An Imidazolium-Based Ionic Liquid as a Model to Study Plasticization Effects on Cationic Polymethacrylate Films

Abstract

Ionic liquids (ILs) have been touted as effective and environmentally friendly agents, which has driven their application in the biomedical field. The study compares the effectiveness of an IL agent, 1-hexyl-3-methyl imidazolium chloride ([HMIM]Cl), to current industry standards for plasticizing a methacrylate polymer. Industrial standards glycerol, dioctyl phthalate (DOP) and the combination of [HMIM]Cl with a standard plasticizer was also evaluated. Plasticized samples were evaluated for stress-strain, long-term degradation, thermophysical characterizations, and molecular vibrational changes within the structure, and molecular mechanics simulations were performed. Physico-mechanical studies showed that [HMIM]Cl was a comparatively good plasticizer than current standards reaching effectiveness at 20-30% w/w, whereas plasticizing of standards such as glycerol was still inferior to [HMIM]Cl even at concentrations up to 50% w/w. Degradation studies show HMIM-polymer combinations remained plasticized for longer than other test samples, >14 days, compared to glycerol <5 days, while remaining more pliable. The combination of [HMIM]Cl-DOP was effective at concentrations >30% w/w, demonstrating remarkable plasticizing capability and long-term stability. ILs used as singular agents or in tandem with other standards provided equivalent or better plasticizing activity than the comparative free standards.

Keywords: green chemistry; ionic liquids; mechanical properties; plasticization; polymeric films; stress/strain properties.

Figure 1

Structure of plasticizers: (a) 1-hexyl 3-methyl imidazolium chloride ([HMIM]Cl); (b) dioctyl phthalate (DOP); (c) glycerol and polymer: (d) Eudragit RL 100 or Poly(ethyl acrylate-co-methyl methacrylate-co-trimethylammonioethyl methacrylate chloride.

Figure 2

Illustration of sample setup within the CellScale BioTester. (a) The setup before any stress is applied; (b) at the end of the test after stress has been applied and deformation can clearly be seen.

Figure 3

FTIR Spectra of (a) DOP, (b) Glycerol, (c) HMIM and (d) HMIM and DOP blend plasticized samples with varying concentrations.

Figure 4

DSC thermogram of (a) DOP, (b) HMIM and (c) HMIM and DOP blend (d) Glycerol plasticized samples. (insert contain reference thermograms).

Figure 5

Images showing the end outcome of the stress–strain tests. End of test images of (a) 50% w/w glycerol (ruptured); (b) Eudragit control film (ruptured); (c) 50% w/w DOP; (d) 50% w/w [HMIM]Cl; (e) 50% w/w blend.

Figure 6

Images show the strain patterns (low strain—green; high strain—orange) before any applied stress (Left) and after the stress test has completed (Right). (a) 50% w/w glycerol (ruptured); (b) Eudragit control film (ruptured); (c) 50% w/w DOP; (d) 50% w/w [HMIM]Cl; (e) 50% w/w blend. Images with source points were generated using the LabJoy software interface accompanying the BioTester equipment.

Figure 7

Effect of plasticizer type and concentration 1, 2 and 5% on the endured stresses and strains as a ramp displacement-type stress test was conducted. The resultant Young’s modulus has been calculated. For all samples, the strain endured was 0.1.

Figure 8

Effect of plasticizer type and concentration 10, 20, 30 and 50% on the endured stresses and strains as a ramp-displacement-type stress test was conducted. The resultant Young’s modulus has been calculated. For all samples, the strain endured was 0.1.

Figure 9

Minimum energy confirmation for (a) IL, (b) EUD and (c) molecular mapping and geometrical positioning of IL and EUD after molecular mechanics simulations in a vacuum.