Mesophase formation and chain rigidity in cellulose and derivatives. 1. (Hydroxypropyl)cellulose in dimethylacetamide

Bioinspired Photonic Materials from Cellulose: Fabrication, Optical Analysis, and Applications

Polysaccharides are a class of biopolymers that are widely exploited in living organisms for a diversity of applications, ranging from structural reinforcement to energy storage. Among the numerous types of polysaccharides found in the natural world, cellulose is the most abundant and widespread, as it is found in virtually all plants. Cellulose is typically organized into nanoscale crystalline fibrils within the cell wall to give structural integrity to plant tissue. However, in several species, such fibrils are organized into helicoidal nanostructures with a periodicity comparable to visible light (i.e., in the range 250-450 nm), resulting in structural coloration. As such, when taking bioinspiration as a design principle, it is clear that helicoidal cellulose architectures are a promising approach to developing sustainable photonic materials. Different forms of cellulose-derived materials have been shown to produce structural color by exploiting self-assembly processes. For example, crystalline nanoparticles of cellulose can be extracted from natural sources, such as cotton or wood, by strong acid hydrolysis. Such "cellulose nanocrystals" (CNCs) have been shown to form colloidal suspensions in water that can spontaneously self-organize into a cholesteric liquid crystal phase, mimicking the natural helicoidal architecture. Upon drying, this nanoscale ordering can be retained into the solid state, enabling the specific reflection of visible light. Using this approach, colors from across the entire visible spectrum can be produced, alongside striking visual effects such as iridescence or a metallic shine. Similarly, polymeric cellulose derivatives can also organize into a cholesteric liquid crystal. In particular, edible hydroxypropyl cellulose (HPC) is known to produce colorful mesophases at high concentrations in water (ca. 60-70 wt %). This solution state behavior allows for interesting visual effects such as mechanochromism (enabling its use in low-cost colorimetric pressure or strain sensors), while trapping the structure into the solid state enables the production of structurally colored films, particles and 3D printed objects. In this article, we summarize the state-of-the-art for CNC and HPC-based photonic materials, encompassing the underlying self-assembly processes, strategies to design their photonic response, and current approaches to translate this burgeoning green technology toward commercial application in a wide range of sectors, from packaging to cosmetics and food. This overview is supported by a summary of the analytical techniques required to characterize these photonic materials and approaches to model their optical response. Finally, we present several unresolved scientific questions and outstanding technical challenges that the wider community should seek to address to develop these sustainable photonic materials.

Thermoresponsive Molecular Brushes with a Rigid-Chain Aromatic Polyester Backbone and Poly-2-alkyl-2-oxazoline Side Chains

A new polycondensation aromatic rigid-chain polyester macroinitiator was synthesized and used to graft linear poly-2-ethyl-2-oxazoline as well as poly-2-isopropyl-2-oxazoline by cationic polymerization. The prepared copolymers and the macroinitiator were characterized by NMR, GPC, AFM, turbidimetry, static, and dynamic light scattering. The molar masses of the polyester main chain and the grafted copolymers with poly-2-ethyl-2-oxazoline and poly-2-isopropyl-2-oxazoline side chains were 26,500, 208,000, and 67,900, respectively. The molar masses of the side chains of poly-2-ethyl-2-oxazoline and poly-2-isopropyl-2-oxazoline and their grafting densities were 7400 and 3400 and 0.53 and 0.27, respectively. In chloroform, the copolymers conformation can be considered as a cylinder wormlike chain, the diameter of which depends on the side chain length. In water at low temperatures, the macromolecules of the poly-2-ethyl-2-oxazoline copolymer assume a wormlike conformation because their backbones are well shielded by side chains, whereas the copolymer with short side chains and low grafting density strongly aggregates, which was visualized by AFM. The phase separation temperatures of the copolymers were lower than those of linear analogs of the side chains and decreased with the concentration for both samples. The LCST were estimated to be around 45 °C for the poly-2-ethyl-2-oxazoline graft copolymer, and below 20 °C for the poly-2-isopropyl-2-oxazoline graft copolymer.

Visual Appearance of Chiral Nematic Cellulose-Based Photonic Films: Angular and Polarization Independent Color Response with a Twist

Hydroxypropyl cellulose (HPC) is a biocompatible cellulose derivative capable of self-assembling into a lyotropic chiral nematic phase in aqueous solution. This liquid crystalline phase reflects right-handed circular polarized light of a specific color as a function of the HPC weight fraction. Here, it is demonstrated that, by introducing a crosslinking agent, it is possible to drastically alter the visual appearance of the HPC mesophase in terms of the reflected color, the scattering distribution, and the polarization response, resulting in an exceptional matte appearance in solid-state films. By exploiting the interplay between order and disorder, a robust and simple methodology toward the preparation of polarization and angular independent color is developed, which constitutes an important step toward the development of real-world photonic colorants.

Robust organohydrogel with flexibility and conductivity across the freezing and boiling temperatures of water

A tough double-network (DN) organohydrogel, obtained by simply soaking a poly(2-acrylamido-2-methylpropane sulfonic acid)/polyacrylamide (PAMPS/PAAm) hydrogel in an ethylene glycol solution of lithium chloride, retains high mechanical performance, flexibility (-80 to 120 °C) and conductivity (-20 to 120 °C), paving the way towards broad applications.

Optical tracking of relaxation dynamics in semi-dilute hydroxypropylcellulose solutions as a precise phase transition probe

Phase separation of thermo-responsive polymers in solution is a complex process, whose understanding is essential to screen and design materials with diverse technological applications. Here we report on a method based on dynamic light scattering (DLS) experiments to investigate the phase separation of thermo-responsive polymer solutions and precisely define the transition temperature (T_PS). Our results are applied on hydroxypropylcellulose (HPC) solutions as an important biosourced green water-soluble polymer. As determined by DLS, the amplitudes of the fast and slow modes of relaxation dynamics evolve as temperature gets closer to the phase transition point eventually leading to phase separation. The evolution of relaxation modes with temperature is markedly different for concentrations below the overlap concentration (c*) (dilute regime), above c* (semi-dilute regime) and above the entanglement concentration (c_e). In the three cases though, the fast and slow mode amplitudes undergo a sharp transition in a narrow temperature range, defining accurately the phase separation locus. The results agree with turbidimetric analysis for the phase transition determination but with a better precision. Our results also show that the one-phase dynamics and phase separation dynamics in the two-phase region are only in continuity for c > c_e, revealing mechanistic details about the HPC phase separation process. Above T_PS we identify a temperature range where the intensity autocorrelation function has a single-exponential shape. In the latter regime, we monitor the growth kinetics of polymer domains and provide clues to rationalize the stabilizing effects of the interfaces leading to the arrested-like phase separation behavior observed for HPC.

Tailored Viscosity Reduction in Aqueous Hydroxypropylcellulose Solutions

Polysaccharides, including cellulosic derivatives such as hydroxypropylcellulose (HPC), are used extensively in the pharmaceutical industry due to their biocompatibility and self-organization properties. While drug delivery vehicles employing, for instance, aqueous polysaccharide gels are desirable, production of such gels can be hindered by tremendously high solution viscosity. In this work, we describe a novel means by which to reduce the viscosity and enhance the solubility of aqueous HPC. Here, the efficacy of several additives which promote HPC viscosity reduction through molecular salting-in is examined by dynamic rheology and light/electron microscopy.

Isotropic-nematic phase equilibrium and phase separation of kappa-carrageenan in aqueous salt solution: experimental and theoretical approaches

The behavior of chiral-nematic and isotropic phases of helical kappa-carrageenan in aqueous solution of sodium iodide was compared with that of the anisotropic biphasic phase that contains both these phases. On the basis of birefringence, rheology, chemical analysis, average molecular weight, and polydispersity index measurements, we derived a number of characteristic differences as well as similarities between these phases, over a range of polysaccharide concentrations obtained by the dilution of each phase. For example, we assessed the critical concentration of an isotropic-anisotropic transition (C(i)), the temperature of the anisotropic-isotropic phase shift during thermal heating-cooling cycles, and the viscosity changes due to the phase shift and due to the diminishing of the helical conformation. We also demonstrated how the different phases and their dilutions behave under the effect of shear and frequency of oscillation and how the viscoelastic properties vary in each phase and discussed the isotropic and anisotropic liquid crystal controlling behavior mechanisms. From a theoretical point of view, we propose to combine the wormlike chain model for semiflexible polyelectrolytes interacting via both hard-core and electrostatic repulsion to assess the concentration of isotropic-nematic transition, to assess the coexistence concentration range, and to determine the effects of charge by applying the effective diameter and a twisting effect.