Great progress in the filtration performance of microfiltration membranes has been achieved in the past few years. As a consequence the structures of the respective membranes have become more and more intricate. New microscopic characterization methods are therefore necessary to reveal the membrane structure in full detail. The environmental scanning electron microscope (ESEM) has the potential for also studying microfiltration membranes in their wet state. The dynamics of the wetting and drying of the membrane surface can be imaged at sub-µm resolution. Simultaneous temperature measurements at both membrane surfaces, macroscopic parameters, provide information about the progress of these pro­cesses in the membrane interior, thus complementing the microscopic results gained from the images.

Introduction

The ESEM is a versatile tool, which besides imaging the membrane surface, also enables three dimensional (3D) analysis of the inner membrane structure. For this purpose a slice and view tool can be mounted into the microscope chamber. The interior membrane structure can also be imaged by cutting thin slices off the membrane in an automated manner and subsequently recording the residual blockface. This provides detailed information on membrane morphology such as pore connectivity and specific surface area [1].

But the filtration performance of membranes is also strongly influenced by other parameters, e.g. the chemical properties of the pore surface. This requires the dynamic behavior of the membrane to be investigated during the wetting and drying process, which can also be done in the ESEM [2].
In addition to the conventional high vacuum (HV) mode, this instrument offers a pressure range between 0.1 and 20 torr [3]. In this study water vapor was used as chamber gas. ESEM analysis of the wetting and drying process of the membrane surface requires chamber pressures above and below the dew point. To enable work at sufficiently low pressures, the experimental setup needs to be cooled. A special cooling stage designed for the investigation of flat membranes was developed. The membrane is fixed between two cooling clamps, which are cooled to a temperature of 4°C (fig.

1). As the chamber pressure exceeds the dew point (6 torr), water starts condensing at the cooling clamps, consequently wetting the membrane. By reducing the pressure below the dew point the membrane begins to dry. The temperatures of both surfaces of the membrane were measured using two micro thermocouples (Type T, COCO-001 from Omega, Newport) [fig. 1, insert a]. The number and size of dry and wet pores can be determined by simultaneous imaging of the membrane surface.

Materials

The investigation was carried out on poly­ethersulfone (PES) membranes, widely used in waste water treatment, the beverage industry and many areas of bio­technology due to their resistance to many aggressive substances.

One must be aware, however, that organic materials are especially prone to irradiation damage. This is even more critical when additionally water is present, because electron irradiation of water creates many highly mobile free radicals, which too can attack the organic material [4]. But for PES membranes irradiation damage was observed only at very high magnifications.

The new generation of microfiltration membranes offers a very asymmetric pore structure. Figure 2 shows the cross-section of a Dura PES 450 membrane (Membrana Wuppertal, Germany). The bright areas represent the membrane matrix; the dark areas represent the pores. The membrane consists of the separation layer and a backup layer. The separation layer consists of small pores and is responsible for the filtration process, whereas the backup layer should provide mechanical stability and protect the separation layer from outside damage. The wide pore structure of the backup layer offers high penetration and a great "loading volume", which results in low fouling tendency. This special structure provides optimal flow rates due to the thin separation layer, necessary for many filtration applications [5].

The asymmetric pore structure is provided by a specially controlled production process. The roll side of the membrane often has much larger pores than the air side. The flow of the liquid inside the membrane is determined by the separation layer, due to the small size of its pores and their correspondingly high capillary forces. The ESEM, however, can only image the membrane surface, because the penetration depth of the electrons at the energies used is only a few µm. However, the temperature characteristics recorded at the membrane surfaces during the wetting and drying process give additional information about both the interior structure of the membrane and the distribution of the water inside the membrane.