Structural Evolution of Polylactide Molecular Bottlebrushes: Kinetics Study by Size Exclusion Chromatography, Small Angle Neutron Scattering, and Simulations.

Structural evolution from poly(lactide) (PLA) macromonomer to resultant PLA molecular bottlebrush during ring opening metathesis polymerization (ROMP) was investigated for the first time by combining size exclusion chromatography (SEC), small-angle neutron scattering (SANS), and coarse-grained molecular dynamics (CG-MD) simulations. Multiple aliquots were collected at various reaction times during ROMP and subsequently analyzed by SEC and SANS. These complementary techniques enable the understanding of systematic changes in conversion, molecular weight and dispersity as well as structural details of PLA molecular bottlebrushes. CG-MD simulation not only predicts the experimental observations, but it also provides further insight into the analysis and interpretation of data obtained in SEC and SANS experiments. We find that PLA molecular bottlebrushes undergo three conformational transitions with increasing conversion (i.e., increasing the backbone length): (1) from an elongated to a globular shape due to longer side chain at low conversion, (2) from a globular to an elongated shape at intermediate conversion caused by excluded volume of PLA side chain, and (3) the saturation of contour length at high conversion due to chain transfer reactions.

Assembly and organization of poly(3-hexylthiophene) brushes and their potential use as novel anode buffer layers for organic photovoltaics.

Buffer layers that control electrochemical reactions and physical interactions at electrode/film interfaces are key components of an organic photovoltaic cell. Here the structure and properties of layers of semi-rigid poly(3-hexylthiophene) (P3HT) chains tethered at a surface are investigated, and these functional systems are applied in an organic photovoltaic device. Areal density of P3HT chains is readily tuned through the choice of polymer molecular weight and annealing conditions, and insights from optical absorption spectroscopy and semiempirical quantum calculation methods suggest that tethering causes intrachain defects that affect co-facial π-stacking of brush chains. Because of their ability to modify oxide surfaces, P3HT brushes are utilized as an anode buffer layer in a P3HT-PCBM (phenyl-C61-butyric acid methyl ester) bulk heterojunction device. Current-voltage characterization shows a significant enhancement in short circuit current, suggesting the potential of these novel nanostructured buffer layers to replace the PEDOT:PSS buffer layer typically applied in traditional P3HT-PCBM solar cells.

Poly(3-hexylthiophene) Molecular Bottlebrushes via Ring-Opening Metathesis Polymerization: Macromolecular Architecture Enhanced Aggregation.

We report a facile synthetic strategy based on a grafting through approach to prepare well-defined molecular bottlebrushes composed of regioregular poly(3-hexylthiophene) (rr-P3HT) as the conjugated polymeric side chain. To this end, the exo-norbornenyl-functionalized P3HT macromonomer was synthesized by Kumada catalyst transfer polycondensation (KCTP) followed by postpolymerization modifications, and the resulting conjugated macromonomer was successfully polymerized by ring-opening metathesis polymerization (ROMP) in a controlled manner. The P3HT molecular bottlebrushes display an unprecedented strong physical aggregation upon drying during recovery, as verified by several analyses of the solution and solid states. This remarkably strong aggregation behavior is attributed to a significant enhancement in the number of π-π interactions between grafted P3HT side chains, brought about due to the bottlebrush architecture. This behavior is qualitatively supported by coarse-grained molecular dynamics simulations.

Surface morphology diagram for cylinder-forming block copolymer thin films.

We investigate the effect of the ordering temperature (T) and film thickness (h(f)) on the surface morphology of flow-coated block copolymer (BCP) films of asymmetric poly(styrene-block-methyl methacrylate). Morphology transitions observed on the ordered film surface by atomic force microscopy (AFM) are associated with a perpendicular to a parallel cylinder BCP microphase orientation transition with respect to the substrate with increasing h(f). "Hybrid" surface patterns for intermediate h(f) between these limiting morphologies are correspondingly interpreted by a coexistence of these two BCP microphase orientations so that two "transitional" h(f) exist for each T. This explanation of our surface patterns is supported by both neutron reflectivity and rotational SANS measurements. The transitional h(f) values as a function of T define upper and lower surface morphology transition lines, h(fu) (T) and h(fl) (T), respectively, and a surface morphology diagram that should be useful in materials fabrication. Surprisingly, the BCP film surface morphology depends on the method of film formation (flow-coated versus spun-cast films) so that nonequilibrium effects are evidently operative. This morphological variability is attributed primarily to the trapping of residual solvent (toluene) within the film (quantified by neutron reflectivity) due to film vitrification while drying. This effect has significant implications for controlling film structure in nanomanufacturing applications based on BCP templates.