Melt-crystallized polymers

Fig. 1-8 Structural organization within a spherulite in melt-crystallized polymer.

Where a melt-crystallized polymer has been processed by drawing, rolling or other means to produce an aligned structure in which lamellae as well as polymer chains have discernible order, a pseudocrystalline unit cell is present. Provided that this unit cell contains elements of the crystals as well as the boundaries between crystals and that it is entirely typical of the material as a whole then it could be considered as a RVE within the meaning defined above. The lamella crystal itself sometimes considered as embedded in an amorphous matrix would not seem to be an acceptable RVE for reasons similar to those advanced against the Takayanagi model, namely that its modulus is dependent upon the surface tractions. The boundaries between lamella crystals in the matrix must be included in an acceptable RVE. 

It is much more common for melt-crystallized polymers to show a spherulitic type of structure. In this morphology, the crystallites are packed into spheres, except where... 

Because of chain folding, melt-crystallized polymers are not as strong as they could be. One can envisage that under a load a sample will at some point yield, with chains in the amorphous domains becoming oriented in the draw direction while the lamellar arms of the spherulite undergo shear and whole sections are pulled ont. This process is illustrated in Figure 8-65. 

As pointed out above, the semicrystalline polymer can be considered as a two-phase composite of amorphous regions sandwiched between hard crystalline lamellae (Fig. 4.2(a)). Crystal lamellae ( c) are normally 10-25 nm thick and have transverse dimensions of 0.1-1 pm while the amorphous layer thickness, a, is 5-10 nm. As mentioned in the previous section, melt-crystallized polymers generally exhibit a spherulitic morphology in which ribbon-like lamellae are arranged radially in the polycrystalline aggregate (Bassett, 1981). Since the indentation process involves plastic yielding under the stress field of the indenter, microhardness is correlated to the modes of deformation of the semicrystalline polymers (see Chapter 2). These... 

C), it has been observed that its crystallization from the melt is enhanced [103-106], Melt crystallized polymers nucleated with n-s polymer-CD-ICs crystallize more rapidly, evidence greater levels of crystallinity, higher melt crystallization temperatures, and semicrystalline morphologies characterized by crystals which are smaller and more uniformly distributed than in un-nucleated pure bulk samples. 

Consequently, Rmax must begin to decrease with a further increase of pn beyond some value of pn to continue to satisfy Equation 6.36. In fact, the decrease of Rmax is likely to begin at a lower value of pn than the upper limit suggested by Equation 6.36, since xc does not increase to 1 with increasing pn. An amorphous fraction always remains in a melt-crystallized polymer. 

An experimental intensity curve as a function of the scattering vector is produced in SAXS. From the isotropic patterns of a melt-crystallized polymer, a slice is taken and, when projected in the plane I q) against q, it shows a scattering maximum with a wide statistical distribution. The evaluation of structural parameters from the intensity curve requires the whole scattering curve. Nonetheless, the lower and upper ends of the curve cannot be determined because of the nature of the scattering process. Therefore, mathematical approximations are used to access both ends of the curve. According to the characteristics of the dispersion function measured experimentally, the intensity curve is divided into three parts, which are described in the following sections. 

Whereas crystallization from dilute solutions may result in the formation of single polymer crystals, this perfection is not achieved when deahng with polymers cooled from the melt. The basic characteristic feature is still the lamellar-hke crystallite with amorphous surfaces or interfaces, but the way these are formed may be different, based on the careful investigation of melt-crystallized polymers using neutronscattering techniques. The two models that have been proposed to deseribe the fine structure of these lamellae and their surface characteristics in semicrystalline polymers differ mainly in the way the chains are thought to enter and leave the ordered lamellae regions. These are ... 

Hoffman, J. D. (1983) Regime III crystallization in melt-crystallized polymers the variable cluster model of chain folding, Polymer, 24, 3-26. 

The DSC curves at the bottom of Fig. 6.96 represent an analogous set of crystallization experiments, but grown from the melt. TTie melt-crystallized polymer shows higher perfection in both the annealing peak and the melting peak for the primary crystals. Ultimately, both peaks are expected to come together. 

Hoffman JD, Lauritzen JI (1961) Crystallization of bulk polymers with chain folding theory of growth of lamellar spherulites. J Res Natl Bur Stand 65A 297-336 Hoffman JD, Guttman CM, DiMarzio EA (1979) On the problem of crystallization of polymers from the melt with chain folding. Faraday Discuss Chem Soc 68 177-197 Hoffman JD (1983) Regime in crystallization in melt-crystallized polymers The variable cluster model of chain folding. Polymer 24 3-26... 

Fig. 7. Linear polyethylene (Rigidex 9) crystallized at 0-5 GPa while cooling at 1 Kmin . Note the splaying development of thick lamellae similar to that of melt-crystallized polymers with much thinner lamellae. Between the very thick leunellae are thinner layers comprising shorter molecules. Replica of an...

Plotting log G + li /R(T,. - TJ against tjfT AT, Straight lines result. As an example, the result for isotactic polypropylene is shown in Figure 4.18. According to the nucleation theories of melt-crystallized polymer developed at different... 

Polymer fiber technology is a field of research branching out into many diverse applications. Fashion, healthcare, smart materials, electronics and load bearing elements both on then-own and in composite materials are just a selection of some of the areas where they can be found. Regardless of their application, the smallest practical size of any pol5rmer fiber can be specified as approximately 50 nm. This can be inferred from the fact that the typical size of a polymer crystallite (lamella) has dimensions in the order of 5 to 50 nm [4]. Below this value (the case of a polymer crystallite) the structure has become so small that it takes on the form of an ordered array of atoms rather than that of a fiber. However, in most melt crystallized polymers it is these lamellae that aggregate and grow linearly to form fibrils [4]. 

Measurements have been made up to the q region 0.02—0.2 A the curves of q Ivs.q i20K now been obtained for PS, PE, " PP, and PEO. This information has been used to evaluate models of the structure of semi-crystalline polymers by Yoon and Flory and they were able to show that in the case ofPE the calculations based on an irregular re entry model fitted the experimental data. Similar calculations by Fischer on PEO also supports irregular re-entry. Sadler and Keeler have studied solution-grown PE crystals and obtained information of the trajectory within the crystals. They concluded that adjacent fold re-entry occurs with the latter but not with melt-crystallized polymer. 

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