The morphology, nucleation, and crystallization of polyethylene/carbon nanotubes nanocomposites
were studied. The nanocomposites were prepared by in-situ polymerization of ethylene on carbon nanotubes
(CNT) whose surface had been previously treated with a metallocene catalytic system. The effects of composition
(5−22% CNT) and structure of the nanotube (single, double, or multiwall, i.e., SWNT, DWNT, and MWNT)
were evaluated, and an excellent nucleating effect on polyethylene matrix was found regardless of the CNT type
in comparison to neat high-density polyethylene (HDPE) prepared under identical conditions. The CNT were
found to be more efficient in nucleating the HDPE than its own crystal fragments, a result obtained by self-nucleation studies. Differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) results
showed that under both isothermal and dynamic crystallization conditions the crystals produced within the
nanocomposite HDPE matrix were more stable than those produced in neat HDPE or in physical blends prepared
by melt mixing of HDPE and untreated CNT. The remarkable stability of the crystals was reflected in melting
points up to 5 °C higher than neat HDPE and concomitant thicker lamellae. The changes induced on HDPE by
CNT are due to the way the nanocomposites were prepared; since the macromolecular chains grow from the
surface of the nanotube where the metallocene catalyst has been deposited, this produces a remarkable nucleating
effect and bottle brush morphology around the CNT. Isothermal crystallization kinetics results showed that the
in-situ nanocomposites crystallize much faster at equivalent supercoolings than neat HDPE because of the nucleating
effect of CNT. Wide-angle X-ray scattering studies demonstrated that the crystalline structure of the HDPE matrix
within the in-situ-polymerized HDPE/CNT nanocomposites was identical to that of neat HDPE and did not change
during isothermal crystallization, keeping its orthorhombic unit cell.
New results of dielectric spectroscopy in semicrystalline nylon-6 samples with different moisture contents are presented by using a wide frequency (10 -2 -3 × 10 6 Hz) and temperature range (133-433 K). The dielectric absorption spectra in frequency domain are decomposed in Cole-Cole distributions corresponding to the local γ and β modes, two segmental R modes, and a high-intensity Maxwell-Wagner-Sillars relaxation. The presence of two segmental relaxations in the isothermal runs, attributed to the plasticized and the unplasticized R mode, is interpreted as the manifestation of two different length and time scales of cooperative motions. A quantitative comparison between the results obtained for the wet and dry samples, such as relaxation times variation, activation energies, Vogel-Tammann-Fulcher parameters, dielectric increments, and distribution widths, is presented for each mode and shows how the progressive drying of the sample during the experiment affects all these quantities.
Biodegradable poly(butylene succinate)/polylactide (PBS/PLA)
blends
with various blending ratios were prepared by melt mixing for morphological
and rheological studies. Dynamic rheological measurements were performed
on the blend systems and the viscoelastic responses were analyzed
with several emulsion models. The results show that the PBS/PLA is
an immiscible blend system with very narrow cocontinuous region and
high percolation threshold. The phase inversion point could be precisely
predicted by the small-amplitude oscillatory shear (SAOS) response.
The Palierne model gave a better description of the viscoelastic response
of PBS/PLA blends than that of the Gramespacher and Meissner (G-M)
model. In addition, the interfacial tension between the two polymers
was measured by several techniques, such as surface property characterizations,
deformed drop retraction, and rheological approaches. The differences
observed by these various methodologies were then further explored.
Moreover, the melting and crystallization behaviors of the blends
were also studied in order to reach a deeper insight into the relations
between the phase behavior and the macroscopic thermal properties
of the PBS/PLA blends.
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