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Semicrystalline Polymers Research

Areas of interest:
  • Electrospun Nanofibers
  • Confinement Effects in Semicrystalline Polymers
  • Semicrystalline Polymer Nanocomposites

Semicrystalline polymers constitute the largest group of commercially useful polymers. These polymers exists as viscous liquids at temperatures above the melting point of the crystals. Upon cooling, crystals nucleate and grow to fill the available volume. The reason these materials are called "semicrystalline" is that some fraction of the polymer remains un-crystallized, or, amorphous when the polymer is cooled to room temperature. The amorphous polymer becomes trapped between the growing crystals. As a result of the highly entangled nature of the polymer chains, the movement of the amorphous polymer becomes restricted.

The Cebe research group is actively investigating the constraints exerted by the crystals upon the mobility of the amorphous phase. We utilize research tools that measure the mobility of molecules, for example, dielectric relaxation spectroscopy (DRS) and temperature-modulated differential scanning calorimetry (TM-DSC). DRS measures the response of electric dipoles to an oscillating electric field. The real and imaginary parts of the complex dielectric function are calculated and their variation with semicrystalline polymer structure is assessed. In the molten state, dipole moments on the amorphous polymer chains are free to move; once crystals form the mobility of the dipoles is reduced, and this is seen as a shift in the relaxation frequency spectrum. Complementary structural information is needed to determine the size of crystals, their arrangement and perfection, and the overall degree of crystallinity. Structural information is obtained from wide and small angle X-ray scattering, atomic force microscopy (AFM) and scanning electron microscopy, and thermal analysis.

Bin Mao at AFM
Graduate student, Bin Mao, adjusts the atomic force microscope.
single crystal PVF2
AFM phase contrast image of a single lamellar crystal of PVF2 grown from dilute solution, showing roughly hexagonal habit, with overgrowths.
Qian Ma at BNL
Graduate student, Qian Ma, shown outside the experimental hutch at beamline X27C, at the Brookhaven National Synchrotron Light Source (NSLS).
Wide-angle X-ray of PVF2
Wide angle X-ray Scattering data taken at NSLS show the impact of nanosilicates on the crystal phase of PVF2. Beta phase is preferred as silicate content increases.
An AFM image is shown for a single lamellar crystal of PVF2, which in certain crystallographic phases is piezoelectric. Single crystals can only be grown from very dilute solutions, where it is possible to avoid the density of chain entanglements found in melt crystallized polymers. We are using AFM to study the morphology of these crystals, and electron diffraction and X-ray diffraction will be used to confirm the crystallographic phase.

Other areas of research in semicrystalline polymers include the confinement effects brought by electro-spinning semicrystalline polymers into very thin fibers, and the impact of nano-additives such as silicates or carbon nanotubes.

Huipeng wins travel grant
Graduate student, Huipeng Chen (third from left) was one of the winners of a travel grant from the North American Thermal Analysis Society. Huipeng presented a lecture about his work on thermal analysis of semicrystalline polymer blends.
Georgi at APS meeting
Prof. Georgi Georgiev (Assumption College) and Tufts graduate students, Huipeng Chen, Lei Yu, and Xiao Hu, are shown at Huipeng's poster, presented at the American Physical Society March Meeting in Denver.
Dr. Marek Pyda, visistor from Poland
Dr. Marek Pyda, visiting from Rzeszow University of Technology in Poland, describes heat capacity data taken in the Cebe Lab's thermal analysis facility.
Nylon-6 heat capacity
Heat capacity vs. temperature is shown for Nylon 6 semicrystalline polymer. Careful analysis of the solid and liquid state heat capacity baselines allows the fraction of mobile amorphous phase to be calculated with high accuracy.

This program is funded by the National Science Foundation through the Polymers Program of the Division of Materials Research, grant DMR-0602473, and previously through NSF grant DMR-0100646 and NASA grant NAG8-1167.