Tomohisa Norisuye

Small angle neutron scattering studies on structural inhomogeneities in polymer gels: irradiation cross-linked gels vs chemically cross-linked gels

A comparison of network structure in a solvent was made for two types of poly(N-isopropylacrylamide) gels cross-linked by chemical reaction with N,N 0 -methylenebisacrylamide (BIS) (chemical gels) and by g-ray irradiation (g-ray gels). The cross-linking density dependence for these gels was examined by small angle neutron scattering (SANS). The SANS results indicated an increase of frozen inhomogeneities with an introduction of cross-links for both chemical and g-ray gels. However, it was found that the effect of cross-linking is much stronger in the chemical gels than in the g-ray gels. The differences in the structure were successfully interpreted by a statisticalmechanical theory of gels proposed by Panyukov– Rabin (Phys. Rep. 269 (1996) 1). The degree of polymerization between cross-links, N, was a decreasing function of cross-linking content for both types of gels, while that for the g-ray gels was a weak function of irradiation dose. Quantitative analyses on BIS concentration and g-ray dose dependence led to an experimental evidence of the existence of cross-linking saturation threshold. q 2002 Elsevier Science Ltd. All rights reserved.

Static inhomogeneities and dynamic fluctuations of temperature sensitive polymer gels

The static inhomogeneities and dynamic fluctuations in polymer gels have been studied by light scattering as a function of the gel preparation temperature, Tprep, and the cross-link density. The light scattered intensity of polymer gels was decomposed to two parts; the intensities due to the thermal fluctuations, IF, and the frozen inhomogeneities, IC. The analysis employing ensemble (··E) and time averaging (··T) of light scattered intensity disclosed that IFT (the thermal fluctuations) depends neither on the cross-link density nor the temperature of gel preparation, but on the temperature of observation. On the other hand, ICE (the frozen inhomogeneities) is a function of both temperatures at preparation and at observation. This clearly shows that gel is matter with dynamic fluctuations superimposed on the static frozen inhomogeneities. The latter are built-in inhomogeneities due to topological constraints of the gel by cross-linking formation.

Time-resolved light scattering study on the gelation process of poly(N-isopropyl acrylamide

The generation process of poly(N-isopropyl acrylamide) (PNIPA) gels has been studied by time-resolved light scattering. The gelation was initiated by adding a redox initiator and cross-linker (N,N′-methylenebisacrylamide, BIS). The scattered intensity was observed at a fixed angle of 60° as a function of polymerization/gelation time, t. The scattered intensity had an abrupt rise at t ≈ 20 min, then reached a plateau value having relatively large fluctuations. In the case of the corresponding PNIPA solutions, prepared without BIS, similar behaviour was observed, although the pulse height and the plateau intensities were significantly lower. In both cases, the abrupt intensity rise disappeared when the monomer concentration, C, was reduced to 88 mM or lower. In addition, a gel was not formed for C ≤ 88 mM even in the presence of BIS. It was concluded by viscometry and the monomer concentration dependence experiment that (i) the abrupt intensity rise corresponds to the gelation threshold and (ii) and so-called chain overlap concentration can be estimated by the appearance of the abrupt intensity rise (in this particular case, 88 ≤ C∗ ≤ 131 mM. The abrupt rise in the scattered intensity was also observed by small-angle neutron scattering, suggesting that the gelation threshold can be observed in a wide range of momentum transfer space.

Autocatalytic phase separation and graded co-continuous morphology generated by photocuring

Interpenetrating polymer networks (IPNs) with spatially graded co-continuous structures were constructed by photo-cross-linking a homogeneous mixture of photo-reactive polystyrene and methyl methacrylate monomer. For a given thickness, irradiation with weak ultraviolet (UV) light results in a co-continuous morphology uniform throughout the sample. However, as the irradiation intensity increases to some certain extent, spatially graded co-continuous morphology in the micrometre scales emerges due to the significant effect of the gradient of the light intensity along the irradiation direction. These 3-dimensional graded structures were observed by using laser scanning confocal microscopy (LSCM) and were analyzed by digital image analysis. The depth dependence of these graded structures can be well expressed by a power law with an exponent depending strongly on the irradiation intensity. The time-evolution of the graded morphology was monitored at different depths of the same irradiated sample. It was found that the phase separation does not follow conventional laws of kinetics, but instead exhibits the autocatalytic behavior, reflecting the effects of the heat produced by the photopolymerization of MMA monomer on the phase separation process. The experimental data obtained in this study suggest a method of producing polymeric materials with spatially graded structures in the micrometer scales by solely changing the irradiation intensity.

Influences of wetting and shrinkage on the phase separation process of polymer mixtures induced by photopolymerization

Phase separation induced by photopolymerization was investigated for a mixture composed of a polystyrene derivative and methyl methacrylate (MMA) monomer. The photopolymerization kinetics of MMA under irradiation with 405 nm visible light was in situ monitored by Fourier-transform infrared spectroscopy (FT-IR). Laser-scanning confocal microscopy (LSCM) combined with 2D-fast Fourier transform (2D-FFT) was utilized to observe and analyze the morphology resulting from the photopolymerization. By using LSCM together with static light scattering, it was found that the phase separation can be induced via either the spinodal decomposition or the nucleation-and-growth processes depending upon the irradiation intensity. A wide variety of stationary morphologies ranging from bi-continuous, salami structures to tri-layer morphology were separately obtained by changing the irradiation intensity. It was also found that the shrinkage of the sample associated with the photo-polymerization plays an important role in the resulting morphology. In addition, the autoacceleration behavior of the polymerization plays a key role in the phase separation kinetics. These experimental results indicate that photopolymerization can be used as an efficient tool not only to control the wetting process during the phase separation, but also to design polymer materials with various morphologies which would be utilized as templates for various practical applications.

Simultaneous observation and analysis of sedimentation and floating motions of microspheres investigated by phase mode–dynamic ultrasound scattering

A high frequency dynamic ultrasound scattering technique was developed to evaluate the mean velocity and the velocity fluctuations of micron-sized beads in highly turbid suspensions. In contrast to the previous study, scattering phase was fully utilized to investigate the dynamics of mixtures consisting of settling and floating microspheres. The velocities of the particles moving upward and downward with respect to gravity were simultaneously measured by a single acquisition. Instantaneous velocities determined by the time derivative of the scattering phase were evaluated as functions of the evolution time and position of the scatterer. (1) The velocity analysis in the time domain, (2) the phase analysis without phase unwrapping, and (3) the effects of noise filtering were discussed. The results were compared with those derived from amplitude mode–dynamic ultrasound scattering.

Collective motion of microspheres in suspensions observed by phase-mode dynamic ultrasound scattering technique

Compared with a nano-sized particle, dynamics of a micron-sized particle in a liquid is often associated with sedimentation (or floating) due to its relatively large mass. The motion of more than two particles is dominated by the hydrodynamic interactions, which are known to persist over a fairly long range, e.g., several millimeters, in suspensions. The particle size may be obtained from the dynamic ultrasound scattering (DSS) technique by the analysis of velocity fluctuations, whose origin is believed to take root in the particle-number fluctuations among temporally formed domains involving collective motion of particles with a certain cut-off length. In this study, such collective particle motion in highly turbid solutions was visualized by means of the phase-mode DSS technique with a single element transducer. Quantitative agreement between the velocity fluctuations obtained by the phase- and conventional amplitude-mode analyses was confirmed, followed by examination of the concentration and the particle size dependences on the dynamic structures induced by the long-ranged interactions. It was found that the phase mode-DSS was a promising method to evaluate the time-dependent structures of the micro-particles in highly turbid suspensions.

Sound velocity and attenuation coefficient of hard and hollow microparticle suspensions observed by ultrasound spectroscopy

Size and elastic properties of micro-particles suspended in liquid can be acoustically determined by ultrasound attenuation and velocity measurements with the aid of elastic scattering theories and a dispersion relation. While quantitative evaluation for hard micron-sized spheres using the theories is available in literature, that for hollow particles is not yet achieved. In this study, we show that the shell thickness and the elastic modulus of hollow particles can be quantitatively evaluated by ultrasound spectroscopy. Several kinds of microparticles including polystyrene rigid particles, polydivinylbenzene rigid particles, borosilicate hollow particles, and phenolic-resin hollow particles were examined as a function of the particle concentration.

Origin of the anomalous decrease in the apparent density of polymer gels observed by multi-echo reflection ultrasound spectroscopy.

Multi-echo reflection ultrasound spectroscopy (MERUS), which enables one to simultaneously evaluate the attenuation coefficient α, the sound velocity v and the density ρ, has been developed for measurements of elastic moduli. In the present study, the technique was further developed to analyze systems undergoing gelation where an unphysical decrease in the apparent density was previously observed after polymerization. The main reason for this problem was that the shrinkage accompanying the gelation led to a small gap between the cell wall and the sample, resulting in the superposition of the reflected signals which were not separable into individual components. By taking into account the multiply reflecting echoes at the interface of the gap, the corrected densities were systematically obtained and compared with the results for the floating test. The present technique opens a new route to simultaneously evaluate the three parameters α, v and ρ and also the sample thickness for solid thin films.

Investigating the existence of bulk nanobubbles with ultrasound

Nanobubbles are expected to dissolve in milliseconds. Experimental evidence of nanobubbles that were stable for days had thus been first received with circumspection. If the large number of experimental confirmations has now made clear that surface nanobubbles could exist, bulk nanobubbles are still subject to debate. When observations are reported, the main problem is to make sure the observed particles are really made of gas. We show that ultrasound is an ideal tool for investigating the existence of bulk nanobubbles: 1) it is sensitive to minute quantities of gas, 2) it allows one to determine the bubble size distribution, 3) it discriminates unambiguously between gaseous and solid/liquid inclusions. To illustrate the efficiency of ultrasonic detection, we performed size measurements of bubbles produced by a commercial nano-/microbubble generator. No nanobubble was detected with this device. It would be insightful to use ultrasonic detection in experimental situations for which stable nanobubbles were reported.