HDPE lamellae

PP-MA/EVAl PE-MA/PA-6 Maleic anhydride grafted PP HDPE lamellas for permeability barrier Kamal et al. 1995 Garmabi and Kamal 1995... 

The SEM images showed that the minority HD PE phase existed as isolated islands scattered in the matrix of the EOC. The HDPE lamellae appeared bright in the SEM images and extended to a length of several hundred nanometers. 

Cavitation is often a precursor to craze formation [20], an example of which is shown in Fig. 5 for bulk HDPE deformed at room temperature. It may be inferred from the micrograph that interlamellar cavitation occurs ahead of the craze tip, followed by simultaneous breakdown of the interlamellar material and separation and stretching of fibrils emanating from the dominant lamellae visible in the undeformed regions. The result is an interconnected network of cavities and craze fibrils with diameters of the order of 10 nm. This is at odds with the notion that craze fibrils in semicrystalline polymers deformed above Tg are coarser than in glassy polymers [20, 28], as well as with models for craze formation in which lamellar fragmentation constitutes an intermediate step [20, 29] but, as will be seen, it is difficult to generalise and a variety of mechanisms and structures is possible. 

Fig. 31 a. Shear bands (S) and "bright lines" (BL) in the compressive region of a HDPE-bend specimen. The axis of the compressive stress is perpendicular to the direction of the lamellae b Intersections of shear bands with "bright lines acting as sites for crack initiation (Courtesy W. Rose, Erlangen)... 

HDPE/PA-6 methacrylic acid/isobutyl acrylate lamellas, post extrusion calendering/ elongation Gonzalez-Nunez et at, 1993... 

Figure 10.6. Hydrocarbon permeability of HDPE containers in terms of hydrocarbon weight loss as a function of time, a HDPE, b HDPE/PA = 80 20, the PA being in the form of dispersed domains, c HDPE/PA = 80 20, the PA being in the form of lamellae [Subramanian, 1990].

We will now discuss the crystallization of polyethylene in more detail to illustrate some of the details previously mentioned. A linear polyethylene molecule, HDPE, will crystallize in an extended chain conformation in the crystallite if its molecular weight is below about 10,000 daltons. Above this molecular weight, the polymer forms folded chain lamellae, as shown in Fig. 3.40. The crystalline regions are closely packed chains that loop back on themselves. The region above and below the crystalline region is composed of two portions. That closest to the crystallite is... 

HDPE has an enormous range of ESC resistance. Figure 10.21 shows creep rupture data for blow moulding and pipe grades under extreme conditions, of 80 °C in 2% Arkopal 110 surfactant solution, which allow the reasonably rapid selection of promising candidate materials. The samples have a 10 X 10 mm cross section, with a circumferential, 1.6 mm deep, razor blade notch. The samples with the longest lives have more tie links between the crystal lamellae. 

The starting morphology of the compression-molded and annealed HDPE that is spherulitic and quite similar to that of Nylon-6 shown in Fig. 9.2 laeked any macroscopie initial orientational anisotropy on a seale larger than that of the spherulites, which was reflected by a featureless initial WAXS pattern. The initial spacing of the crystalline lamellae together with their attached amorphous components is given by the SAXS image of Fig. 9.7(a). Its uniform circular shape, as viewed in all directions, reflects an isotropic distribution of orientations of... 

Whatever the morphology of a semi-crystalline polymer such as HDPE, the irreducible structural unit involved in the plastic flow is a pair of coupled crystalline lamellae and its complement of an amorphous layer in between, where the ratio of the thickness of the crystalline lamellae to that of the pair of crystalline and amorphous layers is, to a first approximation, equal to the crystallinity of the polymer. 

Fig. 9.21 The dependence of the compressive-flow stress of HDPE at 293 K at a strain rate of 7 = 10 s on the lamella thickness compared with theoretical models (from Argon et al. (2005) courtesy of Elsevier).

Fig. 9.22 Micrographs of two etched, chain-extended samples of HDPE with lamellae of large thicknesses (a) X — l > nm and (b) — 156 nm (from Kazmierczak et al. (2005) courtesy of Elsevier)...

The semi-crystalline HDPE being modeled is initially of a sphernlitic morphology described in Chapter 2. It is made up of a 3D packing of crystalline lamellae and their attached amorphous layers as idealized in Fig. 9.25(a). The basic elements of the spherulite are two-phase composite inclusions that consist of integrally coupled crystalline lamellae and their associated amorphous layers between lamellae. Owing to their large aspect ratio, the composite inclusions are modeled as infinitely extended sandwiches with a planar crystalline/amorphous interface as shown in Fig. 9.25(b). Each composite inclusion I is characterized by its interface normal rt and the relative fractional thicknesses and / = 1 / of... 

Identical forms of preferentially oriented crystallization of PE on flat (104) surfaces of calcite crystals were also observed with the same range of oriented crystallization in HDPE (Bartczak et al. 1999c). In these experiments, AFM observations furnished dramatic visual evidence of end-on stacking of crystalline lamellae on calcite surfaces as shown in Fig. 13.36 in the form of star shaped-origins of clusters of lamellae similar to the TEM observations of Fig. 13.31 and the discoid-shaped clusters in Nylon-6. 

---