title: Crystalline Morphology and Polymorphic Phase Transitions in Electrospun Nylon 6 Nanofibers.
abstract: Uniform nylon 6 nanofibers with diameters around 200 nm were prepared by electrospinning. Polymorphic phase transitions and crystal orientation of nylon 6 in unconfined (i.e., as-electrospun) and a high T(g) (340 degrees C) polyimide confined nanofibers were studied. Similar to melt-spun nylon 6 fibers, electrospun nylon 6 nanofibers also exhibited predominant, meta-stable gamma crystalline form, and the gamma-crystal (chain) axes preferentially oriented parallel to the fiber axis. Upon annealing above 150 degrees C, gamma-form crystals gradually melted and recrystallized into the thermodynamically stable alpha-form crystals, which ultimately melted at 220 degrees C. Release of surface tension accompanied this melt-recrystallization process, as revealed by differential scanning calorimetry. For confined nanofibers, both the melt-recrystallization and surface tension release processes were substantially depressed; gamma-form crystals did not melt and recrystallize into alpha-form crystals until 210 degrees C, only 10 degrees C below the T(m) at 220 degrees C. After complete melting of nano-confined crystals at 240 degrees C and recrystallization at 100 degrees C, only alpha-form crystals oriented perpendicular to the nanofiber axis were obtained. In the polyimide-confined nanofibers, the Brill transition (from the monoclinic alpha-form to a high temperature monoclinic form) was observed at 180-190 degrees C, which was at least 20 degrees C higher than that in unconfined nylon 6 at approximately 160 degrees C. This, again, was attributed to the confinement effect.
authors.affiliation: Department of Chemistry, South Dakota School of Mines and Technology, Rapid City, South Dakota 57701.
authors.forename: Yi
authors.initials: Y
authors.lastname: Liu
authors.forename: Li
authors.initials: L
authors.lastname: Cui
authors.forename: Fangxiao
authors.initials: F
authors.lastname: Guan
authors.forename: Yi
authors.initials: Y
authors.lastname: Gao
authors.forename: Nyle E
authors.initials: NE
authors.lastname: Hedin
authors.forename: Lei
authors.initials: L
authors.lastname: Zhu
authors.forename: Hao
authors.initials: H
authors.lastname: Fong