N-Sulfonyl Urethanes to Design Polyurethane Networks with Temperature-Controlled Dynamicity

Abstract

Metrics & MoreArticle Recommendations * sı Supporting Information ABSTRACT: (Re)processing of cross-linked polyurethanes (PUs) is often energy intensive and inefficient since dissociation of urethane linkages at elevated temperatures generates highly reactive isocyanate moieties that can react with a wide range of nucleophiles. In this study, we first show with a small molecule study that the introduction of N-sulfonyl urethane bonds leads to dynamic covalent exchange reactions under much milder conditions compared to regular urethane groups. Then, these exchangeable N-sulfonyl urethane motifs have been introduced, in relatively small amounts (5, 10, and 20%), in a cross-linked PU matrix in an attempt to facilitate plastic flow at lower temperatures. Rheological analysis of the elastomeric dissociative networks revealed an interesting double relaxation behavior, even for temperatures between 150 and 100 °C, which could be described by a Maxwell model with two elements, which can be related to the activated and less activated urethane bonds. Finally, the (re)processability of these sulfonyl urethanes containing PUs was demonstrated through multiple cutting and hot pressing cycles and the corresponding materials showed a good retention of thermal properties.

Covalent Adaptable Networks through Dynamic N,S-Acetal Chemistry: Toward Recyclable CO2-Based Thermosets

“…However, most studies on CANs revealed that single-element Maxwell or stretched exponential models were generally accurately fitting the relaxation experiment data . On the other side, multielement Maxwell could be effectively used to describe materials with kinetically different exchange processes. , At each temperature, three characteristic relaxation times (τ 1 , τ 2 , and τ 3 ) were extracted. τ 1 and τ 2 showed to be independent from temperature changes and were both of relatively short time scale (0.1–0.7 s for τ 1 and 0.7–15 s for τ 2 ) (Figures S40–S42; Table S3).…”

Trialkylsulfonium-Based Reprocessable Polyurethane Thermosets

“…It also provides the material with enhanced mechanical strength through intermolecular interactions (such as H-bonding). , The other elastomeric phase is predominantly constituted by a low- T g amorphous polymer caused by a Desmodur 3600N and gives a transition at lower temperatures in cooling curves. Interestingly, based on the chemistry and ratio of the monomers involved, (micro)­phase separation can be minimized. , Also, it is important to note that urethane linkages themselves are well-known to exhibit some dynamic covalent behavior at elevated temperatures (typically >150 °C), even in the absence of catalysts. ,, Leading to regenerated isocyanates that can participate in multiple side reactions and alcohol moieties that can be irreversibly alkylated at elevated temperatures, thereby deactivating dynamic exchange. For this reason, although aliphatic urethane linkages are considered more stable than their aromatic counterparts, reprocessing of the PUTS-materials has been carried out at 140 °C.…”

“…43,46 Also, it is important to note that urethane linkages themselves are well-known to exhibit some dynamic covalent behavior at elevated temperatures (typically >150 °C), even in the absence of catalysts. 39,47,48 Leading to regenerated isocyanates that can participate in multiple side reactions and alcohol moieties that can be irreversibly alkylated at elevated temperatures, thereby deactivating dynamic exchange. For this reason, although aliphatic urethane linkages are considered more stable than their aromatic counterparts, reprocessing of the PUTS-materials has been carried out at 140 °C.…”

Reprocessable Polyurethane Foams Using Acetoacetyl-Formed Amides

“…In 2010, the concept of covalent adaptable networks (CANs) was introduced to the scientific community by Bowman and Kloxin. − Unlike traditional thermosets, CANs can be reshaped or repaired under certain external stimuli (e.g., heat and light) owing to the presence of reversible or exchangeable bonds. , CANs are classified into associative or dissociative types according to the dynamic chemistry used. , Associative CANs make use of dynamic chemistries for which the bond-breaking and bond-making processes happen simultaneously (e.g., S N 2 reactions), leading to a constant cross-link density over different processing temperatures. , On the other hand, dissociative CANs contain dynamic chemistries for which those two processes are happening in a consecutive way (e.g., Diels–Alder reactions). , Numerous dynamic chemistries have been investigated to prepare CANs such as disulfides, siloxanes, vinylogous urethanes, and many more. − …”