A molecular thermodynamic model for the swelling of thermo-sensitive hydrogels

A new molecular thermodynamic model for describing the swelling behavior of thermo-sensitive hydrogels was developed. The model consists of two terms. One is the contribution of the mixing of hydrogel network and water, which is dependent on the local polymer concentration and the interaction between polymer segment and solvent. A closed packed lattice model for polymer solution developed by Yang et al. was adopted for this term. The other is the elastic contribution derived from the network elasticity, which is dependent on the cross-linking degree of gel network. The elastic Gibbs energy model based on the Gaussian chain model developed by Flory was adopted. The model equation has two parameters. One is an energy parameter ? reflecting the interaction between water and gel network, the other is a size parameter V^* that represents the cross-linking degree of the hydrogel. When the energy parameter ? is expressed as a quadratic of inverse temperature, this model can describe the swelling equilibrium behavior of neutral thermo-sensitive hydrogels quite well. The influences of model parameters were discussed in details. The experimental swelling curves of two kinds of polyacrylamide-based gels were correlated and good agreement was obtained.     

Dual cross-linked networks hydrogels with unique swelling behavior and high mechanical strength: based on silica nanoparticle and hydrophobic association

A series of physically cross-linked hydrogels composed poly(acrylic acid) and octylphenol polyoxyethylene acrylate with high mechanical strength are reported here with dual cross-linked networks that formed by silica nanoparticles (SNs) and hydrophobic association micro-domains (HAMDs). Acrylic acid (AA) and octylphenol polyoxyethylene acrylate with 10 ethoxyl units (OP-10-AC) as basic monomers in situ graft from the SNs surface to build poly(acrylic acid) hydrophilic backbone chains with randomly distributed OP-10-AC hydrophobic side chains. The entanglements among grafted backbone polymer chains and hydrophobic branch architecture lead to the SNs and HAMDs play the role of physical cross-links for the hydrogels network structure. The rheological behavior and polymer concentration for gelation process are measured to examine the critical gelation conditions. The correlation of the polymer dual cross-linked networks with hydrogels swelling behavior, gel-to-sol phase transition, and mechanical strength are addressed, and the results imply that the unique dual cross-linking networks contribute the hydrogels distinctive swelling behavior and excellent tensile strength. The effects of SNs content, molecular weight of polymer backbone, and temperature on hydrogels properties are studied, and the results indicate that the physical hydrogel network integrity is depended on the SNs and HAMDs concentration.     

Polyethylene networks crosslinked in solution: Preparation,elastic behavior,and oriented crystallization. I. Crosslinking in solution

A study has been made of the crosslinking of linear polyethylene in solution. Networks containing a low number of trapped entanglements and elastically ineffective chain ends were prepared by crosslinking high molecular weight linear polyethylene in 1,2,4-trichlorobenzene solutions with dicumyl peroxide at 120°C. No syneresis was observed during crosslinking except at high peroxide concentrations. The networks were characterized by swelling measurements, infrared spectroscopy, and differential scanning calorimetry. The crosslinking efficiency, calculated from swelling, was found to be proportional to the square of the polymer volume fraction. The proportionality constant was 0.8, indicating close to unit efficiency for undiluted polymer. Chemical modification of the polyethylene chains by attachment of peroxide and solvent fragments was of the order of one foreign unit per elastically active network chain, depending on peroxide and polymer concentration. Sol–gel analysis indicated that no chain scission occurred. These results are shown to be consistent with a “cage” mechanism for crosslinking. The possible topological consequence of this mechanism, preferential crosslinking of entanglements, is discussed. The concentration of trapped entanglements was also found to be proportional to the square of the polymer volume fraction. The proportionality constant corresponds to a molecular weight between entanglements of 4000 for the undiluted polymer, which is close to the value of 4200 found for networks prepared from the undiluted polymer. Since the results obtained are based mainly on the use of the swelling equation, different aspects of the applicability of this equation for the evaluation of the crosslinking process are discussed. As regards the reference dimensions, which should be known for a quantitative application of the elastic theory, the results strongly support the use of the dimensions of the network chains after completion of crosslinking.     

Modification of cellulose nanocrystal-reinforced composite hydrogels: effects of co-crosslinked and drying treatment

The synthesis platform of composite hydrogels containing rigid reinforcing filler cellulose nanocrystals (CNCs) and polymer matrix polyacrylamide (PAM) has been proposed (Yang et al. in Cellulose 20:227–237, 2013). The features of CNCs as multifunctional crosslinkers and flexible polymer chain entanglements contributed to the unique arrangement of CNC/PAM clusters with reversible network structures. In this article, the chemical crosslinking agent N,N′-methylene-bisacrylamide (BIS) was added to obtain the dual crosslinked networks, and the mechanical properties of the resulting co-crosslinked hydrogels were examined by tailoring the CNC and BIS concentrations. The results indicated that the homogeneous dispersion of CNCs throughout the polymer matrix was disturbed in the presence of BIS, and the covalent crosslinkers led to weakness and brittleness of the hydrogels. Some new entanglements within the networks were formed after a simple drying treatment, which was verified by the greater tensile strength compared with the as-prepared ones. The mechanism for the formation of these new entanglements was ascribed to the irreversible rearrangement of the CNC/PAM network structure, whereas for co-crosslinked hydrogels no strength increment was observed after the drying treatment.     

Shear and extensional rheology of entangled polymer melts: Similarities and differences

This work extends our previous understanding concerning the nonlinear responses of entangled polymer solutions and melts to large external deformation in both simple shear and uniaxial extension. Many similarities have recently been identified for both step strain and startup continuous deformation, including elastic yielding, i.e., chain disentanglement after cessation of shear or extension, and emergence of a yield point during startup deformation that involves a deformation rate in excess of the dominant molecular relaxation rate. At a sufficiently high constant Hencky rate, uniaxial extension of an entangled melt is known to produce window-glass-like rupture. The present study provides evidence against the speculation that chain entanglements tie up into "dead knots" in constant-rate extension because of the exponentially growing chain stretching with time. In particular, it is shown that even Instron-style tensile stretching, i.e., extending a specimen by applying a constant velocity on both ends, results in rupture. Yet, in the same rate range, the same entangled melt only yields in simple shear, and the resulting shear banding is clearly not a characteristic of rupture. Thus, we conclude that chain entanglements respond to simple shear in the manner of yielding whereas uniaxial extension is rather effective in causing some entanglements to lock up, making it impossible for the entanglement network to yield at high rates.     

高分子熔体和浓溶液流变性能的研究

基于多重缠结网络结构模型和高分子链上缠结点在流动中可进行动态解缠和再缠结的多重蠕动机理,用统计力学和动力学相结合的方法,分别计算出了缠结链组的末端距分布函数;处于缠结状态下高分子链构象统计分布函数;受力下聚合物熔体粘弹性形变自由能和解除外力下高分子挤出体可回复性粘弹性形变自由能,提出了高分子挤出体可回复形变的粘弹性分子理论。推导出的高分子熔体的回忆函数、简单剪切流下的本构方程和物料函数,并采用一种新的方法测定出物料的四种参数： η0、 GN0、 n′和 a。对于高分子挤出体,可回复性粘弹性形变由快速弹性形变和慢速粘弹性形变两者组成,当把两种形变量的复合结构参数－分子链的反式构象分数引入两种形变自由能表达式后,就从理论上得到了可回复形变量同挤出胀大比间的定量表达式,从而建立起一个具有分子链结构参数的新的挤出胀大比方程,可回复形变量同挤出条件（温度、挤出速率和量以及口模长径比不同的挤出机）和树脂结构特征（分子量及分布）的关系式以及在特殊情形下的简化表达式,并用几种高分子熔融体系的挤出胀大比和可回复性形变量的实验数据对理论进行验证,理论方程同实验数据较好的符合。     

Swelling of N-isopropyl acrylamide hydrogels in aqueous solutions of sodium chloride

Hydrogels are three-dimensional networks of hydrophilic polymer chains. Hydrogels can absorb/desorb water and hydrophilic solutes. This behavior is called swelling/shrinking, as it is accompanied by a volume change. The amounts of absorbed substances depend on the structure of the hydrogel and the composition of the coexisting liquid phase. This paper deals with experimental investigations of the swelling behavior of nonionic, chemically crosslinked, synthetic hydrogels of N-isopropyl acrylamide. The swelling equilibrium of some hydrogels in aqueous solutions of sodium chloride was investigated at 298 K. The experimental results are presented, discussed and correlated/predicted with a thermodynamic model which combines an expression for the Gibbs energy of a liquid phase with an expression for the Helmholtz energy of an elastic network.     

Stochastic entangled chain dynamics of dense polymer solutions

We propose an adjustable-parameter-free, entangled chain dynamics model of dense polymer solutions. The model includes the self-consistent dynamics of molecular chains and solvent by describing the former via coarse-grained polymer dynamics that incorporate hydrodynamic interaction effects, and the latter via the forced Stokes equation. Real chain elasticity is modeled via the inclusion of a Pincus regime in the polymer's force-extension curve. Excluded volume effects are taken into account via the combined action of coarse-grained intermolecular potentials and explicit geometric tracking of chain entanglements. We demonstrate that entanglements are responsible for a new (compared to phantom chain dynamics), slow relaxation mode whose characteristic time scale agrees very well with experiment. Similarly good agreement between theory and experiment is also obtained for the equilibrium chain size. We develop methods for the solution of the model in periodic flow domains and apply them to the computation of entangled polymer solutions in equilibrium. We show that the number of entanglements Π agrees well with the number of entanglements expected on the basis of tube theory, satisfactorily reproducing the latter's scaling of Π with the polymer volume fraction φ. Our model predicts diminishing chain size with concentration, thus vindicating Flory's suggestion of excluded volume effects screening in dense solutions. The predicted scaling of chain size with φ is consistent with the heuristic, Flory theory based value.     

高分子水凝胶在医学领域的研究进展

高分子水凝胶是具有三维网络结构的一种新型材料,吸水溶胀后质地柔软,与生物体组织相似,生物相容性和生物可降解性良好,具有一定的力学性能,因此在医学领域具有重要的应用。本文对高分子水凝胶在医学领域的研究热点进行了归纳总结,并重点阐述了高分子水凝胶在药物输送、组织工程支架、伤口敷料和生物传感器等医学领域应用的最新研究进展,并对其未来发展趋势进行了展望。     

Energy storage in polymer chains under stress

The local conformation and storage of energy in individual polymer chains during a deformation of a bulk polymer sample are examined by the computer simulation of a relatively simple model. It is shown that as the interaction between the chain atoms and surrounding medium increases, rotational angle motion is suppressed during the deformation, and large amounts of energy are stored in backbone bond angle and bond length distortions. The relationship of this phenomena to T_g and the implications for chain relaxation are discussed.     

A highly coarse-grained model to simulate entangled polymer melts

We introduce a highly coarse-grained model to simulate the entangled polymer melts. In this model, a polymer chain is taken as a single coarse-grained particle, and the creation and annihilation of entanglements are regarded as stochastic events in proper time intervals according to certain rules and possibilities. We build the relationship between the probability of appearance of an entanglement between any pair of neighboring chains at a given time interval and the rate of variation of entanglements which describes the concurrence of birth and death of entanglements. The probability of disappearance of entanglements is tuned to keep the total entanglement number around the target value. This useful model can reflect many characteristics of entanglements and macroscopic properties of polymer melts. As an illustration, we apply this model to simulate the polyethylene melt of C(1000)H(2002) at 450 K and further validate this model by comparing to experimental data and other simulation results.     

Mechanical structures in polymer melts. II. Roles of entanglements in viscosity and elastic turbulence

Current theories of polymer flow processes often sacrifice realistic molecular models for simplicity of their mathematical equations. An analysis of what might happen to molecules of more realistic sizes and shapes under shear flow, shows the importance of the rapid Brownian motion of chain segments, the elastic deformations of polymer random coils, and the dissipation of this elastic random coil energy by the relatively slow slippage of the chains past each other at a few entanglements where steric hindrance causes long relaxation times. This makes the energy loss depend on the time at each local deformation, and not on the overall shear rate. At high shear rates this model leads to “cluster flow” and low loss cyclic deformations, rather than the high loss processes of steady-state shear. This model gives reasonable qualitative explanations for many anomalous flow properties, and it has predicted new effects that have since been observed.     

Model polyurethane elastomers prepared from noncrystallizable poly(propylene oxide) chains

Elastomers of controlled molecular structure were prepared from hydroxyl-terminated atactic poly(propylene oxide) (PPO) chains having number-average molecular weights M_n in the range 800–4360 g mole^?1. The chains were end-linked into noncrystallizable trifunctional networks using a specially prepared aromatic triisocyanate. The networks thus obtained were studied with regard to their stress–strain isotherms in the unswollen state, in elongation at 25°C, and with regard to their equilibrium swelling in benzene at 61°C. Values of the modulus in the limit at high deformation were in good agreement with corresponding results previously obtained on networks of poly(dimethylsiloxane) (PDMS). This is of considerable importance since use of the widely used “plateau modulus” as a measure of interchain entangling would suggest that the networks of PPO would have a much higher density of such entanglements than would the corresponding networks of PDMS. The close similarity between the moduli of the two types of networks therefore argues against the idea that such entanglements make large contributions to the equilibrium elastomeric properties of a polymer network. These values of the high deformation modulus are also in good agreement with recent molecular theories as applied to the nonaffine deformation of a “phantom” network. The values of the low deformation modulus were considerably smaller than the values predicted for an affine deformation, however, suggesting that the junction points were not firmly embedded in the network structure. This is presumably due to the relatively low degree of chain-junction entangling in the case of relatively short network chains. The swelling equilibrium results were in very good agreement with the new theory of network swelling developed by Flory.     

电场敏感的智能性水凝胶*

电场敏感水凝胶是一类在电刺激下可以溶胀、收缩或弯曲的智能性水凝胶,其主要特点是可以将电能转化为机械能。本文对近年来已见报道的电场敏感水凝胶的研究进行了较为详细的综述。同时,对电场敏感水凝胶的响应机理、影响水凝胶响应性的因素以及其在能量转换装置、人工肌肉等方面的应用也作了相应的介绍。     

Relating fracture energy to entanglements at partially miscible polymer interfaces

A new model has been developed to calculate the areal chain density of entanglements (Σ_eff) at partially miscible polymer–polymer interfaces. The model for Σ_eff is based on a stochastic approach that considers the miscibility of the system. The values agree between Σ_eff calculated from the model and literature values for the reinforced interfaces. Using Σ_eff calculated from the model, the interfacial width, and the average distance between entanglements, an equation for the fracture energy of nonreinforced polymer interfaces is proposed. This equation is used to model the transition from chain pullout to crazing. As a function of system miscibility, the model for Σ_eff also accurately predicts a maximum in mode I fracture energy (G_c) as a result of the transition from gradient‐driven to miscibility‐limited interdiffusion, which is observed experimentally. As Σ_eff increases, the fracture energy increases accordingly. Compared with a recent model developed by Brown, the new model correctly predicts a reduced G_c (attributed to chain pullout) when the interfacial width is less than the average distance between entanglements. Theoretical predictions of the change in fracture energy with respect to interfacial width agree with the experimental measurements. Finally, it is postulated that the use of a miscibility criterion for G_c may reveal the universal nature of the pullout to crazing transition. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2292–2302, 2002     

A Sequential Debonding Fracture Model for Hydrogen-Bonded Hydrogels

Hydrogen bonds are known to play an important role in prescribing the mechanical performance of certain hydrogels such as polyether-based polyurethanes. The quantitative contribution of hydrogen bonds to the toughness of polymer networks, however, has not been elucidated to date. Here, a new physical model is developed to predict the threshold fracture energies of hydrogels physically crosslinked via hydrogen bonds. The model is based on consecutive and sequential dissociation of hydrogen-bonded crosslinks during crack propagation. It is proposed that the scission of hydrogen bonds during crack propagation allows polymer strands in the deformation zone to partially relax and release stored elastic energy. The summation of these partial chain relaxations leads to amplified threshold fracture energies which are 10–45 times larger than those predicted by the classical Lake–Thomas theory. Experiments were performed on a hydrophilic polyurethane hydrogel where urea additions were used to control the density of hydrogen bonds. The measured fracture energies were in good agreement with the calculated values. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1287–1293     

New neutral and ionic poly(ethylene oxide) networks by radical polymerization

New opportunities resulting from a turn to radical polymerization in the synthesis of poly(ethylene oxide) (PEO) networks are discussed and exemplified. Several series of such networks have been prepared by radical homo‐ and copolymerization in aqueous media of “macromonomers”, i.e. partly methacrylated poly(ethylene glycol) (PEG) of varied molecular weight (MW ≅ 2000‐12000) and functionality (f_n ≅ 1.25‐1.8). This family of gels as a whole has the volume swelling degree Q in the range of 10 to 200 ml/ml. The hydrogels are characterized by means of Q, elastic modulus, swelling pressure, and with the use of some probes. The swelling behaviour of neutral hydrogels of this kind is briefly resumed. The multifunctional junctions formed in the propagation reaction of methacrylate end groups determine their main peculiarity. Anomalous elastic behaviour of the swollen networks prepared at high concentration of polymer has been observed and attributed to the network chains stretching of the same nature as in polymer stars or brushes. The junctions' functionality (F ≈ 20‐300) is evaluated from these data as well as from MW of the soluble models of network junctions. The PEO networks with charged units in junctions have been obtained by copolymerization of macromonomers with some ionic (meth)acrylic monomers. These gels display all the polyelectrolyte features, e.g. enhanced Q values in water (up to 50‐70) and, contrary to neutral PEO gels, the strong dependence on salt content. However, the osmotic contribution of mobile ions into swelling is shown to be low due to localization of charges in the junctions. The hydrogels that combine PEO and polymethacrylic acid chains capable of interpolymer complexation have been prepared and studied. They show much higher swelling in pure water (Q up to 200), strong deswelling by NaCl, and very sharp drop in swelling (ca. two order in Q) at pH ≈ 4.5‐5.5 due to complexation.     

Ionizable hydrogels of new type based on poly(ethylene glycol) phosphates

Partly charged poly(ethylene oxide) networks have been prepared by the cure reaction of multifunctional poly(ethylene glycol) phosphate precursors with the diglycidyl ether of triethylene glycol as a crosslinking agent. These new hydrogels display all the features of swelling behaviour characteristics of polyelectrolyte networks. The degree of volume swelling of the hydrogels varies from 16–95 (in distilled water) to 11–45 (in 0.1 M sodium chloride solution) and 7–20 ml/ml (in 0.52 M potassium sulfate as a Θ-solvent). Average chain length, ionic group content, and structure of gels are evaluated from the swelling data.The gelation point occurs at much higher crosslinking ratios and overall P-OH groups conversion than those predicted from the precursor functionality. The role of possible side reactions and some kinetic reasons for the ‘delayed’ gelation are discussed.