Amorphous zones

The qua si-crystalline stmcture of natural starch granules causes them to be insoluble in water at normal room temperature and gives them relative resistance to carbohydrases other than a-amylase and glucoamylase unless the granules become swollen. Three-dimensional arrangements of crystalline and amorphous zones in starch granules have been suggested (2). 

There are thus no solvents at room temperature for polyethylene, polypropylene, poly-4 methylpent-l-ene, polyacetals and polytetrafluoroethylene. However, as the temperature is raised and approaches F , the FAS term becomes greater than AH and appropriate solvents become effective. Swelling will, however, occur in the amorphous zones of the polymer in the presence of solvents of similar solubility parameter, even at temperatures well below T. ... 

Crystalline structures have a much greater degree of molecular packing and the individual lamellae can be considered as almost impermeable so that diffusion can occur only in amorphous zones or through zones of imperfection. Hence crystalline polymers will tend to resist diffusion more than either rubbers or glassy polymers. 

Low-density polyethylene has a gas permeability in the range normally expected with rubbery materials Table 5.11). This is because in the amorphous zones the free volume and segmental movements facilitate the passage of small molecules. Polymers of the Phillips type (density 0.96 g/cm ) have a permeability of about one-fifth that of the low-density materials. 

The commercial poly-(4-methypent-1-ene) (P4MP1) is an essentially isotactic material which shows 65% crystallinity when annealed but under more normal conditions about 40%. For reasons given later the material is believed to be a copolymer. In the crystalline state P4MP1 molecules take up a helical disposition and in order to accommodate the side chains require seven monomer units per two turns of the helix (c.f. three monomers per turn with polypropylene and polybut-I-ene). Because of the space required for this arrangement the density of the crystalline zone is slightly less than that of the amorphous zone at room temperature. 

Perhaps the most astounding property of this material is the high degree of transparency. This arises first because both molecules and crystals show little optical anisotropy and secondly because crystalline and amorphous zones have similar densities. They also have similar refractive indices and there is little scatter of light at the interfaces between amorphous and crystalline zones. 

In the case of crystalline polymers it may be that solvents can cause cracking by activity in the amorphous zone. Examples of this are benzene and toluene with polyethylene. In polyethylene, however, the greater problem is that known as environmental stress cracking , which occurs with materials such as soap, alcohols, surfactants and silicone oils. Many of these are highly polar materials which cause no swelling but are simply absorbed either into or on to the polymer. This appears to weaken the surface and allows cracks to propagate from minute flaws. 

In the solid state, small N0 species can diffuse through the amorphous zones, and clearly become associated with the oxidized domains in these amorphous zones. Thus the local N0 concentration adjacent to an -00H site (before photo-cleavage) is anticipated to be much greater than expected from the overallI NO1 concentration, and so increase the effectiveness of the scavenging process (reaction 7) as compared to the propagation step (reaction 2). 

How should such rigid domain coupling work In principle domains can only be rigidly coupled by a bridge of hard-phase material which has a different density. We know that the polyester hard-phase is semicrystalline. So the observation is indicative for a structure in which the hard domains are subdivided into crystalline and amorphous zones. 

Microcrystalline cellulose (MCC) is obtained by a controlled acid treatment intended to destroy the molecular bonding in the amorphous zones of cellulose. Usually HC1 or H2SO4 are used at 110°C for 15 min over native cellulose or regenerated cellulose. Colloidal gels are thus obtained showing thixotropy. MCC is used in the preparation of pharmaceutical compressed tablets due to its binding and disintegration properties. 

Macromolecules in solution, melt, or amorphous solid states do not have regular conformations, except for certain very rigid polymers described in Section 4.6 and certain polyolefin melts mentioned on page 139. The rate and ease of change of conformation in amorphous zones are important in determining solution and melt viscosities, mechanical properties, rates of crystallization, and the effect of temperature on mechanical properties. 

At room temperature, PP is close to its Tg(0-25°C) and well above its normal brittle-ductile transition temperature ( -30°C). However the presence of surface cracks in the photo-oxidized film is apparently sufficient to promote brittle failure at room temperature. According to the Griffith crack theory, once a critical crack length has been exceeded, a critical crack velocity is required to propagate the crack. If this velocity is not exceeded, cold drawing of the amorphous zones ensues. 

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