Rubber transition

A somewhat similar thing happens in many polymers at the glass-rubber transition that we mentioned in Chapter 6. Below the transition these polymers are much more brittle than above it, as you can easily demonstrate by cooling a piece of rubber or polyethylene in liquid nitrogen. (Many other polymers, like epoxy resins, have low Gc values at all temperatures simply because they are heavily cross-linked at all temperatures by covalent bonds and the material does not flow at the crack tip to cause blunting.)... 

Fig. 23.7. A modulus diagram for PMMA. It shows the glassy regime, the gloss-rubber transition, the rubbery regime and the regime of viscous flow. The diagram is typical of linear-amorphous polymers.

The solidity of gel electrolytes results from chain entanglements. At high temperatures they flow like liquids, but on cooling they show a small increase in the shear modulus at temperatures well above T. This is the liquid-to-rubber transition. The values of shear modulus and viscosity for rubbery solids are considerably lower than those for glass forming liquids at an equivalent structural relaxation time. The local or microscopic viscosity relaxation time of the rubbery material, which is reflected in the 7], obeys a VTF equation with a pre-exponential factor equivalent to that for small-molecule liquids. Above the liquid-to-rubber transition, the VTF equation is also obeyed but the pre-exponential term for viscosity is much larger than is typical for small-molecule liquids and is dependent on the polymer molecular weight. 

Most PHAs are partially crystalline polymers and therefore their thermal and mechanical properties are usually represented in terms of the glass-to-rubber transition temperature (Tg) of the amorphous phase and the melting temperature (Tm) of the crystalline phase of the material [55]. The melting temperature and glass transition temperature of several saturated and unsaturated PHAs have been summarized in Table 2. 

The temperature dependence of the compliance and the stress relaxation modulus of crystalline polymers well above Tf is greater than that of cross-linked polymers, but in the glass-to-rubber transition region the temperature dependence is less than for an amorphous polymer. A factor in this large temperature dependence at T >> TK is the decrease in the degree of Crystallinity with temperature. Other factors arc the reciystallization of strained crystallites ipto unstrained ones and the rotation of crystallites to relieve the applied stress (38). All of these effects occur more rapidly as the temperature is raised. 

A simple relationship was not found between shrinkage and glass - rubber transitions of both peach and apricot tissue (Campolongo, 2002 Riva et al., 2001, 2002). Even when sorbitol use increased AT (= T — 7g ) values, both the color and the structure showed the highest stability. The fact that sorbitol performed better than sucrose indicates that the chemical nature of the infused solute is more important than its glass transition temperature in preventing structural collapse, in accordance with the results reported by del Valle et al. (1998). 

The two main transitions in polymers are the glass-rubber transition (Tg) and the crystalline melting point (Tm). The Tg is the most important basic parameter of an amorphous polymer because it determines whether the material will be a hard solid or an elastomer at specific use temperature ranges and at what temperature its behavior pattern changes. 

The name could suggest that thermosets become harder at temperature increase on the contrary they soften just as all polymers at their glass-rubber transition, though their stiffness remains much higher than that of a rabber. 

Plasticisers are added to a polymer to reduce the glass rubber transition temperature drastically (e.g. with PVC), so that the polymer behaves as a rubber at ambient temperature rather than as hard glassy thermoplast. 

An atactic structure is in both cases not crystallisable. Atactic PP is because of its glass-rubber transition temperature (Tg = -15 °C) rubbery and technically of no use. Isotactic PP is able to crystallise and can, therefore, be used in practice. For PS atacticity is no objection its properties as a glassy polymer are retained up to its Tg (95 °C). 

Since the glass-rubber transition is characterised by large chain parts becoming mobile (e.g. 50 monomer units), we can from a single Tg only conclude that the blend is homogeneous on that scale at a smaller scale (of a few links) a two-phase system may still be present. 

When a chain with M= 200,000 g/mole is linked to other chains at four points, the average molar mass between cross-links, M., amounts to 40,000. The mass of one unit is 4x12 + 6x1 =54 g/mole so the number of units between cross-links is about 740. At the glass-rubber transition no whole chains obtain free mobility, as a result of the entanglements, but chain parts of 30 to 100 monomer units. The chemical cross-links, therefore, hardly contribute to the restriction in chain mobility the increase in Tg will, therefore, be negligible. 

The majority of synthetic polymers can be thin sectioned by microtomy for transmitted light purposes [2]. For optimum results, sectioning should be carried out at a temperature just below the glass/rubber transition temperature, Tg. 

Staverman,A.J. Thermodynamic aspects of the glass-rubber transition. Rheol. Acta 5,283 (1966). 

---