Crystallization and Melting Simulations of Oligomeric α1 Isotactic Polypropylene

Abstract

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.

Melting Point and Solid–Liquid Coexistence Properties of α1 Isotactic Polypropylene as Functions of Its Molar Mass: A Molecular Dynamics Study

“…It can be seen from Figure 9 that it follows the melting process of the systems in question, its change displaying first-order transition characteristics. The order parameter was calculated as in our previous paper, 12 using the equation…”

“…In order to construct the unit cell with all the chains from crystallographic data, 49 including the hydrogens connected to the backbone atoms, the same procedure was used as in our previous work. 12 The construction of the hydrogens again followed ref 51. III.B. Force Field.…”

“…Construction of Crystalline Configurations of Finite Molar Mass and Generation of Sandwich Model Systems. Both the crystalline configurations of finite molar mass and the generation of sandwich model systems followed exactly the same procedures as discussed in our previous paper (ref 12), sections III.D and III.E.…”

Strength of Alkane–Fluid Attraction Determines the Interfacial Orientation of Liquid Alkanes and Their Crystallization through Heterogeneous or Homogeneous Mechanisms

“…Alkanes are modeled with two different united-atom (UA) force fields: PYS [14][15][16] and OPLS [17,18]. These force fields have been widely used to investigate the structure, interfacial properties and phase transitions of alkanes [4,12,[19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37]. Water is modeled with the monatomic water model, mW [38], which has been extensively used to study the structure, thermodynamics, interfacial properties, and phase transitions of water .…”

Effects of composition on microstructure and crystallization behavior for impact polypropylene copolymer investigated by restructuring the complex core-shell dispersed particles in ternary blends

“…where σ e is the free energy for folding surface, Δz is the length of three folded monomer units (helical structure, Δz = 0.65 nm) [38] , Δh f is the heat of fusion per unit volume (166.608 J/cm 3 for HPP [13] ), and T m 0 and T m are equilibrium melting point and melting point respectively. The calculated lamellar thicknesses are listed in Table 2.…”