Effect of Increasing Molecular Weight

A careful study of the above tables and figure 12.1 will indicate why a rubber chemist spends a lot of time in designing compounds with various cross link densities for oil field service as well as for other critical applications. It can also be observed that tear strength, fatigue life and toughness, all important requirements for oil field, rubber seals pass through an optimum at low cross link density and fall off with increase in cross link, whereas the most important sealing properties such as hysteresis and compression set improve with increased cross link. 

In high pressure sealing in the oil field service, it should be noted that hardness, modulus, extrusion resistance and resistance to blistering under rapid decompression can all be increased with an improvement in cross link density. However the elongation is reduced, the elasticity diminished and so the sealing potential is lessened. 

The aggressive environment is more complex for the following reasons  

The combinations of C02 and H2S, with and without water, create aqueous and non-aqueous electrolytes. 

An understanding of Lewis acid-base interactions is necessary to appreciate the problem potential. (Note  

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Figure 5.78 Qualitative effects of increasing molecular weight and crosslink density on modulus where is any reference temperature. Reprinted, by permission, from F. Rodriguez, Principles of Polymer Systems, 2nd ed., p. 221. Copyright 1982 by Hemisphere Publishing Corporation.

Figure 8 shows the effect of increasing molecular weight for a series of normal dialkyl phthalates at 75 p.h.r. An increase in the alkyl chain length increases the transition and critical temperatures. The examples concern a series of normal alkyl esters apart from the ditridecyl phthalate (DTDP) which is added to show the effect of high molecular weight. 

Figure 8. Effect of increasing molecular weight for a series of normal dialkyl phthalates...

FIGURE 16.10 Effect of increasing molecular weight between cross-links on swelling ratio and equilibrium pol3mier fraction of a neutral hydrogel. Simulations conducted for a dextran-based gel using the parameter values in Table 16.2. 

As expected, the effect of increasing molecular weight of the trace components is to raise the dew point of the mixture. Also in accord with the results for Gas 1, although dew points can be raised to summertime air temperatures by the presence of traces characterized by nCio and heavier, the effect of irreversible condensation on composition is apt to be very small. For example, snppose a sample of Gas 2 is stored at 12 atmospheres and trace materials characterized by nCio are present in amount 0.01%. From Table 7 we see even if this sample was cooled to -10 C and irreversible condensation occurred, this gas used to calibrate a chromatograph would be accurate to within 0.7 BTU/SCF, 0.05%, for calculation of heating value. If this same gas were used to calibrate a calorimeter, the maximum possible reduction of heating value due to irreversible condensation would be from 1147.91 to 1146.59 BTU/SCF, a loss of 1.3 BTU/SCF or 0.12%. 

Figure 1.12. The logarithm of the shear modulus (measured at 10 sec) vs. temperature. Effects of increased molecular weight and crosslinking are also illustrated.

A simple example of the relationship between "structure" and "properties" is the effect of increasing molecular weight of a polymer on its physical (mechanical) state a progression from an oily liquid, to a soft viscoelastic solid, to a hard, glassy elastic solid. Even seemingly minor rearrangements of atomic structure can have dramatic effects as, for example, the atactic and syndiotactic stereoisomers of... 

How the size of organic molecules influences their effectiveness as corrosion inhibitors has been investigated many times. However, the results are not consistent enough to permit formulation of a general rule regarding the effect of increasing molecular weight. 

FIGURE 9.14 Qualitative effects of increasing molecular weight and cross-linking on the master curve. 

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