Magnetic Resonance Imaging Analysis of Molecular Mobility during Dissolution of Poly(vinyl alcohol) in Water

Understanding the role of molar mass and stirring in polymer dissolution

When a dry soluble polymer is put in contact with a large quantity of solvent, it swells and forms a transient gel, and eventually, yields a dilute solution of polymers. Everyday lab experience shows that when the molar mass is large, namely tens of times larger than entanglement mass, this dissolution process is slow and difficult and may require stirring. Here, in agreement with previous results, we found that the time needed to turn a dry grain into a dilute solution is not limited by water diffusion in the glassy or semi-crystalline dry polymer, but rather by the life-time of the transient gel made of entangled chains. In addition, we shed new light on the dissolution process by demonstrating that, in contrast to theoretical predictions, the gel life-time is not governed by reptation. We show instead that swelling is simply controlled by the osmotic pressure and the gel permeability until the overlap concentration is reached within the gel. At this stage, the gel turns into a dilute solution in which polymers are dispersed by natural convection. The observed dependence of the dissolution process on the molar mass therefore originates from the molar mass dependent overlap concentration. Under stirring, or forced convection, the polymer gel disappears at a higher critical concentration that depends on the shear rate. We suggest a description of the experimental data which uses the rheological flow curves of the solutions of the considered polymer. Inversely, dissolution times of polymer powders under stirring can be inferred from classical rheological measurements of the polymer solutions at varied concentrations.

Advances in mechanistic understanding of release rate control mechanisms of extended-release hydrophilic matrix tablets

Approaches to characterizing and developing understanding around the mechanisms that control the release of drugs from hydrophilic matrix tablets are reviewed. While historical context is provided and direct physical characterization methods are described, recent advances including the role of percolation thresholds, the application on magnetic resonance and other spectroscopic imaging techniques are considered. The influence of polymer and dosage form characteristics are reviewed. The utility of mathematical modeling is described. Finally, how all the information derived from applying the developed mechanistic understanding from all of these tools can be brought together to develop a robust and reliable hydrophilic matrix extended-release tablet formulation is proposed.

Biodegradable poly(vinyl alcohol) foams supported by cellulose nanofibrils: processing, structure, and properties

In order to capture savings in energy and ambitious environmental targets, biodegradable composite foams of poly(vinyl alcohol) (PVA) supported by cellulose nanofibrils (CNF) were prepared through unidirectional freeze-drying technology. Effects of the content of CNF, the solid content of the precursor suspension, and the quenching temperature on the microstructure and properties of the composite foams were investigated by scanning electron microscopy (SEM), compressive testing, X-ray diffraction (XRD) analysis, water uptake, and biodegradation tests. Results show that the incorporation of CNF preferably at a weight ratio of 30 wt % greatly enhanced the mechanical strength and modulus, energy absorption, water resistance, and dimensional stability of the composite foams because of the rigid and semicrystalline nature of CNF as well as regular and compact pore structures. Furthermore, the biodegradation tests performed in a simulated aerobic compost environment suggested that the involvement of CNF significantly accelerated the pace of biodegradation of the composite foams. Hence, we provided some meaningful information on the biomimetic cellular composite foams with controllable morphs and properties by varying the freeze-drying process and composition.

Self- and mutual-diffusion coefficients measurements by 31P NMR 1D profiling and PFG-SE in dextran gels

31P NMR 1D profiling was successfully introduced to measure macroscale mutual-diffusion coefficients (D(m)) of phosphate ions in dextran gels. Series of 1D profiles describing the phosphate concentration along cylindrical dextran gels were acquired at different times. These profiles that included over 600 points could be fitted using equations derived from Fick's law, with D(m) as the single fitting parameter. Release and penetration profiles were recorded providing two alternative approaches for allowing the determination of D(m). The D(m) values were compared with microscale self-diffusion coefficients (D(s)) measured by pulsed field gradient spin echo (PFG-SE) technique. D(m) values, measured between 25 and 45 degrees C, were systematically lower than D(s). The experimental diffusion time and the associated diffusion length of D(s) (60 ms, 10 microm) are short compared to those of D(m) (up to 18 h, 50 mm). These scale differences are considered to be the origin of different D(s) and D(m) and provide information relative to the network in these gels.

MRI investigation of hydration and heterogeneous degradation of aliphatic polyesters derived from lactic and glycolic acids: a controlled drug delivery device

Magnetic Resonance Imaging experiments have been used to investigate the degradation of drug-loaded poly(glycolic lactic acid) (PGLA) 50:50 cylinders. Spatial variation in the rate of degradation throughout the polymer cross-section has been observed non-invasively using quantitative imaging of penetrant concentration, T(2) and self-diffusion coefficient. This spatial variation in the rate of degradation was attributed to the quicker degradation in the inner region of the sample due to autocatalysis by carboxyl end groups generated upon ester bond cleavage.

Model for water transport into powdered xanthan combining gel swelling and vapor diffusion

Water ingress into xanthan powder compressed to various packing densities has been studied using nuclear magnetic resonance stray field imaging (STRAFI). A foot is observed ahead of the main water ingress front which is attributed to vapor transport around the particles. The main development of the reported work is an analytical model which describes the coupling of vapor transport through the pore space and liquid transport through the progressively swelling gel which gradually occludes the vapor path. Good agreement between theory and experiment is found over a wide range of packing densities with the model requiring only one adjustable parameter, the water diffusivity in the gel measured in a constant polymer mass reference frame. It is suggested that the results are of considerable relevance to situations where the polymer is produced at low concentration by bacteria such as in the rhizosphere and aerial bio films.