(32c) Reprocessable Polyhydroxyurethane Networks Reinforced with Polyhedral Oligomeric Silsesquioxanes (POSS)

Thermosets have many advantages over thermoplastics including improved heat stability, solvent resistance, and enhanced physical properties. However, the recycling of thermosets remains a key challenge because conventional, permanently cross-linked thermosets cannot be melt-reprocessed or reshaped. In recent years, covalent adaptable networks (CANs) have been developed to address this problem. CANs contain dynamic linkages or structures which allow the materials to undergo network reconfiguration and to achieve recyclability under appropriate conditions. Polyhydroxyurethane (PHU), which is synthesized from aminolysis reaction of cyclic carbonates, is an environmentally friendly alternative to conventional isocyanate-based polyurethanes. In recent years, many studies have explored the inherent dynamic nature of cross-linked PHU and its effectiveness in developing CANs. It has been found that PHU networks derived from five-membered cyclic carbonates rearrange via both transcarbamoylation exchange chemistry and reversible cyclic carbonate aminolysis reaction. In addition, with appropriate catalysis, such PHU networks can undergo multiple reprocessing steps with full property recovery associated with cross-link density.

Fabrication of polymer composites by filler incorporation is a common strategy to enhance the performance of polymeric materials. This approach has been applied to CANs to enhance their mechanical properties. However, when CANs are used as the composite matrix, the restricted chain mobility induced by the adhesion of polymer chains onto the filler surface can impede topological rearrangement which is essential for relaxation-related properties such as reprocessability. Under such circumstances, surface-modified nanofillers with relatively small surface-to-volume ratios are desirable to balance the stress relaxation and mechanical property enhancement of dynamic network composites. Polyhedral oligomeric silsesquioxanes (POSS) comprise cage-like organosilicon core and external functional groups attached to each silicon atom. Since the average POSS core diameter is only ~0.5 nm, POSS are considered to be the smallest possible particles of silica. POSS are important building blocks to obtain organic–inorganic nanocomposites with enhanced properties because the reactive functional groups attached to the core provide a unique advantage for POSS molecules to be covalently bonded to the matrix and thus prevent filler aggregation.

In this study, we have developed reprocessable POSS-reinforced PHU network nanocomposites. The PHU matrix is synthesized by reacting difunctional polyetheramine JEFFAMINE D-400 with tris(4-hydroxyphenyl)methane tricarbonate. Cyclic-carbonate-terminated POSS (POSS-CC) was employed as covalently attached nanofillers and was incorporated into neat PHU matrix at 0, 5, and 10 wt% loading. 4-Dimethylaminopyridine catalyst is added into the reaction mixture to facilitate the dynamic chemistries during the reprocessing process. Reprocessing of neat PHU networks and PHU−POSS network composites was performed by cutting the materials into small pieces and compressing them into films using a high-temperature compression mold.

The effect of covalent incorporation of POSS on thermomechanical properties, reprocessability, and stress relaxation of dynamic PHU networks was investigated by various characterization techniques. Thermogravimetric analysis results indicate that the decomposition temperature (T_d) corresponding to a 5 % weight loss for PHU−POSS network composites increases with increasing POSS loading, which suggests that the thermal stability of PHU networks is enhanced by incorporating POSS as nanofillers. The recovery of thermomechanical properties of PHU−POSS network composites after each reprocessing step was characterized by dynamic mechanical analysis (DMA). According to the DMA results, PHU−POSS network composites exhibit significantly enhanced storage modulus at the rubbery plateau region compared to neat PHU networks. With up to 10 wt% POSS loading, PHU−POSS network composites can undergo at least two reprocessing cycles with 100% property recovery associated with cross-link density. These results suggest that the incorporation of POSS as nanofillers is an effective approach to enhance the mechanical properties while retaining the excellent reprocessability of dynamic PHU networks.

The ability to relax external stress under appropriate conditions is a defining characteristic of dynamic polymer networks. We have characterized the effect of POSS incorporation on stress relaxation of dynamic PHU networks by DMA. Stress relaxation tests at 140−170 °C were conducted under a strain of 5%. Regardless of the loading size of POSS, at all tested temperatures, PHU−POSS composites relax significantly more slowly than neat PHU networks. However, with increasing temperature, the difference in stress relaxation rate between neat PHU networks and PHU−POSS network composites is diminished, possibly because at high enough temperatures, the stress relaxation process is achieved predominantly by dissociative reverse cyclic carbonate aminolysis reaction, so all systems are liquid-like and equally flowable. Kohlrausch−Williams−Watts fitting result of the stress relaxation data indicates that with higher POSS loading, the distribution of relaxation times becomes broader, and the relaxation mode becomes more complex.

In summary, taking advantage of the inherent dynamic nature of PHU networks, we have developed reprocessable PHU−POSS network nanocomposites and investigated the impact of POSS molecules as reinforcing fillers on thermomechanical properties, reprocessability, and stress relaxation of dynamic PHU networks. PHU−POSS network composites exhibit improved thermal stability and significantly enhanced rubbery plateau modulus compared to neat PHU networks. Although the incorporated POSS molecules retard the stress relaxation process of the neat PHU matrix, with up to 10 wt% POSS loading, PHU−POSS network composites still exhibit excellent reprocessability and can undergo at least two reprocessing cycles with 100% recovery of cross-link density. This study highlights the advantages and effectiveness of POSS molecules for fabricating high-performance dynamic network composites with excellent reprocessability.