Defects in Polymer Crystals
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Defects in Polymer Crystals Bemhard Wunderlich and Stefan N. Kreitmeier Introduction The development of early knowledge about crystals of flexible macromolecules* (usually called simply "polymers") and their defects was reviewed in Reference 1. Much of the initial research followed the information gathered on
crystals of small molecules, metals, salts, and ceramics.2 Very quickly, however, the special nature of polymers became obvious. It was suggested some time ago to describe semicrystalline polymers with the help of zero- to three-dimen-
* Flexible macromolecules, the subject of this article, are to be distinguished from rigid macromolecules (such as diamond, metals, salts, and poly-p-phenylene) and small molecules (molecules of less than 1,000 atoms, such as water, paraffins, and argon).1
Crystalline Diffraction of PET Fibers ° 07J
Total X-Ray Diffraction of PET Fibers ° 0 7 0.06
(computed from the total X-ray pattern by Fourier filtering)
(absorption, 0 0 5 polarization, Compton, corrected) 0.03-I Sample: 1X90 0.02
0.04
0.06
S (nm"1)
22% of total
0.02
0.04
0.06
S (nnrT1)
0.02
0.04
0.06
S (nm"')
Intermediate z Phase X-Ray (nm-1) Diffraction 007
Isotropic Component in PET
0.06
(anisotropic part of the noncrystalline X-ray pattern)
(Isotropic part of the noncrystalline X-ray pattern) 42% of total
QQS
0.04 0.03 0.02 0.01
0.02
0.04
0.06
S (nrn -1 )
36% of total
Figure 1. Diffraction pattern of a polyfethylene terephthalate) (PET) fiber and the separated patterns from the three constituent phases.
MRS BULLETIN/SEPTEMBER 1995
17
Defects in Polymer Crystals
Often the sizes of the different phases can be as small as the nanometer scale.** One-dimensional or line defects, also called dislocations, became obvious in polymers with the first observations of solution-grown crystals of polyethylene that showed frequent growth spirals.4 Because of the molecular chain structure, many of these dislocations are sessile (immobile) and cannot participate, for example, in deformation mechanisms. Until recently, zero-dimensional or point defects were understood largely because of molecular-mechanics calculations.5 During the last five years, it has become possible in our research group, through the efforts of D.W. Noid, B.G. Sumpter, and G.L. Liang, to simulate crystals of polymers with up to 30,000 atoms, using a molecular-dynamics (MD) approach, thus enabling us to study the point defects.6 During this time, approximately 8,000 h of CPU time from the local supercomputers was used. On the basis of this work, a new approach to the description of properties of semicrystalline polymers is possible and will be discussed in this article.
Micro- and Nanophase Structures (Three-Dimensional Defects) In nonpolymeric crystals, the structure-insensitive properties like density, heat content, and modulus can be approximated by the ideal, defect-free crystal structure. This is not possible for crystals of flexible polymers. An added level of organization must be recognized, that of the "crystallinity." Struc
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