Polymers, crystalline

Some crystalline polymers, e.g., polyolefin polymers such as poly-(ethylene), poly(propylene) (PP) and poly(l-butene), and polyfester) polymers such as polyfethylene terephthalate), and poly(amide) (PA) polymers have a slow rate of crystallization after heat forming. Consequently, the molding cycle when they are processed is too long, and as crystallization proceeds even after molding is complete, the molded product is sometimes deformed. In addition, there is the disadvantage that these crystalline polymer compound materials formed large spheroid crystals, so their mechanical strength and transparency is poor. 

These disadvantages are due to the crystalline properties of the crystalline polymer compounds. The solution is to form fine crystals very rapidly. In order to do this, the crystallization temperature may be increased, or crystal nucleating agents and crystallization promoters may be added (6). 

1 The frlnged-micelle model of semi-crystalline polymers 

Early studies of the structure of semi-crystalline polymers by Bunn and Alcock (1945) and Bryant (1947), using X-ray diffraction, showed considerable line broadening in addition to typical crystallographic diffraction rings in bulk 

Numerous account of the dielectric properties of partially crystalline polymers are available [3,12,14,17,44,45]. Two classes partially crystalline polymers are important, those (rf hi crystallinity, such as polyethylmre, i-polypropylene and polyoxymethylene, and those having only a medium degree of crystallinity, such as the nylons and polyethylene terephthalate (up to 50% crystallinity). Multiple relaxations are observed, e.g. lightly oxidized and lightly chlorinated polyethylenes have, in descending order of temperature, and relaxations. 

These have been documented by Ashcraft and yd [46] and others [3,4,5]. The process in polyethylenes was first explained by Frdhlich [47] using a chain-twist-assisted rotational model in an alkane crystaL Subsequently Hoffinan et aL [44] and Williams et al. [48] extended the theoretical model and applied it successfully to polyethylenes and alkanes of different chain lengths. Further development of the chain-twist-assisted rotation and model was made by Mans- 

For accounts of DRS studies of other crystalline polymers, including poly-oxymethylene, polyvinylidene difluoride, polyvinyl fluoride and the nylons, the reader is referred to the text by McCnim et al. [3], the reviews [12-14,17,23,44, 45] and references therein. In all cases multiple dielectric relaxations are observed, arising from motions within crystals, on crystal surfaces and in the constrained amorphous regions within crystals. These processes are also observed in NMR and mechanical relaxation studies of such polymers. 

The most direct evidence of the crystallinity in polymers is provided by x-ray diffraction studies. The x-ray patterns of many crystalline polymers show both sharp features associated with regions of three-dimensional order, and more diffuse features characteristic of molecularly disordered substances like liquids. The occurrence of both types of feature is evidence that ordered regions (called crystallites) and disordered regions coexist in most crystalline polymers. X-ray scattering and electron microscopy have shown that the crystallites are made up of lamellae which are built-up of folded polymer chains as explained below. 

It is evident from the ductility and strength of polymers that the ties between lamellae must be stronger than the van der Waal s forces holding neighboring, parallel fold planes together. Evidently some molecules (tie molecules) must participate in the growth of two or more adjacent lamellae, thereby providing relatively short molecular links between the lamellae. 

The formation of such fibrilar links between lamellae has actually been demonstrated [20]. 

The Avrami equation [Eq. (2.15)], which was originally proposed in the general context of phase changes, has provided the starting point for many studies of polymer crystallization and spherulitic growth. It relates the fraction of a sample still molten, 9, to the time, t, which has elapsed since crystallization began. The temperature must be held constant. 

For a given system under specified conditions, Z and n are constants and, in theory, they provide information about the nature of the crystallization process. Taking logarithm twice in succession gives 

These materials exhibit complicated behaviour, which depends on the degree of crystallinity and the detailed morphology. In general, relaxations can occur within the amorphous phase, within the crystalline phase, within both phases or be associated with specific details of the morphology, e.g. with the movement of chain folds. For these reasons the detailed interpre- 

It is not easy in mechanical measurements to vary the frequency over a wide range, as is possible for measurements of dielectric relaxation, so that direct determination of relaxation strengths as defined in section 7.6.1 is not usually possible. A further complication is that the amorphous and crystalline regions are coupled together mechanically, so that they do not contribute independently to the spectrum of relaxations 

The data shown in fig. 7.18 for polytetrafluoroethylene (PTFE) provide a good example of how the variation of shear modulus and tanS with temperature can depend strongly on the degree of crystallinity of the sample. Because tan 3 for the y relaxation increases as the crystallinity decreases and tan 3 for the p relaxation decreases, the former is associated with the amorphous material and the latter with the crystalline material. The behaviour of tan 3 for the a relaxation strongly suggests that, in this polymer, it is associated with the amorphous regions. 

If a polymer molecule has a sufficiently regular structure it may be capable of some degree of crystallisation. The factors affecting regularity will be discussed in the next chapter but it may be said that crystallisation is limited to certain linear or slightly branched polymers with a high structural regularity. Well-known examples of crystalline polymers are polyethylene, acetal resins and polytetrafluoroethylene. 

From a brief consideration of the properties of the above three polymers it will be realised that there are substantial differences between the crystallisation of simple molecules such as water and copper sulphate and of polymers such as polyethylene. The lack of rigidity, for example, of polyethylene indicates a much lower degree of crystallinity than in the simple molecules. In spite of this the presence of crystalline regions in a polymer has large effects on such properties as density, stiffness and clarity. 

The properties of a given polymer will very much depend on the way in which crystallisation has taken place. A polymer mass with relatively few large spherulitic structures will be very different in its properties to a polymer with far more, but smaller, spherulites. It is thus useful to consider the factors affecting the formation of the initial nuclei for crystallisation (nucleation) and on those which affect growth. 

Homogeneous nucleation occurs when, as a result of statistically random segmental motion, a few segments have adopted the same conformation as they would have in a crystallite. At one time it was considered that the likelihood of the formation of such nuclei was greatest just above the transition temperature 

Because polymers have a very low thermal conductivity, compared with metals, cooling from the melt proceeds unevenly, the surface cooling more 

In the future, there may be applications in the area of nonlinear optical components and, for side-group LCPs, in held-orientation devices (Donald and Windle, 1992). 

As with small-molecule materials, liquid crystallinity in polymers generally occurs at conditions intermediate between those for which isotropic liquid and crystalline solid states 

Rigid units and flexible connectors on main chain 

In main-chain LCPs, molecular flexibility can be distributed more-or-less uniformly along the chain, as is the case for PBLG, HPC, or Vectra A, or it can be concentrated in flexible spacers, as in OQO(phenylsulfonyl)lU (see Fig. 11-2). The former are called persistently flexible molecules, and are often modeled by the worm-like chain, with a uniform bending modulus, while for the latter, a reasonable model might be the freely jointed chain (see Fig. 11-3 and Section 2.2.3.2). For a recent discussion of the phase behavior and dynamics of worm-like chains, see Sato and Teramoto (1996). 

In the case of isotactic polypropylene, the methyl groups dictate a helical conformation to the backbone and this results in a regular structure which is able to crystallize readily. 

HCH3 HCH3H3CH HCH3 atactic poly (propylene) 

Maximum usage temperature Higher Lower High 

Their UL (Underwriters Laboratory) continuous-use rating for electrical properties is as high as 240°C (464°F), and for mechanical properties it is 220°C (428°F) permiting products to be exposed to intermittent temperatures as high as 315°C (600°F) without affecting performance properties. Their resistance to high-temperature flexural creep is excellent, as are their fracture-toughness characteristics. 

If required, the glass transition temperature can be reduced by adding low molar mass liquids. These act as plasticizers. The most common polymer that is plasticized is poly(vinylchloride) (PVC), to which phtha-lates are added to soften it for use in transparent films and wrappings (these additives are currently raising concern because they persist in the environment). 

7 CRYSTALLINE POLYMERS 2.7.1 Melt Versus Solution Crystallization 

The best method for preparation of single crystals of polymers is to grow them from dilute solution. Usually such crystals are small, typically a few micrometres across, and are plate-like with a regular shape that reflects that of the crystal unit cell. The thickness is typically 10 nm. By analysis of electron diffraction patterns from single crystals of polyethylene (the first polymer to be crystallized in this way), it was shown that the polymer 

The folded chains arrange themselves in layers, and this is the unit of the next level of structure, crystal lamellae. In general, there is significant disorder of the folding, as sketched in Fig. 2.24, where some folds are tight 

The lamellae in turn can arrange themselves into a number of su-perstuctures depending on molar mass, the crystallization temperature, whether the growth is confined to a surface or is in bulk and whether the polymer is oriented before or during crystallization. These superstructure morphologies are the third level of hierarchical structure. Considering 

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Recently, a unique approach for using the correlation fiinction method has been demonstrated to extract morphological variables in crystalline polymers from time-resolved syncluotron SAXS data. The principle of the calculation is based on two alternative expressions of Porod s law using the fonu of interference fiinction [33. 36]. This approach enables a continuous estimate of the Porod constant, corrections for liquid scattering... 

For crystalline polymers, the bulk modulus can be obtained from band-structure calculations. Molecular mechanics calculations can also be used, provided that the crystal structure was optimized with the same method. 

No polymer is ever 100% crystalline at best, patches of crystallinity are present in an otherwise amorphous matrix. In some ways, the presence of these domains of crystallinity is equivalent to cross-links, since different chains loop in and out of the same crystal. Although there are similarities in the mechanical behavior of chemically cross-linked and partially crystalline polymers, a significant difference is that the former are irreversibly bonded while the latter are reversible through changes of temperature. Materials in which chemical cross-linking is responsible for the mechanical properties are called thermosetting those in which this kind of physical cross-linking operates, thermoplastic. 

The loop formed by the chain as it emerges from the crystal, turns around, and reenters the crystal may be regarded as amorphous polymer, but is insufficient to account for the total amorphous content of most crystalline polymers. 

Crystalline fructose Crystalline nylons Crystalline platelets Crystalline polymers Crystalline polypropyL Crystalline Si Crystalline silica... 

Polyoxyethylene. Synthetic polymers with a variety of compositionaHy similar chemical stmctures are as follows. Based on polarity, poly(oxymethylene) (1) would be expected to be water soluble. It is a highly crystalline polymer used in engineering plastics, but it is not water-soluble (see... 

The sorption behavior of perfluorocarbon polymers is typical for nonpolar partially crystalline polymers (89). The weight gain strongly depends on the solubihty parameter. Litde sorption of substances such as hydrocarbons and polar compounds occurs. 

Unlike most crystalline polymers, PVDF exhibits thermodynamic compatibiUty with other polymers (133). Blends of PVDF and poly(methyl methacrylate) (PMMA) are compatible over a wide range of blend composition (134,135). SoHd-state nmr studies showed that isotactic PMMA is more miscible with PVDF than atactic and syndiotactic PMMA (136). MiscibiUty of PVDF and poly(alkyl acrylates) depends on a specific interaction between PVDF and oxygen within the acrylate and the effect of this interaction is diminished as the hydrocarbon content of the ester is increased (137). Strong dipolar interactions are important to achieve miscibility with poly(vinyhdene fluoride) (138). PVDF blends are the object of many papers and patents specific blends of PVDF and acryflc copolymers have seen large commercial use. 

Rigid-rod polymers are often Hquid crystalline polymers classified as lyotropic, such as the aramids Nomex and Kevlar, or thermotropic Hquid crystalline polymers, such as Vectran. 

P. W. Lenz, Recent Advances in Fiquid Crystalline Polymers, Elsevier Apphed Science Pubhshets, Denmark, 1983. 

Second, in the early 1950s, Hogan and Bank at Phillips Petroleum Company, discovered (3,4) that ethylene could be catalyticaHy polymerized into a sohd plastic under more moderate conditions at a pressure of 3—4 MPa (435—580 psi) and temperature of 70—100°C, with a catalyst containing chromium oxide supported on siUca (Phillips catalysts). PE resins prepared with these catalysts are linear, highly crystalline polymers of a much higher density of 0.960—0.970 g/cnr (as opposed to 0.920—0.930 g/cnf for LDPE). These resins, or HDPE, are currentiy produced on a large scale, (see Olefin polymers, HIGH DENSITY POLYETHYLENE). 

The Arrhenius relationship (eq. 5) for crystalline polymers or other transitions, where E is the activation energy and R the gas constant (8.3 J/mol), is as follows ... 

Since 1980, mthenium tetroxide, RuO, has been used for staining a number of heterophase polymers for tern (221) it seems to be a more versatile staining agent than OsO. For instance, in SAN modified with acrylate mbber, where the mbber phase is fully saturated, an excellent contrast between the mbber and the matrix has been achieved (222). Crystalline polymers have been stained with RuO (223), and excellent cra2e stmctures have been revealed (221). The stain may be prepared by dissolving RuCl - 3H2O in aqueous sodium hypochlorite for immediate use (224). 

Polymer crystals most commonly take the form of folded-chain lamellae. Figure 3 sketches single polymer crystals grown from dilute solution and illustrates two possible modes of chain re-entry. Similar stmctures exist in bulk-crystallized polymers, although the lamellae are usually thicker. Individual lamellae are held together by tie molecules that pass irregularly between lamellae. This explains why it is difficult to obtain a completely crystalline polymer. Tie molecules and material in the folds at the lamellae surfaces cannot readily fit into a lattice. 

Crystalline polymers undergo a discontinuous decrease in volume when cooled through (Fig. 4). This can lead to nonuniform shrinkage and warping in molded objects. On the other hand, it also causes the polymer to "lock on" to reinforcing fibers, eg, glass (qv), so that crystalline thermoplastics benefit much more than amorphous thermoplastics from fiber reinforcement. 

Master curves can also be constmcted for crystalline polymers, but the shift factor is usually not the same as the one calculated from the WLF equation. An additional vertical shift factor is usually required. This factor is a function of temperature, partly because the modulus changes as the degree of crystaHiuity changes with temperature. Because crystaHiuity is dependent on aging and thermal history, vertical factors and crystalline polymer master curves tend to have poor reproducibiUty. 

R. A. Weiss and C. K. Ober, ed., Eiquid-Crystalline Polymers, American Chemical Society, Washington, D.C., 1990. 

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