title: 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.
authors.affiliation: Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, United States.
authors.forename: Choonghyun
authors.initials: C
authors.lastname: Sung
authors.forename: Ajay
authors.initials: A
authors.lastname: Vidyasagar
authors.forename: Katelin
authors.initials: K
authors.lastname: Hearn
authors.forename: Jodie L
authors.initials: JL
authors.lastname: Lutkenhaus
mesh_terms: D000180:Acrylic Resins / Q000737:chemistry*; D002152:Calorimetry, Differential Scanning; D006860:Hydrogen Bonding; D008567:Membranes, Artificial; D011092:Polyethylene Glycols / Q000737:chemistry*; D011109:Polymethacrylic Acids / Q000737:chemistry*; D013696:Temperature
chemical_list: D000180:Acrylic Resins; D008567:Membranes, Artificial; D011109:Polymethacrylic Acids; C030613:polymethacrylic acid; D011092:Polyethylene Glycols; C006903:carbopol 940