Correlation between Fragility and Surface T_g of Polymers

The fragility of glass describes how rapidly its molecules slow down as it cools near its glass transition temperature. In nanoscale films, polymer glasses with higher fragility experience larger reductions in their T_g compared to those with lower fragility. We investigated whether this is due to the free surface of the polymers, which can cause the surface T_g (T_g^surf) to decrease relative to the bulk T_g. By measuring the T_g^surf of various polymers, we found that the shift in T_g^surf relative to the bulk T_g increased with fragility. This suggests that more fragile polymers are more susceptible to the free surface effect. We explain this using the concept of energy landscape, as it is used to explain the different slowdown rates between strong and fragile glass-formers at T_g.