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The appearance of the opaque poly(acrylamide) gels. The mole fraction of the cross-linker is 0.01, 0.03, 0.05 and 0.1 from left to right. The total concentration of the pre-gel solution is fixed at 700 mM.
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Polymer gels are known to be opaque when the preparation conditions, such as the reaction temperature and the composition, are changed. The increase of the opaqueness of the gel suggests strongly the change of network structure. Here, we are going to review the recent studies on the structure and the frictional study of the opaque poly(acrylamide)...
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Figure 1 shows the appearance of poly(acrylamide) gels that are prepared at various concentrations of the cross-linker under constant total concentration of the gels. It is clear from the image that the opacity of the gels increases with the concentration of the cross-linker. Since the opacity of the gels is static, that is the opacity of the gels is independent of time, some large structure is frozen into the polymer network of the gels. This phenomenon itself was well known even more than forty years ago. Although several studies have been made, the details of the phenomenon, as well as the actual three-dimensional structure of the gel has not been clarified until recently. Meanwhile, the understanding of the physics of gels has continued extensively after the discovery of the volume phase transition in gels [3]. It was eventually recognized that the friction between the polymer network of the gel and the solvent is a characteristic property of the gel, and it plays an essential role during the phase transition. Then, systematic studies of the frictional properties of the gels were made [4–6]. It is shown in these studies that the friction coefficient between the polymer network of the gel and the solvent is quite sensitive to the structure of the polymer network. One can easily expect that the opacity of the gel is caused by the emergence of the density fluctuations within the gel, and then, it is frozen into the polymer network of the gel. The structure of the opaque gels and their transport properties have been also studied by several researchers [7,8]. The transport properties of the gels, however, have never been discussed in terms of the real-space structure of the gels. The main reason for this may be due to the fact that the structure of the gels is so complex, and hence, usual scattering techniques, such as X-ray, neutron and/or light, are helpless to solve the real-space structure of the gels. We, however, recognize that in the case of opaque gels, the optical microscope technique can be really useful to study the structure of the gels, because the typical length scale of the structure is the same order of magnitude as the wavelength of the light. In this review, therefore, we present the structural study of an opaque gel by using the confocal laser scanning microscope (CLSM) technique. Then, the frictional property of the opaque gel is discussed in terms of the resolved real-space structure of the gel ...
Citations
... The image in Figure 4B shows that the opacity of the gel increases with the content of PMDA. This substantiates previous findings in the literature demonstrating that the gels of the higher concentration region of the cross-linker are opaque [63]. Opaque gels are characterized by a heterogeneous network structure where particle aggregates are sufficiently large to scatter light [64]. ...
The cross-linking density influences the physicochemical properties of cyclodextrin-based nanosponges (CD-NSs). Although the effect of the cross-linker type and content on the NSs performance has been investigated, a detailed study of the cross-linking density has never been performed. In this contribution, nine ester-bridged NSs based on β-cyclodextrin (β-CD) and different quantities of pyromellitic dianhydride (PMDA), used as a cross-linking agent in stoichiometric proportions of 2, 3, 4, 5, 6, 7, 8, 9, and 10 moles of PMDA for each mole of CD, were synthesized and characterized in terms of swelling and rheological properties. The results, from the swelling experiments, exploiting Flory–Rehner theory, and rheology, strongly showed a cross-linker content-dependent behavior. The study of cross-linking density allowed to shed light on the efficiency of the synthesis reaction methods. Overall, our study demonstrates that by varying the amount of cross-linking agent, the cross-linked structure of the NSs matrix can be controlled effectively. As PMDA βCD-NSs have emerged over the years as a highly versatile class of materials with potential applications in various fields, this study represents the first step towards a full understanding of the correlation between their structure and properties, which is a key requirement to effectively tune their synthesis reaction in view of any specific future application or industrial scale-up.
... In previous mechanical measurements of the friction coefficient in macroscopic poly(acrylamide) hydrogels using confocal laser scanning microscopy a decrease of f with increasing crosslinker content and the accompanying increase of structural heterogeneities was observed. [69][70][71] It was argued that the motion in regions of low crosslinker density contributes more to the friction coefficient than the motion in the region of higher polymer and crosslinking density. ...
Poly(N-isopropylacrylamide) microgel particles were prepared via a "classical" surfactant-free precipitation polymerization and a continuous monomer feeding approach. It is anticipated that this yields microgel particles with different internal structures, namely a dense core with a fluffy shell for the classical approach and a more even crosslink distribution in the case of the continuous monomer feeding approach. A thorough structural investigation of the resulting microgels with dynamic light scattering, atomic force microscopy and small angle neutron scattering was conducted and related to neutron spin echo spectroscopy data. In this way a link between structural and dynamic features of the internal polymer network was made.
... The friction of gel thus calculated becomes smaller than the actual values of the friction. The details are given in the previous reports [43,44]. Figure 7 are also plotted in this figure (solid circles). ...
Gel becomes an important class of soft materials since it can be seen in a wide variety of the chemical and the biological systems. The unique properties of gel arise from the structure, namely, the three-dimensional polymer network that is swollen by a huge amount of solvent. Despite the small volume fraction of the polymer network, which is usually only a few percent or less, gel shows the typical properties that belong to solids such as the elasticity. Gel is, therefore, regarded as a dilute solid because its elasticity is much smaller than that of typical solids. Because of the diluted structure, small molecules can pass along the open space of the polymer network. In addition to the viscous resistance of gel fluid, however, the substance experiences resistance due to the polymer network of gel during the transport process. It is, therefore, of importance to study the diffusion of the small molecules in gel as well as the flow of gel fluid itself through the polymer network of gel. It may be natural to assume that the effects of the resistance due to the polymer network of gel depends strongly on the network structure. Therefore, detailed study on the transport processes in and through gel may open a new insight into the relationship between the structure and the transport properties of gel. The two typical transport processes in and through gel, that is, the diffusion of small molecules due to the thermal fluctuations and the flow of gel fluid that is caused by the mechanical pressure gradient will be reviewed.
The science of gel draws much attention after the discovery of the volume phase transition of gel. Among others, the information on the dynamics of gel is of importance to understand the kinetic behaviors of the volume phase transition of the gel. It is well established that the dynamics of gel is governed mainly by the collective diffusion of the polymer network of gel and the collective diffusion itself is determined by the balance between two forces. One is the elastic force due to the deformation of three-dimensional polymer network of gel, and the other is the frictional drag force between the polymer network of gel and the gel fluid. In early stage of the study in gels, the elastic properties of gel attract much attention, and, hence, considerable effort has been devoted to clarify the elastic behaviors of gel. The elastic properties of various gels under various experimental conditions are gained and reported so far. In contrast, much attention has not been paid to the frictional properties of gel since the experimental method in obtaining the reliable values of the friction coefficient has not been established until recently. The systematic studies on the frictional properties of gel begun only recently. Here, we would like to overview the earlier studies on the frictional property of gels including why the frictional study of gel is difficult and how we can solve the difficulty to obtain the reliable values of the friction coefficient of gel. The recent advancement on the frictional study of colloid gel will also be reviewed.
