Crystalline Flexural

Flexural modulus increases by a factor of five as crystallinity increases from 50 to 90% with a void content of 0.2% however, recovery decreases with increasing crystallinity. Therefore, the balance between stiffness and recovery depends on the appHcation requirements. Crystallinity is reduced by rapid cooling but increased by slow cooling. The stress—crack resistance of various PTFE insulations is correlated with the crystallinity and change in density due to thermal mechanical stress (118). 

Detailed modifications in the polymerisation procedure have led to continuing developments in the materials available. For example in the 1990s greater understanding of the crystalline nature of isotactic polymers gave rise to developments of enhanced flexural modulus (up to 2300 MPa). Greater control of molecular weight distribution has led to broad MWD polymers produced by use of twin-reactors, and very narrow MWD polymers by use of metallocenes (see below). There is current interest in the production of polymers with a bimodal MWD (for explanations see the Appendix to Chapter 4). 

Data are for fully crystalline material apart from electrical data for PEEKK It may be noted that the amorphous SG of PEEK is 1.265. The modulus data is flexural for PEEK and tensile for PEEKK. 

Describe the seven different classes of polyethylene and explain how the structural differences affect the tensile strength, crystallinity and flexural modulus of each of the materials. 

Because as-spun fibers of Me-HQ/Cl-PEC showed a higher modulus than those of Ph-HQ/Cl-PEC, the flexural modulus of injection molded specimens of Me-HQ/Cl-PEC exhibited a higher flexural modulus than those of Ph-HQ/Cl-PEC due to the liquid crystalline state. Although flexural-fractured injection molded specimens of Me-HQ/Cl-PEC exhibited highly oriented fibrils, Ph-HQ/ Cl-PEC no longer displayed fibrils because of the lack of liquid crystallinity. 

A good applications-oriented measure of the use temperature for a ma-teral is the heat distortion or heat deflection test (HDT). The HDT is described by ASTM-D648 as the temperature at which a sample of defined dimensions (5 X Vi X Vs (or Va) in.) deflects under a flexural load of 66 or 264 psi placed at its center. In case of a largely amorphous polymer, the HDT temperature is typically slightly (10 to 20 °C) lower than the Tg as determined by DSC or DTA, whereas with more-crystalline polymers, it more closely correlates with the Tm. The HDT temperature is a useful indicator of the temperature limits for structural (load-supporting) applications. A loaded cantilever beam is used in another heat deflection test called the Martens method. 

Type C2H4 mole% Tg/T , Crystallinity % Tensile strength kg cm " Izod impact strength kg cm cm " 23 °C Elongation at break % Flexural modulus kg cm Hardness, Brittle shore A temperature T... 

Thus the structural and mechanical data are mutually compatible and point to an Increase In the order of the hard domains as a result of use of the PEDA additive, comparable to that which Is achieved by annealing without use of the additive. At present we can only speculate on the mechanism whereby the PEDA leads to the higher order. The crystalline reflections are those of homopoly (MDI/EG) and hence the PEDA chains must be outside the crystalline regions, but the flexural modulus does Increase Indicating some hard domain association. Nevertheless, the PEDA could be Intimately Involved with the hard domains. The amine groups are much more reatlve than the hydroxyls of the chain extender and polyol, and hence the first chemical reaction will Involve PEDA and MDI. The MDI-PEDA-MDI units may serve as nucleatlon agents for the formation of the hard domains, perhaps because of their low solubility In the reaction mixture. 

The contribution of the crystalline high melting nylon 6 blocks and soft-block hard-phase separation are also reflected in the resistance to heat sag exhibited by NBC. Table IV shows the heat sag at 163°C as these values are related to the flexural modulus. Even at the very low end of the modulus spectrum, sag values were quite low. 

For those copolymers where other physical property data was available, an attempt was made to correlate the run number with the flexural modulus and the tensile strength at yield. Results are given in Figures 9 and 10 for a limited number of points. This data should be obtained from carefully annealed samples or ones with similar thermal histories however, a clear trend is seen in both figures. The run number does relate to crystallinity and to attendant physical properties such as flexural modulus and tensile strength. 

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