Marek W. Urban

Self-repairing oxetane-substituted chitosan polyurethane networks.

Polyurethanes have many properties that qualify them as high-performance polymeric materials, but they still suffer from mechanical damage. We report the development of polyurethane networks that exhibit self-repairing characteristics upon exposure to ultraviolet light. The network consists of an oxetane-substituted chitosan precursor incorporated into a two-component polyurethane. Upon mechanical damage of the network, four-member oxetane rings open to create two reactive ends. When exposed to ultraviolet light, chitosan chain scission occurs, which forms crosslinks with the reactive oxetane ends, thus repairing the network. These materials are capable of repairing themselves in less than an hour and can be used in many coatings applications, ranging from transportation to packaging or fashion and biomedical industries.

Self-healing polymeric materials

Inspired by nature, self-healing materials represent the forefront of recent developments in materials chemistry and engineering. This review outlines the recent advances in the field of self-healing polymers. The first part discusses thermodynamic requirements for self-healing networks in the context of conformation changes that contribute to the Gibbs free energy. The chain flexibility significantly contributes to the entropy changes, whereas the heat of reaction and the external energy input are the main contributors to enthalpy changes. The second part focuses on chemical reactions that lead to self-healing, and the primary classes are the covalent bonding, supramolecular assemblies, ionic interactions, chemo-mechanical self-healing, and shape memory polymers. The third part outlines recent advances using encapsulation, remote self-healing and the role of shape memory polymers. Recent developments in the field of self-healing polymers undeniably indicate that the main challenge will be the designing of high glass transition (Tg) functional materials, which also exhibit stimuli-responsive attributes. Build-in controllable hierarchical heterogeneousness at various length scales capable of remote self-healing by physical and chemical responses will be essential in designing future materials of the 21st century.

Repairing Polymers Using Oscillating Magnetic Field

Repair of physically separated thermoplastic polymers containing γ-Fe2 O3 nanoparticles without sacrificing their mechanical properties is achieved by applying an oscillating magnetic field. As γ-Fe2 O3 nanoparticles oscillate at the frequency of the magnetic field, localized amorphous flow occur, and a permanent repair of physically separated polymeric films is achieved.

Dynamic materials: The chemistry of self-healing.

A reversible covalent reaction in which two oxygen-insensitive radicals combine to form a carbon–carbon bond provides the mechanism by which a polymer gel can self-heal at room temperature without the need for any external stimulus.

Facile UV-healable polyethylenimine–copper (C2H5N–Cu) supramolecular polymer networks

This study focuses on the development of facile polyethylenimine–copper (C2H5N–Cu) supramolecular polymer networks which upon mechanical damage are capable of reversible UV-induced self-repairs by the reformation of Cu–N coordination bonds. The chemical changes responsible for self-healing that leads to network remodeling include the formation of C2H5N–Cu complexes which, upon UV absorption, induce charge transfer between σ(N) bonding and dx2–y2(Cu) antibonding orbitals. The primary structural component responsible for network remodeling is the C2H5N–Cu coordination complex center that, upon UV exposure, undergoes square-planar-to-tetrahedral (D4h → Td) transition. The energy difference between dx2–y2 and dxz/yz orbitals during process change decreases significantly, enabling the σ(N) → dx2–y2(Cu) charge transfer and leading to energetically favorable Cu–N geometries. Manifested by virtually no temperature changes during UV-initiated self-healing, a unique feature of this network is the high efficiency of the damage–repair cycle resulting from the reversible conversion of electromagnetic radiation to chemical energy.

Attachment of Ampicillin to Expanded Poly(tetrafluoroethylene): Surface Reactions Leading to Inhibition of Microbial Growth

The broad spectrum antibiotic, ampicillin (AM), was reacted to expanded poly (tetrafluoroethylene) (ePTFE) surfaces and resulted in the formation of antimicrobial surfaces effective against gram-positive, Staphylococcus aureus, Bacillus thuringiensis, and Enterococcus faecalis, and gram-negative, Escherichia coli, Pseudomonas putida, and Salmonella enterica bacteria. These ePTFE surface modifications were accomplished by utilization of microwave maleic anhydride (MA) plasma reactions leading to the formation of acid groups, followed by amidation reactions of heterofunctional NH 2/COOH-terminated polyethylene glycol (PEG). The final step, the attachment of AM to the PEG spacer, was achieved by amidation reactions between COOH-terminated PEG and NH 2 groups of AM. This approach protects the COOH-AM functionality and diminishes the possibility of hydrolysis of the antimicrobial active portion of AM. These studies also show that approximately 90% of AM molecules are still covalently attached to PEG-MA-ePTFE surfaces after exposure to the bacteria solutions. Even after a 24 h period, the AM volume concentration changes only from 2.25 to 2.04 microg/m3, and depending upon the bacteria type, the bacteria suspensions containing AM-PEG-MA-ePTFE specimens retain 85-99% of their initial optical density.

Depth-Profiling Studies of Double-Layer PVF2-on-PET Films by Fourier Transform Infrared Photoacoustic Spectroscopy

Photoacoustic Fourier transform infrared spectroscopy (PA/FT-IR) is used to study films of poly(vinylidine fluoride) (PVF2) and poly(ethylene terephthalate) (PET). It is shown that PVF2 films over PET change the thermal diffusion length such that the sample PVF2-on-PET becomes optically transparent and thermally thick. By changing the mirror velocity of the Michelson interferometer it is possible to obtain spectra at various sample depths. The intensity of the carbonyl band of the lower PET film changes as a function of the mirror velocity. The log-log plot of these quantities gives a slope of −3/2, which agrees with theoretical predictions for thermally thick and optically transparent samples.

Distribution of melamine in melamine/polyester coatings ; FT-IR spectroscopic studies

Abstract Studies of polyester/melamine films using ATR FT-IR spectroscopy indicate that the melamine distribution across the films is uniform unless the hydroxyl content is low and the cure temperatures are relatively high. With polyester resins having an OH number as low as 6, curing at 250 °C with 30 w/w% melamine gives rise to melamine enrichment at the film-air interface. Enrichment is believed to result from melamine self-condensation near the interface. Although the distribution is a complex function and depends upon the amount of acid catalyst, the hydroxyl number of polyester, the film thickness and the reaction rate difference between the film-air and film-substrate interfaces, the primary factor leading to self-condensation is the loss of amine at the surface. Apparently, these factors play a key role in melamine self-condensation near the film-air interface and may control such properties as the crosslink density and the modification of properties such as flexibility and stain resistance.

Interfacial studies of crosslinked urethanes: Part II. The effect of humidity on waterborne polyurethanes; a spectroscopic study

In this study we examined isocyanate crosslinking reactions at the film-air (F-A) and film-substrate (F-S) interfaces of waterborne two-component (2K) urethanes as a function of depth and relative humidity (RH) using attenuated total reflectance (ATR) FTIR spectroscopy. Analysis of water/isocyanate reactions revealed urea formation which further interacts with water via H-bonding. Quantitative analysis of isocyanate consumption showed that the rates of film formation for waterborne polyurethanes are faster than equivalent solvent-borne polyurethanes. These studies showed that isocyanate consumption increases at high relative humidities. Examination of isocyanate consumption as a function of depth from the F-A and F-S interfaces showed that the isocyanate concentration increases when going from 0.65 to 1.14 µm.

Evaluation and Analysis of Attenuated Total Reflectance FT-IR Spectra Using Kramers-Kronig Transforms

Two commonly used correction algorithms that were originally developed by Bertie and Dignam and their co-workers for quantitative analysis of ATR spectra are examined. It is shown that the Dignam theory is more suitable for strong ATR bands, whereas the Bertie algorithm is more applicable for weak bands. On the basis of analysis of possible deviation sources determined in the Bertie and Dignam theories, a new ATR algorithm is developed and analyzed. With this approach, the initial estimate of the absorption index spectrum is obtained by KK correlation of the intensity and the phase of the complex Fresnel reflection coefficient. An iterative process that minimizes the difference between the true and calculated reflectivity spectra while maintaining the KK relation between refractive and absorption indices is used in conjunction with the Maclaurin numerical KKT method. Such an approach appears to improve accuracies for both weak and strong ATR bands.