Thin Polymer Films Out of Thermodynamic Equilibrium

Abstract

The fabrication process of a thin polymer film, involving the transition from a (dilute) solution to a dry glass, is believed to have an impact on film properties, often related to the poorly entangled out-of-equilibrium chain conformations and corresponding residual stresses. Thus, a central question concerns the possibility of relaxation of such chains even in the glassy state. Physical ageing of polymer films at temperatures below the glass transition (Tg, bulk) was found to lead to a progressive decrease in dewetting velocity, which is indicative of a decreasing residual stress or an increasing modulus with ageing time. Dewetting velocity showed a close to exponential decay with ageing time, defining a characteristic relaxation time. Variation of relaxation times with temperature seems to follow an Arrhenius temperature dependence. This suggests a process of a faster segmental relaxation mechanism, which is sufficient to relax part of the residual stresses. The relaxation dynamics of residual stresses appears to be in close agreement with some earlier reports, concerning relaxation solely at the surface of polymer thin films. This intriguingly invokes the possibility to correlate the surface mobility in thin polymer film with non-equilibrium conformation of polymer chains. The relaxation behaviour of such films, both above and below the Tg, bulk, is notably faster than the -relaxation time and reptation time of the bulk polymer. In addition, films prepared from solutions close to the thetatemperature were aged for varying times at room temperature and characterized by dewetting. The characteristic relaxation time of ageing varied strongly with the quality of the solvent, which is attributed to distorted chain conformations in the as-cast films. This signifies quite clearly the non-equilibrium nature of thin polymer films, possibly causing some of their unexplained properties, such as fast relaxation.

Interestingly, the process of physical ageing of such films at elevated temperatures close but still below the Tg,bulk can also lead to crack formation. We observed cracking during cooling these films to room temperature. For long chain polymers, AFM inspection inside the cracking patterns showed craze nano/microstructures consisting of voids and fibrils. Short chain polymers (of a length comparable to the entanglement length) did not show such crazes, rather formed simple cracks. For higher molecular weight polystyrene, a systematic study showed a progressive increase of craze growth velocity with ageing time whereas for the same films the dewetting velocity decreased with ageing. While the increase in craze growth velocity might indicate an increase in total stress inside the film upon a temperature jump, the decrease of dewetting velocity is the signature of relaxation of residual stresses.
 
In summary, our study invokes the possibility to correlate several of the long standing puzzling properties of thin polymer films with respect to the nonequilibrium conformations of polymer chains inside the film.