Local Chain Alignment via Nematic Ordering Reduces Chain Entanglement in Conjugated Polymers

Abstract

Chain entanglements govern the dynamics of polymers and will therefore affect the processability and kinetics of ordering; it follows that through these parameters chain dynamics can also affect charge transport in conjugated polymers. The effect of nematic coupling on chain entanglements is probed by linear viscoelastic measurements on poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and poly-((9,9-dioctylfluorene-2,7-diyl)-alt-(4,7-di(thiophene-2-yl)-2,1,3-benzothiadiazole)-5′,5″-diyl) (PFTBT) with varying molecular weights. We first verify the existence of nematic phases in both PFTBT and PCDTBT and identify nematic−isotropic transition temperatures, T IN , between 260 and 300 °C through a combination of differential scanning calorimetry, polarized optical microscopy, temperature-dependent X-ray scattering, and rheology. In addition, both PCDTBT and PFTBT show a glass transition temperature (T g ) and T IN , whereas only PFTBT has a melting temperature T m of 260 °C. Comparing the molecular weight dependence of T IN with theoretical predictions of nematic phases in conjugated polymers yields the nematic coupling constant, α = (550 ± 80 K)/T + (2.1 ± 0.1), and the long-chain limit T IN as 350 ± 10 °C for PFTBT. The entanglement molecular weight (M e ) in the isotropic phase is extracted to be 11 ± 1 kg/mol for PFTBT and 22 ± 2 kg/mol for PCDTBT by modeling the linear viscoelastic response. Entanglements are significantly reduced through the isotropic-to-nematic transition, leading to a 10-fold increase in M e for PFTBT and a 15-fold increase for PCDTBT in the nematic phase.

Glass Transition Phenomenon for Conjugated Polymers

“…By introducing even bulkier building blocks to the backbone structure, the sidechain content is further reduced meanwhile the chain rigidity is increased, which consequently raises the T g . For example, as listed in Table , inclusion of a rigid fused ring benzothiadiazole (BT) unit to F8 backbone results in a much higher T g of ≈135 °C for poly(9,9‐dioctylfluorene‐ co ‐benzothiadiazole) (F8BT); and poly(2,7‐(9,9‐dioctyl‐fluorene)‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)) (APFO‐3 or PFTBT) features a lower T g of approximately 130 °C compared with the T g of poly(2,7‐(9,9‐dioctyl‐fluorene)‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐diphenyl‐quinoxaline)) (APFO‐18) and poly(2,7‐(9,9‐dioctyl‐fluorene)‐alt‐5,5(2,3,6,7‐tetraphe‐nyl‐9,10‐dithien‐2‐yl pyrazino[2,3‐g] quinoxaline)) (APFO‐Green9), which are around 142 °C and 177–192 °C, respectively, because of the bulkier building blocks in the latter two cases.…”

Glass transition temperature from the chemical structure of conjugated polymers

“…Below T g , the storage moduli of conjugated polymers are on the order of one GPa, making the materials too stiff and too brittle for flexible devices 1 . At higher temperatures, the storage modulus decreases orders of magnitude to~10 MPa for a semicrystalline polymer, or~1 MPa for an entangled amorphous polymer 1,2 . Therefore, one of the key design requirements for soft or stretchable electronics is to ensure that T g lies below room temperature 3,4 .…”

Liquid Crystal Ordering on Conjugated Polymers Film Morphology for High Performance

“…In addition, the arrangement of molecules into some type of order could effectively reduce the extent of entanglement . The formation of liquid crystallinity reduces the entanglement between polymers by reducing the critical entanglement molecular weight . This phenomenon has been shown in many studies, both in thermotropic liquid crystal systems and lyotropic liquid crystal systems.…”

“…This phenomenon has been shown in many studies, both in thermotropic liquid crystal systems and lyotropic liquid crystal systems. Enrique D. Gomez and coworkers determined the entanglement molecular weight of a conjugated polymer PCDTBT melt in a thermotropic liquid crystalline state and in the isotropic state through rheological measurements and demonstrated the improvement of Me and reduced entanglement in the liquid crystalline state . Kazuo Akagi et al used a type of liquid crystalline solvent to achieve entanglement‐free polyacetylene, and long and single nanofibers could be attained by molecular assembly in solution .…”