Polymer fiber surface roughness

The values of the surface free energy for the polymer matrix can be obtained from classical contact angle measurements. In the case of fibers, surface roughness and the presence of surface... 

Natural fibers also act as nucleating agents and enhance the crystallization rate of polymer as reported for PP. The fiber surface roughness of the fiber and thermal stresses were found to facilitate the growth of transcrystallinity on cotton fibers, whereas bamboo fibers induced significant amount of beta form crystals and transcrystalline growth of maleated PP. 

The superior mechanical properties of biocomposites with appropriate alkali-treated natural fibers are ascribed to the increased fiber-matrix adhesion on removing the natural and artificial impurities, thereby forming roughened surfaces. As described earher, alkali treatment may lead to fiber fibrillation, resulting in the increase surface area contactable with the polymer. Such surface roughness and increased aspect ratio by fibrillation offer better natural fiber-polymer matrix bonds [4]. In another paper [143], the effect of the addition of silane-treated- and untreated talc as filler on the mechanical properties of PLA/recycled newspaper cellulose fiber/talc hybrid composites was evaluated. 

Hence, based on the parameters available, three different ablation rates were studied corresponding to a weak, a medium, and a full ablation according to laser profilometry (Fig. 20.5). The weakly ablated surface possesses the same surface roughness as an untreated sample (Sa=0.6pm). On the contrary, the medium and the fuUy ablated ones exhibit partial and total removal of the polymer layer at the surface of the composite, respectively. In the case of total ablation, the resulting roughness is directly related to the kind of fiber weaving used for reinforcement. 

Thus the theory has proved a useful one in the sense that it has stimulated the development of new surface treatments for metals, polymers and fibers and has assisted in giving an understanding of their efficacy. There has perhaps been a tendency, now that the theory is again respectable, to invoke mechanical effects somewhat uncritically wherever an increase in surface roughness has been observed. A more detailed review of these developments may be found in references [18] and [44]. 

The interface is a region at least several molecular layers thick with properties intermediate between those of the fiber and matrix phases and arises due to the peculiar restrictions on molecular motions in this zone. Matrix molecules may be anchored to the fiber surface by chemical reaction or adsorption and determine the extent of interfacial adhesion. Fiber modification reduces hydrophilicity of the fiber and improves the physical/chemical interactions between the fiber and matrix. Treatment makes the surface of the fiber very rough and provides better mechanical interlocking with the polymer matrix. 

Commercial applications have been proposed that use high average power free-electron lasers to heat the surface of polymers for enhancements to the surface morphology. This uses infrared at 5.8 to 6.2-/rm wavelengths where high absorption results from carbonyl-related molecular absorption bands. By enhancing surface roughness in polyester and nylon fibers, the fabrics can be made softer, hydrophilic, and the material more readily accepts dyes. 

Generally, the most profitable dispersion method is to mix the nanopaxticles into the solvent and then add and mix in the polymer destined for electrospinning. For example, Hou et al. [117] mixed SWCNTs in DMF with bath sonication. The process took 12 h to reach adequate dispersion. After ultrasonication, the polymer was added and stirred further for 12 h with a magnetic mixer. Hou et al. [117] pointed out that if the CNT concentration is too high, the fiber surface is rough because CNTs are not completely embedded in the fiber in this case. TEM micrographs of CNT embedded polyamide fibers can be seen in Figure 10.30 [118]. 

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