**3.4 Effect on adhesion of PEG 1500**

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

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

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

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

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

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,

(Ra=24 nm).

of the fabric surface.

structure is observed

leaving a smoother (Ra =52 nm).

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

water contact angle of the plasma treated fabric (Takke, 2009).

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

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 surface (see Fig. 16b).

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 layer of PEG covers the scaly bumps.

Atomic Force Microscopy – For Investigating Surface Treatment of Textile Fibers 253

Fig. 16. Tapping Mode AFM Topographical images of (a) an untreated-cleaned PET fiber, (b) a PEG-1500 coated untreated-cleaned PET fiber, (c) a plasma treated PET fiber at 60 KJ/m², (d) a PEG-1500 coated plasma treated PET fiber subjected to wash fastness test at

PEG molecules are still attached to the plasma treated PET fiber surface and cannot be removed by washing. The chemical or physico-chemical interaction/bonding between the free PEG and the polar species of the plasma treated PET results in an increased adhesion between PEG and PET fabric surface. It is also probable that the increase surface roughness

AFM seems an invaluable tool to detect all surface morphological modifications taking place during plasma treatment of a PET fabric surface. It confirms that plasma treatment not only adds polarity to the PET surface, but also, depending on the treatment power used, etching of the PET surface by plasma or a reorganisation of the PET surface takes place. AFM also shows the changes that takes place after ageing in presence or in absence of light, and when the plasma treated fabric is subjected to high temperature aqueous conditions. It confirms that the loss of polarity during ageing is also accompanied by morphological changes. AFM

(Ra =108 nm, as shown by AFM images) would enhance this adhesion.

90°C (Takke, 2011).

**4. Conclusions** 

Fig. 15. Tapping mode topographic image of (a) a PET plasma-treated sample at 60 kJ/m2, (b) followed by immersion in hot water at 90°C during 30 minutes, (c) after having been subjected to a 20 day-ageing without light and (d) ageing with light

Immobilisation of PEG on both untreated –cleaned PET and plasma treated PET lead to a hydrophilic fabric surface with similar water contact angle (~50°) in both cases. However, wash fastness test carried out at room temperature on PEG coated PET fabrics shows that without plasma treatment there is an increase in water contact angle of the PET fabric with an increase in washing cycles: the hydrophilic PEG molecules adsorbed at the PET surface are gradually desorbed with the washing time. This would mean that only weak cohesive forces exist between the cleaned untreated PET surface and the PEG molecules.

However, a plasma treatment of the PET fabric prior to PEG adsorption improves adhesion between PEG and PET-polyester fabric, since the water contact angle value of the PEG coated fabric remains unchanged with washing time at R.T.P and at 80°C. Therefore, the

Fig. 16. Tapping Mode AFM Topographical images of (a) an untreated-cleaned PET fiber, (b) a PEG-1500 coated untreated-cleaned PET fiber, (c) a plasma treated PET fiber at 60 KJ/m², (d) a PEG-1500 coated plasma treated PET fiber subjected to wash fastness test at 90°C (Takke, 2011).

PEG molecules are still attached to the plasma treated PET fiber surface and cannot be removed by washing. The chemical or physico-chemical interaction/bonding between the free PEG and the polar species of the plasma treated PET results in an increased adhesion between PEG and PET fabric surface. It is also probable that the increase surface roughness (Ra =108 nm, as shown by AFM images) would enhance this adhesion.
