Monitoring the hygrothermal response of poly(vinyl methyl ether) submicron films using AFM

Films with submicron thickness of the thermoresponsive polymer poly(vinyl methyl ether) (PVME) are prepared by spin-coating and measured with AFM. The change of film thickness, work of adhesion (WOA), and modulus of elasticity (MOE) are monitored below and above the lower critical solution temperature (LCST) in controlled atmosphere. The film thickness decreases above the LCST at high relative humidity, but not at low humidity. Simultaneously, the MOE increases by 3 orders of magnitude due to conformational transition. The WOA shows a hysteretic behavior due to changes in surface wettability and film compressive strength. Hygrothermal cycling is found to facilitate the relaxation of initially arrested entanglements. The proposed hypothesis of hygrothermally induced partial disentanglement is further supported with cyclic DSC measurements of concentrated aqueous PVME solutions. This work is an important contribution to understand the local and macroscopic behavior of thermoresponsive polymers in dry and humidified atmosphere.