Session X47: Focus Session: Polymers for Energy Storage and Conversion - Nanostructures and Phase Separated Morphologies

Nanostructured Organometallic Polymers for Enzymatic Bioenergy

The development of efficient enzymatic biofuel cell is a subject of considerable studies in past decades for potential applications such as biomedical devices and microchip systems. One of the key challenges in advancing the technology lies in the power densities of the system. Limitations have been arisen from the buried redox active sites within enzyme structure and poor interplay between redox reactions. In present study, a glucose oxidase is employed as a model enzyme and ferrocene-containing organometallic block copolymers are chosen for the electron mediators. Wiring of glucose oxidase into electrode surface was successfully achieved by cross-linked networks of organometallic polymers and remarkably, catalytic current densities of the fabricated electrodes have proven be a sensitive function of the morphologies of electron mediators. Different nanoscale morphologies, i.e., bicontinous structure, nanowires, and nanoparticles, have been derived and the use of bicontinous morphology confirms 2-50 times improved catalytic current response than the values obtained from other morphologies. The bio-sensing ability of the fabricated electrode with structural optimization was also exploited and good sensitivity is obtained at the physiological concentration of glucose in blood.   

Comparison of polymer-fullerene heterojunction morphology to bimolecular recombination kinetics

One of the most important physical processes limiting the practical power conversion efficiency of bulk heterojunction (BHJ) organic photovoltaic devices is the bimolecular recombination of holes and electrons. Reduced recombination would permit the use of thicker BHJ layers, enabling greater light absorption without a penalty in device current. A few polymer light absorbers, when combined in a BHJ with a fullerene electron acceptor, exhibit recombination that is slower than Langevin-type, but the origins of this behaviour are not understood. This talk will describe our effort to determine whether slower-than-Langevin recombination can be attributed to features of the nanoscale morphology or crystalline microstructure within the BHJ film. In comparing BHJ films made from two silole-based monomers, one with Langevin recombination and one with slower-than-Langevin, we find many aspects of the BHJ material structure such as order, orientation, and nanoscale domain size and shape, to be surprisingly similar. We will compare the two materials and emphasize opportunities in data analysis and new measurements to determine whether a morphological basis underlies different recombination kinetics.   

Confinement effects on P3HT-PCBM Morphology for Bulk Hetero-Junction Polymer Solar Cells

Controlling morphology of the bulk hetero-junction in solar cell development is the key aspect for higher efficiency. We controlled the morphologies of phenyl-C61-butyric acid methyl ester (PCBM) and poly(3-hexylthiophene) (P3HT) blend thin films by tunable surface energy polydimethylsiloxane (PDMS) elastomer confinement. The advantages of replacing air-polymer interface with PDMS-polymer interface are its flexibility, easy detachability, facile tunability of surface energy by UVO exposure. We hypothesize that the confined annealing will suppress PCBM crystallinity, control crystallinity of P3HT, direct the vertical segregation of PCHT-PCBM blend, and thus influence the composition distribution of P3HT and PCBM at the interfaces. The annealed films were characterized by dark field optical microscopy, GISAXS, GIWAXS, AFM, and SANS. PCBM crystallization was indeed suppressed in the films during confinement annealing. On the other hand the phase separated interpenetrating network and favorable crystallinity of the P3HT evolved. The optimum surface energy of the confining PDMS yields the best structural features of the film. The morphology developed under different PDMS surfaces is an important step towards improvement of efficiency of OPV's.   

The Role of P3HT Crystallization in the Morphological Development in P3HT/PCBM Thin Films Using Neutron Scattering

Small angle neutron scattering (SANS) provides an important method to characterize the morphology of PCBM/P3HT organic photovoltaics (OPVs), which is essential to improving the efficiency of plastic solar cell devices. Our recent SANS results indicate that fullerene derivatives and conjugated polymers employed in OPVs are significantly miscible, up to $\sim $20{\%}. In this work, the morphology of PCBM/P3HT composite \textit{thin films} is investigated via SANS and analyzed to document their morphology and miscibility. These results indicate that both 20 and 50 vol{\%} PCBM as-cast films exhibit relatively small low-Q scattering, suggesting an overall homogeneous mixture. After annealing at 150\r{ }C for 30 minutes, P3HT undergoes further crystallization in both mixtures. However, the low-Q scattering of the 20 vol{\%} PCBM sample remains low, indicating the film remains homogenous. On the other hand, 50{\%} PCBM sample undergoes phase separation between amorphous PCBM and P3HT. These results therefore exemplify the importance of P3HT crystallization in the structure development of OPV active layers, showing that the crystallization of P3HT is not sufficient to induce phase separation.   

Understanding the Behavior of Poly(3-hexylthiophene) at Liquid/Vacuum Interfaces

Among semiconducting polymers used in opto-electronic devices, poly(3-hexylthiophene) (P3HT) is one of the better candidates because of its good electrical properties and ease of processing. The performance of these devices strongly depends on the structural, morphological, dynamic and interfacial properties of P3HT. Using molecular dynamics simulation and utilizing two different models - all-atom and united-atom - we have studied the dynamic and structural properties of free-standing P3HT thin films at liquid/vacuum interfaces. To quantify these properties, the temperature and chain-length dependence of surface roughness, interfacial width, surface tension, torsional defects, and several other surface properties have been investigated. The results we obtained from all-atom and-united atom models are in reasonable agreement.   

Understanding Schroeder's Paradox

Schroeder's paradox is a well known, but not fully understood, phenomenon that exists in many polymers and gels. Essentially, the uptake of solvent in the polymer depends on the interaction with the boundary phase. Nafion, a polymer of interest for many electrochemical energy applications, is a classic example where the water uptake almost doubles by placement in liquid water versus saturated water vapor. In this talk, we examine the origin of this paradox through examination of Nafion morphology and water-uptake time constants using experiments in various solvents, vacuum, and small-angle X-ray scattering techniques. The results show that the interface controls the water uptake (even in bulk membranes) and that the interfacial morphology depends on the interactions of the different polymer moieties with the external environment including its density and dielectric constant. In addition, interactions with solid phases will be discussed which show similar impact on water uptake depending on whether they are hydrophilic or hydrophobic. Understanding the morphological changes and their associated impact on membrane properties is critical for optimizing polymers for use in energy applications.   

Ions in block copolymers: Effects on thermodynamics, structural changes and electric field induced alignment.

Block copolymers with added ions which selectively dissolve in one block are of interest as nanostructured polymeric ion conductors. In the microphase separated state such a system offers the possibility to simultaneously optimize different properties which would normally exclude each other. One block, being in the solid state, can give mechanical strength while the other block, typically in the liquid state, could be designed to achieve good ion transport. Oriented structures are especially interesting. We present two sets of experiments dealing with fundamental ion induced effects in block copolymers. It is generally observed that the addition of salt to a block copolymer leads to a strong increase of the order-disorder transition temperature and an increased domain spacing, i.e. conformational changes of the polymers. Based on a detailed analysis of small angle scattering data of two different copolymers (PS-b-PEO, PS-b-PVP) close to the order-disorder transition three contributions to the structural changes can be distinguished: an increased incompatibility between the different monomers, the additional volume of the added salt, and chain stretching due to coordination between polymer and salt. At the phase transition, i.e. at constant incompatibility $\chi$N, for low concentrations the increase in domain size is quantitatively explained by the volume of the added salt, at higher concentrations in addition chain stretching sets in. Structural and thermodynamic effects are considerably stronger in PEO than in P2VP. In a second experiment we study the effects of electric field induced interfacial polarization caused by the added salt. Impedance spectroscopy combined with orientation experiments at high fields enable a quantitative analysis of ionic polarization and a direct demonstration of its aligning effect on the interfaces. Field induced orientation effects are much stronger if the ionic charges come into play in comparison to much weaker dielectric effects. We present a physical model accounting for the differences.   

Does filler surface chemistry impact filler dispersion, polymer dynamics and conductivity in nanofilled solid polymer electrolytes?

We study the impact of nanofiller surface chemistry on filler dispersion, polymer dynamics and ionic conductivity in acidic $\alpha $-Al$_{2}$O$_{3}$ filled PEO+LiClO$_{4}$ solid polymer electrolytes (SPEs).SPEs are the key to light-weight and high energy density rechargeable Li ion batteries but suffer from low room temperature ionic conductivity. Addition of ceramic nanofillers improves conductivity of SPEs and their surface chemistry influences extent of conductivity enhancement. The ionic conductivity of acidic $\alpha $-Al$_{2}$O$_{3}$ filled SPE is enhanced for salt concentrations at and below eutectic, while neutral $\gamma $-Al$_{2}$O$_{3}$ filler enhances conductivity only at eutectic composition. Li ion motion is coupled to segmental mobility of polymer and we study how this is affected by addition of $\alpha $-Al$_{2}$O$_{3}$ using quasi-elastic neutron scattering. Aggregation extent of nanoparticles in SPE matrix, a less explored factor in filled SPEs, can affect segmental mobility of polymer. This can vary with surface chemistry of particles and we quantify this using small angle neutron scattering. All measurements are performed as a function of Li concentration, nanoparticle loading and temperature.   

Regular Arrays of Germanium Nanoparticles Assisted by Thermoset Polymer Composites for High Capacity Lithium Ion Battery

In recent years Li-batteries have attracted significant interests for a variety of applications such as portable electronics and electric vehicle (EV) batteries due to their high energy densities. Key challenges in advancing the technology lie in specific energy density, the long term cycle properties, and durability at elevated temperature. In present study, we were motivated to prepare high capacity Li-battery by creating regular arrays of germanium nanoparticles (GeNPs, 1600 mAh/g) to replace commercial graphite anode (370 mAh/g). Thermoset polymers were employed to prepare GeNPs/polymer composites with tunable NP loadings and spacings, followed by carbonization process to prepare GeNPs/carbon composite anode material. Due to the large volume change of GeNPs with charge/discharge cycles, the regular arrays of GeNPs are turned out to be a crucial parameter in obtaining enhanced cyclability. The GeNPs/carbon anode materials were cycle tested in a half cell configuration using Lithium foil as a counter electrode and lithium salt doped PS-PEO block copolymers as electrolytes. High capacity and rate capability were achieved, which demonstrate the role of nano-sized and regularly-arrayed anode active materials in obtaining the improved battery performance.   

Disorder -- Order Transitions in Humidified Block Copolymer Electrolytes Studied by \textit{In Situ }SAXS

The relationship between water uptake, proton conductivity and morphology in the dry and hydrated state for a series of poly(sulfonated styrene-\textit{block}-ethylene) was investigated. Specifically, the disorder-to-order transition (DOT) and hydrated morphology was characterized by \textit{in situ} humidity controlled small angle X-ray scattering (SAXS). The enhanced resolution afforded by SAXS allows for better characterization of the DOT than previous studies which have relied upon neutron scattering. The transition to an ordered state is found to display a coexistence of ordered and disordered states over a broad range of relative humidity values.   

Effects of lithium salts on the lamellar phase of diblock copolymers

We study the effects of lithium salts on the lamellar phase of AB diblock copolymers by means of the self-consistent field theory. We consider a model in which the A and B blocks have different dielectric constants and account for the tight binding of Li$^+$ to one of the blocks, the preferential solvation energy of anions in the higher-dielectric polymer domain, the translational entropy of anions, and change in the $\chi$ parameter due to the binding of Li$^+$. We study the effect of the strong polymer-Li$^+$ binding on the distribution of the salt ions. In particular, we show that local charge separation near the interface of the higher- and lower- dielectric polymers largely arises from the effect of the Born energy. We also examine the relationship between two definitions of the effective $\chi$ parameter, one based on the lamellar spacing and one based on the structure factor in the disordered phase, and show that these two definitions generally do not coincide.   

Structure of Block Copolymer Hydrogel Formed by Complex Coacervate Process

Complex coacervation occurs when oppositely charged polyelectrolytes associate in solution, forming dense micron-sized droplets. Hydrogels with coacervate block domains were formed by mixing two ABA and A'BA' triblock copolymer solutions in water where the A and A' blocks are oppositely charged. Small-angle neutron scattering (SANS) was used to investigate the structure of hydrogels formed by ABA triblock copolymers (A block: poly(allyl glycidyl ether) functionalized with guanidinium (A) or sulfonate (A'), B block: poly(ethylene oxide)). By using an appropriate fitting model, structural information such as coacervate core block radius and water volume fraction w can be extracted from SANS data. The results reveal that w in the coacervate core block was significantly higher than in conventional triblock copolymer hydrogels where microphase separation is driven by the hydrophobicity of the core-forming blocks.