Structure and properties of urea-crosslinked star poly[(ethylene oxide)-ran-(propylene oxide)] hydrogels

Isocyanate-terminated six armed star shaped macromers with a statistical copolymer backbone consisting of 80% EO and 20% PO have previously demonstrated excellent protein and cell repellence as nano-layered surfaces. In this study, various macromers are mixed with water and provide a spectrum of materials that range from particles to uniform hydrogels. Due to hydrophobic end groups, 3 kDa molecular weight macromers result in micro and nano-particles, while 18 kDa macromers completely dissolve and consequently uniform, transparent, high water content hydrogels are formed. Oriented channels may be induced into these hydrogels through the controlled freezing of water in the preformed hydrogel.     

Characterization of well‐defined poly(ethylene glycol) hydrogels prepared by thiol‐ene chemistry

Considering the large number of applications for hydrogels, a better understanding of the relation between molecular structure and mechanical properties for well‐defined hydrogel is essential. A new library has been compiled of poly(ethylene glycol) polymers (PEG) of different length end functionalized with diallyl, dithiol, and dimethacrylate, and crosslinked with complementary trifunctional crosslinkers. In this study, the hydrogels were initially analyzed by FT‐Raman and NMR to study the conversion ratio of the functional groups. The effects of solvent type, solid content concentration, curing time and length of the PEG chains on the final leaching, swelling and tensile properties of the hydrogels were studied. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Poly(ethylene oxide) hydrogels as semi-permeable membranes for an artificial pancreas

Poly(ethylene oxide) (PEO) hydrogels were synthesized in organic solvents, or for better results in water, via the free-radical homopolymerization of α,ω-methacryl-oyloxy PEO macromonomers. Their characteristics (amount of extractable material, equilibrium swelling degree, uniaxial compression modulus) could be controlled by the polymerization parameters (precursor molar mass, macromonomer concentration, polymerization time). In aqueous media, the hydrophobic end-standing polymerizable methacrylic units of the macromonomers self-organize, and their polymerization leads to networks with better mechanical properties than those prepared with the same macromonomers but in organic solvents. In vitro tests confirmed their good biocompatibility: almost no adhesion of cells was evident. It was confirmed that glucose diffuses through these hydrogels. Insulin diffusion was also studied but found to be more complex.

Schematic representation of a device for an artifical pancreas based on a vascular system.     

NMR study of the structure and properties of poly(MMA-co-VP) crosslinked hydrogels

¹H- and ¹³C-NMR techniques were used to study the microscopic structure of NMA/VP copolymer hydrogels. Evidence was obtained for a plasticization effect of MMA chains by VP. An original ¹H-NMR approach revealed the existence of several types of water with various degree of bounding to the polymer network, a conclusion that is corroborated by a complementary ¹³C-NMR study. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 3619–3625, 1997     

Compression elastic modulus of neutral and protonated poly(N-vinylimidazole) hydrogels

The compression modulus of poly(N-vinylimidazole) (PVI) hydrogels synthesized by cross-linking polymerization in aqueous solution, was measured at room temperature in several related systems: i) just after polymerization, ii) swollen at equilibrium in deionized water, iii) swollen in HCl (aq) (pH=2.5), iv) swollen in HCl (pH=2.5) and 1 M NaCl (aq) solution and v) swollen in H₂SO₄ (pH=2.5) (aq) solution. Samples of the first and second groups are neutral whereas hydrogels of the other three groups are ionic because of protonation of basic imidazole groups. The experimental results were fitted with the Erman-Monnerie theory, applied to compression measurements for the first time, to determine the phantom modulus, [f_ph*], and the parameter κ_G which measures the constraining role of entanglements on the fluctuations of chains between knots.     

Radiation-chemical synthesis of crown-containing poly(ethylene oxide) hydrogels. Swelling behavior and crown ether retention

Poly(ethylene oxide) hydrogels containing physically immobilized dicyclohexano-18-crown-6 (DCH18C6) were synthesized by radiation-chemical cross-linking. DCH18C6 concentration in initial polymer solution has insignificant effect on the gelation dose at the weight ratio of crown ether to polymer lower than 0.2. The higher molecular weight of PEO results in lower gelation dose and higher gel fraction. An increase in absorbed dose leads to a noticeable decrease in the swelling ratio corresponding to a denser hydrogel network. At the same time the absorbed dose in the range of 5–100 kGy has no pronounced effect on the DCH18C6 retention.     

Degradable poly(ethylene oxide) hydrogels formed by crosslinking with tert‐butylperoxybenzoate

Poly(ethylene oxide) (PEO, number‐average molecular weight: 2,000,000) was crosslinked by reaction with t‐butylperoxybenzoate in the melt. Upon swelling in water, the resulting hydrogels were acidic and suggested clear evidence of spontaneous hydrolysis that continued over periods of several weeks to give clear and low‐viscosity aqueous solutions of PEO oligomers. In contrast, in neutral media the gels did not show any signs of hydrolysis. As shown by UV, IR, and size exclusion chromatographic analysis, the PEO hydrolysis products consist of benzoic acid and hydroxyl‐ and carboxyl end‐functionalized low‐molecular‐weight PEOs. This is consistent with the acid‐catalyzed hydrolysis of acetal‐, orthoester‐, and similar end‐functionalized PEOs formed by radical coupling of various PEO radicals with benzoate, alkoxy, and other radicals. Titration of the hydrolysis mixtures indicated that the total molar amount of acid exceeds that of the maximum amount of benzoic acid produced during gel formation. However, the amount of benzoic acid equaled this maximum amount. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 520–527, 2003     

Sequential interpenetrating poly(ethylene glycol) hydrogels prepared by UV‐initiated thiol–ene coupling chemistry

Poly(ethylene glycol) (PEG)‐diallyls, ranging from 2 to 8 kDa, were successfully reacted with a trifunctional thiol crosslinker via thiol–ene coupling reaction to construct four different primary PEG hydrogels. These systems were used as scaffolds for the preparation of a library of sequential interpenetrating networks (SeqIPNs). The solid content of the secondary networks varied between 21 and 34% and was dependent on the length of the absorbing PEGs. The gel fractions for the IPNs were above 85%. Additionally, the lowest degree of swelling was found for the IPN based on 2‐kDa PEG (315%), whereas the 8‐kDa PEG IPN exhibited a value of 810%. The SeqIPN strategy facilitated hydrogel systems that cover a larger domain of tensile modulus (192–889 kPa) when compared with single hydrogel networks (175–555 kPa). © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Model polyurethane networks prepared from poly(ethylene oxide) and poly(tetramethylene oxide)

Polyurethane elastomers of known degrees of cross-linking were prepared from hydroxylterminated poly(ethylene oxide) (PEO) and poly(tetramethylene oxide) (PTMO) chains having numberaverage molecular weights in the range 880–6820 g mol^?1. The chains were end-linked into “model” trifunctional networks using a specially prepared aromatic triisocyanate. The networks thus obtained were studied with regard to their stress-strain isotherms in both the unswollen and swollen states, in elongation at 25°, and with regard to their equilibrium swelling in benzene at 57.9°. Values of the modulus in the limit at high deformation were in good agreement with corresponding results previously obtained on trifunctional networks of poly(dimethylsiloxane) (PDMS). Since PEO has a much higher value of the plateau modulus in the uncross-linked state, this agreement indicates that inter-chain entanglements do not contribute significantly to the equilibrium modulus of an elastomeric network. These values of the high deformation modulus are also in good agreement with recent molecular theories as applied to the non-affine deformation of a “phantom” network. The swelling equilibrium results were in very good agreement with the new theory of network swelling developed by Flory.     

Thermogelling hydrogels of poly(ε-caprolactone-co-D,L-lactide)–poly(ethylene glycol)–poly(ε-caprolactone-co-D,L-lactide) and poly(ε-caprolactone-co-L-lactide)–poly(ethylene glycol)–poly(ε-caprolactone-co-L-lactide) aqueous solutions

Thermogelling poly(ε-caprolactone-co-D,L -lactide)–poly(ethylene glycol)–poly(ε-caprolactone-co-D,L -lactide) and poly(ε-caprolactone-co-L -lactide)–poly(ethylene glycol)–poly(ε-caprolactone-co-L -lactide) triblock copolymers were synthesized through the ring-opening polymerization of ε-caprolactone and D,L -lactide or L -lactide in the presence of poly(ethylene glycol). The polymerization reaction was carried out in 1,3,5-trimethylbenzene with Sn(Oct)₂ as the catalyst at various temperatures, and the yields were about 96%. The molecular weights and polydispersities (M_w/M_n) by gel permeation chromatography were in the ranges of 5140–6750 and 1.35–1.45, respectively. The differential scanning calorimetry results showed that the melting temperatures of the poly(ε-caprolactone) components were between 30 and 40 °C. By the subtle tuning of the chemical compositions and microstructures of these triblock copolymers, the aqueous solutions underwent sol–gel transitions as the temperature increased, with the suitable lower critical solution temperature in the range of 17–28 °C at different concentrations. Transesterification in the polymerization process generated the redistribution of sequences, which remarkably affected the sol–gel transition temperature. The amphiphilic copolymers formed micelles in aqueous solutions with a diameter of 62 nm and a critical micelle concentration of about 0.032 wt % at 20 °C. Micelles aggregated as the temperature increased, leading to gel formation. The sol–gel transition was studied, with a focus on the structure–property relationship. It is expected to have potential applications in drug delivery and tissue engineering. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4091–4099, 2007     

Water soluble poly(ethylene oxide) functionalized norbornene polymers

The synthesis of three different poly(ethylene oxide) macromonomers with a norbornene and oxanorbornene end group is presented. The macromonomers were polymerized to comb‐polymers by ring‐opening metathesis polymerization (ROMP) using Grubbs' Catalyst G3 to produce water soluble polymers with polydispersities between 1.04 and 1.30 and molecular weights between 14,000 and 50,000 g/mol. Characterization by static and dynamic light scattering reveals that the comb‐polymers with norbornene backbone are molecularly disperse in aqueous solution, while the oxanorbornene‐backbone polymers form small water‐soluble aggregates. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2640–2648, 2008     

Synthesis and solution behavior of comb‐like terpolymers with poly(ethylene oxide) macromonomer

A novel macromonomer: p‐vinylbenzyl‐terminated octylphenoxy poly(ethylene oxide) (polymerization number: 18) (VBPEO) was synthesized. The comb‐like acrylamide‐based terpolymers (PVEA) were synthesized by aqueous free‐radical copolymerization technique using acrylamide (AM), VBPEO and sodium 2‐acrylamido‐2‐methylpropane sulphonate (NaAMPS). The macromonomer and PVEA polymer were characterized with Fourier transform infrared (FTIR) spectroscopy and proton nuclear magnetic resonance (¹H NMR). The polymers exhibited self‐assembly behavior in water and the brine solutions. The polymers could be employed in the oil reservoirs of either the high or medium to low permeability due to the low intrinsic viscosities. The optimum NaAMPS feed amount could not only increase the water solubility, but also facilitate the intermolecular associations. Implementing VBPEO into the polymer greatly increased the rigidity of molecular chains, resulting in a high apparent viscosity of the PVEA in water and the brine solutions. The PVEA brine solutions exhibited both excellent uni‐ and bi‐valent cation resistance, salt‐ and heat‐thickening twice, pseudoplastic behavior, and thixotropy. The SEM morphologies showed that the expanded polymer bundles as well as the associated structures with huge sizes were formed for PVEA in water and that the continuous network structures were still formed in the PVEA brine solutions. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1241–1250, 2010     

Effect of poly(ethylene oxide) on the structure and properties of poly(vinyl alcohol)

To improve the drawability of poly(vinyl alcohol) (PVA) thermal products, poly(ethylene oxide) (PEO), a special resin with good flexibility, excellent lubricity, and compatibility with many resins, was applied, and the Fourier transform infrared spectroscopy, dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), and wide‐angle X‐ray diffraction (WXRD) were adopted to study the hydrogen bonds, water states, thermal properties, crystal structure, and nonisothermal crystallization of modified PVA. It was found that PEO formed strong hydrogen bonds with water and PVA, thus weakened the intra‐ and inter‐hydrogen bonds of PVA, changed the aggregation states of PVA chains, and decreased its melting point and crystallinity. Moreover, the interactions among PVA, water, and PEO retarded the water evaporation and made more water remain in the system to plasticize PVA. The existence of PEO also slowed down the melt crystallization process of PVA, however, increased the nucleation points of system, thus made more and smaller spherulites formed. The weakened crystallization capability of PVA and the lubrication of PEO made PVA chains to have more mobility under the outside force and obtain high mechanical properties. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1946–1954, 2010     

Modular material system for the microfabrication of biocompatible hydrogels based on thiol–ene‐modified poly(vinyl alcohol)

Novel modifications of the synthetic polymer poly(vinyl alcohol) (PVA) were developed for application in the field of biomedical engineering. PVA was modified with allyl succinic anhydride, norbornene anhydride as well as with γ‐thiobutyrolactone to produce macromers with reactive ene and thiol groups, respectively. Cytotoxicity studies have shown that the material exhibits almost no cell‐toxicity, when used in concentrations of 1 and 0.1 wt % for 24 h. The obtained macromers were photocrosslinked via thiol–ene chemistry. Storage stability of the macromer mixtures with different concentrations of pyrogallol as stabilizer were investigated. Photorheometry was employed to optimize mixtures concerning reactivity based on their thiol‐to‐ene ratio, photoinitiator concentration, and macromer content. The crosslinked hydrogels were studied concerning their swellability. To form hydrogels with cellular structure two‐photon‐polymerization (2PP) was employed. Processing windows for 2PP of selected mixtures were determined. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2060–2070     

Photopolymerized injectable RGD-modified fumarated poly(ethylene glycol) diglycidyl ether hydrogels for cell growth

In this study, photopolymerized hydrogels of fumarated poly(ethylene glycol) diglycidyl-co- poly(ethylene glycol) diacrylate have been synthesized and modified with cell adhesion peptide, Arg-Gly-Asp (RGD). The structural and mechanical properties of the hydrogels are found to be poly(ethylene glycol) diacrylate (PEGDA) dependent. The percentage of gelation is increased from 72 to 89 wt.-% when the amount of the crosslinker co-monomer (PEGDA) in the hydrogel formulation is increased from 20 to 40 wt.-%. In the present case, the equilibrium mass swelling is decreased from 216 to 93%. The viscosities of the uncured formulations have also been measured and likewise, the results were influenced by the increasing amount of PEGDA that reduced the value from 83 to 36 cP. The compressive modulus of the prepared hydrogels was improved with the addition of the PEGDA. Cell growth experiments have been performed by comparing the properties of the hydrogels with and without RGD units. The results show that RGD units enhance the adhesion of cells to the surface of the hydrogels. SEM-EDS studies reveal that nitrogen and calcium are produced on the osteoblast-seeded surface of the scaffold within the culture time period. [Figure: see text].     

Swelling behavior and network structure of hydrogels synthesized using controlled UV‐initiated free radical polymerization

N‐vinyl‐2‐pyrrolidone (VP) and 2‐hydroxyethyl methacrylate (HEMA) copolymeric gels have been synthesized using UV‐initiated photopolymerization to understand their characteristic behavior for development as a bioengineering material, specifically for tissue expansion. The properties of the gels have been investigated by systematic variation of the monomer feed composition and initiator and crosslinker concentrations as well as UV irradiation intensity, which was controlled by various photomasks. The swelling kinetics and network characteristics for the various hydrogels were investigated through the observation of gel swelling behavior in saline solutions and compression modulus determination of the fully swollen hydrogels. The equilibrium swelling ratio (q_e) of the gels increased as expected with increasing VP content and decreasing crosslinker concentration. However, it was found that as the amount of initiator or UV intensity increased, unexpectedly q_e also increased, which indicates a network structure with decreasing effective crosslink density (ν_e) (or increasing average molecular weight between crosslinks (M_c)). Based on this anomalous swelling behavior and thermal analysis of the gels, a molecular structure is proposed consisting of increasing number of dangling chain ends within the polymer network. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1450–1462, 2008     

New aspects in chemistry and characterization of poly(ethylene oxide) hydrogels

To extend the variability of poly(ethylene oxide) (PEO) hydrogels in their swelling behaviour, ability to bind various subjects, and possible applications, some new synthetic approaches have been elaborated recently: (i) PEO networks with ionic and reactive groups were prepared by the reaction of P-OH groups of poly(oligoethylene glycol phosphate)s with diepoxide. The swelling degree of these hydrogels is strongly affected by salts, what is typical of polyelectrolyte networks. The structure parameters of the network evaluated from the swelling data are discussed. (ii) A series of hydrogels with controlled structure was obtained by radical polymerization of methacrylate PEO macromonomers of various molecular weights and average functionality (0.99–1.85) in aqueous media. The networks prepared are characterized by means of elastic modulus and swelling pressure measurements as well as with the help of microparticles of liquid-crystalline dispersion of DNA as probes highly sensitive to the network structure and state.     

Viscoelasticity and tack of poly(vinyl pyrrolidone)–poly(ethylene glycol) blends

The adhesive properties of blends of high molecular weight poly(vinyl pyrrolidone) (PVP) and low molecular weight poly(ethylene glycol) (PEG) were systematically investigated with a probe test and correlated with their viscoelastic properties. The material parameters that were varied were the PEG content (31–41 wt %) and the hydration rate. The 36% PEG showed the best balance of properties for a pressure‐sensitive adhesive. At low debonding rates, the debonding took place through the formation of a fibrillar structure, whereas at high debonding rates, the debonding was brittle. This transition was attributed to the breakage and reformation of hydrogen bonds between PVP units and OH groups on PEG during the large strain of the polymer chains in elongation. This transition was observed, albeit shifted in frequency, for all three compositions, and the characteristic relaxation times of the hydrogen‐bonded network were estimated. A comparison between the tack properties of the adhesives and their linear viscoelastic properties showed a very strong decoupling between the small‐strain and large‐strain properties of the adhesive, which was indicative of a pronounced deviation from rubber elasticity in the behavior of the blends. This deviation, also seen during tensile tests, was attributed to the peculiar phase behavior of the blends. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2395–2409, 2002     

From poly(N‐isopropylacrylamide)‐block‐poly(ethylene oxide)‐block‐poly(N‐isopropylacrylamide) triblock copolymer to poly(N‐isopropylacrylamide)‐block‐poly(ethylene oxide) hydrogels: Synthesis and rapid deswelling and reswelling behavior of hydrogels

Poly(N‐isopropylacrylamide)‐block‐poly(ethylene oxide)‐block‐poly(N‐isopropylacrylamide) (PNIPAAm‐b‐PEO‐b‐PNIPAAm) triblock copolymer was synthesized via the reversible addition‐fragmentation chain transfer/macromolecular design via the interchange of xanthate (RAFT/MADIX) process with xanthate‐terminated poly(ethylene oxide) (PEO) as the macromolecular chain transfer agent. The successful synthesis of the ABA triblock copolymer inspired the preparation of poly(N‐isopropylacrylamide)‐block‐poly(ethylene oxide) (PNIPAAm‐b‐PEO) copolymer networks with N,N′‐methylenebisacrylamide as the crosslinking agent with the similar approach. With the RAFT/MADIX process, PEO chains were successfully blocked into poly(N‐isopropylacrylamide) (PNIPAAm) networks. The unique architecture of PNIPAAm‐b‐PEO networks allows investigating the effect of the blocked PEO chains on the deswelling and reswelling behavior of PNIPAAm hydrogels. It was found that with the inclusion of PEO chains into the PNIPAAm networks as midblocks, the swelling ratios of the hydrogels were significantly enhanced. Furthermore, the PNIPAAm‐b‐PEO hydrogels displayed faster response to the external temperature changes than the control PNIPAAm hydrogel. The accelerated deswelling and reswelling behaviors have been interpreted based on the formation of PEO microdomains in the PNIPAAm networks, which could act as the hydrophilic tunnels to facilitate the diffusion of water molecules in the PNIPAAm networks. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012