Effect of Thickness on the Thermal Properties of Hydrogen-Bonded LbL Assemblies

Abstract

Layer-by-layer (LbL) assemblies have attracted much attention for their functional versatility and ease of fabrication. However, characterizing their thermal properties in relation to the film thickness has remained a challenging topic. We have investigated the role of film thickness on the glass transition temperature (T(g)) and coeffecient of thermal expansion for poly(ethylene oxide)/poly(acrylic acid) (PEO/PAA) and PEO/poly(methacrylic acid) (PEO/PMAA) hydrogen-bonded LbL assemblies in both bulk and ultrathin films using modulated differential scanning calorimetry (modulated DSC) and temperature-controlled ellipsometry. In PEO/PAA LbL films, a single, well-defined T(g) was observed regardless of film thickness. The T(g) increased by 9 °C relative to the bulk T(g) as film thickness decreased to 30 nm because of interactions between the film and its substrate. In contrast, PEO/PMAA LbL films show a single glass transition only after a thermal cross-linking step, which results in anhydride bonds between PMAA groups. The T(g), within error, was unaffected by film thickness, but PEO/PMAA LbL films of thicknesses below ~2.7 μm exhibited a small amount of PEO crystallization and phase separation for the thermally cross-linked films. The coefficients of thermal expansion of both types of film increased with decreasing film thickness.

Highly selective multilayer polymer thin films for CO2/N2 separation

“…It should be noted that this method requires a free‐standing multilayer assembly and the 20 BL film is thinnest that can be prepared. These results agree well with previous findings, where the PEO crystallinity decreases as the film thickness increases . It is clear that LbL assembly suppresses the crystal formation in PEO/PMAA films due to the interdiffusion of polyelectrolytes …”

“…A melting peak near 62 °C is also observed for all the multilayer films, which is associated with PEO. Polymer crystals are relatively impermeable to gasses, and PEO is known to be semi‐crystalline in hydrogen‐bonded multilayer thin films . The degree of crystallinity ( X c ) of PEO in the multilayer assembly was calculated using X c = Δ H m /Δ H °, where Δ H m is the melting enthalpy of PEO in the film and Δ H ° is the melting enthalpy of 100% crystalline PEO (188 J g −1 ) .…”

“…Polymer crystals are relatively impermeable to gasses, and PEO is known to be semi-crystalline in hydrogen-bonded multilayer thin films. 54 The degree of crystallinity (X c ) of PEO in the multilayer assembly was calculated using X c 5 DH m / DH8, where DH m is the melting enthalpy of PEO in the film and DH8 is the melting enthalpy of 100% crystalline PEO (188 J g 21 ). 55 QCM results indicate that the weight fraction of PEO within the multilayer assembly is 27 wt %, which makes the crystallinity of PEO in the films 5.5%, 4.3%, and 3.8%, for 20, 30, and 40 BL films, respectively.…”

“…where the PEO crystallinity decreases as the film thickness increases. 54 It is clear that LbL assembly suppresses the crystal formation in PEO/PMAA films due to the interdiffusion of polyelectrolytes. 56 Gas Separation Behavior CO 2 and N 2 transmission rates (TR) of PEO 2 /PMAA 2 films as a function of BL number are shown in Figure 6(a).…”

Organic Thermoelectric Multilayers with High Stretchiness

“…The thickness profile was dictated by the employment of PAA in each deposition cycle, which induced "in-and-out" diffusion of PEO and PAA during assembly. As is well documented, PEO/PAA films assembled at a pH between 2 and 3 grow exponentially during which an immense amount of diffusive intermixing takes place during deposition instead of staying at the position where they were initially deposited [52,54]. For the remainder of this paper PEO/DWNT-PAA (0.1 wt%) refers to a sample constructed from DWNT dispersed in 0.1 wt% PAA, as distinguished from PEO/DWNT-PAA (0.2 wt%) in which DWNT is stabilized in 0.2 wt% PAA.…”