Titolo della tesi: CALORIMETRIC AND SPECTROSCOPIC CHARACTERIZATION OF BULK AND CONFINED GLASS-FORMING SYSTEMS
Glass formation is a phenomenon that transcends chemical composition. It can occur in materials of every chemical type, including covalent, ionic, molecular, metallic, and hydrogen bonded compounds[1]. Because, in theory, any pure substance or mixture can generate a glass if the cooling rate is high enough[2,3] there is no advantageous method for classifying glass-formers according exclusively to their chemical composition. While common substances like silica and borates are well-known glass-formers, the array of glassy materials that are industrially or academically relevant is immense. Organic molecules, metals, and polymers are just a few examples. This diversity highlights the intricate interplay between molecular structure, intermolecular forces, and cooling conditions that govern glass formation. Among them, polymers possess unique and intriguing properties Unlike many other glass-formers that require high cooling rates to bypass crystallisation and enter in the glassy state, several polymers often achieve this transition at relatively low or, in other cases cooling rates approaching to zero[4,5]. This characteristic makes polymers ideal for experimental studies, as they do not demand the extreme conditions necessary for other glass-forming materials.
The ability of polymers to form glasses under mild cooling conditions can be attributed to their macromolecular structure[6]. Nevertheless, macromolecules exhibit notable distinctions from conventional molecules. They consist of repeated units of unique chemical motifs, referred to as monomers, interconnected in long chains, which have limited flexibility due to inter- and intra-macromolecular barriers. Theoretically, crystallisation of a polymer is contingent upon the fulfilment of three criteria: chemical regularity, regioregularity, and stereoregularity[7]. Chemical regularity requires that the monomer units composing the polymer chain possess uniform chemical composition. Regioregularity mandates a consistent arrangement of substituents. Finally, stereoregularity necessitates a uniform spatial arrangement of atoms or groups throughout the polymer chain.
If all three criteria are satisfied, a slow cooling from the melt can lead to the formation of a semicrystalline structure, characterised by the coexistence of crystalline and amorphous domains. Conversely, if the cooling rate is equal to or exceeds the critical cooling rate, crystallisation can be avoided, resulting in a fully amorphous material. This is the case of poly(L-lactide) (PLA), a biobased polyester that is a prototypical example of a polymer that can exhibit both crystalline and amorphous phases. In fact, its ability to crystallise or remain fully amorphous is highly dependent on the processing conditions, particularly on the cooling rate from the melt. Indeed, the glass can be obtained at a relatively low cooling rate and its Tg above room temperature (Tg=55-60 °C) favours the study of the glassy state. PLA crystallisation behaviour is of significant interest due to its impact on the material’s mechanical and optical properties, thermal stability, and biodegradability. Semicrystalline PLA typically exhibits higher tensile strength and modulus compared to its amorphous counterpart. In this thesis, PLA is one of the primary polymeric glass-forming system under investigation. The research focuses on understanding the multi-step relaxation pathway of PLA amorphous phases to crystal development during isothermal heat treatment close to the glass transition temperature. Following the enthalpic evolution and conformational variations of polymeric chains, a subtle thermal transition occurring prior to the onset of cold crystallisation was identified. The latter, named super-cooled liquid transition, appears to drive the crystallisation process through to the formation of an amorphous phase enriched in low energy conformers. This phase facilitates the nucleation process, thereby promoting the subsequent development of crystals.
The other side of the coin, if a polymer fails to meet any of the three criteria listed above, namely chemical regularity, regioregularity, or stereoregularity, it will be incapable of crystallisation. Under these circumstances, the polymer remains completely amorphous whatever is the cooling rate applied. This is exemplified by poly(4-chlorostyrene) (P4ClS) a synthetic polymer lacking both in regioregularity and stereoregularity. A part of the project conducted during this thesis aimed to elucidate the physical aging kinetics of P4ClS and other glass-forming systems after temperature down-jumps in the glassy state across a broad temperature range. Conventional understanding attributes the decrease in free energy of an out-of-equilibrium system to molecular relaxation, manifested as physical aging through super-Arrhenius α-relaxation. However, our findings suggest an additional molecular mechanism that assists α-relaxation during physical aging, offering new insights into the relaxation dynamics of glassy polymers.
Finally, the effects of dimensional confinement on polymer nanoparticles represent the last part of this thesis project. Polysulfone (PSU), a high-temperature resistant amorphous thermoplastic, was selected as the polymer of interest. PSU is renowned for its exceptional mechanical and thermal properties, high glass transition temperature, and chemical inertness, making it a suitable material for various high-tech applications. To prepare PSU nanoparticles, experimental conditions were optimized using the nanoprecipitation method. Subsequently, the morphological and thermal properties were characterized. Dynamic Light Scattering (DLS), Scanning Electron Microscopy (SEM), and Differential Scanning Calorimetry (DSC) were employed as primary analytical techniques. DLS and SEM facilitated the assessment of the size distribution and morphology, while DSC provided valuable insights into the thermal behaviour.
1. Bragg, W. The glassy state. in Structural Chemistry of Glasses (ed. Rao, K. J.) 13–76 (Elsevier Science Ltd, Oxford, 2002).
2. Li, Y., Ng, S. C., Ong, C. K., Hng, H. H. & Gob, T. T. Glass forming ability of bulk glass forming alloys. Scr Mater 36, 783–787 (1991).
3. Turnbull, D. Under what conditions can a glass be formed? Contemp Phys 10, 473–488 (1969).
4. Merrick, M. M., Sujanani, R. & Freeman, B. D. Glassy polymers: Historical findings, membrane applications, and unresolved questions regarding physical aging. Polymer (Guildf) 211, (2020).
5. Cangialosi, D., Boucher, V. M., Alegría, A. & Colmenero, J. Physical aging in polymers and polymer nanocomposites: Recent results and open questions. Soft Matter vol. 9 8619–8630 (2013).
6. Roth, C. B. Polymer Glasses. (CRC Press, 2017).
7. Corradini, P., Auriemma, F. & De Rosa, C. Crystals and crystallinity in polymeric materials. Acc Chem Res 39, 314–323 (2006).