Critical examination of chemically modified hybrid thermosets: Synthesis, characterization and mechanical behavior in the plateau regime of polyaminosiloxane-nitrile-DGEBA

Highlights

• Polyaminopropyl siloxane was modified with acrylonitrile via aza-Michael addition.

• Control on A1:A2 amine ratios allows tuning miscibility with epoxy resin and Tg.

• Two low temperature relaxations are linked by an isosbestic.

• Rubber modulus are the highest found for epoxy-amine systems.

Abstract

Poly(3-aminopropylmethylsiloxane) has been modified with acrylonitrile via aza Michael addition and a broad range of modified oligomers have been prepared with CN:NH₂ ratio ranging between 0.1 and 1. NMR and FTnIR analysis reveals that acrylonitrile modification proceeds without formation of tertiary amines. Modified oligomers have been characterized by DSC and analysis of Tg reveals that the adducts are self-associated probably due to weak hydrogen bonding and dipole interactions. The modified oligomers with a modification degree higher than 0.4 were miscible with DGEBA. The low and high temperature relaxations of the cured thermosets have been measured by DMTA. In addition to the commonly observed β₂ relaxation, a new β₁ process linked by an isosbestic point to β₂ has been found. β₁ is attributed to an extended segment comprising the pendant propionitrile group as well as the aminopropyl segment that connects tertiary amines to the polysiloxane backbone. Elastic modulus as well as the α relaxation can be tuned from high Tg and high rubbery modulus to low Tg and high damping thermosets changing the nitrile content. The experimental network structure obtained from elastic measurements and the Tg were related through well established structure-property relations.

Introduction

The structure and properties of epoxy/siloxane reactive blends is currently an open question in the field of advanced thermosetting materials. Both components are immiscible and phase segregation limits the morphology and phase composition, size and distribution. Several efforts to improve the compatibility by introducing reactive groups in the silicon-based component have been compiled in a recent review [1]. Common approaches include the functionalization of the inorganic siloxane backbone with reactive functional end-groups [2] (epoxy, amine, acrylate or isocyanate), the synthesis of siloxane block copolymers with a resin compatible segment [3] or the introduction of pendant reactive groups [4]. This last solution was adopted by our group in earlier reports on the synthesis of poly(3-aminopropylmethyl siloxane) (PAMS) and its curing with DGEBA based epoxy resins [5], [6].

Crosslinking agents for epoxy resins based on polyfunctional aminosiloxanes render very interesting polymer–polymer hybrid systems in which structure changes dramatically with both chemical conversion [5], [6] and polymerization protocol [7]. They belong to the class of hybrid thermosetting blends and present a number of distinctive features. Contrary to conventional thermosets, a phase separated system consisting of PAMS spherical domains surrounded by the continuous epoxy matrix is formed upon mixing both reactants. In spite of their thermodynamic incompatibility, both components begin to react and, depending on the specific epoxy precursor and curing protocol, homogeneous (at the micron scale) or partially homogeneous thermosetting polymers are formed [7], [8]. Another distinctive characteristic is related with their modification. They can be modified by physical mixing with conventional toughening agents, such as butadiene homo- and copolymers yielding a fine morphology [8]; but if a small amount of PMMA is used, a polymer with an extremely high affinity for the epoxy precursor, an aphron-like morphology is obtained in which the reactant mixture self organizes into polyhedral aphrons excluding the PMMA towards the interphase [9].

However, the most interesting feature is probably concerned with their high functionality which, at the same time, increases the Young's modulus in the rubbery state, increases hydrophobicity [10] and switches gelation towards lower conversion fixing the final morphology well before limiting conversion. This high functionality can be tuned reacting some of the pendant amino groups with molecules that may impart specific properties while the remaining provide enough crosslinking density to give high performance thermosets. An earlier successful attempt was done reacting some amino groups with 2,3-Epoxypropyl phenyl ether [11] aiming to delay gel conversion and to increase miscibility with DGEBA. However, the Tg and viscosity of the modified oligomers notably increased due to the bulkiness of the pendant group limiting the suitability of this approach.

In this work, we present a new alternative for modification of PAMS using the well-known aza-Michael addition reaction of aliphatic primary amines to acrylonitrile [12], which, as we will show, proceeds extremely well in this particular case yielding modified PAMS oligomers with highly polar cyano groups. Firstly we will present the synthesis and characterization of these adducts demonstrating the absence of double additions, i.e. the modified oligomers contain a controlled ratio between primary and secondary amines. Next, the adducts will be used to prepare a broad range of organo-inorganic thermosets differing in the amount of nitrile groups, i.e., different crosslinking densities, finding that for CN:NH₂ ratios higher than 0.4, homogeneous systems at the micron scale are formed. The paper then focuses on the analysis and critical interpretation of the low and high temperature relaxations and in their elastic behavior. The high T relaxations will be explained using the concentration of elastically active network chains obtained from elastic measurements.