Thermal and Atmospheric Stability of Tungsten (VI) Chloride: Applications to High-Temperature Self-Healing Polymer Systems Authors: Daniel Rey & Jason Kamphaus

Outline What is a self-healing polymer? Why use a tungsten (VI) chloride catalyst? Evaluate the thermal and atmospheric stability of the tungsten (VI) chloride catalyst Incorporate tungsten (VI) chloride into high-temperature self-healing polymers Problems, results, and conclusions Motivation for future work Here’s what I will discuss today. First, I will motivate why we want to use tungsten hexachloride as a catalyst in self-healing polymers. Then I will discuss how we determined the thermal and atmospheric stability of the catalyst. Next I will lay out the work done thus far to incorporate tungsten hexachloride into high-temperature-cure self-healing polymers I will address the problems we’re facing as well as the results from and conclusions from the data we’ve gathered. And finally I will lay out the framework for my future work with this catalyst.

How a self-healing polymer works

Why use tungsten (VI) chloride? Temperature Stability Melting point of WCl6 is 275oC which is ~125oC higher than 1st Generation Grubbs’ Catalyst Cost Approximately 1/40th the cost per gram of 1st generation Grubbs’ catalyst Drawbacks: Tungsten (VI) chloride readily deactivated by water High-density solid may cause dispersion problems in low-density polymers However, two major drawbacks to the tungsten catalyst are that it is very reactive with water and it is a high-density solid which may cause dispersion problems in lower density polymers.

Evaluate Atmospheric Stability Fill specimen vials in glove box with 100mg of RWCl6 Expose vials to atmosphere, place vials in vacuum oven Oven programmed to ramp from room temperature and hold at either 121⁰C or 177⁰C for 3 hours Vials removed from oven, injected with 1mL healing solution and allowed to sit for 24 hours Vials placed under vacuum to remove excess monomer Percent polymer yield calculated We wanted to test the reactive properties of our catalyst in response to different atmospheric and thermal conditions and decided that a polymer yield method would most adequately answer the question of whether the catalyst remains active after exposure to high temperature. To test this, we filled several specimen vials in the glove box with 100mg of RWCl6 and placed the vials in the vacuum oven after exposing it to the appropriate atmospheric conditions. The oven was programmed to ramp from room temperature and hold at either 121C or 177C for 3 hours. These times and temperatures were chosen to reflect the cure cycles of a standard industrial thermo set polymer. The vials were then removed from the oven and injected with 1mL of exoDCPD phenylacetelyne healing solution and allowed to sit for 24 hours before measuring the percent polymer yield. Here are our results. Fig 1: Vials filled with RWCl6 are placed in a jar and sealed.

As expected, the catalyst under inert atmospheric conditions produced the highest polymer yields. Brief exposure was simulated by uncapping the jars for roughly 30 seconds before resealing them and placing them in the oven. Also as expected, the catalyst exposed to air emerges form the cure cycle severely deactivated. It is surprising, however that temperature does not seem to have any effect on the catalyst reactivity. We expected a drop in catalyst reactivity to correlate with temperature as well as atmosphere. To further test this we devised a second experiment to measure the thermal stability of the catalyst.

Evaluate Thermal Stability About 50 mg of RWCl6 was sealed in vials, in a nitrogen environment and heated Every half hour 3 vials were removed Injected with 0.5 mL of healing solution Allowed to cure for 24 hours Vials were placed in a vacuum and the residual monomer was driven off for 24 hours Polymer yield measurements were made The setup to evaluate the thermal stability is almost identical to the previous setup, with a few minor changes. First, we have substituted the smaller 2ml vials for the 20ml scintillation vials because we had some problems with tungsten crystals forming on the inside surface of the jar. Secondly, because we are using smaller vials we fill them with only 50mg RWCl6. The vials are filled under inert atmosphere, placed into the oven at either 121C or 177C, and vials are removed 3 at a time every half hour. The vials are then injected and allowed to cure for 24 hours before measuring polymer yield. Fig 2: Small specimen vials filled with 50mg RWCl6.

The results we obtained show no evident time-dependant temperature correlation to catalyst reactivity. As you can see by the error bars the standard deviations in the mean were rather large suggesting a high degree of experimental error. I would like to note that inert control samples that were not heated gave polymer yields of about 95%. Also, air exposed control samples gave polymer yields around 5% effectively 0 regardless of whether they were heated or not. The most important conclusion from this data is that, under inert conditions, the catalyst remains active after 8 hour exposure at 177C.

Incorporate RWCl6 into High-Temperature Self-Healing Polymer Tried two high-temperature-cure polymer systems: Epon 828 with Epon Curing Agent W© (amine) Epon 828 with Methyl Tetrahydrophthalic Anhydride Problems with high-temp systems: GPMS mold breakdown after 1-2 cure cycles Specimens would have thin crack planes Density issues (WCl6 would settle during cure) Resin tough to degas Solution: Use Jason’s room-temperature cure system Epon 828 w/Curing agent 3046 Post-cure the TDCB specimens at high temperatures 121°C and 177°C The next step is to incorporate the catalyst into a high-temperature cure self-healing polymer. I initially tried two high temperature cure polymer systems using epon 828 and Curing Agent W, an amine, and Methyl Tetryhydrophthalic Anhydride. Here I ran into a ton of processing problems. The silicon rubber molds would break down after one or two cure cycles. Also, mold expansion would cause the specimens to have extremely thin crack planes. Pressed for time I abandoned high-temperature-cure polymer systems and decided make specimens with the room-temperature-cure system Jason was working with and subject them to a high-temperature-post-cure

Catalyst Embedded in Polymer TDCB Specimen Testing Catalyst Embedded in Polymer Create short pre crack with razor blade Test sample and record load-displacement Polymer

Self-Activated Results 12 wt% RWCl6 Samples Healed in Lab Atmosphere High -Temp Post-Cure Heal Ratio (Samples Healed/Total Samples) Average PH / PV None 0.63 0.71 4 Hours 121°C 0.52 0.38 4 Hours 177°C 0.15 Inconsistent healing results Polymer not fully cured after 48 hours Results not improving after increasing catalyst concentration to 15 wt% Most likely due to high lab humidity Solution…? The results from testing the specimens were extremely inconsistent. In the chart I am displaying 2 ratios. The Heal Ratio is the ratio of the number of samples which showed adequate healing (more than 5 N) to the number of samples tested. The next column displays the average peak load ratio for ONLY the samples that exhibited adequate healing. The polymer is not fully cured after 48 hours and the consistency of the results did not improve after increasing the catalyst concentration. Past experience suggests that high lab humidity is causing these anomalies so we decide to begin doing the virgin cracking and healing in the glove box.

Virgin Crack and Healing in N2 Glove Box Consistent healing data Exhibits high peak failure loads and non-linear healing Problem: We no longer have virgin fracture data how do we analyze the data? Proposed Solution: Compare healed data with a baseline of virgin data from similar samples (i.e. same catalyst weight% and post-cure). My proposed solution is to compare the healed data with a baseline average of virgin data from samples with similar manufacturing characteristics; meaning the same catalyst loading and post-cure.

Self-Activated Results, High-Temp Post Cure Weight % Catalyst Healing Conditions PV PH No Post Cure   12wt% Glove Box NA … Lab 114 ± 65 N 32 ± 24 N 15wt% 82 ± 22 N 180 ± 60 N 4.31 N 4 Hours @ 121°C 51 ± 8 N 75 ± 19 N 36 ± 15 N 68 ± 21 N 76 ± 8.8 N 11 ± 3.8 N 4 Hours @ 177°C No Healing 69 ± 13 N The results for the samples cured at 121C for 4 hours suggest that all the samples made a full recovery. The results from the non-post cured samples are rather anomalous because the baseline virgin fracture data for these, highlighted in red, consists of only two samples both of which had very high peak failure. But the bottom line is that the 100% of the samples, even the ones subjected to a high temperature post cure, made a good recovery.

Results and Conclusions Under inert conditions RWCl6 catalyst remains active even after 8 hours at 177°C Consistent results: Good non-linear healing up to at least 121°C No healing after 4 hours at 177°C Conclusion: Inconsistent results from healing in the lab atmosphere were caused by catalyst deactivation during the period between the virgin fracture and injection due to HIGH LAB HUMIDITY! A control sample injected in the glove box and placed on the lab bench to cure tested to a peak fracture load of 82.5 N The diffusion of moisture into the polymer matrix is slow enough that the polymer remains protected while it heals The conclusions we can draw from this is that the inconsistent results from the healing in the lab atmosphere were caused by catalyst deactivation during the period between the virgin fracture and injection due to high lab humidity. Also, since a sample injected in the glove box and healed out of the glove box exhibited excellent healing we can conclude that the diffusion of moisture into the polymer matrix is slow.

Future Work Determine why the catalyst deactivates in the polymer after a 4 hour post cure at 177°C Test in-situ samples in glove box after high temperature cure cycle Determine the humidity level at which the catalyst deactivates Further investigate the potential of polymer-coating the catalyst as a means of protecting it against deactivation In the future I would like to obtain more data from samples healed in the glove box including virgin data at 15wt% to iron out some of the statistical anomalies. I would also like to test the healing response in-situ samples after a high temperature postcure. I would like to determine the threshold of humidity exposure at which the catalyst deactivates as well as further investigate the potential of polymer-coated catalyst as a means of protecting it against deactivation.

Acknowledgements Jason Kamphaus Dr. Scott White NASA Illinois Space Grant Consortium

Experimental Calibration of 828/3046 Epoxy Calibration of new resin system performed for neat material and epoxy with 15 wt% capsules Compliance vs. crack length measurements were made using several specimens Modulus determined by DMA experiments (Eneat=2.6 GPa and E15%Caps=1.95 GPa) m values determined for each sample type (mneat=0.78 N-1 and m15%Caps=0.80 N-1) α values varied by less than 1% (αneat=12.8 x 103 m-3/2, α15%Caps=12.9 x 103 m-3/2)

Tungsten catalyst initiates healing mechanism Tungsten (VI) Chloride Phenyl Acetylene Exo-DCPD Polymer ROMP polymerization of exo-DCPD with WCl6 Phenyl acetylene reacts with tungsten to form an activated metal carbene