Processing-structure-property relationships in cellulose nanocrystal/poly(ethylene-co-vinyl alcohol) composites

Cellulose nanocrystals (CNCs) are nanoparticles of renewed interest in the composites community. Their abundance, renewable source material, and expected mechanical properties have motivated a large number of studies to understand how to use them effectively in composite applications. While these materials have desirable properties, they also present some challenges when considering them as a reinforcement for polymer composites. Specifically, they are not inherently compatible with most polymers, and they have a relatively low thermal decomposition temperature. Both of these factors inform the choice of suitable polymer matrices. The objective of this work is to explore possible materials selection and processing strategies for using them in nanocomposites. In this research, CNCs were paired with two different poly(ethylene-co-vinyl alcohol) (EVOH) polymers and processed using three complementary strategies to understand the processing-structure-property relationships of these materials. The different EVOH polymers had different amounts of the comonomers, ethylene and vinyl alcohol. Through these studies, it was observed that a combined solution and melt processing method produced materials with the highest levels of CNC dispersion and most favorable viscoelastic properties. Additionally, the matrix choice influenced the levels of dispersion observed. The CNCs would be expected to have a higher level of interaction with the EVOH polymer containing more vinyl alcohol, and the results indicated that this expectation was correct. The nanocomposite containing CNCs and the EVOH polymer containing more vinyl alcohol showed structuring of the matrix with CNC addition. These changes in dispersion and component interactions manifested themselves in the viscoelastic properties measured with dynamic mechanical analysis. Changes in the glass transition temperature and storage modulus below the glass transition temperature were observed. Overall, these results provide insight into how nanocomposite design parameters and processing strategies can be combined to produce improved properties, expanding the application space of CNC composites

Auxetic behavior of fiber networks: Paper and nonwoven fabrics

Auxetic materials/structures have been subjects of scientific curiosity due to their counterintuitive response to deformation; specifically, they exhibit a negative Poisson’s ratio. While some natural materials are auxetic, scientists and engineers have sought to produce this property in synthetic materials/structures with a goal of improving properties such as damping and producing composites with mechanically interlocking interfaces. The objective of this research was to more fully understand the out-of-plane auxetic response of engineered fiber structures, specifically paper [1] and nonwoven fabrics [2, 3]. Several paper samples and handsheets were tested to quantify their auxetic response and understand how processing and structure affected their auxeticity. The results showed that paper structures possessed a wide distribution of Poisson’s ratio values, both positive and negative. The difference in the values of Poisson’s ratio suggested a strong correlation with the fiber-network structure and the processing conditions employed during papermaking. The nonwoven fabrics studied were needle-punched nonwoven fabrics. These fabrics contained pillars of entangled fibers that were oriented roughly parallel to the thickness direction, and they were not auxetic in their as-produced state. To produce an auxetic response, a post-processing heat compression step was explored to orient more of the fibers in the plane and produce additional fiber junction points. Large, negative values for Poisson’s ratio were observed for the fabrics after heat compression. Of the available processing parameters, temperature was found to affect the auxetic response most strongly, by increasing the compression set of the fabric and, as a result, the auxetic response. Overall, these results showed some commonality between the two systems studied in that fiber network structure and fiber junctions worked together to produce an out-of-plane auxetic response. Additionally, these structural attributes may provide general design guidelines for producing these auxetic fiber structures from other materials, leading to their use in composites. References 1. Verma, P., M.L. Shofner, and A.C. Griffin, Deconstructing the auxetic behavior of paper. Physica Status Solidi (b) – Basic Solid State Physics, 2014. 251(2): 289-296. 2. Verma, P., M.L. Shofner, A. Lin, K.B. Wagner, and A.C. Griffin, Inducing out-of-plane auxetic behavior in needle-punched nonwovens. Physica Status Solidi (b) – Basic Solid State Physics, 2015. 252(7): 1455-1464. 3. Verma, P., M.L. Shofner, A. Lin, K.B. Wagner, and A.C. Griffin, Induction of auxetic response in needle-punched nonwovens: Effects of temperature, pressure and time. Physica Status Solidi (b) – Basic Solid State Physics, 2016. 253(7): 1270-1278

Performance of chemically modified reduced graphene oxide (CMrGO) in electrodynamic dust shield (EDS) applications

Electrodynamic Dust Shield (EDS) technology is a dust mitigation strategy that is commonly studied for applications such as photovoltaics or thermal radiators where soiling of the surfaces can reduce performance. The goal of the current work was to test the performance of a patterned nanocomposite EDS system produced through spray-coating and melt infiltration of chemically modified reduced graphene oxide (CMrGO) traces with thermoplastic high-density polyethylene (HDPE). The EDS performance was tested for a dusting of lunar regolith simulant under high vacuum conditions (~10-6 Torr) using both 2-phase and 3-phase configurations. Uncapped (bare) devices showed efficient dust removal at moderate voltages (1000 V) for both 2-phase and 3-phase designs, but the performance of the devices degraded after several sequential tests due to erosion of the traces caused by electric discharges. Further tests carried out while illuminating the dust surface with a UV excimer lamp showed that the EDS voltage needed to reach the maximum cleanliness was reduced by almost 50% for the 2-phase devices (500 V minimum for rough and 1000 V for smooth), while the 3-phase devices were unaffected by the application of UV. Capping the CMrGO traces with low-density polyethylene (LDPE) eliminated breakdown of the materials and device degradation, but larger voltages (3000 V) coupled with UV illumination were required to remove the grains from the capped devices.Comment: 22 pages, 7 figure

Measurement & characterization of the interfacial zone in nanotube/polymer

Issued as final reportClark Atlanta Universit

Microstructural Design in Polymer Nanocomposites: Effects of Matrix Crystallinity and Interfacial Chemistry

Dr. Meisha L. Shofner presented a lecture at the Nano@Tech Meeting on September 25, 2012 at 12 noon in room 1116 of the Marcus Nanotechnology Building.Dr. Meisha L. Shofner is an Assistant Professor in the School of Materials Science and Engineering at Georgia Institute of Technology, joining the faculty following post-doctoral training at Rensselaer Polytechnic Institute. She received her B.S. in Mechanical Engineering from The University of Texas at Austin and her Ph.D. in Materials Science from Rice University. Prior to beginning graduate school, she was employed as a design engineer at FMC in the Subsea Engineering Division, working at two plant locations (Houston, Texas and the Republic of Singapore), and she is a registered Professional Engineer in Georgia. Dr. Shofner currently serves as the secretary of the TMS Composite Materials Committee and as a member of ASME’s Nanoengineering for Energy and Sustainability Steering Committee. At Georgia Tech, Dr. Shofner’s research group is concerned with structure-property relationships in polymer nanocomposite materials and with producing structural hierarchy in these materials for structural and functional applications. This research has been recognized by the Ralph E. Powe Junior Faculty Enhancement Award from Oak Ridge Associate Universities and the Solvay Advanced Polymers Young Faculty Award.Runtime: 49:50 minutesStructure-property research in polymer nanocomposites has often focused on producing systems that are homogeneously dispersed in order to capitalize on the large amount of specific surface area available from nanoparticles. However, inhomogeneous dispersion is often obtained and in some cases has been deliberately sought to enhance functional properties through the formation of particle networks. In this research, we are exploring matrix-mediated methods for directing nanoparticle dispersion. Specifically, we are examining dispersion behavior of calcium phosphate nanoparticles of different shapes in semi-crystalline polymer matrices. Our results have shown that nanoparticle arrangement is influenced significantly by the matrix morphology. In matrices with moderate levels of crystallinity, high levels of nanoparticle dispersion are attainable and reinforcement behavior is temperature dependent, similar to amorphous matrices. However at higher crystallinity levels, nanoparticles have a strong tendency to aggregate into larger structures whose shape is related to the native nanoparticle shape. This tendency can be mitigated by changing the surface chemistry through copolymer compatibilization. Experimental results concerning the effect of particle aggregation and shape on polymer crystalline structure, thermal transitions and mechanical properties are presented to more fully understand nanocomposite structure-property relationships from the perspective of the polymer matrix

Tensile Mechanical Properties of Polypropylene Composites Fabricated by Material Extrusion

In the material extrusion additive manufacturing process, a thin filament of material is deposited in a layer-by-layer manner to fabricate a three dimensional part. The filament deposition pattern can result in voids and incomplete bonding between adjacent filaments in a part, which leads to reduced mechanical properties. Further, the layer-by-layer deposition procedure typically results in mechanical property anisotropy, with higher properties in the layer compared to those across layers. The study reported in this paper explored various polypropylene composite formulations to address these issues: low residual stress and warpage, good mechanical properties, and reduced anisotropy. The reduction in anisotropy will be the focus of this paper as a function of thermal properties and process variable settings. A series of process simulation models was developed to explore ranges of thermal properties and process settings, which provided insights into tensile specimen behaviors. Results demonstrate that anisotropy can be reduced almost completely if the material can be formulated to have low crystallinity, low coefficient of thermal expansion, and moderate to high thermal conductivity (for a polymer).Mechanical Engineerin