The crystalline morphologies of poly(bisphenol A hexane ether) (BA-C6) films were investigated using atomic force microscopy (AFM). The glass transition temperatures (T g s) of the BA-C6 films on silicon wafers were determined using an ellipsometer, and the results indicated that the T g increased dramatically when the thickness of the films was smaller than about 30 nm. The lamellar orientation of the BA-C6 films with thicknesses of about 33 and 970 nm was studied at temperatures varying between the T g and the melting point of the polymer (T m ). In the 33 nm films, we found that the edge-on lamellae predominantly formed at the temperatures close to the bulk T g (T g ∼ 35 °C), while some edge-on and mostly flat-on lamellae developed at the temperatures near T m (T m ∼ 98 °C). In addition to temperature, the film thickness was found to have significant influence on the observed lamellar orientation at the surface. The concentration of the edge-on lamellae increased as the film thickness increased. These observations were explained on the basis that polymer chain mobility is strongly affected by the thickness of the films, temperature, and the interactions between the polymer and substrate. A three-layer T g model was proposed for supported polymer films.
The dynamic growth processes of lamellae and spherulites of a polymer (BA-C8) synthesized by condensation polymerization of bisphenol A and 1,8-dibromooctane were directly observed using an atomic force microscope at 25 ( 1 °C. The phase shift measured at the tip of growing lamellae is lower than the phase shifts of the other regions of the lamellae. This result indicates that the growth front of a lamella is softer and contains more defects than the developed ones. The growth process of lamellae and the formation of spherulites were studied. A lamella was observed to breed more lamellae by inducing adjacent secondary nuclei and to develop into a lamellar sheaf and finally into a spherulite. The lamellae were preferentially oriented edge-on with respect to the surface of the BA-C8 polymer thin films. The detailed observations on the formation of the secondary nuclei indicated that the branching of the lamellae was only a temporary growth stage of the secondary lamellae. At the branch point, the secondary lamellae could propagate in both backward and forward directions with respect to the growth direction of the parent lamellae.
Since polymer spherulites were observed using optical microscopy and single polymer crystals were prepared from solution, 1,2 the internal structure of polymer spherulites has been investigated widely with electron microscopy. [3][4][5][6][7][8][9] For most crystalline polymers, it is generally accepted that spherulites are developed from a single crystal through unidirectional growth, and the spherical shape is attained through continuous splaying apart and occasional branching of lamellae, 5 though there are ongoing debates concerning the details of the branching in the literature. More recently, atomic force microscopy (AFM) has been utilized to study the organization of spherulites and the crystallization process of polymers. 10,11 A copolymer, poly(bisphenol A-co-octane), 12 was synthesized with the following structure:This polymer is attractive for analyses of the formation of nuclei and growth of lamellae because its glass transition temperature is close to room temperature, and it crystallizes at room temperature at a rate that allows imaging of the crystallization process by AFM without a hot stage. The glass transition temperature, melting point, and number-average molecular weight for this polymer were respectively measured to be 6.9°C, 83.5°C, and 5.7 × 10 3 g/mol. The crystallized sample exhibited several X-ray diffraction peaks at 2θ ) 5.6°, 15.2°, 18.6°, and 19.8°. A thin film of about 300 nm was prepared by spin-coating the polymer solution on a silicon chip, and the crystallization process was observed directly under tapping-mode AFM phase imaging.Embryos of the lamellae, as fine dots, with a dimension smaller than 10 nm, appeared and disappeared on the surface of the initially amorphous polymer film during scanning (Figure 1a,b). Figure 1a shows the presence of two embryos. In Figure 1b, which was obtained approximately 8.7 min after the image shown in Figure 1a, one of the embryos disappeared. We believe that this is the first experimental evidence in a polymer to show that, as predicted by thermodynamics, embryos smaller than a critical dimension are unstable and may not ultimately grow. The initial nucleating lamella grows along the length of the lamella at both ends (Figure 1c). During the growth stage, it breeds more lamellae (Figure 1d). When their lengths are longer than 0.5-1.0 µm, the lamellae begin to form branches (Figure 1e), and some lamellae splay apart from each other (Figure 1f). As a result of continual splaying and branching of the lamellae, the initial lamellae gradually develop into a lamella sheaf and a spherulite skeleton (Figure 1g,h). Figure 1. Nucleation, growth, and formation of a spherulite: (a) formation of lamella embryos; (b) disintegration of one of the lamella embryos; (c) growth of a lamella embryo; (d) breeding of more lamellae from the initial nucleating lamella; (e) branching of the growing lamellae; (f) splaying apart of the lamellae; (g) a lamella sheaf; and (h) a spherulite skeleton. 8240Macromolecules 1999, 32,[8240][8241][8242] 10.
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