Type of Document Dissertation Author Lin, Zhiyuan URN etd-12242009-093107 Title Nanocomposite-based Lignocellulosic Fibers Degree PhD Department Wood Science and Forest Products Advisory Committee
Advisor Name Title Renneckar, Scott H. Committee Chair Case, Scott W. Committee Member Frazier, Charles E. Committee Member Loferski, Joseph R. Committee Member Zink-Sharp, Audrey G. Committee Member Keywords
- layer-by-layer assembly
- lignocellulosic fibers
- fiber surface modification
- polyelectrolyte adsorption
Date of Defense 2009-12-16 Availability restricted AbstractThe formation of layered nanoparticle films on the surface of wood fibers is reported in this study. The layer-by-layer (LbL) assembly technique was comprehensively investigated as a non-covalent surface modification method for lignocellulosic fiber. Nanocomposite-based lignocellulosic fibers were successfully fabricated by sequential adsorption of oppositely charged poly(diallydimethylammonium) chloride (PDDA) and clay nanoparticles in a number of repeated deposition cycles. Nanocomposite fibers displayed layered structure as indicated by the electrokinetic potential studies and scanning electron microscopy (SEM) analysis.
Layer-by-layer films of PDDA and clay impacted the thermal stability of wood fibers. Average degradation temperature at 5 and 10% weight loss for modified fibers with 4 bi-layers increased by up to ~24 and ~15°C, respectively. Significant char residue formed for the LbL modified fibers after heating to 800°C, indicating that the clay-based coating may serve as a barrier, creating an insulating layer to prevent further decomposition of the material.
Layer-by-layer film formation on wood fibers was investigated as a function of parameters related to fiber composition and solution conditions (ie. presence of lignin, salt concentration and pH). Elemental analysis of modified fibers revealed that PDDA adsorption to the fibers was reduced for all solution conditions for the samples with the highest content of lignin. Upon extracting the non-covalently attached lignin, the samples showed the greatest amount of PDDA adsorption, reaching to 1.5% of total mass, under neutral solution conditions without the presence of added electrolyte. Furthermore, the influence of both the amount of PDDA adsorbed onto the fiber surface and electrokinetic potential of modified fibers on subsequent multilayer formation was quantified. Under select fiber treatments, great amount of PDDA/clay (up to ~75% total mass for only 4 bi-layers) was adsorbed onto wood fibers through the LbL process, giving these high surface area fibers nanocomposite coatings.
LbL modified fibers were melt compounded with isotactic polypropylene (PP) and compression molded into test specimens. The effect of LbL modification as a function of the number of bi-layers on composite performance was tested using the tensile, flexural, dynamic mechanical and thermal properties of fiber reinforced thermoplastic composites. LbL modified fiber composites had similar modulus values but significantly lower strength values than those of unmodified fiber composites. However, composites composed of LbL modified fibers displayed increased elongation at break, increasing by more than 50%, to those of unmodified samples. DSC results indicated that crystallization behavior of PP is promoted in the presence of wood fibers. Both unmodified and LbL modified fibers are able to acts as nucleating agents, which cause an increase of the crystallinity of PP. Moreover, results from tensile and flexural strength, dynamic mechanical analysis and water absorption tests revealed that the material (PDDA or clay) at the terminal (outer) layer of LbL modified fiber influences the performance of the composites.
These findings demonstrate control over the deposition of nanoparticles onto lignocellulosic fibers influencing terminal surface chemistry of the fiber. Further investigation into using renewable fibers as carriers of nanoparticle films to improve fiber durability, compounding with thermoplastics that have higher melt processing temperatures, and tailoring terminal surface chemistry to enhance adhesion is justified by this research.
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