Thiol–norbornene materials: Approaches to develop high Tg thiol–ene polymers

The ability to prepare high T_g low shrinkage thiol–ene materials is attractive for applications such as coatings and dental restoratives. However, thiol and nonacrylated vinyl materials typically consist of a flexible backbone, limiting the utility of these polymers. Hence, it is of importance to synthesize and investigate thiol and vinyl materials of varying backbone chemistry and stiffness. Here, we investigate the effect of backbone chemistry and functionality of norbornene resins on polymerization kinetics and glass transition temperature (T_g) for several thiol–norbornene materials. Results indicate that T_gs as high as 94 °C are achievable in thiol–norbornene resins of appropriately controlled chemistry. Furthermore, both the backbone chemistry and the norbornene moiety are important factors in the development of high T_g materials. In particular, as much as a 70 °C increase in T_g was observed in a norbornene–thiol specimen when compared with a sample prepared using allyl ether monomer of analogous backbone chemistry. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5686–5696, 2007     

Influence of the secondary functionality on the radical‐vinyl chemistry of highly reactive monoacrylates

The impact of secondary functionalities on the radical‐vinyl chemistry of monoacrylates characterized by secondary functionalities that dramatically enhance their polymerization rate was elucidated utilizing experimental and computational techniques. Firstly, bulk interactions affecting the acrylate reactivity towards photopolymerization were removed by polymerizing at 5 wt % monomer in 1,4‐dioxane. Following deconvolution of bulk interactions impacting reactivity towards photopolymerization, a linear correlation between average polymerization rates and Michael addition reaction rate constants was observed on a logarithmic scale. This result indicates that the presence of the secondary functionality intramolecularly alters the monomer chemistry in a manner which impacts both of these distinct reaction types in a similar manner. These monomers exhibited reduced activation energies in both Michael addition and photopolymerization reactions as compared to hexyl acrylate. Reduction up to 20 ± 8 kJ mole^?1was observed for Michael addition reactions and 12 ± 1 kJ mole^?1 for photopolymerization reactions, thereby explaining the higher reactivity of the acrylates characterized by the secondary functionalities. Cyclic voltammetry experiments conducted to investigate the nature of the acrylic double bonds indicated that the rapidly polymerizing acrylates are more readily reduced as compared to traditional acrylates. Further, a distinct monotonic correlation of the irreversible cathodic peak potentials of the (meth)acrylates to photopolymerization and Michael addition reactivity was observed. The computationally estimated acrylic LUMO energies characterized by the secondary functionalities (?2.3 eV to ?2.7 eV) were also found to be lower relative to hexyl acrylate (?2.2 eV). © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4859–4870, 2009     

Neutron scattering study on CeAgAs2

CeAgAs₂, an HfCuSi₂ like layered pnictide, orders antiferromagnetically at T_N=6.2(1) K. The ordering process was monitored in neutron diffraction experiments in the temperature range 10 K≥T≥3.5 K. At T=4 K the lattice parameters are a=5.7438(1) Å, b=5.7696(1) Å and c=21.0067(2) Å. The diffraction pattern of the antiferromagnetic phase with a propagation vector k=[0,0,0] point towards ferromagnetically ordered moments in Ce layers stacked along [001], the individual layers are coupled antiferromagnetically with a +− −+type sequence. The alignment of moments within the Ce layers cannot be determined reliably from the experimental data so that two different structure models are discussed. The proposed metamagnetic transition was confirmed by diffraction experiments applying an external magnetic field at T<T_N. In the interval 4 K≤T≤6 K, a relatively small field of μ₀H≈0.3 T suffices to fully suppress the antiferromagnetic ordering. The effect is completely reversible yet subject to hysteresis: After switching off the external field at any T<T_N the magnetic reflections gain their original intensity within several 10 min indicating the restoring of the antiferromagnetic phase.     

Design, Development and Evaluation of Monovinyl Acrylates Characterized by Secondary Functionalities as Reactive Diluents to Diacrylates

This study focuses on the design and development of novel monovinylic (meth)acrylate monomers with enhanced polymerization kinetics and the evaluation of their performance as reactive diluents in diacrylate systems. Novel (meth)acrylic monomers characterized by several new secondary functionalities are developed in this study and are shown to exhibit reactivities 10-70 fold greater than traditional monoacrylates such as hexyl acrylate. These monomers were designed based on our understanding of interactions between monomer structure, polymerizations kinetics, and polymer properties. Performance of these monovinyl monomers as reactive diluents is also investigated in this study. Copolymerization of these monomers with diacrylates enhanced both the reactivity and the mechanical properties of the diacrylate system. Specifically, while copolymerization of a diacrylate system with traditional monoacrylates such as hexyl acrylate decreases the overall reactivity of the system, its copolymerization with the novel monomers led to comonomer mixtures, that were 30-50% more reactive than either of the individual components, with initial polymerization rates increased by as much as 2 times the more reactive component. Further, the copolymerization of these novel monovinyl systems with diacrylates also enabled formation of polymers with enhanced mechanical properties over the corresponding diacrylates including a more homogeneous network structure as indicated by a glass transition temperature that was narrowed by up to 55 % while increasing the glass transition temperature by as much as 10°C.     

Normal-state hourglass dispersion of the spin excitations in FeSexTe(1-x)

We use cold neutron spectroscopy to study the low-energy spin excitations of superconducting (SC) FeSe0.4Te0.6 and essentially nonsuperconducting (NSC) FeSe0.45Te0.55. In contrast with BaFe2-x(Co,Ni)xAs2, where the low-energy spin excitations are commensurate both in the SC and normal state, the normal-state spin excitations in SC FeSe0.4Te0.6 are incommensurate and show an hourglass dispersion near the resonance energy. Since similar hourglass dispersion is also found in the NSC FeSe0.45Te0.55, we argue that the observed incommensurate spin excitations in FeSe(1-x)Tex are not directly associated with superconductivity. Instead, the results can be understood within a picture of Fermi surface nesting assuming extremely low Fermi velocities and spin-orbital coupling.     

Improvement of sensor performance of high-TC thin filmplanar SQUID gradiometers by ion beam etching

The sensor performance of galvanically coupled Y₁Ba₂Cu₃O_7-x (YBCO) dc SQUID gradiometers on 24° bicrystal substrates has been improved by thickness reduction in the region of the grain boundary Josephson junctions using ion beam etching. The prepared etching mask allows the reduction of the critical current by more than one order of magnitude while the SQUID inductance is slightly increased. This treatment shifts the SQUID parameter β_L from values above 10 to the proposed optimum around 1. The authors observed with decreasing critical current and increasing normal resistance a reduced I_CR_N product with values between 300 and 400 μV at 150-nm film thickness changing to values near 150 μV at 50-nm film thickness. Despite this fact, the white flux noise level as well as the low-frequency noise is reduced. With their galvanically coupled 4×8 mm² dc SQUID gradiometer the authors obtained a white noise level of 4.2 μΦ₀/√Hz corresponding to a field gradient sensitivity of 430 fT/cm√Hz at 77 K after the trimming process     

A fine-grained Ethernet performance model

We have developed a novel analytical Ethernet performance model that contains a complete formulation of system response time from the initiation of user requests for service to the delivery of the service to the user. The estimated response time includes simultaneous input bus delay (requests for service from multiple users), simultaneous output delay (responses to requests from multiple servers), considering time lost to collisions and subsequent backoff time, time lost to deferring transmission due to a busy channel, and transmission time. In addition, we include server disk input and output queueing times and server and user disk access times. By modeling all epochs of collision, backoff, and transmission events, we have developed a way of providing a level of detail and accuracy that other analytic models of Ethernet performance do not provide. In addition to using the model for estimating response time as a function of number of users and servers, the model is used to determine the number of servers that would be appropriate for a given range of number of users.     

SERS: a versatile tool in chemical and biochemical diagnostics

Raman spectroscopy is a valuable tool in various research fields. The technique yields structural information from all kind of samples often without the need for extensive sample preparation. Since the Raman signals are inherently weak and therefore do not allow one to investigate substances in low concentrations, one possible approach is surface-enhanced (resonance) Raman spectroscopy. Here, rough coin metal surfaces enhance the Raman signal by a factor of 10⁴–10¹⁵, depending on the applied method. In this review we discuss recent developments in SERS spectroscopy and their impact on different research fields.     

Reliability modeling for safety-critical software

Software reliability predictions can increase trust in the reliability of safety critical software such as the NASA Space Shuttle Primary Avionics Software System (Shuttle flight software). This objective was achieved using a novel approach to integrate software-safety criteria, risk analysis, reliability prediction, and stopping rules for testing. This approach applies to other safety-critical software. The authors cover only the safety of the software in a safety-critical system. The hardware and human-operator components of such systems are not explicitly modeled nor are the hardware and operator-induced software failures. The concern is with reducing the risk of all failures attributed to software. Thus, safety refers to software-safety and not to system-safety. By improving the software reliability, where the reliability measurements and predictions are directly related to mission and crew safety, they contribute to system safety. Software reliability models provide one of several tools that software managers of the Shuttle flight software are using to assure that the software meets required safety goals. Other tools are inspections, software reviews, testing, change control boards, and perhaps most important-experience and judgement