Crystalline Polymer
Crystalline polymers are a type of polymer with a highly ordered molecular structure, resulting in a regular and repeating pattern. This arrangement gives them a distinct crystalline appearance and properties such as stiffness, strength, and high melting points. The ordered structure of crystalline polymers makes them more resistant to deformation and provides them with unique mechanical and thermal characteristics.
5 Key excerpts on "Crystalline Polymer"
eBook - ePub- Marianne Gilbert(Author)
- 2016(Publication Date)
- Butterworth-Heinemann(Publisher)
...Also, crystalline structures contain a hierarchy of structures which are not produced when smaller molecules crystallize. In spite of these differences, the presence of crystalline regions in a polymer has large effects on such properties as density, stiffness, and clarity. Crystalline Polymers should strictly be referred to as semicrystalline. Regular and sufficiently flexible molecules produce a structure that contains some 3-dimensional order, or crystallinity. SemiCrystalline Polymers are typically opaque because they contain two distinct phases with different densities, hence different refractive indices. Their mechanical properties depend on their transition temperatures. The essential difference between the traditional concept of a crystal structure and Crystalline Polymers is that the former is a single crystal while the latter is polycrystalline, unless crystals are grown from very dilute solutions. A single crystal implies a crystalline particle grown without interruption from a single nucleus and relatively free from defects. The term polycrystallinity refers to a state in which clusters of single crystals are involved, developed from the more or less simultaneous growth of many nuclei. The resulting conglomerate may possess no readily discernible symmetry. Polycrystallinity occurs not only in polymers but also in metals and, unless care is taken, in the large-scale commercial crystallization of materials such as sucrose and sodium chloride. Structures associated with the crystallinity in polymers are complex. Crystals themselves are smaller and less perfect than in conventional crystalline materials, and are generally known as crystallites. Within the crystallites are a number of other, smaller ordered structures; the crystallites themselves aggregate into larger structures, known as spherulites. 3.3.2. Crystallinity in Polymers The crystalline state is the lowest energy state, and as such, the most stable form of a material...
eBook - ePub- Leslie H. Sperling(Author)
- 2015(Publication Date)
- Wiley-Interscience(Publisher)
...Starch and bread are discussed in Section 14.3, and silk fibers in Section 14.4; both are semicrystalline materials. The hierarchical structure of polymers has been reviewed by Baer and co-workers (167,168). They emphasize that the occurrence of crystallinity in synthetic polymers requires a sufficiently stereoregular chemical structure, so that the chain molecules can pack closely in parallel orientation. Structure at the nanometer level is generally determined by the pronounced tendency of these chains to crystallize by folding back and forth within crystals of thin lamellar habit. Chain folding itself is a kinetic phenomenon. Given sufficient annealing time, the lamellar thickness increases until the chains are all straight at thermodynamic equilibrium. When crystallization occurs in flowing solutions or melts, fine fibrous crystals may be produced that consist of highly extended chains oriented axially. This will be recognized as the basis for the supermolecular structure in fibers. Even after prolonged crystallization from the melt, there always remains an appreciable fraction of a disordered phase known as amorphous polymer. If the polymer can crystallize, the appearance of amorphous material is also a kinetic phenomenon, which at equilibrium should disappear. However, the physical entanglements and already existing crystalline structure may slow further crystallization down to substantially zero. While amorphous materials are significantly less organized than their crystalline counterparts, entanglements, branches, and cross-links significantly control their properties. When two polymers are blended, they generally phase-separate to create a super-molecular morphology, which may be of the order of tens of micrometers, while graft and block copolymers exhibit morphologies of the order of hundreds of angstroms. 6.12 HOW DO YOU KNOW IT’S A POLYMER? Suppose that a polymer scientist watches a demonstration...
eBook - ePubFundamentals of Polymer Science
An Introductory Text, Second Edition
- Michael M. Coleman, Paul C. Painter(Authors)
- 2019(Publication Date)
- CRC Press(Publisher)
...Second, even low molecular weight materials form crystals that contain defects and these can profoundly affect properties. We shouldn’t think that crystalline materials have a completely perfect structure. (Anybody who buys a diamond usually learns this very quickly.) Finally, in low molecular weight materials a molecule is smaller than the size of a unit cell. In polymers this is not so and individual chains pass through many unit cells. The details of the determination of polymer crystal structure are beyond the scope of what we want to cover here. We will simply observe that X-ray crystallography played a key role in early studies of polymers and the establishment of the macromolecular hypothesis and proceed to describe two or three typical examples of polymer unit cell structures. This will make it immediately clear why some polymers crystallize and others do not. Figure 7.22 shows a representation of the unit cell of polyethylene. There are three things you should notice. First, as we have just mentioned, only a small part of each chain lies in a unit cell. Accordingly, a knowledge of the arrangement of chains in the unit cell is a sort of local knowledge, in the sense that we do not know what sections of the rest of the chain are doing. Are all the segments also in the crystal or are some in those amorphous regions that we know are also present in polymers? Second, the chains are in the preferred, minimum energy, all trans or zig-zag conformation. This is generally, but not always, the rule for polymer crystals, particularly if there are several conformations of almost equal energy. Finally, the crystal structure is close packed, as one might expect if intermolecular attractions are to be maximized. This means that defects, such as short chain branches, generally cannot be accommodated in a crystal lattice (some small defects are occasionally incorporated into certain polymer crystals, but these naturally distort the lattice)...
eBook - ePubPlastics
Microstructure and Engineering Applications
- Nigel Mills, Mike Jenkins, Stephen Kukureka(Authors)
- 2020(Publication Date)
- Butterworth-Heinemann(Publisher)
...Introduction This chapter considers semi-Crystalline Polymers. Knowledge of the crystallization behaviour of polymers is important as it underpins the mechanisms by which the shapes of plastics products are set during processing. Crystallizable polymers must be cooled so that they are given sufficient time to crystallize. The order of presentation in this chapter is that of increasing size scale: bonding in the crystal unit cell, the shape of lamellar crystals, the microstructure of spherulites, the nature of the amorphous interlamellar phase, the overall crystallinity and the processes of crystallization. Methods to determine the degree of crystallinity are outlined including density measurements, calorimetry and X-ray scattering. Orientation in polymers is also considered briefly. 4.2. Structure and shape 4.2.1. Crystal lattice and unit cell Before discussing crystal structures, the concepts of a unit cell, motif and symmetry operator must be defined. A unit cell is a building block of the crystal lattice; it is stacked in a regular pattern to fill space. In crystallographic terminology, the unit cell is repeated by translation by h a + k b + l c, where h, k and l take all integer values and a, b and c are the lattice vectors. The unit cell for polyethylene (PE) is orthorhombic; the lattice vectors are orthogonal, but their lengths are unequal. Most other polymer unit cells have lower symmetry; that of polypropylene (PP) is triclinic, with non-orthogonal lattice vectors of non-equal length. A motif is a group of atoms repeated by symmetry operators to make the unit cell. The motif for PE is the CH 2 group, whereas for PP it is the atoms – CH 2 – CH(–CH 3)– from the monomer. In Fig. 4.1 the motif in PE is shown as a group of three dark atoms...
eBook - ePub- Michael Jaffe, Joseph D. Menczel(Authors)
- 2020(Publication Date)
- Woodhead Publishing(Publisher)
...H. Mark (Guth and Mark, 1934 ; Mark, 1936) suggested that the tensile modulus of polymers should correlate with both the chemical and physical structures of the macromolecule. It was further recognized that maximum property levels would be achieved when all of the molecular chain backbone bonds of the polymer were lined up in the direction of measurement. Such an extended chain morphology has been demonstrated with gel spun polyethylene (Smith and Lemstra, 1980 ; Kavesh et al., 2006) and with nematogenic polyamides and polyesters (Jaffe et al., 2018). It is known that polymers can exhibit all the mesophases (liquid crystalline phases) associated with low molar mass molecule liquid crystals (Jaffe, 1987). Polymer liquid crystalline behavior can be a function of main-chain or side-chain chemistry, and polymers may exhibit liquid crystalline properties in the melt (thermotropic) or in solution (lyotropic) or in both. For detailed discussions of the chemistry, physics, and applications of liquid Crystalline Polymers (LCPs), see the extensive open (Jaffe, 1987, 1991, 2018) and the extensive patent literature (Dupont, Celanese). Only main-chain LCPs exhibiting nematic behavior in the fluid state have found fiber applications. All of the LCPs are composed of stiff, highly aromatic monomers and are characterized by domains of high local orientation in the solid state (orientation function >0.95). If processed into fibers, the locally oriented domains are transformed into a single domain of high global molecular orientation parallel to the fiber direction. LCP fibers are characterized by very high-specific tensile properties (property divided by density) when compared to metals or ceramics. The highly anisotropic nature of oriented LCP fibers causes inherent weakness in shear and compression, limiting their use almost exclusively to applications in tension...
