The effect of matrix molecular weight on the kinetics of formation of end-adsorbed polystyrene layers from the melt

Review and reproducibility of forming adsorbed layers from solvent washing of melt annealed films

Recent studies suggest chain adsorption in the melt may be responsible for a number of property changes in thin films by making correlations between the residual adsorbed layer thickness h_ads(t) measured after a given solvent washing procedure as a function of annealing time t of the film at an elevated temperature prior to this solvent rinse. This procedure, frequently called "Guiselin's experiment", refers to the thought experiment proposed in a 1992 theoretical treatment by Guiselin that assumed chain segments in contact with the surface are irreversibly adsorbed whereby unadsorbed chains could be washed away by solvent without disturbing the adsorbed substrate contact points in the melt. In the present work, we review this recent literature, identifying and experimentally testing a common protocol for forming adsorbed layers h_ads(t) from solvent washing melt films. We find h_ads(t) curves to be far less reproducible and reliable than implied in the literature, strongly dependent on solvent washing and substrate cleaning conditions, and annealing at elevated temperatures is unnecessary as densification of films sitting at room temperature makes the glassy film harder to wash off, leaving behind h_ads of comparable thickness. This review also summarizes literature understanding developed over several decades of study on polymer adsorption in solution, which experimentally demonstrated that polymer chains in solution are highly mobile, diffusing and exchanging on the surface even in the limit of strong adsorption, contradicting Guiselin's assumption. Preformed adsorbed layers of different thicknesses h_ads are shown to not affect the average glass transition temperature or physical aging of 30 nm thick films. In summary, a number of open questions and implications are discussed related to thin films and polymer nanocomposites.

Nonsolvent annealing polymer films with ionic liquids

Neutron reflectometry has been used to determine the interface structure and swelling of thin polymer films, when annealed in contact with a series of 1-alkyl-3-methylimidazolium ionic liquids (ILs). By choosing immiscible polymer/IL combinations, we have established that thin polymer films can be annealed for several hours in contact with ILs at temperatures well above the glass transition temperature and that this nonsolvent annealing environment can be exploited to direct self-assembly in polymer films. The ingress of IL into polymer films was quantified in terms of the swelling up to 10%. The polymer/IL interfacial width generally also increased from 0.9 nm up to ∼3 nm, but there was remarkably little correlation between interfacial width and swelling. For one combination of polymer and IL (deuterated PMMA and Bmim-BF(4)) the interfacial width decreased slightly with increasing temperature, consistent with LCST behavior for this system. All of the ILs tested had a profound influence the distribution of carboxy-end-functionalized deuterated polystyrene, "dPS-COOH", in blended films with polystyrene homopolymers. The ILs promoted dPS-COOH adsorption at the film/IL interface and the simultaneous rapid desorption at the film silicon-oxide interface. The rate of desorption was found to correlate with the swelling behavior of the polymer with respect to the IL anion species: PF(6)(-) < Br(-) < Cl(-) < BF(4)(-), suggesting that the polymer films are plasticized by the IL as it penetrates the film.

pH-controlled polymer surface segregation

A new approach to promoting and controlling polymer surface functionalization with acidic or basic polar functional groups is demonstrated and evaluated. Blended polymer films were annealed under pH-buffered conditions, and polar end-functional groups were found to promote surface segregation of the functional polymers. Surface segregation of carboxylic acid (COOH)-functionalized polystyrene increases dramatically with increasing pH from 1.9 to 9.4, whereas the opposite behavior is seen for amine (NH2)-functionalized polystyrene. Neutron reflectometry and nuclear reaction analysis were used to obtain surface excess values for the functional polymers. Subsequent SCFT analysis of the composition versus depth profiles indicates that the affinity of each functional group for the polymer surface changes by about 3k(B)T over this pH range.