Poly (N-Isopropylacrylamide) microgel-based optical devices for humidity sensing

Optical sensors for environmental humidity have been constructed from poly (N-isopropylacrylamide)-co-acrylic acid (pNIPAm-co-AAc) microgels. The devices were constructed by first depositing a monolithic layer of pNIPAm-co-AAc microgels on a Au-coated glass substrate followed by the addition of another Au layer on top. The resultant assembly showed visual color, and exhibited multipeak reflectance spectra. We found that the thickness of the device's microgel layer depended on environmental humidity, which corresponded to a change in the device's optical properties. Specifically, at low humidity the microgel layer was collapsed, while it absorbed water from the atmosphere (and swelled) as the humidity increased. Additionally, we investigated how the deposition of the hygroscopic polymer poly (diallyldimethylammonium chloride) (pDADMAC) onto the microgel layer (prior to final Au layer deposition) influenced the devices humidity response. We found that the devices were more sensitive to humidity as the number of pDADMAC layers in the device increased. Finally, we evaluated the device performance at various temperatures, and found that the sensitivity was enhanced at low temperature, although the response was more linear at elevated temperature.     

Conductivity spectra of polyphosphazene-based polyelectrolyte multilayers

Polyelectrolyte multilayers are built up from ionically modified polyphosphazenes by layer-by-layer assembly of a cationic (poly[bis(3-amino-N,N,N-trimethyl-1-propanaminium iodide)phosphazene] (PAZ+) and an anionic poly[bis(lithium carboxylatophenoxy)phosphazene] (PAZ-). In comparison, multilayers of poly(sodium 4-styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) are investigated. Frequency-dependent conductivity spectra are taken in sandwich geometry at controlled relative humidity. Conductivity spectra of ion-conducting materials generally display a dc plateau at low frequencies and a dispersive regime at higher frequencies. In the present case, the dispersive regime shows a frequency dependence, which is deviating from the typical behavior found in most ion-conducting materials. Dc conductivity values, which can be attributed to long-range ionic transport, are on the order of sigmadc = 10-10-10-7 S.cm-1 and strongly depend on relative humidity. For PAZ+/PAZ- multilayers sigmadc is consistently larger by one decade as compared to PSS/PAH layers, while the humidity dependence is similar, pointing at general mechanisms. A general law of a linear dependence of log(sigmadc) on relative humidity is found over a wide range of humidity and holds for both multilayer systems. This very strong dependence was attributed to variations of the ion mobility with water content, since the water content itself is not drastically dependent on humidity.     

Absolute humidity measurement utilizing alpha-ray absorption

Changes in air density due to humidity were measured by a scintillation detector with alpha-particles. The distance between the scintillator and an alpha-ray source of 241 Am, 3.7 MBq (100 microCi), was fixed at 25 mm which was a little shorter than the range of alpha-particles from the source. The measured absolute humidities were in a range of 7.9 g/m3 to 52.2 g/m3 at temperatures of 35 degrees C and 45 degrees C and under atmospheric pressure. The counting rate of alpha-particles in an absolute humidity of 31.7 g/m3 (80% in relative humidity) at 35 degrees C increased 28% compared with that in dry air. From experimental results and theoretical calculation, the counting rate difference between humid air and dry air was shown to be almost proportional to the absolute humidity in air. The absolute humidity can be measured with an accuracy of +/- 3 g/m3, that is +/- 5% in relative humidity at 45 degrees C.     

Effects of humidity on Young's modulus in poly(methyl methacrylate)

Tensile tests on poly (methyl methacrylate) (PMMA) were conducted to clarify the effects of humidity and strain rate on tensile properties, particularly Young's modulus. Prior to the tensile tests, specimens were kept under various humidity conditions at 293 K, which were the same as the test conditions, for a few months to adjust the sorbed water content in the specimens. The tensile tests were performed under each humidity condition at three different strain rates (approximately 1.4 × 10^?3, 1.4 × 10^?4, and 1.4 × 10^?5 s^?1). Stress‐strain curves changed with humidity and strain rate. Young's moduli were also measured at small applied stresses (below 6.7 MPa) under various humidity conditions at 293 K. Young's modulus decreases linearly with increasing humidity and a decreasing logarithm of strain rate. These results suggest that Young's modulus of PMMA can be expressed as a function of two independent parameters that are humidity and strain rate. A constitutive equation for Young's modulus of PMMA was proposed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 460–465, 2002; DOI 10.1002/polb.10107     

Hydrolysis study of organic acid anhydrides by differential thermal analysis--II. Maleic anhydride and trimellitic anhydride

Differential thermal analysis has been used to follow the hydrolysis of maleic anhydride (MA) and trimellitic anhydride (TMA). On exposure of MA to an atmosphere of 96 % relative humidity, maleic acid is produced, hydrolysis being complete in 21 hr at 22 degrees, but no hydrolysis occurs at a relative humidity of 50%. When exposed to an atmosphere of 96% relative humidity, TMA is quite stable for short periods, but hydrolyses slowly, the production of trimellitic acid being complete in 113 hr at 22 degrees.     

Preparation and characterization of humidity sensor based on poly(3-alkyl)pyrrole containing phosphonic acid groups

Poly(3-alkyl)pyrroles containing phosphonic acid groups with different alkyl segment lengths were chemical synthesized and the properties were measured by FTIR and UV-Vis spectroscopy. FTIR and UV-Vis results showed that the synthesized polymers were in a low doping level through chemical polymerization. By spin-coating on the surface of substrates, the polymer can be used as a humidity sensor. The change of DC electric current of the polypyrroles varies with the chain length of the alkyl substituents. The capacitance increases with the increase of the humidity and resistance decreases with the increase of humidity. This result is different from that of polypyrrole without alkyl substituents due to the influence of the phosphonic acid group. The sensor showed the resistive-type at high relative humidity, and the capacitivetype at the low relative humidity. The sensor exhibited very fast response to the change of environment humidity.     

Adsorption of a diverse set of organic vapors on quartz, CaCO3, and alpha-Al2O3 at different relative humidities

Adsorption constants of a diverse set of 50 organic vapors have been measured on quartz (SiO(2)), CaCO(3), and alpha-Al(2)O(3) at different relative humidities at 15 degrees C. For nonpolar compounds we found an exponential decrease of the adsorption constants on a given mineral between 40 and 97% relative humidity. Extrapolated to 100% relative humidity, the adsorption constants of nonpolar compounds on the different minerals coincide and agree with those measured on a bulk water surface. The adsorption constants of polar compounds also decrease with increasing humidity up to 90%, but between 90% and 100% they increase again. We speculate that this effect is due to a change in the orientation of the water molecules that form the surface at which the organic vapors adsorb at this high humidity. The compound variability in the adsorption constants of all compounds on a given surface at a given relative humidity could be described rather well with a linear free energy relationship using Abraham's solvation parameters for the van der Waals and electron-donor/acceptor properties of the compounds. The remaining deviation between fitted and experimental data was found to be systematic, which indicated that an optimized parameter set for the used compounds could still considerably improve the fit.     

Novel,tadpole-shaped,polyhedral oligomeric silsesquioxane containing sulfonated block copolymer for humidity sensing

Due to the “trade-off” effect between the high water adsorption and low stability under high Relative Humidity of polymer matrix, fabrication of resistive-type polymer-based humidity sensors with a wide impedance response and excellent stability in high relative humidity remains a great challenge. Aim at solving that, a novel polymeric humidity sensing matrix, specifically a tadpole-shaped, polyhedral oligomeric silsesquioxane (POSS) containing block copolymers (BCPs) of POSS-poly(methyl methacrylate)-polystyrene (POSS-PMMA-SPS) were proposed. This novel BCP was synthesized using atom transfer radical polymerization (ATRP) employing a two-step approach, and following post sulfonation, a series of sulfonated BCPs (POSS-PMMA-SPS) with different sulfonation degree was obtained. The subject humidity sensors were produced using different sulfonated BCPs employing a dip-coating technique, and three wide-impedance response humidity sensors were produced. Each of these sensors exhibited an excellent humidity-sensing response of more than 10⁴ within the humidity range from 11% to 95% RH. In particular, the humidity sensor S-6 that had a proper degree of sulfonation presented a relatively fast response (t_90% of 11 s and 80 s in both the water adsorption and desorption processes), and superior repeatability for more than 30 days.     

The sorption and desorption of water in lactitols

Summary Anhydrous lactitols (A1, α- and β-lactitol), lactitol monohydrate, lactitol dihydrate and lactitol trihydrate were kept for varying times in atmospheres of different relative humidity at 20°C in equivalent size plastic desiccators. The relative humidities (8-95%) were maintained with saturated salt solutions and drying agents (silica gel and phosphorous pentoxide). The composition of the samples was monitored by thermogravimetry, differential scanning calorimetry and X-ray powder diffraction. According to these measurements both lactitol monohydrate and lactitol dihydrate were substantially stable under the conditions used. Lactitol monohydrate converts to lactitol dihydrate at the highest relative humidity used. All phases of anhydrous lactitol convert into a form of lactitol monohydrate but not to lactitol dihydrate, even at the highest relative humidity used. At a high relative humidity lactitol trihydrate easily loses part of its crystal water and converts partly to lactitol dihydrate. At a lower relative humidity, the phase forming from trihydrate is difficult to identify.     

Humidity sensitive properties of alkoxysilane-crosslinked polyelectrolyte using sol-gel process

New trialkoxysilyl group-containing copolymers for humidity-sensitive polyelectrolytes were prepared by copolymerization of [2-(methacryloyloxy)ethyl]trimethyl ammonium chloride (METAC), 3-(trimethoxysilyl)propyl methacrylate (TSPM) and 2-ethylhexyl acrylate (2-EHA). They were self-crosslinkable copolymers composed of different contents of METAC/TSPM/2-EHA = 4/1/5 and 4/2/4. The resistance varied from 10(7) to 10(3)[capital Omega] between 20% RH and 95% RH, which was required for a humidity sensor operating at ambient humidity. Temperature dependence, hysteresis, response time, water durability and long-term stability at high temperature and humidity were also measured and estimated.     

Atomic force microscopy of DNA at high humidity: irreversible conformational switching of supercoiled molecules

Three topologically different double-stranded DNA molecules of the same size (bps) have been imaged in air on mica using amplitude modulation atomic force microscopy (AM AFM) under controlled humidity conditions. At very high relative humidity (>90% RH), localized conformational changes of the DNA were observed, while at lower RH, the molecules remained immobile. The conformational changes occurred irreversibly and were driven principally by superhelical stress stored in the DNA molecules prior to binding to the mica surface. The binding mechanism of the DNA to the mica (surface equilibration versus kinetic trapping) modulated the extent of the conformational changes. In cases where DNA movement was observed, increased kinking of the DNA was seen at high humidity when more surface water was present. Additionally, DNA condensation behavior was also present in localized regions of the molecules. This study illustrates that changes in the tertiary structure of DNA can be induced during AFM imaging at high humidity on mica. We propose that AM AFM in high humidity will be a useful technique for probing DNA topology without some of the drawbacks of imaging under bulk solution.     

Dynamic holography in bacteriorhodopsin/gelatin films: effects of light-dark adaptation at different humidity

This work examines the kinetics of dynamic holography responses in light-adapted and dark-adapted bacteriorhodopsin (BR) films at different humidity. We have demonstrated that the kinetics of the diffraction efficiency in wild type BR films is quite different in dark-adapted and light-adapted samples. The holographic recording kinetics, which depends on the duration of incubation in the dark after light adaptation at different humidity values, was studied in depth. A specially designed miniature cell containing a BR film was mounted inside the holographic set up to allow controlled humidity changes over a broad range. The diffraction efficiency kinetics at humidity values of 96-99% were quite different from the kinetics at 60-93% humidity. We found that humidity values of 90-93% were most optimal for dynamic holography recording using a gelatin film containing BR. In agreement with a calculation of the wavelength-dependent changes of the refractive index for dark-adapted and light-adapted BR samples using the Kramers-Kronig relation, the maximum difference in the refractive index and thus in the diffraction efficiency for dark-adapted and light-adapted BR films takes place at 630 nm, close to the wavelength of the He-Ne laser used.     

Highly sensitive and ultrafast response surface acoustic wave humidity sensor based on electrospun polyaniline/poly(vinyl butyral) nanofibers

Polyaniline (PANi) composite nanofibers were deposited on surface acoustic wave (SAW) resonator with a central frequency of 433 MHz to construct humidity sensors. Electrospun nanofibers of poly(methyl methacrylate), poly(vinyl pyrrolidone), poly(ethylene oxide), poly(vinylidene fluoride), poly(vinyl butyral) (PVB) were characterized by scanning electron microscopy, and humidity response of corresponding SAW humidity sensors were investigated. The results indicated that PVB was suitable as a matrix to form nanofibers with PANi by electrospinning (ES). Electrospun PANi/PVB nanofibers exhibited a core–sheath structure as revealed by transmittance electron microscopy. Effects of ES collection time on humidity response of SAW sensor based on PANi/PVB nanofibers were examined at room temperature. The composite nanofiber sensor exhibited very high sensitivity of ∼75 kHz/%RH from 20 to 90%RH, ultrafast response (1 s and 2 s for humidification and desiccation, respectively) and good sensing linearity. Furthermore, the sensor could detect humidity as low as 0.5%RH, suggesting its potentials for low humidity detection. Attempts were done to explain the attractive humidity sensing performance of the sensor by considering conductivity, hydrophilicity, viscoelasticity and morphology of the polymer composite nanofibers.     

Effects of humidity on environmental stress cracking behavior in poly(methyl methacrylate)

Environmental stress cracking (ESC) in poly(methyl methacrylate) under different humidity conditions has been investigated. Constant stress‐intensity factor (K) ring‐type specimens were prepared, and all specimens were equilibrated at five different humidity conditions for about two years. ESC tests were carried out under the same humidity as specimens had been stored. Acoustic emission (AE) signals during ESC tests were also measured to examine the crack‐growth behavior. The threshold K value (K_th) tended to increase with increasing humidity. At a relative humidity (RH) of 11%, crack growth occurred gradually until 40 ks under a K value of 0.70 MPam^1/2, and then the crack‐growth rate began to increase and AE events were observed. A laser microscopic observation indicated that the crack extended by the coalescence between a main crack and a microcrack ahead of the main crack tip. AE signals generated are considered to be associated with the coalescence. At 98% RH, an incubation period where no crack growth was observed existed under a K value of 0.94 MPam^1/2, but the crack began to grow suddenly after that incubation period. This suggests that the craze at the crack tip may become weaker with increasing loading time under high humidity. Although the crack‐growth rate at 98% RH was higher than that at 11% RH, no AE events were observed. This suggests that the crack extended stably in the craze at a crack tip, and sorbed water may make the craze growth easy. All the results suggest that two different ESC mechanisms are activated depending on sorbed water that are varied by humidity. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 1–9, 2002     

Effect of temperature and humidity on coarsening behavior of Au nanoparticles embedded in liquid crystalline lipid membrane

Coarsening behavior of the Au nanoparticles produced by thermal evaporation of Au onto a liquid crystalline lipid (1,2-dioleoyl-3-trimethylammonium-propane, DOTAP) membrane was investigated by subjecting the nanoparticle-embedded DOTAP membrane to two different annealing conditions (at 100 °C under no humidity and at 20 °C and 80% relative humidity). Although the coarsening rate was relatively slow because of the low temperature (from 5.6 nm in the as-deposited state to ~7 nm after 30 h), it was identified that at 100 °C without humidity the Au nanoparticles resulted in shape refinement whereas the high humidity at 20 °C induced self-organization of the nanoparticles into a monolayer. It was also found that annealing in both cases tended to segregate the lipid molecules from the nanoparticle array and forced the nanoparticles into a tighter area. In the case of the high-humidity sample, the lipid segregation eventually led to extensive coalescence of the Au nanoparticles.     

Nanostructure formation in polymer thin films influenced by humidity

The influence of relative humidity (RH) during the film preparation on the surface morphology and on the material distribution of the resulting technical polymer blend films consisting of poly (methyl methacrylate) (PMMA) and poly (vinyl butyral) (PVB) is investigated by atomic force microscopy. Both pure polymers and polymer blends with different compositions of PVB/PMMA dissolved in tetrahydrofuran (THF) were used. Polymer films prepared under dry conditions (RH < 20%) are compared with those that have the same polymer composition but were prepared under increased humidity conditions (RH > 80%). The films consisting of the pure polymers showed a nonporous surface morphology for low‐humidity preparation conditions, whereas high‐humidity preparation conditions lead to porous PVB and PMMA films, respectively. These pores are explained as the result of a breath figure formation. In the case of the polymer blend films containing both polymers, porous or phase‐separated surface structures were observed even at low‐humidity conditions. A superposition of the effects of phase separation and breath figure formation is observed in the case of polymer blend films prepared under high‐humidity conditions. Atomic force microscopy (AFM) images taken before and after the treatment with ethanol as a selective solvent for PVB indicate that PMMA is deposited on top of a PVB layer in the case of the low‐humidity preparation process whereas for high‐humidity conditions the silicon substrate is covered with a PMMA film. Copyright © 2006 John Wiley & Sons, Ltd.     

Electrical and humidity characterization of m-NA doped Au/PVA nanocomposites

The Meta-Nitroaniline (m-NA) doped (by varying weight percentage (wt. %)) gold/polyvinyl alcohol (Au/PVA) nanocomposites were synthesized using gold salt and hydrazine hydrate (HH) by in situ process. The composite was coated on ceramic rods having two end electrodes by drop casting method for studying their electrical behavior at different relative humidity (RH) levels, ranging from 4 to 95% RH at room temperature. The optimized wt. % was used to prepare coatings of various thicknesses (20-40 μm) of the films. As the humidity decreases, the resistance increases. The low humidity sensing characteristic can be tailored by varying wt. % of m-NA and thicknesses of the nanocomposite films. The resistive-humidity sensor shows two regions of sensitivity having highest sensitivity for lower RH. The sensor response and recovery time is about 6-10 s and 52 s respectively. The dynamic range of variation of the resistance allows a promising use of the films as a humidity sensor. The material was characterized by X-ray diffraction (XRD) and impedance spectroscopy at 60% RH.     

Sheep Wool Humidity under Electron Irradiation Affects Wool Sorptivity towards Co(II) Ions

The effect of humidity on sheep wool during irradiation by an accelerated electron beam was examined. Each of the samples with 10%, 53%, and 97% relative humidity (RH) absorbed a dose of 0, 109, and 257 kGy, respectively. After being freely kept in common laboratory conditions, the samples were subjected to batch Co(II) sorption experiments monitored with VIS spectrometry for different lapses from electron beam exposure. Along with the sorption, FTIR spectral analysis of the wool samples was conducted for cysteic acid and cystine monoxide, and later, the examination was completed, with pH measuring 0.05 molar KCl extract from the wool samples. Besides a relationship to the absorbed dose and lapse, the sorptivity results showed considerable dependence on wool humidity under exposure. When humidity was deficient (10% RH), the sorptivity was lower due to limited transformation of cystine monoxide to cysteic acid. The wool pre-conditioned at 53% RH, which is the humidity close to common environmental conditions, demonstrated the best Co(II) sorptivity in any case. This finding enables the elimination of pre-exposure wool conditioning in practice. Under excessive humidity of 97% RH and enough high dose of 257 kGy, radiolysis of water occurred, deteriorating the sorptivity. Each wool humidity, dose, and lapse showed a particular scenario. The time and humidity variations in the sorptivity for the non-irradiated sample were a little surprising; despite the absence of electron irradiation, relevant results indicated a strong sensitivity to pre-condition humidity and lapse from the start of the monitoring.     

Properties of nitrocellulose‐coated and polyethylene‐laminated chitosan and whey films

Chitosan (chitosan acetic acid salt) and whey (65% protein) films were coated with a nitrocellulose lacquer or laminated with polyethylene to enhance their water resistance and gas barrier properties in humid environments. The barrier properties were measured by the Cobb₆₀ test and water‐vapor (100% relative humidity) transmission and oxygen (90% relative humidity) permeability tests. Mechanical properties were obtained with tensile tests. Packaging properties were studied with crease and folding tests. The Cobb₆₀ test revealed that the coated films were resistant to liquid water, at least for a short exposure time, if the coating thickness was at least 10–17 μm. Water‐vapor transmission rates comparable to those of polyethylene‐laminated films were obtained for coated chitosan at a coating thickness of 5–7 μm. The coated films possessed low oxygen permeability despite the high humidity. Coated films dried for 3 weeks showed oxygen permeabilities at 90% relative humidity that were similar to values for dry ethylene‐co‐vinyl alcohol at 0% relative humidity. The lacquer partly penetrated the whey films, and this led to excellent adhesion but poor lacquer toughness. The lacquer coating on chitosan was tougher, and it was possible to fold these films 90° without the coating fracturing if the coating thickness was small. The coated whey films were readily creasable. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 985–992, 2001     

Crosslinked and quaternized poly(4-vinylpyridine)/polypyrrole composite as a potential candidate for the detection of low humidity

Poly(4-vinylpyridine) was crosslinked and quaternized with 1,4-bromobutane to form a polyelectrolyte humidity sensitive film on an interdigitated gold electrode, which was further coated with a layer of polypyrrole by a facile method of vapor phase polymerization. The composite so prepared was characterized by UV-vis spectroscopy and scanning electron microscopy. The investigations on the humidity sensitive properties of the composite revealed that it exhibited an impedance as low as 10(5) Omega even at 0%RH due to the existence of intrinsic conducting polypyrrole, thus conquering the difficulties in measuring low humidity with resistive-type humidity sensors. The impedance of the composite changed linearly with humidity in the range of 0-60%RH with good sensitivity. In addition, its response time (t(90%)) for adsorption and desorption between 33% and 97%RH was estimated to be 33 s and 110 s, respectively, and a hysteresis of 5%RH was observed. All these suggest it is promising as a sensitive material for low humidity detection. The effect of concentration and ratio of oxidizing agent to doping agent, polymerization temperature of pyrrole on the humidity sensitive properties of the composite have been investigated. A sensitive mechanism of the composite was proposed by taking into account the contribution of both the intrinsic electronic conduction and ionic conduction.