A readily modified polyethersulfone with amino-substituted groups: Its amphiphilic copolymer synthesis and membrane application

Polymer 53 (2012) 350e358

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A readily modified polyethersulfone with amino-substituted groups: Its amphiphilic copolymer synthesis and membrane application Zhuan Yi a, b, Liping Zhu a, b, *, Liang Cheng a, b, Baoku Zhu a, b, Youyi Xu a, b a

MOE Key Laboratory of Macromolecule Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, 38# Zheda Road, Hangzhou 310027, PR China b The Engineering Research Center of Membrane and Water Treatment Technology (Ministry of Education), Zhejiang University, Hangzhou 310027, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 June 2011 Received in revised form 13 October 2011 Accepted 25 November 2011 Available online 2 December 2011

An amino-substituted polyethersulfone (PES) was synthesized by the polycondensation of a functional monomer bis(3-amino-4-hydroxyphenyl) sulfone with bis(4-fluorophenyl) sulfone. The amine groups incorporated into PES were employed as anchors to immobilize the chain transfer agents of reversible addition-fragmentation polymerization (RAFT). The resultant macro chain transfer agent was used to initiate the polymerization of the hydrophilic monomers N-isopropyl acrylamino (NIPAAm) and N, Ndimethylamino-2-ethyl methacrylate (DMAEMA), respectively. The gel permeation chromatography (GPC) results confirmed the successful synthesis of the amphiphilic copolymers PES-g-PNIPAAm and PES-g-PDMAEMA, and these two copolymers were perhaps the few examples of amphiphilic copolymer synthesized via a radical polymerization from PES main chains. The amino-substituted PES seemed a versatile precursor that showed a potential of functionalization via various strategies including click chemistry, atom transfer radical polymerization and RAFT polymerization. The synthesized amphiphilic copolymers were finally used as additives to improve the hydrophilicity and the filtration performances of PES membranes. Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved.

Keywords: Aminoesubstituted polyethersulfone Polymer membranes Amphiphilic copolymer

1. Introduction Free radical polymerization is robust in polymer synthesis, and a vast of materials has been produced through this process. During the past decades, the applications of the synthetic techniques had grown explosively, and the “living”/controlled polymerization especially attracted much attention [1,2]. Among the most popular controlled radical polymerization methods, atom transfer radical polymerization [3,4], stable free radical (mostly nitroxide) mediated polymerization [5e7], and reversible addition-fragmentation chain transfer polymerization [8,9] were found. Usually, these methods were conducted to design novel macromolecules with controlled composition, molecular weight, and desirable functional groups. For commercially available polymers, the controlled radical polymerization methods were also powerful in their modifications or functionalization. For instance, the poly(vinylidene fluoride) (PVDF), a commonly found membrane material, had been proved

* Corresponding author. MOE Key Laboratory of Macromolecule Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, 38# Zheda Road, Hangzhou 310027, PR China. Tel./fax: þ86 571 87953011. E-mail addresses: [email protected], [email protected] (L. Zhu).

an effective ATRP initiator of the synthesis of amphiphilic copolymer with the aid of the headehead conjunctions (defect sites) at the main chains [10e12]. Similar process had also been conducted for the PVDF-contained copolymer poly(vinylidenefluoride-cochlorotrifluoroethylene) P(VDF-co -CTFE), while the secondary chlorines in the copolymer were found serve as the initial sites though PVDF moieties were also exist in the copolymer [13]. However，the secondary chlorines in polyvinylchloride were too strong to initiate the ATRP polymerization, and foreign initiators were needed to be immobilized firstly via a wet chemical modification before the smooth modification of PVC [14,15]. Compared with its ATRP process, the click chemistry of polyvinylchloride was much easier conducted via a nucleophilic substitution of the chloride with azide sodium [16,17]. In addition, functional groups such as hydroxy in polymers were useful and widely made used of for polymers with no exist initiate sites, and ATRP or RAFT chains transfer agent (or initiator) could be facilely immobilized to these polymers via an esterification reaction, and the cellulose material was a generally found example [18]. The “initiator immobilized to” method was also applicable to the polymers without hydroxyl groups, while the reactive groups of hydroxyl groups and others could be produced via pretreatments such as irradiation and plasma modification [19e21]. Aromatic polymers such as

0032-3861/$ e see front matter Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2011.11.053

Z. Yi et al. / Polymer 53 (2012) 350e358

poly(ether imide) [22], poly(phthalazinone ether sulfone ketone) [23] and polysulfone [24,25] could also be modified via the controlled radical polymerization by introducing initial sites to the polymer through a nucleophilic substitution. However, the modification methods described above can not conducted smoothly on polyethesulfone (PES) because the high resonance effect of the sulfone groups in the polymers made PES a polymer with stable chemical structure, and incorporation of reactive groups to PES became difficult. In previous publications, the hydrophilic modification of PES was mostly carried out by using concentrated H2SO4 [26] or chlorosulfonic acid [27] as both solvents and reactive reagent to introduce sulfonated groups, while polymer degradation was always resulted in when PES suffered from these solvents. More importantly, these processes could not introduce functional groups that could serve as initial sites for the free polymerizations of hydrophilic monomers, which fact was perhaps a real reason that limited the PES based amphiphilic copolymers to few examples. Although the bromination of PES showed the potential of incorporation of Br for the further modification via the click chemistry, the degradation of molecular weight was also observed [28]. The reason of the above collections that presented the polymer degradation was the modifications were generally conducted with the commercial PES as starting material. As alternative method to obtain poly(aryiene ether sulfone)s containing sulfonated groups with higher molecular weight, Samperri [29] and Kim [30] showed a different method via condensed polymerization by using monomers as starting material. The hydrophilic poly(aryiene ether sulfone) produced via polycondensation method indicated a predictable molecular weight compared with the direct sulfonation of the commercial PES. Advantage of the method was the exactly known of the substituted position and the substitution degrees of the polymer because the structure of the feed monomer was definite. The sulfonated group was not a versatile group that could be used for the initial site immobilization, and the PES was also different from the structure of the poly(aryiene ether sulfone) described above. But it could be deduced from the molecular structure of the PES that the bis(4-fluorophenyl) sulphone or 4, 40 sulfonydiphenol can be used as one of the starting material, and the ideal structure of the other monomer might be 4, 40 -sulfonydiphenol or bis(4-fluorophenyl) sulphone containing hydroxyl and amine groups, since the hydroxyl or amine groups are reactive groups and much easier modified compared with sulfonated group. To design the experiment, we used bis(3-amino-4-hydroxyphenyl) sulfone as one of the starting monomer and the PES was synthesized via a condensed polymerization. The amino groups introduced to the polymer will endow PES with potential of facile modification via free radical polymerizations. In the present work, only RAFT modification was described by immobilization a carbonyl terminated RAFT agent to PES, and the resultant macro chain transfer agent was then used to initial the polymerization of hydrophilic monomers. The synthesis process of amino-substituted PES as well as the amphiphilic copolymer was investigated by the nuclear magnetic resonance, gel permeation chromatography and fourier transform infrared spectroscopy. At last, the synthesized polymers were tested as additives to improve the hydrophilicity of PES membranes, and membrane performance including contact angle, water flux as well as the morphology of membranes was investigated. 2. Experimental section 2.1. Materials Bis(3-amino-4-hydroxyphenyl) sulfone (99%), bis(4fluorophenyl) sulphone (99%) and 4, 40 -sulfonydiphenol (99%)

351

were all supplied by Sigma Aldrich and used without purification. Chain transfer agent S-Ethyl-S0 -(a, a’-dimethyl-a’’-acetic acid) trithiocarbonate (EMP) was synthesized following a previous publication [31]. N, N0 -dicyclohexylcarbodiimide (DCC, 99%) and 4, 6dimethyl-2-pyridinamine (DMPA, 98%) were purchased from Aladdin and used directly. N, N-dimethylamino-2-ethyl methacrylate (DMAEMA, Sigma Aldrich, 99%) was distillated from CaH2 prior to use. N-isopropyl acrylamino (NIPAAm, Sigma Aldrich, 99%) was recrystallized from hexane and toluene. Azobisisobutyronitrile (AIBN) was supplied by Shanghai Chemical Regent Company and recrystallized from ethanol. N, N-dimethyl acetamino (DMAc), toluene and other solvents were all dried by stirring with CaH2 and then distilled. Membrane used polyethersulfone (A100, Mw ¼ 53,500) was purchased from Solvay Company and dried in 90  C for 24 h before using. 2.2. Synthesis of an amino-substituted polyethersulfone In a representative procedure, bis(4-fluorophenyl) sulphone (12.7 g, 50.0 mmol), bis(3-amino-4-hydroxyphenyl) sulfone (5.61 g，20.0 mmol), 4, 40 -sulfonydiphenol (7.51 g, 30.0 mmol), DMAc (125 ml), toluene (100 ml) and K2CO3 were introduced to a three-neck round bottom flask equipped with a DeaneStark device. After 30 min nitrogen charging, the reaction mixture was transferred to an oil bath preheated to at 155  C, and last for 4.5 h under nitrogen atmosphere. The reaction was then heated to 180  C and last for 8.5 h. Water formed during the reaction was removed as an azeotrope with toluene. The reacted mixture was cooled to room temperature and precipitated into an excess of ether. After volatilization of the ether, the reserved monomers and K2CO3 in the product were removed by dispersing the polymer into de-ionized water and stirring. The resultant polymer was finally collected by filtration and fully drying in a vacuum oven. 2.3. Immobilization of the CTA Amino-contained PES (1.5 g) and RAFT agent S-Ethyl-S0 -(a, a’dimethyl-a’’-acetic acid) trithiocarbonate with 3 times excess (compared with the amount of amino groups in PES) were dissolve into 30 ml DMAc. After fully dissolving, the solution was cooled to 0  C with an ice bath, DCC and DMPA dissolved in dichloromethane were dropped to the mixture through a funnel, the reaction was then slowly heated to room temperature and terminated after 24 h process. The insoluble matter in mixture was removed by filtration, and the homogenous solution was then precipitated into an excess of ethanol. The CTA-immobilized PES was collected and further purified by several cycles dissolving in DMAc and precipitation in ethanol. 2.4. RAFT polymerization of the NIPAAm Typically, N-isopropyl acrylamino (NIPAAm) (2.85 g, 25.2 mmol), PES-CTA (0.2 g), Azobisisobutyronitrile (2.65 mg, 0.016 mmol) and DMAc (6.0 g) were added to a 25 ml round bottom flask equipped with a magnetic stir bar. The flask was sealed with a rubber septum, and the contents were purged with nitrogen for 30 min at room temperature. The flask was subsequently immersed in an oil bath preheated to the designed temperature, and the polymerization was preceded for 3 h before quenching in an ice bath. The raw product was obtained by precipitating the mixture into hot water, and monomer and homopolymer reserved in the product was further removed by dispersing the polymer into cool water and filtrated with the cellulose membranes (with pore size of 0.22 mm). The resultant polymer was collected and dried in a vacuum oven.

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2.5. RAFT polymerization of the DMAEMA

2.8. Polymer characterization

N, N-dimethylamino-2-ethyl methacrylate (3.3 g, 21 mmol), PES-CTA (0.2 g), Azobisisobutyronitrile (1.8 mg, 0.011 mmol) and DMAc (4.0 g) were added to a 25 ml round bottom flask equipped with a magnetic stir bar. The flask was sealed with a rubber septum, and the contents were purged with nitrogen for 30 min at room temperature. The flask was then transferred to an oil bath preheated to 70  C, and the polymerization was allowed to proceed for 5 h before being quenched by a rapid cooling in an ice bath. The product was obtained by precipitating the mixture into a KCl solution (1M/L). The salt and homopolymer reserved in the product was removed by dispersing the polymer into de-ionized water and filtrated with the cellulose membranes (with pore size of 0.22 mm). The resultant polymer was dried in a vacuum oven for 72 h to a constant weight. The evolution of molecular weight with the polymerization time was tracked by taking out the samples at the designed periods, and analyzed by GPC.

Gel permeation chromatography (GPC) of polymer was conducted at 30  C in DMF containing 0.5 wt% sodium nitrate with a flow rate of 1 ml/min, using a Waters 510 HPLC pump, Waters Styragel columns, and a Waters 410 differential refractometer (Millipore Corp., Bedford, MA). PMMA was used as a calibration standard. 1H NMR was performed in deuterated DMSO, using a Bruker DPX 400 spectrometer. The fourier transform infrared spectroscopy (FITR, Nicolet NEXUS 670) spectra were recorded with a dissolution of 4 cm1, and the wave-numbers was ranged from 700 to 4000 cm1.

2.6. Membrane fabrication The membranes in the present work were all prepared via the traditional non-solvent induced phase separation (NIPS) method using the synthesized amphiphilic copolymers as additives. PES and amphiphilic copolymer were co-dissolved in DMAc with a concentration of 20 wt%. The homogeneous solution was then left at room temperature for 24 h to completely release of air bubbles, the solution was subsequently cast to a glass plate with a casting knife of 250 mm, and immersed into a mixed coagulation bath constituted with DMAc and water (with value ratio of 40/60). The temperature of coagulation bath was 40  C. The compositions of casting solution were listed in Table 1. For the membranes with PES-g-PDMAEMA as additives, the amounts of the modifier in blend membranes were 10, 15 and 20 wt%, respectively, and the concentration of the PES-g-PNIPAAm in fabricated membranes were set as 15 and 20 wt%.

2.7. Membranes characterization Membranes hydrophilicity was evaluated with an OCA20 contact angle system (Dataphysics Instruments with GmbH, Germany). The reported values were averaged from five determinations conducted at random locations. The cross-section morphologies of membranes were inspected by scanning electron microscope (SEM, SIRION-100, FEI CO., Ltd) with an acceleration voltage of 25.0 kV after being sputtered with a thin gold layer, the membrane thickness was calculated with the aid of the Image Pro Plus software (Version 5.0). The water flux and rejection were tested with a cross-filtration system and the pressure was set as 0.10 Mpa. The concentration of bovine serum albumin (BSA) in feed solutions was 1 ㎎/ml (pH ¼ 7.4). Table 1 The compositions of the casting solutions for the membranes fabrication. Sample

M1 M2 M3 M4 M5 M6

Polymer (20 wt%)

Solvent (80 wt%)

Additive 1

Additive 2

PEG4000

PES

DMAc

0 2 3 4 0 0

0 0 0 0 3 4

2 0 0 0 0 0

18 18 17 16 17 16

80 80 80 80 80 80

Additives 1 and 2 are PES-g-PDMAEMA and PES-g-PNIPAAm, respectively.

3. Results and discussion 3.1. Synthesis of amino-substituted PES Fig. 1 shows the representative procedure for the synthesis of amino-substituted polyethersulfone. The bis(4-fluorophenyl) sulphone is used instead of the other bis(aryi halides) since its higher reactivity when fluorine serves as the leaving group [32]. The bis(3amino-4-hydroxyphenyl) sulfone is an function monomer which is supposed to be incorporated into PES backbones simultaneously with 4, 40 -sulfonydiphenol, and the substitution degrees of amino groups in PES can be tuned by changing the feed ratio of bis(3amino-4-hydroxyphenyl) to 4,40 -sulfonydiphenol in the polycondensation. As presented in Table 2, the synthesized PES is observed showing different affinity to three solvents that are water, ethanol and ether, which show the solubility parameter of 23.2, 12.7 and 7.4 (Mpa)0.5, respectively, and this different affinities of synthesized product to designed solvents implies the different polarity of PES when ratios of the bis(3-amino-4-hydroxyphenyl) sulfone in polymer vary [33], and the amino groups from the polymers are thought to be a reason that is responsible for its polarity (or hydrophilicity). When the condensed polymerization (Ⅳ) is conducted in the absence of the 4, 40 -sulfonydiphenol, namely, the polymer is synthesized with only bis(3-amino-4hydroxyphenyl) sulfone (BAPS) and bis(4-fluorophenyl) sulphone (FPS), the resultant product can be dissolved both in methanol and water while precipitates from diethyl. While when the proportion of the bis(3-amino-4-hydroxyphenyl) in the feed monomer decreases to 20 mol% (Ⅵ), the resultant polyethersulfone becomes insoluble in water and remains same solubility in the methanol. This reduced hydrophilicity is further observed when the ratio of bis(3-amino-4-hydroxyphenyl) in polyethersulfone is decreased to 10 mol% (Ⅶ), and the polymer synthesized under this condition shows a poor dispersion both in methanol and water. The hydrophilic PES was obtained by the sulfonation of commercial PES in previously publications [26,27], while in the present work we show another method to endow PES with hydrophilicity via introduction of amide groups to PES polymer. The different polarity of the synthesized polymer has implied the successful incorporation of the bis(3-amino-4-hydroxyphenyl) into polyethersulfone. Actually, the intact of amide group is also found in the NMR spectra that are shown in Fig. 2, and the newly appeared chemical shifts at 6.9, 7.0 and 7.7 ppm compared with neat PES can be assigned to protons in BAPS monomers. The clearly observed peak around 5.6 ppm is ascribed to the amino protons in the BAPS repeat units [33], confirming the intact of the amino groups in the synthesized polymers. The spectrum of PES without substitution is shown and serves as a reference. By analysis the NMR spectra, we find that the contents of the BAPS in the polyethersulfone is 25.3 mol% (Ia/Ib), and which result is approximate to the feed ratio of the monomers (20 mol%) in the polymerization. Herein, the consistence of the polymer constitution with the feed

Z. Yi et al. / Polymer 53 (2012) 350e358

353

Fig. 1. Scheme illustration of the synthesis process for the amino-substituted polyethersulfone and the immobilization of the RAFT chain transfer agent.

monomers may be understood that the BAPS incorporated into the synthesized polymer can be tuned by adjusting the feed ratio of the starting monomers, and the substituted degrees of amino groups in the synthesized PES is controllable. Except for the adjusting the hydrophilicity of the synthesized PES, the molecular weight can be simultaneously tuned when the feed ratio of the bis(3-amino-4-hydroxyphenyl) to 4,40 -sulfonydiphenol is different. For example, the PES with the same structure as the commercial PES can be synthesized with bis(4-fluorophenyl) sulphone and 4,40 -sulfonydiphenol, while molecular weight degradation is observed when reaction time is prolonged to 20 h, and the result indicates the thermal degradation of the PES will resulted in when the reaction time is too long (These two polymerizations are shown by the polymerization of I and Ⅱ). To protect the synthesized polymer from degradation, the polycondensation in the present work is conducted within 10 h. Under the designed polymerization conditions, the PES with the highest amino substitutions can be synthesized by the alternant condensation of the bis(4-fluorophenyl)sulphone and bis(3-amino-4-hydroxyphenyl), while gelation is found when the reaction lasts for 8.5 h at the temperature of 190  C. But the gelation can be avoided when the temperature is lowered to 160  C, and the synthesized polymer presents the molecular weight of 21,766 g/mol and PDI of 1.76 (shown by the polymerizations of Ⅳ). When the bis(3-amino-4-

hydroxyphenyl) is partly substituted by 4,40 -sulfonydiphenol and the ratio of bis(3-amino-4-hydroxyphenyl) in monomer decreases to 20 mol%, the polymerization can be processed under a higher temperature, and the resultant polymer shows a molecular weight of 22,601 g/mol. The successful synthesis of the PES is further found when the BAPS is set as 10 mol% under the similar condition, and the product presents the MW of 23,838 g/mol and polydispersity index of 1.84 (shown by the Ⅷ). It is worth noting that the polydispersity index (PDI, Mw/Mn) of the amino-substituted PES synthesized in the present work are approximate to the product of polymerization of I and Ⅱ, which result implies that the aminosubstituted PES are perhaps linear structured and the gelation is not resulted in. Actually, the amino-contained monomers are also used in the previous publications in the synthesized aminofunctionalized poly(arylene ether ketone)s, and no side reactions were inspected when the polymerization were conducted under the same temperature as that was adopted in the present work [34]. The polymerization of Ⅷ is similarly conducted with Ⅵ while the reaction time is shorted to 4 h, and the resultant polymer shows a lower PDI (Mn ¼ 9,947 g/mol) compared with the same polymerization that processes for 8 h. In the current work, the polymer from Ⅷ is served as a precursor for the further modification.

Table 2 Solubility and molecular weights of the synthesized polyethersulfone.

The amino groups in the synthesized PES are readily reacted with RAFT agent containing carboxylic groups, thus the immobilization of initial sites to the polymer. Fig. 3 indicates the FT-IR spectra of the amino-substituted PES (a) and PES immobilized with RAFT agents (b). As presented in the spectra, the synthesized PES with amino groups at main chains shows a strong adsorption around 3350 cm1, which can be ascribed to the stretching of eNeH [34,35] （the moisture is obviously observed because of the hydrophilicity and water adsorption of the synthesized polymer）. However, when amino groups are being reacted with S-Ethyl-S0 -(a, a’-dimethyl-a’’-acetic acid) trithiocarbonate (EMP), newly appeared adsorptions at 1754.8 cm1 and 1640 cm1 are found. In universally found publications, the adsorptions around 1750 cm1 and 1640 cm1 are assigned to different cases that: a, the adsorption

ID

Ⅰ Ⅱ Ⅲ Ⅳ Ⅵ Ⅶ Ⅷ

Monomer (mmol)

Conditions

Solubility

Mn/PDI

A

B

C

Tem ( C)

Time (h)

Ethanol Water

Ether

6.8 6.8 6.8 6.8 7.0 7.0 7.0

/ / 6.8 6.8 2.8 1.4 2.8

6.8 6.8 / / 4.2 5.6 4.2

190 190 190 160 180 180 180

7.0 20.0 8.5 8.5 8.5 8.5 4.0

   O O Turbid Turbid

Turbid Turbid     

   O Turbid Turbid Turbid

59,315/1.76 23,415/1.73 Gelation 21,766/1.75 22,601/1.76 23,838/1.84 9,947/1.41

Monomers: A: bis(4-fluorophenyl)sulphone; B: bis(3-amino-4-hydroxyphenyl) sulfone; C: 4,40 -sulfonydiphenol. The symbols  and O mean the insoluble and soluble, respectively.

3.2. Immobilization of the RAFT agent

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Fig. 3. The FT-IR spectra of (a) Amino-substituted polyethersulfone, PES-NH2; (b) CTAimmobilized PES, PES-CTA; (c) The PES-NH2 being reacted with 2-bromopropionic acid, PES-Br; (d) The PES-NH2 being reacted with BOC, PES-BOC.

adsorption at the 1743 and 1655 cm1 again, and the result consists well with the simultaneous appearing of the two adsorptions observed in the spectra of PES immobilized with RAFT agent and BOC. Compared with the amino-contained polyethersulfone precursor, the NMR spectrum of CTA-immobilized polyethersulfone presents new peaks at 1.35, 1.75 and 3.35 ppm, and they are assigned to three protons dedicated in the insert images in Fig. 4. The good consistence of the result with the literature indicates the successful immobilization of the RAFT agent [31]. Actually, after immobilization of the CTA, the peak around 5.6 ppm is still visible， and the result consist well with the data from the integration intensity of Ie/Ib that 45.3 mol% of the amino groups has reacted with EMP, and the result indicates that nearly 1.86 CTAs are immobilized in per 10 repeat units in the PES. The incompletely reaction of the amino groups is also observed in the FT-IR spectra by showing the reserved adsorption at 3350 cm1 that are found for PES contained amino groups. Fig. 2. The representative NMR spectra of amino-substituted polyethersulfone(x ¼ 3, y ¼ 2, shown in Table 1 by No. 4), and the spectra of PES without substitution is shown as a reference.

around 1750 cm1 is independently assigned to ester bonds [36]; b, the adsorption around 1650 cm1 is ascribed to amino I band [37]; c, the 1750 cm1 and 1650 cm1 are simultaneously resulted from the adsorption of amino I band [38]; d, the 1750 cm1 and 1650 cm1 are assigned to the ester and amide bonds that are simultaneously exist in the product. To find out that the adsorption at 1754 cm1 is a result of the amide bond formation rather than the ester bonds, we respectively conducted the amidation of the synthesized PES with 2-bromopropionic acid and di-tert-butyl dicarbonate (BOC). For the latter case, the eNH2 is specified reacted and amide I band will be produced. Not surprisingly, the adsorptions at 1762 and 1652 cm1 are observed appearing as a pair again when the PES is reacted, the similar adsorption implies the success immobilization of the RAFT agent and the formation of amino bonds that are found in spectra b. This result is further confirmed when the amino-substituted PES is reacted with 2bromopropionic acid, in which case the spectrum shows the

Fig. 4. The NMR spectrum of the CTA-immobilized polyethersulfone (x ¼ 3, y ¼ 2, shown in Table 1 by no. Ⅷ).

Z. Yi et al. / Polymer 53 (2012) 350e358

3.3. Synthesis of the amphiphilic polymer The successful immobilization of the CTA to PES is confirmed by the increases of molecular weight from 9,947 to 10,845 g/mol after the reaction. The PES immobilized with CTA can be used as macro chain transfer agent to initiate the polymerization of hydrophilic monomers, and the PES-based amphiphilic copolymers are synthesized (shown in Fig. 5). In the present work, the hydrophilic monomers NIPAAm and DMAEMA are tested as examples and the polymerization conditions are shown in Table 3. Clearly it is found that, compared with the PES-CTA (Mn ¼ 10,845 for PES-CTA), the copolymer NIPAAm-0 shows a slight molecular weight increase of 460 g/mol after 16 h reaction, indicating that the polymerization has not been effectively initiated under this condition (in which experiment the mole ratio of CTA to AIBN is 1.63, based on NMR result). However, the successful polymerization is found when the ratio of CTA to AIBN is decreased to 1.06 by increase the feeding amount of AIBN, and the molecular weight increases with the increase of polymerization time. As shown in Table 3, the molecular weight of the copolymer NIPAAm-1, NIPAAm-2 and NIPAAm-3 are 11,611, 15,333 and 31,448 g/mol, respectively. It is worth noting that the product shows a little increase of the polydispersity in the time period of 2 h, but the polydisperisity index (PDI) increases dramatically for the polymerization time conducted for 3 h, which perhaps results from the inter- or intra- coupling of molecules. The hypothesis is confirmed by the GPC curves that mutil-distributions are found for the NIPAAm-3 (in Fig. 6). Though the product shows a broad molecular distribution and resulted from the radicale radical coupling, the radical coupling under this conditions is suggested to remain in a low level because the copolymer NIPAAm3 dissolves well in solvents. The successful polymerization is also found for the monomer DMAEMA, and the product of DMAEMA-5 shows a molecular weight of 21,802 g/mol and PDI of 1.86. The polymerization inhibition or slow propagation is again found when the ratio of CTA to AIBN is 2.0, in which case the mass ratio of PESCTA to AIBN is 200:1.5. In previous experiment, the cause for this inhibition may either be associated with the leaving group of the initial RAFT agent or with the slow fragmentation of the initial intermediate macro-RAFT radical [39].

355

Since the macro chain transfer agent is a product from the polycondensation, the polymer itself shows a broad molecular weight and makes the analysis of the polymerization kinetics become complex. However, it will be found that the CTAimmobilized PES presents a better controlled manner for the NIPAAm when the conversion remains in a low level, and this result is observed for the NIPPAm-1 and NIPAAm-2 that show the PDI of 1.48 and 1.50, respectively (The PDI of PES-CTA is 1.48, and MW is 10,845). While coupling is found when the polymerization is prolonged to 3 h, under which conditions the monomer conversion is assessed to be 32% (based on NMR of the purified polymer).The increasing length of the grafting chains are supposed to increase the hindrance of the polymers diffusing, which will cause a higher probability for the radical-coupling of molecules. Actually, a dramatic increase of the viscosity observed during the experiment also confirms the explanation that the hindrance of diffusing increased when polymerization proceed. However, a poor controlled manner is clearly found for the DMAEMA-5 and DMAEMA-05, especially considered that the DMAEMA-05 shows a molecular increase of about 1,400 g/mol while the PDI increased obviously from 1.48 to 1.65. The phenomena was also found in the previous publications that RAFT agent S-Ethyl-S0 -(a, a’-dimethyl-a’’acetic acid) trithiocarbonate (EMP) shows a better controlled manner for the propenyl monomers compared with that of the methacrylate based monomers [31]. Based on the results from GPC, the weight percents of the hydrophilic component in NIPAAm-3 and DMAEMA-5 copolymers are assessed to be 29.3 and 50.3 wt %, respectively. 3.4. TGA of the synthesized copolymer The thermal stability of the polymer synthesized in the current work has been analyzed by the TGA, and the results are shown in Fig. 7. As a control sample, the PES without substitution shows a one-step degradation from the temperature of about 492  C [40], while the polymer with amino-substitution at the main chains shows the degradation from temperature of 410  C, and this decrease in the thermal stability compared with virgin PES can be ascribed to the disruption of the resonance effect at the main chain [26,27]. Though the three polymer PES-NH2, PES-g-PDMAEMA and PES-g-PNIPAAm are dried under the same condition as that of the PES，moisture and weight loss is determined at the temperature lower than 100  C，and the result may be resulted from the water bonded to the polymers because of their hydrophilicity. Amphiphilic copolymer PES-g-PDMAEMA and PES-g-PNIPAAm show the PES degradation at the temperature of 407  C, while this degradation is perhaps an apparent decomposition of the PES as well as the main chain from the hydrophilic blocks, because PDMAEMA homopolymer was reported to present two decompositions at 279.9 and 415  C, and they were usually assigned to the linkage of ester group and the degradation of main chain methacrylate in PDMAEMA, respectively [41]. The obvious decomposition starting from 270  C in the current spectrum can be ascribed to the degradation of the PDMAEMA incorporated to PES-g-PDMAEMA, and the results also indicates the successful synthesis of the copolymer. The literature had found that the PNIPAAm homopolymer showed a one-step degradation at the temperature of 420  C [42], and this degradation is just overlapped with the degradation of the PES. 3.5. Hydrophilicity of the blend membranes

Fig. 5. Scheme illustration of the synthesis process for the amphiphilic copolymers PES-g-PNIPAAm and PES-g-PDMAEMA.

The synthesized amphiphilic copolymers are tested as hydrophilic modifiers for PES membranes. Fig. 8 shows the static contact angle of the blend membranes prepared in the present work. Compared with the neat PES membrane, the modified membranes

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Table 3 Polymerization conditions for the synthesis of amphiphilic copolymer. ID

Monomer (g)

PES-CTA (g)

AIBN (mg)

DMF (g)

Temp ( C)

Time (h)

Mn

Mw/Mn

NIPAAm-0 NIPAAm-1 a NIPAAm-2 NIPAAm-3 b DMAEMA-5 DMAEMA-05

2.85 2.85 2.85 2.85 3.30 3.30

0.280 0.200 0.200 0.200 0.200 0.200

2.60 2.85 2.85 2.85 1.80 1.50

6.00 6.00 6.00 6.00 4.00 4.00

75 65 65 65 70 70

16 1 2 3 5 5

11,309 11,611 15,333 31,448 21,802 12,242

1.48 1.48 1.50 2.10 1.86 1.66

a and b: These two copolymers were used as membrane additives in the present work.

shows obviously improved hydrophilicity, and the blend membranes contains 10 and 20 wt% PES-g-PNIPAAm present the initial contact angle of 77 and 72 , respectively ( denoted by b and c in Fig. 7, respectively). Even obvious hydrophilicity improvement is found for membranes modified by PES-g-PDMAEMA, and the initial contact angle of 65 and 56 are respectively found for the membranes contains 15 and 20 wt% additives (denoted by d and e, respectively). This different modification efficiency is thought to be mainly resulted from the different hydrophilic contents in the two copolymers, and the contents of the hydrophilic component in PESg-PNIPAAm is assessed to be a half of that in PES-g-PDMAEMA (shown in Table 3). For porous membranes, the surface contact angle is a comprehensive result from surface roughness and hydrophilic contents [43]. Similarity in the present work, the wettability of blend membranes is suggested to be enhanced by the increased pores size and amounts because of the pore-forming ability of amphiphilic copolymer [43,44]. The obviously improved hydrophilicity of blend membranes compared with neat PES membranes indicates that the grafting copolymers are effective modifiers for PES membranes modification.

Fig. 9 shows the cross-section morphologies of blend membranes fabricated in this work. It is firstly found that the blend membranes become porous with increasing content of the amphiphilic copolymer in membranes (from B to D). Besides the large finger-like pores, the sub-pores in the space that separate the finger pores become larger simultaneously, which is another evidence of the pore-forming effect of amphiphilic copolymer and

it is usually ignored (the macroviod or large pore are always focused). Although pore-forming effects are both found for the PEG4000 and the amphiphilic copolymer, the amphiphilic polymers present the difference by showing the obvious formation of the smaller pores located in the space between large pores. The sub-pores formation is easily understood from the aspect of the amphiphilic structure for the synthesized copolymer. When the phase separation of cast films is induced, the hydrophilic components (PDMAEMA or PNIPAAm) are tended to assemble together because of the incompatibility of hydrophilic component with the PES, while dramatic migration of hydrophilic chains is suppressed since of its linkage by the hydrophobic chain. As the third component, DMAc is a solvent that either assembles with the hydrophilic parts (sub-pores) or gather together independently to form a separated phase (larger pores) when the phase separation of the cast solution starts, and which two parts correspond to the formation of the sub-pores as well as the larger pores. Because of the absence of the entanglement between PEG and PES, PEG is mainly tended to gather with DMAc to form solvent-riched phases and only larger pores are formed. The increased pores in separated space between larger pores are also thought to increase the interconnecting of pores, and the filtration resistance decreases. Although the polymer concentrations of the casting solutions are the same, the thicknesses of the as-formed membranes are different. As shown in Table 4, the pure membrane shows the least thickness of 97.6  1.46 mm, while blend membranes with the PES-g-PDMAEMA as modifier reveal the gradually increasing values, and they show the results of 133.4  1.24, 138.3  0.54 and 174.8  1.21 mm for membranes with additives concentration of 10, 15 and 20 wt%, respectively. This increase of the thickness of blend membranes is ascribed to the

Fig. 6. Representatives GPC results of amphiphilic copolymer PES-g-PDMAEMA and PES-g-PNIPAAm.

Fig. 7. The TGA spectra of the polymer PES, PES-NH2, PES-g-PDMAEMA and PES-gPNIPAAm.

3.6. Membranes structure and filtration performance

Z. Yi et al. / Polymer 53 (2012) 350e358

Fig. 8. Static contact angles of blending membranes prepared by using amphiphilic PES-g-PNIPAAm (10 and 20 wt% for b and c, respectively) and PES-g-PDMAEMA (15 and 20 wt% for d and e, respectively) as additives, and results of neat PES membrane is also shown (a).

357

much higher molecular weight of the amphiphilic additives (Mn ¼ 21,802 g/mol) compared with that of the polyethylene glycol 4000. The increased thickness compared with neat PES membranes is also found for membranes modified by PES-gPNIPAAm that shows thickness of 105.5  1.34 and 107.4  1.36 mm for membranes containing 10 and 20 wt% additives, respectively. But the membranes flux seems not obviously influenced when the thickness increases, and the modified membranes shows obviously higher flux compared with that of the pure membrane. It is also worth noting that no permeability is determined for pure PES membrane under the pressure of 0.1 Mpa while water permeates easily for blend membranes under the same pressure. Except for the hydrophilicity improvement (shown in Fig. 8), the increased porosity is also responsible for the improved flux of blend membranes [45], and higher porosity indicates the decreased permeation resistance. The explanation can be easily viewed from the pore size as well as the amount in cross-section that is shown in SEM images. The PNIPAAm and PDMAEMA are well-found thermal-responsive polymers, and the amphiphilic copolymer with these constitutions will show similar stimulate-responsive performance when they are introduced into membranes [46,47], and the switch-off behavior of blend membranes is being conducted and will be reported later.

Fig. 9. Cross-section morphologies of blend membranes fabricated in the present work (A: neat PES; B, C, D: with 10, 15, 20 wt% PES-g-PDMAEMA, respectively; E, F: with 10 and 20 wt% PES-g-PNIPAAm, respectively).

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References

Table 4 Membranes thickness and the filtration performance. Membranes a

PES 10 wt% 20 wt% 10 wt% 15 wt% 20 wt% a

PES-g-PNIPAAm PES-g-PNIPAAm PES-g-PDMAEMA PES-g-PDMAEMA PES-g-PDMAEMA

Thickness (mm) 97.6 105.5 107.4 133.4 138.3 174.8

     

1.46 1.34 1.36 1.24 0.54 1.21

Flux (L/m2.h)

Rejection (%)

18.76 82.4 110.2 17.53 115.3 126.7

99.0 99.0 97.5 98.1 97.0 96.4

determined under the pressure of 0.2 Mpa with a cross-filtration device.

4. Conclusions In the present work, we described a facile method to synthesize an amino-substituted polyethersulfone (PES) that could be served as a precursor for the immobilization of initial sites for the ATRP, RAFT, click chemistry and free radical polymerization. The substitution degrees of amino in PES could be facilely tuned by changing the feed ratio of the BAPS monomer to the bis(4-fluorophenyl) sulphone. The synthesized polymer was reacted with the RAFT agent to result in a PES-based macro chain transfer agent, and two amphiphilic copolymers grafted from PES via radical polymerization were described. The macro-CTA showed a good polydispersity evolution with NIPAAm monomer, while tended to de-activate with DMAEMA monomer. Finally, the amphiphilic polymer PES-gPDMAEMA and PES-g-PNIPAAm were examined as hydrophilic modifiers of PES membranes. It was found that the modifiers were effective in modification and the initial contact angle could be obviously reduced to 56 . The water fluxes of modified membranes increased several times while the retentions of the BSA did not changed greatly. The amino-substituted PES copolymers shows a potential as a versatile polymer precursor that can be grafted with various polymer chains by ATRP, click chemistry and free radical initial sites etc., which offers a broad platform for the functionalization of PES. The candidates of the amphiphilic modifiers for the hydrophilic modification of PES membranes are also extended greatly.

Acknowledgments The authors acknowledge the financial support from the National Basic Research Program of China (973 Program of China, Grant no. 2009CB623402), the National Nature Science Foundation of China (Grant No.50803054), the National High Technology Research and Development Program of China (863 Program of China, Grant No. 2009AA062902), Zhejiang Provincial Sci & Tech Plan of China (Grant No. 2010C31028), and the Key Innovation Team for Science and Technology of Zhejiang Province, China (2009R50047).

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