Relaxation and transport of polymer chains on surfaces during electrospray deposition*

Electrospray deposition (ESD) provides a versatile means of fabricating polymer thin films. The morphology and properties of such films depend on kinetic processes that occur during the deposition. Such deposition occurs in a dynamic environment by sequential buildup of polymer chains that relax and diffuse after delivery to a heated substrate from charged droplets of initially dilute solution. We examine the deposition kinetics by observing the evolution of surface coverage with time during ESD of a homopolymer and a block copolymer. The kinetics are modeled using a 1^st order adsorption equation with a term, S, that incorporates the effect of droplet spreading over time. The kinetics depend strongly on molar mass, M, with S ~ 1/M². We interpret S in terms of an effective polymer diffusivity, and find that the temperature dependence is well-described using the WLF equation with an unmodified T_g. These results indicate that the mobility of the polymer is important in determining the coverage kinetics and are consistent with a picture in which substrate coverage evolves by the delivery of material to the substrate from the reservoirs of already deposited droplets as a rate limiting step, alongside ongoing arrival of new material from the ESD feed. These results are corroborated by a stochastic random deposition simulation that further elucidates the relevance of diffusion in describing the coverage kinetics of films formed by ESD.