Photopolymerization Kinetics of Pigmented Systems Using a Thin-Film Calorimeter

A thin-film calorimeter was used to evaluate the polymerization rates of pigmented photopolymerizable systems. As a result of a wide linear response range, which is more than an order of magnitude greater than that of the photo-DSC, the thin-film calorimeter is capable of measuring both small and large signals accurately. It was used to measure the polymerization exotherms of thin-films of a photocurable acrylate monomer with added pigment as well as commercial UV curable pigmented ink formulations

Activity of topical antimicrobial agents against multidrug-resistant bacteria recovered from burn patients

Background: Topical antimicrobials are employed for prophylaxis and treatment of burnwound infections despite no established susceptibility breakpoints, which are becoming vital in an era of multidrug-resistant (MDR) bacteria. We compared two methods of determining topical antimicrobial susceptibilities. Methods: Isolates of Pseudomonas aeruginosa, methicillin-resistant Staphylococcus aureus (MRSA), extended spectrum beta-lactamase (ESBL) producing Klebsiella pneumoniae, and Acinetobacter baumanii-calcoaceticus (ABC) from burn patients were tested using broth microdilution and agar well diffusion to determine minimum inhibitory concentrations (MICs) and zones of inhibition (ZI). Isolates had systemic antibiotic resistance and clonality determined. MDR included resistance to antibiotics in three or more classes. Results: We assessed 22 ESBL-producing K. pneumoniae, 20 ABC (75% MDR), 20 P. aeruginosa (45% MDR), and 20 MRSA isolates. The most active agents were mupirocin for MRSA and mafenide acetate for the gram-negatives with moderate MICs/ZI found with silver sulfadiazene, silver nitrate, and honey. MDR and non-MDR isolates had similar topical resistance. There was no clonality associated with resistance patterns. Conclusion: Despite several methods to test bacteria for topical susceptibility, no defined breakpoints exist and standards need to be established. We recommend continuing to use silver products for prophylaxis against gram-negatives and mafenide acetate for treatment, and mupirocin for MRSA

Design and Performance of a Thin-Film Calorimeter for Quantitative Characterization of Photopolymerizable Systems

A thin-film calorimeter (TFC) was designed for the quantitative characterization of photopolymerizable systems. A detailed description of its construction indicates the ease with which a TFC can be assembled and the flexibility inherent in its design. The mechanics of operation were optimized to yield a significantly faster instrument response time than other calorimetric methods such as photodifferential scanning calorimetry (photo-DSC). The TFC has enhanced sensitivity, more than an order of magnitude greater linear response range to changes in light intensity than that of the photo-DSC, resulting in the ability to measure both smaller and larger signals more accurately. The photopolymerization exotherm curves are reproducible and can be collected over a broad range of film thicknesses. © 2005 American Institute of Physics

Influence of the Alkene Structure On the Mechanism and Kinetics of Thiol-Alkene Photopolymerizations With Real-Time Infrared Spectroscopy

The effect of the chemical structure on the reactivity of alkenes used in thiol-ene photopolymerizations has been investigated with real-time infrared spectroscopy. Model studies of thiol-ene photoreactions with various monofunctional hydrocarbon alkenes and the monofunctional thiol ethyl-3-mercaptopropionate have been performed to identify and understand structure-reactivity relationships. The results demonstrate that terminal enes react very rapidly with thiol, achieve complete conversion, and are independent of the aliphatic hydrocarbon substituent length. Disubstitution on a single carbon of a terminal ene significantly reduces the reactivity, whereas substitution on the carbon a to the terminal ene has a minimal influence on the reactivity. Internal trans enes display reduced reactivity and a lower overall conversion and deviate from the standard thiol-ene reaction mechanism because of steric strain induced by 1,3-interactions. The reactivity and conversion of internal trans enes decrease as the substituents on the ene become larger, reaching a minimum when the substituent size is greater than or equal to that of propyl groups. Internal cis enes react rapidly with thiol; however, they undergo a fast isomerization-elimination reaction sequence generating the trans ene, which proceeds to react at a reduced rate with thiol. The reactivity of cyclic enes is dictated by ring strain, stereoelectronic effects, and hydrogen abstractability. The reactivity trends in the model studies have been used to explain the photopolymerization mechanism and kinetics of a series of multifunctional thiol-ene systems. (C) 2004 Wiley Periodicals, Inc

The Effect of Monomer Structure on Oxygen Inhibition of (Meth)acrylates Photopolymerization

Oxygen inhibition during the free-radical photopolymerization of various monomers containing ether groups was investigated using photo-DSC and real-time FTIR (RTIR). Methacrylates and acrylates containing different number and types of ether groups were selected to determine the effects of ether group concentration and structure on oxygen inhibition. Also, the oxygen sensitivity of the free-radical polymerization of methacrylate and acrylate was compared. Compared to acrylates, methacrylates are much less sensitive to oxygen. Model poly(tetramethylene oxide) and poly(propylene glycol) system were evaluated to determine the effect of ether groups on polymerization kinetics in air. The polymerizations of the (meth)acrylates containing ether groups were found to be relatively insensitive to oxygen inhibition. The reduction of oxygen inhibition occurs by a series of chain transfer/oxygen scavenging reactions. (C) 2004 Published by Elsevier Ltd

Photopolymerization of Pigmented Thiol-ene Systems

Photopolymerization kinetics and optical properties of pigmented thiol-ene coatings were investigated using photo-DSC, real-time FTIR, colorimetry, and AFM. Pigment has no deleterious effect on the unique ability of thiol-ene systems to photopolymerize in air. When trimethylolpropane tris-(3-mercaptopropionate) is incrementally added to tripropylene glycol diacrylate with and without calcium lithol rubine, a red organic pigment, the photopolymerization rate in nitrogen steadily decreases due to a shift in the polymerization mechanism from an acrylate homopolymerization to a thiol-ene copolymerization. However, the photopolymerization rate of pigmented and nonpigmented systems in air significantly increases with increasing thiol concentration, ultimately reaching a maximum at approximately 35 mole percent trifunctional thiol. The increase in rate is due to chain transfer from the non-reactive peroxy radical to the thiol. Thiol groups reduce oxygen inhibition to a greater degree than standard additives such as N-methyldiethanolamine, N-vinyl pyrrolidinone, and thioether containing trifunctional vinyl esters. For a typical acrylate based pigmented photocurable system, greater than 10 wt% photoinitiator is required to achieve a photopolymerization rate equivalent to a comparable thiol-ene system with 1 wt% photoinitiator in air. AFM and colorimetric data indicate that addition of trifunctional thiol has no deleterious effect on pigment dispersion and may in fact increase dispersion quality. (C) 2004 Elsevier Ltd. All rights reserved

In Situ Characterization of Photopolymerizable Systems Using a Thin-Film Calorimeter

A thin-film calorimeter (TFC) was used to provide quantitative characterization of photopolymerizable systems. Photopolymerization exotherms measured using the TFC are compared with real-time infrared (RTIR) spectroscopic results. The TFC successfully monitored the in-situ polymerization kinetics of thin/thick films because of its increased sensitivity/resolution over traditional calorimetric instrumentation. Reproducible polymerization exotherms of trimethylolpropane triacrylate were measured on films as thin as 163 nm. Flexibility in the sample cell construction enables the characterization of both volatile reactants (closed cell) and the effect of oxygen inhibition (open cell). Several molecular systems were evaluated. First, exotherms for the reaction of a series of volatile hexene monomers with monofunctional thiol illustrate the effect of ene location on alkene reactivity. Second, vinyl acrylate, a volatile difunctional monomer with a unique polymerization mechanism, was shown to homopolymerize significantly faster than either of its monofunctional analogues. Additionally, the rapid instrument response time of the TFC allowed qualitative analysis of non-steady-state polymerization kinetics of vinyl acrylate

Influence of Hydrogen Bonding on Photopolymerization Rate of Hydroxyalkyl Acrylates

The relationship between the photopolymerization rate of hydroxyalkyl acrylates and their structure has been investigated. The polymerization rates of hydroxyalkyl acrylates are significantly higher than those observed for typical monofunctional acrylate monomers and rival those of multifunctional monomers. By polymerizing at several temperatures, it was shown that the enhanced rates are directly proportional to the degree of hydrogen bonding. Apparently, termination rates are greatly reduced by hydrogen bonding, which is present in both the unpolymerized monomer and the final polymer film