Is a fiber made of polymer

Abstract:The development of novel materials that meet the growing demands of industrial applications is an important aspect of polymer chemistry, especially in the field of synthetic fibers. The processing of nanoparticles in polymers has become the focus of interest. The use of carbon nanotubes (CNTs) in polymer matrices is very promising. CNTs are anisotropic cylindrical macromolecules that can be viewed as nano-scale fibers. They consist of carbon arranged in a graphitic structure. The interest in CNTs is based on their outstanding properties such as high mechanical strength and electrical conductivity. The potential of these particles can also be expanded considerably by chemical functionalization or the introduction of a metal into the interior. As part of this doctoral thesis, fundamental investigations into the incorporation of multi-walled carbon nanotubes (MWNTs) into synthetic fibers were carried out. One point was investigations into a chemical functionalization of the MWNTs, which should prevent agglomeration during the spinning process. In view of the higher electrical conductivity of the particles, a metal was introduced into the interior of the MWNTs used. With these, investigations were carried out with regard to the melt and wet spinning process. Silver-containing MWNTs could be produced by introducing a solution of silver nitrate and then reducing it. MWNTs could be functionalized via the in situ polymerization of styrene or acrylonitrile in the sense of MWNT / polymer composites. These functionalized MWNTs were embedded in polymer matrices and spun into fibers: MWNT / polystyrene composites were processed into fibers by melt spinning in a PET matrix. The polystyrene-functionalized MWNTs showed a reduced tendency to agglomeration than untreated MWNTs. SEM images confirmed that the MWNTs are aligned in the direction of the longitudinal axis of the fiber during the spinning process, despite the functionalization by the polystyrene in the PET matrix. However, the embedded MWNTs cause the fibers to become brittle, since the MWNTs act like defects between the PET molecule chains. MWNT / polyacrylonitrile composites could be spun into fibers from a spinning solution in a PAN matrix using the wet spinning process. In these fibers, too, the functionalized MWNT were oriented in the direction of the fibers. The fibers are difficult to stretch, but have a high modulus of elasticity. Due to the high anisotropy of CNTs, their spatial orientation in the fiber has an impact on the fiber properties. Therefore, the alignment of the CNTs was measured quantitatively. This is particularly important in PAN precursors for carbon fibers. CNTs are discussed as nuclei for the formation of the graphitic structure characteristic of carbon fibers and could therefore be used as additives. For this reason, PAN fibers containing very low levels of MWNTs were investigated. This was done through Raman measurements with linearly polarized laser radiation. Since the intensity of the MWNTs in the PAN matrix was too low, the nitrile group of the polyacrylonitrile was used as a reference point. In this way, conclusions could be drawn about the relationship between the spatial orientation of the MWNTs in the fiber and their influence on the fiber properties. The mechanical properties of the fibers could be related to the degree of depolarization. This is important because the mechanical properties have a great influence on the process control in the manufacture of carbon fibers. Furthermore, the influence of MWNTs on the oxidation and carbonization of PAN fibers was investigated with MWNTs. The presence of MWNTs and especially of metal-containing MWNTs in the fiber had a positive effect on the formation of the graphitic structure. The investigations show the diverse influence of MWNTs on PAN precursors for carbon fibers: The mechanical properties of PAN fibers can be influenced by knowing the spatial orientation of the MWNTs. In addition, the carbonization or the formation of the graphitic structure can be controlled by the presence of MWNTs, and particularly silver-containing MWNTs. Additional irradiation with accelerated electrons before carbonization intensifies this effect. Taking these factors into account could enable both a simplified process control during production and an optimization of the properties of carbon fibers.
The developement of new materials which are sufficient to the increasing requirements of industrial applications is an important aspect in polymer chemistry, especially in the field of man-made fibers. In particular adding nanoparticles to polymer matrices has become the spotlight of investigations. Carbon nanotubes (CNTs) are one of the most promising particles. These are anisotropic, cylindrical macromolecules which can be considered as nano-scale fibers consisting of carbon with a graphitic structure. The interest in CNTs is based upon their outstanding properties like high tensile strength and electrical conductivity. The possibilities for applications of these particles can even be enlarged by chemical functionalization or placing metals in the inner core of CNTs. Here fundamental aspects of man-made fibers containing Multi Walled Carbon Nanotubes (MWNTs) were investigated. One aspect was the chemical functionalization of MWNTs to suppress the agglomeration during the spinning process. To reach higher electrical and thermal conductivity, metal particles were placed in the inner cavity of the MWNTs. Investigations were carried out on the melt spinning and wet spinning process. Silver-filled MWNTs were produced by inserting a solution of silver nitrate into opened MWNTs and adjacent reduction of the silver nitrate to silver. MWNTs could be functionalized in the form of MWNT / polymer composites by an in situ polymerisation of styrene and polyacrylontrile. These functionalized MWNTs were spin by adding them to polymer matrices: MWNT / polystyrene composites were added to polyethylene terephthalate (PET) and spun by melt spinning. The MWNT / PS composites showed a reduced tendency to form agglomerates compared to pure MWNTs. SEM pictures proofed that the MWNTs were oriented in the direction of the fiber axis despite of the polystyrene. However, the presence of MWNTs or MWNT / PS composites caused an embrittlement of the PET fibers compared to a pure PET fiber because the MWNTs act as defects between the molecular chains of the PET. For MWNT / polyacrylonitrile-composite the matrix polymer was polyacrylonitrile (PAN). Fibers were produced by wet spinning. Also in these fibers the functionalized MWNTs were approximately oriented parallel to the fiber axis. The PAN fibers containing MWNT / PAN composites were brittle but were found to have high Young’s moduli and could resist high tensile stress. Due to the high anisotropy of CNTs their three-dimensional orientation within the fiber has influence on the fiber properties. Therefore the orientation of the MWNTs was measured quantitatively. Especially concerning precursors for carbon fibers this can be essential to know: CNTs in PAN precursors are discussed as seed for the formation of the graphitic structure which is characteristic for carbon fibers. For this reason CNTs could serve as additives in precursors for carbon fibers. Hence investigations were carried out on PAN fibers containing very small amounts of MWNTs via polarized Raman spectroscopy. Because the intensity of the MWNTs in the PAN matrix was too low to be detected, the band of the nitrile group was investigated. In this way, it was possible to draw conclusions from the orientation of CNTs in the PAN fiber to the fiber properties. Mechanical fiber properties correlate with the degree of depolarization. These mechanical properties have strong influence on the process management in the production of carbon fibers. In addition the influence of MWNTs on the oxidation and carbonization process was looked at in detail. The presence of MWNTs and especially metal-filled MWNTs had a positive effect on the formation of the graphitic structure. These investigations revealed that CNTs show great promise for additives in PAN precursors for carbon fibers: The PAN fiber can be influenced in its mechanical properties by the three-dimensional orientation of the MWNTs within the fiber. The carbonization and formation of the graphitic structure respectively, can be controlled by adding MWNTs or silver-filled MWNTs to the precursor material. Additional irradiation with accelerated electrons of the precursor before carbonization enlarges this effect. Considering these facts could enable to develop a simplified process management and optimized properties of carbon fibers.