Macromolecular defects

Molecular defects as well as the real polymer morphology tend to decrease the strength. Some of these molecular, macromolecular, and morphological defects are illustrated in the schematic overview of Fig. 1.20. The right part shows macromolecular defects, such as chain ends with weak entanglements, overstressed bonds, and others, which initiate the first nanovoids and local stress concentrations. The left part illustrates morphological defects, for example, interlamellar and inter-spherulic defects in semicrystalline polymers, filler particles, and phase-separated particles in blends and composites. If the size of such defects is above a critical size, they initiate crack propagation and fracture. 

Fig. 13 a, b. Possible stacking of macromolecular bilayers in the P form crystals of s-PS. The regular succession of bilayers ABAB. .. gives rise to the ordered P" modification (a) defects, corresponding to pairs of bilayers of the kind AA or BB, would characterize the disordered P modification an AA defect is reported in (b). The symbols (/) and ( ) indicate the orientation of the lines connecting the adjacent phenyl rings of each chain inside the macromolecular bilayers A and B, respectively [29]... 

B. Wunderlich, Macromolecular Physics, Crystal Structure, Morphology and Defects, Vol. 1, Academic Press, New York, 1973. 

Wunderlich B (1973) Macromolecular physics, vol 1. Crystal structure, morphology, defects. Academic, New York, p 68... 

These traps, (Fig. 6) and similar effects in the motion of holes and other charges through polymers, would eventually be correlated also with such structural probes as positron lifetimes in macromolecular solids. Extensive recent studies of positron lifetime are based on positronium decay. In this, the lifetime of o-positronium (bound positron-electron pair with total spin one) is reduced from about 140 nanoseconds to a few nanoseconds by "pick-off annihilation" in which some unpaired electron spins in the medium cause conversion quenching of orthopositronium to para-positronium. The speed of the t2 effect is supposed, among other things, to represent by pick-off annihilation the presence of defects in the crystalline lattice. In any case, what amounts to empty space between molecules can then be occupied by orthopositronium.(14,15,16) It is now found in linear polyethylene, by T. T. Wang and his co-workers of Bell Laboratories(17) that there is marked shift in positron lifetimes over the temperature range of 80°K to 300°K. For... 

B. WunAetlvda., Macromolecular Physics Vol.l (Crystal Structure, Morphology, Defects), Academic Press (1973). 

Administered to mouse embryo cultures in vitro, trimethylamine was teratogenic, causing neural mbe defects and inhibiting embryonic growth. Trimethylamine may exert these effects by reducing macromolecular synthesis. Repeated intraperitoneal injections of trimethylamine hydrochloride in pregnant mice caused fetotoxicity only at maternally toxic doses. ... 

As modified so far the polyethylenimines, in contrast to enzymes, are weak in structural specificity toward substrates. This need not be a defect, however, for these macromolecular catalysts do not have to operate in a cellular environment and hence need not be subject to constraints designed to maintain the stability of a very complex, integrated biochemical network. Nevertheless, circumstances may arise where substrate specificity may be an essential requirement. We have some ideas on how this might be achieved with these relatively elastic macromolecular frameworks. For example, preliminary experiments show that we can attach —SH groups covalently to the polymer. It should be possible thereafter to add to the polymer solution an inhibitor with a structure analogous to the potential substrate and then to expose the solution to air... 

As well known, the crystalline structure of polymers is generally characterized by a comparatively high proportion of defects compared with the case of low molecular weight substances these defects may be due either to chemical faults or may be simply attributed to the mechanism of crystallization. As a consequence, this means that most polymers can contain a small proportion of extraneous units in the crystal state. However, we will not consider as real cases of macromolecular isomorphism those having a concentration of either component below 5%. 

It is not an easy task to define inhomogeneities in the structure of a polymer network. Every system will exhibit the presence of defects and fluctuations of composition in space when the scale of observation becomes smaller and smaller. A hierarchy of structures exists, from atomic dimensions to the macroscopic material. A scheme of different scale levels used to describe linear and crosslinked polymer structures is shown in Fig. 7.2. Inhomogeneities described in the literature for polymer networks are ascribed to permanent fluctuations of crosslink density and composition, with sizes varying from 10 nm up to 200 nm. This means that their size lies in the range of the macromolecular scale. 

Macromolecular 10-100 nm Network chains/strands, crosslinks network defects (dangling chains) Macro- molecular science Rubber elasticity, solvent swelling... 

Aging does not modify the value of E significantly (except in the close vicinity of Tg), so that aging effects on fracture properties are relatively low in the brittle regime, except if defects are created. This is the case in polyesters (osmotic cracks), where durability is controlled by this process rather than by chain scission or any other structural change at the molecular or macromolecular scale. 

Mechanical Criteria. There is a big difference in the behavior of initially ductile and initially brittle materials. Ductility is sharply linked to the macromolecular scale structure, whereas in brittle materials (or more generally in the brittle regime for any polymeric material), the properties (including ultimate ones) depend essentially on the molecular scale structure and on the size of eventual defects. This difference can be easily illustrated in the case of an amorphous linear polymer, but the reasoning would be the same for a thermoset. 

It is seen that free radical micromolecular or macromolecular initiators have been successfully employed for the synthesis of di-, tri- or multiblock copolymers. However, once again, the structure of these block copolymers depends upon the termination step of the polymerization, and especially on the recombination or disproportionation of macroradicals produced. Besides, such a method also generates homopolymers. Separation and purification of these different structures are usually very difficult or even impossible. Moreover, the copolymers obtained usually exhibit a broad polydispersity, a defect inherent in the classical radical process. 

---