Nodular Structure of Polymers in the Membrane

Abstract

Important membrane surface properties include the size of nodules and nodule aggregates, the shape of pores, the pore size and pore size distribution, and the surface roughness. In this chapter, the focus will be on nodules and nodular aggregates since AFM seems most suitable for those. Moreover, there is evidence that nodular structure has some relationship to membrane performance. The phase contrast imaging technique in AFM can distinguished between the amorphous and crystalline phase. A solid formed by the solidification of a chemical and having a highly atomic structure is called a crystal, which has a regular structure and size. On the other hand, a nodule is a mass of polymer molecule agglomerates that are entangled with each other. At a lamellar crystal level, the morphology and crystalline structure are deduced by TEM or X-ray studies. However, the resolution of the AFM can go beyond that easily available with TEM imaging of polymers. At a higher resolution, AFM can give better results and in some cases has revealed unpredicted surface structures. AFM presents surface structures in real space, whereas structural information can be deduced from diffraction data (small angle X-ray scattering or small angle neutron scattering) only in interplay with structural models. Asynthetic polymermay be described as crystalline if it contains regions of threedimensional ordering on atomic (rather than macromolecular) length scales, usually arising from intramolecular folding and/or stacking of adjacent chains. The stacks formed by the folding of chains are called lamellae. Sometimes part of the chain is included in this crystal and part of it isn’t. Lamellae are not neat and tidy, but sloppy, with chains hanging out everywhere. The synthetic polymer may consist of both a crystalline and an amorphous region.The crystalline portion is in the lamellae, and the amorphous portion is outside the lamellae. The degree of crystallinity is expressed in terms of a weight fraction or volume fraction of crystalline material. To examine lamellae and other nanometer-scale structures in polymer materials, it is necessary to achieve high-resolution imaging on the submicron scale.This is easily achieved using Nanoscope® MultiMode™ and Dimension™ 3100 AFMs (Digital Instruments, VeecoMetrology Group, Santa Barbara, CA) under ambient conditions. The necessary prerequisite for high-resolution imaging is a sharp tip. Tapping mode is particularly important for this purpose due to its ability to image softmaterials such as most polymers without sample alteration. Low-force imaging or light tapping allows imaging of top surface features with lateral resolution determined by the small tip contact area (2–3 nm). Imaging with elevated forces or hard tapping allows visualization of subsurface structures and differentiation of crystalline and amorphous regions. Height images yield the true three-dimensional topography of the sample surface; the deflection mode is useful for a sharp contrast of the features imaged. Images of the surface of a nodule can expose the lamellar or crystalline phases.The phase contrast imaging technique can be distinguished between the crystalline and amorphous phase. Nodules are defined as spherical cellswith a diameter of a fewhundred angstroms that are compacted irregularly at the membrane surface. They can also be observed underneath themembrane surface when a cross-sectional picture is taken. Each nodule contains several tens of thousands ofmacromolecules. Schultz and Asunmaawere the first to report the observation of nodules on the surface of an ultrathin cellulose acetatemembrane by electronmicroscope [1]. Figure 4.1 shows the picture taken by them.The nodular structure of the membrane surface is clearly seen with an average nodular diameter of 188 ± 3 Å.The same authors also took a picture of an asymmetric cellulose acetate membrane and found that it, too, had a nodular structure. Panar et al. [2] then observed the close monolayer packing of micelles with diameters from 400 to 800 Å when a cross-sectional picture of an asymmetric aromatic polyamidehydrazide membrane was taken (Fig. 4.2).The top monolayer covers a support layer where the spherical micelles are irregularly packed with void spaces of 75–100 Å. They attributed the formation of the nodules to the micellar structure that was initially present at the surface of the polyamidehydrazide solution. Nodular structures were found not only in the ultrathin and asymmetric membranes but also at the surface of thin film composite (TFC) membranes. Cadotte reported that nodules were closely packed at the surface of a fully aromatic polyamide TFC membrane prepared by the in situ polycondensation reaction between m-phenylene diamine and trimesoyl chloride [3, 4].