Fengji Yeh

Effect of miscible polymer diluents on the development of lamellar morphology in poly(oxymethylene) blends

The development of lamellar morphology in poly(oxymethylene) (POM) and its miscible blends was studied by synchrotron time-resolved small-angle X-ray scattering (SAXS), during primary and secondary crystallization at temperatures near 150°C. The blends contained two different diluents: poly(vinyl 4-hydroxy styrene) [common name poly(vinyl phenol), (PVP)], which had a high glass temperature (Tg = 150°C), and styrene-co-hydroxy styrene oligomer (PhSO), which had a low glass temperature (Tg = −37°C). The SAXS data were analyzed by correlation function analysis to extract several lamellar parameters: long period (L), lamellar crystalline thickness (lc), amorphous layer thickness (la), and invariant (Q). The variation in Q defined the region where spherulites quickly grew and filled the entire space, and was referred to as the primary crystallization dominant regime. A rapid drop in L and lc was observed at early times, and this can be explained by defective lamellar stacks filling in space between primary stacks, as secondary crystals form during the nominal primary crystallization dominant regime. Lamellar thickening with time in the long-time secondary crystallization region was observed in neat POM and the blend with 10 % low Tg diluent, while this process was inhibited with the high Tg diluent due to the higher Tg of the interlamellar species. A decrease in la at long times confirmed the lamellar thickening. We refer to the lamellar thickening process as a type of secondary crystallization. Interlamellar inclusion or trapping was detected to different degrees with the high Tg diluent, while exclusion was found for the low Tg diluent.

Nanostructures of polyelectrolyte gel–surfactant complexes

Small-angle X-ray scattering was used to investigate the nanostructures of complexes formed by slightly crosslinked anionic copolymer gels of poly(sodium methacrylate-co-N-isopropylacrylamide) [P(MAA/NIPAM)] with cetyltrimethylammonium bromide (CTAB), and didodecyldimethylammonium bromide (DDAB), respectively, at room temperature (∼ 23°C). Several highly ordered supramolecular structures were observed in the polyelectrolyte gel–surfactant complexes. In P(MAA/NIPAM)–CTA systems, in sequence with decreasing charge density of the P(MAA/NIPAM) copolymer chains, structures of the Pm3n space group cubic, face-centered cubic close packing of spheres, and hexagonal close packing of spheres were determined at a charge content of ≥ 75, 67, and 50%, respectively. The spheres and rods in these structures were the spherical and cylindrical micelles formed by the self-assembly of CTA cations with their paraffin chains inside. Both the aggregation number and the size of the micelles decreased with a decreasing charge density of the copolymer chains. In the P(MAA/NIPAM)–DDA systems, the bilayer lamellar structures formed at charge contents ≥ 75% transferred to bicontinuous cubic structures of the Ia3d space group at charge contents of 50–67%. The rods in the Ia3d cubic structures were formed by the self-assembly of double-tailed DDA cations with polar moieties inside. The formation of these highly ordered structures were driven by both electrostatic and hydrophobic interactions of the charged copolymer chains/surfactants and the surfactants/surfactants inside the charged gels. The structures became less ordered by further decreasing the charge content of the P(MAA/NIPAM) chains.