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

Comparison of Methods for Determining the Mechanical Properties of Semiconducting Polymer Films for Stretchable Electronics

This paper describes a comparison of two characterization techniques for determining the mechanical properties of thin-film organic semiconductors for applications in soft electronics. In the first method, the film is supported by water (film-on-water, FOW), and a stress–strain curve is obtained using a direct tensile test. In the second method, the film is supported by an elastomer (film-on-elastomer, FOE), and is subjected to three tests to reconstruct the key features of the stress–strain curve: the buckling test (tensile modulus), the onset of buckling (yield point), and the crack-onset strain (strain at fracture). The specimens used for the comparison are four poly­(3-hexylthiophene) (P3HT) samples of increasing molecular weight (<i>M</i>_n = 15, 40, 63, and 80 kDa). The methods produced qualitatively similar results for mechanical properties including the tensile modulus, the yield point, and the strain at fracture. The agreement was not quantitative because of differences in mode of loading (tension vs compression), strain rate, and processing between the two methods. Experimental results are corroborated by coarse-grained molecular dynamics simulations, which lead to the conclusion that in low molecular weight samples (<i>M</i>_n = 15 kDa), fracture occurs by chain pullout. Conversely, in high molecular weight samples (<i>M</i>_n > 25 kDa), entanglements concentrate the stress to few chains; this concentration is consistent with chain scission as the dominant mode of fracture. Our results provide a basis for comparing mechanical properties that have been measured by these two techniques, and provide mechanistic insight into fracture modes in this class of materials

Ionotactile Stimulation: Nonvolatile Ionic Gels for Human–Machine Interfaces

We report the application of a nonvolatile ionic gel as a soft, conductive interface for electrotactile stimulation. Materials characterization reveals that, compared to a conventional ionic hydrogel, a glycerol-containing ionic gel does not dry out in air, has better adhesion to skin, and exhibits a similar impedance spectrum in the range of physiological frequencies. Moreover, psychophysical experiments reveal that the nonvolatile gel also exhibits a wider window of comfortable electrotactile stimulation. Finally, a simple pixelated device is fabricated to demonstrate spatial resolution of the haptic signal

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

Metallic Nanoislands on Graphene for Monitoring Swallowing Activity in Head and Neck Cancer Patients

There is a need to monitor patients with cancer of the head and neck postradiation therapy, as diminished swallowing activity can result in disuse atrophy and fibrosis of the swallowing muscles. This paper describes a flexible strain sensor comprising palladium nanoislands on single-layer graphene. These piezoresistive sensors were tested on 14 disease-free head and neck cancer patients with various levels of swallowing function: from nondysphagic to severely dysphagic. The patch-like devices detected differences in (1) the consistencies of food boluses when swallowed and (2) dysphagic and nondysphagic swallows. When surface electromyography (sEMG) is obtained simultaneously with strain data, it is also possible to differentiate swallowing <i>vs</i> nonswallowing events. The plots of resistance <i>vs</i> time are correlated to specific events recorded by video X-ray fluoroscopy. Finally, we developed a machine-learning algorithm to automate the identification of bolus type being swallowed by a healthy subject (86.4%. accuracy). The algorithm was also able to discriminate between swallows of the same bolus from either the healthy subject or a dysphagic patient (94.7% accuracy). Taken together, these results may lead to noninvasive and home-based systems for monitoring of swallowing function and improved quality of life

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