Crystallization isothermal

Two main factors influence the rate of crystallization at any given temperature (1) the rate of nucleation and (2) the subsequent rate of growth of these nuclei to 

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Thermodynamic Properties. The thermodynamic melting point for pure crystalline isotactic polypropylene obtained by the extrapolation of melting data for isothermally crystallized polymer is 185°C (35). Under normal thermal analysis conditions, commercial homopolymers have melting points in the range of 160—165°C. The heat of fusion of isotactic polypropylene has been reported as 88 J/g (21 cal/g) (36). The value of 165 18 J/g has been reported for a 100% crystalline sample (37). Heats of crystallization have been determined to be in the range of 87—92 J/g (38). 

Other crystallization parameters have been determined for some of the polymers. The dependence of the melting temperature on the crystallization temperature for the orthorhombic form of POX (T = 323K) and both monoclinic (T = 348K) and orthorhombic (T = 329K) modifications of PDMOX has been determined (284). The enthalpy of fusion, Aff, for the same polymers has been determined by the polymer diluent method and by calorimetry at different levels of crystallinity (284). for POX was found to be 150.9 J/g (36.1 cal/g) for the dimethyl derivative, it ranged from 85.6 to 107.0 J/g (20.5—25.6 cal/g). Numerous crystal stmcture studies have been made (285—292). Isothermal crystallization rates of POX from the melt have been determined from 19 to —50 C (293,294). Similar studies have been made for PDMOX from 22 to 44°C (295,296). 

Figure 4 DSC melting endotherms of P7MB after isothermal crystallization at 135°C, starting from the isotropic melt [10]. The curves correspond to 0, 3,6, and 35 min of crystallization time, from bottom to top. Scanning rate 5°C/min.

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... 

As a first example of applying the techniques described in section 2 let us look at the chain motion of linear polyethylene (LPE). A detailed study of a perdeuterated sample, isothermally crystallized from the melt, has been carried out in our laboratory24,25,44). Since all of this work is published and, in fact, has been reviewed extensively17 we can restrict ourselves to stating the main conclusions here ... 

Fig. 14.2H NMR spectra of LPE, isothermally crystallized from the melt at 396 K (Mw as 100000, Mw/Mn as 10, Merck, Darmstadt) at 55 MHz obtained from a complex FT of the solid echo for various temperatures...

Figure 7. Dynamic shear response (expressed as G, G , J" and tanS) of isotactic polypropyienes of varying crystallinity (torsion pendulum, 1Hz, ref 6). O 65%, O 56%, A 50%, achieved by annealing of quenched specimen ( 45%) + 68%, isothermally crystallized. Curves are calculated from parameterized equation (8).

Figure 8. Relaxation strength versus crystallinity in isotactic polypropylenes of Figure 7. Unrelaxed low temperature modulus Q)> relaxed y modulus (A), relaxed 0 modulus (0), relaxed a modulus ( >). Filled symbols are for the isothermally crystallized (68%) specimen. 

Figure 11 Left Spherulites of a Ziegler-Natta isotactic poly(propylene) with Mw = 271,500 g/mol and mmmm — 0.95, isothermally crystallized at 148°C. Right Banded spherulites of a linear polyethylene with Mw = 53,600 g/mol slowly cooled from the melt.

Fig. 7 Snapshots of the simulation systems for Cl, C2, and C3 after an isothermal crystallization following the quenching from the infinite temperature to a temperature of 2.857 Ec/kB [84]...

Fig. 11 Monomer distributions of 32-mers with Ef/Ec = 0.1 at Ec/k /T =0.174 vs. variable crystalline-stem lengths changing with time during isothermal crystallization at a specific temperature. The evolution time is denoted by the numbers (times 1000 Monte Carlo cycles) near the curves. The curves are shifted vertically with an interval of 300 for clarity. We can see that with time the peak shifts from one third to half of the chain length [56]...

Isothermal crystallization was carried out at some range of degree of supercooling (AT = 3.3-14 K). AT was defined by AT = T - Tc, where Tj is the equilibrium melting temperature and Tc is the crystallization temperature. T s was estimated by applying the Gibbs-Thomson equation. It was confirmed that the crystals were isolated from each other by means of a polarizing optical microscope (POM). 

For the study of the M dependence of I and V of FCCs, film samples (0.1 mm thick) were isothermally crystallized from the melt into the orthorhombic (= ordered immobile) phase at atmospheric pressure. The range of AT was 10 -15 K. In order to observe isolated single crystals, observation was limited to the earlier stage of crystallization. 

For the entanglement study, FCCs were isothermally crystallized at atmospheric pressure and ECSCs with different l were isothermally crystallized from the melt to the hexagonal (disordered mobile) phase at... 

Fig. 4 Crystallization procedure indicated by T against time t. ECSCs or FCCs once melted are kept at Tmax for 5 min and then isothermally crystallized at a Tc...

A T = 5.5-13 K under high pressure, P = 0.4 GPa. I was estimated by transmission electron microscopy (TEM). As schematically shown in Fig. 4, ECSCs or FCCs once melted were kept at Tmax = 160 °C for 5 min at atmospheric pressure. After that, samples were isothermally crystallized at various Tcs. Hereafter, we abbreviate these processes as ECSCs-melt-FCSC or FCCs-melt-FCSC, respectively, where FCSC means folded chain single crystal. The range of AT was about 10-14 K. 

Isothermal crystallization was observed by means of SAXS and a polarizing optical microscope (POM, OLYMPUS, BX or BHS-751-P). The SAXS experiment was carried out using synchrotron radiation on the beam line BL40B2 of SPringS (SP8) at JASRI in Harima and at the BL-10C small angle installation of the Photon Factory (PF) at KEK in Tsukuba. 

In observing the time dependent changes in birefringence and stress-optical coefficient, for elongated samples at 25 C, it was found that the rate of crystallization of high trans SBR s was very much faster, some 10 times more rapid, than that for NR (8). This is consistent with the reported rates of isothermal crystallization for NR (2.5 hours at -26°C) and for 807. trans-1,4 polybutadiene (0.3 hours at -3°C) in the relaxed state (12). 

The isothermally crystallized monomers were polymerized with 2 4 Mrad Y-ittadiation. The polymer film was removed from the substrate after C, Pt coating by dissolving the salt. A JEOL JEM 100B electron microscope was used to examine the samples. 

Figure 2.30 X-ray powder diffraction patterns of sPP samples isothermally crystallized from melt at indicated temperatures.172 Indices of 211 and 020 reflections are given for unit cell of form I of Figure 2.29a.

Figure 2.40 X-ray powder diffraction patterns of iPP samples prepared with metallocene catalyst186 and isothermally crystallized form the melt at the indicated temperatures. The (130)a and (117)y reflections at 20 values of 18.6° and 20.1°, respectively, typical of a- and y-forms of iPP, respectively, are also indicated.

Turnbull and Cech [58] analyzed the solidification of small metal droplets in sizes ranging from 10 to 300 xm and concluded that in a wide selection of metals the minimum isothermal crystallization temperature was only a function of supercooling and not of droplet size. Later, it was found that the frequency of droplet nucleation was indeed a function of not only crystallization temperature but also of droplet size, since the probability of nucleation increases with the dimension of the droplet [76]. However, for low molecular weight substances the size dependence of the homogeneous nucleation temperature is very weak [77-80]. 

Figure 3 shows a plot of the volume normalized nucleation time constant as a function of isothermal crystallization temperature for PEO droplets, taken from the work of Massa and Kalnoki-Veress [84]. As expected, droplets of different volumes have the same value of r V. The inset in Fig. 3 is a plot consistent with classical nucleation theory (see Eqs. 1, 4) only the last four data points correspond to the work of Massa and Kalnoki-Veress. The first... 

Fig. 4 Reported dynamic crystallization temperatures for PEO isolated spheres as a function of their volume (bottom x-axis) and diameter (top x-axis). The inset shows dynamic (symbols with vertical bars) and isothermal crystallization (vertical dashed bars) temperature ranges for PEO spheres, see text...

The overall isothermal crystallization kinetics of polymers can be described by the Avrami equation [88-90] ... 

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