Self-assembly of block copolymers during hollow fiber spinning: an in situ small-angle X-ray scattering study

We investigated the self-assembly of block copolymers during hollow fiber membrane (HFM) fabrication by conducting in situ small angle X-ray scattering (SAXS) and ex situ scanning electron microscopy (SEM) studies. SAXS enables us to follow the structural rearrangements after extrusion at different distances from the spinning nozzle. The kinetics of the spinning process is examined as a function of the composition of block copolymer solutions and the spinning parameters. We studied the influence of the extrusion rate on the block copolymer microdomains and their self-assembly in weakly segregated and ordered solutions. The addition of magnesium acetate (MgAc2) leads to the ordering of micelles in the block copolymer solution already at lower polymer concentrations and shows an increased number of micelles with larger domain spacing as compared to the pristine solution. The SAXS data show the effect of shear within the spinneret on the self-assembly of block copolymers and the kinetics of phase separation after extrusion. It is observed that the ordering of micelles in solutions is decreased as indicated by the loss of crystallinity while high extrusion rates orient the structures perpendicular to the fiber direction. The structural features obtained from in situ SAXS experiments are correlated to the structure in the block copolymer solutions in the absence of shear and the morphologies in flat sheet and HF membranes obtained by ex situ SEM. This allows a systematic and comparative study of the effects varying the microdomain ordering within different block copolymer solutions and the formed membrane structures.

Setting the Stage for Fabrication of Self-Assembled Structures in Compact Geometries: Inside-Out Isoporous Hollow Fiber Membranes

Fabrication of evaporation-induced self-assembled structures on easily accessible surfaces is an established strategy, while achieving such microphase-separated structures in compact geometries has been a long-standing goal. The requirement of comparatively less concentrated block copolymer (BCP) solution to pass through the compact geometries significantly reduces the stimulations required for self-assembly. The high polymer relaxation rates and decreased thermodynamic driving forces, as well as high capillary suction of dilute solutions in the porous substrates, complicates the BCP self-assembly and fabrication of the uniform coated layer, respectively. In this study, highly permeable robust poly(ether sulfone) hollow fiber membranes (PES HFM) with an inner diameter of approximately 1 mm are selected as compact geometries, and the isoporous structures are developed on top of ≤10 μm thin coated layer. This fabrication process introduces a technologically favored inside-out configuration for isoporous composite HFM with large bore diameters.

Influences of Crystal Anisotropy in Pharmaceutical Process Development

Crystalline materials are of crucial importance to the pharmaceutical industry, as a large number of APIs are formulated in crystalline form, occasionally in the presence of crystalline excipients. Owing to their multifaceted character, crystals were found to have strongly anisotropic properties. In fact, anisotropic properties were found to be quite important for a number of processes including milling, granulation and tableting. An understanding of crystal anisotropy and an ability to control and predict crystal anisotropy are mostly subjects of interest for researchers. A number of studies dealing with the aforementioned phenomena are grounded on over-simplistic assumptions, neglecting key attributes of crystalline materials, most importantly the anisotropic nature of a number of their properties. Moreover, concepts such as the influence of interfacial phenomena in the behaviour of crystalline materials during their growth and in vivo, are still poorly understood. The review aims to address concepts from a molecular perspective, focusing on crystal growth and dissolution. It begins with a brief outline of fundamental concepts of intermolecular and interfacial phenomena. The second part discusses their relevance to the field of pharmaceutical crystal growth and dissolution. Particular emphasis is given to works dealing with mechanistic understandings of the influence of solvents and additives on crystal habit. Furthermore, comments and perspectives, highlighting future directions for the implementation of fundamental concepts of interfacial phenomena in the rational understanding of crystal growth and dissolution processes, have been provided.