The influence of chain structure on the equilibrium melting temperature of poly(vinylidene fluoride)
Tóm tắt
Three poly(vinylidene fluoride) whole polymers were fractionated according to their head‐to‐head concentrations. Their melting temperatures and fusion properties were studied. Although small but significant differences were found among the fractions obtained from a given parent polymer, a wide range in chain compositions was not obtained. The equilibrium melting temperatures were determined by extrapolating the dependence of the observed melting temperature on the crystallization temperature. A critical analysis is given of this extrapolation method as applied to poly(vinylidene fluoride) and the results are compared with literature reports. The problems involved in explaining the dependence of the equilibrium melting temperatures on the structural irregularities of the chain are given. Possible reasons for the relatively high level of crystallinity that is observed, for what is essentially a copolymer, are also discussed.
Từ khóa
Tài liệu tham khảo
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We should note that this value of δHuwas determined for the α crystalline form. The conventional application of this value to the γ form involves an uncertain error.
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R. G.AlamoandL.Mandelkern to be published.
The calculated crystallinity levels are based on δHu= 1600 cal/mol of repeating unit. This quantity was determined from melting point depressions with diluent on the assumption that the crystalline phase(α‐from) remains pure. If the structural irregularities enter the lattice on the equilibrium basis in significant amount and vary with composition then the value deduced for (1‐λ)δHcould be slightly altered. Hence the values given for (1‐λ)δHcan only be considered approximate.
Ferguson R. C., 1984, Polymer Preprints, ACS a Division of Polym. Chem., 25, 340
Similar observations in the unfractionated polymer studied here also lead to much higher values than have been reported.
A melting temperature versus composition relation has been reported which attempts to account for the case where counits are present in the crystal lattice on either an equilibrium or nonequilibrium basis.48‐49This theoretical development is based on composition rather than sequence distribution. The fact that the repeating units are covalently liked one to the other is not taken into account. Therefore the theory can only be applied to monomeric systems and is not valid for copolymers.