Solution properties and molecular-weight distribution of nylon 11

Abstract

Molecular, hydrodynamic, and thermodynamic properties of nylon 11 in meta cresol and 1,1,1,2,2,2-hexafluor-2-propanol (HFIP) solutions, as well as its distribution of molecular weights were investigated by means of viscosimetry, conductimetric titration of end groups, light scattering, and fractionation by successive precipitation. The studies were carried out on commercial samples as such (Mn = 14,900, Mw = 29,400) and on specimens prepared by solid-state postpolymerization of the former (Mn = 35,000–43,000, Mw = 91,000–104,000). The results show the expected normal or Flory–Schulz distribution of molecular weights on the commercial sample (U = 1.03), and a broadened distribution on the postpolymerized one (U = 1.42), in agreement with previous observations on solid-state postpolymerization of other polyamides. The intrinsic viscosity of the individual fractions was determined experimentally and the weight-average molecular weights were calculated from the data of the fractionation (number-average molecular weight and the mass fraction of polymer on each individual fraction) by means of an iterative numerical procedure. The parameters of the Mark–Houwink equation were, then, derived from the data of a large number of samples, including that corresponding to the whole, unfractionated polymers, spanning a range of about 100,000 units of molecular weight. The value of the exponent (a = 0.69) for solutions in meta cresol corresponds to the behavior of a linear, flexible macromolecule in a good-solvent medium. The solutions in HFIP employed for the light-scattering studies, on the other hand, display high values of the second virial coefficient (A2 = 7.8 × 10−3 − 5.6 × 10−3 mol mL/g), suggesting that HFIP is a good solvent for nylon 11.