Dynamics of Magnetic Alignment in Rod–Coil Block Copolymers

Abstract

The dynamics associated with magnetic field alignment of a model rod–coil block copolymer poly(2,5-di(2′-ethylhexyloxy)-1,4-phenylenevinylene)-b-polyisoprene (PPV–PI) have been investigated using a combination of time-resolved in situ small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Alignment is observed over a wide range of field strengths (0.2–7 T); however, the highest field strengths studied produce the highest degree of alignment. Experiments examining alignment of a disordered sample, cooled into the ordered state in the presence of a magnetic field, show that alignment mostly occurs during nucleation and growth of the block copolymer nanostructure. The slower secondary processes of defect annihilation and grain rotation progress are necessary in producing extremely highly aligned samples. At the highest field strength, due to the increased order–disorder transition temperature (TODT), selective ordering is likely observed at temperatures near the order–disorder transition leading to nucleation of aligned block copolymer grains, resulting in faster and a higher degree of alignment. Additionally, at these high field strengths the alignment process appears to have a more complex defect production and removal process than at low field strengths. At low field strengths isotropic nucleation occurs, and then preferential growth of aligned block copolymer grains is primarily responsible for alignment. Finally, an optimum alignment temperature is observed where the thermodynamic driving force for alignment, thermal disordering processes, and the kinetic effects governing block copolymer growth and defect removal are balanced.