At break of polymers

The influence of the photooxidative degradation on the elongation at break of polymers is also very dramatic. This effect has been attributed to the degradation of the macromolecules but also to the formation of physical and chemical defects on the surface. ... 

Effect of Glass Fiber Reinforcement in Elongation at Break of Polymers... 

FIGURE 38.5 Elongation at break of polypropylene-maleic anhydride grafted polypropylene (PP-MA-g-PP)-rubber powder composites as a function of rubber powder content. (Reprinted from Shanmugharaj, A.M., Kim, J.K., and Ryu, S.H., Polymer Test., 24, 739, 2005. Courtesy of Elsevier, U.K.)... 

The elongation at break of a sample is the strain at which at which it breaks. This value varies widely depending on polymer type and processing conditions. Glassy amorphous polymers typically exhibit low elongations at break because their chains cannot slide past one another. In rubbery amorphous polymers the situation is somewhat different. High molecular weight... 

Table 13.12 gives the numerical values of the strength properties of a series of polymers. The data on the tensile strength are graphically reproduced in Fig. 13.62, where crmax (i.e. the tensile strength at break of brittle (linear) polymers and the tensile strength at yield of ductile (linear) polymers) is plotted as a function of E, the tensile modulus. As an approximation the following empirical expression may be used (drawn line) ... 

Polyacrylamide (30% hydrolysed) is an anionic polymer which can induce flocculation in kaolinite at very low concentrations. Restabilisation occurs by overdosing, probably by the mechanism outlined in Fig. 7.32. Dosages of polymer which are sufficiently large to saturate the colloidal surfaces produce a stable colloidal system, since no sites are available for the formation of interparticle bridges. Under certain conditions, physical agitation of the system can lead to breaking of polymer-suspension bonds and to a change in the state of the system. 

Determination of the residual antioxidant content in polymers by HPLC and MAE is one way to determine the amoimt needed for reasonable stabilization of a material, and also to compare different antioxidants and their individual efficiencies. During ageing and oxidation of PE, carboxyhc acids, dicarboxylic acids, alcohols, ketones, aldehydes, n-alkanes and 1-alkenes are formed [86-89]. The carboxyhc acids are formed as a result of various reactions of alkoxy or peroxy radicals [90]. The oxidation of polyolefins is generally monitored by various analytical techniques. GC-MS analysis in combination with a selective extraction method is used to determine degradation products in plastics. ETIR enables the increase in carbonyls on a polymer chain, from carboxylic acids, dicarboxyhc acids, aldehydes, and ketones, to be monitored. It is regarded as one of the most definite spectroscopic methods for the quantification and identification of oxidation in materials, and it is used to quantify the oxidation of polymers [91-95]. Mechanical testing is a way to determine properties such as strength, stiffness and strain at break of polymeric materials. 

Distinct increases in bonds sensitive to the helical conformer of the PP backbone are also visible (for example at 997 and 841 cm-1). These changes have previously been observed in photo-oxidized samples and correlate with the backbone-scission-induced restructuring of the polymer, y-irradiation of PP film also caused a sharp drop in the elongation at break of the films (Table 1). Tensile strength was little effected. For the same total dose, both the oxidation product quantities and relative proportions as well as the elongation at break were markedly dependent on dose rate (Table 1). 

Polyamides and polyesteramides are more recent arrivals to the commercial biodegradable polymer field. Copolymers of either glycine or serine with e-aminocaproic acid are biodegradable. For example, biodegradable polyaspartic acid was synthesized (95% yield) at low cost [Koskan, 1992]. A copolymer of butylene-adipate and e-caprolactam was recently introduced by Bayer as BAK 1095. The material has T = 125°C, density of 1070 kg/m, tensile modulus of 180 MPa, maximum strain at break of 400%, tensile stress at break of 25 MPa, and it fully degrades in 300 days under the ASTM standard conditions. 

At room temperature, the mechanical properties of PLA are close to the one of PS but smaller than the one of PET (Table 8.5). Polyolefins present reduced stress at yield compared to PLA but the strain at break of LDPE and HOPE are much higher than the one of PLA. Compared to another biobased polymer, poly(hydroxybutyrate) (PHB), PLA shows better mechanical properties with higher modulus of elasticity and stress at yield. 

The deformation of a polymer tensile specimen is rather different from that of a metal. Polymers neck, and the neck region itself will extend in a ribbon until the material ultimately tears (Figure 10.22). The term percentage elongation at fracture used for metals is generally replaced by percentage elongation at break for polymers. 

According to electron spin resonance data, free radicals are produced at chain ends even before a macroscopic break occurs. The free radical concentration depends only on the extension, and not on the tensile stress. Concentrations of lO -lO free radicals/cm are generally observed. Since free radical concentrations of only about 10 free radicals/cm occur on the surface, free radicals must form in the test sample interior, that is, from the breaking of polymer chains. In addition, chemical decomposition products are produced by a ductile break, but not by a brittle fracture. 

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