Ganesh Narayanan

Fabrication and Characterization of Poly(ε-caprolactone)/α-Cyclodextrin Pseudorotaxane Nanofibers.

Multifunctional scaffolds comprising neat poly(ε-caprolactone) (PCL) and α-cyclodextrin pseudorotaxanated in α-cyclodextrin form have been fabricated using a conventional electrospinning process. Thorough in-depth characterizations were performed on the pseudorotaxane nanofibers prepared from chloroform (CFM) and CFM/dimethylformamide (DMF) utilizing scanning electron microscopy (SEM), transmission electron microscopy (TEM), rheology, differential scanning calorimetry (DSC), thermogravimetric analyses (TGA), wide-angle X-ray diffraction (WAXD), and Instron tensile testing. The results indicate the nanofibers obtained from chloroform retain the rotaxanated structure; while those obtained from CFM/DMF had significantly dethreaded during electrospinning. As a consequence, the nanowebs obtained from CFM showed higher moduli and lower elongations at break compared to neat PCL nanowebs and PCL/α-CD nanowebs electrospun from CFM/DMF.

Poly(ε-caprolactone) Nanowebs Functionalized with α- and γ-Cyclodextrins

The effects of alpha- and gamma-cyclodextrins (α- and γ-CDs) on the thermal and crystal nucleation behavior of electrospun poly(ε-caprolactone) (PCL) nanofibers have been investigated. PCL/CD composite nanofibers were obtained for the first time by electrospinning the mixture from chloroform/N,N-dimethylformamide (60:40). Scanning electron microscopy analyses indicated that neat PCL nanofibers have an average diameter of 400 nm, which increases with the addition of CDs. The presence of CDs on or in the electrospun PCL fibers in the electrospun mats was investigated using Fourier transform infrared spectroscopy, thermogravimetric analysis, and wide-angle X-ray diffraction analysis. Differential scanning calorimetry showed that the PCL/CD composite fibers exhibit higher crystallization temperatures and sharper crystallization exotherms with increased CD loading, indicating the ability of CDs to nucleate PCL crystallization. Water contact angle (WCA) measurements indicate an inverse relationship between WCA and α- or γ-CD concentration up to 30% loading. Phenolphthalein absorption tests were performed to study the kinetics of their inclusion complex (IC) formation with CDs. Unexpectedly, γ-CD-functionalized nanowebs performed better than α-CD. This might be because at elevated loadings some α-CDs may have threaded over PCL chains and formed ICs, whereas γ-CD did not. With their encapsulation capabilities and their lowered hydrophobicity, PCL/CD composite fibers might have potential uses in medical applications, in particular as wound odor absorbants in dressings, because it is well known that CDs can form ICs with these odorants, thereby effectively removing them.

Estimation of the poly (ε-caprolactone) [PCL] and α-cyclodextrin [α-CD] stoichiometric ratios in their inclusion complexes [ICs], and evaluation of porosity and fiber alignment in PCL nanofibers containing these ICs.

This paper describes the utilization of Proton-Nuclear Magnetic Resonance spectroscopy (1H NMR) to quantify the stoichiometric ratios between poly (ε-caprolactone) [PCL] and α-cyclodextrin (α-CD) present in their non-stoichiometric inclusion complexes [(n-s)-ICs]. This paper further describes the porosity and fiber alignment of PCL nanofibers nucleated by the [(n-s)-ICs] during electrospinning. 1H NMR indicated that the two non-stoichiometric inclusion complexes utilized in this study had differing stoichiometric ratios that were closely similar to those of the starting ratios used to make them. Studies on porosity and fiber alignments were conducted on the scanning electron microscope images using ImageJ. The data indicates that both fiber alignment as well as porosity values remain almost the same over all the samples. Thus we can conclude the improvement in mechanical properties was due only to the loading of the ICs, and their subsequent interaction with bulk unthreaded PCL.

Correlation of the stoichiometries of poly(ε-caprolactone) and α-cyclodextrin pseudorotaxanes with their solution rheology and the molecular orientation, crystallite size, and thermomechanical properties of their nanofibers

Pseudorotaxane nanofibers based on biomedical polymers, such as poly(e-caprolactone) (PCL), and α-cyclodextrins (α-CD) open new horizons for a variety of biomedical applications. From our recent report on pseudorotaxane suspensions and nanofibers, we inferred several unique characteristics, such as improvements in mechanical properties and a several-fold increase in viscosity values at low concentrations. In this report, through in-depth characterization employing techniques, such as rheological measurements, we investigate the reasons behind the unusual viscosity values observed in their suspensions. Additionally, combining rheology and TEM, we examine the phenomena responsible for the fiber diameter variation within a single nanofiber. Although, both 2D-wide angle X-ray diffraction patterns and selected area electron microscopy showed poor molecular orientation of the polymer chains along the fiber axis in the pseudorotaxanes, we attribute the observed higher modulus values to the denser nature and crystal packing of the PCL chains emanating from the surfaces of the columnar host α-CD crystals in the pseudorotaxanes, which was evidenced by crystallite size analyses. Finally, dynamic mechanical analyses illustrated that the interlacing of polymer chains protruding from the columnar α-CD cavities have a profound impact on the mechanical properties of these composites.

Reorganizing Polymer Chains with Cyclodextrins

During the past several years, we have been utilizing cyclodextrins (CDs) to nanostructure polymers into bulk samples whose chain organizations, properties, and behaviors are quite distinct from neat bulk samples obtained from their solutions and melts. We first form non-covalently bonded inclusion complexes (ICs) between CD hosts and guest polymers, where the guest chains are highly extended and separately occupy the narrow channels (~0.5–1.0 nm in diameter) formed by the columnar arrangement of CDs in the IC crystals. Careful removal of the host crystalline CD lattice from the polymer-CD-IC crystals leads to coalescence of the guest polymer chains into bulk samples, which we have repeatedly observed to behave distinctly from those produced from their solutions or melts. While amorphous polymers coalesced from their CD-ICs evidence significantly higher glass-transition temperatures, Tgs, polymers that crystallize generally show higher melting and crystallization temperatures (Tms, Tcs), and some-times different crystalline polymorphs, when they are coalesced from their CD-ICs. Formation of CD-ICs containing two or more guest homopolymers or with block copolymers can result in coalesced samples which exhibit intimate mixing between their common homopolymer chains or between the blocks of the copolymer. On a more practically relevant level, the distinct organizations and behaviors observed for polymer samples coalesced from their CD-ICs are found to be stable to extended annealing at temperatures above their Tgs and Tms. We believe this is a consequence of the structural organization of the crystalline polymer-CD-ICs, where the guest polymer chains included in host-IC crystals are separated and confined to occupy the narrow channels formed by the host CDs during IC crystallization. Substantial degrees of the extended and un-entangled natures of the IC-included chains are apparently retained upon coalescence, and are resistant to high temperature annealing. Following the careful removal of the host CD lattice from each randomly oriented IC crystal, the guest polymer chains now occupying a much-reduced volume may be somewhat “nematically” oriented, resulting in a collection of randomly oriented “nematic” regions of largely extended and un-entangled coalesced guest chains. The suggested randomly oriented nematic domain organization of guest polymers might explain why even at high temperatures their transformation to randomly-coiling, interpenetrated, and entangled melts might be difficult. In addition, the behaviors and uses of polymers coalesced from their CD-ICs are briefly described and summarized here, and we attempted to draw conclusions from and relationships between their behaviors and the unique chain organizations and conformations achieved upon coalescence.

Aliphatic Polyester Nanofibers Functionalized with Cyclodextrins and Cyclodextrin-Guest Inclusion Complexes

The fabrication of nanofibers by electrospinning has gained popularity in the past two decades; however, only in this decade, have polymeric nanofibers been functionalized using cyclodextrins (CDs) or their inclusion complexes (ICs). By combining electrospinning of polymers with free CDs, nanofibers can be fabricated that are capable of capturing small molecules, such as wound odors or environmental toxins in water and air. Likewise, combining polymers with cyclodextrin-inclusion complexes (CD-ICs), has shown promise in enhancing or controlling the delivery of small molecule guests, by minor tweaking in the technique utilized in fabricating these nanofibers, for example, by forming core–shell or multilayered structures and conventional electrospinning, for controlled and rapid delivery, respectively. In addition to small molecule delivery, the thermomechanical properties of the polymers can be significantly improved, as our group has shown recently, by adding non-stoichiometric inclusion complexes to the polymeric nanofibers. We recently reported and thoroughly characterized the fabrication of polypseudorotaxane (PpR) nanofibers without a polymeric carrier. These PpR nanofibers show unusual rheological and thermomechanical properties, even when the coverage of those polymer chains is relatively sparse (~3%). A key advantage of these PpR nanofibers is the presence of relatively stable hydroxyl groups on the outer surface of the nanofibers, which can subsequently be taken advantage of for bioconjugation, making them suitable for biomedical applications. Although the number of studies in this area is limited, initial results suggest significant potential for bone tissue engineering, and with additional bioconjugation in other areas of tissue engineering. In addition, the behaviors and uses of aliphatic polyester nanofibers functionalized with CDs and CD-ICs are briefly described and summarized. Based on these observations, we attempt to draw conclusions for each of these combinations, and the relationships that exist between their presence and the functional behaviors of their nanofibers.

Novel cellulose-collagen blend biofibers prepared from an amine/salt solvent system

Cellulose/collagen biofibers were produced from ethylene diamine/potassium thiocyanate binary solvent system, with methanol as a coagulant. The dynamic viscosity of the solutions decreased with the gradual increase in the collagen content up to 40%. The elemental analysis showed incorporation of collagen into cellulose matrix, thereby demonstrating some degree of interaction with the cellulose matrix. The chemical and thermal analysis further revealed an intermolecular interaction between cellulose and the protein and improved thermal stability, respectively. Furthermore, the electron microscopy images mostly exhibited fibrillar morphology with no visible phase separation, indicating compatibility between the two phases. Moreover, biofibers containing higher cellulose content showed higher crystallinity, tensile, and birefringence properties of the composite fibers.

Host–Guest Polymer Complexes

Alan E. Tonelli 1,*, Ganesh Narayanan 1 ID and Alper Gurarslan 2 1 Fiber & Polymer Science Program College of Textiles, North Carolina State University, Campus Box 8301, 2401 Research Drive, Raleigh, NC 27695-8301, USA; gnaraya@ncsu.edu 2 Faculty of Textile Technologies and Design, Istanbul Technical University, Inonu Cad. No 65 Gumussuyu, Beyoglu, Istanbul 34437, Turkey; gurarslan@itu.edu.tr * Correspondence: atonelli@ncsu.edu; Tel.: +1-919-515-6588

Functional Nanofibers Containing Cyclodextrins

The process of obtaining nanofibers from polymer solutions has been reported in the literature for the past two decades. However, only in the past few years, have nanofibers containing cyclodextrins (CDs) or their inclusion compounds (ICs) with low or high molecular weight compounds been extensively reported. These nanofibers exhibit superior properties compared to those of their neat polymer nanofibers. It has been observed that with the simple addition of CDs, marked increases in crystallinity, crystallizability, small molecule encapsulation capability, lowered hydrophobicity, and other surface functionalities can be achieved. In this chapter, an in-depth discussion of cyclodextrins, their structures, and inclusion complexes will be provided. Various strategies utilized to obtain those nanofibers functionalized with CDs or their ICs will be discussed. CD based technologies offer green alternatives for designing scaffolds with specific improved properties for growing cells and tissues. For example, increased small molecule encapsulation or release capability can be achieved, as well stronger nanofibers can be produced. Due to their excellent biocompatibility, biodegradability, and abundant availability, CDs and their ICs, offer excellent opportunities for producing functionalized nanofibers, which have not yet been extensively reported. For this reason, much of our focus in this chapter will concentrate on various strategies for future research in nanofibers functionalized with CDs and their ICs.

Formation of Cellulose and Protein Blend Biofibers

Cellulose and proteins are potential polymers for developing biodegradable materials for high value-added applications. A combination with these natural polymers could be useful to enhance the properties of final materials and to extend their application areas. In particular, blend biofibers that are degradable and sustainable can be engineered from a mixture of cellulose and proteins, such as soy protein, silk fibroin, collagen, etc. In a binary polymeric blend, the compatibility of cellulose and proteins is influenced by the characteristics of each polymer in the employed solvent system as well as processing conditions. Therefore, utilizing solvents that can dissolve cellulose and proteins, and coagulants that are non-solvents for both polymers is of importance. In this book chapter, the formation and characteristics of blend biofibers from these polymers will be discussed.