Nanometer smooth, macroscopic spherical cellulose probes for contact adhesion measurements

Reevaluation of the adhesion between cellulose materials using macro spherical beads and flat model surfaces

Interactions between dry cellulose were studied using model systems, cellulose beads, and cellulose films, using custom-built contact adhesion testing equipment. Depending on the configuration of the substrates in contact, Polydimethylsiloxane (PDMS) film, cellulose films spin-coated either on PDMS or glass, the interaction shows three distinct processes. Firstly, molecular interlocking is formed between cellulose and cellulose when there is a soft PDMS thin film backing the cellulose film. Secondly, without backing, no initial attraction force between the surfaces is observed. Thirdly, a significant force increase, ∆F, is observed during the retraction process for cellulose on glass, and there is a maximum in ∆F when the retraction rate is increased. This is due to the kinetics of a contacting process occurring in the interaction zone between the surfaces caused by an interdigitation of a fine fibrillar structure at the nano-scale, whereas, for the spin-coated cellulose surfaces on the PDMS backing, there is a more direct adhesive failure. The results have generated understanding of the interaction between cellulose-rich materials, which helps design new, advanced cellulose-based materials. The results also show the complexity of the interaction between these surfaces and that earlier mechanisms, based on macroscopic material testing, are simply not adequate for molecular tailoring.

The Use of Model Cellulose Materials for Studying Molecular Interactions at Cellulose Interfaces

Despite extensive research on biobased and fiber-based materials, fundamental questions regarding the molecular processes governing fiber-fiber interactions remain unanswered. In this study, we introduce a method to examine and clarify molecular interactions within fiber-fiber joints using precisely characterized model materials, i.e., regenerated cellulose gel beads with nanometer-smooth surfaces. By physically modifying these materials and drying them together to create model joints, we can investigate the mechanisms responsible for joining cellulose surfaces and how this affects adhesion in both dry and wet states through precise separation measurements. The findings reveal a subtle balance in the joint formation, influencing the development of nanometer-sized structures at the contact zone and likely inducing built-in stresses in the interphase. This research illustrates how model materials can be tailored to control interactions between cellulose-rich surfaces, laying the groundwork for future high-resolution studies aimed at creating stiff, ductile, and/or tough joints between cellulose surfaces and to allow for the design of high-performance biobased materials.

Elucidating the fine-scale structural morphology of nanocellulose by nano infrared spectroscopy

Nanoscale infrared (IR) spectroscopy and microscopy, enabling the acquisition of IR spectra and images with a lateral resolution of 20 nm, is employed to chemically characterize individual cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs) to elucidate if the CNCs and CNFs consist of alternating crystalline and amorphous domains along the CNF/CNC. The high lateral resolution enables studies of the nanoscale morphology at different domains of the CNFs/CNCs: flat segments, kinks, twisted areas, and end points. The types of nanocellulose investigated are CNFs from tunicate, CNCs from cotton, and anionic and cationic wood-derived CNFs. All nano-FTIR spectra acquired from the different samples and different domains of the individual nanocellulose particles resemble a spectrum of crystalline cellulose, suggesting that the non-crystalline cellulose signal observed in macroscopic measurements of nanocellulose most likely originate from cellulose chains present at the surface of the nanocellulose particles.

A Numerical Model for Understanding the Development of Adhesion during Drying of Cellulose Model Surfaces

Adhesion is crucial for the development of mechanical properties in fibre-network materials, such as paper or other cellulose fibre biocomposites. The stress transfer within the network is possible through the fibre-fibre joints, which develop their strength during drying. Model surfaces are useful for studying the adhesive strength of joints by excluding other parameters influencing global performance, such as geometry, fibre fibrillation, or surface roughness. Here, a numerical model describes the development of adhesion between a cellulose bead and a rigid surface using an axisymmetric formulation, including moisture diffusion, hygroexpansion, and cohesive surfaces. It is useful for studying the development of stresses during drying. A calibration of model parameters against previously published contact and geometry measurements shows that the model can replicate the observed behaviour. A parameter study shows the influence of cohesive and material parameters on the contact area. The developed model opens possibilities for further studies on model surfaces, with quantification of the adhesion during pull-off measurements.

Understanding the Drying Behavior of Regenerated Cellulose Gel Beads: The Effects of Concentration and Nonsolvents

The drying behavior of regenerated cellulose gel beads swollen with different nonsolvents (e.g., water, ethanol, water/ethanol mixtures) is studied in situ on the macroscopic scale with an optical microscope as well as on nanoscale using small-angle/wide-angle X-ray scattering (SAXS/WAXS) techniques. Depending on the cellulose concentration, the structural evolution of beads during drying follows one of three distinct regimes. First, when the cellulose concentration is lower than 0.5 wt %, the drying process comprises three steps and, regardless of the water/ethanol mixture composition, a sharp structural transition corresponding to the formation of a cellulose II crystalline structure is observed. Second, when the cellulose concentration is higher than 5.0 wt %, a two-step drying process is observed and no structural transition occurs for any of the beads studied. Third, when the cellulose concentration is between 0.5 and 5.0 wt %, the drying process is dependent on the nonsolvent composition. A three-step drying process takes place for beads swollen with water/ethanol mixtures with a water content higher than 20%, while a two-step drying process is observed when the water content is lower than 20%. To describe the drying behavior governed by the cellulose concentration and nonsolvent composition, a simplified phase diagram is proposed.

Structure Development of the Interphase between Drying Cellulose Materials Revealed by In Situ Grazing-Incidence Small-Angle X-ray Scattering

The nano- to microscale structures at the interface between materials can define the macroscopic material properties. These structures are extremely difficult to investigate for complex material systems, such as cellulose-rich materials. The development of new model cellulose materials and measuring techniques has opened new possibilities to resolve this problem. We present a straightforward approach combining micro-focusing grazing-incidence small-angle X-ray scattering and atomic force microscopy (AFM) to investigate the structural rearrangements of cellulose/cellulose interfaces in situ during drying. Based on the results, we propose that molecular interdiffusion and structural rearrangement play a major role in the development of the properties of the cellulose/cellulose interphase; this model is representative of the development of the properties of joint/contact points between macroscopic cellulose fibers.

Fabrication of Cellulose-Based Biopolymer Optical Fibers and Their Theoretical Attenuation Limit

Currently, almost all polymer optical materials are derived from fossil resources with known consequences for the environment. In this work, a processing route to obtain cellulose-based biopolymer optical fibers is presented. For this purpose, the optical properties such as the transmission and the refractive index dispersion of regenerated cellulose, cellulose diacetate, cellulose acetate propionate, and cellulose acetate butyrate were determined from planar films. Cellulose fibers were produced using a simple wet-spinning setup. They were examined pure and also coated with the cellulose derivatives to obtain core-cladding-structured optical fibers. The cellulose-based optical fibers exhibit minimum attenuations between 56 and 82 dB m^-1 at around 860 nm. The ultimate transmission loss limit of the cellulose-based optical fibers was simulated to characterize the attenuation progression. By reducing extrinsic losses, cellulose-based biopolymer optical fibers could attain theoretical attenuation minima of 84.6 × 10^-3 dB m^-1 (507 nm), 320 × 10^-3 dB m^-1 (674 nm), and 745.2 × 10^-3 dB m^-1 (837 nm) and might substitute fossil-based polymer optical fibers in the future.

Facile preparation of all-cellulose composites from softwood, hardwood, and agricultural straw cellulose by a simple route of partial dissolution

In this study, we report a novel, facile, and green method that was used for creating a new all-cellulose composite (ACC) based on inorganic molten salt solvent. Three representatively native fibers from softwood (Pinus kesiya), hardwood (Eucalyptus globulus), and agricultural straw (Zea mays) were selected to verify the effect of the method. The welded sheets were thoroughly characterized and compared. Cellulose sheets from the pine exhibited excellent mechanical properties (σb 16.94 MPa) and thermal stability (T_max 265 °C) after the welding process, while the corn stalk sheets displayed more robust and thermostable features than the eucalyptus. The welding technique using inorganic metal salt hydrate provides a promising and convenient route to obtain firm sheet-materials with micro- or nano-structures from nature fibers.

The use of model cellulose gel beads to clarify flame-retardant characteristics of layer-by-layer nanocoatings

Layer-by-Layer (LbL) assembled nanocoatings are exploited to impart flame-retardant properties to cellulosic substrates. A model cellulose material can make it possible to investigate an optimal bilayer (BL) range for the deposition of coating while elucidating the main flame-retardant action thus allowing for an efficient design of optimized LbL formulations. Model cellulose gel beads were prepared by dissolving cellulose-rich fibers followed by precipitation. The beads were LbL-treated with chitosan (CH) and sodium hexametaphosphate (SHMP). The char forming properties were studied using thermal gravimetric analysis. The coating increased the char yield in nitrogen to up to 29 % and showed a distinct pattern of micro intumescent behavior upon heating. An optimal range of 10-20 BL is observed. The well-defined model cellulose gel beads hence introduce a new scientific route both to clarify the fundamental effects of different film components and to optimize the composition of the films.

Nacre-Inspired Polymeric Materials with Body Heat-Responsive Shape-Memory Effect, High Optical Transparence, and Balanced Mechanical Properties

In this work, inspired by the hierarchical architecture of nacre, we have fabricated poly(propylene carbonate) (PPC)/thermoplastic polyurethane (TPU) alternating multilayer films via layer-multiplying coextrusion. Based on the glass transition at around 37 °C of PPC, the multilayer films exhibited an outstanding body heat-responsive shape-memory effect (SME) with high shape fixation and recovery ratios (96.1 and 93.6%), much better than the conventional cocontinuous blend with the same compositions. It was revealed that the high phase continuity and abundantly two-dimensional interfaces both capable of promoting stress transferring and load distribution maximally contributed to the SME. Furthermore, the multilayer films showed a superior recovery stress storage capacity and the force generated by shape recovery allowed automatic expansion of the spiral in 37 °C water and efficient lifting of a load 880 times its weight. Different from the opacity of the blend, a high optical transparence was observed in the multilayers because of the parallel assembly of transparent PPC and TPU enabling light to directly pass through the films. Besides, the nacre-like films had layer debonding and layer stepwise breaking during stretching, resulting in a 90% increase in tensile strength, a 70% increase in elongation at break, and onefold improvement in yield stress, compared with those of the blend. Our approach paves a new way for developing bioinspired structural materials with excellent optical, mechanical, and shape-memory properties, which can be extended to different amorphous polymers and elastomers. Also, the materials presented herein have great potential in applications of biomedical devices and soft robotics.

Development of mechanical properties of regenerated cellulose beads during drying as investigated by atomic force microscopy

The mechanical properties as well as the size changes of swollen cellulose beads were measured in situ during solvent evaporation by atomic force microscopy (AFM) indentation measurement combined with optical microscopy. Three factors are proposed to govern the mechanical properties of the cellulose beads in the swollen state and during drying: (i) the cellulose concentration, (ii) the interaction between the cellulose entities, (iii) the heterogeneity of the network structure within the cellulose beads.

Macro- and Microstructural Evolution during Drying of Regenerated Cellulose Beads

The macro- and microstructural evolution of water swollen and ethanol swollen regenerated cellulose gel beads have been determined during drying by optical microscopy combined with analytical balance measurements, small-angle X-ray scattering (SAXS), and wide-angle X-ray scattering (WAXS). Two characteristic length scales, which are related to the molecular dimension of cellulose monomer and elongated aggregates of these monomers, could be identified for both types of beads by SAXS. For ethanol swollen beads, only small changes to the structures were detected in both the SAXS and WAXS measurements during the entire drying process. However, the drying of cellulose from water follows a more complex process when compared to drying from ethanol. As water swollen beads dried, they went through a structural transition where elongated structures changed to spherical structures and their dimensions increased from 3.6 to 13.5 nm. After complete drying from water, the nanostructures were characterized as a combination of rodlike structures with an approximate size of cellulose monomers (0.5 nm), and spherical aggregates (13.5 nm) without any indication of heterogeneous meso- or microporosity. In addition, WAXS shows that cellulose II hydrate structure appears and transforms to cellulose II during water evaporation, however it is not possible to determine the degree of crystallinity of the beads from the present measurements. This work sheds lights on the structural changes that occur within regenerated cellulose materials during drying and can aid in the design and application of cellulosic materials as fibers, adhesives, and membranes.

Thermodynamics of the Water-Retaining Properties of Cellulose-Based Networks

Noncrystalline cellulose-based gel beads were used as a model material to investigate the effect of osmotic stress on a cellulosic network. The gel beads were exposed to osmotic stress by immersion in solutions with different concentrations of high molecular mass dextran and the equilibrium dimensional change of the gel beads was studied using optical microscopy. The volume fraction of cellulose was calculated from the volume of the gel beads in dextran solutions and their dry content and the relation between the cellulose volume fraction and the total osmotic pressure was thus obtained. The results show that the contribution to the osmotic pressure from counterions increases the water-retaining capacity of the beads at high osmotic pressures but also that the main factor controlling the gel bead collapse at high osmotic strains is the resistance to the deformation of the polymer chain network within the beads. Furthermore, the osmotic pressure associated with the deformation of the polymer network, which counteracts the deswelling of the beads, could be fitted to the Wall model indicating that the response of the cellulose polymer networks was independent of the charge of the cellulose. The best fit to the Wall model was obtained when the Flory-Huggins interaction parameter (χ) of the cellulose-water system was set to 0.55-0.60, in agreement with the well-established insolubility of high molecular mass β-(1,4)-d-glucan polymers in water.

Adhesion properties of regenerated lignocellulosic fibres towards poly(lactic acid) microspheres assessed by colloidal probe technique

In the field of polymer reinforcement, it is important to understand the interactions involved between the polymer matrix and the reinforcing component. This paper is a contribution to the fundamental understanding of the adhesion mechanisms involved in natural fibre reinforced composites. We report on the use of the colloidal probe technique for the assessment of the adhesion behaviour between poly(lactic acid) microspheres and embedded cross-sections of regenerated lignocellulosic fibres. These fibres consisted of tailored mixtures of cellulose, lignin and xylan, the amount of which was determined beforehand. The influence of the chemical composition of the fibres on the adhesion behaviour was studied in ambient air and in dry atmosphere. In ambient air, capillary forces resulted in larger adhesion between the sphere and the fibres. Changing the ambient medium to a dry nitrogen atmosphere allowed reducing the capillary forces, leading to a drop in the adhesion forces. Differences between fibres of distinct chemical compositions could be measured only on freshly cut surfaces. Moreover, the surface energy of the fibres was assessed by inverse gas chromatography. Compared to fibres containing solely cellulose, the presence of lignin and/or hemicellulose led to higher adhesion and lower surface energy, suggesting that these chemicals could serve as natural coupling agents between hydrophobic and hydrophilic components.

Carbohydrate gel beads as model probes for quantifying non-ionic and ionic contributions behind the swelling of delignified plant fibers

Macroscopic beads of water-based gels consisting of uncharged and partially charged β-(1,4)-d-glucan polymers were developed to be used as a novel model material for studying the water induced swelling of the delignified plant fiber walls. The gel beads were prepared by drop-wise precipitation of solutions of dissolving grade fibers carboxymethylated to different degrees. The internal structure was analyzed using Solid State Cross-Polarization Magic Angle Spinning Carbon-13 Nuclear Magnetic Resonance and Small Angle X-ray Scattering showing that the internal structure could be considered a homogeneous, non-crystalline and molecularly dispersed polymer network. When beads with different charge densities were equilibrated with aqueous solutions of different ionic strengths and/or pH, the change in water uptake followed the trends expected for weak polyelectrolyte gels and the trends found for cellulose-rich fibers. When dried and subsequently immersed in water the beads also showed an irreversible loss of swelling depending on the charge and type of counter-ion which is commonly also found for cellulose-rich fibers. Taken all these results together it is clear that the model cellulose-based beads constitute an excellent tool for studying the fundamentals of swelling of cellulose rich plant fibers, aiding in the elucidation of the different molecular and supramolecular contributions to the swelling.

Probing the adhesion properties of alginate hydrogels: a new approach towards the preparation of soft colloidal probes for direct force measurements

The adhesion of alginate hydrogels to solid surfaces was probed by atomic force microscopy (AFM) in the sphere/plane geometry. For this purpose a novel approach has been developed for the immobilization of soft colloidal probes onto AFM-cantilevers, which is inspired by techniques originating from cell biology. The aspiration and consecutive manipulation of hydrogel beads by micropipettes allows the entire manipulation sequence to be carried-out in situ. Hence, any alteration of the hydrogel beads upon drying can be excluded. The adhesive behaviour of alginate hydrogels was first evaluated by determining the distribution of pull-off forces on self-assembled monolayers (SAMs) terminating in different functional groups (-CH₃, -OH, -NH₂, -COOH). It was demonstrated that solvent exclusion plays practically no role in the adhesion process, in clear difference to solid colloidal probes. The adhesion of alginate beads is dominated by chemical interactions rather than solvent exclusion, in particular in the case of amino-terminated SAMs. The data set acquired on the SAMs provided the framework to relate the adhesion of alginate beads on recombinant spider silk protein films to specific functional groups. The preparation of soft colloidal probes and the presented approach in analysing the adhesive behaviour is not limited to alginate hydrogel beads but can be generally applied for probing and understanding the adhesion behaviour of hydrogels on a wide range of substrates, which would be relevant for various applications such as biomedical surface modification or tissue engineering.