Relationships between thermally induced residual stresses and architecture of epoxy-amine model networks

Abstract

ABSTRACT:The capability of epoxy-amine resins to develop residual stresses was studied as a function of temperature and network architecture. These residual stresses were induced while cooling epoxy-glass bilayers from temperatures higher than the network glass transition temperature, T g . This behavior was the result of the marked differences (␣ r Ϫ ␣ g ), in linear thermal expansion coefficient of the two components, as evidenced by the measurement of ␣ r for the epoxy networks under study. Various network architectures were selected, resulting from variation of (1) the chemical nature of both epoxide and curing agent, (2) the nature and relative amount of the chain-extensor agent, and (3) the stoichiometric ratio. Three ranges of cooling temperature were observed systematically: first, the range of temperatures above T g , where no stress has been detected, then an intermediate temperature range (from T g to T*), where stresses develop quite slowly, and finally, the low temperature range (T Ͻ T*), where a linear increase in stress accompanies the decrease of temperature. The two latter regimes were quantitatively characterized by the extent, T g Ϫ T*, of the first one and by the slope, SDR, of the second one. T g Ϫ T* values were shown to be governed by the T g of the network: the higher the T g , the larger the gap between T g and T*. This result was interpreted by accounting for the variation of relaxation rate at T g from one network to the other. It was also shown that a semiempirical relationship holds between SDR and T g : SDR decreases monotonically as T g increases. By inspecting the effects of network architecture in more details, it turned out that SDR is governed by the Young's moduli, E r (T Ϫ T g ), of the epoxy resins in the glassy state: the lower E r (T Ϫ T g ), the lower SDR in a series of homologous networks. As E r (T Ϫ T g ) values are known to be related to the characteristics of the secondary relaxation ␤, which depends, in turn, on crosslink density, SDR values were finally connected to the amplitude of the ␤ relaxation processes. This finding was corroborated by the measurements on an antiplasticized dense network. Finally, data relative to thermoplastic-filled networks showed that the addition of thermoplastic reduces the development of residual stresses, whatever the system, is homogeneous or biphasic.

Structure–Property Relationships in Epoxy-Amine Networks of Well-Controlled Architecture

“…In contrast with the other quantities discussed in this paper, the thermal expansion coefficient in the glassy state is just slightly affected (with the exception of R-F /2.92) by the architectural characteristics of the networks (table 2). By the way, it is worth noting that the values of α r above T g , not discussed here, are much higher [5] and are architecture-dependent, in agreement with the reported dependence of the viscoelastic coefficients C…”

“…The procedure, detailed in a recent paper [5], was derived from that used by Schirrer and co-workers in an earlier work [12]. Experiments were performed by using an MTS 810 servohydraulic testing system, equipped with a temperature chamber covering the range from −120 • C to 250 • C and with high-sensitivity load cell and displacement gauge.…”

“…If difunctional epoxides are involved, the side reaction of formation of interchain ether linkages by further opening of epoxide rings by the hydroxypropyl units is not detectable with the available sensitivity of 13 C NMR and FTIR measurements [1]. Therefore, this situation offers a good opportunity to design and prepare series of epoxy networks of controlled architecture [2][3][4][5]. The rigidity of dense (stoichiometric) networks can be varied (figure 1) by choosing monomers of appropriate molecular structure.…”

Highly compact co-poly(amide-imide)s from polycondensation of an imide-modified derivative of l-aspartic acid

“…Similar confirming results were achieved from variation curves of the modulus and tan δ with temperature. A considerable point in the modulus thermograph is a slowly decreasing change in storage modulus in the glass transition temperature, representing a tentative conclusion about the nature of the matrix as a compact and amorphous structure with a small free volume (18,19). The thermo-analytical data for the treated polymers are summarized in Table 2.…”

The Analysis of Photocuring Behavior of Polyester Acrylate by Means of Differential Photo Calorimeter (DPC) and Rigid-Body Pendulum Rheometer (RPT)

“…Due to the fact that the reaction changes by means of different mixtures of oligomer polyol, monomer, photoinitiator, illuminant, and operating environment [7] ; DPC [8][9][10][11][12] , IR [13][14][15][16][17][18][19][20] , and RPT [21] have been adopted to show the behavior of the photopolymer. On the other hand, the reaction of reverting the rate of photocuring polyester is controlled by kinetics and dynamics theory, and the relationship with the dynamics rate represents the concentration of photocuring resin and photoinitiator, collision, and effective collision rate [22] this kind of molecules have.…”