Session V18: Focus Session: Properties of Block Copolymers

Thermodynamics, Structure and Transport in Model Fuel Cell Membranes

Polymer electrolyte membranes (PEM), used to conduct protons from the anode to the cathode of hydrogen fuel cells, are open systems that exchange water with the surrounding air. Proton conductivity is closely coupled to the presence of contiguous hydrated channels within the membrane. In an attempt to understand the underpinnings of the morphology of these systems, the phase behavior of model PEMs comprising block copolymers in equilibrium with humidified air was studied as a function of the relative humidity of the surrounding air, ion content of the copolymer, and temperature. At low humidity, the copolymers exhibit an order-to-disorder transition as a function of increasing temperature. At high humidity, however, increasing temperature results in a disorder-to-order transition. \textit{In-situ} small angle neutron scattering experiments on the open block copolymer system, when combined with water uptake measurement indicate that the disorder-to-order transition is driven by an increase in the partial molar entropy of the water molecules in the ordered phase relative to that in the disordered phase. This is in contrast to most systems wherein increasing entropy results in stabilization of the disordered phase. The coupling between entropy and proton conductivity will be discussed.   

Water Uptake and Proton Conductivity of Asymmetric Poly(styrenesulfonate-block-methylbutylene) Copolymers

The effect of chain architecture on water uptake and proton conductivity of poly(styrenesulfonate-block-methylbutylene) (PSS-PMB) copolymers at equilibrium with moist air is studied as a function of temperature, and relative humidity of the air. Symmetric and asymmetric PSS-PMB copolymers were synthesized by anionic polymerization of poly(styrene-block- isoprene) copolymers, followed by hydrogenation of the polyisoprene block and sulfonation of the polystyrene block. Previous studies have shown that symmetric PSS-PMB block copolymers in the presence of humid air (relative humidity$>$50\%) are excellent proton conductors at temperatures as high 90 C. Current work is focused on water uptake and conductivity measurements on asymmetric PSS-PMB block copolymers with PSS as the minor component.   

Temperature dependent charge transport properties of poly(3-hexylthiophene) block poly(styrene) copolymer field-effect transistor

Regioregular poly-3-hexythiophene (rr-P3HT) is considered to be one of the most promising and well studied organic semiconductor. Recently attention has been focused in developing di-block copolymers of rr-P3HT by attaching non conjugated blocks which allows one to tune the electrical properties of rr-P3HT. To properly utilize these new types of materials it is necessary to understand the relationship between their molecular structure and electronic transport properties. In this talk we present electronic transport characteristics of poly(3-hexylthiophene) block poly(styrene) copolymer (rr-P3HT-b-PS) field effect transistor at various temperatures. We show that the current voltage characteristic at different temperature follow SCLC type conduction mechanism with $I\propto V^n$, where n varies from 1.5 to 2.2. We also show that the room temperature hole mobility is $\sim $3.5$\times $10$^{-5}$ cm$^{2}$/Vs, and that the mobility decreases with decreasing temperature. The temperature dependent mobility follow activated hoping process. The space charge limited current along with the low mobility of the devices indicates that the charge transport is limited by the insulating polystyrene segment of the di-block polymer.   

Morphology and Dynamic Mechanical Properties of Styrene Containing Tri-Block Copolymers for Electromagnetic Wave Interaction Applications

Styrene containing triblock copolymers, namely poly(styrene-ethylene/butylene-styrene) (SEBS) and poly(styrene-butadiene-styrene)] (SBS), were selectively modified by attaching polar groups to facilitate the in-growth of an inorganic component. In case of SEBS, the styrene block was sulfonated, and in SBS, the butadiene block was hydroxylated. The extent of modification was determined by analytical and spectroscopic methods. This presentation shows the morphology and dynamical mechanical properties of both block copolymers before and after modification. Nanocomposites of these block copolymers were prepared by inclusion of magnetic metal oxides \textit{via} an \textit{in-situ} precipitation and self assembly processes and their morphology and dynamical mechanical properties were studied. Magnetic properties of these polymers filled with iron oxide nanoparticles were measured using an alternating gradient magnetometer (AGM) at room temperature to observe the magnetic hysteresis.   

The Lyotropic Phase Behavior of Diblock Copolymers Swollen with Ionic Liquids

The lyotropic phase behavior of poly(1,2-butadiene-$b$-ethylene oxide) diblock copolymers (PB-PEO) has been studied upon addition of two ionic liquids, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([BMI][PF$_{6}$]). The copolymer/ionic liquid samples ranged from dilute to concentrated, and were characterized via small angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (cryo-TEM). At moderate to high concentrations, SAXS patterns corresponding to the classical copolymer microstructures of body-centered cubic lattices of spheres, hexagonally packed cylinders, and lamellae were observed. Additionally, at several concentrations, coexisting microstructures and what is thought to be a disordered network microstructure were observed. At low concentration, the morphology of the block copolymer micelles ($i.e. $spheres, cylinders, and vesicles) was used as a qualitative gauge of the ionic liquid solvent quality, and it was concluded that for PB-PEO, [BMI][PF$_{6}$] behaves as a more selective solvent than [EMI][TFSI].   

Structure and Thermodynamics of Block Copolymers Doped with Ionic Liquids

Incorporation of ionic liquids into block copolymers is of interest for applications such as high temperature fuel cell membranes and polymer processing. These applications take advantage of ionic liquids' attractive physiochemical properties, such as low vapor pressure and high thermal stability. We investigate the structure and thermodynamics of poly(styrene-$b$-2-vinylpyridine) (PS-PVP) block copolymers doped with an ionic liquid consisting of imidazole and bis(trifluoromethanesulfonyl)amide (HFTSI). Using small angle X-ray scattering (SAXS), we demonstrate that increased ionic liquid doping leads to swelling of lamellar nanodomains in a symmetric PS-PVP block copolymer. At high ionic liquid loadings, we observe break up of the lamellar domains into hexagonally perforated lamellae. We characterize the effect of ionic liquid loading on the order-disorder transition (ODT) temperature of PS-PVP. We observe depression of the PS-PVP ODT temperature with increasing loading of the ionic liquid.   

Controlling the morphology of liquid crystalline block copolymers: interfacial and liquid crystal content effects.

By systematically controlling the covalent attachment of side chain liquid crystals to a block copolymer backbone, the morphology of both the liquid crystalline (LC) mesophase and the phase segregated BCP microstructures can be precisely manipulated. A wide range of morphologies can be achieved from a single block copolymer backbone during a one step LC attachment reaction. The anchoring of the smectic LC mesophase to the inter-material dividing surface (IMDS) is a key driving force in determining the morphologies for both the bulk and in thin films. In thin films the orientation of the morphology is determined by the minimization of the surface energy with the substrate and air interfaces and the anchoring of the LC mesophase to the substrate. These competing effects can be utilized to manipulate the orientation of as-cast and thermally annealed thin films. Additionally, by controlling the LC content, the mechanical properties of this system can be tailored over a several orders of magnitude. The tune-ability demonstrated in this system will allow for custom design and tailoring of material properties for specific applications such as electromechanical and mechano-optical devices.   

Self-assembly of side chain liquid crystalline block copolymers

We present a new model based on self-consistent field theory (SCFT) approach to characterize the self assembly behavior in side-chain liquid crystalline block copolymers. Our model considers a micromechanical representation of flexible coil-coil diblock copolymers, with rod-like units grafted to one of the blocks. We present results which elucidate self-assembly arising from the interplay between block copolymer microphase separation and the orientational ordering of the rod segments. Our numerical results are in very good agreement with reported experimental observations. Many of the traditional flexible diblock copolymer microphases are also predicted to occur for side chain liquid crystalline polymers, with smectic ordering accompanying within the microphases. The equilibrium phase morphologies are observed to depend on the molecular weight of the copolymer, the length of the rod units, the relative volume fractions of each block, and the energetic and orientational interactions between different components. Moreover, for the parameters considered in this article, microphase separation was observed to be a requisite for developing orientational ordering between mesogenic units   

Structural Formation Process of Microphase Separated Films with Liquid Crystalline Phase Transition

Ordered nanostructures arising from the microphase separation of block copolymers have driven one to fabricate nanofunctional materials as fundamental technology of the coming electronic and photonic materials. Thin films of a series of newly designed amphiphilic block copolymer consisting of hydrophilic polyethylene oxide (PEO) and hydrophobic polymethacrylate with azobenzene-mesogen in side-chain (PMA(Az)) show highly ordered microphase separation with PEO cylinders perpendicularly oriented to the film surface. In the present report, we investigated a structural formation process of the microphase separated films by temperature controlled atomic force microscopy (AFM) and grazing incidence small angle X-ray scattering (GISAXS). These measurements revealed that homeotropic alignments of Az liquid crystalline layers predominated the cylinder orientation, which corresponded to a $<$110$>$ direction of body centered cubic structure under annealing condition, in disagreement with cylinder orientation of order-order transition of traditional block copolymers.   

Rod-to-Coil Transition in Polypeptide/$\pi$-Conjugated Polymer/Polypeptide Triblock Copolymers

Self-assembly in the bulk of a series of hybrid triblock copolymers formed by a poly(9,9-dihexylfluorene-2,7-diyl) (PHF) internal block and two poly($\gamma $-benzyl-L-glutamate) (PBLG) external blocks has been studied. Since the $\alpha $-helical secondary structure of the PBLG block may be either maintained or suppressed depending on the solvent casting history, the PBLG-PHF-PBLG copolymers exhibit two different conformations: a rod-rod-rod or coil-rod-coil configuration, respectively. To provide insight on the influence of molecular architecture on self-aggregation of these systems, three copolymers with different block ratio were investigated in both conformations using small- and wide-angle scattering techniques and transmission electron microscopy. Time-resolve photoluminescence measurements were performed on the same samples to explore the effect of morphology on photophysical properties. The observed photoluminescence spectra and dominant excited lifetimes of the poly(9,9-dihexylfluorene-2,7-diyl) block were found to differ markedly in rod-rod-rod and coil-rod-coil configurations and were correlated to the morphology of the self-assembled triblock copolymers.   

The effect of chain stiffness on the morphology of diblock copolymers

One of the most interesting and challenging problems in the science of materials concerns the structure and dynamics of the morphology of block copolymers. These materials generally consist of two or more homopolymer chains or blocks that are covalently bonded to each other to form a single polymer chain. Variation in the stiffness of the different block segments can directly affect the morphology of the system and may thus result in a very rich phase behavior. In the present study, the microphase separation of symmetric diblock copolymers with variable block stiffness and chain length is studied using coarse-grained molecular dynamics simulations. In the lamellar phase, the equilibrium lamellar spacing and orientation of the block segments in the system are found to depend on the relative stiffness between the two block segments. As the chain length of the block segments increases, the morphology changes from the expected lamellar appearance to a cylindrical one.   

Self-assembled OLEDs from rod-coil block copolymers

High efficient OLEDs tend to be made of many stacked layers including layers for hole transport, emission , and electron transport, which are produced via a very tedious sequence of high vacuum steps. Since conjugated rod-coil block copolymers form layer structures due to rod-coil repulsions and rod-rod interactions, they are an alternate route towards multi-layer devices which can be solution processed in one single step. A functional conjugated rod-coil block copolymer, poly(alkoxyphenylene vinylene-b-oxadiazole (PPV-b-OX), incorporates a hole transporting/emissive rod and an electron transporting coil. Grazing Incidence X-ray scattering is used to demonstrate the layered structure of the resulting self-assembled block copolymer film relative to the substrate (electrode). A multi-layer thin film self-assembled from PPV-b-OX shows significant improvement in luminescence and efficiency over pure PPV and PPV/OX blend devices. The correlation between details of thin film structure including lamellar spacing, orientation, and number of layers and device performance will also be discussed.   

Self-assembly of linear rod-coil block copolymers

Rod-coil block copolymer systems have attracted a great deal of attention due to their rich phase behavior. The spontaneous ordering of coil-rod block copolymers is due to the mutual repulsion of the dissimilar blocks and the packing constrains imposed by the connectivity of each block, while the stiff rigid conformation of the rod segment imparts orientation organization. However, few reports investigate rod-coil block copolymers in three-dimension space. In this work, the self-assembly of linear rod-coil block copolymers is studied by applying self-consistent-field lattice techniques in three-dimension space. The stiffness influences on the self-assembly and the possible orientations of the rods in different structures are focused on for rod-coil diblock copolymers; the interfacial grafting density of the separating rod and coil segments is found exerting important influences on the phase behavior of symmetric coil-rod-coil triblock cpolymers; the influences of the intramolecular interactions between the two rods of the symmetric rod-coil-rod triblock copolymer chain on the self-assembly are studied.