Transition secondary

Plots of loss modulus or tan 5 vs temperature for polymers give peaks at energy absorbing transitions, such as the glass transition and low temperature secondary transitions (Fig. 20). Such plots are useful for characterizing polymers and products made from them. 

Because the polymer is polar it does not have electrical insulation properties comparable with polyethylene. Since the polar groups are found in a side chain these are not frozen in at the Tg and so the polymer has a rather high dielectric constant and power factor at temperatures well below the Tg (see also Chapter 6). This side chain, however, appears to become relatively immobile at about 20°C, giving a secondary transition point below which electrical insulation properties are significantly improved. The increase in ductility above 40°C has also been associated with this transition, often referred to as the 3-transition. 

Aging of samples also leads to brittleness but at the present time there appears to be no information on how this affects either free volume or the secondary transitions. 

The rigid structure of the polymer molecule leads to a material with a high Tg of 208°C. There is also a secondary transition at -116°C and the small molecular motions that this facilitates at room temperature give the polymer in the mass a reasonable degree of toughness. 

Monnerie, L., Halary,. L. and Kausch, H.-H. Deformation, Yield and Fracture of Amorphous Polymers Relation to the Secondary Transitions. Vol. 187, pp. 215-364. 

Most polymers show small secondary glas.s transitions below the main glass transition (3..37,71 -76). These secondary transitions can be important in determining such properties as toughness and impact strength. These transitions are discussed in more detail in later chapters. 

The polymer may show a secondary transition in the glassy region as a result of small movements of parts of the chain or side groups (example PC). 

For a semi-crystalline polymer the E-modulus shows between Tg and (in which region it is already lower than below Tg), a rather strong decrease at increasing T, whereas with amorphous polymers, which are used below Tg, the stiffness is not much temperature dependent (apart from possible secondary transitions). The time dependency, or the creep, shows a similar behaviour. 

A value for E of about 3 GPa is normal for an amorphous polymer in the glassy state, unless below Tg a strong secondary transition occurs such as with PC, so that the E-modulus at ambient temperature is significantly lower. 

With an amorphous thermoplastic the brittleness temperature is about Tg, unless a secondary transition temperature Tgec occurs in the glassy region in that case the brittleness temperature may be in the neighbourhood of Tstc-... 

Secondary Transitions in Glassy Polymers and Methods of Their Determination 120... 

Secondary transitions in glassy polymers are closely associated with limited molecular mobility, i.e. with the rotational and vibrational motions of relatively short chain sections. The motional units may be identified with sequences of the main chains consisting of four to six groups, or with side drains and their parts. Generally, it is believed that... 

Of the thermodynamic quantities just mentioned, only the determination of the expansion coefficient or other quantities reflecting its change have assumed practical importance for the identification of secondary transitions in glassy polymers. The most efficient methods for the investigation of the dynamics and intensity of molecular motions have so far been those based on the interference between molecular motion and the oscillating magnetic, electric or mechanical force field. In recent years, methods which employ various probes or labels in the study of molecular mobility have increasingly been used. 

Secondary transitions of glassy polymers can be studied by means of the following... 

The secondary transitions of real polymers exhibit a broad distribution of relaxation times which originates from the distribution of activation energies1,291 (while in the... 

The site model appears to provide an adequate description of basic features of the dynamics of molecular motions in secondary transitions. On the other hand, however, attempts to calculate and interpret the extent of energy losses, i.e. the relaxation strength, in terms of the barrier model have so far been less successful. Theories employing a model with the same depth of potential energy minima1,76,77 do not seem suitable... 

---