Amorphous polymers secondary transitions

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. 

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. 

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. 

The consequences of secondary transition motions on the mechanical properties (deformation, yield and fracture) of amorphous polymers are addressed in a second paper in this volume [1]. 

Deformation, Yield and Fracture of Amorphous Polymers Relation to the Secondary Transitions... 

In another paper in this issue [1], the molecular motions involved in secondary transitions of many amorphous polymers of quite different chemical structures have been analysed in detail by using a large set of experimental techniques (dynamic mechanical measurements, dielectric relaxation, H, 2H and 13C solid state NMR), as well as atomistic modelling. 

The purpose of this paper is to investigate the mechanical properties (plastic deformation, micromechanisms of deformation, fracture) of several amorphous polymers considered in [1], i.e. poly(methyl methacrylate) and its maleimide and glutarimide copolymers, bisphenol A polycarbonate, aryl-aliphatic copolyamides. Then to analyse, in each polymer series, the effect of chemical structure on mechanical properties and, finally, to relate the latter to the motions involved in the secondary transitions identified in [ 1] (in most cases, the p transition). 

Bisphenol A polycarbonate (BPA-PC), whose the chemical structure is shown in Fig. 66a, has very interesting fracture properties, exhibiting quite a high toughness for a pure amorphous polymer. At a very low temperature (- 100 °C at 1 Hz) it presents a secondary fi transition, shown in Fig. 67, which has been analysed in detail in [1] (Sect. 5). 

The goal of this investigation of the mechanical properties of amorphous polymers (plastic deformation, micromechanisms of deformation, fracture) was to analyse the influence of secondary transition motions on these properties. 

In comparing the E-values of the various polymers, the best starting point is formed by amorphous polymers in the glassy state for this group E is mostly about 3 to 3.2 GPa (e.g. PVC, PS, PMMA), except when, below room temperature, a secondary transition occurs, such as with PC E 2.1 GPa). 

Higher values can be reached for semi-crystalline polymers below Tg the crystalline phase is stiffer than the glassy amorphous phase (e.g. PEEK, E 4 GPa). Semicrystalline polymers above Tg have, however, a much lower E-value, such as PE (0.15 to 1.4 GPa) and PP 1.3 GPa) E is, in these cases, strongly dependent on the degree of crystallinity and on the distance to Tg. Sometimes a low modulus is also found for semi-crystalline polymers below Tg, due to the effect of one or more secondary transitions a strong example is PTFE (E = 0.6 GPa ). 

Amorphous, glassy polymers, used far below their Tg, are cold-brittle if no other mechanisms are active an example is PS. If a polymer has been improved in impact strength by the addition of a rubbery phase (high-impact PS or PVC, ABS etc.), then the cold-brittleness temperature is related to the Tg of the added rubber. If the polymer shows a secondary transition in the glassy region (such as PC), then this governs the brittleness temperature. 

The first secondary transition below Tg, the so called fj-relaxation, is practically important. This became evident after Struik s (1978) finding that polymers are brittle below Tp and establish creep and ductile fracture between Tp and Tg. The p-relaxation is characteristic for each individual polymer, since it is connected with the start of free movements of special short sections of the polymer chain. In view of more recent data of Tp Boyer s relation, Eq. (6.29), is very approximate and fails completely for amorphous polymers with high Tg s (e.g. aromatic polycarbonates and polysulphones). Some rules of thumb may be given for a closer approximation. 

From the preceding discussion it is clear that ageing does substantially affect the glass-rubber transition. The effect of ageing on secondary transitions has been the subject of many studies (see, e.g. Howard and Young, 1997). From Struik s observations on a large number of amorphous polymers it has become clear that thermal history does not affect the secondary transitions (Struik, 1987). 

We have seen already (Sect. 13.4.7) that every amorphous material (including that in semi-crystalline polymers) becomes brittle when cooled below the first secondary transition temperature (Tp) and becomes ductile when heated above the glass transition point (Tg). Between these two temperatures the behaviour - brittle or ductile - is mainly determined by the combination of temperature and rate of deformation. 

In the glass transition region, the storage modulus of an amorphous polymer drops by a factor of I000, and tan 6 is generally one or more. (The tan S in Fig. 11-18a is less than this because the polymer is oriented and partially crystalline.) In addition to Tg, minor transitions are often observed at lower temperatures, where the modulus may decrease by a factor of 2 and tan S has maxima of 0.1 or less. These so-called secondary transitions arise from the motions of side groups or segments of the main chain that are smaller than those involved in the displacements associated with Tg. Secondary transitions increase in temperature... 

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