Amorphous bending deformation

In this section we will consider polydimethylsiloxane (PDMS) as an example of the type of work that is possible with amorphous polymers. The structure and INS spectrum of PDMS are shown in Fig. 10.21a [40]. The repeat unit shown in Fig. 10.21b was used to model the spectrum using the Wilson GF matrix method [41]. The major features are reproduced skeletal bending modes below 100 cm", the methyl torsion and its overtone at 180 and 360 cm respectively, the coupled methyl rocking modes and Si-0 and Si-C stretches at 700-1000 cm and the unresolved methyl deformation modes 1250-1500 cm. The last are not clearly seen because the intensity of the methyl torsion results in a large Debye-Waller factor, so above 1000 em or so, most of the intensity occurs in the phonon wings. 

High temperature mechanical characterization was performed on the PAIC compositions with Al/Si = 0.05 and 0.1. The elastic modulus has been measured in air at various temperatures between 800 and 1400°C by four point bend tests (40 X 20 mm) with a 0.2 mm min deformation rate. E has been calculated, through the standard equation valid for rectangular bars, by measuring the displacement with a LVDT. All the ceramic samples obtained by pyrolysis at 1000°C have been pre-annealed at 1400°C for 1 h in argon atmosphere, before the high temperature tests. This treatment lead to the crystallization of microcrystalline pSiC with a minor amount of aSiC. Aluminum atoms are present both as a solid solution of AI2OC in aSiC and in the residual amorphous phase. 

Fig. 24. Schematic representation of the possible deformation processes of a stack of crystal lamellae (a) the initial state, (b) interlamellar shear, (c) interlamellar separation, (d) intralamellar block shear, (e) intralamellar fine shear (not shown bending and rotation of lamellae), and (f) cavitation within the amorphous regions.

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