Mechanisms for flow and forced high elasticity of cross-linked polymers

Abstract

A quantitative model is proposed for flow and forced high elasticity of cross-linked polymers based on the cluster model of amorphous state structure. The flow process is seen as the loss of displacement stability by the clusters, while forced high elasticity is related to the mecham'cal devitrification of the sofrpack matrix. Fractal analysis was used to show a turbulent regime for the forced high elasticity of cross-linked polymers.The mechanisms for fluidity and cold flow have been studied intensively in light of their theoretical and practical importance. However, despite the vast number of investigations in this area, a complete understanding of these processes has not yet been achieved. To a large extent, this is the case for amorphous and, especially, cross-linked polymers. Various approaches from a dislocation-disclination model [1] to a molecular kinetics model [2] have been employed to describe these processes. Such a broad range of approaches is a function of the nature of amorphous glassy polymers, which are intermediate in structure between liquids and true solids [3]. In our view, further progress in this field for cross-linked polymers is inhibited by at least two circumstances: 1) an extreme exaggeration of the role of the network of chemical cross-links and 2) the lack of a quantitative structural model.At present, the idea of the existence of regions of local order in amorphous polymers first proposed by V. A. Kargin is accepted almost without discussion although direct experimental evidence for such regions has not been obtained due to their small dimensions. The cluster model [4] presupposes that the structure of amorphous polymers entails regions of local order (clusters) surrounded by a softpack matrix. In turn, the clusters consist of several collinear dense-packed segments of various macromolecules, whose length is equal to the length of the statistical segment. Furthermore, two types of clusters are assumed to exist in the softpack matrix: clusters with a small number of segments and, thus, low thermal stability and clusters with a large number of segments and high thermal stability [4]. Evidence for two types of clusters is found in the two-step glass formation process [5], which occurs at the glass transition temperature of the softpack regions T s' and the glass transition temperature of the polymer itself Tg. In this case, Tg' is about 50 K below Tg. In the devitrification (thawing) process, which is seen as decomposition of the regions of "frozen" local order [6], the more stable clusters are lost at Tg', while the more stable clusters decompose at Tg. By analogy with crystallites, the less stable clusters should have smaller dimensions, i.e, contain a smaller number of segments. The value of T s' clearly corresponds to T 2 in the model of Filyanov [2], in which T2'divides two temperature ranges characterized by different behavior of the mechanical properties. This f'mding was attributed by Filyanov [2] to different deformation mechanisms. In the present work, a mechanism is propos...

Fractal analysis of macromolecules

“…The second stage is characterised by self-organisation of mesoclusters. 126 As applied to the glass transition of polymers, these processes can be specified in the following way. The first stage starts at the temperature of so-called liquid ± liquid transition T ll , in which regions of dynamic short-lived local order are formed.…”

Structure and properties of polymers in terms of the fractal approach

“…90 The evolution of DS induced by various types of treatment of the polymer has been considered. 64,93,94 Figure 15 shows the dependence of the cluster functionality F on the curing agent : oligomer ratio (K st ) for epoxy polymers in deformed and nondeformed states. The most characteristic distinction between the plots being compared is the substantial increase in F after the yield point.…”

“…The most characteristic distinction between the plots being compared is the substantial increase in F after the yield point. The simultaneous decrease in V cl and increase in F upon deformation of polymers up to the yield point 63,64 implies decomposition of unstable clusters characterised by relatively low F to leave only stable clusters with large F. 95 The decay of unstable clusters induces mechanical devitrification of the loosely packed matrix; this explains the plastic behaviour of the polymer in the plateau of forced rubber-like elasticity (cold flow). 96,97 The opposite situation is observed during heat ageing, namely, the local order (V cl ) increases and the molecular mobility of chains is suppressed (D decreases).…”

A theoretical description of the oxidation kinetics of polymer melt in fractal mediums

“…Typical examples would be curves of sorption, 3 positron annihilation, 4 kinetic curves of conversion (degree of reaction vs. time) at polymer synthesis, 5 stress-strain curves, 6 etc. Second, the dependence of a process rate on time by itself assumes the fractality of the medium in which the process occurs.…”