Mechanical relaxation mechanism of epoxide resins cured with acid anhydrides. II. Effect of the chemical structure of the anhydrides on the β relaxation mechanism

The mechanism of low-temperature mechanical relaxation of acid-anhydride-cured epoxide resins has been investigated in detail. One mechanical relaxation, denoted as the β relaxation, is observed from ?80 to ?50°C for all epoxide resin systems cured with aromatic, alicyclic, and aliphatic anhydrides. The β relaxation increases in peak height and shifts to higher temperature with increasing molecular volume of the diester segments formed by the reaction of acid anhydrides. From these results, it is concluded that the β relaxation for anhydridecured systems is due to the motion of the diester segment, and that the intensity and peak position of the β relaxation depend on the molecular volume of this segment. Moreover, it was shown that the tensile impact strength of the anhydride-cured systems is governed by the intensity of the β relaxation of these systems when the parameters T_g and v of these systems are nearly constant.     

Relaxation mechanism of epoxide resin cured with acid anhydrides. III. Effect of alkyl side chains on mechanical and dielectric β relaxations

Mechanical and dielectric β relaxations were investigated for bisphenol-A-type epoxide resin cured with succinic anhydrides with and without substituent linear or branched alkyl side chains. The dielectric β relaxation is due solely to the motion of the diester segment which is formed in the network from the reaction with the acid anhydride. The mechanical β relaxation is attributed to the motion of both the diester segment and another nonpolar segment, i.e., the linear alkyl side chain. Thus it is concluded that the long alkyl side chain in the diester segment not only restricts the motion of that segment, but also introduces additional motion of the side chain itself.     

Mechanical relaxation mechanism of epoxide resins cured with aliphatic diamines

The mechanism of low-temperature relaxations in bisphenol-A-type epoxide resins cured with aliphatic diamines, with aliphatic diamines in the presence of salicylic acid as an accelerator, and with tertiary amines was investigated to compare the dynamic mechanical properties and the chemical structure of these networks. Mechanical relaxations are observed at about ?140 and ?60°C. The former relaxation is denoted the γ relaxation and the latter the β relaxation. The β relaxation of the cured epoxide resins containing hydroxyether groups is a sum of contributions from the relaxation of these groups and of other parts of the network structure. A new relaxation due only to the motion of the hydroxyether group can be estimated from the difference of tanδ curves between the aminecrosslinked and ether-crosslinked systems. The γ relaxation is attributed to the motion of a polymethylene sequence consisting of at least four carbon atoms.     

Mechanical relaxations in episulfide network polymers

A variety of condensation network polymers have been prepared by the reaction between amine, episulfide, and epoxide monomers. The mechanical relaxations occurring in these systems have been examined using a torsion pendulum and the role of hydrogen bonding in the mechanism of the β relaxation is shown to be insignificant. The chemical reaction between amine and episulfide groups has been investigated by IR spectroscopy and is shown to parallel the reaction between amine and epoxide groups. However, steric and electronic factors are suggested to decrease the extent of reaction when aromatic amines are involved. In the case of networks prepared from blends of episulfide and epoxide monomers, measurements of the gel time, together with the mechanical behavior around the glass transition, indicate that either interpenetrating or two-phase networks are formed. This is postulated to be a consequence of the high reactivity of the episulfide ring compared to the epoxide ring. The blending of small amounts of episulfide monomer with the epoxide monomer prior to curing may provide an effective method for lowering gel times without reducing the crosslink density and its dependent physical properties.     

The β relaxation behavior of bisphenol-type resins

The β relaxation of a phenoxy resin and a number of related uncured and cured bisphenol-type epoxide resins was studied using a torsion pendulum. Two molecular mechanisms are proposed to contribute to the β relaxation in the range from ?55 to ?80°C. One is the motion of the hydroxyether group and the other is related to motions of the crosslink itself. The β-relaxation peak temperature was found to increase with the concentration of hydroxyether groups in the matrix. Motion of the crosslinks in the matrix gives rise to a contribution to the β peak at ?80°C. In addition, motion of the diphenyl propane group is proposed to result in a relaxation process at ca. ?110°C.     

A mechanistic study of the reaction between furfural-acetone resins and polyamines

We have studied the reactions of furfural-acetone resins and their main components, i.e., monoand difurfurylidene acetones, with aliphatic polyamines and organic acid anhydrides. It is shown that furfural-acetone resins can be hardened under the action of polyamines and do not react with acid anhydrides. probable mechanism of the reaction with polyamines is proposed.     

The β relaxation in the dynamic mechanical spectrum of polyester networks containing various structural units

The β relaxation has been observed in the dynamic mechanical spectra of a range of styrene-unsaturated polyester networks. The influences on the β relaxation of the polyester molecular weight, the nature of the diol and saturated dioic acid units and the presence of water were investigated. From these studies, the relaxation is shown to arise from the motion of the saturated dioic acid-diol grouping in the polyester segments, and appears to be closely connected with the glass transition of the uncrosslinked polyester. Studies of the influence of the styrene concentration in the network suggest that the bulky styrene units may restrict the motion of some of the potential relaxation sites.     

Effects of tertiary amine accelerators on curing of epoxide resins

The effects of seven tertiary amine accelerators on curing of bisphenol-type epoxide resins using azelaic acid as a curing agent have been investigated. The structure of the cured resins is characterized and reaction and structure schemes are proposed. The reaction mechanism and the resulting structure of the resin depend on the basicity of the accelerator. With increasing accelerator basicity crosslinking in the cured resin increases. Characterization results indicate that the network structure consists of ether bonds or a mixture of ether and ester bonds; the linear structure consists of only ester bonds. The structure and, therefore, the properties of the cured epoxide resin may thus be regulated by selection of the amine basicity.     

Curing epoxy resins with anhydrides. Model reactions and reaction mechanism

The mechanism of acid curing of epoxy resins catalyzed with tertiary amines was investigated by using model systems composed of phenylglycidyl ether and benzoic acid or acetic acid anhydrides in the presence of benzyldimethylamine. The reaction was studied by NMR spectrometry, liquid chromatography, and ozone absorption. The main findings are that (1) the tert-amine is bound chemically and irreversibly during the reaction under the formation of a quaternary ammonium salt and (2) 1-phenyloxypropanediol-2,3-dibenzoate or diacetate is the main reaction product. The suggested reaction mechanism involves initiation in which the tertiary amine reacts with the epoxy group, giving rise to a zwitterion that contains a quaternary nitrogen atom and an alkoxide anion; the latter immediately reacts with the anhydride and quaternary salt is formed. In a later stage the carboxy anion of the quaternary salt reacts first with the epoxy group, then with the anhydride. By this reaction diester is formed and the carboxy anion is regenerated.     

Mechanical and dielectric relaxations of epoxide resins containing spiro-ring structure

Low-temperature relaxation behavior was investigated for a bisphenol-A and three spiroring type epoxide resins cured with aromatic and spiro-ring-type diamines. A new well-defined relaxation denoted here as the β′ relaxation was observed from 50 to 100°C for the spiro-ring-type resin systems in both mechanical and dielectric measurements. The peak height and the activation energy of the β′ relaxation were entirely independent of the degree of curing. It is concluded that the β′ relaxation is due to the motion of p-phenylene group adjacent to the spiro ring. The β′ relaxation was not observed in the bisphenol-A-type resin system.     

Dynamic mechanical and dielectric properties of phosphonylated polypentenamers and their hydrogenated derivatives

Dynamic mechanical and dielectric properties of substituted polypentenamers with phosphonate side groups and their hydrogenated derivatives have been studied. Methyl esters, acids, and salts were investigated at two concentrations, 6.5 and 11.1 mole percent. In the unhydrogenated derivatives, one principal relaxation, labeled β, is observed mechanically in the temperature range from ?160 to 100°C. This β relaxation arises from micro-Brownian segmental motion accompanying the glass transition. Its temperature is substantially affected by the substituent concentration while its breadth is affected by the chemical nature of the substituent. An extended “rubbery plateau” region exists in the acid and salt derivatives. The dielectric results generally reinforce the mechanical assignments. In the hydrogenated derivatives, three relaxations labeled α, β, and γ in order of decreasing temperature occur mechanically in this temperature range. The temperatures at which the α and β relaxations occur depend greatly on the chemical nature of the substituents, the substituent concentration, and the thermal history of the sample; while the γ relaxation appears to be independent of these variables. Suggested assignments for the relaxations observed in these polymers, based on the dual glass transition theory of Boyer for semicrystalline polymers, have been proposed. The dielectric results are consistent with the proposed assignments.     

Investigation of local order in unreacted DGEBA epoxy resin monomers by light scattering

The existence of local order in two epoxy resins of the diglycidyl ether of bisphenol-A (DGEBA) type has been investigated using Rayleigh scattering and Brillouin spectroscopy. The resins differ in their molecular weight distributions and their relative concentrations of epoxide and hydroxyl groups. The complementary use of both techniques in elucidating the thermal behavior of local order is illustrated, and the use of the latter technique to study thermal acoustic phonons and hypersonic relaxation is discussed. Both techniques independently show that molecular aggregates exist in each resin system. The scattering-envelope dissymmetry shows that the resin with the high epoxide/hydroxyl group ratio contains aggregates up to 20 nm in size, and the low-ratio resin exhibits sizes up to 70 nm. These aggregates are thermally unstable in the temperature range studied (293–443 K). Dissolution in chloroform shows that these aggregates are reduced in size and that further structural changes occur which are dependent on solvent concentration. Aggregate volume fractions were determined for a range of aggregate size. Brillouin spectroscopy indicated that both resins exhibit hypersonic relaxation in the temperature range studied. The complex longitudinal moduli of the resins were superimposable under a WLF temperature transformation comparable to the difference in their static glass transition temperatures. Molecular aggregate size, number, and stability are related to the epoxide/hydroxyl ratio of the resins and the degree of intermolecular hydrogen bonding.     

Spectroscopic,Mechanical and Dynamic-Mechanical Studies on the Photo-Aging of a Tetrafunctional Epoxy Resin

The photo-oxidative degradation of a densely cross-linked epoxide/diamine network based on tetraglycidyl-4,4′ diaminodiphenylmethane (TGDDM) and 4,4′ diaminodiphenyl sulphone (DDS) has been investigated by FTIR spectroscopy, dynamic-mechanical analysis (DMA) and compressive mechanical tests. The FTIR measurements allowed us to monitor the degradation process of the different groups present in the TGDDM/DDS network and to obtain reliable kinetic data. On this basis the most likely photo-degradation mechanisms were proposed. Dynamic-mechanical measurements and mechanical compressive tests were used to gain an insight in the effect of the photo-oxidative degradation on the relaxation processes of the epoxy network and on the mechanical performances.     

MECHANICAL RELAXATION AND INTERMOLECULAR INTERACTION IN EPOXY RESINS/POLY (ETHYLENE OXIDE) BLENDS CURED WITH PHTHALIC ANHYDRIDE

The miscibility of the blend,composed of a bisphenol A epoxy resins (Diglycidyl etherof bisphenol A) (DGEBA) and poly(ethylene oxide) (PEO) and crosslinked by phthalicanhydride (PA) was studied using dynamic mechanical method. Single glass transitiontemperatures intermediate between the two pure components were observed for all blendlevels. The secondary relaxation mechanism should relate to not only diester linkage, butalso hydroxyether structural unit in the system. Fourier transform infrared spectroscopy(FTIR) is applied to study the curing reaction and intermolecular specific interaction of thesystem. The results indicate the PEO participates the crosslinking reaction, accelerates thecuring reaction and make the reaction more perfect. The shifts of the hydroxyl band andcarbonyl band demonstrate the presence of the intermolecular interaction in the curedblend. Moreover, the molecular interaction between the side hydroxyl in the hydroxyetherunits and the ether bond in PEO macromolecules is stronger.     

Water transport in epoxy-aliphatic amine networks - Influence of curing cycles

In this work, polyepoxide network based on an aliphatic amine hardener were cured under different curing temperatures and their water sorption characteristics was systematically investigated as function of cure conditions. Attention was focused on the analysis of equilibrium properties, swelling behaviour and thermomechanical properties.For the undercured systems (epoxide conversion around 0.90), additional crosslinking reactions take part during water immersion, even at 20 °C. This effect was found to be mainly related to plasticizing of the network by water molecules.Local dynamics and plasticization effects were also monitored by dynamic mechanical analysis (DMA). The changes in the glass transition for the two well-crosslinked epoxies were greater than those predicted by considering the free volume changes only. The β process was significantly affected by the absorbed water. A decrease of the average relaxation time with moisture uptake was observed. The disruptions of the α and β relaxations were found to be reversible after re-drying of the wet material for the nearly fully cured epoxies.The fully crosslinked network was found to absorb slightly more water and at a greater rate than a slightly less crosslinked network (conversion around 0.98). This anomalous behaviour was analysed and a tentative explanation was presented in this paper.     

Catalytic double carbonylation of epoxides to succinic anhydrides: catalyst discovery, reaction scope, and mechanism

The first catalytic method for the efficient conversion of epoxides to succinic anhydrides via one-pot double carbonylation is reported. This reaction occurs in two stages: first, the epoxide is carbonylated to a beta-lactone, and then the beta-lactone is subsequently carbonylated to a succinic anhydride. This reaction is made possible by the bimetallic catalyst [(ClTPP)Al(THF)2]+[Co(CO)4]- (1; ClTPP = meso-tetra(4-chlorophenyl)porphyrinato; THF = tetrahydrofuran), which is highly active and selective for both epoxide and lactone carbonylation, and by the identification of a solvent that facilitates both stages. The catalysis is compatible with substituted epoxides having aliphatic, aromatic, alkene, ether, ester, alcohol, nitrile, and amide functional groups. Disubstituted and enantiomerically pure anhydrides are synthesized from epoxides with excellent retention of stereochemical purity. The mechanism of epoxide double carbonylation with 1 was investigated by in situ IR spectroscopy, which reveals that the two carbonylation stages are sequential and non-overlapping, such that epoxide carbonylation goes to completion before any of the intermediate beta-lactone is consumed. The rates of both epoxide and lactone carbonylation are independent of carbon monoxide pressure and are first-order in the concentration of 1. The stages differ in that the rate of epoxide carbonylation is independent of substrate concentration and first-order in donor solvent, whereas the rate of lactone carbonylation is first-order in lactone and inversely dependent on the concentration of donor solvent. The opposite solvent effects and substrate order for these two stages are rationalized in terms of different resting states and rate-determining steps for each carbonylation reaction.     

Effect of Functionality of Oligophenolic Hardener on Structural and Kinetic Features of Formation and on Properties of Epoxy-Phenolic Cross-Linked Polymers

Kinetics of the reaction of resorcinol diglycidyl ether with novolac phenol-formaldehyde resins of various functionalities were studied over a wide temperature range by isothermal calorimetry. The effects of the functionality of oligophenols on curing of epoxy-phenolic composites, structure of the resulting polymer at the chemical, topological, and supramolecular levels, its mechanical and relaxation properties, and the mechanism of its fracture were analyzed.     

Epoxy/modifier block copolymers

The curing of epoxy resins with anhydrides in the presence of dihydroxyl-terminated polyester or polyether modifiers produces block copolymer structures in which one “block” is the crosslinked epoxy/anhydride network and the other block is the linear modifier segment. The morphology of the cured system is dependent upon modifier molecular weight. The critical molecular weight of polycaprolactone and poly(propylene oxide) is 3000–5000. Below this level, single morphological phase systems are obtained, but two-microphase systems result above this level. This behavior is displayed by several epoxy resins. Two-phase systems display a superior balance of heat distortion temperature and impact strength, thus providing tough systems with greater elevated temperature capabilities than are obtained with single-phase systems.     

Mechanical and dielectric relaxations of epoxide resins containing the spiro-ring structure. II. Effect of the introduction of methoxy branches on low-temperature relaxations of epoxide resins

Mechanisms for low-temperature relaxations of three spiro-ring-type epoxide resin systems with and without methoxy branches were investigated by comparison with those of a bisphenol A-type resin system. In the spiro-ring-type epoxide resin systems, two well-defined relaxation peaks, denoted as the β and β′ relaxations, and a shoulder peak were observed at about ?70, +60, and 0°C, respectively. The magnitude of the β relaxation was decreased by the introduction of methoxy branches on the phenylene group. This phenomenon could be interpreted as a result of the formation of hydrogen bonds between the hydroxy-ether group and methoxy branch. Moreover, it was concluded that the β′ relaxation and the shoulder peak are due to the motion of the p-phenylene group adjacent to the spiro-ring and of the hydroxy-ether group blocked by the hydrogen bond, respectively.     

Anisotropy of mechanical and dielectric relaxation in oriented poly(ethylene terephthalate)

The anisotropy of the α and β relaxations in oriented poly(ethylene terephthalate) has been studied by dynamic mechanical and dielectric relaxation measurements. The α relaxation shows considerable mechanical anisotropy but gives rise to an isotropic dielectric process. The β relaxation, on the other hand, shows pronounced dielectric anisotropy but very little mechanical anisotropy. The implication of these results with regard to possible interpretations of the relaxations are discussed.