Rubber glass 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.)... 

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). 

It is worth noticing that some small molecule additives, called antiplasticisers , are able to significantly decrease the amplitude of the secondary transition (the first transition appearing at a lower temperature than the glass-rubber transition) and, consequently, to affect the material properties. 

It is well known that the glass-rubber transition is characterised by the gradual development of cooperative segmental motions, when approaching the glass-rubber transition from higher temperatures. As a consequence, the temperature dependence of the frequency of the segmental motions, at temperature T and frequency /f, is described by the Wiliams, Landel, Ferry (WLF) expression ... 

For amorphous polymers, the glass-rubber transition is usually referred as the a transition, the solid state transitions (frequently called sub-Tg or secondary transitions) being designated by /3, y, S,... A typical example is shown in Fig. 1 with the temperature dependence of the mechanical loss modulus, E", which exhibits several peaks corresponding to the various transitions. 

In the case of any transition occurring at a temperature lower than the glass-rubber transition temperature, Tg, the frequency of the involved process has a temperature dependence which obeys an Arrhenius law ... 

It is clear from the described characteristics that the secondary transitions have specific features, different from those of the glass-rubber transition. Furthermore, to determine these characteristics, as well as the existence of single or multiple motional processes, measurements have to be performed at various temperatures and frequencies. 

The a transition with its maximum at 116 °C, corresponding to the glass-rubber transition processes. 

In the plastic flow process, large displacements of the whole chain are performed to reach the involved strain values. Such displacements are analogous to those that happen during polymer flow above the glass-rubber transition (a transition) temperature. These motions are performed by segmental backbone conformation changes with intra- and intermolecular cooperativity specific of the a transition. They will be called a transition motions, in order distinguish them from the /3 transition motions analysed in [ 1],... 

As mentioned in Sect. 2.2.1.2, the yield point originates from a conformational change of the main chain, leading to an increase of conformations corresponding to what would happen at a temperature above the glass-rubber transition temperature. 

The thin film technique described in Sect. 2 has been applied to PMMA [37, 38] from room temperature to the glass-rubber transition temperature. 

---