Thermokinetic analysis of polymorphous transformations in the crystallization and orientation of flexible‐chain polymers

Elyashevich, G. K.; Poddubny, V. I.; Bezprozvannykh, A. V.

doi:10.1002/actp.1990.010410302

Cited by 4 publications

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“…However, the T m is equal to 176°C for PVDF and 275°C for TFE-E CP. This means that the higher T m of the latter is due to its lower flexibility in comparison to PVDF and its specific capacity for polymorphism [4].…”

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Correlation of the chemical composition, structure, and mechanical properties of oriented fluoropolyolefins

Bezprozvannykh

2008

Fibre Chem

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Structural and mechanical studies of fluorine-containing polymers were conducted. It was found that the mechanical characteristics of oriented materials are a function of the composition of the copolymer, intermolecular interactions, and chain flexibility.The group of fluorine-containing polymers includes products with very varied properties: rigid plastics, thermoplasts, and elastomers; insoluble and nonswelling polymers and copolymers; materials that withstand prolonged irradiation; fibres stronger than highly alloyed steel; corrosion-resistant coatings; films and fibres with unique dielectric properties.The structure of polytetrafluoroethylene (PTFE) chains causes them to twist into a stable coil, which in a block is similar to semirigid cylinders with excess real free volume even in close packing. The weak intermolecular interaction, which predetermines the unique triboscopic and adhesive properties, results in a number of crystallization and melting anomalies. The main one is that even almost totally crystallized PTFE is difficult to distinguish from the tactoid mesophase and is subject to plastic flow much lower than the melting point (T m ). This a priori prevents obtaining high strength and modulus, although the fluidity can be used for a relatively convenient manufacturing process based on extrusion from the solid phase.In contrast to PTFE, copolymers based on perfluorinated or incompletely substituted fluoromonomers are typical amorphous-crystalline materials. The morphology of the fibres and films made from them can easily be regulated both with the usual copolymerization methods or by incorporation of massive side groups, as well as by changing the extrusion conditions. This is the basic advantage of these polymers over PTFE.The rheological properties of melts and solutions of hydrogen-containing fluoropolymers are due to the important intermolecular interactions of the hydrogen bond type and the appearance of developed spatial networks, which significantly enhances the non-Newtonian character of flow in them in comparison to perfluorinated polymers.Two types of positioning of trans-chains are possible in polyvinylidene fluoride (PVDF), the copolymer of vinylidene fluoride (VDF) and tetrafluoroethylene (TFE) containing 3-20 mole % TFE, and the copolymer of TFE with ethylene (TFE-E): a planar trans-conformation of the "comb" type (a) and a trans-conformation with formation of a H bond (b). Hydrogen atoms valently bound with carbon atoms participate in the formation of a hydrogen bond in incompletely fluorinated copolymers and simultaneously react with the unshared electron pairs in the fluorine atoms.The interaction energy (E) was calculated in the atom-atom approximation [1] with consideration of the electrostatic effect (E es ) on the constituent unit (CH 2 -CF 2 ) 2 at calculated equilibrium distances of 4.3 and 4.5 Å for conformation a and b:where r ik is the distance between atoms a i and b k , α ik , B ik are constants which are a function of the species of the atoms; V ik is the overall interact...

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confidence: 98%