Titolo della tesi: 2-(hydroxyimino)aldehyde and triarylmethane mechanophores as novel functional groups for multi-stimuli responsive polymers and mechanochromic materials
Stimuli-responsive polymers are macromolecules able to modify their structure and properties in response to external physical or chemical stimuli. These characteristics make these polymers very interesting in many application fields such as sensors, catalysis, controlled and targeted release of drugs, environmental remediation, chemical-mechanical actuators, damage detection. In this thesis work, two types of novel functional groups were obtained and incorporated in polymeric structures to achieve stimuli-responsive macromolecules. In the main part of the work, 2-(hydroxyimino)aldehyde groups (HIA; -C(=NOH)CHO) were included in the hydrophobic block of an amphiphilic block copolymer to obtain multi-responsive macromolecules that are be able to form polymeric micelles. The HIA groups are located in the micelle core. Additionally, triarylmethane mechanophores were synthesized ad incorporated into polymeric structures to obtain rubbery mechanochromophore materials.
The HIA functional group is sensitive to pH, light and metal ions. Therefore, its incorporation in macromolecular structures is expected to afford multi-stimuli responsive materials of great versatility. Here, the RAFT synthesis of several A-B type amphiphilic block copolymers of oligo(ethylene glycol) methacrylate (block A, hydrophilic, thermosensitive) and 4-[(hydroxyimino)aldehyde]butyl methacrylate (HIABMA) (block B, hydrophobic, multi-stimuli sensitive) is reported. The pH-sensitivity of the pHIABMA block is employed to drive the formation of polymeric micelles (PIM method: pH induced micellization) and the outcome is compared to that of the standard solvent displacement technique (DMF / H2O) through a combination of light scattering, TEM, SAXS and UV-Vis measurements. Specifically, PIM method promotes the formation of monodisperse, stable and reversible core-shell micelles also from polymers that fail to form nanoaggregates or yield loose nanostructures with the solvent displacement technique. As a result, pH-induced micelles are thermoresponsive, whereas those obtained by solvent displacement lack this useful property.
Thanks to the complexing capacity of the polymeric HIA function it is possible to prepare nanoaggregates with a core consisting of Cu2+ complexed by PHIABMA, both starting from aqueous solutions with preformed PIM micelles and from the polymer solubilized in organic solvent. Acidification of the Cu2+-loaded nanoaggregates leads to copper release and it is possible to separate polymer and metal ion by centrifugal filtration or dialysis, yielding a recyclable system.
As regards the response to the light stimulus, the HIA groups photoisomerizes chemoselectively to cyclobutanol oximes (CBO) through Norrish-Yang reaction (365 nm) when irradiation is conducted on solutions in DMSO. When irradiation is performed on aqueous micellar suspensions, there is no evidence of this photoisomerization as the HIA group disappears, nor has any other Norrish-type product been identified. However, the micelles are stabilized through probable crosslinking.
Aiming at chemically modifying the polymers chain ends, the terminal thiocarbonylthio group has been removed by nucleophilic substitution, hoping to reveal and trap the resulting –SH moieties. Whether terminal removal was successful, the resulting polymeric chain ends were mostly unreactive, showing limited dimerization through disulphide links and no significant reactivity towards other substrates.
The second project carried out at Adolphe Merkle Institute of University of Fribourg (Switzerland) concerns the synthesis of two novel triarylmethane mechanophores, 4,4’-((((4-vinylbenzyl)oxy)(2-((4-vinylbenzyl)oxy)phenyl)methylene)bis(4,1-phenylene))dimorpholine and 4,4'-((((4-vinylbenzyl)oxy)(4-((4-vinylbenzyl)oxy)phenyl)methylene)bis(4,1-phenylene))dimorpholine indicated with the acronym M.O and M.P respectively. Rubbery poly(methyl acrylate-co-2-hydroxyethyl acrylate) P(MA-co-HEA) networks featuring M.O and M.P motifs as cross-linkers are then synthesized and characterized through thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and tensile test with in situ transmittance measurements.
The comparison between the absorption spectrum of acid-activated carbocations M.O+ and M.P+ in solution and the transmission spectrum of polymer networks subjected to elongation demonstrates the heterolytic bond cleavage and selective carbocation formation upon mechanical stimulation. Comparison of the stress-strain and ΔTransmittance curves as a function of the applied strain shows that M.O is activated earlier, suggesting that the mechanophore featuring the ortho substitution could be applicable in signalling mechanical deformation at lower elongation.