Semicrystalline polymers have a metastable nanophase structure, which is determined by a competition among self-organization, crystallization and vitrification, and is established during material processing. Early investigations of semicrystalline polymers based their description on a two-phase model, where the two phases, an amorphous and a crystalline one, have nanometer dimensions in one or more directions. More detailed analyses revealed that an intermediate fraction is present at the interface between the crystals and the surrounding melt. The intermediate fraction is non-crystalline, and includes portions of macromolecules whose mobility is hindered by the near crystalline structures. It is generally named "rigid amorphous fraction" (RAF), being its mobility lower than that of the unstrained amorphous phase, which is addressed as "mobile amorphous fraction" (MAF).
Our study on the three-phase structure of isotactic poly(1-butene) (PB-1) revealed that amorphous and crystalline phases in PB-1 are largely coupled, with points of coupling lying on the boundary between the phases. These act as nanoscopic stress-transfer, with remarkable effects on mechanical properties of the material. Hence, for a complete description of the mechanical behavior, it is necessary to account for the role played by all the three nanophases. As shown in the enclosed figures, a correlation of the Young's modulus with the solid fraction at the temperature of analysis, composed of crystalline and rigid amorphous parts, was proposed. This is expected to hold not only for isotactic poly(1-butene), but also other semicrystalline polymers containing a considerable rigid amorphous fraction.
Analysis on the complex melting behavior of cis-1,4-polybutadiene (PBD) probed a close relation between crystal fusion and devitrification of the rigid amorphous fraction. The overall multiple melting behavior of PBD is affected by the physical state of the RAF, since the partial melting-recrystallization processes that cause multiple melting can take place only if the rigid amorphous fraction attains sufficient mobility to allow development of perfected crystals with improved thermal stability that melt at higher temperatures.
The research project was partially financed by MIUR.
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