Dynamic light scattering (DLS) and rheological measurements were performed on aqueous silk fibroin solutions extracted from the middle division of Bombyx mori silkworm over a wide range of polymer concentration C from 0.08 to 27.5 wt %. DLS results obtained in the dilute region of C less than 1 wt % are consistent with a model that an elementary unit is a large protein complex consisting of silk fibroin and P25 with a 6:1 molar ratio. Rheological measurements in the dilute C region reveal that those units (or clusters) with the hydrodynamic radius of about 100 nm form a network extending over the whole sample volume with small pseudoplateau modulus mainly by ionic bonding between COO(-) ions of the fibroin molecules and divalent metallic ions such as Ca(2+) or Mg(2+) ions present in the sample and also that, after a yield stress is reached, steady plastic flow is induced with viscosity much lower than the zero-shear viscosity estimated from creep and creep recovery measurements by 4-6 orders of magnitude. Angular frequency omega dependencies of the storage and the loss shear moduli, G'(omega) and G' '(omega), measured in the linear viscoelastic region, indicate that all solutions possess the pseudoplateau modulus in the low omega region and samples become highly viscoleastic for C greater, similar 4.2 wt %. Above C = 11.2 wt % another plateau appears at the high omega end accompanied by a distinct maximum of G' ' in the intermediate omega region. The relaxation motion with tau = 0.5 s corresponding to the maximum of G' ' is one of characteristic properties of the fibroin solutions in the high C region. Thermorheological behaviors of the solution with C = 27.5 wt % show that the network structure formed in the MM part of the silk gland is susceptible to temperature and a more stable homogeneous network is realized by raising the temperature up to T = 65 degrees C.

Marked effects of protonation (ionization) of tetradecyldimethylamine oxide on the viscoelastic properties of the micelle solutions were found. The effect strongly suggests the short-range attractive interaction between the headgroups of the nonionic (deprotonated) and the cationic (protonated) species. The zero shear viscosity reached a maximum at the half-ionized state (the degree of ionization R ) 0.5) and the value was larger than that of the nonionic species (R ) 0) or the cationic species (R ) 1) by more than 2 orders of magnitude. At a surfactant concentration C of 0.1 mol/kg, approximately single Maxwell behavior was observed as R approached 0.5 from either side. For the half-ionized micelles (R ) 0.5) in 0.1 mol/kg NaCl solutions, the steady-state compliance J e 0 decreased with C with an exponent of 2.1 ( 0.2, suggesting the presence of an entangled network of flexible threadlike micelles. The relaxation time, on the other hand, exhibited a nonlinear dependence on C. It was about 0.1 s and remained nearly constant in the range C > 0.1 mol/kg (regime I), whereas it increased with C in the range of C < 0.09 mol/kg (regime II) with an exponent slightly larger than 1. The single Maxwell behavior was observed in regime I. The regime shift was not controlled by the ratio C/m s , m s representing the NaCl concentration. Effects of NaCl concentration and the temperature on the viscoelastic properties were also examined at R ) 0.5. Cryo-transmission electron micrographs clearly showed a highly entangled network in the solution for R ) 0.5, while much smaller micelles for R ) 0. Contrary to the expectation from the rheological results, a highly entangled network was also observed in the solution for R ) 1.

Dynamic light scattering (DLS) measurements were performed on aqueous solutions of native silk fibroin extracted from three parts, the posterior (MP), the middle (MM), and the anterior parts (MA), of the middle division (M) of the silk gland of the Bombyx mori silkworm to study the dynamics and aggregation properties of silk fibroin. In the MP part, fibroin molecules are present as aggregates (or clusters) being composed of several large protein complexes or elementary unit (EU), which are further associated to make a large assembly connected via divalent metallic ions. In the MM part, such clusters of EU take more compact structure, and finally in the MA part, clusters disappear, but EUs are more or less aligned to keep the assembly, and the EU takes the conformation of wormlike cylinder capped with hemispheres at both ends. The overall conformational change in solution structure was interpreted as being due to the change in ionic environment in the solution. DLS study was also performed on regenerated silk fibroin solutions, which revealed that fibroin is present as a single molecule dominantly and their association behavior seems completely different from that of native samples and does not depend on types and concentration of added metallic ions.