Rubbery amorphous

The chains that make up a polymer can adopt several distinct physical phases the principal ones are rubbery amorphous, glassy amorphous, and crystalline. Polymers do not crystallize in the classic sense portions of adjacent chains organize to form small crystalline phases surrounded by an amorphous matrix. Thus, in many polymers the crystalline and amorphous phases co-exist in a semicrystalline state. 

Rubbery amorphous polymers behave this way, and not like liquids, because their chains are not entirely free to slide past one another. The principal factors that limit long range... 

Rubbery amorphous polymers do not hold their shape well unless they are permanently crosslinked. If automobile tires were not crosslinked, they would be a soft sticky mess that would flow under the weight of the car. For this reason, we rarely encounter rubbery amorphous polymers that are not crosslinked. 

As polymers solidify from the molten state, their free volume decreases and their organization increases. Solid polymers fall into one of three classes rubbery amorphous, glassy amorphous, and semicrystalline, which we introduced in Chapter 1. 

The properties of a rubbery amorphous polymer form a continuum ivith those of the polymer in its molten state. Rubbery amorphous polymers exhibit the same range of motions as molten polymers, but they happen much slower, due to reduced thermal motion and the associated decrease in free volume. 

Solid polymers can adopt a wide variety of structures, all of which are derived from the three basic states rubbery amorphous, glassy amorphous, and crystalline. Either of the amorphous states can exist in a pure form. However, crystallinity only occurs in conjunction with one of the amorphous states, to form a semicrystalline structure. 

In the molten state polymers are viscoelastic that is they exhibit properties that are a combination of viscous and elastic components. The viscoelastic properties of molten polymers are non-Newtonian, i.e., their measured properties change as a function of the rate at which they are probed. (We discussed the non-Newtonian behavior of molten polymers in Chapter 6.) Thus, if we wait long enough, a lump of molten polyethylene will spread out under its own weight, i.e., it behaves as a viscous liquid under conditions of slow flow. However, if we take the same lump of molten polymer and throw it against a solid surface it will bounce, i.e., it behaves as an elastic solid under conditions of high speed deformation. As a molten polymer cools, the thermal agitation of its molecules decreases, which reduces its free volume. The net result is an increase in its viscosity, while the elastic component of its behavior becomes more prominent. At some temperature it ceases to behave primarily as a viscous liquid and takes on the properties of a rubbery amorphous solid. There is no well defined demarcation between a polymer in its molten and rubbery amorphous states. 

The density of the rubbery amorphous state is only slightly higher than that of the molten state, the difference being attributable to reduced thermal motion of its chains. In this loosely packed condition, the polymer incorporates a substantial amount of molecular scale void... 

Secondary crystallization occurs most readily in polymers that have been quench-cooled. Quenched samples have low degrees of crystallinity and thus have relatively large volumes of amorphous material. A pre-requisite for secondary crystallization is that the amorphous regions must be in the rubbery amorphous state. Increased temperature accelerates the rate of secondary crystallization. The new volumes of crystallinity that form during secondary crystallization are generally quite small, amounting to less than 10% of the crystalline volume created during primary crystallization. 

Define the terms rubbery amorphous, glassy amorphous, crystalline and semicrystalline. 

Figure 8.4 shows generic load versus elongation curves for rubbery amorphous, glassy amorphous, and semicrystalline polymers. In each case, the effect of extension on a dogbone specimen is shown at various points along the curve. 

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... 

Why do the tensile behaviors of rubbery amorphous, glassy amorphous and semicrystalline polymers differ as they do ... 

Styrene co-butadiene is a rubbery amorphous polymer with a glass transition temperature well below room temperature. Polystyrene co-butadiene is an important component of several commercial families of plastic that contain polystyrene blocks. 

The rates of different chemical reactions in irradiated polymers are dependent on physical as well as chemical factors. Polymers may have crystalline and glassy or rubbery amorphous regions. 

Blends of LDPE with ethylene styrene interpolymers (ESI, see Section 3.2) also have a complex microstructure. The semi-crystalline LDPE is immiscible with the amorphous ESI, which has a glass transition temperature (Tg) just above room temperature. Consequently there are rigid crystalline regions and rubbery amorphous LDPE, mixed on a 0.1 pm scale, together with regions of leathery ESI on a 5 to 10 pm scale (71). 

The mass fraction of the narrow component that corresponds to the rubbery noncrystalline amorphous phase is as small as 0.003-0.006. The mass fraction does not increase appreciably with increasing temperature, but stays almost unchanged up to 70 °C. Hence, it is concluded that solution-grown samples do not actually comprise a rubbery amorphous phase. This conclusion is confirmed by high-resolution solid-state 13C NMR with more detailed information. 

Spin-Lattice and Spin-Spin Relaxations. In order to determine the content of these crystalline and noncrystalline resonances, the longitudinal and transverse relaxations were examined in detail. It was first confirmed that the noncrystalline resonance of all samples is associated with Tic in an order of 0.45-0.57 s. Hence, the noncrystalline component of all samples comprises a monophase, in as much as judged only by Tic. However, it was found that the noncrystalline component of drawn samples generally comprises two phases with different T2C values amorphous and crystalline-amorphous interphases. The dried gel sample does not include rubbery amorphous material it comprises the crystalline and rigid noncrystalline components. However, the rubbery amorphous phase with T2C of 5.5 ms appears by annealing at 145 °C for 4 minutes. For the orthorhombic crystalline component, three different Tic values, that suggest the distribution of crystallite size, were recognized for each sample, as normal for crystalline polymers [17,54, 55]. The Tic and T2C of all samples examined are summerized in Table 6. 

D. W. van Krevelen, Properties of Polymers, 3rd ed., Elsevier, New York, 1972, pp 44—54. (The data used here, Table 4.3 on p. 44, are titled Molar volumes of rubbery amorphous polymers at 25° C. They need not necessarily represent crosslinked elastomers. We are making an... 

The rubbery amorphous state of polymers has the greatest correspondence with the liquid state of organic compounds. So it may be expected that the molar volume per structural unit of polymers in this state can be predicted by using the averaged values of the group contributions mentioned in Table 4.5 (Van Krevelen and Hoftyzer, 1969). 

TABLE 4.6 Molar volumes of rubbery amorphous polymers at 25 °C... 

There may be at least three transitions in the glassy state below Tg viz. in the temperature ranges from 0.5Tg to 0.8Tg, from 0.35Tg to 0.5Tg and at very low temperatures (4-40 K). Between Tg and Tm transitions may be observed in the rubbery amorphous state and in the crystalline state. Even in the liquid state of the polymer transitions may be observed, e.g. the temperature of melting of "liquid crystals". 

The decrease of En with increasing T may be explained by the extra free volume created by thermal expansion. This was suggested by Batchinski in 1913 already. Several attempts have been made to formulate a joint temperature function for polymer melts and rubbery amorphous polymers on this basis. Doolittle (1951) formulated the equation ... 

---