a function of the crystallization temperature. 

The morphology and the isothermal radial growth rate of PEO spherulites in the blends were studied on thin films of these samples using a Reichert polarizing microscope equipped with a Mettler hot stage. The films were first melted at 85° C for 5 minutes, following which they were rapidly cooled to a fixed crystallization temperature T and the radius of the growing spherulites was measured as a function of time. 

Fig. 2, Radial growth rate G of spherulites in pure PEG and PEO/PMMA blends as a function of crystallization temperature.

FIC U RE 12 Plot of radial growth rate of PTT spherulites as a function of T as discussed in Hong et al., (2002) (modified from Hong and co-workers (2002)). 

Ozawa proposed to study the overall crystallization kinetics from several simple DSC scanning experiments (Ozawa 1971). Assuming that when the polymer sample is cooled from To with a fixed cooling rate a = dT/dt, both the radial growth rate v T) of the spherulites and the nucleation rate 1(T) will change with temperature. For a sphemlite nucleated at time t, its radius at time t will be... 

More importantly, the crystallization kinetics of all samples of different molar mass displays the characteristic discontinuity due to the different radial growth rates of a - and a-spherulites. Independent of the molar mass, the transition from growth of a -crystals to growth of a-crystals occurs at 100-120 C [28]. 

As mentioned above, PLA should be addressed as a random copolymer rather than as a homopolymer the properties of the former depend on the ratio between L-lactic acid and D-lactic acid units. A few studies describe the influence of the concentration of D-lactic acid co-units in the PLLA macromolecule on the crystallization kinetics [15, 37, 77-79]. The incorporation of D-lactic acid co-units reduces the radial growth rate of spherulites and increases the induction period of spherulite formation, as is typical for random copolymers. In a recent work, the influence of the chain structure on the crystal polymorphism of PL A was detailed [15], with the results summarized in Figure 5.13. It shows the influence of D-lactic acid units on spherulite growth rates and crystal polymorphism of PLA for two selected molar mass ranges. 

For a spherulite, the radial growth rate G can be calculated by applying the Hofifman-Lauritzen theory (Lauritzen and Hoffman 1960)... 

The Avrami equation represents only the initial portions of polymer crystallization correctly. The spherulites grow outward with a constant radial growth rate until impingement takes place when they stop growth at the intersection, as illustrated in Figures 6.13 and 6.22. Then a secondary crystallization process is often observed after the initial spherulite growth in the amorphous interstices (85). 

Usually spherulites grow with a constant radial growth rate. The crystallization rate, as given by dp/dt, is then proportional to the total area of free, i.e. non-touching spherulite surfaces. For a growth rate u and a fixed number... 

Fig. 4.16. Temperature dependence of the radial growth rate u of spherulites in isotactic polystyrene left), polyamid 6 (center) and poly(tetramethyl-p-silpheylene siloxane) (right). Data from different authors taken from [39]...

Figure 16.9 Radial growth rates of spherulites and the induction periods for spherulite growth of pure phase and blends of stereocomplexationable polymers as estimated from the respective POM images. Reprinted with permission from Ref [79] 2011, Nature Publishing Group.

Different morphologies can be obtained, depending on the crystallization conditions and on the crystalline forms. From the melt, SPS crystallizes according to a spherulitic morphology. Two different types of spherulites are observed sheaf-like spherulites with a fibrosity of a few micrometers, and round spheruhtes, which are 50 pm in diameter. They all show positive birefringence and have the same isothermal radial growth rate [66,67]. 

The radial growth rates of polymeric spherulites are expressed as a function of undercooling from their equilibrium melting point according to the well-known equation of Hoffman and Lauritzen [4,5],... 

Fig. 5.38. Temperature dependence of the radial growth rate of spherulites of PeCL as measured in a polarizing microscope. Representation by Eq. (5.24) with Ta = 1350K, Tv = —110°C and Tg = 443K the zero growth temperature is T,g = (77 1)°C [57]...

Figure 10.36 (a) Radial growth rate, G, of PBT spherulites in PBT/HTHIO blends isother-maUy crystallized from the isotropic melt at Tc = 478 K ( ) and = 483 K ( ), as a fnnction of HTHIO content, (b) Overall crystallization rate, l/fos, of PBT in PBT/HTHIO blends crystallized at various T versus HTHIO content. Reprinted from Pracella et al. [144], Copyright 1989, with permission from John Wiley Sons. 

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