Recyclability of Vitrimer Materials: Impact of Catalyst and Processing Conditions

Abstract

With sustainability at the forefront of material research, recyclable polymers, such as vitrimers, have garnered increasing attention since their introduction in 2011. In addition to a traditional glass-transition temperature (T _g), vitrimers have a second topology freezing temperature (T _v) above which dynamic covalent bonds allow for rapid stress relaxation, self-healing, and shape reprogramming. Herein, we demonstrate the self-healing, shape memory, and shape reconfigurability properties as a function of experimental conditions, aiming toward recyclability and increased useful lifetime of the material. Of interest, we report the influence of processing conditions, which makes the material vulnerable to degradation. We report a decreased crosslink density with increased thermal cycling and compressive stress. Furthermore, we demonstrate that shape reconfigurability and self-healing are enhanced with increasing compressive stress and catalyst concentration, while their performance as a shape memory material remains unchanged. Though increasing the catalyst concentration, temperature, and compressive stress clearly enhances the recovery performance of vitrimers, we must emphasize its trade-off when considering the material degradation reported here. While vitrimers hold great promise as structural materials, it is vital to understand how experimental parameters impact their properties, stability, and reprocessability before vitrimers reach their true potential.

Figure 1

As the catalyst concentration increases, the number of transesterification reactions increases per unit time and per unit volume. This increase in catalyst concentration results in a direct increase in self-healing and shape reconfigurability while also making the material susceptible to property degradation as a function of processing conditions. All chemical structures are depicted in the far-left panel, where bisphenol A diglycidyl ether is an epoxy resin, sebacic acid is a crosslinker, and 1,5,7-triazabicyclo[4.4.0]dec-5-ene is the catalyst molecule. The dynamic covalent reaction site (ester activated via TBD) is seen as the geometric structures (white and green) in the far-right panel.

Figure 2

Representative mechanical testing data are shown comparing pristine vitrimer materials with those that have been damaged and self-healed. All samples contained a 5 mol % catalyst concentration, and a minimum of five samples were tested to achieve normal statistical distribution. (a) Samples are healed in a hot press at 200 °C for 1 h, with varying pressures; unsurprisingly, increasing the pressure increases the degree of healing and recovery of mechanical properties. (b) Samples are healed in a hot press at 200 °C under 0.67 MPa of pressure, with varying times. The length of time appears to have a negligible impact on the degree of self-healing; this trend is reasonable as both times exceed the stress relaxation constant for these materials, as previously reported.

Figure 3

(a) Changes in the T_g are determined via DSC for a variety of catalyst concentrations after a series of HP. (d) Similar results are reported for samples after a series of temperature sweep experiments cycled with HP. The sample size is four to ensure statistical significance; bars denoted with * indicate statistical significance (p < 0.05). The temperature sweep results are reported for samples with no catalyst (b–c) and for samples with a 5 mol % catalyst concentration (e–f) as the storage modulus (b,e) and tan(δ) curves (c,f).

Figure 4

(a) Changes in the T_g are determined via DSC for samples after a series of temperature sweep experiments cycled with annealing. The sample size is four to ensure statistical significance; bars denoted with * indicate statistical significance (p < 0.05). (d) The molecular weight between crosslinks is reported for each cycle when the temperature sweeps are cycled between HP or A for samples with and without a catalyst. The temperature sweep results are reported for samples with no catalyst (b–c) and for samples with a 5 mol % catalyst concentration (e–f) as the storage modulus (b,e) and tan(δ) curves (c,f).

Figure 5

Shape memory demonstrations are performed for samples with either no catalyst (a) or a 5 mol % catalyst concentration (b). The scale bars are 5 mm. (c) A series of five shape memory cycles are performed where the temperature and force are controlled, and the strain is recorded; even numbered cycles are shaded gray for clarity. (d) Shape memory results are shown for a variety of catalyst concentrations with two applied forces (0.5 and 1.0 N force). In all cases, the change in strain increases with increasing force on the sample. (e) Shape memory results are compiled demonstrating that the change in strain increases with increasing catalyst concentration. (f) These results are quantified and validated by the decrease in the storage modulus of the rubbery regime with increasing catalyst concentration.

Figure 6

Shape reconfigurability demonstrations are performed for samples with either no catalyst (a) or a 5 mol % catalyst concentration (b) where the demonstrations are unsuccessful in the absence of a catalyst. The scale bars are 5 mm. (c) A series of alternating shape memory and shape reconfigurability cycles are performed where the temperature and force are controlled, and the strain is recorded; all shape memory cycles are shaded light blue for clarity. (d) Results are shown for a variety of catalyst concentrations where the applied force during shape reconfigurability cycles is either incrementally increasing or held constant. (e) In all cases, the permanently embedded strain increases with increasing force on the sample and catalyst concentration. When the force is kept constant, there is a slight increase in embedded strain. The first three bars for each sample type are recorded from the upper graph of Figure 6d, while the last three bars for each sample type are recorded from the lower graph of Figure 6d. (f) Shape reconfigurability tests are repeated where samples with either a 1 mol % or a 3 mol % catalyst concentration are heated near their T_v of 256 and 219 °C, respectively.