Gels are everywhere in modern life – consumer care products, food, coatings, inks, packaging, biomedical technologies, and more. These various samples contain internal structures that link different components, resulting in deformation or fracture like a solid rather than flowing like a liquid. However, not all gels are made of covalently-bonded polymer networks, as non-chemical bonding mechanisms are a growing area. Here we explore the various physical assemblies that are considered to be gels or behave like gels but differ from traditional chemical crosslinks.
Polymer Melts with Hydrogen Bonding
For polymer melts, hydrogen bonds between functional groups can induce temporary networks between polymer chains. (1) These types of gels can be chemically tuned to associate or dissociate upon exposure to temperature extremes or changes in pH based on the availability for hydrogen bonding. If there are enough hydrogen bonding sites to reach the effective gel point, gelation can be observed through increased elastic modulus in an oscillatory test or a double relaxation peak in time-temperature superposition frequency sweeps. (1) Unlike conventional covalently bonded polymer networks, hydrogen bonded crosslinks can reform after being broken apart, which serves as an advantage for self-healing, chemical sensing, and drug delivery. (1)
Polymer Solutions with Hydrogen Bonding
Polyacrylonitrile (PAN) chains in dimethyl sulfoxide (DMSO) solutions have been shown to be capable of forming hydrogen bonded networks for dilute concentrations as low as 0.1−5%, whereas chain interaction should not even occur until 0.5%. (2) This phenomenon is very rare, as dilute solutions typically do have enough polymer to form any type of structure. The particular polymer-solvent combination forms a gel at low concentration due to hydrogen bonding between the functional groups along PAN chains and the sulfur atoms in DMSO. (2) When water was added to the gel, the water molecules acted like links between the PAN and DMSO, further increasing the gel connectivity. (2) However, at higher concentrations of PAN, this gelation mechanism deteriorates on account of polymer chain entanglement dominating the physical interactions. (2)
Polymer Melt Entanglement
A common substance that is not always recognized as a gel has a flexible solid
structure made from entangled elastomer chains in a melt-like state. This is chewing gum - the favorite entanglement gel of consumers. The gel-like behavior originates from overlapping but not bonded polyisobutylene polymers that string together waxes and fillers. (3) Traditional gel rheology has been recorded for gums under low strains, but researchers have found that large amplitude oscillatory strain (LAOS) is an effective way to simulate the extreme chewing and stretching of chewing gum. (3)
Concentration-dependent physical crosslinks formed by hydrogen bonds and entanglements can be utilized to improve products as an alternative to covalent chemistry. Next week’s blog will take a closer look at more physical gelation phenomena, including ionic crosslinks, aggregation, and colloidal gelation.
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Gold, B. J., et al., “The microscopic origin of the rheology in supramolecular entangled polymer networks”, Journal of Rheology, 2017, 61, 1211.
Malkin, A., et al., “Rheological Evidence of Gel Formation in Dilute Poly(acrylonitrile) Solutions”, Macromolecules, 2013, 46, 257.
Martinetti, L., et al., “A critical gel fluid with high extensibility: The rheology of chewing gum”, Journal of Rheology, 2014, 58, 821.