Confinement Effects on Cross-Linking within Electrostatic Layer-by-Layer Assemblies Containing Poly(allylamine hydrochloride) and Poly(acrylic acid)

Abstract

Thermal cross-linking is widely used to impart stability or improved mechanical properties to layer-by-layer (LbL) assemblies. However, the kinetics of thermal cross-linking within LbL films is not well understood. Furthermore, because LbL films are generally ultrathin (<100 nm), the influence of confinement on cross-linking kinetics is potentially substantial. Using temperature-controlled ellipsometry, differential scanning calorimetry, and thermal gravimetric analysis, we are able to accurately track amide cross-linking within poly(allylamine hydrochloride)/poly(acrylic acid) LbL films. The rate of amidation is strongly influenced by film thickness and surface chemistry, which indicates that the observed "confinement effects" are primarily related to the catalytic contribution of hydroxyl groups present on the substrate's surface. The analytical techniques presented herein highlight new ways to access thermochemical information within ultrathin LbL assemblies.

Simultaneous characterization of physical, chemical, and thermal properties of polymeric multilayers using infrared spectroscopic ellipsometry

“…They prepared ultra-thin PS films and their ellipsometric results of the change in thickness vs. temperature showed two linear regions of thermal expansivity, which they related to the polymer glass and melt temperatures. They report that the point that divides two regions was associated to the Tg of the polymer [69]. Relying on these results and the behavior registered over the increase and expansion of the thickness of the polymeric multilayers formed by the layer-by-layer technique, we have attributed this behavior to the Tg of the multilayers.…”

“…Lutkenhaus and her group studied the thermal properties of LbL films of poly(diallyldimethylammonium chloride)/poly(styrene sulfonate) (PDAC/PSS) [69]. They found that different thermal transitions of the multilayers can be observed depending on the properties of the solution used to prepare the multilayers; for example, the ionic force or whether they are hydrated or not.…”

“…Therefore, this mobility allows the polymers to rearrange, modifying the topographic environment and hence the characteristics measured. Lutkenhaus, in another work [69], where 5, 6, and 8 layer pairs of poly (allylamine hydrochloride)/ poly(acrylic acid) were fabricated, monitored through temperature-controlled ellipsometry the changes in thickness of the multilayers. They initially obtained at 25 °C a thickness of the multilayers of 59 ± 3 nm, 119 ± 1 nm, and 183 ± 2 nm, for 5, 6, and 8 layer pairs, respectively.…”

“…They observed that when less multilayers are formed, the decrease in thickness is greater, obtaining a decrease of 14 ± 1 %, 12.18 ± 0.01 %, and 9 ± 4 %, for 5, 6, and 8 layer pairs, respectively, up to a change of 275 °C. They attributed the decrease in thickness of the multilayers to the loss of mass or to an increase in the density of the multilayers [69]. In another work, Lutkenhaus [70], found that for 200 layer pairs of poly(allylamine hydrochloride)/ poly(acrylic acid), the reduction of thickness of the LbL film is the result of the formation of the anhydride and the amidation.…”

Covalent Layer‐by‐Layer Assembly Using Reactive Polymers

“…For example, some PEMs can exhibit limited stability upon changes in pH, ionic strength, and temperature (or upon exposure to other ''harsh'' environments) that can disrupt the weak ionic interactions that stitch these materials together [18][19][20][21][22][23][24]. Several groups have sought to address this limitation by developing approaches to the covalent cross-linking of PEMs by postfabrication thermal treatment [25][26][27][28][29], exposure to UV radiation [27,[30][31][32], or by treatment of preassembled PEMs with chemical cross-linking agents [33][34][35][36][37][38]. These approaches can lead to PEMs with increased stability, but thermal treatment can also alter other important film properties and these approaches are, in general, limited to the cross-linking of PEMs with appropriate combinations of mutually reactive functional groups (e.g., amine and carboxylic acid functionalities) that may or may not be present depending on the structure of the polyelectrolytes used to fabricate a film.…”