Cure kinetics of glycidyl azide polymer (GAP) with propargyl esters, bis-propargyl ether, and 4,4'-dicyanohepta-l,6-diyne, a possible energetic curing agent for azide polymers

摘要

Dipolarophiles related to bis-propargyl esters and ethers are known as alternative curing agents to isocyanates for hydroxyl-terminated azide polymers: they are less ecotoxic and allow a curing reaction to take place by 1,3-dipolar cycloaddition in the presence of moisture without undesirable side-effects [1]. However, the cycloaddition reaction of dipolarophiles with energetic azide polymers involves higher heat generation than with isocyanates, creating a potential hazard due to the possibility of thermal runaway and degradation of the polymer if the resulting heat cannot be adequately dissipated from the system [2]. To better understand this reaction and form accurate predictions towards the scale-up of this process with various dipolarophiles, the curing kinetics of glycidyl azide polymers (GAP) through 1,3-dipolar cycloadditon were studied via differential scanning calorimetry (DSC) for four compounds: two propargyl esters (bis-propargyl oxalate, BPO and bis-propargyl malonate, BPM), one ether (bis-propargyl ether, BPE), and one dialkyne (4,4'-dicyanohepta-l,6-diyne, DCHD), all of which were synthesized and characterized. The effects of higher alkyne-to-azide-ratios (C7CH/N3) and of the chemical composition of the dipolarophiles on the reaction kinetics were studied and it was found that the Kissinger activation energy (Ea) of the curing reaction for the oxalate ester showed an increase of up to 32% (68.0 kJ/mol to 89.9 kJ/mol) for mixtures of higher C7CH/N3 ratios while the other esters were less affected over the same range (BPM 72-80 kJ/mol, BPS 71-83 kJ/mol). Moreover, the presence of two strong electron-withdrawing nitrile groups in the structure of DCHD did not lead to a remarkably high reactivity (Ea = 73-80 kJ/mol) and the most consistently reactive compound was found to be BPE (70-73 kJ/mol).