Convergence of dissipation and impedance analysis of quartz crystal microbalance studies

Investigation of Rare Earth Element Binding to a Surface-Bound Affinity Peptide Derived from EF-Hand Loop I of Lanmodulin

Bioinspired strategies have been given extensive attention for the recovery of rare earth elements (REEs) from waste streams because of their high selectivity, regeneration potential, and sustainability as well as low cost. Lanmodulin protein is an emerging biotechnology that is highly selective for REE binding. Mimicking lanmodulin with shorter peptides is advantageous because they are simpler and potentially easier to manipulate and optimize. Lanmodulin-derived peptides have been found to bind REEs, but their properties have not been explored when immobilized on solid substrates, which is required for many advanced separation technologies. Here, two peptides, LanM1 and scrambled LanM1, are designed from the EF-hand loop 1 of lanmodulin and investigated for their binding affinity toward different REEs when surface-bound. First, the ability of LanM1 to bind REEs was confirmed and characterized in solution using circular dichroism (CD), nuclear magnetic resonance (NMR), and molecular dynamics (MD) simulations for Ce(III) ions. Isothermal titration calorimetry (ITC) was used to further analyze the binding of the LanM1 to Ce(III), Nd(III), Eu(III), and Y(III) ions and in low-pH conditions. The performance of the immobilized peptides on a model gold surface was examined using a quartz crystal microbalance with dissipation (QCM-D). The studies show that the LanM1 peptide has a stronger REE binding affinity than that of scrambled LanM1 when in solution and when immobilized on a gold surface. QCM-D data were fit to the Langmuir adsorption model to estimate the surface-bound dissociation constant (Kd) of LanM1 with Ce(III) and Nd(III). The results indicate that LanM1 peptides maintain a high affinity for REEs when immobilized, and surface-bound LanM1 has no affinity for potential competitor calcium and copper ions. The utility of surface-bound LanM1 peptides was further demonstrated by immobilizing them to gold nanoparticles (GNPs) and capturing REEs from solution in experiments utilizing an Arsenazo III-based colorimetric dye displacement assay and ultraviolet-visible (UV-vis) spectrophotometry. The saturated adsorption capacity of GNPs was estimated to be around 3.5 μmol REE/g for Ce(III), Nd(III), Eu(III), and Y(III) ions, with no binding of non-REE Ca(II) ions observed.

Aggregation States and Segmental Dynamics of Poly(methyl methacrylate) in Nanofiber Mats

Nanofiber mats composed of polymers, having a large surface-to-volume ratio and high porosity, have been widely applied in the environmental and biomedical fields but fundamental knowledge on the polymer chains in the mats seems to be limited. We here report the aggregation states and segmental dynamics of poly(methyl methacrylate)s (PMMAs) with different stereoregularities in electrospun nanofiber mats. Attenuated total reflectance Fourier transform infrared (ATR/FTIR) spectroscopy revealed that, in the case of atactic PMMA (at-PMMA), the population of the trans-trans conformation of the main chain part, which allows carbonyl groups of the side group to interact affirmatively with each other, increased in the electrospun nanofiber mat. On the other hand, in the case of isotactic PMMA (it-PMMA), the skeletal conformation was unchanged even in the nanofiber mat. As a result of the aggregation states of PMMA chains, the glass-transition temperature (T_g) of the electrospun nanofiber mats increased and remained unchanged from the corresponding bulk value for at- and it-PMMA, respectively. These findings should be useful for designing materials and devices composed of electrospun nanofibers.

Modification of a Polymer Surface by Partial Swelling Using Nonsolvents

Surface modification without changing the physical properties in the bulk is of pivotal importance for the development of polymers as devices. We recently proposed a simple surface functionalization method for polymer films by partial swelling using a nonsolvent and demonstrated the incorporation of poly(2-methoxyethyl acrylate) (PMEA), which has an excellent antibiofouling ability, only into the outermost region of a poly(methyl methacrylate) (PMMA) film. We here extend this technology to another versatile polymer, polystyrene (PS). In this case, PS and PMEA have different solubility parameters making it difficult to select a suitable solvent, which is a nonsolvent for PS and a good solvent for PMEA, unlike the combination of PMMA with PMEA. Thus, such a solvent was first sought by examining the swelling behavior of PS films in contact with various alcohols. Once a mixed solvent of methanol/1-butanol (50/50 (v/v)) was chosen, PMEA chains could be successfully incorporated at the outermost region of the PS film. Atomic force microscopy in conjunction with neutron reflectivity revealed that chains of PMEA incorporated in the PS surface region were well swollen in water. This leads to an excellent ability to suppress the adhesion of platelets on the PS film.

Data evaluation for surface-sensitive label-free methods to obtain real-time kinetic and structural information of thin films: A practical review with related software packages

Interfacial layers are important in a wide range of applications in biomedicine, biosensing, analytical chemistry and the maritime industries. Given the growing number of applications, analysis of such layers and understanding their behavior is becoming crucial. Label-free surface sensitive methods are excellent for monitoring the formation kinetics, structure and its evolution of thin layers, even at the nanoscale. In this paper, we review existing and commercially available label-free techniques and demonstrate how the experimentally obtained data can be utilized to extract kinetic and structural information during and after formation, and any subsequent adsorption/desorption processes. We outline techniques, some traditional and some novel, based on the principles of optical and mechanical transduction. Our special focus is the current possibilities of combining label-free methods, which is a powerful approach to extend the range of detected and deduced parameters. We summarize the most important theoretical considerations for obtaining reliable information from measurements taking place in liquid environments and, hence, with layers in a hydrated state. A thorough treamtmaent of the various kinetic and structural quantities obtained from evaluation of the raw label-free data are provided. Such quantities include layer thickness, refractive index, optical anisotropy (and molecular orientation derived therefrom), degree of hydration, viscoelasticity, as well as association and dissociation rate constants and occupied area of subsequently adsorbed species. To demonstrate the effect of variations in model conditions on the observed data, simulations of kinetic curves at various model settings are also included. Based on our own extensive experience with optical waveguide lightmode spectroscopy (OWLS) and the quartz crystal microbalance (QCM), we have developed dedicated software packages for data analysis, which are made available to the scientific community alongside this paper.

Formation and Stability of Nontoxic Perovskite Precursor

Strontium, calcium, and magnesium silicate hydrate phases are synthesized by the reaction between silica and solution of metal hydroxides. The kinetics of the reaction is recorded using a quartz crystal microbalance (QCM), continuously monitoring the change in frequency and dissipation energy. Based on QCM results, it is shown that properties of solutions like the pH-value or the type of ions play a pivotal function on the rate-determining stage of the reaction, the thickness of the diffuse layer, the formation of carbonates, as well as the kinetics of the formed phases. Further properties of the reaction products are investigated using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and infrared spectroscopy (IR). With the help of thermogravimetric analysis (TGA) and temperature-dependent X-ray diffraction (XRD), we investigate how our synthesized phases can be turned into MSiO₃ structures. Finally, the Goldschmidt rules for perovskites structures show that this might be an attractive way for new and nontoxic phases in the future.

Charged Polystyrene Nanoparticles Near a SiO2/Water Interface

Quartz crystal microbalance with dissipation (QCM-D) monitoring is used to investigate the adsorption processes at liquid-solid interfaces and applied increasingly to characterize viscoelastic properties of complex liquids. Here, we contribute new insights into the latter field by using QCM-D to investigate the structure near the interface and the high-frequency viscoelastic properties of charge-stabilized polystyrene particles (radius 37 nm) dispersed in water. The study reveals changes with increasing ionic strength and particle concentration. Replacing water with a dispersion is usually expected to give rise to a decrease in frequency, f. Increases in both f and dissipation, D, were observed on exchanging pure water for particle dispersions at a low ionic strength. The QCM-D data are well-represented by a viscoelastic model, with viscosity increasing from 1.0 to 1.3 mPa s as the particle volume fraction changes from 0.005 to 0.07. This increase, higher than that predicted for noninteracting dispersions, can be explained by the charge repulsion between the particles giving rise to a higher effective volume fraction. It is concluded that the polystyrene particles did not adhere to the solid surface but rather were separated by a layer of pure dispersion medium. The QCM-D response was successfully represented using a viscoelastic Kelvin-Voigt model, from which it was concluded that the thickness of the dispersion medium layer was of the order of the particle-particle bulk separation, in the range of 50-250 nm, and observed to decrease with both particle concentration and addition of salt. Similar anomalous frequency and dissipation responses have been seen previously for systems containing weakly adherent colloidal particles and bacteria and understood in terms of coupled resonators. We demonstrate that surface attachment is not required for such phenomena to occur, but that a viscoelastic liquid separated from the oscillating surface by a thin Newtonian layer gives rise to similar responses.

In situ viscoelastic properties and chain conformations of heavily hydrated carboxymethyl dextran layers: a comparative study using OWLS and QCM-I chips coated with waveguide material

Hydration, viscoelastic properties and dominant structure of thin polymer layers on the surface of waveguide material were evaluated using optical waveguide lightmode spectroscopy (OWLS) and quartz crystal microbalance (QCM) methods. The fundamentally different principles of the two applied label-free biosensors enable to examine analyte layers from complementary aspects, e.g. to determine the amount of bound water in hydrated layers. In this study, a new QCM instrument with impedance measurement (QCM-I) is introduced. Its specially designed sensor chips, covered by thin film of waveguide material, supply identical surface as used in OWLS sensors, thus enabling to perform parallel measurements on the same type of surface. Viscoelastic analysis of the measured data was performed by our evaluation code developed in MATLAB environment, using the Voinova's Voigt-based model. In situ deposition experiments on the ultrathin films of poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) were conducted for instrumental and code validation. Additionally, a novel OWLS-QCM data evaluation methodology has been developed based on the concept of combining hydration and viscoelastic data with optical anisotropy results from OWLS measurements. This methodology provided insight into the time-dependent chain conformation of heavily hydrated nano-scaled layers, resulting in unprecedented structural, hydration and viscoelastic information on covalently grafted ultrathin carboxymethyl dextran (CMD) films. The measured mass values as well as hydration and viscoelastic properties were compared with the characteristics of PLL-g-PEG layers.

Truncated CPSF6 Forms Higher-Order Complexes That Bind and Disrupt HIV-1 Capsid

Cleavage and polyadenylation specificity factor 6 (CPSF6) is a human protein that binds HIV-1 capsid and mediates nuclear transport and integration targeting of HIV-1 preintegration complexes. Truncation of the protein at its C-terminal nuclear-targeting arginine/serine-rich (RS) domain produces a protein, CPSF6-358, that potently inhibits HIV-1 infection by targeting the capsid and inhibiting nuclear entry. To understand the molecular mechanism behind this restriction, the interaction between CPSF6-358 and HIV-1 capsid was characterized using in vitro and in vivo assays. Purified CPSF6-358 protein formed oligomers and bound in vitro-assembled wild-type (WT) capsid protein (CA) tubes, but not CA tubes containing a mutation in the putative binding site of CPSF6. Intriguingly, binding of CPSF6-358 oligomers to WT CA tubes physically disrupted the tubular assemblies into small fragments. Furthermore, fixed- and live-cell imaging showed that stably expressed CPSF6-358 forms cytoplasmic puncta upon WT HIV-1 infection and leads to capsid permeabilization. These events did not occur when the HIV-1 capsid contained a mutation known to prevent CPSF6 binding, nor did they occur in the presence of a small-molecule inhibitor of capsid binding to CPSF6-358. Together, our in vitro biochemical and transmission electron microscopy data and in vivo intracellular imaging results provide the first direct evidence for an oligomeric nature of CPSF6-358 and suggest a plausible mechanism for restriction of HIV-1 infection by CPSF6-358.IMPORTANCE After entry into cells, the HIV-1 capsid, which contains the viral genome, interacts with numerous host cell factors to facilitate crucial events required for replication, including uncoating. One such host cell factor, called CPSF6, is predominantly located in the cell nucleus and interacts with HIV-1 capsid. The interaction between CA and CPSF6 is critical during HIV-1 replication in vivo Truncation of CPSF6 leads to its localization to the cell cytoplasm and inhibition of HIV-1 infection. Here, we determined that truncated CPSF6 protein forms large higher-order complexes that bind directly to HIV-1 capsid, leading to its disruption. Truncated CPSF6 expression in cells leads to premature capsid uncoating that is detrimental to HIV-1 infection. Our study provides the first direct evidence for an oligomeric nature of truncated CPSF6 and insights into the highly regulated process of HIV-1 capsid uncoating.

Unfound Associated Resonant Model and Its Impact on Response of a Quartz Crystal Microbalance in the Liquid Phase

Quartz crystal microbalance (QCM) is an important tool to detect in real time the mass change at the nanogram level. However, for a QCM operated in the liquid phase, the Sauerbrey equation is usually disturbed by the changes in liquid properties and the longitudinal wave effect. Herein, we report another unfound associated high-frequency resonance (HFR) model for the QCM, with the intensity 2 orders of magnitude higher than that of the fundamental peak in the liquid phase. The HFR model exhibits obvious impact on the response of QCM in the thickness-shear model (TSM), especially for overtones. The frequency of HFR peak is decreased dramatically with increasing conductivity or permittivity of the liquid phase, resulting in considerable additional frequency shifts in the TSM as baseline drift. Compared to that with a faraway HFR peak, the overlapping of HFR peak to a TSM overtone results in the frequency shifts of ±50-70 kHz with its intensity enhancement by 3 orders of magnitude in the later. The HFR behavior is explained by an equivalent circuit model including leading wire inductance, liquid inductance, and static capacitance of QCM. Taking into account the HFR model, the positive frequency shifts of the QCM at high overtones during the cell adhesion process is understandable. Combining the TSM and HFR is an effective way to improve the stability of QCM and provides more reliable information from the responses of QCM. The HFR may have potential application in chemical and biological sensors.

Modified Sauerbrey equation: a facile method to quantitatively probe the conformation of isolated molecules at solid–liquid interfaces

It is found that a quartz crystal microbalance signal is proportional to the product of mass and intrinsic viscosity of molecules at solid–liquid interfaces, with a constant coefficient. This relationship provides a convenient way to semi-quantitatively probe the conformation of a discrete polymer at solid–liquid interfaces.

Comparison of the Responsivity of Solution-Suspended and Surface-Bound Poly(N-isopropylacrylamide)-Based Microgels for Sensing Applications

In this submission, the phase transition behavior for poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-co-AAc) microgels and their assemblies was investigated as a function of temperature and pH using UV-vis spectroscopy (to probe light scattering behavior) and quartz crystal microbalance with dissipation (QCM-D) measurements. PNIPAm-co-AAc microgels were "painted" onto Au-coated glass substrates (for UV-vis) and the Au electrode of a QCM crystal to generate monolayers. The subsequent deposition of another Au layer on top of the pNIPAm-co-AAc microgel layer yields what is known as an etalon. UV-vis/QCM-D measurements revealed that the temperature and pH responsivities for the microgel assemblies match well with their solution behavior. UV-vis spectroscopy shows that the transmittance of the microgel monolayers decreased with increasing solution temperature at pH 3.0. At pH 6.5, the AAc groups in the microgels were deprotonated, leading to strong Coulombic repulsive forces inside the microgels that prevented their collapse and lead to minimal change in the transmitted light intensity. However, QCM-D analysis reveals more complex behavior as it is sensitive to the viscosity/viscoelasticity and thickness changes of the microgel layer, which ultimately depends on the microgel chemical composition and the interaction of the etalon's Au layer with the crystal. The maximum sensitivity to temperature is 0.8 × 10^-3 °C·Hz^-1, which is the most sensitive pNIPAm microgel-based QCM temperature sensor thus far reported in the literature. Finally, we exploit this new understanding to characterize the pH and ionic strength of a solution using pNIPAm-co-XAAc microgel-based etalon coated crystals. The research results and the sensing demonstration can inspire new and improved sensor designs for a variety of analytes.

Interactions between Polyelectrolyte Brushes and Hofmeister Ions: Chaotropes versus Kosmotropes

We have investigated the interactions between the positively charged poly[2-(methacryloyloxy)ethyltrimethylammonium chloride] (PMETAC) brushes and the Hofmeister anions and the interactions between the negatively charged poly(3-sulfopropyl methacrylate potassium) (PSPMA) brushes and the Hofmeister cations using a combination of quartz crystal microbalance with dissipation and spectroscopic ellipsometry. A V-shaped anion series is observed in terms of the ion-specific interactions between the PMETAC brushes and the Hofmeister anions. We have found that the chaotropic and kosmotropic anions interact with the PMETAC brushes in different manners. The ion-specific interactions between the PMETAC brushes and the chaotropic anions are dominated by the direct interactions between the anions and the positively charged quaternary ammonium group via ion pairing mediated by ionic hydration strength or polarizability, whereas the ion-specific interactions between the PMETAC brushes and the kosmotropic anions are dominated by the competition for water molecules between the anions and the brushes. The ion-specific interactions between the PMETAC brushes and the anions have significant influences on both the hydration and the conformation of the brushes. The cations exhibit weaker specific ion effects on the PSPMA brushes in comparison with the specific anion effects on the PMETAC brushes.

Preconditioning of model biocarriers by soluble pollutants: a QCM-D study

Preconditioning of a biocarrier surface is the first step in triggering biofilm formation in attached-growth bioreactors. However, the quantification and control of this step as influenced by solution conditions and biocarrier properties have been rarely explored. In this paper, deposition behaviors of soluble pollutants on the model biocarriers polystyrene (PS) and polyamide (PA) were performed using a quartz crystal microbalance with dissipation monitoring (QCM-D). Three types of wastewater from municipal and industrial wastewater treatment plants and 12 synthetic wastewaters with different configurations of model macromolecules (bovine serum albumin and sodium alginate) and ionic compositions (Na(+) and Ca(2+)) were prepared. Results showed that high organic contents (protein and humic acid) in real wastewater increased deposition compared to the impact of ions on the two types of carriers. For synthetic wastewater, an interesting phenomenon was observed in that the presence of Ca(2+) can transform a thin and rigid adlayer into a denser and viscoelastic one on the surface of PS with low organic contents, yet a viscoelastic adlayer can directly form on PS and an increase in the ionic strength hinders deposition in the presence of high organic contents. The deposition of solutes on PA produces a thicker and viscoelastic adlayer that is strengthened an elevated concentration of organic materials. Additionally, a weakening effect of Ca(2+) on deposition was revealed under high ionic strength. This is the first demonstration of control strategies for preconditioning hydrophilic and hydrophobic biocarriers under different water quality conditions and has important implications for the design of a start-up process for biofilm formation in attached-growth bioreactors.

A quartz crystal microbalance as a tool for biomolecular interaction studies

A quartz crystal microbalance was successfully applied to quantitatively analyze biomolecular interactions using a poly(ethylene glycol) matrix and equations for impedance analysis of frequency changes at multiple overtones.

Swelling enhancement of polyelectrolyte brushes induced by external ions

It has been observed previously that when permanently charged polyelectrolyte brushes are exposed to an external salt solution, they shrink when the salt level is high enough. In this study, we observed an enhanced swelling process before the salt concentration reached that limit. We systematically investigated a few strong polyelectrolyte brush systems, including sodium polystyrene sulfonate (PSSNa), poly([2-(methacryloyloxy)ethyl] trimethylammonium chloride) (PMETAC) and potassium poly(3-sulfopropyl methacrylate) (PSPMA) with different molecular weights and grafting density using a combination of methods, including ellipsometry, quartz crystal microbalance with dissipation (QCM-D) and atomic force microscope (AFM). The swelling enhancement is expressed by the thickening of the brush layer at moderate salt concentrations, accompanied by the decrease of refractive index, the increase of the amount of solvent inside the brushes and an increase in retardation time. A scenario is proposed in which the counterions penetrate into the brushes driven by the external salt ions, they disrupt and break up the previously formed multiplets due to the dipole-dipole interaction by the ion-pairs on the polymer chain. This process results in the release of the bound segments and the stretching of the polymer chains.

A proton shelter inspired by the sugar coating of acidophilic archaea

The acidophilic archaeons are a group of single-celled microorganisms that flourish in hot acid springs (usually pH < 3) but maintain their internal pH near neutral. Although there is a lack of direct evidence, the abundance of sugar modifications on the cell surface has been suggested to provide the acidophiles with protection against proton invasion. In this study, a hydroxyl (OH)-rich polymer brush layer was prepared to mimic the OH-rich sugar coating. Using a novel pH-sensitive dithioacetal molecule as a probe, we studied the proton-resisting property and found that a 10-nm-thick polymer layer was able to raise the pH from 1.0 to > 5.0, indicating that the densely packed OH-rich layer is a proton shelter. As strong evidence for the role of sugar coatings as proton barriers, this biomimetic study provides insight into evolutionary biology, and the results also could be expanded for the development of biocompatible anti-acid materials.

Solidified liquid layer model expands the application fields of quartz crystal microbalance

The application of a quartz crystal microbalance (QCM) in liquid is hindered by the complexity of data analysis. Recently, a "solidified liquid layer" (SLL) model has been proposed to simplify the data analysis. Here, missing evidence to support the SLL model is provided: 1) the SLL model is responsive to the density change of the liquid environment, 2) thickness values from the SLL model (T(SLL) ) are in agreement with values measured by ellipsometry. The SLL model predicts that a 0.18 nm change of T(SLL) will lead to a 1 Hz signal, which is the resolution that most commercial QCMs could easily achieve. Using the SLL model, Au-S bond breakage has been successful. Biosensor applications are also being designed according to the SLL model. It is believed that with these results, the SLL model will bring QCM back to the radar screen of scientists.

A new method for wideband characterization of resonator-based sensing platforms

A new approach to the electronic instrumentation for extracting data from resonator-based sensing devices (e.g., microelectromechanical, piezoelectric, electrochemical, and acoustic) is suggested and demonstrated here. Traditionally, oscillator-based circuitry is employed to monitor shift in the resonance frequency of the resonator. These circuits give a single point measurement at the frequency where the oscillation criterion is met. However, the resonator response itself is broadband and contains much more information than a single point measurement. Here, we present a method for the broadband characterization of a resonator using white noise as an excitation signal. The resonator is used in a two-port filter configuration, and the resonator output is subjected to frequency spectrum analysis. The result is a wideband spectral map analogous to the magnitude of the S21 parameters of a conventional filter. Compared to other sources for broadband excitation (e.g., frequency chirp, multisine, or narrow time domain pulse), the white noise source requires no design of the input signal and is readily available for very wide bandwidths (1 MHz-3 GHz). Moreover, it offers simplicity in circuit design as it does not require precise impedance matching; whereas such requirements are very strict for oscillator-based circuit systems, and can be difficult to fulfill. This results in a measurement system that does not require calibration, which is a significant advantage over oscillator circuits. Simulation results are first presented for verification of the proposed system, followed by measurement results with a prototype implementation. A 434 MHz surface acoustic wave (SAW) resonator and a 5 MHz quartz crystal microbalance (QCM) are measured using the proposed method, and the results are compared to measurements taken by a conventional bench-top network analyzer. Maximum relative differences in the measured resonance frequencies of the SAW and QCM resonators are 0.0004% and 0.002%, respectively. The ability to track a changing sensor response is demonstrated by inducing temperature variations and measuring resonance frequency simultaneously using the proposed technique in parallel with a network analyzer. The relative difference between the two measurements is about 5.53 ppm, highlighting the impressive accuracy of the proposed system. Using commercially available digital signal processors (DSPs), we believe that this technique can be implemented as a system-on-a-chip solution resulting in a very low cost, easy to use, portable, and customizable sensing system. In addition, given the simplicity of the signal and circuit design, and its immunity to other common interface concerns (injection locking, oscillator interference, and drift, etc.), this method is better suited to accommodating array-based systems.

Surface and bulk collapse transitions of thermoresponsive polymer brushes

We elucidate the sequence of events occurring during the collapse transition of thermoresponsive copolymer brushes based on poly(di(ethyleneglycol) methyl ether methacrylate) chains (PMEO2MA) grown by atom-transfer radical polymerization (ATRP). The collapse of the bulk of the brush is followed by quartz crystal microbalance measurements with dissipation monitoring (QCM-D), and the collapse of its outer surface is assessed by measuring equilibrium water contact angles in the captive bubble configuration. The bulk of the brush collapses over a broad temperature interval (approximately 25 degrees C), and the end of this process is signaled by a sharp first-order transition of the surface of the brush. These observations support theoretical predictions regarding the occurrence of a vertical phase separation during collapse, with surface properties of thermoresponsive brushes exhibiting a sharp variation at a temperature of T(br)(surf). In contrast, the bulk properties of the brush vary smoothly, with a bulk transition T(br)(bulk) occurring on average approximately 8 degrees C below T(br)(surf) and approximately 5 degrees C below the lower critical solution temperature (LCST) of free chains in solution. These observations should also be valid for planar brushes of other neutral, water-soluble thermoresponsive polymers such as poly(N-isopropylacrylamide) (PNIPAM). We also propose a way to analyze more quantitatively the temperature dependence of the QCM-D response of thermoresponsive brushes and deliver a simple thermodynamic interpretation of equilibrium contact angles, which can be of use for other complex temperature-responsive solvophilic systems.

Determination of viscoelastic property in polyethylene crystallization using a quartz crystal resonator

A new generalized relationship between the viscoelastic properties of an overlayer placed on the electrode interface of a quartz crystal resonator and its resonant characteristic is developed from the mechanics of the quartz movement. The relationship is used to estimate the viscoelastic properties from the experimentally measured resonant characteristic. It is utilized in the estimation of viscosity and elastic shear modulus of a polyethylene overlayer during its crystallization. The measurements are compared with the viscosity and elastic shear modulus of a polyethylene melt measured using a rheometer. It is found that the development of this study is useful in the determination of viscoelastic property of polymer materials by measuring the resonant frequency and conductance of the polymer overlayer placed on the resonator electrode.