In this study, coatings of cross-linked gold nanoparticles (AuNPs) on flexible polyethylene (PE) substrates were prepared via layer-by-layer deposition and their application as strain gauges and chemiresistors was investigated. Special emphasis was placed on characterizing the influence of strain on the chemiresistive responses. The coatings were deposited using amine stabilized AuNPs (4 and 9 nm diameter) and 1,9-nonanedithiol (NDT) or pentaerythritol tetrakis(3-mercaptopropionate) (PTM) as cross-linkers. To prepare films with homogeneous optical appearance, it was necessary to treat the substrates with oxygen plasma directly before film assembly. SEM images revealed film thicknesses between ∼60 and ∼90 nm and a porous nanoscale morphology. All films showed ohmic I-V characteristics with conductivities ranging from 1 × 10⁻⁴ to 1 × 10⁻² Ω⁻¹ cm⁻¹, depending on the structure of the linker and the nanoparticle size. When up to 3% strain was induced their resistance increased linearly and reversibly (gauge factors: ∼20). A comparative SEM investigation indicated that the stress induced formation and extension of nanocracks are important components of the signal transduction mechanism. Further, all films responded with a reversible increase in resistance when dosed with toluene, 4-methyl-2-pentanone, 1-propanol or water vapor (concentrations: 50-10 000 ppm). Films deposited onto high density PE substrates showed much faster response-recovery dynamics than films deposited onto low density PE. The chemical selectivity of the coatings was controlled by the chemical nature of the cross-linkers, with the highest sensitivities (∼1 × 10⁻⁵ ppm⁻¹) measured with analytes of matching solubility. The response isotherms of all film/vapor pairs could be fitted using a Langmuir-Henry model suggesting selective and bulk sorption. Under tensile stress (1% strain) all chemiresistors showed a reversible increase in their response amplitudes (∼30%), regardless of the analytes' permittivity. Taking into consideration the thermally activated tunneling model for charge transport, this behavior was assigned to stress induced formation of nanocracks, which enhance the films' ability to swell in lateral direction during analyte sorption.

Summary: Droplet polymerization of N-vinyl-2-pyrrolidone (NVP) and sodium acrylate (NaAA) was carried out in an acoustic levitator for different ambient temperatures and relative humidities. The resulting particle morphologies were compared to particles of two crystalline systems (mannitol and ammonium sulfate) and two disperse systems (silica and styrene -butyl acrylate dispersion), all obtained under similar drying conditions. NVP was found to form a higher amount of crystals with lower temperature and relative humidity, contrary to the behaviour observed for mannitol and ammonium sulfate. The processes of both, polymerization and drying of NaAA lead to similar morphologies at low temperatures and humidities, probably caused by the precipitation of NaAA during polymerization.

Summary In order to optimize the spray polymerization of acrylic acid and sodium acrylate, an acoustic levitator was efficiently utilized to model the system. The acoustic levitator simulates the fall of a droplet in a spray tower. A particle morphology map of SEM images was designed to reproduce certain particle surfaces by varying the drying conditions and the initiator system, which were adjusted for a polymerization. Thus, a droplet of an aqueous monomer mixture of acrylic acid and sodium acrylate was positioned in an acoustic levitator and the polymerizations were initiated with two redox systems, one with ammonium peroxodisulfate and the other one with potassium peroxodisulfate, and were compared to each other. The redox system can beneficially initiate the polymerization at low temperature (>40 °C). By means of a camera and a light source, shadowgraphy was used to analyze the droplet drying kinetics.

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