In response to increased demand for fuel efficiency and sustainability, companies are seeking advancements in tire technologies to reduce fuel consumption and greenhouse gas emissions. There are several factors that affect fuel efficiency in tires. The tire tread is of most interest and relevance today, as it is responsible for the grip and wear issues, thereby requiring the highest performance. Silica is added as a reinforcing filler to the tread compounds to achieve a lower rolling resistance, while maintaining high grip and wear resistance. Due to the polarity difference between hydrophilic silica and hydrophobic rubber, industries mostly use silane agents to aid the dispersion.
Such dispersion methods involve surface modification of silica in situ but provide very little control over the dispersion states achieved. Polymer grafted nanoparticles have recently attracted attention due to their ability to control and optimize dispersion. Tuning parameters like grafting density and ratio of matrix to graft chain length results in various dispersion states, which in turn have a direct correlation with reinforcement. However, most of their applications have been restricted to plastics, especially in the melt state. In this dissertation, we extended the use of grafted nanoparticles to polyisoprene rubber composites, where the grafted polymer and elastomer matrix don’t have the same chemical microstructure. We explored wide range of morphologies, ranging from well dispersed, connected networks, strings, sheets and small clusters. After studying the relative importance of entropic vs enthalpic effects on the self-assembly of nanoparticles, we observed how the morphologies evolved on crosslinking. The different morphologies studied were independent of the state of crosslinking and the chemical composition of the matrix, with minor changes occurring in samples where the nanoparticles were well dispersed.
We next investigated the mechanical properties of the composites in two critical areas: linear regime and nonlinear regime. In each area, we performed systematic studies on the different morphologies in order to isolate the morphology that shows the optimal properties under different conditions. We demonstrated that nanoparticle dispersion states play a very important role in mechanical properties and are sensitive to the state of the polymer. While the connected network morphology shows the maximum reinforcement in the melt state, it is the aggregated sheets morphology at a lower grafting density that shows the highest reinforcement in the crosslinked state. This morphology at intermediate grafting density also shows the highest strain dependence in the Payne effect measurements. To understand the reinforcement mechanism in the linear and nonlinear measurements, we attempted to explore the underlying dynamics through NMR and XPCS. The evaluation crosslink density of the rubber matrix using NMR suggests that we do not have a fully developed immobilized layer and there is no variation in crosslink density in the polymer phase across the composites. This eliminates the popular theory that immobilized polymer layer in the vicinity of nanoparticles is responsible for reinforcement in rubber composites.
Our research shows that filler network plays a crucial role in determining the dynamic properties of filled elastomers. At the intermediate grafting density region where sheets morphology form, the particles are aggregated with particle-particle contacts while having enough graft chains to entangle and crosslink. We believe that the crosslinked grafts connect the different sheets in the composite and aid in an improved stress transfer. This causes the filler network to percolate at much lower NP loadings. This lower percolation threshold for sheets morphologies explains the high reinforcement in the linear regime and large modulus drop in Payne effect measurements at lower nanoparticle loadings as compared to the other morphologies. Furthermore, the macroscale mechanical properties and results of XPCS derived microscopic dynamics align well. We observed that when the particles are dispersed they show faster dynamics as compared to the aggregated morphologies above the percolation threshold. At lower nanoparticle loadings, all the morphologies show similar dynamics, emphasizing the importance of percolation.
In summary, we have used a combination of different techniques to understand the underlying mechanism of rubber reinforcement. We have identified which dispersion states affect reinforcement in the linear and nonlinear regime under different conditions. We combined macroscale mechanical testing with nanoscale dynamic measurements to draw a holistic picture on the dependence of reinforcement on nanoparticle dispersion states.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/c3zx-xn65 |
| Date | January 2023 |
| Creators | Dhara, Deboleena |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
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