Effect of Polymer-Surface Mobility on Adhesion in Poly(Methyl Methacrylate)-Tape System
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The interaction between two polymer layers, especially adhesion between them, plays an important role in polymer processing and other applications. Detailed knowledge of the molecular structure and dynamics of polymer interfaces, and how they relate to macroscopic mechanical properties, should help designers construct more functional systems. Unfortunately, there have been few studies where both molecular and macroscopic studies have been performed on similar systems. In previous studies from our group, we have probed the dynamics of poly(methyl acrylate) (PMA) and thermal behavior of poly(methyl methacrylate) (PMMA) on silica. These studies helped us paint a picture for strongly bound molecules on silica, where a motional gradient perpendicular to the surface was observed. More mobile, lower Tg, material was found at the air-polymer interface, while less mobile, higher Tg material was found at the polymer-substrate interface. A previous study from our group showed that the glass-transition temperature of PMMA changes with the thickness of the polymer layer on silica. by examining the thermal behavior of the polymer with modulated differential scanning calorimetry (MDSC), it was observed that the glasstransition temperature increases as the thickness of the polymer layer decreases. Blum and Lin have also used the deuterium NMR to probe the dynamics of bulk and silica-absorbed poly(methyl acrylate)-d3 (PMA-d3). It was found that, an increase in the absorbed amount of polymer increased the mobility of polymer at the air-polymer interface and a decrease in the absorbed amount of polymer, decreased the polymer mobility. It is not known how this change in mobility relates to the mechanical properties of polymers, especially adhesion between two polymer layers. Of the various methods available for measuring adhesion-related properties, peeling is considered to be the most convenient. Current theories consider peeling to be the principal mode of separation of an adhesive from the substrate. It is proposed that adhesion strength is proportional to the surface free energy. Research has been done to find the adhesion strength between films and substrates, but there are some questions which remain unanswered like: Does adhesion strength depend on the thickness of the polymer layer? Does adhesion strength relate to the mobility of polymer segments on the surface? in the present report, we summarize some of our macroscopic studies, namely peel tests, on systems similar to those previously studied. We find that the mobility of the polymer chains at the interface play an important role in determining the peel strength between the two polymer layers. We decided to use a 90° peel angle method. At this high angle, the lateral stress in the film is very negligible, thereby reducing the lateral elongation of the film. In addition, at this angle, there is no slip at the interface. Our studies show that as the polymer film thickness decreases, the force required to initiate fracture between two polymer surfaces increases and the fracture energy also increases proportionately. We have also observed how fracture energy changes with a change in film thickness.