Graft Polymers: From Dendrimer Hybrids to Latex Particles

The research presented focused on the synthesis and the characterization of graft polymers, of interest either as model systems or for large-scale applications. The materials selected as substrates for grafting reactions were carbosilane dendrimers, linear and branched polystyrenes, and cross-linked polystyrene latex particles. The synthesis of dendrimer-arborescent polymer hybrids was thus achieved by derivatization of the carbosilane dendrimers with dichlorosilane moieties and coupling with 1,4-polybutadiene side chains with Mn ≈ 1000. A second derivatization and coupling reaction with Mn ≈ 1500, 5000, or 30000 side chains yielded hybrid polymers with narrow molecular weight distributions (Mw/Mn ≤ 1.16). In the second part of the thesis, a procedure for the large-scale (100-g) synthesis of arborescent styrene homopolymers and copolymers incorporating poly(2-vinylpyridine) segments is presented. End-capping of the polystyryllithium chains with 1,1-diphenylethylene in the presence of LiCl, followed by the addition of 3 – 6 equivalents of 2-vinylpyridine per side chain, eliminated side reactions and led to grafting yields of up to 95 %. A systematic investigation of the solution properties of polyelectrolytes obtained by protonation of the poly(2-vinylpyridine) arborescent copolymers with a strong acid (trifluoroacetic acid) is also presented. The relative importance of the electrostatic repulsion and the elastic deformation forces on molecular expansion was investigated by examining the solution properties of the copolymers as a function of structure, protonation level, and the presence of salts in polar solvents (methanol, DMF, H2O). The viscosity of the arborescent copolymer solutions was also found to be much lower than for linear P2VP samples under the same conditions. In the last part of the thesis, the synthesis of model filler particles was achieved by grafting polyisoprene chains onto cross-linked polystyrene latex particles derivatized with acetyl coupling sites. These substrates, which can be viewed as an extreme case of a dense (hard-sphere) arborescent polymer structure, were used to investigate the influence of filler-matrix polymer interactions on the rheological behavior of filled polyisoprene samples. The influence of the filler structure on the rheological behavior of the blends was examined by dynamic mechanical analysis in terms of frequency-dependent complex viscosity, storage modulus, and damping factor. All the blends exhibited enhanced complex viscosity, storage modulus, and decreased damping factor values relative to the matrix polymer

Depletion gels from dense soft colloids: Rheology and thermoreversible melting

Truzzolillo, D., Vlassopoulos, D., Munam, A., & Gauthier, M. (2014). Depletion gels from dense soft colloids: Rheology and thermoreversible melting. Journal of Rheology, 58(5), 1441–1462. https://doi.org/10.1122/1.4866592Upon addition of small nonadsorbing linear polymers, colloidal glasses composed of large hard spheres melt and eventually revitrify into the so-called attractive glass regime. We show that, when replacing the hard spheres by star polymers representing model soft particles, a reentrant gel is formed. This is the result of compression and depletion of the stars due to the action of the osmotic pressure from the linear homopolymers. The viscoelastic properties of the soft dense gel were studied with emphasis on the shear-induced yielding process, which involved localized breaking of elements with a size of the order of the correlation length. Based on these results, a phenomenological attempt was made at describing the universal rheological features of colloid/nonadsorbing polymer mixtures of varying softness. The star gel was found to undergo thermoreversible melting, despite the fact that conventional hard-sphere depletion gels are invariant to heating. This phenomenon is attributed to the hybrid internal microstructure of the stars, akin to a dry-to-wet brush transition, and is characterized by slow kinetics, on the time scale of the osmotic gel formation process. These results may be useful in finding generic features in colloidal gelation, as well as in the molecular design of new soft composite materials.Financial support from the EU (ITN-COMPLOIDS FP7-234810, FP7 Infrastructure ESMI, GA 262348 and FP7-SMALL-Nanodirect CP-FP-213948) and the Natural Science and Engineering Research Council of Canada (NSERC) is gratefully acknowledged

Virtual Pseudonym-Changing and Dynamic Grouping Policy for Privacy Preservation in VANETs

Location privacy is a critical problem in the vehicular communication networks. Vehicles broadcast their road status information to other entities in the network through beacon messages to inform other entities in the network. The beacon message content consists of the vehicle ID, speed, direction, position, and other information. An adversary could use vehicle identity and positioning information to determine vehicle driver behavior and identity at different visited location spots. A pseudonym can be used instead of the vehicle ID to help in the vehicle location privacy. These pseudonyms should be changed in appropriate way to produce uncertainty for any adversary attempting to identify a vehicle at different locations. In the existing research literature, pseudonyms are changed during silent mode between neighbors. However, the use of a short silent period and the visibility of pseudonyms of direct neighbors provides a mechanism for an adversary to determine the identity of a target vehicle at specific locations. Moreover, privacy is provided to the driver, only within the RSU range; outside it, there is no privacy protection. In this research, we address the problem of location privacy in a highway scenario, where vehicles are traveling at high speeds with diverse traffic density. We propose a Dynamic Grouping and Virtual Pseudonym-Changing (DGVP) scheme for vehicle location privacy. Dynamic groups are formed based on similar status vehicles and cooperatively change pseudonyms. In the case of low traffic density, we use a virtual pseudonym update process. We formally present the model and specify the scheme through High-Level Petri Nets (HLPN). The simulation results indicate that the proposed method improves the anonymity set size and entropy, provides lower traceability, reduces impact on vehicular network applications, and has lower computation cost compared to existing research work

Glassy States in Asymmetric Mixtures of Soft and Hard Colloids

© 2013 American Physical Society, available at: Truzzolillo, D., Marzi, D., Marakis, J., Capone, B., Camargo, M., Munam, A., … Vlassopoulos, D. (2013). Glassy States in Asymmetric Mixtures of Soft and Hard Colloids. Physical Review Letters, 111(20). https://doi.org/10.1103/PhysRevLett.111.208301By employing rheological experiments, mode coupling theory, and computer simulations based on realistic coarse-grained models, we investigate the effects of small, hard colloids on the glassy states formed by large, soft colloids. Multiarm star polymers mimic hard and soft colloids by appropriately varying the number and size of their arms. The addition of hard colloids leads, depending on their concentration, to either melting of the soft glass or the emergence of two distinct glassy states. We explain our findings by depletion of the colloids adjacent to the stars, which leads to an arrested phase separation when the repulsive glass line meets the demixing binodal. The parameter-free agreement between experiment, theory, and simulations suggests the generic nature of our results and opens the route for designing soft-hard colloidal composites with tunable rheology.This work has been supported by the EU (ITN-COMPLOIDS Grant No. 234810) and by the J. S. Latsis Foundation (Grant No. 0839-2012)

A distributed mix-context-based method for location privacy in road networks

Preserving location privacy is increasingly an essential concern in Vehicular Adhoc Networks (VANETs). Vehicles broadcast beacon messages in an open form that contains information including vehicle identity, speed, location, and other headings. An adversary may track the various locations visited by a vehicle using sensitive information transmitted in beacons such as vehicle identity and location. By matching the vehicle identity used in beacon messages at various locations, an adversary learns the location history of a vehicle. This compromises the privacy of the vehicle driver. In existing research work, pseudonyms are used in place of the actual vehicle identity in the beacons. Pseudonyms should be changed regularly to safeguard the location privacy of vehicles. However, applying simple change in pseudonyms does not always provide location privacy. Existing schemes based on mix zones operate efficiently in higher traffic environments but fail to provide privacy in lower vehicle traffic densities. In this paper, we take the problem of location privacy in diverse vehicle traffic densities. We propose a new Crowd-based Mix Context (CMC) privacy scheme that provides location privacy as well as identity protection in various vehicle traffic densities. The pseudonym changing process utilizes context information of road such as speed, direction and the number of neighbors in transmission range for the anonymisation of vehicles, adaptively updating pseudonyms based on the number of a vehicle neighbors in the vicinity. We conduct formal modeling and specification of the proposed scheme using High-Level Petri Nets (HPLN). Simulation results validate the effectiveness of CMC in terms of location anonymisation, the probability of vehicle traceability, computation time (cost) and effect on vehicular applications

Green IoT: An Investigation on Energy Saving Practices for 2020 and Beyond

Internet of Things (IoT) is an emerging concept, which aims to connect billions of devices with each other. The IoT devices sense, collect, and transmit important information from their surroundings. This exchange of very large amount of information amongst billions of devices creates a massive energy need. Green IoT envisions the concept of reducing the energy consumption of IoT devices and making the environment safe. Inspired by achieving a sustainable environment for IoT, we first give the overview of green IoT and the challenges that are faced due to excessive usage of energy hungry IoT devices. We then discuss and evaluate the strategies that can be used to minimize the energy consumption in IoT, such as designing energy efficient datacenters, energy efficient transmission of data from sensors, and design of energy efficient policies. Moreover, we critically analyze the green IoT strategies and propose five principles that can be adopted to achieve green IoT. Finally, we consider a case study of very important aspect of IoT, i.e., smart phones and we provide an easy and concise view for improving the current practices to make the IoT greener for the world in 2020 and beyond