Semi-crystalline microstructures

In the case of semi-crystalline PET, comparing the TEM photographs and the measured spherulite sizes, it can be assumed that the individual reactive particles should be distributed in within the spherulitic structure. Concerning the non-reactive one it is highly probable, knowing the small size of the semi-crystalline microstructure, that the modifier clusters remain outside the spherulites. 

PET fibers in final form are semi-crystalline polymeric objects of an axial orientation of structural elements, characterized by the rotational symmetry of their location in relation to the geometrical axis of the fiber. The semi-crystalline character manifests itself in the occurrence of three qualitatively different polymeric phases crystalline phase, intermediate phase (the so-called mes-ophase), and amorphous phase. When considering the fine structure, attention should be paid to its three fundamental aspects morphological structure, in other words, super- or suprastructure microstructure and preferred orientation. 

Blends of LDPE with ethylene styrene interpolymers (ESI, see Section 3.2) also have a complex microstructure. The semi-crystalline LDPE is immiscible with the amorphous ESI, which has a glass transition temperature (Tg) just above room temperature. Consequently there are rigid crystalline regions and rubbery amorphous LDPE, mixed on a 0.1 pm scale, together with regions of leathery ESI on a 5 to 10 pm scale (71). 

Polyolefin foams are easier to model than polyurethane (PU) foams, since the polymer mechanical properties does not change with foam density. An increase in water content decreases the density of PU foams, but increases the hard block content of the PU, hence increasing its Young s modulus. However, the microstructure of semi-crystalline PE and PP in foams is not spherulitic, as in bulk mouldings. Rodriguez-Perez and co-workers (20) showed that the cell faces in PE foams contain oriented crystals. Consequently, their properties are anisotropic. Mechanical data for PE or PP injection mouldings should not be used for modelling foam properties. Ideally the mechanical properties of the PE/PP in the cell faces should be measured. However, as such data is not available, it is possible to use data for blown PE film, since this is also biaxially stretched, and the texture of the crystalline orientation is known to be similar to that in foam faces. 

One of the most important subjects of applied polymer science is the understanding of the deformation mechanisms and the fracture properties of semi-crystalline polymers. At the same time, it is one of the most diffictdt to study, and the amount of research in this area is high (see e.g. One of the complications experienced with semi-crystalline polymers stems from the fact that they are composed of crystalline and amorphous phases, arranged in a diversity of microstructures. These are generally... 

In a final chapter a closely related phenomenon, the formation of shear bands in semi-crystalline polymers under compressive load will be described. It is attempted to discuss under which conditions shear bands are formed in semi-crystalline materials and how they interact with each other or with certain microstructural features, finally leading to crack initiation and shear fracture of the bulk polymer. 

In comparing the shear fracture surfaces of amorphous and semi-crystalline polymers, it appears that the features in both cases are quite similar (Fig. 39a -c ). This indicates that, under comparable conditions, the local stress field rather than details of the crystalline-amorphous microstructure of the polymers tested determines the operating deformation mechanism. Only secondary effects arise from the morphology of the cry stalline material. 

As with other natural fibres, silk has a hierarchical microstructure - about five anti-parallel (f-sheets, each with around 12 chains, aggregate to form parallel, crystalline microfibrils (approximately 10 nm in diameter), bundles of which make up fibrillar elements (roughly 1 p,m across), which in turn associate to comprise the individual fibroin filaments (7-12 xm) at each level of organisation, the ordered elements are embedded within amorphous matrices derived from the non-crystalline components. Once again, then, the behaviour of the structural composite can be understood in terms of the semi-crystalline array of its component parts. 

All materials belong to the class of semi-crystalline thermoplastic polymers. Characteristic appearances of spherulitic microstructures of the polymers are shown in Figures 4 and 5 for the examples of POM and PA66. 

This chapter describes the microstructures of the main types of polymer, concentrating on features used later to explain physical properties. The order of magnitude of elastic moduli for rubbers, glassy polymers and polymer crystals will be related to their molecular mobility and inter-molecular forces. These values will be used in Chapter 4 to predict the moduli of semi-crystalline polymers. 

The tensile stress-strain curves, for the four microstructural types, cover the range from elastomers to typical semi-crystalline thermoplastics (Fig. 3.23). The lowest crystallinity material is a competitor with thermoplastic elastomers . 

Even a single polymer can have a composite structure. Here, the phase geometry and mechanical properties are considered for polymers that separate into two amorphous phases. Block copolymers usually have sufficient block lengths to allow micron-scale phase separation. Later on, we have considered smaller scale microstructures caused by the spinodal decomposition of polyurethanes. Semi-crystalline polymers will be considered in Section 4.6. 

This has the same form as Eq. (7.1) the renamed constants are a Young s modulus , and a viscosity 17. It is not possible to directly link these constants to the modulus of the crystalline phase and the viscosity of the amorphous inter-layers in a semi-crystalline polymer. Hence, the Voigt model is an aid to understanding creep, and relating it to other viscoelastic responses, rather than a model of microstructural deformation. 

The shape of the mastercurve is related to the polymer microstructure. That for polystyrene at 100 °C (Fig. 7.5b) shows a transition from a glassy compliance at Is to a rubbery one at times exceeding 10 s. It continues to 10 ° s, so it can be used for extrapolation to times longer than those accessible by experiment. Time-temperature superposition for semi-crystalline polymers, such as polyethylene, may be successful for a limited temperature range, i.e. 20°C-80°C. As polyethylene starts to recrystallise if heated within 50 °C of T, and residual stresses may start to relax, data for higher temperatures will not superimpose. 

For semi-crystalline polymers, the average orientation function 2 for the crystal c axes can be calculated from X-ray diffraction measurements (Chapter 3). Figure 8.15 shows how 2 increases linearly with the draw ratio, for polypropylene fibres and films, while the spherulitic microstructure survives. At 2 = 0.9, where the spherulites are destroyed and replaced by a microfibrillar structure, there is an increase in the slope of the 2 versus true strain relationship. It is impossible to achieve perfect c axis orientation... 

Light scattering can be marked in semi-crystalline polymers with a spher-ulitic microstructure for example, unpigmented polyethylene appears milky and opaque. However, it is negligible when the diameter, of inclusions in a matrix, are smaller than 10% of the wavelength of the light (Fig. 11.16). 

Fig. 6.19 Illustration of microstructure evolution upon cold-drawing of semi-crystalline polymers...

Agarwal, S. and Speyere, C. (2010) Degradable blends of semi-crystalline and amorphous branched poly(caprolactone) effect of microstructure on blend properties. Polymer, 51 (5), 1024-1032. 

Umare and co-workers prepared a series of bis-phenolate complexes of titanium, 20-21, which were not active in solution for LA ROP, but in solvent free conditions showed acceptable activity (Table 7.2, entry 50-54) [22]. The isolated semi-crystalline polymers showed to have isotactic microstructure. 

Final properties of an injection-molded product, made of semi-crystalline polymers, strongly depend on the flow-induced microstructure (crystallinity, morphology, orientation, etc.). Particularly, the enhancement in polymer crystallization rate crucially changes the soUdiiication behavior, and the oriented microstructure leads to a local anisotropy in thermal and mechanical properties. 

The intensity of the glass transitions in semi-crystalline polycarbonate measured by DSC, has been found to be governed by the extent of primary crystallization. Crystallization induced by annealing, by organic solvents, and in the presence of plasticizers have all been reported. In this last paper, using the kinetic theory of Hoffmann and Lauritzen, it was found possible to determine concentrations of seeds inducing crystallization. Texturization in the form of surface roughness has also been found to cause microstructural order. ... 

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