Date of Award

12-2007

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Chemical Engineering

Advisor

Ogale, Amod A

Committee Member

Hirt , Douglas E

Committee Member

Harrison , Graham M

Committee Member

Lickfield , Gary C

Abstract

Nano-modifiers are typically three orders of magnitude smaller in size than their micro-counterparts. At this scale, interactions between matrix molecules and the nano-modifiers can lead to novel physical and chemical properties of the resulting nanocomposites. Carbon nanofibers (CNFs) are a class of nano-modifiers that has received significant attention recently because they have superior electrical conductivity and mechanical properties with a high aspect ratio (length over diameter). Several recent research studies have focused on enhancing electrical and mechanical performance of composites in the presence of CNFs. However, the influence of CNFs on the structure of the polymer matrix is important in understanding the role CNFs have on the properties of nanocomposites, but this has not been thoroughly examined. Therefore, crystalline and orientational structure of CNF/polymer composites was investigated in this study.
First, the microstructure of two different grades of CNFs, MJ (experimental) and PR (commercial), was investigated as a function of different thermal treatments. Using Raman spectroscopy and XRD analysis, an enhancement of crystallite size was observed after heat treatment at 2200°C. The crystallite thickness increased from 1.6±0.1 nm to 10.9±0.5 nm for MJ fibers and from 3.1±0.3 nm to 11.7±0.4 nm for PR fibers. Also, an increase in thermal oxidation stability for heat-treated CNFs was observed. BET adsorption isotherms showed a significant reduction of specific surface area of MJ fibers after the heat treatment, resulting from a decrease of pore volume. However, even after heat treatment, MJ fibers possessed a rougher surface than did PR fibers. The role of such nano-texture was studied on two distinct types of polymeric matrices: flexible-chain and semi rigid-rod polymers.
Linear low density polyethylene (LLDPE), a flexible-chain polymer, is widely used for packaging applications because of its film-forming properties and good barrier characteristics. However, LLDPE has a poor electrical conductivity, which results in poor EMI/ESD shielding. Therefore, CNFs were incorporated into LLDPE to improve electrical conductivity. The Electrical percolation threshold was observed at approximately 15 wt% MJ (MJ15) and 30 wt% PR (PR30). Tensile modulus increased from 110 MPa for pure LLDPE to 200 MPa and 300 MPa for MJ15 and PR15, respectively. However, the tensile strength remained fairly unchanged at about 20 MPa. Strain-to-failure decreased from 690% for pure LLDPE to 460% and 120% for MJ15 and PR15, respectively. This indicates that the interfacial bonding of LLDPE matrix with MJ fibers is better than that with PR fibers, possibly due to rougher surface of the MJ fibers.
Crystallization behavior of LLDPE nanocomposites was investigated in the presence of three types of CNFs (MJ, PR, and PRCVD). During non-isothermal crystallization studies, all three crystalline melting peaks for LLDPE matrix were observed in the presence of PRCVD fibers up to 15 wt% content. However, at only 1 wt% MJ fibers, the disappearance of the intermediate melting peak was observed. The broad melting peak at the lower temperature became larger, suggesting an increase in the relative content of thinner lamellae in the presence of MJ fibers. The larger and the rougher surface of MJ fibers observed from the nano-textural study contributes toward the different crystallization behavior of MJ/LLDPE composites. TEM micrographs of nanocomposites revealed transcrystallinity of LLDPE on the surface of CNFs. Further, a broader distribution of LLDPE lamellar thickness was observed in TEM images of MJ composites.
The third major component of this research project was a study on the role of CNFs on a thermotropic liquid crystalline polymer (TLCP) matrix possessing a semi rigid-rod molecular structure. A variation of anisotropy of the TLCP was investigated in the presence of CNFs. Electrical percolation threshold was observed at approximately 5 wt% MJ fibers. Decrease of tensile modulus and strength was observed for composites. For a given type of flow, wide angle X-ray diffraction studies showed a decrease in Herman's orientation parameter from 0.85 for pure V400P to 0.71 for 5 wt% MJ composites. Thus, the presence of CNFs led to a reduction of the overall anisotropy of the nematic phase in the nanocomposite.
The disruption of molecular orientation of TLCPs was inferred by SEM and TEM analysis. SEM micrographs revealed a fibrillar structure for pure TLCPs at a macro-scale. However, this structure was not observed in composites at the same scale although micro-size fibrils were found with the addition of PR fibers. TEM micrographs displayed banded structures of pure TLCPs, but these structures were not significant in the vicinity of PR fibers. These results indicate that CNFs can help to reduce the severe anisotropy that is otherwise observed for TLCPs. In summary, this study establishes the significant role of CNFs as nano-modifiers that help modify the texture of the matrix while serving to enhance specific properties, such as electrical conductivity, for the nanocomposites.

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