Conformational analysis and dielectric relaxation of polycarbosilanes and related materials

The conformational characteristics of poly(dimethylsilmethylene), poly(dimethylsilethene), poly(dimethylsilethane) and a related material, poly(2,2,5,5-tetramethyl-1-oxa-2,5-disilapentane), have been investigated using the method of molecular mechanics. In this method, a quantitative analysis of the factors affecting the nature and magnitude of the bond rotation potentials governing their conformational behaviour has been undertaken. Along with their structural data, the results obtained were employed to calculate a variety of conformationally-dependent properties for these polymers, including the characteristic ratio, the dipole moment ratio and the mean-square radius of gyration. In addition, the dielectric relaxation behaviour of two samples of poly(2,2,5,5-tetramethyl-1-oxa-2,5-disilapentane) with molar masses Mw = 28000 and Mw = 46000 respectively, have been studied as a function of temperature (179K-205K) and frequency (100-105Hz). Activation energies for the α-relaxation process and Davidson-Cole empirical distribution factors have been calculated.

Design, synthesis and RAFT polymerisation of a quinoline-based monomer for use in metal-binding composite microfibres

Metal-binding polymer fibres have attracted major attention for diverse applications in membranes for metal sequestration from waste waters, non-woven wound dressings, matrices for photocatalysis, and many more. This paper reports the design and synthesis of an 8-hydroxyquinoline-based zinc-binding styrenic monomer, QuiBoc. Its subsequent polymerisation by reversible addition–fragmentation chain transfer (RAFT) yielded well-defined polymers, PQuiBoc, of controllable molar masses (6 and 12 kg mol−1) with low dispersities (Đ, Mw/Mn < 1.3). Protected (PQuiBoc) and deprotected (PQuiOH) derivatives of the polymer exhibited a high zinc-binding capacity, as determined by semi-quantitative SEM/EDXA analyses, allowing the electrospinning of microfibres from a PQuiBoc/polystyrene (PS) blend without the need for removal of the protecting group. Simple “dip-coating” of the fibrous mats into ZnO suspensions showed that PQuiBoc/PS microfibres with only 20% PQuiBoc content had almost three-fold higher loadings of ZnO (29%) in comparison to neat PS microfibres (11%).

Nonaqueous capillary zone electrophoresis of synthetic organic polypeptides

Poly(Nε-trifluoroacetyl-l-lysine) was used as a model solute to investigate the potential of nonaqueous capillary electrophoresis (NACE) for the characterization of synthetic organic polymers. The information obtained by NACE was compared to that derived from size exclusion chromatography (SEC) experiments, and the two techniques were found to be complimentary for polymer characterization. On one hand, NACE permitted (i) the separation of oligomers according to their molar mass and (ii) the separation of the polymers according to the nature of the end groups. On the other hand, SEC experiments were used for the characterization of the molar mass distribution for higher molar masses. Due to the tendency of the solutes (polypeptides) to adsorb onto the fused-silica capillary wall, careful attention was paid to the rinsing procedure of the capillary between runs in order to keep the capillary surface clean. For that purpose, the use of electrophoretic desorption under denaturating conditions was very effective. Optimization of the separation was performed by studying (i) the influence of the proportion of methanol in a methanol/acetonitrile mixture and (ii) the influence of acetic acid concentration in the background electrolyte. Highly resolved separation of the oligomers (up to a degree of polymerization n of ∼50) was obtained by adding trifluoroacetic acid to the electrolyte. Important information concerning the polymer conformations could be obtained from the mobility data. Two different plots relating the effective mobility data to the degree of polymerization were proposed for monitoring the changes in polymer conformations as a function of the number of monomers.

Synthesis and characterisation of interfacial layers in organic solar cells

The quest for renewable energy sources has led to growing attention in the research of organic photovoltaics (OPVs), as a promising alternative to fossil fuels, since these devices have low manufacturing costs and attractive end-user qualities, such as ease of installation and maintenance. Wide application of OPVs is majorly limited by the devices lifetime. With the development of new encapsulation materials, some degradation factors, such as water and oxygen ingress, can almost be excluded, whereas the thermal degradation of the devices remains a major issue. Two aspects have to be addressed to solve the problem of thermal instability: bulk effects in the photoactive layer and interfacial effects at the photoactive layer/charge-transporting layers. In this work, the interface between photoactive layer and electron-transporting zinc oxide (ZnO) in devices with inverted architecture was engineered by introducing polymeric interlayers, based on zinc-binding ligands, such as 3,4-dihydroxybenzene and 8-hydroxyquinoline. Also, a cross-linkable layer of poly(3,4-dimethoxystyrene) and its fullerene derivative were studied. At first, controlled reversible addition-fragmentation chain transfer (RAFT) polymerisation was employed to achieve well-defined polymers in a range of molar masses, all bearing a chain-end functionality for further modifications. Resulting polymers have been fully characterised, including their thermal and optical properties, and introduced as interlayers to study their effect on the initial device performance and thermal stability. Poly(3,4-dihydroxystyrene) and its fullerene derivative were found unsuitable for application in devices as they increased the work function of ZnO and created a barrier for electron extraction. On the other hand, their parental polymer, poly(3,4-dimethoxystyrene), and its fullerene derivative, upon cross-linking, resulted in enhanced efficiency and stability of devices, if compared to control. Polymers based on 8-hydroxyquinoline ligand had a negative effect on the initial stability of the devices, but increased the lifetime of the cells under accelerated thermal stress. Comprehensive studies of the key mechanisms, determining efficiency, such as charge generation and extraction, were performed by using time-resolved electrical and spectroscopic techniques, in order to understand in detail the effect of the interlayers on the device performance. Obtained results allow deeper insight into mechanisms of degradation that limit the lifetime of devices and prompt the design of better materials for the interface stabilisation.