Mechanical properties of ultrathin polymer films investigated by a nanobubble inflation technique: Surface tension, geometery and molecular architecture effects



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It is very important to understand the mechanical properties of polymers at the nanoscale with the continuing demand of decreasing the size of circuit in the electronic industry. There has been considerable research on the confinement effect in thin films with a variety of techniques, often with conflicting results. Most of the previous work has been done in a pseudo-thermodynamic mode, where the glass transition temperature (Tg) is taken to be the break in the temperature dependence of a property [e.g. ellipsometry, Brillouin scattering]. A method based on the determination of a dynamic property, the absolute biaxial creep compliance, has been developed by O’Connell and McKenna. The method is a scaled down version of the classic bubble inflation technique. They found that the Tg of polystyrene (PS) decreases with film thickness, while it doesn’t change for poly(vinyl acetate) (PVAc). The most surprising finding is that the rubbery plateau compliance decreases dramatically for both materials. These results are unexplained though it has been suggested that the observed stiffening at the nanometer size scale could be attributed to surface tension. In this thesis, we investigated a new material (poly (n-butyl methacrylate) (PBMA)) that shows significantly different behavior from PVAc or PS and that provides new evidence that the stiffening of the rubbery plateau region in ultrathin polymer films is a nanoconfinement effect. We developed the stress-strain analysis and energy balance approach to separate the surface tension contribution to the observed rubbery stiffening. We found that the surface tension contribution for PBMA is much larger than that of PVAc. The rubbery stiffening of PBMA is much less than PS and PVAc. The surface tension of PBMA doesn’t change with decreasing film thickness. Further, the geometry effect in the nanobubble inflation technique was investigated by comparing the creep behavior of circular bubbles with that of rectangular bubbles. The accuracy of the analytical approximate solutions was evaluated by comparing with the finite element (FE) analysis for simulation of the inflation of rectangular bubbles. We found that the shape of the bubble obtained from the experiment is consistent with that of FE. We also found that the reduction of Tg and the rubbery plateau compliance for rectangular bubbles are consistent with those of circular bubbles. So geometry is not the reason for the observed stiffening effect. Next, we investigated the molecular architecture effect in the nanobubble inflation technique by comparing the creep behavior of linear PS with that of the three-arm star PS. Both the reduction of Tg and the stiffening in the rubbery region for star PS is consistent with those of linear PS. In the last part of this thesis, the capability of the nanobubble inflation technique to investigate the yield and fracture behavior of ultrathin films was demonstrated. Stepped pressure is applied to ultrathin films until it broke. We found that for 33nm film, it transit from brittle failure to yield with increasing temperature. The yield stress decreases with increasing temperature. For 22nm film, it always failed without yield.



Nanobubble, Thin films, Viscoelasticity