Polymerization kinetics of polyacrylamide gels II. Effect of temperature

The temperature at which polyacrylamide gels are polymerized strongly affects the structure and physical properties of this matrix. The suggested temperature of 0–4 °C has been found to produce highly turbid, highly porous and unelastic gels. At temperature of 25 °C of higher, the gels became progressively transparent, less porous and more elastic. This phenomenon is strongly pronounced in N,N′‐methylenebisacrylamide (Bis) gels and progressively decreases for N,N′‐(1,2‐dihydroxyethylene)bisacrylamide (DHEBA) and N,N′‐bisacrylylcystamine (BAC) gels. It has been attributed to the formation of hydrogen bonds among cross‐linker molecules, which are presumably stabilized by low temperature and progressively broken at higher temperatures. The most extensively H‐bonded compounds seem to be Bis molecules, since they can form four H‐bonds/molecule, followed by DHEBA (in which inter‐molecular H‐bonds have to compete with intra‐molecular H‐bonds) and finally by BAC, which is sparingly H‐bonded at 2 °C, and fully uncomplexed at 30°C. It is suggested that polymerization at 0–4°C should be abandoned, since it leads to unhomogeneous and unreproducible pore size matrices. Since H‐bonds in Bis molecules are fully disrupted only at 60°C (a temperature unsuitable for gel polymerization, since it produces short chains and unelastic matrices), it is also suggested that the use of Bis should be discontinued in favour of better cross‐links, such as DHEBA. In all cases, best polymerization conditions are obtained at 25–30°C. For all three cross‐linkers, homogeneous gels are obtained in presence of 8 M urea or in pure formamide as solvent, since both agents fully disrupt H‐bonds.