VTechWorks Repository :: Browsing by Author "Bortner, Michael J."

Browsing by Author "Bortner, Michael J."

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4D-Printing with Cellulose Nanocrystal Thermoplastic Nanocomposites: Mechanical Adaptivity and Thermal Influence
Seguine, Tyler William (Virginia Tech, 2021-05-24)
This thesis is concerned with fused filament fabrication (FFF) of cellulose nanocrystal (CNC) and thermoplastic polyurethane (TPU) nanocomposites, focusing on preliminary optimization of a processing window for 3D printing of mechanically responsive composites and the influence of temperature on mechanical adaptivity, thermal stability, and rheology. CNC thermoplastic nanocomposites are a water responsive, mechanically adaptive material that has been gaining interest in additive manufacturing for 4D-printing applications. Using a desktop FlashForge Pro 3D printer, we first established a viable processing window for a nanocomposite comprising 10 wt% CNCs in a thermoplastic urethane (TPU) matrix, formed into a filament through the combination of masterbatch solvent casting and single screw extrusion. Printing temperatures of 240, 250, and 260°C and printing speeds of 600, 1100, and 1600 mm/min instituted a consistent 3D-printing process that produced characterizable CNC/TPU nanocomposite samples. To distinguish the effects of these parameters on the mechanical properties of the printed CNC/TPU samples, a design of experiments (DOE) with two factors and three levels was implemented for each combination of printing temperature and speed. Dynamic mechanical analysis (DMA) highlighted 43 and 66% increases in dry-state storage moduli values as printing speed increases for 250 and 260°C, respectively. 64 and 23% increases in dry-state storage moduli were also observed for 600 and 1100 mm/min, respectively, as temperature decreased from 260 to 250°C. For samples printed at 240°C and 1600 mm/min, it was determined that that parameter set may have fallen out of the processing window due to inconsistent deposition and lower dry-state storage moduli than what the slower speeds exhibited. As a result, the samples printed at 240°C did not follow the same trends as 250 and 260°C. Further analysis helped determine that the thermal energy experienced at the higher end printing temperatures coupled with the slower speeds decreased the dry-state storage moduli by nearly 50% and lead to darker colored samples, suggesting CNC degradation. Isothermal thermogravimetric analyses (TGA) demonstrated that the CNC/TPU filament would degrade at relative residence times in the nozzle for all the chosen printing temperatures. However, degradation did not eliminate the samples' ability to mechanically adapt to a moisture-rich environment. DMA results verified that mechanical adaptivity was persistent for all temperature and speed combinations as samples were immersed in water. However, for the higher temperatures and slower speeds, there was about a 15% decrease in adaptability. Optimal parameters of 250°C and 1600 mm/min provided the highest dry-state storage modulus of 49.7 +/- 0.5 MPa and the highest degree of mechanical adaptivity of 51.9%. To establish the CNC/TPU nanocomposite's use in 4D printing applications, shape memory analysis was conducted on a sample printed at the optimal parameters. Multiple wetting, straining, and drying steps were conducted to highlight 76% and 42% values for shape fixity and shape recovery, respectively. Furthermore, a foldable box was printed to serve as an example of a self-deployable structure application. The box displayed shape fixity and recovery values of 67% and 26%, respectively, further illustrating significant promise and progress for CNC/TPU nanocomposites in 4D-printed, shape adaptable structures. Further analysis of the effect of degradation during FFF of the CNC/TPU nanocomposite was conducted using rotational rheometry, Fourier-Transform Infrared Spectroscopy (FTIR), and polymer swelling experiments. A temperature ramp from 180 to 270°C showed a significant increase in complex viscosity (h*) at the chosen printing temperatures (240, 250, and 260°C). Moreover, h* of neat TPU suddenly increases at 230°C, indicating a potential chemical crosslinking reaction taking place. 20-minute time sweeps further verified that h* increases along with steady increases in storage (G') and loss (G'') moduli. From these results, it was hypothesized that crosslinking is occurring between CNCs and TPU. Preliminary characterization with FTIR was used to probe the molecular structure of thermally crosslinked samples. At 1060 and 1703 cm-1, there are significant differences in intensities (molecular vibrations) as the temperature increases from 180 to 260°C related to primary alcohol formation and hydrogen bonded carbonyl groups, respectively. The hypothesis is the disassociation of TPU carbamate bonds into soft segments with primary alcohols and hard segments with isocyanate groups. The subsequent increasing peaks at 1060 and 1703 cm-1 may indicate crosslinking of CNCs with these disassociated TPU segments. To quantify potential crosslinking, polymer swelling experiments were implemented. After being submerged in dimethylformamide (DMF) for 24 hours, CNC/TPU samples thermally aged for 15 minutes at 240, 250, and 260°C retained their filament shape and did not dissolve. The 240 and 250°C aged samples had relatively similar crosslink densities close to 900 mole/cm3. However, from 250 to 260°C, there was about a 36% increase in crosslink density. These results suggest that crosslinking is occurring at these printing temperatures because both CNCs and TPU are thermally degrading into reactive components that will lead to covalent crosslinks degradation. Additional characterization is needed to further verify the chemical structure of these CNC/TPU nanocomposites which would provide significant insight for CNC/TPU processing and 3D printing into tunable printed parts with varying degrees of crosslinking.
Accelerated Durability Characterization of Laminated Polycarbonate Systems
Riddle, Samuel George (Virginia Tech, 2024-08-27)
Accelerated Testing Method to Estimate the Lifetime of Polyethylene Pipes
Kalhor, Roozbeh (Virginia Tech, 2017-02-14)
The ability to quickly develop predictions of the time-to-failure under different loading levels allows designers to choose the best polymeric material for a specific application. Additionally, it helps material producers to design, manufacture, test, and modify a polymeric material more rapidly. In the case of polymeric pipes, previous studies have shown that there are two possible time-dependent failure mechanisms corresponding to ductile and brittle failure. The ductile mechanism is evident at shorter times-to-failure and results from the stretching of the amorphous region under loading and the subsequent plastic deformation. Empirical results show that many high-performance polyethylene (PE) materials do not exhibit the brittle failure mechanism. Hence, it is critical to understand the ductile mechanism and find an approach to predict the corresponding times-to-failure using accelerated means. The aim of this study is to develop an innovative rupture lifetime acceleration protocol for PE pipes which is sensitive to the structure, orientation, and morphology changes introduced by changing processing conditions. To accomplish this task, custom fixtures are developed to admit tensile and hoop burst tests on PE pipes. A pressure modified Eyring flow equation is used to predict the rupture lifetime of PE pipes using the measured mechanical properties under axial tensile and hydrostatic pressure loading in different temperatures and strain rates. In total, the experimental method takes approximately one week to be completed and allows the prediction of pipe lifetimes for service lifetime in excess of 50 years.
Additive Manufacturing Filled Polymer Composites for Environmental Contaminants: Material Extrusion Processing, Structure and Performance
Kennedy, Alan James (Virginia Tech, 2023-12-18)
Research interest in Additive Manufacturing (AM) as an enabling technology for customizable parts is rapidly expanding. While much AM research focus is on high performance feedstocks and process optimization to obtain parts with improved mechanical properties, interest in the environmental applications of AM has recently increased. The lower cost and greater accessibility AM is leading to novel environmental research solutions in wastewater treatment and toxicity reduction by capitalizing on the increased affordability and accessibility of 3D printing (3DP) technologies for customizable, high surface area structures. The novelty and focus of this dissertation is exploration of Material Extrusion (MatEx) based Fused Filament Fabrication (FFF) of filled polymer composites as a disruptive technology enabler for deployable and retrievable structures in environmental media for adsorption, destruction and toxicity reduction of harmful chemicals. This dissertation addresses research questions that generally answer, "why AM for environmental applications?". The inherent layer-by-layer design provides larger surface area structures for interaction with contaminated media. Polylactic acid (PLA) was selected due to its green sources and biocompatibility relative to synthetic polymers and its wide processing window allowing shear thinning and "printability" despite the elevated viscosity and modulus of highly filled composites. The filler selected for contaminant adsorption was microporous zeolite, which has affinity for ammonia, radionuclides and Per- and Polyfluorinated Substances (PFAS). The filler selected for contaminant destruction was photocatalytic TiO2 nanoparticles which can degrade organic chemicals, harmful algal bloom toxins and PFAS. A preliminary research hurdle was overcome by demonstrating that immobilization of zeolite and TiO2 in a PLA binder matrix did not prevent adsorption or free radical release, respectively. The first major research objective involved investigation of high surface area printed PLA-zeolite geometries with different zeolite loadings and found that while ammonia was reduced, there were diminishing returns with increased loading in terms of mass standardized adsorptive performance due to insufficiently exposed zeolite. The research solution leveraged AM print process parameters to increase the macroporosity of the printed composite structure to create voids and channels allowing water infiltration and chemical adsorption to zeolite. Faster printing of larger roadways generated macrostructural voids that were maintained by extrusion at lower temperature for rapid solidification. The second research objective involved compounding different loadings and dispersion states of TiO2 in PLA to demonstrate immobilization of TiO2 closer to UV-light penetration water improves photocatalysis. Higher 32% w/w TiO2 loadings were heavily agglomerated and more difficult to print process due to high viscosity, rapid liquid-solid transition (G'>G") and particle network recovery during printer retractions, leading to nozzle clogging. Lower 20% w/w loading was more conducive to larger production printing due to lower viscosity, longer viscosity recovery times for retractions and thus generally a wider processing window. While altering twin screw processing parameters reduced TiO2 agglomerates in filaments, leading to increases in crystallinity (due to seeding effects and chain scission) and lower viscosity recovery, photocatalytic performance was not significantly improved. Evidence presented showed that larger particle agglomerates were more toward the inside of printed surfaces and thus less available to UV-light irradiation. This location of larger particles is supported by previous theoretical and empirical investigations showing larger particles migrate at a faster velocity away from the outer walls of confined extrudates within non-Newtonian flow fields due to normal forces, leaving more smaller particles toward outer surfaces. This research provided novel contributions to the environmental and AM research communities and pioneered a convergence of these fields into an interdisciplinary community of practice focused on better characterization and processing in environmental applications to improve structure-environmental property relationships. Future research should build on these findings to enhance performance through multi-functional materials that adsorb and destroy contaminants. The reactive surface area should be further increased through by high surface area designs and print parameter optimized porous structures providing a continuum of meso- to microporosity as confirmed by chemical flux and mass transfer studies for additional AM technologies (e.g., Direct Ink Write).
Additive Manufacturing of Commercial Polypropylene Grades of Similar Molecular Weight and Molecular Weight Distribution
Nour, Mohamed Imad Eldin (Virginia Tech, 2024-06-12)
Filament-based material extrusion additive manufacturing (MEAM) is an established technique in additive manufacturing (AM). However, semicrystalline polymers, such as polypropylene (PP), have limited commercial use in MEAM processes in the past due to their rapid crystallization kinetics and the subsequent effect on the integrity of the generated structures. The rapid crystallization of PP can be controlled by formulating blends of PP with hydrocarbon resins to enable longer re-entanglement times for interlayer adhesion. While the topic of formulating PP blends/composites with other materials to improve the printability has been investigated, variation in properties of commercial PP grades, of similar molecular weight (MW) and molecular weight distribution (MWD), on printability is still to be investigated. Those commercial PP grades can have wide variation in properties such as Melt Flow Index (MFI), additive content, and polymer architecture which can impact material properties relevant to printability. To investigate the effect of properties of commercial PP on their printability and mechanical performance, different commercial PP grades, with different properties, are blended with a fixed loading of hydrogenated resins, and the consequent effects on the mechanical properties of MEAM generated PP structures are studied via mechanical analysis. Tensile strength and the extent of interlayer adhesion in the 3D printed blends are characterized through rheological measurements. These measurements emphasize the importance of the relative location of the storage/loss modulus crossover point via small oscillatory frequency sweeps. We specifically show that a relatively higher crossover frequency will correlate with improved interlayer adhesion and reduced warpage in printed structures. However, this improvement is accompanied by a tradeoff, resulting in inferior tensile strength and an increased degree of print orientation anisotropy.
Advanced Thermoplastic Nanocomposite Melt Processing Using an Improved Supercritical Carbon Dioxide Pretreatment for the Nanomaterial
Quigley, John (Virginia Tech, 2014-06-10)
Polymer nanocomposites have been proposed as lightweight replacements for traditional composite materials in various applications. Montmorillonite (MMT) and carbon nanotubes (CNTs) are two nanofillers which have accrued significant interest in the past 20 years due to their superior mechanical and electrical properties, respectively. However, efficient dispersion of the nanofiller and damage to CNTs prevent widespread utilization of these materials in polymer nanocomposites. Novel methods of nanocomposite generation combining the use of supercritical carbon dioxide (scCO2) with melt compounding have been investigated to overcome these issues. The focus of this work is on developing the scCO2 treatment of nanomaterial for thermoplastic nanocomposite generation. First, the supercritical carbon dioxide aided melt blending method was applied to nanoclay nanocomposites of Nylon 6/ and organoclay where the polymer may interact with the nanoclay surface. Second, the effect of scCO2 processing of CNTs was investigated with special consideration to the processing variables. Finally, a study was carried out to analyze the electrical conductivity of polycarbonate nanocomposites generated using CNTs deagglomerated by scCO2 processing. The initial focus of this dissertation is the use of supercritical carbon dioxide (scCO2) as a processing aid in the generation of nylon 6 nanocomposites in which the nylon 6 may interact with the nanoclay surface. Wide-angle X-ray diffraction, transmission electron microscopy, rheology, and tensile tests were carried out to investigate the effect of processing with scCO2 on the final composite morphology and properties. It was observed that mechanical properties of composites prepared with the scCO2 aided melt blending method were similar to or higher than those reported in the literature for samples prepared with twin screw compounding. At 7.6 wt% nanoclay the modulus value reaches 4.75 +/- 0.194 GPa which is one of the highest increases (1.7 GPa) reported for these materials processed at intermediate concentrations. Beyond 7.6 wt% the improvement due to scCO2 processing matched that of direct blending. The next objective of this work is to develop a method for the deagglomeration of commercially available multi-walled carbon nanotubes (MWCNTs) by manipulating processing variables and observing carbon nanotube aspect ratios after deagglomeration. High levels of deagglomeration of Baytubes C 150 P and Nanocyl NC-7000 MWCNT agglomerates were observed, resulting in 30 fold and 50 fold decreases in bulk density, respectively, with median agglomerate sizes < 8 um in diameter. These results were obtained while retaining the aspect ratio of the as-received nanomaterial, irrespective of the MWCNT agglomerate morphology. It was found that the supercritical temperature and pressure of 40 deg C and 7.86 MPa were the optimal temperature and pressure for maximum deagglomeration without damaging the MWCNTs. The final goal of this work is to apply the scCO2 aided melt blending process to generate polycarbonate/ carbon nanotube (CNT) nanocomposites with enhanced electrical conductivity and improved dispersion while maintaining the aspect ratio of the as-received CNTs. Different degrees of scCO2 processed Baytubes C 150 P CNT were benignly deagglomerated with scCO2 resulting in 5 fold (5X), 10X, and 15X decreases in bulk density from the as-received CNTs. The CNT were melt compounded with polycarbonate using single screw melt extrusion and compression molded into plaques. A surface conductivity of 4.8 x 10-8 +/- 2.0 x 10-9 S was observed for samples prepared with the scCO2 aided melt blending at 15X scCO2 processing. Electrical percolation thresholds as low as 0.83 wt% were observed for composites prepared with 15X CNTs using the scCO2 aided melt blending method, while concentrations as high as 1.5% are required without scCO2 processing. The percolation concentration in nanocomposites prepared with 15X scCO2 processing and single screw extrusion is competitive with values reported for similar nanocomposites generated using twin screw melt compounding in the literature. Optical microscopy, transmission electron microscopy, and rheology were used to investigate the dispersion and mechanical network of CNTs in the nanocomposites. The dispersion of CNTs generally improved with scCO2 processing compared to direct melt blending but was found to be significantly worse than that of twin screw melt compounded nanocomposites from the literature. Because the percolation thresholds are similar with substantially different extents of dispersion, the importance of maintaining longer CNTs during nanocomposite generation is emphasized.
Advancing Elastomers to Additive Manufacturing Through Tailored Photochemistry and Latex Design
Scott, Philip Jonathan (Virginia Tech, 2020-07-08)
Additive manufacturing (AM) fabricates complex geometries inaccessible through other manufacturing techniques. However, each AM platform imposes unique process-induced constraints which are not addressed by traditional polymeric materials. Vat photopolymerization (VP) represents a leading AM platform which yields high geometric resolution, surface finish, and isotropic mechanical properties. However, this process requires low viscosity (<20 Pa·s) photocurable liquids, which generally restricts the molecular weight of suitable VP precursors. This obstacle, in concert with the inability to polymerize high molecular weight polymers in the printer vat, effectively limits the molecular weight of linear network strands between crosslink points (Mc) and diminishes the mechanical and elastic performance of VP printed objects. Polymer colloids (latex) effectively decouple the relationship between viscosity and molecular weight by sequestering large polymer chains within discrete, non-continuous particles dispersed in water, thereby mitigating long-range entanglements throughout the colloid. Incorporation of photocrosslinking chemistry into the continuous, aqueous phase of latex combined photocurability with the rheological advantages of latex and yielded a high molecular weight precursor suitable for VP. Continuous-phase photocrosslinking generated a hydrogel scaffold network which surrounded the particles and yielded a solid "green body" structure. Photorheology elucidated rapid photocuring behavior and tunable green body storage moduli based on scaffold composition. Subsequent water removal and annealing promoted particle coalescence by penetration through the scaffold, demonstrating a novel approach to semiinterpenetrating network (sIPN) formation. The sIPN's retained the geometric shape of the photocured green body yet exhibited mechanical properties dominated by the high molecular weight latex polymer. Dynamic mechanical analysis (DMA) revealed shifting of the latex polymer and photocrosslinked scaffold T_g's to a common value, a well-established phenomenon due phasemixing in (s)IPN's. Tensile analysis confirmed elastic behavior and ultimate strains above 500% for printed styrene-butadiene rubber (SBR) latexes which confirmed the efficacy of this approach to print high performance elastomers. Further investigations probed the versatility of this approach to other polymer compositions and a broader range of latex thermal properties. Semibatch emulsion polymerization generated a systematic series of random copolymer latexes with varied compositional ratios of hexyl methacrylate (HMA) and methyl methacrylate (MMA), and thus established a platform for investigating the effect of latex particle thermal properties on this newly discovered latex photoprocessing approach. Incorporation of scaffold monomer, N-vinyl pyrrolidone (NVP), and crosslinker, N,N'-methylene bisacrylamide (MBAm), into the continuous, aqueous phase of each latex afforded tunable photocurability. Photorheology revealed higher storage moduli for green bodies embedded with glassy latex particles, suggesting a reinforcing effect. Post-cure processing elucidated the necessity to anneal the green bodies above the T_g of the polymer particles to promote flow and particle coalescence, which was evidenced by an optical transition from opaque to transparent upon loss of the light-scattering particle domains. Differential scanning calorimetry (DSC) provided a comparison of the thermal properties of each neat latex polymer with the corresponding sIPN. Another direction investigated the modularity of this approach to 3D print mixtures of dissimilar particles (hybrid colloids). Polymer-inorganic hybrid colloids containing SBR and silica nanoparticles provided a highly tunable route to printing elastomeric nanocomposite sIPN's. The bimodal particle size distribution introduced by the mixture of SBR (150 nm) and silica (12 nm) nanoparticles enabled tuning of colloid behavior to introduce yield-stress behavior at high particle concentrations. High-silica hybrid colloids therefore exhibited both a shear-induced reversible liquid-solid transition (indicated by a modulus crossover) and irreversible photocrosslinking, which established a unique processing window for UV-assisted direct ink write (UV-DIW) AM. Concentric cylinder rheology probed the yield-stress behavior of hybrid colloids at high particle concentrations which facilitated both the extrusion of these materials through the UV-DIW nozzle and the retention of their as-deposited shaped during printing. Photorheology confirmed rapid photocuring of all hybrid colloids to yield increased moduli capable of supporting subsequent layers. Scanning electron microscopy (SEM) confirmed well-dispersed silica aggregates in the nanocomposite sIPN's. DMA and tensile confirmed significant reinforcement of (thermo)mechanical properties as a result of silica incorporation. sIPN's with relative weight ratio of 30:70 silica:SBR achieved maximum strains above 300% and maximum strengths over 10 MPa. In a different approach to enhancing VP part mechanical properties, thiol-ene chemistry provided simultaneous linear chain extension and crosslinking in oligomeric diacrylate systems, providing tunable increases to Mc of the photocured networks. Hydrogenated polybutadiene diacrylate (HPBDA) oligomers provided the first example of hydrocarbon elastomer photopolymers for VP. 1,6-hexanedithiol provided a miscible dithiol chain extender which introduced linear thiol-ene chain extension to compete with acrylate crosslinking. DMA and tensile confirmed a decrease in T_g and increased strain-at-break with decreased crosslink density. Other work investigated the synthesis and characterization of first-ever phosphonium polyzwitterions. Free radical polymerization synthesized air-stable triarylphosphine-containing polymers and random copolymers from the monomer 4-(diphenylphosphino) styrene (DPPS). ³¹P NMR spectroscopy confirmed quantitative post-polymerization alkylation of pendant triarylphosphines to yield phosphonium ionomers and polyzwitterions. Systematic comparison of neutral, ionomer, and polyzwitterions elucidated significant (thermo)mechanical reinforcement by interactions between large phosphonium sulfobetaine dipoles. Broadband dielectric spectroscopy (BDS) confirmed the presence of these dipoles through significant increases in static dielectric content. Small-angle X-ray scattering (SAX) illustrated ionic domain formation for all charged polymers.
Continuous flow synthesis of a pharmaceutical intermediate: a computational fluid dynamics approach
Armstrong, Cameron T.; Pritchard, Cailean Q.; Cook, Daniel W.; Ibrahim, Mariam; Desai, Bimbisar K.; Whitham, Patrick J.; Marquardt, Brian J.; Chen, Yizheng; Zoueu, Jeremie T.; Bortner, Michael J.; Roper, Thomas D. (2019-03-01)
Continuous flow chemistry has the potential to greatly improve efficiency in the synthesis of active pharmaceutical ingredients (APIs); however, the optimization of these processes can be complicated by a large number of variables affecting reaction success. In this work, a screening design of experiments was used to compare computational fluid dynamics (CFD) simulations with experimental results. CFD simulations and experimental results both identified the reactor residence time and reactor temperature as the most significant factors affecting product yield for this reaction within the studied design space. A point-to-point comparison of the results showed absolute differences in product yield as low as 2.4% yield at low residence times and up to 19.1% yield at high residence times with strong correlation between predicted and experimental percent yields. CFD was found to underestimate the product yields at low residence times and overestimate at higher residence times. The correlation in predicted product yield and the agreement in identifying significant factors in reaction performance reveals the utility of CFD as a valuable tool in the design of continuous flow tube reactors with significantly reduced experimentation.
Crystallization Behavior, Tailored Microstructure, and Structure-Property Relationships of Poly(Ether Ketone Ketone) and Polyolefins
Pomatto, Michelle Elizabeth (Virginia Tech, 2024-04-08)
This work investigates the influence of microstructure and cooling and heating rates on the physical and chemical properties of fast crystallizing polymers. The primary objectives were to 1) utilize advanced methodologies to accurately determine the fundamental thermodynamic value of equilibrium melting temperature (Tmo) for the semi-crystalline polymer poly(ether ketone ketone) (PEKK), 2) increase understanding of the influence of microstructure (random versus blocky) of functionalized semi-crystalline polymers on physical and chemical properties, and 3) understand the influence of additive manufacturing process parameters on semi-crystalline polymer crystallization and final properties. All objectives utilized the advanced characterization technique of fast scanning calorimetry (FSC) using the Mettler Toledo Flash DSC 1. The first half of this work focuses on the high-performance semi-crystalline aromatic polymer poly(ether ketone ketone) (PEKK) with a copolymerization ratio of terephthalate to isophthalate moieties (i.e., T/I ratio) of 80/20. Due to the fast heating and cooling rates of the Flash DSC, PEKK underwent melt-reorganization upon heating at slow heating rates. This discovery resulted in utilizing a Hoffman-Weeks linear extrapolation of the zero-entropy production temperature to establish a new equilibrium melting temperature of 382 oC. Additionally, a new NMR solvent, dichloroacetic acid, was discovered for PEKK, allowing for comprehensive NMR analysis of PEKK for the first time. Diphenyl acetone (DPA) was discovered as a novel, benign gelation solvent for PEKK, enabling heterogeneous gel-state bromination and sulfonation to afford blocky microstructures. The gel state functionalization process resulted in a blocky microstructure with runs of pristine crystallizable PEKK retained within the crystalline domains, and amorphous domains containing the functionalized PEKK monomers. The preservation of the pristine crystalline domains resulted in enhanced physical and chemical properties compared to the randomly functionalized analogs. Additionally, heterogeneous gel state functionalization of PEKK gels prepared from different solvents and gelation temperatures resulted in differences in crystallization behavior between blocky microstructures of the same degree of functionalization. This result demonstrates that the blocky microstructure can be tuned through controlling the starting gel morphology. The second half of this work focuses on understanding the influence of cooling and heating rates on the melting, crystal morphology, and crystallization kinetics on isotactic polypropylene (iPP), iPP-polyethylene copolymers (iPP-PE), and iPP/iPP-PE blends and using this information to gain understanding of how these polymers crystallize during the additive manufacturing processes of powder bed fusion (PBF) and material extrusion (MatEx). The crystallization kinetics of iPP, iPP-PE copolymers, and iPP/iPP-PE blends exhibited bimodal parabolic-like behavior attributed to crystallization of the mesomorphic crystal polymorph at low temperatures and the α-form crystal at high temperatures. Incorporation of non-crystallizable polyethylene fractions both covalently and blended as a secondary component, resulted in decreasing crystallization rates, inhibition of crystallization, and decreased crystallizability. Additionally, the non-isothermal crystallization behavior of these systems shows that the non-crystallizable fractions influence the crystal nucleation density and temperature at which polymorphic crystallization occurs. Utilizing in-situ IR thermography in the PBF system, the heating and cooling rates observed for a single-layer PBF print were used to mimic the PBF process by FSC. Partial melting in the printing process leads to self-seeding and increased crystallization onset temperatures upon cooling, which influences the final part melting morphology. Nucleation from surrounding powder and partially melted crystals greatly influences the crystallization kinetics and crystal morphology of the final part. Utilizing rheological experiments and process-relevant cooling rates observed in the MatEx process, the miscibility of iPP/iPP-PE blends influenced the nucleation behavior and crystallization rates, subsequently leading to differences in printed part properties.
Customized blends of polypropylene for extrusion based additive manufacturing
Das, Arit; Shanmugham, Nishanth; Bortner, Michael J. (Wiley, 2022-11)
Filament-based material extrusion (MatEx) additive manufacturing has garnered great interest due to its simplicity, customizability, and cost-effectiveness. However, MatEx of semicrystalline polymers is still largely relegated to prototyping applications. Major issues involving volumetric shrinkage and warpage of the printed parts must be addressed in order to employ them for printing functional parts. Moreover, the crystallization behavior and rheology of the polymer are dependent on the MatEx processing conditions. In the current work, the printability of blends of isotactic polypropylene with a soft, low crystallinity propylene based homopolymer is evaluated. Addition of the homopolymer resulted in an increase in the crystallization window of the blends by similar to 6 degrees C that had a profound impact on the interlayer adhesion and residual stress state. The shear-dependent melt flow behavior inside the printing nozzle as well as the interlayer chain diffusion and interlayer welding on the print bed were investigated. Rheological characterizations also indicate sufficient dispersion and miscibility of the homopolymer in the neat polypropylene matrix. The incorporation of the homopolymer as an additive significantly improved the dimensional accuracy of the printed parts through better dissipation of the entrapped residual stresses during MatEx. Moreover, the degree of mechanical anisotropy of the parts was significantly lower than that obtained using many 3D printable grade polymers. The findings from this study can be leveraged in toolpath planning, process parameter optimization, and new feedstock development, highlighting current limitations as well as providing valuable insights into necessary processing modifications in order to enable MatEx of next generation semicrystalline polymers.
Designing Multiphase Step-Growth Polymers for Advanced Technologies: From Electromechanical Transducers to Additive Manufacturing
White, Benjamin Tyler (Virginia Tech, 2021-05-28)
The synthesis and characterization of step-growth polymers with novel monomers provided materials with tailored properties for emerging technologies. Specifically, multiphase materials (i.e., microphase separated block copolymers) exploit the synergistic relationship of combining polymers with disparate thermal and mechanical properties. The introduction of intramolecular interactions such as hydrogen and ionic bonding into these polymers further tailored their properties for applications including elastomers, electromechanical transducers, and additive manufacturing (AM). A review of recent literature revealed the material properties required for polymeric materials in electromechanical transducers, which aided in the design of polymers for this application. An isocyanate-, catalyst-, and solvent-free approach facilitated the synthesis of segmented polyureas with tunable thermal and mechanical properties. These materials found use as high dielectric elastomers and water-soluble polymers for extrusion-based AM dependent on the backbone composition. Vat photopolymerization (VP) AM served as a technique to 3D printed novel unsaturated polyester resins (UPR). Incorporating a phosphonium ionic liquid as a reactive diluent replaced styrene and reduced the volatility of commonly used UPRs. VP successfully provided 3D structures from these UPRs that demonstrated limited ionic conductivities. An extensive review of the literature surrounding the structure-property relationships of charged block copolymers with varying architectures helped to inform the synthesis of novel, cationic step-growth polymers. The synthesis of a new phosphonium IL facilitated the synthesis of a segmented polyurethane containing a phosphonium-functionalized soft segment for the first time. This phosphonium polyurethane exhibited ionic conductivities comparable to literature examples of block copolymers used for ionic polymer transducers, which suggests that these materials may serve for this application as well. Carbonyldiimidazole provides a novel route towards synthesizing imidazolium ionenes with unique backbone structures. The coupling of poly(ethylene glycol) dibromides with a bis-carbonylimidazole monomer and a commercial aliphatic dibromide led to the formation of segmented imidazolium ionenes. These polymers exhibited significant atmospheric water uptake as well as water solubility. However, the physical properties of the materials suggested that the synthetic procedure resulted in low molecular weights. Suggested future work provides methods for circumventing this issue and proposes next steps for all the projects discussed herein.
Effect of Dispersion on Rheology and 3D Printing of Chitosan-Graphene-Titanium Dioxide Composites
Alidu, Mariama (Virginia Tech, 2024-08-06)
Three-dimensional printing is renowned for its ability to produce complex geometries. By utilizing a pressure-driven additive manufacturing (AM) process called direct ink write (DIW) with polymer composite ink, it is possible to create parts with tailored internal microstructures that enhance surface area and particle-particle adsorption kinetics for water remediation applications. However, DIW of particle-filled systems faces challenges, particularly nozzle clogging. This paper explores the relationship between dispersion of aggregate size, torsional rheology, and the capacity to print relatively highly particle-filled systems. Various characterization methods, including torsional rheology, dynamic light scattering (DLS), and field emission scanning electron microscopy (FESEM) were employed utilizing a chitosan-graphene-titanium dioxide (CS-G-TiO2) polymer composite ink composed of TiO2 nanoparticles (1 wt.% to 25 wt.%), graphene (1 wt.%), and chitosan (5 wt.% to 9 wt.%) to evaluate the effect of ultrasonication techniques (bath vs. probe) on aggregate size. Probe-sonicated dispersions showed a more narrow monodispersed and unimodal aggregate size distribution with a primary average aggregate size of 255 nm. In contrast, bath-sonicated dispersions exhibited a moderately polydispersed, trimodal distribution with modes centered at 90 nm, 295 nm, and 5.6 μm. Non-Newtonian rheological parameters such as yield stress, complex viscosity, storage, and loss moduli were higher for the probe-sonicated CS-G-TiO2 composite ink than for the bath-sonicated CS-G-TiO2 composite ink. This increase is likely attributed to enhanced particle interactions, which allow for greater CS adsorption. These findings offer valuable insights into optimizing formulations for desired rheological properties in DIW printing. The results enable the direct ink writing of intricate geometries with high surface areas and less shape distortion, providing significant insights into processing similar multi-component slurry-based composite inks for DIW.
The Effect of Topography on Surface Behavior of Pseudomonas aeruginosa
Chang, Yow-Ren (Virginia Tech, 2019-10-17)
Bacterial biofilms are communities of micro-organisms encased a self-produced extracellular matrix. While they form readily in a nature, biofilm formation in man-made systems have economic and health consequences. Prior research demonstrated that topographical features comprised of uniform, micro-meter sized particles hindered the biofilm formation of Pseudomonas aeruginosa (P. aeruginosa), an opportunistic human pathogen. The goal of the present work is to 1) further develop a potential anti-biofilm coating by improving its robustness and 2) study the mechanism(s) by which surface topography hinders biofilm formation. The robustness of a topographical coating comprised of an array of silica particles is improved by the introduction of silica bridges through a sol-gel reaction. To study the mechanism(s), specifically, we hypothesized that the motion, or surface motility, of P. aeruginosa is hindered by the presence of micro-meter scale obstacles via physical obstruction. To test this, we analyzed the behavior of single P. aeruginosa cells at micron-scale spatial resolutions using time-lapse fluorescence microscopy, image analysis, and particle tracking techniques. We fabricated various types of micron-scale topography with curvature (particle arrays) and recti-linear features (vertical steps) and varied the critical dimension within the range of 0.5 – 10 µm which spans the dimensions of a typical P. aeruginosa cell. We found that there was a threshold feature size of 1-2 µm at which bacterial surface motility is drastically impacted. On positively curved topography (particle arrays), we found that the frequent obstacles reduced the average speed of a bacterium from 6.2  0.3 µm per 5 min on a flat surface to 2.1  0.3 µm per 5 min on an array of 2 µm particles. Furthermore, we observed that bacteria often move in-between particles, suggesting that bacteria have difficulty climbing over tall obstacles. To further investigate P. aeruginosa's ability to cope with topography, we examined the effect of recti-linear features (vertical steps) on surface motility. We found that step heights > 0.9 µm drastically reduced the probability of crossing and that the average speed when approaching the step is reduced by a factor of 2. Interestingly, we find that bacteria have a slight preference to traverse down which is against the direction of gravity in our system. In summary, these results offer insights into how a surface motile bacterium copes with a topographical surface. Our data indicate that the topography of a surface can impede the surface motility of bacterium and thus, may be an important mechanism by which topography prevents biofilm formation.
Enabling New Material and Process Capabilities for Ultraviolet-Assisted Direct Ink Write Additive Manufacturing via Exploration of Material Rheology and Reactivity
Rau, Daniel Andrew (Virginia Tech, 2022-05-24)
Ultraviolet-Assisted Direct Ink Write (UV-DIW) is a material extrusion additive manufacturing (AM) technology in which a viscous ink, often at room temperature, is selectively extruded through a translating nozzle to selectively deposit material. The extruded ink is solidified via UV irradiation (photocuring) and three-dimensional parts are created by repeating the process in a layer-by-layer fashion. UV-DIW is an attractive AM technology due to its ability to (1) extrude highly viscous inks (i.e. >10,000 Pa·s if ink exhibits shearthinning behavior) (2) the promise of leveraging the broad photopolymer material library and chemistries established for other AM technologies capable of processing photopolymers and (3) the promise of processing a wide range of inks, which enables the fabrication of metal, ceramic, polymer, bio-based, and multi-material parts. Currently, the technology faces a few shortcomings including (1) limited material selection for UV-DIW due to requirement for inks to be photocurable and limited mechanical properties of photocurable materials (2) lack of feature resolution and topological complexity of printed parts and (3) lack of material screening models providing robust definition of the material requirements (e.g., viscosity, cure time, strength) for successful UV-DIW printing. To address these shortcomings, the goal of this work is to gain a fundamental understanding of the rheological and reactive properties required for successful Ultraviolet-Assisted Direct Ink Write (UV-DIW). The first approach to answering the fundamental research question is establishing the existing rheology experiments used to characterize DIW inks and the relationships between rheology and printability. An in-depth literature review of the techniques and relationships was compiled to better understand ink requirements for successful printing (Chapter 2). This broad survey is not limited to only UV-DIW, but includes all variations of DIW. The first part of the review provides a summary of the rheological experiments that have been used to characterize a wide variety of DIW inks. The second part of this review focuses on the connections between rheology and printability. This survey helps identify the required rheological properties for successful printing that is then used throughout the rest of this work. Additionally, this review identifies shortcomings in current work and proposes areas for future work. From this exhaustive literature review, a systematic roadmap was developed that investigators can follow to quickly characterize the printability of new inks, independent of that ink's specific attributes (Chapter 3). The roadmap simplifies the trends identified in literature into a brief and intuitive guide to the rheology experiments relevant to DIW printing and the relationship between those experiment and printing results. The roadmap was demonstrated by evaluating the printability of two inks: (1) a silicone ink with both yield-stress and reactive curing behavior and (2) urethane acrylate inks with photocuring behavior. Experimental printing studies were used to support the conclusions on printability made in the roadmap. The second main approach focuses on the development of three novel UV-DIW inks to address the current limited material selection for UV-DIW and help better understand the rheological and reactive properties required for successful printing. For the three novel UVDIW inks, the iterative process of ink synthesis, analysis of ink rheology, and printability evaluation was detailed. Data from the development process contributed to gaining a fundamental understanding of how rheology and reactivity affect printability. The three inks each had novel rheological properties that impacted their printing behavior: (1) photocuring (2) yield-stress behavior + photocuring and (3) photocuring + thermal curing. Additionally, each ink had unique properties that expands material selection for UV-DIW including (1) an all-aromatic polyimide possessing a storage modulus above 1 GP a up to 400 °C (Chapter 4), (2) a styrene butadiene rubber (SBR) nanocomposite with elongation at break exceeding 300 % (Chapter 5), and (3) a dual-cure ink enabling the printing of inks containing over 60 vol% highly opaque solids (Chapter 6). The third approach details the development of two UV-DIW process models to better understand the process physics of the UV-DIW process and give insight to the properties of a successful ink. The first process model uses data from photorheology experiments to model how a photocurable ink spreads upon deposition from the nozzle, accounting for transient UV curing behavior (Chapter 7). This model allows for the rapid evaluation of an ink's behavior during the solidification sub-function of UV-DIW solely based on its rheology, without the time-consuming process of trial-and-error printing or complex computer simulations. The second process model combines modeling with a novel experimental method that uses a UV photorheometer to accurately characterize the relationship between cure depth and UV exposure for a wide range of photopolymers (Chapter 8). This model helps understand an inks photocuring behavior and ensure a sufficient cure depth is produced to adhere to the previous layer in UV-DIW printing. Lastly, two UV-DIW process modifications are introduced to address research gaps of printing high resolution features and limited material selection. A hybrid DIW + Vat Photopolymerization system is presented to improve the feature size and topographical complexities of parts, while still retaining UV-DIW's ability to print with very high viscosity photoresins (Chapter 9). A high temperature Heated-DIW system is presented to heat inks to over 300 °C and ultimately enable printing of poly(phenylene sulfide) aerogels (Chapter 10). In enabling the DIW of poly(phenylene sulfide) aerogels, the production of ultra-lightweight thermally insulating components for applications in harsh environments is enabled. With the use of additive manufacturing, hierarchical porosity on the macroscale is enabled in addition to the meso-scale porosity inherent to the aerogels.
Establishing the Conditions for Stable Extrusion of Melt Spun Polyacrylonitrile with Water Based Plasticizers
Yu, Jianger (Virginia Tech, 2019-06-18)
Polyacrylonitrile (PAN) fiber is one of the most important synthetic fibers in the world because it is a precursor to carbon fiber. Compared to the traditional solution spinning process, the melting spinning process of PAN is less costly and can further reduce the price of PAN fiber. This dissertation is concerned with the objective of establishment of conditions (temperature, plasticizer type, and plasticizer composition) that a PAN copolymer is able to be stable melt spun with water based plasticizers. More specifically, PAN/water/acetonitrile (70/15/15) mixture is considered as reference sample in this study because it was proposed in a BASF patent in which it was claimed it could be stably melt spun. We are looking for a more benign plasticizer so that the use of acetonitrile can be avoided and PAN can still be stably melt spun. To achieve this objective, the first step is to measure the melting point (Tm) of PAN copolymer with various plasticizers and compositions by using differential scanning calorimetry (DSC). The results indicate the Tm of PAN copolymer can be reduced to around 160 oC with water only as a plasticizer, which is lower than the degradation temperature of PAN (180 oC). Moreover, using a water/ethanol mixture and water/acetonitrile as plasticizers can further reduce the melting point of PAN to 150 oC and 135 oC, respectively. The second step is conducting rheological measurements on the PAN/plasticizers mixture. A pressure chamber was designed and attached to the capillary rheometer in order to prevent the foaming and evaporation of plasticizers during the viscosity experiments. Both steady-shear and time-dependent viscosity measurements were conducted. The rheological measurement results indicate that PAN can keep stable for more than 120 minutes with all plasticizers under 170 oC, and it starts to degrade in 60 minutes at 180 oC, except samples plasticized with 30 wt% of water (which keep stable for 120 minutes as well). The steady-shear viscosity results indicate the shear-thinning behavior is observed for the PAN/plasticizer mixtures at a temperature ranging from 170 oC to 190 oC and provide the fundamental viscosity data which can be applied to the extrusion process. In conclusion, the rheological measurements show PAN/Water (70/30 wt%) at 180 oC and PAN/EtOH/Water (70/15/15) at 170 oC are two potential systems for carrying out the PAN melt spinning process. Scanning electron microscopy (SEM) images were taken for the reference state and potential conditions. These images show that the copolymer strands have more and larger voids when plasticized with water only compared to those plasticized with water/acetonitrile and water/ethanol mixture. In this case, PAN/EtOH/Water (70/15/15) at 170 oC is considered to be the most benign system for that PAN melt spinning.
Factors Affecting Fiber Orientation and Properties in Semi-Flexible Fiber Composites Including the Addition of Carbon Nanotubes
Herrington, Kevin D. (Virginia Tech, 2015-09-24)
Within this research, factors affecting the orientation of injection molded long fiber composites in an end-gated plaque were investigated. Matrix viscosity was found to have a small effect on fiber orientation. The impact matrix viscosity had on orientation was dependent on fiber loading. At lower fiber loadings, the higher viscosity material had a more asymmetric orientation profile throughout the samples and less of a shell-core-shell orientation. At higher fiber loadings, there were few differences in orientation due to matrix viscosity. Fiber concentration was found to have a larger influence on fiber orientation than matrix viscosity. Increased fiber concentration led to a lower degree of flow alignment and a broader core region at all locations examined, following the trend previously reported for short fiber composites. The orientations of three different fiber length distributions of glass fiber (GF) were compared. The longer fibers in the fiber length distribution were shown to have a disproportionate effect on orientation, with weight average aspect ratio being better than number average aspect ratio at indicating if the GF and CF samples orientated comparably. To improve properties transverse to the main flow direction, the super critical carbon dioxide aided deagglomeration of multi-walled carbon nanotubes (CNTs) was used to create injection molded multiscale composites with CNT, CF, and polypropylene. The addition of CNTs greatly improved the tensile and electrical properties of the composites compared to those without CNTs. The degree of improvement from adding CNTs was found to be dependent on CF concentration, indicating that the CNTs were most likely interacting with the CF and not the polymer. A CNT concentration of 1 wt% with a tenfold degree of expansion at 40 wt% CF proved to be optimum. A large improvement in the tensile properties transverse to the flow direction was found implying that the CNTs were not highly flow aligned. Tensile and electrical properties began to fall off at higher CNT loadings and degrees of expansion indicating the importance of obtaining a good dispersion of CNTs in the part.
Fundamental Understanding and Functionality of Silicon Oxycarbide
Yang, Ni (Virginia Tech, 2021-01-07)
Silicon oxycarbide (SiOC) is a unique polymer-derived ceramic (PDC) containing silicon, oxygen, and carbon atoms in the form of an amorphous network structure. The phase separation of SiOC is determined by polymeric precursors, pyrolysis temperatures, and atmosphere, which results in different compositions and microstructures. Because of its unique properties (high thermal stability, corrosion resistance, among others), SiOC has numerous applications in fields such as additive manufacturing, lithium-ion batteries, and advanced optics. In the SiOC system, SiO2 nanoclusters can be removed through the etching process, to create nanopores for increasing the surface area. By introducing the SiO2-forming filler (perhydropolysilazane) into SiOC, more SiO2 nanodomains with an average size of 1.72 nm were generated for an ultrahigh surface area of ~2100 m2/g material. Meanwhile, the distributions of domain wall thickness and pore distribution can be calculated by our modified model, to further understand the pore formation. The formation of porous SiOC ceramics with ultrahigh surface areas is greatly desired in numerous applications. Transition metal-containing SiOC composites have more functional properties over pure SiOC and receive more attention in different areas. High-temperature resistant TiC/SiOC was successfully synthesized by pyrolysis of polysiloxane (PSO) and titanium isopropoxide at 1200-1400 °C in argon. It had the first reported conductivity of >1000 S/m for TiC/SiOC ceramics. Nickel-containing SiOC magnetoceramics with soft ferromagnetism was fabricated from a base PSO with the addition of nickel 2,4‐pentanedionate. The effect of water vapor on the phase evolution of Ni/SiOC composites was studied at different pyrolysis temperatures, and the formation of nickel silicides was suppressed by the effect of water vapor during the pyrolysis. Our investigation showed the catalysts from transition metals induced the generation of metal silicides, silicon carbide, and turbostratic carbon with the catalytic activity corresponding to Fe > Co > Ni, which agrees with the activation energy calculation. Also, the phase separation of SiOC was proved to be predominant than local carbothermal reduction. In addition to these findings, a novel approach was developed through the Gibbs free energy minimization method to predict the phase content in PDCs with transition metal additives. And this work provides useful guidance to fabricate the transition metal-containing SiOCs with the desired phase content. Last, the state-of-the-art 4D-STEM technique, collaborated with Lawrence Berkeley National Laboratory, was applied to SiOC ceramics containing amorphous phase. The results showed that 4D-STEM is a valid approach to characterize the nanostructure of the amorphous phase as well as the crystallites. It solves the problem of analyzing SiOC materials at nanoscale due to the disordered atomic arrangement and properties.
Generation of Multi-Scale Thermoplastic Composites for Use in Injection Molding and Fused Filament Fabrication
Han, Jier Yang (Virginia Tech, 2021-01-07)
Thermoplastic composites that have been reinforced by thermotropic liquid crystalline (TLCP) fibrils in the microscale and by nanoparticles in the nanoscale are defined as multi-scale wholly thermoplastic composites (WTCs). Multi-scale WTCs have been proposed as lightweight replacements with high performance for some traditional glass fiber (GF) and carbon fiber (CF) reinforced composites materials in various applications. TLCPs are known for performing mechanical properties similar to those of the lower end of CF but significantly better than those of GF. To enhance the mechanical properties of TLPC reinforced WTCs, carbon nanotubes (CNTs) are considered being used as a secondary enhancement in WTCs. CNTs have gathered significant interest in the last 30 years because of their high aspect ratio, high mechanical properties, and other high-performance properties. The focus of this work is on investigating the processing conditions of generating in situ injection-molded multi-scale WTCs, then extending the technology to dual-extrusion and fused filament fabrication (FFF) and obtain high-performance multi-scale WTC products. This dissertation initially focused on investigating the processing conditions, in particular mixing histories and processing temperature profiles, of generating in situ injection-molded multi-scale WTCs, which consist of a representative TLCP, scCO2 aided exfoliated CNTs, and the thermoplastic matrix polyamide 6 (PA 6). The supercooling behavior of the TLCP and thermal stability of PA 6 are studied by applying the rheological methods of small amplitude oscillatory shear (SAOS). Multiple mixing histories with CNTs and processing temperature profiles are analyzed based on the criterion of maximizing tensile properties of multi-scale WTCs and minimizing thermal degradation of the matrix. Under the optimum processing conditions, the in situ injection-molded multi-scale WTCs exhibit a 26% and 34% tensile modulus and strength enhancement, compared to the in situ injection-molded WTCs with no CNTs. Scanning electron micrograph (SEM) images were used to understand the enhancement. The second part of this work is to extend the scCO2 aided in situ multi-scale WTCs processing technology to dual-extrusion and FFF. Multi-scale WTC filaments, which consists of TLCP, CNTs, and polyamide copolymer (PAc), are generated by dual-extrusion, and 3D printed into rectangular specimens in FFF. The 1 wt% CNTs reinforced multi-scale WTC filaments generated by the means of dual-extrusion exhibit 225% and 80% improvement in tensile modulus and strength, respectively, compared to the WTC filaments with no CNTs. In FFF, 40 wt% TLCP/1 wt% CNT/PAc 3D printed specimens with filament laid in longitudinal direction exhibited excellent tensile modulus and strength of 38.92 GPa and 127.16 MPa, respectively. The well-dispersed exfoliated CNTs show high alignment with TLCP microfibrils in the multi-scale WTC filaments and their laid-down specimens, which causes the significant tensile modulus enhancement. Bridging elements are discovered between TLCP fibrils and PAc matrix to improve interfacial adhesion, which is attributed to the well-dispersed exfoliated CNTs. Finally, the significant improvements in tensile properties attributed to scCO2 aided exfoliated CNTs in WTCs are verified on the multi-scale WTCs based on polypropylene (PP). Moreover, additional tensile properties improvements for exfoliated CNTs reinforced multi-scale WTCs are obtained with the use of maleic anhydride grafted polypropylene (MAPP). With 1 wt% CNTs and 16 wt% MAPP dual reinforcement, 20 wt% TLCP reinforced WTCs based on polypropylene (PP) exhibit 265%, 274%, and 182% improvement in the tensile modulus of the filaments, laid up specimens in the concentric pattern and laid up specimens in ±45° rectilinear pattern, respectively. The dual reinforcement also improves the tensile strength of 20 wt% TLCP reinforced WTC filaments by up to 73%. The high alignment between TLCP fibrils and CNTs are confirmed in the multi-scale WTCs based on PP. Besides the bridging elements attributed to CNTs found in the second part of this work, SEM images show that CNTs are partially trapped in TLCP fibrils.
Generation of Recyclable Liquid Crystalline Polymer Reinforced Composites for Use in Conventional and Additive Manufacturing Processes
Chen, Tianran (Virginia Tech, 2021-05-21)
The application of glass fiber reinforced composites has grown rapidly due to their high strength-to-weight ratio, low cost, and chemical resistance. However, the increasing demand for fiber reinforced composites results in the generation of more composite wastes. Mechanical recycling is a cost-effective and environmentally-friendly recycling method, but the loss in the quality of recycled glass or carbon fiber composite hinders the wide-spread use of this recycling method. It is important to develop novel composite materials with higher recyclability. Thermotropic liquid crystalline polymers (TLCPs) are high-performance engineering thermoplastics, which have comparable mechanical performance to that of glass fiber. The TLCP reinforced composites, called in situ composites, can form the reinforcing TLCP fibrils during processing avoiding the fiber breakage problem. The first part of this dissertation is to study the influence of mechanical recycling on the properties of injection molded TLCP and glass fiber (GF) reinforced polypropylene (PP). The processing temperature of the injection molding process was optimized using a differential scanning calorimeter (DSC) and a rheometer to minimize the thermal degradation of PP. The TLCP and GF reinforced PP materials were mechanically recycled up to three times by repeated injection molding and grinding. The mechanical recycling had almost no influence on the mechanical, thermal, and thermo-mechanical properties of TLCP/PP because of the regeneration of TLCP fibrils during the mold filling process. On the other hand, glass fiber/PP composites decreased 30% in tensile strength and 5% in tensile modulus after three reprocessing cycles. The micro-mechanical modeling demonstrated the deterioration in mechanical properties of GF/PP was mainly attributed to the fiber breakage that occurred during compounding and grinding. The second part of this dissertation is concerned with the development of recyclable and light weight hybrid composites through the use of TLCP and glass fiber. Rheological tests were used to determine the optimal processing temperature of the injection molding process. At this processing temperature, the thermal degradation of matrix material was mitigated and the processability of the hybrid composite was improved. The best formulation of TLCP and glass fiber in the composite was determined giving rise to the generation of a recyclable hybrid composite with low melt viscosity, low mechanical anisotropy, and improved mechanical properties. Finally, TLCP reinforced polyamide composites were utilized in an additive manufacturing application. The method of selecting the processing temperature to blend TLCP and polyamide in the dual extrusion process was devised using rheological analyses to take advantage of the supercooling behavior of TLCP and minimize the thermal degradation of the matrix polymer. The composite filament prepared by dual extrusion was printed and the printing temperature of the composite filament that led to the highest mechanical properties was determined. Although the tensile strength of the TLCP composite was lower than the glass fiber or carbon fiber composites, the tensile modulus of 3D printed 60 wt% TLCP reinforced polyamide was comparable to traditional glass or carbon fiber reinforced composites in 3D printing.
Generation of Thermotropic Liquid Crystalline Polymer (TLCP)-Thermoplastic Composite Filaments and Their Processing in Fused Filament Fabrication (FFF)
Ansari, Mubashir Qamar (Virginia Tech, 2019-03-11)
One of the major limitations in Fused Filament Fabrication (FFF), a form of additive manufacturing, is the lack of composites with superior mechanical properties. Traditionally, carbon and glass fibers are widely used to improve the physical properties of polymeric matrices. However, the blending methods lead to fiber breakage, preventing generation of long fiber reinforced filaments essential for printing load-bearing components. Our approach to improve tensile properties of the printed parts was to use in-situ composites to avoid fiber breakage during filament generation. In the filaments generated, we used thermotropic liquid crystalline polymers (TLCPs) to reinforce acrylonitrile butadiene styrene (ABS) and a high performance thermoplastic, polyphenylene sulfide (PPS). The TLCPs are composed of rod-like monomers which are highly aligned under extensional kinematics imparting excellent one-dimensional tensile properties. The tensile strength and modulus of the 40 wt.% TLCP/ABS filaments was improved by 7 and 20 times, respectively. On the other hand, the 67 wt.% TLCP/PPS filament tensile strength and modulus were improved by 2 and 12 times, respectively. The filaments were generated using dual extrusion technology to produce nearly continuously reinforced filaments and to avoid matrix degradation. Rheological tests were taken advantage of to determine the processing conditions. Dual extrusion technology allowed plasticating the matrix and the reinforcing polymer separately in different extruders. Then continuous streams of TLCP were injected below the TLCP melting temperature into the matrix polymer to avoid matrix degradation. The blend was then passed through a series of static mixers, subdividing the layers into finer streams, eventually leading to nearly continuous fibrils which were an order of magnitude lower in diameter than those of the carbon and glass fibers. The composite filaments were printed below the melting temperature of the TLCPs, and the conditions were determined to avoid the relaxation of the order in the TLCPs. On printing, a matrix-like printing performance was obtained, such that the printer was able to take sharp turns in comparison with the traditionally used fibers. Moreover, the filaments led to a significant improvement in the tensile properties on using in FFF and other conventional technologies such as injection and compression molding.

Browsing by Author "Bortner, Michael J."

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