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Relatively few polymer gels are known to show volume phase transition where the gels undergo an abrupt change in the degree of swelling by passing through a three-phase equilibrium. Characteristic for such transition is the existence of van der Waals (vdW) loop on the dependence of solvent chemical potential versus polymer concentration. For the χ-induced transition, the existence of vdW loop is determined by the concentration dependence of the interaction function. It is shown that expansive mechanical strains can assist in development of the vdW loop. Systems characterized by continuous change of the degree of swelling transform upon such strain into ones where the degree of swelling changes much and abruptly. Also, expansive modes of strain can make the transition wider and more robust in gels where transition is already observed under free swelling condition. The possibility to induce the volume phase transition by external stresses can be utilized for finding other stimuli sensitive gels, strengthening of gel response, and in modeling of properties of gel constructs by Finite Element Method.

The history of volume phase transition of responsive gels from its theoretical prediction to experimental discovery was described and the major role of mixing Gibbs energy function in theoretical models was stressed. For detailed analysis and fine tuning of the volume phase transition, the generalized Flory–Huggins model with concentration and temperature dependent interaction function coupled with Maxwell construction as a tool is very suitable. Application of expansive stresses can uncover the potential of various swelling gels for volume phase transition. Experimentally, the abrupt, equilibrium-controlled phase transition is often hard to achieve due to passage of gel through states of mechanical instability and slow relaxation processes in macroscopic objects.

Methacrylate hydrogels are unique synthetic materials known for their capability to serve as multifunctional eye-implants, practically without duration and compatibility limits. We introduce a novel strategy consisting in toughening of a macroporous microstructure using the interpenetrating network concept, which improves the commonly preferred preparation way of hydrogel based on photopolymerization. The method proceeds at ambient temperature and can be used in situ. Scattering of irradiation generated by the microstructure considerably enhances the polymerization rate. This acceleration effect was quantified by careful optical analysis and is important for in situ applications. Crosslinked IPN hydrogels of 2-hydroxyethyl methacrylate (HEMA) as the first network and glycerol methacrylate (GMA) as the second network based on this new design were studied and compared with IPNs prepared from non-porous PHEMA gels. Surprisingly, a relatively high swelling capacity was achieved with this new design and the Young’s modulus increased from 4 kPa for parent PHEMA network to 380 kPa for the PHEMA–PGMA IPNs and to 980 kPa for the PHEMA–PHEMA IPNs. The IPN hydrogels were strong and resisted mechanical load. The reinforcement of the mechanically poor macroporous network by swelling in another hydrophilic monomer and subsequent polymerization presents a new concept of preparation of strong microstructured IPNs (MIPNs).

The aza-Michael reaction is the addition of an amine to an electron deficient C=C double bond. This reaction is also used in the synthesis of precursors of polymeric networks. In this study, we paid attention to the kinetics and mechanism of the aza-Michael addition of cyclohexylamine (CHXA) to diethyl maleate (DEM) performed as a solvent-free, catalyst-free reaction and to concurrent reactions. In situ Raman spectroscopy, NMR spectroscopy and gas chromatography/mass spectrometry have shown the occurrence of three simultaneous reactions: (i) the aza-Michael addition of CHXA to DEM leading to diethyl 2-(cyclohexylamino)succinate, (ii) isomerization of DEM to diethyl fumarate (DEF), and (iii) the aza-Michael addition of CHXA to DEF formed by the reaction (ii). All of these reactions proceed with third order kinetics, first order in DEM or DEF and second order in CHXA. We propose a kinetic model that allows kinetic constants to be estimated. Furthermore, a numerical solution of the set of differential equations confirms the expected kinetic equations of reactions (i) and (ii) and gives values of rate constants comparable to the estimated ones. A DFT mechanistic study illustrates the structure of the reaction intermediates and transition states of all reactions and explains the contribution of the second amine molecule in the reaction mechanism.

Summary The majority of cross-linked systems of practical importance are manufactured from pre-prepared precursors which are characterized by distributions in the type and number of functional groups and molecular weights. For understanding of the effects of these distributions on network formation and properties and designing new systems, a theoretical insight is necessary that takes these distributions into account. The article lists some examples where the theoretical insight has helped optimize the cross-linking process and application properties.

Summary Swelling of cross-linked polymer constructs in which swelling of network is constrained by adhering rigid substrate or another cross-linked polymer (coatings, core-shell particles, and particulate composites of rigid filler in cross-linked matrix) is investigated. The equilibrium state was modeled analytically or by the Finite Element Method. A Gibbs energy function was selected which respects the finite extensibility of network chains. Also, the additive contribution to the Gibbs energy of swollen systems due to mixing was reformulated. The mixing of cross-linked polymer with solvent is considered without deforming the network chains, instead of mixing of solvent with an infinite uncross-linked polymer molecule.

In certain real situations, polymer networks cannot swell freely but are subjected to constraints by action of external forces or geometric restrictions. In this contribution, the restriction to swelling by adhesion of a cross-linked polymer film deposited on a rigid substrate is considered. Such constraints prevent swelling in plane, and the osmotic force concentrates on extension of network chains in the direction normal to the surface. We analyze existing rubber elasticity models taking into account finite extensibility of network chains and derive the elastic contribution to the chemical potential of the solvent in swollen networks. For the mixing contribution to the change of the Gibbs energy, it is more appropriate to consider the network containing cross-links, but with undeformed network chains than an un-cross-linked polymer of infinite degree of polymerization, because continuing formation of bonds within an infinite molecule further decreases the entropy. The additional term is proportional to ...

High-performance cross-linked polymeric materials are now prepared from preformed precursors typical by distributions of molecular weights, number and reactivities of functional groups, and specific architectures. This makes theoretical treatment of networks evolution and their final structure difficult. This paper describes kinetically controlled cross-linking of a precursor formed from polyfunctional cores by arm extension by which molecular weight distribution develops and new groups of different reactivity are formed. These precursors are then cross-linked with a polyfunctional cross-linker. If the precursor groups react independently, a random (binomial) distribution of reactive groups results and the gel point conversion and other network parameters are independent of the differences in reactivity the groups with the cross-linker. If the condition of random (binomial) distribution is not met (fixed numbers of groups or substitution effect in the precursor molecules), this independence does not exist...

Swelling and dynamic mechanical behavior in a broad frequency range of homogeneous and porous poly(2-hydroxyethyl methacrylate) gels were studied in relation to their matrix structure, gel morphology and nature of the swelling medium. Very lightly cross-linked gels showed up relatively large mechanical losses caused by slow relaxation of hydrophobic physical associates still proceeding at frequency 10 −3 Hz. The macroporous gels of fused-spheres type morphology contained many dangling aggregates not bearing the stress which caused concentration of the stresses in the load bearing paths through the matrix. This is the reason for the sudden drop of storage modulus. In the α-relaxation region, the loss factors for homogeneous and porous gels merged into a single master curve. Reswelling of gels in a poor solvent caused a decrease of the swelling degree and shifted their α-relaxation to lower frequencies; reswelling in good solvent shifted the maximum of α-relaxation to higher frequencies.

Composite hydrogels consisting of nanofibrous bacterial cellulose (BC) embedded in a biocompatible polymeric matrix of various methacrylates were synthesized by UV polymerization using the ever-wet technique. The effect of monomer(s) type and ratio, system dilution at polymerization, monomer(s) hydrophilicity, crosslink density and cellulose/hydrogel ratio was investigated. The effect of BC reinforcement on equilibrium swelling depends on whether the neat gel swells more when brought into contact with water. The major improvement achieved by introduction of 1%2% BC concerns mechanical properties. Compared with neat gels, the storage shear modulus G' increased by a factor 10-20, and the loss part G? also rose significantly. The compression modulus ranged from 2 to 5.5 MPa for composites swollen to equilibrium (20-70 wt% water). The BC-hydrogel composites are considered for application in the tissue engineering area.

The swelling and rheological behaviour of hydrogels of morphology varying from non-porous to highly porous was investigated. The hydrogels were prepared by redox free radical copolymerization of 2-hydroxyethyl methacrylate with 0.1 to 5 mol% of di(ethylene glycol) dimethacrylate in the presence of water varying from 40 to 80 wt%. Various compositions led to clear, turbid or macroporous gels. The morphology of the gels was characterized using optical microscopy and cryoscan electron microscopy. The oscillatory shear and creep of swollen gels revealed that there was a pronounced difference between homogeneous or microheterogeneous and macroporous gels with communicating pores. The achievement of optimum conditions for the correct determination of shear modulus was also analysed. Copyright © 2011 Society of Chemical Industry

The effect of the presence of diluent during network formation on the gel-point conversion, αcrit, and on the equilibrium elastic modulus was studied using polyurethane networks from star-shaped polyols and a star-shaped triisocyanate. The dependence of αcrit on reciprocal concentration of functional groups, 1/c0, extrapolated well for 1/c0 0 to the value calculated corresponding to the ring-free system. The decrease of the equilibrium modulus and the concentration of elastically active network chains (EANC) in dependence on the volume fraction of polymerizable compounds (solids) was curved downwards and extrapolated to the limiting dilution of the system at which no gel was formed when the conversion of functional groups was 100%. Some samples exhibited phase separation in the form of macrosyneresis which affected . The continuous change of gel volume as a result of phase separation was obtained by solving an integral equation respecting the thermodynamic stability of the system. The change of the concentration of EANCs obtained from equilibrium modulus was translated into the intermolecular conversion calculated using the branching theory. The intermolecular conversion depended linearly on the shift of the gel-point conversion.

Three types of amino-functionalized calixarenes, i.e. the diaminocalix[4]arenes 5,17-diaminomethyl-25,26,27,28-tetrapropoxycalix[4]arene, 25,27-diaminoethoxy-26,28-dihydroxycalix[4]arene and 5,11,17,23-tetra-tert-butyl-25,27-diaminoethoxy-26,28-dihydroxycalix[4]arene, were synthesized and incorporated as comonomers into the backbones of aromatic polyimides. As a first step, polyimide precursors (poly(amic acid)s) were prepared by condensation reaction of diamine with dianhydride at the stoichiometric ratio. The diamine component was composed of synthesized diaminocalix[4]arene and commercial 4,4′-oxydianiline combined in various molar ratios. The dianhydride used was 4,4′-oxydiphthalic anhydride. The poly(amic acid)s were characterized using intrinsic viscosity measurements and their chemical composition was determined using 1H NMR spectroscopy. The precursors were then transformed into the polyimides using a thermal treatment. Thermal and dynamic mechanical behaviour, wide-angle X-ray diffraction and solubility of the resulting polyimide films in selected solvents were evaluated. The structure–property relationship of the novel types of synthesized polyimides is discussed in terms of the calixarene monomer used and the fraction of it incorporated into the polymer backbone. Copyright © 2010 Society of Chemical Industry

Bi-, tri-, and tetrafunctional star-shaped polyol precursors were cross-linked with a star-shaped triisocyanate and the effect of the presence of diluent during network formation on the gel-point conversion, αcrit, and on the equilibrium elastic modulus was studied. The dependence of αcrit on reciprocal concentration of functional groups, 1/c0, was fairly linear and extrapolated for 1/c0 → 0 well to the value calculated for the ring-free system using the branching theory. The decrease of the equilibrium modulus and of the associated concentration of elastically active network chains (EANC) in dependence on the volume fraction of polymerizable compounds (solids), memory factor ϕ20, was curved downward and extrapolated to the limiting dilution of the system at which no gel was formed at full conversion of functional groups. Some highly diluted samples exhibited phase separation in the form of macrosyneresis (expulsion of excess solvent as bulk phase) which affected the memory factor ϕ20. The situation was a...

Formation of highly-branched off-stoichiometric polymers from A x +B y monomers has been described theoretically using a statistical branching theory in which the irreversibility of bond formation was respected. The critical molar ratio (CMR) of A to B groups, i.e., such excess of groups of one type beyond which the system does not gel and only soluble highly-branched polymers are formed, was calculated. CMR depends on the functionality of the components and on the reactivity of functional groups involved in formation of bonds. For an increasing excess of A groups over B groups, soluble A-functional polymers of decreasing molecular weight are formed. When monofunctional monomer is added, the number-average molecular weights and degrees of polymerization as well as the number of functional groups per molecule remain relatively low even at the CMR when the gel point is reached. When one A group in the A3 monomer has higher reactivity than the remaining two and also in B2 where the reactivities of B groups are different, the value of CMR decreases from 2 towards 1.5, which is the limiting case. Important is that with increasing differences in reactivity the range of molecular weights can be extended and the polydispersity can be suppressed. Functional highly-branched polymers of reasonable molecular weight and polydispersity can be obtained, which in their application can compete with conventional hyperbranched polymers. The use of the information obtained in this description of crosslinking of the highly-branched off-stoichiometric polymers is outlined.

Changes in solvent activity and thermodynamic stability of a system undergoing crosslinking in the presence of a solvent are modeled by combination of branching theory with thermodynamics of polymer solutions and swelling of a crosslinked polymer. The system is considered as a quasiternary one in which the solvent, polydisperse soluble polymer (sol) and crosslinked network, respectively, represent the components. Crosslinking brings about growth of polymer structures but also changes in polymer-solvent interactions. The results are compared with onset of phase separation and changes in evaporation rates from films formed from a hydroxy-functional star oligomer and triisocyanate.

A growing group of polyfunctional precursors of polymer networks contains more than one branch point (cross-link). The bonds they extend lead either to another branch point or to a functional group. When such precursors are cross-linked, these branch points are gradually activated to reach the status of elastically active cross-links (EAC), i.e., such cross-links from which three or more bonds extend that have infinite continuation. The contribution of EAC's to the concentration of elastically active network chains (EANC) increases gradually with conversion of functional groups and depends on precursor architecture. Cross-linking of specially assembled precursors, functional dendrimers, and hyperbranched polymers is treated theoretically using the stochastic theory of branching processes based on generation of structures from units in different reaction states. Each bond extending from a branch point is considered as to whether the issuing subtree is finite or whether an infinite continuation exists. Seve...

Network formation from A x + B y precursors (A + B → A - B) with functionality and molecular weight distributions is described by the statistical theory of branching processes. Network formation is described in terms of sol and gel fractions, dangling chains, elastically active network chains (EANC), elastically active crosslinks, free chain ends and branch points partitioned between sol and gel. Various definitions of an EANC are considered. The general relations are applied to a special case of A1 + A2 + A3/B2 system. It is show how the size and weight fractions of dangling chains can be varied indepently by varying the functionality or molecular weight distributions. This is demonstrated experimentally analyzing corresponding polyetherurethane networks prepared from mixtures of polyoxypropylene polyols. The width of the main transition region correlates with the fraction of material in dangling chains. The dependence of the equilibrium shear modulus on the concentration of EANC's indicates much weaker intermolecular interactions for networks with many short dangling chains compared with systems having few longer dangling chains.

Novel aromatic azo-containing pH-sensitive hydrogels were synthesized by a “one-step” polymer−polymer reaction of two precursors. Precursor A was synthesized by copolymerization of a new aromatic azo-containing monomer, N-(1-aminohexyl)-4-[[4-(methacryloylamino)phenyl]azo]benzamide (MA-AZO-NH2), with N,N-dimethylacrylamide, N-tert-butylacrylamide, and acrylic acid. Precursor B is a copolymer similar to precursor A that contains N-methacryloylglycylglycine p-nitrophenyl ester (MA-GG-ONp) instead of MA-AZO-NH2. Aromatic azo bonds of MA-AZO-NH2 in the polymer and in hydrogels were degradable in the presence of fresh rat cecum contents. Control hydrogels were synthesized by cross-linking polymer precursor B with a low molecular weight aromatic azo-bond-containing diamine. Modeling of network structure for both novel and control hydrogels was accomplished using a statistical branching theory (theory of branching processes). The theoretical values and experimental data were compared. Conclusions were made for t...

Crosslinking is a very important process in coating film formation and the structure of the formed network determines the application properties of the coating film. In this review, general features of crosslinking and evaluation of network structure are described. Network formation theories and their applicability to special chemical systems are analyzed. The kinetics of network formation is usually controlled by chemical reactivity of functional groups. A transition to the regime controlled by segmental mobility is typical for film formation especially for ambient temperature drying. Formation of inhomogeneities in multicomponent systems determined by composition and group reactivities and/or by thermodynamic segregation may also play an important role. In the last section, special features of formation of crosslinked structured from designed precursors — polyfunctional molecules with designed backbone architectures, such as telechelic polymers, stars, combs, microgels, dendrimers, or hyperbranched polymers are reviewed. Introduction of precursors of special architecture allows us not only to modify the processing and material properties of the film, but also brings about new problems in explaining and modeling the formation and properties of coating films.

A series of thirteen polyurethanes were synthesiszed using blends of monofunctional, difunctional, and trifunctional poly(propylene oxides) reacted with 4,4'-diphenylmethane diisocyanate in a stoichiometric ratio. Experimental measurements were made of glass transition temperature, sol fraction, and dynamic mechanical properties. Computer simulations based on the theory of branching processes were made on the same systems. The glass transition temperature was correlated with the number average functionality of the components. Dynamic mechanical data was fitted to the Havriliak-Negami model. A correlation was demonstrated between the measured rubbery modulus and the concentration of elastically active network chains.

A mean-field theoretical approach was developed to model gelation of solutions of hydrophilic polymers with grafted peptide motifs capable of forming associates of coiled-coil type. The model addresses the competition between associates engaged in branching and cyclization. It results in relative concentrations of intra- and intermolecular associates in dependence on associate strength and motif concentration. The cyclization probability is derived from the model of equivalent Gaussian chain and takes into account all possible paths connecting the interacting motifs. Examination of the association-dissociation equilibria, controlled by the equilibrium constant for association taken as input information, determines the fractions of inter- and intramolecularly associated motifs. The gelation model is based on the statistical theory of branching processes and in combination with the cyclization model predicts the critical concentration delimiting the regions of gelled and liquid states of the system. A comparison between predictions of the model and experimental data available for aqueous solutions of poly[N-(2-hydroxypropyl)methacrylamide] grafted with oppositely charged pentaheptad peptides, CCE and CCK, indicates that the association constant of grafted motifs by four orders of magnitude lower than that of free motifs. It is predicted that at the critical concentration of each motif of about 6×10(-7) mol/cm(3), about half of motifs in associated state is engaged in intramolecular bonds.