Poly during isothermal crystallization

Answers to these questions can be obtained if one performs small angle scattering measurements during isothermal crystallization. Such measurements have been performed on different materials. The results were observed to depend strongly on the material used. So, for example, with increasing crystallization time, the long period increased with polyethylene, it decreased with polyethylene terephthalate and it stayed constant with poly-P-hydroxybutyrate. 

A series of morphological images captured during isothermal crystallization of poly(p-diox-anone), PDS, at 85°C by (a) AFM, (b) HSOM, and (c) SALS. 

Figure 9.8 Evolution of K" z) during isothermal crystallization of i-poly(propylene-co-octene) at 125.5 °C (b) for different times t and during subsequent heating up to the melt (a). Reprinted with permission from [15]. Copyright 1999 lOP Publishing Ltd.

Figure 133 Polarized optical micrograph of PLA composite with 5 wt% of hemp fibers during isothermal crystallization at 116°C. Reprinted from Masirek, R., et al. Composites of poly(L-lactide) with hemp fibers morphology and thermal and mechanical properties. J. Appl. Polym. Sci. 2007,105,255-268, with kind permission from WUey, Copyright 2007.

Figure 32 Crystallization kinetics for the poly(ethylene oxide) block in triblock terpolymers with a rubbery end biock (poiybutadiene-Wocfr-polystyrene-b/oc/f-poly(ethylene oxide)) or a crystalline end block (polyethylene-b/oc/f-polystyrene-Wock -poiy(ethylene oxide)) (a) deveiopment of the relative crystallinity with crystallization time during isothermal crystallization at 49.5 °C, and (b) inverse of experimentai crystaiiization haif-time as a function of crystallization temperature. Reprinted with permission from Boschetti-de-Fierro, A. etal. Macromol. Chem. Phys. 2008,209,476- 87. ...

For some polymers such as isotactic polystyrene (iPS), polycarbonate, and PHB, reversing heat capacity from TMDSC equals baseline heat capacity for temperatures near glass transition. For other polymers also in isothermal TMDSC experiments latent heats may contribute to the measured reversing heat capacity at the generally low frequencies available by TMDSC. Figure 11 shows the development of measirred reversing heat capacity during isothermal crystallization of polyamide 12. Similar observations were made for PE, poly(c-caprolactone) (PCL), and polyetheretherketone (PEEK) to name a few. 

The rate at which these spherulites grow is also affected by tacticity. Measurements were made of the Isothermal rate of spherulitic growth on a hot stage of a microscope. For this purpose, a sequence camera was used to photograph automatically the growing spherulites during crystallization. Dilatometric measurements were also made on these polymers in order to determine the rate of isothermal crystallization from the melt. The fraction of the polymer which had crystallized at any time was calculated from the measured density and the known values of the density of crystalline and amorphous poly(propylene oxide) (8,2,... 

Figure 930 Multiple melting peaks of poly(lactide acid) recorded during heating (a) after isothermal crystallization at 130°C for 5 hours followed by annealing at 120°C and 145°C forlO hours, (b) after isothermal crystaUization at 130°C for 5 hours.

Here, i/gph is the hardness value of the spherulites, is the hardness of the amorphous interspherulitic regions, and is the volume fraction of crystallized spherulites. During primary crystallization, Hgph remains constant and hardness is directly proportional to the volume occupied by the spherulites (12). The hardness variation in the course of isothermal crystallization of PET and poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) has been shown to follow Avrami law (13,14). 

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