Chain Conformation in Polymer Glassy Films Under Residual Stress

The evolution of chain conformation in polymer glassy films under residual stress was investigated. Poly n-butyl methacrylate (PnBMA) was selected as the model polymer. The polymers were labeled at the two chain ends with a fluorescence donor and acceptor, separately. By measuring the fluorescence resonance energy transfer (FRET) efficiency between them, the end-to-end distance of the polymer chains was determined. Polymer thin films fabricated by spin-casting without thermal annealing were taken as the model sample with residual stress stored inside. It was found that the PnBMA chains are elongated as the molecular source of residual stress, as expressed by the increase in the scaling exponent, changing from 0.5 under zero residual stress to 0.8 at 8.5 MPa residual stress. Single-molecule fluorescence defocused microscopy was used to track the rotational motion of the fluorescent probes, revealing that the donor's rotation is suppressed in the presence of the acceptor, with larger suppression at shorter distance.