1
Reinforcing Poly(ε-caprolactone) Nanofibers with Cellulose
Nanocrystals
Justin O. Zoppe
1
, Maria S. Peresin
1
, Youssef Habibi
1
, Richard A. Venditti
1
and Orlando J.
Rojas
1,2,
*
1
Department of Forest Biomaterials, North Carolina State University, Raleigh, North Carolina,
27695-8005
2
Helsinki University of Technology, Laboratory of Forest Products Chemistry, P.O. Box 6300,
Helsinki, Finland, FI-02015 HUT
*Corresponding author, email address: ojrojas@ncsu.edu
Table of Content
2
Abstract
We studied the use of cellulose nanocrystals obtained after acid hydrolysis of ramie
cellulose fibers to reinforce poly(ε-caprolactone) nanofibers. Chemical grafting with low
molecular weight polycaprolactone diol onto the cellulose nanocrystals was carried out in an
attempt to improve the interfacial adhesion with the fiber matrix. Grafting was confirmed via
infrared spectroscopy and thermogravimetric analyses. The polymer matrix consisted of
electrospun nanofibers that were collected as nonwoven webs. The morphology as well as
thermal and mechanical properties of filled and unfilled nanofibers were elucidated by scanning
electron microscopy, differential scanning calorimetry, and dynamic mechanical analysis,
respectively. Addition of CNXs into PCL produced minimal changes in the thermal behavior of
the electrospun fibers. However, a significant improvement in the mechanical properties of the
nanofibers after reinforcement with unmodified cellulose nanocrystals was confirmed. Fiber
webs from PCL reinforced with 2.5% unmodified CNXs showed ca. 1.5-fold increase in
Young’s modulus and ultimate strength compared to PCL webs
Compared to the case of grafted nanocrystals, the unmodified ones imparted better
morphological homogeneity to the nanofibrillar structure. The grafted nanocrystals had a
negative effect on the morphology of nonwoven webs in which individual nanofibers became
annealed during the electrospinning process, and therefore could not be compared neat poly(ε-
caprolactone) nonwovens. A rationalization for the different effects of grafted and unmodified
cellulose nanocrystals in reinforcing poly(ε-caprolactone) nanofibers is provided.
3
Keywords: Cellulose Nanocrystals, Nanocrystalline Cellulose, cellulose Whiskers, Surface
Grafting, Electrospinning, Fiber reinforcement, Nanofibers, Nanocomposites, Nanoparticles,
Poly(ε-caprolactone).
4
Introduction
Manufacture and use of nanofiber-based scaffolds have recently attracted interest in
biomedical applications, especially for tissue engineering (1-6). One of the main goals in this
area is to create biocompatible and biodegradable scaffolds with mechanical and biological
properties similar to those of extracellular matrices (ECM) so as to facilitate surgical implant and
promote tissue regeneration (7). Natural and synthetic polymers have been examined for this
purpose, including collagens, chitosan, hyaluronic acid and biodegradable polyesters such as
polylactic acid, polyglycolic acid, and polycaprolactone, among others. Synthetic, biodegradable
poly(ε-caprolactone), or PCL, has been shown to be particularly useful when used in the
production of electrospun fibers to mimic ECM (8-10). However, there are a few challenges that
need to be overcome in the application of PCL, including effects brought about by its
hydrophobicity, which can potentially prevent living cell adhesion, mobility and also limit
mechanical strength (required to ensure structural integrity) (11). In order to reduce its surface
energy, PCL is usually blended with hydrophilic polymers, thereby facilitating cell adhesion (5).
On the other hand, improvement of the mechanical and thermal properties of PCL-based
scaffolds can be accomplished by reinforcing them with suitable fillers (12).
Recently, efforts to increase mechanical properties of PCL nanofiber webs have been
reported by addition of carbon nanotubes (CNTs) (13,14). Although introduction of CNTs
yielded composite fibers with increased mechanical properties, manufacture protocols required
demanding processing conditions (15); therefore, it is anticipated that such types of composites
can be quite costly. In addition, CNTs posses some degree of toxicity (in vivo and in vitro),
predominately due to the presence of transition metal catalysts and have been shown to cause
some cytotoxicity at elevated concentrations (16).
5
Use of biological materials such as proteins from egg shells have been also reported as a
reinforcing agent for PCL-based nanofibers (17). Though the mechanical and interfacial
properties of the composites were improved, the incorporation of such soluble polymers in the
PCL matrices required special manufacturing protocols.
Incorporation of cellulose nanocrystals (CNXs) into polymer nanofibers would be
advantageous over CNTs because the precursor cellulose in CNXs is obtained from an abundant
bioresource. Also, CNXs are less expensive, simple to produce, and, most importantly, are
biocompatible and biodegradable. Excellent chemical and thermomechanical properties of CNXs
have been reported (18,19), making them suitable candidates as a reinforcing, disperse phase in
polymer matrices (20-23). CNXs can be produced from a variety of cellulose sources including
wood, cotton, sisal, ramie, tunicate, fungi or bacteria (24). Bacterial CNXs have been
successfully incorporated into hydrophilic poly(ethylene oxide) nanofibers, and an increase in
modulus, strength, and strain at break were observed (17). Recently, we have successfully used
CNXs as a reinforcing phase in composites made from synthetic hydrophobic matrices (25).
In this work we propose a novel combination of biodegradable CNXs from natural fibers
as reinforcing material in PCL nanofibers via electrospinning. The resulting composites were
developed to overcome the otherwise low mechanical strength of neat PCL nonwovens. The
grafting of low molecular weight polycaprolactone diol chains onto the surfaces of cellulose
nanocrystals and their incorporation in electrospun PCL matrices were also investigated. The
effect of such functionalization was addressed in an effort to improve their compatibility with the
continuous phase. Finally, the main structural and thermomechanical features of the developed
composite nanofibers were addressed for various CNXs loadings.
