In this study the cryo-unit is introduced as a new and useful instrument to investigate water-containing specimens with the SEM. Often water-containing biological specimens are studied, but in our case we used water-swollen polymer membranes. The results show that application of a cryo-unit permits the study of this material at low temperatures up to magnifications of about 10 000 times, while other techniques failed to give reproducible results

Jongh, M.A. de

Koenhen, D.M.

Roesink, H.D.W.

Smolders, C.A.

de Jongh, M.A.

University of Twente Research Information

Ultramicroscopy 3 (1978) 325-328 0 North-Holland Publishing Company SHORT NOTE A RAPID PREPARATION TECHNIQUE FOR STUDYING HIGHLY WATER-SWOLLEN MEMBRANES WITH A SCANNING ELECTRON MICROSCOPE (SEM) SUPPLIED WITH A CRYO-UNIT H.D.W. ROESINK, D.M. KOENHEN, M.A. DeJONGfiand C.A. SMOLDERS Twente University of Technology, Dept. of Chemical Engineering, Enschede, The Netherlands Received 26 June 1978 In this study the cryo-unit is introduced as a new and useful instrument to investigate water-containing specimens with the SEM. Often water-containing biological specimens are studied, but in our case we used water-swollen polymer mem- branes. The results show that application of a cryo-unit permits the study of this material at low temperatures up to mag- nifications of about 10 000 times, while other techniques failed to give reproducible results. 1. Introduction In our laboratory we are studying phase-separation phenomena of polymer solutions in contract with non-solvents. As is known from the literature [ 11, these phenomena play an important role during the formation of asymmetric membranes. These mem- branes consist of an extremely thin selective layer (0, I-O, 2 pm) backed by a porous support structure. The waterflux of these membranes is relatively high, while a good rejection of dissolved materials (e.g. salts) is maintained [2]. In our case we used highly water-swollen membranes. Cross-sections of water- containing polymer membranes are usually studied with a SEM using sample preparation techniques such as freeze-drying and solvent-substitution [3,4]. These techniques, however, are rather time-consuming. In addition we have found that artifacts can be intro- duced when, after the freeze-drying step, the sample in its dry state is brought to room temperature. In this study we investigate the use of a cryo-unit (freeze-shock, followed by cryogenic breakening and observing the specimen at low temperatures) and the effect of defrosting treatments on the SEM results. 2. Apparatus The SEM (JEOL, JSM-Us) is equipped with a cryo- unit (JEOL, CRU). The normal preevacuation cham- ber is exchanged by a longer one, with a liquid nitro- gencooled specimen table. In this pre-evacuation chamber there is a knife for producing a fresh speci- men surface (figs. 1 and 2). There is also a defroster, a heating element (30 W) to remove the ice grown on the specimen surface. The specimen stage in the microscope can also be cooled with liquid nitrogen. Fig. 1. Cryo-unit. (1) defrosting heater, (2) specimen frac turing knife, (3) specimen fracturing pin, (4) specimen exchange chamber cold stage, (5) refrigerant tank. 325 326 H.D. IV. Roesink et al. / SEM smdy olwater-swollen membranes Fig. 2. Cryo-unit with SEM. (1) thermometer, including heater power supply, (2) specimen exchange rod, (3,4) specimen mani- pulating rods. Fig. 3. Iced surface; specimen immediately in microscope (accelerating voltage 10 kV). 3. Sample preparation Sample preparation is very simple and takes only a few minutes; the wet specimen is put into the speci- men holder, and then put into liquid nitrogen to freeze the specimen immediately (a so-called freeze- shock). As soon as possible the specimen is brought into the cooled pre-evacuation chamber, where a fresh cross-section is obtained. The image of the specimen now shows a very iced surface (fig. 3). There are two methods for removing this ice: l by sublimation in the microscope; this takes three to four hours. l by using the defroster in the pre-evacuation chamber; since structural damage might occur, the rate and intensity of defrost must be determined experimentally for the specimen under study. 4. Results In this work we used highly water-swollen, parti- ally ionic modifications of styrene-isoprene-styrene (SE) membranes. SIS is a thermoplastic rubber, con- sisting of glass-like domains of polystyrene (Z’s = H.D. W. Roesink et al. / SEM shtdy of water-swollen membranes 327 100°C) embedded in a rubber-like matrix of poly- isoprene (Ts = -100°C). Modification of the poly- isoprene middle block, to introduce ionic groups, will not destroy this rubber-like character at room tem- perature. In water this ionic membrane will be rather swollen [S] , producing a certain swelling tension in the polymer as a whole. To study specimens with a SEM it is necessary to remove the water, without damaging the swollen structure in which the water prevents the relaxation of the tension. All water can be removed by freeze- drying; that is, by sublimation of the ice at -1 30°C in a high vacuum. However, to prevent charging up of the sample in the microscope, coating with a charge- conducting layer (carbon/gold) is required. To apply this layer it is necessary to bring the specimen to room temperature; the structure may then collapse partly and may therefore have a denser appearance. The results are seen in figs. 4 and 5. Another method is air-drying. Now relaxation of the tension in the rubber network and additional capillary forces cause a dense structure without any pores remaining in the substructure (see fig. 6). The reason is that when the temperature is kept low (= -1OO’C) during the electron microscopic investiga- tion, the polymer remains below its glass-transition Fig. 5. Structure after freeze-drying (coal/gold, 25 kV). point, and the “wet structure” will be retained. When the temperature rises above the glass-transition point of polyisoprene, the tension in the structure can relax, producing a denser structure or a collapse of the structure. Fig. 4. Structure after freeze-drying (coal/gold, 15 kV). Fig. 6. Structure after air-drying (10 kV). 328 H.D. W. Roesink et al. / SEMstudy of water-swollen ttrembranes The cryo-unit offers the possibility of studying the specimen at low temperatures (= -130°C); at these temperatures the structure is not damaged and is . comparable with the “wet structure”. A further advantage of this method is that no coating of a charge-conducting layer is necessary, since the ice in the porous support structure prevents charging up of the specimen at low voltages (6-10 kV). The top layer of the ice can be removed easily by the defros- ter. 5. Conclusions For highly swollen rubber-like structures there is no alternative way of handling than keeping the speci- men at temperatures below -100°C. In comparison with freeze-drying we conclude that use of the cryo- unit means an enormous gain of time. When using the defroster for a short period (1 S-30 s) to free the sur- face from ice, no effect on the structure can be found. References [l] D.M. Koenhen, M.H.V. Mulder, C.A. Smolders. J. Appl. Polymer Sci. 21 (1977) 199. [2] S. Loeb and S. Sourirajan, Adv. Chem. Ser. 38 (1962) 117. [3] M.A. Hayat, Principles Techn. Scanning Electron Micros- copy l(1974) 113. [4] R.E. Kesting, M. Engdahl and W.J. Stone, Macromol. Sci. A3 (1969) 157. [5] P.M. Van der Velden, Thesis, Twente University of Technology, Enschede (1976). 

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A rapid preparation technique for studying highly water-swollen membranes with a scanning electron microscope (SEM) supplied with a cryo-unit

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In this study the cryo-unit is introduced as a new and useful instrument to investigate water-containing specimens with the SEM. Often water-containing biological specimens are studied, but in our case we used water-swollen polymer mem-branes. The results show that application of a cryo-unit permits the study of this material at low temperatures up to mag-nifications of about 10 000 times, while other techniques failed to give reproducible results. 1

H. D. W. Roesink

D. M. Koenhen

M. A. Dejongfi

C. A. Smolders

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A RAPID PREPARATION TECHNIQUE FOR STUDYING HIGHLY WATER-SWOLLEN MEMBRANES WITH A SCANNING ELECTRON MICROSCOPE (SEM) SUPPLIED WITH A CRYO-UNIT

https://research.utwente.nl/en/publications/ab3c6a9b-bcef-4432-9695-d4355c8e3794

A rapid preparation technique for studying highly water-swollen membranes with a scanning electron microscope (SEM) supplied with a cryo-unit

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