Scattering by linear, branched, and copolymer chain molecules for large scattering vectors

Supertough Lignin Hydrogels with Multienergy Dissipative Structures and Ultrahigh Antioxidative Activities

Hydrogels derived from lignin are typically weak and contain only a small amount of lignin, which limits their broad application prospects. In the present work, a novel lignin/poly(N,N-dimethylacrylamide) (PDMA) hydrogel with a high lignin content, superb toughness, and ultrahigh antioxidative performance is constructed by employing a facile dissolve-dry-swell solvent exchange method. Through this process, lignin and PDMA are self-assembled into a multienergy dissipative structure containing rigid lignin-rich domains. Precisely, the PDMA chains both interpenetrated inside and adhered on the surface of these domains through hydrophobic associations. This structure enables the lignin hydrogels to dissipate energy efficiently during the fracture process. At an optimized ultrahigh lignin content of 58% (dry weight basis), the prepared lignin hydrogel exhibited remarkable mechanical properties, such as a high elastic modulus (2.5 MPa), tensile strength (2.5 MPa), and super tensile strain (11.3), and an extremely high fracture energy above 16 000 J m^-2. In addition, the tough lignin hydrogel exhibited a commendable antioxidant property and nontoxicity. All these advantageous properties provide the lignin/PDMA hydrogels with the potential for use in biomedical materials applications.

Formation of Nanofibrous Structure in Biopolymer Aerogel during Supercritical CO2 Processing: The Case of Chitosan Aerogel

Supercritical drying is widely considered as the gold standard to produce aerogels that preserve the microstructure of the gels, but we have found this is not always the case. Chitosan aerogel, one of the emerging biopolymer aerogels, was prepared by chemical cross-linking gelation, followed by solvent exchange with methanol and supercritical drying using CO₂. Small-angle X-ray scattering analysis shows that the structure of the wet gel, which consists of Gaussian chains of individual molecular strands, converts into a nanofibrous network during CO₂ processing. In situ observation reveals a drastic shrinkage of the gel in CO₂, demonstrating that physical coagulation caused by the low affinity between chitosan and CO₂ is the main structure-forming step. These results challenge the common perception of supercritical drying: it is no longer an inactive drying method, but rather an active nanostructure forming a tool to produce porous biopolymer materials with tailored structure and properties.

Nanoconfinement and Chemical Structure Effects on Permeation Selectivity of Self-Assembling Graft Copolymers

Permeation of small molecule solutes through thin films is typically described by the solution-diffusion model, but this model cannot predict the effects of nanostructure due to self-assembly or additives. Other models focusing on diffusion through isolated nanopores indicate that confining permeation to channels slightly larger than the size of the solute can lead to an increased influence of solute-pore wall interactions on permeation rate. In this study, we analyze how differences in polymer nanostructure affect the relative contributions of solute size and polymer-solute interactions on transport rate. We compared the diffusion rates of several small molecules through two polymer thin films: A cross-linked, homogeneous film of poly(ethylene glycol phenyl ether acrylate) (PEGPEA) and a graft copolymer with a poly(vinylidene fluoride-co-chlorotrifluoroethylene) (P(VDF-co-CTFE)) backbone and PEGPEA side chains that self-assemble into continuous ∼1-3 nm PEGPEA domains through which transport occurs. We correlated these rates with the size of each solute and its chemical affinity to PEGPEA, as measured by the difference between their solubility parameters. Diffusion rate through the homogeneous polymer film was controlled by solute size, whereas diffusion rate through the copolymer was strongly controlled by the difference between the solubility parameters. Furthermore, permeation selectivity between two selected molecules was 2.5× higher for the nanostructured copolymer, likely enhanced by the nanoconfinement effects. These initial results indicate that polymer self-assembly is a promising tool for designing polymeric membranes that can differentiate between solutes of similar size but differing chemical structures.

Evidence for Complex Molecular Architectures for Solvent-Extracted Lignins

Lignin, an abundant, naturally occurring biopolymer, is often considered "waste" and used as a simple fuel source in the paper-making process. However, lignin has emerged as a promising renewable resource for engineering materials, such as carbon fibers. Unfortunately, the molecular architecture of lignin (in vivo and extracted) is still elusive, with numerous conflicting reports in the literature, and knowledge of this structure is extremely important, not only for materials technologies, but also for production of biofuels such as cellulosic ethanol due to biomass recalcitrance. As such, the molecular structures of solvent-extracted (sulfur-free) lignins, which have been modified using various acyl chlorides, have been probed using small-angle X-ray (SAXS) and neutron (SANS) scattering in tetrahydrofuran (THF) solution along with hydrodynamic characterization using dilute solution viscometry and gel permeation chromatography (GPC) in THF. Mass spectrometry shows an absolute molecular weight ≈18-30 kDa (≈80-140 monomers), while GPC shows a relative molecular weight ∼3 kDa. A linear styrene oligomer (2.5 kDa) was also analyzed in THF using SANS. Results clearly show that lignin molecular architectures are somewhat rigid and complex, ranging from nanogels to hyperbranched macromolecules, not linear oligomers or physical assemblies of oligomers, which is consistent with previously proposed delignification (extraction) mechanisms. Future characterization using the methods discussed here can be used to guide extraction processes as well as genetic engineering technologies to convert lignin into value added materials with the potential for high positive impact on global sustainability.