What Controls the Lamellar Orientation at the Surface of Polymer Films during Crystallization?

Abstract

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.

Crystalline and Spherulitic Morphology of Polymers Crystallized in Confined Systems

“…Confinement of polymer systems can be generally classified into three categories according to their confined environments, and they are one, two and three dimensions, respectively. Confined crystallization of polymers has been found in a variety of systems, such as polymer ultrathin films [1][2][3][4][5], polymer blends [6][7][8][9][10], block copolymers [11][12][13][14], polymer droplets [15][16][17], self-assembled polymer nanostructures [18][19][20][21][22], polymers segregated inside nanoporous templates [23][24][25][26][27][28][29] and polymer nanocomposites [30][31][32][33]. In the past few years, the crystallization of polymers or polymer segments confined in ultrathin films (thickness <100 nm), miscible polymer blends and block copolymers has been widely studied for various systems.…”

Polymer surface structures determined using ToF-SIMS

“…At such a surface, the polymer crystals have two basic orientations: flat-on and edge-on orientations. Figure 1 is a schematic diagram showing these two orientations (Wang et al 2008). Wang et al (2008) found that edge-on and flat-on lamellae tend to develop at the surface of a polymer film at low and high temperatures, respectively.…”

“…Figure 1 is a schematic diagram showing these two orientations (Wang et al 2008). Wang et al (2008) found that edge-on and flat-on lamellae tend to develop at the surface of a polymer film at low and high temperatures, respectively. At temperatures above the glass transition temperature (T g ) of a polymer, edge-on lamellae are mostly observed at the surface.…”

“…The crystallization of poly(bisphenol-A-co-ether octane) (BA-C8) at room temperature, which has been studied using AFM (Li et al 1999, Lei et al 2002, Wang et al 2008, started with the formation of a nucleus that grew in length to become a founding lamella, as shown in Figure 2A-F. The synthesis and the structure of BA-Cn polymers are shown in Scheme 1.…”

“…The edge-on and the flat-on lamellae can be predominantly produced at low and high crystallization temperatures, respectively (Wang et al 2008). If BA-C8 polymers films were crystallized at increasing temperature, the surface morphology of these films would change from the edge-on orientation to the flat-on orientation.…”