Chapter 4 Methods for the characterisation of porous structure in membrane materials

3D high spatial resolution visualisation and quantification of interconnectivity in polymer films

A porous network acts as transport paths for drugs through films for controlled drug release. The interconnectivity of the network strongly influences the transport properties. It is therefore important to quantify the interconnectivity and correlate it to transport properties for control and design of new films. This work presents a novel method for 3D visualisation and analysis of interconnectivity. High spatial resolution 3D data on porous polymer films for controlled drug release has been acquired using a focused ion beam (FIB) combined with a scanning electron microscope (SEM). The data analysis method enables visualisation of pore paths starting at a chosen inlet pore, dividing them into groups by length, enabling a more detailed quantification and visualisation. The method also enables identification of central features of the porous network by quantification of channels where pore paths coincide. The method was applied to FIB-SEM data of three leached ethyl cellulose (EC)/hydroxypropyl cellulose (HPC) films with different weight percentages. The results from the analysis were consistent with the experimentally measured release properties of the films. The interconnectivity and porosity increase with increasing amount of HPC. The bottleneck effect was strong in the leached film with lowest porosity.

Characterisation of pore structures of pharmaceutical tablets: A review

Traditionally, the development of a new solid dosage form is formulation-driven and less focus is put on the design of a specific microstructure for the drug delivery system. However, the compaction process particularly impacts the microstructure, or more precisely, the pore architecture in a pharmaceutical tablet. Besides the formulation, the pore structure is a major contributor to the overall performance of oral solid dosage forms as it directly affects the liquid uptake rate, which is the very first step of the dissolution process. In future, additive manufacturing is a potential game changer to design the inner structures and realise a tailor-made pore structure. In pharmaceutical development the pore structure is most commonly only described by the total porosity of the tablet matrix. Yet it is of great importance to consider other parameters to fully resolve the interplay between microstructure and dosage form performance. Specifically, tortuosity, connectivity, as well as pore shape, size and orientation all impact the flow paths and play an important role in describing the fluid flow in a pharmaceutical tablet. This review presents the key properties of the pore structures in solid dosage forms and it discusses how to measure these properties. In particular, the principles, advantages and limitations of helium pycnometry, mercury porosimetry, terahertz time-domain spectroscopy, nuclear magnetic resonance and X-ray computed microtomography are discussed.

Characterisation of Low-K Dielectric Films by Ellipsometric Porosimetry

Ellipsometric porosimetry (EP) is a simple and effective method for the characterization of the porosity (volume of both open and close pores), average pore size, specific surface area and pore size distribution (PSD) in thin porous films deposited on top of any smooth solid substrat e. Because a laser probe is used, small surface area can be analyzed. Therefore, EP can be used on patterned wafers and it is compatible with microelectronic technology. This method is a new version of adsorption (BET) porosimetry. In situ ellipsometry is used to determine the amount of adsorptive which adsorbed/condensed in the film. Change in refractive index is used to calculate of the quantity of adsorptive present in the film. EP also allows the study of thermal stability, adsorption and swelling properties of low-K dielectric films. Room temperature EP based on the adsorption of vapor of some suitable organic solvents and method of calculation of porosity and PSD is discussed. Examination of the validity of Gurvitsch rule for various organic adsorptives (toluene, heptane, carbon tetrachloride and isopropyl alcohol) has been carried out to assess the reliability of measurements of pore size distribution by the ellipsometric porosimetry.

Characterization of a molecular sieve coating using ellipsometric porosimetry

Ellipsometric porosimetry was used to determine the adsorption isotherms of toluene, methanol, and water on b-oriented Silicalite-1 coatings with a thickness of less than ca. 250 nm and to obtain adsorption kinetics. The adsorption isotherms are of sufficient quality to reveal several aspects of the pore structure such as the adsorbate capacity and the adsorbate/framework affinity. The use of a combination of different molecular probes in ellipsometric porosimetry to elucidate the molecular accessibility of Silicalite-1 pores is demonstrated. It is shown that ellipsometric porosimetry is an appropriate technique for probing the influence of aging of the Silicalite-1 coating and of planarization polishing on the porosity, pore accessibility, and adsorbate/framework affinity.