Measuring the Glass Transition Temperature of Conjugated Polymer Films with Ultraviolet–Visible Spectroscopy

The glass transition temperature (<i>T</i>_g) of a conjugated polymer can be used to predict its morphological stability and mechanical properties. Despite the importance of this parameter in applications from organic solar cells to wearable electronics, it is not easy to measure. The <i>T</i>_g is often too weak to detect using conventional differential scanning calorimetry (DSC). Alternative methodse.g., variable temperature ellipsometryrequire specialized equipment. This paper describes a technique for measuring the <i>T</i>_g of thin films of semicrystalline conjugated polymers using only a hot plate and an ultraviolet–visible (UV–vis) spectrometer. UV–vis spectroscopy is used to measure changes in the absorption spectrum due to molecular-scale rearrangement of polymers when heated past <i>T</i>_g, corresponding to the onset of the formation of photophysical aggregates. A deviation metric, defined as the sum of the squared deviation in absorbance between as-cast and annealed films, is used to quantify shifts in the absorption spectra. The glass transition is observed as a change in slope in a plot of the deviation metric versus temperature. To demonstrate the usefulness of this technique, a variety of semiconducting polymers are tested: P3BT, PBTTT-C14, F8BT, PDTSTPD, PTB7, PCDTBT, TQ1, and MEH-PPV. These polymers represent a range of solid-state morphologies, from highly ordered to predominantly amorphous. A successful measurement of <i>T</i>_g depends on the ability of the polymer to form photophysical aggregates. The results obtained using this method for P3BT, PBTTT-C14, F8BT, and PDTSTPD are in agreement with values of <i>T</i>_g that have been reported in the literature. Molecular dynamics simulations are used to show how the morphology evolves upon annealing: above the <i>T</i>_g, an initially kinetically trapped morphology undergoes structural rearrangement to assume a more thermodynamically preferred structure. The temperature at which onset of this rearrangement occurs in the simulation is concomitant with the spectroscopically determined value of <i>T</i>_g

Stretchable and Degradable Semiconducting Block Copolymers

This paper describes the synthesis and characterization of a class of highly stretchable and degradable semiconducting polymers. These materials are block copolymers (BCPs) in which the semiconducting blocks are based on the diketo­pyrrolopyrrole (DPP) unit flanked by furan rings and the insulating blocks are poly­(ε-caprolactone) (PCL). The combination of stiff conjugated segments with flexible aliphatic polyesters produces materials that can be stretched >100%. Remarkably, BCPs containing up to 90 wt % of insulating PCL have the same field-effect mobility as the pure semiconductor. Spectroscopic (ultraviolet–visible absorption) and morphological (atomic force microscopic) evidence suggests that the semiconducting blocks form aggregated and percolated structures with increasing content of the insulating PCL. Both PDPP and PCL segments in the BCPs degrade under simulated physiological conditions. Such materials could find use in wearable, implantable, and disposable electronic devices

Measurement of Cohesion and Adhesion of Semiconducting Polymers by Scratch Testing: Effect of Side-Chain Length and Degree of Polymerization

Most advantages of organic electronic materials are enabled by mechanical deformability, as flexible (and stretchable) devices made from these materials must be able to withstand roll-to-roll printing and survive mechanical insults from the external environment. Cohesion and adhesion are two properties that dictate the mechanical reliability of a flexible organic electronic device. In this paper, progressive-load scratch tests are used for the first time to correlate the cohesive and adhesive behavior of poly­(3-alkylthiophenes) (P3ATs) with respect to two molecular parameters: length of the alkyl side chain and molecular weight. In contrast to metrological techniques based on buckling or pull testing of pseudofreestanding films, scratch tests reveal information about both the cohesive and adhesive properties of thin polymeric films from a single procedure. Our data show a decrease in cohesion and adhesion, that is, a decrease in overall mechanical robustness, with increasing length of the side chain. This behavior is likely due to increases in free volume and concomitant decreases in the glass transition temperature. In contrast, we observe increases in both the cohesion and adhesion with increasing molecular weight. This behavior is attributed to an increased density of entanglements with high molecular weight, which manifests as increased extensibility. These observations are consistent with the results of molecular dynamics simulations. Interestingly, the normal (applied) forces associated with cohesive and adhesive failure are directly proportional to the average degree of polymerization, as opposed to simply the molecular weight, as the length of the alkyl side chain increases the molecular weight without increasing the degree of polymerization