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Description:
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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 . |