Rheology of LDPE-Based Semiflexible Fiber Suspensions M. Keshtkar, M.-C. Heuzey, P.J. Carreau Center for Applied Research on Polymers and Composites (CREPEC), Chemical Engineering Department, Ecole Polytechnique, Montreal, Quebec H3C 3A7, Canada Molten LDPE suspensions containing fibers of different flexibilities have been investigated in simple shear and small and large amplitude oscillatory shear (LAOS) flow. The suspensions exhibited viscosity and normal stress overshoots in stress growth experiments, and the magnitude and width of the overshoots became larger as the fiber flexibility increased. LAOS was used to help understanding the relationship between stress growth and fiber orientation. For all composites, the stress signal decreased with time in LAOS, and this behavior was more pronounced in the case of the more rigid fibers. The energy dissipated per LAOS cycle was evaluated for each composite, and it showed that less energy was dissipated as fiber flexibility decreased. In addition, the dissipated energy decreased with time and this has been interpreted in terms of a reduction of fiber contacts. The first normal stress difference showed a nonsinusoidal periodic response, and fast Fourier transform analysis indicated the presence of a first harmonic corresponding to the applied frequency for the fiber-filled systems, in addi- tion to the second harmonic observed for the neat LDPE. It resulted in asymmetrical strain-normal force Lissajou curves for the suspensions, with this asym- metry being more pronounced in the case of the more rigid fibers. This has been attributed to a more exten- sive fiber orientation for the latter. POLYM. COMPOS., 31:1474–1486, 2010. ª 2009 Society of Plastics Engineers INTRODUCTION The mechanical properties of fiber-reinforced compo- sites are strongly dependent on microstructure and fiber orientation. The structure itself is highly affected by mate- rial characteristics such as fiber properties, component interactions, suspending fluid properties, but also by the imposed flow field. Understanding the relationships between rheology, structure, and macroscopic properties can be extremely useful in the design and optimization of processes and composite properties [1]. One aspect that may impact fiber orientation is fiber flexibility. The flexi- bility may vary with fiber properties such as stiffness and aspect ratio. The rheology of fiber-reinforced matrices is quite complex due to several factors like fiber–fiber, fiber–wall, fiber–matrix interactions and phenomena such as fiber breakage and migration, and many investigations have been conducted to understand the relationships between rheology and microstructure. However, among these studies very few have focused on the role of fiber flexibility. Using a Couette geometry and fiber suspensions of various stiffnesses, aspect ratios, suspending fluid viscosities, and applied shear rates, Forgacs and Mason [2] observed that as the stiffness decreased or aspect ratio increased—hence larger flexibility—the fibers tended to bend and not follow the orbits predicted by the Jeffery model. The fibers went instead through what has been called ‘‘flexible orbits.’’ They also found that the critical stress to bend the fibers was: _ cg m ð Þ crit ffi E b ðln 2r  1:75Þ 2r 4 ð1Þ where g m is the viscosity of the suspending fluid (matrix) and E b the bending modulus of the rod, E b  2E Y , with E Y the Young modulus of the fiber, and r is the apparent aspect ratio of the fibers, which is the ratio of fiber length to diameter. The results of Forgacs and Mason [2] indicated that axial forces imposed by shear flow can bend fibers with low modulus or high-aspect ratio, decrease the apparent aspect ratio and shorten the period of rotation. These results stress the important role of fiber flexibility on the dynamics and orientation of fibers in suspensions. Recently, Keshtkar et al. [3] showed that by increasing fiber flexibility (reducing the fiber Young modulus or increasing its aspect ratio), the viscosity and normal stress differences increased for model suspensions of fibers in silicone oil, especially in the semiconcentrated regime. To obtain information about the microstructure of suspensions, oscillatory shear flow experiments are Correspondence to: M.-C. Heuzey; e-mail: marie-claude.heuzey@polymtl.ca Contract grant sponsor: Natural Sciences and Engineering Research Council of Canada (NSERC-CIAM Program). DOI 10.1002/pc.20934 Published online in Wiley InterScience (www.interscience.wiley.com). V V C 2009 Society of Plastics Engineers POLYMERCOMPOSITES—-2010