Dramatic Tunability of the Glass Transition Temperature and Fragility of Low Molecular Weight Polystyrene by Initiator Fragments Located at Chain Ends

Abstract

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.

Poly(alanine-nylon-alanine) as a bioplastic: chemoenzymatic synthesis, thermal properties and biological degradation effects

“…The chain-end structures, such as hydroxyl group or ethyl ester, are known to affect the thermal properties, especially, in the case of lower molecular weight polymers. 31 In the current study, we detected two types of the C-terminal structures, namely, carboxylic acid and ethyl ester ends. However, the most of the C-terminal end was ethyl ester, and hence the effect of the chain end structure was not significant.…”

Influence of RAFT end‐groups on the water swelling of poly( N ‐propyl methacrylate)

“…PPMA‐DD: M n = 3.0 × 10 4 Da, Đ = 1.05). This similarity in molecular weight is critical as the molecular weight can dramatically influence the impact of end groups on physical properties …”

“…For example in thin films, the end group from initiator fragments significantly impacts the thickness dependence of the glass transition temperature . These end‐group effects tend to decrease as the molecular weight of the polymer increases with the impact of the end group becoming vanishingly small at sufficiently high molecular weight …”

Insights into the Structural Dynamics of PLGA at Terahertz Frequencies

“…Here T g,∞ is the glass transition temperature for an ideal polymer chain of infinite length, K is a proportionality constant, and M n is the number average MW of the polymer. 43 The equation reflects that a polymer of low MW has more free volume than a polymer of high MW, since the low MW polymer must have a greater concentration of chain ends for the same mass of sample. Consequently, an increase in the concentration of chain ends is accompanied by a decrease in T g .…”