Perspectives on the Lindman Hypothesis and Cellulose Interactions

In the history of cellulose chemistry, hydrogen bonding has been the predominant explanation when discussing intermolecular interactions between cellulose polymers. This is the general consensus in scholarly textbooks and in many research articles, and it applies to several other biomacromolecules' interactions as well. This rather unbalanced description of cellulose has likely impacted the development of materials based on the processing of cellulose-for example, via dissolution in various solvent systems and regeneration into solid materials, such as films and fibers, and even traditional wood fiber handling and papermaking. In this review, we take as a starting point the questioning of the general description of the nature of cellulose and cellulose interactions initiated by Professor Björn Lindman, based on generic physicochemical reasoning about surfactants and polymers. This dispute, which became known as "the Lindman hypothesis", highlights the importance of hydrophobic interactions in cellulose systems and that cellulose is an amphiphilic polymer. This paper elaborates on Björn Lindman's contribution to the subject, which has caused the scientific community to revisit cellulose and reconsider certain phenomena from other perspectives.

Role and importance of solvents for the fractionation of lignocellulosic biomass

Lignocellulosic biomass is one of the most important renewable materials to replace carbon-based fossil resources. Solvent-based fractionation is a promising route for fractionation of biomass into its major components. Processing is governed by the employed solvent-systems properties. This review sheds light on the factors governing both dissolution and potential reactivities of the chemical structures present in lignocellulose, highlighting how proper understanding of the underlying mechanisms and interactions between solute and solvent help to choose proper systems for specific fractionation needs. Structural and chemical differences between the carbohydrate-based structural polymers and lignin require very different solvents capabilities in terms of causing and eventually stabilizing conformational changes and consequent activation of bonds to be cleaved by other active components in the. A consideration of potential depolymerization events during dissolution and energetic aspects of the dissolution process considering the contribution of polymer functionalities allow for a mapping of solvent suitability for biomass fractionation.

In situ 13 C solid-state polarization transfer NMR to follow starch transformations in food

Convenience food products tend to alter their quality and texture while stored. Texture-giving food components are often starch-rich ingredients, such as pasta or rice. Starch transforms depending on time, temperature and water content, which alters the properties of products. Monitoring these transformations, which are associated with a change in mobility of the starch chain segments, could optimize the quality of food products containing multiple ingredients. In order to do so, we applied a simple and efficient in situ ¹³ C solid-state magic angle spinning (MAS) NMR approach, based on two different polarization transfer schemes, cross polarization (CP) and insensitive nuclei enhanced by polarization transfer (INEPT). The efficiency of the CP and INEPT transfer depends strongly on the mobility of chain segments-the time scale of reorientation of the CH-bond and the order parameter. Rigid crystalline or amorphous starch chains give rise to CP peaks, whereas mobile gelatinized starch chains appear as INEPT peaks. Comparing ¹³ C solid-state MAS NMR experiments based on CP and INEPT allows insight into the progress of gelatinization, and other starch transformations, by reporting on both rigid and mobile starch chains simultaneously with atomic resolution by the ¹³ C chemical shift. In conjunction with ¹ H solid-state MAS NMR, complementary information about other food components present at low concentration, such as lipids and protein, can be obtained. We demonstrate our approach on starch-based products and commercial pasta as a function of temperature and storage.

Solid State NMR a Powerful Technique for Investigating Sustainable/Renewable Cellulose-Based Materials

Solid state nuclear magnetic resonance (ssNMR) is a powerful and attractive characterization method for obtaining insights into the chemical structure and dynamics of a wide range of materials. Current interest in cellulose-based materials, as sustainable and renewable natural polymer products, requires deep investigation and analysis of the chemical structure, molecular packing, end chain motion, functional modification, and solvent–matrix interactions, which strongly dictate the final product properties and tailor their end applications. In comparison to other spectroscopic techniques, on an atomic level, ssNMR is considered more advanced, especially in the structural analysis of cellulose-based materials; however, due to a dearth in the availability of a broad range of pulse sequences, and time consuming experiments, its capabilities are underestimated. This critical review article presents the comprehensive and up-to-date work done using ssNMR, including the most advanced NMR strategies used to overcome and resolve the structural difficulties present in different types of cellulose-based materials.

Revisiting the dissolution of cellulose in H3PO4(aq) through cryo-TEM, PTssNMR and DWS

Cellulose can be dissolved in concentrated acidic aqueous solvents forming extremely viscous solutions, and, in some cases, liquid crystalline phases. In this work, the concentrated phosphoric acid aqueous solvent is revisited implementing a set of advanced techniques, such as cryo-transmission electronic microscopy (cryo-TEM), polarization transfer solid-state nuclear magnetic resonance (PTssNMR), and diffusing wave spectroscopy (DWS). Cryo-TEM images confirm that this solvent system is capable to efficiently dissolve cellulose. No cellulose particles, fibrils, or aggregates are visible. Conversely, PTssNMR revealed a dominant CP signal at 25 °C, characteristic of C-H bond reorientation with correlation time longer than 100 ns and/or order parameter above 0.5, which was ascribed to a transient gel-like network or an anisotropic liquid crystalline phase. Increasing the temperature leads to a gradual transition from CP to INEPT-dominant signal and a loss of birefringence in optical microscopy, suggesting an anisotropic-to-isotropic phase transition. Finally, an excellent agreement between optical microrheology and conventional mechanical rheometry was also obtained.

Surface-Initiated Controlled Radical Polymerization Approach to In Situ Cross-Link Cellulose Nanofibrils with Inorganic Nanoparticles

This paper investigates a strategy to convert hydrophilic cellulose nanofibrils (CNF) into a hydrophobic highly cross-linked network made of cellulose nanofibrils and inorganic nanoparticles. First, the cellulose nanofibrils were chemically modified through an esterification reaction to produce a nanocellulose-based macroinitiator. Barium titanate (BaTiO₃, BTO) nanoparticles were surface-modified by introducing a specific monomer on their outer-shell surface. Finally, we studied the ability of the nanocellulose-based macroinitiator to initiate a single electron transfer living radical polymerization of stearyl acrylate (SA) in the presence of the surface-modified nanoparticles. The BTO nanoparticles will transfer new properties to the nanocellulose network and act as a cross-linking agent between the nanocellulose fibrils, while the monomer (SA) directly influences the hydrophilic-lipophilic balance. The pristine CNF and the nanoparticle cross-linked CNF are characterized by FTIR, SEM, and solid-state ¹³C NMR. Rheological and dynamic mechanical analyses revealed a high dregee of cross-linking.

New Insights on the Role of Urea on the Dissolution and Thermally-Induced Gelation of Cellulose in Aqueous Alkali

The gelation of cellulose in alkali solutions is quite relevant, but still a poorly understood process. Moreover, the role of certain additives, such as urea, is not consensual among the community. Therefore, in this work, an unusual set of characterization methods for cellulose solutions, such as cryo-transmission electronic microscopy (cryo-TEM), polarization transfer solid-state nuclear magnetic resonance (PTssNMR) and diffusion wave spectroscopy (DWS) were employed to study the role of urea on the dissolution and gelation processes of cellulose in aqueous alkali. Cryo-TEM reveals that the addition of urea generally reduces the presence of undissolved cellulose fibrils in solution. These results are consistent with PTssNMR data, which show the reduction and in some cases the absence of crystalline portions of cellulose in solution, suggesting a pronounced positive effect of the urea on the dissolution efficiency of cellulose. Both conventional mechanical macrorheology and microrheology (DWS) indicate a significant delay of gelation induced by urea, being absent until ca. 60 °C for a system containing 5 wt % cellulose, while a system without urea gels at a lower temperature. For higher cellulose concentrations, the samples containing urea form gels even at room temperature. It is argued that since urea facilitates cellulose dissolution, the high entanglement of the cellulose chains in solution (above the critical concentration, C*) results in a strong three-dimensional network.

Cationic hydrophobicity promotes dissolution of cellulose in aqueous basic solution by freezing–thawing

Hydrophobic cations accumulate at the cellulose interface, favouring the physical dissolution of cellulose in aqueous quaternary ammonium hydroxides.