Thermosets are chemicals made up of multiple subunits that are covalently bonded to one another (cross-linked). The many bonds make a material tough. Thermosets are important in the production of polymers, such as rubber or plastics. But, because thermosets are so sturdy, they are not degradable or recyclable. The authors use thermoset polydicyclopentadiene (pDCPD) for this study. They find that within a subunit or comonomer, there can be bonds that can be broken. If this is the case, the thermoset can be degraded in a controlled manner when the comonomer is added to the polymer. The material still remains sturdy. However, making the cross-link cleavable does not make thermostats degradable. Thus, it is possible to create a recyclable material by engineering which bonds can be cleaved.
The authors aim to present a way to make thermosets degradable.
The researchers created a reverse gel-point model to predict how much comonomers are necessary to cleave bonds and cause degradation. The model depends on the number of crosslinks and functional groups that can be crosslinked. The model indicates that the polymer can be degraded when the number of cleavable bonds is similar to the number of crosslinks. However, the model is only an estimate since many assumptions are made. For example, the model assumes that all bonds can react equally and that cleavable bonds are distributed randomly. Intramolecular reactions are not considered.
Cleavable bond location controls degradability
The researchers used silyl ether monomers (iPrSi) as the comonomers. These monomers can make larger structures with norbornene derivatives. The copolymerization is ring-opening metathesis polymerization (ROMP). They applied iPrSi in the context of DCPD. The norbornene parts of pDCPD successfully copolymerized with iPrSi. The resulting polymer has strands with cyclopentene side chains as possible crosslinking sites. The new silyl ether linkages can be cleaved. The researchers can predict where the cleavages occurred. iPrSi is also inexpensive to make. The researchers also incorporated additional crosslinks with silyl ether linkage (SiXL). Based on the model, the researchers predict the additional crosslinks will make the polymer harder to degrade (Figure 1).
The researchers had three polymers to test: pDCPD, pDCPD with iPrSi, and pDCPD with SiXL. They exposed each of the groups to tetrabutylammonium fluoride (TBAF), which breaks silyl ether bonds. The normal pDCPD nor the pDCPD with SiXL did not grade at all. The pDCPD with iPrSi degraded after four hours. The researchers used shear rheology, another method of cleavage. This experiment confirmed the results of the TBAF experiment. This means that the silyl ether linkages are more important in thermoset degradation than the crosslinks.
Functional evaluation of degradable pDCPD
In pDCPD with 10% or 20% iPrSi-doped pDCPD, the polymer had the same strength as pDCPD alone. However, the strength of the polymer decreased at 33% or 50% iPrSi-doped pDCPD. This was confirmed in several tests. They calculated Young’s modulus, which tells how stiff a material is. 10% iPrSi-doped pDCPD also had the same efficacy as normal pDCPD at stopping small projectiles in a ballistics test.
The researchers find that hydrofluoric acid can dissolve iPrSi-doped pDCPD. However, extremely acid or basic substances cannot dissolve iPrSi-doped pDCPD because it is so hydrophobic. However, pDCPD can be doped with 10% EtSi to better hydrolyze in solutions. This shows that researchers can adjust the materials to control the degradation rates.
The researchers exposed iPrSi and EtSi doped pDCPD to synthetic seawater and lasers. They wanted to determine if plastic waste may be released into the environment due to degradation of these thermosets. When the pDCPD was degraded, the particles were under 5 nm. This is smaller than microplastics, which are usually on the millimeter or micrometer scale. This suggests that this degradation process may be ideal since it will not be toxic to the environment.
Characterization and reprocessing of iPrSi-doped pDCPD degradation products
Using various spectroscopy methods, the researchers confirmed that the cyclopentene functional groups are what allow for cleavage. These cleavages occur through intramolecular reactions. The cleavage reaction also supports that the polymer formation is through secondary metathesis reactions, which is when various ions, radicals, or atoms are exchanged. The more iPrSi that is put in pDCPD, the smaller the size of the degraded product.
Any unreacted parts of the crosslinked polymer can be recycled to create new pDCPD derivatives. The researchers combined degraded products from 10% iPrSi-doped pDCPD. The new product had the same strength as normal pDCPD. Moreover, pDCPD recycling may apply to carbon fibre recovery. Carbon fibre is usually difficult to recover if mixed with pDCPD. However, if the researchers put together 10% iPrSi-doped pDCPD with carbon fibre, the carbon fibre can be recovered. This process does not affect the makeup of the carbon fibre.
The researchers used the chemical process of curing to create the polymers. They shot steel microparticles of various sizes and speeds during the ballistics experiment. NMR spectroscopy was primarily used to analyze the functional groups in the polymers.
Thermosets can be engineered to be degradable and recyclable. Silyl ether linkages within the monomer rather than the cross-links allow for cleavage. The findings may apply to other polymers to make them degradable.