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43011-G10
Deformation and Failure of Model Diblock Copolymer Nanocomposites
Scientific Accomplishments
This project is focused on the fundamental mechanisms involved in the development of mechanical properties in thin films of polymer/nanoparticle composites. As outlined in the original proposal, we aim to answer two questions:
1) How does nanoparticle packing density control the mechanism of deformation and failure in nanocomposite thin films?
2) What role does confinement of nanoparticle distribution play in the deformation and failure processes of these materials?
In accordance with our proposal, we have used this funding to understand the impact of nanoparticle volume fraction on the deformation and failure processes in glassy nanocomposite films.
We studied model materials comprised of CdSe nanoparticles (diameter = 5 nm) that are uniformly dispersed in a polystyrene (PS) homopolymer matrix (MW =126,000 g/mol). The CdSe nanoparticles are modified with surface ligands of short PS chains (MW=1000 g/mol). These nanoparticles are provided as part of a collaboration with Professor Todd Emrick in the Polymer Science & Engineering Department at the University of Massachusetts Amherst. This model material allows us to isolate and understand entropic contributions to the deformation and failure of glassy nanocomposites.
In our first year of funding, we focused on the impact of the nanoparticles on crazing, the primary deformation mechanism leading to failure in PS homopolymer. We performed detailed morphological analysis of the change in nanoparticle distribution as a function of applied strain and nanoparticle loading. We found that the spatial distribution of nanoparticles during craze formation and propagation is altered in polymer nanocomposites. TEM micrographs reveal an assembly and alignment of nanoparticles along the path of the precraze, premature crazes that are split into multiple, small “secondary crazes”, and mature crazes with microstructures based on incorporated nanoparticle clusters that exist between craze fibrils, not within the fibrils.
At an optimal volume fraction of only 0.68%, the failure strain of the nanocomposite thin film is nearly 100% greater than the failure strain of unmodified homopolymer. During the second year of funding, we conducted a series of experiments and analyzed the energetic growth processes of the crazes to understand the origin of this effect. We concluded that two competing mechanisms define the optimal volume fraction for enhanced ductility. The first mechanism is related to the decrease in cross-tie fibrils in the mature craze due to the presence of excluded nanoparticles and incorporated nanoparticle clusters. The second mechanism is the limitation of macroscopic failure strain due to rigid particles that increase the local strain of a composite material.
During the second year, we also quantified the elastic modulus and glass transition temperature (Tg) as a function of nanoparticle loading. The Tg is depressed as a function of nanoparticle weight fraction. This depression is related to two mechanisms: 1) the presence of short chain ligands attached to the nanoparticle surface for particle/matrix compatibility; and 2) the influence of rigid, enthalpically-neutral nanoparticles on the configurational entropy of neighboring matrix chains. To quantify the relative importance of these contributions, we compared the properties of the polymer/nanoparticle composites to blends of our matrix PS chains with short PS chains, similar to the ligands attached to the nanoparticle surfaces. These blends confirm that the constraint on the local packing of neighboring matrix chains significantly impacts the Tg of the nanocomposite. We used classical Fox-Flory relations for Tg and our measured data to estimate the extent of the nanoparticles' impact on the matrix. This impact on the configurational constraint of polymer chains neighboring distributed nanoparticles also leads to a decrease in the elastic modulus of these materials. A predictive model for this effect has not been established at this point, but there is no question on the importance of these fundamental results for understanding the true “nano” effect on mechanical properties.
Personnel Accomplishments
In addition to obtaining these previously undocumented experimental results and developing models to provide fundamental insight into nanocomposite mechanical properties, the PI and graduate student supported by this grant have gained significantly. We have published four manuscripts on these results in Macromolecules, and a fifth publication in Polymer Reviews. The PI has delivered multiple presentations on this research, including presentations at the ACS National Meeting, the APS National Meeting, the Deformation, Yield, and Fracture Conference in Rolduc Netherlands, and the Polymer Physics Symposium in Suzhou, China. The graduate student has given multiple presentations at regional and national conferences (e.g. American Physical Society's March Meeting), including an invited presentation at the Adhesive and Sealant Council's annual meeting. He also received the best poster award for this work at the US-Taiwan Soft Materials Workshop held in Taipei in January 2007. This past summer the graduate student successfully defended his Ph.d based on this research and began as a staff scientist at Proctor & Gamble.
