Controlling Wettability in Paper by Atmospheric-Pressure Microplasma Processes to Be Used

-Supplementary material-

Controlling wettability in paper by atmospheric-pressure microplasma processes to be used in µPAD fabrication

Lars Hecht1), Jens Philipp2), Kai Mattern1), Andreas Dietzel1), Claus-Peter Klages2,*)

1) Technische Universität Braunschweig, Institute of Microtechnology (IMT), Alte Salzdahlumer Str. 203, D-38124 Braunschweig

2) Technische Universität Braunschweig, Institute of Surface Technology (IOT), Bienroder Weg 54 E,

D-38108 Braunschweig

*) [email protected] Phone: +49 531 2155 510 Fax: +49 531 2155 900,

Online Resource 3

1. Plasma polymerization - parameter determination

The definition of the best suitable parameter set used for the plasma polymerization process is based on the experimental results which are presented in the following.

1.1 c-C4F8 concentration, treatment time and generator mode

For the first experiments the setup already described in Online Resource 1 was used.

The process-specific details (for example varied parameters and their ranges, etc.) are already described in chapter 2.2.2 Plasma polymerization of our contribution with the exception that the generator was operated in both continuous wave and pulsed mode (pulse/pause-ratio of 1/10 ms) to evaluate the feasibility of the two different generator modes. Furthermore the setup was purged for 2 min using an argon gas flow containing the precursor amount of interest before plasma ignition. Plasma polymerization took place with at a peak voltage of 2 kV and plasma ignition was observed at 1.6 kV peak voltage. Whatman Filter Paper Grade 6 as substrate and the electrode design with the comb structures was used. The applicability of the coated samples was tested by dipping them completely into DI- water colored with fuchsine. Table 1 and Table 2 show a selection of the colored samples which were taken into account for the definition of suitable process parameters. Table 1 Pictures from the top side and bottom side of samples which were treated with argon gas flow containing c-C4F8 (0.25 %, 0.5 % and 1 %, respectively) for 15 s using continuous wave mode and pulsed mode.

c-C4F8 (%) 0.25 0.5 1 Treatment time (s) Top Bottom Top Bottom Top Bottom 15 (continuou s wave mode)

15 (pulsed mode)

Table 2 Pictures from the top side and bottom side of samples which were treated with a constant concentration of 1 % c-C4F8 for a duration of 30 s and 60 s, respectively, using continuous wave mode and pulsed mode.

Treatment 30 30 60 time (continuous wave mode) (Pulsed mode) (Pulsed mode) c-C4F8 (s) (%) Top Bottom Top Bottom Top Bottom

Results:

The experiments reveal that the best fiber coatings abilities are given at an argon gas flow containing 1 % c-C4F8 compared to samples with a c-C4F8 concentration of 0.25 % or 0.5 %, respectively, as it is shown exemplarily in Table 1. Also a pulsed excitation mode with an applied pulse/pause-ratio of 1/10 ms results in a better pattern transfer of the electrode structure onto the sample surface compared to samples which were treated under identically conditions using continuous wave excitation mode (see Table 1).

Longer treatment times seemed to improve the quality of the coating at the top and bottom side of the sample. However, at higher treatment times the hydrophilic space between polymerized structures seemed to shrink for experiments done under continuous wave excitation mode. In pulsed excitation mode some undesired polymerization seems to occur in the center of the sample - this coating does not appear to be as strong as the in the areas defined by the structured electrode (see Table 2)

1.2 Experiments with an optimized reactor type The gas inlet of the experimental setup was optimized prior to further experiments because of the coating growth which could be observed in the center of the sample during the polymerization with pulsed excitation mode. The central position of the gas inlet was figured out to be problematically for the gas delivery. The reactor setup for the coating process presented in our contribution uses an optimized porous electrode layout which enables a more efficient gas delivery into the micro cavities of the substrate (Whatman Filter Paper Grade 6). Two gas inlets on the bottom side of a polycarbonate adapter-plate lead the process gas into a 5 mm wide groove, which is connected with the rear edge of the porous electrode over their entire length (see. Fig. 2).

Fig. 2 Bottom view of the optimized polycarbonate adapter-plate supporting two gas inlets.

This optimized setup was used for following experiments. Based on the results from the first experimental series pulse/pause-ratio of 1/10 ms, precursor concentration of 1 % C4F8 and 2 min purge time (using argon containing the precursor), respectively, were kept at a constant value. The varied parameters were treatment time, burning peak voltage and purge time, respectively. The samples were colored after plasma polymerization using the test dye described in our contribution. A selection of coated samples with varied parameter configurations is presented in Table 3 and Table 4.

Table 3 Pictures from the top side and bottom side of samples which were coated by plasma polymerization of 1 % C4F8 in argon using an applied peak voltage in the range from 2 kV to 3.5 kV and treatment time durations of 60 s and 120 s.

Burning 2 2.75 3.5 voltage (kV) Treatment Top Bottom Top Bottom Top Bottom time (s)

120 Table 4 Pictures from the top side and bottom side of samples which were coated by plasma polymerization of 1 % C4F8 in argon using 2.75 kV applied peak voltage, a pulse/pause-ratio of 1/10 ms and treatment time 120 s, respectively. Before plasma polymerization the setup was purged with argon (containing 1 % C4F8 or not) for varied durations of purge times.

Purge 0 20 time (Plasma 1 (min) (Pure argon + 3 min polymerization directly (Argon with 1 % C4F8) argon with 1 % C4F8) Process started) parameter s Top Bottom Top Bottom Top Bottom

1% C4F8, 1/10 ms, 5.5 kV, 120 s

Results:

As the main result of the variation of the applied burning peak voltage and the treatment time it was determined that an applied peak voltage of 2.75 kV for 120 s generates the best barrier effect as it is shown in Table 3. At lower peak voltages the barrier effect was less defined especially on the bottom side and at higher peak voltages the hydrophilic space between the small channels of the meander structure was reduced (see colored sample pictures in Table 3)

In order to improve the coating on the top side and also achieve a more effective coating on the bottom side of the sample the purge time was varied as well. Substrates purged for 20 min with pure argon and then for additional 3 min with argon containing 1% c-C4F8 before plasma polymerization achieved the best results compared to lower purge times with different purge gas configurations as it is presented in Table 4.