7 Defects

possible local disturbances in a crystal.
1. Electrons and electron holes
governing conductivity of crystals, of little importance for most polymers.
2. Excitons
F-2 Surface defects
The surface defects can be discussed in the following four
groups:
1. Free surface
Parameters: Depth of layer of surface Distribution of folds Concentration and position of chain ends in the surface Frequency, length and location of isolated loose chain ends and loops
A right-handed closed circuit (Burgers circuit) in an ideal crystal is imaged on the defect crystal about the dislocation vector. The circuit will be open in the defect crystal. The Burgers vector points from the beginning to the end of the circuit.
Dnislocation in solution grown
polyethylene, revealed by moiré fringes. (electron micrograph)
F-4 Point defects
The list of point defects is rather large; it contains all
4. Foreign atoms on interstitial or substitutional sites
in macromolecular crystals only possible as low molecular impurity or in connection with copolymerization.
Besides the parameters
of free surface, and a description of tie molecules is needed as seen in figure.
Tie-molecules between grain boundaries in polyethylene made visible by crystallizing from paraffin solution and late removal of the paraffin (50%)
How edge dislocations can be made visible through moiré patterns
Upper
Lower
An optical analog to the double
diffraction phenomenon which causes moiré fringes in electron micrographs of double layers of crystals. One extra fringe is created (or destroyed) for each dislocation.
X-ray diffraction reveals the existence of
subgrain boundaries in polymer crystals.
Illustration of a subgrain boundary
Distance between etra planes b≈dφ
Dislocations in macromolecular crystals
Screw dislocations of a giant Burgers
vector, found as growth features in solution grown poly(dipropyl siloxane) (0.05%, from n-butyl acetate). Similar screw dislocations with a Burgers vector of the repeating unit cell are also expected; however, they would contribute little to accelerate crystal growth.
Left-handed and right-handed
screw dislocations, as well as terraces
consisting of pairs of screw dislocation of opposite handedness can be seen.
Burgers vector =fold-length 80-90 Å
Assumption 15000 mw, 1020
ends/cm3 average separation between chain ends= 22 Å, there successive planes are drawn, little “crystallinity” is left.
The need of an “amorphous defect” on the example of poly(ethylenepropylene), a copolymer [1.08-CH3 branches per 100 chain atoms]. Nitric acid etches the copolymer crystal in a manner similar to the homopolymer; It removes much more weight and branches. This extra weight must thus have been centered about the branches in the form of amorphous defects. Each branch has caused many CH2-groups to crystallize poorly so that they are amenable to HNO3 attack.
Crystallization of Linear Macromolecules
F. Microscopic defects
F-1 Type of defect
The
defect solid state description of other than macromolecules was developed by the mid-1950’s. macromolecular crystals were not too well understood at that time. Presently, one can for the first time make an effort to include macromolecular crystals into a general scheme of description of defect crystals.
Morphological defect
parameters like surface steps; changes in perfection, collapse features, fold sectors, etc.
2. Grain boundaries
In other than macromolecular single crystals angles above 30 ˚; In macromolecular crystals in addition, fold surfaces at smaller angle. The fold surfaces have a particularly large amorphous layer;
3. Subgrain boundaries
Crystals contacting each other at angles of less than 30 ˚, but no fold surface; The crystalline register is kept over certain lengths within the crystal; Extra planes of motifs are terminated at distances b≈d where d is the ≈dφ, ≈d lattice parameter and φ the contact angle
5. Chain disorder
special defect in macromolecular crystals— kinks; jogs; torsion; ends; folds.
6. Amorphous defects
Special defect in macromolecular crystals, makes the connection between “amorphous regions”
Point defects
Side and tБайду номын сангаасp view of a kink in a paraffin crystal
Example of the enormous
disruption in a polymer crystal if all chain ends would occur inside the crystal. Similar disruptions would occur for all other points defects. Their concentration must be less to get the type of perfection observed.