While block copolymer self-assembly offers an elegant route for the self-assembly of structures on the 10-100 nm length scale, many functional polymers, such as helical proteins and conjugated semiconducting polymers, do not follow these classical self-assembly behaviors.   Nonetheless, new applications in organic photovoltaics and biotechnology rely on controlled nanoscale morphologies incorporating biological and semiconducting polymers.   Due to extended conjugation or hydrogen-bonded helical structure, such functional polymers have rodlike shapes rather than the classical Gaussian coil chain shape; incorporation of these rodlike polymers into block copolymers results in an interplay between liquid crystalline interactions and microphase separation.  To understand the thermodynamics of functional rod-coil block copolymers, we have synthesized a model system with accessible phase transitions that we have used establish a universal bulk phase diagram and investigate self-assembly in thin films.

Phase space in rod-coil block copoymers is four dimensional; the dimensionless variables characterizing the system include the traditional repulsive interaction between the two blocks and the coil volume fraction as well as the strength of the liquid crystalline interaction and the geometrical ratio of the rod and coil block sizes.  The aligning liquid crystalline interactions favor microphases with little curvature, resulting in a stable lamellar phase at low temperatures in most block copolymers.  At high coil fraction and high geometric asymmetry between the rod and coil, hexagonal packing of rectangular rod aggregates is observed.  At higher temperatures, the rod and coil blocks become miscible and nematic and isotropic phases are observed.   Independent experiments measure the strengths of liquid crystalline interactions between the rods and repulsions between the rod and coil, allowing the phase diagram to be converted to a universal set of coordinates applicable to all rod-coil diblock copolymers. 

Many applications of rod-coil block copolymers, such as organic photovoltaics, require both the creation of nanoscale structure and on the details pattern formation in thin films.   The thin film state introduces additional complexities due to surface energy of the interfaces and geometric confinement normal to the substrate.  Selective segregation of the coil block to the supported film interface orients lamellae preferentially parallel to the substrate in films, and perpendicularly oriented lamellae form defects between parallel grains.  The lamellae have high bending moduli due to the liquid crystalline interactions between rods.  The high bending moduli and in-plane liquid crystalline packing of the rod blocks lead to unusual grain shapes, non-traditional defect structures, and the potential for new handles in controlling long range order.  

Publications:

14.  “Crystalline Structure in Thin Films of DEH-PPV Homopolymer and PPV-b-PI Rod-Coil Block Copolymers.”  B.D. Olsen, D. Alcazar, V. Krikorian, M.F. Toney, E.L. Thomas, and R.A. Segalman.  Macromolecules in press.

13.  “Square Grains in Asymmetric Rod-Coil Block Copolymers.”  B.D. Olsen and R.A. Segalman.  Submitted.

12.  “Hierarchical Structure Control in Block Copolymers with Magnetic Fields.”  Y. Tao, H. Zohar, B.D. Olsen, and R.A. Segalman.  Nano Letters 2007, 7, 2742-2746.

11.  “Non-Lamellar Phases in Asymmetric Rod-Coil Block Copolymers at Increased Segregation Strengths.  B.D. Olsen and R.A. Segalman.  Macromolecules 2007, 40, 6922-6929.

10.  “Domain Size Control by Self-Assembly of Rod-Coil Block Copolymers and Homopolymers Blends.”  Y. Tao, B.D. Olsen, V. Ganesan, and R.A. Segalman.  Macromolecules 2007, 40, 3320-3327.

9.  “Thin Film Structure of Symmetric Rod-Coil Block Copolymers.”  B.D. Olsen, X. Li, J. Wang, R.A. Segalman.  Macromolecules 2007, 40, 3287-3295.

8.  “Phase Transitions in Asymmetric Rod-Coil Block Copolymers.”  B.D. Olsen and R.A. Segalman.  Macromolecules 2006, 39, 7078-7083.

7.  “Higher Order Liquid Crystalline Structure in Low-Polydispersity DEH-PPV.”  B.D. Olsen, S.-Y. Jang, J.M. Lüning, and R.A. Segalman.  Macromolecules 2006, 39, 4469-4479.

6.  “Polymeric Nanocoatings by Hot-Wire Chemical Vapor Deposition (HWCVD).”  K.K.S. Lau, Y. Mao, H.G. Pryce Lewis, S.K. Murthy, B.D. Olsen, L.S. Loo, and K.K. Gleason.  Thin Solid Films 2006, 501, 211-215.

5.  “Structure and Thermodynamics of Weakly Segregated Rod-Coil Block Copolymers.”  B.D. Olsen and R.A. Segalman.  Macromolecules 2005, 38, 10127-10137.

4.  “Peptide Attachment to Vapor Deposited Polymeric Thin Films.”  S.K. Murthy, B.D. Olsen, and K.K. Gleason.  Langmuir 2004, 20, 4774-4776.

3.  “Effect of Filament Temperature on the Chemical Vapor Deposition of Fluorocarbon-Organosilicon Copolymers.”  S.K. Murthy, B.D. Olsen, and K.K. Gleason.  J. App. Poly. Sci. 2004, 91, 2176-2185.

2.  “Making Thin Polymeric Materials, Including Fabrics, Microbicidal and Also Water-Repellent.”  J. Lin, S.K. Murthy, B.D. Olsen, K.K. Gleason, A.M. Klibanov.  Biotechnology Letters 2003, 25, 1661-1665.

1.  “Initiation of Cyclic Vinylmethylsiloxane Polymerization in a Hot-Filament Chemical Vapor Deposition Process.”  S.K. Murthy, B.D. Olsen, and K.K. Gleason.  Langmuir 2002, 18, 6424-6428.