Spiral crack propagation confined to a surface zone of a cross-linked polymer

The purpose of this letter is to report an unusual mode of crack propagation observed in the course of dynamic mechanical analysis (DMA) of crosslinked samples prepared by radiation polymerization of liquid triethylene glycol dimethacrylate [H2C=C(CH3)CO2CH2CH2OCH2-]2, which was confined between glass sheets. Samples of 7.0 cm × 0.4 cm × 0.07 cm were cut which had no flaws visible to the naked eye. They were subjected to a small sinusoidal strain which alternated between frequencies of 1.1, 11, and l l 0 H z , on an Autovibron apparatus (IMASS, Hingham, MA) during heating from 100 ° C at 2.5°Cmin -1 [1]. Unusual fracture modes were observed in the case of samples prepared with doses in the range 0.4 to 2.0 Mrad. This dose range results in samples which are above the gel point (near 0.15 Mrad) but are short of the maximal cross-linking which can be achieved by doses > 2.0 Mrad. In the region above the glass transition temperature (Tg ~ 120 ° C) some samples suddenly disintegrated into numerous fragments. This seemed surprising inasmuch as the maximal strain energy imposed in DMA testing is small. Typically in the past, fragmentation has been reported in cases where large amounts of energy are dissipated, as in Griffith's work on high-strength glass fibres [2] or as a result of impacts or explosions [3, 4]. Fragmentation seemed less surprising when testing was interrupted, just short of fracture, and still intact samples found to be pre-cracked as illustrated by views of parts of two samples (Fig. 1). The upper sample in Fig. 1 has a simple symmetry which made it easy to discern that a single crack propagated around the perimeter of the sample, alternating from one surface zone to the other in a segmented spiral. The continuity of the crack around the edge of the sample is illustrated in Fig. 2. The dark zones correspond to disruption of the surface zones in which the crack is propagating. Judging from the depth of the dark zones it appears that the crack depth is considerable. Returning to the appearance of Fig. 1 it should be pointed out that, due to the transparency of the sample, little or no contrast is apparent between crack segments on the front and rear surfaces. In order to emphasize this distinction the spiral pattern has been sketched (Fig. 2). Apart from fracture by fragmentation, a minority of the samples failed by other modes mostly involving delamination. In extreme cases considerable lengths of samples were cleaved through their thickness, giving two films in place of one. Evidence for a laminar structure was obtained by fracturing samples by bending, thereby revealing a sandwich-like cross-section (Fig. 4). Following up this lead, localized delamination was also noticed at the base of cracks (see lower specimen in Fig. 1). This effect is shown at a higher magnification by focusing on a crack to a depth of about one-third of the sample thickness (Fig. 5). The river markings, a characteristic of brittle fracture, indicate the direction of crack propagation in opposite directions away from the crack segment. In order to check whether the results reported above were a special feature of the conditions of DMA testing, some samples were heated in a vacuum whilst under a continuous tension produced by small loads. Spiral cracking was observed but it was much less regular and extensive than shown in Fig. 1. Further, delamination was more limited and largely replaced by extensive spalling. These results indicate that the DMA testing conditions result in more orderly fracture patterns but that they are not essential. From the above observations it would appear that heating results in samples in which a crack propagates more readily in a surface zone than in the interior. In this respect such samples seem to be in just the opposite