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Development of High-Performance, Lightweight, Low-Cost Carbon Fibers for Sustainability

(Georgia Institute of Technology, 2021-12-14) Ramachandran, Jyotsna

This dissertation focuses on the development of high-performance carbon fibers for light weighting automobiles to achieve sustainable developmental goals (SDG) of clean energy, climate action and infrastructure. Replacing steel and aluminum used in current day automobiles by carbon fibers leads to low-weight structures (theoretical reduction by ~70 % and 35% for steel and aluminum, respectively) with higher fuel savings and lower emissions. Two pathways were studied to design high performance carbon fibers that could have the potential for such replacement and their widespread use in the future. The first approach towards the goal involved designing porous carbon fibers with low-density and multifunctional capability using blends of poly(acrylonitrile) (PAN) with sacrificial polymers and block copolymers. The second route explored the use of low-cost precursor, asphaltene, to derive carbon fibers. Porous carbon fibers from gel-spun blends of PAN with sacrificial polymers were the initial systems studied to characterize the mechanical performance upon introduction of porosity. Orientation imparted during gel-spinning of the PAN-sacrificial polymer blends was synergistically combined with the micro-phase separation of the polymer pairs, to tailor the porous morphology of these fibers and their resultant mechanical properties. From five sacrificial polymers, three systems including poly(acrylic acid) (PAA), poly(methyl methacrylate) (PMMA), poly(styrene-co-acrylonitrile) (SAN), were selected due to their intermediate and higher degree of compatibility with PAN, discerned through UV-vis spectroscopy. Porous morphology of the different carbon fibers produced was characterized through scanning electron microscopy (SEM) and the differences shown due to the sacrificial polymers studied. The variation in pore size caused by the differences in compatibility between PAN and the sacrificial polymer was evaluated experimentally through blend rheology and theoretically using interaction parameter values. The tensile properties of porous carbon fibers from the three systems were compared to that of the non-porous PAN based carbon fibers processed under similar conditions. Specific tensile modulus of the porous carbon fibers derived from PAN-PAA and PAN-PMMA was found to be 15 to 40 % higher than that for the PAN based carbon fibers. Additionally, influence of sacrificial polymer content (5-20 wt%) on the mechanical, structural and morphological properties was studied for the PAN-SAN fiber systems with porous channeled morphology. This study showed that gel-spinning of bicomponent PAN/PMMA-b-PAN fibers provided a versatile means for tuning the mechanical and electrochemical properties of porous carbon fibers, thus allowing for their potential use as structural energy storage materials. Herein, by gel-spinning polymer precursors of poly(acrylonitrile) (PAN) and poly(methyl methacrylate)-block¬-poly(acrylonitrile) (PMMA-b-PAN), we produced a series of carbon fibers and systematically studied the morphological, mechanical, and electrochemical properties. Porous carbon fibers with block copolymer (BCP) in the sheath exhibited the best tensile properties with a strength of 1.1 GPa, modulus of ~190 GPa, and electrochemical capacitance of 11 F/g at 10 mV/s when pyrolyzed at 1315 °C under tension. Without tension and at a pyrolysis temperature of 800 °C, the fibers with BCP as both the sheath and core components achieved the highest electrochemical capacitance of 70 F/g at 10 mV/s and highest surface area of 264 m2/g. The characteristic correlation length of PMMA-b-PAN was calculated through thermodynamically governed computational method and compared with the pore size in the experimentally obtained carbon fibers. Through the second approach, conditions to gel-spin blends of low-cost precursor of asphaltene with PAN were established. Functionalization of asphaltenes with nitric acid (f-Asp) enabled their complete solubility in dimethylacetamide (DMAc) unlike the as-received asphaltene powder and further resulted in obtaining relatively homogeneous spinning solutions with PAN. Mechanical properties and structural parameters of precursor f-Asp/PAN fibers with their blend ratio ranging from 30/70 to 60/40, were studied. Carbon fibers with maximum tensile strength and modulus of 1.1 GPa and 181 GPa, were obtained from the gel-spun f-Asp/PAN blends. The study suggests that carbon fibers with reasonable mechanical properties can be derived from low-cost asphaltene through the scalable route of solution/gel-spinning. The two approaches studied in the dissertation present ways to develop high-performance carbon fibers that would have the potential to light weighting automobiles and thus, steer towards a future of achieving SDG.
Development of High-Performance Carbon Fibers: Accelerating Paradigm Shifts

(Georgia Institute of Technology, 2021-08-18) Shirolkar, Narayan Satish

This work details the development of three technological pathways to accelerate paradigm shifts in the way carbon fibers are developed and manufactured. The first two, viz. hollow carbon fibers and small diameter carbon fibers, provide a comprehensive understanding of the process, structure, and property relationship for these continuous carbon fibers. The third pathway provides insights into the challenges and opportunities to employ machine learning models to predict carbon fiber properties by leveraging experimental data and accelerate the improvement in tensile properties in a cost-efficient manner. Multifilament continuous hollow carbon fiber tows with a honeycomb cross-section have been produced using a gel-spun bicomponent islands-in-a-sea precursor with polyacrylonitrile (PAN) as the sea component and polymethylmethacrylate (PMMA) as the sacrificial island component. Over 80% improvement in tensile strength has been achieved for these fibers compared to the previously reported batch processed hollow carbon fibers, along with a manufacturing scale up from single filament to 740 filament tow. The effect of precursor and carbon fiber manufacturing parameters on the structure and tensile properties of the hollow carbon fibers has been studied. Furthermore, mechanical properties of hollow carbon fiber-epoxy composites have been tested and compared with commercial aerospace grade carbon fiber composites. The effect of adhesion between the fiber and epoxy matrix, alignment of fibers in the composite along the testing direction, and various testing environments, on the composite mechanical properties has been explored. The properties of hollow carbon fibers and their composites show great promise to replace conventional aerospace grade carbon fibers in the foreseeable future. Continuous multifilament carbon fiber tows with 2-3μm fiber diameter have been developed from a PAN (island) - PMMA (sea) bicomponent precursor. These small diameter carbon fibers have tensile strength as high as 5.1 GPa and tensile modulus as high as 434 GPa in different trials. The size of the defects in these fibers is estimated to be in the range of 35-70 nm. The role of smaller diameter in improving the tensile properties of these fibers is explored and the nano scale defects in these fibers have been characterized. Finally, the efficacy of four supervised machine learning techniques, in establishing a mathematical relationship to model the continuous stabilization and carbonization process and predicting the tensile strength and modulus of the fibers, based on the manufacturing process parameters, has been investigated. The data set consisted of 600 data points with 31 features each. The results indicate that machine learning can be used to approximate the underlying function describing the effect of the manufacturing process parameters on the carbon fiber tensile properties This thesis develops a comprehensive understanding of the three technologies that can each accelerate the development high performance structural carbon fibers. Pursuing these studies separately or in conjunction with each other will likely bring about a paradigm shift in the way high performance carbon fibers and composites are developed.
Advanced Dispersion Strategies of Carbon Nanofillers and their use to enhance Mechanical and Electrical Properties of Polyacrylonitrile Fibers

(Georgia Institute of Technology, 2020-12-06) Arias Monje, Pedro Jose

This study focuses on making next generation of polyacrylonitrile fibers containing carbon nanofillers, namely carbon nanotubes (CNTs) and carbon black (CB). Mechanically strong and electrically conducting poly(acrylonitrile) (PAN) fibers were obtained by incorporating up to (a) 15 wt% single wall carbon nanotubes (SWNTs) and (b) 15 wt% carbon black (CB) and 2 wt% multiwall carbon nanotubes (MWNTs). These fibers with tensile modulus of up to 32.1 GPa and electrical conductivity of 2.2 S/m rival some intrinsically electrically conducting polymer fibers without doping. Nanocomposite carbon fibers with up to (a) 25 wt% SWNTs and (b) 24 wt% carbon black and 3 wt% MWNTs were also produced, and it is shown that CNT inclusion improves tensile modulus, while the inclusion of CB can be used to lower the carbon fiber cost, while lowering the mechanical properties. Stretchable PAN fibers with up to 60 wt% CB were also produced by increasing the diameter of the CB particles. Fibers with high SWNT loading of 15 wt% were possible by wrapping the SWNTs with poly(methyl methacrylate) (PMMA). The mechanism of PMMA wrapping of SWNTs was studied experimentally and theoretically (using molecular dynamic simulation). It is shown that PMMA wrapping can be used to increase filler-matrix interaction in the polymer fiber. It is further shown that PMMA wrapping is not detrimental to the filler-matrix interaction in the resulting carbon fiber. This is despite the fact that PMMA does not have carbon yield. Effect of the carbon nanotubes and carbon black fillers on PAN solution/dispersion rheology has been studied. The effect of these fillers on fiber processability and fiber structure is also comprehensively studied. Research also includes stabilization and carbonization of the conductive CB/PAN nanocomposite fibers via Joule Heating to obtain low-cost carbon fibers.
STUDIES ON HIGH-PERFORMANCE THERMOSETS AND THEIR INTERFACE AND INTERPHASE WITH CARBON-NANOTUBES

(Georgia Institute of Technology, 2020-12-03) Kirmani, Mohammad Hamza Hamza

Materials with higher strength to weight ratio than the current state of the art (SOA) carbon fiber reinforced plastics (CFRP) are desired by NASA to support affordable space exploration, including human travel to mars and beyond. The carbon nanotube-polymer (CNT-polymer) composites are expected to have significantly better mechanical properties than the current SOA CFRP and qualify as a potential system for achieving the target mechanical properties in materials required to support human travel to mars. CNT containing polymer composites, however, have some limitations, one of which is the load transfer at the CNT- polymer interface. The interface plays a critical role in determining the overall macroscale properties of the composite. While, significant attention has been directed to this end the CNTs in the composites have not yet reached their full potential. There are several aspects of the CNT-polymer composites which can help create the next generation of high strength and lightweight materials, to help support human travel to mars and beyond. These include, (a) improving the fracture toughness of the polymer resin, (b) understanding and optimizing the CNT-polymer interactions, (c) understanding the effects of CNTs on the polymer cure reactions, which consequently can alter the mechanical properties of the composite, (d) modifying the CNT-polymer interface-interphase through surface treatment and sizing, (e) understanding the effects of amorphous carbon on the CNT-polymer interface-interphase. Herein, the first part of the dissertation focuses on the effect of processing on the molecular structure and the properties of a multi-component aerospace grade bismaleimide (BMI) resin, containing no CNTs, and is discussed in Chapter 2. Materials in nature such as nacre that are made of mechanically inferior building blocks exhibit extreme toughness at the macro scale because of the geometry and arrangement of their constituents. Taking a cue from these systems, we have investigated whether the molecular rearrangement in a heterogeneous BMI system can alter toughness at the macro scale. To this end, a multicomponent BMI system is processed by using (a) a melt and cast (termed Melt) approach and (b) a dual asymmetric centrifuge based high-speed shear mixing (termed HSSM) approach to enforce molecular rearrangement. FTIR, Raman, and NMR spectroscopies have been used to study the molecular rearrangement upon HSSM processing. Small-angle X-ray scattering has been used to study the effect of processing on the molecular arrangement of the BMI. The second part of this dissertation focuses on the structure, process and properties of CNT modified BMI, with tailored interface-interphase and is discussed in Chapter 3. With the recent large-scale production and availability of the CNT macro-assemblies in the yarn, tape and sheet forms, CNT-polymer composites could now be prepared through conventional CFRP manufacturing techniques such as filament winding. It is however expected that the resin dominated properties, such as the inter and intra laminar fracture toughness in these CNT- polymer composites would still remain relatively weak, as they have been for the CFRPs. Modifying the resin with CNTs is an attractive route for further improving the resin properties. Herein, CNT- BMI nanocomposites using three different CNTs and via two different processing routes, have been prepared and studied. The third part of this dissertation focuses on the effects that the CNT have on the cure of the BMI, as well as the effects that the cure of BMI has on the CNTs, in the nanocomposites containing up to 40 wt% CNTs, and is discussed in Chapter 4. CNTs can interact with the BMI system through the NH-π, π-π, CH-π, and OH-π, non-covalent interactions. The individual components of the BMI however can have exclusive non-covalent interactions with the CNTs. For example, in a BMI system containing 4,4'- bismaleimidodiphenylmethane (BDM) and diallylbisphenol A (DABA) components, only the BDM component contains the maleimide functional group which can potentially interact with the CNTs through the NH-π bonding, while only the DABA component, containing the OH functional group can potentially interact with the CNTs through the OH-π interactions. The potential for the preferential stacking of the different BMI components around the CNTs, can have important implications on the cure behavior of the BMI in the nanocomposite and consequently on the overall mechanical properties of the nanocomposite. Herein, two different types of CNTs in the sheet form: unbaked and baked (termed as UB and B CNT), have been employed. The effects of the varying CNT content on the inter-CNT spacing, cure reactions of the BMI, compression of CNTs and the thermomechanical properties of the nanocomposites have been investigated. The thermomechanical results and the theoretical calculations have then been used to estimate the interphase thickness of the CNT- BMI nanocomposites. The fourth part of this dissertation focuses on sizing and tailoring the CNT- BMI interface - interphase using a carbon fiber sizing, and is discussed in Chapter 5. Sizing of carbon and glass fibers is a critical step in the manufacturing of their respective composites with polymers and has led to improved interfacial shear strength (IFSS), inter-laminar shear strength (ILSS) and fracture toughness of the composites. As the CNT-polymer composites could now be prepared through conventional CFRP manufacturing techniques such as filament winding, the question is, could we integrate another critical step of the conventional CFRP manufacturing, i.e., ‘sizing’, to the CNT-polymer composite preparation to tailor the CNT-polymer interface-interphase? To be able to answer that question, we first need to understand the sizing-CNT interactions and reactions. To this end, herein, the effects that (a) CNTs, (b) the degree of functionalization and defects (DOFD) in the CNTs and (c) the sizing content, have on the sizing cure reaction and cure kinetics have been evaluated. CNTs with three different DOFD have been employed. The sizing coated CNTs have then been used to prepare nanocomposite films with a high- performance aerospace grade bismaleimide (BMI) resin. Overall three different types of CNT- BMI interface-interphase have been prepared and studied in nanocomposites containing 60 wt% CNTs: (a) pristine CNT- BMI, (b) functionalized CNT- BMI, and (c) sizing coated functionalized CNT- BMI. The effect of CNT, CNT functionalization and sizing coated CNTs on the BMI cure reactions, thermomechanical properties and the molecular heterogeneity and hierarchy of the nanocomposites have been studied and discussed. Finally, CNTs may contain amorphous carbon, among other impurities which consequently could interfere with the interfacial interactions of the CNT and the polymer. While such impurities are expected to have a negative effect on the polymer-CNT interface, quantitative evidence of the extent of such negative effects is lacking. Herein, the effect that the amorphous carbon and the baking of CNTs to remove the amorphous carbon have, on the interfacial stress transfer with the polyurea matrix has been studied and discussed in Chapter 6. During CNT synthesis, by products such as amorphous carbon may be formed which consequently could interfere with the interfacial interactions of the CNT and the polymer. While such impurities are expected to have a negative effect on the polymer-CNT interface, quantitative evidence of the extent of such negative effects is lacking. Herein, the difference in interfacial straining has been studied in composites of polyurea with two types of CNT sheets: (a) sheets containing amorphous carbon (termed as unbaked CNT sheet) and (b) sheets that are thermally treated to remove amorphous carbon (termed as baked CNT sheet). The understanding of the effects of the amorphous carbon and the baking treatment, based on the CNT- polyurea system should be translatable to other CNT-polymer system, including the CNT-BMI system. It is expected that these studies will provide guidance for the manufacturing of CNT, or CNT and carbon fiber hybrid based laminates that will ultimately meet NASA mechanical property goals.
Development of fundamental understanding of the cure kinetics of benzoxazine epoxy blends

(Georgia Institute of Technology, 2020-01-13) Maffe, Adam Paul

This study attempts to bridge the gap between the current fundamental understanding of benzoxazines on the monomer level and their macro scale thermo-mechanical properties. Bisphenol-A based benzoxazine (Bz) was blended with di- and tri-functional epoxies to reduce viscosity for processing, and their resulting thermal and mechanical properties were characterized. Additionally, the formation of inter-molecular and intra-molecular hydrogen bonds was investigated within a Bz-epoxy two component system. Activation energy, heat of reaction, degradation temperature, hydrogen bonding characterization and thermo-mechanical characterization were studied using a differential scanning calorimeter, dynamic mechanical analyzer, thermogravimetric analysis, Fourier transform infrared spectroscopy and quasistatic tensile testing. Preliminary results show a synergistic increase in Tg of the blends, for both di- and tri-funcitonal epoxy blends. Surprisingly, while the two components exhibit Tg’s of ~ 150-170 oC, the blended systems consistently exhibited a Tg in the range of 210-250 oC. This work aims to expand upon thermal and mechanical characterization data generated by our collaborator Ehsan Barjasteh for the benzoxazine – di-functional epoxy system, as well as explore a new benzoxazine – tri-functional epoxy-based system. Our underlying motivations in this study are to identify the origins of the synergistic increase in Tg upon blending through various thermo-mechanical characterization methods and in-situ FT-IR analysis of cure kinetics, as well as identifying the compositions and functionality which exhibit the most desirable combination of thermal and mechanical properties.
Processing, structure, and properties of polyacrylonitrile-nanocellulose composite films and fibers

(Georgia Institute of Technology, 2018-07-24) Luo, Jeffrey

Cellulose is the most abundant biopolymer on earth with an estimated production of 1.5 x 1012 tons per year. With the high production, cellulose could be utilized to increase our sustainability and reduce our dependence on synthetic polymers made from oil. Recently nanocellulose in the form of cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs) as potential fillers have been gaining significant interest. This is due to nanocellulose’s high mechanical properties, high surface area, biodegradability, biorenewability, and low toxicity.Polyacrylonitrile (PAN) is a synthetic polymer and has uses in clothing, home furnishings, and filtration membranes. PAN is also the predominant precursor for carbon materials such as carbon fiber. The effect of CNC loading on the mechanical, thermal, optical, and structural properties of PAN-CNC films was studied. Films were made successfully with up to 40 wt% CNC loading. These composite films demonstrated the same optical transparency, and an increase in mechanical properties when compared to neat PAN. The effect of CNC loading on rheology was also looked at, and it was found that the addition of CNCs increases the viscosity at low shear rates. It was also found that the viscosity of the PAN-CNC suspensions increased with time. The stabilization kinetics of the PAN-CNC films, and structural changes of the films were investigated. It was found that the addition of CNCs reduced the cyclization activation energy. It was also found that the reaction rate of cyclization and oxidation could increase with the addition of CNCs. The thermal stability of CNCs was determined to be higher in the composite films than in neat form, which was examined by wide angle x-ray diffraction and Fourier transform infrared spectroscopy. A method to make sulfonated CNFs that has the same surface chemistry as CNCs made by sulfuric acid hydrolysis is developed. This sulfonation of CNFs improves the dispersibility of CNFs in solvent. The method developed in this study can produce sulfonated CNFs faster the other reported sulfonation method. This technique is also faster than the most common CNF functionalization, TEMPO oxidation. The chemical stability of TEMPO CNFs and sulfonated CNFs in water and dimethylformamide was also investigated. The characterization of PAN-CNF fibers and the effect of processing on the resulting mechanical properties was studied. Two different methods of functionalization were performed on the CNFs, TEMPO oxidation and amination, and the reinforcement effect of the different functionalizations were studied. The aminated CNFs provided more mechanical reinforcement than the TEMPO CNFs. This is attributed to the difference between surface chemistry, and the better interaction between the amine groups on the aminated CNFs and PAN than the carboxylic acid groups on the TEMPO CNFs.
Structure, processing, and properties of carbon nanotube composite with polypropylene

(Georgia Institute of Technology, 2018-01-22) Wang, Po-Hsiang

Having high aspect ratio and structural similarity to the macromolecular building blocks, carbon nanotubes (CNTs) have demonstrated their great potential in tailoring the physical properties, e.g. conformation, crystallization, rheological, electrical and thermal characteristics etc., of the polymers. However, achieving good CNT dispersion, while also achieving good interfacial properties, remains a challenge, especially in non-polar polymers such as polyethylene (PE) and polypropylene (PP). In this study, the CNT modified PP with an engineered interphase was manufactured using a co-solvent process where a homogeneous PP layer was bonded non-covalently on acid functionalized multiwall carbon nanotubes (f-MWNTs). Unlike conventional melt blending, which simply compounds the neat polymer and pristine CNTs, or preparing the nanocomposite with covalently modified CNTs, e.g. via in-situ polymerization of macromolecules, this approach provides a practical way not only to obtain good CNT dispersion but also to alter the polymer physical properties by using small amount of CNTs (less than 1 wt%). This research systematically investigates the polymer-CNT interaction when the interphase is tailored. Three types of interphase were studied: (i) The PP/f-MWNT system using co-solvent process. (ii) The maleic anhydride grafted polypropylene (MA-g-PP)/f-MWNT system using co-solvent process. (iii) The PP/pristine MWNT (p-MWNT) system using melt process. The f-MWNT based systems, termed as master batches, were further melt blended with neat polymers to the target f-MWNT concentrations (0.001 wt% to 1 wt%) followed by injection molding. Mechanical properties, including the tensile and impact testing, as well as rheological, crystallization and the structure of the interphase were studied. It was shown that CNTs serve as a strong nucleating agent for templated polymer crystal growth. With addition of 1 wt % f-MWNT, an 152 % increase in PP impact strength was achieved in the PP/f-MWNT system. On the other hand, addition of pristine (unmodified) MWNTs yielded no statistical difference in the impact strength between the nanocomposite and neat PP. Strong adhesion between PP and f-MWNTs owing to the presence of interphase was verified by Raman spectroscopy as well as SEM and was shown to be absent between PP and p-MWNTs. Also, the structure of this interphase was characterized by thermal analysis and wide-angle X-ray diffraction. It was found that the polymer at the interphase exhibits higher melting temperature, suggesting higher crystal perfection and/or larger crystals, with respect to the bulk polymer. With a designed thermal treatment, the span of this interphase can be further increased as demonstrated by the formation of columnar crystals surrounding CNTs. Abnormal temperature dependence of shear viscosity was observed when temperature of the polymer melt was increased from 190 °C to 220 °C. Overall, this research demonstrates that tailored CNT/polymer interphase is needed for achieving high performance nanocomposites and improving the polymer-CNT interaction.
Studies on polyacrylonitrile/cellulose nanocrystals composite precursor and carbon fibers

(Georgia Institute of Technology, 2017-11-10) Chang, Huibin

In this study, PAN fibers containing up to 40 wt% CNC were spun by gel-spinning technology. The structure, morphology, and mechical properties of PAN/CNC precursor and their carbon fibers have been studied. The results show that the H2O/dimethyl formamide (DMF) co-solvent disperses individual CNCs more effectively than pure H2O or pure DMF, especially at high CNC concentrations. The addition of CNCs improves the tensile modulus of PAN fibers at a low draw ratio. The fully drawn PAN/CNC fibers containing even the 40 wt% CNCs exhibit the same strain to failure as the fully drawn PAN fibers. The high orientation of CNCs in PAN fibers was observed by Raman spectroscopy, in which the 1095 cm−1 Raman band of CNCs shows a two-fold symmetry under vertical/vertical (VV) mode and a four-fold symmetry under vertical/horizontal (VH) mode. With the addition of 40 wt% CNC, the activation energy of cyclization and crosslinking reactions of PAN is reduced by 17.5 and 19%, respectively. The carbon fibers made from PAN containing 20 wt% CNCs show the tensile strength of 2.3 GPa and the tensile modulus of 252 GPa, which are comparable to the PAN based carbon fiber properties (tensile strength of 1.9 GPa and tensile modulus of 251 GPa) processed under the same conditions.
Effect of poly(methyl methacrylate) wrapping on the structure and properties of CNT films, and polymer/CNT films and fibers

(Georgia Institute of Technology, 2017-02-24) Bakhtiary Davijani, Amir Ahmad

Carbon nanotubes (CNTs) exhibit high electrical and thermal conductivity and good mechanical properties, making them suitable fillers for composites. Their effectiveness as a filler is affected by their state of aggregation. Various solvents, surfactants, and processing techniques have been studied to improve CNT dispersion in polymers. However, prior to this work there is no suitable solution for achieving good CNT dispersion. In this study, a novel process has been developed that prevents CNT aggregation. Ordered helical wrapping of poly (methyl methacrylate) (PMMA) has been achieved on single wall carbon nanotubes (SWNTs). PMMA wrapped SWNT dispersions in dimethylformamide (DMF) are found to be stable for over three months at room temperature. Ordered PMMA wrapping has been confirmed by X-ray diffraction, and the wrapping behavior has also been verified using molecular modeling. PMMA only wraps on SWNTs with diameter of ~1 nm and not on larger diameter CNTs, such as few wall and multi wall carbon nanotubes. PMMA wrapped SWNT dispersions have also been characterized using UV-vis and Raman spectroscopy which confirm exfoliation of PMMA wrapped SWNTs. The novel finding has been successfully leveraged for electrical energy storage and mechanical reinforcement. SWNT buckypapers, typically have a surface area of about 650 m2/g. Using PMMA wrapping, SWNT buckypapers with surface area as high as 950 m2/g have been processed. These buckypapers exhibited significantly higher energy storage performance when used as electrodes in electrochemical supercapacitor. At a given power density, the energy density of the high surface electrodes was more than four times higher than the best value reported in the literature to-date for carbon nanotube or graphene electrodes. Wrapping SWNTs with PMMA in buckypaper increased the modulus and tensile strength by a factor of 5.9 and 3.7, respectively, compared to pristine SWNT buckypaper. Stress transfer studies on buckypapers revealed that while non-wrapped SWNTs experienced negligible stress during deformation, PMMA wrapped SWNTs took up to ~1 GPa stress before mechanical failure of the buckypaper. The modulus of composite films with PMMA wrapped SWNTs was 75 % higher than non-wrapped SWNT films. The effect of PMMA wrapping on thermomechanical properties and electrical conductivity of composite films is also reported. PMMA wrapped and non-wrapped SWNTs were incorporated in PAN fibers and the effect of PMMA wrapping on mechanical properties, and stress transfer was characterized. The stress transfer analysis of SWNTs in precursor fiber indicated 45 % higher interfacial shear strength in PMMA wrapped SWNTs compared to non-wrapped SWNTs. PMMA wrapping effectively debundled SWNTs in the PAN fibers as evidenced by Raman spectroscopy of the precursor fiber. SEM images of the carbon fiber fracture surface revealed 60% reduction in fibril size when PMMA wrapped SWNTs were used instead of non-wrapped SWNTs.
Gel spun polyacrylonitrile based carbon fibers containing lignin and carbon nanotubes

(Georgia Institute of Technology, 2017-01-13) Liu, Hsiang-Hao Clive

Forestry bioproduct lignin has been long proposed as an ideal material for carbon fiber precursor due to its abundance, renewability, high carbon yield and cost-effectiveness as the biorefinery by-product. However, little success of lignin based commercial carbon fibers had been reported due to the barriers such as varying chemical structure, complex thermal stabilization process, poor miscibility with other polymers that resulted in unmanageable processing and limited mechanical performance. In this research, systematic investigations are conducted on the lignin/polyacrylonitrile (PAN) blends in solution, fiber structure, and during thermal stabilization and carbonization process to valorize biorefinery waste lignin for its potential towards green manufacturing and renewability of carbon fibers. Dynamic shear rheology study of PAN with various lignin content in solutions was carried out to investigate the potential boundary conditions for lignin incorporation, and to characterize the interactions of PAN and lignin with respect to fiber processing. The findings have shown that increasing incorporation of lignin in the PAN solution promotes fluid-solid transition during coagulation, and reduces solution viscosity, yield stress, relaxation time, and thermo-reversibility. The rheology findings along with gel spinning technology been successfully leveraged to address un-favored porous structure of PAN/lignin blends reported in the literature. Single-component PAN/lignin and PAN/lignin/carbon nanotubes (CNT) blend fibers, as well as PAN-sheath and PAN/lignin-core bi-component fiber are manufactured. The presence of lignin in spinning dope is shown to reduce the fiber maximum draw ratio. Since fibers manufactured with higher draw ratio can render smaller diameter to reduce number of defects per unit volume in the fiber structure, the decrease in maximum draw ratio limits the improvements on the derived carbon fiber mechanical properties. With the aid of bi-component fiber spinning, PAN sheath provides protection and endurance to fibers towards higher draw ratios. The presence of lignin and CNT is shown to affect the precursor fiber structure and to alter the structural reordering process during carbonization. In thermal stabilization process, a critical step in carbon fiber conversion, lignin was shown to reduce PAN cyclization, oxidation, and crosslinking reaction activation energies and increase the corresponding reaction rates. This renders a potential to reduce time and energy consumption during carbon fiber manufacturing. With ≥ 30 wt.% of lignin incorporation, batch-processed PAN/lignin carbon fibers exhibit tensile strength of 2.11 GPa and tensile modulus of 260 GPa, exceeding low-cost carbon fiber performance target (1.72 GPa, 172 GPa) set by the U.S. Department of Energy and the best PAN/APL carbon fiber mechanical performance reported in the literature to-date. With proper processing parameters, PAN sheath, and PAN/APL core bi-component carbon fibers with porous core (translates to ~24% overall porosity) can be manufactured as an approach for low-density carbon fibers.