现代化工

现代化工 ›› 2025, Vol. 45 ›› Issue (4) : 90-95. DOI: 10.16606/j.cnki.issn0253-4320.2025.04.017

科研与开发

海绵状结构聚醚砜膜的制备及性能表征

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Preparation and performance characterization of sponge-like polyethersulfone membrane

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摘要

为了制备高通量海绵状结构聚醚砜平板膜,利用浊点滴定法确定聚醚砜(PES)/N,N-二甲基乙酰胺(DMAc)/一缩二乙二醇(DEG)三元体系浊点相图,通过改变铸膜液邻近比α和膜厚度对PES平板膜结构进行调控。结果表明,随着邻近比α的增大,PES平板膜表面由致密变为多孔结构,膜截面由指状孔结构变为海绵状结构,膜的纯水通量逐渐增大;增大膜厚度,PES平板膜截面结构先保持海绵状结构,后变为指状孔结构。

Abstract

To prepare high-flux sponge-like polyethersulfone (PES) flat membrane,the cloud point titration method is used to determine the cloud point phase diagram of polyethersulfone/N,N-dimethylacetamide/diethylene glycol (PES/DMAc/DEG) ternary system.The structure of PES flat membrane is regulated through varying the proximity ratio (α) of the casting solution and the membrane thickness.Study findings show that as α increases,the surface of PES flat membrane transitions from dense to porous structure,and the membrane cross-section changes from finger-like pore structure to sponge-like structure,resulting in gradually increasing in pure water flux of the membrane.As the membrane thickness increases,the cross-section structure of PES flat membrane initially maintains the sponge-like structure,but later transforms into a finger-like pore structure.

Graphical abstract

关键词

浊点 / 相转化 / 海绵状结构 / 聚醚砜 / 相图

Key words

cloud point / phase inversion / sponge-like structure / polyethersulfone / phase diagram

Author summay

黄启顺(2000-),男,硕士生,研究方向为分离膜的制备及应用研究,qshuang0324@163.com。

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ext=[AuthorCompanyExt(id=1241353549147238764, tenantId=1226480274814496768, journalId=1226484258170171394, articleId=1240719988673180184, companyId=1241353549138850155, language=EN, country=null, province=null, city=null, postcode=null, companyName=null, departmentName=null, remark=Functional Membrane Science and Engineering Research Center, State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China), AuthorCompanyExt(id=1241353549151433069, tenantId=1226480274814496768, journalId=1226484258170171394, articleId=1240719988673180184, companyId=1241353549138850155, language=CN, country=null, province=null, city=null, postcode=null, companyName=null, departmentName=null, remark=华东理工大学化学工程联合国家重点实验室,功能膜科学与工程研究中心,上海 200237)])])] 黄启顺,魏永明,彭阳峰. 海绵状结构聚醚砜膜的制备及性能表征[J]. , 2025, 45(4): 90-95 DOI:10.16606/j.cnki.issn0253-4320.2025.04.017

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聚醚砜(PES)是一种常见的用于微滤膜(MF)和超滤膜(UF)制造的聚合物材料,具有良好的化学、氧化和水热稳定性以及高机械强度等特性,广泛应用于废水处理、膜蒸馏、污染物去除、气体分离、复合膜支撑层等方面^[1⇓⇓-4]。

非溶剂^[5]、助溶剂^[6]、纳米颗粒^[7]和聚合物^[8]等常作为铸膜液添加剂用于改善膜的形貌结构和性能,起到致孔剂的作用,增加铸膜液黏度或加速相转化过程。添加剂分为有机添加剂和无机添加剂两类,有机添加剂主要有聚乙烯吡咯烷酮(PVP)^[9]和丙酸(PA)^[10]等;无机添加剂主要有无机盐(如LiCl)^[11]、金属氧化物(如TiO₂)^[12]和纳米颗粒(如碳纳米管、沸石、贵金属纳米颗粒)^[13⇓-15]等。

理想的不对称膜由薄的表面皮层和相互连接的多孔海绵状子结构组成,高互连性的多孔子结构是降低传输阻力的关键,其机械性能也优于指状结构^[16]。铸膜液组成、非溶剂添加剂、膜厚度、凝固浴组成、膜基底、空气湿度和环境温度等因素都对膜结构有显著影响^[17]。采用浸没沉淀相转化法制备的PES平板膜通常呈现非对称指状孔结构,为了制备具有高通量的海绵状结构聚醚砜膜,考察了铸膜液邻近比α和膜厚度对膜结构和渗透性能的影响。

1 实验部分

1.1 实验药品及仪器

药品:聚醚砜(PES),M_w=58 000 Da,巴斯夫新材料有限公司生产;N,N-二甲基乙酰胺(DMAc)、一缩二乙二醇(DEG)、牛血清白蛋白(BSA),分析纯,上海泰坦科技股份有限公司生产;去离子水,实验室自制。

仪器:自动刮膜机,HLKGM3125型,苏州圣恳自动化科技有限公司生产;刮膜刀,BEVS 1806型,珠海天创仪器有限公司生产;场发射扫描电子显微镜(SEM),NOVA SEM 450型,美国FEI公司生产。

1.2 PES/DMAc/DEG三元浊点相图的测定

准确称取一定量的PES粉末,置于100 mL的烧杯中,配置PES/DMAc混合溶液共30 g,用磁力搅拌器在室温下搅拌6 h得到均一透明的PES/DMAc混合溶液,记录初始质量为m₀。用DEG溶液进行滴定,直至溶液刚好变浑浊且搅拌1 min后不再澄清,记录此时烧杯的质量为m₁,m₁-m₀即为体系达到浊点时DEG的添加质量。浊点计算式如下:

(1)

m P E S ∶ m D M A c ∶ m D E G = m P E S ∶ m D M A c ∶ (m 1 - m 0)

其中:m_PES、m_DMAc、m_DEG分别为PES、DMAc、DEG的质量,g;m₀为滴定初始烧杯质量,g;m₁为滴定终点烧杯质量,g。

1.3 铸膜液的配置及PES平板膜的制备

按照一定比例准确称量PES和DMAc放于 250 mL三口烧瓶中,在室温下用电动搅拌器在 400 r/min的转速下持续搅拌6 h得到均一透明溶液,再向其中加入一定量的DEG,持续搅拌18 h得到均一透明的铸膜液。将其取下于室温环境下静置24 h以上脱泡。采用浸没沉淀相转化法制备PES平板膜,用去离子水作为凝胶浴,在室温条件下进行刮膜。

1.4 表征方法及性能测试

1.4.1 扫描电子显微镜(SEM)分析

利用扫描电子显微镜(SEM)观察平板膜的表面和截面特征。平板膜先在60℃烘箱内烘干6 h以上,去除水分,再经液氮脆断,在真空环境下喷金,进行扫描电镜拍摄。

1.4.2 渗透性能

利用实验室自制错流装置测量平板膜的纯水通量。在25℃条件下将平板膜于0.2 MPa预压 30 min,待通量稳定后将测试压力调整到0.1 MPa,然后用量筒量取一定时间内的渗透液,记录其体积。纯水通量计算式为:

(2)

F l u x = V / (S × Δ t)

式中:Flux为纯水通量,L/(m²·h);V为渗透液体积,L;S为膜有效面积,m²;Δt为渗透液所用时间,h。

将去离子水换成1 g/L的BSA溶液继续运行20 min后,利用紫外-可见分光光度计在280 nm波长下测量原料液和渗透液的吸光度,通过BSA标准曲线计算BSA浓度。BSA截留率计算式为:

(3)

R = [(c f - c p) / c f] × 100 %

式中:R为截留率,%;c_f为原料液的质量浓度,g/L;c_p为渗透液的质量浓度,g/L。

2 结果与讨论

2.1 PES/DMAc/DEG三元体系浊点相图

25℃下PES/DMAc/DEG三元体系的浊点相图如图1所示。DEG的加入破坏了溶液原有的热力学稳定状态,随着三元体系中DEG含量的增加,溶液逐渐从热力学稳定状态转变为不稳定状态,导致体系发生液-液分相。

Boom等^[18]认为,当聚合物和非溶剂强烈不相容,且非溶剂的加入只会导致体系发生液-液分相,不存在结晶现象时,由一种聚合物、一种溶剂和一种非溶剂组成的三元体系的浊点组成符合一种线性函数关系。从Flory-Huggins理论推导出线性浊点关系式(LCP)为:

(4)

l n (ϕ 1 / ϕ 3) = b l n (ϕ 2 / ϕ 3) + a

式中:ϕ₁为非溶剂的质量分数,%;ϕ₂为溶剂的质量分数,%;ϕ₃为聚合物的质量分数,%。

根据Flory-Huggins理论,对25℃下PES/DMAc/DEG三元体系的浊点数据进行线性拟合,结果如图2所示。从图2中可以看出,ln(m_DEG/m_PES)与ln(m_DMAc/m_PES)具有很好的线性关系,拟合得到的R²达到0.998 0。

对于只存在液-液分相的体系,LCP关系式也适用于高浓度聚合物体系^[19]。通过线性方程可以计算出高浓度聚合物状态下溶液的浊点组成,并绘制出双节线相图,如图3所示。

2.2 邻近比α对膜结构和渗透性能的影响

邻近比α为铸膜液体系中非溶剂和溶剂的质量比与该体系下相同聚合物浓度浊点组成中非溶剂和溶剂的质量比的比值^[20],其计算式为:

(5)

α = (w N S A / w S o l) / (w N S A, c l / w S o l, c l)

式中:w_NSA为铸膜液中非溶剂的质量分数,%;w_Sol为铸膜液中溶剂的质量分数,%;w_NSA,cl为该聚合物浓度下该体系浊点组成中非溶剂的质量分数,%;w_Sol,cl为该聚合物浓度下该体系浊点组成中溶剂的质量分数,%。

邻近比α代表铸膜液组成接近浊点的程度,邻近比α为0代表铸膜液组成中只含有聚合物和溶剂;邻近比α为1代表铸膜液组成恰好为浊点组成,刚好达到液-液分相的临界点。邻近比α的不断增大代表铸膜液中的非溶剂成分在不断上升,铸膜液的组成也在不断地接近浊点。不同邻近比α的铸膜液组成如表1所示。

不同邻近比α的PES平板膜的表面SEM图如图4所示。从图4中可以看出,随着邻近比α从0增加到0.90,膜表面孔径逐渐增大。当邻近比α分别为0、0.35和0.55时,P0、P35和P55的表面较为致密、孔径较小,无明显开孔。此时,铸膜液中非溶剂添加量少,形成的平板膜具有较致密表层;当邻近比α为0.75时,铸膜液组成逐渐靠近双节线,液-液分相速度加快,分相界面非溶剂含量更多,膜表面出现明显的多孔结构;当邻近比α为0.90时,铸膜液更容易达到热力学不稳定状态,发生瞬时液-液分相,在膜表面形成多孔结构。

不同邻近比α的PES平板膜的截面SEM图如图5所示。从图5中可以看出,当邻近比α分别为0、0.35和0.55时,P0、P35和P55的截面为典型的不对称指状孔膜结构,由致密的表面皮层和贯穿整个截面的指状孔及大空穴组成。这些大空穴会降低膜的耐压性和机械强度,易形成薄弱部位。较小的邻近比α导致铸膜液组成距双节线较远,容易形成表面皮层,阻碍溶剂和非溶剂的交换,导致内部延迟分相,聚合物稀相长大,形成空穴结构;当邻近比α为0.75时,铸膜液组成接近双节线,上部更易达到液-液分相区从而固化成膜,抑制致密皮层的形成,生成海绵状结构。但是厚的海绵状结构也会影响溶剂和非溶剂的交换,导致下部难以在较短时间内达到液-液分相区,仍有大空穴结构生成;当邻近比α为0.90时,少量的溶剂和非溶剂交换即可导致膜内液-液分相,形成交联的海绵状结构,P90截面为连续的海绵状结构,无指状孔和大空穴。

不同邻近比α的平板膜的渗透性能如图6所示。从图6中可以看出,随着邻近比α的增加,PES平板膜的纯水通量逐渐增大,BSA截留率先基本保持不变后下降。当邻近比α从0增大到0.35时,纯水通量从37.2 L/(m²·h)增大到2 000 L/(m²·h),主要与致孔剂DEG的加入有关。当邻近比α分别为0、0.35、和0.55时,P0、P35和P55对BSA均有较好的分离效果;当邻近比α为0.75时,膜内部出现海绵状结构,表面皮层开孔率变大,P75的BSA截留率降至66.8%;当邻近比α为0.90时,膜截面完全为海绵状结构,表面皮层有明显大孔,纯水通量最大,BSA截留率为24.6%。

2.3 膜厚度对膜结构和渗透性能的影响

选择邻近比α为0.90的铸膜液配方(PES质量分数为16.00%、DMAc质量分数为45.86%、DEG质量分数为38.14%),通过调整刮膜刀间隙至150、200、250 μm和300 μm,考察了膜厚度对膜结构和渗透性能的影响。刮膜刀间隙对PES平板膜厚度的影响如表2所示。

不同厚度平板膜的表面SEM图如图7所示。从图7中可以看出,随着膜厚度增加,膜表面开孔率先增加后减小。P90-15、P90-20和P90-25的表面开孔较多,而P90-30表面较致密,无明显大孔结构。这是因为当铸膜液厚度较小时,更容易发生瞬时液-液分相形成多孔性皮层;随着铸膜液厚度的增加,液-液分相延迟,则得到较致密的表面皮层。

不同厚度的PES平板膜的截面SEM图如图8所示。从图8中可以看出,随着膜厚度的增加,膜截面的海绵状结构减少,指状孔出现。当平板膜的厚度超过100 μm时,海绵状结构逐渐减少,指状孔结构逐渐出现并贯穿整个膜截面,仅上部少量区域保留海绵状结构。原因在于刮在玻璃板上较薄的铸膜液在凝胶浴中发生相转化时,溶剂与非溶剂的传质阻力较小,有利于海绵状孔的形成。较厚的铸膜液在发生相转化时,由于表层固化,内部传质阻力增加,阻碍了下部靠近玻璃板的铸膜液中溶剂与非溶剂的交换,从而导致指状孔结构的形成。

刮在玻璃板上的不同厚度的平板膜的渗透性能如图9所示。从图9中可以看出,P90-15、P90-20、P90-25和P90-30的纯水通量呈先上升后下降趋势,BSA截留率先下降后升高。P90-20的表面开孔较多,内部存在连续贯通的海绵状结构孔,减少了水分子透过时的阻力,纯水通量最大,BSA截留率最小。P90-30的表面皮层较为致密,纯水通量最小,BSA截留率最大。

3 结论

(1)通过浊点滴定法准确测定25℃下PES/DMAc/DEG三元体系的浊点组成,并基于Flory-Huggins理论进行线性拟合,ln(m_DEG/m_PES)与 ln(m_DMAc/m_PES)呈良好的线性关系,R²达0.998 0。

(2)随着邻近比α从0增大到0.90,PES平板膜表面由致密变为多孔结构,截面指状孔结构减少,海绵状结构增加。当邻近比α为0.90时,膜截面为完全的海绵状结构,纯水通量显著提高,BSA截留率下降。

(3)随着膜厚度的增加,PES平板膜表面的开孔率先增加后减小,截面由完全的海绵状结构逐渐转变为指状孔结构。50~80 μm厚度的膜截面为完全的海绵状结构,100 μm厚度的膜截面下部出现大空穴,150 μm厚度的膜截面主要为指状孔结构。

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