Intrinsic viscosity of polymers of low molecular weight

Experimental evidence concerning the dependence of the intrinsic viscosity [η] on molecular weight M in the low molecular weight range (from oligomers to M = 5 × 10 4 ) has been collected in a variety of solvents for about ten polymers, i.e., polyethylene, poly(ethylene oxide), poly(propylene oxide), polydimethylsiloxane, polyisobutylene, poly(vinylacetate), poly(methyl methacrylate), polystyrene, poly‐α‐methylstyrene, and some cellulose derivatives. In theta solvents, the constancy of the ratio [η]Θ/ M 0.5 extends down to values of M much lower than those predicted by current hydrodynamic theories. In good solvents, and on decreasing M , the polymers examined, with the exception of polyethylene and some cellulose derivatives, show a decrease in the exponent a of the Mark‐Houwink equation [η] = KM a . This upward curvature gives rise to the existence of a more or less extended linear region where the equation [η] = K 0 M 0.5 is obeyed. Below the linear range, i.e., for even shorter chains, the exponent a can increase, i.e., polydimethylsiloxane, or decrease below 0.5, i.e., poly(ethylene oxide), depending on the particular chain properties. These different dependences have been discussed in terms of: ( a ) variations of thermodynamic interactions with molecular weight; ( b ) variations of conformational characteristics (as for instance the ratio) 〈r0 2 / nl 2 〉, where 〈r0 2 〉 is the unperturbed mean square end‐to‐end distance and n is the number of bonds each of length l ; ( c ) hydrodynamic properties of short chains.