Exploring Pathways to Equilibrate Langmuir Polymer Films

Abstract

Focusing on the phase-coexistence region in Langmuir films of poly­(l-lactide), we investigated changes in nonequilibrated morphologies and the corresponding features of the isotherms induced by different experimental pathways of lateral compression and expansion. In this coexistence region, the surface pressure Π was larger than the expected equilibrium value and was found to increase upon compression, i.e., exhibited a nonhorizontal plateau. As shown earlier by using microscopic techniques [Langmuir 2019, 35, 6129–6136], in this plateau region, well-ordered mesoscopic clusters coexisted with a surrounding matrix phase. We succeeded in reducing Π either by slowing down the rate of compression or through increasing the waiting time after stopping the movement of the barriers, which allowed for relaxations in the coexistence region. Intriguingly, the most significant pressure reduction was observed when recompressing a film that had already been compressed and expanded, if the recompression was started from an area value smaller than the one anticipated for the onset of the coexistence region. This observation suggests a “self-seeding” behavior, i.e., pre-existing nuclei allowed to circumvent the nucleation step. The decrease in Π was accompanied by a transformation of the initially formed metastable mesoscopic clusters into a thermodynamically favored filamentary morphology. Our results demonstrate that it is practically impossible to obtain fully equilibrated coexisting phases in a Langmuir polymer film, neither under conditions of extremely slow continuous compression nor for long waiting times at a constant area in the coexistence region which allow for reorganization.

Reversible Dehydration–Hydration of Poly(ethylene glycol) in Langmuir Monolayers of a Diblock Copolymer Inferred from Changes in Filament Curvature

“…By compressing the Langmuir film from A 1 to A 4 , significant changes in the order of this arrangement were observed as a function of increasing surface density (see Figure ). Interestingly, in addition to the distinctive differences discussed below, the morphology of the filaments was somewhat similar to the one observed for the homopolymer P l LA 1840 (see also Figure S2 of the Supporting Information) . Thus, it is quite reasonable to assume that the formation of filaments was influenced or even governed by the P l LA block.…”

“…Therefore, the kinetics of compression and expansion as well as constraints imposed by the geometry of the Langmuir trough will affect the hysteresis loops. 18 However, the study presented here clearly shows that the observed changes in morphology were reversible and were controlled by surface density.…”

“…It cannot be expected that expansion at the finite rate chosen in our experiments allowed for immediate and complete reversal (equilibration) of structural and morphological changes involved in the corresponding phase transition. Therefore, the kinetics of compression and expansion as well as constraints imposed by the geometry of the Langmuir trough will affect the hysteresis loops . However, the study presented here clearly shows that the observed changes in morphology were reversible and were controlled by surface density.…”

“…Interestingly, in addition to the distinctive differences discussed below, the morphology of the filaments was somewhat similar to the one observed for the homopolymer PLLA 1840 (see also Figure S2 of the Supporting Information). 18 Thus, it is quite reasonable to assume that the formation of filaments was influenced or even governed by the PLLA block. This assumption is additionally supported through the comparison of the isotherms of the diblock copolymer mPEG 2000 -b-PLLA 1700 with the isotherms of the homopolymers PLLA 1840 and mPEG 2050 presented in Figure S3 of the Supporting Information.…”

“…In particular, under conditions that allowed the molecules to approach equilibrium, filaments were observed whose heights and widths suggested an ordered, probably (liquid) crystalline state. 18 In contrast, the PEG block exhibits a more complex behavior at the air−water interface. Based on the results published over many years by several research groups and also derived from our measurements, we assume that above a minimum surface density, PEG blocks became gradually immersed into the subphase.…”