Isothermal Bulk Crystallization Kinetics

The DSC crystallization isotherms of pure iPP, pure PB-1, and blends compared at the same demonstrate that the overall crystallization rate constant progressively decreases with increase in the amount of the diluent component in the sample. 

The isotherms of crystallization were obtained by plotting the crystalline weight fraction at time t Xt) versus t. The half-time of crystallization, to.5 (defined as the time taken for half of the crystallinity to develop), is obtained from the curves and plotted against blend composition, for some T, in Fig. 6.2. 

The reduction in the overall crystallization rate of iPP in the iPP/HOCP blends was more drastic to the one engendered by the addition of PB-1 as the second component. 

The addition of HOCP to PB-1 causes a striking reduction in the overall crystallization rate. The overall kinetic rate constant K was calculated by using the Avrami equation (38)  

The values of n and K were derived, for each T, from the slope and the intercept, respectively, of the straight lines obtained by plotting logjQ[ ln(l — X,)] versus logio t (Fig. 6.3). The value of the Avrami exponent n was about 3 for pure iPP, pure PB-1, and all blends for aU the investigated Tc. This indicates a three-dimensional growth of crystalline units developed by heterogeneous nucleation (39). 

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Isothermal Bulk Crystallization Kinetics of Isotactic Polypropylene Component... 

The classic Avrami Equation (1) is used to study the isothermal bulk crystallization kinetics of PLA. 

Jaffe melted and isothermally recrystallized fibers spun under different stress levels. The bulk crystallization kinetics was measured with both DSC and optical microscopy techniques. Table 3.30 lists the samples studied. The film samples were included to ensure the absence of spurious DSC effects due to fiber packing. The samples were heated from 50°C to the melt temperature at 80°C/min. The melt temperatures ranged from 170 to 230°C. The samples were held in the melt for a specified time and then cooled to the 130°C crystallization temperature at 40°C/min. 

The kinetics of melt-crystallization of PLA has been analyzed by a number of research groups [14, 35, 39, 71-75]. Isothermal bulk crystallization rates were determined in a wide temperature range from 70 to 165 C [71,72]. The maximum crystaUization rate is observed around 100 C, and the most peculiar behavior is a discontinuity in the phase change kinetics around 110-120°C, an example of which is shown in Figure 5.10. Figure 5.10a reports the half-time of crystallization of PLA as function of the isothermal crystallization temperature. The data set shows a broad minimum around 108 C and a step/discontinuity around 116-118°C, indicated by the arrow. The sudden variation in crystallization rate... 

The crystallization of polymer in bulk as well as in solution is initiated by nucle-ation followed by growing of spheralites (Mandelkem, 2002). A common fundamental approach to study isothermal crystallization kinetics is the heuristic Avratni phase transition theory (Avrami, 1939 1940 1941). 

The equilibration process in oxide systems can be monitored by changes in a bulk crystal property that is nonstoichiometiy sensitive, such as weight, Aw, electrical conductivity, Ao, or thermopower, AS. The kinetics of the re-equilibration process, provoked by isothermal changes of p(02), are schematically illustrated in Figure 4.25. The rate of the equilibration is determined by chemical diffusion involving the transport of defects under a gradient of chemical potential (ambipolar diffusion). Chemical diffusion coefficients may be determined from the relevant solutions of Pick s second law for long and short times, respectively... 

Hoffman JD, Lauritzen J (1961) Crystallization of bulk polymers with chain folding theory of growth of lamellar spherulites. J Res Natl Bur Stand A65(4) 297-336 Hoffman JD, Miller RL (1997) Kinetic of crystallization from the melt and chain folding in polyethylene fractions revisited theory and experiment. Polymer 38(13) 3151-3212 Hoffman JD, Weeks JJ (1965) X-Ray study of isothermal thickening of lamellae in bulk polyethylene at the crystallization temperature. J Chem Phys 42(12) 4301-4302... 

There are several noteworthy and important features that are characteristic of polymer crystallization from dilute solution. A derived Avrami equation fits the isotherms over almost the complete transformation. Except for very low molecular weights, this is contrary to the results for bulk crystalUzation.(37) The reason for this difference and the crystallinity level that is attained will be discussed shortly. The crystallization of polymer from dilute solution can be considered to be ideal, since the kinetics resemble the crystallization of low molecular weight substances. 

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