The free radical copolymerization of nadic anhydride (NA) and styrene (St) at 80°C has been investigated for the first time. The copolymerization was done in toluene using benzoyl peroxide as initiator. The monomer reactivity ratios were calculated from the copolymer composition (estimated from the acid value) by using Fineman–Ross, Kelen–Tüdös, and Extended Kelen–Tüdös methods. The reactivity ratios (rNA, = 0.34, rST = 0.84) implied a good copolymerizability for the monomer pair and existence of an azeotropic composition at fNA = 0.195. The unsaturation in nadic anhydride exhibited good copolymerizability despite the fact that the double bond is isolated and allylic in nature. The usual retardation by the allylic protons was not significantly felt in this case, as the allylic protons were located at a bridge head posing geometric restrictions for stabilization of the allylic free radicals. However, an increase in concentration of NA in the feed decreased the overall rate of copolymerization. Based on the reactivity ratio, the probability for formation of sequences of NA in its copolymer with styrene was computed for a few compositions.
The present work focuses on the evolution in the mechanical properties of an unsaturated polyester resin (UPR) on blending with itaconimide‐end terminated polyethers, namely, polypropylene glycol (I‐PPG), polyethylene glycol (I‐PEG), and polytetra methylene oxide (I‐PTMO). Blends of an unsaturated polyester (UPR, based on propylene glycol, terephthalic, and maleic acids) resin with different loading of itaconimide end‐capped telechelics were investigated for their mechanical and thermal properties. Blending with these additives enhanced the mechanical and thermal properties of the crosslinked UPR. The impact strength and fracture toughness values were improved by more than 100% by small quantities of the additives. The improvement in fracture properties was correlatable primarily to a decrease in overall crosslink density. The distribution of the polyether chains in the cured matrix as dictated by the reactivity ratios of styrene and polyether macromer was found to have a role in deciding the properties. The properties were found to be the best for the blend toughened with I‐PPG with a molecular weight 2000 g/mole at a loading of 2.5 parts per hundred parts. On comparison with the resin blended with a maleimide‐encapped polyether of same molecular weight, the itaconimide end‐capped polyether was found to provide a better toughening of the UPR matrix. This could a priori be explained based on a difference in distribution of the end‐capped polyether as a consequence of the difference in the copolymerization behavior of itaconimide and maleimide functionalized telechelics toward UPR. The itaconimide enters into a random copolymerization with styrene and the probability for formation of the continuous sequences of the itaconic group is about 60%. This will permit polyether segments to come close enough to form micro or even sub‐micron clusters of the polyether which eventually forms the micro crystallites of poly ether that act as a crack stopper. This possibility cannot be envisaged in maleimides which forms invariably an alternating sequence with styrene. The morphological features as reflected in scanning electron microscopic analyses tallied with these observations. This work could help identify the ways for obviating the inherent brittleness of the UPR systems.
Segmented block copolymers from different grades of hydroxyl terminated liquid natural rubber (HTNR) (Mn 3000, 8800, 10,000, and 17,000) and polypropylene oxide (PPO) (Mn 1000, 2000, 3000, and 4000) have been synthesized and characterized by spectral analysis, thermal analysis, scanning electron microscopy (SEM), and mechanical testing. The glass transition temperature of NR block was found to be at about −64°C, which is independent of the PPO whose transition is around 15°C. The thermogravimetric analysis (TGA) shows that the thermal degradation of the samples proceeded in two steps characteristic of the immiscible components. The inability of PPO segments to provide physical crosslinking and the subsequent formation of hard domains is reflected in the low tensile properties and tear properties. The amorphous nature of the PPO phase and its immiscibility with NR phase are evidenced by the SEM studies. The effect of molecular weight of PPO as well as HTNR on the properties of the block copolymers has also been discussed. © 2006 Wiley Periodicals, Inc. JAppl Polym Sci 103: 909–916, 2007
Chain-end functionalized polybutadiene polymers have widespread application in composite solid propellants (CSP). Curing of these polymers is effected using the reactions at the terminal groups with isocyanates or aziridines if the functional groups are hydroxyl or carboxyl respectively. The high toxicity of isocyanates and aziridines demands alternate cure methods. A facile reaction, devoid of any side reactions is the most desirable one. The large number of double bonds in polybutadienes is favorable for 1, 3-dipolar addition reaction with an azide to yield triazolines. The mechanistic aspects of the uncatalyzed and copper-mediated azide-alkene reaction have not been explored previously. The present study focuses on elucidation of the reaction using model compounds of polybutadiene namely trans 3-hexene, cis-3 hexene and 3-methyl pentene which mimic the microstructure of polybutadienes. The paper presents the elucidation of the mechanism using density functional theory (DFT) calculations, detailed reaction pathway and its experimental validation using Fourier transform infrared (FTIR) spectroscopy and 13 C nuclear magnetic resonance (NMR) spectroscopy. DFT studies indicate that the activation barrier of 63.8 to 85 kJ/mol for the uncatalyzed reaction. In the copper catalyzed reaction, it diminishes to the range of 18.1 to 33.0 kJ/mol. The thermal decomposition aspects of the cured triazoline system were evaluated using thermogravimetric-mass spectrometer (TG-MS). The binder undergoes single stage decomposition in the temperature regime of 278 C-534 C which is lower than that reported for polyurethane-polybutadienes. The decomposition reaction yields more volatile products like nitrogen, carbon dioxide, 1,4 butadiene and 4-vinylcyclohexene, conducive for propellant applications.
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