Polymer lamella

In bulk-crystallized polymers, lamellae are often organized into spheruHtes, spherical stmctures which grow outward from a point of nucleation, typically to about 0.01 mm in diameter. SpheruHtes are in some ways similar to the grain stmcture in metals. They can make a polymer brittle and also reduce transparency. 

Electron diffraction measurements indicate that polymer chains are generally oriented normal or very nearly normal to the plane of the lamellae [13,14]. As the molecules in the polymer are at least 1000 A long and the lamellae are only about 100 A thick, the most plausible explanation is that the chains are folded [13,15], Figure 22.3 illustrates the proposed models of the fold surface in polymer lamellae [13,16],... 

Fig. 22.3 Two-dimensional representations of models of the fold surface in polymer lamellae (a) sharp folds, (b) loose loops, (c) switchboard model, and (d) a combination of all.

They correspond to the features observed by Perahia et al. [60, 61]. Using more concentrated solutions, Mullen and Rabe observed the formation of these structures on a smaller scale, and dense nanoribbons were observed by a combination of transmission electron microscopy and AFM. These features are approximately 1 nm wide and several hundred nm long, and represent single polymer lamellae in which the wing span of 12a dictates the width of the lamellae. In simple terms, each of the structures in Figure 16c would represent one or several polymer chains stacked on top of one another. 

For a discussion of the melting of polymer lamellae see Wunderlich B (1980) Macro-molecular Physics. Vol 3, Crystal Melting. Academic Press, New York. 

Two final points need to be made about secondary nucleation. First, that screw-dislocation defects, described in more detail in Sect. 5.3, prodnce indesttnctible secondary nuclei for growth on top of the fold surfaces of polymer lamellae. This surface would otherwise be inactive for further growth and restrict polymer crystals to single lamellae (see Chap. 5). An example of a series of screw dislocations is shown in Fig. 3.72 on the example of poly(oxyethylene) of 6,000 molar mass grown... 

Although a number of nucleating agents are used for simple polymers, such as polyethylene (PE), and for iPP and sPP, the real nature of the interactions has been worked out only in the last ten years or so. These interactions are essentially of epitaxial character, i.e. do not involve chemical reactions. For polymers with a linear envelope (such as PE), the epitaxial relationship involves, as a rule, the chains lying flat on the substrate (with the helix axis parallel to the substrate). The major dimensional match involves the interchain distance in the contact plane the latter may differ for different substrates. As a consequence, the polymer lamellae stand edge on on the substrate [1]. 

The above insight, gained soon after the discovery of polymer single crystals, and the subsequent discovery of chain folding (1-3), has mapped out the route taken by much of the research into polymer crystallization over the subsequent decades. It soon became clear that the thickness of polymer lamellae was controlled by the supercooling at which they were crystallized and defined by the kinetics of crystallization. The crystal thickness, or alternatively the thickness of each new crystalline layer in a growing crystal, is the one that grows the fastest (4,5) rather than the one that is at equilibrium (6). There is now a wealth of information available on the crystallization of many polymers, as well as several theories that aim to predict the crystallization rates, crystal shapes, and lamellar thickness. 

Fig. 14. A schematic diagram representing the growth front of a polymer lamella. The polsmier chain is represented by a series of boxes (stems) with defined surface energies.

In particular, in the area of nucleation and crystallization of polymers one can anticipate in the near future significant improvements in quantitative real-space investigation on processes occurring at the nanometer level. To date, the nucleation of polymer lamellae from the melt has been observed in some systems (176) however, the limited spatial resolution allowed only to observe the presence and not the shape/precise dimensions of stable, metastable, or unstable nuclei. 

Fig. 11 Friction force micrograph of PE lamella on mica left, friction forces increase from dark to bright contrast) and laterally resolved adhesion forces right, dark - 80 nN, bright -50nN pull-off force) collected in the so-called force-volume (FV) mode (see Sect. 5.2.2). Since the measurement was carried out in air, the forces are dominated by capillary forces. The contrast in the friction force micrograph is related to the orientation of the folds on the polymer lamella siuface, which are oriented along the crystal edge in each sector. (Reprinted with permission from [158]. Copyright 1999 American Chemical Society)...

Kinetics of Crystaiiization The free energy barrier to a stem laying down properly on a growing polymer lamella is given by AF. The quantity AF is proportional to the length of the stem, 1. The growth rate of the crystal, G, depends exponentially on AFIkT. Then,... 

The general requirements for the preparation of block copolymers by chemical modification at the fold surfaces of polymer lamellas are as follows ... 

The most successful methods for the preparation of block copolymers from polymer lamellas to date are apparently those involving addition reactions. The reaction conditions are relatively mild and the reaction is specific. Complete conversion of the functional groups (double bonds) in the folds and noncrystallizing ends apparently takes place whereas for substitution and elimination reactions, this has not been shown to be exclusively the case. 

We have seen in the previous section that objects such as fibres or bundles of polymer chains can act as nucleating surfaces for polymer lamellae. The geometry of the lamellae is normal to the surface as can be seen clearly in Fig. 3.1. As a... 

A typical shish-kebab crystalline structure has been foimd by Maiti and Okamoto (2003) and Kim et al. (2001) in polyamide/organoclay nanocomposite and by Choi and Kim (2004) in PP/EPR/talc nanocomposite where a preferential orientation of polymer lamellae perpendicular to the surface of organoclay layers was inspected by TEM measurements. The unique observation of lamellar orientation (Ml the clay layers was ascribed to nucleation and epitaxial crystallization at the interface between layered silicate and polymer matrix especially the surfaces of clay platelets acted as heterogeneous nucleation sites. Orientation of iPP crystals was also enhanced in rPP/PP-MA/o-MMT injection-moulded parts, especially manufactured by dynamic packing injection moulding (Wang et al. 2005). MMT... 

---