In situ reinforcing composites based on two elastomer matrices very different in melt viscosity, styrene(ethylene butylene)-styrene triblock copolymer (Kraton G1650) and styrene-(ethylene propylene) diblock copolymer (Kraton G1701), and a thermotropic liquid crystalline polymer (TLCP), Rodrun LC3000, were prepared using a twin-screw extruder. The rheological behavior, morphology, mechanical and thermal properties of the blends containing various LC3000 contents were investigated. G1650 was found to have much higher shear viscosity than G1701. All neat components and their blends exhibited shear thinning behavior. Melt viscosity of the blends gradually decreased with increasing LC3000 contents. Despite a large difference in melt viscosity of the two matrices, the results showed that the fibrillar morphology was obtained for both as-extruded strands of LC3000/G1650 and LC3000/G1701 with up to 30 wt % LC3000. At 40 wt % LC3000 or more, the lamellar structure was observed for both types of blends because of the coalescence of liquid TLCP threads that occurred during extrusion. The addition of LC3000 into both elastomer matrices enhanced the tensile modulus considerably whereas the extensibility remarkably decreased. The results obtained from thermogravimetric analysis suggested that an addition of LC3000 into both elastomer matrices improved the thermal resistance significantly in air, but not in nitrogen. (c) 2006 Wiley Periodicals, Inc.