Cross-linked Polymeric Materials Research Articles

Metal-Catalyzed Transesterification for Healing and Assembling of Thermosets

Catalytic control of bond exchange reactions enables healing of cross-linked polymer materials under a wide range of conditions. The healing capability at high temperatures is demonstrated for epoxy-acid and epoxy-anhydride thermoset networks in the presence of transesterification catalysts. At lower temperatures, the exchange reactions are very sluggish, and the materials have properties of classical epoxy thermosets. Studies of model molecules confirmed that the healing kinetics is controlled by the transesterification reaction rate. The possibility of varying the catalyst concentration brings control and flexibility of welding and assembling of epoxy thermosets that do not exist for thermoplastics.

Networking Properties of Cyclodextrin-Based Cross-Linked Polymers Probed by Inelastic Light-Scattering Experiments

An integrated experimental approach, based on inelastic light-scattering techniques, has been here employed for a multilength scale characterization of networking properties of cyclodextrin nanosponges, a new class of cross-linked polymeric materials built up from natural oligosaccharides cyclodextrins. By using Raman and Brillouin scattering experiments, we performed a detailed inspection of the vibrational dynamics of these polymers over a wide frequency window ranging from gigahertz to terahertz, with the aim of providing physical descriptors correlated to the cross-linking degree and elastic properties of the material. The results seem to suggest that the stiffness of cross-linked polymers can be successfully tuned by acting on the type and the relative amount of the cross-linker during the synthesis of a polymer matrix, predicting and controlling their swelling and entrapment properties. The proposed experimental approach is a useful tool for investigating the structural and physicochemical properties of polymeric network systems.

Light-Harvesting Cross-Linked Polymers for Efficient Heterogeneous Photocatalysis

Nonporous, phosphorescent cross-linked polymers (Ru-CP and Ir-CP) were synthesized via Pd-catalyzed Sonogashira cross-coupling reactions between tetra(p-ethynylphenyl)methane and dibrominated Ru(bpy)(3)(2+) or Ir(ppy)(2)(bpy)(+), respectively. The resultant particulate cross-linked polymer (CP) materials have very high catalyst loadings (76.3 wt % for Ru-CP and 71.6 wt % for Ir-CP), and are nonporous with negligibly small surface areas (2.9 m(2)/g for Ru-CP and 2.7 m(2)/g for Ir-CP). Despite their nonporous nature, the insoluble CP materials serve as highly active and recyclable heterogeneous photocatalysts for a range of organic transformations such as aza-Henry reaction, aerobic amine coupling, and dehalogenation of benzyl bromoacetate. An efficient light-harvesting mechanism, which involves collection of photons by exciting the (3)MLCT states of the phosphors and migration of the excited states to the particle surface, is proposed to account for the very high catalytic activities of these nonporous CPs. Steady-state and time-resolved emission data, as well as the reduced catalytic activity of Os(bpy)(3)(2+)-doped Ru-CPs supports efficient excited state migration for the CP frameworks. This work uncovers a new strategy in designing highly efficient photocatalysts based on light-harvesting cross-linked polymers.

Analysis of the interdiffusion of resin monomers into pre-polymerized fiber-reinforced composites

ObjectivesThe aim of the study was to analyze the dissolving depth of adhesive resin monomers into pre-polymerized fiber-reinforced composites (FRC) with either a semi-interpenetrating polymer network (semi-IPN) or a cross-linked polymer (CLP) matrix. MethodsFive unidirectional FRCs were tested, including one semi-IPN FRC (everStick® C&B, StickTech, (ES)) and four CLP-FRCs (GrandTec®, VOCO, (GT); Dentapreg®, ADM, (DP); TenderFiber®, Micerium, (TF); Splint-It®, Pentron Clinical Technologies, (SI)). The FRCs were light-polymerized following manufacturers’ instructions: the oxygen inhibition layers were removed and the adhesive resin (Optibond-FL®, KerrHawe) was labeled with a fluorescent dye (Rhodamine-B-isothiocyanate), which was then applied to the FRCs (5min) and light-polymerized (40s). Specimens were then prepared for confocal laser scanning microscopy. Three FRC strands per group were sectioned orthogonally to the direction of fibers, thus forming nine slices in each group. Four images were taken from each slice and the depths of adhesive penetration were measured in four sites per image (n=144 measurements per group). SignificanceDissolving depths were 17.28 (3.04)μm (ES), 12.58 (2.94)μm (SI), 7.57 (1.91)μm (TF), 3.27 (0.73)μm (DP) and 2.55 (0.63)μm (GT). Samples were normally distributed. Differences between groups were analyzed by ANOVA (PostHoc Scheffé) showing four subgroups (p=0.05). The infiltration layers detected were either continuous/homogenous (ES, TF, DP GT) or discontinuous/insular (SI). ConclusionThe adhesive resin monomers were able to diffuse significantly deeper into pre-polymerized semi-IPN specimens than into CLP materials. Semi-IPN specimens showed a homogenous and comparatively deeper layer of infiltration. The diffusion capabilities of secondary-IPN formation might increase the opportunity to establish a good bond between pre-polymerized FRC and new resin.

Resins with “Nano-Raisins”

Thermosensitive hydrogels are materials which globally shrink/swell in water when the surrounding temperature crosses the lower critical solution temperature (LCST). We demonstrate here a novel class of cross-linked polymeric materials, which do not shrink/swell in water globally, but nevertheless reveal a hydrogel-like, stimuli-responsive behavior. In particular, they demonstate a positive thermosensitive release of the embedded fluorescent dye significantly modulated when temperature crosses the LCST. Using staining with copper, transmission electron microscopy and energy dispersive X-ray analysis, we show that this effect is associated with nanogel "raisins" dispersed in such materials (e.g., polymer nanofibers). Shrinkage of individual nanogel "raisins" at elevated temperatures increases nanoporosity via increased exposure of the existing nanopores to water, or formation of new nanopores/nanocracks in the overstretched polymer matrix in the vicinity of shrinking nanogel "raisins". As a result, the release rate of the embedded dye from the nanofibers increases at elevated temperatures. We suggest that similar functional materials with embedded nanogel "raisins" will find applications in nanofluidics and as drug carriers for controlled drug release.

Multilayer structured polymer light emitting diodes with cross-linked polymer matrices

Currently, there is great interest in manufacturing multilayer polymer light emitting diode (PLED) structures via low-cost solution-based spin-casting or printing methods. The difficulty with this approach is that solvent from freshly deposited films often dissolves the underlying layers. This letter demonstrates that fully operational multilayer PLED structures can be fabricated via a solution process by embedding the hole transport material in cross-linked inert polymer matrices that protect the functional material while subsequent layers are deposited using the same solvent. The resulting devices exhibited greatly improved quantum efficiency compared with devices that did not employ cross-linked polymer matrices.

Engineering of bio-hybrid materials by electrospinning polymer-microbe fibers

Although microbes have been used in industrial and niche applications for several decades, successful immobilization of microbes while maintaining their usefulness for any desired application has been elusive. Such a functionally bioactive system has distinct advantages over conventional batch and continuous-flow microbial reactor systems that are used in various biotechnological processes. This article describes the use of polyethylene oxide(99)-polypropylene oxide(67)-polyethylene oxide(99) triblock polymer fibers, created via electrospinning, to encapsulate microbes of 3 industrially relevant genera, namely, Pseudomonas, Zymomonas, and Escherichia. The presence of bacteria inside the fibers was confirmed by fluorescence microscopy and SEM. Although the electrospinning process typically uses harsh organic solvents and extreme conditions that generally are harmful to bacteria, we describe techniques that overcome these limitations. The encapsulated microbes were viable for several months, and their metabolic activity was not affected by immobilization; thus they could be used in various applications. Furthermore, we have engineered a microbe-encapsulated cross-linked fibrous polymeric material that is insoluble. Also, the microbe-encapsulated active matrix permits efficient exchange of nutrients and metabolic products between the microorganism and the environment. The present results demonstrate the potential of the electrospinning technique for the encapsulation and immobilization of bacteria in the form of a synthetic biofilm, while retaining their metabolic activity. This study has wide-ranging implications in the engineering and use of novel bio-hybrid materials or biological thin-film catalysts.

Molecular Simulation of Cross-Linked Epoxy and Epoxy−POSS Nanocomposite

We present an efficient method for the creation of atomistic model structures of cross-linked polymer matrices. The method consists of preparation of a physical mixture of the monomer and the cross-linker molecules in the box followed by a single-step polymerization of the entire mixture. For this purpose, the simulated annealing algorithm is used to identify pairs of reacting atoms that are spatially close. The technique is used to create five structures of cross-linked epoxy as well as cross-linked epoxy−POSS (i.e., polyhedral oligomeric silsesquioxane) nanocomposite. The models so generated are characterized with respect to the density, volume−temperature behavior, and the detailed molecular structure. Our results show that incorporation of POSS particles (at 5 wt %) in the cross-linked epoxy resin leads to a weak tendency for lowering the coefficient of volume thermal expansion but does not cause a measurable change in the glass transition temperature.

Photo‐crosslinking of poly[ethene‐stat‐(methacrylic acid)] functionalised with maleimide side groups

AbstractBACKGROUND: Photo‐crosslinkable polymers are well known and commercially applied as photoresists. But so far they have not been applied as membrane materials for separation processes. They would offer certain advantages in membrane fabrication over conventional crosslinked polymer materials. Therefore, in this work, a poly[ethene‐stat‐(methacrylic acid)] (PEMAA) which is a potential membrane polymer for different separation problems was functionalised with photo‐crosslinkable maleimide side groups.RESULTS: It has been shown that PEMAA can be used as basic polymer material and a conversion with 3‐hydroxypropylmaleimide is possible in order to obtain a photo‐crosslinkable polymer. Investigation of the crosslinking mechanism was performed using stationary infrared and UV‐visible spectroscopy as well as nanosecond transient spectroscopy absorption measurements of a rotating film. Intense transient absorption of the maleimide‐esterified PEMAA occurs at 250 nm in the film pointing to maleimide anion formation and crosslinking via an ionic dimerisation mechanism.CONCLUSION: It is found that crosslinking reactions can be observed spectroscopically in situ using a maleimide‐functionalised PEMAA. Furthermore, experiments can be performed in the liquid phase (polymer in solution) as well as in the solid phase (polymer film) using a rotating polymer film sample. Maleimide anion formation and crosslinking via an ionic dimerisation mechanism can be investigated by variation of the polymer structure as well as the structure of the maleimide side groups. Copyright © 2009 Society of Chemical Industry

Structural Investigations on Hybrid Polymers Suitable as a Nanoparticle Precipitation Environment

Cross-linked hybrid polymer matrices with tunable hydrophilic/hydrophobic areas were prepared from end-group functionalized poly(ethylene oxides) (PEO) and poly(methylhydrido siloxanes) (PMHS) or t...

Synthesis and Properties of Methacrylic-Functionalized Tween Monomer Networks

Tween surfactants possess very interesting properties such as biodegradability, biocompatibility, and low toxicity. The synthesis of acrylate monomers by means of the chemical modification of polysorbate surfactants Tween 20, 40, and 60 with unsaturated groups is described. Monomers were obtained as a result of the reaction of methacrylic anhydride with different grades of Tween surfactants. Further polymerization was carried out in tetrahydrofuran, dimethylformamide, and a mixture of water-tetrahydrofuran. Physicochemistry properties of the polymer networks were investigated, and the obtained results reveal that they strongly depend on the type of solvent used during the polymerization, as well as on the concentration of the casting solution. In particular, our study demonstrated that, depending on the solvent boiling point, i.e., the facility to remove the solvent from the polymer matrix, it is possible to predict properties of the network morphology. Moreover, in vitro studies on controlled release were accomplished to demonstrate the possibility of utilizing these new materials as drug delivery systems. All resulting networks represent a novel class of cross-linked polymeric materials useful both in pharmaceutical and chemical applications.

Dynamics of solid-state extrusion of viscoelastic cross-linked polymeric materials

A mathematical model for the solid-state extrusion of a compressible polymeric material is developed. The novel feature of the model is the consideration of the actual rheologic behavior of the material and crosslink formation kinetics. Two types of boundary conditions at the press plunger are considered. The system of the governing equations is solved using Lagrangian coordinates. The generalized Maxwell model is used to describe the viscoelastic behavior of the material. The results of numerical experiments discussed in the study show that an unstable material flow can be observed both for high and low extrusion velocities.

Spontaneous crosslinking of poly(1,5‐dioxepan‐2‐one) originating from ether bond fragmentation

AbstractThe spontaneous reaction of unsaturated double bonds induced by the fragmentation of ether bonds is presented as a method to obtain a crosslinked polymer material. Poly(1,5‐dioxepan‐2‐one) (PDXO) was synthesized using three different polymerization techniques to investigate the influence of the synthesis conditions on the ether bond fragmentation. It was found that thermal fragmentation of the ether bonds in the polymer main chain occurred when the synthesis temperature was 140 °C or higher. The double bonds produced reacted spontaneously to form crosslinks between the polymer chains. The formation of a network structure was confirmed by Fourier transform infrared spectrometry and differential scanning calorimetry. In addition, the low molar mass species released during hydrolysis of the DXO polymers were monitored by ESI‐MS and MALDI‐TOF‐MS. Ether bond fragmentation also occurred during the ionization in the electrospray instrument, but predominantly in the lower mass region. No fragmentation took place during MALDI ionization, but it was possible to detect water‐soluble DXO oligomers with a molar mass up to approximately 5000 g/mol. The results show that ether bond fragmentation can be used to form a network structure of PDXO. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7258–7267, 2008

Synthesis and Characterization of a Single-Component Thermally Remendable Polymer Network: Staudinger and Stille Revisited

Two new single-component, highly cross-linked polymeric materials have been developed that are capable of undergoing repeated cycles of mending. On the basis of the thermally reversible Diels−Alder (r-DA) cycloaddition reaction, these materials are comprised of a dicyclopentadiene core which acts as both diene and dienophile in the r-DA reaction. Polymer specimens have been prepared from two monomeric units, monomers 400 and 401, and the thermal and mechanical properties of these materials have been studied via differential scanning calorimetry, dynamic mechanical analysis, and three-point bending, compression, and fracture tests. After fracture, these hard, colorless, transparent materials are capable of thermal mending at 120 °C, effectively healing cracks formed in the specimen.

Multifunctional sorbents for the extraction of pesticide multiresidues from natural waters

In this work multifunctional sorbents, based on surfactant-coated mineral oxides, are assessed for the simultaneous extraction/preconcentration of pesticide multiresidues from aqueous environmental samples. Seventeen pesticides, representative of all the common groups (triazines, phenylureas, carbamates, azols, anilides, chloroacetanilides, organophosphorous, phenoxyacids, aryloxy acids and phenols), are selected for this study. The sorbents assessed are pure sodium dodecyl sulphate (SDS) and mixed tetrabutylammonium (TBA)–SDS hemimicelles and/or admicelles adsorbed onto alumina. Because of their multifunctional character, these sorbents provide different retention mechanisms (i.e. hydrophobic, ionic and/or π-cation interactions), which highly contribute to the efficient retention of pesticides with different polarities and acidities (bases, neutrals and acids). In addition, the low volume of eluent required for complete elution of analytes (typically 1–2 mL) avoided the need of using time-consuming and tedious evaporation steps that generally are needed when cross-linked polymeric resins or carbon materials are used as sorbents. The performance of two sorbents, i.e. SDS and TBA–SDS mixed hemimicelles/admicelles, for the admicellar solid-phase extraction (ASPE) of pesticide multiresidue was comparatively investigated. The latter was selected on the basis of the higher breakthrough volumes permitted, the lower volume of eluent required and the higher sample and eluent flow rates allowed. The proposed ASPE/LC/UV approach provided detection limits lower than 100 ng L −1 for the determination of the 17 pesticides tested. Recoveries from spiked (at the ng L −1 level) river and underground water samples was quantitative for most of the pesticides tested.

Thermoluminescence - An optical technique for in-situ measurement of cross-linking by-products during degassing of power cables

This paper presents a novel optical technique that has been developed to determine the concentration of by-products in chemically cross-linked polymeric materials. Underground power cables, especially of the high-voltage and extra high-voltage class, must be subjected to thermal treatment or conditioning called degassing in order to reduce the concentration of the cross-linking by-products to a negligible level. Degassing causes a reduction and redistribution of the by-products in the insulation of freshly cross-linked cables. It is advantageous to degas cross-linked power cables and reduce the by-products to a low level and stable level so that the electrical tests, performed prior to cable installation, would reveal the true condition of the insulation

Fabrication of Large, Robust Nanomembranes from Diverse, Cross-Linked Polymeric Materials

Fabrication of Large, Robust Nanomembranes from Diverse, Cross-Linked Polymeric Materials

Combinatorial-mold imprint lithography: A versatile technique for fabrication of three-dimensional polymer structures

A two-step fabrication technique based on nanoimprint lithography is described for the fabrication of three-dimensional micro- and nanostructures. By combining simple two-dimensional geometries from two molds, complex and useful three-dimensional structures are obtained. The careful selection of mold geometries constitutes a simplified and efficient approach toward building up desirable three-dimensional structures without resorting to the use of a sacrificial process or components. Three-dimensional structures fabricated for a variety of specific applications are presented using both thermoplastic and cross-linked polymer materials.

Methacryloylpropyl-βcyclodextrin and vinylpyrrolidone copolymers: Synthesis and characterization as potential chiral selector

The free radical copolymerization of vinylpyrrolidone (VP) with 2-hydroxy-3-methacryloyloxypropyl β-cyclodextrin (βW7 MAHP), a derivative of hydroxypropyl β-cyclodextrin (CD) substituted by polymerizable methacryloyl groups, is carried out in water by varying the molar ratio of the comonomers. It is found that the higher the molar ratio of βW7 MAHP to VP, the larger the molar mass of the water-soluble copolymer. A size exclusion chromatography analysis coupled with multiangle laser light scattering detection (SEC–MALLS) suggests that VP-rich copolymers with a mass-average molar mass (MM) of about 2.5 × 104 g mol−1 may consist of βW7 MAHP dimers, trimers, and oligomers containing few CD units whereas βW7 MAHP rich copolymers with an MM of about 5.5 × 106 g mol−1 are likely to be dominated by crosslinked polymer materials. βW7 MAHP-co-VP copolymers coated on porous silica are used as high-performance liquid chromatography chiral selectors. The effects of the structural features of the guest molecule and the characteristics of the chiral stationary phases on the retention and resolution are evaluated. SEC–MALLS detection shows that, by varying the comonomer feed, copolymers with different molar masses and macromolecular structures are formed. The chiral separation ability of Copo VP is evaluated toward enantiomers having one or more aromatic rings as a function of the amount of copolymer adsorbed onto the silica surface, the βW7 MAHP content, and the concentration of solute. It is clear that the column combining the greater amounts of adsorbed copolymer and βW7 MAHP exhibits better resolving power. Moreover, the size, geometry, and functionality of the guest molecule are important factors that strongly affect the enantioselectivity. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 2364–2374, 2005

Cyclopentadienyl-functionalised polyhedral silsesquioxanes as building blocks for new nanostructured materials

This work is to design new organosilicon precursors, which contain both siloxane and cyclopentadienyl functionalities and use these compounds as building blocks for new nanomaterials. Polyhedral silsesquioxanes, (RSiO 1.5) n ( n = 8 and 10) with cyclopentadienyl (R = –C 5H 5 or –(CH 2) 3–C 5H 5) functionalities have been prepared by hydrolytic condensation of the correspondent silicon organic precursors RSiCl 3 or RSi(OC 2H 5) 3. The products with cyclopentadienyl groups have been used to prepare new cross-linked 3D polymeric materials by Diels–Alder reaction. The compounds have been characterised by multinuclear ( 1H and 29Si) NMR, ESI MS and IR spectroscopy, UV/Vis spectrometry, gel permeation chromatography and scanning electron microscopy. The approach above seems to be very promising for the preparation of new nanostructured materials.