Polymers semicrystalline

Amorphous polymers of commercial importance include polymers which are glassy or rubbery at room temperature. Many amorphous thermoplastics, such as atactic polystyrene and poly (methyl methacrylate), form brittle glasses when cooled from the melt. The glass transition temperature, Tg or glass-rubber transition, is the temperature above which the polymer is rubbery and can be elongated and below which the polymer behaves as a glass. Thermal analysis of amorphous polymers shows only a glass transition temperature whereas crystalline poly- 

Semicrystalline polymers exhibit a melting transition temperature (Tm), a glass transition temperature (Tg) and crystalline order, as shown by x-ray and electron scattering. The fraction of the crystalline material is determined by x-ray 

Spherulitic chains folded at right angles to main axis 

Semicrystalline polymers constitute an important group of polymers with a very broad range of applications, in particular, the family of polyolefins, including poly-ethylenes and polypropylenes, is one of the most prominent commodity plastics that make up a great part of the world s plastic market. The rapid development of new catalysts has allowed for the tailored design of macromolecules with defined semicrystalline morphologies and thus defined properties. Besides these commodity plastics, there are a number of technical and functional polymers that have a typical semicrystalline structure (e.g., PEEK, PVDE, PTFE). 

Consequently, semicrystalline polymers can be only partially crystalline, consisting of highly ordered domains and regions of randomly arranged chains. In other words, they consist of a crystalline fraction and an amorphous phase. This phase separation often results in a lamellar structure that is typical of semicrystalline polymers. 

The development of a hierarchical architecture, that is, the existence of more than only one distinct morphological feature, is a universal aspect of the semicrystalline polymers. The scale of relevant structural details ranges from some nanometers to millimeters and is controlled by the architecture of the macromolecules and the processing history. The easiest example of a macromolecule is a linear arrangement of CH2 groups in high-density polyethylene (HDPE). Constitutions of some typical semicrystalline polymers are collected in Table 2.1. 

There is also a larger variety of structures due to the existence of short and long CH2 side chains in PE (low-density PE, LDPE and linear low-density PE, LLDPE) or different tacticity in PP (iso-, syndiotactic). Some types of morphology appear under specific conditions of crystallization or preparation. In general, some of the most influential factors are 

To characterize semicrystalline polymers, the measurement and qualitative description of several morphological parameters are necessary, including 

Commercially important glassy polymers are polymers which are crystallizable but which may form as amorphous materials. These noncrystalline polymers are formed by rapid cooling of a polymer from above the melting transition tem- 

Semicrystalline polymers exhibit a melting transition temperature (Tm), a glass transition temperature f Tg) and crystalline order, as shown by. . lay and electron scattering. The fraction of the crystalline material is determined by x-ray diffraction, heat of fusion and density measurements. Major structural units of semi-crystalline polymers are the platelet-like crystallites, or lamellae, and the dominant feature of melt crystallized specimens is the spherulite. The 

When a polymer is melted and then cooled it can recrystallize, with process variables, such as temperature, rate of cooling, pressure and additives. 

The general morphology of crystalline polymers is now well known and understood and was described by GeU [5], Keller [6], Wunderlich [7], Grubb [8], Uhlmann and Kolbeck [9], Bassett [10,11], and Seymour [12]. The work of Keller and his group has been reviewed by Bassett [13]. More recent edited books by Bassett [14], Ciferri [15], and Ward [16, 17], among others, provide excellent updates on current knowledge in the area of polymer morphology. 

Plastic deformation in glassy polymers and in rubber toughened polymers is due to crazing and shear banding. Crazing is the formation of thin sheets perpendicular to the tensile stress direction that contain fibrils and voids. The fibrils and the molecular chains in them are aligned parallel to the tensile stress direction. 

A schematic of the spherulite structure is shown in Fig. 1.1 [16]. The structure [16,18,20, 23] consists of radiating fibrils with amorphous material, additives, and impurities between the fibrils and between individual spherulites. Although the shape of the growing spherulite 

Given AG, one can estimate the critical stress for the formation of a dislocation (Tc. Usually, it is assumed that AU x SOkT, which gives a jc comparable with experimentally determined values of the shear yield stress, ty. The main objection to the dislocation model is that the predicted temperature dependence of the yield stress is much weaker than that observed experimentally, particularly at higher T, and more recent efforts to describe yield have been based on thermally activated helical motions of polymer chains within their crystals, believed to be associated with the a relaxation, but which may occur below Tm [27). 

In an isotropic polycrystalline polymer whose microstructure consists of stacked lamellae arranged in the form of spherolites, the slip systems activated depend on the local orientation of the lamellae with respect to the applied stress and, as deformation proceeds, these orientations are modified. To calculate the evolution of the crystalline texture, one can consider the polymer to behave as a crystalline aggregate. Although the entropic contribution of chain orientation in the amorphous regions may also need to be considered, the major contribution to work hardening in tension is rotation of the slip planes toward the tensile axis, so that the resolved shear stress in the slip direction diminishes. This results in a fiber texture in the limit of large deformations, such that the crystallites are oriented with their c axis (the chain axis) parallel to the stretch direction. Despite the relative success of such models, they do not explicitly address the micro-mechanisms involved in the transformation of the spherulitic texture into a fiber texture. One possibility is that the 

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Lamellar morphology variables in semicrystalline polymers can be estimated from the correlation and interface distribution fiinctions using a two-phase model. The analysis of a correlation function by the two-phase model has been demonstrated in detail before [30,11] The thicknesses of the two constituent phases (crystal and amorphous) can be extracted by several approaches described by Strobl and Schneider [32]. For example, one approach is based on the following relationship ... 

Polyethylene (PE) is a genetic name for a large family of semicrystalline polymers used mostiy as commodity plastics. PE resins are linear polymers with ethylene molecules as the main building block they are produced either in radical polymerization reactions at high pressures or in catalytic polymerization reactions. Most PE molecules contain branches in thek chains. In very general terms, PE stmcture can be represented by the following formula ... 

Polyamides, often also lefeiied to as nylons, are liigli polymers which contain the amide repeat linkage in the polymer backbone. They are generally characterized as tough, translucent, semicrystalline polymers that ate moderately low cost and easily manipulated commercially by melt processing. 

Poly(phenylene sulfide) (PPS) is another semicrystalline polymer used in the composites industry. PPS-based composites are generally processed at 330°C and subsequently cooled rapidly in order to avoid excessive crystallisation and reduced toughness. The superior fire-retardant characteristics of PPS-based composites result in appHcations where fire resistance is an important design consideration. Laminated composites based on this material have shown poor resistance to transverse impact as a result of the poor adhesion of the fibers to the semicrystalline matrix. A PPS material more recently developed by Phillips Petroleum, AVTEL, has improved fiber—matrix interfacial properties, and promises, therefore, an enhanced resistance to transverse impact (see PoLYAffiRS containing sulfur). 

Figure 8.6. A diagrammatic view of a semicrystalline polymer showing both chain folding and interlamellar entanglements. The lamellae are 5-50 nm thick (after Windle 1996).

Typically, a semicrystalline polymer has an amorphous component which is in the elastomeric (rubbery) temperature range - see Section 8.5.1 - and thus behaves elastically, and a crystalline component which deforms plastically when stressed. Typically, again, the crystalline component strain-hardens intensely this is how some polymer fibres (Section 8.4.5) acquire their extreme strength on drawing. 

In a semicrystalline polymer, the crystals are embedded in a matrix of amorphous polymer whose properties depend on the ambient temperature relative to its glass transition temperature. Thus, the overall elastic properties of the semicrystalline polymer can be predicted by treating the polymer as a composite material... 

The SFA, originally developed by Tabor and Winterton [56], and later modified by Israelachvili and coworkers [57,58], is ideally suited for measuring molecular level adhesion and deformations. The SFA, shown schematically in Fig. 8i,ii, has been used extensively to measure forces between a variety of surfaces. The SFA combines a Hookian mechanism for measuring force with an interferometer to measure the distance between surfaces. The experimental surfaces are in the form of thin transparent films, and are mounted on cylindrical glass lenses in the SFA using an appropriate adhesive. SFA has been traditionally employed to measure forces between modified mica surfaces. (For a summary of these measurements, see refs. [59,60].) In recent years, several researchers have developed techniques to measure forces between glassy and semicrystalline polymer films, [61-63] silica [64], and silver surfaees [65,66]. The details on the SFA experimental procedure, and the summary of the SFA measurements may be obtained elsewhere (see refs. [57,58], for example.). 

The model has also been found to work well in describing the mechanics of the interface between the semicrystalline polymers polyamide 6 and polypropylene coupled by the in-situ formation of a diblock copolymer at the interface. The toughness in this system was found to vary as E- where E was measured after the sample was fractured (see Fig. 8). The model probably applied to this system because the failure occurred by the formation and breakdown of a primary craze in the polypropylene [14],... 

Creton, C., Kramer, E.J., Hui, C.-Y. and Brown, H.R., Failure mechanisms of polymer interfaces reinforced with block copolymers. Macromolecules, 25, 3075-3088 (1992). Boucher et al., E., Effects of the formation of copolymer on the interfacial adhesion between semicrystalline polymers. Macromolecules, 29, 774-782 (1996). 

These differences on the stress-strain behavior of P7MB and PDTMB show the marked influence of the mesomorphic state on the mechanical properties of a polymer. When increasing the drawing temperatures and simultaneously decreasing the strain rate, PDTMB exhibits a behavior nearly elastomeric with relatively low modulus and high draw ratios. On the contrary, P7MB displays the mechanical behavior typical of a semicrystalline polymer. 

PTEB-Q) to the annealed ones, owing to the presence of the crystalline phase. Moreover, the temperature of the peak increases with the annealing, as well as the broadness of the relaxation. These results suggest that the liquid crystalline phase gives raise to an a relaxation similar to that of amorphous polymers despite the existence of the two-dimensional order characteristic of smectic mesophases, and it changes following the same trend than that of semicrystalline polymers. 

Fig. 12.13C-NMR spectrum of erythrodiisotactic poly(l,2-dimethyltetramethylene) at 75.47 MHz and 303 K. a) in solution of CDC13, b) CP-MAS spectrum of the semicrystalline polymer in the bulk. Chemical shifts given at the signals refer to TMS = 0 ppm. (Ref.20))...

The present review is devoted to recent advances, mainly concerning the characterization of semicrystalline polymers, specifically polyethylene (PE) by means of microindentation hardness. The author has chosen to organize the chapter as follows ... 

When the polymeric material is compressed the local deformation beneath the indenter will consist of a complex combination of effects. The specific mechanism prevailing will depend on the strain field depth round the indenter and on the morphology of the polymer. According to the various mechanisms of the plastic deformation for semicrystalline polymers 40 the following effects may be anticipated ... 

Aliphatic polyesters are low-melting (40-80°C) semicrystalline polymers or viscous fluids and present inferior mechanical properties. Notable exceptions are poly (a-hydroxy acid)s and poly (ft -hydroxy acid)s. 

Wholly aromatic polyesters, in which both R1 and R2 are aromatic, are either high-7 amorphous polymers or veiy high melting semicrystalline polymers that often exhibit liquid crystalline properties. 

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