7 research outputs found
Engineering polymer-nanocellulose systems towards sustainable devices
This thesis explores nanocellulose-polymer systems for use in organic electronics applications. Each chapter provides an understanding of the interactions between polymers and cellulose nanocrystals (CNCs) that give rise to desirable morphologies and optoelectronic properties, which can be used in the design of advanced electronic materials. Chapter 2 presents a proof-of-concept study looking at the compatibility and templating ability of CNCs with a water-soluble semiconducting polymer. The liquid crystal phase behavior of the blended system and its effect on semiconducting polymer alignment are explored. Chapter 3 expands on this work; a temperature-switchable liquid crystal template is developed by grafting a thermoresponsive polymer from the ends of the CNCs. The effect of the polymer grafting morphology on the degree and reversibility of the thermal “switch” is studied. Finally, Chapter 4 revisits the topic of semiconducting polymer organization. CNCs are grafted with polymers of varying surface energies and are blended with a benchmark semiconducting polymer. The effect of the CNC surface chemistry on the self-assembly of the semiconducting polymer is investigated. Chapter 5 summarizes the major findings of these three chapters and provides suggestions for future work.Ph.D
Thermally switchable liquid crystals based on cellulose nanocrystals with patchy polymer grafts
A thermally “switchable” liquid-crystalline (LC) phase is observed in aqueous suspensions of cellulose nanocrystals (CNCs) featuring patchy grafts of the thermoresponsive polymer poly(N-isopropylacrylamide) (PNIPAM). “Patchy” polymer decoration of the CNCs is achieved by preferential attachment of an atom transfer radical polymerization (ATRP) initiator to the ends of the rods and subsequent surface-initiated ATRP. The patchy PNIPAM-grafted CNCs display a higher colloidal stability above the lower critical solution temperature (LCST) of PNIPAM than CNCs decorated with PNIPAM in a brush-like manner. A 10 wt% suspension of the “patchy” PNIPAM-modified CNCs displays birefringence at room temperature, indicating the presence of an LC phase. When heated above the LCST of PNIPAM, the birefringence disappears, indicating the transition to an isotropic phase. This switching is reversible and appears to be driven by the collapse of the PNIPAM chains above the LCST, causing a reduction of the rods' packing density and an increase in translational and rotational freedom. Suspensions of the “brush” PNIPAM-modified CNCs display a different behavior. Heating above the LCST causes phase separation, likely because the chain collapse renders the particles more hydrophobic. The thermal switching observed for the “patchy” PNIPAM-modified CNCs is unprecedented and possibly useful for sensing and smart packaging applications.Peer ReviewedPostprint (updated version
Thermally Switchable Liquid Crystals Based on Cellulose Nanocrystals with Patchy Polymer Grafts
A thermally “switchable” liquid-crystalline (LC) phase is observed in aqueous suspensions of cellulose nanocrystals (CNCs) featuring patchy grafts of the thermoresponsive polymer poly(N-isopropylacrylamide) (PNIPAM). “Patchy” polymer decoration of the CNCs is achieved by preferential attachment of an atom transfer radical polymerization (ATRP) initiator to the ends of the rods and subsequent surface-initiated ATRP. The patchy PNIPAM-grafted CNCs display a higher colloidal stability above the lower critical solution temperature (LCST) of PNIPAM than CNCs decorated with PNIPAM in a brush-like manner. A 10 wt% suspension of the “patchy” PNIPAM-modified CNCs displays birefringence at room temperature, indicating the presence of an LC phase. When heated above the LCST of PNIPAM, the birefringence disappears, indicating the transition to an isotropic phase. This switching is reversible and appears to be driven by the collapse of the PNIPAM chains above the LCST, causing a reduction of the rods' packing density and an increase in translational and rotational freedom. Suspensions of the “brush” PNIPAM-modified CNCs display a different behavior. Heating above the LCST causes phase separation, likely because the chain collapse renders the particles more hydrophobic. The thermal switching observed for the “patchy” PNIPAM-modified CNCs is unprecedented and possibly useful for sensing and smart packaging applications.Peer ReviewedPostprint (updated version
Enhanced Alignment of Water-Soluble Polythiophene Using Cellulose Nanocrystals as a Liquid Crystal Template
Cellulose
nanocrystals (CNCs) are bioderived, rodlike particles that form a
chiral nematic liquid crystal (LC) in water. In this work, CNCs were
used to induce long-range order in a semiconducting polymer, polyÂ[3-(potassium-4-butanoate)
thiophene-2,5-diyl] (PPBT). When mixed with CNCs, it was found that
PPBT was incorporated into the liquid crystal “template”
to form ordered structures with highly birefringent domains, as observed
under polarized light. We show that the π–π interactions
between polymer chains, which contribute considerably to the energetics
of the semiconducting system, are directly influenced by the presence
and packing of the liquid crystal phase. Upon increasing the concentration
of CNCs from the isotropic to chiral nematic regime, we observe a
bathochromic shift in the UV–vis spectra and the emergence
of the 0–0 vibrational peak, suggesting enhanced π–π
stacking leading to chain coplanarization. Furthermore, the chiral
nature of the PPBT/CNC mixture was evidenced by a negative peak in
circular dichroism (CD) spectroscopy, promoting the notion that the
polymer chains followed the helicoidal twist of the chiral nematic
liquid crystal host. At high temperatures, the peak height ratios
and overall intensities of the UV–vis and CD spectra associated
with PPBT decreased as the chiral nematic pitch grew larger in size
High-Throughput Image Analysis of Fibrillar Materials: A Case Study on Polymer Nanofiber Packing, Alignment, and Defects in Organic Field Effect Transistors
High-throughput
discovery of process–structure–property relationships
in materials through an informatics-enabled empirical approach is
an increasingly utilized technique in materials research due to the
rapidly expanding availability of data. Here, process–structure–property
relationships are extracted for the nucleation, growth, and deposition
of semiconducting polyÂ(3-hexylthiophene) (P3HT) nanofibers used in
organic field effect transistors, via high-throughput image analysis.
This study is performed using an automated image analysis pipeline
combining existing open-source software and new algorithms, enabling
the rapid evaluation of structural metrics for images of fibrillar
materials, including local orientational order, fiber length density,
and fiber length distributions. We observe that microfluidic processing
leads to fibers that pack with unusually high density, while sonication
yields fibers that pack sparsely with low alignment. This is attributed
to differences in their crystallization mechanisms. P3HT nanofiber
packing during thin film deposition exhibits behavior suggesting that
fibers are confined to packing in two-dimensional layers. We find
that fiber alignment, a feature correlated with charge carrier mobility,
is driven by increasing fiber length, and that shorter fibers tend
to segregate to the buried dielectric interface during deposition,
creating potentially performance-limiting defects in alignment. Another
barrier to perfect alignment is the curvature of P3HT fibers; we propose
a mechanistic simulation of fiber growth that reconciles both this
curvature and the log-normal distribution of fiber lengths inherent
to the fiber populations under consideration
Versatile Interpenetrating Polymer Network Approach to Robust Stretchable Electronic Devices
The
pursuit of intelligent optoelectronics could have profound
implications on our future daily life. Simultaneous enhancement of
the electrical performance, mechanical stretchability, and optical
transparency of semiconducting polymers may significantly broaden
the spectrum of realizable applications for these materials in future
intelligent optoelectronics, i.e., wearable devices, electronic skin,
stretchable displays, and a vast array of biomedical sensors. Here,
semiconducting films with significantly improved mechanical elasticity
and optical transparency, without affecting the film’s electronic
conductivity even under 100% strain, were prepared by blending only
a small amount (below 1 wt %) of either p-type or n-type commercial
semiconductor polymers. We demonstrate that a self-organized versatile
conjugated polymer film displaying an interpenetrating polymer network
is formed in the semiconducting films and is crucial for the observed
enhancement of elasticity, optical transparency, and charge-carrier
mobility. On the basis of this versatile semiconducting film, we explored
a new practical approach to directly integrate all the stretchable
components for a large area transistor array through solution processing
and a final single, mechanical peel-off step. We demonstrate robust
transistor arrays exhibiting charge carrier mobilities above 1.0 cm<sup>2</sup>/V s with excellent durability, even under 100% strain. We
believe our achievements will have great impact on stretchable optoelectronic
devices for practical applications and represent promising directions
for industry-scale production of stretchable displays and wearable
electronic devices