Temperature‐Dependent Photonic Bandgap in a Self‐Assembled Hydrogen‐Bonded Liquid‐Crystalline Diblock Copolymer

Abstract

We take advantage of self‐assembly in a hierarchically structured, hybrid material to develop photonic bandgaps in the visible which may be systematically tuned by application of thermal or electric fields. Hydrogen bonding between a host polymer and a guest small molecule is used to augment the molecular weight of an appropriately selected coil–coil diblock copolymer to bring the microdomain structure onto the length scale needed for significant interaction with visible light. Further, the use of liquid‐crystal‐mesophase‐forming moieties as the guest hydrogen‐bonding units adds functionality to the system as the optical properties of the liquid‐crystalline domains can be modulated by external stimuli. We use hydrogen bonding to sequester varying amounts of imidazole terminated mesogens within the acid domains of a poly[styrene‐block‐poly‐(methacrylic acid)] (PS‐b‐MAA) diblock copolymer. The resulting PS‐b‐MAA/LC side‐group liquid‐crystalline diblock copolymer possesses a photonic bandgap in the green with the exact location and structure of the gap dependent on the composition of the system. Here, we discuss the structure and optical properties of these materials as a function of their composition and the response of the optical properties to temperature. Varying the order parameter of the LC domains by heating into the isotropic state changes the peak reflectivity by 40 nm, resulting in a color change from green to orange.

Self-assembled polymeric solid films with temperature-induced large and reversible photonic-bandgap switching

“…These facts induce a very strong decrease in the long period of the lamellar structure within a narrow temperature range, as the comb-shaped supramolecular architecture of the P4VP(MSA) 1.0 chains is 'purged' , thus allowing more compact coiling of the polymer. The previous studies, where the bandgap properties were slightly modifi ed due to the temperature-dependent refractive indices 22 , encouraged us to measure the refractive indices of PS, P4VP(MSA) 1.0 and P4VP(MSA) 1.0 (PDP) 1.5 separately as a function of temperature. They changed only a few percent when heated up to ~140 °C and their temperature behaviour was smooth, suggesting that they do not explain the thermal switching of the bandgap.…”

“…Such systems did not show optically responsive or switching behaviour, but it can be expected that phase transitions of self-assembled block copolymers could lead to a useful platform for switchable bandgaps in the solid state. Concepts for switching bandgaps have so far been based, for example, on liquid crystals 22,23 , hydrogels and colloids 14,[24][25][26] , mechanical tuning of plastic opals 13 , and thermochromism 12 . Within the context of the present work, the block-copolymer/liquidcrystal systems are particularly interesting, and reversible changes in optical properties have been demonstrated 22 .…”

Functional liquid-crystalline polymers and supramolecular liquid crystals

“…Photonic materials made from hydrogen-bond donor polymer 16 and imidazolecontaining mesogen 17 show interesting optical properties. 97 Reversible change of optical transmission in the visible region can be observed as a function of temperature because refractive indices of the LC materials change with the order parameter. Fluorescent supramolecular LC polymers with rigid bis(phenylethynyl)benzene moieties were constructed by the formation of hydrogen bonds between two complementary nucleobase-terminated monomers 18 and 19.…”

“…96 On the basis of the supramolecular design described above, functional supramolecular LC polymers have been developed ( Figure 6). 51,[97][98][99][100][101][102][103][104][105] Ferroelectricity can be induced for hydrogenbonded LC polymer 15 exhibiting chiral smectic C phases. 51 Chiral hydrogen-bond acceptors are used to form supramolecular LC polymers that show spontaneous polarization.…”

Transport and Electro‐Optical Properties in Polymeric Self‐Assembled Systems

“…For certain large area applications, simpler techniques could be more suitable. Spontaneous self-assembly of colloids [375], synthetic opals [379][380][381][382][383][384], inverted opals [379,[385][386][387] and block copolymers [218,219,221,378,[388][389][390][391][392][393][394][395][396][397][398][399][400][401][402], on the other hand, allows the preparation of small enough structures. Although self-assembly leads to a well-defined local order and offers a potentially low-cost method for the production of photonic crystals, it is nontrivial to achieve perfectly ordered structures over the macroscopic length scale combining carefully engineered defects.…”