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Polymers are important materials for a number of applications and can be utilized in a number of different ways depending on what the application calls for. The structures of polymers can vary significantly from polymer to polymer and between different hierarchical forms of the same polymer. It has also been established recently by researchers from Rice University that unexpected distortions in the lattice of self-assembled polymers can alter the structure and properties of the polymer.
What is Self-Assembly?
Self-assembly is a common process for many materials, including a number of different polymers. Self-assembly involves taking molecules within a system that are highly disordered and organizing them so that they form an organized structure pattern.
While this can be done manually by scientists for a number of materials, the defining feature of self-assembled polymers (and other self-assembled materials) is that they do this automatically, without external input — and is typically performed by the system chemically manipulating the local interactions and molecules.
There is some ambiguity of terminology: complete systems arranging themselves are referred to as self-assembled materials, but when the process involves molecules arranging themselves, it is termed molecular self-assembly.
Self-assembly usually occurs in polymers (and other materials) to reduce the amount of free energy within the molecular network of the polymer. This, in turn, makes the polymer more stable, as highly energetic and strained molecular systems are more reactive and therefore less stable.
One of the defining features of self-assembly processes is that they don’t rely on covalent interactions to order themselves. Instead, they use non-covalent interactions, i.e. intermolecular interactions, to order, stabilize, and hold the material together.
Self-Assembly in Polymers
Polymers are an ideal class of materials for facilitating self-assembly processes, as there are many types of intermolecular interactions that are present (or can be introduced) in polymers which enables them to order and self-assemble themselves.
The tunable nature of polymers at the synthesis stage means that many different regions — be it hydrophobic/hydrophilic, hydrogen bonding networks, different polymer regions (coblock, grafted, branched etc.), or electron-dense regions — can be all be introduced during the chemical synthesis of the polymers, and these regions are then responsible for facilitating the intermolecular bonding between the polymer backbone chains.
From an overall perspective, it means that many polymers can be chemically designed so that they self-assemble naturally when formed, making polymers a very versatile and interesting material across self-assembly studies.
It should be noted that polymers which self-assemble range from nanopolymers to bulk polymers, and the applications include everything from drug release agents, to electronic components, to being used in nano-lithography methods. So, the application, scope, and usage of self-assembled polymers is a very wide field.
Distortions in Self-Assembled Polymers
All materials experience some kind of distortion at the atomic level, as it very rare for an atomic lattice to be completely free from defects (especially as there are many types). There are a number of defects that can be expected, but when unexpected effects occur within the material, it can throw up some interesting results.
Research into block copolymers over the years has shown that the presence of two different polymers co-existing can cause some issues due to the competing energetics of the two different systems becoming one.
The different energetics within the polymers can range from different polymer chain distortions, to different morphologies, different polymer orientations, chemical mismatch, and interfacial mismatch (i.e. the interface where the two polymers meet), all of which can make the entire system more energetically unfavorable (i.e. a higher energy system), and ultimately, more unstable.
However, the specific effects and issues found with copolymers varies from system to system, with some effects being known, but others being unexpected.
Such unexpected distortions came to the fore recently (2019) when researchers led by Ned Thomas at Rice University analyzed a block copolymer of polydimethylsiloxane (PDMS) and polystyrene. The researchers were expecting to see a perfect double gyroid structure (i.e. an infinitely continuous 3D network), however, what they found was that the structure was not perfect, it was full of distortions.
Instead of being a double gyroid structure, the researchers surprisingly found that the different polymer networks wrap around each other without even touching and are held in place completely via intermolecular attraction.
The copolymer structure tested by the researchers was thought to be one of the 230 spatial arrangements possible if the gyroid was a perfect cube. However, despite the researchers first synthesizing the material back in 1994, it’s only now that they’ve realized that the copolymer adopts this self-assembled intermolecularly bonded network instead of a perfect cube thanks to being able to analyze the images of it ‘slice by slice’ with an electron microscope.
The discovery has shown that the copolymer doesn’t adopt the lowest symmetrical state of a cubic lattice. Instead, the distortions cause it to form the least-symmetrical lattice formation of a triclinic cell, but because the symmetry of the different grains varied from grain to grain, it gives the impression of an average cubic cell, until it was probed in more detail.
The reasons for such distortions occurring has been attributed to the synthetic processes used to make the copolymer. It’s thought that because they form by evaporating from a solvent, the different grains shrink at different rates, and the different orientations of the polymer chains become squashed by shrinkage forces, leading to distortions occurring. These distortions then cause the unit cells to break their usual symmetry to adopt a lower energy and more stable state.
Such a discovery is important, because many of the properties, energies, and band gaps, etc. are based around calculations that use a perfect cubic structure, so when such unexpected distortions occur, some of the properties (especially band gap and energy properties) of certain copolymers could be much different than previously thought.
References and Further Reading
- Nanowerk: https://www.nanowerk.com/nanotechnology-news2/newsid=53914.php
- “Self-assembly as a key player for materials nanoarchitectonics”- Ariga K. et al, Science and Technology of Advanced Materials, 2018, DOI: 10.1080/14686996.2018.1553108
- “Selective directed self-assembly of coexisting morphologies using block copolymer blends”- Black C. T. et al, Nature Communications¸ 2016, DOI: 10.1038/ncomms12366