The crystallization and melting of the α1 form of isotactic polypropylene (iPP) was studied with molecular dynamics (MD) simulations, using a fully flexible force field representing the C and H atoms atomistically and the CH3 groups as united atoms. Initially, a model crystal lattice of iPP of infinite molar mass was generated from experimental diffraction data, adding the pendant and geminal hydrogens. Next, crystal configurations of finite molar mass were created from this initial model lattice, and it was verified that the force field could preserve the crystal structure, keeping intact the monoclinic symmetry of the crystal. The generated configurations were heated under isobaric conditions, until a first-order transition into the melt state took place. The density change upon melting and the enthalpy of melting were estimated as differences between well-equilibrated ensembles of crystal and melt configurations. In order to calculate the equilibrium melting temperature T m, composite (sandwich) configurations consisting of both melt and crystal subdomains in contact with each other were generated and subjected to a series of isothermal−isobaric simulations at a variety of temperatures. T m was determined as that temperature where none of the phases in the sandwich grew at the expense of the other. As proximity to the glass temperature T g made the dynamics of crystal growth very sluggish, a constraining potential inducing helical conformations along the chains was introduced to accelerate crystallization. Using a novel Gibbs−Duhem integration scheme that utilizes data from both sandwich and single-phase solid and liquid simulations, results for equilibrium solid−liquid coexistence were extrapolated to zero constraining potential. An accurate estimate of T m was thereby obtained.

The melting point (T m ) of the α1 form of isotactic polypropylene (iPP) as a function of its molar mass was studied with molecular dynamics (MD) simulations. The 11mer, 32mer, and 47mer systems were simulated, with the latter displaying a T m representative of the long-chain polymer. Adopting a methodology developed in previous work, composite (sandwich) configurations consisting of both melt and crystal subdomains in contact with each other were generated, and T m was determined as that temperature where none of the phases in the sandwich grew at the expense of the other. To deal with the sluggish dynamics of solidification and melting, a constraining potential that drives chains toward helical conformations was added to the Hamiltonian, solid−liquid equilibration was achieved at high temperature in the presence of this potential, and T m was ultimately obtained through gradual removal of the constraining potential in single-phase solid and liquid simulations by Gibbs−Duhem integration. The enthalpy difference Δ fus H and entropy difference Δ fus S between single-phase solid and liquid were obtained in the absence of the constraining potential. During the melting process the up−down configuration of chains, the structure factor, the order parameter describing chain orientation relative to the c crystallographic axis, and motions characteristic of the rotator phase were studied along with the density and enthalpy.

No abstract

No abstract

We study, via united atom molecular dynamics (MD) simulations, the crystallization of six polyethylene (PE) systems, each consisting of alternating layers of molten linear polymer of two different degrees of polymerization, under stretch, while at the same time comparing against the corresponding results from two homogeneous monodisperse systems. The “slab,” out of which the model bidisperse layered systems are formed by applying periodic boundary conditions, is comprised of two different molten film subsystems, both of initial thickness 3.66 nm in the z‐direction, adhering to each other; in the first the chain length is 200 carbon atoms and in the second 400 carbon atoms. Following a short pre‐equilibration at 350 K and 1 bar, a stretching simulation is conducted in the NėxxLyPzzT ensemble, with the engineering strain rate ėxx fixed in the machine (pulling) direction x, parallel to the interfaces; the width of the films Ly fixed in the transverse direction; and the pressure Pzz normal to the interfaces fixed at 1 bar. The temperature during stretching is held constant at T = 300 K, corresponding to more than 100 K subcooling for both molar masses. Deformation to a stretch ratio of Lxx/Lxx,0 = 2.6 within 120 ns is followed by annealing according to two different protocols. In the first protocol, stretching is stopped and the systems allowed to relax in the NLxLyPzzT ensemble under Pzz = 1 bar for 60 ns. In the second protocol, stretching is continued at a rate 10 times lower than the initial one for 60 ns and then stopped, letting the systems relax for 100 ns. Crystallization is observed following both protocols. A nucleus of ordered material emerges early on during the first stage of stretching at a deformation rate of 0.05 nm/ns. The crystalline structure is sharper and more extended following the second, slower protocol. Crystallized macromolecules lie in planes almost parallel to the interface, forming a tilt angle of 20° to 24° with the pulling direction. Long and short chains co‐crystallize into lamellae, which are slightly more extended in the long‐chain regions. This study sheds light on the morphology expected to be developed in monomaterial packaging consisting of oriented bidisperse PE layers, designed to combine low permeability with recyclability and low‐environmental impact.