Self-Healability of Poly(Ethylene-co-Methacrylic Acid): Effect of Ionic Content and Neutralization

Abstract

Self-healing polymers such as poly(ethylene-co-methacrylic acid) ionomers (PEMAA) can heal themselves immediately after a projectile puncture which in turn lowers environmental pollution from replacement. In this study, the thermal-mechanical properties and self-healing response of a library of 15 PEMAA copolymers were studied to understand the effects of the ionic content (Li, Na, Zn, Mg) and neutralization percentage (13 to 78%) on the results. Differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and tensile testing were used to study the thermo-mechanical properties of PEMAA copolymers while the self-healing response was studied using the projectile test. Puncture sites were observed using scanning electron microscopy (SEM) and the healing efficiency was quantitatively measured using the water leakage test. Five different self-healing responses were observed and correlated to ionic content and neutralization. At high neutralization, divalent neutralizing ions (Zn and Mg) that have stronger ionic interactions exhibited brittle responses during projectile testing. PEMAA samples neutralized with Mg and Li at low concentrations had a higher healing efficiency than PEMAA samples neutralized with Zn and Na at low neutralization. The PEMAA copolymers with higher tensile stress and two distinct peaks in the graph of loss factor versus temperature that indicate the presence of sufficient ionic aggregate clusters had improved healing efficiency. By increasing the neutralization percentage from 20% to 70%, the tensile strength and modulus of the samples increased and their self-healability generally increased. Among the investigated samples, the copolymer with ~50% neutralization by Li salt showed the highest healing efficiency (100%). Overall, the strength and elastic response required for successful self-healing responses in PEMAA copolymers are shown to be governed by the choice of ion and the amount of neutralization.

Keywords: ionomer; mechanical properties; poly(ethylene-co-methacrylic acid); projectile test; self-healing efficiency; self-healing polymers; thermal properties.

Figure 1

Ionomer structure with a focus on the ionic aggregate clusters that are tightly packed on the left-hand side. Upon heating, they have a larger margin for movement and can form new thermally reversible crosslinks.

Figure 2

(a) The dog-bone mold and (b) the film mold and (c) the dog-bone specimen, the film, and the rectangular specimen, from top to bottom with their respective dimensions, used for characteristic tests and projectile tests of PEMAA co-polymers.

Figure 3

Schematic of the ballistic test with a pellet gun, a sample placed in a holder.

Figure 4

A DSC thermogram for a PEMAA-53%Li-M122 during a heat-cool-heat cycle at a rate of 5 °C/min.

Figure 5

DSC thermograms of PEMAA-26%Li-122M at different time intervals from the date of injection molding at a time scale from 1 day to about 10 years.

Figure 6

Mechanical aging test of PEMAA-26%Li-122M that shows the effect of aging on the storage modulus of PEMAA using DMA. Days refer to the time between the DMA run and when the sample was injected molded.

Figure 7

(a) Melting temperatures and (b) secondary crystallization temperatures of the PEMAA copolymers obtained from DSC tests.

Figure 8

Storage modulus at 94.5 °C for all PEMAA copolymers under investigation from DMA tests.

Figure 9

The loss factor obtained from DMA run for PEMAA neutralized at various percentages by (a) Li, (b) Mg, (c) Zn, and (d) Na.

Figure 9

The loss factor obtained from DMA run for PEMAA neutralized at various percentages by (a) Li, (b) Mg, (c) Zn, and (d) Na.

Figure 10

Representative graph of tensile stress vs. tensile strain for PEMAA-73%Mg-M122 and PEMAA-21%Mg-M122 and PEMAA-20%Mg-M33.

Figure 11

SEM image (a) shows the high impact response of the PE sample and SEM image (b) shows the circular holes of PEMAA-21%Zn-M122 at the impact site.

Figure 12

SEM image (a) shows the entry side, SEM image (b) shows the exit side, and SEM image (c) shows the diagonal view of the exit side of the door-flap puncture site for PEMAA-20%Li-M122.

Figure 13

(a) Brittle hole from a PEMAA-71%Zn-M190 sample (b) Line fracture from a PEMAA-71%Zn-M190 sample (c) Brittle hole from of PEMAA-73%Mg-M122 sample (d) Line fracture from a PEMAA-73%Mg-M122 sample.

Figure 14

(a) Sealed exit site from a PEMAA-53%Li-M122 sample (b) Sealed entry site from a PEMAA-58%Li-M190 sample, and (c) Line fracture from a PEMAA-58%Li-M190 sample.

Figure 14

(a) Sealed exit site from a PEMAA-53%Li-M122 sample (b) Sealed entry site from a PEMAA-58%Li-M190 sample, and (c) Line fracture from a PEMAA-58%Li-M190 sample.

Figure 15

Healing efficiency obtained using Equation (1) as a function of neutralization percentage and ion type.

Figure 16

Tensile stress at maximum load for all PEMAA polymers at 25 °C in relation to the healing efficiency.

Figure 17

Storage Modulus at 94.5 °C for all PEMAA polymers in relation to the healing efficiency.