Block copolymers are made by joining two or more polymer components into a
single molecule. Due to the incompatibility between the interconnected chains, block
copolymers form microphase separated domains of long range order at length scales of 5
- 100 nm. These materials have been the subject of intense study for the past four decades
and the rich structural and dynamic behavior of block copolymers are still being explored.
In this thesis, the structure, dynamics and mechanical properties of block copolymers and
block copolymer blends were investigated using small-angle X-ray scattering (SAXS),
transmission electron microscopy (TEM), dynamic mechanical spectroscopy (DMS),
differential scanning calorimetry (DSC), and tensile testing.
A new equilibrium block copolymer phase, the Frank-Kasper σ-phase was
discovered in poly(1,4-isoprene-b-DL-lactide) (IL) diblock and poly(styrene-b-1,4-
isoprene-b-styrene-b-ethylene oxide) (SISO) tetrablock copolymer melts. The σ-phase
has tetragonal symmetry (P42/mnm) and possesses 30 spheres per unit cell. This gigantic
crystal, a dodecagonal quasicrystal approximant, structure has been reported primarily in
two heavy metals, numerous metal alloys, and dendrimers. Identification of the σ-phase
in block copolymers provides new evidence regarding the complex nature of packing
spheres on an ordered lattice.
The dynamics ordered sphere-forming block copolymers was studied using SAXS
and rheological techniques. The process of ordering into a body-centered cubic (BCC) morphology from the disordered state and the order-to-order transition (ODT) from the
(metastable) BCC to σ-phase were found to follow nucleation and growth mechanisms.
The IL diblock copolymer phase diagram was investigated as a function of
composition and temperature. IL diblock copolymers are strongly segregated due to a
relatively large Flory-Huggins interaction parameter χ between polyisoprene and
poly(DL-lactide). Fluctuation effects strongly influence the ODT due to the low IL
molecular weights and this was evidenced by thermal signatures in DSC thermograms.
The structure and mechanical properties of poly(DL-lactide-b-1,4-isoprene-b-DLlactide)
(LIL) triblock copolymer thermoplastic elastomer and low molecular weight IL
diblock copolymers, and blends of these materials were studied. While the linear
response is relatively invariant to the molecular architecture and molecular weight, the
extensional properties were dramatically influenced by the triblock content.
Finally, path dependency of microstructures of poly(1,2-butadiene-b-ethylene
oxide) (PB-PEO) non-ionic block copolymer surfactants in oil and water was examined.
Due to an extremely low critical micelle concentration due to the high molecular weight
of the PB-PEO block copolymer surfactant, highly path dependent and long-lasting
metastable microstructures were generated. This result offers new opportunities for the
preparation of target block copolymer microstructures.