**3. Results and discussions**

248 Atomic Force Microscopy – Imaging, Measuring and Manipulating Surfaces at the Atomic Scale

treatment, and these forces remain homogeneous throughout the plasma treated PET fiber

Fig. 13. Application procedure of PEG-1500 on PET fabric by padding, squeezing followed

In this chapter, surface investigation by imaging in the Tapping mode using AFM "Nanoscope III" from Digital Instrument, is presented. This mode was preferred to the AFM /LFM (contact mode AFM), since the Tapping mode overcomes problems associated with friction, adhesion, electrostatic forces which may arise after a plasma treatment, and which

Tapping mode imaging was carried in ambient air using 'Budget sensor' tips from 'Nanoandmore', of length 125 µm, made of Si3 N4 with aluminium coating, and a resonance frequency of 300 kHz. The cantilever which scans the surface is oscillated at or near its resonant frequency using a piezoelectric crystal. The oscillating tip is moved toward the surface until it begins to lightly touch, or tap the surface, significantly reducing the contact time. During scanning, the vertically oscillating tip alternately contacts the surface and lifts off, generally at a frequency of 5000 to 500,000 cycles per second. As the oscillating cantilever begins to intermittently contact the surface, the cantilever oscillation is necessarily reduced due to energy loss caused by the tip contacting the surface. The reduction in oscillation amplitude is used to identify and measure surface features. When the tip passes over a bump in the surface, the cantilever has less room to oscillate and the amplitude of oscillation decreases. Conversely, when the tip passes over a depression, the cantilever has more room to oscillate and the amplitude increases (approaching the maximum free air amplitude). This feed back signal also allows construction of the topographic image. The fiber surface roughness Ra was calculated directly from AFM signals (Feninat, 2001) using

The sessile drop method is useful for measuring water contact angles greater than 90° however for lower contact angles the porous woven fabric immediately absorbs the liquid drop. That is why wettability of textile surfaces was monitored indirectly through a wicking test using a tensiometer. This method allows the measurement of contact angle as well capillarity measurements. The tensiometer used is the 3S from GBX, France. During measurements, a vertically hanging fabric sample of defined dimension is connected to the tensiometer at the weighing position and progressively brought into contact with the

by curing and a final rinsing before drying.

would distort image data (Kailash, 2008).

the software supplied with the Nanoscope III.

**2.3 Water contact angle and capillary measurements** 

surface.

#### **3.1 Effect of varying speed and electrical power on plasma-treatment of PET woven fabrics**

Irrespective of treatment power and speed setting, the water contact angle after plasma treatment was always around 45°, while the untreated PET fabric water contact angle was 80°. Any decrease in speed or increase in Treatment Power did not have a significant impact on this minimum contact angle value. However, tapping mode AFM images of samples (Fig. 14) subjected to an air–plasma treatment at varying Treatment powers "TP" of 0, 12, 24, 60 and 120 kJ/m² show considerable changes in fiber surface morphology.

Fig. 14. Topographic images obtained by AFM in the tapping mode of a) an untreated PET fabric fiber surface, a plasma-treated PET fabric fiber surface at treatment power ''TP'' of (b) 12 kJ/m², (c) 24 kJ/m² , (d) 60 kJ/m² and (e) 120 kJ/m² (Takke, 2009).

Fig. 14 shows that as the treatment power is increased from 0 to 120 kJ/m² there is significant surface restructuring. At very low Treatment Power of 12 kJ/m² at a speed of 10

Atomic Force Microscopy – For Investigating Surface Treatment of Textile Fibers 251

While in absence of light slow ageing initiates a disordering of the surface, ageing in presence of light leads to an organized structure. Without light, the ageing process proceeds slowly as evidenced by the similar water contact angles of the 20 and 1 day samples. As only slight change in the surface topography is observed, the oxidised species should stay on the fiber surface and are not removed by ageing without light. With light, the ageing process seems to be accelerated, both at the outer fabric surface and the inner fabric fiber surface.

Further chemical analysis of the surfaces for example by XPS (X-Ray Photoemission Spectroscopy) would provide clearer information on the effect of light on plasma treated samples, however AFM imaging seem to be an invaluable tool to investigate ageing effect

Plasma treatment alone increases the hydrophilicity of PET fabric, demonstrated by the decrease in water contact angle from 80° to 45°. However, after 30 minutes of immersion in water at R.T.P there is an increase in water contact angle from 42° to 55°, and a considerable decrease in capillary weight from 360 to 80 mg. Thus, the polar species formed as a result of plasma treatment are gradually lost during washing even at R.T.P. Under dyeing conditions, that is washing at 90°C for 30 min, the wettability of the plasma treated PET reaches that of untreated PET fabric (water contact angle=80°) meaning that there is complete loss of all polar species formed by plasma treatment. Surface analysis of plasma treated PET subjected to hot aqueous dyeing conditions shows complete disappearance of the scaly shaped PET surface (Fig. 15b). Only traces or finger prints of the scales remain leaving a smooth surface (Ra = 38 nm). Moreover dyeing of plasma treated PET shows no increase in disperse dye

Atomic Force Microscopy, in the Tapping mode was carried out to better understand the different morphological changes occurring at the PET fabric fiber surface before and after PEG-1500 immobilization with and without a prior plasma treatment of the PET fabric

Figures 16a and 16b show typical topographical images of a cleaned-untreated PET fiber with and without PEG coating respectively. Though the surface roughness of the PET surface with or without PEG coating is almost the same (~70nm), the untreated PET fiber seem quite smooth and homogeneous. However, after application of PEG, many small thin elliptical and irregular shaped deposits, most probably due to PEG, appear at the fiber

After plasma treatment of PET fiber (at 60 kJ/m²) uniform scale-shaped bumps (6 scales per µm in the x-direction) appear on the PET fiber surface (see Fig. 16c) as a result of surface etching caused by plasma treatment. After immobilizing of PEG 1500, big bumps appear in the foreground relief, covering the regular scaly shaped bumps which can no longer be perceived. However, on the upper part of the image (x =0 to 1), the scaly shaped bumps still appear as if the gaps (valleys) in between scales have not been completely filled with PEG in that region, most probably, because either the PEG does not cover them at all, or only a thin

surface. Detailed results have been published elsewhere (Takke, 2010).

**3.3 Effect of PET fabric dyeing conditions (High temperature conditions)** 

on surface structuring.

uptake.

surface (see Fig. 16b).

**3.4 Effect on adhesion of PEG 1500** 

layer of PEG covers the scaly bumps.

m/min, a smoother surface is obtained observable as a decrease in surface roughness, Ra=50 nm, (Fig. 14 b). The smoother surface at this highest speed and low plasma power, may be due to etching of the very upper layer of the PET surface. However, as the Treatment Power is further increased to 24 KJ/m² at a lower speed of 5 m/min, further etching of the PET surface creates disordered bumps (Fig. 14c). Continued increase of TP to 60 KJ/m² at a lower speed of 2 m/min, leads to more uniform scale-like structures (6 scales per µm, Fig. 14d) which increases the surface roughness of the fiber, Ra= 108 nm. Further increase in TP to 120 KJ/m² (at 1 m/min) causes further uniform etching yielding tinier, more organized, uniform and flatter beads (12 beads/µm, Fig. 14e) leading to a decrease in the surface roughness (Ra=24 nm).

The increase in hydrophilicity of the PET fabric surface after the atmospheric-air plasma treatment is probably due to plasma oxidation, which destroys the surface chemical bonds, leading to an increase in polar groups (Borcia, 2003). Plasma treatments generate polymer chain-scissions of the weakest bonds of the polyester, creating very reactive chain-end free radicals. These radicals then react easily with the reactive species (ex: oxygen radicals) present in the plasma generating polar species such as carbonyl, carboxyl and hydroxyl groups, which are polar species capable of increasing the surface energy (Leroux, 2006). We have previously reported using X.P.S measurements (X-Ray Photoemission Spectroscopy) (Leroux, 2009) that there is an increase in concentration of oxygen at the fabric surface after air plasma treatment of the PET fabric, and this would explain the increase in surface energy of the fabric surface.

The plasma treatment not only causes chemical modifications of the PET surface by adding polar groups, but also morphological surface changes that are observed by tapping mode AFM imaging. Although the lower water contact angle (45°) is measured even at low Treatment Power (12 kJ/m²), surface etching of the PET fiber surface continues with increasing Treatment Power, yielding an increased surface roughness. At higher Treatment Powers (60 and 120 kJ/m²) a more organized scale–like surface structure is observed

#### **3.2 Effect of ageing on plasma treated PET sample**

Ageing of atmospheric plasma treated PET woven fabric with TP of 60 KJ/m² in the absence and presence of light was evaluated. In absence of light, very little change in water contact angle was observed. However, there were substantial increase in water contact angle (45° to 73°) and decrease in capillary weight for plasma treated PET fabrics kept in presence of daylight. The increase in contact angle with time, in presence of light, could be due to the loss of surface oxidation species at the plasma treated PET fabric surface. Degradation of peroxide radicals due to UV rays of the daylight could be a major factor in increasing the water contact angle of the plasma treated fabric (Takke, 2009).

Tapping mode AFM imaging carried out after the 20th day of ageing in the absence of light shows that there is deformation of the scale-like structures formed as a result of plasma treatment at 60 KJ/m² observed as a disordered surface structure with big bumps. In presence of light, smaller, flatter and fewer blister-shaped structures, of different sizes, appear as if, the scales formed by plasma treatment had been nearly completely eroded, leaving a smoother (Ra =52 nm).

While in absence of light slow ageing initiates a disordering of the surface, ageing in presence of light leads to an organized structure. Without light, the ageing process proceeds slowly as evidenced by the similar water contact angles of the 20 and 1 day samples. As only slight change in the surface topography is observed, the oxidised species should stay on the fiber surface and are not removed by ageing without light. With light, the ageing process seems to be accelerated, both at the outer fabric surface and the inner fabric fiber surface.

Further chemical analysis of the surfaces for example by XPS (X-Ray Photoemission Spectroscopy) would provide clearer information on the effect of light on plasma treated samples, however AFM imaging seem to be an invaluable tool to investigate ageing effect on surface structuring.
