THE EFFECT OF ADDITIVES ON ELECTROSPUN PAN FIBRES

4.1 INTRODUCTION

The effect of copper salts and copper nanoparticle additives on the crystallinity, molecular orientation structure and thermal properties of electrospun PAN nanofibres, experimental conditions and results are discussed here in Chapter 4. The analytical techniques employed include FTIR, DSC, TGA, SEM, TEM, NMR, ESR and XRD.

The structure of the electrospun polymers are compared with wet spun commercial PAN polymers made from the same polymer grades and with annealed cast films, (with and without additives), and the “as-polymerized” powder. Whilst the electrospun fibres are also wet spun, in this document the term “wet spun” refers to fibres spun by conventional commercial spinning lines.

Then in addition the effect of the inclusion of oligomers and conductive polyaniline blends in PAN on the resultant electrospun fibres is reported in Sections 5.2 and 5.3 respectively.

4.2 EXPERIMENTAL

All PAN electrospinning solution samples containing metal salts or nanoparticles were prepared using an 8% solution of the 120k PAN copolymer in DMAc or the 6% 210k PAN homopolymer also in analytical grade DMAc supplied by Aldrich, unless otherwise stated. The PAN polymer grades are described in detail in Section 3.1.

The polymer was first added to the DMAc solvent and the solution was left to stir at 50 °C overnight. The polymer solution was then cooled and divided into 20 ml aliquots. The weight of copper additive as indicated in Table 4.1 was added to the polymer solution whilst stirring and left to stir overnight. All the salts used were completely soluble in DMAc.

Films were also cast from the copper containing PAN solutions and annealed overnight in an oven at 90°C and then vacuum dried at room temperature for 24 hours.

The samples described in Table 4.1 were spun during the author’s visit at the University of Marburg and the samples described in Table 4.2 were prepared and electrospun at the University of Stellenbosch.. The sample series labeled “MCx” and “SCx” were identical chemically but spun by the author in Marburg and Stellenbosch respectively using identical electrospinning set-ups) to assess the repeatability of the control containing CuCl2 where Cu:CN equals 1:100, 1:75, 1:50, 1:20 and 1:10..

All were spun at 20 kV, at a distance of 20 cm at 30 °C using an Epindorff® yellow plastic tip. The

identified as a salt, which was easy to electrospin as an additive in other polymers (Graeser 2004). The copper acetate salt was found to only dissolve at a Cu:CN ratio of 1:50 if amine ligands were included. The ligand containing solutions spun included tetraethyl-triamine, (TETA) and ethylene-diamine, (ED) obtained from Aldrich and added at a concentration of 0,25% w/w.

Table 4.1 Mass of Copper additive in 20 ml 8% 120k PAN in DMAc solution Cu:CN

Addition of copper (I) salts , CuCl and Cu(Ac) to PAN was found to form a highly viscous gel. The gel was bright green in the case of CuCl in PAN in DMAc. The gel became a spinnable solution when the pH was adjusted to pH = 9 with concentrated ammonia, NH3.

Samples with CuO nanoparticles, 40 to 70 nm, also added to the 120k PAN were prepared.

Problems were experienced with particle agglomeration. A combination of intensive sonication and the addition of amine ligands gave a more homogeneous distribution of the nanoparticles in the electrospun fibres and is discussed further in Section 4.7.

CuO was also formed in-situ from CuNO3 salts which were calcined in carbonized 120k PAN electrospun fibres. The TEM micrographs of the electrospun carbon fibres with CuO are shown in Section 4.7. CuO was additionally deposited as a 3 to 20 nm thick coating on the surface of the PAN nanofibres at room temperature using a process of laser vapor deposition is also discussed further in Section 4.7.

Table 4.2 8% 120k PAN & 6% 210k PAN with Cu- salts (Univ. of Stellenbosch) CODE MW (k) % cm kV Cu:CN Comments general nanoparticle chemistry were discussed in Section 2.4. The sample preparation and experimental procedure relevant to each analytical technique is described in the following sections of Chapter 4.

4.3 STRUCTURE AND ANALYSES OF PAN FIBRES

After the conventional melt, wet or dry spinning of PAN fibres higher levels of orientation can be achieved by post-drawing processes, where after they are subjected to annealing by heating and cooling under tension in a continuous process over rollers. The degree of crystallinity and molecular orientation is critical to the tenacity, chemical and thermal performance of the fibres.

Methods to measure crystallinity and molecular orientation of polymers are listed in Table 4.3.

The methods used and reported in this document are highlighted in Table 4.3. In addition the fibres were characterized by thermal gravimetric analysis, (TGA), and electron microscopy.

Table 4.3 Analytical techniques to measure crystallinity and orientation of fibres

Crystallinity Molecular Orientation

Infra-red Spectroscopy, (FTIR) Infra-red Spectroscopy, (FTIR) Wide Angle X-ray Diffraction, (XRD) Wide Angle X-ray Diffraction, (XRD) Small Angle Neutron Scattering, (SANS) Birefringence

Differential Scanning Colorimetry

(DSC) Solid State cp-MAS Nuclear Magnetic

Resonance Spectroscopy, (NMR)

Density Electron Spin Resonance Spectroscopy,

(EPR) or (ESR)

Polarised Raman or FTIR Spectroscopy The results of researchers using these techniques specifically on PAN and / or electrospun fibres is discussed briefly in Chapter 2 where the results of the analyses on the electrospun PAN fibres and films with additives for each individual technique are presented and discussed.

4.4 INFRARED SPECTROSCOPY STUDY OF ELECTROSPUN PAN:

RESULTS, DISCUSSION AND CONCLUSIONS