https://yunus.hacettepe.edu.tr/~damlacetin/kmu407/index_dosyalar/4.%20makale.pdf

Biodegradable polymers as biomaterials

Recombinant DNA methods were used to create artificial proteins that undergo reversible gelation in response to changes in pH or temperature. The proteins consist of terminal leucine zipper domains flanking a central, flexible, water-soluble polyelectrolyte segment. Formation of coiled-coil aggregates of the terminal domains in near-neutral aqueous solutions triggers formation of a three-dimensional polymer network, with the polyelectrolyte segment retaining solvent and preventing precipitation of the chain. Dissociation of the coiled-coil aggregates through elevation of pH or temperature causes dissolution of the gel and a return to the viscous behavior that is characteristic of polymer solutions. The mild conditions under which gel formation can be controlled (near-neutral pH and near-ambient temperature) suggest that these materials have potential in bioengineering applications requiring encapsulation or controlled release of molecular and cellular species.

https://science.sciencemag.org/content/sci/281/5375/389.full.pdf

Reversible Hydrogels from Self-Assembling Artificial Proteins

A liquid crystal system was used for the fabrication of a highly ordered composite material from genetically engineered M13 bacteriophage and zinc sulfide (ZnS) nanocrystals. The bacteriophage, which formed the basis of the self-ordering system, were selected to have a specific recognition moiety for ZnS crystal surfaces. The bacteriophage were coupled with ZnS solution precursors and spontaneously evolved a self-supporting hybrid film material that was ordered at the nanoscale and at the micrometer scale into approximately 72-micrometer domains, which were continuous over a centimeter length scale. In addition, suspensions were prepared in which the lyotropic liquid crystalline phase behavior of the hybrid material was controlled by solvent concentration and by the use of a magnetic field.

/pdf/ordering-of-quantum-dots-using-genetically-engineered-1x9zmi0dzy.pdf

Ordering of Quantum Dots Using Genetically Engineered Viruses

Additional functionality can be incorporated into liquid crystalline materials by using phase segregation and self-assembly. Intermolecular interactions such as hydrogen bonding and ionic interactions play key roles in the formation of these complex structures. One-, two-, and three-dimensional phase-segregated structures on various scales of length are formed by self-assembly of a variety of partially incompatible molecules. Such structures can enhance anisotropic properties such as ionic conductivity.

https://science.sciencemag.org/content/sci/295/5564/2414.full.pdf

Self-Assembly of Phase-Segregated Liquid Crystal Structures

One of the fascinating subjects in areas such as materials science, nanochemistry, and biomimetic chemistry is concerned with the creation of supramolecular architectures with well-defined shapes and functions. Self-assembly of molecules through noncovalent forces including hydrophobic and hydrophilic effects, electrostatic interactions, hydrogen bonding, microphase segregation, and shape effects has the great potential for creating such supramolecular structures.1-5 An example is provided by rodlike macromolecules whose solutions and melts exhibit liquid crystalline phases such as nematic and/or layered smectic structures with the molecules arranged with their long axes nearly parallel to each other.6,7 The main factor governing the geometry of the supramolecular structures in the liquid crystalline phase is the anisotropic aggregation of the molecules. In contrast, coil-coil diblock molecules consisting of different immiscible segments exhibit a wide range of microphase-separated supramolecular structures with curved interfaces in addition to layered structures.8-11 This phase behavior is mainly due to the mutual repulsion of the dissimilar blocks and the packing constraints imposed by the connectivity of each block. The covalent linkage of these different classes of molecules to a single linear polymer chain (rod-coil copolymer) can produce a novel class of self-assembling materials since the molecules share certain general characteristics of diblock molecules and rodlike liquid crystalline molecules.12-15 The difference in chain rigidity of stiff rodlike and flexible coillike block is expected to greatly affect the details of molecular packing and thus the nature of thermodynamically stable supramolecular structures. This rod-coil molecular architecture imparts microphase separation of the rod and coil blocks into ordered periodic structures even at very low molecular weights relative to flexible block copolymers due to the high stiffness difference between the blocks. As a consequence, the rod-coil copolymer forms supramolecular structures with dimensions as small as few nanometers, which are not common in microphaseseparated flexible block copolymers.16 The supramolecular structures of rod-coil polymers arise from a combination of organizing forces including the mutual repulsion of the dissimilar blocks and the packing constraints imposed by the connectivity of each block, and the tendency of the rod block to form orientational order. Apart from the wide range of different supramolecular structures in nanoscale dimensions, another unique characteristic is that rod segments can endow various functionalities such as photophysical and electrochemical properties to the supramolecular materials. Many of the syntheses of rod-coil diblock and triblock copolymers as well as their interesting supramolecular structures and the intriguing properties of rod-coil copolymers are discussed in excellent books and reviews that have been published by several experts in the field.16-19 Here, we do not want to present a complete overview on reported rod-coil copolymers. Instead, we have highlighted the most recently synthesized rod-coil copolymers and their supramolecular structures.

Supramolecular structures from rod-coil block copolymers.

Solutions and melts of stiff ('rod-like') macromolecules often exhibit nematic liquid crystalline phases characterized by orientational, but not positional, molecular order Smectic phases, in which macromolecular rods are organized into layers roughly perpendicular to the direction of molecular orientation, are rare, owing at least in part to the polydisperse nature (distribution of chain lengths) of polymers prepared by conventional polymerization processes Bacterial methods for polypeptide synthesis, in which artificial genes encoding the polymer are expressed in bacterial vectors, offer the opportunity to make macromolecules with very well defined chain lengths Here we show that a monodisperse derivative of poly(γ-benzyl α,L-glutamate) prepared in this way shows smectic ordering in solution and in films This result suggests that methods for preparing monodisperse polymers might provide access to new smectic phases with layer spacings that are susceptible to precise control on the scale of tens of nanometres

Christoph Kayser

Papers

Smectic ordering in solutions and films of a rod-like polymer owing to monodispersity of chain length