From topological gels to slide‐ring materials

A novel type of gel, called a topological gel, has been recently developed with a supramolecular architecture with topological characteristics. In the topological gel, polymer chains with bulky end groups are neither covalently crosslinked as in chemical gels nor attractively interacting as in physical gels but are topologically interlocked by figure‐eight shaped crosslinks. Hence, these crosslinks can pass along the polymer chains freely to equalize the tension of the threading polymer chains similarly to pulleys; this is called the pulley effect. This concept can be applied not only to gels but also to a wide variety of polymeric materials without solvents. Then, polymeric materials with movable crosslinks are referred to as slide‐ring materials (SRMs) in a wider sense. Here, we review the synthesis, structure, physical properties, and applications of topological gels and SRMs. In particular, slide‐ring elastomers show remarkable scratch‐proof properties for application to coating materials for automobiles, cell phones, mobile computers, golf clubs, and so on. © 2014 The Authors Journal of Applied Polymer Science Published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40509.     

Poly(ethylene glycol) electrolyte gels prepared by condensation reaction

Poly(ethylene glycol) electrolyte gels were prepared by condensation reaction in the presence of tetraethoxysilane. Differential scanning calorimetry and X‐ray diffraction spectroscopy were used to investigate the thermal transition behavior and crystalline structure of polymer gels prepared. Both formation of crosslinks and incorporation of salts or plasticizers reduced the development of crystalline structure of poly(ethylene glycol)s. Cyclic voltammetric and ion‐conducting behaviors of polymer gels were analyzed using potentiostat and impedance spectroscopy, and those were also considerably affected by the crosslinking density of polymers and the concentration of electrolyte salt or plasticizers incorporated. Poly(ethylene glycol) gels possessing certain levels of electrolyte salt and plasticizer were expected to have applications of solid electrolytes for lithium polymer secondary batteries. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 948–956, 2002     

Differential scanning calorimetry of aqueous polymer solutions and gels

The melting phenomena of aqueous polymer solutions and gels have been investigated by differential scanning calorimetry (DSC). The polymers used were synthetic polyacrylamide and poly(vinyl alcohol) samples as well as guar and xanthan gums. By using an empirical relation, the energy measured from the area under the melting peak yielded heats of mixing and sorption, when fitted by an association factor computed from the data. This factor (independent of the concentration) is a measure of the water fraction associated with the polymer and has a definite and characteristic value for a given polymer in water. When a crosslinking agent (potassium pyroantimoniate or chromic nitrate) was added to the water–polymer system, the association factor varied with the polymer concentration; the macromolecular chains thus become less accessible to penetrating water. If a branched gel was obtained owing to the formation of chemical crosslinks, a hump appeared on the melting peak.     

Preparation and moduli of model polymer networks

Polystyrene containing a small number of randomly spaced p-substituted secondary amine groups has been prepared. This polymer has been crosslinked in solution at concentrations in the range 5–20%, by hexamethylene diisocyanate, to form gels. The moduli of these gels were measured and the results compared to theoretical predictions from rubber elasticity theory. The dependence of the moduli on the concentration at which crosslinking takes place has been explained in terms of chemical crosslinks, closed loops and entanglements. The results support a value for the ideal modulus of nkT, where n is the number of crosslinks per cm³.     

Posteffekt bei der Radiolyse von Polystyrol

In polystyrene the major part of crosslinks is produced during irradiation in a “hot reaction”. 10% of the crosslinks are formed in a postreaction. A kinetic analysis of the experimental data showed the latter reaction to be first order. At larger storage times especially at higher temperatures the initial increase of crosslinks is followed by a decrease. This can only be explained if the crosslinks are formed during dissolution. We assume, that the posteffect is due to trapped cyclohexadienyl- and benzylradicals. The first order kinetics is ascribed to a transformation of cyclohexadienyl- to benzylradicals. By dissolution of the polymer after increasing storage time the reaction of cyclohexadienylradicals with benzylradicals, which leads predominantly to disproportionation, is replaced by recombination of two benzylradicals forming crosslinks. At higher temperatures and long storage times also benzylradicals must dissapear in the solid polymer by reactions not forming crosslinks. Additives like Anthracene and Naphthochinon suppress the posteffect. Based on the experimental results a reaction mechanism for the formation of crosslinks during irradiation and in the postreaction is proposed.     

Observations on the structure of a polyacrylamide gel from electron micrographs

We compare the results of electron microscopy from replicas obtained by the freeze-etch technique applied to a polyacrylamide gel and a polyacrylamide solution at the same concentration. The gel differs from the polymer solution in its preparation only by the presence of N,N′-methylene bisacrylamide which crosslinks the linear chains. The comparison is made on samples prepared in a 75% water-25% glycerol solution to minimize crystallite formation during the quenching step. The structure of the polymer solution appears to be much more homogeneous than that of the gel at the same magnification level. Analyses of the micrographs with the help of diffraction patterns indicate that the gel has a much broader distribution of ‘pore’ or ‘mesh’ sizes than the corresponding polymer solution. Some support for the suggestion of aggregation of crosslink molecules to form somewhat denser regions (heterogeneities) in the gel can be deduced from the micrographs obtained from gels prepared in pure water and in a water-glycerol mixture.     

New aspects of nonlinear elasticity of polymer gels and elastomers revealed by stretching experiments in various geometries

This review focuses on the novel features of nonlinear elasticity for a range of gels and elastomers that were recently revealed through experiments using various types of strain such as uniaxial and biaxial stretching. Particular emphasis is placed on the magnitude of the cross‐effect of strains in different directions which can be characterized by unequal biaxial stretching. The studied materials include (1) markedly swollen hydrogels with a water content of 98%, (2) tetra‐arm poly(ethylene glycol) (Tetra‐PEG) gels with nearly uniform network structures without entanglement couplings of network strands, (3) slide‐ring gels with movable crosslinks along network strands and (4) elastomer foams with low densities of polymer solids. The magnitude of the cross‐effect of strains reflects the structural features of networks and crosslinks as well as bulk compressibility for each material. We also summarize recent results on strain‐driven swelling and the accompanying force reduction phenomena under various types of stretching. Remaining issues to be investigated are presented on the basis of existing results. © 2016 Society of Chemical Industry     

Compatibility of plasticizers with poly(vinyl chloride)

The tendency of a plasticizer to resist exudation from poly(vinyl chloride) (PVC) under compressive stress, known technologically as “compatibility,” is treated in terms of a network model, the plasticized composition being thought of as a rubber crosslinked by crystallites. Compatibility is increased by increased solvency (Flory-Huggins χ) and decreased by increasing plasticizer molar volume. A large crosslink density and/or insufficient melting of crosslinks during processing (thermal history) also decreases compatibility. All commercial primary plasticizers are believed to be infinitely miscible with amorphous PVC. Phase separation which occurs is syneresis and not related to any phase diagram. Swelling tests for compatibility and swelling measurements on dilute PVC gels are described. Some general principles relating to gel formation and association in polymer solutions are also discussed.     

Local motion of crosslinks for poly (methyl methacrylate) gels by the fluorescence depolarization method

Poly(methyl methacrylate) (PMMA) gels labeled at crosslinks with anthracene were prepared. Anthracene fluorescence depolarization was monitored to probe the local motion of crosslinks for PMMA gels at different equilibrium swelling states. The relaxation times and the activation energies of local motion were measured for PMMA gels at the swollen states in various solvents through fluorescence anisotropy decays. The local motion of PMMA gel at crosslinks became faster with the increase of swelling ratio. When the swelling ratios were almost the same, the mobility of crosslinks was the same irrespective of the molecular weights between crosslinks. These results indicate that the local motion of crosslinks for PMMA gel is mainly governed by the segment density of network chains in the vicinity of crosslinks. Received: 31 March 1997/Revised: 11 April 1997/Accepted: 21 April 1997     

Mechanical properties of borate crosslinked poly(vinyl alcohol) gels

Boric acid does not introduce crosslinks in poly(vinyl alcohol) solutions, but gelation does occur in the presence of cations. In this experimental study, the dynamic mechanical properties of these gels were determined using test-tube torsion pendulums and an air-bearing torsion pendulum. The modulus at a fixed concentration of polymer and boric acid increases with increasing sodium ion concentration up to the point where the atom ratio of sodium to boron reaches 1. Higher sodium concentrations do not increase the modulus. The log decrement, on the other hand, decreases with increasing sodium concentration continuously without reaching a plateau at the equal atom ratio. Log decrements as low as 0.02 can be measured. The storage modulus depends on the logarithm of borate concentration and on the 4.7 power of poly(vinyl alcohol) concentration. Only a very small portion of the borates in solution take part in effective crosslinks. The activation energy for breaking individual bonds in a function of temperature and the cation to boron ratios. At a fixed cation concentration, this activation energy is more negative with increasing amount of boron ions due to a transformation of monomeric crosslinks into polymeric crosslinks, so that the storage modulus which measures crosslink density decreases as a temperature rises.     

A model for swelling changes in a covalently crosslinked gel caused by unfolding of folded domains

A model describing swelling changes of covalently crosslinked gels caused by unfolding of crosslinks of folded chains built-in in the gel matrix was proposed. By unfolding, a new interacting surface is exposed which may differ from the interacting sites of the gel polymer. Non-ionic as well as ionic interactions are considered. A certain force generated by the swelling pressure acts on the ends of the folded crosslinks and unfolding starts when this force exceeds a certain limiting value. The continuation or arrest of unfolding depends on whether the exposure of the new interacting surface decreases or increases this force. The unfolding process is considered to be irreversible but possible reversibility can be considered. This model relates to biohybrid gels in which the folded domain of the crosslink is composed of elements of the muscle protein titin. The model is based on the statistical-mechanical theory of swelling of crosslinked polymers. A change in external variables such as temperature or degree of ionization can cause an abrupt change in volume of the swollen gel (transition) within a certain range of parameters characterizing the gel and polymer-solvent interactions. Received: 23 January 2001/Accepted: 12 May 2001     

X-Ray-Studies of crystallization in swollen polyvinyl fluoride

Polyvinyl fluoride (PVF) shows a strong crystal interference caused by the lateral distances of the chains, and is highly crystalline. Over the temperature range 0—100°C the degree of crystallinity is about 86%. Gelation of PVF in γ-butyrolactone was studied over practically the whole range of concentrations (from pure PVF to 1% solution). Gel temperatures were found practically independent of concentration (174°C). Gelation is coupled with crystallization. The swollen samples showed no differences in degree of crystallinity and quality of the crystals compared with the pure polymer at the same temperature. Therefore the degree of crystallinity of the gels is solely a function of temperature. The crosslinks of the network are formed by crystalline regions. Only the amorphous regions in the gels can take up solvent. Caused by the high crystallinity the gels have a crumb-like nature.     

Poly(2,4-perfluoroalkylene-6-Perfluoroalkyltriazine)s,new high temperature elastomers

Poly(perfluoroalkyleneimidoylperfluoroalkylenoamidine)s have been prepared by the reaction of perfluoroalkane dinitriles with ammonia or with the diamidines of the dinitriles. The imidoylamidine polymers were treated with anhydrides to give poly(perfluoroalkyIenetriazine)s. Treatment of the imidoylamidine polymer with perfluoroglutaric anhydride or 4-cyanoperfluorobutyryl chloride gave pendant carboxyl or nitrile groups, respectively, on the triazine polymer which served as crosslinking sites. The formation of triazine crosslinks from the nitrile-pendant groups by trimerization with catalysts such as tetraphenyltin has given crosslinked polymers with tensile strengths of 1500 p.s.i., 70% elongation, and with thermal stability aproaching that of the original gum. The milling of catalysts and fillers with the polymer decreased the thermal stability in comparison with the original gum.     

Swelling behavior of physical and chemical DNA hydrogels

DNA hydrogels were prepared from aqueous solutions of double‐stranded DNA (about 2000 base pairs long) by physical and chemical means. Physical gels were obtained via denaturation–renaturation cycle of 5% aqueous DNA solutions between 25 and 90°C. Although physical DNA gels exhibit a high modulus of elasticity, the crosslinks holding the DNA network together are destroyed during the expansion of gels in water or in dilute salt solutions. It was shown that these gels can be used for the controlled release of DNA in aqueous media. Chemical DNA gels formed using ethylene glycol diglycidyl ether crosslinker are stable in water with a wide range of swelling ratios that could be adjusted by the amount of DNA at the gel preparation. Swelling behavior of chemical DNA gels in acetone/water mixtures as well as in aqueous salt solutions is very similar to that of synthetic polyelectrolyte hydrogels. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Brillouin scattering study of gelatin gel using a double passed Fabry-Perot spectrometer

The Brillouin spectrum of gelatin gels has been recorded at room temperature as a function of concentration of gelatin, using a double passed Fabry-Perot spectrometer. It was found that both the Brillouin shifts and line-widths increase linearly with concentration. In the case of the line-widths it was shown that the rate of increase was sensitive to the thermal history of the gels, being greater for gels cooled slowly. Explanation of the results in terms of a relaxation phenomenon is improbable. The observations can however be explained in terms of the structure of the gel. Expressing this structure as a number density of crosslinks, which is dependent on the concentration of gelatin, the increase in Brillouin shift is consistent with the change in bulk properties. The increase in line-width is shown to be caused by the crosslinks scattering the sound waves in the gel.     

Poly(vinyl alcohol) hydrogels. Formation by electron beam irradiation of aqueous solutions and subsequent crystallization

Aqueous solutions of poly(vinyl alcohol) were submitted to varying doses of electron beam irradiation. By modification of the classical Flory-Huggins equations appropriate to the initial state of solution of the polymer, the molecular weight between crosslinks, M_c, was calculated as a function of radiation dose, initial polymer concentration, and temperature. Following crosslinking in the solution state, crystallization was induced by dehydrating the network at temperatures above 90°C. Following dehydration, the polymer network was reequilibrated with water and its tensile properties compared with identically prepared hydrogels not subjected to crystallization by dehydration. Greatly enhanced values of ultimate tensile strength and resistance to tear result from the treatment producing crystallization, compared with those of the crosslinked but not previously dehydrated gels.     

Viscosity of ionomer gels

We study the viscosity behaviour of metal sulphonate- (or metal carboxylate-) containing ionomers in solution with non-polar solvents; at relatively low polymer concentration these ionomer solutions show an unusually large thickening behaviour, due to the association of the metallic groups. By regarding the ionomer system as a polymer solution with transient crosslinks, an expression is derived from the viscosity of the sulphonated polymer in terms of the viscosity of the corresponding non-sulphonated polymer and the average number of crosslinks associated with any chain.     

Using light to control the interactions between self-rotating assemblies of active gels

Polymer gels undergoing the oscillating Belousov-Zhabotinsky (BZ) reaction exhibit an autonomous, periodic swelling and deswelling, where the mechanical oscillations are driven by the chemical reaction within the polymer network. Using computer simulations, we show that these BZ gels can undergo a form of auto-chemotaxis, enabling the gels to spontaneously move in response to self-generated chemical gradients. Focusing on four millimeter-sized pieces of these BZ gels, we show that the pieces can organize into self-rotating clusters that resemble a moving pinwheel or gear. By analyzing the factors that promote the formation of a single self-rotating cluster, we attempt to design systems of multiple, interacting gears. We show that light, which suppresses the oscillations of the gels, can be harnessed to promote the formation of two self-rotating clusters. These studies point to a novel form of photo-chemo-mechanical transduction, where light is utilized to control the conversion of chemical and mechanical energy in the system. Moreover, the interaction between the BZ gel gears reveals a new form of entrainment between these moving units. Namely, their coordinated motion is achieved through chemical coupling or communication, rather than a mechanical coupling. These findings can lead to the formation of chemically “communicating” devices that can be programmed to perform autonomous work through the use of light.     

Die vernetzung von natur- und synthesekautschuk durch schwefel,die vulkanisation als eine zwischenstoffreaktion

In this communication a summary of results on kinetical investigations of the unaccelerated and accelerated vulcanization of natural and synthetic rubber is presented. It is outlined, that the vulcanization processes are complex reactions with respect to sulfur. In a first reaction step cross links with the constitution of an alkyl alkenyl polysulfide are built up. In a second reaction step, which is a consecutive reaction with respect to sulfur, these bridge bonds are degraded, and di- and monosulfidic crosslinks are produced. At the same time cyclic, mostly five membered thioethers along the main polymer chain are formed. In the course of extensive kinetical investigations the sulfur decrease and the formation and degradation of polysulfidic bound sulfur are measured, whereby the reaction of the crosslinks with LiAl-hydride and triphenylphosphin respectively gives the background of the quantitative analysis. The quantitative explanation of the analytical results was performed (i) by means of an empirical formula and (ii) by a strong treatment of the fundamental kinetics of a reaction in which an intermediate compound (the polysulfidic bound sulfur) is involved. It is shown that in special cases (sulfur decrease of first order) an exact calculation and a proof of the sulfur balance and moreover the determination of the sulfur content as well as the concentration of crosslinks produced by a given amount of sulfur can be achieved. It is shown that the sulfur content of the bridge bonds depends on the chemical constitution of the polymer but not on the nature of the accelerating system. An accelerator chiefly governs the special kinetics of the vulcanization reaction.