Poly flaws

Another type of initiator that has been evaluated for increasing polystyrene production rates are the multifunctional peroxides. Examples include 2,2-bis [4,4-bis(tert-butylperoxy)cyclohexyl]propane (I) [9], peroxyfumaric acid, 0,0-te/Y-butyl O-butyl ester (II) [10], ter t-butyl peritaconate (III) [11], and poly (monopercarbonates) (IV) (Figure 7.4) [12]. Although all of these initiators indeed show extremely fast production rates of high MW polystyrene, they all suffer from a flaw, i.e. the polystyrene produced is branched and special precautions must be taken to keep the continuous bulk polymerization reactors from fouling [13]. This is likely why none are currently used commercially for polystyrene manufacture. 

As can be seen in Fig. 10 [29], the fracture toughness in poly crystalline tetragonal zirconia (TZP) and partially-stabilized zirconia (PSZ) appears to reach a maximum. This indicates a transition from flaw-size control of strength to transformation-limited strength. Ranges of fracture toughness values for zirconia composites are given by Richerson [38]. 

Figure 3.23 Types of internal flaws in acrylic polymer fibers. Source Reprinted with permission from Thorne DJ, JAppI Poly Sci, 14, 103-113, 1970. Copyright 1970, John Wiley Sons Ltd.

One of the most common problems encountered by workers in this area is the ubiquity of surface contamination. All materials, even those in the artificial environment of an ultra high vacuum (UHV) chamber, procure a hydrocarbon surface layer. In addition to this ever present contamination, it is found that low level impurities in polymers will often preferentially reside at the surface, typical examples of this form of contamination being poly (dimethyl siloxane), Ratner et al. (1993) and the plasticiser di-octyl phthalate. A layer of any tj pe of surface contaminant can completely alter the observed reacthity of the polymer and ultimately lead to flawed conclusions regarding it s performance as a biomaterial. 

An example of a study conducted using a tensile stage in the SEM is the evaluation of the ductile failure of poly(vinyl chloride). Smith et al. [316] stamped dumbbell shaped pieces of polymer from 1 mm thick sheets and extended them to a neck in an Instron tester. The prestrained pieces were then strained in the SEM. Low accelerating voltage was used for imaging of the uncoated specimens. These experiments showed that, after neck formation, fracture occurs by crack propagation from a flaw or cavity within the surface craze. 

One possible flaw in the poly(vinyl alcohol) calculations is that the shifts might also be directly and differentially affected by H-bonding. However, an independent study of saccharides in water and in DMSO indicates that this is probably not so. Another interesting feature is that, when the rotamers that were calculated to be dominant were reset to be totally dominant, then the CH shift pattern was... 

McCrum [213, 214] recently suggested that the above approach is subject to large errors and based on an irrational premiss. He proposed a new method of thermoviscoelasticity . Smith and Mark [215] have demonstrated McCrum s analysis to be flawed and have shown that the classical thermoelasticity approach is soundly based on theory. Indeed, there is excellent agreement between thermoelastic and viscometric results for poly(l-pentene) [206, 211], polyethylene [151, 154, 155, 211, 216], poly(dimethyl siloxane) [205, 211, 217], poly(ethylene oxide) [211, 218], poly (isobutylene) [207, 211, 216, 219] and poly(H-butyl methacrylate) [220, 221] (Table 6). 

It is known that the temperature at which the brittle-ductile transition occurs for a particular polymer is sensitive to both the structure of the polymer and the presence of surface flaws and notches. The temperature of the transition is generally raised by crosslinking and increasing the crystallinity. Both of these factors tend to increase the yield stress without affecting the brittle stress significantly. The addition of plasticizers to a polymer has the effect of reducing yield stress and so lowers T, . Plasticization is widely used in order to toughen polymers such as poly(vinyl chloride) which would otherwise have a tendency to be brittle at room temperature. 

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