Frequency-Tuned Porous Polyethylene Glycol Films Obtained in Atmospheric-Pressure Dielectric Barrier Discharge (DBD) Plasma

Round 1

Reviewer 1 Report

1. There are many mistakes in the manuscript. "2 ÷ 10 kHz" in the abstract should be "2 and 10 kHz". "N2 and N2+" should be in the upright form, not italic form. However, all the variabes should be in the italic form.

2. Please present the failure temperature of the ethylene glycol monomer, which should be much higher than the plasma temperature.

3. Fig. 6 (c) is not referred in the manuscript. What is the mechanism of plasma-induced monomer fragmentation, mechanical impact or thermal ablation? You can refer to the article "Review on laser-induced etching processing technology for transparent hard and brittle materials, Int J Adv Manuf Technol (2021)" and "Anisotropy of material removal during laser-induced plasma assisted ablation of sapphire, Ceramics International (2022)".

4. Please present more details for the plasma temperature calculation.

Author Response

1. There are many mistakes in the manuscript. "2 ÷ 10 kHz" in the abstract should be "2 and 10 kHz". "N2 and N2+" should be in the upright form, not italic form. However, all the variabes should be in the italic form.

Response to the point 1: All required changes were made in manuscript.

2. Please present the failure temperature of the ethylene glycol monomer, which should be much higher than the plasma temperature.

Response to the point 2: You probably refer to the energy required to break the specific bonds of the EG monomer and not to the effective temperature of thermal degradation of the monomer. In this case, the energy required to break the C-C bond of 347 kJ/mol (3.59 eV) that is much higher that the plasma temperature 2850 K (0.25 eV), avoids the complete decomposition of the monomer. Therefore, the following sentence was introduced: “The maximum plasma temperature of 2850 K (0.25 eV) found for the DBD plasma operating in He is much lower than the energy required to break the C-C bond (3.59 eV) that characterizes the EG monomeric unit, preventing the monomer from completely decomposing.”

3. 6 (c) is not referred in the manuscript. What is the mechanism of plasma-induced monomer fragmentation, mechanical impact or thermal ablation? You can refer to the article "Review on laser-induced etching processing technology for transparent hard and brittle materials, Int J Adv Manuf Technol (2021)" and "Anisotropy of material removal during laser-induced plasma assisted ablation of sapphire, Ceramics International (2022)".

Response to the point 3: The observation regarding Figure 6c (denoted by 7c after the last changes) was considered and we made the required changes. As regarding the remark on the mechanism induced monomer fragmentation, the present work aims to present the change in morphology of DBD plasma-polymerized PEG films with discharge frequency in correlation with plasma properties. For this purpose, frequency-dependent measurements on the properties of the polymerized films as well as the plasma parameters were systematically performed. We cannot comment on the monomer fragmentation mechanism, as it requires more investigations on the volume interaction processes that take place between DBD plasma species using complementary techniques such as space and time-resolved OES, in situ mass spectrometry. We are running a series of experiments by using the former technique.

4. Please present more details for the plasma temperature calculation.

Response to the point 4: The details of plasma temperature calculation are given in Section 2.2. Other examples of DBD plasma temperature calculation using the Boltzmann semi-logarithmic plot can be found in the cited references [30, 37–40].

Reviewer 2 Report

The manuscript  describes the plasma deposition of polyethylene glycol films with dielectric barrier discharges. However the paper is quite confused and the novelty not well explained against the current status of the research. The discussion of the method and of the experiments is lacking essential details. So it is difficult to assess whether the results are significant. In particular the case for studying only the effect of the pulse repetition frequency is not sufficiently motivated.  

 

In view of this, my opinion is that the paper could only be accepted after a major revision.

 

Here are some points that should be clarified by the authors:

1) Abstract: “ the intensity of the nitrogen (?2 and ?2+) spectral bands generated during 21 discharge and the characteristic temperatures (i.e., vibrational Tvib and rotational Trot temperatures) 22 decrease due to the energy losses of the charged particles” the sentence is quite confusing and left without any explanation.

N2 molecule is not a charged particle, so it is not clear why its rotational temperature should be controlled by “energy losses of charged particles”. Please explain or correct it.

2) Abstract: “ atomic spectral features of all the generated species were enhanced”

The meaning is obscure. If you means the intensity of the emission lines of atomic transitions observed in the gas-phase please explain it.

 3) Introduction, line 58-61: The paper restricts itself to investigate the effect of changing then pulse repetition rate, which is only one of the main operating parameters that affects the outcoming. The authors should discuss their choice and also review what is known about other, more relevant points, such as the carrier gas, the concentration of the monomer, the electrode geometry, the intensity of the HV pulses and their duration, … for instance.   

4) Experimental, Line 64-68: The description of the experimental setup is lacking. The essential should be reported here, not in references. Electrode geometry, discharge region respect to the monomer carrying flow, pulse properties. Otherwise Figure1 remains obscure.

5) Experimental, Line 68-70: “The experiments were made at room temperature in a laminar flowing of the helium spectral gas“.  

What is a”spectral gas” ?

6) Fig.1 a is useless. Please show only the times when discharges occur, so that the current pulses could be observed and judged. Reporting a vertical line carries only a limited information. This also benefit understanding of Figure 2.

7) Line 71-72: “When ethylene glycol monomer vapors were introduced into the dis-71 charge gap, an additional helium flow rate of 0.5 l/min was used”. The key information is missing: how much monomer you consume to produce the films and which concentration you have respect to the Helium.

8) Fig. 1b. Please add also a fifth frame of the post discharge at times 0.48/24.48 us.

9) Line 95: “The global emission of DBD plasma was spectrally resolved by using optical emission spectroscopy (OES) technique”.  Again the sentence is wrong. DBD have rich far UV emission as well IR so do not write “global” if you observe the 300-800 nm range.

10)Fig. 3. You should explain which part of the spectrum your ICCD is sensitive to. Together with other missing details on the setup, such as what is the viewfiled respect to the electrode geometry and so on. It is also obscure what is the zero in the delay times. It is the rising front of the discharge currant or some random time.  

11) Line 180-…: You observe quite dominant emission form nitrogen molecules and ions, but you never mentioned the presence on nitrogen in your gas-phase (that should be pure helium). If you have a substantial amount of air in your gas-phase, you should write what is its concentration, since this affects also your polymerization process.

12) Results, line 206-207: “This decrease  in temperature could be attributed to the energy losses of the plasma generated particles, as the inelastic collisions augment with the injection of monomer.”.

I wonder whether Fig.5 is necessary. You report a flat distribution with large error bars. So the effect of pulse frequency is null. The explanation of the changes in the rotational/vibrational temperatures is questionable. You want/have to explain why the population of the N2(C,v=1) state is depleted respect to that of the N2(C,v=0). Both requires about 13 eV to be excited. And you state this depends on the inelastic collision frequency (of the electrons ? the excited molecules ? other components of the gas-phase ?). If you have a comprehension of the excitation or quenching mechanisms in He and He+EG please share with your readers. If you have measurements about electron energies please present it.

Do not write sentences you cannot prove.

13) Figure4: it seems to me that the main effect of increasing the pulse frequency is to increase the number of discharges and so the overall energy provided to the system.

Could you plot the results as intensity of after a single (or fixed number) of pulse.

This would help to understand the effect of the plasma much more.   

Comments for author File: Comments.pdf

Author Response

1. Abstract: “the intensity of the nitrogen (?2 and ?2+) spectral bands generated during 21 discharge and the characteristic temperatures (i.e., vibrational Tvib and rotational Trot temperatures) 22 decrease due to the energy losses of the charged particles” the sentence is quite confusing and left without any explanation. N2 molecule is not a charged particle, so it is not clear why its rotational temperature should be controlled by “energy losses of charged particles”. Please explain or correct it.

Response to the point 1. The second half of the abstract was rewritten: “The determined vibrational (T_vib) and rotational (T_rot) temperatures exhibit a decrease with the introduction of monomer vapors into the discharge gap. For instance, the Trot drops from approximately 475 K to 350 K, and the T_vib falls from 2850 K to 2650 K for a monomer vapor injection rate of 16 µl/min. This was attributed to the energy losses of the plasma generated particles, as the inelastic collisions augment with the injection of monomer. Concurrently with the change in temperature, the discharge current varies significantly for the investigated frequency range and exhibits a drop at high frequencies. This discharge current drop was explained by an enhancement of the recombination rate of charged particles and seems to confirm the prevalence of a plasma-induced monomer fragmentation process at high frequencies.”

2. Abstract: “atomic spectral features of all the generated species were enhanced”. The meaning is obscure. If you means the intensity of the emission lines of atomic transitions observed in the gas-phase please explain it.

Response to the point 2: The original sentence you are referring to (“With increasing frequency, no significant effects were observed on the plasma temperature, while both molecular and atomic spectral features of all the generated species were enhanced.”) was removed from the abstract. The explanation of this increase in intensity with frequency is given below at the response to point 13.

3. Introduction, line 58-61: The paper restricts itself to investigate the effect of changing then pulse repetition rate, which is only one of the main operating parameters that affects the outcoming. The authors should discuss their choice and also, more relevant points, such as the carrier gas, the concentration of the monomer, the electrode geometry, the intensity of the HV pulses and their duration, … for instance.

Response to the point 3: The introduction section of the manuscript was rewritten, including additional information about the context regarding the existing date found in literature, trying to emphasize the novelty of the obtained results. The added information is related to the presentation of the techniques used for the fabrication of polymer films, highlighting the properties of the plasma-polymerized films in relation to them, the polymerization mechanism, applicability, etc.

4. Experimental, Line 64-68: The description of the experimental setup is lacking. The essential should be reported here, not in references. Electrode geometry, discharge region respect to the monomer carrying flow, pulse properties. Otherwise Figure1 remains obscure.

Response to the point 4: The reviewer has right. Indeed, details about the experimental arrangement were missing. We added more details about the performed measurements in Section 2.1 of the manuscript together with a sketch of the set-up.

5. Experimental, Line 68-70: “The experiments were made at room temperature in a laminar flowing of the helium spectral gas“. What is a”spectral gas” ?

Response to the point 5: The appellative "spectral" was withdrawn from the text.

6. 1 a is useless. Please show only the times when discharges occur, so that the current pulses could be observed and judged. Reporting a vertical line carries only a limited information. This also benefit understanding of Figure 2.

Response to the point 6: The Figure 1a (denoted by Figure 2a in the new numerotion) has been modified in accordance with the reviewer's requirements.

7. Line 71-72: “When ethylene glycol monomer vapors were introduced into the dis-71 charge gap, an additional helium flow rate of 0.5 l/min was used”. The key information is missing: how much monomer you consume to produce the films and which concentration you have respect to the Helium.

Response to the point 7: Using 0.5 l/min helium flow for bubbling the ethylene glycol, after 1 hour will consume around 1 ml of monomer, result around 16 µl/min. The following sentence was introduced in Section 2.1: “The ethylene glycol monomer vapors were introduced into the discharge gap at a consumption rate of 16 µl/min by using an additional helium flow rate of 0.5 l/min for monomer bubbling.”

8. 1b. Please add also a fifth frame of the post discharge at times 0.48/24.48 us.

Response to the point 8: The fifth frames were added in the figure for the recording delays with respect to the discharge currents of 0.48 and 24.48 µs, respectively.

9. Line 95: “The global emission of DBD plasma was spectrally resolved by using optical emission spectroscopy (OES) technique”. Again the sentence is wrong. DBD have rich far UV emission as well IR so do not write “global” if you observe the 300-800 nm range.
10. 3. You should explain which part of the spectrum your ICCD is sensitive to. Together with other missing details on the setup, such as what is the viewfiled respect to the electrode geometry and so on. It is also obscure what is the zero in the delay times. It is the rising front of the discharge currant or some random time.

Response to the points 9 and 10: We thank you for the observation. The used word “global” is inappropriate and it was removed, since the ICCD detector allows optical measurements in the spectral range between 300 – 1000 nm, with a maximum sensitivity between 350 – 800 nm. The missing details were added in Section 2.1 (“… ICCD camera produced by Hamamatsu (C8484-05G) that allows optical measurements in the spectral range between 300 – 1000 nm, with a maximum sensitivity between 350 – 800 nm.”).

The view field of ICCD camera covers the entire space between the electrodes. For more clarity a sketch of the electrodes was superimposed on the recorded plasma sequences presented in Figure 2a. The zero-time delay represents the positive rising edge of the positive current peak if we looking to the 1ry plasma region or the negative dropping edge of the negative current peak if we are looking to the 2ry plasma region. The following sentence was modified accordingly: “This signal was triggered by the discharge current pulse, either the positive rising edge or the negative dropping edge, depending on the plasma region that is being investigated.”  

11. Line 180-…: You observe quite dominant emission form nitrogen molecules and ions, but you never mentioned the presence on nitrogen in your gas-phase (that should be pure helium). If you have a substantial amount of air in your gas-phase, you should write what is its concentration, since this affects also your polymerization process.

Response to the point 11: In all experiments, we used high-purity helium gas for both discharge generation and monomer bubbling. Also, the helium gas was blown into the reactor chamber for 5 minutes to remove air impurities prior to the experiment. Nevertheless, a substantial amount of nitrogen remains in the discharge area because no preliminary vacuum pump was used. All these aspects were presented in Section 2.1 (“The experiments were made at room temperature in a laminar flowing of high purity helium gas (99.999%), with a flow rate of 3 l/min. No preliminary vacuum pumping was used. Prior to the experiment, the working gas was blown into the reactor chamber for 5 minutes to remove air impurities. Then, the ethylene glycol monomer vapors were introduced at a consumption rate of 16 µl/min by using an additional helium flow rate of 0.5 l/min for monomer bubbling.”). The presence of nitrogen impurities evidenced by emission spectra was also reported by other authors who used the same DBD configuration and cited it in manuscripts [25, 30] or elsewhere [Hao Zhang, Zimu Xu, Jie Shen, Xu Li, Lili Ding, Jie Ma, Yan Lan, Weidong Xia, Cheng Cheng, Qiang Sun, Zelong Zhang Paul K Chu, Effects and Mechanism of Atmospheric-Pressure Dielectric Barrier Discharge Cold Plasma on Lactate Dehydrogenase (LDH) Enzyme Scientific Reports | 5:10031 | DOI: 10.1038/srep10031].

12. Results, line 206-207: “This decrease in temperature could be attributed to the energy losses of the plasma generated particles, as the inelastic collisions augment with the injection of monomer.” I wonder whether Fig.5 is necessary. You report a flat distribution with large error bars. So the effect of pulse frequency is null. The explanation of the changes in the rotational/vibrational temperatures is questionable. You want/have to explain why the population of the N2(C,v=1) state is depleted respect to that of the N2(C,v=0). Both requires about 13 eV to be excited. And you state this depends on the inelastic collision frequency (of the electrons ? the excited molecules ? other components of the gas-phase ?). If you have a comprehension of the excitation or quenching mechanisms in He and He+EG please share with your readers. If you have measurements about electron energies please present it. Do not write sentences you cannot prove.

Response to the point 12:

Indeed, the determined temperatures show an insensitivity of the plasma temperature to frequency variation. This is the most probably due to the way the spectral measurements were performed, i.e. by time-averaging the optical signal over the used gate width, previously stated in manuscript (“To get deeper insight the kinetics of the plasma species, time-integrated OES measurements were performed within the investigated frequency range.”). See also the response given to comment 13. However, the temperature-frequency dependences are important since it shows that with the addition of monomer the temperature drops all over the investigated frequency range. The paragraph related to the discussion on temperature variations was modified accordingly:

“As the time gap between individual voltage pulses decreases, no significant effects are observed on the plasma temperature, although the previously presented variation of the discharge current with the frequency of the applied voltage (Figure 3) suggests a change in the plasma parameters. This insensitivity of the plasma temperature is probably due to the way the spectral measurements were performed, i.e. the optical signal was time-averaged over the used gate width. Instead, these dependencies clearly show that the introduction of monomer vapors into the discharge led to a decrease of T_rot and T_vib all over the investigated frequency range, along with the optical signal associated to almost all plasma species (except for the He assigned spectral features, see corresponding curves presented in Figure 5c and Figure 5d). For instance, the T_rot drops from approximately 475 K to 350 K (Figure 6a), and the T_vib falls from 2850 K to 2650 K (Figure 6b) with monomer vapors addition. These results agree with the previously presented data related to DBD current recordings (see the discussions related to Figure 3) when the introduction of monomer vapors causes at high frequencies a significant decrease of the discharge current. This drop in electrical current was attributed to an enhancement of the recombination rate of charged particles coming from a plasma-induced monomer fragmentation process. Hence, the decrease in temperature that accompanies the electrical current drop could be attributed to the same process of plasma-induced monomer fragmentation, where the plasma generated particles lose their energies as the inelastic collisions augment with the injection of monomer.”

The paragraph related to the discussion on "the excitation or quenching mechanism" to which you refer was retracted: “According to Masoud and colleagues [37], in a typical DBD plasma the population of vibrational levels is determined by electron impact excitation, and the resulting vibrational temperature represents a quantity sensitive to changes in the energy distribution function of the electrons. Instead, the population of the rotational levels is performed via neutral-neutral or neutral-excited molecule collisions. As a result, the rotational temperature represents an indicator of the kinetic energy of the neutrals in the plasma. This is consistent with explanations proposed for the vibrational and rotational temperatures dropping with the addition of monomer.”

 

13. Figure4: it seems to me that the main effect of increasing the pulse frequency is to increase the number of discharges and so the overall energy provided to the system. Could you plot the results as intensity of after a single (or fixed number) of pulse. This would help to understand the effect of the plasma much more.

Response to the point 13: The reviewer is right; by increasing the pulse frequency, the number of discharges increases per unit time. The plot of the results after a single (or fixed number) of pulses is not possible, since spectral measurements were performed by time-averaging the optical signal within the gate-time used for spectral recordings. An additional comment was added to manuscript to present the variation the intensities of different plasma species with frequency:

“As expected, irrespective of the working gas composition, the intensities of all DBD plasma species increase monotonically with discharge frequency because more discharge pulses are caught during the gate width for which the optical signal was recorded. In-stead, the intensities of the most representative plasma species plotted as a function of discharge frequency (Figure 5c and Figure 5d) follow a drop trend with the addition of monomer, except for the intensities of the He spectral lines that remain constant.”

Reviewer 3 Report

The authors present the correlation between the porosity of the PEG film with the monomer fragmentation and link this fragmentation to the frequency. The topic is relevant and interesting for the field and this correlation can be convincing. However increasing the frequency they increase also the power delivered to the discharge, therefore in order to draw a conclusion on frequency effect also the power effect on fragmentation has to be investigated and presented. Therefore before acceptance, in order to allow their conclusions the authors should add further experiments related to power effects.

In addition to this overall observation I would add some specific issues:

1. maximum light emission usually is close to the cathode, in the article it is not clear if this is the case. The misunderstanding is also due to mismatch between fig 1b where light of the primary discharge is on the top electrode and fig. 3 where is on the bottom electrode. May a skectch of the electrical connections of the apparatus would help the understanding.

2. It is missing the detail on how I,V are recorded

Author Response

The authors present the correlation between the porosity of the PEG film with the monomer fragmentation and link this fragmentation to the frequency. The topic is relevant and interesting for the field and this correlation can be convincing. However increasing the frequency they increase also the power delivered to the discharge, therefore in order to draw a conclusion on frequency effect also the power effect on fragmentation has to be investigated and presented. Therefore before acceptance, in order to allow their conclusions the authors should add further experiments related to power effects.

Authors’ response: Indeed, with the increase in frequency, the power delivered to the discharge increases because the number of discharges per time unit increases. To establish the determining factor that acts on the monomer fragmentation, additional investigations will be considered in a future study following systematic measurements at different power delivered to the discharge (changing the amplitude of applied voltage) for a constant frequency.

1. maximum light emission usually is close to the cathode, in the article it is not clear if this is the case. The misunderstanding is also due to mismatch between fig 1b where light of the primary discharge is on the top electrode and fig. 3 where is on the bottom electrode. May a skectch of the electrical connections of the apparatus would help the understanding.

Response to the point 2: For more clarity more details about the performed measurements were added in Section 2.1 of the manuscript together with a sketch of the set-up (the new Figure 1). Also, a sketch of the electrodes superimposed on the recorded plasma sequences was presented in Figure 2a.

2. It is missing the detail on how I,V are recorded

Response to the point 3: We believe that the new added sketch (Figure 1) clarifies the missing aspects related to the I-V recordings.

Round 2

Reviewer 1 Report

1. Actually, I want to know the degradation temperature of the monomer. Please present more details.

2. If the the monomer fragmentation mechanism is not considered, how to control the fragmentation mechanism during DBD plasma processing?

Author Response

Response to the Reviewer 1.

1. Actually, I want to know the degradation temperature of the monomer. Please present more details.

Response to the point 1:

The maximum plasma temperature, also known as thermal kinetic energy per particle, revealed by OES measurements for the DBD plasma operating in He is much lower than the breaking bound energy of the C-C chemical group in the EG monomeric unit. This rules out a monomer-complete decomposition caused by plasma particles' interaction with injected monomer.

Regarding the prevalence of a thermal effect of DBD exhibited on the polymerized film or on the monomer, the dielectric barrier discharge (DBD) involves the generation of a cold plasma frequently used in the treatment of thermally sensitive biological molecules, like proteins and enzymes [Innovative Food Science and Emerging Technologies 29, 247–254 (2015); RSC Advances, 3(31), 12540–12567(2013)], or in skin therapy [Skin Pharmacol Physiol 33, 69–76 (2020); Frontiers in Oncology 12, 918484 (2022)]. Therefore, it is highly improbable that the temperature of the DBD plasma will reach a level close to the EG's degradation temperature, which is about 280 °C [J. Am. Chem. Soc. 59, 2521-2525 (1937)].

 

2. If the the monomer fragmentation mechanism is not considered, how to control the fragmentation mechanism during DBD plasma processing?

Response to the point 2:

Your observation is right. The understanding of monomer fragmentation mechanism allows to control preservation of the monomer functionality. A new paragraph about the mechanism of ethylene glycol fragmentation given in the literature has been added:

“DBD plasma polymerization involves a complex series of fundamental chemical and physical processes that occur between the plasma-generated species (monomer fragmentation, random recombination, and diffusion of generated fragments) and at the surface/plasma interface [13]. The chemistry in the gas phase related to the fragmentation mechanisms of the monomer is a key issue addressed in the literature, on the understanding of which depend the preservation of the functionality of the monomer or its complete degradation [41,42]. Previous research has revealed a mechanism for EG fragmentation by electronic impact, with a high probability of breaking the EG monomer skeleton at the C-Cs and C-O covalent bounds and the subsequent formation of various types of radicals such as CH*, CH2*, C2H4*, C2H4OH*, CH2OH*, and OH* [43]. Later, by recombination with neutral plasma species, these radicals can form larger molecular fragments and contribute to the growth of the pp-PEG film.”

Reviewer 2 Report

The manuscript  describes the plasma deposition of polyethylene glycol films with dielectric barrier discharges. The paper was substantially revised and it is interesting enough to deserve publication.

Author Response

We would like to thank you for your useful comments and suggestions that greatly improve the quality of the manuscript.

Reviewer 3 Report

The authors addressed all the risen issues and improved widely the paper.

Author Response

We would like to thank you for your useful comments and suggestions that greatly improve the quality of the manuscript.