Topochemistry of cellulose nanofibers resulting from molecular and polymer grafting

The aim of this study was to synthesize hydrophobic cellulose nanofibers (CNFs) using different chemical treatments including polymer and molecular grafting. For polymer grafting, immobilizing poly (butyl acrylate) (PBA) and poly (methyl methacrylate) (PMMA) on CNFs were implemented by the free radical method. Also, acetyl groups were introduced directly onto the CNFs surface by acetic anhydride for molecular grafting. The gravimetric and X-ray photoelectron spectroscopy analysis showed the high grafting density of PMMA on the surface of CNFs. AFM results revealed that molecular grafting created non-uniformity on the CNFs surface, as compared to polymer brushes. In addition, thermodynamic work of adhesion and work of cohesion for the modified CNFs were reduced in water and diiodomethane solvents. Dispersion factor was studied to indicate the dispersibility of CNFs in polar and non-polar media. Dispersion energy was reduced after modification as a result of decreasing interfacial tension and the dispersibility of modified CNFs was improved in diiodomethane.     

A molecular model for epsilon-caprolactam-based intercalated polymer clay nanocomposite: Integrating modeling and experiments

In studying the morphology, molecular interactions, and physical properties of organically modified montmorillonite (OMMT) and polymer clay nanocomposites (PCNs) through molecular dynamics (MD), the construction of the molecular model of OMMT and PCN is important. Better understanding of interaction between various constituents of PCN will improve the design of polymer clay nanocomposite systems. MD is an excellent tool to study interactions, which require accurate modeling of PCN under consideration. Previously, the PCN models were constructed by different researchers on the basis of specific criteria such as minimum energy configuration, density of the polymer clay nanocomposite, and so forth. However, in this article we describe the development of models combining experimental and conventional molecular modeling to develop models, which are more representative of true intercalated PCN systems. The models were used for studying the morphological interactions and physical properties. These studies gave useful information regarding orientation of organic modifiers, area of coverage of organic modifiers over the interlayer clay surface, interaction of organic modifiers with clay in OMMT, interaction among different constituents of PCN, conformational and density change, and actual proportion of mixing of polymer with clay in PCN. We have X-ray diffraction and photoacoustic Fourier transform infrared spectroscopy to verify the model.     

Influence of cation on the cellulose dissolution investigated by MD simulation and experiments

Cellulose is the most abundant natural polymer on the earth, and effective solvents are essential for its wide application. Among various solvents such as alkali/urea or ionic liquids, cations all play a very important role on the cellulose dissolution. In this work, the influence of cation on the cellulose dissolution in alkali/urea via a cooling process was investigated with a combination of MD simulation and experiments, including differential scanning calorimetry (DSC) and NMR diffusometry (PFG-SE NMR). The results of DSC proved that the dissolution of cellulose in both solvents was a process within a temperature range, starting at above 0 °C and completing at low temperature (?5 °C for LiOH/urea and ?20 °C for NaOH/urea), indicating the necessity of low temperature for the cellulose dissolution. Molecular dynamic (MD) simulation suggested that the electrostatic force between OH^? and cellulose dominated the inter-molecular interactions. In our findings, Li⁺ could penetrate closer to cellulose, and displayed stronger electrostatic interaction with the biomacromolecule than Na⁺, thus possessed a greater “stabilizing” effect on the OH^?/cellulose interaction. PFG-SE NMR demonstrated a more significant binding fraction of Li⁺ than Na⁺ to cellulose, which was consistent with MD. These results indicated that the direct interactions existed between the cations and cellulose, and Li⁺ exhibited stronger interaction with cellulose, leading to stronger dissolving power.     

Effects of mussel-inspired co-deposition of 2-hydroxymethyl methacrylate and poly (2-methoxyethyl acrylate) on the hydrophilicity and binding tendency of common hemodialysis membranes: Molecular dynamics simulations and molecular docking studies

Despite advances in the field, hemoincompatibility remains a critical issue for hemodialysis (HD) as interactions between various human blood constituents and the polymeric structure of HD membranes results in complications such as activation of immune system cascades. Adding hydrophilic polymer structures to the membranes is one modification approach that can decrease the extent of protein adsorption. This study conducted molecular dynamics (MD) simulations to understand the interactions between three human serum proteins (fibrinogen [FB], human serum albumin, and transferrin) and common HD membranes in untreated and modified forms. Poly(aryl ether sulfone) (PAES) and cellulose triacetate were used as the common dialyzer polymers, and membrane modifications were performed with 2-hydroxymethyl methacrylate (HEMA) and poly (2-methoxyethyl acrylate) (PMEA), using polydopamine-assisted co-deposition. The MD simulations were used as the framework for binding energy simulations, and molecular docking simulations were also performed to conduct molecular-level investigations between the two modifying polymers (HEMA and PMEA) and FB. Each of the three proteins acted differently with the membranes due to their unique nature and surface chemistry. The simulations show PMEA binds less intensively to FB with a higher number of hydrogen bonds, which reflects PMEA's superior performance compared to HEMA. The simulations suggest PAES membranes could be used in modified forms for blood-contact applications as they reflect the lowest binding energy to blood proteins.     

Improvement of bonding between cellulose and polypropylene by plasma treatment

Plasma treatments can be utilized to upgrade the value of lignocellulosic materials for applications such as biobased composites. Poor adhesion in biobased composites is caused by incompatibility between polar cellulosics and non-polar thermoplastics. Plasma modification of both cellulose and polypropylene was evaluated by a T-peel test for improved compatibility and adhesion between these materials. Oxygen and argon plasmas were used to modify the surface of polypropylene films, while a cyclohexane plasma was used to modify the cellulose surface through deposition of a hydrophobic polymer layer. For plasma treatment of polypropylene, changes in power input had a greater effect on adhesion than changes in pressure. Surface oxidation and increased acid/base characteristics were found on both argon- and oxygen-plasma-treated polypropylene based on ESCA and wetting measurements. With the non-reactive argon plasma the persistence of reactive species, such as free radicals, was very important for enhanced adhesion. The amount of polar carbonyl groups introduced onto the surface was also an important factor for adhesion improvement. Modification of the cellulose (filter paper) surface to a hydrophobic character with a cyclohexane plasma did not improve adhesion to polypropylene.     

Surface modification for nano‐lignocellulose fiber through vapor‐phase‐assisted surface polymerization

This study addresses the inherent issues surrounding surface modification methods of nanofibers and proposes an environmentally friendly and less toxic strategy for the surface modification of hydrophilic nanofiber. From the continuation of our previous work, which discussed the easy production of nanofiber (average size: 127 nm) from oil palm mesocarp fiber (OPMF), in this work, the surface of nanofibers (M‐IL‐OPMF) were modified through vapor‐phase‐assisted surface polymerization (VASP) to improve the affinity of interface between the polymer grafted M‐IL‐OPMF and non‐polar matrix. VASP of ε‐caprolactone was successfully proceeded from the [M‐IL‐OPMF] at 70 °C for 24 h and 72 h, and compositions were estimated to be 35.7% fiber/64.3% polymer and 27.8% fiber/72.2% polymer. To confirm the grafting of PCL, size‐exclusion chromatography (SEC) and Fourier transform infrared (FT‐IR) spectroscopy, thermogravimetry (TG), and dispersibility test in hydrophobic solvent were carried out. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 2575–2580     

Polymer-Filler Interphase Dynamics and Reinforcement of Elastomer Nanocomposites

The effect of polymer-filler interaction on interphase dynamics between filler particles in elastomer nanocomposites and the mechanisms of rubber reinforcement by carbon black (CB) are investigated with different techniques. To determine how polymer-filler interface influences the properties of the system, CB black was modified with the ionic liquid (IL) 1-allyl-3-methylimidazolium chloride (AMIC) and mixed with different, more or less, polar elastomers. For typical diene-elastomers (EPDM, SBR), this modification leads to a decreased polymer-filler coupling strength due to the coverage of active sites at the CB surface by AMIC. This is demonstrated by evaluating the energy site distribution from static gas adsorption isotherms with the polymer analogues gas 1-Butene. However, an improvement of polymer filler coupling was determined in the case of saturated, polar rubbers (HNBR) due to attractive dipolar interactions between the polar units of the polymer and the strongly adsorbed IL at the CB surface. The different couplings affect the polymer-filler interphase dynamics between filler particles, which determines the properties of the filler network and filler-filler bonds. To describe the effect of CB surface modification quantitatively, the Dynamic Flocculation Model (DFM) has been used to calculate polymer- and filler-specific material parameters from cyclic stress-strain measurements. The fitted data deliver a coherent picture of filler-filler- and polymer-filler couplings showing a characteristic dependence on rubber polarity. A confirmation of the effect of surface modification on the strength of filler-filler bonds is obtained by nonequilibrium molecular dynamics (MD) simulations of bond rupture under tension. They also provide indications for a glassy-like behavior of strongly confined polymer layers between attractive walls.     

Atomic force microscopy study of cellulose surface interaction controlled by cellulose binding domains

Colloidal probe microscopy has been used to study the interaction between model cellulose surfaces and the role of cellulose binding domain (CBD), peptides specifically binding to cellulose, in interfacial interaction of cellulose surfaces modified with CBDs. The interaction between pure cellulose surfaces in aqueous electrolyte solution is dominated by double layer repulsive forces with the range and magnitude of the net force dependent on electrolyte concentration. AFM imaging reveals agglomeration of CBD adsorbed on cellulose surface. Despite an increase in surface charge owing to CBD binding to cellulose surface, force profiles are less repulsive for interactions involving, at least, one modified surface. Such changes are attributed to irregularity of the topography of protein surface and non-uniform distribution of surface charges on the surface of modified cellulose. Binding double CBD hybrid protein to cellulose surfaces causes adhesive forces at retraction, whereas separation curves obtained with cellulose modified with single CBD show small adhesion only at high ionic strength. This is possibly caused by the formation of the cross-links between cellulose surfaces in the case of double CBD.     

Molecular modeling study on surface,thermal, mechanical and gas diffusion properties of chitosan

The motivation of this work is to provide reliable and accurate modeling studies of the physical (surface, thermal, mechanical and gas diffusion) properties of chitosan (CS) polymer. Our computational efforts have been devoted to make a comparison of the structural bulk properties of CS with similar type of polymers such as chitin and cellulose through cohesive energy density, solubility parameter, hydrogen bonding, and free volume distribution calculations. Atomistic modeling on CS polymer using molecular mechanics (MM) and molecular dynamics (MD) simulations has been carried out in three dimensionally periodic and effective two dimensionally periodic condensed phases. From the equilibrated structures, surface energies were computed. The equilibrium structure of the films shows an interior region of mass density close to the value in the bulk state. Various components of energetic interactions have been examined in detail to acquire a better insight into the interactions between bulk structure and the film surface. MD simulation (NPT ensemble) has also been used to obtain polymer specific volume as a function of temperature. It is demonstrated that these V–T curves can be used to locate the volumetric glass transition temperature (T_g) reliably. The mechanical properties of CS have been obtained using the strain deformation method. Diffusion coefficients of O₂, N₂, and CO₂ gas molecules at 300 K in CS have been estimated. The calculated properties of CS are comparable with the experimental values reported in the literature. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1260–1270, 2007     

Interactions underpinning the plasticization of a polymer matrix: a dynamic and structural analysis of DMP-plasticized cellulose acetate

This work establishes that the plasticization effect of a classical petrochemical plasticizer, dimethyl phthalate (DMP), on a polymer matrix, cellulose acetate (CA), is due to the development of intermolecular interactions of dipolar type. Plasticized cellulose acetate films are studied with regard to the interactions between the polymer and plasticizer at the macroscopic scale by thermogravimetric analysis and differential scanning calorimetry. At the molecular level, Fourier transform infrared spectroscopy and dielectric relaxation spectroscopy are used to elucidate the nature of interactions that are responsible for the plasticizing effects. These static and dynamic complementary analyses evidenced that DMP does not establish H-bonding interactions with the polymer chains of cellulose acetate but rather weaker interactions of dipolar type. These dipole–dipole interactions that develop between acetyl side groups of CA and the ester phthalate moieties of DMP increase the overall mobility of CA chains and also locally influence the molecular mobility and the water uptake tendency.     

Cellulose-polyamide 66 blends. Part II: Mechanical behavior

Blends of the natural polymer cellulose with a synthetic polymer, polyamide 66, are studied in order to determine if the expected strong interaction between them, due to hydrogen bonds, could improve their mechanical properties such as strength and elongation at break. In a previous work {Part I, J. Polym. Sci. Polym. Phys., 32 , 1437 (1994)}, the preparation technique and the characterization of cellulose-polyamide 66 (PA66) blends were described in detail. Several samples in the composition range between 0 to 70 wt % of PA66 were carefully dried and examined using dynamic mechanical and tensile tests. Based on previous work a new percolation model has been developed. It takes both linear and nonlinear mechanical behaviors into account and allows for the effect of adhesion between material domains. From comparison between experimental and predicted data, it is concluded that a partial miscibility between the amorphous phases of cellulose and PA66 exists and is responsible for a strong adhesion at their interface. Solid-state ¹³C nuclear magnetic resonance has also been used to study these samples and supports the existence of strong interactions between both homopolymers. © 1995 John Wiley & Sons, Inc.     

POLAR INTERACTIONS OF HYALURONIC ACID -EXPERIMENTS AND MOLECULAR DYNAMICS SIMULATIONS-

ABSTRACT

To study the polar interactions of the cartilage component hyaluronate (HA) contact angle measurements of polymer films of sodium hyaluronate and of the free hyaluronic acid with different probe liquids and theoretical investigations with molecular dynamics simulation (MD) on polymer segments in aqueous environment were performed. For the designation of contact angles water, formamid, glycerol and α-bromnaphthalene as probe liquids were used. The surface tension components were calculated on the basis of the theory of van Oss using the Young equation. Experimental investigations were done with air dried layers of the sodium salt of HA whose surface has been formed at the interface to the air resp. glass support. Whereas the surface polymer/air is characterized by small, but non-zero values for γ^? and γ^-the surface polymer/glass tends to have γ^-monopolar properties. In opposite to the salt form of HA a strong repulsion of chains and high γ^- monopolarity was measured for the protonated form.

The molecular dynamics simulation (MD) on HA in water were carried out employing the force field CHARMM and the water model TIP3P. MD trajectories of HA tetramer subunit surrounded by approximately 950 water molecules were produced up to 3 ns. The interaction energies of HA and water, hydrogen bonding, and the orientation of water molecules at different solute atom groups were calculated. On the basis of energy and geometry criteria, the number of hydrogen bonds between the water molecules and the polymer acceptor atoms was determined to be between 10 and 15 per dimer unit.     

Plasma Surface Modification of Biomedical Polymers: Influence on Cell-Material Interaction

Polymers are commonly used in industry because of their excellent bulk properties, such as strength and good resistance to chemicals. Their surface properties are for most application inadequate due to their low surface energy. A surface modification is often needed, and plasma surface modification is used with success the past decades. In the past few years, also plasma surface modification for biomedical polymers has been investigated. For biomedical polymers, the surface properties need to be altered to promote a good cell adhesion, growth and proliferation and to make them suitable for implants and tissue engineering scaffolds. This review gives an overview of the use of plasma surface modification of biomedical polymers and the influence on cell-material interactions. First, an introduction on cell-material interaction and on antibacterial and antifouling surfaces will be given. Also, different plasma modifying techniques used for polymer surface modification will be discussed. Then, an overview of literature on plasma surface modification of biopolymers and the resulting influence on cell-material interaction will be given. After an overview of plasma treatment for improved cell-material interaction, plasma polymerization and plasma grafting techniques will be discussed. Some more specialized applications will be also presented: the treatment of 3D scaffolds for tissue engineering and the spatial control of cell adhesion. Antibacterial and antifouling properties, obtained by plasma techniques, will be discussed. An overview of research dealing with antibacterial surfaces created by plasma techniques will be given, antifouling surfaces will be discussed, and how blood compatibility can be improved by preventing protein adhesion.     

Synthesis,adsorption and adhesive properties of a cationic amphiphilic block copolymer for use as compatibilizer in composites

In this work, the objective was to synthesize a compatibilizer that can electrostatically adsorb onto cellulose fibers, in fiber-based composites, to enhance the interaction between the fibers and non-polar polymer matrices. This physical route to attach the compatibilizer onto and thereby modify a fiber surface is convenient since it can be performed in water under mild conditions. Polystyrene (PS) was used for the high molecular weight, non-polar, block and poly(dimethylamino)ethyl methacrylate (PDMAEMA) was used as the polar block, which was subsequently quaternized to obtain cationic charges. The block copolymer self-assembles in water into cationic micelles and the adsorption to both silicon oxide surfaces and cellulose model surfaces was studied. The micelles spread out on the surface after heat treatment and contact angle measurements showed that the contact angles against water increased significantly after this treatment. AFM force measurements were performed with a PS probe to study the adhesive properties. The adhesion increased with increasing contact time for the treated surfaces, probably due to entanglements between the polystyrene blocks at the treated surface and the probe. This demonstrates that the use of this type of amphiphilic block copolymer is a promising route to improve the compatibility between charged reinforcing materials, such as cellulose-based fibers/fibrils, and hydrophobic matrices in composite materials.     

Understanding interactions between poly(styrene-co-sodium styrene sulfonate) and single-walled carbon nanotubes

Understanding formation mechanisms of hybrids of carbon nanotubes (CNTs) wrapped by polymers and their interactions is critical in modifying solubility of CNTs in aqueous solution and developing new nanotube-based polymer materials. In the present work, we investigate the structural details of poly(styrene-co-sodium styrene sulfonate) (PSS) wrapping around the CNT and the interactions between the PSS chain and the CNT using molecular dynamics (MD) simulations. The fraction of sulfonated groups significantly influences the wrapping conformations of the PSS chain. Due to limited time scale in the MD simulations, two different initial conformations of the chains are introduced to explore the effect of the initial state on the wrapping behavior. When the chains initially wrap around the CNT in a perfect helix manner, more compact pseudo-helical conformations are obtained. For initial straight line arrangement of the chain monomers, the chains adopt looser wrapping conformations. The free-energy analysis and binding interaction of the PSS chain on the CNT surface take a glance on the relationship between the conformational transition of the chain and the energy evolution.     

Effect of cationic polyacrylamides on the interactions between cellulose fibers

The interaction between cellulose fibers in the presence of cationic polyacrylamide (CPAM) was analyzed by rheology as a function of polyelectrolyte concentration, charge density, and molecular weight. CPAM was found to strongly influence the yield stress of cellulose suspensions; low doses of CPAM increased the yield stress, but at higher concentrations the yield stress declined. The charge density of the CPAM was the most significant factor in how yield stress responded to CPAM concentration; this effect was able to be normalized to a master curve by considering only the charged fraction of the polymer. The molecular weight of CPAM samples had some effect at high concentrations, but for lower CPAM doses the yield stress was independent of molecular weight over the range studied. The data suggest that CPAM modifies the interaction between cellulose surfaces via several mechanisms, with electrostatic interactions in the form of charge neutralization and charged patch formation dominating; polymer bridging and steric repulsion also influence the overall balance of forces between interacting cellulose fibers.     

MMA-天然橡胶接枝聚合物表面的酸碱作用

<正> 聚合物-基体界面的酸碱配位作用是界面粘合力形成的重要因素之一,Fowkes,Drago,Matsunaga等曾对参照液体-聚合物体系粘附功的酸碱作用成份的测算方法进行了研究,并测定了乙烯、丙烯酸、纤维素等聚合物表面的酸碱属性。本文应用Fowkes的基本方法,对MMA(甲基丙烯酸甲酯)-天然橡胶接枝聚合物表面的酸碱属性及其对基体的粘合力的影响进行了研究。     

Molecular simulation to discover rheological properties and soil-binding ability of PHPA polymer on montmorillonite surface

Synthetic polymer fluids are increasingly being applied to support excavations in deep foundations. As these fluids are molecularly engineered, their underlying microstructure interaction with in situ soils significantly affect excavation stability and soil dispersion. However, little molecular-scale research has been done on the rheological behavior of partially hydrolyzed polyacrylamides (PHPA) polymer fluids on the clay surface. Molecular models of the clay–polymer systems are constructed using PHPA on montmorillonite (MMT) clay surface. Initial rheological properties and soil-binding ability at different shear rates, temperatures, and polymer concentrations are first studied using molecular dynamics (MD) simulations. It is found that the functional groups of PHPA can interact with the MMT surface and form a viscous film under the atomic interaction of hydrogen bonds, water bridges, and electrostatic attraction. The shear stress, σ increases with the shear rate and follows the power-law model. And the viscosity, η decreases as the shear rate increases, which is consistent with the experimental trend. However, the σ and η decrease with the increase of temperature. And the action mode of PHPA concentration has been identified from the MD perspective. This work provides insight into the molecular mechanism for PHPA's rheology on the clay surface and their interaction.     

Fabrication of random and aligned electrospun nanofibers containing graphene oxide for skeletal muscle cells scaffold

Graphene oxide (GO)‐based materials have been explored in biomedical applications as active engineered materials for diagnosis and therapy. Although a large number of studies have been carried out in the last years, aspects involving the orientation and elongation of cells on GO immobilized on polymeric nanofibers are still scarce. We investigated the interactions between skeletal muscle cells and GO immobilized on random and aligned electrospun nanofibers of poly(caprolactone) (PCL), a biocompatible and biodegradable polymer. Oxygen plasma was employed to modify the nanofiber polymer surface to enhance the interactions between the PCL fibers and GO. Scanning electron microscopy and confocal microscopy revealed the morphology and orientation of skeletal muscle cells (C2C12 cells) on random and aligned GO/PCL nanofibers. The approach employed here is useful to investigate the interaction of skeletal muscle cells with biocompatible polymer nanofibers modified with GO intended for cell scaffolds and tissue engineering.     

Quantification of a tightly adsorbed monolayer of xylan on cellulose surface

The successive extraction and re-adsorption of a linear β-(1 → 4) xylan extracted from microfibrillated birch pulp was investigated using solid-state CP/MAS ¹³C NMR spectroscopy, specific surface area measurements, and atomistic molecular dynamics (MD) simulations. The NMR spectra confirmed that when in contact with cellulose after re-adsorption, the xylan molecules altered their conformation from the classical left-handed threefold structure found in the bulk to a different one, presumably a cellulose-like twofold system for quantities up to the equivalent amount of extracted xylan. Combining these observations with specific surface area measurements and the surface occupied by a xylosyl residue, it was possible to show that the re-adsorbed xylan in the modified conformation occurred only within the first adsorbed layer in direct interaction with the cellulose surface. It is only when an excess xylan was added and after full cellulose surface coverage, that the subsequent deposited layers took the classical threefold organization. Following the variation of xylan conformation in terms of sequential xylan addition allowed quantifying the surface of cellulose accessible for a tight adsorption of xylan, not only for microfibrillated birch cellulose, but for other samples as well. The MD simulations confirmed that xylan in threefold conformation had a weaker affinity for the cellulose surface than its twofold counterpart, thus supporting the hypothesis of the twofold conformation for xylan at the cellulose surface. The MD simulations also showed that in contact with cellulose, the adsorbed xylan was mainly organized as an extended molecular chain aligned parallel to the cellulose chain direction.