Influence of crosslinking and plasticizing on the viscoelasticity of highly filled elastomers

Abstract

International audienceSolid propellants, like all highly filled elastomers, exhibit a complex nonlinear viscoelastic behavior. The aim of this study was to establish the relationships between the structure and properties, which is needed to construct a robust constitutive law for these materials. An extensive design of experiments approach allowed us to quantify the influence of the curing agents and plasticizer molecules on the microstructure of the propellant and its viscoelastic properties. Swelling and gel permeation chromatography measurements described the microstructure of the propellant and prestrained dynamic mechanical analysis (PDMA) characterized the viscoelastic behavior. The curing agents reacted with polymer chain ends participating in the network, in the sol fraction, or in filler–binder links. Consequently, the polymer network was incomplete even in stoichiometric conditions, and a minimum of 10% of the polymer was free in the microstructure. In addition, preswelling the polymer with plasticizer molecules before curing modified the obtained network by decreasing the crosslink density in the binder and increasing it in the vicinity of the filler surface. This study provided new insight into the local deformation mechanisms controlling nonlinearity as measured by PDMA. The nonlinear behavior appeared between 0 and 1.7% prestrain in both the elastic and viscous parts of the behavior. The network reached its maximum extensibility in the elastic part and constrained the sol fraction in this extended mesh for the viscous part. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40392

Microstructure and deformation mechanisms of a solid propellant using 1 H NMR spectroscopy

“…The propellants numbered 0 to 21 (Table 1) correspond to a design of experiment (DoE) allowing us to determine the influence of four composition factors (the filler fraction, the FBBA, the NCO/OH ratio, and the plasticizer content) on the properties of an HTPB-based solid propellant. The principles of building the DoE have been detailed elsewhere [1,6].…”

“…The relations between the nonlinear mechanical behavior at the macroscopic scale and the microstructure at the scale of the fillers or the polymer chains have been explored [1][2][3][4][5][6]. However, direct experimental evidence of the physical mechanisms of deformation at the scale of the polymer network is difficult to obtain.…”

“…As a consequence, the transversal or spin-spin relaxation time T 2 of the spins varies according to the mobility of the molecular segment on which they are situated. Therefore, the 1 H NMR T 2 relaxometry enables the measure of segmental mobility [9]. The measurement commonly results in the continuous spectrum of relaxation times for the protons in the material.…”

“…Consequently, the binder contains a significant amount of sol fraction, that is the fraction of the binder that can be extracted by swelling. The sol fraction is constituted of polymer chains and plasticizer and participate actively in the nonlinearity of the mechanical behavior [1][2][3]. Schematically, the propellant is composed of the fillers, the polymer network, and the sol fraction.…”

Plasticizer effect on network structure and hydrolytic degradation

“…Dw TA ¼ 4 K for the (50/50) sample and 9.3 K for the (25/75) sample, suggesting these extracted networks contain some structural irregularities that increase the polymer mobility. (5) As already addressed in the literature for similar systems [25], the comparison of the moduli for polyester-urethane P, P+TA 50/50 extracted , and P+TA 25/75 extracted suggests that the presence of a plasticizer during polymerization decreases the crosslink density. (6) According to Table 1, plasticization induces an increase of the glassy modulus.…”