POLITECNICO DI MILANODipartimento di Chimica, Materiali e Ingegneria Chimica

“Giulio Natta”

MODIFICATION OF SURFACE PROPERTIES OF ELECTROSPUN POLYAMIDE NANOFIBERS

Chiara Bertarelli

VI Convegno Nazionale sulla Scienza e Tecnologia dei Materiali, Perugia 12/15 June 2007

ELECTROSPINNING

The fibers formation is due to the electric field applied to a needle through which a polymer solution or a melt polymer flows: instability sets in and a polymer jet is extruded from the tip of the needle and reaches a collector. During this fast process a strong elongation of the jet takes place and, causing the shrinkage of the diameter and fast evaporation of the solvent.

SETUP

1) High voltage power supply (KV)

2) Syringe (positive elecrode) through which the polymer solution flows.

3) Infusion pump: provide a constant flow of solution at the tip of the syringe.

4) Target onto which fibers are collected.

Electrospinning TheoryDan Li, Younan Xia, Advanced Materials, 2004, 16, No. 14, July 19Zheng-Ming Huang, Y. Z. Zhang, M. Kotaki, S. Ramakrishna, Comp. Science and Technology, 63, 2003, pp 2223-2253

ELECTROSPINNING

SOLUTION

Polymer concentration Mw of polymer Solvent electrical conductivity Boiling temperature of the solvent Solution viscosity

SETUP PARAMETERS

Applied Voltage Flow Rate Tip-to-collector distance Dimensions of the needle

ENVIRONMENT

Temperature Relative Humidity Pressure

Fibers diameters between 50 nm and few microns Very high Surface Area/Volume ratio Ultra orientation of molecular chains within fiber

ELECTROSPUN FIBERS

Polymer Nanofibers

Conductive “nanowires”

(sensors, actuators, electronic devices)

Antibacterial membranes for

filters

Biomedics

Composites reinforcements

Aerospace(“Solar sails”)

Protective Clothing

In particular, surface fiber morphology, the presence of pores and pores geometry depends on:

- Chemical structure of the polymer- Molecular weight of the polymer- Boiling point of the solvent- Relative humidity

ELECTROSPINNING

Acting on the process parameters modification of mat morphology is allowed

Electrospray Fibers with defects (“beads”) Homogeneous defect-free fibers

MW

hu %T

b

solvent

C. Casper, J.S. Stephens, D.B. Chase, J.F. Rabolt Macromolecules 37, 573 (2004)

Why electrospun Polyamide Fibers?

The Polyamides-X (or XY) are widely used in producing synthetic fibers for apparel,

technical fabrics, and so forth

The PAs are soluble in different solvents (i.e. formic acid, HFIP, ...)

The good solubility allows polyamides to be doped with different additives to enhance

several mechanical, chemical, electrical and physical properties

SOLVENT: HFIP, Formic Acid

Polymer CONCENTRATION 15% → 25% (wt)

VOLTAGE Applied 12 kV → 25 kV

FLOW RATE 0.03 ml/h → 0.3 ml/h

DIST. TIP-to-COLLECTOR 10 cm → 30 cm

PRELIMINARY SPINNING of PA6

ELECTROSPUN PA6 FROM HEXAFLUOROISOPROPANOL (HFIP)

FINAL SET

Defect-free fibersAverage diameter: 1 m

HFIP

20% (wt)

17 kV

0.1 ml/h

20 cm

FINAL SET

Defect-free fibersAverage diameter: 150 nm

Standard deviation limited to less than 25 nm

Formic Acid

20% (wt)

17 kV

0.10 ml/h

20 cm

ELECTROSPUN PA6 FROM FORMIC ACID

CHEMICAL MODIFICATION OF THE SURFACE PROPERTIES:

Many efforts are addressed to modify the surface properties of polymers to improve the performance of these materials and to pave the way to new technological applications

WETTABILITY

Surface morphology Surface chemistry

Fluorinated surface

Direct reaction with fluorine(W.J. Feast et al., J. Polym. Sci. Pert A 13 (1975) 857)

Plasma treatment with fluorine-containing gases(J. Hopkins et al. J. Phys Chem. 99 (1995) 4261;

A Raffaele-Addamo et al. Surf. Coat. Technol. 174 (2003) 86)

Sputtering of fluorocarbon layers(M.E. Ryan et al. J. Phys Chem. 99 (1995) 7060)

Chemical derivatization(R.P. Popat et al. J. Mater. Chem. 5 (1995) 713; H. Shao et al J. Fluorine Chem. 125 (2004) 721)

The IDEA: combining the superficial segregation of fluorinated molecules with electrospinning to obtain hydrophobic nanofibers

PA6 film Electrospun PA6 fibers

θ ~ 50°

The wettability was measured with the contact angle technique on flat surface (planar electrospun non-woven textiles). To take into account the possible effect of morphology on contact angle, the fibrous mat was compared to cast films (doctor blade technique).

WETTABILITY OF ELECTROSPUN POLYAMIDE 6 FIBERS

Formic Acid

20% (wt)

17 kV

0.10 ml/h

20 cm200 nm 1 m

a) Pentafluoro propionic acid (PTP)b) tridecafluoro-1-octanol (TDO) → Slight changes in wettabilityWettability properties non stable

c) 1,3,4-trifluoro-2-(1H,1H-perfluoroundecyloxy)-7-dimethylamino-acridine (PFA)

C-F based molecules

CHEMICAL MODIFICATION OF THE SURFACE PROPERTIES:

Fibers doped with acridine (PFA)

6% 123°

Doping (wt) Contact angle

1% 63°

2% 70°

4% 83°

PA6 fibers doped with small quantity of acridine (PFA)

Doped films vs Doped Fibers

At higher levels (6%) the acridine is more efficient in the fibers where the high specific surface become predominant and amide groups are “shielded”

At very low doping level (1%) the acridine is more efficient for the film, whereas in the fibers it is not sufficient to overcome the H-bonds.

86° (film) vs 63° (fibers) 101° (film) vs 123° (fibers)

105° is near to typical values for PTFE, the reference material for hydrophobic materials(M.R. Yang et al. Mater. Chem. Phys. 50 (1997) 11)

Acridine Segregation towards surface can be thermally induced (heating at T=62°C)

Contact angle of 131° in only 10 minutes with 4% PFA

THERMALLY-INDUCED SEGREGATION

Doping (wt) Contact angle

4% 83°

6% 123°

TIME AGING

Doping (wt)Contact angle

(fibers just produced)

Contact angle

(after 2 months)

2% 70° 124°

6% 123° 127°

HIGHLY-ORIENTED ELECTROSPUN POLYAMIDE FIBERS

By acting on the geometrical configuration of the collector, it is possible to control and produce uniaxially aligned fibers. Two conductive strips separated by a gap of few centimeters was used as collector, thus forcing the fibers to align themselves perpendicular to each edge of the gap.

The molecular orientation of the polymer chains can be studied by polarized infrared spectroscopy

FIBERS ORIENTATION

Comparing IR spectra in the two different polarization (parallel and perpendicular to the fiber direction) a strong dichroism of several absorption bands were observed

Amide II (1540 cm-1) +skeletal CH wagging coupled

(1400-1200 cm-1)

γ (1121 and 973 cm-1) and α (930 cm-1), thus indicating order in crystalline and amorphous parts

N-H stretching (3300 cm-1)

From intensity Ipar

and Iperp

of absorption bands in different polarizations...

... to the average degree of orientation <θ>

Herman's Theory

Dichroic ratios and <cos2θ>, not taking into account that the vibrational modes and the

related transition moments are not fully aligned with the fiber axis

1) The values found are very similar, thus confirming the consistency of the approach.

2) The orientation calculated is underestimated, because the transition moments were considered // (or perpendicular) to the chains axis

ULTRAORIENTED SYSTEM

FIBERS ORIENTATION

Conclusions

1. Polyamide fibers were electrospun from formic acid or hexafluoroisopropanol solution

2. Polymer fibers obtained have smooth surface and homogenous diameters

3. Contact angle measurements were carried out to study the wettability of PA6 before and after doping with an acridine derivative.

4. Comparison between the contact angle values of films and fibers pointed out that morphology does not play a relevant role

5. Modification of wettability of PA6 electrospun nanofibers by means of doping procedure

6. An appreciable increment of the contact angles took place even at low dopant concentration

7. Thermal induced segregation and time aging further reduce PA6 wettability

8. Ultra-oriented fibers were obtained spinning onto a suitably designed collectors

Acknowledgment

Dipartimento di Chimica, Materiali e Ingegneria Chimica - Politecnico di Milano

Prof. Giuseppe Zerbi

Andrea Bianco, Giacomo Iardino

Flavio Granato, B. Broggi, Giulia Suarato, Rossella Castagna (Electrospinning)

Eleonora V. Canesi (P054) ; Giovanni Dassa (P084) (Electrical memories and Photodetectors)

Giorgio Toso, Stephan Hermes, Monica Ginocchio (Photochromic Materials)

Chiara Castiglioni, Mirella Del Zoppo, Matteo Tommasini, Alberto Milani (P053), Daniele Fazzi (PD08), Elena Rodighiero (Modelling)

Luigi Brambilla, Andrea Lucotti, Giorgio Fustella, Roberta Colombo, Anna Ferruggiari (Vibrational Spectroscopy)

Funding of Regione Lombardia through Nano Engineered Textile (NETex) and Italian Scientific and Technological Research Ministry through Nano Engineered Materials and Surface (NEMAS)

Thanks to Mrs. Manuela Gullo for the assistance in contact angle measurements

REFERENCES:

A. Bianco, G. Iardino, A. Manuelli, C. Bertarelli, G. Zerbi, ChemPhysChem, 8, (2007) 510A. Bianco, G. Iardino, C. Bertarelli, L. Miozzo, A. Papagni, G. Zerbi, Appl. Surf. Sci. (2007), doi:10.1016/j.apsusc.2007.04.003

Dipartimento di Scienza dei Materiali - Università degli Studi di Milano Bicocca

Prof. Antonio Papagni, Dr. Luciano Miozzo