Bond disorder, frustration and polymorphism in the spontaneous crystallization of a polymer melt

Abstract

The isothermal, isobaric spontaneous crystallization of a supercooled polymer melt is investigated by molecular-dynamics simulation of an ensemble of fully-flexible linear chains. Frustration is introduced via two incommensurate length scales set by the bond length and the position of the minimum of the non- bonding potential. Marked polymorphism with considerable bond disorder, distortions of both the local packing and the global monomer arrangements is observed. The analyses in terms of: i) orientational order parameters characterizing the global and the local order and ii) the angular distribution of the next-nearest neighbors of a monomer reach the conclusion that the polymorphs are arranged in distorted Bcc-like lattice

Elastic modulus and yield strength of semicrystalline polymers with bond disorder are higher than in atomic crystals

“…Note that in the atomic liquids the size of the jump depends on the final crystalline state, as expected owing to the better packing of the Fcc lattice with respect to the Bcc one. The detailed characterization of the polymorphic structure of the polymer solid is reported elsewhere [19]. Notice that the jump is smaller for polymers (∼ 3.3%) than atomic liquids ( 4.5%) even if the polymer melt has lower density.…”

“…see Fig.3 and ref. [19]. The softer character of the Bcc (ex) solid with respect to the ideal Bcc crystal is apparent.…”

“…As to the polymer systems, a coarse-grained polymer model of N c = 50 linear, fully-flexible, unentangled chains with M = 10 monomers per chain is considered [19]. The total number of monomers is N = 500.…”

“…The local order parameter Q loc l is defined as [40]: Cross correlations between Q loc l and Q glob l with l = 4, 6 proved to be rather useful to characterize the order of the solid phases [19]. Fig.3 plots the crosscorrelations for l = 6 (top) and l = 4 (bottom) for all the solids under study.…”

“…To increase the readability, the plots also present the pairs (Q loc l , Q glob l ) with l = 4, 6 corresponding to the ideal Bcc and Fcc atomic lattices. In addition, since the Bcc lattice is known to be less stable then the Fcc one [19,44,45], the pairs (Q loc l , Q glob l ) with l = 4, 6 of a Bcc excited crystal are also presented. The latter is obtained by heating the ideal Bcc crystal to temperature T = 0.7 with P ≃ 6.5 and, after short equilibration, quenching it at T = 10 −3 , P = 0.…”