Photomechanics of light-activated polymers

Light-activated polymers are an exciting class of modern materials that respond mechanically when irradiated by light at particular wavelengths. While details of the mechanisms that connect the optical excitation to mechanical behavior are complex and differ from material to material, there is sufficient commonality among them to permit the development of a generalized modeling framework to describe the photomechanics. The features shared by light-activated polymers involve light interacting with the material, which triggers photochemical reactions that alter the structure of the crosslinked polymer network. Many such structural alterations result in an evolution of the polymer network, and subsequent macroscopic deformation. When this process is appropriately executed it can enable a photomechanical shape-memory effect. In this paper, we develop a three-dimensional finite-deformation modeling framework to describe the photomechanical response of light-activated polymer systems. This framework integrates four coupled phenomena that contribute to macroscopic photomechanical behavior: photophysics, photochemistry, chemomechanical coupling, and mechanical deformation. The chemomechanical coupling consists of chemically induced structural alterations of the crosslinked network that result in subsequent deformation. We describe this behavior through a decomposition of the crosslinked network into two components consisting of an original network and a photochemically altered network; both evolve during photomechanical deformation. The modeling framework presented in this paper is sufficiently general that it is applicable to light-activated polymer systems that operate with various mechanisms in each of the four areas. Using this modeling approach, we develop constitutive models for two recently developed light-activated polymer systems [Lendlein, A., Hongyan, J., Junger, O., Langer, R., 2005. Light-induced shape-memory polymers. Nature 434 (7035) 879; Scott, T.F., Schneider, A.D., Cook, W.D., Bowman, C.N., 2005. Photoinduced plasticity in crosslinked polymers. Science 308 (5728) 1615]. For the material developed by Scott and his co-workers we validate our model by measuring and numerically simulating photo-induced stress relaxation and bending deformation and obtain good agreement between measurements and predictions. Finally, we use the model to study the effects of photomechanical parameters (applied strain magnitude, irradiation time and intensity, and photoabsorber concentration) and the behavior of the network evolution rule on the material response.     

Finite Rate Chemistry Effects in Highly Sheared Turbulent Premixed Flames

Detailed scalar structure measurements of highly sheared turbulent premixed flames stabilized on the piloted premixed jet burner (PPJB) are reported together with corresponding numerical calculations using a particle based probability density function (PDF) method. The PPJB is capable of stabilizing highly turbulent premixed jet flames through the use of a small stoichiometric pilot that ensures initial ignition of the jet and a large shielding coflow of hot combustion products. Four lean premixed methane-air flames with a constant jet equivalence ratio are studied over a wide range of jet velocities. The scalar structure of the flames are examined through high resolution imaging of temperature and OH mole fraction, whilst the reaction rate structure is examined using simultaneous imaging of temperature and mole fractions of OH and CH₂O. Measurements of temperature and mole fractions of CO and OH using the Raman–Rayleigh–LIF-crossed plane OH technique are used to examine the flame thickening and flame reaction rates. It is found that as the shear rates increase, finite-rate chemistry effects manifest through a gradual decrease in reactedness, rather than the abrupt localized extinction observed in non-premixed flames when approaching blow-off. This gradual decrease in reactedness is accompanied by a broadening in the reaction zone which is consistent with the view that turbulence structures become embedded within the instantaneous flame front. Numerical predictions using a particle-based PDF model are shown to be able to predict the measured flames with significant finite-rate chemistry effects, albeit with the use of a modified mixing frequency.     

Controlling the Spontaneous Precipitation of Silver Nanoparticles in Sol-Gel Materials

The mechanisms responsible for spontaneous silver precipitation in silver-doped sol-gel materials are identified. The chemistry of the solvent phase is found to be the critical factor in controlling this phenomenon. The addition of HCl as catalyst leads to the formation of AgCl and subsequent formation of silver upon light exposure. Another factor leading to silver precipitation is the reducing capability of methanol radicals. Silver precipitation is inhibited by simply washing out the pore solvents by a solvent exchange method.     

Electromagnetically induced transparency in N-level cascade schemes

We examine electromagnetically induced transparency (EIT) in cascade schemes with N levels and N−1 fields. We show that transparency effects are present when N is odd and that destruction of EIT is present on line centre when N is even. We predict multiple dark resonances in such schemes due to multiphoton EIT effects. By examining atomic rubidium we propose methods of achieving such schemes by use of coupling rf fields into hyperfine levels.     

Copper and iron interactions with angiotensin-converting enzyme inhibitors. A study by fast-atom bombardment tandem mass spectrometry

Fast atom bombardment, combined with high-energy collision-induced tandem mass spectrometry, has been used to investigate gas-phase metal-ion interactions with captopril, enalaprilat and lisinopril, all angiotensin-converting enzyme inhibitors.Suggestions for the location of metal-binding sites are presented. For captopril, metal binding occurs most likely at both the sulphur and the nitrogen atom. For enalaprilat and lisinopril, binding preferably occurs at the amine nitrogen. Copyright 1999 John Wiley & Sons, Ltd.     

Analysis of a Complex Gaseous Mixture by TG-MS and TG-FTIR

The thermal decomposition of sodium ethyl xanthate (SEX) was used to compare the techniques of pyrolysis-gas chromatography-mass spectrometry (py-GC-MS), thermogravimetry-Fourier transform infrared spectroscopy (TG-FTIR), and TG-MS. In the py-GC-MS analysis, SEX was pyrolysed at 400°C in an inert atmosphere. Major gases evolved were carbon disulfide, diethyl sulfide, ethanol, and carbonyl sulfide. The TG of SEX exhibited a sharp mass loss at 201°C (42.3%) and a gradual mass loss at 217-325°C (20.8 %). The MS spectra of the evolved gases were complex due to overlapping of molecular, isotope, and fragment ion signals. Using the MS in selected ion monitoring mode, the major gases evolved were found to be carbon disulfide and carbonyl sulfide. The FTIR spectra of the evolved gases displayed vibrational frequencies due to alkanes, carbonyls, carbonyl sulfide, and carbon disulfide. From the analyses it was concluded that py-GC-MS provided unambiguous gas identification. Interpretation of the MS results was reliant on the py-GC-MS results, and the FTIR data was limited to identifying gases with very characteristic vibration frequencies. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of Perfluoroarene–Arene Interactions on the Phase Behavior of Liquid Crystalline and Polymeric Materials

A stabilization of the liquid-crystalline mesophase and thus an enlarged temperature range of the mesogenic phase is achieved by adding perfluorotriphenylene to a chiral liquid-crystalline triphenylene. This mesophase is based on 1:1 perfluoroarene–arene interactions (see picture). In a polymer with triphenylenes as mesogens in the side chains, the addition of perfluorotriphenylene led to crystallization.     

Rotational analysis of the red electronic emission spectrum of molybdenum nitride (MoN)

A system of emission bands in the red region of the optical spectrum has been identified as due to the species MoN. The system was generated by the microwave (2450 MHz) excitation of a flowing mixture of MoCl₅ and molecular nitrogen in a stream of helium but is also observed in a DC arc in air between molybdenum electrodes. One of the Q-form branches has previously been assumed to be an atomic line of Mo I. The system has been assigned as the 0, 0 band of a ⁴Π(a) → X⁴Σ⁻(a) transition, with a large zero-field splitting of the ground ⁴Σ⁻ term (86 cm⁻¹). A preliminary search has been made to detect the presence of MoN in M-type stars.     

New reactions of deoxyvasicinone. Part 4

Analogues of deoxyvasicinone (1) in which the pyrrolo ring is substituted, enlarged, or attached to the a face of the quinazolone system were prepared and several reactions of these analogues with electrophilic reagents have been investigated.