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We present an ab initio molecular dynamics study of the temperature-induced phases of methylammonium lead bromide (MAPbBr 3 ). We confirm that the low-temperature phase is not ferroelectric and rule out the presence of any overall polarization arising from the motion of the individual sublattices. Our simulations at room temperature resulted in a cubic Pm3̅ m phase with no discernible local orthorhombic distortions. At low temperatures, such distortions are shown to originate from octahedral scissoring modes, but they vanish at room temperature. The predicted timescales of MA motion agree very well with experimental estimates, establishing dynamic disordering of the molecular dipoles over several orientational minima at room temperature. We also identify the key modes of the inorganic and organic sublattices that are coupled at all temperatures, mainly through the N−H•••Br hydrogen bonds. Estimated lifetimes of the H bonds correlate well with MA dynamics, indicating a strong connection between these two aspects of organic−inorganic hybrid perovskites. We also confirm that, in addition to the disordering of MA orientations, the transition to the cubic phase is also associated with displacive characteristics arising from both MA and Br ions in the lattice.

Pressure-induced phases of MAPbBr 3 were investigated at room temperature in the range of 0−2.8 GPa by ab initio molecular dynamics. Two structural transitions at 0.7 GPa (cubic → cubic) and 1.1 GPa (cubic → tetragonal) involved both the inorganic host (lead bromide) and the organic guest (MA). MA dipoles behave like a liquid crystal undergoing isotropic → isotropic and isotropic → oblate nematic transitions as pressure confines their orientational fluctuations to a crystal plane. Beyond 1.1 GPa, the MA ions lie alternately along two orthogonal directions in the plane forming stacks perpendicular to it. However, the molecular dipoles are statically disordered, leading to stable polar and antipolar MA domains in each stack. H-Bond interactions, which primarily mediate host−guest coupling, facilitate the static disordering of MA dipoles. Interestingly, high pressures suppress CH 3 torsional motion, emphasizing the role of C−H•••Br bonds in the transitions.

Flex barrier and optical density (OD) are two unique properties for metallized films and aluminum foil. This study investigated the effect of flexing on barrier properties of a laminate comprising metallized films and aluminum foil, as well as OD and adhesive lamination process on the overall multilayer laminate performance. Three (3) barrier layers, namely aluminum foil, metallized polyethylene terephthalate (met OPET) film with a high OD, and standard metallized PET film(met OPET), were laminated with the same printing layer OPET and sealing layer linear low density polyethylene (LLDPE), into OPET/adhesive/foil/adhesive/LLDPE and OPET/adhesive/met OPET/adhesive/LLDPE structures. The oxygen transmission rate (OTR) and water vapor transmission rate (WVTR) were measured before and after flexing. The aluminum foil centered laminate had an excellent oxygen barrier after the 20-cycle flexing test and failed to retain its oxygen barrier after the 270-cycle flexing. Compared to the aluminum foil centered laminate, the metallized film centered laminates were less affected by the flexing on the oxygen barrier. For all laminates, the water vapor barrier was less severely affected by flexing than the oxygen barrier. This study suggests that the ASTMF392 Gelbo D (20- cycle flexing) can determine if aluminum foil and metallized film centered laminates are resistant to flex-formed pinhole failures. A higher OD, a thicker film thickness and lamination process improved laminate’s actual barrier, resulting in lower measured transmission rates versus that predicted using Henry's solubility law and Fick's diffusion law. The laminate’s water vapor transmission rate is 55–74% lower than predicted. The oxygen transmission rate is 15–31% lower than predicted.

In this paper, the influence of molecular structure variations on the mesophase behavior, photocurability, and film properties of polymerizable mesogenic diacrylates is reported. The effect of methoxy or chloro substituent on the mesophase behavior of the monomers, present at the 2‐position of the phenylene core, in comparison to the unsubstituted core, has been studied using thermal and photo‐DSC techniques. These monomers were used in combination with a chiral dopant to form cholesteric mesophases, which was subsequently crosslinked through photopolymerization. Optical microscopy and polarized optical microscopy (POM) were used to study the morphology of monomers as well as the photopolymerized films. Optical images show the influence of chiral dopant on the texture present in the films, both before and after the crosslinking. POM images show the mesophase present in the monomers and the free films. Evaluation of viscoelastic properties of the photocured films demonstrates the influence of mesogenic core substituents on the network transition temperature, modulus, and damping characteristics. Copyright © 2008 John Wiley & Sons, Ltd.