Of in situ composites

III. THE ROLE OF BLENDS MISCIBILITY IN THE GENERATION OF IN SITU COMPOSITES... 

Miscibility or compatibility provided by the compatibilizer or TLCP itself can affect the dimensional stability of in situ composites. The feature of ultra-high modulus and low viscosity melt of a nematic liquid crystalline polymer is suitable to induce greater dimensional stability in the composites. For drawn amorphous polymers, if the formed articles are exposed to sufficiently high temperatures, the extended chains are retracted by the entropic driving force of the stretched backbone, similar to the contraction of the stretched rubber network [61,62]. The presence of filler in the extruded articles significantly reduces the total extent of recoil. This can be attributed to the orientation of the fibers in the direction of drawing, which may act as a constraint for a certain amount of polymeric material surrounding them. 

According to the composite theory, tensile modulus of fiber reinforced composites can be calculated by knowing the mechanical constants of the components, their volume fraction, the fiber aspect ratio, and orientation. But in the case of in situ composites injection molded, the TLCP fibrils are developed during the processing and are still embedded in the matrix. Their modulus cannot be directly measured. To overcome this problem, a calculation procedure was developed to estimate the tensile modulus of the dispersed fibers and droplets as following. 

Palandri J. L. and Reed M. H. (2001) Reconstruction of in situ composition of sedimentary formation waters. Geochim. Cosmochim. Acta 65, 1741-1767. 

Table 5.16. Properties of injection-molded minitensile bars of in situ composites. ...

E. Shiva Kumar, C. Das, K. Banik, and G. Mennig. Viscoelastic properties of in situ composite based on ethylene acrylic elastomer (AEM) and liquid crystalline polymer (LCP) blend. Compos. Sci. Tech., 67(6) 1202-1209, May 2007. 

S. C. Tjong, R. K. Y. Li, and X. Xie. Properties of in situ composites based on semiflexible thermotropic liquid crystalline copolyesteramide and polyamide 66 blends. Polym. J. (Tokyo), 32 907-914,2000. 

L. Pan and B. Liang. A comparative study of in-situ composite fibers reinforced with different rigid liquid crystalline polymers. J. Appl. Polym. Scl, 70 1035-1045, 1998. 

S. Bualek-Limcharoen, 1. Samran, T. Amomsakchai, and W. Meesiri. Effect of compatibUizers on mechanical properties and morphology of in-situ composite film of thermotropic liquid crystalline polymer/polypropylene. Polym. Eng. ScL, 39 312-320, 1999. 

Polymeric materials have relatively large thermal expansion. However, by incorporating fillers of low a in typical plastics, it is possible to produce a composite having a value of a only one-fifth of the unfilled plastics. Recently the thermal expansivity of a number of in situ composites of polymer liquid crystals and engineering plastics has been studied [14,16, 98, 99]. Choy et al [99] have attempted to correlate the thermal expansivity of a blend with those of its constituents using the Schapery equation for continuous fiber reinforced composites [100] as the PLC fibrils in blends studied are essentially continuous at the draw ratio of 2 = 15. Other authors [14,99] observed that the Takayanagi model [101] explains the thermal expansion. 

ELASTIC MODULI OF IN SITU COMPOSITES CONTAINING POLYMER LIQUID CRYSTALS... 

From the viewpoint of composite mechanics, the better performances of composites come from the effective reinforcing effect of the reinforcements and effective stress transferring from the mafiix to the reinforcements. The reinforcing LCP fibrils in in situ composites are generated during the melt processing of LCP blends, which is different fi om conventional glass or carbon fiber-reinforced composites. So the formation of LCP fibrils becomes one of the most important aspects in the preparation of in situ composites, except the common interfacial interaction in fiber-reinforced plastics. 

TABLE 7.5. The Mechanical Properties of in situ Composites (along Machine/Flow Direction). 

TABLE 7.6. The Tensile Properties of Maohine and Transverse Direotions of in situ Composites. 

It should be noted that there is not simply an addition of thermotropic LCPs into fiber-reinforced plastics to get in situ hybrid composites. Bafna et al. used glass fibers to decrease the anisotropy of LCP fibril-reinforced polyetherimide [136]. He et al. improved the processability and mechanical performances of glass fiber-reinforced polypropylene by the addition of LCPs [159]. However, these two works did not actively and purposely generate a reinforced composite with the reinforcements having their diameters at two orders of magnitude. The key point for in situ hybrid composites is the formation of LCP fibrils in the material system. As a combination of in situ composite and hybrid fiber reinforcing, the fabrication of in situ hybrid composites utilizes fabrication... 

D. G. Baird, S. S. Bafna, J. P. DeSouza, and T. Sun, Mechanical properties of in situ composites based on partially miscible blends of polyetherimide and liquid crystalline polymers, Polymer Composites, vol. 14, No. 3, 214-223, June (1993). 

Farmer, S.C., Sayir, A. and Dickerson, P.O. (1993) Mechanical and microstructural characterization of directionally solidified alumina-zirconia eutectic fibers. In Symposium Proceedings of In-Situ Composites, Science and Technology, pp. 167-182. TMS, Warrendale, PA. 

The second section is devoted to self-reinforced composites prepared through the melt processing of thermoplastics and thermotropic LCPs. The individual chapters address various aspects of rheology, processing compatibilization, and performance characteristics of in situ composites. Attention is directed to preparation of fibers and molded products, with the special objective of optimal performance properties through processing. 

Dencheva N, Denchev Z and Oliveira M J (2010) Microstructure studies of in situ composites based on polyethylene/polyamide 12 blends, Macromolecules 43 4715-4726. 

Dencheva N, Denchev Z, Oliveira M J and Funari S S (2010) Microstructure studies of in situ composites based on polyethylene/polyamide 12 blends, Macromolecules 43 4715-4726. Polaskova M, Cermak R, Sedlacek T, Kalus J, Obadal M and Saha P (2010) Extrusion of polyethylene/poljTDropylene blends with microfibrillar-phase morphology, Polym Compos S1 1427-1433. Wang H, Guo J and He Y X (2011) Rheology and thermal properties of polypropylene/poly(phenyl-ene sulfide) microfibrillar composites, Adv Mater Res 194-196 1506-1509. 

Oliveira M J and Funaxi S S, Title Microstructure studies of in situ composites based on PE/PA12 blends, pp. 4715-4726, Copyright (2010), with permission from The American Chemical Society. 

Crevecoeur, G. and Groeninckx, G., Sheet extrusion of in-situ composites Influence of processing parameters, Polym. Eng. Sci. 33 937 (1993). 

---