Manufacturing and characterisation of photoresponsive composites for defence applications

The development of composites with improved mechanical and impact resistance properties has attracted considerable attention within the defence and aerospace industries. These composites are finding applications in vehicle or aircraft structures which are frequently exposed to impact from bird strikes, hailstorm, dropped tools and runway debris. In addition, exposure to extreme conditions such as high levels of UV light exposure or extreme temperatures increases the brittleness of the polymer matrix, ultimately leading either to the loss of mechanical properties such as compression strength or even to the catastrophic structural failure of the composites. The aim of this PhD project was to develop toughened and self-healing composites in which delamination and crack growth can be managed to an acceptable level. This was attempted using a new epoxy-based resin modified with photoresponsive azobenzene. Mechanical properties of the composites, toughened by azobenzene, were improved as a result of the photo-induced changes in the azobenzene structures. Initial works were to identify, synthesise and characterise appropriate photoresponsive resins that were thought to offer composites with enhanced properties. The synthesis of azobenzene acrylic- and epoxy-based polymers were accomplished using conventional wet chemistry. Their properties were triggered by different stimuli and characterised using 1H NMR, UV/Vis and FTIR spectroscopy, DSC, GPC, rheology, and nanoindentation. The azobenzene/acrylic-based copolymer films showed a significant photomechanical behaviour. Nanoindentation analysis demonstrated a maximum increase in stiffness of 19% with an optimum azobenzene loading of 30 mol%. Such an enhancement in stiffness was attributed to the photoinduced reorganisation of the polymer chains by geometrical isomerisation (trans to cis isomers) of the azobenzene chromophores. Analysis of the thin films by optical microscopy demonstrated a healing effect of the indented region under UV irradiation suggesting that this class of polymers can be used as self-healing materials. Ultrasound was also found to trigger cis trans isomerisation in the solid state at a much slower rate (120-150 min) than by visible light (30-60 s). Azobenzene-modified epoxy resins were synthesised by optimising an existing synthetic route and their responses to light and curing behaviour with a common amine hardener (a curing agent for epoxy resin, used to initiate curing/hardening) were assessed. The resulting kinetic reactions were investigated using isothermal (95°C) and dynamic heating scans (30-180°C) in a DSC and by simultaneously monitoring the spectral information using a NIR-FTIR spectrometer. The modified epoxy azobenzene resin proved to be reactive enough to form a network that can withstand temperatures of up to 200°C. The azobenzene-epoxy resins exhibited high dimensional stability, stiffness enhancement and healing behaviour when they were exposed to UV light. Gas pycnometer studies demonstrated constant volume and density values of the resins before and after UV irradiation. Optical and atomic force microscopes were used to assess and quantify the healing effect of damaged azobenzene-based polymer films. An intrinsic healing (73% of the total damaged area) was caused by the UV-induced molecular mobility of azobenzene in a 3D crosslinked network. The influence of UV light and the effect of azobenzene loading on the epoxy-based polymeric matrices were also evaluated after fracture mechanics analysis and it was found that an 11% increase in fracture toughness was observed with 10 mol% azobenzene (without exposure to UV light). On the contrary UV light increased the brittleness of the matrix with higher azobenzene loading. The azobenzene-modified epoxy resin was used to produce glass fibre-reinforced composites. Their photo-induced properties were investigated by compression, impact and post-impact compression testing. The composites exhibited an increase (3-16%) in compressive strength after exposure to UV light due to the trans cis isomerisation. Moreover, it was demonstrated that the introduction of a small fraction of azobenzene (10 mol%) into the composites enhanced their impact resistance by 10% when subjected to high velocity impacts (190 m/s). The absorbed energy of the azobenzene composites which had been previously exposed to UV light was also increasing linearly with the azobenzene loading

Valoon reagoivat atsobentseeniä sisältävät hybridiohutkalvot ALD/MLD-menetelmällä

Photochromic molecules show a change in their absorption spectrum when they are irradiated at a suitable wavelength. The difference is caused by a change in the structure of the molecule and is usually reversible. Such molecules can be used in applications utilizing their optical properties, such as lenses and filters, as well as applications utilizing the changes in their chemical or physical properties, such as optical switches. In this thesis, different photochromic molecules and thin film techniques are compared. In hybrid materials, the efficiency of the photoisomerization reaction depends on the ability of the photochromic molecule to change from one form to another. Free volume around the molecule has a great effect on its photoisomerization. A rigid matrix with a layered structure helps immobilize the molecules and thus improves the photochromic response. As atomic and molecular layer deposition (ALD/MLD) is based on self-limiting reactions by surface saturation, it should provide uniform, conformal thin films with a suitable layered matrix and accurate control of thickness. In the experimental part, the goal was to deposit photoresponsive thin films containing azobenzene using the ALD/MLD method. The organic precursor was azobenzene-4,4’-dicarboxylic acid and the inorganic precursor was trimethylaluminium. Both hybrid and superlattice thin films were deposited on silicon and quartz. The resulting films were amorphous and nearly homogenous, with a slight gradient seen on hybrid films. The growth rate of the hybrid thin films was lower than expected, and the decreased growth rate upon increasing the number of cycles suggested the presence of double surface reactions in the process. The hybrid thin film showed minor irreversible photoresponsivity after UV irradiation. Superlattice films with very thin oxide layers showed absorption bands related to azobenzene, but with increasing oxide thickness the bands disappeared. None of the superlattice films showed any response to UV irradiation. Poor photoresponsivity was attributed to steric hindrance, likely to be partially caused by the double surface reactions. The results were compared to similar hybrid systems found in literature.Fotokromaattisten yhdisteiden absorptiospektri muuttuu, kun niitä säteilytetään sopivalla aallonpituudella. Niiden eroavuus johtuu muutoksessa molekyylin rakenteessa, joka on usein reversiibeli. Kyseisiä yhdisteitä voidaan käyttää sovelluksissa jotka hyödyntävät niiden optisia ominaisuuksia, esimerkiksi linsseissä ja suodattimissa, sekä sovelluksissa jotka hyödyntävät niiden kemiallisia tai fysikaalisia ominaisuuksia, esimerkiksi optisissa kytkimissä. Tässä työssä vertaillaan eri fotokromaattisia yhdisteitä ja ohutkalvotekniikoita. Hybridimateriaaleissa fotoisomerisaation tehokkuus riippuu fotokromaattisen yhdisteen kyvystä muuttua muodosta toiseen. Vapaa tilavuus molekyylin ympärillä vaikuttaa suuressa määrin sen fotoisomerisaatioon. Jäykkä kerrosrakenteinen matriisi auttaa pitämään molekyylit paikallaan ja siten parantaa fotokromaattista vastetta. Atomi- ja molekyylikerroskasvatus (ALD/MLD) perustuu itserajoittuviin reaktioihin, joissa pinnat saturoituvat, joten sillä pitäisi saada kasvatettua tasalaatuisia, pinnan muotoihin mukautuvia ohutkalvoja, joissa on sopiva kerrosrakenteinen matriisi, ja joiden paksuutta voidaan kontrolloida tarkasti. Kokeellisessa osuudessa päämääränä oli kasvattaa atsobentseeniä sisältäviä valoon reagoivia ohutkalvoja käyttäen ALD/MLD-menetelmää. Orgaaninen prekursori oli atsobentseeni-4,4’-dikarboksyylihappo ja epäorgaaninen prekursori oli trimetyylialumiini. Hybridi- ja superhilaohutkalvoja kasvatettiin sekä pii- että kvartsisubstraateille. Kasvatetut kalvot olivat amorfisia ja lähes homogeenisiä. Hybridiohutkalvojen pinnalla nähtiin pieni gradientti. Hybridiohutkalvojen kasvunopeus oli odotettua matalampi, ja pienenevä kasvunopeus syklien määrän kasvaessa viittasi kaksoispintareaktioiden olevan osa kasvuprosessia. Hybridiohutkalvo reagoi UV-säteilytykseen vain hieman ja irreversiibelisti. Superhilaohutkalvoissa, joissa oksidikerrokset olivat hyvin ohuita, nähtiin atsobentseenille ominaiset absorptiovyöt, mutta paksummilla oksidikerroksilla vöitä ei enää nähty. Superhilaohutkalvot eivät reagoineet UV-säteilytykseen lainkaan. Huonon säteilyyn reagoivuuden syyksi epäiltiin steerisiä esteitä, todennäköisesti johtuen osaksi kaksoispintareaktioista. Tuloksia vertailtiin vastaaviin hybridirakenteisiin kirjallisuudessa

Hydrogen bond topology and the ice VII/VIII and Ih/XI proton ordering phase transitions

Ice Ih, ordinary ice at atmospheric pressure, is a proton-disordered crystal that when cooled under special conditions is believed to transform to ferroelectric proton-ordered ice XI, but this transformation is still subject to controversy. Ice VII, also proton disordered throughout its region of stability, transforms to proton-ordered ice VIII upon cooling. In contrast to the ice Ih/XI transition, the VII/VIII transition and the crystal structure of ice VIII are well characterized. In order to shed some light on the ice Ih proton ordering transition, we present the results of periodic electronic density functional theory calculations and statistical simulations. We are able to describe the small energy differences among the innumerable H-bond configurations possible in a large simulation cell by using an analytic theory to extrapolate from electronic DFT calculations on small unit cells to cells large enough to approximate the thermodynamic limit. We first validate our methods by comparing our predictions to the well-characterized ice VII/VIII proton ordering transition, finding agreement with respect to both the transition temperature and structure of the low-temperature phase. For ice Ih, our results indicate that a proton-ordered phase is attainable at low temperatures, the structure of which is in agreement with the experimentally proposed ferroelectric Cmc2_1 structure. The predicted transition temperature of 98 K is in qualitative agreement with the observed transition at 72 K on KOH-doped ice samples

Light-induced reversible modification of the work function of a new perfluorinated biphenyl azobenzene chemisorbed on Au (111)

This work was financially supported by EC through the Marie-Curie ITN SUPERIOR (PITN-GA-2009-238177) and IEF MULTITUDES (PIEF-GA-2012-326666), the ERC project SUPRAFUNCTION (GA-257305), the Agence Nationale de la Recherche through the LabEx project Chemistry of Complex Systems (ANR-10-LABX-0026_CSC), and the International Center for Frontier Research in Chemistry (icFRC). The work in Mons is further supported by the Interuniversity Attraction Poles Programme (P7/05) initiated by the Belgian Science Policy Office, and by the Belgian National Fund for Scientific Research (FNRS). J.C. is an FNRS research director. The synthesis team in Switzerland acknowledges financial support by the Swiss National Science Foundation (SNF) and the Swiss Nanoscience Institute (SNI)

Improved sampling in Monte Carlo simulations of small clusters

In this thesis, improved sampling algorithms are applied to atomic and molecular clusters. The parallel-tempering Monte Carlo procedure is used to characterize the (CO2)n, n = 6, 8, 13, 19, and 38, clusters. The heat capacity curves of the n = 13 and 19 clusters are found to have pronounced peaks that can be associated with cluster melting. In addition, there is evidence of a low temperature "solid -> solid" transition in the case of (CO2)19. The low-energy minima and rearrangement pathways are determined and used to examine the complexity of the potential energy surfaces of the clusters. An algorithm combining the Tsallis generalized ensemble and the parallel tempering algorithm is introduced and applied to a 1D model potential and to Ar38. The convergence of parallel tempering Monte Carlo simulations of the 38-atom Lennard-Jones cluster starting from the Oh global minimum and from the C5v second lowest-energy minimum is also investigated. It is found that achieving convergence is appreciably more difficult, particularly at temperatures in the vicinity of the Oh -> C5v transformation, when starting from the C5v structure. Compared to PTMC, the hybrid algorithm is about 10 times faster for reaching equilibrium in the 1D model potential and is about 3 times faster for reaching equilibrium in the LJ38 system when starting from the second lowest energy minimum. The Wang-Landau free random walk algorithm is also applied to Ar13 and Ar38

Molecular Vibrations and Shape-Selectivity: A Computational Model of Biofuel Precursors in Zeolites

We have used Density Functional Theory (DFT) to model acyclic and cyclic olefins in acidic zeolites. We have studied the impact of host-guest interactions between adsorbed molecules and zeolite frameworks through the lens of molecular vibrations and shape-selectivity. This work considered three zeolite frameworks with varying pore structures and environments: large pore zeolite HMOR and medium pore zeolites HZSM-5 and HZSM-22. A key finding is that for acyclic olefins in acidic zeolites there exists two regimes of host-guest interaction: a strong interaction leading to protonation and a weak interaction between charged guest and zeolite framework. We found that these interactions manifest in the IR spectra such that protonation leads to significant changes in band position for allylic vibrations, vam(C=C─C+), and in contrast these band positions are left substantially unchanged due to the weaker Coulombic interaction. These results indicate that to model acyclic olefins in acidic zeolites one only need to consider the protonated state in the gas phase. We worked in close collaboration with zeolite experimentalists E. Hernandez and F. Jentoft at the University of Massachusetts Amherst Chemical Engineering department to investigate the presence of shape-selectivity during the formation of alkylcyclopentenyl cations from acyclic precursors in acidic zeolites. We incorporated DFT models and configurational sampling to establish band positions associated with allylic stretching vas(C=C─C+) in cyclopentenyl cations. We found that the band position of this stretch was sensitive to the substitution pattern on the allylic system of the ring, such that a methyl substitution instead of a hydrogen at the center carbon (C-2) resulted in a ~ 20 cm-1 red-shift in the IR band. Our collaborative efforts also found that the formation of these alkylcyclopentenyl cations in zeolites is shape-selective; the C-2 methyl-substituted alkylcyclopentenyl cation forms in larger pore HMOR whereas in medium pore zeolites, HZSM-5 and HZSM-22, the C-2 hydrogen-substituted alkylcyclopentenyl appears to be the main product. We performed DFT-based thermodynamics calculations and found that the relative stability of the methyl-substituted alkylcyclopentenyl cation remained unchanged in all three zeolites. This suggests that the formation of these alkylcyclopentenyl cations is not under thermodynamic control. We used DFT calculations to build a microkinetic model of the isomerization of alkylcyclopentenyl cations in HZSM-5 and HZSM-22 zeolites, using both finite-temperature dynamics and zero-Kelvin path methods to compute barriers. We found that the isomerization leading to experimentally relevant alkylcyclopentenyl cations with C-2 methyl (T-type) or hydrogen (K-type) substitutions occurs through a multi-pathway reaction network. We found that the pathways were similar in the two zeolites, but the populations at equilibrium differed such that one T-type product formed in HZSM-5 and two formed in HZSM-22 with evidence of kinetic control of product formation

The Synthesis and Characterization of Novel Ruthenium Terpyridine Complexes: Reactivity with DNA

Side effects associated with platinum-based anticancer therapies have resulted in the search for novel nonplatinum-based metal species. Among the different metal complexes generating interest, rutheniumII complexes have shown great potential as anticancer agents. A series of mixed ligand ruthenium complexes with he formula cis-[Ru(X)(HPB)(OH2)], where X corresponds to either (1) Terpy = 2,2\u27:6\u27,2 -terpyridine; (2) Pheterpy = 4\u27 -Phenyl-2,2\u27:6\u27,2 -terpyridine or (3) Tterpy = 4\u27-Tolyl-2,2\u27:6\u27,2 -terpyridine, and HPB = 2-(2\u27 -hydroxyphenyl)-benozoxazole, have been synthesized and structurally characterized. The DNA binding of the rutheniumII complexes has been studied using several physical methods including absorption spectroscopy and competitive binding displacement assays. The abilit of the complexes to unwind and / or untwist DNA has also been studied using supercoiled pUC18 DNA