Fold surface defects

On 0001 surfaces a rosette pattern is sometimes observed by electron microscopy. This could be decoration by lower-MW PTFE or it might be a result of fold surface defects (see Figure 1.6). It is of note that the hexagonal daughter crystal in this figure developed in an orthogonal relationship to the primary lamella. 

Luminescence lifetime depends upon radiative and nomadiative decay rates. In nanoscale systems, there are many factors that may affect the luminescence lifetime. Usually the luminescence lifetime of lanthanide ions in nanociystals is shortened because of the increase in nomadiative relaxation rate due to surface defects or quenching centers. On the other hand, a longer radiative lifetime of lanthanide states (such as 5Do of Eu3+) in nanocrystals can be observed due to (1) the non-solid medium surrounding the nanoparticles that changes the effective index of refraction thus modifies the radiative lifetime (Meltzer et al., 1999 Schniepp and Sandoghdar, 2002) (2) size-dependent spontaneous emission rate increases up to 3 folds (Schniepp and Sandoghdar, 2002) (3) an increased lattice constant which reduces the odd crystal field component (Schmechel et al., 2001). 

The crosslinks formed in less ordered regions at fold surfaces cause irregularities in the crystal lattice. Near the defect formed, a progression of further linking deeper down in the crystal on continued irradiation may be observed [58]. The result of irradiations is then a crosslinked polymer with a lower extent of crystalline r ons. 

Two final points need to be made about secondary nucleation. First, that screw-dislocation defects, described in more detail in Sect. 5.3, prodnce indesttnctible secondary nuclei for growth on top of the fold surfaces of polymer lamellae. This surface would otherwise be inactive for further growth and restrict polymer crystals to single lamellae (see Chap. 5). An example of a series of screw dislocations is shown in Fig. 3.72 on the example of poly(oxyethylene) of 6,000 molar mass grown... 

Zero-dimensional defects or point defects conclude the list of defect types with Fig. 5.87. Interstitial electrons, electron holes, and excitons (hole-electron combinations of increased energy) are involved in the electrical conduction mechanisms of materials, including conducting polymers. Vacancies and interstitial motifs, of major importance for the explanation of diffusivity and chemical reactivity in ionic crystals, can also be found in copolymers and on co-crystallization with small molecules. Of special importance for the crystal of linear macromolecules is, however, the chain disorder listed in Fig. 5.86 (compare also with Fig. 2.98). The ideal chain packing (a) is only rarely continued along the whole molecule (fuUy extended-chain crystals, see the example of Fig. 5.78). A most common defect is the chain fold (b). Often collected into fold surfaces, but also possible as a larger defect in the crystal interior. Twists, jogs, kinks, and ends are other polymer point defects of interest. 

The technique was first applied to polyethylene and i-polypropylene revealing itself to be sensitive to crystallinity, lamellar orientation, crystal structure, and crystal defects (55). Thus it penetrates between lamellae attacks lamellar side surfaces more than their fold surfaces, a- more than /3-i-polypropylene and removes dislocation cores preferentially. As with chlorosulfonation, it allows systematic study of samples by cutting them open where desired. But it can also reveal the particular character of external or fracture surfaces, neither of which is usually random. 

Morphology affects mechanical properties in two ways. First, the macroscopic property falls below that of the crystal lattice, by some three orders of magnitude for Yoimg s modulus, partly because of disorientation but more significantly because of the interruption of covalent bond sequences by fold surfaces. Second, boimdaries and defects such as cracks, crazes, voids and dislocations can cause or influence 5ueld, deformation, and fracture. A recent review of the effect of morphology on mechanical properties has been published recently (124). 

The changes in Q and Lb can be explained on the basis of crystals perfecting and/or melting during heating. The quenched sample contains a small population of very imperfect crystals which can become perfected through mechanisms such as melt-recrystallization, fold surface smoothing, or rejection of defects from the crystal. These would tend to increase the electron density difference between the crystal... 

Polymer crystallization can be described as a phase transition process from the disorder isotropic melt to the order semicrystalline one. The disorder state is characterized by the randomly coiled chains, while the order state is complex because it is formed by crystalline chain-folded lamellae surrounded by the amorphous chains that constitute the fold surfaces and the interlamellar regions. The amorphous interfaces are formed by entanglements, end groups, bulky substituent groups, and chain defects, all of which cannot be included into the crystalline lattice. Polymers form metastable (thin) lamella separated by intervening amorphous layers and are nearly always semicrystalline. 

There are many other possible types of imperfections in crystalline polymers as well as point-defects and dislocations. The fold surface and chain folds can be considered as defects. It is also possible to consider the grain boundaries between crystals as areas containing defects. 

If folding is a consequence of kinetics, it is an inherent surface defect that compromises the attainment of high degrees of crystallinity even under the most favorable circumstances. Multiple nucleation and topological contraints worsens this flawed situation even more as physical measurements have shown (eg. see situation summary in Ref. 155). Another puhhcation is in press. 

---