Photooxidative degradation and conformational changes in syndiotactic polypropylene/clay nanocomposites

Syndiotactic polypropylene nanocomposites based on layered silicates in various proportions were subjected to prolonged (246 h) ultraviolet (UV)-c irradiation. Fourier transform infrared (FT-IR) spectroscopy was used in order to investigate the molecular alterations of the polymeric matrix during the UV exposure relative to the concentration of nanoclay. It was found that a significant increase of the helical conformation upon the irradiation took place as a result of scissions of the polymeric chains. In addition, a simultaneous increase in the crystallinity was verified by X-ray diffraction (XRD) measurements. Furthermore, a variety of photooxidation products were detected, among them carboxylic acids, ketones, gamma-lactones, and esters. We report in this paper the impact of the clay on the degradation mechanism of syndiotactic polypropylene mostly by the production of additional free radicals. Therefore, the relative intensities of the produced photooxidative species are affected drastically as a function of the concentration of the layered silicate present. Finally, an interaction of the carbonyl groups formed in the polymer upon irradiation with the polar groups of the layered silicates was confirmed both from infrared and UV-visible spectroscopic studies.     

Optimal location of a general position and orientation end-effector's path relative to manipulator's base,considering velocity performance

The objective of the present paper is to introduce an offline algorithm searching for the optimal or suboptimal placement of a robot's base during workcell design, so that its end-effector can perform a position and orientation path following task of a given 3D curved path and orientation, maximizing the manipulator's velocity performance. The global index employed for this velocity performance optimization is the approximation of the minimum manipulator velocity ratio (AMMVR).     

Propane Reactions over Faujasite Structure Zeolites Type-X and USY: Effect of Zeolite Silica over Alumina Ratio, Strength of Acidity and Kind of Exchanged Metal Ion

In this work we prepared various zeolite-X and USY samples partially exchanged with copper, iron and platinum. These samples were characterized by XRD, Chemical Analysis, SEM-EDS, N₂-adsorption–desorption, ammonia-TPD, and tested as catalysts in high temperature (400 and 550 °C) propane transformation. The obtained results revealed the strong effect of Si/Al ratio in faujasite zeolite structure, the number and strength of acid sites and of the presence of different metal ions in countered ion sites, on the catalytic activity and selectivity of zeolite-X and USY. The highest propane dehydrogenation activity was achieved with the platinum-exchanged X zeolite (∼11.2% propylene yield, ∼31% selectivity). On the contrary USY zeolites showed high cracking capability and relatively low dehydrogenation activity excepting the platinum-exchanged sample which yielded notably high aromatization products.     

Microwave curing of epoxy polymers reinforced with carbon nanotubes

The driver for this study is the observation that heating of carbon nanotubes (CNTs) with electromagnetic field can offer a more efficient and cost‐effective alternative in heat transfer for the production of composites. The idea of this study is twofold; CNT can work as microwave (MW) radiation susceptors and they can act as nanoreinforcements in the final system. To test these assumptions, a household oven was modified to control the curing schedule. Polymers with different CNT concentrations were prepared (0.5 and 1.0 wt %). The dispersion of the CNTs in the epoxy was achieved using shear‐mixing dissolver technique. MW and conventionally cured specimens were also produced in a convection oven for reference. Thermal and mechanical tests were used as control point. A curing schedule investigation was further performed to quantify the energy and time‐saving capabilities using CNT and MWs. The presence of CNTs into epoxy matrix has been proven beneficial for the shortening of the curing time. MW‐cured composites showed the same degree of polymerization with the conventionally cured composites in a shorter time period and this time was reduced as the CNT concentration was increased. A good distribution of the CNT is required to avoid hot spot effects and local degradation. Mechanical performance was, in some cases, favored by the use of CNT. The benefit from the use of MWs and CNT could reach at least 40% in terms of energy needed and time without sacrificing mechanical performance. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Joint Power and Time Allocation Scheme with QoS Constraints in Overlay Multi-user Cognitive Radio Networks

This paper studies the case of an overlay cognitive radio network where the primary user leases spectral resources to the secondary user in exchange for cooperation, considering that both type of users have specific quality of service requirements. We investigate the problem of joint power and time allocation for the secondary access during the cooperative phase, with a view to optimizing the effective capacity of the primary user given an average energy constraint for the secondary user. Afterwards, the optimal power allocation of the secondary user for its own transmission phase is investigated in order to maximize the effective capacity of the secondary link. The proposed joint power and time allocation mechanism is compared with an optimal time/constant power allocation scheme and a less sophisticated baseline allocation scheme, i.e. power allocation under constant time and its superiority is proven for various network parameters. The reference model of one primary–one secondary user is extended to a general multi user cognitive radio network through the proposed pairing mechanism based on matching theory. Particularly, considering the remarks of the reference scenario, we propose two different matching schemes (with/without consideration of primary users’ quality of service requirements) and we confirm their superiority compared to other matching mechanisms.     

Multi-Hop Wireless Backhaul Networks above 10 GHz: Connectivity and Critical Density Evaluation

Microwave links are the obvious mobile backhauling solution for many mobile operators. Multi-hops are likely to be necessary in order to ensure connectivity for mobile backhaul solutions. The subject of the present paper is the evaluation of the connectivity of wireless multi-hop backhaul networks assuming high frequency transmissions among the relays. A novel analytical physical propagation and engineering model is presented for the calculation of the connectivity of wireless multi-hop networks that appropriate for operating frequencies above 10 GHz. Assuming equal power transmissions from every node and a random spatial node distribution following the homogeneous Poisson process, we calculate the node isolation probability. Furthermore, we calculate the minimum required node density in order to keep the backhaul network almost connected. The sensitivity of the isolation probability and of the minimum node density on frequency of operation, transmission power and climatic conditions is also investigated. Some useful conclusions are drawn.     

Future Internet of Things: open issues and challenges

Internet of Things (IoT) and its relevant technologies have been attracting the attention of researchers from academia, industry, and government in recent years. However, since the requirements of the IoT are quite different from what the Internet today can offer, several innovative techniques have been gradually developed and incorporated into IoT, which is referred to as the Future Internet of Things (FIoT). Among them, how to extract “data” and transfer them into “knowledge” from sensing layer to application layer has become a vital issue. This paper begins with an overview of IoT and FIoT, followed by discussions on how to apply data mining and computational intelligence to FIoT. An intelligent data management framework inspired by swarm optimization will then given. Finally, open issues and future trends of this field will be addressed.     

Access stratum resource management for reliable u-healthcare service in LTE networks

With the rapid growth of elderly population, ubiquitous healthcare (u-healthcare) service combined with advanced wireless networking technology is gaining popularity. The advance in medical sensor technology has boasted up the u-healthcare market potential. However, the global connectivity with a sufficient level of reliability is still an issue to study for its prototype development. In our previous study (Lee et al. In: Proc BodyNets 2010, Lee et al. in J Commun Netw 13(2):160–166, 2011), the energy efficient packet-level scheduling was studied but the scheduling and management topics in the access stratum, which is the beginning part of end-to-end connection for ubiquitous healthcare service, are still open. In this paper, we study an efficient and adaptive method that can provide u-healthcare user devices with an increased level of access privilege so that the healthcare related user data, which is sometimes delay-critical, can be more favorable admitted to the access stratum. We develop a mathematical formulation for the access control in access stratum domain over the existing 3.9-th generation cellular communication standard, called long term evolution (LTE) and devise an efficient and exact algorithm to solve it. The proposed method has simplicity in implementation and efficiency in operation. In addition, it can improve key performance measures, such as call blocking and call dropping performance.     

On the design of secure user authenticated key management scheme for multigateway‐based wireless sensor networks using ECC

In wireless sensor networks (WSNs), there are many critical applications (for example, healthcare, vehicle tracking, and battlefield), where the online streaming data generated from different sensor nodes need to be analyzed with respect to quick control decisions. However, as the data generated by these sensor nodes usually flow through open channel, so there are higher chances of various types of attacks either on the nodes or on to the data captured by these nodes. In this paper, we aim to design a new elliptic curve cryptography–based user authenticated key agreement protocol in a hierarchical WSN so that a legal user can only access the streaming data from generated from different sensor nodes. The proposed scheme is based upon 3‐factor authentication, as it applies smart card, password, and personal biometrics of a user (for ticket generation). The proposed scheme maintains low computation cost for resource‐constrained sensor nodes, as it uses efficient 1‐way cryptographic hash function and bitwise exclusive‐OR operations for secure key establishment between different sensor nodes. The security analysis using the broadly accepted Burrows‐Abadi‐Needham logic, formal security verification using the popular simulation tool (automated validation of Internet security protocols and applications), and informal security show that the proposed scheme is resilient against several well‐known attacks needed for a user authentication scheme in WSNs. The comparison of security and functionality requirements, communication and computation costs of the proposed scheme, and other related existing user authentication schemes shows the superior performance of the proposed scheme.