Daniel J. Lacks

A model of crystal polarization in β‐poly(vinylidene fluoride)

A model of the crystal polarization of β‐poly(vinylidene fluoride) utilizing an atomic potential energy function based on the shell model of electronic polarization is developed. Lattice constants, crystal polarization, and dielectric constants at finite temperatures are determined through minimization of the Gibbs free energy calculated using consistent quasi‐harmonic lattice dynamics. Molecular dynamics is used to include the effects of thermal oscillations of the dipoles. We find that in going from a single chain in vacuum to a chain packed in the crystal the repeat unit dipole increases by approximately 50% or 0.9 debye. Increasing temperature results in a decrease in polarization due to: (i) an increase in the unit cell volume, (ii) a decrease in the local electric field, and (iii) an increase in the magnitude of dipole oscillations. It is found that the dipole oscillation is consistent with the excitation of a single rotational lattice mode.

Oxazine Ring-Related Vibrational Modes of Benzoxazine Monomers Using Fully Aromatically Substituted, Deuterated, 15N Isotope Exchanged, and Oxazine-Ring-Substituted Compounds and Theoretical Calculations

Polymerization of benzoxazine resins is indicated by the disappearance of a 960-900 cm-1 band in infrared spectroscopy (IR). Historically, this band was assigned to the C-H out-of-plane bending of the benzene to which the oxazine ring is attached. This study shows that this band is a mixture of the O-C2 stretching of the oxazine ring and the phenolic ring vibrational modes. Vibrational frequencies of 3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine (PH-a) and 3-(tert-butyl)-3,4-dihydro-2H-benzo[e][1,3]oxazine (PH-t) are compared with isotope-exchanged and all-substituted compounds. Deuterated benzoxazine monomers, 15N-isotope exchanged benzoxazine monomers, and all-substituted benzoxazine monomers without aromatic C-H groups are synthesized and studied meticulously. The various isotopic-exchanges involved deuteration around the benzene ring of phenol, selective deuteration of each CH2 in the O-CH2-N (2) and N-CH2-Ar (4) positions on the oxazine ring, or simultaneous deuteration of both positions. The chemical structures were confirmed by 1H nuclear magnetic resonance spectroscopy (1H NMR). The IR and Raman spectra of each compound are compared. Further analysis of 15N isotope-exchanged PH-a indicates the influence of the nitrogen isotope on the band position, both experimentally and theoretically. This finding is important for polymerization studies of benzoxazines that utilize vibrational spectroscopy.

Sheared polymer glass and the question of mechanical rejuvenation

There has been much recent debate as to whether mechanical deformation reverses the aging of a material, and returns it to a structure characteristic of the system at a higher temperature. We use molecular dynamics simulation to address this problem by carrying out shear and temperature increase simulation on atactic glassy polystyrene. Our results show explicitly that the structure (as quantified by the torsion population) changes associated with shear and temperature increase are quantitatively--and in some cases qualitatively--different. This is due to the competition between rejuvenation and physical aging, and we show this by carrying out a relaxation simulation. The conclusion agrees with those from previous experiments and simulations, which were suggestive of mechanical deformation moving the system to structures distinct from those reached during thermal treatment.

Implications of the volume dependent convergence of anharmonic free energy methods

The free energies calculated by lattice dynamics in the quasiharmonic approximation and anharmonic perturbation theory are compared with exact results obtained by Monte Carlo simulation. The free energies obtained with the approximate methods are more accurate at smaller volumes. One implication of this result is that inclusion of interactions beyond nearest neighbor increases the accuracy of these methods due to the compression associated with the attractive longer range interactions. Also, these methods will be accurate to higher temperatures when thermal expansion is smaller, as is shown to be the case for the Morse crystal as compared to the Lennard‐Jones crystal.

A comparison of quasi-harmonic lattice dynamics and Monte Carlo simulation of polymeric crystals using orthorhombic polyethylene

The temperature dependence of lattice parameters, elastic constants and other physical properties of crystalline polyethylene at zero pressure in the orthorhombic phase is discussed. Two complementary approaches, self-consistent quasi-harmonic lattice dynamics and Monte Carlo simulation, both of which are predicated on the use of empirical force fields to describe the interatomic potentials, are critically compared. Both techniques are studied in their classical and quantum mechanical versions, to assess the accuracy and limitations of each method. Particular attention is paid to the classical approximation, the onset of anharmonicities in dynamical behavior which are not captured by the quasi-harmonic approximation, and finite size effects. It is shown that quantum effects are important throughout the range of temperatures 0⩽T⩽300 K. At temperatures below about 23 of the melting temperature (i.e., 250 K for polyethylene) the two approaches yield consistent results in both classical and quantum mechanical cases for a given empirical force field, provided that finite size effects are avoided. Above 300 K, anharmonic effects become quite pronounced. The combined treatment of these effects in the framework of path integral Monte Carlo (PIMC) pushes the limits of current computational feasibility, due to simulation sizes required. Guidelines are offered for choosing between classical simulations, quasi-harmonic methods, and full path integral Monte Carlo simulation.The temperature dependence of lattice parameters, elastic constants and other physical properties of crystalline polyethylene at zero pressure in the orthorhombic phase is discussed. Two complementary approaches, self-consistent quasi-harmonic lattice dynamics and Monte Carlo simulation, both of which are predicated on the use of empirical force fields to describe the interatomic potentials, are critically compared. Both techniques are studied in their classical and quantum mechanical versions, to assess the accuracy and limitations of each method. Particular attention is paid to the classical approximation, the onset of anharmonicities in dynamical behavior which are not captured by the quasi-harmonic approximation, and finite size effects. It is shown that quantum effects are important throughout the range of temperatures 0⩽T⩽300 K. At temperatures below about 23 of the melting temperature (i.e., 250 K for polyethylene) the two approaches yield consistent results in both classical and quantum mechanical...

Atomic mobility in a polymer glass after shear and thermal cycles.

Molecular dynamics simulations and energy landscape analyses are carried out to study the atomic mobility of a polymer glass during the physical aging process that follows shear and thermal cycles. The mobility is characterized by the fraction of atoms moving more than their diameter in a given time interval. The mobility is enhanced after a shear or thermal cycle, and this enhancement decays with time. These mobility results are related to the position of the system on the energy landscape, as characterized by the average energy of the energy minima visited by the system; the mobility over longer time scales increases with the average energy of the energy minima visited, but the mobility over shorter time scales does not show a correlation with this average energy. From these results, we conclude that barriers separating metabasins composed of proximate energy minima, rather than barriers between individual energy minima, control the physical aging process. We also show that, after some finite time, the mobility following shear and thermal cycle appears to behave similarly to the mobility without perturbations; however, the system is at different regions of the energy landscape in these two cases, which implies that mobility alone does not characterize the state of the system.

Fold catastrophes and the dependence of free-energy barriers to conformational transitions on applied force.

Applied mechanical force (f) can activate conformational change in molecules by reducing the height of a free-energy barrier (DeltaG(b)). In this paper, molecular dynamics simulations are carried out with umbrella sampling and self-consistent histogram methods to determine free-energy profiles for a coarse-grained model of a protein under an applied force. Applied force is shown to cause fold catastrophes, where free-energy minima are destabilized until they disappear. It is well-known that a fold catastrophe at force f = B implies the scaling DeltaG(b) approximately |B - f|(3/2) in the limit of DeltaG(b) --> 0, but it is not clear whether this scaling is accurate for physically relevant barrier heights. The simulation results show that the fold catastrophe scaling is in fact accurate in the physically relevant regime and that the two-parameter function DeltaG(b) = A(B - f)(3/2) is superior to the two-parameter linear function for parametrizing changes in free-energy barriers with applied force.

Mechanisms for axial thermal contraction in polymer crystals: polyethylene vs isotactic polypropylene

Abstract We show that the mechanism for the negative axial thermal expansion is fundamentally different in polyethylene (PE) and isotactic polypropylene (iPP) crystals. For PE, axial contraction increases the entropy, thereby leading to negative axial thermal expansion. In contrast, for iPP, axial expansion increases the entropy, which alone would lead to positive axial thermal expansion. The negative axial thermal expansion in iPP occurs as an elastic response to the positive transverse thermal expansion, and is based on potential energy effects.

Molecular Dynamics Investigation of the Effects of a Water Surface on the Aggregation of Bent-Core Molecules

Molecular dynamics simulations are used to determine how the presence of a water surface affects the way that bent-core surfactant molecules interact with one another. The simulations are carried out for isolated pairs of bent-core molecules, and for pairs of bent-core molecules on a water surface. The results show that the water surface fundamentally alters the nature of the interaction between the bent-core molecules: a stable complex is formed when the two molecules are on the water surface, but not for an isolated pair of molecules. This difference occurs because the water surface constrains the internal structure and orientation of the molecules, which makes the packing of the molecules into a stable complex more thermodynamically favorable.

Molecular dynamic simulations of self-assembly of amphiphilic comb-like anionic polybenzoxazines.

Fully atomistic molecular dynamic simulations were performed to address the self-assembly of amphiphilic and comb-like polybenzoxazines (iBnXz) in water, with i = 3 (trimer), i = 4 (tetramer); i = 6 (hexamer), i = 8 (octamer), and i = 10 (decamer). Spontaneous aggregation of these comb-like polybenzoxazine molecules into a single micelle occurs in the simulations. The simulations show that molecular size and concentration play important roles in micellar morphology. At an iBnXz concentration of 50 mM, the 3BnXz and 4BnXz molecules aggregate into spherical micelles, whereas the 6BnXz, 8BnXz, and 10BnXz molecules aggregate into cylindrical micelles. The micellar morphology is spherical at low concentrations, but undergoes a transition to cylindrical shape as concentration increases. The transition point depends on the molecular size-both the true size as indicated by molecular weight, as well as an additional effective size dependent on molecular flexibility.