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Rheological behavior of biocomposites of silk fibroin fiber and poly(ε-caprolactone): Effect of fiber network

Rheological behavior was examined for biocomposites of rod-like silk fibroin (SF) fiber and poly(e-caprolactone) (PCL) to investigate an effect(s) of the SF fiber network therein on the mechanical properties. At 160 °C where PCL was a homogeneous melt, linear viscoelastic tests revealed that the SF/PCL composites hardly relax to behave essentially as elastic solids (more precisely, plastic solids before yielding) at low frequencies. The corresponding equilibrium modulus G0 increased strongly with the SF volume fraction ϕSF (G0 ∼ ϕ) and was attributable to the elastic bending of the SF fibers incorporated in the network. The Doi-Kuzuu model for non-Brownian rods was modified for the SF/PCL composites by incorporating the rod–rod contact at equilibrium. The G0 calculated from this model was satisfactorily close to the data, in both ϕSF dependence and magnitude, lending support to the assignment of the composite elasticity to the fiber bending. The storage modulus G′ measured under large-amplitude oscillatory shear (LAOS) was smaller than the linear viscoelastic G′, and this difference between the linear and nonlinear moduli was enhanced for the composites with a larger SF content and at lower frequencies. This nonlinear effect was attributable to a decrease of the effective fiber–fiber contacts sustaining the elasticity under LAOS. Under steady shear, the SF/PCL composites exhibited nonlinear (plastic) flow behavior associated with the stress overshoot, and their apparent viscosity was comparable to/lower than the viscosity of neat PCL matrix. The overshoot became much less significant on application of a second shear immediately after the first shear, while the overshoot was partly recovered after a quiescent rest between the first and second shears. These nonlinear features were attributable to slippage between shear-oriented fibers and PCL matrix. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1957–1970, 2009