Academic and research departmentsSchool of Chemistry and Chemical Engineering.
My research project
Pressure-sensitive adhesives (PSAs) are usually made from viscoelastic, high-molecular weight copolymers, which are fine-tuned by adjusting the comonomer ratios, molecular weights, and cross-link densities to optimize the adhesion properties for the desired end-use. To create a lightly cross-linked network, an ultraviolet (UV) photoinitiator can be incorporated. Here, we present the first use of perfluorophenylazide chemistry to control precisely a polyacrylate network for application as a PSA. Upon UV irradiation, the highly reactive nitrene from the azide moiety reacts with nearby molecules through a C–H insertion reaction, resulting in cross-linking via covalent bonding. This approach offers three benefits: (1) a means to optimize adhesive properties without the addition of an external photoinitiator; (2) the ability to switch off the tack adhesion on demand via a high cross-linking density; and (3) a platform for additional chemical modification. A series of poly(n-butyl acrylate-co-2,3,4,5,6-pentafluorobenzyl acrylate) or poly(PFBA-co-BA) copolymers was synthesized and modified post-polymerization into the photo-reactive poly(n-butyl acrylate-co-4-azido-2,3,5,6-tetrafluorobenzyl acrylate) (azide-modified poly(PFBA-co-BA)) with various molar contents. When cast into films, the azide-modified copolymers with a high azide content achieved a very high shear resistance after UV irradiation, whereas the tack and peel adhesion decreased strongly with increasing azide content, indicating that excessive cross-linking occurred. These materials are thus photo-switchable. However, in the low range of azide content, an optimum probe tack adhesion energy was obtained in films with a 0.3 mol% azide content, where a long stress plateau (indicating good fibrillation) with a high plateau stress was observed. An optimum peel adhesion strength was achieved with 0.5 mol% azide. Thus, the adhesion was finely controlled by the degree of cross-linking of the PSA, as determined by the azide content of the copolymer chain. Finally, as a demonstration of the versatility and advantages of the materials platform, we show an azide–aldehyde–amine multicomponent modification of the azide copolymer to make a dye-functionalized film that retains its adhesive properties. This first demonstration of using azide functionality has enormous potential for functional PSA design.
A novel monomer, 4-azido-2,3,5,6-tetrafluorobenzyl methacrylate (ABMA), enabled the selective and efficient postpolymerization modification of RAFT-made homopolymers and diblock copolymer nanoparticles prepared through polymerization-induced self-assembly (PISA). Poly(ABMA) homopolymers were modified postpolymerization in (near-)quantitative conversions with phosphines to give stable iminophosphoranes and in a multicomponent reaction with phenylacetaldehyde and morpholine, piperidine, or the crosslinker N,N’-dimethylethylene diamine to give the corresponding amidine derivatives in one step. Product polymers were characterized by NMR and FT-IR spectroscopy, size-exclusion chromatography, and differential scanning calorimetry. Unlike its monomer, poly(ABMA) was insoluble in ethanol and enabled the preparation of well-defined spherical, worm-shaped, and vesicular nanoparticles with azide-functional cores through RAFT dispersion polymerization with concurrent PISA. Worm-shaped particles formed physical gels that underwent thermally reversible degelation. Multicomponent modification of spherical nanoparticles with phenyl acetaldehyde and morpholine or piperidine led to (near-)quantitative core modification and, for morpholine, a significant increase in sphere diameter. UV-irradiation of nanoparticles led to crosslinking through the formation of reactive nitrene intermediates which prevented the disassembly of nanoparticles in non-selective solvents, representing a simple and reagent-free crosslinking strategy, and expanding the scope of azide-based polymer chemistry.
Organic azides are a common functional group in polymer chemistry. Upon irradiation, they form nitrenes which are usually exploited for crosslinking based on their “universal” reactivity allowing insertion into nearby C–H bonds. Herein, it is demonstrated that polymer-bound perfluorophenyl azide groups selectively form sulfoximines when irradiated in the presence of an excess of sulfoxide, without observable crosslinking. Poly(4-azido-2,3,5,6-tetrafluorobenzyl methacrylate) (carrying an azide group in every repeat unit) is prepared through azide–para-fluoro postpolymerization modification of a 2,3,4,5,6-pentafluorobenzyl-functional precursor. Samples of the azide-functional polymer are irradiated (𝝀max = 254 nm) in dichloromethane in the presence of varying amounts of five different sulfoxides. Products are characterized by 1H and 19F NMR spectroscopy, Fourier transform infrared spectroscopy, size exclusion chromatography, and differential scanning calorimetry. Irradiations for 7 hwith a 40–70-fold molar excess of sulfoxides gave soluble products with azide-to-sulfoximine conversions of 89–96%. This first example of a selective nitrene-based reaction in the polymer arena shows that sulfoxide solvents must be avoided if crosslinking is desired and provides access to novel sulfoximine-functional materials.
The radical ring-opening polymerization (RROP) of thionolactones provides access to thioester backbone-functional copolymers but has, to date, only been demonstrated on acrylic copolymers. Herein, the thionolactone dibenzo[c,e]oxepane-5-thione (DOT) was subjected to azobisisobutyronitrile (AIBN)-initiated free-radical homopolymerization, which produced a thioester-functional homopolymer with a glass-transition temperature of 95 °C and the ability to degrade exclusively into predetermined small molecules. However, the homopolymerization was impractically slow and precluded the introduction of functionality. Conversely, the reversible addition–fragmentation chain-transfer (RAFT)-mediated copolymerization of DOT with N-methylmaleimide (MeMI), N-phenylmaleimide (PhMI), and N-2,3,4,5,6-pentafluorophenylmaleimide (PFPMI) rapidly produced well-defined copolymers with the tendency to form alternating sequences increasing in the order MeMI ≪ PhMI < PFPMI, with estimated reactivity ratios of rDOT = 0.198 and rPFPMI = 0.0078 for the latter system. Interestingly, defects in the alternating structure were more likely caused by (degradable) DOT–DOT sequences rather than (nondegradable) MI–MI sequences, which was confirmed through the paper spray mass spectrometric analysis of the products from aminolytic degradation. Upon the aminolysis of backbone thioesters, maleimide repeating units were ring-opened, forming bisamide structures. Conversely, copolymer degradation through a thiolate did not result in imide substitution but nucleophilic para-fluoro substitution on PFPMI comonomer units, indicating the ability of DOT–MI copolymers to degrade under different conditions and to form differently functional products. The RROP of thionolactones has distinct advantages over the RROP of cyclic ketene acetals and is anticipated to find use in the development of well-defined degradable polymer materials.
Novel anion exchange membranes (AEMs), based on poly(phenylene oxide) (PPO) chains linked to pendant 1,2-dimethylimidazolium (DIm) functional groups, have been prepared for evaluation in alkaline polymer electrolyte membrane fuel cells (APEFCs). Successful functionalisation of the PPO chains was confirmed using H-NMR and FT-IR spectroscopies. The ionic conductivities of the resulting DIm-PPO AEMs at 30 °C are in the ranges of 10-40 mS cm and 18-75 mS cm at 60 °C. The high ionic conductivities are attributed to the highly developed microstructures of the membranes, which feature well-defined and interconnected ionic channels (confirmed by atomic force microscopy, AFM, measurements). Promisingly, the ion-exchange capacities (IECs) of the DIm-PPO AEM are maintained after immersion in an aqueous KOH solution (2 mol dm) for 219 h at 25 °C; a previously developed monomethyl imidazolium PPO analogue AEM (Im-PPO) showed a significant decline in IEC on similar treatment. This reduction in undesirable attack by the OH conducting anions is ascribed to an increase in steric interference and removal of the acidic C2 proton [in the monomethyl Im-groups] by the methyl group in the DIm cationic ring. Moreover, the maximum power densities produced in simple beginning-of-life single cell H /O fuel cell tests increased from 30 mW cm to 56 mW cm when switching from the Im-PPO AEM (fuel cell temperature = 50 °C) to the DIm-PPO-0.54 AEM (fuel cell temperature = 35 °C) respectively (even with the use of lower temperatures). This journal is © The Royal Society of Chemistry 2013.
The 2,3,4,5,6-pentafluorobenzyl group has become a popular reactive functionality in polymer chemistry because of its high susceptibility to para-fluoro substitution with thiols. Herein, it is demonstrated postpolymerization that the para-fluoride can be substituted using sodium azide and that the resulting 4-azido-2,3,5,6-tetrafluorobenzyl-functional polymers are versatile precursors for a multitude of onward modifications with click-like efficiencies. Quantitative azide–para-fluoro substitution was found for poly(2,3,4,5,6-pentafluorobenzyl methacrylate) and the related Passerini ester–amide (meth)acrylic (co)polymers when heated in DMF with sodium azide to 80 °C for 60–90 min. Conversely, the azidation of poly(2,3,4,5,6-pentafluorostyrene) under similar conditions resulted in ~90% substitution efficiency. Azide-functional (co-)polymers were thermally stable below 100 °C and were subsequently modified with (i) four different alkynes (CuBr, triethylamine, DMF, 55 °C, overnight) to give 1,4-substituted 1,2,3-triazoles in >95% conversions; (ii) potassium thioacetate (DMF, RT, 15 min) with quantitative amidation to the acetanilide derivative; and (iii) DL-dithiothreitol (methanol/DMF, RT, 90 min) resulting in complete reduction of the azides to primary amines, which were subsequently acylated with two different acyl chlorides. Products were characterized by 1H NMR, 19F NMR, and FT-IR spectroscopies, and size exclusion chromatography. Given their adaptability, perfluorophenylazides have large potential as multi-purpose intermediates in polymer and materials chemistry.
A range of quaternised tertiary amine methacrylate-based homopolymers and copolymers were synthesised as mimics of the biopolymers implicated in biosilica formation. These synthetic polymers were evaluated for their ability to catalyse and direct the structure of silica formed by condensation of silicic acid in aqueous solution and at neutral pH. Homo- and co-polymers of differing degrees of quaternisation were studied, while some of the homopolymers also differed in their chain length. All polymers acted as catalysts for the condensation reaction, but at different rates according to their architecture and degree of quaternisation. The resulting silica–polymer hybrids were characterised fully, as were pure silicas obtained by calcination of the hybrids. Some crystallites were present in the hybrids and differences in crystal structure were observed in the calcined silicas, depending on the structure of the polymer, indicating that the polymers exert a structure-directing effect during initial silica formation. The work provides some new insights into structural factors affecting silica growth catalysed by synthetic cationic polymers.