The understanding of the structure-mechanical property relationship for semiconducting polymers is essential for the application of flexible organic electronics. Herein pseudo free-standing tensile testing, a technique that measures the mechanical property of thin films floating on the surface of water, is used to obtain the stress-strain behaviors of two semiconducting polymers, poly(3-hexylthiophene) (P3HT) and poly(2,5-bis(2-decyltetradecyl)-3,6-di(thiophen-2-yl)diketopyrrolo[3,4-c]pyrrole-1,4-dione-alt-thienovinylthiophene (DPP-TVT) donor-acceptor (D-A) polymer. To our surprise, DPP-TVT shows similar viscoelastic behavior to P3HT, despite DPP-TVT possessing a larger conjugated backbone and much higher charge carrier mobility. The viscoelastic behavior of these polymers is due to sub room temperature glass transition temperatures (T ), as shown by AC chip calorimetry. These results provide a comprehensive understanding of the viscoelastic properties of conjugated D-A polymers by thickness-dependent, strain rate dependent, hysteresis tests, and stress-relaxation tests, highlighting the importance of T for designing intrinsically stretchable conjugated polymers.
There remains a lack
of fundamental understanding in the role of
backbone rigidity on the thermomechanical properties of conjugated
polymers. Here, we provide the first holistic approach to understand
the fundamental influence of backbone rigidity on an n-type naphthalene
diimide-based conjugated polymer, denoted by PNDI-Cx, through insertion
of a flexible conjugation break spacer (CBS). CBS lengths are varied
from fully conjugated with zero alkyl spacer (PNDI-C0) to a seven-carbon
alkyl spacer (PNDI-C7), with the CBS engineered into each repeat unit
for systematic evaluation. Solution small-angle neutron scattering
and oscillatory shear rheometry were employed to provide the first
quantitative evidence of CBS influence over conjugated polymer chain
rigidity and entanglement molecular weight (M
e), demonstrating a reduction in the Kuhn length from 521 to
36 Å for fully conjugated PNDI-C0 and PNDI-C6, respectively,
as well as a nearly consistent M
e of ∼15
kDa upon the addition of CBS. Thermomechanical properties, such as
elastic modulus and glass-transition temperature, were shown to decrease
with an increasing length of CBS. An extraordinary ductility, upwards
of 400% tensile strain before fracture, was observed for high-molecular-weight
PNDI-C4, which we attribute to a high number of entanglements and
disruption of crystallization. Furthermore, the deformation mechanism
for PNDI-Cx was studied under strain through X-ray diffraction, polarized
UV–vis spectroscopy, and atomic force microscopy. Overall,
this work sheds light on the important role of backbone rigidity in
designing flexible and stretchable conjugated polymers.
A new
method, “nonvolatile solvent vapor annealing”
(NVASA), has been developed to anneal block copolymers during film
deposition by controlling the solvent drying process. Precise amounts
of high boiling point additive added to the polymer solution briefly
remain in the polymer film after casting, leaving the film in a swollen
state, increasing its chain mobility, and ultimately improving domain
order. We demonstrated the effectiveness of NVASA on several block
copolymer systems and used in situ grazing incidence
small-angle X-ray scattering (GISAXS) to validate the ordering process
during the self-assembly. The simplicity and reproducibility of the
method is attractive for implementation in large-scale manufacturing
processes such as roll-to-roll printing as swell ratio is easily controlled
by the amount of additive used and separate annealing steps are not
needed. This work potentially introduces a new way to quickly and
cost effectively anneal block copolymers.
In
this work, the effect of long-chain branching (LCB) on the tensile
properties of sulfur-cured, unfilled, polypentenamer rubber (PPR)
was investigated. Branched PPR, prepared by ring-opening metathesis
copolymerization of cyclopentene (CP) and dicyclopentadiene (DCPD),
showed improved mechanical strength, demonstrating more than 3 times
higher tensile stress at 500% strain compared to its linear counterpart
(a homopolymer of CP). In situ wide-angle X-ray scattering
showed that branching units caused significant changes in the strain-induced
crystallization (SIC). At low temperatures, linear PPR underwent rapid
SIC after a critical stretch was reached, while branched PPR crystallized
more slowly. However, SIC is not the cause of the enhanced mechanical
strength. Elevated temperature experiments confirmed that even in
the absence of SIC, LCB PPR exhibits a stiffer stress–strain
response. We propose that the stiffer behavior of branched PPR is
caused by a reduction in the chain mobility. The origins of reduced
chain mobility are likely from topological constraints imposed by
the LCB architecture and also from an unintended nanofiller effect
created by microphase separation of DCPD-rich domains. The work described
here is the initial investigation of adding branching units to PPR
to improve the elastomer performance.
A new pore formation process was investigated for the manufacture of composite ultrafiltration membranes. Phase-separated block copolymer (BCP) thin films supported on a compliant macroporous poly(ether sulfone) (PES) support craze under tensile strain, leaving behind pores of predictable size based on the self-assembled nanoscopic domains. The high aspect ratio pores formed in this process were used to create membranes that were highly permeable (959 L/ (m 2 h bar) with near complete rejection of 40 nm diameter gold nanoparticles (AuNP). By use of BCP's inherent ability to cavitate under strain, tedious block removal steps are avoided. Membranes can thus be prepared in a simple, roll-toroll ready, one-step process. In this initial study, BCP craze formation and filtration performance were characterized for various polymer types, molecular weights, and thicknesses. All these factors influenced the BCP's thin film morphology, mechanical performance, deformation mechanism, and ultimately filtration performance. This work demonstrates a possible new path toward achieving scalable, BCP-based ultrafiltration membranes.
Dicyclopentadiene monomer was incorporated into statistical copolymerizations with cyclopentene to determine its influence on the resulting copolymers. These dicyclopentadiene nanodomains acted as crosslinks providing strength to the uncured network.
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