Bhanu Nandan

Mediating polymer crystal orientation using nanotemplates from block copolymer microdomains and anodic aluminium oxide nanochannels

Crystals formed by polymers are typically nanoscale in at least one dimension. The directional control of properties via precise control of the orientations of polymer nanocrystals has strong relevance to technical applications in various areas. Polymer crystals may exhibit preferential orientation when they are formed inside nanoscale domains or pores. Such a confinement-mediated orientation behavior has been a subject of extensive investigation over the past two decades, where a block copolymer in which two or more chemically different sub-chains form a single molecule template system that has received the most attention. In this article, we focus on an overview of the orientation behavior of polymer crystals under the influence of one-dimensional (1-D) and two-dimensional (2-D) confinement templated by the lamellar and cylindrical microdomains of block copolymers, respectively. In the case of lamellae-forming diblock copolymers, both the crystalline–amorphous system (which is composed of one type of nanocrystals) and the more complex double-crystalline diblock (which consists of two types of nanocrystals) are considered. In addition to the templates offered by block copolymers, the preferential orientation of polymer crystals confined in the inorganic anodic aluminium oxide (AAO) nanochannels has also been critically reviewed due to strong relevance to the 2-D confinement effect. Moreover, the significant thermodynamic and kinetic factors governing the crystal orientation behavior have been summarized, which may allow one to understand the strategy for tuning the preferential orientation of polymer nanocrystals under the spatial confinement of different dimensionalities.

Helical Packing of Nanoparticles Confined in Cylindrical Domains of a Self‐Assembled Block Copolymer Structure

Theoretical models predict that a variety of self-assembled structures of closely packed spherical particles may result when they are confined in a cylindrical domain. In the present work we demonstrate for the first time that the polymer-coated nanoparticles confined in the self-assembled cylindrical domains of a block copolymer pack in helical morphology, where we can isolate individual fibers filled with helically arranged nanoparticles. This finding provides unique possibilities for fundamental as well as application-oriented research in similar directions.

Hairy polymer nanofibers via self-assembly of block copolymers

We demonstrate a simple approach to prepare hairy polymer nanorods/nanofibres by exploiting the self-assembly behavior of block copolymers. The hairy nanofibres were prepared from an asymmetric polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer whose self-assembled morphology consisted of PS cylinders dispersed in P4VP matrix. The dissolution of the block copolymer in a P4VP selective solvent resulted in the isolation of individual PS cylinders wrapped with a shell consisting of covalently bound P4VP chains. The diameter of these hairy nanofibres was ∼80 nm, whereas the length was in the range of several hundred nanometers. We further show that such hairy nanofibres provide interesting possibilities for doing further chemistry on their surface. This, in principle, allows one to prepare a range of functional hybrid organic/inorganic nanofibres of diameter less than 100 nm using the hairy polymer nanofibres as template. The organic fraction of the nanofibres can then be further removed by thermal treatment to produce pure inorganic nano-objects.

Control on molecular weight reduction of poly(ε-caprolactone) during melt spinning--a way to produce high strength biodegradable fibers.

Poly(ε-caprolactone) (PCL) is known for its biocompatibility and biodegradability. These features of PCL have resulted into significant academic as well as industrial research interests for use of this polymer in various areas including biomedical and tissue engineering. Three-dimensional porous scaffolds, controlled drug release systems and nerve guides are some of the forms in which this polymer has been used. Despite these forms, fibers made of PCL have not gained much attention due to PCLs low melting point (57-60 °C) and relatively inferior mechanical properties as compared to poly(L-lactide) (PLA). Also the polymer is sensitive to the process conditions of melt spinning which leads to degradation of PCL when subjected to high temperatures in the presence of air or moisture. Here we present an approach in which addition of a bilactone, bis-(ε-caprolactone-4-yl) (BCY), during melt spinning of PCL resulted into monofilament fibers having tenacity as high as 2500 MPa. The cross-linking of PCL which occurred due to BCY transesterification compensated for molecular weight reduction of the polymer under melt spinning conditions. PCL monofilament fibers thus developed have enhanced thermo-mechanical properties and therefore have high potential to be used in tissue engineering applications in the form of sutures, a mesh or a non-woven.

Morphology of electrospun fibers derived from High Internal Phase Emulsions

High Internal Phase Emulsions (HIPEs) are known for their excessive volume of dispersed phase (volume fraction of dispersed phase Φd>0.74) and are primarily used for polymerization of continuous phase monomer(s) thereby generating porous systems in a single step. In the present work, electrospinning of HIPEs formed from aqueous solution of poly(vinyl alcohol) (PVA) dispersed in continuous phase comprised of poly(ε-caprolactone) (PCL) solution in toluene is conducted. Effect of variation in volume fraction of dispersed and continuous phase on fiber morphology was studied. Fibers of co-continuous morphology were obtained due to coalescence and dielectrophoresis of the higher electrically conducting dispersed aqueous phase than toluene containing continuous phase. Removal of PVA was later done by washing of fibers with water to evaluate the presence of two phases in the fibers and relate it to original HIPE morphology of the emulsions. Heterogeneous and surface nucleation of PCL and Brij-58 confined within electrospun fibers of HIPEs was studied in detail and related to the original HIPE structure.

Nanoparticle directed domain orientation in thin films of asymmetric block copolymers

We investigated the thin film morphology of two different asymmetric block copolymers (BCP), polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) and poly(n-pentyl methacrylate)-block-poly(methyl methacrylate) (PPMA-b-PMMA), loaded with pre-synthesized iron oxide nanoparticles (NP). The chemical composition of the BCP constituents determines the strength of the interaction between polymer chains and nanoparticles. In the case of NP/PS-b-P4VP system, the nanoparticles interact preferentially with the P4VP block and hence localize selectively in the P4VP cylindrical microdomains. However, for the NP/PPMA-b-PMMA system, the nanoparticles have no significant preference for the copolymer blocks and segregate at the polymer/substrate interface. Interestingly, this changes the effective substrate surface energy and hence leads to a remarkable change in domain orientation from parallel to perpendicular with respect to the substrate. These results clearly demonstrate the importance of both enthalpic and entropic factors which determine spatial distribution of NP in BCP films and influence domain orientation.

Synthesis of hollow silica nanostructures using functional hairy polymer nanofibers as templates

Hollow silica nanofibers and nanospheres were synthesized from functional hairy polymer nanofibers as sacrificial templates. The polymer nanofibers were isolated from a cylinder-forming polystyrene-block-poly(4-vinylpyridine) block copolymer using a selective-swelling approach.

Electrospun composite matrices of poly(ε-caprolactone)-montmorillonite made using tenside free Pickering emulsions.

The production of composite electrospun matrices of poly(ε-caprolactone) (PCL) using an emulsifier-free emulsion, made with minimal organic solvent, as precursor is reported. Pickering emulsions of PCL were prepared using modified montmorillonite (MMT) clay as the stabilizer. Hydrophobic tallow group of the modified MMT clay resulted in analogous interaction of clay with oil and aqueous phase and its adsorption at the interface to provide stability to the resultant emulsion. Composite fibrous matrices of PCL and MMT were produced using electrospinning under controlled conditions. The fiber fineness was found to alter with PCL concentration and volume fraction of the aqueous and oil phases. A higher tensile strength and modulus was obtained with inclusion of MMT in PCL electrospun matrix in comparison to a matrix made using neat PCL. The presence of clay in the fibrous matrix did not change the cell proliferation efficiency in comparison to neat PCL matrix. Composite fibrous matrices of PCL/MMT bearing enhanced tensile properties may find applications in areas other than tissue engineering for example food packaging and filtration.

Multifunctional core–shell polymer–inorganic hybrid nanofibers prepared via block copolymer self-assembly

We demonstrate a simple and robust approach for preparing multifunctional core–shell hybrid nanofibers via block copolymer self-assembly. The approach utilizes the different chemistry and solubilities of the two blocks of a diblock copolymer and different affinity of functional inorganic nanoparticles towards block copolymer constituents. In the first step, the silver nanoparticles (Ag) modified with short-chain polystyrene (PS) ligand are incorporated in the cylindrical domains of a polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer, constituted of PS blocks. The Ag-loaded cylindrical domains are then isolated as nanofibers by swelling the matrix forming P4VP phase using a selective solvent. The isolated nanofibers exhibit core–shell morphology with the core constituted of Ag-loaded PS phase and shell consisting of P4VP chains. The reactive P4VP shell of the nanofibers is subsequently used as a host for depositing a second type of nanoparticles. The second type of nanoparticles could be either directly synthesized on the P4VP shell or deposited from an aqueous dispersion of pre-synthesized nanoparticles. In this work, gold (Au) and cadmium sulfide (CdS) nanoparticles were deposited on the nanofiber shell. The approach is versatile and, in principle, could be extended to the fabrication of various combinations of targeted functionalities in a single nanofiber with core–shell morphology.

Electrospun composite matrices from tenside-free poly(caprolactone)-grafted acrylic acid/hydroxyapatite oil-in-water emulsions

Composite matrices of poly(ε-caprolactone)-grafted acrylic acid (PCL-g-AA) and hydroxyapatite (HA) were prepared via electrospinning of oil-in-water emulsions. Grafting of varying amounts of AA on PCL was carried out in a twin-screw compounder using benzoyl peroxide as an initiator under inert atmosphere. A solution of PCL-g-AA in toluene, containing HA, comprised the oil phase of the emulsion, while the aqueous phase contained poly(vinyl alcohol) (PVA) as a template polymer. No emulsifier was used in making such emulsions which were found to be stable for more than a month at room temperature. Secondary interactions of AA group of PCL-g-AA with HA and PVA at the oil–water interface provided stability to the emulsion. Uniform composite fibrous matrices were produced from the resultant emulsions under controlled electrospinning conditions. The composite matrices, thus developed using minimal organic solvent, are free from emulsifiers and have high potential to be used in applications including tissue engineering.