Synthesis and characterization of dense and porous cellulose films

Whereas cellulose‐derived polymers are routinely used as membrane materials, the cellulose polymer itself is not directly used to synthesize dense/porous films for membrane applications. Recently, N‐methylmorpholine N‐oxide (NMMO) and dimethylacetamide (DMAc)/lithium chloride (LiCl) have been successfully employed for dissolving unmodified cellulose. This provides a strong rationale for reexamining the possibility of cellulose membrane fabrication using these solvents. By judiciously selecting solvents, casting conditions, and solvent exchange steps, we successfully synthesized dense/asymmetric‐porous cellulose films. The pore size and porosity of the porous films decreased systematically with increasing cellulose concentration. SEM analysis of the cross sections revealed an asymmetric skinned structure with monotonically increasing pore size away from the skin. The measured pore diameters were in the range 1.8–4.8 μm. Mechanical testing indicated that the dense films possessed tensile properties comparable to those of cellulose acetate (CA) films. Though nitrogen permeability values were comparable for cellulose and CA dense films, cellulose film permeability depended upon the type of drying protocol employed. Overall, these results demonstrate that processability need not be a constraint in the use of cellulose polymer for membrane fabrication. In selected applications, cellulose membranes could become a cost‐effective, environmentally friendly alternative to other more commonly employed membrane polymers. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Hemodialysis membrane prepared from cellulose/N‐methylmorpholine‐N‐oxide solution. III. The relationship between the drying condition of the membrane and its permeation behavior

The effects of drying condition on the performance (ultrafiltration rate, diffusive solute permeability, and sieving) of hemodialysis membranes prepared from cellulose/N‐methylmorpholine‐N‐oxide (NMMO) solution (NMMO membrane) and cellulose/cuprammonium solution (cuprammonium membrane; the referential membrane) were studied. The drying condition investigated was the glycerin concentration of the solution, which was used to substitute glycerin for the water in the membrane before the membrane was dried. A lower glycerin concentration in the solution brought about a lower reswelling degree (water content) in the dried membrane in pure water, which resulted in a drop in the performance of the as‐cast membrane. The NMMO membrane had a high water content and a high membrane performance compared with the cuprammonium membrane when both the membranes were treated under the same drying condition. The differences in the performance between both membrane series is discussed on the basis of the results of the observation of the membrane morphology by scanning electron microscopy, the observation of the crystallinity of the membranes by wide‐angle X‐ray diffraction, and the estimation of the pore structure of the membranes. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1671–1681, 2003     

Atomic force microscopy of cellulose membranes prepared from the N‐methylmorpholine‐N‐oxide/water solvent system

Cellulose membranes were obtained by solutions of cellulose being cast into a mixture of N‐methylmorpholine‐N‐oxide (NMMO) and water under different processing conditions. Atomic force microscopy (AFM) was used to investigate the surface structures of the membranes. The AFM method provided information on both the size and shape of the pores on the surface, as well as the roughness of the skin, through a computerized analysis of AFM micrographs. The results obtained showed that the surface morphologies were intrinsically associated with the permeation properties. For the cellulose membranes, increasing the NMMO concentration and the temperature of the coagulation bath led to higher fluxes and lower bovine serum albumin rejection. These were always correlated with higher values of the roughness parameters and larger pore sizes of the membrane surfaces. When the cellulose concentration of the casting solution was 11 wt %, the membrane showed a nodular structure with interconnected cavity channels between the agglomerated nodules. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3389–3395, 2002     

凝固浴组成对NMMO法纤维素膜形貌的影响

为实现对分离用纤维素膜形貌的控制,采用NMMO法,分别以水、甲醇、乙醇和由它们配制的双组分溶液为凝固浴,制备了纤维素膜,观察了干膜的表面和断面形貌,测定了湿膜的孔结构参数,在对比基础上分析了干膜中微孔的形成过程与机制,探讨了纤维素结晶尺度与湿膜平均孔径的相关性和影响因素.研究表明,不同凝固浴制备出的纤维素膜在干态形貌上差别明显.纤维素膜的湿态孔隙率基本不受凝固浴组成影响,而干态下的致密区域在水溶胀后呈现多孔结构,对湿膜平均孔径影响很大.通过改变凝固浴组成,可分别或协同地调控纤维素膜的干态或湿态结构.     

Hemodialysis membrane prepared from cellulose/N‐methylmorpholine‐N‐oxide solution. II. Comparative studies on the permeation characteristics of membranes prepared from N‐methylmorpholine‐N‐oxide and cuprammonium solutions

Two kinds of regenerated cellulose membranes for hemodialysis were prepared from casting solutions of N‐methylmorpholine‐N‐oxide (NMMO) and cuprammonium (denoted NMMO membranes and cuprammonium membranes, respectively). The concentration of cellulose in the casting solution investigated was 6–8 wt %. The permeation characteristics of both membrane series were compared in terms of the ultrafiltration rate (UFR) of pure water, the sieving coefficient (SC) of dextran, and the solute permeabilities of urea, creatinine, and vitamin B₁₂. The UFR and SC of the NMMO membranes were strongly affected by the cellulose concentration of the casting solution, and NMMO was a preferable solvent for the production of cellulose membranes with high performance; the cuprammonium solution gave low‐performance membranes. The pore structures of both types of membranes were estimated with the Hagen–Poiseuille law. The results showed that the NMMO membranes had larger pore radius and smaller pore numbers than the cuprammonium membranes. The differences in the membrane pore structures led to the differences in the performance between the two membrane series. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 333–339, 2003     

Effect of coagulation conditions on fine structure of regenerated cellulosic films made from cellulose/N-methylmorpholine-N-oxide/H2O systems

The important properties of cellulosic fibers in the conditioned state are mainly influenced by fine structure. In particular, the development of new methods of spinning regenerated cellulosic fibers made from a cellulose/N-methylmorpholine-N-oxide (NMMO)/H₂O system require a better understanding of their fine structures in order to explain their special physical properties. The regenerated cellulosic films were made from cellulose/NMMO/H₂O according to the degree of polymerization and solution concentration (wt %) of cellulose and the concentration (wt %) of NMMO in the coagulation bath. The quantification of crystal content was carried out by the resolution of the wide angle X-ray diffraction intensity distribution on the assumption that all diffracted intensities take the form of a symmetrical Gaussian distribution centering at its Bragg angle. The X-ray diffraction patterns resolved into individual integral intensities showed that the polymorphic structure mixed with part cellulose III and II was obtained for only coagulated cellulose films. The degree of crystallinity and apparent crystalline size of regenerated cellulosic films depended on the degree of polymerization, the solution concentration of cellulose, and the concentration of NMMO. The diameter of the microfibril decreased with an increase in the concentration of NMMO. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2681–2690, 1999     

Hemodialysis membrane prepared from cellulose/N‐methylmorpholine‐N‐oxide solution. I. Effect of membrane preparation conditions on its permeation characteristics

Flat hemodialysis membranes were prepared from cellulose/N‐methylmorpholine‐N‐oxide (NMMO) solutions (dope) with different cellulose concentrations (6–8 wt %) by using a phase‐inversion method. The coagulant used was NMMO aqueous solution, of which the NMMO concentration and its temperature were varied in the range of 0 to 50 wt % and 5 to 60°C, respectively. The effects of these preparation conditions on the permeation characteristics, the ultrafiltration rate (UFR) of pure water, and sieving coefficient (SC) of dextran, were investigated. The decrease in cellulose concentration of the dope and the increases in both temperature and NMMO concentration of the coagulant gave a membrane with high UFR. Concerning the SC, the increase of the cellulose concentration and the decreases in both temperature and NMMO concentration gave a good result. Consequently, the membrane having the preferable UFR and SC as a hemodialysis membrane was obtained when the 8 wt % cellulose dope was coagulated in water at 5°C. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2302–2307, 2002     

Investigation of membrane preparation condition effect on the PSD and porosity of the membranes using a novel image processing technique

A totally computerized image processing program package is developed to analyze the SEM images of membrane surface and cross‐section. Pore size distribution and porosity of the fabricated membranes are determined using the proposed image processing procedure. Furthermore, effect of coagulation bath temperature on the morphology and mechanical properties (such as tensile strength, strain break, tensile energy absorbent, and tensile stiffness) of Polysulfone (PSf) membranes are investigated. The results reveal that the mechanical properties are higher when N‐methyl‐2‐pyrrolidone (NMP) is used as solvent. Also, an increase in the coagulation bath temperature caused a monotonous increase in the mean pore size value of Dimethylformamide (DMF)‐based membranes. However, mean pore size curve has a maximum when NMP is used as solvent. Also, porosity of the fabricated membranes increased when coagulation bath temperature increased. For the NMP‐base membranes, pore's diameter was in the range of 0–5 μm. However, DMF‐based membranes have pore size value of smaller than 1 μm when the precipitation medium is kept at 8°C. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39899.     

The Online Measurement of Lyocell Fibers and Investigation of Elongational Viscosity of Cellulose N‐methylmorpholine‐N‐oxide Monohydrate Solutions

In this paper, the development of diameter and surface temperature of Lyocell fibers was measured online. The diameter and tensile force on the spin line in the coagulation bath were traced. The velocity, velocity gradient and the tensile stress profiles development of the fibers in the air gap were studied. The apparent elongational viscosity of cellulose N‐methylmorpholine‐N‐oxide monohydrate (NMMO‐MH) solutions was studied by steady‐state melt spinning theory. The decrease of the fiber diameter was mainly taking place near the spinneret, and the decrease of the diameter became more dramatic with increasing taking‐up speed. The surface temperature of the fibers was also dropping faster with increasing taking‐up speed for the heat transfer coefficient increased. The diameter of the Lyocell fibers almost did not change before and after it entered the coagulation bath. The tensile force on the spin line increases with increasing taking‐up speed and coagulation bath length. The velocity and the tensile stress increase slowly near the spinneret, and then accelerate. The apparent elongational viscosity of cellulose NMMO‐MH solutions decreases with increasing temperature at the same elongation rate and decreases with increasing elongation rate at the same temperature. The fiber of the Lyocell process was not really solidified in the air gap and a gel or rubbery state was formed.     

纤维素/NMMO溶液及其薄膜的制备与性能研究

以水质量分数为13.3%的N-甲基吗啉-N-氧化物(NMMO)为溶剂溶解棉浆粕,制备质量分数为5%～11%的纤维素/NMMO溶液。将所得溶液制备纤维素薄膜,考察了纤维素/NMMO溶液的稳定性,研究了凝固浴温度和组成对纤维素薄膜的成膜性、断面形态及力学性能的影响。结果表明:纤维素/NMMO溶液随着浓度增大,其粘度先增大后减小,再急剧上升;纤维素/NMMO溶液在玻璃介质中稳定性较好,微量Cu~(2+),Fe~(3+)等杂质存在时,其稳定性显著下降;纤维素薄膜随凝固浴温度升高,其透明性、拉伸强度和断裂伸长率均下降;相对于水,含有乙醇和NMMO的凝固浴能减缓双扩散的速度,使纤维素薄膜的拉伸强度略有提高,断裂伸长率出现不同程度下降。     

Physical agglomeration behavior in preparation of cellulose‐N‐methyl morpholine N‐oxide hydrate solutions by simple mixing

The different melting temperatures of N‐methyl morpholine N‐oxide (NMMO) hydrates in the cellulose–NMMO hydrate solution may be explained by the rather different crystal structures of NMMO hydrates, which are determined by the amount of the hydrates. The preparative process of cellulose–NMMO hydrate solution may result in cellulose structural change from cellulose I to cellulose II, depending on the amount of the hydrate. Mixtures of cellulose and NMMO hydrate in a blender was changed from the granules to slurry with increasing mixing time at 60–70°C, which is below the melting point of the NMMO hydrate. In the case of 15 wt % cellulose–NMMO hydrate granules, which were made by mixing for 20 min, the melting points of various NMMO hydrates were obtained as 77.8°C (n = 0.83), 70.2°C (n = 0.97), and 69.7°C (n = 1.23), respectively, depending on the hydrate number. However, the melting points of cellulose–NMMO hydrate slurry and solution were shifted lower than those of cellulose granules, while the mixing time of slurry and solution are 25 and 35 min, respectively. These melting behaviors indicate instantaneous liquefaction of the NMMO hydrate and the diffusion of the NMMO hydrate into cellulose during mixing in a blender. When cellulose was completely dissolved in NMMO hydrate, the crystal structure of cellulose showed only cellulose II structure. In the cellulose–NMMO products of granules or slurry obtained by high‐speed mixing, which is a new preparation method, they still retained the original cellulose I structure. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1687–1697, 2004     

Preparation and characterization of asymmetric polysulfone membrane for separation of oxygen and nitrogen gases

Defect‐free skinned asymmetric gas separation membranes were prepared by a dual bath coagulation method using a wet phase inversion technique. The membranes were cast from polysulfone solution in different solvents such as: dimethyl‐formamid, 1‐methyl‐2‐pyrrolidone, N‐N‐dimethyl‐acetamide (DMAC), and tetrahydrofuran. The mixtures of water/iso‐propanol (IPA), water/propanol, water/ethanol (EtOH), and water/methanol (MeOH) with volume ratio of 80/20 were used as the first coagulation bath. This led to the formation of a dense skin top layer. Distillated water was used as the second coagulation bath. The influences of several experimental variables, such as thickness of the membrane, polymer concentration, type of solvent and nonsolvent, immersion time in IPA 20%, and second coagulation bath temperature on skin layer and sublayer were elucidated. For preparing membrane with higher permeance, the influence of internal nonsolvents and addition of polyvinylpyrrolidone (PVP) as additive were investigated. The membrane performance was tested in terms of gas permeance and selectivity for O₂/N₂ separation. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Influence of the coagulation‐bath temperature on the phase‐separation process of poly(vinylidene fluoride)‐graft‐poly(N‐isopropylacrylamide) solutions and membrane structures

A poly(vinylidene fluoride)‐graft‐poly(N‐isopropylacrylamide) (PVDF‐g‐PNIPAAm) copolymer was synthesized, and flat‐sheet membranes were prepared via the phase‐inversion method with N,N‐dimethylformamide (DMF) as the solvent and water as the coagulation bath. The effects of the coagulation‐bath temperature on poly(vinylidene fluoride) (PVDF)/DMF/water and PVDF‐g‐PNIPAAm/DMF/water ternary systems were studied with phase diagrams. The results showed that the phase‐separation process could be due to the hydrophilicity/hydrophobicity of poly(N‐isopropylacrylamide) at low temperatures, and the phase‐separation process was attributed to crystallization at high temperatures. The structures and properties of the membranes prepared at different coagulation‐bath temperatures were researched with scanning electron microscopy, porosity measurements, and flux measurements of pure water. The PVDF‐g‐PNIPAAm membranes, prepared at different temperatures, formed fingerlike pores and showed higher water flux and porosity than PVDF membranes. In particular, a membrane prepared at 30°C had the largest fingerlike pores and greatest porosity. The water flux of a membrane prepared in a 25°C coagulation bath showed a sharp increase with the temperature increasing to about 30°C. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effect of hypochlorite concentration on properties of posttreated outer‐skin ultrafiltration membranes spun from cellulose acetate/poly(vinyl pyrrolidone) blends

An outer‐skin hollow‐fiber ultrafiltration (UF) membrane was spun from a new dope solution containing cellulose acetate (CA)/poly(vinyl pyrrolidone) (PVP 360K)/N‐methyl‐2‐pyrrolidone (NMP)/water using a wet‐spinning technique. The as‐spun fibers were posttreated with a hypochlorite solution over a range of concentrations for a fixed period of 24 h. The experimental results showed that the pure water flux of the treated membrane increased with increasing hypochlorite concentration. The treated membrane experienced an increased fouling tendency with increasing hypochlorite concentration because the hydrophilicity of the treated membrane decreased as a result of the removal of PVP contents in the membrane matrix after hypochlorite treatment. SEM images revealed that the membrane had an outer dense skin, a porous inner surface, and a spongelike structure, which confirmed that addition of PVP favored the suppression of macrovoids in the membrane. The membrane pore size could be significantly increased when the hypochlorite concentration reached 200 mg/L. It was concluded that hypochlorite treatment provided an additional option to easily alter the pore size of UF membranes. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 227–231, 2005     

Methods for reducing the tendency of lyocell fibers to fibrillate

A study has been made of the influence of various process parameters on the fibrillation characteristics of Lyocell fibers, which are spun from a solution of cellulose in N-methylmorpholine-N-oxide (NMMO). The parameters were air gap length, temperature, and humidity; line speed; draw ratio; polymer solution cellulose and water contents; and coagulation bath concentration and temperature. Fibrillation was induced in the fibers by an ultrasonic treatment and compared by defining a fibrillation index from optical micrographs. By selecting combinations of the parameters described above, fibrillation can either be increased or decreased without significantly affecting the tensile properties of the fibers. A method for spinning nonfibrillable filaments on a laboratory scale is presented. © 1996 John Wiley & Sons, Inc.     

Influence of coagulation bath on morphology of cellulose membranes prepared by NMMO method

To control the morphology of cellulose membranes used for separation, they were prepared by the NMMO method using water, methanol, ethanol and their binary solution as coagulation baths. Morphologies of the surface and cross section of dry membranes were observed. The pore structure parameters of wet membranes were determined. By comparison, the process and mechanism of pore formation in dry membranes were suggested, and the relativity of cellulose crystal size to average pore diameter in wet membranes and their influences were discussed. The results show that the morphology of dry membranes is clearly varied with coagulation baths, while the porosity of wet membranes is almost constant. Porous structures can appear in the compact region of dry membranes due to swelling from water. These pores have a virtual effect on the average pore diameter of wet membranes. By changing the composition of coagulation baths, the microstructure of cellulose membranes in a dry or wet environment can be adjusted separately.     

Effect of mixed solvents on phase inversion of polymeric membranes

The thermodynamics of phase inversion of polyvinylidene fluoride membrane with mixed solvents (N,N‐dimethyl formamide (DMF) and acetone) were modeled using Flory–Huggins theory. The kinetics of phase inversion were studied by measuring solvent concentration in the precipitation bath. A model was proposed to predict the time‐dependent solvent concentration profile in the precipitation bath. Depending on solvent volatility, the duration of the kinetics‐dominated regime and the evaporation‐dominated regime varies. A comparative analysis of thermodynamic and kinetic factors was used to predict membrane morphology and it was observed that the system under consideration was thermodynamics dominated. The membrane porosity exhibited decreasing porosity up to the Ac60 membrane (acetone to DMF ratio 60) and thereafter the membrane sublayer showed small pores. Addition of acetone resulted in increased crystallinity and surface hydrophilicity. The mean flow pore diameter measured using a liquid–liquid porometer decreased from 105 nm for an Ac0 membrane (acetone to DMF ratio 0) to 17 nm for an Ac60 membrane. Correspondingly, the molecular weight cut‐off of the membranes decreased from 135 kDa (for the Ac0 membrane) to 104 kDa (for the Ac60 membrane). The model proposed in this work can be used as a tool to predict the properties of intermediate compositions and prepare tailor‐made membranes with desired properties. © 2020 Society of Chemical Industry     

Influence of coagulation bath on morphology of cellulose membranes prepared by NMMO method

To control the morphology of cellulose membranes used for separation, they were prepared by the NMMO method using water, methanol, ethanol and their binary solution as coagulation baths. Morphologies of the surface and cross section of dry membranes were observed. The pore structure parameters of wet membranes were determined. By comparison, the process and mechanism of pore formation in dry membranes were suggested, and the relativity of cellulose crystal size to average pore diameter in wet membranes and their influences were discussed. The results show that the morphology of dry membranes is clearly varied with coagulation baths, while the porosity of wet membranes is almost constant. Porous structures can appear in the compact region of dry membranes due to swelling from water. These pores have a virtual effect on the average pore diameter of wet membranes. By changing the composition of coagulation baths, the microstructure of cellulose membranes in a dry or wet environment can be adjusted separately. __________ Translated from Journal of Chemical Engineering of Chinese Universities, 2007, 21(3): 398–403 [译自: 高校化学工程学报]     

NMMO溶剂法纤维素海绵的制备及性能研究——成孔剂用量的影响

以N-甲基吗啉-N-氧化物(NMMO)为溶剂、以棉浆粕为原料、以无水硫酸钠为成孔剂制备了纤维素海绵,探讨了成孔剂用量对纤维素海绵形态结构、孔隙率、吸水性、保湿性和拉伸强度的影响。结果表明,随着成孔剂用量的提高,所制得的纤维素海绵平均孔径变大,孔壁变薄,通孔增多,但成孔剂用量过多时,海绵孔壁会产生轻微断裂。此外,随着成孔剂用量的提高,海绵的孔隙率及吸水保湿性提高,拉伸强度则有所下降,综合考虑海绵的性能及成型过程的难易程度等因素,当成孔剂用量为纤维素溶液的3倍为宜,所得纤维素海绵的孔隙结构均匀饱满、综合性能较高。     

Study on the control of pore sizes of membranes using chemical methods: Part III. The performance of carbonates and bicarbonates in the membrane-making process by the chemical reaction

In this paper, polyvinylidene fluoride (PVDF)/polymethyl acrilate (PMMA)/cellulose acetate (CA) blend UF membranes were prepared by chemical reaction introduced phase-inversion method. The results of the experiment show that: (1) The membrane pore size distribution is more uniform due to the presence of carbonates or bicarbonates in the coagulation bath; (2) No more than the stoichiometric ratio amount of carbonates or bicarbonates in the coagulation bath can effectively improve the membrane pore size distribution and make the pore size of membrane more uniform; (3) The membrane prepared by carbonates solution as a working solution in coagulation bath possess superior performance than that by bicarbonates.