Crystal growth rates, blends

Incorporating the concepts discussed above, the equation describing the crystal growth rate in a miscible polymer blend can be expressed as ... 

Nadkami and Jog (1986), Nadkami et al. (1987), and Jog et al. (1993) investigated the crystallization in blends of PPS with three types of HOPE, having a different melt flow index. In contrast to the PPS/PET blends, PPS crystallizes now in a superheated HOPE melt environment. Erom the dynamic cooling experiments, it was found that the presence of the HDPE melt suppresses the crystallization of PPS. The crystal growth rate, G, of PPS was found to remain unchanged, but its nucleation density was reduced as the concentration of HDPE in the blend increased or when the melt viscosity of the HDPE phase decreased. As a consequence, the overall crystallization rate of PPS was found to be retarded. 

Blending PEO with m-PLLA indicates, based on Avrami exponent, kinetic constant and crystalhzation half-time, that the blend has 3-dimerrsiorral growth. The crystallization growth rate increases more than 4 times as compared with PEO. Also, size of spherulites was decreased and their ntrmber increased. m-PLLAs not orrly functioned as nucleating agents but also errhanced the miscibility of the blerrds. ... 

SpheruUte and Single Crystal Growth Rates.—Rate measurements have been made on homopolymers, block copolymers, and blends. - ... 

Emulsifiers are key players in polymorphic transformation studies. They alter the fat surface properties, resulting in changes in crystal size and nature. Early reviews by van den Tempel [52] and Garti [53] showed that many types of emulsifiers tend to reduce the crystal growth rate of natural fat blends. Since then, further work has been performed on the effects of different emulsifiers on fats not only in bulk but also in emulsion systems. Garti and Yano [54] discuss in great detail the progress made in this field in recent years. 

The chapter is divided into five sections. Section 10.2 deals with the thermodynamics of polymer blends the general principles and the main theories on the phase behavior of polymer mixtures are briefly presented. Section 10.3 deals with the properties of miscible blends with crystallizable components. The phase morphology, crystal growth rate, overall crystallization kinetics, and melting behavior of miscible blends are analyzed. The crystallization phenomena in blends with miscibility gap are also described. Then, examples of miscible systems comprising one or two crystallizable components are reported with particular attention to the thermodynamic and kinetic aspects of the crystallization process. 

In order to evaluate the application of modulated-temperature differential scanning calorimetry (M-TDSC) to the study of the crystallisation kinetics of semicrystalline polymers, isothermal crystallisation kinetics in poly(e-caprolactone)-SAN blends are investigated. The temperature dependence of d In G/dT (G =crystal growth rate), determined by M-TDSC agrees approximately with previous experimental data and theoretical values. These were obtained from direct measurements of spherulite growth rate by optical microscopy. Here, theoretical and M-TDSC experimental results show that the d In G/dT versus temperature plots are not sensitive to the noncrystalline component in the poly(e-caprolactone)-SAN blends. 15 refs. 

Conclusively, the calculated Avrami exponents reveal a three-dimensional growth of the crystalline regions for each blend. The rate of crystallization of each blend increased with the decrease in crystallization temperature, and the rate of crystallization of the (PHB80-PET20)/PEN blend was faster than that of the (PHB 80-PET20)/PET blend. 

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. 

Cimmino et al. [25] reported that the radial growth rates of crystallization G, measured in sPS/PPE blends, decrease strongly with increase in PPE content (Figure 20.3). This effect might arise from an increase in the transport free energy of crystalline segments in the melt, due the larger Tg of the blend compared with pure sPS, or to a decreased capability of sPS to nucleate, induced by its dilution in PPE. 

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 crystallization rate constant k) is a combination of nucleation and growth rate constants, and is a strong function of temperature (47). The numerical value of k is directly related to the half time of crystallization, ti/2, and therefore, the overall rate of crystallization (50). For example, Herrera et al. (21) analyzed crystallization of milkfat, pure TAG fraction of milkfat, and blends of high- and low-melting milk-fat fractions at temperatures from 10°C to 30°C using the Avrami equation. The n values were found to fall between 2.8 and 3. 0 regardless of the temperature and type of fat used. For temperatures above 25°C, a finite induction time for crystallization was observed, whereas for temperatures below 25°C, no induction time was... 

This is due to the effects on nucleation and growth rates. Thus, blending method may have serious effects on crystaHizability and crystal size. Experimentally, the presence of a miscible, amorphous polymer in the blend usually slows down, or it even prevents, crystallization of the semicrystalline resin. The enhancement of crystallinity and increase in T on blending have also been reported [Harris and Robeson, 1987 Dumoulin et ai, 1987]. As a result, the T method is far from being fool-proof and the obtained values of Xi2 should be confirmed by other techniques [Utracki, 1989 Groeninckx et al., 1998]. 

The rate of crystal growth in a semicrystalline blend, will depend on the magnitude of k, k2 and AF. At low undercooling, AT = T ° - F, AF is high and hence G is small. However, if the blend Tg approaches or exceeds the melting point (F °) 2 prohibit crystallization regardless the value of AF ... 

Figure 3.16. Surface free energy of folding, versus the volume fraction of the crystallizable component, for blends of PEG (10) with PMMA from spherulite growth rate data (circles) and from overall rates of crystallization data (triangles) (the value between brackets refers to the molecular weight of PEG in kg/mol) [Martuscelli et al., 1984].

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