Physical Ageing of Polystyrene: Does Tacticity Play a Role?

Effect of Polymer Host on Aggregation-Induced Enhanced Emission of Fluorescent Optical Brighteners

Fluorophores displaying concentration-dependent luminescence are becoming increasingly valuable in stress-sensing, tagging, and dyeing applications, including the quantification of recycled content in plastic packaging. In this work, we investigate the effects of the polymer matrix, dye structure, and crystallinity on aggregation-induced enhanced emission (AIEE). We demonstrate that the aggregation threshold required for successful quantification can be adjusted through modulation of guest-host (dye-polymer) interactions and monitored using an array of fluorescence characterization. Modification of guest-host interactions is realized through choice of host, change of guest, and tuning of the crystallinity of the host system. Increasing the number of guest-host interactions and solubility between guest and host, loosely predicted through the calculation of the solubility parameter, increases the aggregation threshold relative to other low-polarity and low-interacting systems. We demonstrate that issues, such as loading level and cost, associated with high aggregation thresholds, can be circumvented by increasing system crystallinity, improving spectral intensities, and subsequent quantification. These insights explore the fundamental understanding of supramolecular interactions that govern dye-polymer systems.

A Phenomenological Model for Enthalpy Recovery in Polystyrene Using Dynamic Mechanical Spectra

This paper studies the effects of annealing time on the specific heat enthalpy of polystyrene above the glass transition temperature. We extend the Tool-Narayanaswamy-Moynihan (TNM) model to describe the endothermic overshoot peaks through the dynamic mechanical spectra. In this work, we accept the viewpoint that the enthalpy recovery behavior of glassy polystyrene (PS) has a common structural relaxation mode with linear viscoelastic behavior. As a consequence, the retardation spectrum evaluated from the dynamic mechanical spectra around the primary Tg peak is used as the recovery function of the endothermic overshoot of specific heat. In addition, the sub-Tg shoulder peak around the Tg peak is found to be related to the structural relaxation estimated from light scattering measurements. The enthalpy recovery of annealed PS is quantitatively described using retardation spectra of the primary Tg, as well as the kinetic process of the sub-Tg relaxation process.

The Effect of Stereoregularity and Adding Irganox 1076 on the Physical Aging Behavior of Poly(1-trimetylsilyl-1-propyne)

The effect of the stereoregularity of poly(1-trimethylsilyl-1-propyne) [PTMSP] (cis-content from 50 to 90%) on physical aging was investigated by measurement of the gas permeability. Films from pure PTMSP as well as those with the addition of the antioxidant Irganox 1076 were exposed to the air. The permeability of pure PTMSP films increases with an increase in cis-stereoregularity and correlates with an increase in interchain distances (according to X-ray analysis). For pure PTMSP films, the most significant aging (up to 50% of permeability drop) was observed for polymers with mixed microstructure, and the slowest aging (10-30% of permeability drop) was observed for polymers with cis-regular structure. For PTMSP films with added Irganox 1076, some decrease in permeability with time is also observed. The addition of Irganox 1076 to PTMSP in mixed as well as cis-enriched configurations visibly slows down aging. In the case of cis-regular PTMSP with a slow aging rate, the introduction of an antioxidant does not provide any advantages. The high stability of cis-regular PTMSP demonstrates the possibility of obtaining more stable membrane materials with the highest equilibrium state of the polymer selective layer prepared by casting solution.

Controlling fine touch sensations with polymer tacticity and crystallinity

The friction generated between a finger and an object forms the mechanical stimuli behind fine touch perception. To control friction, and therefore tactile perception, current haptic devices typically rely on physical features like bumps or pins, but chemical and microscale morphology of surfaces could be harnessed to recreate a wider variety of tactile sensations. Here, we sought to develop a new way to create tactile sensations by relying on differences in microstructure as quantified by the degree of crystallinity in polymer films. To isolate crystallinity, we used polystyrene films with the same chemical formula and number averaged molecular weights, but which differed in tacticity and annealing conditions. These films were also sufficiently thin as to be rigid which minimized effects from bulk stiffness and had variations in roughness lower than detectable by humans. To connect crystallinity to human perception, we performed mechanical testing with a mock finger to form predictions about the degree of crystallinity necessary to result in successful discrimination by human subjects. Psychophysical testing verified that humans could discriminate surfaces which differed only in the degree of crystallinity. Although related, human performance was not strongly correlated with a straightforward difference in the degree of crystallinity. Rather, human performance was better explained by quantifying transitions in steady to unsteady sliding and the generation of slow frictional waves (r² = 79.6%). Tuning fine touch with polymer crystallinity may lead to better engineering of existing haptic interfaces or lead to new classes of actuators based on changes in microstructure.