Elsevier

Polymer

Volume 49, Issue 21, 6 October 2008, Pages 4713-4722
Polymer

Effect of fiber diameter on tensile properties of electrospun poly(ɛ-caprolactone)

https://doi.org/10.1016/j.polymer.2008.08.022Get rights and content

Abstract

The tensile properties of electrospun fibers have not been widely investigated due to the difficulties in handling nanofibers and measuring low load for deformation. In this study, the effect of dimensional confinement on free standing biodegradable poly(ɛ-caprolactone) (PCL) is investigated using electrospinning-enabled techniques and a nanoforce tensile tester. The structural properties such as crystallinity and molecular orientation of the spun fibers are examined using wide angle X-ray diffraction (WAXD). The degree of crystallinity and molecular orientation of fibers are enhanced when the diameter of spun fibers is reduced, resulting in improved mechanical strength and stiffness. It is evident that PCL fibers with decreasing fiber diameter exhibit an abrupt shift in tensile performance in comparison to those derived from non-spun systems. The abrupt shift in tensile strength and stiffness of electrospun PCL fibers occurs at around 700 nm in diameter and illustrates the importance of studying the mechanical behavior of the nanofibers, for the first time, systematically with the aid from electrospinning techniques. This shift cannot be otherwise explained by a noticeable change in Tg, and the gradual increase in crystallinity and molecular orientation.

Introduction

Recent advances in nanostructured biomaterials have led us to revisit the bio-inspired phenomena that are essential to understand the superior performance of heterogeneous materials that occur at the nanometer length scale. Nanostructured biomaterials possess superior mechanical strength despite the comparatively poor performance of their constituents when tested alone [1], [2], [3]. Natural biomaterials are self-assembled in an orderly pattern of organic matrix and mineralized organics in the sub-micrometer and nanometer length scales providing superior strength and fracture resistance [4]. Such material systems include the widely studied nacre, enamel and bone. The structure of polymeric constituents is geometrically confined in the nanometer and micrometer length scales and helps these natural materials to attain the theoretical values for intramolecular and intermolecular boundary separations [1], [2], [3], [4], [5], [6], [7], [8], [9] and, hence, optimizes the mechanical properties of the resulting composites. Despite the widely held belief that the confinement effects of nacre and enamel are derived from the reinforcing minerals or mineral platelets on polymers, there is very little understanding pointing to the possibility that the polymers are spatially confined by the characteristic length such that the fiber diameter approaches that of the radius of gyration of the molecular chains. This lack of understanding is further exacerbated by the fact that simple mechanical properties such as stiffness and strength of plastics are seldom measured with accuracy and reliability when they are made into structures the size of the polymer molecules themselves, which are just tens of nanometers in diameter. Our studies [10], [11] indicate that the macromolecules need to form uniquely different geometric orientations in order for them to appear in dimensions from 10 nm (near the radius of gyration) to 1000 nm like the bulk specimens. This concept is analogical to the well-established plane stress to plane strain transition in measuring the specific essential work of fracture in polymer films [12], [13]. As the specimen ligament approaches zero in length, the material is severely constrained and cannot exhibit profuse plastic deformation, resulting in plane strain, instead of plane stress, fracture toughness assessment. As the polymer molecules are spatially limited in nano-confined state, the nanoscale fibers ought to re-arrange themselves to accommodate dimensional constraints for enhanced mechanical behavior.

In this study, confinement refers only to limiting spatial arrangement of PCL molecules in cylindrical shapes with diameter ranging from two hundred to several hundred nanometers, such as in electrospun nanofibers. Such spatial confinement produces flaw insensitive [6] nanofibers that are compression molded into bulk geometry as compared to a compression molded specimen from pellets. The confinement resulting from the processing condition can affect the material's structure, macromolecular conformation and mechanical characteristics resulting in superior properties [14]. Recently, Arinstein et al. [7] reported an abrupt increase in Young's modulus when the supramolecular structural dimension is comparable to the nanofiber diameter. Their findings challenge the commonly held view that surface/boundary effects are most significant for deformation in nanofibers. Modeling the strength and toughness of nacre, Katti and Katti [8] demonstrated that nanoscale asperities play only a marginal role in the strengthening and toughening of the natural composite. Their results reinforce the notion that confined proteinaceous molecules in between the aragonite platelets play a more substantial role in the synergistic toughness and strength of nacre, which by nature is a polymer–mineral composite. Hence, the mechanical response of a polymeric system within a certain degree of spatial confinement needs to be investigated in detail in order for materials designers to mimic the nanostructure of biomaterials for superior mechanical properties.

In this paper, we utilize the versatile electrospinning technique to produce polymer nanofibers such that we can, for the first time, systematically evaluate the phenomena arising from spatially limited polymers. We test the hypothesis that the mechanical properties of electrospun polymer nanofibers are influenced by the fiber diameter, molecular geometry, molecular orientation and degree of crystallinity. In this study we employ X-ray diffraction techniques to evaluate the effect of electrospinning process on the structures of spun polymer fibers. The change in crystallinity and molecular orientation as functions of fiber diameters and tensile strains is reported. This study demonstrates that the nanometer-scale (250–1000 nm) fibers produced by electrospinning can effectively result in strikingly superior tensile strength and modulus with the same specimen dimensions of the bulk and loading rates.

Section snippets

Materials

The biodegradable polymer, polycaprolactone (PCL) (Mw = 80,000 g/mol) is obtained from Dow Chemical Company (Freeport, TX) and 2% Mg doped hydroxyapatite nanopowder (HAP) of particle size 50–100 nm is purchased from nGimat™ (Atlanta, GA). PCL pellets are dried under vacuum at 40 °C for 24 h and are used to produce spun and non-spun specimens.

Bulk (non-spun) specimen processing

For comparative studies, non-spun samples of PCL are prepared by one-step compression molding. The PCL pellets are compression molded at 130 °C and 275 bar in

Effect of electrospinning on mechanical properties of spun and non-spun samples

The tensile properties of the tested specimens (Samples 1 and 2) are summarized in Table 1. An average of five specimens was tested for each sample type. The representative stress–strain curves of Samples 1 and 2 are shown in Fig. 4. The modulus and tensile strength of Sample 2 are superior to Sample 1 prepared with conventional one-step compression molding of pellets. Their stress–strain curves are drastically different. Sample 2 exhibits an average tensile strength (>58 MPa) more than

Conclusions

The abrupt shift in tensile strength and stiffness of electrospun PCL fibers at around 700 nm reported in this paper illustrates the importance of studying the multiscale transition systematically by applying spatial confinement using the electrospinning techniques. Tensile strength, stiffness and draw ratio of electrospun PCL were increased, in an abrupt fashion, by decreasing the fiber diameter. From the WAXD study and draw ratio results, it is determined that reducing the fiber diameter

Acknowledgements

This work is supported by the National Science Foundation under the CAREER Award CMMI #0746703 and Award DMI # 0520967. The authors wish to thank Dr. T.A. Blackledge and Dr. D.H. Reneker for suggestions and useful discussions during the preparation of this paper. One of us (SCW) acknowledges the support from Dr. Andrew McGill during the tenure as a senior faculty fellow at the US. Naval Research Laboratory. The authors would like to thank the reviewers for constructive comments on improving

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