Non-Fickian Diffusion of Water in Polylactide (original) (raw)

Nonequilibrium Sorption of Water in Polylactide

Macromolecules, 2012

The sorption of water in polylactide (PLA) was measured at various vapor activities (0−0.85) and temperatures using a number of experimental techniques, including quartz spring microbalance (QSM), quartz crystal microbalance (QCM), and in situ time-resolved Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy. Additionally, a prediction of the water sorption isotherm in PLA was obtained with the use of the nonequilibrium lattice fluid (NELF) model, where an excellent agreement between the model prediction and experimental sorption data was observed at different experimental temperatures (all below the glass transition temperature of PLA) for water vapor activities less than 0.65. Results from in situ time-resolved FTIR-ATR spectroscopy revealed that water is present predominately as dimers in PLA at water vapor activities less than 0.65, and the presence of larger hydrogen-bound water clusters was observed at water vapor activities >0.65. This provides a rationale for the deviation between the NELF model prediction and water sorption data at high vapor activities, where the NELF model does not account for the strong self-association interactions present in large hydrogen-bound water clusters. Furthermore, non-Fickian sorption kinetic behavior was observed with all the experimental techniques, and this highlights the nonequilibrium nature of the water−glassy polymer system and provides insight into the variability in the sorption isotherms reported in the literature.

Moisture Sorption, Transport, and Hydrolytic Degradation in Polylactide

Applied Biochemistry and Biotechnology, 2006

Management of moisture penetration and hydrolytic degradation of polylactide (PLA) is extremely important during the manufacturing, shipping, storage, and end-use of PLA products. Moisture transport, crystallization, and degradation in PLA have been measured through a variety of experimental techniques including size-exclusion chromatography, differential scanning calorimetry, and X-ray diffraction. Quartz crystal microbalance and dynamic vapor sorption experiments have also been used to measure moisture sorption isotherms in PLA films with varying crystallinity. A surprising result is that, within the accuracy of the experiments, crystalline and amorphous PLA films exhibit identical sorption isotherms.

Absorption and Diffusion in Bio-Based Polymer Films

2006

Thin bio-based polymer films have been shown to have potential uses in a variety of applications, such as medical coatings, food packaging, and textile fibers. In order to optimize their use, it is essential to understand how these films interact with their environment and how these interactions effect changes in the properties of these films. This paper presents experimental measurements of moisture transport in polylactide (PLA), obtained using a combined Quartz Crystal Microbalance and Heat Conduction Calorimeter (QCM/HCC) and Dynamic Vapor Sorption (DVS). Measurements of moisture sorption in PLA, a moderately hydrophobic aliphatic polyester, versus relative humidity and temperature show that moisture sorption increases with degradation but is only weakly dependent on crystallinity. These results contradict the common assumption that water is exclused from crystalline domains. The composition of the end groups in PLA were modified and no significant end group effect was observed.

Sorption of Water Vapor in Poly(L-Lactic Acid): A Time-Resolved FTIR Spectroscopy Investigation

Frontiers in Chemistry

In this contribution the sorption of water vapor in Poly(L-lactic acid) (PLLA) was studied by time-resolved FTIR spectroscopy. The collected FTIR data were analyzed by complementary approaches such as difference spectroscopy, two-dimensional correlation spectroscopy (2D-COS), and least-squares curve-fitting analysis which provided information about the overall diffusivity, the nature of the molecular interactions among the polymer and the penetrant and the dynamics of the various molecular species. The diffusion coefficient were evaluated as a function of vapor activity and were found in good agreement with previously reported values. The system showed a Fickian behavior with diffusivity increasing with penetrant concentration. Two distinct water species (first-shell and second-shell layers) were detected and quantified by coupling FTIR and gravimetric measurements.

Sorption and transport of water vapor in glassy poly(acrylonitrile)

Polymer Engineering & Science, 1980

Sorption data for H2O in glassy poly(acrylonitrile)(PAN) are presented for a range of relative vapor pressures at temperatures from 20 to 50°C. Simple dual mode sorption, involving “hole‐filling” and molecular solution appears to dominate the low activity region of sorption. Based on the clustering analysis suggested by Zimm and Lundberg, pronounced clustering of penetrant appears to occur above a relative pressure of 0.6. The form of the effective concentration‐dependent diffusion coefficient for H2O in PAN, determined by analysis of steady state permeation data, suggests that water in the microvoids and clusters has a lower mobility than the molecularly dissolved water in the polymer matrix. Time lag measurements at high upstream relative water vapor pressures suggest that the transient state permeation has a non‐Fickian character due to relaxations which occur slowly to accommodate the clustering process.

Liquid−Glassy Polymer Diffusion: Rate-Controlling Step and Diffusion Mechanism

Macromolecules, 2005

We investigate the diffusion mechanism at a liquid-glassy polymer interphase, produced in this case between poly(vinyl methyl ether) (PVME) as the liquid polymer and polystyrene (PS) as the glassy matrix. The evolution of the interphase was directly measured by using confocal Raman microspectroscopy in the depth-profiling mode. Diffusion experiments were performed in the range 85-125°C, with the specific purpose of encompassing the glass transition temperature (T g) of the glassy matrix (PS, 100°C). In this way, direct evidence about the effect of the physical state of the (glassy or liquid) PS matrix on the diffusion modes was obtained. We found that the diffusion experiments performed at temperatures below the matrix T g (liquid-glassy polymer diffusion) are controlled by the same parameters and show the same features as those performed at temperatures above the matrix Tg (liquidliquid polymer diffusion). Furthermore, a Fickean diffusion model developed for liquid-liquid polymer diffusion correlates precisely with the whole set of data, including liquid-glassy polymer diffusion experiments, without invoking case II diffusion theory. It is concluded that the diffusion-controlling step of the process is placed at the liquid PVME-PS interphase. These observations are in marked contrast with interpretations from other authors that used the context of case II to explain the mechanisms that control the evolution of these interphases, an idea often proposed to interpret experimental results for this polymer pair. The origin of the discrepancy is discussed.

Liquid‐Glassy Polymer Diffusion: Effects of Liquid Molecular Weight and Temperature

Macromolecular Chemistry and Physics, 2007

We examine mechanistic aspects of the diffusion between a series of liquid polystyrenes (PS) and a glassy poly(phenylene oxide) (PPO) matrix, through the use of confocal Raman microspectroscopy. The results show that the diffusion kinetics has Fickean characteristics, similar to those found in liquid‐liquid polymer diffusion. No signatures of the linear regime typical of the case‐II diffusion mechanism were found. Overall, these observations are consistent with the claim that case‐II is unlikely to occur in liquid‐glassy polymer diffusion.magnified image

Water effect in the thermal and molecular dynamics behavior of poly(L-lactic acid)

Journal of Thermal Analysis and Calorimetry, 2007

A detailed dielectric characterization of the relaxation modes found in a poly(L-lactic acid), PLLA, film containing 0.4 mass% of water is provided. The sub-glass relaxation process is a superposition of two processes, one highly influenced by water with activation energy of 50 kJ mol -1 , and another one, with longer relaxation times and lower intensity having activation energy of 38 kJ mol -1 . Dried PLLA exhibits an abnormally broad secondary b-relaxation that probably corresponds to the superposition of multiple processes. Upon water sorption the strength of the more mobile process significantly increases being shifted to lower temperatures which allows the detection of the underlying process. The glass transition relaxation process is deviated to higher frequencies almost one decade due to the water plasticizing effect. The reported results show that small quantities of water may have a profound impact in the relaxational features in PLLA, which should be taken in account when considering the properties and performance of this system.

Method to Study Water Diffusion into Polymers

Proceedings

We designed and realized a low cost relative humidity (RH) capacitive sensor having an interdigitated transducer coated with a cheap sensing material working at room temperature (polyimide). Thermally perturbed diffusion of water molecules into the polyimide layer is studied by heating the sensor locally and measuring the sensor capacitance change. The swelling and deswelling model is applied to determine the time constant of involved processes. This simple method, using an evaluation kit developed by our group, to measure the sensor capacitance and to study the diffusion process of water molecules into polyimide could be generalized to analyze the gas diffusion processes into polymer based sensing layer generally used in the field of gas chemical sensors.

Sorption kinetics and equilibrium uptake for water vapor in soft-contact-lens hydrogels

Journal of Biomedical Materials Research Part A, 2006

A gravimetric-sorption technique was used to obtain kinetic and equilibrium adsorption/desorption data for water vapor in four different soft-contact-lens (SCL) polymers at 35°C. The SCL materials are a conventional hydrogel (polymacon) with a low water content at saturation (Ͻ50 wt %); two conventional hydrogels (hilafilcon A and alphafilcon A) with a high water content at saturation (Ͼ50 wt %); and a siloxane hydrogel (balafilcon A). Absorption and desorption equilibrium isotherms (water activity versus water weight fraction) overlap at high water contents, whereas significant hysteresis is observed at low water contents. The hysteresis loop is likely due to trapping of water in the polymer during the desorption process because of a rubber-to-glass transition of the SCL-film surfaces. Sorption data were interpreted using Flory-Rehner theory. The positive Zimm and Lundberg cluster function suggests that water tends to cluster in these SCL materials, except at very low water content. For polymacon and hilafilcon A, Fickian diffusion is observed for all activities for both water sorption and desorption. However, for alphafilcon A and balafilcon A, non-Fickian features appear at intermediate/low activities, in particular during water desorption, suggesting coupling of the diffusion process with polymer-matrix relaxation. The diffusion coefficient increases significantly with water concentration for polymacon and hilafilcon A (from ϳ0.3 ϫ 10 Ϫ8 to 4.0 ϫ 10 Ϫ8 cm 2 /s) because of augmented mixture free volume induced by water sorption, whereas a more complex composition dependence is observed for alphafilcon A and balafilcon A probably as consequence of a combined effect of polymer relaxation, plasticization, and water clustering.