Date on Master's Thesis/Doctoral Dissertation

5-2013

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Mechanical Engineering

Committee Chair

Panchapakesan, Balaji

Author's Keywords

Load transfer; Polymer composites; Interface; Raman spectrum; Young's modulus; X-ray photoelectron spectrum

Subject

Polymeric composites; Polymerization

Abstract

Advanced composites play important roles in the materials sciences, military, space and commercial applications. The desirable load transfer and mechanical strength of reinforced polymers are crucial for developing advanced composites. Owing to their excellent mechanical properties derived from the sp2 bonding structure and the nanoscale size, nano-carbons are attractive materials used for nanoscale reinforcement of polymer composites. This dissertation describes a novel method to develop polymer composites using near infrared (NIR) photon-assisted polymerization and nanoscale reinforcement. We used multi-walled carbon nanotubes (MWNTs), reduced graphene oxide (RGO), and graphene nanoplatelets (GNPs) to make polymer composites, and explored in-situ NIR photon assisted heating of these nano carbons to enhance polymerization of the nano-carbon/polymer interface, thus achieving significant load transfer and improved mechanical properties. To specify, nano-carbon was dispersed into the polymer matrix by shear or evaporation mixing method to attain a uniform distribution in the prepared thin film composite. The thin film was exposed to NIR light during polymerization instead of conventional oven based heating. NIR was effectively absorbed by nano-carbons and also atoms from polymer molecule; the induced photo-thermal heat was transferred into the polymer matrix to induce polymerization of the composite and the covalent bonding between nano-carbons and polymer matrix at the interface. Scanning electron microscope (SEM), Raman spectroscopy, and RSA were used to evaluate the load transfer and mechanical strength of the polymerized composite samples. Investigating first the nanotube/polymer composites synergized by NIR photon-assisted polymerization, large Raman shifts (20 cm-1 wavenumber for up to 80% strains) of the 2D band were recorded for the NIR light polymerized samples and an increase in Young‘s modulus by ~130% was measured for the 1 wt. MWNT/poly(dimethylsiloxane) (PDMS) composites. While at first it was thought that NIR radiation during polymerization heated the nano-carbons inside resulting in strengthening of the nano-carbon/polymer interface, it was seen after further experimentation with graphene reinforcements that other light induced bonding effects apart from heat were also responsible. Raman spectroscopy revealed that mixing graphene in polymer has a profound effect on the G, D and 2D bands. Investigating G bands for pure RGO and GNPs and comparing them with their polymer counterparts showed large shifts in the G band indicating lattice compression. The comparison of the NIR polymerization with the conventional oven based polymerization for both RGO and GNPs revealed large changes in wavenumbers and indicated increased load transfers for the NIR photon-assisted polymerization method. The Full Width Half Maximum (FWHM) data of the NIR treated samples exhibited smaller change at large strains compared to conventionally polymerized samples indicating the minimum slippage in the former. Finally, the stress-strain curves showed more than three times improvement in the Young‘s modulus of the composites fabricated using the NIR treatment in comparison to the conventional baking for both types of graphene. These results are compared to the carbon nanotube (CNT) counterparts in PDMS. The study provided insights on how to use stress-sensitive shifts in Raman spectroscopy for the development of advanced polymer composites. While NIR light induced polymerization showed increased load transfer and mechanical strength of nanotube and graphene polymer composites, investigation into two types of nano-carbon of different dimensionalities yielded extraordinary synergy between nano-carbons. Synergistic effects in binary mixtures of nano-carbon/polymer composites polymerized by NIR photon-assisted polymerization are observed. Small amounts of MWNT0.1 dispersed in RGO0.9/PDMS samples (subscripts represent weight percentage) reversed the sign of the Raman stress-sensitive wavenumbers from positive to negative values demonstrating the reversal of the lattice stress itself on applied uniaxial strain. A wavenumber change from 10 cm-1 in compression to 10 cm-1 in tension, and an increase in the Young‘s modulus of ~103% was observed for the MWNT0.1RGO0:9/PDMS with applied uniaxial tension. Extensive scanning electron microscopy measurement revealed the bridging of MWNT between two graphene plates in polymer composites. Mixing small amounts of MWNTs in RGO/PDMS eliminated the previously reported compressive deformation of RGO and significantly enhanced the load transfer and the mechanical strength of composites in tension. This is a direct indication of the cooperative effects of binary nano-carbons that produces an overall dramatic increase in load transfer (100% change). The orientation order of MWNTs with the application of uniaxial tensile strain directly affected the shift in the Raman wavenumbers (2D-band and G-band) and the load transfer. It is observed that the cooperative behavior of binary nano-carbons in polymer composites resulted in enhanced load transfer and mechanical strength. Such binary compositions could be fundamentally interesting for developing advanced composites such as nano-carbon based mixed dimensional systems. The NIR polymerization could be used to control aspects such as polymer chain entanglement between nano-carbons of different dimensional states, polymer chain lengths, mobility and eventual mechanical and electrical properties. At first it was thought that NIR light based polymerization only heated the nano-carbons and strengthened the interface, further studies using X-ray photoelectron spectroscopy (XPS) suggested other light induced bond formation was also responsible mechanism for improved interfacial strength, load transfer and mechanical properties. XPS data on RGO/polymer composites suggested activation of hydroxyl and carbonyl groups on the RGO that opens the carbon-carbon double bond of the PDMS oligomer thereby assisting in the formation of the C-O bonds between the PDMS matrix and the graphene filler. High absorption of NIR photons causes the free radical reaction between SiH group on PDMS crosslinker and hydroxyl/carbonyl groups on the RGO. The increase in the number of C-O and Si-O bonds at the graphene/polymer interface assists in the improved load transfer and eventual mechanical properties of the composites. This is the first such study which shows direct correlation between bond formation, load transfer and mechanical properties without degrading the interface. While surface chemical functionalization is attractive, past reports have shown that improvement in interfacial adhesion due to surface functionalization of nanotubes does not always promote improvement in mechanical properties. This is due to the surface degradation of nanotubes/graphene during functionalization process. Compared to these techniques, the NIR light induced technique is benign, environmentally friendly and also results in high interfacial shear strength, load transfer and excellent mechanical properties. As a demonstration of applications, PDMS/RGO/PDMS sandwiched structure strain sensor, a demo application of the NIR photon-assisted polymerization was investigated. High sensitivity and high Gauge Factor (GF) are addressed. These results shown in this dissertation suggest that the NIR photon-assisted polymerization can be practically developed as a scalable nanomanufacturing technique to create large panels of advanced composites with strong interface and better mechanical properties compared to conventional oven based heating methods. It also suggests that it is possible to fabricate large-scale flexible smart device like high sensitivity strain sensors.

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