Crystalline Order and Mechanical Properties of As-Electrospun and Post-treated Bundles of Uniaxially Aligned Polyacrylonitrile Nanoﬁber Dorna Esraﬁlzadeh, 2 Rouhollah Jalili, 1 Mohammad Morshed 2 1 ITA-Co Engineering, Unit 7, No. 63, Sheikh e Bahaii Sq, Tehran, 19949–13434 2 Textile Engineering Department, University of Technology, Isfahan, Iran 84154 Received 21 February 2007; accepted 11 October 2007 DOI 10.1002/app.27593 Published online 8 September 2008 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: This study investigates the crystalline order and mechanical properties of as-electrospun and posttreated polyacrylonitrile nanoﬁbers. To keep the nanoﬁbers under tension during the posttreatment, a modiﬁed method of pre- paring bundles such as multiﬁlament yarn was used in which the alignment of the nanoﬁbers and linear density of the bun- dles were controlled successfully. An increase in the nanoﬁb- ers’ diameter from 240 to 500 nm led to the E modulus, ulti- mate strength, and elongation at break of the bundles rising from 836 MPa, 45 MPa, and 38% to 1915 MPa, 98 MPa, and 120%, respectively. The crystallinity index (%) and coherence length of the nanoﬁber bundles were evaluated through wide-angle X-ray diffraction. The mechanical properties and crystalline order of the nanoﬁber bundles were both increased as a result of the posttreatment. Wide-angle X-ray diffraction patterns of annealed bundles showed equatorial diffraction from the (10 10) reﬂection at  5.1 A ˚ and from the (11 20) reﬂection at  3A ˚ . The values of the coherence length, crys- tallinity index (%), ultimate strength, and E modulus of the bundles prepared from 240-nm nanoﬁbers increased from negligible, 2%, 1109 MPa, and 48 MPa to 54 A ˚ , 35%, 2235 MPa, and 95 MPa after annealing at 858C in a mixture of water (95 wt %) and N,N-dimethylformamide (5 wt %), respectively. Ó 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110: 3014–3022, 2008 Key words: ﬁbers; nanotechnology; X-ray; electrospinning; PAN INTRODUCTION Many applications in textiles and as carbon ﬁber pre- cursors have been found for polyacrylonitrile (PAN). PAN homopolymer ﬁbers are only rarely used for ﬁber spinning, and virtually all commercial acrylic ﬁbers are spun from acrylonitrile polymers containing 1–15 wt % comonomers. 1 There are strong intrachain and interchain interactions through secondary bonding because of the large magnitude of the dipole moment of the nitrile groups in PAN ﬁbers. Therefore, upon heating, PAN undergoes a degradation reaction before melting at 320–3308C. 2,3 The spinning processes most commonly used for acrylic ﬁbers involve highly polar solvents, such as N,N-dimethylformamide (DMF), dimethylacetamide, and dimethyl sulfoxide. 4 Recently, PAN nanoﬁbers have been produced by electrospin- ning. The electrospinning technique has been recog- nized as an efﬁcient processing method for manufac- turing nanoscale ﬁbrous structures, which are used in many applications such as ﬁltration, reinforcements in composites, and carbon nanoﬁber precursors. 5 The majority of textile ﬁbers have a morphology that can be described by the classical two-phase model. In this model, discrete crystalline domains of the order of several hundred angstroms are mixed with amorphous domains of similar or smaller size. A high degree of crystallinity and high orientation of the crystalline molecular segments impart high ten- sile strength and modulus to the ﬁbers. The amor- phous phase gives rise to ﬂexibility and dyeability. 6,7 Whether PAN can be described by the classical model is debatable. In the ordered phase, irregularly twisted yet extended atactic polymer chains are packed hexagonally. 7 Because of the strong inter- chain interactions noted previously, this hexagonal columnar phase behaves as a solid, and it is often referred to as a (two-dimensional) crystal. Wide-angle X-ray scattering of drawn PAN ﬁbers has been shown to have two strong equatorial reﬂec- tions (Bragg spacings of d  3.0 A ˚ and d  5.3 A ˚ ), indicating an order perpendicular to the ﬁber axis. This result has frequently been interpreted in terms of hexagonal packing of molecular rods comprising distorted helices or kinked planar zigzags. Some have assumed a single, laterally ordered or paracrys- talline phase, whereas others have proposed a two- phase structure with regions of ordered rods and regions of amorphous material or disordered rods. 7 In the case of nanoﬁbers, their crystalline order may also be of primary importance when these materials are considered for commercial applica- tions. 8 One characteristic feature of the electrospin- Correspondence to: R. Jalili (jaliliar@yahoo.com). Journal of Applied Polymer Science, Vol. 110, 3014–3022 (2008) V V C 2008 Wiley Periodicals, Inc.