Reinforcement with microfibers

Nano-additives are able to functionalize the materials to a greater extent or can even be used to produce hybrid composites [5,6]. Even the fibers can be composites themselves, for example, where microfibers or even nanofibers are reinforced with carbon nanotubes (CNTs) [7,8] or with nanoclay [9,10], etc. In these cases nano-reinforcement is typically embedded into a fiber which becomes a kind of matrix itself. These composite fibers can have excellent tensile properties [7] or can be used for detecting damage in composite structures [11] or can be precursors of carbon fibers [12]. 

Fiber diameters can be decreased to the submicron-scale in several ways in order to get nano-reinforcement. Nanofibers have unique properties primarily due to their small diameter. They are very flexible and have a very high surface to mass ratio. Self-crystalline polymer nanofibers can have different crystallinity compared with microfibers [13,14]. They can have a very high aspect ratio which is preferable to establish a good adhesion with the matrix. 

These continuous fibers are used mostly in hybrid composites in combination with microfibers. In principle it is possible to reinforce composites with only nanofibers but knowledge so far is very limited in this field. 

Caves, J.M., Kumar, V.A., Martinez, A.W., Kim, J., Ripberger, C.M., Haller, C.A., and Chaikof, E.L. (2010) The use of microfiber composites of elastin-like protein matrix reinforced with synthetic collagen in the design of vascular grafts. Biomaterials, 31, 7175 7182. 

Zhu et al. studied polyurethane foams from soy reinforced with cellulose microfibers. They found an increase on the onset degradation temperature of the thermal degradation of polyurethane with the addition of 2 wt % cellulose fibers. They attributed this fact to the insulator effect of cellulose fibers [52]. Navarro-Baena et al. studied shape memory PU based on PLA-PCL-PLA block copolymer and reinforced with both CNCs and PLA grafted CNCs [72]. Aside the increment on the shape memory behavior of the polyurethane-based nanocomposites, they reported an increase on the thermal stability of the PU matrix in particular, they reported that, although CNCs improved the thermal stability of both PCL and PLEA blocks, in particular the thermal stability of the PCL block was improved in the nanocomposites increasing the maximum degradation temperature of about 40 with respect to the PCL block of the neat PU-matrix [72]. 

Additional studies directed at fibrillation of crystalline polymer blends note the potential for in-situ fiber reinforcement. Extruded sheets of PP/poly(e-caprolactone) showed microfiber reinforcement and synergistic modulus/strength properties at high PP levels [290]. Polyamide (PA6) microfiber formation in a PP matrix was observed with PP-g-MA optionally added to... 

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