Evaluation of the Degree of Exfoliation in Poly(ε‐caprolactone)/Organoclay Nanocomposites Based on Viscoelastic Relaxation

Abstract

IntroductionIn the past few decades, layered silicate-based polymer nanocomposites have attracted considerable attention because of their dramatically increased strength, modulus, thermal resistance and gas permeability barrier, which is obtained with a much lower silicate content than that used in conventional filled polymer composites. [1] Layered silicates have layers with thicknesses on the order of 1 nm and have very high aspect ratios (e.g., 10-1 000), and interlayer spacings between the stacked layers of about 1 nm. [2] The distinctive features of the layered silicates result in the nanocomposites having two possible structures, namely intercalated or exfoliated structures. The intercalated nanocomposites show regularly alternating silicate and polymer layers with a repeat distance of Full PaperPoly(e-caprolactone) nanocomposites, PCLOC25A and PCLOC30B, with organoclays (OCs) having nonpolar and polar organic modifiers, respectively, were prepared by the melt mixing method and additional heat treatment. WXRD analysis revealed that both nanocomposites were exfoliated, irrespective of the OC polarity. However, WXRD failed to show the degree of exfoliation of the nanocomposites, because the d 001 peaks disappeared. Thus, dynamic mechanical analysis (DMA) was carried out to compare the degree of exfoliation of the PCL nanocomposites. From DMA, PCLOC30B showed higher elasticity, storage moduli, viscosity, and activation energy than PCLOC25A, indicating that PCLOC30B had a more exfoliated structure than PCLOC25A. This is due to the polar interaction in PCLOC30B, as verified by the plots of a T versus temperature. Thus, it was confirmed that DMA provides an alternative approach to evaluating the degree of exfoliation of nanocomposites. 627 a few nanometers, whereas the individual layers in the exfoliated nanocomposites are irregularly delaminated and dispersed in a continuous polymer matrix. [3] Such structural differences play a key role in the enhancement of the nanocomposite properties. Exfoliated nanocomposites generally have superior mechanical properties to intercalated nanocomposites, because of the larger surface area that exists between the reinforcement phase and the polymer matrix. [4] Giannelis et al. reported that the thermodynamically stable equilibrium states of the nanocomposites, such as the intercalated and exfoliated systems described above, are dependent on various entropic and enthalpic factors. [5] Since the total entropy change in the system was small, the change in enthalpic factors such as the intermolecular interactions determined the structure of the nanocomposites. [5] Thus, the layered silicate structure depended on the establishment of very favorable polymer-surface interactions to overcome the penalty of the confinement condition, and specific interactions, driven by polar interactions or hydrogen bonding, played a significant role in promoting the dispersion of the layered silicate. [6,7] Finally, the silicate structure, depending on polymer-surface interactions, can have influence...

Viscoelastic properties of poly(ε‐caprolactone)/clay nanocomposites in solid and in melt state

“…Applied frequency shift factors are relatively small and comparable to reported values for pure PCL and its various nanocomposites in the similar temperature window . The shift factors used to obtain good superposition for the nanocomposites seem to be independent of the clay loading.…”

Rheological behavior of biocomposites of silk fibroin fiber and poly(ε‐caprolactone): Effect of fiber network

“…Thus, we can unequivocally attribute the elasticity of the composites to the SF fiber network therein. Qualitatively similar elasticity has been seen also for polymer composites containing carbon nanofibers/nanotubes and organoclay 16, 18–20, 28, 29…”

“…Qualitatively similar elasticity has been seen also for polymer composites containing carbon nanofibers/nanotubes and organoclay. 16,[18][19][20]28,29…”

“…16 For organoclay/PCL nanocomposites, linear viscoelastic moduli data have been utilized to evaluate/discuss the clay-PCL miscibility, a degree of exfoliation, and structural changes with thermal history. [17][18][19][20] The miscibility, phase behavior, processing performance, and thermal stability have been rheologically examined also for PCL blended with starch, [21][22][23] poly(styrene-co-acrylonitrile) (SAN), 24 Polylactide (PLA), 25 and polyurethane (PU). 26 Thus, we have conducted linear and nonlinear viscoelastic tests for SF/PCL composites containing a network of SF fibers.…”

“…For example, the rheological analysis demonstrated that multiwalled carbon nanotube/PCL composites (PCLCNs) has a much lower percolation threshold (2–3 vol %) compared to clay‐based nanocomposites and that the percolated nanotube network structure sensitively changes with the shear deformation and the temperature 16. For organoclay/PCL nanocomposites, linear viscoelastic moduli data have been utilized to evaluate/discuss the clay‐PCL miscibility, a degree of exfoliation, and structural changes with thermal history 17–20. The miscibility, phase behavior, processing performance, and thermal stability have been rheologically examined also for PCL blended with starch,21–23 poly(styrene‐ co ‐acrylonitrile) (SAN),24 Polylactide (PLA),25 and polyurethane (PU) 26…”