High purity cyclic PS (c-PS) samples with number-average molecular weight (MW) of 3.4 and 9.1 kg/ mol were synthesized via atom transfer radical polymerization and "click" chemistry with narrow MW distribution. Bulk glass transition temperature (T g ) measured by differential scanning calorimetry exhibited a much weaker MW dependence for c-PS relative to its linear precursor and anionically polymerized linear PS (A-PS). Using ellipsometry and fluorescence spectroscopy, major differences were observed in the T gconfinement effect in c-PS films supported on silicon substrates compared to A-PS. Whereas a large T g reduction with confinement is commonly observed for A-PS supported on silica, within error, no confinement effect is seen in c-PS/3.4k films on Si/SiO x substrates down to 21 nm thickness. Although the c-PS linking group contains nitrogen and oxygen atoms potentially able to undergo hydrogen bonding, T g is invariant with confinement for c-PS/3.4k or slightly reduced for c-PS/9.1k regardless of the level of substrate-surface hydroxyl groups. Ellipsometry indicates that the near elimination of the T gconfinement effect in c-PS originates mainly from a very weak perturbation to T g near the free surface (in comparison to linear PS) rather than a strong perturbation at the polymer−substrate interface. We hypothesize that unlike linear polymers, the packing efficiency of cyclic PS segments, i.e., cyclic PS fragility, is not significantly perturbed by the free surface, which in turn results in at most a very weak T g perturbation at the free surface and an invariance of average T g across the film with confinement.

Antioxidant-rich agro-wastes such as grape pomace waste (GW), turmeric shavings and waste, coffee grounds, and orange peel waste are used as-received for the first time as thermo-oxidative stabilizers for polymer. Relative to neat low density polyethylene (LDPE), a well-dispersed hybrid made by solid-state shear pulverization with 4 wt % GW results in 62 and 44 °C increases in temperatures corresponding to 10 and 20% mass loss in air (T 10% and T 20%), respectively. Such enhancements are superior to those obtained by adding 1 wt % synthetic antioxidant Irganox I1010 to LDPE by melt mixing. Relative to neat LDPE, hybrids with well-dispersed agro-waste exhibit enhanced Young’s modulus, equal or enhanced tensile strength, and relatively small reduction in elongation at break. Reprocessing or recycling sometimes leads to enhanced antioxidant activity: relative to a hybrid before melt extrusion, 92/8 wt% LDPE/TW exhibits major increases in T 10% and T 20% after two and six melt extrusion passes, which is consistent with formation of transformation products with improved antioxidant activity during multiple high-temperature reprocessing cycles. Natural antioxidants are effective in suppressing LDPE chain scission and branching. After ten extrusion passes, neat LDPE exhibits a 16% increase in zero-shear viscosity and reduction in elongation at break from 500% to 280%, whereas hybrids with agro-waste have zero-shear viscosity and elongation at break values close to those of unprocessed hybrids. Isothermal shear flow measurements also show the effectiveness of natural antioxidant in stabilizing LDPE: hybrids exhibit no sign of chain branching during 3000 s of melt flow at 200 °C whereas neat LDPE branches after ∼500 s.

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We used differential scanning calorimetry and spectroscopic ellipsometry to measure the molecular weight (MW) dependence of bulk fragility (m bulk) and spectroscopic ellipsometry to measure the thickness dependences of the glass transition temperature (T g) and fragility (m) in supported thin films of low MW cyclic or ring polymer. The effects of confinement on T g and m of thin polymer films are important in a range of advanced technology applications, including nanoimprinting. It has previously been shown that nanoconfined films of high MW linear polystyrene (PS) exhibit major T g- and m-confinement effects whereas films of low MW cyclic PS (c-PS) show at most a very weak T g-confinement effect. In the absence of chain ends, c-PS exhibits very weak T g,bulk– and m bulk–MW dependences compared to linear PS. Despite low MW c-PS having m bulk values similar to that of high MW linear PS, we found that low MW c-PS films show a very weak m-confinement effect because of a weak free-surface effect; e.g., m for a 27 nm thick film of 3.4 kg/mol c-PS is the same as m bulk within error. Overall, these results support a strong correlation between the susceptibility of fragility perturbation and the susceptibility of T g perturbation caused by MW reduction, chain topology, and/or confinement.

Important, yet unexplored effects of chemically distinct initiator fragments incorporated at chain ends in linear polymer are investigated in depth. Polystyrene (PS) samples of a wide range of molecular weight (MW) were synthesized by conventional free radical polymerization and controlled radical polymerization using seven different initiators and compared with anionically polymerized PS. The initiator fragments incorporated during polymerization have major consequences on the glass transition temperature (T g) and dynamic fragility of low MW PS. For example, with ∼4 kg/mol PS, the T g onset value and fragility can be tuned from ∼334 K and ∼65, respectively, with dodecanethiol and hydrogen atom chain ends to ∼367 K and ∼130, respectively, with cyanopentanoic acid chain ends. A similar high T g and high fragility were measured with isobutyric acid/SG1 nitroxide chain ends. These remarkable effects, with a greater than 30 K difference in T g and a factor of 2 difference in fragility, indicate that chain ends in low MW PS homopolymer play an “outsize” role in comparison to comonomer units in perturbing properties that are sensitive to the density of chain ends. The T g results also provide further direct evidence against any correlation between the MW at which the T g–MW dependence saturates and entanglement MW. Instead, the perturbation of T g by the combined effects of a reduction in MW (increase in chain-end density) and chain-end structure correlates one-to-one within error with the perturbation of fragility. These results suggest that the susceptibility of fragility to be perturbed is key to the susceptibility of T g to be perturbed.