Poly(ethylene sulfide). II. Thermal degradation and stabilization

Abstract

High molecular weight poly(ethylene sulfide) undergoes severe thermal degradation at the high temperatures (220–260°C) required for processing in injection‐molding equipment. Thermal degradation of the polymer is accompanied by gas evolution and a decrease in melt viscosity. Stabilization of poly(ethylene sulfide) can be effectively accomplished by addition of small concentrations of certain 1,2‐polyamines, preferably together with certain zinc salts as coadditives. Use of this stabilizer system inhibits thermal degradation to a remarkable extent, making it possible to mold the polymer at these high temperatures and obtain excellent physical and mechanical properties. Investigation of the thermal degradation process was carried out. The rate at which gases evolved from unstabilized poly(ethylene sulfide) resins of various molecular weights and preparative histories and from model compounds of the same organic backbone structure was measured at temperatures ranging from 220 to 260°C. Rate of gas evolution from the resins, irrespective of chain length or preparation, was found to be constant at 230°C. The evolved gases, analyzed by infrared spectroscopy and gas chromatography, contained ethylene. Nearly identical apparent activation energies were found for the gas evolution reaction from the resin and model compounds. The ΔE* values were in good agreement with ΔE* determined by other techniques, 58 ± 2 kcal/mole. This is about the energy requirement expected for the homolytic cleavage of a carbon–sulfur bond of the type present in a poly(ethylene sulfide) structure. The rate and analytical data indicate that the degradative mechanism at processing (molding) temperatures is primarily due to the organic structure of the polymer. A mechanism of thermal stabilization is proposed in which the polyamine and zinc salt, in presence of molten polymer at processing temperatures, form a two‐centered electron transfer complex, capable of reacting with both radicals of the homolytically cleaved bond, “healing” the scission, so to speak.

Influence of Chain Primary Structure and Topology (Branching) on Crystallization and Thermal Properties: The Case of Polysulfides

“…The crystallizable unit is closely related to PS, but devoid of the methyl group: ethylene sulfide (ES). The regularity of the ES repeating units makes its homopolymer, poly­(ethylene sulfide) (PES), highly crystalline, with a high melting point (i.e., 205–210 °C) , and very poor solubility (only in drastic conditions such as nitrobenzene, >170 °C).…”

“…24−26 In the present work, P(PS-co-ES) were successfully prepared by employing two different protocols of anionic ring-opening copolymerization that provide different primary structures: (1) protocol a, a one-shot addition of all monomers to the initiators, which leads to gradient (blocky) structures (due to the higher reactivity ratio of ES to PS 12 ); and (2) protocol b, repeated additions of the monomer mixture, which produce shorter gradients along the chain; this determines a local ES/ PS ratio that deviates less from that of the feed, and due to the presence of shorter homosequences of the crystallizable unit, the copolymers should exhibit a lower tendency to crystallize. Further, the copolymers obtained through the one-shot addition protocols were also produced in different chain topologies following procedures described in a recent paper: 13 linear macromolecules obtained from a bifunctional initiator (theoretically a 2-armed star topology) were compared to "real" branched polymers with a 4-and 8-armed star structure produced from multifunctional initiators, and to comb polymers (10,15, and 20 arms); the latter were obtained through the polymerization of propargyl episulfide, followed by click reaction with an azide-containing thioacetate, which was finally used to initiate the side-chain copolymerization of ES/ PS. In all cases, the degree of polymerization (DP) was also varied, using DP = 10, 20, or 30 per arm, which for linear polymers corresponds to a total DP of 20, 40, or 60 monomeric units The objectives of the paper are to study the influence of (a) the monomer addition protocol employed during the polymerization, which influences the distribution of comonomeric sequences/sequence lengths and (b) the chain topology (type and number of branches) on the morphology, nucleation, and crystallization of these complex P(PS-co-ES) copolymers.…”

Thermal transformations of organic compounds of divalent sulfur

“…The activation energy for this reaction corresponds to the energy of a homolytic cleavage of the C7S bond in macromolecular chains of polyethylene sulfide. 60 Thermolysis of bis(2-ethoxyethyl) sulfide, 1,1-bis(ethylthio)ethane and 1,3-bis(propylthio)propane involves the initial cleavage of the C7S bonds and occurs in accordance with reactions (1) ± (4). The dissociation energies of the C7S bonds in these compounds have been calculated.…”

Synthesis and characterization of saturated and unsaturated polysulfide polymers and their thermal degradation processes investigated by flash pyrolysis–gas chromatography/mass spectrometry

“…The incorporation of a sulfide linkage into a polymer backbone imparts increased solvent resistance and other properties 2. The insertion of sulfide groups into vinyl polymers, that is, polystyrene and polyethylene (PE), results in poly(styrene sulfide) and poly(ethylene sulfide), respectively, and their synthesis and thermal properties have been reported 3–7. Hydroxy‐terminated polybutadiene (HTPB) is another vinyl polymer (containing unsaturated units in the polymer backbone) and has been widely used in composite solid propellants; its thermal degradation products have also been studied 8–10.…”

“…The thermal degradation studies of the aforementioned saturated polysulfide polymers,3–7 other polysulfide polymers,12–31 saturated vinyl polymers such as PE and polypropylene (PP), and unsaturated vinyl polymers such as polyacetylene32–34 and HTPB8–10, 35 have already been reported in the literature 8–10, 35. The thermal degradation of saturated polysulfide polymers proceeds through intramolecular exchange or back‐biting reactions, forming cyclic and linear products and sometimes exclusively linear products 12.…”