Structure, dynamics and rheology of glass-forming polymers and vitrimers

A glass is an amorphous solid, meaning its atomic or molecular arrangement is disordered, similar to that of a liquid but frozen in place. The glass transition remains a complex and incompletely understood phenomenon, with ongoing debate about its fundamental mechanisms. Unlike metal or small-molecule glasses, polymer glasses exhibit unique chain-length-dependent behaviour due to their interconnected molecular structure. This chain connectivity enables intramolecular motions, such as dihedral rotations, which drive complex dynamics that vary with chain length. Moreover, vitrimers—an innovative class of polymers named for their glass-like (vitreous) behaviour combined with dynamic associative covalent bonds to enable reprocessability while retaining structural integrity. These dynamic cross-links allow stress relaxation and network rearrangement without permanent bond dissociation, bridging the gap between thermosets and thermoplastics. In this thesis, poly(methyl acrylate) (PMA) serves as a versatile model system, excelling both in studies of glass-forming polymer dynamics and as a backbone material in model vitrimers.

In this thesis, low-dispersity PMAs with a wide chain length range were synthesized by controlled/living radical polymerization and the effect of the presence of the alkyl end-group on their behaviour was studied in parallel. Significant multi-regional behaviour was found in the investigation of chain length-dependent glass transition temperature (Tg) and fragility (m) of PMAs. The relationship between ion transport and structural relaxation in PMA was also investigated. In addition, the length-scale of dynamic heterogeneity was investigated for different PMA chain lengths, a topic relatively rare in previous polymer studies.

Moreover, a new class of vitrimers based on poly(methyl acrylate) was also investigated, which utilizes the dioxaborolane metathesis to produce cross-linking. These vitrimers demonstrate a combination of ultra-stretchability (up to ∼ 80 times their own length), mechanical toughness (∼ 40 MJ/m^3), and thermal stability up to T ∼ 250 °C; moreover, the vitrimers demonstrate excellent mechanical damping characterised by a loss factor (tanδ) with a maximum of ∼ 2–3 and an effective value > 0.3 across five orders of magnitude in frequency (0.001–100 Hz), or correspondingly across a T-range of ∼ 35 °C near room temperature (for a probe frequency of 1 Hz). The vitrimers can be successfully re-processed using both a thermo-mechanical and a chemical processing route, and can self-heal at room temperature, making them suitable for sustainable applications. The material properties are directly tuneable by variation of both the amount of cross-linker and by the degree of curing. Thus, this class of vitrimers are promising for applications where stretchability combined with mechanical toughness and/or a high mechanical dissipation is required. Finally, the multiscale relaxation dynamics of the PMA-based vitrimers were investigated. These vitrimers exhibit very sluggish network rearrangement time, on the order of 100 s at 180 °C, which is much longer than the structural (α) relaxation time (∼ 100 s at 20 °C). Their plateau moduli are proportional to the nominal cross-link density but lower than the effective cross-link density, which is related to the enriched region formed by the cross-linker during synthesis. Although the network reorganization activation enthalpies of these vitrimers are in the range of ∼ 60–80 kJ/mol, the activation energies of their more localized secondary β-relaxation (∼ 100 kJ/mol) and γ-relaxation (∼ 30–35 kJ/mol) are very similar to those of PMA.