A tin-containing liquid crystalline polymer with two glass temperatures

A tin-containing liquid crystalline side group polymer was synthesized and characterized. Two glass transitions were detected by calorimetric investigations. The X-ray pattern corresponds to a smectic C order of the side groups and a disordered isotropic main chain. Dielectric measurements show two relaxation ranges which are influenced by the glass transitions and a fast local process. The low frequency mechanism can be related to the reorientation of the side groups and the higher glass transition temperature. The second is connected with the α-relaxation of the main chain and freezes in at lower temperatures.     

On the Effect of Secondary Nucleation on Deracemization through Temperature Cycles

Herein, the pivotal role of secondary nucleation in a crystallization-enhanced deracemization process is reported. During this process, complete and rapid deracemization of chiral conglomerate crystals of an isoindolinone is attained through fast microwave-assisted temperature cycling. A parametric study of the main factors that affect the occurrence of secondary nucleation in this process, namely agitation rate, suspension density, and solute supersaturation, confirms that an enhanced stereoselective secondary nucleation rate maximizes the deracemization rate. Analysis of the system during a single temperature cycle showed that, although stereoselective particle production during the crystallization stage leads to enantiomeric enrichment, undesired kinetic dissolution of smaller particles of the preferred enantiomer occurs during the dissolution step. Therefore, secondary nucleation is crucial for the enhancement of deracemization through temperature cycles and as such should be considered in further design and optimization of this process, as well as in other temperature cycling processes commonly applied in particle engineering.     

Molecular dynamics investigations on polishing of a silicon wafer with a diamond abrasive

During the final stages of polishing silicon wafers, much of the interactions between silicon and diamond abrasive takes place at the silicon asperities. These interactions, leading to material removal, were investigated in a MD simulation of polishing of a silicon wafer with a diamond abrasive under dry conditions. Simulations were conducted with silicon asperities of different geometries, different abrasive configurations, and polishing speeds. Under the conditions of polishing, the silicon atoms from the asperities were found to bond chemically to the surface of the diamond abrasive. Continued transverse motion of the diamond abrasive (relative to the silicon asperity) leads to tensile pulling, necking, and ultimate separation of the silicon asperity material instead of conventional material removal in polishing (chip formation) involving cutting/ploughing, which takes place in the absence of chemical bonding between the abrasive and the asperity material. This phenomenon has not been reported previously in the literature. The thrust and cutting forces initially increase due to the increase in the number of asperity atoms affected finally reaching a maximum. This is followed by a decrease of these forces due to tensile pulling and formation of individual strings followed by ultimate separation or breakage of the final string. The ratio of thrust force (F _z ) to the cutting force (F _x ), i.e. |(F _z /F _x )| was found to increase continuously to a maximum of ～0.8 followed by continuous decrease to ～0.25. This is in contrast to a more or less constant value of ～2 in the case of tools with rounded radii or tools with large negative rake angles, where material is removed in the form of chips ahead of the tool. Three regions of the asperity have been identified that are useful in the development of a phenomenological model for polishing that enables computation of material removal rates: (1) the region directly in front of the abrasive for which the probability of the removal of an asperity atom is close to unity, (2) the distant region where this probability is nearly zero, and (3) an intermediate region from which the probability of removal is close to half.     

Simulation of the gelation process of hydrogel droplets in 3D bioprinting

The aim of this work is to numerically simulate the gelation of a crosslinking polymer, which is a very complex process involving chemical reactions and phase transitions from a viscous fluid to a viscoelastic solid. A phenomenological model is proposed to simulate the gelation process of agarose droplets, considering the thermal boundary conditions. The numerical model is implemented using the finite element package FlexPDE. The temperature- and time-dependent degree of gelation and the deformation of the droplets during the gelation process are investigated. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

APplication of Continuous Thermodynamics to the Stability of Polymer Systems. II

A determinant criterion for the critical state in solutions and mixtures of polydisperse polymers is established within the general framework of Gibbs theory. The treatment continues an earlier paper by considering more general Gibbs free energy relations: The function replacing the x-term in the classic Flory-Huggins equation is permitted to depend on a finite number of moments of the polymer distribution(s) so as to embrace most Gibbs free energy relations of practical use. The new criterion leads to a very large reduction of computer time and of needed storage capacity compared to the traditional Gibbs determinant criterion. Some relations known from the literature are shown to be special cases of the established new criterion.     

Nucleation of Organic Crystals—A Molecular Perspective

The outcome of synthetic procedures for crystalline organic materials strongly depends on the first steps along the molecular self‐assembly pathway, a process we know as crystal nucleation. New experimental techniques and computational methodologies have spurred significant interest in understanding the detailed molecular mechanisms by which nuclei form and develop into macroscopic crystals. Although classical nucleation theory (CNT) has served well in describing the kinetics of the processes involved, new proposed nucleation mechanisms are additionally concerned with the evolution of structure and the competing nature of crystallization in polymorphic systems. In this Review, we explore the extent to which CNT and nucleation rate measurements can yield molecular‐scale information on this process and summarize current knowledge relating to molecular self‐assembly in nucleating systems.