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Polymer 114 (2017) 113e121

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Polymer

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Enhanced anti-ultraviolet, anti-fouling and anti-bacterialpolyelectrolyte membrane of polystyrene grafted with trimethylquaternary ammonium salt modified lignin

Yanli Ma a, *, Junxiu Dai a, Lili Wu b, Guizhen Fang a, Zhanhu Guo b, **

a Material Science and Engineering College, Northeast Forestry University, Harbin, 150040, PR Chinab Integrated Composites Laboratory, Department of Chemical & Biomolecular Engineering, University of Tennessee, Knoxville, TN 37996, USA

a r t i c l e i n f o

Article history:Received 28 January 2017Received in revised form17 February 2017Accepted 26 February 2017Available online 28 February 2017

Keywords:Polystyrene blending membraneLigninTrimethyl quaternary ammonium salt

* Corresponding author.** Corresponding author.

E-mail addresses: myl219@126.com (Y. Ma), zguo1

http://dx.doi.org/10.1016/j.polymer.2017.02.0830032-3861/© 2017 Elsevier Ltd. All rights reserved.

a b s t r a c t

A polyelectrolyte membrane with anti-ultraviolet, anti-fouling, and antibacterial abilities was preparedusing trimethyl quaternary ammonium salt grafting lignin (QAL) and polystyrene (PS) at different ratios.Contact angle measurements indicated that the addition of QAL into the PS blending membrane alteredthe hydrophilicity by trimethyl quaternary ammonium groups and phenolic hydroxyl groups of QAL.Furthermore, the phenylpropane structural unit in the lignin formed double bonds by conjugating withcarbonyl groups and formed p-p by conjugating with phenolic hydroxyl of side chains. For this reason,the produced composite membranes displayed high potential and commercial importance for absorbingultraviolet light. The ultraviolet transmission values of all the PS/QAL blending membranes were close tozero ranging from 200 to 400 nm. The QAL5-PS membrane had less variation in the gas transmission rate(�6.388 cm3/m2/day/atm) than that of PS membrane (�32.9 cm3/m2/day/atm) after 5 circles of bovineserum albumin (BSA) fouling. The inhibition zone of the QAL5-PS membrane was 10.4 ± 0.06 mm, andalmost zero for the neat PS membrane, indicating a much better antibacterial performance of the QAL-PScomposite membranes than that of pure PS membranes.

© 2017 Elsevier Ltd. All rights reserved.

1. Introduction

Polystyrene (PS) has many key features such as very hightransparency, high hardness, ease of processing. However, thedisadvantages of PS including inherent brittleness and very lowimpact strength limit its usability range [1]. Nanocomposites withintroduced unique nanofillers have the potential to expand theapplications of polymers including water cleaning [2e5], electro-magnetic interface (EMI) shielding/absorption [6e9], energy stor-age and saving [10e14], anticorrosive coating [15e17], andintelligent sensing [18e21]. With the same nanocompositesconcept aiming to expand the application of PS membranes, ligninwith similar structure to styrene was chosen as blend materialbecause of its most abundance, renewable, non-toxic, and biode-gradable characters [22]. Approximately 50 million tons of ligninare produced annually from the wood, straw, and paper industry

0@utk.edu (Z. Guo).

[23e25]. Meanwhile, lignin and its derivatives have been incor-porated as fillers in polymers to develop lignin-based compositematerials with improved physical properties. For example, ligninwas used as a reactive reinforcing filler for a soy oil-based poly-urethane biofoam [26]. It served as a plasticizing agent in com-posite films with starch [27,28]. It also enhanced the thermo-mechanical properties of poly (ester/amine) because of itshydrogen bonded/cross-linked polymer networks [29]. Lignin wasalso an additive to the polysulfone layer in various concentrationsto increase the porosity of its substructure [30]. In addition, ligninin the soil can transform into humus after degradation, which helpsto improve the soil fertility. It is a more cost-effective alternative tostarch, cellulose, chitosan, and inorganic clay. The precursors oflignin, such as p-coumaric, ferulic, and sinapic acids, as well asother phenolic acids, possess antimicrobial activity. The antimi-crobial properties of lignin have also been reported [31].

Moreover, the application of PS membrane is limited to someextent due to its poor hydrophilicity. When biomolecules areimmobilized on the hydrophobic PS surface, they suffer consider-able denaturation and lose biological activity due to the stronghydrophobic interaction. In contrast, biomolecules have less

Y. Ma et al. / Polymer 114 (2017) 113e121114

conformational change and retain functional activity when they areimmobilized on the hydrophilic surface. Hydrophilic properties canenhance membrane resistance to organic and biological foulingcaused by substances such as proteins, natural organic matter(NOM), and bacteria, because of hydration via hydrogen bonding[32e34]. As most available membranes from market are hydro-phobic, variousmethods have therefore been developed to enhancethe surface hydrophilicity of membranes for water treatment.Moreover, several covalently modified surfaces are not stableenough to keep the hydrophilicity for a long time [35]. Water-soluble quaternary ammonium salts have been demonstrated toprovide derivatives with an enhanced hydrophilic and antibacterialability. For example, N-[(2-hydroxy-3-trimethyl-ammonium) pro-pyl] chitosan chloride exhibited better water solubility and anti-bacterial properties than chitosan [36]. To improve hydrophilic ofPS membrane, trimethyl quaternary ammonium salt grafted lignin(QAL) has potential to serve as large molecular grafting copolymerto facilitate the phenylpropane structure of lignin for improving itscompatibility with polystyrene membrane. QAL can be synthesizedas a hydrophilic material from lignin and trimethylamine throughthe Mannich reaction [37]. The QAL can be used as a flocculent andbacteriostatic agent [38]. The phenylpropane structures of ligninhas formed carbonyl by conjugating with the benzene groups in theside ch [39]ain, which also can absorb ultraviolet light. The growthof different green plants has a selective absorption of light. Thereare 2 kinds of “light fertilizer”, one is mainly concentrated in the400e460 nm blue, the other is 600e700 nmviolet and orange lightin the visible region. However, ultraviolet rays on most plants andplastic film supplies are harmful, such as wavelength 290e315 nmUV is harmful to plants, the addition of QAL can greatly reduce theabsorption of ultraviolet light and improve aging resistant perfor-mance, and can inhibit sclerotinia sclerotiorum disease caused bybacteria or ash by making full use of the function of alkali lignin[40]. However, to improve the hydrophilic of PS membrane, tri-methyl quaternary ammonium salt grafted lignin (QAL) as largemolecular grafting copolymer to facilitate the phenylpropanestructure of lignin for improving its compatibility with polystyrenemembrane has not been reported yet.

Herein, a novel polyelectrolyte membrane with anti-ultraviolet,anti-fouling, and antibacterial abilities was prepared using tri-methyl quaternary ammonium salt grafting lignin and polystyrene.The influence of trimethyl quaternary ammonium salt graftedlignin (QAL) on the anti-fouling and antimicrobial performance of anovel polyelectrolyte QAL polystyrene membrane was studied. Thesurface properties of modified membranes were analyzed by themethods such as ATR-FTIR, SEM and the measurements of contactangle to reveal the effects of polyelectrolytes with different amountof QAL on the performance of modified membranes. The QAL effectof membrane surface modification on the thermal properties wasalso investigated by TGA and DSC. The antifouling mechanism ofthe membranes modified by QAL and PS membrane were analyzedby gas permeability tester. The antibacterial activities of the PS andQAL-PS membranes were examined quantitatively using the discdiffusion method and colony-forming count method respectively.

2. Experimental

2.1. Materials

Industrial alkali lignin from rice and wheat straws were pur-chased from Tralin paper mill (Shandong, China). According to themanufacturer's report, the alkali lignin had a molecular weight of1903 Da, sugar content of 8.35%, and ash content of 8.96%. Prior touse, it was washed with acidified water to remove the residual saltsand then washed with water to make the pH 7. At this stage, the

total hydroxyl content was 2.95 mmol g�1 and the aliphatic-OH/aromatic-OH ratio was 1:1.02. All other reagents used were ofanalytical grade. Sodium triphosphate and bovine serum albumin(BSA) were obtained from Sigma-Aldrich. Escherichia coli was ob-tained from the Key Laboratory of Bio-based Material Science andTechnology of the Ministry of Education (Harbin, China). Poly-styrene and other reagents were obtained from Aladdin ChemistryCo., Ltd. (Shanghai, China), and a general-purpose grade of poly-styrene GPPS resin for injection molding process had the range ofmolecular weight from 10,000e20,000.

2.2. Methods

2.2.1. Synthesis of trimethyl quaternary ammonium salt graftedlignin (QAL) and preparation of QAL-PS polyelectrolyte membrane

The solution of trimethylamine (33%, 50.2 g) in epichlorohydrin(16 mL) was added dropwise by using a constant pressure, and themixture was continuously stirred for 2 h at �5 �C to make the tri-methyl quaternary ammonium ionmonomer. Alkali lignin (1 g) wasdissolved in 20 mL NaOH solution (0.5 mol/L), the products werenamed alkali lignin solution, and 20 mL aqueous solution ofdifferent contents of trimethyl quaternary ammonium ion mono-mer (0.5, 1, 1.5, 2 and 2.5 g, respectively) was added dropwise andmixed with alkali lignin solution. And then the reaction wasmaintained at 50 �C for 4 h [38,41]. The trimethyl quaternaryammonium salt grafted lignin was obtained. According to nitrogencontent of the products whichwere caused by the differences in thegrafting degree of quaternary ammonium salt on QAL, the productswere named as QAL1 (N%, 4.0 wt%; QA 27.55%/72.45 %L), QAL2 (N%,6.0 wt%; QA 36.32%/63.68% L), QAL3 (N%, 7.24 wt%; QA 40.77%/59.23% L), QAL4 (N%, 8.05 wt%; QA 43.35%/56.65% L), and QAL5 (N%,8.62%; QA 45.04%/54.96% L), respectively. The main chemical re-actions involved in the synthetic process of QAL are shown inScheme 1. To improve the grafting degree of lignin and trimethylquaternary ammonium salt and overcome steric hindrance of thelarge molecular fragments, firstly, trimethylamine and epichloro-hydrin reacted to form quaternized chloropropanols via a quater-nary ammonium reaction, and then glycidyl quaternary ammoniumsalts (GQA) via cyclization in NaOH solution. At last, lignin wasgrafted with GQA by the etherification reaction to get trimethylquaternary ammonium salt grafted lignin (QAL). In detail, QAL(quaternary ammonium ion mole concentration: 0.01 M) andpolystyrene (monomermole concentration: 0.01M)were dissolvedin 50 mL dichloromethane, respectively. The PS solution was addedinto the QAL solution (600 rpm stirring) at a speed of 10 mL$min�1,yielding polyelectrolyte precipitates (QAL-PS PEC) when QAL wasfully neutralized by PS. The precipitates were collected by filtrationand washed three times with deionized water to remove impu-rities, and followed by adding 15% acetone to cast the QAL-PSpolyelectrolyte membrane (Scheme 2) [37]. According to the tri-methyl quaternary ammonium salt content of the QAL-PS poly-electrolyte which was caused by grafting different degrees ofquaternary ammonium salt on QAL, the products were named asQAL1-PS (QAL1 5.56%/94.44% PS), QAL2-PS (QAL2 8.33%/91.67% PS),QAL3-PS (QAL3 10.00%/90.00% PS), QAL4-PS (QAL4, 11.05%/88.95%PS), and QAL5-PS (QAL5 11.99%/88.01% PS), respectively.

2.2.2. Membrane characterizationTo assess the modification efficiency of trimethyl quaternary

ammonium ion monomer, the nitrogen contents (N %) of preparedQAL samples were determined before and after modification. Theexperiments were performed using a Fourier transform infrared(FT-IR) spectrometer (Nicolet 6700, Thermo Scientific, USA) con-nected to an OMNIC operating system (Version 9.0). The FT-IRspectra were collected in the frequency range of 4000 to

Scheme 1. Synthesis of trimethyl quaternary ammonium salt grafted lignin. (a) trimethylamine, (b) epichlorohydrin, (c) trimethyl quaternary ammonium ion monomer, and (d)trimethyl quaternary ammonium salt grafted lignin.

Scheme 2. Preparation of QAL-PS polyelectrolyte membrane. (a) Polystyrene, (b) grafted with trimethyl quaternary ammonium salt modified lignin, (c) trimethyl quaternaryammonium ion monomer, and (d) QAL-PS polyelectrolyte membrane.

Y. Ma et al. / Polymer 114 (2017) 113e121 115

400 cm�1 at a resolution of 2 cm�1.The hydrophilicity of PS and QAL-PS modified membranes was

evaluated using contact angle measurements. The static contactangle measurements were performed via the sessile droplet tech-nique at 25 �C with a FTÅ 200 contact angle analyzer (First TenÅngstroms, Portsmouth, VA). The static apparent contact angle wasmeasured just after depositing 8 mL droplet. Each measurementwas operated 10 times, and then the average value and its errorwere calculated. The scanning electronmicroscope (SEM) images ofthe surface and cross-sectional morphology of the membraneswere captured using a Quanta 200 (12.5 kV, FEI, USA). The drymembrane samples were frozen in liquid nitrogen, and then frac-tured to expose the cross-sectional areas. The samples were gold-coated by the metal spraying machine (Bal-Tec SCD 005, BalzersCo., Ltd., Switzerland) before scanning to provide an electricallyconductive surface.

The tensile mechanical properties of the prepared membranewere measured using tensile equipment (LDX-200, ShanghaiLandmark Packaging Materials Co., Ltd., China), following the pro-cedures outlined in the GB/T methods 1040.3e2006 with anaverage of five measurements taken for each film and with at leasttwo films per formulation. The thickness of the membranes wasmeasured by a digital micrometer (SF-2000, Guanglu MeasuringInstrument Co., Ltd., Guilin, China). Each membrane was cut intorectangle piecewith 150mm in length and 20mm inwidth, and the

pieces were conditioned in a desiccator with a sulphuric acid so-lution at 25 �C with 50% relative humidity (RH) for 72 h. A piece offlat membrane was fixed vertically between a pair of tweezersseparated by a gap of 100 mm, with a crosshead speed of50 mm$min�1. The measurement for each type of membrane wasrepeated three times, and then the average value was reported.

The glass transition temperature (Tg,�C) was determined using adifferential scanning calorimeter (DSC-204, Netzsch, Germany).The samples of approximately 10 mg (±0.002 mg) were tightlyencapsulated in aluminum pans and scanned under a dry nitrogenpurge (50 mL/min). Freshly conditioned films were rapidly cooleddown to 50 �C and scanned between 50 and 500 �C at a heating rateof 10 �C/min. The Tg data were reported as mean values of at leastduplicate samples for each film, usually within ±1 �C. Thermalgravity analysis (TGA) data were acquired using a thermogravi-metric analyzer (TG 209-F3, Netzsch, Germany) at a heating rate of10 �C/min.

The UVevisible spectra of PS and QAL-PS membranes were ac-quired using a UVevisible spectrophotometer (UV 1901, PurkinjeGeneral Co., Beijing, China) with a scan mode in the range of200e800 nm [39].

2.2.3. Characterization of membrane foulingThemembrane surface can be fouled easily by the contaminants

during the process of separation, and the holes of membrane can be

Y. Ma et al. / Polymer 114 (2017) 113e121116

blocked. Thus, the membrane flux will be decreased with pro-longing the running time, and the service life of the membrane willbecome shortened. Pure water flux (PWF) was often taken as anindex to characterize anti-fouling property of membranes underdifferent pressures (generally 20e100 kPa). However, QAL/PS hadtoo weak force among different components to bear pressure ofPWF testing, thus we chosen the gas transmission rate (GTR,cm3�m�2�day�1�atm�1) as an index for testing the anti-foulingproperty of PS and QAL-PS membranes.

The BSA organic foulant standard solution (1 mg/mL) was fed tothe beaker, in which the PS and QAL-PS membranes wereimmersed for 1-h and then cleaned with pure water, then dried forreserve. The GTR of sample was measured with a VAC-VBS gaspermeability tester (Labthink Instruments Co., Ltd., Shandong,China) with nitrogen as the carrier gas (the high pressure chamberof 1� 105 Pa and low pressure chamber of 27 Pa) for a round-shapefilmwith a diameter of 6 cm [28]. Prior to each experiment, samplewas pretreated following the GB/T2918-1998 standard, the mem-branes were conditioned at 50% relative humidity (RH) and 25 �Cfor 90 h. The anti-fouling test of each membrane repeated fivetimes [19].

2.2.4. Antibacterial test of membranes®

The antibacterial activities of the PS and QAL-PS membraneswere examined quantitatively using the disc diffusion method andcolony-forming count method, respectively [39] in Luria-Bertani(LB) agar plates (containing 10 g peptone, 10 g NaCl, 5 g yeastextract and 2% (w/v) agar in 1000 mL distilled water) at 30 �C [42].To develop the inoculums, a single colony of E. coli from the LB agarplate was inoculated into LB broth (LB agar minus agar) and incu-bated for growth under shaking at 150 rad/min and 30 �C. Themembranes were aseptically inoculated with Escherichia coli-beefextract suspension (1 � 107 CFU/mL) [43]. The PS and QAL-PSmembranes were cut into circles with 1 cm in diameter, sterilizedby UV light, placed on E. coli-cultured agar plates and then incu-bated at 37 �C for 24 h [36]. After incubation, a bacterial inhibitionzone was formed around the membrane. The width of the inhibi-tion zone (W) was calculated using Equation (1) [44]:

Fig. 1. SEM images of membranes with the scales of 100 mm: cross-sectional images of (a) PSQAL4-PS membrane, and (f) QAL5-PS membrane. The insets are surface images of the corre

W ¼ ðd1 � d2Þ=2 (1)

where d1 (mm) is the total diameter of the inhibition zone and themembrane, and d2 is the diameter of themembrane (20mm). In thecolony-forming count method, three pieces of PS and QAL-PSmembranes (20 mm) were sterilized by UV light and thenimmersed in 10 mL bacterial suspension in a flask. The solutionwasthen shaken at 200 rpm at 37 �C. After 24 h, 0.2 mL bacterial culturewas taken from the flask and serial dilutions were repeated withphosphate buffer saline (PBS) in each initial sample. Then, 0.1 mLdiluted samples were spread onto agar plates. The plates wereincubated at 37 �C for 24 h, and the number of viable cells wasmanually counted and then multiplied by the dilution factor [45].

3. Results and discussion

3.1. FTIR analysis of QAL

QAL samples were freeze-dried and the IR spectra weremeasured (Supporting Information Fig. S1). The characteristicstretching and bending vibrations of QAL due to N (CH3)3þ such asamide I (3463 cm�1), amide II (3050 cm�1), amide III (2500-2000 cm�1) and amideⅣ (1000-850 cm�1) can be seen in Fig. S1 b-f, the bands at 2960 cm�1 can be attributed to C-H vibrations. Thefeatures around 1612 and 1480 cm�1 arising from the benzeneskeleton indicate the alkali lignin in QAL. The broad featureobserved at around 3450 cm�1 is ascribed to the O-H and N-Hstretching vibrations. These band features confirm the successfulsynthesis of QAL [46,47].

3.2. Membrane morphology and tensile strength

Fig. 1 shows the SEM images of the surface and cross-section ofthe polyelectrolyte membrane. The QAL-PS membrane exhibited adense structure, and the cross-section of the QAL-PS membranewas observed to be rougher with increasing the QAL content insidethe QAL-PS membrane. Furthermore, the porous structure was

membrane, (b) QAL1-PS membrane, (c) QAL2-PS membrane, (d) QAL3-PS membrane, (e)sponding samples.

Fig. 2. Tensile strength of QAL-PS membranes.

Y. Ma et al. / Polymer 114 (2017) 113e121 117

found on the section of the QAL4-PS membranes [46]. It is obviousthat themechanical behavior is greatly influenced by the amount ofQAL. Compared with PS (42.54 MPa), the dispersion of QAL in thepolymer matrix decreased the tensile strength (QAL1-PS, 2.15 MPa;QAL2-PS, 3.48 MPa; QAL3-PS, 6.34 MPa; QAL4-PS, 6.55 MPa; QAL5-PS, 7.57 MPa) (see Fig. 2). The main reason for these changes wasthat the PS units were neutralized with the positively charged QALby ionic-dipole interactions (See in Fig. S2). Among the compositemembranes of QAL-PS, the tensile strength increased withincreasing the amount of QAL. This enhancement in themechanicalproperties is due to the presence of electrostatic attraction betweennegatively charged PS and positively charged QAL molecules, andintermolecular forces in QAL [48].

3.3. Hydrophilic modification of polystyrene membrane

A generally accepted result is that in order to efficiently resistbiofouling, a surface or a porous matrix should be hydrophilic [49].The presence of negatively charged and positively charged mole-cules obviously enhanced the membrane surface hydrophilicity[39]. The polyelectrolytes with negatively or positively chargedgroups could improve the surface hydrophilicity of anion exchangemembrane [50]. To enhance the anti-fouling property, the poly-electrolyte membranes were designed consisting of trimethylquaternary ammonium salt grafting lignin and polystyrene.

The surface hydrophilicity of the PS and QAL/PSmembranes wasdetermined by contact angle analyzer. The water contact anglemeasurements showed a rapid and significant decrease as the QALcontent increased, Fig. 3, due to the existence of hydrophilic tri-methyl quaternary ammonium salt on the PS surface. The watercontact angle measurements showed a rapid and noticeabledecrease as the QAL content increased [27,32,51]. The

Fig. 3. Water contact angles of QAL-PS membranes with different QAL loading. In thetop inset, the Image of static contact on surface of QAL-PS membranes appears here tobe lower than 90� .

polyelectrolyte membrane had the lowest contact angle (10.57�).The hydrophilic membrane surfaces show a better membranefouling resistance [52]. The abundant presence of functional groupssuch as the quaternary ammonium groups and phenolic hydroxylgroups in QAL enhanced the membrane surface hydrophilicity [53].The increased hydrophilicity reduced the interfacial energy be-tween the membrane and water and led to a lower contact angle[40].

3.4. Thermal properties

TGA measurements were performed to evaluate the thermalstability of QAL-PS membranes, Fig. 4. The QAL-PS materialsshowed two main thermal degradation phenomena during pyrol-ysis under N2. One is in the temperature range from 150 to 300 �C,and the other is for the temperatures higher than 300 �C. The firstweight-loss, of approximately 10e30%, may be associated with thebreaking of a- and b-aryl-alkyl-ether linkages, phenol ether (qua-ternary ammonium groups), aliphatic chains, and decarboxylationreactions [28]. The second broad and sharp mass-loss event (above350 �C) may be related to the rupture of carbon�carbon linkagesbetween the lignin structural units and functional groups (phenolichydroxyl groups, carbonyl groups, and benzylic hydroxyl groups).Upon ionic dipole interaction, improved thermal stability at hightemperatures (>300 �C) was observed for the PS materials, irre-spective of the QAL-PS weight ratio. This was also observed by thehigher decomposition temperatures at 20% mass loss found inthese systems compared with neat PS, which yielded slightly morefinal char residue at 500 �C.

Higher decomposition temperatures at 50% and 90% mass-losswere found in these systems compared with the unreacted QAL(see Table 1). By further increasing the temperature, a broad massloss was noted in all QAL-PS materials, which could be associatedwith the degradation of the ionic dipole interaction and the car-bon�carbon structure in QAL [54]. This behavior may be correlatedwith the presence of residual unreacted QAL in the QAL5-PS ma-terial, even after thermal condensation. As more PS than QAL5 wasobserved in this case, the final char residues of PS and QAL5 were 0%and 29% at 500 �C, respectively. These results further confirm thatthe QAL-PS materials with higher QAL content gave an improvedthermal stability with respect to the parent un-crosslinked QALsample. The glass transition temperatures (Tg) of several poly-electrolyte membranes are also depicted as a function of

Fig. 4. TGA and glass transition temperatures of PS and QAL-PS membranes.

Table 1Thermal degradation characteristics of AL, PS, and QAL-PS membranes.

Material T10% (�C) T50% (�C) T80% (�C) T50% (�C) R500 (%) TDTG max (�C)

QAL1-PS 169.5 374 412 374 12.7 412QAL2-PS 187.0 387 417 387 12.2 409QAL3-PS 192.0 390 417 390 13.4 406QAL4-PS 207.0 390 422 390 17.7 406QAL5-PS 224.5 397 424 397 18.3 399AL 148.1 300 500 300 29.0 258PS 332.0 398 412 398 0.0 404

Note: T10%: temperature at 10% mass loss; T50%: temperature at 50% mass loss; T80%:temperature at 80%mass loss; R500: final char residue at 500 �C; TDTG max: maximummass loss rate derivative temperature.

Y. Ma et al. / Polymer 114 (2017) 113e121118

composition, Fig. 4. The order of trimethyl quaternary ammoniumsalt content was QAL1 < QAL2 < QAL3 < QAL4 < QAL5, but adecreasing tendency was observed for the main Tg values of QAL-PSmembranes with increasing the trimethyl quaternary ammoniumsalts of QAL. This phenomenon arose from higher QAL content,meaning more proportion of ionic-dipole interactions among thepolyelectrolyte QAL-PS membranes. However, this interaction forceof the interface was weaker than the winding effect between the PSlarge molecular chains.

3.5. Optical transmittance

To understand the light resistance of the polyelectrolyte mem-branes of QAL-PS after introducing various amounts of QAL, thetransmittance of QAL-PS and PS membranes was measured. TheQAL-PS membrane had a higher UV protection compared with PSbetween 280 and 400 nm. In addition, the light transmittancemeasurements showed a rapid and noticeable decrease as the QALcontent increased, Fig. 5. Generally, the benzene groups are knownto obstruct UV light by p�p* transitions (204 and 254 nm) [55,56].The QAL can absorb UV irradiation and partial visible light, whichcan be attributed to the p�p* transitions and n�p* of the guaiacyl,syringyl, and p-hydroxyphenyl (204, 254, 280, and 350 nm) units oflignin. Thus, QAL-PS membranes can prevent the damage of290e315 nm ultraviolet on plants, while absorb partially helpful600e700 nm violet and orange light, which had a potential appli-cation in the field of agriculture.

3.6. Anti-fouling behavior

The polyelectrolytes exhibit a good resistance to proteinadsorption, similar to a zwitterion, and have a strong hydration ofthe copolymer through ionic solvation. The key to anti-fouling of

Tran

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200

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(a)

400 500

(a)PS (b)QAL1-P (c)QAL2-P (d)QAL3-P (e)QAL4-P (f)QAL5-PS

Wavelength600

(b)PSPSPSPSS

h (nm)

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700

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(e)(d)

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800

Fig. 5. Transmittance studies of PS and QAL-PS membranes.

protein is the water film close to or on the surface of the poly-electrolyte membrane, which is formed by hydrogen bonding ofpositive and negative charges on the surface of QAL-PS membranewith water [40]. Several fouling tests were conducted by testing airpermeability to investigate the fouling trend of the membranesusing BSA as an organic model foulant [32]. The normalized BSAfouling behaviors of QAL-PS and PS membranes are illustrated inFig. 6. In contrast, the GTR value of QAL5-PS membrane (9.60 cm3/m2/day/atm) decreased 83.97% compared with that of pure PS(59.9 cm3/m2/day/atm) before fouling tests (Fig. 6a). In the firstfouling tests, the△GTR of QAL-PS membrane was all less than thatof the PS membrane which decreased up to 11.3 cm3/m2/day/atm.This indicates that the addition of QAL can help PS inhibit the BSApollution (QAL1-PS, �8.6 cm3/m2/day/atm; QAL2-PS, �7.5 cm3/m2/day/atm, QAL3-PS, 8.2 cm3/m2/day/atm, QAL4-PS, �9.8 cm3/m2/day/atm and QAL5-PS, �3.716 cm3/m2/day/atm). And the QAL5-PSmembrane had less △GTR (�6.388 cm3/m2/day/atm) than that ofPS membrane (�32.9 cm3/m2/day/atm) after 5 cycles of BSAfouling. It can be confirmed that the introduction of QAL for QAL-PSpolyelectrolyte membrane can improve the antifouling property ofprotein (Fig. 6b). However, the initial GTR values of QAL1-PS(64.7 cm3/m2/day/atm), QAL3-PS (70.2 cm3/m2/day/atm) and QAL4-PS (79.8 cm3/m2/day/atm) membranes were higher than those ofPS, of which the highest was QAL4-PS membrane (Fig. 6a). Thisresult is in agreement with the phenomena observed in the cross-section SEM images of QAL4-PS membrane which had more poresthan other membranes, Fig. 1. After that, all the membranepermeability decreased gradually as the BSA fouling test continued[57,58].

3.7. Antibacterial activity

The water-soluble quaternary ammonium salts are reported toprovide composites with enhanced antibacterial ability [36]. The

Fig. 6. Air permeability of PS and QAL-PS membranes: (a) effect of number of foulingexperiments; (b) effect of QAL content. 1 represents QAL1-PS, 2 represents QAL2-PS, 3represents QAL3-PS, 4 represents QAL4-PS and 5 represents QAL5-PS in the graph.

Fig. 7. Inhibition zone test of the PS and QAL-PS membranes against E. coli at 24 h (a) PS, (b) QAL1-PS, (c) QAL2-PS, (d) QAL3-PS, (e) QAL4-PS and (f) QAL5-PS.

Y. Ma et al. / Polymer 114 (2017) 113e121 119

positive charge of quaternary ammonium cationic group normallyresults in a poly cationic structure that is absorbed onto thenegatively-charged cell surface of the bacteria. This leads to a sig-nificant alteration of the structure of the outer membranes andcauses the release of a large proportion of proteinaceous materialout from the cell [59]. The inhibition zone can be used to charac-terize the antibacterial effect of antibacterial agents, and the anti-bacterial activity of the antibacterial materials can be displayed bythe diffusion of agar. The criterial is that the antibacterial effect isbetter for an increased diameter of inhibition zone. As QAL-PSmembranes have a cationic group of quaternary ammonium, ithas antibacterial activities against E. coli.

Fig. 7 shows the antibacterial activity of the neat PS and QAL-PSmembranes against E. coli. The concentration of E. coli suspensionused was 2 � 107 CFU/mL. The widths of the inhibition zone forQAL-PS membranes were 4.74 ± 0.04 mm (QAL1-PS),6.67 ± 0.03 mm (QAL2-PS), 8.13 ± 0.09 mm (QAL3-PS), 8.7 ± 0.2 mm(QAL4-PS), and 10.4 ± 0.06 mm (QAL5-PS). Apparently, the QAL5-PSmembrane revealed a noticeable inhibition activity (QAL5-PS;10.4 ± 0.06 mm) compared with the neat PS membrane (zero)because of the higher QAL content of QAL5-PS blending membranesthan other QAL-PS membranes. Additionally, the strain E. coli grewvigorously in the culture vessels outside the inhibition zone ofQAL1-PS, QAL2-PS, and QAL3-PS membranes. The QAL4-PS andQAL5-PS membranes also exhibited antimicrobial activity. TheE. coli did not grow well when the QAL content exceeded 11%. It isdifficult to penetrate into the cell with large molecular weightgrafted trimethyl quaternary ammonium salt. The reason forpolyelectrolyte membranes against E. coli is most likely that thenegatively charged environment on the cell surface of bacteria waschanged by the cations of trimethyl quaternary ammonium salt ofQAL-PS membrane. The study further strengthens the evidencethat quaternary ammonium salt enhanced its antibacterial ability.

4. Conclusions

The trimethyl quaternary ammonium salt grafted lignin (QAL)contains cations of quaternary ammonium which can help toimprove the formation of polyelectrolyte and antimicrobial activity

of E. coli. The cross-section of the QAL-PS membrane was observedto be rougher with increasing the QAL content inside the QAL-PSmembrane. Meanwhile, it also benefits the resistance of proteinadsorption and protecting plant from the ultraviolet rays. Theelectrostatic attraction between negatively charged PS and posi-tively charged QAL molecules as well as the cation contents of tri-methyl quaternary ammonium salt had an important role indetermining the tensile strength and thermal properties. There aretwo reasons influencing the membrane permeability of the BSAfouling test. One is the hydrophilicity of QAL-PS membrane, theother is the pores which are formed by ionic-dipole interactionsbetween negatively charged PS and positively charged QAL. QAL-PSmembrane has a cationic group of quaternary ammonium and thushas antibacterial activities against E. coli. The QAL5-PS membranesrevealed a noticeable inhibition activity (the diameter of inhibitionzone, 10.4 ± 0.06 mm). In general, lignin was introduced into thetarget polymer to reduce the product costs. However, compared tothe unmodified PS membrane, grafting trimethyl quaternaryammonium salt to lignin to prepare a novel polyelectrolyte QALpolystyrene membrane played an important role for hydrophilicproperties, antifouling and antimicrobial ability.

Acknowledgments

Dedicated this paper to Professor Fang Guizhen of NortheastForestry University. May she always be healthy and graceful. Theauthors gratefully acknowledge the financial support provided bythe State Forestry Administration Public Welfare Project of China(201304614), Heilongjiang Province Science Foundation for Youths(No. QC2014C011), and Harbin City Foundation of science andtechnology for innovative talents (2016RQQXJ221). Z. Guo appre-ciates the financial supports from University of Tennessee. L. Wu issupported by the Chinese Scholarship Council (CSC) program.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.polymer.2017.02.083.

Y. Ma et al. / Polymer 114 (2017) 113e121120

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