Relaxation transitions in crystalline polymers

3 Relaxation Transitions in Crystalline Polymers 10 J.l General Introduction 

Semi-crystalline polymers are less sensitive to wide variations of stiffness with temperature than those that are totally amorphous, but even so stiffnesses may vary by an order of magnitude over the useful working range of a given material. Oriented crystalline polymers may additionally show contrasts between extensional and shear deformations, and also angular-dependent changes in relaxation strength. 

Some polymers, notably low-density polyethylene (LDPE), show clearly resolved a, p and Y processes. The high-temperature a relaxation is frequently related to the proportion of crystalline material present, the p process is related to a greatly broadened glass-rubber relaxation and the y relaxation has been associated, at least in part, with the amorphous phase. Other materials - an example to be discussed shortly being that of PET - show only two relaxation processes. In these cases, the a relaxation is akin to the p process in polymers where all three relaxations are evident. 

We shall begin with a brief and simplified discussion of the main features of experimental observations and proceed to consider the interpretation of these features. Three polymers are selected as paradigms PET, which can exist in the wholly amorphous state but also as a partially crystalline polymer polyethylene, which is a high crystalline polymer and a liquid crystalline polymer, the thermotropic copolyester whose mechanical anisotropy was discussed in Section 7.5.4 above. 

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Figure 10.15 Kinks and kink inversion, (a) The conformational sequence... TTTC TG TTT... has parallel offset planar zigzag stems (indicated by arrows) on either side of the GTG portion. The transition TCTG TGTCT (called here kink inversion) creates a mirror image of the kink about the displaced stems, (b) A three-bond crankshaft move is shown at a kink site (as dashed line). This move advances the kink along the chain by 2CH2 units. (Reproduced from Boyd, R.H. (1985) Relaxation processes in crystalline polymers - molecular interpretation - a review. Polymer, 26, 1123. Copyright (1985) Elsevier Ltd.)...

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. 

In crystalline polymers, the principal relaxation process is associated with melting. In polyethylene, a (1-. and 7-transit ions have been identified and, particularly in higb-tlensity polyethylene, the a-transition has been sub-divided into a and a. In ethylene-based polymers, the y-transitions at —120°C is generally associated with the amorphous phase, in particular, with crankshaft motion of methylene sequences. " However, based upon studies of solution grown lamellae, it has also been suggested that this may llien be associated with... 

Crystalline polymers exhibit more mechanical relaxations than amorphous polymers. It is not an overstatement to remark that the greater number of mechanical relaxations in crystalline polymers is the cause of the substantial difference in properties between crystalline and amorphous polymers (4.N.4). For example in linear (LPE) and branched (BPE) polyethylene at temperatures above — 200 "C, there is a sequence of relaxations (see Fig. 4.12). In branched PE the processes are y-relaxation at — 120 C, -relaxation at — lO C, and a-relaxation at 70 C. The presence of a relaxation is detected most easily by the peak in A this is one reason why this parameter is of value. The relaxation observed in creep in linear PE at room temperature and above (shown in Fig. 4.4) is the a-process. The torsion pendulum is a useful tool for yielding quickly a description of temperature regions where creep or stress-relaxation processes are to be expected. In addition, the relaxation temperatures often mark transitions in ductility the polymer becomes increasingly brittle as it is eooled. 

Finally, CR spectra of semi-crystalline polymers may characterize not only the relaxation transitions in the disordered regions but also the transformations occurring in the crystalline regions, namely solid-solid transitions. Figure 12 shows the DSC curve and the CR spectrum of poly(tetrafiuoroethylene) (PTFE). Strongly overlapping endothermic peaks I and II at 25°C and 32°C are observed in the DSC curve. Meantime, distinctly separated peaks I and II at 20°C and 40°C, with... 

As we will illustrate later in the chapter, the various relaxations are influenced by polymer structure, morphology, and environmental factors. For example, the intensity, temperature position, and temperature range of the glass transition in semicrystaUine polymers depends on the degree of crystallinity and morphology of the polymer. These will vary depending on the pro-... 

The potential of ROFTIR has been exploited in a wide variety of polymer systems. These include the orientation processes during elongation and relaxation of polyethylene [299-301, 312] uniaxial deformation phenomena in polypropylene [302, 303, 309-311], PET [304, 305] and polystyrene [306] hard and soft segmental orientations during stretching of polyurethanes [307, 308] and the stress-induced reversible a-P phase transition in crystalline PBT [312-314] and uniaxial deformation of amorphous PBT [315]. 

The glass transition is by far the most important one among the many transitions and relaxations observed in amorphous polymers. It has a drastic effect on the properties and processing characteristics of such polymers. It is important in semicrystalline polymers as well, but its role diminishes in importance with increasing crystalline fraction. 

The assignment of relaxation transitions to molecular or structural processes in crystalline polymers is not as well established as for amorphous plastics. 

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