This Faculty Early Career Development (CAREER) program seeks to quantify the mechanical behavior of polymers subjected to nanoscale confinement and restricted/enhanced macromolecular mobility near hard surfaces. The latter will be designed to simulate the behavior of polymer molecules near pristine and functionalized nanotubes and nanofibers in polymer nanocomposites. The large surface-to-volume ratio in these material systems is such that the hard phase restricts molecular mobility and conformations. As a result, the entire composite may be practically an interphase, with local and effective macroscopic properties being very different from pristine bulk polymers. Experimental nanoscale probe methods will be developed to determine variations in molecular dissipation at the nanoscale and to quantify the collective time-dependent response of surface molecules in ultra-thin polymer films. Measurements of the viscoelastic creep functions and glass transition temperatures as a function of polymer layer thickness will provide the data to assess molecular conformations near hard interfaces. Due to the lack of local information on polymer matrix modification near hard surfaces, the fabrication of strong and tough multifunctional and lightweight polymeric nanocomposites is far from reaching its projected potential. If successful, this program will set the foundations for controlled and efficient design of high performance nanostructured polymers and establish property limits for multiphase polymers. The experimental results from this program will be essential material parameters for molecular dynamics and micromechanics models with applications to nanocomposites, biological systems, nanoimprinting, nanofluidic arrays, etc. The outreach plan of this CAREER award will (a) inspire high school students to study aerospace engineering via an engineering teaching kit that will increase high school student technology awareness in hierarchical materials for aerospace applications, (b) provide hands-on education on nanoscale mechanics to undergraduate and graduate students, and (c) support professionals with nanoscale mechanics laboratory training.
This NSF-CAREER project on Nanoscale Confinement of Polymers focused on the mechanical behavior, including the tensile strength and ductility, of polymeric nanofibers in which the size of the polymer macromolecules becomes comparable to their diameter. In this experimental research program, the molecular size in polystyrene nanofibers was controlled by selecting molecular weights between 13,000 g/mol and 9,000,000 g/mol, while the fiber diameters were varied in the range of 150 – 5,000 nm. The results of a large number of microscale tension experiments with individual polystyrene nanofibers showed for the first time that simple polymers such as polystyrene, which behave as brittle solids at the macroscale, become very ductile and extend by as much as 200% when made in the form of nanofibers with small ratios of nanofiber diameter to molecular size. This tremendous extensibility of nanofibers occurs with a simultaneous increase in tensile strength by a factor of three or more. Such modifications in the mechanical behavior of monolithic materials are unique because conventional man-made homogeneous materials can be processed to increase their strength or their ductility but not both at the same time. It was also shown for the first time that imperfections and flaws distributed on the surface of nanoscale fibers, or deviations from the circular fiber cross-section are not as detrimental as they would be for macroscale fibers. On the contrary, such imperfections promote a particular mechanical behavior during fiber extension which prevents the formation of a stable neck that propagates along the fiber and thus limits the fiber strength. Instead, rough polymer nanofibers or nanofibers with non-circular cross-sections were shown to be flaw tolerant and possess increased load bearing capacity compared to those with smooth surfaces and uniform circular cross-sections, while also demonstrating outstanding ductility. Finally, it was shown that, contrary to their macroscale counterparts, nanoscale polymeric fibers exhibit enhanced capacity for energy dissipation by maintaining their ductility virtually unchanged under fast and ultrafast loading that takes place in the timeframe of microseconds, while at the same time increasing their load bearing capacity by a factor of three. The broader impact of these results lies with the potential of ordinary polymers to be manufactured in much stronger and dramatically more ductile forms, which can increase the total energy dissipation by as much as 4,000% when refined in the form of nanoscale fibers which, in turn, can serve as the building blocks for strong and tough polymer yarns and fabrics.
