Polymeric glass-forming materials are used in construction, textiles, optics, and advanced manufacturing. Glassy relaxation dynamics of polymeric glass-formers generally involve a series of molecular relaxations (α, β, γ), where the α-relaxation relates to the glass transition temperature (Tg). Understanding these relaxations is key to characterising and designing polymer glasses for specific applications. For many polymeric glass formers, the dynamics and structural properties are well known for only a small range of molecular weights (M).
Whilst crucial to our understanding, some relaxations are difficult to measure using standard techniques. Broadband dielectric spectroscopy is a common, powerful technique for probing relaxation mechanisms; however, for systems like polystyrene, this technique cannot resolve certain relaxations. This project investigates how dielectrically active probes may be used to address this issue. These probes were added to polystyrene of various M. The addition of the probes made it possible to track the behaviour of secondary relaxations in PS across a wide range of M. We determine that this is a viable technique to measure the M dependence of secondary relaxations.
Other important aspects which are not yet resolved are how topological entanglements effect dynamics. It is unknown whether the onset of entanglements and the onset of M-independent Tg(M) behaviour are correlated, at least for some polymers. This work addressed this using binary blends, where one component short, non-entangled oligomer, to systematically control the entanglement density and investigate the effects on Tg(M) behaviour. The detailed change in the Tg(M) behaviour upon blending was determined. The results demonstrated that the M characterising changes in Tg(M) within the relevant M-range decrease upon dilution, whereas the characteristic M of entanglements increase.
Finally, hydrogen bonding plays an important role in the relaxation dynamics of hydrogen bonding glass formers, but the role is not well understood. A good example of this is oligopropylene glycol, which exhibits pseudo-polymeric behaviour due to hydrogen bonding chain-ends. A series of oligopropylene glycols with varied chain-ends were studied to address this. Different chain-ends and chain-lengths varied the strength and density of the hydrogen bonding, respectively. Mechanical measurements revealed a rheological response slower than the structural relaxation; this is reminiscent of behaviour observed in some other non-polymeric hydrogen bonding systems. The effects that deuteration of the end hydroxyl group has on the properties were investigated. Deuteration affected the dielectric strength of the relaxations, but no significant effects on the relaxation timescales were observed. Moreover, it is demonstrated that the dielectric strength of the α-relaxation depends only on temperature and not on dynamics, and the behaviour follows a simple exponential form.
