6 research outputs found
Measuring the Glass Transition Temperature of Conjugated Polymer Films with UltravioletâVisible Spectroscopy
The
glass transition temperature (<i>T</i><sub>g</sub>) 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><sub>g</sub> is often
too weak to detect using conventional differential scanning calorimetry
(DSC). Alternative methodsî¸e.g., variable temperature ellipsometryî¸require
specialized equipment. This paper describes a technique for measuring
the <i>T</i><sub>g</sub> of thin films of semicrystalline
conjugated polymers using only a hot plate and an ultravioletâvisible
(UVâvis) spectrometer. UVâvis spectroscopy is used to
measure changes in the absorption spectrum due to molecular-scale
rearrangement of polymers when heated past <i>T</i><sub>g</sub>, 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><sub>g</sub> 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><sub>g</sub> 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><sub>g</sub>, 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><sub>g</sub>
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 stressâstrain
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
stressâstrain 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 polyÂ(3-hexylthiophene)
(P3HT) samples of increasing molecular weight (<i>M</i><sub>n</sub> = 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><sub>n</sub> = 15 kDa), fracture
occurs by chain pullout. Conversely, in high molecular weight samples
(<i>M</i><sub>n</sub> > 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 HumanâMachine 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 diketoÂpyrrolopyrrole (DPP) unit flanked by
furan rings and the insulating blocks are polyÂ(Îľ-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
(ultravioletâvisible 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
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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 polyÂ(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