Dynamic light scattering and dynamic viscoelasticity of poly(vinyl alcohol) in aqueous borax solutions .2. Polymer concentration and molecular weight effects

Dynamic light scattering (DLS) and dynamic viscoelastic (DVE) measurements are made on aqueous borax solutions of three poly(vinyl alcohol) (PVA) samples with weight-average degrees of polymerization DPw = 600, 2100, and 2600. The PVA concentration C of the respective samples is chosen to encompass the threshold concentration C* at which chains start to overlap in pure aqueous solution. The time correlation function of light intensity scattered from the solutions always exhibits the presence of two dominant modes. The decay rate Gamma(f) of the fast mode is proportional to the square of the scattering vector q for all samples, from which the dynamic correlation length xi(H) is estimated. xi(H) is found to change its C and M dependences at the concentration C-N close to C*. The q dependence of Gamma(s) of the slow mode varies from Gamma(s) similar to q(2-3) below C-N to Gamma(s) similar to q(0) with a further increase in C. The dynamic scaling law is applied to the former data. The characteristic time tau(s) (=Gamma(s)(-1)) obtained for C > C-N increases with M and is in good agreement with the mechanical relaxation time tau(M) obtained by fitting DVE data to the Maxwell model with a single relaxation time.