Preparation and characteristics of cross-linked cellulose acetate ultrafiltration membranes with high chemical resistance and mechanical strength

    Preparation and Characteristics of Cross-linked Cellulose Acetate Ultrafiltration Membranes with High Chemical Resistance and Mechanical Strength¡!–¡QUERY id=”Q2” name=”nindab”¿¡ce:para¿Provided supplementary data was not capture as supplementary since they were the same on the normal figures provided. Please check if appropriate.¡/ce:para¿¡/QUERY¿–¿ Ju Sung Lee, Sung Ah. Heo, Hyung Jun Jo, Byoung Ryul Min PII: DOI: Reference:

S1381-5148(15)30086-9 doi: 10.1016/j.reactfunctpolym.2015.12.014 REACT 3605

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3 December 2015 29 December 2015

Please cite this article as: Ju Sung Lee, Sung Ah. Heo, Hyung Jun Jo, Byoung Ryul Min, Preparation and Characteristics of Cross-linked Cellulose Acetate Ultrafiltration Membranes with High Chemical Resistance and Mechanical Strength¡!–¡QUERY id=”Q2” name=”nindab”¿¡ce:para¿Provided supplementary data was not capture as supplementary since they were the same on the normal figures provided. Please check if appropriate.¡/ce:para¿¡/QUERY¿–¿, (2015), doi: 10.1016/j.reactfunctpolym.2015.12.014

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Preparation and Characteristics of Cross-linked Cellulose Acetate

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Ultrafiltration Membranes with High Chemical Resistance and

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Mechanical Strength

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Ju Sung Lee a, Sung Ah Heo a, Hyung Jun Jo a, Byoung Ryul Min a,

Department of Chemical and Biomolecular Engineering, Yonsei University,

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120-749, 50 Yonsei-ro, Seodaemun-gu, Seoul, South Korea

Abstract

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Cellulose acetate (CA) membranes, which are highly hydrophilic, have the advantage of being highly

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resistant to fouling phenomena and economically affordable. However, as material they pose the problem of low chemical resistance and mechanical strength. In this study, cross-linked cellulose acetate (CCA)

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was prepared using trimesoyl chloride (TMC) in order to overcome this material weakness. Cross-linking of CA was confirmed both directly and indirectly using FT-IR and viscosity of dope solution. In particular, viscosity of dope solution gradually increased as TMC concentration was increased, but rose

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sharply from 2.1wt.%. CCA membranes thus prepared showed four times the tensile strength of existing CA membranes, and improved chemical resistance against polar solvents (ethanol, acetone, DMAc). The morphology of membranes was confirmed using SEM, and CCA membranes displayed more even pore size and dense cross-section than those of existing CA membranes. It was also shown that CCA membranes can also significantly reduce compaction phenomena, which may occur in the use of membranes, due to their improved mechanical property and dense structure. CCA membranes showed definite possibility for use in ultrafiltration (UF) by removing over 99.85% of kaolin.

Keywords: cross-liking, cellulose acetate, compaction, mechanical strength, viscosity

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Corresponding author. Tel.: +82 2 2123 2757; fax: +82 2 312 6401 E-mail address: [email protected] (B.R. Min) 1

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1. Introduction

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The world’s water shortage problem is becoming worse with population increase. Various methods such as disinfection, distillation, and media filtration are being researched in order to overcome this

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problem. Membranes have received special attention as they can separate and remove suspended solids, bacteria, virus, and even ions according to pore size using pressure as driving force [1-4]. Membranes have seen drastic improvement and development since Reid and Breton first discovered

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the possibility of using cellulose acetate (CA) membranes for desalting water in 1959 [5-7]. CA is a hydrophilic material with high resistance to fouling and can conveniently be prepared into membranes.

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Membranes prepared using CA have superior characteristics such as high potential flux and good desalting. Due to this advantage, CA membranes have been widely used in technological research and

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commercial membranes, and also have diverse applications in microfiltration (MF), Ultrafiltration (UF),

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nanofiltration (NF), reverse osmosis (RO), and gas separation [8-12]. While various other polymers such as PAN, PI, PU, PES, PSf, PP, and PVDF are now being used as membrane material, CA remains an

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attractive membrane material [13-16]. However, in order to be competitive against these other materials, CA needs to overcome its weaknesses of poor chemical resistance, mechanical strength, thermal resistance, biodegradability, and greater compaction phenomena [17]. Accordingly, there have been

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numerous studies to overcome these weaknesses of CA membranes through modification [18-20]. It is known that polymer modification through cross-linking can improve chemical resistance and mechanical strength of polymers using cross-linking agent to form network among polymer chains [21]. Such cross-linking modification has already been widely applied in NF, RO, pervaporation, and gas separation membranes, but it is difficult to find studies which improve CA characteristics using crosslinking modification [22-25]. This study attempts to improve the physical and chemical characteristics of CA membranes by introducing cross-linking reaction through condensation polymerization of CA and TMC, which is a linking agent. As a result, not only were tensile strength and chemical resistance improved according to degree of cross-linking of the cross-linked cellulose acetate (CCA), but the morphology of membranes denser and compaction phenomena limited. It was confirmed that these were the result of the correlation

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2. Experimental

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2.1 Materials

Cellulose acetate (CA) (Sigma Aldrich, USA) with 30,000g/mol average content and N,Ndimethylacetamide (DMAc) (Sigma Aldrich, USA) solvent were used to prepare CA UF membranes.

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Trimesoyl chloride (TMC) purchased from TCI (Japan) was used as CA cross-linking reagent. PET nonwoven used as support membranes for UF membranes was purchased from Philos (Korea). N-butanol, n-

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hexane (Sigma Aldrich, USA) was used in the solvent exchange method introduced to minimize damage to UF membranes during drying.

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Ethanol, acetone, and DMAc used for chemical resistance testing of UF membranes were purchased

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from Sigma Aldrich (USA). Kaolin used for performance testing of membranes was also purchased from

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Sigma Aldrich (USA). Ultrapure water was used in the preparation of all aqueous solutions.

2.2 Preparation of cross-linked cellulose acetate (CCA)

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Table 1 Composition of dope solution for CA cross-linking and membrane preparation. Primary mixing (1)

Secondary mixing (2)

Sample name

CA (wt.%)

DMAc (wt.%)

TMC (wt.%)

CA

10

90

0

CCA-1

10

89

1

CCA-2

10

88

2

CCA-2.1

10

87.9

2.1

CCA-2.2

10

87.8

2.2

CCA-2.3

10

87.7

2.3

CA dope solution was prepared as a preliminary step to CCA preparation. CA dope solution was first made by mixing CA and DMAc according to the composition in Table 1(1). TMC was later added to the 3

ACCEPTED MANUSCRIPT CA dope solution according to the compositions in Table 1(2). Finally, CCA dope solution was prepared by completely reacting CA and TMC for 2 days. All experiments were conducted at 25℃ and the dope

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solution stirred at 80rpm while dissolving. Fig. 1 is a graphic representation of the aforementioned CCA

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{

25℃, 80rpm, 48 hr

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STEP 1

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STEP 2

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dope solution preparation process.

Cross-linked CA dope solution

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DMAc + Cellulose Acetate

Fig. 1. CCA preparation process using condensation polymerization of TMC and CA.

2.3 Fabrication of cross-linked cellulose acetate (CCA) membranes

Non-solvent induced phase separation (NIPS) method was used in the preparation of CCA membranes. CCA dope solution was coated thinly onto the PET non-woven using 200 ㎛ casting knife. Then the coated non-woven was immediately immersed in coagulation bath with non-solvent (DI water). The CCA membranes were then left to sit for 30 minutes, and rinsed for about 1 day in DI water bath in order to remove solvents. Generally polymer membranes contract during drying due to water evaporation. Solvent exchange method was used in order to prevent such membrane deformation [26, 27]. Membranes rinsed

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ACCEPTED MANUSCRIPT amply with DI water were exchanges first with butanol, then subsequently with hexane. Membranes were finally dried after exchange with hexane, which has low surface tension.

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For accurate experimentation, membranes used for mechanical property and chemical resistance

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experiments were prepared in thin film type form using NIPS method without non-woven. CA

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membranes in the comparison group were also prepared in the same manner.

2.4 Characterization

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2.4.1 ATR-FTIR

Attenuated total reflectance-Fourier transform infra-red spectrophotometer (ATR-FTIR, SPECTRUM

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100, Perkin Elmer, USA) was used to confirm cross-linking of CCA. Membranes were cumulatively

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measured with an average of 32 scans per sample in the 4000cm-1 ~ 600cm-1 wavenumber range.

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2.4.2 Viscosity measurement

Viscometer (LVDV-II+, Brookfield, USA) was used to measure changes in CCA dope solution

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viscosity according to TMC addition in order to confirm cross-linking in CCA dope solution. Measurements were conducted by pouring 5ml of dope solution in small sample adapter using SC4-31

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spindle at 25℃.

2.4.3 Mechanical property Tensile strength and elongation at break of the prepared membranes was measured with UTM (Multi test 1-1, Mecmesin, United Kingdom). Rectangular samples (30mm

10mm) were prepared from each

membrane and measured with a gauge length of 20mm and stretching speed of 50mm/min. Film-type membranes without non-woven were used in order to measure mechanical property precisely. All experiments were carried out at room temperature.

2.4.4 Chemical resistance experiments Chemical resistance of materials was confirmed by weight loss (%) resulting from contact between membranes and solvent. Pre-weighed membranes were immersed in ethanol, acetone, and DMAc for 12 5

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Where

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Chemical resistance was calculated using the following equation:

is the weight (g) of membranes before contact with polar solvent, and

after contact.

their weight

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Film-type membranes without non-woven were used for precise chemical resistance comparison between materials.

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2.4.5 FE-SEM

Surface and cross-section morphology of membranes were observed using field emission scanning

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electron microscopy (FE-SEM, JEOL-6701F, JEOL, Japan) at an accelerating voltage of 5kV. In

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particular, samples were fractured after being immersed in liquid nitrogen for 1 min for cross-section observation. Prepared membranes were coated in platinum (Pt) for 150 seconds before observation due to

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their non-conductivity. The average pore sizes of membranes were calculated by averaging the measurements of all pore sized in SEM images.

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2.4.6 AFM

Atomic force microscopy (AFM, Park Systems, XE-BIO, Korea) was used to compare the surface roughness of prepared CA and CCA membranes. Samples (1cm × 1cm) were prepared and placed on slide glasses. The membrane scanned over a specific area (5.0μm ×5.0μm) by cantilever tapping with arithmetic average roughness (

and root mean square (RMS) roughness

.

2.4.7 Compaction tests of membranes Amicon stirred cell (Model 8050, Millipore, USA) was used in order to confirm compaction of membranes due to water permeation. Pure water was used as feed in this experiment and the amicon stirred cell operated with dead end system at 25℃, 200rpm condition. Compaction of membranes due to water permeation was confirmed with changes in pure water flux over time. Pure water flux was 6

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where

(L) is the volume of permeated pure water, and

(m2)

(hr) the effective surface area and permeation time of membranes, respectively.

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and

(Lm-2hr-1) is pure water flux.

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calculated over measured time intervals using the following equation:

The following equation was additionally used for an accurate comparison of compaction between

where

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membranes due to water flux.

(Lm-2hr-1) is initial momentary pure water flux and

(Lm-2hr-1) momentary pure water

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flux after 120 minutes of continuous pure water permeation. In this study, the effective membrane area

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was 0.00134m2 and driving force of 1bar was maintained with purified nitrogen gas.

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2.4.8 Kaolin rejection of membranes

Kaolin rejection test was conducted in order to confirm the performance of the prepared membranes.

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Measurements were made for 60 minutes using a dead end system with amicon stirred cell at 25℃, 2bar, 200rpm conditions. Suspension of 65±5 NTU with kaolin particles (0.1μm ~ 4μm) was used as feed. The turbidity of solution was measured using a turbidimeter (model 2100P, Hach, USA). The kaolin rejection ratio of membranes was calculated using the following equation.

where

(NTU) is the turbidity of feed solution and

3. Results and discussion

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(NTU) the turbidity of permeate solution.

ACCEPTED MANUSCRIPT 3.1 Confirmation of cross-linking of CA

C=C Peak of aromatic ring

(a)

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140

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160

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1568 cm-1

100 80

(b)

800 cm-1 757 cm-1

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Transmittance (%)

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C-H Peak of aromatic ring

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40 (a) : CCA (TMC 2.3wt.%) (b) : CA

20 0

800

600

-1

Wavenumber (cm )

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4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000

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Fig. 2. (a) ATR-FTIR spectra of CA, (b) CCA (TMC 2.3wt.%).

CCA is formed by condensation polymerization between CA and TMC. Since the Cl in TMC is separated after than phase inversion phase of the condensation polymerization, cross-linking of CCA can

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be confirmed through the C=C, C-H bonds in the aromatic rings of TMC. Fig. 2 is the ATR-FTIR spectra used to confirm the cross-linking of CA reacted with TMC. The sharp peak that can be seen in Fig. 2(b) around 1750cm-1 is a typical acetate peak (C=O) of CA. The smoother peak that can be seen around 3400cm-1 is a O-H peak of CA. Such peaks can also be confirmed for CCA (Fig. 2(a)). However, for the CCA (Fig. 2(a)), additional peaks which cannot be seen for CA (Fig. 2(b)) can also be found around 1560cm-1, 800cm-1, and 755cm-1. Here, the peak around 1560cm-1 is a C=C peak of the aromatic ring and the peaks around 800cm-1 and 755cm-1 are C-H peaks of the aromatic ring. By confirming that the characteristics of CA and TMC can be clearly seen in IR as in Fig. 2(a), it was confirmed that the CA was well cross-linked.

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4000

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3000

2000

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1000 253.7

496.1 502.1

0.5

1 1.5 2 Concentration of TMC (wt.%)

2.5

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0

1,800.0

230.5

0

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Dynamic viscosity [mPa∙s)

3,550.2

Fig. 3. Changes in viscosity according to concentration of TMC (wt.%) added to CA dope solution.

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Another evidence of cross-linking in addition to IR peaks for CCA is the viscosity of dope solution. Fig. 3 is a graph of the change in viscosity of the CA dope solution according to TMC addition. The viscosity of the CA dope solution shows a decline at TMC 1wt.%, but then rises sharply from 2.1wt.%.

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This means that the degree of cross-linking of the CA is dependent on the amount of TMC added and that it approaches gel point after 2.1wt.%. (For TMC addition over 2.3wt.%, the CA dope solution displayed gelation. Meanwhile, the dope solution viscosity was lowest at TMC 1wt.%, which is presumably because of intramolecular cross-linking between TMC and CA at low TMC concentrations which lowers viscosity [28]. These trends in viscosity of CA with TMC addition concentrations are consistent with results of studies with other cross-linking agents [29]. The trends in viscosity of CA dope solution and IR peak characteristics conclusively show that TMC and CA reacted well to cross-link.

3.2Physical and chemical characteristics of CCA

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4000

8

IP

6

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3000

2000

4

Tensile strength (kPa) Elongation at break (%)

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1000

0 0

0.5

1

2

Elongation at break (%)

10

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5000

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Tensile strength (kPa)

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0 1.5

2

2.5

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Concentration of TMC (wt.%)

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Fig. 4. Tensile strength (kPa) and elongation (%) of prepared membranes according to concentration of TMC (wt.%) added to CA dope solution.

Fig. 4 is a graph showing the changes in the tensile strength and elongation of CCA according to changes in TMC concentration. As with the viscosity graph in Fig. 3, there were sharp rises from TMC

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2.1wt.%. In particular, the tensile strength of CCA membranes at TMC 2.3wt.% were about 4 times that of CA membranes. As mentioned, the increase in viscosity is attributable to the degree of crosslinking. Accordingly, the similar trends in tensile strength and viscosity according to TMC concentration shows that tensile strength is also dependent on the degree of cross-linking. Moreover, the decrease in elongation with increase in TMC concentration explains even more clearly that the mechanical property of polymers are dependent on the degree of cross-linking. These results can also be confirmed by earlier studies [30, 31].

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100

Ethanol Acetone DMAc

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60 40

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Weight loss(%)

80

0 1

2 2.1 2.2 Concentration of TMC (%)

2.3

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0

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20

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Fig. 5. Chemical resistance of prepared membranes according to concentration of TMC (wt.%) added to CA dope solution.

Fig. 5 shows the effect degree of CA cross-linking has on chemical resistance. The graph shows no weight loss (%) in ethanol for CA and CCA. CA shows 100% weight loss in both acetone and DMAc. In

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contrast, CCA weight loss in acetone and DMAc shows a decline as TMC concentrations are increased. In the case of CCA-2.3 (TMC 2.3wt.%), weight losses (%) in acetone and DMAc are considerably reduced at 6% and 30.8%, respectively. These results are consistent with the trends for dope solution viscosity and tensile strength, and mean that the high degree of cross-linking by CCA is a critical factor in the chemical resistance of membranes. It is hoped for that such chemical resistance characteristics of CCA membranes can be used for applications not only in water treatment, but also for organic solvent nanofiltration (OSN) fields [32-34].

3.3 Morphology of CCA UF membranes

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Fig. 6. Surface roughness of (a) CA, (b) CCA-1, (c) CCA-2, (d) CCA-2.1, (e) CCA2.2, (f) CCA-2.3 membranes observed through AFM.

Fig. 6 shows the surface roughness images and arithmetic average roughness ( square (RMS) roughness (

, nm), root mean

, nm) values for the CA and CCA membranes as observed by AFM. Here,

Fig. 6(a) is the surface roughness for CA membrane, while Fig. 6(b), Fig. 6(c), Fig. 6(d), Fig. 6(e), and Fig. 6(f) are those of CCA-1, CCA-2, CCA-2.1, CCA-2.2, CCA-2.3 membranes, respectively. Fig. 6(a)

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ACCEPTED MANUSCRIPT shows

,

values for CA membranes of 7.81nm, 9.98nm, which are relatively lower than those for

CCA membranes (12.86nm ~ 22.52nm, 15.36nm ~ 26.17nm). It was confirmed through repeated testing

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that this difference in surface roughness between CA and CCA are attributable to the CA cross-linking

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with TMC. However, comparisons among CCA membranes only showed no clear trends in surface

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roughness according to degree of cross-linking. This implies that trends in CCA surface roughness are

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dependent on factors other than degree of cross-linking, which will require further study.

Fig. 7. FE-SEM images of CA ((a) surface, (b) cross-section) and CCA-2.3 ((c) surface, (d) cross-section).

Fig. 7 shows the surface (a), and cross-section (b) of CA membranes (which are not cross-linked) and surface (c) and cross-section (d) of CCA-2.3 membranes with a high degree of cross-linking. First, in comparing the CA surface (Fig. 7(a)) with CCA-2.3 surface (Fig. 7(c)), the average pore size of CCA-2.3

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ACCEPTED MANUSCRIPT was 26.6nm, smaller than the 49.7nm for CA. The pore size distribution was much more even for CCA2.3 compared to CA. CA and CCA showed significant difference in cross-section in addition to surface.

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The CA cross-section (Fig. 7(b)) showed generally high porosity and finger-like structure, while that for

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CCA-2.3 (Fig. 7(d)) showed both finger-like structure and sponge-like structure, and was much denser overall compared to CA. Such differences in structure between CA and CCA-2.3 can be explained by

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demixing rate. In the phase inversion process using NIPS, demixing rate is an important factor in the structure formation of membranes. In general, instantaneous demixing results in porous membranes while delayed demixing results in non-porous membranes [35-37]. Such demixing rate is affected by the

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mobility of the polymer chain in dope solution, as demixing rate decreases with lower polymer chain mobility.

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Studies conducted by Z. Zhang et al. support this [38]. Z. Zhang et al. studied characterization of membranes according to polymer concentrations in dope solution, and reported that as polymer

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concentrations increases, dope solution viscosity and demixing time in phase inversion process increases.

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Membranes prepared at high polymer concentrations showed relatively smaller pore size and denser structure than those prepared at relatively lower polymer concentrations, and Z. Zhang et al. explained

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that this is because dope solution viscosity obstructs the exchange rate between solvent and non-solvent. Similarly, cross-linking of CA also reduces polymer chain mobility in dope solution while increasing dope solution viscosity. In conclusion, rises in viscosity obstruct exchange between solvent and non-

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solvent, and result in reduced demixing rate in the phase inversion process. For this reason, CCA-2.3 membranes have smaller pore size and form denser structure than CA membranes.

3.4 Characterization of CCA UF membranes

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IP

T

800

SC R

600

CA CCA-1 CCA-2 CCA-2.1 CCA-2.2 CCA-2.3

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400 200

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Pure water flux (LMH)

1000

0 20

40

60 Time (min)

80

100

120

D

0

TE

Fig. 8. Pure water flux of prepared membranes according to degree of CA cross-linking.

In general, compaction occurs widely in various membrane materials, and such phenomenon reduces

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membrane porosity and ultimately membrane flux [39-42]. Fig. 8 is a graph of pure water flux of the prepared membranes measured over 120 minutes. CA membranes showed a higher initial pure water flux

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compared to other membranes (other than CCA-1). This is because of their larger average pore sizes compared to CCA as shown in Fig. 7.

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1.00

1.0

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J120 / J0

0.63

0.5 0.32

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0.73

1.00

0.0 1

2 2.1 2.2 Concentration of TMC (%)

2.3

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Fig. 9. Compaction due to pure water flux of prepared membranes according to concentration of TMC (wt.%) added to CA dope solution.

Fig. 9 is a graph showing compaction of CA and CCA membranes with pure water flux. Despite their high initial pure water flux, flux for CA membranes fell to 32% of initial flux after 2 hours. However, the

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difference in pure water flux for CCA membranes decreased with degree of cross-lining. In particular, pure water flux was steady for CCA-2.2 and CCA-2.3 over time. This compaction limiting effect of CCA membranes is the result of various factors. As already mentioned, CA has a porous membrane structure (Fig. 7(b)) due to instantaneous demixing as a result of low dope solution viscosity. In contrast, CCA has a dense membrane structure due to delayed mixing as a result of high dope solution viscosity. As structure goes from porous to dense, tensile strength and elongation of membranes increases [43]. Furthermore, cross-linking of CA increases the tensile strength of the materials themselves and reduces elongation. (Fig. 4 shows that the reduction of elongation due to dense structure is a greater factor than reduction due to materials.) Therefore, the compaction limitation by CCA membranes is the result of the composite effects of increase in tensile strength due to both structural and material aspects, and reduction in elongation from a material aspect. (CA cross-linked at TMC 1wt.% showed the greatest compaction effect. As shown in

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ACCEPTED MANUSCRIPT Fig. 4, this is because at TMC 1wt.%, it has lowest tensile strength and most porous membrane structure due to low viscosity. As explained, it is presumed that at TMC 1wt.% there is intra intramolecular cross-

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linking of polymer chain due to low TMC concentration, resulting in relatively low viscosity and tensile

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strength [28].)

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Fig. 10. Schematic representation of interrelationship of materials, phase inversion process, and membrane characterization starting from cross-linking process.

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Fig. 10 summarizes this complicated chain of factors, and the interrelationship of chemical resistance, viscosity, materials-related mechanical property, demixing rate, membrane morphology, structure-related

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mechanical property, and compaction.

Table 2 Rejection performance of CA and CCA membranes against kaolin particle (0.1μm ~ 4μm) at suspension of 65±5 NTU. Time (min) Sample name 0

15

30

45

60

CA

65

0.2

0.1

0.1

0.1

CCA-1

65

0.1

0.1

0.1

0.1

CCA-2

65

0.1

0.1

0.1

0.1

CCA-2.1

65

0.1

0.1

0.1

0.1

CCA-2.2

65

0.1

0.1

0.1

0.1

CCA-2.3

65

0.1

0.1

0.1

0.1

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ACCEPTED MANUSCRIPT Table 2 shows the result of kaolin removal performance test of CA and CCA membranes over 60 minutes. In particular, CCA membranes removed over 99.85% of kaolin particles (0.1μm ~ 4μm) at

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kaolin suspension of 65±5 NTU regardless of time, which confirms the possibility of CCA membranes for

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use in UF.

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4. Conclusion

Cross-linking of CA with TMC was confirmed through observation of C=C peak (1560cm-1) and C-H

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peaks (800cm-1, 755cm-1) of aromatic ring using ATR-FTIR. The viscosity of CA dope solution increased with TMC concentration, which additionally confirmed cross-linking of CA.

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Cross-linking among CA chain with TMC also had large effects on mechanical property and chemical resistance of CCA. As the degree of cross-linking increased, CCA showed up to 4 times the tensile

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strength of existing CA, and also high resistance against polar solvents. In particular, CCA-2 showed no

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weight loss (%) in sharp contrast to CA, and the weight loss (%) of CCA-2.3 against acetone and DMAc was considerably lower at 6% and 30.8%, respectively. This implies that CCA may have applications in

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OSN field.

The morphology of CCA and CA membranes were confirmed through AFM and SEM observations. A comparison of surface roughness of CCA and CA membranes by AFM showed that the surface

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roughness of CCA membranes was relatively higher than that of CA membranes. However, there were no clear differences between CCA membranes. However, the difference in morphology using SEM was more pronounced. CCA-2.3 membranes had relatively smaller pore size compared to CA membranes, but also more even pore size distribution and denser cross-section. This is because the demixing rate decreased due to higher CCA dope solution viscosity. The density had a composite effect of increasing tensile strength and lowering elongation due to structural aspects, and as a result minimizing compaction effect. Cross-linked membranes removed over 99.85% of kaolin in performance tests, which confirmed the possibility for their use as UF membranes.

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ACCEPTED MANUSCRIPT Reference

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