Concurrent crystallization

When dealing with miscible blends containing two crystalline components, several modes of crystallization are possible separate crystallization, concurrent crystallization, co-crystallization, etc. Only those blends in which both components are miscible in the melt are considered here (Table 3.3). PET/PBT blends were reported to be an example of separate crystallization [Escala and Stein, 1979 Stein et al., 1981]. A spherulitic crystallization was observed for the neat components as well as for blends with small amounts of one component, and the crystals of the minor component were included within the spherulites of the major component, which results in a coarsening of the spherulitic texture. Transesterihcation is, however, the reason for the homogenous amorphous phase. 

A simultaneous (or concurrent) crystallization can only occur when the crystallization temperature ranges overlap and if the crystallizability of both blend components is similar. Cocrystallization is only possible when the components are isomorphic or miscible in the amorphous as well as in the crystalline phase. In both cases mixed crystals can result, but in the case of concurrent crystallization no changes in crystal strucmre may be induced. Cocrystallization requires chemical compatibihty, close matching of the chain conformations, lattice symmetry and comparable lattice dimensions [Olabisi et al., 1979]. Some examples of miscible polymer blends with two crystalline components are given in Table 3.3 together with the type of crystalhzation. 

It should be noted that not all authors use the same terminology concerning the type of crystallization. Especially the terms cocrystallization and concurrent crystallization are often confused. Since some authors did not examine whether the lattice parameters change are not, it is not possible to decide if they mean cocrystallization or concurrent crystallization. Different molecular weight fractions... 

PP/PA-6 maleated-PP (PP-MA) Thermal analysis and optical microscopy. One or two crystallization peaks were affected by PP-MA. T of PA-6 initially decreased Uien leveled off with PP-MA content, whereas that of PP was not affected. Blends with PP-MA showed concurrent crystallization at the T of PP. Moon et ah, 1994... 

Recent studies (Defieuw 1989) indicate that the crystallization conditions, blend composition, chain rigidity and microstracture, and molecular weight of the components are important. Blends consisting of two crystallizable components (Sect. 3.2.2.2) can exhibit separate crystallization or concurrent crystallization (cocrystallization). 

When dealing with miscible blends containing two crystalline components, several modes of crystallization are possible separate crystallization, concurrent crystallization, cocrystallization, etc. Only those blends in which both components are miscible in the melt are considered here (Table 3.3). PET/PBT blends were reported to be an example of separate crystallization (Escala and Stein 1979 ... 

A simultaneous (or concurrent) crystallization can only occur when the crystallization temperature ranges overlap and if the crystallizability of both blend s... 

The type of added component is also important. Crystallization in the presence of an amorphous component is paralleled to segregation. The segregation can occur into three regions interspherulitic, interfibrillar, and interlamellar, depending on the ratio of the diffusion rate of the amorphous component and of the crystallization rate of the crystallizable component. In blends of two crystallizable polymers, the phenomena such as separate crystallization, concurrent crystallization, and cocrystaUization may take place. 

Thermodynamically speaking, 2-piperidone is slightly more polymerizable than 2-pyrrolidone, but kinetically less reactive. The low kinetic polymerizability has been attributed to the concurrent crystallization of the growing chains and consequent physical termination as well as the relevance of side reactions, allowed to extensively occur because of the slowness of the polymerization. Activators have been found to be always essential for the polymerization of 2-piperidone. Relatively high molar masses of poly(2-piperidone) have been achieved only when quaternary ammonium salts have been used as initiators. The resultant polyamide has a Tm of 283°C, higher than that of poly(2-pyrrolidone), 260 °C, and is thermally much more stable than the latter. Therefore, melt spinning can be safely carried out and fibers with good characteristics are obtained. 

Several examples of miscible blends with both crystallizable components have been reported for polyolefins and olefin copolymers, such as high density polyethylene (HDPE)/linear low density polyethylene (LLDPE), HDPE/very low density polyethylene (VLDPE), LDPE/ VLDPE, and PP/PB, with the occurrence of concurrent crystallization and/or co-crystallization phenomena [19,... 

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