Poly(ethylene oxide) (PEO) is a key material in solid polymer electrolytes, biomaterials, drug delivery devices, and sensors. Through the use of hydrogen bonds, layer-by-layer (LBL) assemblies allow for the incorporation of PEO in a controllable tunable thin film, but little is known about the bulk properties of LBL thin films because they are often tightly bound to the substrate of assembly. The construction technique involves alternately exposing a substrate to a hydrogen-bond-donating polymer (poly(acrylic acid)) and a hydrogen-bond-accepting polymer (PEO) in solution, producing mechanically stable interdigitated layers of PEO and poly(acrylic acid) (PAA). Here, we introduce a new method of LBL film isolation using low-energy surfaces that facilitate the removal of substantial mass and area of the film, allowing, for the first time, the thermal and mechanical characterization that was previously difficult or impossible to perform. To further understand the morphology of the nanoscale blend, the glass transition is measured as a function of assembly pH via differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The resulting trends give clues as to how the morphology and composition of a hydrogen-bonded composite film evolve as a function of pH. We also demonstrate that LBL films of PEO and PAA behave as flexible elastomeric blends at ambient conditions and allow for nanoscale control of thickness and film composition. Furthermore, we show that the crystallization of PEO is fully suppressed in these composite assemblies, a fact that proves advantageous for applications such as ultrathin hydrogels, membranes, and solid-state polymer electrolytes.

International audienceNanoscale patterning of piezoelectric and ferroelectric polymers, such as polyvinylidene fluoride (PVdF) and its copolymers with trifluoroethylene (PVdF-TrFE), is increasingly important in organic electronics, memory, and sensing. The nanoscale processing of polymers can lead to materials behavior that is strikingly different from the bulk because of confinement effects. Here we report the effects of confinement of PVdF-TrFE melt-wetted in porous templates of varying pore diameter. PVdF-TrFE is particularly interesting because it possesses a solid-state Curie transition, where both ferro and nonferroelectric phases crystallize into a paraelectric phase. Using modulated differential scanning calorimetry (MDSC), X-ray diffraction (XRD), and broadband dielectric spectroscopy (BDS), we demonstrate that confined PVdF-TrFE crystallizes into an oriented ferroelectric beta phase. Both melting and crystallization temperatures decrease with decreasing pore diameter, and the Curie temperature is weakly affected. Results imply that nanoconfinement enhances the formation and orientation of the ferroelectric beta phase and could potentially enhance ferroelectricty and piezoelectricity in nanoscale PVdF-TrFE features

The influence of pH and ionic strength on the structure and properties of hydrogen-bonded layerby-layer (LbL) assemblies of poly(ethylene oxide) (PEO) and poly(acrylic acid) (PAA) is explored. The degree of inter-and intramolecular hydrogen bonding is estimated from Fourier-transform infrared spectroscopy, the glass transition temperature is measured using differential scanning calorimetry of bulk free-standing films, and ionic conductivity is studied using electrochemical impedance spectroscopy. Results indicate that (PEO/PAA) LbL films assembled without added salt are sensitive to pH, with a T g decrease (59-26 °C) and intermolecular hydrogen bonding increase (27 to 51% COOH groups bonding with PEO) with increasing assembly pH (2 to 3). Films assembled in the presence of 0.1 M lithium triflate exhibit properties independent of assembly pH (T g ∼ 48 °C and 12% COOH groups bonding with PEO), presumably due to the "screening" of hydrogen bonds. Ionic conductivity is found to range from 10 -6 to 10 -10 S cm -1 , depending on humidity, plasticization, and salt content.

The structure and morphology as well as the mechanism of formation of porous polyelectrolyte multilayers consisting of linear poly(ethylenimine) (LPEI) and poly(acrylic acid) (PAA) have been systematically investigated as a function of pH. The structures obtained exhibit dramatic differences with small changes in the pH of multilayer assembly and pH of postassembly treatment, yielding an observed range of pore sizes from tens of nanometers to micrometers and pore volume fractions from 0 to 77%. The porous phase transition is quite rapid (<20 min), and structures observed include asymmetric membranes and isolated craters. It is thought that asymmetric membranes are achieved due to the high mobility of LPEI, which can exhibit interdiffusion when layered with PAA. To further understand the nature of the porous LbL multilayers, the pores were filled with liquid electrolyte and the impedance response of electrolyte-filled porous multilayers was examined; two time constants or two dry-state room temperature conductivities on the order of 10 -6 and 10 -9 S cm -1 were observed. The asymmetric membrane LbL structure, first reported here, holds many potential applications in terms of filtration, catalysis, drug delivery, etc.

Over recent decades, poly(lactic-co-glycolic acid) (PLGA) based nano- and micro- drug delivery vehicles have been rapidly developed since PLGA was approved by the Food and Drug Administration (FDA). Common factors that influence PLGA particle properties have been extensively studied by researchers, such as particle size, polydispersity index (PDI), surface morphology, zeta potential, and drug loading efficiency. These properties have all been found to be key factors for determining the drug release kinetics of the drug delivery particles. For drug delivery applications the drug release behavior is a critical property, and PLGA drug delivery systems are still plagued with the issue of burst release when a large portion of the drug is suddenly released from the particle rather than the controlled release the particles are designed for. Other properties of the particles can play a role in the drug release behavior, such as the glass transition temperature (Tg). The Tg, however, is an underreported property of current PLGA based drug delivery systems. This review summarizes the basic knowledge of the glass transition temperature in PLGA particles, the factors that influence the Tg, the effect of Tg on drug release behavior, and presents the recent awareness of the influence of Tg on drug delivery applications.