Melt crystallization isothermal

Folded Surface Free Energy in Miscible Polymer Blends by Isothermal Melt Crystallization... 

Combination of WAXS and SAXS is a very efficient way of detailed description of crystallization in blends of semicrystalbne polsmiers. Baldrian and co-workers (203,204) has studied isothermal melt crystallization of blends of low molecular weight poly(ethylene oxide) (PEO) and poly(methyl methacrylate) (PMMA) using time resolved SAXSAVAXS measurements on the ELETTRA synchrotron (205). He has reported the formation of unstable PEO lamellae of nonintegrally folded... 

An example of time- and temperature-dependent SAXS measurements corresponding to isothermal melt crystallization and subsequent heating of PTT is given in Figure 33. The solid lines are the fits to the curves performed with the generalized paraciystalline model/ After correction for the parasitic and liquid-like scattering, as well as for the geometrical factors (e.g Lorentz factor), the curves exhihit more clearly the second-order interference maximum (visible as a shoulder atca. 0.014-0.016 A ) and a faint hroad ripple due to the form factor of crystals with the first minimum at about 0.02 ft is... 

Figure 33 Processed SAXS curves (s (/(s) - B)) corresponding to isothermal melt crystallization of PTT at 205 °C (left) and subsequent heating (right). The fits with the generalized paracrystalline model (solid lines) are offset vertically for clarity. With permission from Ivanov, D. A. Bar, G. Dosifere, M. Koch, M. H. J. Wacromo/eco/es 2008, 41,9224. ...

IMC Isothermal Melt Crystallization rare but effective method of isother-... 

Shieh, Y.-T.and Liu, G.-L. (2007) Temperature-modulated differential scanning calorimetry studies on the origin of double melting peaks in isothermally melt-crystallized poly(L-lactic acid), Journal of Polymer Science Part B Polymer Physics, 45,466-474. 

Figure 5. Influence of the annealing treatment on melting curves of the P(3HB-co-3HH) copolyester containing 14.6 mol% HH units after isothermal melt-crystallization for 3 day at different temperatures. Annealing temperature = (Ts +Tm )/2 time 10 min heating rate 20 °C/min.

Chiu, F., Shen, K., Tsai, S. H. Y, Chen, C. Pre-melting temperature effect on the isothermal melt crystallization and multiple melting behavior of syndiotactic polystyrene. Polym. Eng. Sci., 41(5), 881-889 (2001). 

The crystallization of polymers has been smdied both in solution and from the melt. Crystallization also occurs during polymerization and during cooling from the melt with imposed orientation (e.g., injection molding). In this chapter, we focus particularly on the isothermal melt crystallization under quiescent conditions. 

Fig. 4.20 Lamellar thickness as a function of crystallization temperature for isothermally melt-crystallized polyethylene (after Wunderlich).

Zone refining is one of a class of techniques known as fractional solidification in which a separation is brought about by crystallization of a melt without solvent being added (see also Crystallization) (1 8). SoHd—Hquid phase equiUbria are utilized, but the phenomena are much more complex than in separation processes utilizing vapor—Hquid equiHbria. In most of the fractional-solidification techniques described in the article on crystallization, small separate crystals are formed rapidly in a relatively isothermal melt. In zone refining, on the other hand, a massive soHd is formed slowly and a sizable temperature gradient is imposed at the soHd—Hquid interface. 

Another interesting example is the melt crystallization of s-PS. For the case of rapid cooling from the melt, the hexagonal a form is obtained [7-9], while for low cooling rates or for isothermal crystallizations, the crystalline form which is... 

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.)...

A sample of the polymer to be studied and an inert reference material are heated and cooled in an inert environment (nitrogen) according to a defined schedule of temperatures (scanning or isothermal). The heat-flow measurements allow the determination of the temperature profile of the polymer, including melting, crystallization and glass transition temperatures, heat (enthalpy) of fusion and crystallization. DSC can also evaluate thermal stability, heat capacity, specific heat, crosslinking and reaction kinetics. 

Figure 19. Sample isotherms and interface shapes computed for the QSSM for CZ crystal growth by Atherton et al. (153). The model includes detailed radiation between the surfaces of the melt, crystal, and crucible. Isotherms are spaced at 10 K increments in the melt and 30 K increments in the other...

Fig. 4 Paths to integer folded forms in long alkanes. While solution crystallization gives integer forms (E, F2,...) directly, for melt crystallization this is true only for the E form. Below the melting point of F2, the transient non-integer form (NIF) appears and subsequently transforms isothermally either to the once-folded F2 form or the mixed-integer folded-extended (FE) form, based on layer triplets. Deuterium- labelled chain ends, prepared for neutron scattering experiments, are indicated with circles...

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. 

This shows that by means of synchrotron radiation, for the first time the primary lamellar thickening on melt crystallization in polyethylene could be observed. This enabled also the determination of the true crystal thickness as a function of supercooling AT. Most satisfyingly it was found to be closely identical to that obtained in solution crystallization were no isothermal thickening takes place. In conclusion, the hitherto existing gap between melt and solution crystallization has been removed and the corresponding f versus AT curves brought in coincidence. 

Figure ll-12a presents room temperature adsorption isotherms for some representative activated carbon samples on Technical Grade DMSO, showing the difficulty of reducing UV275 to the desired level of <0.10 without additional purification. Figure ll-12a is a schematic of the melt crystallization-carbon column recycle system which was employed to get around this problem. The higher concentration of impurities in the unfrozen melt altered the equilibrium concentration on the activated carbon. In the steady state (Fig. 1 l-12b) a reasonably sized carbon column could produce effluent suitable for further freeze CrystalUzation, and the yield of the total process was close to 100%. 

Melt crystallization processes are dominated by heat transfer. In the case of melt crystallization in suspension processes, the crystals are surrounded by melt and the system is almost isothermal at the crystallization temperature. The heat of the solidification process is only transported away through the melt. Such a system is often called adiabatic grown, which results in moderate growth rates with rather pure crystals. 

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