The sophisticated use of self-organizing materials, which include liquid crystals, block copolymers, hydrogen- and π-bonded complexes, and many natural polymers, may hold the key to developing new structures and devices in many advanced technology industries. Synthetic materials are usually designed with only one structure-forming process in mind. However, combination of both complementary and antagonistic interactions in macromolecular systems can create order in materials over many length scales. Here polymer materials that make use of competing molecular interactions are summarized, and the prospects for the further development of such materials through both synthetic and processing pathways are highlighted.

https://science.sciencemag.org/content/sci/277/5330/1225.full.pdf

Competing Interactions and Levels of Ordering in Self-Organizing Polymeric Materials

The history of human civilization is accompanied by the development of novel materials, from the period of natural stone, the bronze and iron age, to the modern times of synthetic materials and promising advanced materials. Liquid crystals (LCs) are one of such kind of materials that greatly infl uence our daily life, and which are not limited to displays such as TVs and personal computers. Being an intermediate phase of matter, LCs show characteristics of both controllable mobility of an isotropic liquid and ordered regularity of a crystalline solid. Combined with their photoresponsive properties, photocontrollable LC actuators have enabled a variety of applications in various fi elds, which include fl at panel displays, photonics, photo-driven devices, and more recently nanotechnology. The LC’s unique features provide soft materials in a liquid-crystalline state with interesting properties such as 1) selfassembly, 2) fl uidity with long-range order, 3) molecular and supramolecular cooperative motion (MCM and SMCM), 4) large birefringence and anisotropy in various physical properties (optical, mechanical, electrical and magnetic), 5) alignment change induced by external fi elds at surfaces and interfaces, and 6) deformation of LC elastomers (LCEs) in response to stimuli. [ 1–4 ]

Photocontrollable liquid-crystalline actuators.

Functional nanomaterials based on block copolymer self-assembly

http://nathan.instras.com/documentDB/paper-262.pdf

Photoactive liquid crystalline polymer systems with light-controllable structure and optical properties

A series of liquid crystalline (LC) homopolymers of poly{11-[4-(4-butylphenylazo)phenoxy]undecyl methacrylate}containing an azobenzene mesogen with different degrees of polymerization were synthesized by using the atom transfer radical polymerization (ATRP) method. The homopolymers were prepared with a range of number-average molecular weights from 6100 to 23 500 with narrow polydispersities of less than 1.17. Thermal investigation showed that the homopolymers exhibit monolayer smectic A, smectic C, and an unknown smectic X phases. The transition temperatures increase slightly with the increase of the molecular weights and level off at around 21 500. Novel amphiphilic LC-coil diblock copolymers with a defined length of a flexible poly(ethylene glycol) segment as the hydrophilic coil were also prepared by the ATRP method. The LC-coil diblock copolymers exhibit narrow polydispersities of less than 1.11. Morphologies of the thin films of the block copolymers were investigated by using transmission electron m...

Synthesis, nanostructures, and functionality of amphiphilic liquid crystalline block copolymers with azobenzene moieties

The synthesis and characterization of a family of well-defined liquid crystal−coil (LC−coil) diblock copolymers have been carried out. The block copolymers in this study have been designed to have nearly identical molecular weight azobenzene-containing LC blocks in order to eliminate possible variations in LC behavior caused by the differences in the LC block molecular weight. Quantitative hydroboration chemistry was used to convert the pendent double bonds of an isoprene block to hydroxyl groups to which the mesogenic groups were attached via acid chloride coupling. The LC homopolymer and the block copolymers (LC volume fraction from fLC = 0.82 to fLC = 0.20) all exhibited smectic mesophases with similar clearing transition temperatures. The clearing transition enthalpies strongly depend on the block composition ratio and decrease as the LC block volume fraction decreases. Solvent-casting of a lamellar LC−coil copolymer (SICN5-66/60) resulted in an oriented bulk film in which both the axes of the mesogen...

Molecular Design, Synthesis, and Characterization of Liquid Crystal−Coil Diblock Copolymers with Azobenzene Side Groups

Low-stress creep measurements on a nematic liquid crystal polymer indicate that it is a viscoelastic solid, with a modulus of 100dynes/cm2 and a yield stress of 50dynes/cm2. Both smectics and nematics are viscoelastic solids at very low stress levels, with a modulus that is related to their defect texture. At stress levels somewhat above the yield stress, there is a yielding regime where the deformation rate and defect spacing are power laws in the applied stress. We understand these power laws using the ideas developed long ago by Orowan for the motion of line defects in crystalline solids. The exponents of these power laws are different for nematics and smectics, but the nematic universality class also appears to apply to superplastic metals and ceramics.

Defect-mediated creep of structured materials

Curvature driven relaxation of disclination loops in liquid crystals

Small-angle neutron scattering is utilized to determine the flow induced alignment of a model thermotropic liquid crystalline polymer (LCP) as a function of shear rate and temperature. The results demonstrate that the flow-induced structures in thermotropic liquid crystalline polymers have similarities and differences to those in lyotropic liquid crystalline polymer solutions. The shear rate dependence of the alignment shows that the flow-induced alignment correlates very well to the viscosity behavior of the LCP in the shear thinning regime, while temperature variation results in a change in the extent of alignment within the nematic phase. Relaxation results also demonstrate that the flow-induced alignment remains essentially unchanged for up to an hour after the shear field has been removed. Last, there exists a regime at low shear rate and low temperature where alignment of the LCP molecule perpendicular to the applied shear flow is stable. These results provide important experimental evidence of the molecular level changes that occur in a thermotropic liquid crystalline polymer during flow, which can be utilized to develop theoretical models and more efficiently process thermotropic polymers. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 3017–3023, 1998

Flow-induced structure in a thermotropic liquid crystalline polymer as studied by SANS

Two LC side-group poly(methacrylates) were synthesized, and their melt dynamics were compared with each other and a third, main-chain side-group combined LC polymer. A new route was developed for the synthesis of the poly(methacrylate) polymers which readily converts relatively inexpensive perdeuteromethyl methacrylate to other methacrylate monomers. Self-diffusion data was obtained through the use of forward recoil spectrometry, while modulus and viscosity data were measured using rotational rheometers in oscillatory shear. Diffusion coefficients and complex viscosity were compared to previous experiments on liquid crystal polymers of similar architecture to determine the effect of side-group interdigitation and chain packing on center of mass movement. The decyl terminated LC side-group polymer possessed an interdigitated smectic phase and a sharp discontinuity in the self-diffusion behavior at the clearing transition. In contrast, the self-diffusion behavior of the methyl terminated LC side-group polymer, which possessed head-to-head side-group packing, was seemingly unaffected by the smectic-nematic and nematic-isotropic phase transitions. The self-diffusion coefficients of both polymers were relatively insensitive to the apparent glass transition. The presence of moderately fast sub-T g chain motion was supported by rheological measurements that provided further evidence of considerable molecular motion below T g . The complex phase behavior of the combined main-chain side-group polymer heavily influenced both the self-diffusion and rheological behavior. Differences between the self-diffusion and viscosity data of the main-chain side-group polymer could be interpreted in terms of the defect structure.

Scott Clingman

Papers

Molecular Design, Synthesis, and Characterization of Liquid Crystal−Coil Diblock Copolymers with Azobenzene Side Groups

Defect-mediated creep of structured materials

Curvature driven relaxation of disclination loops in liquid crystals

Flow-induced structure in a thermotropic liquid crystalline polymer as studied by SANS

Effect of polymer architecture on self-diffusion of LC polymers