Sol-gel Transition Point Research Articles

A Novel Approach for the Manufacturing of Gelatin-Methacryloyl.

Gelatin and its derivatives contain cell adhesion moieties as well as sites that enable proteolytic degradation, thus allowing cellular proliferation and migration. The processing of gelatin to its derivatives and/or gelatin-containing products is challenged by its gelation below 30 ∘C. In this study, a novel strategy was developed for the dissolution and subsequent modification of gelatin to its derivative gelatin-methacryloyl (GelMA). This approach was based on the presence of urea in the buffer media, which enabled the processing at room temperature, i.e., lower than the sol-gel transition point of the gelatin solutions. The degree of functionalization was controlled by the ratio of reactant volume to the gelatin concentration. Hydrogels with tailored mechanical properties were produced by variations of the GelMA concentration and its degree of functionalization. Moreover, the biocompatibility of hydrogels was assessed and compared to hydrogels formulated with GelMA produced by the conventional method. NIH 3T3 fibroblasts were seeded onto hydrogels and the viability showed no difference from the control after a three-day incubation period.

Zein inclusion changes the rheological, hydrophobic and mechanical properties of agar/konjac glucomannan based system

The practical characteristics (rheological, hydrophobic, and mechanical) of biopolymer matrix are essential for its application in food. This work discloses how zein incorporation alters these characteristics of agar/konjac glucomannan (KGM) based matrix. The zein particles formed non-intensive interactions with hydrophilic agar/KGM molecules and thus weakened the chain entanglements in the agar/KGM/zein (AKZ) ternary solutions, which allowed reductions in the zero-shear viscosity (from 4800 to 600 mPa s), sol-gel transition point (Tgel, from 39 to 35 °C) and storage modulus (from about 16000 to ∼1000 Pa) of the resulting AKZ ternary gels. Also, such effects tended to enhance the agar aggregation (zein content: <12%) and the zein crystalline structure (zein content: >12%), reducing the dissolution rate and swelling ability of the acquired AKZ films (dehydrated gels) but increasing their Tg (ca. 10 °C higher). The tensile strength and elongation at break of the composite films reached ca. 70–80 MPa and ca. 5%–7%, respectively. The results here may facilitate the development of agar/KGM-based composites (solution, gel or film) with desirable performance.

Increasing xanthan gum content could enhance the performance of agar/konjac glucomannan-based system

The properties (e.g., viscosity, sol-gel transition, and mechanical characteristics) of polysaccharides are crucial for their food applications. This work discloses how xanthan gum (XG) incorporation tailors the performance of an agar/konjac glucomannan (KGM) system. As reflected by rheological and thermal analyses, an increasing content of XG added could strengthen the chain entanglement in the resulting ternary solution with strong KGM-XG binding (an exothermic event). For example, a ternary system with a 16% XG content (AKX-16%) showed simultaneous increases in the zero-shear viscosity (about 5.5 times that of the binary counterpart AK), sol-gel transition point (about 48 °C; 10 °C higher than that of AK), and gel hardness (about 2300 g; 450 g higher than AK). Moreover, the AKX-16% composite showed lower crystallinity and enhanced hydrogen bonding and tensile strength (74 MPa) resulting from gel dehydration. Additionally, increasing the XG content showed a negligible effect on the morphology of the composite matrix, suggesting the excellent compatibility between these different polysaccharides.

An Investigation of the Sol-Gel Transition of Chitosan Lactate and Chitosan Chloride Solutions via Rheological and NMR Studies.

In recent years, intensive research has been carried out on the use of hydrogels obtained from natural polymers, mainly chitosan. These products are increasingly replacing solutions based on synthetic materials in medicine. This publication presents the results of studies on the sol-gel transition of chitosan solutions as the base material for the preparation of thermosensitive hydrogels for potential applications in tissue engineering. The measurements were carried out for systems consisting of chitosan lactate and chitosan chloride solutions using β-glycerol phosphate disodium salt pentahydrate and uridine 5′-monophosphate disodium salt as the cross-linking agents. The sol-gel transition point of the solutions was determined based on the rheological measurements in the cone-plate configuration of the rotational rheometer and experiments performed using the method of nuclear magnetic resonance. The obtained results showed a significant influence of the cross-linking agent on the course of the sol-gel transition of chitosan salt solutions, and the systems that consisted of chitosan lactate seemed to be especially interesting for biomedical applications.

Comparative Study on the Models of Thermoreversible Gelation.

A critical survey on the various theoretical models of thermoreversible gelation, such as the droplet model of condensation, associated-particle model, site–bond percolation model, and adhesive hard sphere model, is presented, with a focus on the nature of the phase transition predicted by them. On the basis of the classical tree statistics of gelation, combined with a thermodynamic theory of associating polymer solutions, it is shown that, within the mean-field description, the thermoreversible gelation of polyfunctional molecules is a third-order phase transition analogous to the Bose–Einstein condensation of an ideal Bose gas. It is condensation without surface tension. The osmotic compressibility is continuous, but its derivative with respect to the concentration of the functional molecule reveals a discontinuity at the sol–gel transition point. The width of the discontinuity is directly related to the amplitude of the divergent term in the weight-average molecular weight of the cross-linked three-dimensional polymers. The solution remains homogeneous in the position space, but separates into two phases in the momentum space; particles with finite translational momentum (sol) and a network with zero translational momentum (gel) coexist in a spatially homogeneous state. Experimental methods used to detect the singularity at the sol–gel transition point are suggested.

Incorporation of κ-carrageenan improves the practical features of agar/konjac glucomannan/κ-carrageenan ternary system

Three materials (agar, konjac glucomannan (KGM) and κ-carrageenan) were used to prepare ternary systems, i.e., sol-gels and their dried composites conditioned at varied relative humidity (RH) (33 %, 54 % and 75 %). Combined methods, e.g., scanning electron microscopy, small-angle X-ray scattering, infrared spectroscopy (IR) and X-ray diffraction (XRD), were used to disclose how κ-carrageenan addition tailors the features of agar/KGM/κ-carrageenan ternary system. As affirmed by IR and XRD, the ternary systems with κ-carrageenan below 25 % (agar/KGM/carrageenan, 50:25:25, m/m) displayed proper component interactions, which increased the sol-gel transition temperature and the hardness of obtained gels. For instance, the ternary composites could show hardness about 3 to 4 times higher than that for binary counterpart. These gels were dehydrated to acquire ternary composites. Compared to agar/KGM composite, the ternary composites showed fewer crystallites and nanoscale orders, and newly-formed nanoscale structures from chain assembly. Such multi-scale structures, for composites with κ-carrageenan below 25 %, showed weaker changes with RH, as revealed by especially morphologic and crystalline features. Consequently, the ternary composites with less κ-carrageenan (below 25 %) exhibited stabilized elongation at break and hydrophilicity at different RHs. This hints to us that agar/KGM/κ-carrageenan composite systems can display series applications with improved features, e.g., increased sol-gel transition point.

Polymeric Hydrogelator-Based Molecular Gels Containing Polyaniline/Phosphoric Acid Systems.

To expand the range of applications of hydrogels, researchers are interested in developing novel molecular hydrogel materials that have affinities for the living body and the ability to mediate electrical signals. In this study, a simple mixing method for creating a novel composite molecular gel is employed, which combines a hydrophilic conductive polymer, a polyaniline/phosphoric acid complex, and a polymer hydrogelator as a matrix. The composite hydrogel showed an improved gel-forming ability; more effective mechanical properties, with an increased strain value at the sol–gel transition point compared to the single system, which may be sufficient for paintable gel; and a better electrochemical response, due to the electrically conducting polyaniline component. These findings demonstrate the applicability of the new composite hydrogels to new potential paintable electrode materials.

Alginate/gelatin-based hybrid hydrogels with function of injecting and encapsulating cells in situ

Multi-network hydrogels with high strength and toughness have attracted increasing attention. Herein, a hybrid hydrogel consisting of alginate, gelatin, and polyacrylamide was constructed with the combination of advantages of natural and synthetic polymers. Alginate grafted with host-guest complex of βCD/Ad-AAm was first prepared, namely Alg-βCD/Ad-AAm, then further crosslink with gelatin methacryloyl (GelMA) to form hydrogel via one-step UV light initiation. The hydrogel produced by this method has more uniform and well-crosslinked networks. The hydrogels demonstrated uniform porosity, adjustable hydrophilicity (water contact angle within 32.7–91.5°), and desired mechanical properties (maximum tensile strain of 242.8%, tensile strength of 75.9 kPa, and Young's modulus of 28.5 kPa). The hydrogel also possessed self-healing ability and pH sensitivity, showing higher mechanical tensile strength at lower pH. The temperature-adjustable viscosity of pre-gel solution (sol-gel transition point of 20.4 °C) endowed it to be 3D printed as a bioink, and the printed scaffold exhibited good resilience and toughness. Moreover, HUVEC, L929, and 3T3 cells were cultured on hydrogel surfaces for 28 days and were enveloped within the hydrogels for 3D culture, indicating excellent cytocompatibility of the hydrogels. Therefore, this hybrid hydrogel system can be used potentially in cell culture scaffold and tissue engineering.

Polyvinyl alcohol inclusion can optimize the sol-gel, mechanical and hydrophobic features of agar/konjac glucomannan system

The practical features (e.g., sol-gel, mechanical and hydrophobic) of biopolymer systems are crucial for their materials applications. This work reveals how polyvinyl alcohol (PVA) inclusion affects the practical features of agar/konjac glucomannan (KGM) system. From rheological analysis, incorporating PVA (especially 6%) enhanced the chain entanglements of resulted ternary solution (A70K24P6) with stabilized sol-gel transition point. Such effect not only increased the zero-shear viscosity (ca. 1.5 times that of agar/KGM counterpart) and structural recovery degree of A70K24P6 solution, but also caused reduced crystallites and simultaneously increased tensile strength, elongation at break and hydrophobicity for A70K24P6 film from solution dehydration. This ternary film exhibited a tensile strength of ca. 105 MPa, an elongation at break of ca. 20%, and a water contact angle of ca. 97.6°. Additionally, incorporating PVA almost unaffected the morphology of film fracture surface. These results are valuable for the design of agar/KGM systems with improved practical features.

Xanthan gum inclusion optimizes the sol-gel and mechanical properties of agar/konjac glucomannan system for designing core-shell structural capsules

Core-shell structural capsules can encapsulate various food ingredients (e.g., nutrients), thus being highly potential for the design of many food products. This work reports how the gum (xanthan, locust bean or gellan) inclusion tailors the practical features of agar/konjac glucomannan (KGM) system for fabricating the core-shell structural capsules. The rheological and textural analyses revealed that xanthan gum inclusion could simultaneously increase the sol-gel transition point and gel strength (ca. 1.6 times higher) of agar/KGM system. The composite gels were dehydrated to generate shell materials. The materials including xanthan or locust bean gum showed stronger chain interactions than did that including gellan gum, accompanied by weakened crystalline features for the different shell materials. Again, compared to the agar/KGM counterpart, the shell materials including xanthan gum could display a similar tensile strength (close to 50 MPa) but an evidently increased elongation at break (ca. 2.5 times higher). Then, these agar/KGM based systems were used to prepare core-shell structural capsules. The capsules based on agar/KGM/xanthan gum materials exhibited the highest roundness and breaking stress. Regarding this, the increased sol-gel point reduced the time required for droplet hardening (to generate core-shell gel balls); such fact, together with the higher gel strength, weakened the deformation of gel balls and thus increased the roundness and breaking stress of capsules (resulting from drying of gel balls).

Cluster growth from a dilute system in a percolation process

The gelation process has been explained in the framework of percolation models. Although the percolation models accurately predict the gelation behaviors above the overlap concentration of prepolymers (c*0), they do not predict the gelation behaviors below c*0. We measured the osmotic pressure during the gelation of a series of model polymer gels, namely, tetra-polyethylene glycol (PEG) gels. The osmotic pressure decreased during the gelation reaction and was constant after the sol–gel transition. These results suggest that the clusters grow and fill the system at the sol–gel transition point. As a result, the gels behaved as semidilute systems regardless of the initial polymer concentration. This representation of the sol–gel transition at the overlap condition of the critical clusters corresponds well to the aggregation process predictrion. These results will help better understanding of general percolation problems in the dilute regime. The osmotic pressure during the gelation process below the overlapping concentration of prepolymers was investigated using a series of model polymer gels, namely, tetra-polyethylene glycol gels. The osmotic pressure decreased during the gelation reaction and was constant after the sol–gel transition, suggesting that the clusters grow and fill the system, at the sol–gel transition point. This representation of the sol–gel transition at the overlapping condition of the critical clusters corresponds well to the aggregation process prediction.

Similarity between Gel and Semi-Dilute Solution

A polymer gel in the air is in equilibrium state, as a closed system. However, when the gel is immersed in a solvent, the volume changes automatically toward the new equilibrium as an open system. The equilibrium is determined as the balance of elastic pressure and osmotic pressure according to Flory-Rehner theorem. Conventional polymer gels swell in good solvent because osmotic pressure is larger than elastic pressure. The swelling brings some drawbacks in actual applications of polymer gels. For example, swelling of hydrogels in vivo compress the surrounding tissues, and causes severe adverse reactions. Thus, understanding and control over swelling is an urgent issue in material science. On the other hand, gel swelling has been investing in the field of polymer physics, especially focusing on the equilibrium-swollen condition. Based on the scaling rules on elastic pressure (Πel) and osmotic pressure (Πos), both pressures balance in the equilibrium-swollen state as, Πel ≈ Πos ~ C 3ν/3ν-1. Here, ν is a parameter indicating the interaction between polymer and solvent. Previous researches have shown that the scaling relationship is consistent with experiments. Based on the consistency, some papers concluded the validity of C* theorem which predicted that a polymer gel maintains a concentration proportional to the overlapping concentration (C*) of polymer solution of the corresponding network strand of the gel. However, these agreements only suggest thatΠos of polymer gel is the same form with that of semi-dilute solution, as pointed out by Urayama. Polymer gels swell toward equilibrium keeping semi-diluted, even if the concentration of the polymer gels is below C* of the network strands. Once polymer gels are fabricated, the networks of the polymer gels behave as polymers in the semi-dilute solution. In the case of gels fabricated from prepolymers such as tetra-PEG gel, sol-gel transition can occur even if the initial polymer concentration (C 0) is far below C*. As for conventional gelling system from small molecular monomer and crosslinker, the initial condition is inherently below C*. Then, whether or not polymer gels are “universally” semi-diluted is an inevitable question. If any polymer gels are semi-diluted system, it is important to understand how the gel from diluted prepolymer solution reaches semi-diluted system during gelation. Despite of the importance, these issues have never been investigated. This situation is in good contrast with viscoelastic change during gelation, which is fully understood based on Winter-Chambon’s criterion. In this research, we measured the Πos of gelling system from diluted prepolymer solutions. Gelation process was reproduced statically using the tetra-PEG system that can control the reaction conversion by controlling mixing ratio of AB-type prepolymer solutions, and Πos was measured sequentially. As a result, we discovered that Πos of critical clusters, which are the polymeric clusters just before the sol-gel transition, determined that of corresponding gels. In addition, the molecular weights of critical clusters depend on C 0 when gelling from the diluted systems which simply suggested that sol-gel transition requires more growth of polymeric clusters at lower C 0. By integrating the result of previous study about fractal dimension of the critical clusters, similarity between gelation thresholds and the C* condition was derived. Based on this suggestion, the picture of gelation from the diluted solution is as follows. Given that the system is initially diluted, overlapping of prepolymers is not preferred at beginning (FIG. a). As the branching reaction proceeds (FIG. b), polymeric clusters grow, and C* of the system decreases. As a result, although C 0 is constant, the system approaches the overlapping condition. In this process, Πos decreases due to the decrease in translational entropy of clusters. The system reaches the overlapping condition, just before sol-gel transition (FIG. c). At the sol-gel transition point, viscosity and stiffness show drastic changes, because they strongly relate to connectivity. In contrast, Πos does not show any apparent changes most likely due to the concentration fluctuation at their overlapping condition is not strongly influenced by further crosslinking. As a result, Πos of gels corresponds to the osmotic pressure just before sol-gel transition point. Figure 1

Unique gelation of chitosan in an alkali/urea aqueous solution

The gelation behavior of chitosan in an alkali/urea aqueous solution was studied by means of rheology, X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). The sol-gel transition temperature of the chitosan alkaline solution strongly depends on chitosan concentration and heating rate. The equilibrium sol-gel transition temperature, Tc, which was obtained by an extrapolation method based on multiwave oscillation, decreases with increasing chitosan concentration, and could be fitted to a parabolic equation, Tc = −4.1 + 48.0w - 11.5w2, where w is the weight fraction of chitosan in the alkaline aqueous solution. At the sol-gel transition point, the relaxation behavior shows a strong dependence on kinetics and the equilibrium critical relaxation exponent ne decreases as chitosan concentration increases. The FESEM images verified that the chitosan gel is formed as a fibrillar network and the fibrillar network is denser at a higher polymer concentration or a lower heating rate, leading to an irreversibility of the sol-gel transition of chitosan in the alkaline aqueous solution. A schematic diagram has been proposed to describe the change of gel network structure with chitosan concentration.

Experimental Observation of Two Features Unexpected from the Classical Theories of Rubber Elasticity.

Although the elastic modulus of a Gaussian chain network is thought to be successfully described by classical theories of rubber elasticity, such as the affine and phantom models, verification experiments are largely lacking owing to difficulties in precisely controlling of the network structure. We prepared well-defined model polymer networks experimentally, and measured the elastic modulus G for a broad range of polymer concentrations and connectivity probabilities, p. In our experiment, we observed two features that were distinct from those predicted by classical theories. First, we observed the critical behavior G∼|p-p_{c}|^{1.95} near the sol-gel transition. This scaling law is different from the prediction of classical theories, but can be explained by analogy between the electric conductivity of resistor networks and the elasticity of polymer networks. Here, p_{c} is the sol-gel transition point. Furthermore, we found that the experimental G-p relations in the region above C^{*} did not follow the affine or phantom theories. Instead, all the G/G_{0}-p curves fell onto a single master curve when G was normalized by the elastic modulus at p=1, G_{0}. We show that the effective medium approximation for Gaussian chain networks explains this master curve.

Probe Diffusion of Sol–Gel Transition in an Isorefractive Polymer Solution

The sol–gel transition of tetrafunctional polymers with mutual reactive end-groups was investigated by analyzing the dynamics of probe particles via dynamic light scattering. The dynamics of probe particles was exclusively observed by matching the refractive index of the solvent and the polymers. The sol–gel transition point, decreasing of sol fraction and increasing of gel fraction with the reaction, the onset of formation of closed structure inside branched polymer clusters, and a piece of evidence for the decrease of the local viscosity in postgel regime were observed via the dynamics of probe particles. In addition, a scaling relationship ηeff ∼ e–1.13±0.06 was found in a wide range of cross-linking conversion (p) before the gel point, where ηeff is the effective viscosity estimated from probe particles’ dynamics and e ≡ |p – pc|/pc is the relative distance from the sol–gel transition point (pc is the cross-linking conversion at gel point).

Rheological Studies of Physical Soy Protein Gels

Many naturally-occuring gels exists in the biological systems such as foods. Because of their biodiversibily and delicate nature, their properties can be used by the food industry to achieve a variety of physicochemical functions. The gel utilized in this study, soy protein isolates (SPI) and concentrates (SPC), all have unique rheological properties which play an important role in their strength function of foods. The objective of this study is to gain insights as to how the rheological properties of these bio-gels are designed to control the gel-strength of these materials. The concentration dependence of dynamic moduli G’ and G” of soy protein isolates (SPI) and soy protein concentrates were studied in solutions prepared under different conditions. Three-level preliminary fractional experiments were undertaken to study effects of temperature, pH, and concentration on the gel-strength of these products. Protein concentration, pH and temperature appear to have an impact on the strength of these bio-gels. An investigation of the viscoelastic behavior of SPI solution at the sol-gel transition point was mainly considered by applying a rheological testing procedure. The data demonstrated a power law frequency dependence of the viscoelastic functions G΄(ω) and n*(ω) at the gel point. At low temperatures and high concentrations we get weaker gel-strengths than at high temperatures and low concentrations. Lowering the concentration and keeping the temperature constant seem to have little effect on the gel strength but points towards to becoming stronger gels.

Sol-gel transition behavior near critical concentration and connectivity

A polymer cross-linking system undergoes a phase transition from liquid to solid at a critical point, which is called the sol-gel transition. The sol-gel transition is often understood in the context of the lattice-based percolation model. Two parameters govern the sol-gel transition including the connectivity and polymer concentration. In this study, we independently tuned these parameters and experimentally accessed the sol-gel transition point as a function of connectivity and concentration using a model gel system (tetra-polyethylene glycol gel). The connectivity required to percolate the system continuously increased as the polymer concentration decreased, which is completely different from that predicted by the site-bond percolation model. The viscoelastic behavior at the critical points indicates that the fractal dimension of the percolation clusters deviated from the prediction of the lattice-based percolation model as the polymer concentration decreased. These results indicate that the lattice assumption cannot be applied for a gelling system prepared far below the overlapping concentration. A polymer cross-linking system undergoes a phase transition from liquid to solid at a critical point, which is called the sol-gel transition. We independently tuned the intrinsic parameters governing the sol-gel transition, that is, the connectivity and polymer concentration, using a model gel system (tetra-polyethylene glycol gel), and examined the model predictions. The lattice-based percolation model can be applied only for systems prepared around the overlapping concentration. In the case far below the overlapping concentration, the aggregation model well reproduced the transition behavior.

Dynamical transition of heat transport in a physical gel near the sol-gel transition.

We experimentally study heat transport in a gelatin solution near a reversible sol-gel transition point where viscosity strongly depends on temperature. We visualize the temperature field and velocity field using thermochromic liquid crystals and polystyrene latex particles, respectively. During the initial stages of heating, we find that heat transport undergoes a dynamical transition from conductive to convective. Subsequently, during later stages, we observe that the transport dynamics are much more complex than conventional thermal convections. At the sample’s surface we observe the formation of stagnant domains, which lack fluid flow. Their formation is not due to the effects of local cooling. We determine that it is the dynamics of these stagnant domains that induce convective-conductive-convective transitions.

Preparation and characterization of chitosan gel beads crosslinked by organic titanium

Chitosan (CTS) gel was successfully prepared by organic titanium (OT) ionic crosslinking agent. The viscoelastic properties of CTS gel was investigated through rheological measurement. The sol-gel transition point (t cr ) of gel prepared with CTS (22.5 g•L−1) and OT (1.5 %) was determined at 339 s, with the increases of either CTS or OT concentration, the gelation became faster and the sol-gel transition time became shorter. The optimal chitosan gel beads were prepared with 35 g•L−1 chitosan and 7 % organic titanium (OT) at room temperature for 1 h reaction. The SEM data indicated that the gel beads had special shape with rough and poriferous surface. The effective grain size range was 1.0–1.2 mm. It was confirmed by TG data that the gel beads performed better in thermal stability than original chitosan. Swelling experiments showed that the swelling ability of gel beads strongly depended on the pH value of buffer solution. On the whole, the modified CTS/OT gel beads showed high adsorption capacity, good mechanical properties, strong acidic and alkali resistances, it could serve as potential drug deliver and the absorbent for heavy metal ions.

Evidence of ZrO2 sol–gel transition by gelation time and viscosity

Sol–gel transition is a stage of the sol that evolves towards the formation of a gel-like network and that exhibits gradually a mobility loss. Hence, the measurement of both viscosity and gelation time is expected to evidence the sol–gel transition occurrence. ZrO2 sols were prepared using zirconium oxychloride octahydrated as precursor material. The structures of the sols before and after gelation were characterized by TEM. Gelation time depends on the concentrations of the reactants and the aging temperature. Viscosity increases linearly with the gelation time firstly, and then increases exponentially nearby the sol–gel transition point. TEM results show the gel is granular before the sol–gel transition point, and appears as brain-like structure after this point. The determination of the sol–gel transition point is to enable the optimum aging time of a sol and to perform the next operations which determine the desired shape of gel (bulk, film, powder, etc.).