2 research outputs found
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><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
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