# Low-Intensity Magnetic-Field-Directed Lattice Symmetry Transition to Induce the Centered Rectangular Cylinders in Diblock Copolymer/Magnetic Nanoparticle Nanocomposite Films

## TL;DR

This paper shows how a low-intensity magnetic field can change the structure of a polymer film by altering its lattice symmetry.

## Contribution

A new method for inducing a centered rectangular lattice in nanocomposite films using low-intensity magnetic fields.

## Key findings

- A 350 mT magnetic field transformed hexagonally packed cylinders into centered rectangular cylinders.
- Applying the magnetic field to unoriented structures led to lamellar phase formation.
- The CR phase was kinetically stabilized, while the lamellar phase was thermodynamically stable.

## Abstract

This study demonstrates a strategy to control lattice
symmetry
in a polystyrene-block-poly­(methyl methacrylate)
(PS-b-PMMA) diblock copolymer hybridized with a small
fraction of NH2-tethered Fe3O4 magnetic
nanoparticles incorporated within the cylindrical PMMA microdomains.
A low-intensity magnetic field (= 350 mT) was applied following large-amplitude
oscillatory shear alignment, transforming the shear-aligned hexagonally
packed cylinders (HEX) into centered rectangular cylinders (CR) stabilized
through microdomain reorientation that relieved chain crowding and
stretching of the PS blocks. In contrast, applying the magnetic field
to unoriented HEX induced reorganization into lamellae aligned parallel
to the field. The lamellar phase represented the thermodynamic equilibrium
state, whereas the CR phase was a kinetically stabilized metastable
structure governed by the prealigned framework. These findings highlight
low-intensity magnetic manipulation as an effective “noncontact
tweezer” for tuning lattice symmetry in block copolymers via
the interplay of magnetic anisotropy and initial microdomain orientation.