Glass Transition Temperature of Conjugated Polymers by Oscillatory Shear Rheometry

Abstract

The stiff backbones of conjugated polymers can lead to a rich phase behavior that includes both crystalline and liquid crystalline phases, making measurements of the glass transition challenging. In this work, the glass transitions of regioregular poly(3-hexylthiophene-2,5-diyl) (RR P3HT), regiorandom (RRa) P3HT, and poly((9,9-bis(2-octyl)-fluorene-2,7-diyl)-alt-(4,7-di(thiophene-2-yl)-2,1,3benzothiadiazole)-5′,5″-diyl) (PFTBT) are probed by linear viscoelastic measurements as a function of molecular weight. We find two glass transition temperatures (T g 's) for both RR and RRa P3HT and one for PFTBT. The higher T g , T α , is associated with the backbone segmental motion and depends on the molecular weight, such that the Flory−Fox model yields T α = 22 and 6 °C in the long chain limit for RR and RRa P3HT, respectively. For RR P3HT, a different molecular weight dependence of T α is seen below M n = 14 kg/mol, suggesting this is the typical molecular weight of intercrystal tie chains. The lower T g (T αPE ≈ −100 °C) is associated with the side chains and is independent of molecular weight. RRa P3HT exhibits a lower T α and higher T αPE than RR P3HT, possibly due to a different degree of nanophase separation between the side chains and the backbones. In contrast, PFTBT only exhibits one T g above −120 °C, at 144 °C in the long chain limit.

Glass Transition Phenomenon for Conjugated Polymers

“…In the leathery regime, in which the glass transition occurs, the Young’s modulus can vary by several orders of magnitude. Figure b shows the response of dynamic shear moduli for a regiorandom poly(3‐hexylthiophene) (P3HT) in the frequency space (low frequency equivalents to high temperature), where similar magnitude of decrease in shear moduli is observed . Hence, it is pivotal to study the T g of polymers, both commodity polymers and conjugated polymers, in order to determine the proper operating temperature range for desired applications.…”

“…b) Dynamic shear moduli for a regiorandom poly(3‐hexylthiophene) (P3HT) (weight average molecular weight of 16.7 kg mol −1 and PDI of 1.15) at reference temperature of 0 °C. Adapted with permission . Copyright 2017, American Chemistry Society.…”

“…For a material that has greater electrical conductivity, the low‐frequency conductivity slope can mask the dielectric relaxation response of the material, which results in the difficulties in the data analysis . Hence, the application of BDS on measuring T g for conjugated polymers is limited …”

The Critical Role of Electron‐Donating Thiophene Groups on the Mechanical and Thermal Properties of Donor–Acceptor Semiconducting Polymers

“…The side chain transition peak was more pronounced as observed in the loss modulus curve. [62,63] Both bulk and thin film techniques suggested the incorporation of this electron donating group into the polymer structure greatly altered its backbone dynamics, and reflected in their macroscopic mechanical properties. [39] From Figure 2a-c, we observed a noticeable increase of backbone T g from −3.96 °C (DPP-T) to 11.95 °C (DPP-T2), and 18.98 °C (DPP-T3) as the number of thiophene units increased (Table 1).…”

Fluorinated Head‐to‐Head Dialkoxybithiophene: A New Electron‐Donating Building Block for High‐Performance Polymer Semiconductors

“…TGA data reveal that the decomposition onset with a 5% weight loss (T d ) under N 2 is 326 °C ( Figure S5, Supporting Information), probably due to the breakup of CO bond. [53,54] DSC measurement shows no distinctive thermal transitions in the temperature range of 30-320 °C ( Figure S6, Supporting Information).…”