Amorphous polymers behaviour

One further problem consists in the probability of an inhomogeneous molecular system (which unambiguously the amorphous glassy polymer is) being considered as a two-phase system. If there is such a probability, this will allow application of the so-called composite models to the description of the amorphous polymer behaviour (see Chapter 9). These models are developed well and used successfully, for example, for the description of artificial two-phase systems, including filled systems. These two problems were discussed in paper [30]. 

In the case of an amorphous polymer the glass transition temperature will define whether or not a material is glass-like or rubbery at a given temperature. If, however, the polymer will crystallise, rubbery behaviour may be limited since the orderly arrangement of molecules in the crystalline structure by necessity limits the chain mobility. In these circumstances the transition temperature is of less consequence in assessing the physical properties of the polymer. 

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. 

In comparison to a conventional polymer and l.c. mentioned above, we will now discuss the PVT behavior of a l.c. side chain polymer, which has linked mesogenic moieties as side chains, and is very similar to the previous monomer. The experimental results are shown in Fig. 5. It is obvious, that the phase behavior of the l.c. polymer differs from that of a 1-l.c. and amorphous polymer. At high temperature we observe a transformation from the isotropic polymer melt into the l.c. phase, indicated by the jump in the V(T) curve. At low temperatures no crystallisation is observed but the bend in the curves signifies a glass transition. Obviously the phase behaviour is determined by the combination of l.c. and polymer properties. 

Before analysing the mechanical behaviour of amorphous polymers, it is useful to briefly give information on their molecular characteristics, the main descriptors used for plastic deformation and fracture, the micromechanisms of deformation, and some of the experimental procedures. 

Fig. 2 Typical stress-strain curves for amorphous polymers, a Elastic, anelastic, strain softening, and plastic flow regions can be seen, b Plastic flow occurs at the same stress level as required for yielding so strain softening does not exist, c Strain hardening occurs very close to yielding, suppressing both strain softening and plastic flow behaviour...

A convenient technique for studying the crack tip craze propagation in amorphous polymers deals with optical interferometry. It has been applied to the examination of PMMA behaviour at various temperatures and crack speeds under conditions of stable propagation [44,45]. 

In this final section, emphasis will be placed on the relationship between the behaviour of the homopolymer and that of heterogeneous systems containing interfaces. Thus, in Sect. 4.2, rather than dwell on the (albeit very important) technological aspects of welding, the discussion centres on the extent to which studies of interfaces might help in understanding the fundamentals of fracture in semicrystalline polymers, as they have in the case of interfaces between amorphous polymers [137]. 

Finally when the temperature increases several degree over the Tg (D zone) the mobility of the molecules is larger and the application of the force produce a creep of the chains relative to the other chains. This is the general behaviour of linear amorphous polymers. 

A second important event was the development by Hosemann (1950) of a theory by which the X-ray patterns are explained in a completely different way, namely, in terms of statistical disorder. In this concept, the paracrystallinity model (Fig. 2.11), the so-called amorphous regions appear to be the same as small defect sites. A randomised amorphous phase is not required to explain polymer behaviour. Several phenomena, such as creep, recrystallisation and fracture, are better explained by motions of dislocations (as in solid state physics) than by the traditional fringed micelle model. 

The curve of the tensile modulus versus temperature for the amorphous polymer shows five regions of elastic behaviour (Fig. 13.4) ... 

Dimensional stability is one of the most important properties of solid materials, but few materials are perfect in this respect. Creep is the time-dependent relative deformation under a constant force (tension, shear or compression). Hence, creep is a function of time and stress. For small stresses the strain is linear, which means that the strain increases linearly with the applied stress. For higher stresses creep becomes non-linear. In Fig. 13.44 typical creep behaviour of a glassy amorphous polymer is shown for low stresses creep seems to be linear. As long as creep is linear, time-dependence and stress-dependence are separable this is not possible at higher stresses. The two possibilities are expressed as (Haward, 1973)... 

FIG. 13.44 Typical tensile creep behaviour of a glassy amorphous polymer (a modified PMMA at 20 °C), where strain is plotted vs. log time, for various values of tensile stress. From bottom to top 10, 20, 30, 40, 50 and 60 MPa. From Haward (1973). Courtesy Chapmann Hall. 

The small-strain viscoelastic behaviour of all amorphous polymers is similar, so that in a limited region it can be described by a single universal formula... 

Above Tg the stress relaxation and the creep behaviour of amorphous polymers obey the "time-temperature superposition (or equivalence) principle". 

Hopfenberg and Frisch (1969) succeeded in describing all observed behavioural features for a given polymer-penetrant system in a diagram of temperature versus penetrant activity, which seems to be of general significance for amorphous polymers. It is reproduced in Fig. 18.13. 

An interesting example of the difference in drawing behaviour between amorphous and crystalline yam is the drawing of crystalline poly(ethylene terephthalate). It is often stated that crystalline PETP cannot be drawn. It is tme that the material breaks if drawn at a temperature of 80 °C, which is a drawing temperature normal for the amorphous polymer. Mitsuishi and Domae (1965), however, were able to draw crystalline PETP to a draw ratio of 5.5 at a temperature of 180 °C. 

Figure 10 presents the kinetic trans-cis photoisomerization process, under UV irradiation in the solid state, hi this case, significant differences appear between samples behaviour, as a function of the nucleobase chemical structures. It is interesting to note that, in the case of azo-polysiloxane substituted with adeiune (sample 2 -Table 1), the behaviours in the solid state and in solution are similar. That means that the polysiloxane chain flexibility, combined with the amorphous polymer ordering assure enough free volume for the trans-cis isomerization process. 

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