Polyamide-hydrazide membrane

AROMATIC POLYAMIDE-HYDRAZIDE MEMBRANES °Jt 90 C/0-l6mln OPERATING PRESSURE ... 

Figure II. Effect of film casting conditions on the performance of resulting (a) cellulose acetate (12) and (h) aromatic polyamide-hydrazide membranes. The latter data are tho.se of O. Kutowy and S. Sourirafan (not yet published).

Figure 6 shows the skin structure of a polyamide-hydrazide that has been freeze-dried. Unlike the Figure 3 micrograph of an air-dried polyamide—hydrazide membrane that has a homogeneous appearance, the freeze-dried membrane shows a micellar structure. 

Figure 3. Fracture surface of an alr-drled polyamide-hydrazide membrane. Reproduced from Ref. 18. Copyright 1973 American Chemical Society.

Nodules are defined as spherical cells with a diameter of a few hundred angstroms that are compacted irregularly at the membrane surface. They can also be observed underneath the membrane surface when a cross-sectional picture is taken. Each nodule contains several tens of thousands of macromolecules. Schultz and Asunmaa were the first to report the observation of nodules on the surface of an ultrathin cellulose acetate membrane by electron microscope [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 A. 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 A when a cross-sectional picture of an asymmetric aromatic polyamide-hydrazide 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 A. They attributed the formation of the nodules to the micellar structure that was initially present at the surface of the polyamidehydrazide solution. 

Figure 4,7. Electron micrographic picture of the cross-section of an asymmetric aromatic polyamide-hydrazide membrane. (Reproduced from [63] with permission.)...

Strathmann et al.20) examined the water and salt sorption and the homogeneity of water distribution in various polymers and indicated that the uniformity of water distribution in the polymer is an important parameter controlling reverse osmosis desalination efficiency. As summarized in Table 2, the average number of water molecules included in a cluster is 1.4 to 2.9 for the superior barrier materials such as aromatic polyamides, polyamide-hydrazide, and polybenzimidazopyrrolone, while the number for the other polymer membranes is larger than 5. 

McKinney et al. 49,50) described the reverse osmosis properties of asymmetric membranes prepared from the following polyamides and polyamide-hydrazides (7, 8, or 9). 

The polyamide-hydrazide 7 was prepared by solution polymerization in anhydrous dimethylacetamide from terephthaloyl chloride and p-amino-benzhydrazide at ca. 10 °C. The polyamide 8 resulted from the polycondensation of m-phenylenediamine with isophthaloyl chloride at —20 °C, whereas 9 was prepared by the reaction of terephthaloyl chloride with the complex diamine l,3-bis(3-aminobenzamide)benzene at —20 °C. The water flux and salt rejection through these membranes were summarized in Table 5. The polyamide-hydrazide (7) membranes were prepared from polymer solutions containing 6 7% polymer (Mv 3 34,000) by casting on glass plates. The material was placed in an oven for 30 60 min and coagulated in deionized... 

Mohamed, N. A., and Al-Dossary, A. O. H. (2003), Structure-property relationships for novel wholly aromatic polyamide-hydrazides containing various proportions of para-phenylene and mefa-phenylene units. Part III Preparation and properties of semi-permeable membranes for water desalination by reverse osmosis separation performance, Eur. Polym. /., 39,1653-1667. 

Structure Level I. Structure Level I variations for aromatic polyamides are broad. The wide range of segmental structures possible with these polymers is what makes them so interesting for membrane science. The discussion of Structure Level I will be limited to some representative segmental units in polyamides, polyhydrazides and polyamide-hydrazides. Structures and abbreviations for some typical diamines that are condensed with mixtures of isophthaloyl chloride (l) and terephthaloyl chloride (T) to give the aromatic polyamides discussed in this paper are shown in Table III. 

The preference of the 1,3 orientation for the diamine used for polyamide synthesis carries over to the 010 and 1,3 BO diamines used for synthesis of polyhydrazides and polyamide-hydrazides, respectively. One of the reasons that the 1,3 BO-l/T polyamide-hydrazides give such good RO membranes is that the ring symnetry is lowered a second time by having different amine groups in the 1,3 positions on the benzene ring. The effect of Structure Level II on RO properties will be apparent when we discuss Structure Level III. 

The flux of 0.03 gfd for the homogeneous polyamide membrane was more than two orders of magnitude too low for commercial desalination. The flux was increased 175 fold with no decrease in salt rejection by casting the membrane with asynmetric morphology. Even higher fluxes, up to 3.5 times that observed for the asymmetric MPD-l/T (100-70/30) polyamide membrane, were obtained with asymmetric membranes cast from polyhydrazides and polyamide-hydrazides. Permeation properties for the three types of aromatic polyamides are shown in Table IX. The RO properties of this group of membranes illustrate the combined effects of Structure Levels I, II and III on membrane performance. 

Figure 1 shows the top edge of cross-section of a polyamide-hydrazide asymmetric gel membrane. The surface shows a structure formed from a closely packed monolayer of polyamide-hydrazide micelles of about 400 to 600 A diameter. 

Figure 3 shows a fracture surface of a dried polyamide-hydrazide gel membrane. Fusion of the micelles to give a typically homogeneous, bulk phase is clearly evident from this micrograph. 

Many polymeric materials are used for membrane fabrication. The most widely used polymers are cellulose acetate and its derivatives, aromatic polyamide, polyamide hydrazide, polysulfone, polyphenylene, polypropylene, etc. Among these cellulose acetate, aromatic polyamide, and polysulfone are more commonly used. In addition, some membranes are made from glasses, ceramics, and metal oxides. 

Fig. 4.2. Top edge of cross section of polyamide-hydrazide asymmetric gel membrane taken by SEM. Reprinted from Polymeric Gas Separation Membranes by R.E. Resting and A.R. Fritzsche, p 248. Copyright 1993, with kind permission from Wiley...

The patents 9 47) filed by Richter and Hoebn in du Pont include many aromatic and heterocyclic condensed polymers for asymmetric reverse osmosis membranes, for example, the condensed polymers 4 from 3-arninobcnzhydrazide, 4-amino-benz-hydrazide (5), isophthaloyl chloride, and terephthaloyl chloride (6). The membranes were cast from polyamide solutions containing 0 3 % LiCl in dimethylacetamide,... 

---