Nuclear Magnetic Resonance: structural characterisation of polymers and biochars

Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool to investigate inner properties of matter and it is used for both fundamental and applied research. If not-zero nuclear spins are embedded in a magnetic field, a detectable magnetization is formed. Its subsequent manipulation allows the study of scientifically interesting microscopic parameters: they can mostly be correlated to macroscopic properties which need to be tuned for industrial applications. On top, polymer science and porous media research exploit NMR peculiarities a lot. Particularly, low-field (LF) time domain (TD) 1H NMR can provide valuable information about bulk, nanostructured and polymer films rapidly, accurately and cheaply. Additionally, NMR-active noble gases combine their almost null reactivity with a high sensitivity to the local environment: they are effective probes to check the presence of pores, their dimensions and degree of accessibility. Firstly, since theories link the NMR-accessible residual dipolar coupling constants (Dres) and their distribution to macroscopic crosslink density (CLD) of vulcanized elastomers, LF TD 1H-NMR was used to evaluate the CLD of polyisoprene/polybutadiene (IR/BR) rubber blends at their optimum curing time. Compared to the ideal case where the two homopolymers evolve independently, small but still significant experimental differences in Dres values suggesting the vulcanizing agents may act differently on the two polymer phases were collected. Introducing new experimental variables (experimental temperature, different vulcanization times) we tried to enhance a single phase only and measure their CLD separately: if ever enhanced, CLD differences in the two phases were however below the technique sensitivity. Additionally, a direct evaluation of the Dres distributions and CLD on IR, BR single-polymer rubber vulcanized at different times in undercuring conditions was performed. The compared analysis of NMR and equilibrium swelling data allowed to propose a new model for rubber network growth with time: the crosslinks are not formed randomly in the sample, but instead islands with a precise morphology are formed and then they merge together. LF TD 1H NMR techniques were applied on polymeric nanostructured systems as well. On top, changes in measured T1 values can be correlated to the activation of different molecular motions. Mostly, we investigated the T1 change in oxygen-free divinylbenzene (DVB)-crosslinked polystyrene (PS) nanoparticles (NPs) due to temperature and different amount of crosslinker: we eventually differentiated the collective motion of the main chain from the PS and DVB phenyl rings ones. Similarly, a precise quantification of the amount of protons which are rigid (fr) and mobile was performed on poly(n-butylacrylate)/polystyrene (PBA/PS) core-shell NPs. A fr temperature-dependent study and its direct comparison with the synthesis conditions at temperatures far away from both PS and PBA glass transition temperatures proved the two phases of PBA@PS NPs behaved as two independent homopolymers. Again, a comparison of the experimental fr values with the theoretical ones of two and three-components BEVA ®731-replacement mixture for cultural heritage applications helped understanding the improved adhesive properties of the three-components mixture over the two-components ones: only in this case a perfect miscibility was obtained. Lastly, high-field 129Xe NMR was applied to study the morphology of differently activated pecan and almond shells-based biochars: on top, 1D conventional and pressure-dependent spectra proved the biochars structure is porous with an average dimensions of around 10Å. T1 and 2D exchange measurements probed the pores accessibility. It strongly depended on the activation conditions: only the pecan-based biochars cured with the higher airflow or the longer-cured, rain-washed almond-based ones showed 129Xe exchange, proving them effective as filtration systems.