Crystallization-driven surface segregation processes for polymer blends and copolymers
by Zhuo-Lin Cheung
M.Phil. Chemical Engineering
xvi, 96 leaves : ill. ; 30 cm
Three different polymer systems were studied, 1) a blend of poly(bisphenol A octane) (BA-C8) and Poly(4,4'(hexfluoroisopropylidene)diphenol octane) (6FBA-C8), 2) a copolymer containing both BA-C8 and 6FBA-C8 and 3) the blend of poly (ε-caprolactone) (PCL) and poly(vinyl chloride) (PVC). The results showed that the segregation mechanisms of the component to the surfaces of above systems are controlled by crystallization....[ Read more ]
Three different polymer systems were studied, 1) a blend of poly(bisphenol A octane) (BA-C8) and Poly(4,4'(hexfluoroisopropylidene)diphenol octane) (6FBA-C8), 2) a copolymer containing both BA-C8 and 6FBA-C8 and 3) the blend of poly (ε-caprolactone) (PCL) and poly(vinyl chloride) (PVC). The results showed that the segregation mechanisms of the component to the surfaces of above systems are controlled by crystallization.
A miscible polymer blend with 80 weight percentage of BA-C8 and 20 weight percentage of 6FBA-C8 was prepared and studied. Using an atomic force microscope (AFM), BA-C8 was shown to form small spherulites. The surface morphology is different from that of the pure BA-C8. The surface composition as a function of time was studied using contact angle measurements, X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The low surface energy component, 6FBA-C8, segregated to the spherulites surface. The segregation of the low surface component in this miscible was driven by crystallization.
A copolymer containing 18 units of BA-C8 and 1 unit of 6FBA-C8 was synthesized and studied. The morphology of the copolymer surface, as observed by using AFM, was similar to that of the BA-C8 except the crystallization rate of the copolymer was slower. However, in this case, ToF-SlMS and XPS results as well as contact angle measurements indicated that. The surface energy of the copolymer increased and the fluorine content decreased as the copolymer crystallized. This unusual phenomenon was caused by the transformation of the polymer chains from random coils to a regular structure (lamellae). The lower surface energy component, 6FBA-C8, was pinned to fixed positions in the crystal structure.
The surface composition and morphology of a blend of PCL/PVC, which formed banded spherulites under a selected range of temperature and composition, were studied. Ridges and valleys, which were shown to form one the blend surface, were shown by AFM to be edge-on and flat-on, respectively. The ToF0SlMS images revealed that the fluorine and oxygen concentrations were higher in the valley and ridge regions, respectively. This positional segregation was caused by the difference in surface energy between edge-on and flat-on lamellae. The surface energy of PVC is higher than that of edge-on lamellae but lower than that of flat-on lamellae. As a result, an alternate ring-type distribution of PVC was observed on the surface of the blend.
The above examples clearly show that crystallization is a key driving force that determines the surface composition of blends and copolymers which contain a crystallizable component.