Rubbery behavior

At the transition between glassy and rubbery behavior, a distinct relaxation occurs. From one viewpoint, the molecules have enough time to jostle into more relaxed conformations from another, they have enough thermal energy to do so. 

In the rubbery plateau, a new impediment to movement must be overcome entanglements along the polymer chain. In discussing the effects of entanglements in Chap. 2, we compared them to crosslinks. Is it any surprise, then, that rubbery behavior similar to that shown by cross-linked elastomers characterizes this region ... 

The plateau compliance is characteristic of rubbery behavior where chain entanglements play the role of effective crosslinks. 

With T as the independent variable, the transition between glassy and rubbery behavior can be read directly at Tg. Note that Tg is about 100° lower for poly(methyl acrylate) than for poly(methyl methacrylate). 

Time and energy can be saved if one recognizes that there is only one qualitative difference between a linear and a tridimensional polymer the existence in the former and the absence in the latter of a liquid state (at a macroscopic scale). For the rest, both families display the same type of boundaries in a time-temperature map (Fig. 10.1). Three domains are characterized by (I) a glassy/brittle behavior (I), (II), a glassy/ductile behavior, and (III) a rubbery behavior. The properties in domain I are practically... 

Finally, some fluids that undergo viscosity changes on shearing are elastic as well. These are termed viscoelastic fluids. These materials have properties of both a liquid and a solid. An excellent example is silicone putty (e.g., Silly Putty), which shows three different types of behavior depending upon the shear rate. If a piece of this material is suspended (gravity, a low shear force), it will slowly flow downward like a very viscous fluid. If it is sheared fasten it has rubbery behavior. You can observe this by rolling some of it into a ball... 

Soft flexible rubbery behavior depends on long flexible polymer molecules in the form of random coils. Strength, heat and chemical resistance depend on attachment between the coils. Conventional rubber chemistry uses vulcanization, permanent thermoset primary covalent cross-links, usually by sulfiir plus metal oxides, to hold the coils together but this makes processing more difficult, and recycling very difficult. In the past 40 years, this technology has been supplemented by the... 

Glass transition temperature (Tg) A point below which an amorphous polymer behaves as glass does It is very strong and rigid but brittle. Above this temperature it exhibits leathery or rubbery behavior. 

Knot et al. (51) converted soybean oil to several monomers for use in structural applications. They prepared rigid thermosetting resins by using free radical copolymerization of maleates with styrene. The maleates are obtained by glycerol trans-esterification of the soybean oil, followed by esterification with maleric anhydride. They also synthesized several TAG-based polymers and composites and compared their properties. It was found that the moduli and glass transition temperature (Tg) of the polymers varied and depended on the particular monomer and the resin composition. They proposed that the transition from glassy to rubbery behavior was extremely broad for these polymers as a result of the TAG molecules acting both as cross-linkers as well as plasticizers in the system. 

A polymer has maximum potential for damping vibrations in the temperature and frequency range covered by the transition region where properties change from glassy to rubbery behavior as shown in Figure 1. 

Both Tg and Tm are important parameters that serve to characterize a given polymer. While Tg sets an upper temperature limit for the use of amorphous thermoplastics like poly(methyl methacrylate) or polystyrene and a lower temperature limit for rubbery behavior of an elastomer-like SBR rubber or 1,4-cij-polybutadiene, Tm or the onset of the melting... 

The viscosity of the clear and colorless charge increased rapidly during the first three-four minutes and stirring became difficult after about 24 hours. After two days the products exhibited rubbery behavior (very rapid recovery upon releasing the strain of stretched samples). The rubbery masses were completely soluble in THF at room temperature. 

At this point, we had the first four of the seven characteristic features of A-B-A thermoplastic elastomers, as shown in the box. That is, we were completely confident that we had a three-block polymer, rubbery behavior with high tensile strength in the unvulcanized state, and also complete solubility. We concluded from these properties that these polymers were two-phase systems. We then generated the essentials of the two-phase, domain theory and visualized the physical structure illustrated schematically in Figure 1. We also visualized applications in footwear, in injection-molded items, and in solution-based adhesives. Positive confirmation of the two-phase structure quickly followed, by detection of two separate glass transition temperatures, as well as observation of the thermoplasticlike reversibility of bulk- and... 

Thermoplastic polyolefin blends can be produced as quite soft materials (as low as 55 Shore A) or as harder (above 95 Shore A), flexible products. They are particularly useful when a combination of good weatherabi1ity and rubbery behavior is required. Applications include automotive parts, weather stripping, hose, and sporting goods. Because of their good dielectric and insulating properties, polyolefin blends may be particularly useful in wire and cable insulation. 

Retained solvents may act as plasticizers (i.e., unless the solvent evaporates completely, the deposited resin may not have the same properties as the original solid in its pure state). Retained solvent lowers the glass-transition temperature. If the Tg of the mixture is at or below room temperature, rubbery behavior will result, and consolidant effectiveness will be reduced. Acryloid B72 seemed to retain solvent in films cast from acetone and toluene solutions and then air-dried (12). Further study showed that Acryloid B72 may retain measurable amounts of solvent even after 30 days at room temperature (16, 17). Results are shown in Table II. 

Only high-molecular grades show sufficient rubbery behavior above their melting point to enable a thermoforming process. PA, PET, and poly(butylene terephthalate) (PBT), however, will be very difficult to shape using this technique. 

The glass transitions of FPE-2 and -3 (as Indicated by loss modulus maxima, not shown) are 270 and 280°C, respectively. There are also various sub-Tg transitions apparent for each polymer. The peak damping value (tan 6 > 1.0) observed for FPE-3 is comparable to that for polymers which exhibit a sharp transition from glassy to rubbery behavior. 

Small Low Relatively flexible rubbery behavior, high permeabiiity. Ex PE, PP... 

Whether the polymer is totally amorphous or partially crystalline, the material will be glassy (hrittle) or ruhher-like (soft) depending on its temperature with respect to Tg. If an amorphous polymer is at a temperature helow Tg, it will be brittle and will show properties of a glassy material for example, it will fracture more easily. As the temperature of the sample increases and approaches Tg, it adopts a leathery behavior and its elastic modulus decreases. When the sample has reached several degrees above Tg, it shows a clear rubbery behavior and is easily deformable. If the temperature is increased even more, the polymer reaches liquid flow behavior. If the polymer is semicrystalline, it exhibits similar behavior, but when it reaches the melting temperature the crystals will break up, and the polymer will then reach the melted liquid state. This behavior is illustrated in Fig. 3.45 where the elastic modulus is plotted versus temperature. 

---