Data from: pHDependent friction of Polyacrylamide hydrogels
Data files
Sep 12, 2023 version files 402.77 KB

2023_Chau_TribLetters_Data.zip

README.md
Abstract
Polyacrylamide hydrogels are widely used in biomedical applications due to their tunable mechanical properties and charge neutrality. Our recent tribological investigations of polyacrylamide gels have revealed tunable and pHdependent friction behavior. To determine the origins of this pHresponsiveness, we prepared polyacrylamide hydrogels with two different initiating chemistries: a reductionoxidation (redox)initiated system using ammonium persulfate (APS) and N,N,N’N’tetramethylethylenediamine (TEMED) and a UVinitiated system with 2hydroxy4’(2hydroxyethoxy)2methylpropiophenone (Irgacure 2959). Hydrogel swelling, mechanical properties, and tribological behavior were investigated in response to solution pH (ranging from ~ 0.34 to 13.5). For polyacrylamide hydrogels in sliding contact with glass hemispherical probes, friction coefficients decreased from µ = 0.07 ± 0.02 to µ = 0.002 ± 0.002 (redoxinitiated) and from µ = 0.05 ± 0.03 to µ = 0.003 ± 0.003 (UVinitiated) with increasing solution pH. With hemispherical polytetrafluoroethylene (PTFE) probes, friction coefficients of redoxinitiated hydrogels similarly decreased from µ = 0.06 ± 0.01 to µ = 0.002 ± 0.001 with increasing pH. Raman spectroscopy measurements demonstrated hydrolysis and the conversion of amide groups to carboxylic acid in basic conditions. We therefore propose that the mechanism for pHresponsive friction in polyacrylamide hydrogels may be credited to hydrolysisdriven swelling through the conversion of side chain amide groups into carboxylic groups and/or crosslinker degradation. Our results could assist in the rational design of hydrogelbased tribological pairs for biomedical applications from acidic to alkaline conditions.
README
Data from peerreviewed article:
Title: pHDependent Friction of Polyacrylamide Hydrogels
Journal: Tribology Letters
Authors: Allison L. Chau, Conor D. Pugsley, Madeleine E. Miyamoto, Yongkui Tang, Claus D. Eisenbach, Thomas E. Mates, Craig J. Hawker, Megan T. Valentine, and Angela A. Pitenis
Corresponding author: Angela Pitenis, apitenis@ucsb.edu
File List
A) Fig_2a.csv
B) Fig_2b.csv
C) Fig_3a.csv
D) Fig_3b.csv
E) Fig_5_FigS4.csv
F) Fig_S1.txt
G) Fig_S2.csv
H) Fig_S3.csv
I) Fig_S5a.csv
J) Fig_S5b.csv
K) Fig_S5c.csv
L) Fig_S5d.csv
M) Fig_S5e.csv
N) Fig_S5f.csv
O) Fig_S5gcsv
P) Fig_S5h.csv
Q) Fig_S6.csv
R) Fig_S7a.csv
S) Fig_S7b.csv
T) Friction_Irgacure.csv
U) Friction_TEMED_APS.csv
V) Modulus_Irgacure.csv
W) Modulus_TEMED_APS.csv
X) Swelling_Irgacure.csv
Y) Swelling_TEMED_APS.csv
Z) Volume_change_TEMED_APS.csv
FIGURE 2: friction and reduced elastic modulus data
A) Fig_2a.csv: friction coefficient vs. solution pH
 row 1: x and y axis, corresponding to solution pH (x) and friction coefficient average (y) and friction coefficient standard deviation (+/ y)
 row 2: initiating system (redox [TEMED + APS] or UV initiated [Irgacure]) and probe material (glass or PTFE)
 row 3 and beyond: data
 dataset 1: redox (TEMED + APS) hydrogels with glass probe  columns A + B + C
 dataset 2: redox (TEMED + APS) hydrogels with PTFE probe  columns A + D + E
 dataset 3: UV initiated (Irgacure) hydrogels with glass probe  columns A + F + G
 cells with n/a: data was not gathered for those conditions (pH = 2, pH = 10)
 Notes: data from Friction_Irgacure.csv and Friction_TEMED_APS.csv
B) Fig_2b.csv: reduced elastic modulus vs. solution pH
 row 1: x and y axis, corresponding to solution pH (x) and reduced elastic modulus average (y) and reduced elastic modulus standard deviation (+/ y)
 row 2: units for corresponding axes
 row 3: initiating system (redox [TEMED + APS] or UV initiated [Irgacure])
 row 4 and beyond: data
 dataset 1: redox (TEMED + APS) hydrogels with glass probe  columns A + B + C
 dataset 2: UV initiated (Irgacure) hydrogels with glass probe  columns A + D + E
 cells with n/a: data was not gathered for those conditions (UV initiated gels, pH = 13.5)
 Notes: data from Modulus_Irgacure.csv and Modulus_TEMED_APS.csv
FIGURE 3: swelling data
C) Fig_3a.csv: water content vs. solution pH
 row 1: x and y axis, corresponding to solution pH (x) and water content average (y) and water content standard deviation (+/ y)
 row 2: initiating system (redox [TEMED + APS] or UV initiated [Irgacure])
 row 3 and beyond: data
 dataset 1: redox (TEMED + APS) hydrogels  columns A + B + C
 dataset 2: UV initiated (Irgacure) hydrogels  columns A + D + E
 cells with n/a: data was not gathered for those conditions (UV initiated gels, pH = 13.5)
 Notes: data from Swelling_Irgacure.csv and Swelling_TEMED_APS.csv
D) Fig_3b.csv: volume change vs. solution pH
 row 1: x and y axis, corresponding to solution pH (x) and volume change average (y) and volume change standard deviation (+/ y)
 row 2: initiating system (redox [TEMED + APS])
 row 3 and beyond: data
 dataset 1: redox (TEMED + APS) hydrogels  columns A + B + C
 Notes: data from Volume_change_TEMED_APS.csv
FIGURE 5: Raman spectroscopy data (same data as shown in Figure S4)
E) Fig_5_FigS4.csv: Raman spectroscopy normalized intensity vs. wavenumber
 row 1: x and y axis, corresponding to wavenumber (x) and normalized intensity (y)
 row 2: units for corresponding axes
 row 3: solution (0.5 M HCl, DI water, or 0.5 M NaOH) or hydrogel type (polyacrylamidecoacrylic acid [P(AAmcoAA)]) with AA concentration (1, 5, 6, 9, or 12 wt)
 row 4 and beyond: data
 dataset 1: redox (TEMED + APS) hydrogels in 0.5 M HCl  columns A + B
 dataset 2: redox (TEMED + APS) hydrogels in DI water  columns C + D
 dataset 3: redox (TEMED + APS) hydrogels in 0.5 M NaOH  columns E + F
 dataset 4: P(AAmcoAA) hydrogel with 1 wt AA in DI water  columns G + H
 dataset 5: P(AAmcoAA) hydrogel with 5 wt AA in DI water  columns I + J
 dataset 6: P(AAmcoAA) hydrogel with 6 wt AA in DI water  columns K + L
 dataset 7: P(AAmcoAA) hydrogel with 9 wt AA in DI water  columns M + N
 dataset 8: P(AAmcoAA) hydrogel with 12 wt AA in DI water  columns O + P
FIGURE S1: surface topography map information
F) Fig_S1.txt > surface topography map information
 information about how the surface topography map was processed as well as the sample identifier (internal use)
FIGURE S2: xray photoelectron spectroscopy data
G) Fig_S2.csv > xray photoelectron spectroscopy counts vs. binding energy
 row 1: x and y axis, corresponding to binding energy (x) and counts per second (y)
 row 2: units for corresponding axes
 row 3: sample (acrylamide [AAm] powder, ammonium persulfate [APS] powder, bisacrylamide [MBAm] powder, or polyacrylamide [PAAm] hydrogel)
 row 4 and beyond: data
 dataset 1: AAm powder  columns A + B
 dataset 2: APS powder  columns C + D
 dataset 3: MBAm powder  columns E + F
 dataset 4: PAAm hydrogel (argon etched for 105 s)  columns G + H
FIGURE S3: Raman spectroscopy data
H) Fig_S3.csv > Raman spectroscopy normalized intensity vs. wavenumber
 row 1: x and y axis, corresponding to wavenumber (x) and normalized intensity (y)
 row 2: units for corresponding axes
 row 3: solution (0.5 M HCl, DI water, or 0.5 M NaOH) or hydrogel type (polyacrylamidecoacrylic acid [P(AAmcoAA)]) with AA concentration (1, 5, 6, 9, or 12 wt)
 row 4 and beyond: data
 dataset 1: redox (TEMED + APS) hydrogels in 0.5 M HCl for three gels  columns A (x) + B (y for gel 1) + C (y for gel 2) + D (y for gel 3)
 dataset 2: redox (TEMED + APS) hydrogels in DI water for three gels  columns E (x) + F (y for gel 1) + G (y for gel 2) + H (y for gel 3)
 dataset 3: redox (TEMED + APS) hydrogels in 0.5 M NaOH for three gels  columns I (x) + J (y for gel 1) + K (y for gel 2) + L (y for gel 3)
 dataset 4: P(AAmcoAA) hydrogel with 1 wt AA in DI water for three gels  columns M (x) + N (y for gel 1) + O (y for gel 2) + P (y for gel 3)
 dataset 5: P(AAmcoAA) hydrogel with 5 wt AA in DI water for three gels  columns Q (x) + R (y for gel 1) + S (y for gel 2) + T (y for gel 3)
 dataset 6: P(AAmcoAA) hydrogel with 6 wt AA in DI water for three gels  columns U (x) + V (y for gel 1) + W (y for gel 2) + X (y for gel 3)
 dataset 7: P(AAmcoAA) hydrogel with 9 wt AA in DI water for three gels  columns Y (x) + Z (y for gel 1) + AA (y for gel 2) + AB (y for gel 3)
 dataset 8: P(AAmcoAA) hydrogel with 12 wt AA in DI water for three gels  columns AC (x) + AD (y for gel 1) + AE (y for gel 2) + AF (y for gel 3)
FIGURE S5: Raman band deconvolution data
I) Fig_S5a.csv > Raman band deconvolution curve fitting  PAAm gels at pH = 0.34
 row 1: x and y axis, corresponding to wavenumber (x) and normalized intensity (y)
 row 2: units for corresponding axes
 row 3: data indentifier (peak number, cumulative fit, or measurement)
 row 4 and beyond: data
 dataset 1: peak fit 1  columns A + B
 dataset 2: peak fit 2  columns C + D
 dataset 3: peak fit 3  columns E + F
 dataset 4: peak fit 4  columns G + H
 dataset 5: peak fit 5  columns I + J
 dataset 6: peak fit 6  columns K + L
 dataset 7: peak fit 7  columns M + N
 dataset 8: peak fit 8  columns O + P
 dataset 9: cumulative peak fit  columns Q + R
 dataset 10: measurement  columns S + T
J) Fig_S5b.csv > Raman band deconvolution curve fitting  PAAm gels at pH = 7
 row 1: x and y axis, corresponding to wavenumber (x) and normalized intensity (y)
 row 2: units for corresponding axes
 row 3: data indentifier (peak number, cumulative fit, or measurement)
 row 4 and beyond: data
 dataset 1: peak fit 1  columns A + B
 dataset 2: peak fit 2  columns C + D
 dataset 3: peak fit 3  columns E + F
 dataset 4: peak fit 4  columns G + H
 dataset 5: peak fit 5  columns I + J
 dataset 6: peak fit 6  columns K + L
 dataset 9: cumulative peak fit  columns M + N
 dataset 10: measurement  columns O + P
K) Fig_S5c.csv > Raman band deconvolution curve fitting  P(AAmcoAA) gel with 1 wt AA
 dataset 1: peak fit 1  columns A + B
 dataset 2: peak fit 2  columns C + D
 dataset 3: peak fit 3  columns E + F
 dataset 4: peak fit 4  columns G + H
 dataset 5: peak fit 5  columns I + J
 dataset 6: peak fit 6  columns K + L
 dataset 7: peak fit 7  columns M + N
 dataset 8: peak fit 8  columns O + P
 dataset 9: cumulative peak fit  columns Q + R
 dataset 10: measurement  columns S + T
L) Fig_S5d.csv > Raman band deconvolution curve fitting  P(AAmcoAA) gel with 5 wt AA
 dataset 1: peak fit 1  columns A + B
 dataset 2: peak fit 2  columns C + D
 dataset 3: peak fit 3  columns E + F
 dataset 4: peak fit 4  columns G + H
 dataset 5: peak fit 5  columns I + J
 dataset 6: peak fit 6  columns K + L
 dataset 7: peak fit 7  columns M + N
 dataset 8: peak fit 8  columns O + P
 dataset 9: cumulative peak fit  columns Q + R
 dataset 10: measurement  columns S + T
M) Fig_S5e.csv > Raman band deconvolution curve fitting  P(AAmcoAA) gel with 6 wt AA
 dataset 1: peak fit 1  columns A + B
 dataset 2: peak fit 2  columns C + D
 dataset 3: peak fit 3  columns E + F
 dataset 4: peak fit 4  columns G + H
 dataset 5: peak fit 5  columns I + J
 dataset 6: peak fit 6  columns K + L
 dataset 7: peak fit 7  columns M + N
 dataset 8: peak fit 8  columns O + P
 dataset 9: cumulative peak fit  columns Q + R
 dataset 10: measurement  columns S + T
N) Fig_S5f.csv > Raman band deconvolution curve fitting  P(AAmcoAA) gel with 9 wt AA
 dataset 1: peak fit 1  columns A + B
 dataset 2: peak fit 2  columns C + D
 dataset 3: peak fit 3  columns E + F
 dataset 4: peak fit 4  columns G + H
 dataset 5: peak fit 5  columns I + J
 dataset 6: peak fit 6  columns K + L
 dataset 7: peak fit 7  columns M + N
 dataset 8: peak fit 8  columns O + P
 dataset 9: cumulative peak fit  columns Q + R
 dataset 10: measurement  columns S + T
O) Fig_S5g.csv > Raman band deconvolution curve fitting  P(AAmcoAA) gel with 12 wt AA
 dataset 1: peak fit 1  columns A + B
 dataset 2: peak fit 2  columns C + D
 dataset 3: peak fit 3  columns E + F
 dataset 4: peak fit 4  columns G + H
 dataset 5: peak fit 5  columns I + J
 dataset 6: peak fit 6  columns K + L
 dataset 7: peak fit 7  columns M + N
 dataset 8: peak fit 8  columns O + P
 dataset 9: cumulative peak fit  columns Q + R
 dataset 10: measurement  columns S + T
P) Fig_S5h.csv > Raman band deconvolution curve fitting  PAAm gels at pH = 13.5
 dataset 1: peak fit 1  columns A + B
 dataset 2: peak fit 2  columns C + D
 dataset 3: peak fit 3  columns E + F
 dataset 4: peak fit 4  columns G + H
 dataset 5: peak fit 5  columns I + J
 dataset 6: peak fit 6  columns K + L
 dataset 7: peak fit 7  columns M + N
 dataset 8: peak fit 8  columns O + P
 dataset 9: peak fit 9  columns Q + R
 dataset 10: cumulative peak fit  columns S + T
 dataset 11: measurement  columns U + V
FIGURE S6: Raman linear calibration curve data
Q) Fig_S6.csv > Raman linear calibration curve data
 Table 1: ratio of peaks vs. acrylic acid mol percent (A1A9, E1E9)
 Column A: hydrogel sample (polyacrylamide [PAAm] or poly(acrylamidecoacrylic acid) [P(AAmcoAA)] and solution (DI water, HCl, or NaOH)
 Column B: acrylic acid (AA) mol% average
 Column C: acrylic acid (AA) mol% standard deviation
 Column D: ratio of peak areas (peak at 1555 cm^1/peaks at 1670 cm^1 and 1716 cm^1) average
 Column E: ratio of peak areas (peak at 1555 cm^1/peaks at 1670 cm^1 and 1716 cm^1) standard deviation
 cells with n/a: no standard deviation for those conditions (P(AAmcoAA) gels)
 Table 2: ratio of peaks vs. ratio of acrylic acid/amide (G1G9, J1J9)
 Column A: ratio of acrylic acid mol/amide mol average
 Column B: ratio of acrylic acid mol/amide mol standard deviation
 Column C: ratio of peak areas (peak at 1555 cm^1/peaks at 1617 cm^1) average
 Column D: ratio of peak areas (peak at 1555 cm^1/peaks at 1617 cm^1) standard deviation
 cells with n/a: no standard deviation for those conditions (P(AAmcoAA) gels)
FIGURE S7: switchable friction data
R) Fig_S7a.csv > friction coefficient vs. solution pH  switchable friction data
 Table 1: raw data for hydrogel with TEMED + APS initiating system (A2A7, K2K7)
 Column A: sample identifer (internal use)
 Column B: solution pH (pH = 0.35, 7, or 10)
 Column C + D: normal force average and standard deviation
 Column E + F: friction force average and standard deviation
 Column G + H: kinetic friction coefficient average and standard deviation
 Column I: cycle # where analysis begins (internal use)
 Column J: number of cycles analyzed
 Column K: range of x position analyzed on the friction force loop (internal use)
 Table 2: raw data for hydrogel with Irgacure initiating system (A9A14, K9K14)
 Column A: sample identifer (internal use)
 Column B: solution pH (pH = 0.35, 7, or 10)
 Column C + D: normal force average and standard deviation
 Column E + F: friction force average and standard deviation
 Column G + H: kinetic friction coefficient average and standard deviation
 Column I: cycle # where analysis begins (internal use)
 Column J: number of cycles analyzed
 Column K: range of x position analyzed on the friction force loop (internal use)
 Table 3: data for Figure S7a (A19A23, E19E23)
 Column A: solution pH
 Column B + C: friction coefficient average and standard deviation for the hydrogel with TEMED + APS initiating system
 Column D + E: friction coefficient average and standard deviation for the hydrogel with Irgacure initiating system
S) Fig_S7b.csv > reduced elastic modulus vs. solution pH  TEMED + APS initiated gels
 Table 1: elastic modulus for PAAm sample in pH = 0.35 (0.5 M HCl) (B2B24, J2J24)
 Column B: sample identifier (internal use)
 Column C: position
 Column D: cycle number
 Column E: maximum force reached during analysis
 Column F: maximum contact pressure reached during analysis
 Column G: maximum contact radius reached during analysis
 Column H: estimated reduced elastic modulus using the Hertz model
 Column I + J: averages and standard deviation of reduced elastic modulus per position
 cells with n/a: no cycle data at the specified position
 Table 2: elastic modulus for PAAm sample in pH = 7 (DI water) (L2L24, T2T24)
 Column L: sample identifier (internal use)
 Column M: position
 Column N: cycle number
 Column O: maximum force reached during analysis
 Column P: maximum contact pressure reached during analysis
 Column Q: maximum contact radius reached during analysis
 Column R: estimated reduced elastic modulus using the Hertz model
 Column S + T: averages and standard deviation of reduced elastic modulus per position
 Table 3: elastic modulus for PAAm sample in pH = 10.01 (0.001 M NaOH) (V2V24, AD2AD24)
 Column V: sample identifier (internal use)
 Column W: position
 Column X: cycle number
 Column Y: maximum force reached during analysis
 Column Z: maximum contact pressure reached during analysis
 Column AA: maximum contact radius reached during analysis
 Column AB: estimated reduced elastic modulus using the Hertz model
 Column AC + AD: averages and standard deviation of reduced elastic modulus per position
 cells with n/a: no cycle data at the specified position
 Table 4: data for Figure S7a (A28A33, C28C33)
 Column A: solution pH
 Column B + C: elastic modulus average and standard deviation for the hydrogel with TEMED + APS initiating system
Compilation of raw friction coefficient data
T) Friction_Irgacure.csv
 Table 1: raw data for hydrogel with UV (Irgacure) initiating system at pH = 0.34 (0.5 M HCl) with glass probe (A1A9, K1K9)
 Column A: hydrogel type (polyacrylamide)
 Column B: sample identifier (internal use)
 Column C + D: normal force average and standard deviation
 Column E + F: friction force average and standard deviation
 Column G + H: kinetic friction coefficient average and standard deviation
 Column I: cycle # where analysis begins (internal use)
 Column J: number of cycles analyzed
 Column K: range of x position analyzed on the friction force loop (internal use)
 Table 2: raw data for hydrogel with UV (Irgacure) initiating system at pH = 2 (0.01 M HCl) with glass probe (A12A19, K12K19)
 Column A: hydrogel type (polyacrylamide)
 Column B: sample identifier (internal use)
 Column C + D: normal force average and standard deviation
 Column E + F: friction force average and standard deviation
 Column G + H: kinetic friction coefficient average and standard deviation
 Column I: cycle # where analysis begins (internal use)
 Column J: number of cycles analyzed
 Column K: range of x position analyzed on the friction force loop (internal use)
 Table 3: raw data for hydrogel with UV (Irgacure) initiating system at pH = 7 (DI water) with glass probe (A22A29, K22K29)
 Column A: hydrogel type (polyacrylamide)
 Column B: sample identifier (internal use)
 Column C + D: normal force average and standard deviation
 Column E + F: friction force average and standard deviation
 Column G + H: kinetic friction coefficient average and standard deviation
 Column I: cycle # where analysis begins (internal use)
 Column J: number of cycles analyzed
 Column K: range of x position analyzed on the friction force loop (internal use)
 Table 4: raw data for hydrogel with UV (Irgacure) initiating system at pH = 10 (0.001 M NaOH) with glass probe (A32A39, K32K39)
 Column A: hydrogel type (polyacrylamide)
 Column B: sample identifier (internal use)
 Column C + D: normal force average and standard deviation
 Column E + F: friction force average and standard deviation
 Column G + H: kinetic friction coefficient average and standard deviation
 Column I: cycle # where analysis begins (internal use)
 Column J: number of cycles analyzed
 Column K: range of x position analyzed on the friction force loop (internal use)
U) Friction_TEMED_APS.csv
 Table 1: raw data for hydrogel with TEMED + APS initiating system at pH = 0.34 (0.5 M HCl) with glass probe (A2A10, K2K10)
 Column A: hydrogel type (polyacrylamide)
 Column B: sample identifier (internal use)
 Column C + D: normal force average and standard deviation
 Column E + F: friction force average and standard deviation
 Column G + H: kinetic friction coefficient average and standard deviation
 Column I: cycle # where analysis begins (internal use)
 Column J: number of cycles analyzed
 Column K: range of x position analyzed on the friction force loop (internal use)
 Table 2: raw data for hydrogel with TEMED + APS initiating system at pH = 2 (0.01 M HCl) with glass probe (A12A19, K12K19)
 Column A: hydrogel type (polyacrylamide)
 Column B: sample identifier (internal use)
 Column C + D: normal force average and standard deviation
 Column E + F: friction force average and standard deviation
 Column G + H: kinetic friction coefficient average and standard deviation
 Column I: cycle # where analysis begins (internal use)
 Column J: number of cycles analyzed
 Column K: range of x position analyzed on the friction force loop (internal use)
 Table 3: raw data for hydrogel with TEMED + APS initiating system at pH = 7 (DI water) with glass probe (A21A28, K21K28)
 Column A: hydrogel type (polyacrylamide)
 Column B: sample identifier (internal use)
 Column C + D: normal force average and standard deviation
 Column E + F: friction force average and standard deviation
 Column G + H: kinetic friction coefficient average and standard deviation
 Column I: cycle # where analysis begins (internal use)
 Column J: number of cycles analyzed
 Column K: range of x position analyzed on the friction force loop (internal use)
 Table 4: raw data for hydrogel with TEMED + APS initiating system at pH = 10 (0.001 M NaOH) with glass probe (A30A37, K30K37)
 Column A: hydrogel type (polyacrylamide)
 Column B: sample identifier (internal use)
 Column C + D: normal force average and standard deviation
 Column E + F: friction force average and standard deviation
 Column G + H: kinetic friction coefficient average and standard deviation
 Column I: cycle # where analysis begins (internal use)
 Column J: number of cycles analyzed
 Column K: range of x position analyzed on the friction force loop (internal use)
 Table 5: raw data for hydrogel with TEMED + APS initiating system at pH = 13.5 (0.5 M NaOH) with glass probe (A39A46, K39K46)
 Column A: hydrogel type (polyacrylamide)
 Column B: sample identifier (internal use)
 Column C + D: normal force average and standard deviation
 Column E + F: friction force average and standard deviation
 Column G + H: kinetic friction coefficient average and standard deviation
 Column I: cycle # where analysis begins (internal use)
 Column J: number of cycles analyzed
 Column K: range of x position analyzed on the friction force loop (internal use)
 Table 6: raw data for hydrogel with TEMED + APS initiating system at pH = 0.34 (0.5 M HCl) with PTFE probe (N2N10, Y2Y10)
 Column N: hydrogel type (polyacrylamide)
 Column O: sample identifier (internal use)
 Column P: date/attempt (internal use)
 Column Q + R: normal force average and standard deviation
 Column S + T: friction force average and standard deviation
 Column U + V: kinetic friction coefficient average and standard deviation
 Column W: cycle # where analysis begins (internal use)
 Column X: number of cycles analyzed
 Column Y: range of x position analyzed on the friction force loop (internal use)
 Table 7: raw data for hydrogel with TEMED + APS initiating system at pH = 7 (DI water) with PTFE probe (N12N19, Y12Y19)
 Column N: hydrogel type (polyacrylamide)
 Column O: sample identifier (internal use)
 Column P: date/attempt (internal use)
 Column Q + R: normal force average and standard deviation
 Column S + T: friction force average and standard deviation
 Column U + V: kinetic friction coefficient average and standard deviation
 Column W: cycle # where analysis begins (internal use)
 Column X: number of cycles analyzed
 Column Y: range of x position analyzed on the friction force loop (internal use)
 Table 8: raw data for hydrogel with TEMED + APS initiating system at pH = 13.5 (0.5 M NaOH) with PTFE probe (N21N28, Y21Y28)
 Column N: hydrogel type (polyacrylamide)
 Column O: sample identifier (internal use)
 Column P: date/attempt (internal use)
 Column Q + R: normal force average and standard deviation
 Column S + T: friction force average and standard deviation
 Column U + V: kinetic friction coefficient average and standard deviation
 Column W: cycle # where analysis begins (internal use)
 Column X: number of cycles analyzed
 Column Y: range of x position analyzed on the friction force loop (internal use)
Compilation of raw elastic modulus data
V) Modulus_Irgacure.csv
 Table 1: raw data for hydrogel sample 1 with Irgacure initiating system at pH = 0.34 (0.5 M HCl) (A2A24, I2I24)
 Column A: hydrogel type (polyacrylamide)
 Column B: position
 Column C: cycle number
 Column D: maximum force reached during analysis
 Column E: maximum contact pressure reached during analysis
 Column F: maximum contact radius reached during analysis
 Column G: estimated reduced elastic modulus using the Hertz model
 Column I + J: averages and standard deviation of reduced elastic modulus per position
 Table 2: raw data for hydrogel sample 2 with Irgacure initiating system at pH = 0.34 (0.5 M HCl) (K2K24, S2S24)
 Column K: hydrogel type (polyacrylamide)
 Column L: position
 Column M: cycle number
 Column N: maximum force reached during analysis
 Column O: maximum contact pressure reached during analysis
 Column P: maximum contact radius reached during analysis
 Column Q: estimated reduced elastic modulus using the Hertz model
 Column R + S: averages and standard deviation of reduced elastic modulus per position
 Table 3: raw data for hydrogel sample 3 with Irgacure initiating system at pH = 0.34 (0.5 M HCl) (U2U24, AC2AC24)
 Column U: hydrogel type (polyacrylamide)
 Column V: position
 Column W: cycle number
 Column X: maximum force reached during analysis
 Column Y: maximum contact pressure reached during analysis
 Column Z: maximum contact radius reached during analysis
 Column AA: estimated reduced elastic modulus using the Hertz model
 Column AB + AC: averages and standard deviation of reduced elastic modulus per position
 Table 4: raw data for hydrogel sample 1 with Irgacure initiating system at pH = 2 (0.01 M HCl) (A27A49, I27I49)
 Column A: hydrogel type (polyacrylamide)
 Column B: position
 Column C: cycle number
 Column D: maximum force reached during analysis
 Column E: maximum contact pressure reached during analysis
 Column F: maximum contact radius reached during analysis
 Column G: estimated reduced elastic modulus using the Hertz model
 Column I + J: averages and standard deviation of reduced elastic modulus per position
 Table 5: raw data for hydrogel sample 2 with Irgacure initiating system at pH = 2 (0.01 M HCl) (K27K49, S27S49)
 Column K: hydrogel type (polyacrylamide)
 Column L: position
 Column M: cycle number
 Column N: maximum force reached during analysis
 Column O: maximum contact pressure reached during analysis
 Column P: maximum contact radius reached during analysis
 Column Q: estimated reduced elastic modulus using the Hertz model
 Column R + S: averages and standard deviation of reduced elastic modulus per position
 Table 6: raw data for hydrogel sample 3 with Irgacure initiating system at pH = 2 (0.01 M HCl) (U27U49, AC27AC49)
 Column U: hydrogel type (polyacrylamide)
 Column V: position
 Column W: cycle number
 Column X: maximum force reached during analysis
 Column Y: maximum contact pressure reached during analysis
 Column Z: maximum contact radius reached during analysis
 Column AA: estimated reduced elastic modulus using the Hertz model
 Column AB + AC: averages and standard deviation of reduced elastic modulus per position
 Table 7: raw data for hydrogel sample 1 with Irgacure initiating system at pH = 7 (DI water) (A52A74, I52I74)
 Column A: hydrogel type (polyacrylamide)
 Column B: position
 Column C: cycle number
 Column D: maximum force reached during analysis
 Column E: maximum contact pressure reached during analysis
 Column F: maximum contact radius reached during analysis
 Column G: estimated reduced elastic modulus using the Hertz model
 Column I + J: averages and standard deviation of reduced elastic modulus per position
 Table 8: raw data for hydrogel sample 2 with Irgacure initiating system at pH = 7 (DI water) (K52K74, S52S74)
 Column K: hydrogel type (polyacrylamide)
 Column L: position
 Column M: cycle number
 Column N: maximum force reached during analysis
 Column O: maximum contact pressure reached during analysis
 Column P: maximum contact radius reached during analysis
 Column Q: estimated reduced elastic modulus using the Hertz model
 Column R + S: averages and standard deviation of reduced elastic modulus per position
 Table 9: raw data for hydrogel sample 3 with Irgacure initiating system at pH = 7 (DI water) (U52U74, AC52AC74)
 Column U: hydrogel type (polyacrylamide)
 Column V: position
 Column W: cycle number
 Column X: maximum force reached during analysis
 Column Y: maximum contact pressure reached during analysis
 Column Z: maximum contact radius reached during analysis
 Column AA: estimated reduced elastic modulus using the Hertz model
 Column AB + AC: averages and standard deviation of reduced elastic modulus per position
 Table 10: raw data for hydrogel sample 1 with Irgacure initiating system at pH = 10 (0.001 M NaOH) (A77A99, I77I99)
 Column A: hydrogel type (polyacrylamide)
 Column B: position
 Column C: cycle number
 Column D: maximum force reached during analysis
 Column E: maximum contact pressure reached during analysis
 Column F: maximum contact radius reached during analysis
 Column G: estimated reduced elastic modulus using the Hertz model
 Column I + J: averages and standard deviation of reduced elastic modulus per position
 Table 11: raw data for hydrogel sample 2 with Irgacure initiating system at pH = 10 (0.001 M NaOH) (K77K99, S77S99)
 Column K: hydrogel type (polyacrylamide)
 Column L: position
 Column M: cycle number
 Column N: maximum force reached during analysis
 Column O: maximum contact pressure reached during analysis
 Column P: maximum contact radius reached during analysis
 Column Q: estimated reduced elastic modulus using the Hertz model
 Column R + S: averages and standard deviation of reduced elastic modulus per position
 Table 12: raw data for hydrogel sample 3 with Irgacure initiating system at pH = 10 (0.001 M NaOH) (U77U99, AC77AC99)
 Column U: hydrogel type (polyacrylamide)
 Column V: position
 Column W: cycle number
 Column X: maximum force reached during analysis
 Column Y: maximum contact pressure reached during analysis
 Column Z: maximum contact radius reached during analysis
 Column AA: estimated reduced elastic modulus using the Hertz model
 Column AB + AC: averages and standard deviation of reduced elastic modulus per position
 Column AD + AE: averages and standard deviations of the three hydrogel samples at their specified solution pH
W) Modulus_TEMED_APS.csv
 Table 1: raw data for hydrogel sample 1 with TEMED + APS initiating system at pH = 0.34 (0.5 M HCl) (A2A24, I2I24)
 Column A: hydrogel type (polyacrylamide)
 Column B: position
 Column C: cycle number
 Column D: maximum force reached during analysis
 Column E: maximum contact pressure reached during analysis
 Column F: maximum contact radius reached during analysis
 Column G: estimated reduced elastic modulus using the Hertz model
 Column I + J: averages and standard deviation of reduced elastic modulus per position
 Table 2: raw data for hydrogel sample 2 with TEMED + APS initiating system at pH = 0.34 (0.5 M HCl) (K2K24, S2S24)
 Column K: hydrogel type (polyacrylamide)
 Column L: position
 Column M: cycle number
 Column N: maximum force reached during analysis
 Column O: maximum contact pressure reached during analysis
 Column P: maximum contact radius reached during analysis
 Column Q: estimated reduced elastic modulus using the Hertz model
 Column R + S: averages and standard deviation of reduced elastic modulus per position
 Table 3: raw data for hydrogel sample 3 with TEMED + APS initiating system at pH = 0.34 (0.5 M HCl) (U2U24, AC2AC24)
 Column U: hydrogel type (polyacrylamide)
 Column V: position
 Column W: cycle number
 Column X: maximum force reached during analysis
 Column Y: maximum contact pressure reached during analysis
 Column Z: maximum contact radius reached during analysis
 Column AA: estimated reduced elastic modulus using the Hertz model
 Column AB + AC: averages and standard deviation of reduced elastic modulus per position
 Table 4: raw data for hydrogel sample 1 with TEMED + APS initiating system at pH = 2 (0.01 M HCl) (A27A46, I27I46)
 Column A: hydrogel type (polyacrylamide)
 Column B: position
 Column C: cycle number
 Column D: maximum force reached during analysis
 Column E: maximum contact pressure reached during analysis
 Column F: maximum contact radius reached during analysis
 Column G: estimated reduced elastic modulus using the Hertz model
 Column I + J: averages and standard deviation of reduced elastic modulus per position
 Table 5: raw data for hydrogel sample 2 with TEMED + APS initiating system at pH = 2 (0.01 M HCl) (K27K46, S27S46)
 Column K: hydrogel type (polyacrylamide)
 Column L: position
 Column M: cycle number
 Column N: maximum force reached during analysis
 Column O: maximum contact pressure reached during analysis
 Column P: maximum contact radius reached during analysis
 Column Q: estimated reduced elastic modulus using the Hertz model
 Column R + S: averages and standard deviation of reduced elastic modulus per position
 Table 6: raw data for hydrogel sample 3 with TEMED + APS initiating system at pH = 2 (0.01 M HCl) (U27U46, AC27AC46)
 Column U: hydrogel type (polyacrylamide)
 Column V: position
 Column W: cycle number
 Column X: maximum force reached during analysis
 Column Y: maximum contact pressure reached during analysis
 Column Z: maximum contact radius reached during analysis
 Column AA: estimated reduced elastic modulus using the Hertz model
 Column AB + AC: averages and standard deviation of reduced elastic modulus per position
 cells with n/a: no cycle data at the specified position
 Table 7: raw data for hydrogel sample 1 with TEMED + APS initiating system at pH = 7 (DI water) (A49A71, I49I71)
 Column A: hydrogel type (polyacrylamide)
 Column B: position
 Column C: cycle number
 Column D: maximum force reached during analysis
 Column E: maximum contact pressure reached during analysis
 Column F: maximum contact radius reached during analysis
 Column G: estimated reduced elastic modulus using the Hertz model
 Column I + J: averages and standard deviation of reduced elastic modulus per position
 cells with n/a: no cycle data at the specified position
 Table 8: raw data for hydrogel sample 2 with TEMED + APS initiating system at pH = 7 (DI water) (K49K71, S49S71)
 Column K: hydrogel type (polyacrylamide)
 Column L: position
 Column M: cycle number
 Column N: maximum force reached during analysis
 Column O: maximum contact pressure reached during analysis
 Column P: maximum contact radius reached during analysis
 Column Q: estimated reduced elastic modulus using the Hertz model
 Column R + S: averages and standard deviation of reduced elastic modulus per position
 Table 9: raw data for hydrogel sample 3 with TEMED + APS initiating system at pH = 7 (DI water) (U49U71, AC49AC71)
 Column U: hydrogel type (polyacrylamide)
 Column V: position
 Column W: cycle number
 Column X: maximum force reached during analysis
 Column Y: maximum contact pressure reached during analysis
 Column Z: maximum contact radius reached during analysis
 Column AA: estimated reduced elastic modulus using the Hertz model
 Column AB + AC: averages and standard deviation of reduced elastic modulus per position
 Table 10: raw data for hydrogel sample 1 with TEMED + APS initiating system at pH = 10 (0.001 M NaOH) (A75A94, I75I94)
 Column A: hydrogel type (polyacrylamide)
 Column B: position
 Column C: cycle number
 Column D: maximum force reached during analysis
 Column E: maximum contact pressure reached during analysis
 Column F: maximum contact radius reached during analysis
 Column G: estimated reduced elastic modulus using the Hertz model
 Column I + J: averages and standard deviation of reduced elastic modulus per position
 Table 11: raw data for hydrogel sample 2 with TEMED + APS initiating system at pH = 10 (0.001 M NaOH) (K75K94, S75S94)
 Column K: hydrogel type (polyacrylamide)
 Column L: position
 Column M: cycle number
 Column N: maximum force reached during analysis
 Column O: maximum contact pressure reached during analysis
 Column P: maximum contact radius reached during analysis
 Column Q: estimated reduced elastic modulus using the Hertz model
 Column R + S: averages and standard deviation of reduced elastic modulus per position
 cells with n/a: no cycle data at the specified position
 Table 12: raw data for hydrogel sample 3 with TEMED + APS initiating system at pH = 10 (0.001 M NaOH) (U75U94, AC75AC94)
 Column U: hydrogel type (polyacrylamide)
 Column V: position
 Column W: cycle number
 Column X: maximum force reached during analysis
 Column Y: maximum contact pressure reached during analysis
 Column Z: maximum contact radius reached during analysis
 Column AA: estimated reduced elastic modulus using the Hertz model
 Column AB + AC: averages and standard deviation of reduced elastic modulus per position
 Table 13: raw data for hydrogel sample 1 with TEMED + APS initiating system at pH = 13.5 (0.5 M NaOH) (A97A119, I97I119)
 Column A: hydrogel type (polyacrylamide)
 Column B: position
 Column C: cycle number
 Column D: maximum force reached during analysis
 Column E: maximum contact pressure reached during analysis
 Column F: maximum contact radius reached during analysis
 Column G: estimated reduced elastic modulus using the Hertz model
 Column I + J: averages and standard deviation of reduced elastic modulus per position
 Table 14: raw data for hydrogel sample 2 with TEMED + APS initiating system at pH = 13.5 (0.5 M NaOH) (K97K119, S97S119)
 Column K: hydrogel type (polyacrylamide)
 Column L: position
 Column M: cycle number
 Column N: maximum force reached during analysis
 Column O: maximum contact pressure reached during analysis
 Column P: maximum contact radius reached during analysis
 Column Q: estimated reduced elastic modulus using the Hertz model
 Column R + S: averages and standard deviation of reduced elastic modulus per position
 Table 12: raw data for hydrogel sample 3 with TEMED + APS initiating system at pH = 13.5 (0.5 M NaOH) (U97U119, AC97AC119)
 Column U: hydrogel type (polyacrylamide)
 Column V: position
 Column W: cycle number
 Column X: maximum force reached during analysis
 Column Y: maximum contact pressure reached during analysis
 Column Z: maximum contact radius reached during analysis
 Column AA: estimated reduced elastic modulus using the Hertz model
 Column AB + AC: averages and standard deviation of reduced elastic modulus per position
 Column AD + AE: averages and standard deviations of the three hydrogel samples at their specified solution pH
Compilation of raw swelling data
X) Swelling_Irgacure.csv
 Table 1: raw data for hydrogels with Irgacure initiating system at pH = 0.34 (0.5 M HCl) (A2A7, L2L7)
 Column A: hydrogel type (polyacrylamide)
 Column B: acrylic acid mol% (0%)
 Column C: acrylamide mol% (100%)
 Column D: sample identifier (internal use)
 Column E: swollen mass (g)
 Column F: dried mass (g)
 Column G: q = swollen mass/dried mass ratio
 Column H: Q = ((swollen mass  dried mass)/swollen mass)*100
 Column I + J: q average and standard deviation of the three samples after 2 weeks of drying
 Column K + L: Q average and standard deviation of the three samples after 2 weeks of drying
 Table 2: raw data for hydrogels with Irgacure initiating system at pH = 2 (0.01 M HCl) (A9A14, L9L14)
 Column A: hydrogel type (polyacrylamide)
 Column B: acrylic acid mol% (0%)
 Column C: acrylamide mol% (100%)
 Column D: sample identifier (internal use)
 Column E: swollen mass (g)
 Column F: dried mass (g)
 Column G: q = swollen mass/dried mass ratio
 Column H: Q = ((swollen mass  dried mass)/swollen mass)*100
 Column I + J: q average and standard deviation of the three samples after 2 weeks of drying
 Column K + L: Q average and standard deviation of the three samples after 2 weeks of drying
 Table 3: raw data for hydrogels with Irgacure initiating system at pH = 7 (DI water) (A17A22, L17L22)
 Column A: hydrogel type (polyacrylamide)
 Column B: acrylic acid mol% (0%)
 Column C: acrylamide mol% (100%)
 Column D: sample identifier (internal use)
 Column E: swollen mass (g)
 Column F: dried mass (g)
 Column G: q = swollen mass/dried mass ratio
 Column H: Q = ((swollen mass  dried mass)/swollen mass)*100
 Column I + J: q average and standard deviation of the three samples after 2 weeks of drying
 Column K + L: Q average and standard deviation of the three samples after 2 weeks of drying
 Table 4: raw data for hydrogels with Irgacure initiating system at pH = 10 (0.001 M NaOH) (A25A30, L25L30)
 Column A: hydrogel type (polyacrylamide)
 Column B: acrylic acid mol% (0%)
 Column C: acrylamide mol% (100%)
 Column D: sample identifier (internal use)
 Column E: swollen mass (g)
 Column F: dried mass (g)
 Column G: q = swollen mass/dried mass ratio
 Column H: Q = ((swollen mass  dried mass)/swollen mass)*100
 Column I + J: q average and standard deviation of the three samples after 2 weeks of drying
 Column K + L: Q average and standard deviation of the three samples after 2 weeks of drying
Y) Swelling_TEMED_APS.csv
 Table 1: raw data for hydrogels with TEMED + APS initiating system at pH = 2 (0.01 M HCl) (A1A6, L1L6)
 Column A: hydrogel type (polyacrylamide)
 Column B: acrylic acid mol% (0%)
 Column C: acrylamide mol% (100%)
 Column D: sample identifier (internal use)
 Column E: swollen mass (g)
 Column F: dried mass (g)
 Column G: q = swollen mass/dried mass ratio
 Column H: Q = ((swollen mass  dried mass)/swollen mass)*100
 Column I + J: q average and standard deviation of the three samples after 2 weeks of drying
 Column K + L: Q average and standard deviation of the three samples after 2 weeks of drying
 Table 2: raw data for hydrogels with TEMED + APS initiating system at pH = 10 (0.001 M NaOH) (A10A15, L10L15)
 Column A: hydrogel type (polyacrylamide)
 Column B: acrylic acid mol% (0%)
 Column C: acrylamide mol% (100%)
 Column D: sample identifier (internal use)
 Column E: swollen mass (g)
 Column F: dried mass (g)
 Column G: q = swollen mass/dried mass ratio
 Column H: Q = ((swollen mass  dried mass)/swollen mass)*100
 Column I + J: q average and standard deviation of the three samples after 2 weeks of drying
 Column K + L: Q average and standard deviation of the three samples after 2 weeks of drying
 Table 3: raw data for hydrogels with TEMED + APS initiating system at pH = 0.34 (0.5 M HCl), pH = 7 (DI water), and pH = 13.5 (0.5 M NaOH)
 Column A: solution pH (pH = 0.34 [0.5 M HCl], pH = 7 [DI water], or pH = 13.5 [0.5 M NaOH])
 Column B: sample identifier (internal use)
 Column C: swollen mass (mg)
 Column D: dried mass (mg)
 Column E: Q = ((swollen mass  dried mass)/swollen mass)*100
 Column F + G: Q average and standard deviation of the three samples at the specified pH
Z) Volume_change_TEMED_APS.csv
 Column A: solution pH (pH = 0.3 [0.5 M HCl], pH = 7 [DI water], pH = 13.5 [0.5 M NaOH])
 Column B: sample identifier (internal use)
 Column C: hydrogel diameter (mm) after polymerization before swelling (triplicate measurements per sample)
 Column D: hp = hydrogel height (mm) after polymerization before swelling (triplicate measurements per sample)
 Column E: mp = hydrogel mass (g) after polymerization before swelling
 Column F: hydrogel diameter (mm) after swelling for 14 days in specified solution (triplicate measurements per sample)
 Column G: hs = hydrogel height (mm) after swelling for 14 days in specified solution (triplicate measurements per sample)
 Column H: ms = hydrogel mass (g) after swelling for 14 days in specified solution
 Column I: Vp = hydrogel volume (mm^3) after polymerization before swelling (assuming V = pi*r^2*h)
 Column J: Vs = hydrogel volume (mm^3) after swelling for 14 days in specified solution (assuming V = pi*r^2*h)
 Column K: volume change = ((VsVp)/Vp)*100
 Column L: Q = (hs/hp)^3
 Column M: mass change = ((msmp)/mp)*100
 Column N + O: volume change average and standard deviation of the three samples at the specified pH
 cells with n/a: mass data was only collected once per sample
#############################################################################################
MATLAB CODES
MATLAB Codes: all codes are under MIT license please cite the code if a significant portion has been used
A) Tribometer_Indent_Analysis_v5 > uses natsortfiles function
B) Compiling_Friction_Data_v7 > uses natsortfiles function
ANALYZING TRIBOMETER DATA
A) Tribometer_Indent_Analysis_v5: opens microtribometer indentation data from Excel files and fits the data with the Hertz contact mechanics model using the lsqcurvefit function
B) Compiling_Friction_Data_v7: opens microtribometer friction data from Excel files and calculates the friction coefficient from the specified xrange in the friction force loop over a specified number of cycles
FUNCTIONS TO DOWNLOAD
 Author: Stephen Cobeldick
 Download from: https://www.mathworks.com/matlabcentral/fileexchange/47434naturalorderfilenamesort
 Citation: Stephen23 (2023). NaturalOrder Filename Sort (https://www.mathworks.com/matlabcentral/fileexchange/47434naturalorderfilenamesort), MATLAB Central File Exchange. Retrieved 2021.
 Function of codes: reads and sorts through the files to analyze
 Functions: A) natsortfiles B) natsort
 Notes: natsortfiles calls natsort
Methods
Microindentations and sliding experiments were conducted with a custombuilt linear reciprocating tribometer. Data was analyzed and processed with the following Matlab codes (Tribometer_Indent_Analysis_v5 and Compiling_Friction_Data_v7).
Raman spectroscopy was conducted with a confocal Raman microscope (Horiba Jobin Yvon T640000) with an excitation wavelength of 488 nm and laser power of 400 mW. A 50x objective lens was used to focus the laser light 10 µm below the sample surface. Spectra data was obtained with a resolution of 0.66 cm^{1}.
Xray photoelectron spectroscopy was conducted with a Thermo Scientific ESCALAB^{TM} Xi+. A monochromatic Al/Kα xray radiation with a pass energy of 100 eV, dwell time of 50 ms, and spot size of 650 µm was used. Spectra were obtained after 105 s of etching using a 6 kV Ar1000+ cluster ion source.
Usage notes
All data is in .csv or .txt format and can be opened with any opensource software.