Polymers, brittle

Poly(vinyl chloride). PVC is a hard, brittle polymer that is self-extinguishing. In order to make PVC useful and more pHable, plasticizers (qv) are added. More often than not the plasticizers are flammable and make the formulation less flame resistant. Flammability increases as the plasticizer is increased and the relative amount of chlorine decreased, as shown in Table 7. The flame resistance of the poly(vinyl chloride) can be increased by the addition of an inorganic flame-retardant synergist. 

To any one concerned with the production of plasticized resins, compatibility has long meant the ability of two or more materials to mix with each other to form a homogeneous composition of useful desired plastic properties [44,45]. A plasticizer, therefore, can be defined as a material that will soften and make inherently rigid and even brittle polymers flexible. Plasticizers are also sometimes... 

Although pyrrole can be chemically synthesised [239-241,267,268], electropolymerisation is easily achieved and is the most common preparative method. This was first reported by Dall Olio et al. [242] who prepared brittle polymer films on Pt by... 

Little is known about the variation of the critical stress ", with structure and temperature. For the polyethylene discussed abovedecreased from 620 psi at 22X to 39general trend with all polymers. Turner (84) found that the value of (r(. for polyethylenes increased by a factor of about 5 in going from a polymer with a density of 0.920 to a highly crystalline one with a density of 0.980. Reid (80,81) has suggested that for rigid amorphous polymers. ", should be proportional t° (Tt - T) For brittle polymers, the value of ", may be related to the onset of crazing. 

Biaxially oriented films, made by stretching in two mutually perpendicular directions, have reduced creep and stress relaxation compared to unoriented materials. Part ot the effect is due to the increased modulus, but for brittle polymers, the improved behavior can be due to reduced crazing. Biaxial orientation generally makes crazing much more difficult in all directions parallel to the plane of the film. 

Phenoxy resins can also be used for the modification of PET and PBT. Phe-noxy resins such as Paphen PKFE (Inchem Corporation) are polyhydroxyether materials with pendant hydroxyl groups that can react with the PET. Paphen phenoxy resins are reactive modifiers that can modify and upgrade brittle polymers. Such resins contain 6 % of secondary hydroxyl groups. The latter serve as reactive sites for reaction with polyesters, polyamides, polycarbonates, epoxies and phenolics. 

Figure 5.58 The stress-strain behavior of brittle polymer (curve A), ductile polymer (curve B), and highly elastic polymer (curve C). Reprinted, by permission, from W. Callister, Materials Science and Engineering An Introduction, 5th ed., p. 475. Copyright 2000 by John Wiley Sons, Inc.

Figure 3.3 shows representative stress-strain curves for a variety of polymeric materials. At normal use temperatures, such as room temperature, rigid polymers such as polystyrene (PS) exhibit a rapid increase in stress with increasing strain until sample failure. This behavior is typical of brittle polymers with weak interchain secondary bonding. As shown in the top curve in Figure 3.3, the initial stress-strain relation in such polymers is approximately linear and can be described in terms of Hooke s law, i.e., S = Ee, where E is Young s modulus, typically defined as the slope of the stress-strain plot. At higher stresses, the plot becomes nonlinear. The point at which this occurs is called the proportional limit. 

Although some polymers may be satisfactory when used under the stress of static loads, they may fail when subjected to impact. The impact resistance, or resistance to brittle fracture, is a function of the molecular weight of a polymer. Thus uhmwpe is much more resistant to impact failure than general purpose high-density polyethylene (hdpe). The impact resistance of brittle polymers is also increased by the addition of plasticizers. Thus polyvinyl chloride (PVC), plasticized by relatively large amounts of dioctyl phthalate, is much less brittle than unplasticized rigid PVC. 

The toughness of brittle polymers, such as rigid PVC, may be improved by the addition of a semicompatible elastomer such as a polyethyl acrylate graft copolymer. Such composites are harder than polymers plasticized with liquid plasticizers. 

PS has a high index of refraction (1.592) and hence has excellent transparency to visible light. PS is a brittle polymer with a glass transition temperature rgof 100 C, a heat deflection temperature of 90 C, and a solubility parameter of 9.1 H. 

Thus different phase behaviors of polyrotaxanes induced different thermal transitions. One-phase or two-phase materials can be obtained simply by proper choice of the components. The easy introduction of highly flexible cyclic components such as crown ethers with low T% surely expands the applications of otherwise brittle polymers into the low temperature range and also improves elasticity. The plasticizing effect of the crown ether is different from that of a normal plasticizer, because the cyclic is permanently connected to the backbone and no migration can occur. 

Plasticizers. Plasticizers are materials that soften and flexibilize inherently rigid, and even brittle polymers. Oiganic esters are widely used as plasticizers in polymers (97,98). These esters include the benzoats, phthalates, terephthalates, and trimellitates, and aliphatic dibasic acid esters. For example, triethylene glycol bis(2-ethylbutyrate) [95-08-9] is a plasticizer for poly (vinyl butyral) [63148-65-2], which is used in laminated safety glass (see Vinyl polymers, poly(vinyl acetals)). Di(2-ethylhexyl)phthalate [117-81-7] (DOP) is a preeminent plasticizer. Variation of acid and/or alcohol components) modifies the efficacy of the resultant ester as a plasticizer. In phthalate plasticizers, molecular sizes of the alcohol moiety can be varied from methyl to tridecyl to control permanence, compatibility, and efficiency branched (eg, 2-ethylhexyl, isodecyl) for rapid absorption and fusion linear (C6—Cl 1) for low temperature flexibility and low volatility and aromatic (benzyl) for solvating. Terephthalates are recognized for their migration resistance, and trimellitates for their low volatility in plasticizer applications. 

Another important factor in relation to impact strength is illustrated in Figure 5. A brittle polymer and a tough polymer, polystyrene and ABS respectively, are used as an example. Their impact strengths differ by a factor of 32, with impact strengths of 0.25 ft lb/inch for polystyrene and 8 ft lbs/inch for ABS. The addition of fibrous glass is the great... 

For a given rate of chain scission, the failure properties of an initially brittle polymer decrease at a considerably slower rate than for an initially ductile polymer. 

It is clear that if a relative end-life criterion (e.g., pf = Po/2) is chosen, the ductile polymer will systematically appear less stable than the brittle polymer. However, from the practical point of view, this distinction has no sense. 

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