Membrane and Desalination Technologies - TCE Moodle Website

Preparation of Polymeric Membranes 57
Fig. 2.10 Chemical
structure of
polycarbonate.
3.8. Polyamide
Polyamides (PA) are synthetic amides in which the amide group (–NHCO–) is an integral
group of the main polymer chain, which include aliphatic and aromatic polyamides. The basic
aliphatic polyamides are referred as nylons, which chemical structures are shown in Fig. 2.11.
Nylon 6, nylon 4,6 and nylon 6,6 are semicrystalline polymers, they are important engineering
thermoplastics and possess good thermal stability and mechanical strength (14). The
porous polyamide membranes have been commercialized for many years by immersion
precipitation. Nylons are resistant to many organic solvents such as aliphatic, aromatic
hydrocarbons as well as some linear alcohols and aprotic liquids such as NMP and DMF.
Now polyamides are frequently used as the thin dense layer in composite membranes for
reverse osmosis and nanofiltration, which are formed by interfacial polymerization of diamines
and diacylchlorides. Polyamide membranes overcome some of the pH limitations of cellulosic
membranes. However, the chlorine tolerance is lower that cellulose based membranes.
3.9. Polyimide
O C
O
Polyimides (PIs) are generally known as polymers with excellent thermal stability because
of their high glass transition temperature, which demonstrate high thermal and chemical
stability. Polycondensation of aromatic dianhydrides with aromatic diamines is to form a
soluble poly (amic acid). The poly(amic acid) is shaped into the polyimide product by
dehydration step. P84 copolyimide is an amorphous commercial copolyimide from Lenzing.
Its solvent resistance to many organic solvents is excellent, thus it is mainly used for the
preparation of solvent-resistant nanofiltration membranes. Whereas it is well soluble in
amides and it is also not resistant to aqueous alkaline solutions. The Tg of P84 copolyimide
is 315 C, which can be used at a high temperature (above 200 C). Matrimide 5218 is another
commercial polyimide, which is manufactured by Ciba-Geigy. It is originally developed for
use in microelectric industry. Now it is also used as a material for making gas separation and
nanofiltration membranes. The chemical structures of copolyimide P84 and Matrimid 5218
are shown in Fig. 2.12.
CH 3
CH 3
H O
H
H O
O
* N CH2 C
x-1
* * N CH2 x
N C CH2 y-2
C *
n
n
Fig. 2.11 Chemical structures of basic aliphatic polyamides (nylon x or nylon x,y).
C
O