Layer-by-layer (LbL) assemblies are remarkable materials, known for their tunable mechanical, optical, and surface properties in nanoscale films. However, questions related to their thermal properties still remain unclear. Here, the thermal properties of a model LbL assembly of strong polyelectrolytes, poly(diallyldimethylammonium chloride)/poly(styrene sulfonate) (PDAC/PSS), assembled from solutions of varying ionic strength (0-1.25 M NaCl) are investigated using quartz crystal microbalance with dissipation (QCM-D) and modulated differential scanning calorimetry. Hydrated exponentially growing films (assembled from 0.25 to 1.25 M NaCl) exhibited distinct thermal transitions akin to a glass transition at 49-56 °C; linearly growing films (assembled without added salt) did not exhibit a transition in the temperature range investigated and were glassy. Results support the idea that exponentially growing films have greater segmental mobility than that of linearly growing films. On the other hand, all dry LbL assemblies investigated were glassy at room temperature and did not exhibit a T(g) up to 250 °C, independent of ionic strength. For the first time, thermal transitions such as T(g) values can be measured for LbL assemblies using QCM-D by monitoring fluctuations in changes in dissipation, allowing us to probe the film's internal structure as a function of film depth.

The swelling behavior of a surface-tethered poly(N-isopropylacrylamide) (polyNIPAAm) network in D2O was characterized with neutron reflection and compared to the demixing behavior of linear poly(NIPAAm) in solution. The surface-tethered network was fabricated by cross-linking a 250 Å thick film of a copolymer comprised of NIPAAm and 3 mol % methacroylbenzophenone. As the temperature was varied between 15 and 29 °C, the thickness of the poly(NIPAAm) network in aqueous solution decreased gradually from 1100 to 750 Å. In this regime, the network lay entirely in the single phase region of the phase diagram for linear poly(NIPAAm). At approximately 30 °C, the network entered the two-phase region of the phase diagram and collapsed along the tie line to a thickness of 339 Å. Above 30 °C, the thickness of the network adjusted to the binodal curve of the phase diagram. The agreement between the swelling discontinuity in the surface-tethered network and the two-phase region of uncross-linked poly(NIPAAm) suggests that confinement does not alter the miscibility gap of poly(NIPAAm). With the use of a temperature, T, and polymer volume fraction, φ, Flory interaction parameter, χ(T,φ), that is tabulated from the demixing data of the linear poly(NIPAAm), the swelling behavior of the surface-tethered network could be modeled using the mean-field Flory−Rehner theory modified for uniaxial swelling.

Layer-by-layer (LbL) assemblies have remarkable potential as advanced functional materials with applications in energy and biomedical related areas. However, very little is known about their thermal and viscoelastic properties owing to the inherent difficulty in their accurate measurement. Here we report on the thermal behavior of a model LbL system containing weak polyelectrolytes poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) as a function of assembly solution pH. Quartz crystal microbalance with dissipation (QCM-D) and modulated differential scanning calorimetry (DSC) indicate that hydrated PAH/PAA LbL assemblies undergo a thermal transition that is akin to a glass transition for most assembly pH’s investigated, with the exception being the case where both polyelectrolytes are fully charged. The nonmonotonic dependence of the glass transition temperature of the PAH/PAA LbL system with respect to assembly pH is discussed in relation to the film’s hydration, composition, film-growth mechanism (linear vs exponential), and ion-pairing density.

A hydrated, surface-tethered polymer network capable of fivefold change in thickness over a 25-37 degrees C temperature range has been demonstrated via neutron reflectivity and fluorescence microscopy to be a novel support for single lipid bilayers in a liquid environment. As the polymer swells from 170 to 900 A, it promotes both in- and out-of-plane fluctuations of the supported membrane. The cushioned bilayer proved to be very robust, remaining structurally intact for 16 days and many temperature cycles. The promotion of membrane fluctuations offers far-reaching applications for this system as a surrogate biomembrane.