Strain-induced crystallites

Strength retention in any elastomer over an extended range of temperature is usually ascribed to the presence of a dispersed phase which results from the presence of strain-induced crystallites or the presence of a small particle-size particulate filler (with a chemically active surface). It is the domain structures in a urethane elastomer that are considered to provide these functions. 

Undeformed NR forms spherulitic crystallites below 0°C. The tempera-ture/time induced crystallites generally form as folded chain lamellae, whereas the morphology of the strain/stress induced crystallites has been reported to be various fibrils, fibrils and folded lamellae, and shish-kebabs. " Strain-induced crystallites and their orientation have been indicated as the key reasons for the sharp increase in modulus that accompanies strain-induced crystallization. 

This figure allows also to follow the retracting process, in whieh the crystalline reflections disappeared gradually, returning to the initial isotropie pattern after total relaxation of the sample. Ringlike (unoriented) crystalline reflections of NR were not observed during the cyclic deformation process, allowing to conclude that strain-induced crystallites always appear in the form of oriented crystals. ... 

Density is also found to increase in this region, thus providing additional evidence of crystallisation. Certain synthetic elastomers do not undergo this strain-induced crystallisation. Styrene-butadiene, for example, is a random copolymer and hence lacks the molecular regularity necessary to form crystallites on extension. For this material, the stress-strain curve has a different appearance, as seen in Figure 7.12. 

Potentially of equal importance is the relationship between strain and catalyst stability. A calculation of the contribution to the total free energy of a catalyst crystal caused by the presence of strain-inducing microscopic precipitates50 showed that the extra free energy increases with the size of the crystal and inhibits it from sintering. This theory is an interesting one since it provides a mechanism which the catalyst scientist can exploit in his search for stable, high surface-area materials. The theory predicts the equilibrium crystallite size of the iron crystals of an ammonia synthesis catalyst with acceptable accuracy. 

The stress-strain curve for unfilled NR exhibits a large increase in stress at higher deformations. NR displays, due to its uniform microstructure, a very unique important characteristic, that is, the ability to crystallise under strain, a phenomenon known as strain-induced crystallization. This phenomenon is responsible for the large and abrupt increase in the reduced stress observed at higher deformation corresponding, in fact, to a self-toughening of the elastomer because the crystallites act as additional cross-links in the network. This process can be better visualized by using a Mooney-Rivlin representation, based on the so-called Mooney-Rivlin equation ... 

One can conclude that the microindentation technique allows the strain-induced polymorphic transition in PBT to be followed. The observed rather abrupt variation in H (within 2-4% of external deformation) makes the method competitive with respect to sensitivity to other commonly used techniques such as WAXS, infrared spectroscopy, Raman spectroscopy, etc. (Tashiro Tadokoro, 1987). Furthermore, by applying the additivity law it is possible to calculate the microhardness of completely crystalline PBT, comprising crystallites of the /6-type, as = 122 MPa. This technique can also be used to examine the stress-induced polymorphic behaviour of PBT in copolymers and blends as will be demonstrated in the following sections. 

Comparison of the curves presented in Fig. 6.9 for homo-PBT (a), the blend (b) and the copolymer of PBT with PEG (c) allows one to draw the following conclusions the initial drop in H at around e = 5% (Fig. 6.8) originates from the strain-induced polymorphic transition in the crystallites comprising only homo-PBT segments. The second change in H at around s = 25-30% is related to the strain-induced polymorphic transition in the PBT crystallites comprising PBT segments from its multiblock copolymer PEE. 

The result shown in Fig. 6.8 that the two species of crystallites respond to the mechanical field in sequence - first the homo-PBT crystallites and later those arising from PEE, means that the homo-PBT crystals are probably dispersed within PEE in such way that they experience the mechanical field from the very beginning of loading. Moreover, one can assume that in the blend some internal stress and/or strain pre-exists since the strain-induced polymorphic transition starts even at lower... 

Thus, the observation of two sharp well defined, and clearly separated on the deformation scale, strain-induced polymorphic transitions convincingly demonstrates that the two populations of PBT crystallites of differing origin, undergo the mechanical loading not simultaneously but in two steps first those comprising homo-PBT (at s = 2-3%), followed by crystallites belonging to PEE (at s = 25%). 

In summarizing the results from the last three sections, one can conclude that the systematic variation of microhardness under strain performed on (a) homo-PBT (Section 6.2.1), (b) its multiblock copolymer PEE (Section 6.2.2) and (c) on blends of both of these (this section) is characterized by the ability of these systems to undergo a strain-induced polymorphic transition. The ability to accurately follow the strain-induced polymorphic transition even in complex systems such as polymer blends allows one also to draw conclusions about such basic phenomena as cocrystallization. In the present study of a PBT/PEE blend two distinct well separated (with respect to the deformation range) strain-induced polymorphic transitions arising from the two species of PBT crystallites are observed. From this observation it is concluded that (i) homo-PBT and the PBT segments from the PEE copolymer crystallize separately, i.e. no cocrystallization takes place, and (ii) the two types of crystallites are not subjected to the external load simultaneously but in a sequential manner. 

In this expression, similar to Equation (84a), the first term is the strain of the isotropic matrix given by Equation (94). The second term is the strain induced in crystallite by the matrix and is given by the Eshelby" theory for an ellipsoidal inclusion. The tensor lifg) accounts for the differences between the compliances of the inclusion and of the matrix and has the property ty = 0. To calculate the peak shift. Equation (105) is replaced in Equation (67b), which is further replaced in Equation (83). Analytical calculations can be performed only for a spherical crystalline inclusion that has a cubic symmetry. For the peak shift an expression similar to Equation (91) is obtained but with different compliances. According to Bollenrath el the compliance constants in Equation (91) must be replaced as follows ... 

To minimize shrinkage and moisture absorption in hot-fill bottles, the first step is to heat-set the bottle. This is a process in which the bottle crystallinity, mainly in the sidewalls, is increased to about 30% under stress to maintain the bottle shape. The crystallization that is desired is strain-induced crystallization, which results in smaller crystallites that do not scatter light. The preform weight for hot-fill bottles is higher also, to help stabilize the finish area. Illustrations of the different preform weights commonly used in industry are in Table 12.1. 

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