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Dynamic viscoelasticity of melts of semi-aromatic liquid crystalline polymer (LCP) and its silicone carbide (SiC) whisker-filled compounds was measured to analyze quantitatively their nonlinear viscoelastic properties by Fourier expansion of the shear stress wave. For LCP the tertiary terms, G'3 and G″3, are considerably smaller than their primary terms, G'1 and G″1, i.e. their viscoelasticity is almost linear. Viscoelastic behaviour of the silicone carbide whisker-filled LCP system was compared with that of the carbon fiber (CF)- and glass fiber (GF)-filled LCP systems presented in [18]. The CF/LCP system shows a dependence of the oscillatory angle upon its G'i and G″i, making both accordingly decrease with increasing i from 1 to 3 and 5. The G'3 and G″3 values of the polymer system, compared to its G'1 and G″1 counterparts, are sufficiently large not to be negligible, suggesting that the system's viscoelasticity is nonlinear. Furthermore, the SiC whisker-filled LCP system was examined for the dependence of the ratios G'i/G'1 and G″i/G″1 (i = 3, 5), and Gi/G1 (i = 3, 5, 7) as indicators of the degree of viscoelastic nonlinearity upon angular frequency. The results did not show any significant difference between G'3/G'1 and G'5/G'1 values, or between G″3/G″1 and G″5/G″1 values. The ratio Gi/G1 (i = 3, 5, 7), representing the nonlinearity of viscoelasticity of the system, however, is higher with an increase in the oscillatory angle.

Linears and non-linears viscoelastic properties play an important role in polymer processing. On addition of fillers (whether fibrous or particulate) the material's behaviour becomes very complicated. This paper presents the steady–state and dynamic viscoelastic properties of short carbon-fibre filled polypropylene (CF/PP) melts measured by a cone-plate type rheogoniometer. Storage and loss moduli were analyzed quantitatively by the Lissajous figures procedure (L method) as well as by Fourier series expansion (F method) to determine their respective higher order terms as the indicators of nonlinear viscoelasticity. The dependence of these components on fibre content and oscillatory angle, q was investigated. The results were compared with similar analytical data for CF filled liquid crystalline polymer (CF/LCP) melts. In the presently researched systems the CF content affects the viscosity, and at 15 and 20% an apparent yield stress appears. The complex viscosity is different for pure PP and its filled systems when measured as a function of θ. While for the unfilled polymer the viscoelasticity can be considered linear, for CF/PP some non-linearity must be taken into account, especially at higher value of θ. In general, the filled systems show complex viscoelastic behaviour with some irregularities, e.g. some of the higher order terms of the storage modulus are negative, which was not seen in the previous study of fibre filled LCP systems.