How do you make a material that has the elasticity of a rubber band and the thermal insulation of a Styrofoam cup? Connect two distinct polymer chains – poly(isoprene) and poly(styrene) – end to end like a series of children’s building blocks. The result is an appropriately named “block copolymer” that boasts the properties of both materials and is commonly used in the tires of automobiles and the soles of athletic shoes.
But the most impressive trait of a block copolymer is its ability to self-assemble. Imagine, for example, dropping a mixture of poly(isoprene) and poly(styrene) on the floor. The two incompatible blocks will “phase separate” like oil and water. However, connect the ends of the two polymers together and the material usually can assemble into a well-defined material with nanoscale structure.
This natural assembly is extremely valuable for emerging nanoscale applications, including materials for fuel cells, lithium ion batteries, and organic photovoltaics.
In order to take full advantage of a block copolymer’s molecular architecture, researchers are looking for ways to control the interactions between polymer blocks through means such as high temperatures and selective solvents.
To do this, University of Delaware researchers are exploring tuning with tapering, which increases the compatibility of the polymer by smoothing out the chemical interface between the two blocks.
Tapering the interface is thought to reduce the “penalty” of mixing between the two very different blocks. The result is a lower processing temperature, which is easier and cheaper to achieve, and greater chemical compatibility.
To test this idea, the researchers synthesized a number of tapered poly(isoprene-b-styrene) block copolymer samples at the University of Delaware: samples with different taper lengths (from 15-35 percent tapered material) with either normal or inverse tapering.
The samples were then heated and cooled while researchers observed the changes in the material with small-angle x-ray scattering at NSLS and Argonne’s Advanced Photon Source, and transmission electron microscopy and dynamic mechanical analysis at the University of Delaware.
Their results proved that researchers can tune the compatibility of block copolymers (via tapering) with no detriment to the ordering or mechanical properties of the material. The research also revealed that inverse tapering gives the greatest increase in compatibility.
N. Singh, M.S. Tureau, T.H. Epps, III, “Manipulating Ordering Transitions in Interfacially Modified Block Copolymers,” Soft Matter, 5, 4757 (2009).
Top: Cartoon of density profile and segment distribution along model
polymer chains as a function of position, where purple represents
isoprene and blue represents styrene, illustrating the difference
between random, gradient, block, and tapered block copolymers.
Middle: Synchrotron-SAXS data for a P(I-SI-S) tapered diblock copolymer. Specimens were annealed at 210 degrees C and then cooled to room temperature for data acquisition. The integral moduli values are characteristic of two-domain lamellae. The inset shows a transmission electron microscopy image of a P(I-SI-S) specimen. The sample is stained by OsO4 vapor to enhance contrast.
Bottom: University of Delaware researchers Thomas Epps (left) and Maeva Tureau