Hierarchical fractal‐structured allophanate‐derived network formation in bulk polyurethane synthesis

Abstract

Polyurethanes are among the most applied and researched polymers worldwide. Nevertheless, polyurethane synthesis is accompanied by a side-reaction occurring between isocyanate groups and the secondary nitrogen of already formed urethane groups, leading to the formation of crosslinking allophanates. This inevitably requires the development of highly diagnostic direct analytical methods that can be performed in the solid state of the polymer. The present research focused on the direct investigation and diagnostic determination of the chemical structure formation in bulk polyurethane synthesis, using a combination of Fourier transform infrared and solidstate 13 C nuclear magnetic resonance analysis. Polyurethane syntheses were performed in bulk and designed as to obtain significantly strong diagnostic analytical measurements signals for the accurate identification of each of the investigated chemical structures. The present research results led to the conclusive analytical identification of allophanate formation during polyurethane synthesis. In addition, the occurrence of a new reaction mechanism was discovered in the present research. It was demonstrated in the present research that this newly described reaction occurs via the further reaction of the allophanate secondary nitrogen with an isocyanate group, the reaction creating a tertiary nitrogen and an additional reactive secondary nitrogen, and so on, in a consecutive step progression, leading to the formation of a 3-dimensional hierarchical fractal-like crosslinked polymeric structure. Solid-state 13 C nuclear magnetic resonance analysis results were highly consistent with the Fourier transform infrared results. The discovery of this newly described reaction can facilitate the optimization of industrial processes and potentially opens a new door to the development of a vast variety of biomedical and nanotechnology applications.

Urethane Formation with an Excess of Isocyanate or Alcohol: Experimental and Ab Initio Study

“…These IR spectral data may be used to monitor the components that take part in the reaction [34], although assignment of these peaks can be difficult due to the multicomponent reaction mixtures, as well as the overlap amongst the IR peaks corresponding to similar functional groups (e.g., allophanate, biuret and urethane). Proper peak assignment for allophanates is still under debate [35].…”

Hydroxyl‐terminated polybutadiene based waterborne polyurethane acrylate emulsions: Synthesis, characterization, and damping property

“…Absorption intensity of CO group in line c of Figure 3 increased comparing to line a, b illustrated PA was synthesized successfully. The absorption peak for free NCO group at 2000–2300 cm −1 could not find from line a, b, and c demonstrated the completion of the reaction 30 . From line c in Figure 3, intensity of peak at 1640, 965, and 909 cm −1 decreased relative to line b indicated part of double bonds of the HTPB were involved in polymerization of acrylate.…”

“…The absorption peak for free NCO group at 2000-2300 cm −1 could not find from line a, b, and c demonstrated the completion of the reaction. 30 From line c in Figure 3, intensity of peak at 1640, 965, and 909 cm −1 decreased relative to line b indicated part of double bonds of the HTPB were involved in polymerization of acrylate. 3.3 | The water absorption rate of the WPUA films Figure 4 presented the water absorption of films.…”

Water Swelling Behavior of Poly(ethylene glycol)-Based Polyurethane Networks

“…The absence of NCO peaks in the ATR-FTIR spectra indicates a complete disappearance of NCO groups in the network. For the network formation conditions used ( T = 125 °C for 1 h) resulting in T g on the order of −40 °C, allophanate formation − (reaction of an NCO group from a tHDI molecule with the N–H group of a urethane connected to another tHDI entity; see SI-III) has taken place, as proven by swollen state 13 C NMR. The formation of such allophanates gives rise to effectively five-functional cross-linking junctions, which, apart from the overall decrease of network junctions upon combination of junctions, will influence the swelling properties of the networks and needs to be taken into account when calculating the M c .…”