59th Annual Report on Research 2014

The goal of this proposal is to synthesize and evaluate conducting polymer (CP) derivatives that can be systematically varied in order to establish fundamental relationships between the chemical structure of CPs and associated dopant molecules and their ability to undergo electromechanical actuation. The proposed polymers are designed to specifically address current issues of (1) retention of the anionic dopant leading to device degradation, and (2) inefficient actuation due to low ion mobility through the bulk polymers used in standard CP-based actuator devices.

Project 1: The chemical stability of the cationic, conductive form of CPs is tied to the continued presence of stabilizing anionic dopants. Therefore, the aim of this project is to synthesize a new class of CPs that contain an interpenetrating network of polymeric sulfonic acid dopants. To synthesize such materials, a small molecule dopant that contains a polymerizable alkene (vinyl benzene sulfonic acid, VBS) was added to the CP polymerization mixture, and the resulting polymer properties were compared to samples synthesized in the presence of small molecule (p-toluene sulfonic acid, pTSA) and polymeric variants (poly(styrene sulfonate), PSS). For our initial study, properties of polypyrrole (PPy) samples synthesized in water via electropolymerization or chemical oxidation were evaluated. Electropolymerized films were soaked in water to allow residual monomer to leech out prior to removal from the electrode. Chemical polymerization produced insoluble powders that were rinsed thoroughly then vacuum dried prior to being pressed into pellets.

The SEM images of the films and pellets produced are shown in Figure 1. Consistent with literature reports, electropolymerized PSS-doped films exhibited a smoother surface, while the pTSA films were much rougher and had the largest nodules. Elemental analysis using EDX showed that the mean sulfur content was similar for all samples (6-7wt%). Chemically polymerized samples all contained aggregates of distinct, rounded particles. Mean particle size was similar for the PSS and pTSA samples (0.40 ± 0.11 vs. 0.43 ± 0.09 μm), while the VBS particles were smaller (0.32 ± 0.06 μm), and more jagged. Contrary to the data collected for electropolymerized films, the PSS-doped pellet exhibited much higher sulfur content than VBS or pTSA-doped pellets (11.2% vs. 5.2% and 5.1% respectively), likely due to the difficulty in rinsing away the excess due to the large size of the polymer. The film and pellet morphologies of the VBS-doped samples are intermediate between the PSS and pTSA samples, suggesting good incorporation of the VBS dopant into the PPy.

Figure 1. SEM images of electropolymerized films and pellets (scale is the same for all images).

However, the conductivity of the VBS samples is surprisingly low (Figure 1). We are currently looking into the cause, and reaction conditions are being further optimized to increase the conductivity. We are also evaluating other polymerizable dopants including vinyl and allyl sulfonic acid, and 2-acrylamido-2-methyl-1-propanesulfonic acid. These molecules were chosen to compare how small aliphatic vs. aromatic vs. bulky aliphatic sulfonic acid groups will effect the physical and mechanical properties of the final composite materials.

Project 2: The aim of this project is to establish a relationship between CP chemical structure and the ionic mobility, and in turn determine how these properties affect the ability of the polymer to undergo actuation. To carry out this study, our first goal is to synthesize a series of linear random copolymers containing hydroxymethylEDOT (EDOT-OH) and PEO segments of varying length, and further tailor the mechanical properties via crosslinking of reactive side chains.

Synthesis of a series of copolymers containing PEDOT-OH and PEO segments was initially proposed as shown in Scheme 1. We were able to synthesize bithiophene-terminated macromonomers linked by ethylene glycol segments containing 4, ~67 and ~220 repeat units using DIC-activated esterification reactions. However, copolymerization of this macromonomer with either EDOT or EDOT-OH proved troublesome using both chemical and electrochemical methods. Attempted co-polymerization reactions with the shorter PEO derivatives resulted in insoluble powders that had poor incorporation of the PEO-containing macromonomer, while the larger PEO derivatives produced only short oligomeric species with poor conductivity.

Scheme 1. Initial and revised copolymer synthesis route.

Since it has proved difficult to produce the desired copolymers using our initial synthesis method, we have redesigned our scheme to utilize direct arylation polymerization reactions that have recently appeared in the literature. Here the only requirement is that one monomer be dibrominated, but the reaction is otherwise tolerant of a variety of functionalized monomers, potentially allowing for facile incorporation of the PEO-containing macromonomers. Protection of the EDOT-OH monomers with triisopropylsilane (TIPS) has been found to increase reaction compatibility, and provide additional solubility, which was an issue in our initial scheme. Initial polymerization results have been very promising, so this route will be further pursued this year.

In addition to the synthesis of PEDOT-OH / PEO copolymers, we have been evaluating different methods to crosslink the reactive –OH side chains of PEDOT-OH in order to manipulate the mechanical properties of the polymer. We have successfully synthesized pyrrole and thiophene-based crosslinkers (Figure 2) that contain monomers connected by a hexaethylene glycol spacer. As shown in Table 1, addition of 10% of the pyrrole crosslinker to a pyrrole electropolymerization reaction dramatically increases the mechanical properties of the resulting films, and only slightly increases the resistivity. Current work is underway to repeat this characterization with films synthesized with the thiophene-based crosslinker, and evaluate the effects on actuation performance of both.

Figure 2. Pyrrole and EDOT-OH crosslinkers.

Table 1. Properties of polypyrrole with and without 10% crosslinker added

Film	% Strain at break	Ultimate tensile strength (MPa)	Young's Modulus (MPa)	Resistivity (Ohms/sq)
Plain polypyrrole	3.2	3.8	117	9.3 ± 1
10% crosslinker	9.2	10.5	265	17.1 ± 2

Student Impact

Since July 2013, this project has provided research opportunities for 4 undergraduates and one masters student. Six posters in total have been presented by these students on this work; three at WWU, and three at regional and national meetings, most notably the Materials Research Society meeting in San Francisco in April, 2014.