Epoxy polymers amorphous

Cycloahphatic diamines react with dicarboxyUc acids or their chlorides, dianhydrides, diisocyanates and di- (or poly-)epoxides as comonomers to form high molecular weight polyamides, polyimides, polyureas, and epoxies. Polymer property dependence on diamine stmcture is greater in the linear amorphous thermoplastic polyamides and elastomeric polyureas than in the highly crosslinked thermo set epoxies (2—4). 

Experimental results are presented that show that high doses of electron radiation combined with thermal cycling can significantly change the mechanical and physical properties of graphite fiber-reinforced polymer-matrix composites. Polymeric materials examined have included 121 °C and 177°C cure epoxies, polyimide, amorphous thermoplastic, and semicrystalline thermoplastics. Composite panels fabricated and tested included four-ply unidirectional, four-ply [0,90, 90,0] and eight-ply quasi-isotropic [0/ 45/90]s. Test specimens with fiber orientations of [10] and [45] were cut from the unidirectional panels to determine shear properties. Mechanical and physical property tests were conducted at cold (-157°C), room (24°C) and elevated (121°C) temperatures. 

The load-displacement curves for C(T) tests of the neat EpoxyH were almost linear until the final unstable fracture. The fracture toughness value in 77K-LNj was 210 J/m and that in RT-air was 120 J/m. Thus the toughness increased by 1.8 times by changing the test environment from RT-air to 77K-LN. Brown and co-workers have found that amorphous polymers crazed in 77K-LNj, but not in a helium or vacuum at about 78K [20-22]. They have also reported that the stress-strain behavior of all polymers, amorphous and crystalline, is affected by at low temperatures [22]. Kneifel has reported that the fracture toughness of epoxy in 77K-LNj is higher than that in RT-air and 5K, and that the reason for this is the reduced notch effect by plastic deformation [23]. Then, the increase of the fracture toughness of the neat EpoxyH in this study is probably caused by the similar effect. 

Although epoxies are mainly classified as thermosets, it is also possible to produce linear epoxy polymers using comonomers with two reactive sites per molecule. These linear polymers behave as thermoplastics and can be amorphous or semicrystalline. They exhibit some outstanding optical and barrier properties. Similarly, PUs can be either thermoplastics or thermosets depending on the number of reactive sites per molecule of monomers and comonomers. 

It has been established that the structure of an amorphous polymer is fractal with a fractal dimension (df) (2 < df < 3). Therefore we ought to expect, that the fluctuation free volume should also have fractal properties. The purpose of this chapter is to investigate the fractality of the fluctuation free volume in glassy polymers and epoxy polymers are... 

As can be seen in Figure 15.2, the dependence (15.3) is correct for the epoxy polymers investigated and confirms the self-similarity of a cluster of microvoids of fluctuation free volume. The interval of the scale of the self-similarity, with allowance for correlations between Df and fractal dimension of the structure of the polymer df may be assumed. This interval coincides with a similar interval for the structure of an amorphous polymer which is distributed from several units up to several tens of Angstrom (5-50 A) [1, 4]. 

Polymer mechanical properties are one from the most important ones, since even for polymers of different special-purpose function a definite level of these properties always requires [20]. Besides, in Ref [48] it has been shown, that in epoxy polymers curing process formation of chemical network with its nodes different density results to final polymer molecular characteristics change, namely, characteristic ratio C, which is a polymer chain statistical flexibility indicator [23]. If such effect actually exists, then it should be reflected in the value of cross-linked epoxy polymers deformation-strength characteristics. Therefore, the authors of Ref [49] offered limiting properties (properties at fracture) prediction techniques, based on a methods of fractal analysis and cluster model of polymers amorphous state structure in reference to series of sulfur-containing epoxy polymers [50]. 

The authors of Ref [9] conducted cross-linked polymers microhardness description within the frameworks of the fractal (structural) models and the indicated parameter intercommunication with structure and mechanical characteristics clarification. The epoxy polymers structure description is given within the frameworks of the cluster model of polymers amorphous state structure [10], which allows to consider polymer as natural nanocomposites, in which nanoclusters play nanofiller role (this question will be considered in detail in chapter fifteen). 

As it has been noted above, at present it is generally acknowledged [2], that macromolecular formations and polymer systems are always natural nanostructural systems in virtue of their structure features. In this connection the question of using this feature for polymeric materials properties and operating characteristics improvement arises. It is obvious enough that for structure-properties relationships receiving the quantitative nanostructural model of the indicated materials is necessary. It is also obvious that if the dependence of specific property on material structure state is unequivocal, then there will be quite sufficient modes to achieve this state. The cluster model of such state [3-5] is the most suitable for polymers amorphous state structure description. It has been shown, that this model basic structural element (cluster) is nanoparticles (nanocluster) (see Section 15.1). The cluster model was used successfully for cross-linked polymers structure and properties description [61]. Therefore, the authors of Ref [62] fulfilled nanostmetures regulation modes and of the latter influence on rarely cross-linked epoxy polymer properties study within the frameworks of the indicated model. 

It is known that the introduction of adamantane fragments in epoxy polymers exercises an essential influence on their characteristics. In papers [19,20] the effect of such network structures is examined in the example of EP modified by adamantane acids. The interpretation of the results obtained in [19, 20] within the frameworks of the cluster model of the amorphous state structure of polymers [5, 6] allows to suppose availability of two types of clusters in the studied EP stable ones, formed by main chain segments, and unstable ones, formed at the expense of the interaction of adamantane fragments. The authors of papers [21-23] studied the problem of how much the indicated notions corresponded to the real structure of the studied EP. This can be carried out with the aid of the methods of [3], based on the study of wide angle X-ray halos. 

Therefore, the results stated in the present section have shown that the wide angle X-ray diffractometry and mechanical testing using the cluster model of the amorphous state structure of polymers attraction as a theoretical basis allows profound analysis of the structural organisation of complex polymer systems to be carried out, for example, such as epoxy polymers, modified by network fragments. 

It is obvious that the fraction c of macromolecules segments able to form clusters should reduce during the formation of clusters and, accounting for the condition = const, for each epoxy polymer, the value of should also decrease. This explains the availability of unstable clusters, which have smaller values of [5, 6, 91], in the amorphous state structure of polymers. This also explains the self-similarity of the structure of a cluster within the scales range of its existence. Let us note one more important circumstance - the system of Equations 5.12 and 5.13 is applicable to all studied epoxy polymers and the system of Equations 5.14 and 5.15 is applicable to each separately [89]. 

The experimental observation of the same Gaussian statistics of polymer chains in 0-solvent and condensed state is the main objection against local order availability in amorphous state polymers [105]. The equality of distances between macromolecules or subchains ends in the indicated states is considered as one of the pieces of evidence of this rule. Boyer [106] demonstrated schematically the possibility of local order existence at fulfilment of the indicated condition. However, strict confirmation of such a possibility was not obtained. Therefore the authors of paper [107] confirmed analytically Boyer s concept on the example of two series of epoxy polymers (EP-1 and EP-2). 

In paper [43] acceleration of the stress relaxation process was found at loading of epoxy polymers under the conditions similar to those described above (Figure 6.8, curves 2-4). The authors [43] explained the observed effect by the partial rupture of chemical bonds. In order to check this conclusion in paper [39] repeated tests on compression of samples, loaded up to the cold flow plateau and then annealed at T < T, were carried out. It has been established that in the diagram o-e tooth of yield is restored. This can occur at the expense of the restoration of unstable clusters, since the restoration of failed chemical bonds at T < is scarcely probable. In this connection it is also necessary to note that yield tooth suppression as a result of preliminary plastic deformation was observed earlier for linear amorphous polymers, for example, polycarbonate [44], for which the chemical bonds network is obviously absent. 

Mechanical properties of polymers are among the most important, since a certain level of these properties is always required even for polymers of different special-purpose functions [50]. In papers [38, 51] it has been shown that the curing process of the chemical network of epoxy polymers with the formation of nodes of various density results in a change in the molecular characteristics, particularly the characteristic ratio C. If such an effect actually exists, then it should be reflected in the deformation-strength characteristics of crosslinked epoxy polymers. Therefore the authors [49] offered methods of prediction of the limiting properties (properties at fracture), based on the notions of fractal analysis and the cluster model of the amorphous state structure of polymers, with reference to a series of sulfur-containing epoxy polymers [52, 53] (see also Section 5.4). 

The authors of paper [76] showed the distinction of micro- and macroexpansion in amorphous polymers and explained it by a certain degree of ordering of chain macromolecules. In other words, the authors [76] found interconnection of thermal expansion and supramolecular structure for a number of amorphous polymers. However, the quantitative structural model for absence of the amorphous state does not allow similar interconnection details to be more precise. Therefore the authors [77] carried out the study of interconnection for amorphous epoxy polymers EP-1 and EP-2 of thermal expansion and structure, for the description of which the cluster model [8, 9] was used. 

It is significant that the reinforcement degree corresponds to a class of polymer forming a nanocomposites matrix. The largest values of / are obtained for polymers whose chains are able to stretch on the silicate platelet surface (rigid-chain polyimide, crystallising polypropylene and thermotropic liquid crystalline polyester), intermediate values for polymers whose chains are able to stretch only partly (polycarbonate, poly (butylenes terephthalate) and amorphous polyamide-6) and the smallest values for nanocomposites on the basis of epoxy polymer, the capability of chains stretching of which decreases sharply because of the availability of transverse covalent bonds network [30]. 

Figure 8.8 The dependences of the permeability to gas coefficient P, normalised over amorphous phase volume contents for oxygen (1) and nitrogen (2) on epoxy polymer contents c p for nanocomposites HDPE/EP [31]...

The treatment of an epoxy polymer as a natural nanocomposite or quasi-two-phase system [8] puts in the foregroimd the interaction of such system components, which for nanocomposites is expressed first of all in an interfacial regions formation [2-4]. Let us note that in the reinforcement process (increase in the elasticity modulus of the nanocomposite in comparison with the matrix polymer) interfacial regions play the same role as the nanofiller [2-4], Such a reinforcement mechanism is due to the formation of nanocomposites with inorganic nanofiller [1-A and the structure of natural nanocomposites (linear amorphous polymers) [9] in three-dimensional Euclidean space. Therefore in paper [10] the study of structure formation conditions for crosslinked epoxy polymers, treated as natural nanocomposites, was carried out within the frameworks of fractal analysis. 

In Figure 9.1 the comparison of dimensions and for the studied EP is adduced. Their good correspondence indicates unequivocally that their loosely packed matrix, which serves simultaneously as a natural nanocomposite matrix, is the fractal space where the nanocluster structure of epoxy polymers is formed. Since for linear amorphous polymers = 3 [9], i.e., their nanostructure formation is realised in three-dimensional Euclidean space, then the conclusion that chemical crosslinking network availability in the considered EP serves as the indicated distinction cause is obvious enough. In Figure 9.2 the dependence of on crosslinking density is... 

In Figure 9.14 the dependence K (D ) is adduced, which has shown linear decay with growth in and at = 3, i.e., at nanostructure formation in Euclidean space, K = 0 and the structure of epoxy polymers does not undergo changes (formation of nanoclusters) in its creation process. Let us note that such treatment is confirmed by the data for particulate-filled polymer nanocomposites, for which the structure formation proceeds in Euclidean space and the polymer matrix dimension of nanocomposites is constant and equal to this parameter for a matrix polymer [40]. The similar, but weaker, dependence K (D ) was found for a linear amorphous polymer (polycarbonate, a dashed line in Figure 9.14), which is due to the absence of such a powerful factor as chemical crosslinking nodes network. 

As the studies of amorphous linear polymers considered as natural nanocomposites have shown, their elasticity modulus is a linear increasing function of the relative fraction of nanoclusters [48]. Such behaviour of the elasticity modulus of the indicated polymers confirms the treatment of nanoclusters as nanofillers (reinforcing elements). The authors of paper [49] carried out a similar analysis of elasticity modulus behaviour in compression tests for crosslinked epoxy polymers. 

The ratio in the left-hand part of Equation 926 should be considered as a reinforcement degree of crosslinked epoxy polymers treated as a natural nanocomposite. Let us note again that a loosely packed matrix is a structure reinforcing element for a crosslinked polymer, unlike for linear amorphous polymers. Nevertheless, for both indicated classes of polymers the reinforcement degree is defined equally, namely as polymer and loosely packed matrix elasticity moduli ratio and in both cases the nanoclusters are assumed as a nanofiller. Confirmation of this postulate can be obtained within the frameworks of the model [50] where three basic cases of the dependence of the reinforcement degree of the composites EJE (where E and E are elasticity moduli of the composite and the matrix polymer, respectively) on filler content (p were considered. It has been shown that the following basic types of the dependence EJ exist ... 

The authors of paper [60] gave the description of the microhardness of crosslinked epoxy polymers within the frameworks of fractal (structural) models and elucidated the indicated parameter interconnection with structure and mechanical characteristics. Description of the structure of the epoxy polymers is given within the frameworks of the cluster model of the amorphous state structure of polymers [5-7], which allows polymers to be considered as natural nanocomposites in which nanoclusters play the role of nanofiller. 

In Figure 9.33 the dependence a p(D ) for the considered epoxy polymers is adduced, which has an expected character. The growth in the thermal expansion linear coefficient with an increase in molecular mobility level is observed. In Figure 9.33 a solid straight line shows the similar dependence oc(D p) for amorphous aromatic polyamide (phenylone S-2). As one can see, this straight line corresponds well to the data for the considered epoxy polymers. This means that irrespective of the class of polymers, their thermal expansion coefficient is defined by the molecular mobility level, which in paper [62] is characterised by the dimension... 

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