Guiduk Yu

Effects of polymeric additives on the crystallization and release behavior of amorphous ibuprofen

Some polymeric additives were studied to understand their effects on the amorphous phase of ibuprofen (IBU), used as a poorly water soluble pharmaceutical model compound. The amorphous IBU in bulk, as well as in nanopores (diameter ∼24 nm) of anodic aluminum oxide, was examined with the addition of poly(acrylic acid), poly(N-vinyl pyrrolidone), or poly(4-vinylphenol). Results of bulk crystallization showed that they were effective in limiting the crystal growth, while the nucleation of the crystalline phase in contact with water was nearly instantaneous in all cases. Poly(N-vinyl pyrrolidone), the most effective additive, was in specific interaction with IBU, as revealed by IR spectroscopy. The addition of the polymers was combined with the nanoscopic confinement to further stabilize the amorphous phase. Still, the IBU with addition of polymeric additives showed sustained release behavior. The current study suggested that the inhibition of the crystal nucleation was probably the most important factor to stabilize the amorphous phase and fully harness its high solubility.

Freezing and Melting in Nanopores

In two-dimensionally confined spaces such as nanopores or nanotubes, freezing and melting occurs differently from the bulk phase transition. While bulk materials are crystallized mainly via crystal growth, crystallization in nanopores is dominated by nucleation, and the growth of the crystal is restricted due to the imposed spatial constraint. Different crystallization mechanisms result in different crystal structures and physical properties. Under nanoscopic cylindrical confinement, the crystals are oriented to a certain favorable direction with nucleation being dominant, and the crystal orientation can shift upon the variation of dominant crystallization mechanism. The melting temperatures (T m) are also influenced by the reduced dimension of crystal and interfacial interaction between crystallizable components and their environment in nanopores.

Phase transition of block copolymer/homopolymer binary blends under 2D confinement

Constraints imposed by nanometer scale confinement lead to the changes in bulk equilibrium behavior of block copolymers (BCPs). Cylindrical pores with diameter corresponding to the equivalent length of several copolymer chains have been employed to investigate the influence of two-dimensional (2D) confinement on the behavior of BCPs. Herein, we reported the microdomain transition behavior of block copolymer/homopolymer (AB/A) binary blends in cylindrical confinement. Lamellae forming poly(styrene-b-butadiene) (PS-b-PBD) and PS homopolymers (hPS) were drawn into the pores of anodized aluminum oxide (AAO) and isolated by selective etching of the AAO templates. Concentric ring morphologies of neat PS-b-PBD in the cylindrical confinement varied with on the addition of hPS. Given the volume fraction of homopolymers, the phase behavior of polymer blends was dependent on the molecular weight of homopolymers. When the ratio between the molecular weight of hPS and PS block was much lower than unity (α«1) (i.e., wet brush regime), the concentric ring structure was transformed into helical or spherical structure depending on the volume fraction of hPS. For α≈1 (i.e., dry brush regime), additional hPS chains were localized at the center of concentric ring where the entropic penalty of copolymer chains induced by confinement is maximized. Over the capability of hPS in the BCP domains in radial axis, the phase transition into microemulsions occurred in the dry brush regime. In both wet and dry brush regimes, the effect of hPS on the phase transition of BCP was significantly enhanced in the nanoscale 2D confinement compared to the bulk state due to the loss of conformational entropy of polymer chains.

Polymers in Nanotubes

Differently from small molecules like gas or liquid molecules, polymers show unique properties that are dependent on the size (degree of polymerization) of the molecules. As geometric constraints on the nanoscale are imposed upon polymers, their physical behavior also tends to deviate from that in bulk. Especially in cylindrical nanopores, the imposed curvature and two-dimensional nanoconfinement are mostly considered to cause the deviation in the physical behavior. Since the size of a polymer chain generally ranges from few nanometers to few tens of nanometers in bulk, the nanoscopic geometric constraint influences the static as well as dynamic behavior in the molecular scale of polymers such as mobility, crystallization mechanism, phase behavior. In this chapter, it is generally discussed how cylindrical nanoconfinement affects the physical behavior of polymers.

From circular to triangular alumina nanopore arrays via simple replication

We found inverse-hexagonal packing pattern from self-assembled anodic aluminum oxide and exploited the pattern to obtain triangular pore array. By replicating the curved interface between aluminum and porous alumina, we fabricated a pattern with the opposite packing structure as well as the inversed pattern curvature. Anodization from the replicated structure formed triangular pores in inverse-hexagonal packing, whereas that from the original pattern produces circular pores in hexagonal packing. Our finding highlights the importance of the curvature as well as packing structure of pre-patterns in pore formation and achievement in the control via a simple replication process.