(6cf) Self-Assembly of Functional Rod-Coil Block Copolymers

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.