Conjugated Polymer Thesis

This semi-empirical correlation requires only one adjustable parameter on the relative difference in atomic mobility between a conjugated and a non-conjugated atom and predicts the Tg ± 25 °C with 95% confidence for nearly all conjugated polymers with alkyl side chains.Other attempts of correlating Tg with side chain mass fraction and packing length are also illustrated but deemed as unsatisfactory because of the limited applicability in only certain groups of conjugated polymers.As the molecular weight of pure RR P3HT decreases, the possibility of forming tie chains in RR P3HT decreases, eventually leading to isolated crystal domains and viscoelastic liquid behavior at long times.

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This method then allows systematic investigation of the effects of molecular weight and side chain regioregularity of P3HT on Tg.

In addition, both side chain and backbone Tgs are identified for P3HT, while only the backbone Tg is found for PFTBT.

Although conjugated polymer is promising for applications in stretchable electronics, no rational design exists due to the lack of characterization of the fundamental properties of conjugated polymers and their interrelationships with chemical structures and performances.

Regarding the mechanical performance, the fundamental properties include the glass transition temperature (Tg), the entanglement molecular weight (Me) and the tie chain density (ρx), which dictate the stretchability of a homopolymer in different states (i.e., glassy, semicrystalline, melt).

The molecular weight dependence of backbone Tg is modeled by the Flory-Fox equation, yielding Tg = 22 °C, 6 °C and 144 °C in the long chain limit for RR P3HT, RRa P3HT and PFTBT, respectively.

Furthermore, for RR P3HT, a different molecular weight dependence of Tg is seen below Mn = 14 kg/mol, suggesting this is the typical molecular weight of intercrystalline tie chains, based on our hypothesis that RR P3HT has 16 K higher Tg than RRa P3HT due to stretched tie chains.

Specifically, with hexyl side chains added onto the thiophene rings of PFTBT and PCDTBT, both PFT6BT and PCT6BT appear to be entirely amorphous within the temperature range, where PFTBT and PCDTBT show strong evidence of nematic phase.

This behavior suggests that the nematic ordering in conjugated polymers may profoundly depend on the propensity of π-π stacking, which can be distorted by significant side chain steric hindrance, thus supporting the possible nematic phase in RR P3HT but not in RRa P3HT.

Because nematic phase is ubiquitous in the conjugated polymer, the role of nematic coupling on entanglement molecular weight (Me) is explored with multiple molecular weights of PFTBT and poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT).

We first verify the existence of nematic phases in both PFTBT and PCDTBT and identify nematic-isotropic transition temperatures, TNI, between 260 and 300 °C via a combination of differential scanning calorimetry, polarized optical microscopy, temperature dependent wide-angle X-ray scattering, and rheology.

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