Two kinds of b-nucleating agents, named a rare earth complex (WBG) and a N,N 0 -dicyclohexylterephthalamide (TMB5), were introduced into isotactic polypropylene (iPP), and their effect on crystallization and melting behavior of iPP was comparatively investigated. Wide angle X-ray diffraction measurements revealed that both the two additives were highly effective in inducing b modification. At their respective optimum concentrations of 0.08 wt % for WBG and 0.06 wt % for TMB5, the relative amount of bform calculated by Turner-Jones equation both exceeds 92%. However, the isothermal crystallization kinetics investigated by differential scanning calorimetry demonstrated that WBG had more pronounced effect than TMB5 in accelerating the overall crystallization rate. The LauritzenHoffman theory analysis also revealed that WBG was more effective not only in increasing the nucleus number but also in accelerating the growth rate of crystallization. After completing isothermal crystallization process, the subsequent melting behavior examination suggested that the addition of WBG expanded the upper limit temperature of forming b modification, and therefore was more effective in delaying the b-a transformation than TMB5.
A molecular framework based on guanidinium cations and 1,2,4,5-tetra(4-sulfonatophenyl)benzene (TSPB), an aromatic tetrasulfonate with nominal 2-fold and mirror symmetry, exhibits three crystallographically unique one-dimensional channels as a consequence of molecular symmetry and complementary hydrogen bonding between the guanidinium (G) ions and the sulfonate (S) groups of TSPB. Unlike previous GS frameworks, this new topology is sufficiently flexible to permit reversible release and adsorption of guest molecules in large single crystals through a cyclic shrinkage and expansion of the channels with retention of single crystallinity, as verified by single crystal X-ray diffraction. Moreover, the G4TSPB framework permits guest exchange between various guest molecules through SCSCTs as well as exchange discrimination based on the size and character of the three different channels. The exchange of guest molecules during single crystal-single crystal transformations (SCSCT), a rare occurrence for hydrogen-bonded frameworks, is rather fast, with diffusivities of approximately 10(-6) cm(2) s(-1). Rapid diffusion in the two channels having cross sections sufficient to accommodate two guest molecules can be explained by two-way or ring diffusion, most likely vacancy assisted. Surprisingly, rapid guest exchange also is observed in a smaller channel having a cross-section that accommodates only one guest molecule, which can only be explained by guest-assisted single-file unidirectional diffusion. Several single crystals of inclusion compounds can be realized only through guest exchange in the intact framework, suggesting an approach to the synthesis of single crystalline inclusion compounds that otherwise cannot be attained through direct crystallization methods.
The crystalline structure, morphology, and nonisothermal crystallization behavior of isotactic polypropylene (iPP) with and without a novel rare earth-containing b-nucleating agent (WBG) were investigated with wideangle X-ray diffraction, polar optical microscopy, and differential scanning calorimetry. WBG could induce the formation of the b form, and a higher proportion of the b form could be obtained by the combined effect of the optimum WBG concentration and a lower cooling rate. The content of the b form could reach more than 0.90 in a 0.08 wt % WBG nucleated sample at cooling rates lower than 5 C/min. Polar optical microscopy showed that WBG led to substantial changes in both the morphological development and crystallization process of iPP. At all the studied cooling rates, the temperature at which the maximum rate of crystallization occurred was increased by [8][9][10][11] C in the presence of the nucleating agent. An analysis of the nonisothermal crystallization kinetics also revealed that the introduction of WBG significantly shortened both the apparent incubation period for crystallization and the overall crystallization time.
The role of conformational flexibility in topological enforcement of several crystalline materials based on hydrogen-bonded two-dimensional guanidinium-sulfonate (GS) networks is demonstrated by using a series of organopolysulfonates that prompt the formation of either lamellar or cylindrical architectures. Whereas flexible organopolysulfonate linkers decorated with flexible arms self-assemble into lamellar architectures, rigid organopolysulfonates linkers enforce the formation of hydrogen-bonded cylinders with intercylinder spacing governed by the size of the linker. Specifically, hexagonal cylindrical structures generated from trisulfonates with three-fold molecular symmetry are the topological equivalent of the cylindrical hexagonal phases reported previously for guanidinium organomonosulfonate inclusion compounds, but neighboring cylinders are now connected through covalent nodes provided by the trisulfonates rather than dispersive interactions between the arene rings of the organomonosulfonates. Organopolysulfonates with moderate conformational freedom, however, can generate both lamellar and cylindrical structures, depending on the guest molecules encapsulated by the host framework. These observations illustrate that the crystal architecture (i.e., lamellar vs cylindrical) and the shape of GS cylinders can be regulated in a predictable way by the molecular symmetries and conformational constraints of the organopolysulfonates building blocks.
The azo dye, sunset yellow, is a prototypical, chromonic liquid crystal in which assembly in aqueous solution at high volume fraction leads to lyotropic mesophases with a "package of properties distinct in almost every aspect" (Lydon, J. Curr. Opin. Colloid Interfac. Sci. 2004, 8, 480). In particular, the isotropic to nematic transition in such phases, the consequence of stacking of dye molecules in chains, is difficult to bring into correspondence with athermal theories for rigid rods as well as modifications that consider chain interactions with one another. Chromonic mesogens, small molecules that stack to form lyotropic liquid crystals, prompt structural questions that have yet to be answered; a full understanding of structure should inform colligative properties. Herein, the single crystal structure of a guanidinium salt of the sunset yellow dianion, a known chromonic mesogen, is reported. The compound crystallizes as a dihydrate, tetrahydrofuran solvate in the orthorhombic space group Pnna, with a = 6.8426 (5) Å, b = 20.048(1) Å, c = 21.466(2) Å. The sunset yellow molecules, point group approximately C s , are disordered about a crystallographic diad axis. The structure is informative because pairwise interactions in the disordered crystal structure show a remarkable correspondence with the stereochemistry of sunset yellow molecules in solution and in the liquid crystal phase. The solution structure is here simulated by the combination of molecular dynamics, metadynamics, and quantum chemical computations. The comparable disorder in the fluid and solid states suggests the possibility that stacked aggregates adhere to growing crystals intact. Computations evaluated proposals that stacking faults and branching pointslower the X-ray correlation lengths while preserving extended structures. Evidence is found for stacking faults but not branches. The solution stereochemistry and stereodynamics has implications for the geometry of long rods, for which understanding is a prerequisite for reckoning properties of vexing chromonic mesophases.
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