氧化多壁碳纳米管、PEG及PVP添加剂对PVDF膜性能的影响

氧化多壁碳纳米管、PEG及PVP添加剂对PVDF膜性能的影响

作者：徐海朋 陈兴凡 张旋 郎万中

来源：《上海师范大学学报·自然科学版》2016年第01期

Abstract：Introducing hydrophilic additives is an effective method to improve the structure and permeation performance of polyvinylidene fluoride （PVDF） membranes.In this work，oxidized multiwall carbon nanotubes （OMWNTs） accompanied with polyethylene glycol/polyvinyl pyrrolidone （PEG/PVP） additives were employed to modify PVDF membranes.The effects of additives on the structure，permeation performances，hydrophilicity and crystallization behavior of the prepared PVDF membranes were investigated in detail.The results indicate that the addition of OMWNTs can enhance the permeation flux，hydrophilicity and mechanical property of PVDF membranes.The pure water flux（PWF） attains 222.9±12.5 L·m-2·h-1·bar-1 and 256.9±14.8 L·m-2·h-1·bar-1 by adding 0.6 wt.% OMWNTs accompanied with 5% PEG 200 and 3% PVP in the cast solution，respectively.PEG200 additive can enhance the dispersion of OMWNTs，and accelerate the exchange rate between solvent and nonsolvent.PVP working as poreforming agent promotes the growth of pores of PVDF membranes and pure water permeation flux.

Key words：polyvinylidene fluoride （PVDF） membrane； ultrafiltration； multiwall carbon nanotubes （MWNTs）

CLC number： TQ 31 Document code： A Article ID： 10005137（2016）01004109 1 Introduction

In the recent years，polyvinylidene fluoride （PVDF） has become a very popular material to prepare microfiltration/ultrafiltration（MF/UF） membranes for its superior thermal，chemical stability and oxidation resistance etc.[1-4].However，due to its strong hydrophobicity，PVDF membranes are easily fouled by natural organic matters （NOMs），proteins and other substances.To overcome the shortcomings，various ways were explored to modify PVDF membranes.Among these methods，blending modification is widely used because it makes preparation and modification be accomplished in a single step.And numerous nanofillers were employed to modify PVDF membranes[5-8].

For its high specic surface area，high chemical stability and easy possessing property，multiwall carbon nanotubes （MWNTs） also attracted considerable attentions in membrane

field[9].Zhao et al.[10]successfully prepared PVDF/MWNTs membranes and the blending membranes exhibited improved hydrophilicity，pure permeation flux（PWF） and higher bovine serum albumin （BSA） rejection.Ma et al.[11]investigated the effects of oxidized MWNTs （OMWNTs） on

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PVDF membranes and found that oxygencontaining groups of OMWNTs played a critical role in determining the morphologies and performances of PVDF membranes.However，there are two important issues to retard the application of MWNTs in polymer composites[12].First，it tends to aggregate in the casting solution due to the strong Vander Waals forces.Moreover，smooth surface of CNTs and lack of interfacial bonding will result in weak interfacial adhesion between CNTs and polymer matrix[13].So，lots of works were done to enhance the dispersion and interfacial adhesion[14-16].

In this work，OMWNTs were used as fillers to improve PVDF membrane

performances.Polyethylene glycol（PEG） and/or polyvinyl pyrrolidone （PVP） worked as an auxiliary is used to enhance the dispersion of OMWNTs.The influences of different content of OMWNTs，PEG on the membrane performances were studied in detailed. 2 Experimental 2.1 Materials

PVDF in powder form was supplied by Shanghai 3F New Material Co.Ltd.（China）.Raw multiwall carbon nanotubes （RMWNTs，diameter 10～20 nm，length 10～15 μm，purity > 97.0wt.%） were purchased from Shenzhen Nanotech Port Co.Ltd.（China）.Nmethyl2pyrrolidone （NMP） and ethanol were provided by Shanghai Chemical Agent Co.Ltd.（China）.PVP （K30，MW=58，000） in powder form and PEG200 were purchased from Shanghai Aladdin Chemical Agent Co.Ltd.（China）.Deionized water （DI） was selfproduced by a reverse osmosis （RO） system.BSA （MW=67 000） was purchased from Shanghai Bio Life Sci.&Tech.Co.，Ltd.（China）.All reagents were not further purified before used. 2.2 Preparationand characterization of OMWNTs

Nitric acid vaporoxidation method was adopted to prepare OMWNTs，the detailed preparation procedures and characterizations were presented in our recent works[17-18]. 2.3 Membrane preparation

ThePVDF/OMWNTs membranes were prepared by phase inversion method.Different amounts of polymer and solvent were mixed and stirred at 70℃ for 24 h to get homogenous dope solutions.The detailed parameters for membrane preparation were summarized in Table 1.

The membranes were cast on a glass plate.The cast intrinsic films were quickly immersed into water which was used as coagulation bath.After peeled off from the plate，the obtained membranes were immersed into deionized water bath for 48 h to remove the residual solvent. 2.4 Characterizations of PVDF/OMWNT membranes

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2.4.1 Morphologies

Themorphologies of PVDF/OMWNTs membranes were tested by a field emission scanning electron microscopy （FESEM，Hitachi S4800，Japan）.The crosssections were obtained by

fracturing the membranes in liquid nitrogen.The samples were placed onto a copper holder and coated with gold.The morphologies of PVDF/OMWNTs membranes were observed under different magnifications.

2.4.2 Dynamic contact angle

A contact angle analyzer （KRSSDSA30，German） was used to detect the dynamic

contactangles of PVDF/OMWNTs membranes.A drop of 3.0 μL DI water was dropped on the top surfaces of PVDF/OMWNTs membranes at room temperature.A camera enabling image capture at 1 frames/s was coupled with the machine. 3 Results and discussion 3.1 Membrane morphologies

The FESEM images of the crosssections，top surfaces and bottom surfaces of the resultant PVDF/OMWNTs membranes are shown in Figs.1-3.From Fig.1，all the membranes present an asymmetric structure with fingerlike structures near top surface which are the typical structure of instantaneous precipitation process[19].M1 and M2 exhibit spherulite grains near the bottom surface which are different from other membranes.This is the result of PVDF nuclei growth.For M1 and M2 membrane，the intrinsic hydrophobic properties of PVDF retard the rate of phase demixing rare without PEG or PVP auxiliary[20].Therefore，PVDF nuclei have enough time to growth.For M3-M5，the fingerlike structure grows bigger and spherulite structure turns into spongelike structure near the bottom surface with the increase of OMWNTs content.From Fig.1，the pore size in the

spongelike structure of M6 and M7 is bigger than other membranes.It verifies that PVP can induce the formation of fingerlike structure and macrovoids for its high molecular weight[21]. 3.2 Dynamic contact angle

The hydrophilicity of PVDF/OMWNTs membranes was tested by contact angle measurements and the results are shown in Fig.4 and Table 2.According to Fig.4 and Table 2，the final contact angle increases from 69.7° for M1 to 82.7° for M2 with the addition of PEG200.That is because PEG200 can accelerate the exchange rate between solvent and nonsolvent.Thus，the top surface of M2 is denser than that of M1，which results in the higher contact angle for M2.Meanwhile，the contact angle increases from 62.5° for M6 to .2° for M7 which is similar to M1 and M2.With the addition of OMWNTs from 0.0 wt.% to 0.6 wt.% for M2-M5，the contact angle decreases from 82.7° to 71.4°，because OMWNTs can form hydrogen bonds with water molecules on membrane surface.For M6 and M7，the macrovoids on the surface benefit for the permeation of water into membranes.So，their contact angles are smaller than those of other membranes.

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3.3 Permeation performances

As we all know，many parameters such as porosity，interconnection of cavities，surface pore size and surface hydrophilicity are closely related to the PWFs of membranes[8].The PWFs and BSA rejections of prepared PVDF membranes are shown in Table 2.It can be seen that M2 shows relatively high BSA rejection and low PWF than M1 for the addition of PEG 200 which is similar to the difference between M6 and M7.For M2～M5，the PWFs increase from 73.7±2.7 L·m-2·h-1·bar-1 for M2 to 222.9±12.5 L·M-2·H-1·bar-1 for M5 with the gradual addition of OMWNTs.Meanwhile，the BSA rejections are also improved with the increase of OMWNTs in the membranes.The reason is that the higher viscosity of dope solutions with higher OMWNTs content may result in denser membrane structure and can improve the BSA rejections of membranes.It can be concluded that OMWNTs can significant by improve the permeation performance of PVDF membranes.Also，the changes of PWFs and BSA rejections are highly highly in accordance with the morphological variations of PVDF/OMWNTs membranes shown in Figs.1-3. 3.4 Crystallization behaviors

The crystallizat behaviors of PVDF/OMWNT membranes were detected by ATRFTIR and WXRD （Figs.5～6）.ATRFTIR spectra of M5 and M6 exhibit a peak at 16 cm-1 which is ascribed to the existence of PVP in membrane matrix[22].For each sample，there are welldefined absorption bands at 840，877，1171，1180，1276，and 1403 cm-1，which are the indications of β phase of PVDF crystal[23].Meanwhile，it can be drawn that all PVDF membranes demonstrate similar XRD patterns.The peaks at around 18.6°±0.2°[24]，20.6°±0.2°[25]and 36.5°±0.3°[26]are referred to the characteristic diffractions of β phase. 3.5 Mechanical properties

The introduced additives have different effects on the mechanical properties of PVDF/OMWNTs membranes for their critical effects on membrane structures.The mechanical properties of

PVDF/OMWNTs membranes are shown in Table 3.It can be seen that the tensile strength increases from 0.59±0.02 Mpa for M2 to 1.91±0.05 Mpa for M5.The results verify that OMWNTs can improve the mechanical properties of PVDF membranes.It may due to the fillers sufficiently bound to PVDF chains and the decreased thickness of spongylike structures which are shown in Fig.1.Comparing M2 to M1 and M7 to M6，we found that the addition of PEG has no promoting effect on mechanical properties of PVDF membranes. 4 Conclusions

In this work，OMWNTs accompanied with PEG/PVP additives are used to modify PVDF membranes.The structures，permeation performances，hydrophilicity，crystallization behaviors and mechanism properties of the prepared PVDF membranes are explored.According to the results，the addition of OMWNTs can enhance the permeation performances，hydrophilicity and mechanical properties of PVDF membranes.The PWF attains 222.9±12.5 L·m-2·h-1·bar-1 and BSA rejection is

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68.6±1.0% with 0.6 wt.% OMWNTs in the cast solution.PEG200 additive can enhance the dispersion of OMWNTs in cast solution，and accelerate the exchange rate between solvent and nonsolvent.PVP working as poreforming agent promotes the growth of pores of PVDF membranes and pure water permeation flux. References：

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摘要：通过引入亲水性添加剂是改善膜聚偏氟乙烯（PVDF）膜亲水性和膜结构的一种有效方法.在本研究中，利用氧化碳纳米管（OMWNTs）和聚乙二醇（PEG）及聚乙烯吡咯烷酮（PVP）修饰PVDF膜.对添加剂对PVDF膜的结构、渗透性能、亲水性、结晶行为等影响进行了详细研究.结果表明：加入OMWNTs可以提高PVDF膜的渗透通量、亲水性和机械性能.当铸膜液中添加0.6% OMWNTs和5% PEG 200或者0.6% OMWNTs和3% PVP时，所制PVDF膜渗透通量分别达到222.9±12.5 L·m-2·h-1·bar-1和 256.9±14.8 L·m-2·h-1·bar-1.添加剂PEG200可以强化OMWNTs的分散性，并能够促进相转化过程中溶剂与非溶剂间物质交换.PVP作为致孔剂能够促进孔生长和纯水通量提高.

关键词：聚偏氟乙烯膜； 超滤； 多壁碳纳米管 （责任编辑：郁 慧）

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