2.3.1
Contact Angle
Contact angle measurements were taken with two different systems throughout the course of this research. These systems and the methods will be detailed in the following sections.
2.3.1.1 CAM100 Instrument
The CAM100 contact angle meter (KSV Instruments Ltd., Finland) was used as a quick method to determine if a sample had been treated. This system was operated manually and a short video was taken using a CCD camera controlled by CAM100 software (KSV Instruments Ltd., Finland), 10 frames at 1 frame per second (fps) were captured. The centre of a sample was positioned approximately beneath the needle. A ≈0.3µl drop of deionised water was forced from the needle tip. The sample stage was then raised until the drop contacted the sample. When the stage was lowered the drop remained on the sample surface. After a baseline was manually set the contact angle was automatically calculated by the CAM100 software using a tangent method. The mean contact angle for each set of 10 frames was calculated.
2.3.1.2 DSA100 Instrument
Initial spatially resolved contact angle was performed using the DSA100 drop shape analysis system (Krüss GmBH, Germany). In this system 0.2µl drops of deionised water were dispensed from a needle and syringe via DSA3 software (Krüss GmBH, Germany). Water drops were placed across the horizontal axis of a treated sample at a resolution of 2.5mm using a micro-step x-y stage controlled by the DSA3 software. 10 frames at 1fps were captured using a CCD camera controlled by the DSA3 software. The baseline was set manually and images were analysed by the DSA3 software using the circle method. The mean contact angle for each set of 10 frames was calculated.
2.3.1.3 DSA100m Instrument Modification
DSA100m “micro-drop” modification to the Krüss DSA100 was also conducted. This modification included additional lenses on the camera to improve magnification, an LED illuminator which provided an illumination source closer to the sample and piezoelectric
dosing unit capable of dispensing drops in the picolitre range [220]. The volume of deionised water dispensed was controlled by altering the signal voltage and pulse time sent to the piezoelectric dosing unit; 80V and 200µs were used for these experiments. Drops were placed upon samples with a spatial resolution of 250µm or 500µm depending on experiment. A video was recorded at 25fps and began recording as soon as the drop was dispensed. The first frame containing a steady drop on the sample surface was measured using the circle method. Subsequent frames of each video were not measured as water drops very quickly evaporated, altering the contact angle.
Contact angle profiles were taken across the horizontal axis of samples through the centre of the treatment (example in Figure 2.18). As the 1.2mm thick PS was cut by hand samples were not always exactly 10x10mm. Therefore, the focus of the plasma jet was aligned to be centred 5mm from the bottom of the sample, and 5mm in from the left edge of the sample. The mean contact angle corresponding to each position along the profile was calculated. 2- D contact angle maps were taken by placing drops in a grid pattern with spatial resolution of 250µm or 500µm between points (example in Figure 2.19).
Figure 2.18 Illustration depicting an example of drop positions for a contact angle profile with a 0.5mm resolution on a 10x10mm sample.
Figure 2.19 Illustration of the drop positions for a 2-D contact angle map. Spatial resolution of drops was 500µm.
2.3.2
X-Ray Photoelectron Spectroscopy (XPS)
The spectrometer used in this study was designed in Liverpool and constructed in Manchester by KSV, UK. The instrument used for these studies has been described previously[221]. The x-ray source was a “multicrystal Al Kα monochromator on a 0.7m Rowland circle which subtends a solid angle of ~0.1 sterradian at the specimen thus providing high sensitivity and high resolution (< 0.5eV) XPS capability at 1486.6eV” [221]. The sample holder accommodated relatively large sample sizes (12 × 12 mm2) and was mounted on an x-y-z stage which also had the capability of being rotated through 360°. The x-ray ‘spot’ focused on a sample surface was ≈1.5 × 7mm2 in size.
Single broad scan or high resolution spectra were taken in the centre of the sample. For spatially resolved XPS samples were translated through the x-ray ‘spot’ along the horizontal axes of samples, and broad scans were taken every mm. The edge of the sample was determined by the presence of spectra from the sample holder. Broad scan and high resolution spectra were acquired at pass energies of 100eV and 50eV respectively. The C1s of all spectra were calibrated to a binding energy of 285eV. All peak components were fitted with a 70% Guassian: 30% Lorentzian function and the position restricted by ±0.5eV. Full width at half maximum (FWHM) was limited between 0.5-2eV.
2.3.3
Atomic Force Microscopy (AFM)
A Multimode 8 AFM with NanoScope V controller was used in normal tapping mode with a silicon cantilever of spring constant 40N/m. A 500 x500nm2 region was scanned at a rate of 1Hz with 512 samples/line with the NanoScope software (ver. 8.10). Using the NanoScope analysis software (ver. 1.4). scans were subjected to 0th order plane fit, to remove centre data and 0th and 1st order flattening, to centre data and remove tilt. The mean roughness (Ra) and root mean square roughness (Rq) for each parameter were averaged from 4 scans in the centre of the samples. All equipment, probes and software were supplied by Bruker, UK.