AFM studies on detailed structures and dynamic growth of crystals in BA-Cn polymer films
by Yong Wang
Ph.D. Chemical Engineering
xxxv, 298 leaves : ill. ; 30 cm
Using AFM, detailed structures and dynamic growth of crystals in films of poly(bisphenol A alkyl ether) (BA-Cn) (n = 6, 8, 10, 12) were studied. Several important fundamental issues have been addressed....[ Read more ]
Using AFM, detailed structures and dynamic growth of crystals in films of poly(bisphenol A alkyl ether) (BA-Cn) (n = 6, 8, 10, 12) were studied. Several important fundamental issues have been addressed.
Regarding the construction of spherulites through continuous lamellar branching and splaying, three mechanisms for branching were proposed and confirmed: 1) screw dislocations; 2) induced nucleation of partially trapped polymer chains in the parent lamellae; 3) stresses induced by the crystallization of the parent lamellae in their nearby regions. It was found that a relatively small range of branching (splaying) angles between daughter and parent lamellae is necessary for the formation of spherulites.
Totally different from traditional banded spherulites, a novel self-organized crystalline structure containing concentric rings (or bands) was presented. This concentric-ringed crystalline structure consists of crystals only with the flat-on orientation. The formation of a depletion zone due to specific volume decrement between the crystal and the melt and the diffusion of polymer chains to the fold surface of the flat-on lamellae were found to lead to the formation of the structure. A model for the formation of the concentric-ringed crystalline structure was proposed.
Surface crystallization of polymer films at temperatures below bulk Tg of the polymer was observed at (near) the film surface, revealing that the polymer/air and polymer/substrate interfaces in the supported polymer films play dominant roles in the crystallization behavior within films. On the one hand, due to the requirement of small surface energy of the system, edge-on and flat-on nuclei (lamellae) are preferred at the polymer/air and polymer/substrate interfaces, respectively. On the other hand, crystallization in films is initiated and accelerated by the presence of the polymer/air and polymer/substrate interfaces.
The consistency of AFM observations compared with conventional OM results in both crystal growth kinetics and morphologies was tested. Because the well-known Lauritzen-Hoffman theory was found not to be the best mode to use to analyze the cold crystallization of BA-Cn (n = 8) polymer, a new expression for lamellar growth kinetics was proposed based on a simple kinetic model - attachment-and-detachment of crystallization units onto the crystal rough surfaces without the secondary nucleation step during crystallization.