16 Search Results

Many structure/property relationships of hydrolyzed poly(ester urethane) (PEU) – a thermoplastic – have been reported. Examples include changes in molecular weight vs. elongation at break and crosslink density vs. mechanical strength. However, the effect of molecular weight (or molar mass) reduction on some physical, thermal, and chemical properties of hydrolyzed PEU have not been reported. Therefore, a large set of hydrolyzed PEU (Estane®5703) samples were obtained from two aging experiments: 1) accelerated aging conducted under various environments (air, nitrogen, moisture) and at 64 °C and below for almost three years, and 2) natural aging conducted under ambient conditions for moremore » than three decades. The hydrolyzed samples were characterized via multi-detection gel permeation chromatography (GPC), thermogravimetric analysis (TGA), modulated differential scanning calorimetry (mDSC), UV–vis spectroscopy, nuclear magnetic resonance (NMR), and Fourier-transform infrared (FTIR) spectroscopy techniques. Hydrolysis of ester linkages in the soft-segments decreases both the molecular weight (M_w) and the melting point (T_m) of Estane (from ~55 °C to 39 °C). Aging above this T_m, increased mobility of polymer chains and water diffusivity in the PEU matrix alter the PEU degradation pathway from those expected at aging temperatures below this T_m and have significant bearing on the critical molecular weight (M_C) at which the physical, chemical, thermal, and mechanical properties of Estane change abruptly. While a M_C value of 20 kDa is found for PEU hydrolysis at mild temperatures (e.g., as low as 39 °C), the value of M_C increases with increasing aging temperatures. To complement the existing structure/property relationships reported in the literature, more correlations are obtained, which include the effect of M_w on polydispersity, intrinsic viscosity (Mark-Houwink equation), UV extinction coefficient, and dn/dc (GPC analysis) values. Furthermore, we seek to bolster previously reported aging models for PEU by developing a practical model with which the extent of degradation and material performance can be predicted based on aging under different temperature ranges both above and below the melting point of Estane.« less

The hydrolysis of thermoplastic poly(ester urethane) (PEU) is convoluted by its block copolymer phase structure and competing hydrolytic sensitivities of multiple functional groups. The exact pathways for water ingress, water interaction with the material and ultimately the kinetics and order of functional group hydrolysis remain to be refined. Additional diagnostics are needed to enable deeper insight and deconvolution of material changes. In combination with GPC results, a promising analytical technique – two-dimensional correlation spectroscopy (2D-COS) – has been reviewed and applied to analyze FTIR spectra of hydrolyzed PEUs aged under various conditions, such as exposure time, temperature, and relative humidity.more » 2D-COS allows the complex role of water with distinct intermediate steps to be established, plus it emphasizes the initial stages of PEU hydrolysis at more susceptible functional groups. As a complication for the raw material, ATR IR detected some talc on the surface of commercial PEU beads and pressed sheets thereof, which can interfere with water ingress and thereby retards PEU hydrolysis, particularly in its natural form or moderate aging at lower temperatures (e.g., below the melting point of PEU). As aging temperature increases above the melting temperature, even traces of water trapped inside the PEU are sufficient to initiate the hydrolysis, which then progresses strongly with increasing temperatures. Feedback from 2D-COS analysis confirms that PEU hydrolysis starts at esters in the soft-segments before those in the urethane linkage become susceptible. Only when the molecular weight of PEU is below a critical molar mass (M_c) will the hydrolysis occur in parallel in the hard-segments since protective morphological phase structures are then absent. The current observations demonstrate unexpected behavior that may result from 'unknown' additives in polymer degradation, the temporal and group-specific hydrolysis of PEU as a function of locally available water molecules, the order of reactivity of susceptible functional groups, and the importance of changes in molecular weight coupled with the phase structure of the polymer.« less

Understanding the degradation behavior of nitroplasticizer (NP) and the subsequent production of nitro-organics is crucial for both environmental monitoring and material development. A nontargeted approach via LC-QTOF-MS was employed to thoroughly study the degradation mechanism of NP in its late aging stage. Both positive and negative modes of ESI were performed to increase the compound coverage. To shed light on the fragmentation behavior of NP degradants (e.g., compounds containing a high density of NO₂ moieties and oxygen sites) in the positive mode, which is rarely reported, the high-resolution tandem MS information on precursor ions at m/z 251(+), 254(+), 266(+), andmore » 270(+) and a pair of isomeric ions at m/z 284(+) was investigated to extract their common diagnostic ions and dissociation channels, including the neutral loss of 2,2-dinitropropanol, nitro-nitrite rearrangement, homolytic cleavage of NO₂, and simple inductive cleavage. Additionally, leveraging the sensitivity for nitroaromatics in the negative polarity, negative ions m/z 182(–) and 233(–) are identified as dinitroaniline and dinitronaphthol, respectively, which confirm the secondary hydrolysis pathway of the antioxidant (e.g., N-phenyl-2-naphthylamine) postulated in our previous work. In addition to earlier findings, the detection of these eight degradants further supports the evidence of increased acid concentration and aging temperatures in the late-stage NP environment, which contribute to intricate degradation behaviors in different aging environments.« less

A critical review is given for our past 10 years of research on the thermal degradation of a eutectic mixture of bis(2,2-dinitropropyl) acetal and formal (BDNPA/F), known as nitroplasticizer (NP). Additional experimental and theoretical studies presented here thoroughly investigate the degradation mechanisms of NP as it thermally aged over several years. On the experimental front, a 44-month long experiment was conducted under various aging conditions. To further probe reaction pathways that initiate NP degradation, a 5-month aging experiment was also added. The aged NP samples were fully analyzed using FTIR and NMR spectroscopy, KF titration, TGA, LC-QTOF mass spectrometry, andmore » IC. These results show that NP degradation occurs in two major stages. In the low acidity early stage, HONO elimination followed by decomposition is a key degradation mechanism and leads to the formation of small molecules (e.g., NO, NO⁺, NO₂, NO₂⁺, H₂O, N₂O, NO₃^-, H₃O⁺, HNO_x) and NP residuals. While HONO, NOx and NO2+ are scavenged by N-phenyl-2-naphthylamine (PBNA) through an initial nitrosation followed by sequential nitration reactions, water accumulates and acidity increases. In the later stage, acid-catalyzed hydrolysis dominates to produce 2,2-dinitropropanol (DNPOH), organic acids, aldehydes, hydrolyzed products of PBNA, and other products. Theoretical calculations using density functional theory (DFT) performed on full molecules of BDNPA/F show that HONO elimination is more energetically favorable than NO₂ homolysis and NP hydrolysis and is the initial step in NP degradation under neutral environment and at moderate temperatures. DFT calculations also show that the energetics of NP hydrolysis strongly depend on the polarity of the local environment as well as the proton concentration. In-depth analyses of FTIR data and LC-QTOF data confirm the presence of NO₂⁺ and hydrolyzed products of PBNA in the proposed stages of NP aging. Finally, time-temperature superposition (TTS) analysis was applied to the concentration profiles of dinitro-/trinitro-PBNA and DNPOH species over aging time under various conditions; and the lifetimes of NP stability are predicted accordingly.« less

Antioxidants are additives and used for inhibiting oxidative degradation of many polymeric materials. Likewise, to maintain long-term stability of nitroplasticizer (NP) used in the formulation of a polymer-bonded energetic material, 0.1 wt.% of an arylamine antioxidant, n-phenyl-β-naphthylamine (PBNA) was added. Recently, the detection of PBNA nitration has brought forth the mechanistic complexity of antioxidant chemistry and created a path to understand how NP naturally degrades over time. Through a short-term aging experiment and with optimized LC-QTOF analysis, a reaction between PBNA and nitrous acid (HONO) that precedes the nitration event has been found, known as nitrosation. The identification of nitroso-PBNAmore » has solidified the HONO elimination mechanism in NP degradation and provided new insight on the role of this arylamine antioxidant. It also shows how stabilizer reactions with key intermediate degradation species can be a contributing process in polymer degradation.« less

In the thermal aging of nitroplasticizer (NP), the produced nitrous acid (HONO) can decompose into reactive nitro-oxide species and nitric acid (HNO₃). These volatile species are prone to cause cascaded deterioration of NP and give rise to various acidic constituents. To gain insight on the early stage of NP degradation, an adequate method for measuring changes in the concentrations of HONO, HNO₃, and related acidic species is imperative. The typical assessment of acidity in nonaqueous solutions (i.e., acid number) cannot differentiate acidic species and thus presents difficulty in the measurement of HONO and HNO₃ at a micromolar concentration level. Usingmore » liquid–liquid extraction and ion chromatography (IC), we developed a fast and unambiguous analytical method to accurately determine the concentration of HONO, HNO₃, acetic/formic acids, and oxalic acid in aged NP samples. Given by the overlay analysis results of liquid chromatography coupled with quadrupole time-of-flight mass spectrometry and IC, the prominent increase of produced HONO after the depletion of antioxidants is the primary cause of HNO₃ formation in the late stage of NP degradation, which results in the acid-catalyzed hydrolysis of NP into 2,2-dinitropropanol and acetic/formic acids. Our study has demonstrated that the aging temperature plays a crucial role in accelerating the formation and decomposition of HONO, which consequently increases the acidity of aged NP samples and hence accelerates the hydrolyzation of NP. Therefore, to prevent NP from undergoing rapid degradation, we suggest that the concentration of HNO₃ should be maintained below 1.35 mM and the temperature under 38 °C.« less

In the eutectic mixture of bis(2,2-dinitropropyl) acetal (BDNPA) and bis(2,2-dinitropropyl) formal (BDNPF), also known as nitroplasticizer (NP), n-phenyl-β-naphthylamine (PBNA), an antioxidant, is used to improve the long-term storage of NP. PBNA scavenges nitrogen oxides (e.g., NO_x radicals) that are evolved from NP decomposition, hence slowing down the degradation of NP. Yet, little is known about the associated chemical reaction between NP and PBNA. Herein, using liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF), we thoroughly characterize nitrated PBNA derivatives with up to five NO₂ moieties in terms of retention time, mass verification, fragmentation pattern, and correlation with NP degradation. The propagationmore » of PBNA nitration is found to depend on the temperature and acidity of the NP system and can be utilized as an indirect, yet reliable, means of determining the extent of NP degradation. At low temperatures (<55 °C), we find that the scavenging efficiency of PBNA is nullified when three NO₂ moieties are added to PBNA. Hence, the dinitro derivative can be used as a reliable indicator for the onset of hydrolytic NP degradation. At elevated temperatures (≥55 °C) and especially in the dry environment, the trace amount of water in the condensed NP (<700 ppm) is essentially removed, which accelerates the production of reactive species (e.g., HONO, HNO₃ and NO_x). In return, the increased acidity due to HNO₃ formation catalyzes the hydrolysis of NP and PBNA nitro derivatives into 2,2-dinitropropanol (DNPOH) and nitrophenol/dinitrophenol, respectively.« less

Eutectic bis(2,2-dinitropropyl) acetal - bis(2,2-dinitropropyl) formal mixture, nitroplasticizer (herein called NP) has historically been produced by either the ter Meer or oxidative nitration synthesis process, wherein 2,2-dinitropropanol (DNPOH) is produced as an intermediate step in both processes. Therefore that DNPOH, could be present in NP either as a production or hydrolysis degradation product is worth investigation. Here, we synthesized DNPOH, validated the synthesis using NMR, and identified DNPOH in aged NP using liquid chromatography tandem time of flight – quadrupole mass spectrometry (LC-QTOF). Using these results, for the first time we positively identify that DNPOH is absent from NP aftermore » its production, but is present as a degradation product through hydrolysis from a thermo-chemical aging profile of NP. To hydrolyze NP, prerequisite is the presence of both water and acid. Despite the presence of water in NP, DNPOH is only generated in the late stage of the aging process, when acid concentration is sufficiently high. It has been previously shown both theoretically and experimentally that a primary step of NP degradation is HONO elimination followed by decomposition, wherein nitric acid, nitrous acid, and water are produced (NP→NP’+2HONO and 2HONO→NO+NO₂+H₂O). It is shown from this reaction series that water slows HONO decomposition and therefore in small quantities, 100 s of ppm, water actually stabilizes NP against hydrolysis by equilibrium and reducing acidity.« less

The effect of water concentration on the aging behavior of blend components in plastic bonded explosive (PBX) 9501 is investigated when samples were aged up to 24 months under various conditions. Additionally, the blend components studied here are: poly(urethane ester) (Estane®5703) (Estane), nitroplasticizer (NP), and antioxidant Irganox 1010 (Irg1010). The experimental results reveal that NP is prone to thermally degrading and producing H₂O, NO_x, and HNO_x species, which are the predominant species to consume Irg1010 during PBX 9501 aging under inert environment. As Irg1010 is completely consumed, Estane degrades through oxidation and NP addition, in addition to well anticipated hydrolysis.more » The competition among hydrolysis, oxidation, and NP addition results in non-monotonical changes in the molecular weight of Estane over the aging process.« less

To study the thermal stability of ethylene/vinyl acetate/vinyl alcohol (EVA-OH) composite with filler particles, systematic thermogravimetric analysis (TGA) investigation was conducted in both nonisothermal and isothermal modes. The effect of polymer concentration on the aging behavior of EVA-OH was investigated in the nitroplasticizer (NP) environment. Fourier transform infrared spectroscopy was used to probe chemical structural changes in the EVA-OH polymer before and after thermal aging. The results suggest that filler addition accelerates the rate of NP exudation out of the EVA-OH composites and deteriorates the thermal stability of NP at moderate temperatures (up to 70°C). The degradation of NP, inmore » turn, accelerates the degradation of EVA-OH polymer in its composite form through oxidation and hydrolysis indicating the importance of antioxidants in such phase blends.« less