Tensile properties branching, effects

Among the main molecular structural variables in EPDMs that are stipulated by catalyst systems and that affect the vulcanizate tensile properties we may mention molecular weight (MW) and MWD, degree of unsaturation (LG=C 1) and its distribution in the polymer, composition (C S) and monomer sequence length distribution along molecular chains, and long-chain branching if present. Effect of... 

The effects of SCB and side groups are similar. They disrupt the ability of the polymer to crystallize. If the disruption is not complete, the added bulkiness will make the rate of crystallization slow down. SCB has little effect on the flow properties of a polymer, but LCB has a profound effect. We will discuss this more when we look at the differences in behavior between HDPE, LDPE, and LLDPE in Chapter 4. For now, we can illustrate the effects of branching by comparing LDPE with HDPE. The densities differ, the tensile properties differ, and the elastic character of the polymers differs greatly, even though both are made from the same monomer. 

Measurement of low-temperature flexibility was significantly advanced by the method of Clash and Berg. This assessment of torsional modulus was adopted as an ASTM procedure (41). By its use, it can be shown that plasticizing with linear molecular structures enhances the low-temperature flex most efficiently (i.e., lower plasticizer concentration has a more pronounced effect) chain branching and ringed aromatic moieties show lower efficiency. Increasing any compatible plasticizer s concentration in the composition lowers the temperature at which brittleness develops. However, this formulation approach may sacrifice the optimum in other properties, such as tensile strength, modulus, or hand. 

It was mentioned that the incorporation of nanoclay had only a minor effect on melt strength however it increased the tensile modulus. On the other hand, Cole-Cole plot of the melts showed that the chain extender can promote the development of chain branching by time. The use of an epoxy based multifunctional chain extender resulted in significant enhancement of the melt strength and processability of the blends even at 30 wt% PBSA (Fig. 7.39b). These blends also exhibited interesting mechanical properties (Eslami and Kamal 2013a). 

The effect of short-chain (LLDPE) and long-chain (LDPE) branching in PE alters the physical and mechanical properties and melt rheology. For the same density, LLDPE has a higher melting point, tensile strength and flexural modulus. Because film is the major outlet for LDPE, this means that thinner film (down-gauging) and hence less material can be used for comparable applications. However, because LLDPE is stiffer, heavy-duty sacks are still made of LDPE. 

Higher tensile strength is obtained with the higher molecular weight materials. The effect of branching on the physical properties of polysulfide liquid oligomers is shown in Table 2 [35]. 

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