Polyethylene melt crystallized

The effect of diluents on the melting behavior of polyethylenes has also been studied by Ke (168). A comparison between the melting transitions of solution and melt-crystallized polyethylene has been made. The measurement of the melting and second-order transitions of polyethylene tereph-thalate by DTA has been studied by Scott (169). Rudin et al. (170) measured the oxidation resistance of various polymers and rubbers by a DTA method. A comparison of the melting and freezing curves before and after oxidation provided the indication of the extent to which the polymer had been damaged or oxidized. 

SANS and SAXS from Melt-Crystallized Polyethylene... 

FIGURE 23.5. (Q) vs Q for SAXS and SANS data from melt crystallized polyethylenes. 

Morphologies of Deformation. Figure 2 shows a series of calculated stress-strain curves for melt-crystallized polyethylene at three different molecular weight values (M = 1900, 9500, and 250,000). The lowest molecular weight sample exhibits a low elongation at break and brittle failnre. As the molecnlar weight is increased (M = 9500), the stress-strain curve exhibits a so-called yield... 

Fig. 3. Dependence of morphology on molecular weight. 7bp Deformation morphologies obtained from the model for melt-crystallized polyethylene at increasing values of the molecular weight. From left to right M = 1900, M = 8500, M = 20,000, and M = 250,000. Bottom Experimental morphologies obtained at comparable molecular weights. Reprinted with permission from Ref 8. Copyright (1987) American Chemical Society.

Fig. 5. Calculated dependence of the maximum draw ratio on testing conditions for melt-crystallized polyethylene with M = 143,000. (a) Dependence on temperature at constant elongation rate (6.25 min ) (b) Dependence on elongation rate at constant temperature (130°C). Reprinted with permission from Ref 12. Copyright (1988) American Chemical Society.

Fig. 4. Melt-crystallized polyethylene lamellae (a) linear polymer crystallized at 130°C as a planar crystal in which successive layers spiraling around the central (etched-out) giant screw dislocation are not in contact (b) ridging along 6 in a 17,000 mass fraction of linear poljrmer crystallized at 129°C (c) an ethyl-branched copolymer crystallized at 123°C showing a central S-profile, asymmetrically placed screw dislocations and new layers diverging therefrom. From Ref. 66.

Melt-Crystallized Polymers Upon crystallization from the melt, an entirely different result emerges. Experiments by Sadler and Keller (120-122) showed that nearly random stem reentry was most likely that is, some type of switchboard model was correct. Quantitative calculations by Yoon and Flory (125-127) and by Dettenmaier et al. (128,129) on melt-crystallized polyethylene (130) and isotactic polypropylene (131) also showed that adjacent reentry should occur only infrequently on cooling from the melt. 

Bassett DC, Hodge AM, Olley RH (1979) Lamellar morphologies in melt-crystallized polyethylene. Faraday Discuss Chem Soc 68 218-224... 

Attempts have been made to determine the relationship between the structure and mechanical properties in semicrystalline polymers. If melt-crystallized polyethylene is subjected to plane-strain... 

Morin FG, Delmas G, Gilson DFR. Variable-Temperature Solid-State C NMR Studies of Nascent and Melt-Crystallized Polyethylene. Macromolecules 1995 28 3248. 

Figure 15. Maximum draw versus molecular weight of melt-crystallized polyethylene, under optimized conditions. Data from Ward et al. (30). Also plotted are the calculated average maximum draw ratios for isolated polyethylene chains. Note the dramatic discrepancy between the calculated and experimentally observed maximum draw ratios.

Melt crystallized polyethylenes can vary in cr3retallinity from 40 to almost 100%. The low crystallinity samples are usually branched (l%h pressure polyethylene), while the high crystallinity samples are more linear. [But see Magill, Pollack, and Wyman (1965) for low crystallinity linear polyethylene.] Also, it becomes more difficult to crystallize samples with high molecular weights. 

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

---