Facile Alignment of Amorphous Poly(ethylene oxide) Microdomains in a Liquid Crystalline Block Copolymer Using Magnetic Fields: Towards Ordered Electrolyte Membranes
Edited by Qichao Hu
October 4th, 2010
The hydrophilic Poly(ethylene oxide) (PEO) polymer is water-soluble and semicrystalline, and has many applications. One of the main applications is used as a solid polymer electrolyte in lithium batteries. When lithium salts are introduced to the PEO polymer, the lithium ions are complexed by the ether oxygen atoms, and can migrate along the PEO side chains and result in ion conducting electrolyte.
However, due to the crystalline nature of PEO, ion transport is slow, since the lithium ion conductivity is related to the segmental motion of the PEO side chains. The effect of increasing ion concentration in PEO is unclear, while it suppresses the crystallinity, it also increases the glass transition temperature of the polymer. Thus amorphous and liquid-like PEO is desired for high conductivity polymer electrolyte. However, a certain mechanical strength is also needed in lithium batteries to overcome the dendrite growth problem. As a consequence, one approach of using PEO-based polymer as an electrolyte in lithium battery, is to use block copolymer, where microphase separation between PEO and another block results in a copolymer that is liquid on the nano-scale, but solid on the macro-scale.The local liquid nature enables descent ion conductivity, and the global solid nature enables sufficient mechanical stability.
In order to have good ion conductivity, the ion pathway provided by the PEO polymer needs to be contiguous and nonconvoluted. This is difficult to achieve with conventional uncontrolled self-assembly process, where defect density is high and there is little long-range order.
This paper introduces a technique that can control the ordering in these materials. The technique involves using magnetic field to align the block copolymers. This technique is particularly appealing for large length-scale (> 1 mm) samples supported by thin film substrates (1-100 um). It replies on the magnetic anisotropy provided by the liquid crystalline PEO. The diamagnetic anisotropy which originates from the smectic ordering of the mesogens in the liquid crystalline block, drives the alignment. This work shows that in a block copolymer between PEO and poly(acrylic acid) PAA and doped with LiClO4, magnetic fields can be used to align the PEO-based diblock copolymer. A schematic of the block polymer is shown the figure below.
The PEO undergoes and order to disorder transition around 65C. When PAA is added to the system, an interpolymer complex between PAA and PEO is formed. This formation improves the weak segregation between the low molar mass PEO and MA/LC domains, and result in sharper Bragg peaks and more prominent scattering. When 10% PAA is added, the oder to disorder temperature increased to 70C. When 20% PAA is added, further improvement in the order of the system is observed. However, at this PAA concentration, no order to disorder transition is observed, suggesting that further addition of PAA to the block copolymer has little effect on the domain spacing.
When about 1% of LiClO4 is added to the copolymer, further improvement in the segregation between the EO and MA/LC domains is observed. The order to disorder transition is increased to 110C.
When a vertically applied magnetic field is applied, the alignment of the block copolymer is shown in the figure below.
Here the mesogens are coassembled with the mesogens in the LC block, which are covalently bonded. These are arranged with their long axis along the field and parallel to the interface with the PEO domains.
The setup including the magnetic field direction and the polymer is shown in the figure below.
We can see that the magnetic alignment depends on the anisotropy of magnetic susceptibility of the LC mesophase and the orientation of the mesogens. The alignments that satisfy both the anchoring constraint and magnetic anisotropy are degenerate. There are infinite possible orientation with normals in the xy plane. However, among these degenerate sets, only the one with its normal perpendicular to the X-ray beam contributes to the scattering.
This paper introduces a new technique "rotational annealing", which breaks the degeneracy. Here a rotation is imposed to the sample, and selects a unique system configuration. This is possible because the planar anchoring of the mesogens is degenerate, and is selected because it is the lowest energy state. A significant increase in the number of scattering lamellae due to the breaking of the degeneracy is observed.
While optimization in the cooling rate and rotation speed is needed to improve the alignment process, the ability to align large length scale self-assembled structures is useful for many applications. Magnetically driven alignment technique is particularly useful where strong flow fields or intricate topologically constraints are not possible. Moreover the ability to align PEO-based polymer has potential application for solid polymer lithium batteries.