There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Experimental NMR techniques designed to recouple weak homonuclear and heteronuclear dipolar coupling interactions under magic-angle spinning conditions are discussed. Developments and applications of such dipolar recoupling techniques are reviewed, covering the period 1994-early 1999. The various homonuclear and heteronuclear dipolar recoupling pulse sequences are illustrated and explained within the framework of average Hamiltonian theory.

Dipolar recoupling under magic-angle spinning conditions

Absorbed moisture can degrade the physical properties of an epoxy resin, jeopardizing the performance of an epoxy-based component. Although specific water- epoxy interactions are known to be very important in determining transport behavior, the role of network topology is not clear. In this article, a series of epoxies in which the topology is systematically varied (and the polarity held constant) is used to explore how topology influences the kinetics of moisture transport. The topology is quantified via the positron annihilation lifetime spectroscopy technique in terms of the size and volume fraction of electron density heterogeneities 5- 6 A in diameter, a dimension comparable to the 3-A kinetic diameter of a water molecule. Surprisingly, the volume fraction of such nanopores does not affect the diffusion coefficient (D) of water in any of the resins studied. For temperatures at and below 35 °C, there is a mild exponential dependence of D on the average nanopore size observed. Otherwise, the kinetics of moisture transport do not appear to depend on the nanopores. However, the initial flux of moisture into the epoxy does appear to correlate with the intrinsic hole volume fraction. That this correlation persists only in the initial stages of absorption is partially understood in terms of the ability of the water to alter the nanopore structure; only in the initial stages of uptake are the nanopores, as quantified in the dry state, relevant to transport. The role of specific epoxy-water interactions are also discussed in terms of transport kinetics. The lack of a correlation between the topology and transport sug- gests that polar interactions, and not topology, provide the rate-limiting step of trans- port. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 776 -791, 2000

https://deepblue.lib.umich.edu/bitstream/handle/2027.42/35005/15_ftp.pdf;sequence=1

Contributions of the nanovoid structure to the kinetics of moisture transport in epoxy resins

The function of synthetic and natural macromolecularsystems critically depends on the packing and dynamics of the individual components of a given system. Not only can solid-state NMR provide structural information with atomic resolution, but it can also provide a way to characterize the amplitude and time scales of motions over broad ranges of length and time. These movements include molecular dynamics, rotational and translational motions of the building blocks, and also the motion of the functional species themselves, such as protons or ions. This Account examines solid-state NMR methods for correlating dynamics and function in a variety of chemical systems.In the early days, scientists thought that the rotationalmotions reflected the geometry of the moving entities. They described these phenomena as jumps about well-defined axes, such as phenyl flips, even in amorphous polymers. Later, they realized that conformational transitions in macromolecules happen in a much more complex way. Because the indivi...

Solid-state NMR in macromolecular systems: insights on how molecular entities move.

Flexible and skin-attachable vibration sensors have been studied for use as wearable voice-recognition electronics. However, the development of vibration sensors to recognize the human voice accurately with a flat frequency response, a high sensitivity, and a flexible/conformable form factor has proved a major challenge. Here, we present an ultrathin, conformable, and vibration-responsive electronic skin that detects skin acceleration, which is highly and linearly correlated with voice pressure. This device consists of a crosslinked ultrathin polymer film and a hole-patterned diaphragm structure, and senses voices quantitatively with an outstanding sensitivity of 5.5 V Pa−1 over the voice frequency range. Moreover, this ultrathin device (<5 μm) exhibits superior skin conformity, which enables exact voice recognition because it eliminates vibrational distortion on rough and curved skin surfaces. Our device is suitable for several promising voice-recognition applications, such as security authentication, remote control systems and vocal healthcare. Though skin-attachable vibration sensors are promising for voice recognition applications, current technologies do not meet key performance requirements. Here, the authors report a flexible skin-attachable sensor with high sensitivity and flat frequency response over the vocal frequency range.

/pdf/an-ultrathin-conformable-vibration-responsive-electronic-wl2egriwji.pdf

An ultrathin conformable vibration-responsive electronic skin for quantitative vocal recognition

This paper deals with the determination of the main characteristics and the assignment of the molecular motions leading to the solid-state transitions observed in amorphous polymers at temperatures lower than the glass-rubber transition. First, the specific features of these secondary transitions (β, y, δ,...) are briefly described. Then, the behaviour of various polymer systems is analysed: poly(cycloalkyl methacrylates) and their intracycle motions, poly(ethylene tere-phthalate) and bisphenol A polycarbonate β transition and the effect of small molecule antiplasticisers, aryl-alkyl polyamide transitions, the β transition of poly(methyl methacrylate) and its maleimide as well as glutarimide random copolymers. Additionally linear polymers, aryl-aliphatic epoxy systems, with or without antiplasticisers, illustrate the case of crosslinked amorphous polymers. Whereas all the systems considered were investigated by dynamic mechanical measurements and 13 C solid-state nuclear magnetic resonance experiments, some systems were also studied by other techniques such as dielectric relaxation and molecular modelling. Poly(methyl methacrylate) and bisphenol A polycarbonate constitute the best examples of the level of description of the molecular motions involved in the β transition and, in particular, the nature and extent of the cooperativity which develops in the high-temperature part of the transition. This is achieved by combining all the experimental and modelling techniques presently available.

Investigation of solid-state transitions in linear and crosslinked amorphous polymers

Carbon-13 rotational-echo double-resonance (REDOR) NMR with 15N or 2H dephasing, combined with 15N REDOR NMR with 13C dephasing, has been obtained for a fully cross-linked epoxy resin prepared from a nominally uniform mixture of two parts of diglycidyl ether of Bisphenol A, one part of hexamethylenediamine, and 19% (by weight) antiplasticizer made from a carbonyl-13C-labeled aromatic acetamide. The antiplasiticizer contains the hydroxypropyl ether moiety of the epoxy repeat unit. A partially cross-linked resin was formed from five parts of epoxide, one part of hexamethylenediamine, three parts of hexylamine, and 19% (by weight) antiplasticizer. Labels were introduced into the resin by replacing both methyl groups of the isopropylidene moiety of Bisphenol A with CD3 groups and by using [15N2]hexamethylenediamine and [15N]hexylamine (and their unlabeled counterparts) in various combinations. The antiplasticizer 13C-carbonyl carbon is 4.9 ± 0.5 A from an amine 15N (with no preference for free or cross-linked...

Location of the Antiplasticizer in Cross-Linked Epoxy Resins by 2H, 15N, and 13C REDOR NMR

Etude du mecanisme de decomposition spinodale dans la region monophasique instable et des mecanismes de nucleation et de croissance dans les regions metastables. Etude de l'effet de la viscosite du melange et de la temperature de separation de phase sur la cinetique. Les resultats montrent que dans le regime de nucleation et de croissance, la mobilite des chaines est un facteur important

Kinetics of phase separation in binary mixtures of polystyrene and poly(vinyl methyl ether) as studied by fluorescence emission of labeled polystyrene

The chemical substitution of amine nitrogens in cured, 15 N-labeled epoxy resins has been determined by a combination of rotational-echo double-resonance 13 C NMR and dipolar rotational spin-echo 15 N NMR. Amine-nitrogen substitution is at least 90% (that is, no more than 10% of all nitrogens have a directly bonded hydrogen) for resins formed from stoichiometric amounts of epoxide and either hexamethylenediamine or a 1:3 molar mixture of hexamethylenediamine and hexylamine. This direct measure of curing and cross-linking is consistent with indirect Fourier-transform infrared estimates of curing based on the disappearance of the deformation band of the epoxide ring.

Determination of the Extent of Reaction of Amine Cross-Linked Epoxy Resins by Solid-State 13C and 15N NMR

This paper compares the results yielded by two methods of small-angle X-ray scattering data analysis for semicrystalline polymer blends. The first method is based on the use of a theoretical modeling for isotropic samples and a subsequent curve fitting. The second one is a more familiar method, based on the calculation of the linear one-dimensional correlation function. The experimental material considered for this purpose deals with 
a series of semi-crystalline blends of poly(vinylidene fluoride) and poly(methyl methacrylate), with a PVDF content covering the range 50 wt%–100 wt%. The results obtained by both calculation methods are systematically confronted to the crystallinity degrees deduced from wide angle X-ray scattering patterns.

/pdf/quantitative-analysis-of-semicrystalline-blends-saxs-data-3wd9m1rd1i.pdf

Jean Louis Halary

Papers

Investigation of solid-state transitions in linear and crosslinked amorphous polymers

Location of the Antiplasticizer in Cross-Linked Epoxy Resins by 2H, 15N, and 13C REDOR NMR

Kinetics of phase separation in binary mixtures of polystyrene and poly(vinyl methyl ether) as studied by fluorescence emission of labeled polystyrene

Determination of the Extent of Reaction of Amine Cross-Linked Epoxy Resins by Solid-State 13C and 15N NMR

Quantitative Analysis of Semicrystalline Blends SAXS Data: Theoretical Modeling versus Linear Correlation Function