Abstract
Microfibrillar reinforced composites (MFC) are prepared from polymer blends of immiscible partners having different melting points, T-m. The essential stages of MFC preparation are: (i) blending, (ii) extrusion, (iii) drawing, and (iv) annealing at constant strain above T-m of the lower-melting component and below T-m of the higher-melting one. During the drawing microfibrils are created (fibrillization step), and in the subsequent annealing process melting of the lower-melting component occurs (isotropization step), with preservation of the microfibrillar structure of the higher-melting component. The mechanical properties of MFC are about the same or slightly better than those of commercial short glass fibre-reinforced thermoplastics when the starting blend comprises two condensation polymers. In such cases a self-compatibilization effect is also observed due to the interchange chemical reactions resulting in the formation of a copolymer at the phase boundaries. MFCs involving polyolefins, such as polypropylene (PP) or polyethylene (PE), as matrices and poly(ethylene terephthalate) (PET) as the reinforcing component show the same morphological and structural characteristics as the other MFC types. Their mechanical parameters (tensile strength and Young's modulus) are up to five times higher compared to the neat PP or PE matrices. Compared to short grass fibre-reinforced composites (30% by wt), having the same matrices, the MFCs have approximately the same Young's moduli and tensile strengths about 3-4 times higher, with a much better (up to 10 times) deformation ability.
A conclusion is drawn that polymer-polymer composites of the MFC type are very illustrative examples for final property enhancements through the development of special microstructures during polymer processing.