Rippled Sheets: The Early Polyglycine Days and Recent Developments in Nylons.

The rippled sheet structure is a remarkable insight due to Pauling and Corey, that supplements the pleated sheet structure of homochiral proteins introduced in 1951. Whereas the pleated sheet was immediately adopted by the scientific community, the rippled sheet has remained more confidential since it applies only to blends of poly(L-peptides) and poly(D-peptides). The present account tells the intimate but patchy relationship developed by the author with the rippled sheet. In the 1970s, twenty years after Pauling and Corey's proposal, the rippled sheet was recognized as a valid model for the sheet structure of the achiral polyglycine, polyglycine I, which helped improve the structure of Bombyx mori silk fibroin. Very recently, pleated and rippled sheets were found to account for unsolved crystal structures of a variety of nylons. These structures help to explain a mysterious high temperature "Brill transition" first reported in nylon 6-6 by Brill in 1942.

About the Crystallization of Abiotic Coded Matter.

The crystallization of digitally encoded polyurethanes was studied by electron diffraction. A series of oligomers with different primary structures was analyzed in this work. They all form hydrogen-bonding-directed lamellar single crystals with a base-centered orthorhombic unit cell. Although crystal morphology was the same in all cases, the digital coding of the oligomers has a small influence on the intersheet distance in the crystals. The crystal lattices allow calculation of the volume occupied by one basic information unit, which is in the range 148-188 Å3. Interestingly, this volume is about 3× smaller than that occupied by a coded nucleotide in a DNA double helix. Furthermore, crystallization of blends of oligourethanes with different coded primary structures was investigated. Oligomers with drastically different monomer compositions form structures that are not cocrystals but more probably segregated crystals containing distinct domains of different composition.

Oriented Overgrowths of Poly(l-Lactide) on Oriented Isotactic Polypropylene: A Sequence of Soft and Hard Epitaxies.

The crystallization behavior of an amorphous poly(l-lactide) (PLLA) layer deposited on uniaxially oriented isotactic polypropylene (iPP) substrate is been studied by atomic force microscopy (AFM) and electron microscopy combined with electron diffraction. A patterned PLLA structure with two fixed lamella and chain orientations is observed. Electron diffraction demonstrates that the major lamellar set is oriented with molecular chains perpendicular to the chain direction of the iPP. The minor lamellar set is inclined at ≈64° to both the iPP chain axis direction and the lamellae of the major set as judged from both the bright field electron micrograph and the AFM image. The orientation of the main set is explained in terms of "soft" epitaxy or graphoepitaxy, in which PLLA chains oriented parallel to the ditches of the iPP substrate caused by alternatively arranged crystalline and amorphous regions. The minor set is due to a homoepitaxy of PLLA with parallelism of the helical paths. The orientation of this minor set of lamellae therefore depends on and can help determine the chirality-l or d-of the PLA investigated.

Helical Polyacetylene-Based Switchable Chiral Columnar Phases: Frustrated Chain Packing and Two-Way Shape Actuator.

Chiral columns formed by a helical cis-polyphenylacetylene (PPA) derivative P1 are reversibly switched during a phase transition between two chiral columnar phases: the frustrated Φh (3D-SL) phase containing four chains at low temperature and a hexagonal columnar phase Φh at high temperature, accompanied by a simultaneous conformational change. The helix-helix transition along the PPA backbone during the Φh (3D-SL) -Φh transition makes the uniaxially oriented P1 capable of reversibly and reproducibly elongating (132 %) upon heating and contracting upon cooling, exhibiting the behavior of a two-way shape actuator.

Manipulation of Self-Assembled Nanostructure Dimensions in Molecular Janus Particles.

The ability to manipulate self-assembly of molecular building blocks is the key to achieving precise "bottom-up" fabrications of desired nanostructures. Herein, we report a rational design, facile synthesis, and self-assembly of a series of molecular Janus particles (MJPs) constructed by chemically linking α-Keggin-type polyoxometalate (POM) nanoclusters with functionalized polyhedral oligomeric silsesquioxane (POSS) cages. Diverse nanostructures were obtained by tuning secondary interactions among the building blocks and solvents via three factors: solvent polarity, surface functionality of POSS derivatives, and molecular topology. Self-assembled morphologies of KPOM-BPOSS (B denotes isobutyl groups) were found dependent on solvent polarity. In acetonitrile/water mixtures with a high dielectric constant, colloidal nanoparticles with nanophase-separated internal lamellar structures quickly formed, which gradually turned into one-dimensional nanobelt crystals upon aging, while stacked crystalline lamellae were dominantly observed in less polar methanol/chloroform solutions. When the crystallizable BPOSS was replaced with noncrystallizable cyclohexyl-functionalized CPOSS, the resulting KPOM-CPOSS also formed colloidal spheres; however, it failed to further evolve into crystalline nanobelt structures. In less polar solvents, KPOM-CPOSS crystallized into isolated two-dimensional nanosheets, which were composed of two inner crystalline layers of Keggin POM covered by two monolayers of amorphous CPOSS. In contrast, self-assembly of KPOM-2BPOSS was dominated by crystallization of the BPOSS cages, which was hardly sensitive to solvent polarity. The BPOSS cages formed the crystalline inner bilayer, sandwiched by two outer layers of Keggin POM clusters. These results illustrate a rational strategy to purposely fabricate self-assembled nanostructures with diverse dimensionality from MJPs with controlled molecular composition and topology.

Toward Controlled Hierarchical Heterogeneities in Giant Molecules with Precisely Arranged Nano Building Blocks.

Herein we introduce a unique synthetic methodology to prepare a library of giant molecules with multiple, precisely arranged nano building blocks, and illustrate the influence of minute structural differences on their self-assembly behaviors. The T8 polyhedral oligomeric silsesquioxane (POSS) nanoparticles are orthogonally functionalized and sequentially attached onto the end of a hydrophobic polymer chain in either linear or branched configuration. The heterogeneity of primary chemical structure in terms of composition, surface functionality, sequence, and topology can be precisely controlled and is reflected in the self-assembled supramolecular structures of these giant molecules in the condensed state. This strategy offers promising opportunities to manipulate the hierarchical heterogeneities of giant molecules via precise and modular assemblies of various nano building blocks.

Single Crystals of the Frustrated ß-Phase and Genesis of the Disordered α'-Phase of Poly(l-lactic acid).

The metastable, frustrated ß-phase of poly(l-lactic acid) (PLLA) had been obtained so far only by drawing fibers from amorphous PLLA or the stable α-phase. This phase has now been obtained by crystallization in thin films in the form of snow-flake-like crystals produced at high Tc (140 °C). They display the characteristic single crystal diffraction pattern of frustrated polymers with a three-chains, trigonal unit-cell. Also, hko electron diffraction patterns indicate that crystallization of PLLA at 90 °C takes place in a transient, frustrated ßPLLA phase that converts rapidly to α'PLLA. The disorder in α'PLLA stems from the incompatible cell symmetries of the trigonal, three-chains ßPLLA and orthorhombic, two-chains αPLLA unit-cells. The disorder in α'PLLA involves shifts of domains along the chain axis and three azimuthal orientations 120° apart that both reflect the initial trigonal/hexagonal symmetry of the parent ßPLLA. Recognition of this solid-state transformation provides a logical framework that explains many unusual characteristics of the α'PLLA phase (structural disorder, lamellar thickness and possibly growth rate vs Tc).

Two-dimensional nanocrystals of molecular Janus particles.

This paper describes a rational strategy to obtain self-assembled two-dimensional (2D) nanocrystals with definite and uniform thickness from a series of molecular Janus particles based on molecular nanoparticles (MNPs). MNPs are 3D framework with rigid shapes. Three different types of MNPs based on derivatives of polyhedral oligomeric silsesquioxane (POSS), [60]fullerene (C60), and Lindqvist-type polyoxometalate (POM) are used as building blocks to construct these amphiphilic molecular Janus particles by covalently connecting hydrophobic crystalline BPOSS with a charged hydrophilic MNP. The formation of 2D nanocrystals with an exact thickness of double layers of molecules is driven by directional crystallization of the BPOSS MNP and controlled by various factors such as solvent polarity, number of counterions, and sizes of the MNPs. Strong solvating interactions of the ionic MNPs in polar solvents (e.g., acetonitrile and dimethylformamide) are crucial to provide repulsive interactions between the charged outlying ionic MNPs and suppress further aggregation along the layer normal direction. The number of counterions per molecule plays a major role in determining the self-assembled morphologies. Size matching of the hydrophobic and ionic MNPs is another critical factor in the formation of 2D nanocrystals. Self-assembly of rationally designed molecular Janus particles provides a unique "bottom-up" strategy to engineer 2D nanostructures.

Analysis of the structure and morphology of crystalline polymers by electron microscopy imaging and diffraction: a personal journey.

Crystalline polymers form complex spherulitic morphologies upon crystallization from the melt. The spherulites are made of thin lamellae, frequently twisted, in which the long chains span the lamellar thickness and are folded back and forth at the lamellar surface. Investigation of the crystalline and lamellar structures becomes, however, possible by using single crystals, the size and thickness of which are well adapted for transmission electron microscopy. Imaging in bright and dark field modes, electron diffraction and specific decoration techniques have made it possible to reach a detailed understanding of lamellar and surface structures as well as the crystallization processes. These insights are transferable to the more complex bulk architectures and thus help analyze some of their most striking features. Electron microscopy turns out to be a major tool and in many cases the only possible tool to reach such molecular and even sub-molecular understanding. Some of these insights are illustrated, based mostly on work performed in our laboratory. They involve some original experimental techniques that should be useful in materials science.

Morphology Diagram of Single-Layer Crystal Patterns in Supercooled Poly(ethylene oxide) Ultrathin Films: Understanding Macromolecular Effect of Crystal Pattern Formation and Selection.

A series of single-layer crystal patterns were observed in ultrathin films of 10 poly(ethylene oxide) fractions of molecular weights ranging from 2.02k to 932.0k g/mol. Morphology transitions between these different crystal patterns were quantitatively identified, and a morphology diagram with respect to supercooling and molecular weight dependencies was constructed. This will foster understanding of the macromolecular effects on the crystal pattern formation and selection critically associated with the parameters of molecular diffusion length and growth anisotropy.

Breaking symmetry toward nonspherical Janus particles based on polyhedral oligomeric silsesquioxanes: molecular design, "click" synthesis, and hierarchical structure.

The design, synthesis, and self-assembly of a series of precisely defined, nonspherical, polyhedral oligomeric silsesquioxane (POSS)-based molecular Janus particles are reported. The synthesis aims to fulfill the "click" philosophy by using thiol-ene chemistry to efficiently install versatile functionalities on one of the POSS cages. In such a way, both the geometrical and chemical symmetries were broken to create the Janus feature. These particles self-organize into hierarchically ordered supramolecular structures in the bulk. For example, the Janus particle with isobutyl groups on one POSS and carboxylic groups on the other self-assembles into a bilayered structure with head-to-head, tail-to-tail arrangements of each particle, which further organize into a three-dimensional orthorhombic lattice. While the ordered structure in the layers was lost upon heating via a first-order transition, the bilayered structure persisted throughout. This study provides a model system of well-defined molecular Janus particles for the general understanding of their self-assembly and hierarchical structure formation in the condensed state.

Onset of tethered chain overcrowding.

We proposed an approach to precisely control the density of tethered chains on solid substrates using PEO-b-PS and PLLA-b-PS. As the crystallization temperature Tx increased, the PEO or PLLA lamellar crystal thickness d(L) increased as well as the reduced tethering density sigma; of the PS chains. The onset of tethered PS chains overcrowding in solution occurs at sigma(*) approximately 3.7-3.8 as evidenced by an abrupt change in the slope between (d(L))(-1) and Tx. This results from the extra surface free energy created by the tethered chain that starts to affect the growth barrier of the crystalline blocks.

Nucleation control in polymer crystallization: structural and morphological probes in different length- and time-scales for selection processes.

Thermodynamically, polymer crystallization is a first-order transition that involves overcoming an energy barrier. Building a molecular kinetic model that links this macroscopic concept with experimental observations has been and still remains a difficult issue. It requires a physical picture that can show how a three-dimensionally random linear macromolecule is converted to a chain-folded crystalline state despite the loss of entropy in the process. There are a number of dynamic molecular pathways during polymer crystallization, and previous analytical models have used a 'mean-field' approach. In polymer crystallization, every macromolecule has to go through several selection processes on different length- and time-scales. In this article, we try to identify these selection processes and lay down some basic principles of polymer crystallization. Experimental observations on stem configurations, helical conformations, crystal structures, fold lengths, global macromolecular conformations and lamellar single-crystal morphologies have been used as probes to identify these selection processes.