The thermal analysis of gel-spun ultra-high molar mass polyethylene fibers

Author

Andreas Boller

Date of Award

12-1996

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

Bernhard Wunderlich

Committee Members

S.D. Alexandratos, G. Bhat, M. Dadmun, J. Spruiell

Abstract

The thermal analytical properties of gel-spun ultra-high molar mass polyethylene (UHMMPE) fibers were studied with differential scanning calorimetry (DSC), thermo-optical analysis (TOA), and thermo-mechanical analytical (IMA) methods. The thermal analysis data were combined with the results from full-pattern x-ray diffraction analysis, small-angle x-ray scattering, powder x-ray diffraction, and solid state ¹³C nuclear magnetic resonance experiments. At room temperature, these fibers show mainly the common orthorhombic and a small amount of monoclinic crystals, in addition to an intermediate, oriented fraction and a minimal amount of amorphous phase. The structure parameters of the orthorhombic phase change slightly with the processing history. The chains of the intermediate phase have a largely trans-conformation and are oriented preferentially parallel to the fiber axis, but are disordered laterally. The mobility (correlation time) of the carbon atoms of the intermediate phase is higher than that of the crystalline phase by one order of magnitude, but lower than that of the amorphous phase by one order of magnitude. The intermediate phase can contribute significantly to the meridional x-ray diffraction peaks 00l, but not to the equatorial peaks hk0. A simple structural model is proposed to reconcile the experimental results. This dissertation contains the first published account of heat capacities of UHMMPE fibers. The results from these heat capacity measurements show that the post-drawing of the gel-spun fibers creates structures which have the same heat capacity, C_p throughout the range of 175 to 400 K The non-post-drawn fiber, in contrast, shows a step-increase in heat capacity in the temperature range from 260 to 290 K. Comparisons with the vibrations-only and the experimental, 100%-crystalline, extended-chain polyethylene heat capacities lead to the conclusion that the intermediate, non-crystalline material has a steady increase of C_p from 180 to 290 K. This increase is interpreted as the mesophase glass transition. The increase from 290 to 360 K is solely due to the increase in gauche conformations. At 360 K the locally reversible melting of the noncrystalline intermediate starts. The mass dependent DSC experiments revealed that melting occurred in one step at 412 K, opposed to the often cited multiple-stage melting. The one-step melting was confirmed by optical observation of the melting of UHMMPE fibers in a microscopy hot-stage and thermomechanical analysis. This one-step melting behavior combines the isotropization of the crystalline and the mesophase material into one transition. The two phases are therefore highly connected and interdependent. It is concluded that such behavior requires a nano-phase separation of the mesophase and the crystalline phase. The mesophase model suggested by the experimental data allows a correlation of the structure to properties.

Recommended Citation

Boller, Andreas, "The thermal analysis of gel-spun ultra-high molar mass polyethylene fibers. " PhD diss., University of Tennessee, 1996.
https://trace.tennessee.edu/utk_graddiss/9672