Temperature shift, crystallization

The heat-treatment for PTFE containing the various amount of fluorinated-pitch [PTFE / fluorinated-pitch polymer-blend PTFE / FP] was carried out at 350 °C + 5 °C for 30min. Figure 2 shows the thermal properties of heat-treated PTFE with fluorinated-pitch. The melting and the crystallization temperatures shift to lower temperatures by addition of fluorinated-pitch. And also, the heat of crystallization (AHc) decreased with increasing of amount of fluorinated-pitch. 

The cold crystallization temperature shifts to the low-temperature side with increasing spinning speed. In contrast, the melting peak temperature shifts to the high-temperature side with increasing spinning speed. 

Figure 5.149 (right) shows the profile of melt and crystallization behavior prior and subsequent to irradiation for a PA 6-GF30. Irradiation causes a shift in melt temperature to lower temperatures because the crystalline structure is disturbed. Only amorphous zones can be crosslinked. The crosslinking points act as interruption points for a higher order level. At the same time, melt enthalpy is reduced and crystallization temperature shifts to lower temperatures [720]. 

We now consider hydrogen transfer reactions between the excited impurity molecules and the neighboring host molecules in crystals. Prass et al. [1988, 1989] and Steidl et al. [1988] studied the abstraction of an hydrogen atom from fluorene by an impurity acridine molecule in its lowest triplet state. The fluorene molecule is oriented in a favorable position for the transfer (Figure 6.18). The radical pair thus formed is deactivated by the reverse transition. H atom abstraction by acridine molecules competes with the radiative deactivation (phosphorescence) of the 3T state, and the temperature dependence of transfer rate constant is inferred from the kinetic measurements in the range 33-143 K. Below 72 K, k(T) is described by Eq. (2.30) with n = 1, while at T>70K the Arrhenius law holds with the apparent activation energy of 0.33 kcal/mol (120 cm-1). The value of a corresponds to the thermal excitation of the symmetric vibration that is observed in the Raman spectrum of the host crystal. The shift in its frequency after deuteration shows that this is a libration i.e., the tunneling is enhanced by hindered molecular rotation in crystal. 

As-received UD90 was oxidized for 2, 6,17, 26, and 42 h at 430°C, which is the temperature for slow ND oxidation as discussed in Sect. 5.3.3 [94]. Figure 12.18a shows the HRTEM images of two ND powders oxidized at 430°C for 2 and 42 h, respectively. The weight loss due to oxidation was 13% and 74% after 2 and 42 h, respectively. While oxidation for 2 h removes mainly amorphous carbon and other non-diamond species [95], longer oxidation times result in selective oxidation of smaller crystals, thus shifting the size distribution toward larger values. 

For polyethylene, the glass-transition temperature is far below the crystallization temperature and time-temperature shifting satisfies an Arrhenius form, with activation energy Ea = 6.5 kcal/mol for high-density polyethylene. 

Fig. 10. Rheograms (phase shift angle, 8) and DSC thermograms for an 80% palm stearin blend in sesame oil. The graphs show the 1,10, and 30°C/min cooling rates used. Crystallization temperature 33°C. The dotted line is the induction time of crystallization by DSC.

Figure 5 Heat flow curves obtained on heating and cooling at a rate of0.833 mK s for the mercury-pressurized MDPE. The base lines were shifted for the sake of clarity on the pressure effect on the melting/crystallization temperatures...

Once the amorphous gel is heated, 170 NMR shows that both OZr3 and OZr4 environments remain, although there is an increase in line width. There is also an increase in the isotropic chemical shift of the peaks, especially the OZr4 peak, which moves from 303 to 321 ppm as the gel starts to crystallize. At the crystallization temperature (approximately 360°C), EXAFS results show that there is a distinct change in the structure, with the oxygen correlation now better fit by two closely spaced shells and a large increase in the coordination number to 12 associated with the Zr-Zr correlation. This is likely because the particles are nanocrystalline. 

The synergistic crosslinking was carried out, that is, EB-crosslinking treatment was performed after thermo-chemical crosslinking treatment of PTFE with fluorinated-pitch (1.8 wt% additive). The thermal properties of obtained crosslinked PTFE were examined by DSC analysis shown in Table II. The melting and the crystallization temperatures of radiation-crosslinked PTFE (RX-PTFE), thermo-chemical-crosslinked PTFE (CX-PTFE), synergetic-crosslinked PTFE (thermo-chemical and radiation SX-PTFE) shift to lower temperatures, compared with PTFE. 

Crystallization Kinetics. Typical isothermal crystallization curves, which were measured using cast films of both types of copolymers, are presented in Figures 9 and 10. At the early stage of crystallization, the effect of the noncrystalline PS block on the rate curves was only shifts of the degree of crystallinity [1 — A(t)] vs. time curve along the time axis. However, the extent of shift does not correspond to PS content in the block copolymers. The change in crystallization temperature also causes the crystallization curves to shift. At the initial stage of crystallization, these rate curves could be superimposed on the rate curve of the homo-PTHF (Mn = 6 X 104), and an Avrami exponent... 

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