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Crystal Lattice Contents are copyrighted (有版權). Do not use, distribute, or copy the contents of the lecture notes without permission. Do not use outside class room purposes without permission!!! Advanced Polymer Physics Lecture notes by Prof. E. M. Woo adapting from: Textbook of Physical Polymer Science (Ed: L. H. Sperling, 3rd Ed) Chap 6: Part-I [Sections 6.1~6.3] Polymers in Crystalline State + Basic of Crystal Lattices + XRD methods -Lecture notes 1 Introduction • The development of crystallinity in polymers depends on the regularity of structure in the polymer (see Chapter 2). Thus, isotactic and syndiotactic polymers usually crystallize, whereas atactic polymers, with a few exceptions (where the side groups are small or highly polar), do not. • Regular structures also appear in the polyamides (nylons), polyesters, and so on, and these polymers make excellent fibers. • Nonregularity of structure first decreases the melting temperature and finally prevents crystallinity. Mers of incorrect tacticity (see Chapter 2) tend to destroy crystallinity, as does copolymerization. Thus statistical copolymers are generally amorphous. • Blends of isotactic and atactic polymers show reduced crystallinity, with only the isotactic portion crystallizing. • Under some circumstances block copolymers containing a crystallizable block will crystallize; again, only the crystallizable block crystallizes. • Factors that control the melting temperature include polarity and hydrogen bonding as well as packing capability. • Table 6.1 lists some important crystalline polymers and their melting temperatures (1). 2 Table 6.1 Properties of selected crystalline polymers (1) 3 6.1 GENERAL CONSIDERATIONS In this chapter the study of crystalline polymers is undertaken. • The crystalline state is defined as one that diffracts X-rays and exhibits the first- order transition known as melting. • A first-order transition normally has a discontinuity in the volume-temperature dependence, as well as a heat of transition, ∆Hf, also called the enthalpy of fusion or melting. • The most important second-order transition is the glass transition, Chapter 8, in which the volume-temperature dependence undergoes a change in slope, and only the derivative expansion coefficient, dV/dT, undergoes a discontinuity. There is no heat of transition at Tg, but rather a change in the heat capacity, ∆Cp. • Polymers crystallized in the bulk, however, are never totally crystalline, a consequence of their long-chain nature and subsequent entanglements. • The melting (fusion) temperature of the polymer, Tf, is always higher than the glass transition temperature, Tg. • Thus, the polymer may be either hard and rigid or flexible – depending on its Tm, Tg. o • An example of the latter is ordinary polyethylene, which has a Tg of about -80 C and a melting temperature of about +139oC. At room temperature it forms a leathery product as a result. 4 6.1.1 Polymers in Crystal - Historical Aspects • Historically the study of crystallinity of polymers was important in the proof of the Macromolecular Hypothesis, developed originally by Staudinger. • In the early 1900s when X-ray studies were first applied to crystal structures, scientists found that the cell size of crystalline polymers was of normal size (about several Angstoms on a side). • In the case of ordinary sized organic molecules, each cell was found to contain only a few molecules. If polymers were composed of long chains, they asked Staudinger, how could they fit into the small unit cells? • The density of such a material would have to be 50 times lead!! • The answer, developed by Mark and co-workers (2-7), and others, was that the unit cell contains only a few mers that are repeated in adjacent unit cells. The molecule continues in the adjacent axial directions. • Mark showed that the bond distances both within a cell and between cells were consistent with covalent bond distances (1.54Å for carbon-carbon bonds) and inconsistent with the formation of discrete small molecules. • For these and many other advances (see Section 3.8) and a life-long leadership in polymers (he died at age 96 in 1992), Herman Mark was called the Father of Polymer Science. 5 Examples in polysaccharide cellulose • One of the first structures to be determined was the natural polysaccharide cellulose. In this case, the repeat unit is cellobiose, composed of two glucoside rings. • In the 1980s, l3C NMR experiments established that native cellulose is actually a composite of a triclinic parallel-packed unit cell called cellulose-Iα [produced by bacteria], and a monoclinic parallel-packed unit cell called cellulose-Iβ [produced by higher plants]. [both are native cellulose] • Experimentally, the structures are only difficultly distinguishable via X-ray analysis (7a,7b). • Figure 6.1 (3) [next] illustrates the general form of the cellulose unit cell. 6 Figure 6.1 The crystalline structure of cellulose. (a) The unit cell of native cellulose, or cellulose I, as determined by X-ray analysis (2,10,11). Cellobiose units are not shown on all diagonals for clarity. The volume of the cell is given by V= abc sin β. (b) Unit cell dimensions of the four forms of cellulose. (c) Known pathways to change the crystalline structure of cellulose. The lesser known form of cellulose x is also included (10) 7 Cellulose structure - conferring rigidity to plant cells. Cellulose has high Tm. Cellulose requires a temperature of 320 °C and pressure of 25 MPa to become amorphous in water.[14] - Without water, cellulose decomposes without showing melting (very high T ). m Note: - I, II, III, and IV Natural cellulose is cellulose I, with structures Iα and Iβ. Cellulose produced by bacteria and algae is enriched in Iα while cellulose of higher plants consists mainly of Iβ. Cellulose in regenerated cellulose fibers is cellulose II. The conversion of cellulose I to cellulose II is irreversible, suggesting that cellulose I is metastable and cellulose II is stable. With various chemical treatments it is possible to produce the structures cellulose III and cellulose IV.[15] 8 • When vinyl, acrylic, and polyolefin polymers were first synthesized, the only microstructure then known was atactic. Most of these materials were amorphous. • Scientific advancement waited until Ziegler's work on novel catalysts (8), together with Natta's work on X-ray characterization of the stereospecific polymers subsequently synthesized (9), when isotactic and syndiotactic crystalline polymers became known. • For this great pioneering work, Ziegler and Natta were jointly awarded the 1963 Nobel Prize in Chemistry (see Appendix 5.3). • The general class of these catalysts are known today as Ziegler-Natta catalysts. 9 Some commercially useful semicrystalline polymers Of course, crystalline polymers constitute many of the plastics and fibers of commerce. • Polyethylene (PE) is used in films to cover dry-cleaned clothes, and as water and solvent containers (e.g., wash bottles). • Polypropylene (PP) makes a highly extensible rope, finding particularly important applications in the marine industry. • Polyamides (nylons) and polyesters are used as both plastics and fibers. Their use in clothing is world famous. • Cellulose, mentioned above, is used in clothing in both its native state (cotton) and its regenerated state (rayon). The film is called cellophane. 10 6.1.2 Melting Phenomena • The melting of polymers may be observed by any of several experiments. For linear or branched polymers, the sample becomes liquid and flows. However, there are several possible complications to this experiment, which may make interpretation difficult. First of all, simple liquid behavior may not be immediately apparent because of the polymer's high viscosity. • If the polymer is cross-linked, it may not flow at all. • It must also be noted that amorphous polymers soften at their glass transition temperature, Tg , which is emphatically not a melting temperature but may resemble one, especially to the novice (see Chapter 8). • If the sample does not contain colorants, it is usually hazy in the crystalline state because of the difference in refractive index between the amorphous and crystalline portions. On melting, the sample becomes clear, or more transparent. • The disappearance of crystallinity may also be observed in an OM microscope, for example, between crossed Nicols – i.e., POM. • The sharp X-ray pattern characteristic of crystalline materials gives way to amorphous halos at the melting temperature, providing one of the best experiments. 11 • Another important way of observing the melting point is to observe the changes in specific volume with temperature. Since melting constitutes a first-order phase change, a discontinuity in the volume is expected. • Ideally, the melting temperature should give a discontinuity in the volume, with a concomitant sharp melting point. In fact, because of the very small size of the crystallites in bulk crystallized polymers (or alternatively, their imperfections), most polymers melt over a range of several degrees (see Figure 6.2) (12) [next]. • The melting temperature is usually taken as the temperature at which the last trace of crystallinity disappears. This is the temperature at which the largest and/or most perfect crystals are melting. • The volumetric coefficients of expansion in Figure 6.2 can be calculated from α = (1/V)(dV/dT)p. • Alternatively, the melting temperature can be determined thermally. Today, the differential scanning calorimeter (DSC) is popular, since it gives the heat of fusion as well as the melting
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