The most revealing indicator for oxidative processes or state of degraded plastics is usually carbonyl formation, a key step in materials degradation as part of the carbon cycle for man-made materials. Hence, the identification and quantification of carbonyl species with infrared spectroscopy have been the method of choice for generations, thanks to their strong absorbance and being an essential intermediate in carbon oxidation pathways. Despite their importance, precise identification and quantification can be challenging and rigorous fully traceable data are surprisingly rare in the existing literature. An overview of the complexity of carbonyl quantification is presented by the screening of reference compounds in solution with transmission and polymer films with ATR IR spectroscopy, and systematic data analyses. Significant variances in existing data and their past use have been recognized. Guidance is offered how better measurements and data reporting could be accomplished. Experimental variances depend on the combination of uncertainty in exact carbonyl species, extinction coefficient, contributions from neighboring convoluting peaks, matrix interaction phenomena and instrumental variations in primary IR spectral acquisition (refractive index and penetration depth for ATR measurements). In addition, diverging sources for relevant extinction coefficients may exist, based on original spectral acquisition. For common polymer degradation challenges, a relative comparison of carbonyl yields for a material is easily accessible, but quantification for other purposes, such as degradation rates and spatially dependent interpretation, requires thorough experimental validation. All variables highlighted in this overview demonstrate the significant error margins in carbonyl quantification, with exact carbonyl species and extinction coefficients already being major contributors on their own.
A series of networks is introduced with systematically varied network heterogeneity and high overall values of average glass transition temperature (Tg), based on polymerization of rigid acrylate and aromatic thiol monomers. The curing behavior, chain dynamics, and microstructure of these networks were investigated through a combination of dynamic mechanical analysis and infrared spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and x-ray scattering, respectively. The maximum Tg achieved during cure can be related to the breadth of the mechanical loss tangent, as others have previously suggested, as well as the temperature dependence of the chain dynamics in the network as monitored by 1H NMR. In addition, the microstructures of the networks are characterized by periodic, fractal microgels with characteristic length scales of ca. 20–40 nm. Intriguingly, this structural motif persists in the more homogeneous networks exhibiting comparatively narrow glass transitions and chain dynamics, indicating that dynamically homogeneous networks can still exhibit significant compositional heterogeneity at the mesoscale.
The current COVID-19 pandemic has resulted in globally constrained supplies for face masks and personal protective equipment (PPE). Production capacity is limited in many countries and the future course of the pandemic will likely continue with shortages for high quality masks and PPE in the foreseeable future. Hence, expectations are that mask reuse, extended wear and similar approaches will enhance the availability of personal protective measures. Repeated thermal disinfection could be an important option and likely easier implemented in some situations, at least on the small scale, than UV illumination, irradiation or hydrogen peroxide vapor exposure. An overview on thermal responses and ongoing filtration performance of multiple face mask types is provided. Most masks have adequate material properties to survive a few cycles (i.e. 30 min disinfection steps) of thermal exposure in the 75°C regime. Some are more easily affected, as seen by the fusing of plastic liner or warping, given that preferred conditioning temperatures are near the softening point for some of the plastics and fibers used in these masks. Hence adequate temperature control is equally important. As guidance, disinfectants sprayed via dilute solutions maintain a surface presence over extended time at 25 and 37°C. Some spray-on alcohol-based solutions containing disinfectants were gently applied to the top surface of masks. Neither moderate thermal aging (less than 24 h at 80 and 95°C) nor gentle application of surface disinfectant sprays resulted in measurable loss of mask filter performance. Subject to bio-medical concurrence (additional checks for virus kill efficiency) and the use of low risk non-toxic disinfectants, such strategies, either individually or combined, by offering additional anti-viral properties or short term refreshing, may complement reuse options of professional masks or the now ubiquitous custom-made face masks with their often unknown filtration effectiveness.
The current COVID-19 pandemic is stretching both the global supply for face masks and personal protective equipment (PPE). Production capacity is severely limited in many countries. This is a call for the R&D community, particularly to those in the polymer degradation and stability field. We have not only an opportunity but an obligation to engage and collaborate with virology and bio-medical experts. We require comparative R&D for extended, reuse and recyclability options. There is urgent need for large scale institutional approaches and methods that can be quickly applied locally by non-experts with limited resources.
Park, Jong R.; Van Guyse, Joachim F.R.; Podevyn, Annelore; Bolle, Eleonore C.L.; Bock, Nathalie; Linde, Carl E.; Celina, Mathias C.; Hoogenboom, Richard; Dargaville, Tim R.
Four drug-conjugated poly(2-alkyl-2-oxazoline) (PAOx) networks with different hydrophobicity were synthesized via copolymerization of either 2-methyl-, 2-ethyl-, 2-propyl- or 2-butyl-2-oxazoline with the functional monomer, 2-dec-9-enyl-2-oxazoline. The incorporation of a labile ester linkage between the polymer and the drug benazepril allowed for sustained drug release over periods of months with the release rates strongly depending on the hydrophobicity of the polymer pendant groups. Drug loading of 13 ± 2 wt% was used with 10 mol% crosslinking sites simply by tuning the thiol-ene stoichiometry. The networks exhibited negligible cell toxicity but cell repulsion was observed for hydrogels based on poly(2-methyl-2-oxazoline) and poly(2-ethyl-2-oxazoline) while those based on poly(2-n-propyl-2-oxazoline) and poly(2-n-butyl-2-oxaoline) showed cell adhesion. These results suggest that PAOx networks have great potential as drug delivery devices for long-lasting drug release applications.
The TED-GC–MS analysis is a two-step method. A sample is first decomposed in a thermogravimetric analyzer (TGA) and the gaseous decomposition products are then trapped on a solid-phase adsorber. Subsequently, the solid-phase adsorber is analyzed with thermal desorption gas chromatography mass spectrometry (TDU-GC–MS). This method is ideally suited for the analysis of polymers and their degradation processes. Here, a new entirely automated system is introduced which enables high sample throughput and reproducible automated fractioned collection of decomposition products. The fractionated collection together with low temperatures reduces the risk of contamination, improves instrumental stability and minimizes maintenance efforts. Through variation of the two main parameters (purge gas flow and heating rate) it is shown how the extraction process can be optimized. By measuring the decomposition products of polyethylene it is demonstrated that compounds with masses of up to 434 Da can be detected. This is achieved despite the low temperature (˜40 °C) of the solid-phase adsorber and the low thermal desorption temperature of 200 °C in the TDU unit. It is now shown that automated TED-GC–MS represents a new flexible multi-functional method for comprehensive polymer analyses. Comparable polymer characterization was previously only achievable through a combination of multiple independent analytical methods. This is demonstrated by three examples focused on practical challenges in materials analysis and identification: The first one is the analysis of wood plastic composites for which the decomposition processes of the polymer and the bio polymer (wood) could be clearly distinguished by fractionated collection using sequential adsorbers. Secondly, a fast quantitative application is shown by determining the weight concentrations of an unknown polyolefin blend through comparison with a reference material. Additionally, the determination of microplastic concentrations in environmental samples is becoming an increasingly important analytical necessity. It is demonstrated that with TED-GC–MS calibration curves showing good linearity for the most important precursors for microplastic, even complex matrix materials (suspended particulate matter) can be successfully analyzed.
The application, continued performance, and degradation behavior of polymers often depends on their interaction with small organic or gaseous volatiles. Understanding the underlying permeation and diffusion properties of materials is crucial for predicting their barrier properties (permeant flux), drying behavior, solvent loss or tendency to trap small molecules, as well as their interaction with materials in the vicinity due to off-gassing phenomena, perhaps leading to compatibility concerns. Further, the diffusion of low M w organics is also important for mechanistic aspects of degradation processes. Based on our need for improved characterization methods, a FTIR-based spectroscopic gas/volatile quantification setup was designed and evaluated for determination of the diffusion, desorption and transport behavior of small IR-active molecules in polymers. At the core of the method, a modified, commercially available IR transmission gas cell monitors time-dependent gas concentration. Appropriate experimental conditions, e.g. desorption or permeation under continuous flow or static gas conditions, are achieved using easily adaptable external components such as flow controllers and sample ampoules. This study presents overview approaches using the same IR detection methodology to determine diffusivity (desorption into a static gas environment, continuous gas flow, or intermittent desorption) and permeability (static and dynamic flow detection). Further, the challenges encountered for design and setup of IR gas quantification experiments, related to calibration and gas interaction, are presented. These methods establish desorption and permeation behavior of solvents (water and methanol), CO 2 off-gassing from foam, and offer simultaneous measurements of the permeation of several gases in a gas mixture (CO 2 , CO and CH 4 ) through polymer films such as epoxy and Kapton. They offer complementary guidance for material diagnostics and understanding of basic properties in sorption and transport behavior often of relevance to polymer degradation or materials reliability phenomena.
The use of self-assembling, pre-polymer materials in 3D printing is rare, due to difficulties of facilitating printing with low molecular weight species and preserving their reactivity and/or functions on the macroscale. Akin to 3D printing of small molecules, examples of extrusion-based printing of pre-polymer thermosets are uncommon, arising from their limited rheological tuneability and slow reactions kinetics. The direct ink write (DIW) 3D printing of a two-part resin, Epon 828 and Jeffamine D230, using a self-assembly approach is reported. Through the addition of self-assembling, ureidopyrimidinone-modified Jeffamine D230 and nanoclay filler, suitable viscoelastic properties are obtained, enabling 3D printing of the epoxy–amine pre-polymer resin. A significant increase in viscosity is observed, with an infinite shear rate viscosity of approximately two orders of magnitude higher than control resins, in addition to, an increase in yield strength and thixotropic behavior. Printing of simple geometries is demonstrated with parts showing excellent interlayer adhesion, unachievable using control resins.
A new approach is presented for conducting and extrapolating combined environment (radiation plus thermal) accelerated aging experiments. The method involves a novel way of applying the time-temperature-dose rate (t-T-R) approach derived many years ago, which assumes that by simultaneously accelerating the thermal-initiation rate (from Arrhenius T-only analysis) and the radiation dose rate R by the same factor x, the overall degradation rate will increase by the factor x. The dose rate assumption implies that equal dose yields equal damage, which is equivalent to assuming the absence of dose-rate effects (DRE).Aplot of inverse absolute temperature versus the log of the dose rate is used to indicate experimental conditions consistent with themodel assumptions, which can be derived along lines encompassing so-called matched accelerated conditions (MAC lines). Aging trends taken along MAC lines for several elastomers confirms the underlying model assumption and therefore indicates, contrary to many past published results, that DRE are typically not present. In addition, the MAC approach easily accommodates the observation that substantial degradation chemistry changes occur as aging conditions transition R-T space from radiation domination (high R, low T) to temperature domination (low R, high T). The MAC-line approach also suggests an avenue for gaining more confidence in extrapolations of accelerated MAC-line data to ambient aging conditions by using ultrasensitive oxygen consumption (UOC) measurements taken along the MAC line both under the accelerated conditions and at ambient. From UOC data generated under combined R-T conditions, this approach is tested and quantitatively confirmed for one of thematerials. In analogy to the wear-out approach developed previously for thermo-oxidative aging, the MAC-line concept can also be used to predict the remaining lifetimes of samples extracted periodically from ambient environments.
This report catalogues the results of a project exploring the incorporation of organometallic compounds into thermosetting polymers as a means to reduce their residual stress. Various syntheses of polymerizable ferro cene derivatives were attempted with mixed success. Ultimately, a diamine derivative of ferrocene was used as a curing agen t for a commercial epoxy resin, where it was found to give similar cure kinetics and mechanical properties in comparison to conventional curing agents. T he ferrocen e - based material is uniquely able to relax stress above the glass transition, leading to reduced cure stress. We propose that this behavior arises from the fluxional capacity of ferrocene. In support of this notion, nuclear magnetic resonance spectroscopy indicates a substantial increase in chain flexibility in the ferrocene - containing network. Although t he utilization of fluxionality is a novel approach to stress management in epoxy thermosets, it is anticipated to have greater impact in radical - cured ther mosets and linear polymers.
Physical stress relaxation in rubbery, thermoset polymers is limited by cross-links, which impede segmental motion and restrict relaxation to network defects, such as chain ends. In parallel, the cure shrinkage associated with thermoset polymerizations leads to the development of internal residual stress that cannot be effectively relaxed. Recent strategies have reduced or eliminated such cure stress in thermoset polymers largely by exploiting chemical relaxation processes, wherein temporary cross-links or otherwise transient bonds are incorporated into the polymer network. Here, we explore an alternative approach, wherein physical relaxation is enhanced by the incorporation of organometallic sandwich moieties into the backbone of the polymer network. A standard epoxy resin is cured with a diamine derivative of ferrocene and compared to conventional diamine curing agents. The ferrocene-based thermoset is clearly distinguished from the conventional materials by reduced cure stress with increasing cure temperature as well as unique stress relaxation behavior above its glass transition in the fully cured state. The relaxation experiments exhibit features characteristic of a physical relaxation process. Furthermore, the cure stress is observed to vanish precipitously upon deliberate introduction of network defects through an increasing imbalance of epoxy and amine functional groups. We postulate that these beneficial properties arise from fluxional motion of the cyclopentadienyl ligands on the polymer backbone.
When diethanolamine (DEA) is used as a curative for a DGEBA epoxy, a rapid “adduct-forming” reaction of epoxide with the secondary amine of DEA is followed by a slow “gelation” reaction of epoxide with hydroxyl and with other epoxide. Through an extensive review of previous investigations of simpler, but chemically similar, reactions, it is deduced that at low temperature the DGEBA/DEA gelation reaction is “activated” (shows a pronounced induction time, similar to autocatalytic behavior) by the tertiary amine in the adduct. At high temperature, the activated nature of the reaction disappears. The impact of this mechanism change on the kinetics of the gelation reaction, as resolved with differential scanning calorimetry, infrared spectroscopy, and isothermal microcalorimetry, is presented. It is shown that the kinetic characteristics of the gelation-reaction of the DGEBA/DEA system are similar to other tertiary-amine activated epoxy reactions and consistent with the anionic polymerization model previously proposed for this class of materials. Principle results are the time-temperature-transformation diagram, the effective activation energy, and the upper stability temperature of the zwitterion initiator of the activated gelation reaction. It is established that the rate of epoxide consumption cannot be generically represented as a function only of temperature and degree of epoxy conversion. The complex chemistry active in the material requires specific consideration of the dilute intermediates in the reaction sequence in order to define a model of the reaction kinetics applicable to all time-temperature cure histories.
The aim of this study was to alter polymerization chemistry to improve network homogeneity in free-radical crosslinked systems. It was hypothesized that a reduction in heterogeneity of the network would lead to improved mechanical performance. Experiments and simulations were carried out to investigate the connection between polymerization chemistry, network structure and mechanical properties. Experiments were conducted on two different monomer systems - the first is a single monomer system, urethane dimethacrylate (UDMA), and the second is a two-monomer system consisting of bisphenol A glycidyl dimethacrylate (BisGMA) and triethylene glycol dimethacrylate (TEGDMA) in a ratio of 70/30 BisGMA/TEGDMA by weight. The methacrylate systems were crosslinked using traditional radical polymeriza- tion (TRP) with azobisisobutyronitrile (AIBN) or benzoyl peroxide (BPO) as an initiator; TRP systems were used as the control. The monomers were also cross-linked using activator regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP) as a type of controlled radical polymerization (CRP). FTIR and DSC were used to monitor reac- tion kinetics of the systems. The networks were analyzed using NMR, DSC, X-ray diffraction (XRD), atomic force microscopy (AFM), and small angle X-ray scattering (SAXS). These techniques were employed in an attempt to quantify differences between the traditional and controlled radical polymerizations. While a quantitative methodology for characterizing net- work morphology was not established, SAXS and AFM have shown some promising initial results. Additionally, differences in mechanical behavior were observed between traditional and controlled radical polymerized thermosets in the BisGMA/TEGDMA system but not in the UDMA materials; this finding may be the result of network ductility variations between the two materials. Coarse-grained molecular dynamics simulations employing a novel model of the CRP reaction were carried out for the UDMA system, with parameters calibrated based on fully atomistic simulations of the UDMA monomer in the liquid state. Detailed metrics based on network graph theoretical approaches were implemented to quantify the bond network topology resulting from simulations. For a broad range of polymerization parameters, no discernible differences were seen between TRP and CRP UDMA simulations at equal conversions, although clear differences exist as a function of conversion. Both findings are consistent with experiments. Despite a number of shortcomings, these models have demonstrated the potential of molecular simulations for studying network topology in these systems.