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Polyaspartates are industrially important resins formed by reactions of N-substituted 2‑aminosuccinates (or, synonymously, N-substituted aspartates) with isocyanates. To obtain sufficiently crosslinked networks, applicable as coatings, bis-aspartates with two secondary NH groups and isocyanates with more than two NCO groups in their molecules must be used. There are two major problems connected with these preparations: (i) bis-aspartates which are mostly synthesized by the aza-Michael addition of NH2 groups of a suitable diamine to C=C bonds of a maleate ester almost always contain a certain amount of a fumarate ester (usually a few percent) as a result of the Z→E isomerization, unavoidably accompanying the addition; this deteriorates the quality of the resulting product; (ii) during the reaction of bis-aspartate with isocyanate, hydantoin rings are often formed to some extent in addition to the desirable three dimensional structures; the presence of these rings decreases the attainable network density. In this review, those properties, syntheses, and reactions of hydantoin are summarized which could be useful for finding conditions to avoid the formation of hydantoin rings. Fulltext of this article is available on the website of this Journal.

In situ1H NMR characterization of copolymerization reactions of various 2-oxazoline monomers at different molar ratios offers detailed insight into the build-up and composition of the polymer chains.

X-ray phase-contrast imaging (PCI) is sensitive to the phase shift of X-rays induced by the sample. This is advantageous for low X-ray absorption samples, such as polymers, biomaterials, tissues, and scaffolds. We show propagation-based phase-contrast X-ray computed tomography (CT) imaging [propagation-based imaging (PBI)] in a specific configuration with the wide-cone angle (>50°), polychromatic X-ray source, and a flat-panel detector. We demonstrate PBI on measurements with a polymer composite. The experiments show a tradeoff between high signal-to-noise ratio (SNR) acquisition at very large cone angles and low-SNR acquisition at large propagation distances. The degree of data quality improvement by phase retrieval increases for high propagation distances. We show the application of PBI on macroporous synthetic hydrogels, which represents an important type of material with a complex 3-D morphology in the field of polymer science. With the use of the above-described experimental configuration, it is possible to visualize the hydrogels and segment the structure of the sample in tomographic data. The segmented sample can be used for morphology characterization, such as the description of internal space or determination of specific surface area (SSA).

In the after-fire restoration of the buildings belonging to the culture heritage, the use of salvaged scorched wood elements is desirable but often leading to the long-term malodorous, irritating, ...

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.

Summary Hydrogelation triggered by host-guest anion recognition is difficult to achieve due to the challenging anion recognition in water and de novo gelator design. Herein, we report such a hydrogel system, based on bambus[6]uril anion receptor, BU. Albeit not a gelator per se, BU formed hydrogels selectively in the presence of iodides or perchlorates. Anion recognition by BU governed the hydrogelation, indicated by the NMR, ATR-IR, and single-crystal X-ray. Gelation mechanism and lamellar nature of the network structure were elucidated by SAXS, cryoSEM, and holotomography methods. Rheological characterization revealed the hydrogels were stable and relatively strong. In physiological saline solutions, the hydrogels released cationic cargo by cation metathesis, leading to a gel-to-gel transformation. Choline derivatives, as model drugs, exhibited slow release from the hydrogels. Our simple hydrogel system may inspire the design of smart materials based on host-guest anion recognition, where the release or sequestration of specific charged species are desirable.

Open macroporosity is crucial for scaffolds in tissue engineering. Porogen-templating method is an attractive approach for fabrication of macroporous hydrogels, however, the effect of shape and amount of template particles on imprinted structure has not yet been quantitatively established. We present a mathematical model for simulating the formation of paths percolating through distributed cubical particles as a function of the filling volume. The model was used to select the fraction of NaCl particles as templates for preparation of hydrogels with communicating pores. Hydrogels were prepared from 2-hydroxyethyl methacrylate (HEMA) copolymerized with 2-ethoxyethyl methacrylate (EOEMA), [2-methacryloyloxy)ethyl]trimethylammonium chloride (MOETACl) or ionizable methacrylic acid (MANa) to modulate swelling, surface and mechanical properties of gels. Micro-CT analysis of swollen samples proved a highly-interconnected pore structure. Charged hydrogels swelled more and their apparent elastic modulus G′ was below 1 kPa. For PHEMA and P(HEMA/EOEMA) hydrogels, G′ was 5 and 80 kPa, respectively. Within two-week in vitro studies, MG63 osteoblasts proliferated fastest on P(HEMA/EOEMA) showing the lowest swelling and the highest elastic modulus, whereas cell growth was impaired on positively charged P(HEMA/MOETACl). The mathematical simulation of cubical particle packing in hydrogels and micro-CT data in swollen state provided evidence of an extensive void communication in 3D.

(1) Background: Macroporous hydrogel scaffolds based on poly [N-(2-hydroxypropyl) methacrylamide] are one of the widely studied biocompatible materials for tissue reparation and regeneration. This study investigated the morphological changes during hydrogel characterization which can significantly influence their future application. (2) Methods: Three types of macroporous soft hydrogels differing in pore size were prepared. The macroporosity was achieved by the addition of sacrificial template particles of sodium chloride of various sizes (0&ndash, 30, 30&ndash, 50, and 50&ndash, 90 &micro, m) to the polymerizing mixture. The 3D structure of the hydrogels was then investigated by scanning electron microscopy (SEM) and laser scanning confocal microscopy (LSCM). The SEM was performed with specimens rapidly frozen to various temperatures, while non-frozen gels were visualized with LSCM. (3 and 4) Results and Conclusion: In comparison to LSCM, the SEM images revealed a significant alteration in the mean pore size and appearance of newly formed multiple connections between the pores, depending on the freezing conditions. Additionally, after freezing for SEM, the gel matrix between the pores and the fine pores collapsed. LSCM visualization aided the understanding of the dynamics of pore generation using sodium chloride, providing the direct observation of hydrogel scaffolds with the growing cells. Moreover, the reconstructed confocal z-stacks were a promising tool to quantify the swollen hydrogel volume reconstruction which is not possible with SEM.

The exact knowledge of hydrogel microstructure, mainly its pore topology, is a key issue in hydrogel engineering. For visualization of the swollen hydrogels, the cryogenic or high vacuum scanning electron microscopies (cryo-SEM or HVSEM) are frequently used while the possibility of artifact-biased images is frequently underestimated. The major cause of artifacts is the formation of ice crystals upon freezing of the hydrated gel. Some porous hydrogels can be visualized with SEM without the danger of artifacts because the growing crystals are accommodated within already existing primary pores of the gel. In some non-porous hydrogels the secondary pores will also not be formed due to rigid network structure of gels that counteracts the crystal nucleation and growth. We have tested the limits of true reproduction of the hydrogel morphology imposed by the swelling degree and mechanical strength of gels by investigating a series of methacrylate hydrogels made by crosslinking polymerization of glycerol monomethacrylate and 2-hydroxyethyl methacrylate including their interpenetrating networks. The hydrogel morphology was studied using cryo-SEM, HVSEM, environmental scanning electron microscopy (ESEM), laser scanning confocal microscopy (LSCM) and classical wide-field light microscopy (LM). The cryo-SEM and HVSEM yielded artifact-free micrographs for limited range of non-porous hydrogels and for macroporous gels. A true non-porous structure was observed free of artifacts only for hydrogels exhibiting relatively low swelling and high elastic modulus above 0.5 MPa, whereas for highly swollen and/or mechanically weak hydrogels the cryo-SEM/HVSEM experiments resulted in secondary porosity. In this contribution we present several cases of severe artifact formation in PHEMA and PGMA hydrogels during their visualization by cryo-SEM and HVSEM. We also put forward empirical correlation between hydrogel morphological and mechanical parameters and the occurrence and intensity of artifacts.

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.

The macroporous synthetic poly(2-hydroxyethyl methacrylate) (pHEMA) hydrogels as 3D cellular scaffolds with specific internal morphology, so called dual pore size, were designed and studied. The morphological microstructure of hydrogels was characterized in the gel swollen state and the susceptibility of gels for stem cells was evaluated. The effect of specific chemical groups covalently bound in the hydrogel network by copolymerization on cell adhesion and growth, followed by effect of laminin coating were investigated. The evaluated gels contained either carboxyl groups of the methacrylic acid or quaternary ammonium groups brought by polymerizable ammonium salt or their combinations. The morphology of swollen gel was visualized using the laser scanning confocal microscopy. All hydrogels had very similar porous structures - their matrices contained large pores (up to 102 μm) surrounded with gel walls with small pores (100 μm). The total pore volume in hydrogels swollen in buffer solution ranged between 69 and 86 vol%. Prior to the seeding of the mouse embryonal stem cells, the gels were coated with laminin. The hydrogel with quaternary ammonium groups (with or without laminin) stimulated the cell growth the most. The laminin coating lead to a significant and quaternary ammonium groups. The gel chemical modification influenced also the topology of cell coverage that ranged from individual cell clusters to well dispersed multi cellular structures. Findings in this study point out the laser scanning confocal microscopy as an irreplaceable method for a precise and quick assessment of the hydrogel morphology. In addition, these findings help to optimize the chemical composition of the hydrogel scaffold through the combination of chemical and biological factors leading to intensive cell attachment and proliferation.

Summary Objectives To distinguish the layers of the vocal fold at the submacroscopic level and determine their boundaries, thereby creating a basis for the construction of a digital 3D model of the human vocal folds. Study design The submacroscopic delineation of individual layers of fixed vocal ligaments based on their structural differences. Methods Following tasks were performed: (1) Submicroscopic dissection of the vocal folds fixed in a solution with a low concentration of fixation substance (in this case, the muscular parts of the vocal folds were removed); (2) Using the CT and micro-MRI methods, we determined the position of the dense parts of the vocal folds; and (3) Using a modified plastination method, we preserved macroscopically natural appearance of all ligamentous and muscular layers. Results The vocal ligament is composed of several volumes of connective tissue. It is surrounded by layers of fibrous material permeated by liquid. Individual fibers stretch all the way to the fibrous casing (fascia) of the vocal muscle. The vocal fold layer surrounding the ligament externally has a stratified character. Conclusions According to our findings, we infer that this ligament is a complex of several fibrous bundles which are surrounded by a thin layer of connective tissue. Below the surface of epithelium of the vocal fold run several separate bands which are closely adjacent to it. Therefore, we propose using the term ligamentous complex involving closely adjacent structures, instead of the vocal ligament only. We feel that it better reflects the functional and structural character of the whole formation.

Modification of combustible materials, against easy ignition and combustion is an effort of many researchers. Modification of traditional materials, wood and textiles from natural mate-does it for their natural character quite complicated. Despite this complexity, it is one of the ways within the fire prevention.

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.

Degradable poly(2-hydroxyethyl methacrylate) hydrogels were prepared from a linear copolymer (Mw = 49 kDa) of 2-hydroxyethyl methacrylate (HEMA), 2-(acethylthio)ethyl methacrylate (ATEMA), and zwitterionic 2-methacryloyloxyethyl phosphorylcholine (MPC). The deprotection of ATEMA thiol groups by triethylamine followed by their gentle oxidation with 2,2'-dithiodipyridine resulted in the formation of reductively degradable polymers with disulfide bridges. Finally, a hydrogel 3D structure with an oriented porosity was obtained by gelation of the polymer in the presence of needle-like sodium acetate crystals. The pore diameter and porosity of resulting poly(2-hydroxyethyl methacrylate-co-2-(acethylthio)ethyl methacrylate-co-2-methacryloyloxyethyl phosphorylcholine) [P(HEMA-ATEMA-MPC)] hydrogels varied between 59 and 65 μm and between 70 and 79.6 vol % according to Hg porosimetry, and complete degradation of these materials was reached in 86 days in 0.33 mmol solution of l-cysteine/L in phosphate buffer. The cross-linked P(HEMA-ATEMA-MPC) hydrogels were evaluated as a possible support for human mesenchymal stem cells (MSCs). No cytotoxicity was found for the un-cross-linked thiol-containing and protected P(HEMA-ATEMA-MPC) chains up to a concentration of 5 and 1 wt % in α-minimum essential medium, respectively.

Precursors of cross-linked polymer systems are blends of many compounds, some of them forming series (distributions) of compounds of increasing molecular weights, type and number of functional groups, or some other property. The distributions may arise from impurities in the raw materials, or a result of side reactions. In most cases, the distributions are generated intentionally; functional copolymers, hyperbranched polymers, off-stoichiometric highly branched polymers, chain extended systems, or precursors prepared in several stages can serve as examples. The distributions affect processing and materials properties. We show ways to generate these distributions from bond formation kinetics and reaction mechanisms. Statistical branching theories based on assemblage of branched molecules and gel structure from building units in different reaction states are used to model the evolution of the cross-linked system.

The factors and theoretical approaches to equilibrium swelling of cross-linked polymer systems were analyzed in relation to the application of the equilibrium swelling method for characterization of structure of cross-linked coating films. Special attention was paid to the possibility of characterizing the cross-link density and its changes during film formation. For swelling of highly cross-linked networks (coating films) in good solvents, the finite extensibility of network chains was respected. The effect of adhesion of the film to the substrate was also considered. If equilibrium swelling method (swell test) is used for characterizing changes in cross-link density of drying (reacting) films, the results are often not meaningful for several reasons such as (a) dependence of interaction parameter on conversion; (b) if the sample is in its glassy state and contains unreacted functional groups, increase of conversion occurs during the test when swelling induces transition from the glassy to the rubbery state.

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 ...

Self-inflating soft tissue expanders represent a valuable modality in reconstructive surgery. For this purpose, particularly synthetic hydrogels that increase their volume by swelling in aqueous environment are used. The current challenge in the field is to deliver a material with a suitable protracted swelling response, ideally with an induction period (for sutured wound healing) followed by a linear increase in volume lasting several days for required tissue reconstruction. Here, we report on synthesis, swelling, thermal, mechanical and biological properties of novel hydrogel tissue expanders based on poly(styrene-alt-maleic anhydride) copolymers covalently crosslinked with p-divinylbenzene. The hydrogels exerted hydrolysis-driven swelling response with induction period over the first two days with minimal volume change and gradual volume growth within 30 days in buffered saline solution. Their final swollen volume reached more than 14 times the dry volume with little dependence on the crosslinker content. The mechanical coherence of samples during swelling and in their fully swollen state was excellent, the compression modulus of elasticity being between 750 and 850 kPa. In vitro cell culture experiments and in vivo evaluation in mice models showed excellent biocompatibility and suitable swelling responses meeting thus the application requirements as soft tissue expanders.

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.

In order to create a soft tissue surplus, implantable volume expanders are often utilized in dental surgery. Implanted tissue expanders should gradually increase their volume, exerting a constant pressure on the surrounding tissue for weeks. Current tissue expanders are based predominantly on externally inflatable balloons or on osmotically active tissue expanders that use soft hydrogels wrapped in perforated plastic coatings, which limit fluid entry and swelling. We have designed and examined tissue expanders based on the controlled rate expansive hydrogels synthesized from copolymers of selected methacrylates and N-vinylpyrrolidone, cross-linked with a combination of non-degradable (glycol dimethacrylates) and hydrolytically degradable (N,O-dimethacryloylhydroxylamine) cross-linkers. These copolymers have close-to-linear volume expansion rates (up to 6-9 times their original volume) and exert an increasing swelling pressure in vitro. The anesthetic benzocaine has been incorporated into the hydrogels, and kinetic release experiments have shown that most of the drug (90%) was released within 48 h. Our proposed hydrogel expanders are homogeneous and have suitable mechanical properties, thus simplifying the surgical manipulations required. Further studies will be needed to completely evaluate their biocompatibility and tissue response to the implants.

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...

A network formation from an OH-functional star oligomer and two isocyanates (HDI and HDI-trimer) was investigated by changes in the critical molar ratio of OH groups $$(r_{\rm OH})_{\rm c}$$ as a function of dilution. The critical ratio $$(r_{\rm OH})_{\rm c}$$ is such critical excess of OH groups at which the critical conversion of NCO at the gel point groups reaches 100%. The dilution effect was interpreted by pronounced cyclization, which was higher in the case of the HDI-trimer compared with the HDI.

In this article, we report on the rheological properties of agarose aqueous solutions and gels. Viscosity curves were determined for homogeneous agarose aqueous solutions at different temperatures (from 68 to 38 8C) to study the viscosity behavior as the systems undergo gelation. The gelation phenomenon of agarose solutions was also investigated by shear oscillation experiments and differential scanning calorime- try. The gelation and melting temperature as a function of agarose concentration were determined together with the gelation and melting enthalpies. The results obtained were interpreted using the two-step model describing the gelation of agarose in water.

We present an investigation of the preparation of highly porous hydrogels based on biodegradable synthetic poly(α-amino acid) as potential tissue engineering scaffolds. Covalently cross-linked gels with permanent pores were formed under cryogenic conditions by free-radical copolymerization of poly[N(5)-(2-hydroxyethyl)-L-glutamine-stat-N(5)-(2-methacryloyl-oxy-ethyl)-L-glutamine] (PHEG-MA) with 2-hydrohyethyl methacrylate (HEMA) and, optionally, N-propargyl acrylamide (PrAAm) as minor comonomers. The morphology of the cryogels showed interconnected polyhedral or laminar pores. The volume content of communicating water-filled pores was >90%. The storage moduli of the swollen cryogels were in the range of 1-6 kPa, even when the water content was >95%. The enzymatic degradation of a cryogel corresponded to the decrease in its storage modulus during incubation with papain, a model enzyme with specificity analogous to wound-healing enzymes. It was shown that cryogels with incorporated alkyne groups can easily be modified with short synthetic peptides using azide-alkyne cycloaddition "click" chemistry, thus providing porous hydrogel scaffolds with biomimetic features.

The new generation clear coats introduced in the automotive industry have improved scratch and mar resistance. It is believed that those properties are related to the introduction of new binder technologies leading to higher film cross-link densities. This was relatively easy to accomplish in the automotive industry because baking temperatures are well above the glass-transition temperature (Tg) of the final clear coat film. In car repair coatings, the cross-linking takes place under conditions where the intermediate and/or final film Tg values are above the curing temperature. Such formulations result in vitrification during the drying process, a transition during which chemical cure and solvent release are slowed down substantially. The chemical cure can be improved through more reactive and/or higher functional binders, but this shifts the gel point to shorter times in the drying cycle which may lead to more solvent entrapment, resulting in low film hardnesses and paint defects. This paper describes the overall properties of 2K-refinish clear coats based on linear versus branched, random and graft acrylic polyols. The authors will try to correlate overall properties with theoretical predictions and practical results.

Gel point conversions and equilibrium tensile moduli of polyester-based polyurethane networks were determined for different dilutions during network formation. Also, the limiting dilution beyond which the network was not formed at full conversion of functional groups was determined. It ranged between 92 and 94% of diluent. The scaling dependences against the reference states – the ring-free case and the limiting dilution case – showed that the excluded volume effects were operative and structural changes associated with cyclization affected the rates of both the intramolecular and the intermolecular reactions.

Cross-linking hydroxy-functional stars and copolymer precursors with a triisocyanate and associated structural changes were studied theoretically and experimentally. The model of network formation was based on the statistical generation of structures from units which were in different reaction states. The differences in gel point conversion and cross-link density were interpreted in terms of precursor functionality and functionality distribution, and differences in the reactivity of functional groups. Hydroxy-functional copolymer systems containing precursors of the same number-average functionality gel at lower conversions than stars which are monodispersed mainly due to functionality distribution. The structure of copolymers characteristic of several internal cross-links contributes more to the concentration of elastically-active network chains than stars with a single cross-link. Cyclization accompanies intermolecular cross-linking; its intensity is higher for copolymer systems compared with stars.

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...

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.

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.