A new model of protein adsorption kinetics derived from simultaneous measurement of mass loading and changes in surface energy

We describe a novel technology based on changes in the resonant frequency of an acoustically actuated surface and use it to measure temporal changes in the surface energy gamma (N m(-1)) of an elastomeric polymer membrane due to the adsorption of macromolecules from aqueous solution. The resonant elastomeric surface-tension (REST) sensor permits simultaneous determination of mass loading kinetics and gamma(t) for a given adsorption process, thereby providing a multivariable data set from which to build and test models of the kinetics of adsorption at solid-liquid interfaces. The technique is used to measure gamma(t) during the adsorption of either sodium dodecyl sulfate (SDS) or hen egg-white lysozyme (HEWL) onto an acrylic polymer membrane. The adsorption of SDS is reversible and is characterized by a decrease in gamma over a time period that coincides with that required for the mass loading of the membrane. For the adsorption of HEWL labeled with Alexa Fluor 532 dye, gamma continues to change long after the surface concentration of labeled HEWL, measured by using the elastomeric polymer membrane as an optical waveguide, reaches steady state. Gradual but significant changes in gamma(t) are observed as long as the concentration of protein in the bulk solution, c(b), remains nonzero. HEWL remains adsorbed to the membrane when c(b) = 0, but changes in gamma(t) are not observed under this condition, indicating that the interaction of bound protein molecules with those free in solution contribute to the prolonged change in the surface energy. This observation has been used to define a new model for the kinetics of globular protein adsorption to a solid-liquid interface that includes a mechanism by which the molecules in the bulk can facilitate the desorption of a sorbate molecule or change the energetic states of adsorbed molecules and, thus, the overall surface energy. The model is shown to capture the unique features of protein adsorption kinetics, including the relatively fast mass loading, the much more gradual change in surface energy that does not cease until the protein is removed from the bulk, the rapid desorption of an incubation-time-dependent fraction of bound protein when the protein is removed from the bulk, and the fixing of the residual surface concentration and surface energy at constant values once the removal of reversibly bound protein and free protein is complete.     

Photooxidation of poly[methyl(phenyl)silylene] and effect of photostabilizers

Poly[methyl(phenyl)silylene] (PMPSi) samples were irradiated in air atmosphere using either a weatherometer ATLAS Ci 3000+ or a mercury discharge lamp HBO 200. The kinetics of photodegradation was determined by FTIR and UV spectroscopies. The absorption changes in the siloxane, carbonyl and hydroxyl regions were monitored and analyzed. It is assumed that photodegradation of neat PMPSi in air atmosphere is an irreversible dual photoprocess consisting of primary photolysis followed by the photooxidation to oxygenated products such as siloxanes and species containing hydroxyl and carbonyl groups. Degradative changes can be retarded by the addition of photostabilizers. Triazine-based phenolic UV absorber protecting the polymer by the excited-state intramolecular proton transfer mechanism, bifunctional stabilizer (consisting of UV absorber and HAS functions) and the combination of triazine-based phenolic UV absorber with HAS was rather effective in retardation of photodegradation of PMPSi.     

Confinement effect in diffusion-controlled stepwise polymerization by Monte Carlo simulation

Diffusion-controlled stepwise polymerization of a linear polymer confined in nanoscopic slits is simulated through a Monte Carlo approach. A noticeable influence of the confinement on the kinetics is found. The confinement modifies both the spatial pair distribution function and the diffusive properties of the polymers. As a consequence, the confined system can show either faster or slower reaction kinetics with respect to the bulk system, depending on the strength of intermolecular interactions. The predicted polydispersity of the polymer is in agreement with recent theories of diffusion-controlled stepwise polymerization, and can be slightly affected by the confinement.     

Application of the transverse differential swelling stress model to the interpretation of case II diffusion kinetics

Attention is called to an early observation of Case II diffusion kinetics during penetration of a liquid into a uniaxially prestretched polymer film along the axis of stretching. Application of a previously developed transverse differential swelling stress model to these experimental conditions is described. It is shown that the model can predict Case II diffusion kinetics in a physically meaningful manner and will also explain the transition of the kinetics of the above system into normal or hindered diffusion regimes as either the degree of stretching or its direction relative to that of penetration is changed.     

Polystyrene comb polymers built on cellulose or poly(styrene-co-2-hydroxyethylmethacrylate) backbones as substrates for the preparation of structured honeycomb films

Comb polymers were prepared using the RAFT process via a Z-group approach. Trithiocarbonate RAFT agents were covalently bound to a polymer backbone via ester linkages. Two different types of comb polymer backbones were prepared, based on either modified cellulose or statistical poly(styrene-co-2-hydroxyethylmethacrylate) backbones. The subsequent polymerization of styrene from these multi-RAFT-functional backbones displayed some unusual kinetics with the rate of polymerization increasing concomitantly with the trithiocarbonate concentration. The molecular weight of each polystyrene branch increased with conversion, however, deviation from the theoretical molecular weight was detected. The polystyrene comb polymers were utilized in a casting process to prepare highly regular honeycomb structured porous films using breath figures. In general, the regularity was found to increase with increasing number of branches on a backbone and with increasing length of the polystyrene branch.     

Synthesis and application of cinnamate-functionalized rubber for the preparation of UV-curable films

A butyl rubber derivative that can be cured upon exposure to UV light in the absence of additional chemical additives was developed. This polymer was prepared by the reaction of hydroxyl-functionalized butyl rubber with cinnamoyl chloride to provide a cinnamate functionalized rubber. The cinnamate content was varied by starting with derivatives prepared from butyl rubber containing either 2 or 7 mol% isoprene. The kinetics of the cross-linking was studied by UV–visible spectroscopy and it was found to vary according to the film thickness. The changes in gel content and volume swelling ratio with irradiation time were dependent on the cinnamate content. Toxicity studies suggested that the cross-linked materials do not leach toxic molecules. The approach was also applied to obtain cross-linked films of butyl rubber-poly(ethylene oxide) graft copolymers, leading to surfaces that resisted the adhesion and growth of cells. Thus the approach is versatile and is of particular interest when non-leaching coatings of cross-linked butyl rubber are desired for biomedical or other applications.     

Laser flash photolysis of benzophenone in polymer films

With a nanosecond laser we studied flash photolysis of benzophenone (BP) dissolved in four different polymer films. We measured kinetics of decay of a triplet state of benzophenone (3)BP as well as kinetics of decay of benzophenone ketyl free radicals BPH(?). Polymer matrices have plenty of reactive C-H bonds, and the hydrogen abstraction by (3)BP leads to a formation of geminate pair which either recombines into molecular products or dissociates. Decay kinetics of (3)BP is well described by dispersive kinetics and in particular by the kinetic law suggested in Albery, W. J.; et al. J. Am. Chem. Soc. 1985, 107, 1854. We observed a broader distribution of rate constants in hard films. It was observed that the decay kinetics of transients radicals in the "hard" polymers is quite satisfactory described by the same law for dispersive kinetics. Kinetics of radicals decay in "soft" polymers is satisfactorily described as a diffusion-enhanced reaction. Effect of a hardness of polymer matrix on the measured kinetic parameters is discussed.     

Polymer surfactant kinetics using surface plasmon resonance spectroscopy dodecyltrimethylammonium chloride/polyacrylic acid system

Kinetics of polymer surfactant interactions and the effect of surfactant binding on the conformational dynamics of the polymer were explored in this work using surface plasmon resonance spectroscopy. Polyacrylic acid was modified with thiol to varying degrees so as to force the polymer to form different loop sizes upon adsorption on the gold SPR sensor surface. Dodecyltrimethylammonium chloride in solution was flowed over the polymer-coated sensor surface and the binding was followed in real time. It was found that control of the loop size of the polymer on the solid surface enabled in turn the control of surfactant binding, with the largest loop allowing the maximum amount of surfactant to bind and vice versa. The kinetic plot of the binding showed three distinct segments. The first segment followed convective-diffusive kinetics. The second and third segments followed first-order kinetics with the second rate being significantly faster than the first one. Careful analysis of the second segment showed that it is possible to divide it into two different segments, each following a first-order kinetics, with the second rate being slightly slower than the first one suggesting a gradual slow down of the reaction due to convolution from the polymer conformational changes. Mechanistically, the sudden increase in the rate for the third segment of surfactant binding implies that the polymer matrix is opening up so as to incorporate more surfactant molecules. This was attributed to the formation of charged double surfactant species the repulsive interaction of which prevented the polymer network from imploding. Studies using unmodified polymers suggested the possibility of sudden conformational rearrangement in the polymer network, with progress in surfactant binding. Furthermore, the reflectance of the SPR spectrum was found to increase upon surfactant binding, implying that there is a decreased efficiency of coupling of the incident radiation into the surface plasmon mode of the metal, which suggests that the surfactant actually penetrated the polymer matrix.     

Conformational dynamics of poly(acrylic acid). A study using surface plasmon resonance spectroscopy

The conformational dynamics of poly(acrylic acid) induced by pH change is reported here. Poly(acrylic acid) immobilized on gold surface was exposed to pH changes, and the conformational changes thus induced were followed in real time using surface plasmon resonance spectroscopy. The temporal profile of the stretching-coiling phenomenon showed a minimum point, which was proposed to be arising due to the contradictory behavior of two different property changes in the polymeric system. Normally surface plasmon resonance (SPR) response would be a convoluted effect of the thickness and refractive index changes, but the behavior observed here, where the SPR response is predominantly governed by either one of the two, is unique and to the author's knowledge is a feature that is observed for the first time. Analysis of the kinetics of the angle change revealed that it takes longer for the polymer to stretch than it takes for it to collapse, with the kinetic rate constants varying by at least an order of magnitude. The SPR angle change as well as the kinetic constants increased linearly with molecular weight. Effect of Ca2+ was studied, and it was found that the polymer was locked in its conformation due to the binding of the multivalent cations.     

Thermal oxidation of ester-modified isotactic polypropylene

The relationship between the thermal oxidation of isotactic PP samples modified by esters miscible and immiscible with the polymer and the structure of these samples has been studied. An analysis of kinetic features of oxygen uptake, buildup of oxidation products, and changes in the mechanical properties and structural parameters of PP in the course of oxidation has shown that the effect of a modifier on the kinetics of thermal oxidation of the polymer depends on compatibility of an additive and a polymer matrix. The addition of ester that is partially miscible with PP accelerates oxidation. In the case of an immiscible ester, the effect is quite the reverse. This phenomenon is rationalized by the fact that the phase state of the system determines changes in the initial structure of the polymer matrix and, hence, manifestation of structural effects in the reaction kinetics and the participation of additives in chain reactions of cooxidation with PP.     

“Raft” Formation by Two‐Dimensional Self‐Assembly of Block Copolymer Rod Micelles in Aqueous Solution

Block copolymers can form a broad range of self‐assembled aggregates. In solution, planar assemblies usually form closed structures such as vesicles; thus, free‐standing sheet formation can be challenging. While most polymer single crystals are planar, their growth usually occurs by uptake of individual chains. Here we report a novel lamella formation mechanism: core‐crystalline spherical micelles link up to form rods in solution, which then associate to yield planar arrays. For the system of poly(ethylene oxide)‐block‐polycaprolactone in water, co‐assembly with homopolycaprolactone can induce a series of morphological changes that yield either rods or lamellae. The underlying lamella formation mechanism was elucidated by electron microscopy, while light scattering was used to probe the kinetics. The hierarchical growth of lamellae from one‐dimensional rod subunits, which had been formed from spherical assemblies, is novel and controllable in terms of product size and aspect ratio.     

Model experiments for a molecular understanding of polymer crystallization

Time‐resolved real‐space observations of morphology and pattern formation resulting from crystallization of ultrathin films of low‐molecular‐weight poly(ethylene oxide) (PEO) or diblock copolymers containing PEO shed light on the mechanisms of how polymer crystals are formed. We used simple but restricted geometries like thin films of controlled thickness or confinement resulting from block copolymer mesotructures. Under such conditions, we were able to relate the observed morphology and its temporal evolution directly to molecular processes and the kinetics of crystal growth. We demonstrate that changes in the morphology with time are due to different thermal histories and are the consequence of the mestable nature of polymer crystals. Information about the nucleation process was obtained by examining crystal formation in 12‐nm small spherical cells of a block copolymer mesostructure. We discuss the advantages of thin‐film studies for a better understanding of polymer crystallization. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1869–1877, 2003     

Controlled release of goserelin from microporous polyglycolide and polylactide

Two microporous biodegradable polyesters, i.e., PGA and PDLLA, were obtained by solid-state polymerization reaction from the sodium salts of the corresponding alpha-hydroxycarboxylic acids after washing out the by-product sodium chloride. The polymers were shaped by cold uniaxial pressing, by hot uniaxial pressing, and by extrusion at elevated temperature. Due to the special microporosity of the polymers, the introduction of drugs is possible at moderate temperature. The release kinetics of the model drug Phe and of the anti-tumor drug goserelin (an LH-RH agonist) from compacted polymer samples were fast (approx. 2 d). The release kinetics of goserelin were corrected for the decomposition of the drug. External coatings with PDLLA or PLLA obtained by immersion in polymer solution strongly slowed down the release kinetics in the case of the PDLLA coating, giving an almost linear release during 100 d. A coating with PLLA was unsuitable to slow down the release kinetics.     

Slow adsorption reaction between arsenic species and goethite (alpha-FeOOH): diffusion or heterogeneous surface reaction control

The slow stage of phosphate or arsenate adsorption on hydrous metal oxides frequently follows an Elovich equation. The equation can be derived by assuming kinetic control by either a diffusion process (either interparticle or intraparticle) or a heterogeneous surface reaction. The aim of this study is to determine whether the slow stage of arsenic adsorption on goethite is more consistent with diffusion or heterogeneous surface reaction control. Adsorption kinetics of arsenate and dimethylarsinate (DMA) on goethite (alpha-FeOOH) were investigated at different pH values and inert electrolyte concentrations. Their adsorption kinetics was described and compared using Elovich (Gamma vs ln time) plots. Desorption of arsenate and DMA was studied by increasing the pH of the suspension from pH 4.0 to pH 10.0 or 12.0. The effective particle sizes and zeta-potential of goethite were also determined. Effective particle size increased rapidly as the pH approached pH(IEP), both in the absence and presence of arsenic. Inert electrolyte concentrations and pH had no effect on the slow stage of arsenate adsorption on goethite, while the kinetics of DMA adsorption on goethite was influenced by both parameters. The slow stage of arsenate adsorption on goethite follows an Elovich equation. Since effective particle size changes with both pH and inert electrolyte concentrations, and effective particle size influences interparticle diffusion, the arsenate adsorption kinetics indicate that the slow adsorption step is not due to interparticle diffusion. DMA also has complex adsorption kinetics with a slow adsorption stage. DMA desorbed completely and rapidly when the pH was raised, in contrast to the slow adsorption kinetics, indicating that the slow adsorption step is not due to intraparticle diffusion. The slow adsorption is not the result of diffusion, but rather is due either to the heterogeneity of the surface site bonding energy or to other reactions controlling arsenic removal from solution.     

Kinetics of polymeric radical oxidation with molecular oxygen in poly(methyl methacrylate) films

In poly(methyl methacrylate) films, the kinetics of the oxidation of polymeric radicals and azobenzenenitrenes with molecular oxygen dissolved in the polymer is studied. The free radicals are produced at 77 K by irradiating the polymer with UV light, fast electrons, or γ rays. The concentration of oxygen is varied from 4.5 × 10¹⁸ to 3.1 × 10¹⁹ cm^?3; the temperature of the reaction, from 90 to 130 K. The reaction is carried out in excess oxygen. The kinetics of radical oxidation is shown to be independent of the type of radiation that stimulates the formation of radicals and coincides with the kinetics of the oxidation of azobenzenenitrenes, which are uniformly dissolved in the polymer. It is concluded that the structure of the polymer in the vicinity of the radicals is virtually the same as the structure of the polymer bulk. The activation energy of the oxygen diffusion coefficient calculated according to the radical oxidation kinetics amounts to ～30 kJ/mol.     

Micromechanical cantilever technique: a tool for investigating the swelling of polymer brushes

Polymer brush coatings are well-known for their ability to tailor surface properties in a wide range of applications from colloid stabilization to medicine. In most cases, the brushes are used in solution. Consequently, efforts were expended to experimentally investigate or theoretically predict the swelling behavior of the brushes in solvents of different qualities. Here, we show that the micromechanical cantilever (MC) sensor technique is a tool to perform time-resolved physicochemical investigations of thin layers such as polymer brushes. Complementary to scattering techniques, which measure the thickness, the MC sensor technique provides information about changes in the internal pressure of the brushes during a swelling and deswelling process. We show that the kinetics of both swelling and deswelling are dependent on solvent quality. Comparing the measured data with its thickness evolution, which was calculated based on the Flory-Huggins theory, we found that only the first 10% of the thickness increase of the polymer brush results in a significant pressure increase inside the polymer brush layer.     

Phase separation of polymer mixtures induced by polarization‐selective chemical reactions: spatial symmetry breaking and mode selection

Phase separation of polystyrene/poly (vinyl methyl ether) (PS/PVME) blends was induced and controlled by irradiation with linearly polarized light. The PS component was made photosensitive by chemically labeled with either anthracene or trans‐stilbene. The former was used to crosslink the PS component whereas the latter induces phase separation by changing polymer segmental volumes. The phase separation and reaction kinetics were observed and discussed in terms of mode‐selection process.     

Sorption studies that reflect sequential physical changes in polymer-liquid systems from saturation to virtual dryness in a glassy state

The changes in macromolecular architecture that occur sequentially when a polymer is allowed to swell to saturation in a test-liquid and then evaporated from its gel-saturated state through its rubbery transition and finally down to a glassy state at virtual dryness are described in detail. The influence of molecular structure of the sorbed liquid and temperature on the kinetics of evaporation during each part of the above cycle is discussed. The relevance of these results with respect to the models proposed by polymer physicists to describe thermoreversible gelation and polymer relaxation in the glassy state is also discussed.     

Specific feature of water vapor sorption by chitosan medicated films

Sorption properties of chitosan-based medicated films were studied. IR and UV spectroscopies were used to confirm that chitosan chemically interacts with medicinal preparations. It is assumed that structural changes occurring in the polymer matrix either as a result of introduction of medicinal substances, or in the course of isothermal annealing of films lead to significant departures of the fundamental aspects of the sorption process from the classical Fick’s mechanism.     

Interaction of inorganic salts with polar polymers. I. Physical properties of phenoxy–calcium thiocyanate mixtures

Calcium thiocyanate is appreciably soluble in “Phenoxy” polymer. Solutions of this salt have significantly different physical properties compared to the pure polymer. The glass-transition temperature T_g is increased, and the kinetics of the glass transition are affected. The melt viscosity and its temperature dependence are increased. The viscosity changes are predicted from the changes in T_g and thermal expansion coefficients, in contrast to ionomers, in which clustering or domain formation cause viscosity to increase. Mechanical properties of the glassy polymers are also affected by the presence of dissolved salt. The most striking effect is an increased resistance to stress cracking by polar organic liquids. This may be related to the T_g increase, or to changes in solubility parameter, as indicated by insolubility of the salt solutions in solvents for the pure polymer. Increased water sorption and electrical conductivity are also results of salt incorporation.