Surface methacrylation and graft copolymerization of ultrafine cellulose fibers

Ultrafine fibrous (? from 100 to 450 nm) cellulose membranes were generated by electrospinning of cellulose acetate [degree of substitution (DS): 2.45, weight‐average molecular weight: 30,000 Da], followed by alkaline deacetylation. Reaction of these ultrahigh surface‐area cellulose fibers with methacrylate chloride (MACl) produced activated surfaces without altering the fiber morphology. Surface methacrylation of these fibers was confirmed by the acquired hydrophobicity (θ_water = 84°) as compared to the originally hydrophilic (θ_water = 56°) cellulose. Changing the MACl:OH molar ratios could vary the overall DS of methacrylation. The very low overall DS values indicate the surface nature of the methacrylation reaction. At a DS of 0.17, the thermal properties of the surface methacrylated cellulose resemble those of cellulose derivatives at much higher DS values, an unusual behavior of the ultrafine fibers. The methacrylated cellulose could be further copolymerized with vinyl monomers (methyl methacrylate, acrylamide, and N‐isopropylacrylamide) as linear grafts or three‐dimensional (3D) networks. The morphology of cellulose fibers and the interfiber pore structure were not altered at 15–33% graft levels. This study demonstrates that either linear or 3D networks of vinyl polymers could be efficiently supported on ultrafine cellulose fibrous membranes via surface methacrylation. Through these surface reactions the chemical, thermal, and liquid wetting and absorbent properties of these ultrafine fibrous membranes were significantly altered with no change to the fiber dimensions or interfiber pore morphology. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 953–964, 2003     

Surface Esterification of Cellulose by Vapor-Phase Treatment With Trifluoroacetic Anhydride

Vapor-phase esterification of cellulose was achieved by exposing filter paper and tunicate cellulose film to mixed vapor of trifluoroacetic anhydride (TFAA) and acetic acid (AcOH) or TFAA and acetic anhydride (Ac₂O) at room temperature. Both treatments gave high hydrophobicity to the specimens. The hydrophobicity of TFAA/AcOH-treated sample was resistant to water immersion, but that of TFAA/Ac₂O-treated sample was lost due to hydrolysis. In spite of high hydrophobicity, the bulk degree of substitution for acetyl groups was as low as 0.07, indicating localization of acetyl groups on the fibril surfaces. Detection of ester groups by X-Ray photoelectron spectroscopy corroborated this feature. Infrared spectroscopy of tunicate cellulose revealed that the esterification occurred preferentially to the weakly hydrogen-bonded hydroxyl groups. This vapor-phase process is potentially useful in modifying cellulosic materials without immersing in solvents or liquid reagents.     

Bi-phobic cellulose fibers derivatives via surface trifluoropropanoylation

The surface modification of cellulose fibers with 3,3,3-trifluoropropanoyl chloride (TFP) was studied in a toluene suspension. The characterization of the modified fibers was performed by elemental analysis, Fourier transform infrared (FTIR), 13C-solid-state NMR, X-ray diffraction, thermogravimetry, and surface analysis (XPS, ToF-SIMS, and contact angles measurements). The degree of substitution (DS) of the ensuing trifluoropropanoylated fibers ranged from less than 0.006 to 0.30, and in all instances the fibers' surface acquired a high hydrophobicity and lipophobicity resulting from a drastic reduction in its energy. The hydrolytic stability of these cellulose derivatives was also evaluated and shown to be permanent in time in the presence of neutral water, still appreciable in basic aqueous solution at pH 9, but, as expected quite poor at pH 12.     

Surface characterization by XPS, contact angle measurements and ToF-SIMS of cellulose fibers partially esterified with fatty acids

The topochemistry of the controlled heterogeneous esterification of cellulose fibers with fatty acid chlorides of different chain length, both in swelling and non-swelling media, was assessed by X-ray Photoelectron Spectroscopy (XPS), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and contact angle measurements. On the one hand, the results provided by the combined use of these three powerful techniques showed unambiguously the occurrence of the reaction at the fibers' surface and, on the other hand, the XPS results showed that the surface coverage with the fatty acid moieties increased with their chain length, but was only modestly affected by the degree of substitution (DS), suggesting that when the esterification yield was increased (higher DS values), an in-depth reaction also occurred, particularly when DMF was used as a cellulose swelling medium, involving the OH groups buried below the fibers' surface.     

Novel cellulose derivatives. V. Synthesis and thermal properties of esters with trifluoroethoxy acetic acid

Esters of cellulose with trifluoroethoxy acetic acid (TFAA) were prepared in homogeneous phase using a mixed anhydride with p‐toluenesulfonic acid. Esters with low degree of substitution (DS), and with DS rising from 0 to 3, had hydrophobic character that prevented the usual association with moisture, which is otherwise typical of cellulose esters with low DS. Cellulose trifluoroethoxy acetate (CT) had T_g's declining by about 40 °C per DS‐unit (from 160 to 41 °C) as DS rose from 1 to 3. Mixed esters, cellulose derivatives with acetate and trifluoroethoxy acetate substituents (CAT), exhibited glass‐to‐rubber and melting transitions by DSC. A linear relationship between both T_g and T_m with respect to DS was recorded with the T_g and T_m separated by 30° to 40 °C. This is consistent with cellulose esters described elsewhere. Surprisingly, the T_g's of CT and CAT were found to be identical when the DS was equivalent to the DS of the fluoro substituents (DS_F). © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 486–494, 2000     

Preparation and properties of a new type of comb-shaped, amphiphilic cellulose derivative

Comb-shaped, amphiphilic O-(2- hydroxy-3-butoxypropyl) cellulose (HBPC) was prepared by a homogeneous reaction of cellulose with butyl glycidyl ether (BGE) in a 10% (w/w) LiCl--DMAc solution. It was found that: (a) the molar substitution (MS) of water-soluble HBPC ranges from 0.4 to 1.0, and is nearly equal to its degree of substitution (DS), indicating that the HBPC derivatives obtained are comb-shaped polymers; (b) the water-soluble HBPC shows a thermally reversible sol-gel transition in aqueous solution; and (c) the derivative having a DS of 0.6 shows surface activity with critical micelle concentration (cmc) in the order of 0.8 g/l and surface tension of 31.5 dyn/cm     

Ultrafine fibrous cellulose membranes from electrospinning of cellulose acetate

Three solvents, that is, acetone, acetic acid, and dimethylacetamide (DMAc), with a range of solubility parameter δ, surface tension γ, viscosity η and boiling temperature were used to generate mixtures for electrospinning cellulose acetate (CA) (degree of substitution, DS = 2.45). Although none of these solvents alone enables continuous formation of fibers, mixing DMAc with either acetone or acetic acid produced suitable solvent systems. The 2:1 acetone:DMAc mixture is the most versatile mixture because it allows CA in the 12.5–20% concentration range to be continuously electrospun into fibrous membranes. These CA solutions have η between 1.2 and 10.2 poise and γ around 26 dyne/cm and produce smooth fibers with diameters from 100 nm to ～1 μm. Fiber sizes generally decrease with decreasing CA concentrations. The nature of the collectors affects the morphology as well as packing of fibers. Fibers collected on paper have more uniform sizes, smooth surfaces, and fewer defects, whereas fibers collected on water are more varied in size. Electrically conductive solid collectors, such as Al foil and water, favor more tightly packed and less porous membranes. Porous collectors, like paper and copper mesh, produce highly porous membranes. The pores in membranes collected on the Al foil and paper are much better interconnected in the planar directions than those in membranes collected on water. There is evidence that electrospinning induces order in the fibers. Deacetylation of CA membranes is more efficient and complete in NaOH/ethanol than in aqueous NaOH, producing DS values between 0.15 and 2.33 without altering fiber surfaces, packing, or organization. The fully regenerated cellulose membranes are similarly hydrophilic as commodity cellulose fibrous matrices but absorb nearly 10 times as much water. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2119–2129, 2002     

Readily hydrolysable cellulose esters as intermediates for the regioselective derivatization of cellulose; II. Soluble,highly substituted cellulose trifluoroacetates

Cellulose trifluoroacetate (CTFA) with DS values of 1.5 and 2.1 and DP values ranging from 170 to 800 have been prepared free from impurities of the reaction mixture (weakly bound trifluoroacetic acid) and of the procedure of isolation (diethyl ether). The CTFAs are soluble in DMSO, DMF, pyridine, and THF and thermostable up to 250 °C. A convenient synthetic method for CTFAs with DS 1.5 involves the acylation of cellulose with mixtures of trifluoroacetic acid (TFA) and trifluoroacetic anhydride (TFAA, 33% v/v) at room temperature for 4 h and subsequent treatment of the crude polymer at 150 °C and 80 Pa for 40 min. The preparation of CTFAs with DS-values up to 2.1 requires the addition of chloroform and 16 h reaction time.¹³ C-NMR studies as well as HPLC analyses after methylation and chain degradation show a preferred trifluoroacetylation of the primary hydroxy groups of the cellulose. The extent of depolymerization during the trifluoroacetylation was investigated for various cellulose materials. The cleavage of the trifluoroacetyl groups is possible by treating the derivative with a protic medium like water. Total hydrolysis of CTFA dissolved in DMF with water (room temperature) takes 6 min. The first paper on this topic was concerned with cellulose formates (Schnabelrauchet al., 1992).     

Cold NaOH/urea aqueous dissolved cellulose for benzylation: Synthesis and characterization

Cold NaOH/urea aqueous dissolved cellulose was studied for the synthesis of benzyl cellulose by etherification with benzyl chloride. By varying the molar ratios of benzyl chloride to OH groups in cellulose (1.5–4.0) and reaction temperatures (65–70 °C), benzyl cellulose with a degree of substitutions (DS) in the range of 0.29–0.54 was successfully prepared under such mild conditions. The incorporation of benzyl groups into cellulose was evidenced by multiple spectroscopies, including FT IR, ¹H NMR, ¹³C NMR, CP/MAS ¹³C NMR and XRD. In addition, the thermal stability and surface morphology of the benzyl cellulose was also investigated with regard to the degree of substitution. The results indicated that the benzyl cellulose product with a low DS (0.51) in the present study reached the same solubility in many organic solvents as compared to those prepared in heterogeneous media. After benzylation, the sample decomposed at a lower temperature with a wider temperature range, which indicated that the thermal stability of benzyl cellulose was lower than that of the native cellulose. In addition, benzylation resulted in a pronounced reduction in crystallinity as well as a fundamental alteration of morphology of the native cellulose.     

Trimethylsilylcellulose synthesis revisited

A simplification of the overall method of producing trimethylsilylcellulose solutions in common organic volatile solvents is reported. Microcrystalline cellulose was derivatized aiming its solubilization in tetrahydrofuran and toluene to obtain ultra-thin films of cellulose by spin-coating. The main simplifications are: 1) elimination of a swelling step before dissolution in the binary solvent N,N-dimethylacetamide/LiCl by using microcrystalline cellulose; 2) derivatization of cellulose –OH groups with –Si(CH₃)₃ groups was successful with no additional catalyst, like trimethylsilyl chloride, the degree of substitution (DS) being 2.2 ± 0.1. The DS at the extreme surface of the films was computed from the X-ray Photoelectron Spectroscopy data: using solely C 1s fitted component areas and, alternatively, from both C 1s and Si 2p areas. The DS estimated with a technique specific of the surface is close to the bulky one obtained from gravimetric measurements reported in the literature for materials synthesized with catalyst. The topographic uniformity is shown by Atomic Force Microscopy.     

Determination of Surface Functional Groups in Lignocellulosic Materials by Chemical Derivatization and ESCA Analysis

Simple and convenient methods for determining surface chemical composition of lignocellulosic materials are described. The methods are based on vapor phase fluorine surface derivatization with either trifluoro acetic anhydride (TFAA), tri-fluoro ethanol (TFE) or pentafluorophenyl hydrazine (PFPH) and subsequent Electron Spectroscopy for Chemical Analysis (ESCA). Model cellulosic surfaces with well defined functionalities were used to optimize the derivatization reaction conditions. Detection and accessibility of surface hydroxyl functional groups were investigated in cotton and regenerated cellulose as models. Carboxymethyl cellulose (CMC) was used as a model surface for detection and quantification of carboxylic acid groups. Theoretical conversion curves for derivatization reactions were calculated and used to evaluate the extent of the reactions on the model surfaces. It was found that the conversion was higher for the regenerated cellulose and CMC than for cotton. The protocols developed using the model surfaces were applied to a case study on wood fibers with different degrees of complexity, namely dissolving and chemithermomechanical (CTMP) pulp. Untreated and oxygen-plasma modified pulps were compared with respect to the surface composition of functional groups. According to the derivatization reactions, functionalities containing oxygen were significantly increased on the plasma-treated samples. The effect of the treatment was found to be dependent on the type of pulp. Fluorine derivatization is shown to be an unambiguous method for clear assessment of the chemical functionalities of cellulosic surfaces.     

Multi-technique surface characterization of bio-based films from sisal cellulose and its esters: a FE-SEM, μ-XPS and ToF-SIMS approach

Bio-based films were prepared from LiCl/DMAc solutions containing sisal cellulose esters (acetates, butyrates and hexanoates) with different degrees of substitution (DS 0.7–1.8) and solutions prepared with the cellulose esters and 20 wt% sisal cellulose. A novel approach for characterizing the surface morphology utilized field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS) and contact angle analysis. XPS and ToF-SIMS were a powerful combination while investigating both the ester group distribution on the surface and effects of cellulose content on the film. The surface coverage by ester aliphatic chains was estimated using XPS measurements. Fibrous structures were observed in the FE-SEM images of the cellulose and bio-based films, most likely because the sisal cellulose chains aggregated during dissolution in LiCl/DMAc. Therefore, the cellulose aggregates remained after the formation of the films and removal of the solvent. The XPS results indicated that the cellulose loading on the longer chain cellulose esters films (DS 1.8) increased the surface coverage by ester aliphatic chains (8.2 % for butyrate and 45 % for hexanoate). However, for the shortest ester chains, the surface coverage decreased (acetate, 42 %). The ToF-SIMS analyses of cellulose acetate and cellulose hexanoate films (DS 1.8) revealed that the cellulose ester groups were evenly distributed across the surface of the films.     

Synthesis of carboxyl cellulose sulfates with regioselective sulfation and regiospecific oxidation using cellulose trifluoroacetate as intermediates

Synthesis of cellulose sulfates (CSs) and carboxyl cellulose sulfates (COCSs) with regioselectively or regiospecifically distributed functional groups within anhydroglucose units was reported. CS with regioselectively distributed sulfate groups at 2,3-O- or 2,6-O-position were homogeneously synthesized and cellulose trifluoroacetate (CTFA) was used as intermediates. The trifluoroacetyl groups were detected primarily at 6-O-position and their distributions could be altered by changing the amount of trifluoroacetyl anhydride (TFAA). Various sulfating agents were used for further homogeneous sulfation of CTFA. The total degree of sulfation (DS_S) and the distribution of sulfate groups within the repeating units were affected by the amount of TFAA, the type and amount of sulfating agents. Subsequent homogenous 4-acetamide-TEMPO or TEMPO-mediated oxidation of CS led to COCS with carboxyl groups regiospecifically distributed at C6 position, which may be interesting structural mimics for natural occurring heparin.     

Sisal cellulose and its acetates: generation of films and reinforcement in a one-pot process

This study was undertaken to evaluate both the properties of cellulose acetate films as a function of their degree of substitution (DS) and the possibility of generating reinforcements during film preparation. Sisal was selected for the entire study, among other reasons, because it is a rapidly growing source of cellulose. Cellulose acetates with various DS values were prepared in a homogeneous medium (dimethylacetamide/lithium chloride as the solvent system) and characterized. In DMAc/LiCl, cellulose and cellulose acetate films (mixed or not mixed with sisal cellulose) were successfully prepared and characterized. The films with high DS values exhibited lower hygroscopicity, a distinct morphology (scanning electron microscopy images), and lower tensile strength. In some cases, the films prepared from acetates/cellulose exhibited higher tensile strength and/or storage modulus than the acetate films. This result suggested a reinforcing action of the auto-organized cellulose chains that enabled the generation of both a film and reinforcement in a one-pot process.     

纳米纤维素晶须的表面醋酸酯化改性及复合材料的力学性能

采用硫酸水解法制备纳米纤维素晶须, 再以冰醋酸为分散介质, 浓硫酸为催化剂, 醋酸酐为酯化剂对纳米纤维素晶须进行不同程度醋酸酯化改性, 得到醋酸酯化的纳米纤维素. 采用红外光谱(FTIR)、 X射线光衍射(XRD)、 透射电子显微镜(TEM)和X射线光电子能谱(XPS)等手段对改性产物进行分析和表征. 结果表明, 改性纳米纤维素晶须中醋酸酯基的平均取代度过小或过大时均不适宜用作复合材料的增强相. 当改性纳米纤维素晶须中醋酸酯基的平均取代度为0.05时, 醋酸酯化反应只发生在纳米纤维素晶须的表面. 此时, 晶须能在丙酮中稳定悬浮, 表现出流动双折射现象, 并保持了改性前纳米纤维素晶须的棒状形态和高结晶度. 将这种改性后的纳米纤维素晶须作为增强相与醋酸纤维素通过溶液浇铸法制成纳米复合膜, 结果显示, 与空白醋酸纤维素膜相比, 添加改性纳米纤维素晶须后, 纳米复合膜的拉伸强度、 杨氏模量和断裂伸长率都得到了提高. 在玻璃化转变区间纳米复合膜储能模量的降低幅度小于空白膜.     

Nanofiber cellulose di- and tri-acetate using ferric chloride as a catalyst promoting highly efficient synthesis under microwave irradiation

An efficient method for the generation of cellulose di- and tri-acetate nano-structures is obtained through testing ferric chloride hydrate (FeCl₃·6H₂O) as a valuable Lewis acid catalyst with acetic anhydride under microwave irradiation. Our target was to evaluate the effects of the reaction conditions on the products' properties such as surface area and particle size distribution. It was found that changes in the degree of substitution (DS), the surface area, the degree of polymerization and the particle size distribution of the products correlated with reaction conditions. Cellulose tri-acetate nanofibers with DS of 2.94 with 98.03% yield was prepared using 200 mg of FeCl₃·6H₂O, 25 ml of Ac₂O and 4 minutes of microwave irradiation. Also, cellulose di-acetate nanofibers were prepared with DS values ranged between 2.37 and 2.72 with yield ranged between 78.92 and 90.58%. The percentage of acetyl groups (Ac%) as well as the BET specific surface area, total pore volume, mean pore diameter, mono layer volume and the mean particle size of the products were determined. The maximum specific surface area obtained for the acetylated cellulose was about ten times larger than that measured for the commercial cotton cellulose and about six times larger than that of the commercial cellulose acetate. The lowest mean particle size (34.90 nm) was about eleven times smaller than the mean particle size of the commercial cellulose acetate (394 nm). The present work has proved that FeCl₃·6H₂O was a highly active catalyst for the esterification of cellulose with unexpected yields and for the formation of nanofibers with low molecular weight.     

PHOTOINDUCED SURFACE RELIEF GRATINGS ON AZOCELLULOSE FILMS

Surface relief gratings (SRG) on films of azobenzene–modified cellulose (azocellulose) with ultrahigh molecular weight were holographically recorded in a single step. Surface modulation depth of the SRGs increased with the degree of substitution (DS) of the azobenzene chromophores in the cellulose as well as with the grating spacing. Large surface modulations over 500 nm on all samples were obtained with a grating spacing of 4 μm. A linear increase of photoinduced birefringence with the azobenzene chromophore concentration in these photodynamic polymers was also observed. The maximum birefringence of 0.11 was achieved for an azocellulose polymer with a DS of 0.99.

     

Structural analysis of polymer-brush-type cellulose β-ketoesters by molecular dynamics simulation

The reason for anomalous NMR patterns of cellulose β-ketoesters, which were prepared by reaction between cellulose and ketene dimers having long alkyl chains (AKDs) under homogeneous conditions using a cellulose solvent system, was studied by molecular dynamics simulation. Cellulose/AKD β-ketoester models with degree of substitution (DS) 2.0 and degree of polymerization (DP) 5, 10, 20 or 40, and cellulose/fatty acid ester models with DS 3.0 and DP 5, 10, 20 or 40 were assembled in the simulation. The calculated results were compared with those obtained by NMR and conformation analyses of the actually prepared cellulose derivatives. The molecular dynamics simulation data showed that the average velocities of anhydroglucose units in cellulose/AKD β-ketoesters were approximately one tenth of those in cellulose/fatty acid esters. Thus, cellulose chains in the cellulose/AKD β-ketoesters are extremely restricted in motion by the β-ketoester substituents. The solid-like behavior of cellulose chains in cellulose/AKD β-ketoesters in solution state is, therefore, explainable by strong restriction in motion of cellulose chains by long, branched and bulky substituents introduced into cellulose hydroxyls in high densities via β-ketoester bonds.     

Heterogeneous modification of various celluloses with fatty acids

Heterogeneous modification of various types of cellulose (microcrystalline cellulose, cellulose whiskers and regenerated cellulose) was performed with long-chain fatty acids by an esterification reaction. The differences in reactivity between the celluloses were studied as well as the influences of the chain length and double bond content of the fatty acids. The success of the modification reaction and the structure of modified samples were studied with diverse characterization methods. Surface modification changed the thermal stability of cellulose by decreasing the degradation temperature but also made the pyrolysis curve two-stepped due to the double bonds in the fatty acid chain. It was observed that the nature of the fatty acid affected the degree of substitution (DS). The longer the fatty acid chain was, the lower was the DS. Fatty acids with increased double bond content gave decreased DS. Regenerated cellulose seemed to have the highest surface reactivity due to the distinct morphological structure, which also led to a much lower quantity of fatty acids attached to the structure than for other modified cellulose particles. The mixture of tall oil fatty acids behaved in the same manner as the commercial fatty acids, proving to be an excellent “green” choice for this kind of application.     

Synthesis of cellulose benzoates under homogeneous conditions in an ionic liquid

The ionic liquid 1-allyl-3-methylimidazolium chloride (AmimCl) as a reaction medium was studied for the synthesis of cellulose benzoates by homogeneous acylation of dissolved cellulose with benzoyl chlorides in the absence of any catalysts. Cellulose benzoates with a degree of substitution (DS) in the range from about 1 to 3.0 were accessible under mild conditions. The DS of cellulose derivatives increased with the increase of the molar ratio of benzoyl chloride/anhydroglucose unit (AGU) in cellulose, reaction time, and reaction temperature. Benzoylation of cellulose with some 4-substituted benzoyl chlorides including 4-toluoyl chloride, 4-chlorobenzoyl chloride and 4-nitrobenzoyl chloride was also readily carried out under mild conditions. Furthermore, regioselectively substituted mixed cellulose esters were synthesized in this work. All products were characterized by means of FT-IR, ¹H-NMR, and ¹³C-NMR spectroscopy. In addition, at the end of benzoylation of cellulose, the ionic liquid AmimCl was easily recycled. When the recycled AmimCl was used as the reaction media, the cellulose benzoate with a similar DS was obtained under comparable reaction conditions.