Spherulites crystallization rates

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,... 

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]. 

It is of course important to note that the overall rate of crystallization is not only determined by the growth rate of the spherulites, but also by the amount of nuclei being present in the system. This possibility is used as an effective method to influence the total crystallization rate of commercial polymeric materials in a controlled manner and to influence the size of spherulites and thus the physical properties of finished articles made from semicrystalline polymers. 

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. 

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. 

Rate of spherulitic crystallization with chain folds in polychlorotri-fluoroethylene. J. Chem. Phys. 37, 1723 — 1741 (1962). 

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-... 

The effects of morphology (i.e., crystallization rate) (6,7, 8) on the mechanical properties of semicrystalline polymers has been studied without observation of a transition from ductile to brittle failure behavior in unoriented samples of similar crystallinity. Often variations in ductlity are observed as spherulite size is varied, but this is normally confounded with sizable changes in percent crystallinity. This report demonstrates that a semicrystalline polymer, poly(hexamethylene sebacate) (HMS) may exhibit either ductile or brittle behavior dependent upon thermal history in a manner not directly related to volume relaxation or percent crystallinity. 

Figure 20.3 Spherulite growth rate (G) for sPS/PPE and sPS/PVME blends as a function of the crystallization temperature Tci ( ) sPS ( ) sPS/PPE 90 10 ( ) sPS/ PPE 80 20 (A) sPS/PVME 80 20 ( ) sPS/PVME 70 30 ( ) sPS/PVME 50 50. Reprinted from Polymer, vol. 34, Cimmino S., Di Pace E., Martuscelli E., Silvestre C., sPS based blends crystallization and phase structure , p. 2799, Copyright 1993, with permission from Elsevier Science.

The second important result is related to the change in the crystallization rate due to the shear flow. Since the rate of addition of polymer to a spherulite is roughly proportional to the diffusivity of the molecules, variations of the diffusion coefficient... 

Spherulitic crystals also are often found in glass ceramics. Spherulitic growth, or branching growth from a central point, is common in viscous systems where ionic mobility is the rate-limiting step. Models developed for polymer systems apply to glass ceramics and qualitatively explain the tendency to form spherulites. ... 

This helps to confirm that nucleation, crystallization rate, and spherulite size are strongly influenced by the presence of fillers. It is still uncertain what role a filler plays in the mechanism of nucleation. 

Figure 4. Computed distributions for samples partly crystallized at 125°C and quenched. DL = diffusion coefficient of additive in liquid pm2 sec"1 DS = back diffusion coefficient G = spherulite growth rate pm sec"1.

The influence of the SAN copolymer composition on the spherulitic growth rate of PCL has been studied at a hxed crystallization temperature by Kressler et al. [1992, 1993]. A minimum has been observed at about 20 wt% AN in SAN for several compositions (see Figure 3.9), due to a minimum in the value of the interaction parameter, at the same copolymer composition that is responsible for a reduced chain mobility. 

The first value is calculated from the spherulitic growth rate data of PEG(10)/PMMA. The second one from the overall crystallization data of PEG(10)/PMMA. The third one from the spheruhtic growth rate data of PEG(2)/PMMA [Martuscelli and Demma, 1980] (the value between brackets refers to the molecular weight of PEG in kg/mol)... 

The crystallization rate is retarded for all regimes, but the extent of hindrance increases from regime 1 to 3. It should be noted that the diffusion of the crystalline polymer occurs on a lamellar scale (about 10 nm), whereas the diffusion of the amorphous component, induced by demixing, takes place on a spherulitic scale (10-20/normal processing conditions,... 

The discussion on the crystallization behavior of neat polymers would be expected to be applicable to immiscible polymer blends, where the crystallization takes place within domains of nearly neat component, largely unaffected by the presence of other polymers. However, although both phases are physically separated, they can exert a profound influence on each other. The presence of the second component can disturb the normal crystallization process, thus influencing crystallization kinetics, spherulite growth rate, semicrystalline morphology, etc. 

Long et al. [1991] investigated the crystaUiza-tion behavior in blends of PP with LLDPE. They found the crystallization temperature of the PP matrix, T, to decrease slightly upon the addition of LLDPE. However, the degree of crystaUinity, X, and the spheruUte growth rate, G, were not affected. The authors concluded that the overall crystaUiza-tion rate of PP in the matrix decreased due to a decreasing primary nuclei density. The latter was confirmed in O. M. experiments by the increased size of the PP spherulites upon the addition of LLDPE. However, Zhou and Hay [1993] reported that with the addition of LLDPE to PP, the crystallization rate remained similar as for the PP homopolymer. 

Plaris et al. [1993] investigated also the same blend system and reported that blending had a pronounced effect on the lamellar morphology. Furthermore, the isothermal crystallization experiments indicated that the spherulite growth rate, G, and the nucleation density of the PP phase were enhanced. The authors suggested that these observations could be related to the formation of additional nucleation sites, which arise from the polymer-polymer interfaces created by the blending. 

Although most often also here the crystal growth rate is not affected, some authors have reported that finely dispersed solidified domains can increase the melt-viscosity of the matrix in such a way that the crystallization rate becomes depressed. Again, the matrix component will crystallize around its bulk temperature. The above mentioned phenomena can eventually alter the spherulite size and shift the T of the matrix on average by 5 to 10°C. The melting behavior remains normally unaffected. 

The presence of HOCP considerably slows down the melt crystallization process of PB-1. Therefore, the adopted values, lowered by increasing the HOCP fraction, provided similar rates of crystallization for pure PB-1 and blends. Previous calculations from the spherulite growth rate and from the overall kinetic rate constant showed that the number of nuclei per unit volume was similar for samples crystallized at equal undercoolings. Had we used a constant value of T, there would have... 

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