Polylactide (PLA)-based nanocomposites

In this article, the effects of size and confinement at the nanometre size scale on both the melting temperature, Tm, and the glass transition temperature, Tg, are reviewed. Although there is an accepted thermodynamic model (the Gibbs–Thomson equation) for explaining the shift in the first-order transition, Tm, for confined materials, the depression of the melting point is still not fully understood and clearly requires further investigation. However, the main thrust of the work is a review of the field of confinement and size effects on the glass transition temperature. We present in detail the dynamic, thermodynamic and pseudo-thermodynamic measurements reported for the glass transition in confined geometries for both small molecules confined in nanopores and for ultrathin polymer films. We survey the observations that show that the glass transition temperature decreases, increases, remains the same or even disappears depending upon details of the experimental (or molecular simulation) conditions. Indeed, different behaviours have been observed for the same material depending on the experimental methods used. It seems that the existing theories of Tg are unable to explain the range of behaviours seen at the nanometre size scale, in part because the glass transition phenomenon itself is not fully understood. Importantly, here we conclude that the vast majority of the experiments have been carried out carefully and the results are reproducible. What is currently lacking appears to be an overall view, which accounts for the range of observations. The field seems to be experimentally and empirically driven rather than responding to major theoretical developments.

https://iopscience.iop.org/article/10.1088/0953-8984/17/15/R01/pdf

Effects of confinement on material behaviour at the nanometre size scale

Research pertaining to conductive polymers has gained significant traction in recent years, and their applications range from optoelectronics to material science. For all intents and purposes, conductive polymers can be described as Nobel Prize-winning materials, given that their discoverers were awarded the Nobel Prize in Chemistry in 2000. In this review, we seek to describe the chemical forms and functionalities of the main types of conductive polymers, as well as their synthesis methods. We also present an in-depth analysis of composite conductive polymers that contain various nanomaterials such as graphene, fullerene, carbon nanotubes, and paramagnetic metal ions. Natural polymers such as collagen, chitosan, fibroin, and hydrogel that are structurally modified for them to be conductive are also briefly touched upon. Finally, we expound on the plethora of biomedical applications that harbor the potential to be revolutionized by conductive polymers, with a particular focus on tissue engineering, regene...

Conductive Polymers: Opportunities and Challenges in Biomedical Applications

1. Introductory Concepts 2. Polarization and Static Dielectric Constant 3. Dielectric Loss and Relaxation-I 4. Dielectric Loss and Relaxation-II 5. Experimental Data (Frequency Domain) 6. Absorption and Desorption Currents 7. Field Enhanced Conduction 8. Fundamental Aspects of Gaseous Breakdown-I 9. Fundamental Aspects of Electrical Breakdown-II 10. Thermally Stimulated Processes 11. Space Charge in Solids Dielectrics

Dielectrics in electric fields

Poly (lactic acid) blends: Processing, properties and applications.

AC Conductivity of Selectively Located Carbon Nanotubes in Poly(ε-caprolactone)/Polylactide Blend Nanocomposites

The existence of a distribution of relaxation times has been widely used to describe the relaxation function versus frequency in glass-forming liquids. Several empirical distributions have been proposed and the usual method is to fit the experimental data to a model that assumes one of these functions. Another alternative is to extract from the experimental data the discrete profile of the distribution function that best fits the experimental curve without any a priori assumption. To test this approach a Monte Carlo algorithm using the simulated annealing is used to best fit simulated dielectric loss data, ${\ensuremath{\varepsilon}}^{\ensuremath{''}}(\ensuremath{\omega}),$ generated with Cole-Cole, Cole-Davidson, Havriliak-Negami, and Kohlrausch-Williams-Watts (KWW) functions. The relaxation times distribution, $G(\mathrm{ln}(\ensuremath{\tau})),$ is obtained as an histogram that follows very closely the analytical expression for the distributions that are known in these cases. Also, the temporal decay functions, $\ensuremath{\varphi}(t),$ are evaluated and compared to a stretched exponential. The method is then applied to experimental data for $\ensuremath{\alpha}$-polyvinylidene fluoride over a temperature range $233 \mathrm{K}l~Tl~278 \mathrm{K}$ and frequencies varying from 3 MHz to 0.001 Hz. These data show the existence of two relaxation processes: the fast segmental ${\ensuremath{\alpha}}_{a}$ process associated with the glass transition and a ${\ensuremath{\alpha}}_{c}$ mode, which is slower and due to changes in conformation that can occur in the crystalline regions. The experimental curves are fitted by the simulated annealing direct signal analysis procedure, and the relaxation times distributions are calculated and found to vary with temperature. The decay function is also evaluated and it shows clearly its bimodal character and a good agreement with a KWW function with a temperature dependent $\ensuremath{\beta}$ for each mode. The relaxation plots are drawn for each mode and the Vogel-Tammann-Fulcher and Arrhenius parameters are found. The fragility parameter for polyvinylidene flouride (PVDF) is found to be 87, which characterizes this polymer as a relatively structurally strong material.

Distribution of relaxation times from dielectric spectroscopy using Monte Carlo simulated annealing: Application to α-PVDF

New results of dielectric spectroscopy in semicrystalline nylon-6 samples with different moisture contents are presented by using a wide frequency (10-2−3 × 106 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 α 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 α 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.

Water Absorption Effect on the Dynamic Properties of Nylon-6 by Dielectric Spectroscopy

Polycarbonate/poly(e-caprolactone) (PC/PCL) blends are found to be miscible when extruded samples are studied without any further thermal treatment. PCL crystallizes in blends containing 60% or les...

Miscibility and Crystallization in Polycarbonate/Poly(ε-caprolactone) Blends: Application of the Self-Concentration Model

The behavior of the various relaxation modes in poly(e-caprolactone) (PCL), is studied by Broad Band Dielectric Spectroscopy in the frequency range from 3×10−3 to 1.8×109 Hz and from 133 to 313 K. The experimental trace of the dielectric loss as a function of the angular frequency, e″(ω), is analyzed by best fitting a sum of Cole–Cole distributions corresponding to the γ and β local modes and to the α relaxation which is the dielectric manifestation of the dynamic glass transition. The kinetic parameters of the three predominant relaxations are determined and relaxation plots describing the temperature dependencies of the relaxation times are given as a function of 1/T. These relaxation plots are insensitive within experimental errors, either to the molecular weight or to the water concentration. The hydration level (<1%) only affects the intensities of the local processes and no plasticization effect is observed. At temperatures higher than those recorded for the α mode a fourth intense process, α′ is ob...

Alfredo Bello

Papers

AC Conductivity of Selectively Located Carbon Nanotubes in Poly(ε-caprolactone)/Polylactide Blend Nanocomposites

Distribution of relaxation times from dielectric spectroscopy using Monte Carlo simulated annealing: Application to α-PVDF

Water Absorption Effect on the Dynamic Properties of Nylon-6 by Dielectric Spectroscopy

Miscibility and Crystallization in Polycarbonate/Poly(ε-caprolactone) Blends: Application of the Self-Concentration Model

Study of dielectric relaxation modes in poly(ε-caprolactone): Molecular weight, water sorption, and merging effects