Dynamic properties of rubber.
On the other hand, reinforcing fillers like carbon black introduce a completely new source of hysteresis. This addition to the viscous nature of rubber is associated with the breakdown and reformation of the network structure of the filler. The hysteresis generated in this manner has a different relationship to temperature and frequency; and is highly affected by variations in strain amplitude. Caution must therefore be exercised when using the superposition principle on filled elastomers.
Effect of chemical structure
The dynamic properties of an elastomer are determined by its Tg (ref. 17). This also includes the temperature range of the Tg. The Tg is a measure of the ability of a polymer to maintain rotational motion. A polymer that has the least structural features to interfere with this motion has the lowest Tg. Any structural feature that resists rotation makes it necessary to have a higher input energy or temperature to maintain the motion. Any of the following parameters can affect rotational motion (ref 18):
* Flexibility of the main chain;
* Bulkiness of side groups;
* Stearic hindrance;
* Flexibility of side groups;
For ease of rotational motion, the backbone of a polymer must be very flexible. The presence of a rigid structure in the backbone reduces the degrees of freedom and restricts rotational motion.
The addition of a double bond or cyclic group to polyethylene increases the Tg from -120[degrees]C to -100[degrees]C and 105[degrees]C for polybutadiene and polyvinyl formal respectively (table 1). The addition of large side groups to the main chain also restricts rotational motion. The examples in table 2 show the effect of adding methyl, benzene and carbazole groups to the backbone of polyethylene. Polyethylene has a Tg of -120[degrees]C, polypropylene a Tg of -10[degrees]C, polystyrene a Tg of 100[degrees]C and polyvinyl carbazole a Tg of 208[degrees]C. The amount of change in the Tg increases with increased bulkiness of the side group.
The addition of a side group, in such a steric position that it hinders rotation, also increases the Tg. The example in table 3 shows that the Tg of poly (o-methyl styrene) is 24[degrees]C higher than the Tg of poly (p-methyl styrene).
Flexible side groups are thought to act as lubricants and make rotational motion easier. The examples in table 4 show that the flexible n-butyl group on poly (P-n-butyl styrene) cause it to have a much lower Tg than either poly (P-tert butyl styrene) or poly (P-methyl styrene). This lubricating effect increases with the linear length of the side group up to about [C.sub.8] Longer lengths form entanglements with neighboring polymer chains and thus retard rotational motion.
The addition of two opposing side groups to a polymer maintains symmetry and does not imp rotational motion as much as the addition of a single side group. The examples given in table 5 show that polyvinylidene chloride with opposing chlorine groups has a much lower Tg (-17[degrees]C) than polyvinyl chloride (87[degrees]C) which has only a single chlorine. The same is found when comparing the Tg's of polypropylene (-10[degrees]C) and polyisobutylene (-65[degrees]C).
The addition of polar groups increases the dipole moment and sets up intermolecular forces of attraction. These in turn increase the resistance to rotational mo on. The examples in table 6 show that changing the methyl group of polypropylene to a chlorine group changes the Tg from -10[degrees]C to 87[degrees]C. Changing it further to a nitrile group increases the Tg to 103[degrees]C. The same is found when comparing the Tg's of polyisoprene (-73[degrees]C) and polychloroprene (-50[degrees]C).
Homogeneous vs. heterogeneous polymers
Based on the sequence of monomer(s) units in the polymer chain, polymers can be divided into two types, namely homogeneous and heterogeneous. Homogeneous polymers contain a random sequence of monomer units while heterogeneous polymers contain a long chain of one monomer followed by a long chain of another monomer. There are also varying degrees of homogeneity and heterogeneity. A typical homogeneous polymer like polybutadiene is synthesized from a single monomer but can have three possible structures in the backbone. Polybutadienes consisting of all cis, trans or 1, 2-vinyl structures have Tg's of -100[degrees]C, -100[degrees]C and 0[degree]C, respectively. If these structures are present in the polymer backbone in a random sequence, the Tg of the polymer can be calculated as follows ref. 19):
Tg = [V.sub.1] [Tg.sub.1]+[V.sub.2] [Tg.sub.2]+[V.sub.3] [Tg.sub.3] where [V.sub.1] = volume of 1st structure
[V.sub.2] = volume of 2nd structure
[V.sub.3] = volume of 3rd structure
[Tg.sub.1] = Tg of 1st structure (degreeKelvin)
[Tg.sub.2] = Tg of 2nd structure (degreeKelvin)
[Tg.sub.3] = Tg of 3rd structure (degreeKelvin)
Therefore, a polybutadiene containing 25% cis, 25% trans and 50% vinyl structure would have a Tg of -50degreesC.
If a second monomer such as styrene is added to polymerization, there are four possible structural units. Since polystyrene has a Tg of 100degreesC, it has a large effect in increasing the Tg of the copolymer. The polymer is considered homogeneous as long as the structural units maintain a random sequence. It also has a single Tg which forms the basis of the polymer's dynamic properties.
The homogeneity of a copolymer, however, can decrease when one monomer is more reactive than the other. In such a case the polymer formed at the beginning of the polymerization is richer in the more reactive monomer, and richer in the less reactive monomer, at the end of the polymerization. Such a copolymer has a variation in the ratio of the two monomers in going from one polymer chain to another. This causes a large increase in the width of the transition region. The effect of this broadening of the transition region is shown in figure 21 (ref 20). Here, the shear (elastic) modulus and the damping (tan [delta]) are plotted vs. temperature for two copolymers of vinyl chloride-methylacrylate having widely varying levels of homogeneity. The least homogeneous copolymer (B) has a much wider transition region. In some cases the transition region can extend into the temperature range in which the polymer is being tested or utilized. It then has a very large effect on the dynamic properties.
Heterogeneous copolymers are quite different. They consist of a long chain of one homogeneous structure followed by a long chain of a second homogeneous structure. Blocked or grafted polymers are examples of heterogeneous copolymers. Blocked styrene-butadiene copolymer consists of a long continuous chain of polystyrene attached to a long continuous chain of polybutadiene. The resultant copolymer has two distinct glass transition regions, one corresponding to polystyrene and one corresponding to polybutadiene. The same is also observed with a grafted copolymer; where a second polymer is polymerized off the backbone of a pre-existing polymer. Polymethyl methacrylate (PMMA) for example, can be grafted onto the backbone of natural rubber forming a copolymer having two separate Tg's (-73[degrees]C for natural rubber and 125[degrees]C for PMMA). Which polymer, in a hetero geneous copolymer, has the most influence on dynamic prop erties depends on which forms the continuous and the discontinuous phases. If, for example, the PMMA grafted natural rubber is cast from a solvent in which the natural rubber has less solubility, the PMMA forms the continuous phase, with nodules of natural rubber dispersed in it. If the natural rubber has greater solubility in the solvent, then the exact opposite occurs. When the PMMA is the continuous phase, the copolymer is rigid and a Tg for both the natural rubber and the PMMA are observed. It can actually function as a high impact resistant plastic. When the natural rubber is the continuous phase, it functions as a soft polymer with the PMMA acting as a hard filler. The Tg for the natural rubber is readily observed but the Tg of the PMMA is difficult to find because of the softness of the natural rubber. This, of course, is based on mechanical measurements. Both could be detected using DSC techniques. In any case, the dynamic properties would change drastically depending which part of the copolymer forms the continuous and discontinuous phases.
[17.] K.R. Beck, R. Korsmeyer and R.J. Kunz, Journal of Chem. Ed., 61, p. 668, (1984). [18.] L.E. Nielsen, "Mechanical properties of polymers," Rheinhold, New York, (1962), pp. 15-25. [19.] L.A. Wood, J. Polymer Science, 28, p. 319, (1958). [20.] L.E. Nielsen, J. Am. Chem. Soc., 75, p. 1435, (1953).
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|Title Annotation:||Tech Service; part 5|
|Date:||Jan 1, 1995|
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