Thin films, crazing

Once a few neighboring fibrils have broken down, a large void is formed in the craze. The presence of such large voids can be inferred from observations of fracture surfaces and TEM micrographs of crazes in thin films Craze breakdown to form large... 

As an indication of the changes in deformation modes that can be produced in ionomers by increase of ion content, consider poly(styrene-co-sodium methacrylate). In ionomers of low ion content, the only observed deformation mode in strained thin films cast from tetra hydrofuran (THF), a nonpolar solvent, is localized crazing. But for ion contents near to or above the critical value of about 6 mol%, both crazing and shear deformation bands have been observed. This is demonstrated in the transmission electron microscope (TEM) scan of Fig. 3 for an ionomer of 8.2 mol% ion content. Somewhat similar deformation patterns have also been observed in a Na-SPS ionomer having an ion content of 7.5 mol%. Clearly, in both of these ionomers, the presence of a... 

As one example, in thin films of Na or K salts of PS-based ionomers cast from a nonpolar solvent, THF, shear deformation is only present when the ion content is near to or above the critical ion content of about 6 mol% and the TEM scan of Fig. 3, for a sample of 8.2 mol% demonstrates this but, for a THF-cast sample of a divalent Ca-salt of an SPS ionomer, having only an ion content of 4.1 mol%, both shear deformation zones and crazes are developed upon tensile straining in contrast to only crazing for the monovalent K-salt. This is evident from the TEM scans of Fig. 5. For the Ca-salt, one sees both an unfibrillated shear deformation zone, and, within this zone, a typical fibrillated craze. The Ca-salt also develops a much more extended rubbery plateau region than Na or K salts in storage modulus versus temperature curves and this is another indication that a stronger and more stable ionic network is present when divalent ions replace monovalent ones. Still another indication that the presence of divalent counterions can enhance mechanical properties comes from... 

The combined effects of a divalent Ca counterion and thermal treatment can be seen from studies of PMMA-based ionomers [16]. In thin films of Ca-salts of this ionomer cast from methylene chloride, and having an ion content of only 0.8 mol%, the only observed deformation was a series of long, localized crazes, similar to those seen in the PMMA homopolymer. When the ionomer samples were subject to an additional heat treatment (8 h at 100°C), the induced crazes were shorter in length and shear deformation zones were present. This behavior implies that the heat treatment enhanced the formation of ionic aggregates and increased the entanglement strand density. The deformation pattern attained is rather similar to that of Na salts having an ion content of about 6 mol% hence, substitution of divalent Ca for monovalent Na permits comparable deformation modes, including some shear, to be obtained at much lower ion contents. 

Crosby AJ, Easolka MJ, Beers KL (2004) High-throughput craze studies in gradient thin films using ductile copper grids. Macromolecules 37 9968-9974... 

Wellinghoff.S.T., Baer,E. Crazing in ultra-thin films of atactic polystyrene. Unpublished manuscript. 

Fig. 11 Craze in commercial polystyrene showing the characteristic steps nucleation through void formation in a pre-craze zone, growth of the fibrillar structure of the widening craze by drawing-in of new matrix material in the process zone, and final breakdown of the fibrillar matter transforming a craze into a crack (the crack front is more advanced in the center of the specimen, shielded by a curtain of unbroken fibrils marked by the arrow). The fibril thickness depends—of course—on the molecular variables, the strain rate-stress-temperature regime of the crazing sample and on its treatment (preparation, annealing) and geometry (solid, thin film) for PS typical values of between 2.5 and 30 nm are found [1,60,61]...

Fig. 5 TEM micrographs of thin films a craze in MGIM76 at 0 °C (From [51]), b an active zone at the craze-bulk interface in polystyrene (From [21])...

TEM micrographs of crazes in thin films have been used to determine the extension ratio, k, of the polymer chains along the craze [19]. At the crack tip a higher extension ratio is observed, which remains in the midrib. Otherwise, the measured k is identical along the craze and depends on the polymer being... 

Fig. 28 Temperature dependence of the strain to craze for PMMA thin films at a strain rate of 2 x 10-3 s-1...

In the case of BPA-PC, the thin film investigation of deformation micromechanisms (Sect. 4.2) shows that CDCs occur around 60 °C. So, it is unlikely that the craze at the crack tip occurring at - 20 °C, or above, could be a CDC. The observed MW dependence of failure originates from the above described mechanism with CSCs. 

It is likely that the craze at the crack tip over the low temperature range a corresponds to CSC, in agreement with the observation of CSCs during the thin film deformation of the same samples (as described in Sect. 5.2.1). As the strength required to break a chemical bond is temperature-independent and the CSC stress presents a weak decrease with increasing temperature, toughness would decrease weakly with temperature, as actually observed. 

Descriptions of craze thickening are based on the observed crazes at the tip of a stationary crack for creep tests [29,30] and on observations of crazes in thin films by transmission electron microscopy (TEM) or small-angle X-ray scattering (SAXS) [31,32],... 

The ESCR performance of a resin is not easily modeled. A laboratory technique for the preparation of thin films of HIPS materials for the study of deformation processes by microscopy allows the deformation process to be better understood. The transmission electron microscope (TEM) allows direct visualization of the crazes themselves in thin films. For good contrast between the crazes and the bulk polystyrene, thin, cast films from 0.5 to 2 p,m are required, and also staining of the rubber phase with a heavy atomic species to provide contrast between the rubber and the polystyrene. Another intricacy of this method requires a solution of the HIPS material in a 65 35 methyl ethyl ketone-toluene solution to prevent significant swelling of the rubber particles during the preparation process. 

Stereo-transmission electron microscopy of craze tips has shown that the meniscus instability is the operative craze tip advance mechanism in a wide variety of glassy polymers Figure 4 shows a craze tip in a thin film of a styrene-acrylonitrile copolymer (PSAN). The void fingers are clearly visible. No isolated voids can be... 

Verheulpen-Heymanshas measured the fibril volume fraction profile along isolated crazes in polycarbonate using an optical technique whereas Trent, Palley and Baer have measured it in isolated polystyrene crazes in thin films by comparing craze displacements measured from the displacement of bars of an evaporated metal grid intersecting the craze thicknesses. They use TEM of the unstressed film to make the measurements. Both groups find that Vf is independent of craze thickness. 

Fig. 14a—c. Craze microstructure in a a thick film (1.2 nm), b a moderately thick film (0.45 lun) and c a very thin film (0.1 tun). From Ref.courtesy Chapman and Hall... 

It was shown recently by Brown that in PS the craze structures are similiar in form for thin and thick films and that, however, the fibrils are about three times larger in thin films. 

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