The thermoelastic behaviour of semicrystalline and of glassy poly(ether-ether-ketone)

A thermoelastic evaluation, based on simultaneous measurements of the mechanical work and of the concomitant heat of deformation by a stretching micro calorimeter, was performed on semicrystalline and glassy PEEK. The objective of this study was to utilize the sensitive technique to detect differences that would account for observed effects of micro structure on mechanical performance. A clear difference was detected beyond a 0.6% strain, where the behaviour of glassy PEEK began to exhibit inelastic features such as yielding and plastic deformation. This difference between the glassy and the semicrystalline polymers was considered the reason for the superior mechanical fatigue and fracture properties produced by the latter micro structure.     

Filament-wound composite soft tissue prostheses:controlling compliance and strength by water absorption and degradation

This paper deals with the effect of water absorption on the mechanical performance of selectively biodegradable filament-wound composite soft tissue prostheses. It shows how the water absorption phenomenon can be utilized as a new concept of material design, in contrast to the current view that associates moisture absorption by composite materials exclusively with damage. Harnessing water absorption to property design of vascular grafts has two advantages. The first is the controlled increase of the compliance of the graft during healing, aiming to reach a final stage of isocompliance with the host artery. The second is the water-induced biodegradation which enables regulation of the mural porosity during healing of an initially impervious graft. Filament-wound composite vascular prostheses comprising partially biodegradable matrices, different compositions and various winding angles are studied. Water absorption and material degradation are expressed in terms of relative weight gain/loss, which in turn is correlated with changes in the compliance and in the ultimate strength of the grafts.     

Hybrid pulse position modulation/ultrashort light pulse code-division multiple-access systems .I. Fundamental analysis

We propose a hybrid pulse position modulation/ultrashort light pulse code-division multiple-access (PPM/ULP-CDMA) system for ultrafast optical communications. The proposed system employs spectral CDMA encoding/decoding and PPM with very short pulse separation. The statistical properties of the encoded ULP are investigated with a general pulse model, and the performance of the proposed PPM/ULP-CDMA system is investigated. It is shown that we can improve upon the performance of other ULP-CDMA systems, such as those using on-off keying, by employing PPM. It is also shown that we can improve the performance of the proposed system by increasing the effective number of chips, by increasing the number of PPM symbols, and by reducing the ULP duration. The performance analysis shows that the aggregate throughput of the proposed PPM/ULP-CDMA system could be over 1 Tb/s.     

Folded free-space polarization-controlled multistage interconnection network

We present a folded free-space polarization-controlled optical multistage interconnection network (MIN) based on a dilated bypass-exchange switch (DBS) design that uses compact polarization-selective diffractive optical elements (PDOE's). The folded MIN design has several advantages over that of the traditional transparent MIN, including compactness, spatial filtering of unwanted higher-order diffraction terms leading to an improved signal-to-noise ratio (SNR), and ease of alignment. We experimentally characterize a folded 2 x 2 switch, as well as a 4 x 4 and an 8 x 8 folded MIN that we have designed and fabricated. We fabricated an array of off-axis Fresnel lenslet PDOE's with a 30:1 SNR and used it to construct a 2 x 2 DBS with a measured SNR of 60:1. Using this PDOE array in a 4 x 4 MIN resulted in an increased SNR of 120:1, highlighting the filtering effect of the folded design.     

Analysis of spatial-temporal converters for all-optical communication links

We analyze parallel-to-serial transmitters and serial-to-parallel receivers that use ultrashort optical pulses to increase the bandwidth of a fiber-optic communication link. This method relies on real-time holographic material for conversion of information between spatial and temporal frequencies. The analysis reveals that the temporal output of the pulses will consist of chirped pulses, which has been verified experimentally. When the signal pulses are transmitted along with a reference pulse, the distortions of the received signal, caused by dispersion and other factors in the fiber, are canceled because of the phase-conjugation property of the receiver. This self-referencing scheme simplifies the receiver structure and ensures perfect timing for the serial-to-parallel conversion.     

Interfacial Bonding and Thermal Expansion of Fibre-Reinforced Composites

The present work examines the effect that the fibre-matrix bond strength has on longitudinal and transverse thermal expansivities of unidirectional glass fibre and carbon fibre-reinforced epoxies. It is shown that the weakening of the interfacial bond results in an increase of the longitudinal coefficient of thermal expansion, and a decrease of the transverse one. The results are explained in terms of the reinforcement efficiency factor, according to a theory proposed elsewhere.⁴     

Orientated crystallization in drawn thermoplastic polyimide modified by carbon nanofibers

The solid state crystallization in drawn thermoplastic polyimide films is studied as a function of draw ratio (DR) under the effect of vapor grown carbon fiber nanoinclusions. The nucleating effect of the nanoinclusions coupled with the orientation effect of drawing generates a unique orientated layered lamellar structure, characteristic of smectic‐like mesophase. The degree of draw induced orientated crystallization increases with the content of nanoinclusions and with the DR, and is reflected in the mechanical behavior of the film. Generally, the Young's modulus and the yield point of the drawn crystalline films in the drawing direction are significantly higher compared with the noncrystalline counterparts. POLYM. ENG. SCI., 2009. © 2008 Society of Plastics Engineers     

Improving the oxygen barrier properties of polyethylene terephthalate by graphite nanoplatelets

Enhancement of the oxygen gas barrier properties of polyethylene terephthalate (PET), used in the packaging industry, is the main objective here. For this purpose, nanocomposites of PET containing graphite nanoplatelets (GNPs) were prepared by melt compounding. The effects of the nanocomposites' structural morphology on oxygen gas permeability were analyzed using a range of thermal, microscopic, and mechanical characterization techniques. The investigated nanocomposite films exhibited GNP exfoliated morphology and good mixing with PET, as well as uniform dispersion within the polymer. All nanocomposite films were shown to possess superior oxygen barrier properties and improved thermal and dimensional stability compared with the plain PET films. In the best case, for 1.5 wt % GNP, the oxygen permeation was reduced by more than 99%. The improved barrier properties are attributed to the direct effect of the GNPs and to their induced increase of degree of crystallinity. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Using “last-minute” sales for vertical differentiation on the Internet

In Internet-based commerce, sellers often use multiple distribution channels for the sale of standard consumer goods. We study a model of second-degree price discrimination in which a monopolist sells to risk-averse buyers. The seller uses two channels that differ in their risk attributes. In one channel prices and qualities are fixed and availability is assured. In the second channel, the seller offers a joint distribution of prices and qualities and may not guarantee availability. We characterize optimal two-channel selling policies. We show that it can be optimal to offer multiple identical items in a random sale event. However, the seller cannot benefit by offering two distinct quality levels in a sale event that is held with probability smaller than one. We use the model to offer explanations for the observed behavior of online sellers and discuss implementation issues in recent e-commerce environments.