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Separation properties and surface morphology of polyacrylonitrile membranes D Paul, H Kamusewitz, H.-G Hicke and H Buschatz GKSS Forschungszentrum Geesthacht GmbH, Institut fur Chemie, Abt Membranforschung, KantstralJe 55, D-0-1530 Teltow, Germany The formation of polyacrylonitrile membranes by phase inversion has been studied The surface morphology, as investigated by atomic force microscopy, and the separation performance in ultrafiltration were correlated with the concentration of the polymer in the casting solution and a post-treatment by annealing Trenneigenschaften und Oberflachenmorphologie von Polyaciylnitril-Membranen Polyacrylnitril- Membranen, hergestellt aus GieRIosungen mit unterschiedlicher Polymerkonzentration und thermisch nachbehandelt, zeigen eine Korrelation zwischen Herstellungsbedingungen und der mit Hilfe der Atom-Kraftfeld-Mikro-skopie abgebildeten Oberflachenmorphologie beziehungsweise den Trenneigenschaften bei der Ultrafiltration I Introduction Polyacrylonitrile (PAN) is a well known polymer material for manufacturing textile fibres and membranes PAN membranes are frequently used for ultrafiltration but as support for composite membranes [I] The formation by casting of PAN polymer solution [2, 31 or polymer blend solution [4] is well known The aim of this investigation was to study the dependence of the surface morphology and the separation properties of membranes on the formation conditions Therefore, the concentration of PAN in the solvent D M F was changed from 10 to 17.5 O h in the casting solution Additionally, the influence of annealing of the PAN membranes to the separation performance in the process of ultrafiltration is investigated The porous structure of polymer material may be characterized i.e by mercury high pressure porosimetry, electron and light microscopy, X-ray small angle scattering, or low temperature gas adsorption [5] More recently, by application of scanning tunnel microscopy and atomic force the tip deflection is minimized (constant force) and the vertical motion of the piezoelectric transducer is recorded The measurement range for z is constant (< 2.7 nm) All scans were recorded as 512 x 512 pixel images [lo] Results and discussion The performance of polymer membranes is influenced by varying their physical structure or by varying operation conditions in the separation equipment The effect of the manufacturing parameters of the polymer membranes from PAN is reflected in the separation properties Using constant ultrafiltration conditions, the water flux J, of PAN membranes decreases, if the polymer concentration in the casting solution increases (Table 1) This result confirms a lower porosity of the membrane skin layer by using a casting solution with higher polymer concentration [4, 11, 121.From the hydrodynamic equivalent radii of various test substances and their rejection in ultrafiltration experi-ments, we can calculate a mean pore diameter d,, = 15 nm for the membrane TV microscopy the surface structure of microfilter polymer membranes was studied [6,7,8] The scope of this note is to investigate the surface roughness and the small pores of an ultrafiltration membrane using the high resolution atomic force microscopy (AFM) Table Parameters of PAN membranes; support: PETP nonwoven ,fabric Experitn cn ta I Property water flux TV1 TV2 1900 J, (llhm2) 480 Membranes were casted by using polymer solutions of 10, 15 and 17.5 wt% PAN (molecular weight M , ca 30000 g/mol) in D M F onto PETP nonwoven fabrics as support (membranes TVI, TV2, TV3) The membranes were annealed for 10 at 90 'C (membranes TVl/T, TV2/T, TV3/T) The membrane performance in the pressure-driven process ultrafiltration is described by the parameters water flux J,, permeate flux J , and rejection R for poly(ethy1ene glycol) (PEG, different molecular weight, g/l) at 0.3 MPa by constant conditions in the separation apparatus The mean pore diameters dsoof the membrane were calculated from the smallest molecularweight components retained at R = 50 Yo [9] The surface morphology of the membranes was observed by atomic force microscopy (AFM Nanoscope 111, Digital Instruments Inc.) The samples were investigated with a 800 nm scan size at room temperature in the height mode I n this mode, the feedback gain to the z-axis (vertical, height) piezoelectric transducer is set at a high value so that Acta Polymer., 43, 353-355 (1992) VCH Verlagsgesellschaft mbH, D-6940 Weinheim, 1992 Membrane TV3 TVIIT 250 TV21T TV3lT 500 2000 250 pore diameter &, (nm)(calc.) 21 17 15 20 18 0.1 0.2 14 roughness AFM z (nm) 0.7 0.4 0.2 0.2 AFM images of the membranes show a graduated roughness (Fig 1) The medium height difference z measured by AFM technique differs from 0.7 nm (Membrane TVI) to 0.2 nm (TV3) A tendency of lowering the frequency of the height pattern of the membranes with the polymer concentration in the casting solution can be shown High resolution AFM imaging of the surface of the membrane TV3 makes visible a secluded narrow valley (Fig 2) which could be discussed as an entrance of a pore The pore walls are not vertical Caused by the tip geometry (the shape of the tip is an approximate pyramid), the bottom ot the pore could not be recognized 0323-764819210612-0353$3.50 + ,2510 3.53 Fig Top view of a pore of PAN membranes (Casting solution 17.5 O/o wto/o PAN, scan size: 50 n m ) (2a) Dried membrane; (2b) annealed and dried membrane Fig Surface of PAN membranes (scan size: 800 nm) ( l a ) Casting solution 10 wt% PAN; (Ib) Casting solution 15 wt% PAN; (lc) Casting solution 17.5 wt% PAN In comparison with the nonannealed samples, after annealing the PAN membranes reveal also a decrease of water permeability at polymer concentration used in the casting solution (Tab 1) The pore diameter of the membrane skin layer is not reduced by annealing For such membranes, a lower roughness in AFM images is observed than for nonannealed material (Fig 3).The observed profile indicates a slope down.According to the lower Tg(wet PAN membrane ca 60-70 “C; dry PAN ca 100 “C) than the annealing temperature, a “melting down” of the tops of the surface occurs Height differences decrease and appear to be almost independent on the manufacturing conditions (variation of the polymer concentration in the casting solution) of the membranes 354 Paul, Kamusewitz, Hicke, Buschatz The surface and subsurface structure of the PAN membranes are key factors in determining their separation performance Indeed, the pore diameter of the skin layer and consequently the separation properties are indepen-dent on the annealing and correlate only with the casting conditions A direct correlation between surface roughness and separation properties was not observed However, it is imaginable that by high roughness the plugging is increased and the membrane performance is impaired Acknowledgement The authors thank Mrs G Peters for technical assistence This work was partly supported by the Funds of Chemical Industry, Germany Acta Polymer., 43, 353-355 (1992) Fig Surface of annealed PAN membranes (scan size: 800 nm) (3a) Casting solution 10 wt% PAN; (3b) casting solution 15 wt% PAN; (3c) casting solution 17.5 wt% PAN R ryeren ces [l] R Raiitenbach and R Albrecht: Membrane Processes John Wiley & Sons, Chichester/New York/Brisbane, 1989, p 36 [2] V: Grobe et al.: Acta Polymerica 40 (1989) 137 [3] S Petrov et al.: J Membr Sci 64 (1991) 183 [4] W Albrecht et al.: Acta Polymerica 43 (1992) 165 [5] Grosse et al.: Acta Polymerica 43 (1992) 151 [6] P Diet; et al.: J Membr Sci 65 (1992) 101 [7] A Chahboun et al.: J Membr Sci 67 (1992) 295 [8] A.K frifzsche et al.: J Membr Sci 68 (1992) 65 [9] C/7 Ei.ro/d: Dissertation TU Dresden 1991 [lo] Digital Instruments Inc 6780 Cortona Drive, Santa Barbara, California: Nanoscope I11 Manual (1991) [ l l ] H Daurzrnhrrg et al.: Acta Polymerica 40 (1989) 177 [12] H.-G Hicke and D.Paul: Filtr & Separ (1991) 376 Received Scptetnber 14 1992 Acta Polymer., 43, 353-355 (1992) Separation properties and surface morphology of polyacrylonitrile membranes 355