Use of Polystyrene-Block-Poly(MethylacrylateRandom-Octadecylate)-Block-Polystyrene as an
On Demand Adhesive
Schaun Ginesi
Department of Chemical Engineering
Honors Research Project
Submitted to
The HonorsCollege
Approved:
Accepted:
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Table of Contents
Executive Summary ................................................................................................................................. 3
Introduction & Purpose ........................................................................................................................... 4
Background ............................................................................................................................................. 5
Experimental Procedure .......................................................................................................................... 8
Results & Discussion .............................................................................................................................. 13
Conclusion & Recommendations ........................................................................................................... 23
References ............................................................................................................................................ 26
2
Executive Summary
Initial testing of a triblock copolymer with polystyrene end blocks and a poly(methylacrylaterandom-octadecylacrylate) mid-block(PS-b-PMA-r-PODA-b-PS) was performed using the JohnsonKendall-Roberts (JKR) method for measuring work of adhesion and adhesion strength to determine its
feasibility for use as an on demand adhesive. Further descriptions of the polymer tested and the JKR
test method are given in the Background section of this report. Testing of the PS-b-PMA-r-PODA-b-PS
was performed at three temperatures, refrigerated, room temperature and elevated, to provide results
on both sides of two thermal transition temperatures. The testing and calculations outlined in the
Experimental Procedure section allowed for calculations of the system’s modulus from loading and
unloading, the work of adhesion and adhesion strength, respectively. For PS-b-PMA-r-PODA-b-PS the
average system‘s modulus were determined from loading to be 2.25x106± 2.15x105N/m2at the
refrigerated temperature, 2.43x106 ± 1.81x105N/m2at room temperature, and 1.15x106±
1.08x106N/m2at the elevated temperature. The measured average work of adhesions of the polymer
and a soft silicone lens at the refrigerated, room, and elevated temperatures were found to be 24.23 ±
5.05mJ/m2, 27.43 ± 4.31mJ/m2, and 154.84 ± 87.93 mJ/m2respectively. These values represent the
thermodynamic minimum interaction energy of the two surfaces at the given temperature. For the
same conditions respectively the average adhesion strengths were found to be 185.37 ± 34.83mJ/m2,
120.29 ± 14.82mJ/m2, and 698.72 ± 115.74mJ/m2. The adhesion strength is the actual interaction
energy that must be overcome when separating the two surfaces in contact. In ideal situations this
value should be equal to the work of adhesion. The adhesion strength is the value that is of greatest
concern for this project based on the possible applications. When looking at these values it can be seen
that there is a significant increase in the adhesion strength, over four and a half times larger, when
increasing the temperature from room temperature to the elevated one. Analysis of variance (ANOVA)
testing verified that this increase was not the result of random sample variation with greater than 99.9%
confidence. There is also a change in the adhesion strength when moving from room temperature to
the refrigerated temperature. This change was less significant than when going to the elevated
temperature, but ANOVA testing verified with greater than 95% confidence that this was not due to
random sample variation. It is believed that these changes in adhesion strength are the result of the
thermal transitions undergone by the polymer when changing temperature and that the transition going
from room temperature to the elevated temperature is more significant in effecting adhesive
propertied. Because of the large increase in adhesive strength that is generated in reaction to an
outside stimulus, PS-b-PMA-r-PODA-b-PS has shown adhesive on demand properties in this testing.
Based on this project’s results it is believed that PS-b-PMA-r-PODA-b-PS is a feasible material for use in
an on demand adhesive application. Further testing and research is needed to confirm its usability in
specific applications.
3
Introduction &Purpose
The goal of this project is to use a polymer material with specific cross linking properties to
apply for use as an adhesive. Properties of the adhesive will be variable depending on the cross linking
of the polymer chain. Ultimately it is desired to create an adhesive that only has adhesive properties
within a set temperature range, and beyond that is not adhesive. Doing this will allow the polymer to
have the adhesive properties when it is desired, or on demand.
One current possible application for a material with these properties is for the use as a tape that can be
applied while the polymer has adhesive properties, or is sticky, and then removed when the properties
are changed. This would allow for the removal of the tape with little to know damage to the substance
to which it was applied. A tape like this can be used in the medical industry for application to sensitive
skin such as that of new born babies or burn victims. The tape could then be applied to the skin to hold
on instruments or other important medical equipment without concern for damage of the skin upon
removal. A tape with on demand adhesion properties has potential for use in almost any area in which
tapes are applied for temporary use. This project has many potential applications and benefits to the
greater community.
4
Background
This project was centered on a polymer material where the crosslink density can be varied in a
stepwise manner across a thermal transition. The polymer that was tested was a triblock co-polymer
with long alkyl side chains on the middle block of the copolymer. Both end groups of this polymer were
polystyrene, and the midblock consisted of poly(methylacrylate-random-octadecylacrylate)(PS-b-PMA-rPODA-b-PS). Figure 1 shows the structure of the polymer used. The nature of this block copolymer
allowed for the polystyrene end blocks to physically crosslink the midblock which bridges the domain of
the glassy end blocks. The random octadecylacrylate side chains on the midblock were able to
crystallize at room temperature which created a secondary physical linking within the network. This
polymer has already been published on as a shape memory polymer in Synthesis and Characterization of
aPoly(styrene-block-methylacrylaterandom-octadecylacrylate-block-styrene) ShapeMemory ABA
Triblock Copolymer by Fei and Cavicchi. In both applications, shape memory and adhesive on demand,
it was the random octadecylacrylateside chains that made the desired behavior possible. The ability of
the side chains to crystallize, with transition temperatures Tc=19°C and Tm=32°C (Fei & Cavicchi, 2010),
allowed for a melting or crystallization transition to occur. This thermal transition created a change in
the properties of the material.
Figure 1 (Fei & Cavicchi, 2010)
Adhesion testing for this project was done via the contact Johnson-Kendall-Roberts (JKR)
method of measuring contact mechanics. The JKR method measures the work of adhesion of a
semispherical elastic lens to a thin film substrate. Using the JKR method the lens was pressed onto the
5
thin film substrate using a cantilever while a microscope measured the contact area of the two and a
balance measured the force applied to the substrate. The setup for this configuration is shown in Figure
2. In Figure 2 the microscope, substrate and balance are labeled. The orange semispherical section is
the lens, the lens is attached to the gray bar that is the cantilever, and the contact area measured by the
microscope can be seen as the flat part of the lens touching the substrate.
Figure 2 (Chaudhury)
Work of adhesion was calculated from the measurements of mass, which was used to estimate
the force applied, from the balance and contact area from the lens. Equation 1 was used to generate
the correlation between force, contact area and work of adhesion. Further explanation of how work of
adhesion was calculated from Equation 1 is given in the Experimental Procedure section of this report.
Equation 1
In Equation 1
a= radius of contact area
R= radius of curvature of the lens
E= overall Young’s modulus of system
P= applied load
W= work of adhesion
As will be further explained in the Experimental Procedure section of this report, multiple
measurements were taken with different materials to confirm measurement procedure was valid. One
of the materials used as a substrate was Poly(N-isopropyl acrylamide) (pNIPAM) which is a thermoresponsive polymer used commonly in bioengineering applications (Cooperstein & Canavan, 2009).
6
There is a transition of pNIPAM at around 32°C (Cooperstein & Canavan, 2009), which causes the
properties of the polymer to change over this temperature. Below this temperature pNIPAM is
hydrophilic, while above this temperature the polymer is hydrophobic (Cooperstein & Canavan, 2009).
This change affects the adhesive properties of the polymer among other properties.
To determine if the differences between the different conditions were significant or caused by
random deviation in the testing, analysis of variance (ANOVA) tests were performed on the data. The
ANOVA test compares the variation within a group of data to the variation between two groups of data
by calculating an F statistic. The F statistic is calculated based on the average value of the data, the
variance of the data, and the number of data points. For a given confidence level a critical F statistic
(Fcrit) exists such that if F is not greater than Fcrit the null hypothesis (that there is no statically significant
difference between the two sets of data) fails to be rejected. In other words, if F is greater than F crit it
can be said to the set confidence level that there is a significant difference between the two sets of data.
The critical F statistic is calculated from the location of a value calculated using the confidence level and
number of data points on a set distribution which is constant. Summaries of the results of ANOVA tests
run are given in the Results & Discussion section. This allowed for the significance of the difference
between the different conditions to be determined to a known confidence level.
7
Experimental Procedure
Work of adhesion was measured using the JKR method of measuring surface contact mechanics.
As outlined in the Background section, this method involves the use of a semispherical lens pressed onto
a substrate with a thin film of the test material. For all tests run for this project the lens used was made
of polydimethyl siloxane (PDMS). A microscope was set up over the lens so that the contact area could
be clearly seen. The microscope output was sent to a computer screen for viewing and a program
allowed the diameter of the contact area to be measured in pixels. The measurement in pixels was
converted to meters by using a conversion factor found by measuring the number of pixels
corresponding to 1mm on a ruler. Once this was completed, all measurements of contact diameter
were taken using the computer program and recorded as the number of pixels to the nearest whole
pixel. The applied load was measured using an electric balance under the thin film substrate and
cantilever setup and measured the mass applied to the nearest .0001 g. Prior to the lens contacting the
substrate the balance was zeroed. Once contact was made, the reading on the balance increased above
zero corresponding to the applied force. Measurements were recorded as the mass given on the
balance and converted to force by multiplying by the acceleration of gravity. Values for diameter and
mass on the balance were recorded and converted to the desired values of radius and applied force in
the correct units of meters and Newton’s respectively as an ordered pair. Measurements were taken at
ten different applied forces and the corresponding values were recorded. This allowed for the creation
of a ten point plot of the ordered pairs. Using Equation 1, this plot allowed for the calculation of Young’s
modulus and work of adhesion.
Equation 1
Equation 2
In Equation 2
x= the independent variable
y= the dependent variable
m= the slope of the line
b= the intercept on the y-axis
8
Comparing Equations 1 & 2, it can be seen that Equation 1 is of the same form as Equation 2 if
certain parameters and variables are grouped in the correct way. It is also observed that Equation 2 is
the general equation of a line. Taking both of these observations to be true, then Equation 1 can be
plotted using the ordered pairs for radius of contact area and applied load as a straight line. This is done
by setting the following groupings equal to the variables and parameters in Equation 2.
Equation 3
Equation 4
Equation 5
Equation 6
Plotting x vs. y derived in Equations 3 & 4, a line was generated. A linear regression was used to
fit a straight line to this plot with a corresponding equation of the same for of Equation 2. The equation
for this line gave the values of m and b for the given set of ordered pairs. Next Equations 5 & 6 were
rearranged so that Young’s modulus (E) of the system and work of adhesion (W) could be calculated for
the material using the m and b from the corresponding equation.
Equation 7
Equation 8
Equations 7 & 8 were used to find the Young’s modulus in Newton’s per meter squared or Pascal
(N/m2 or Pa) and work of adhesion in millijoules per square meter (N/m2) for each material at each
condition. Each value received corresponds to one set of ten ordered pairs of radius of contact area and
applied force. For each material at each condition several runs were performed giving values of E and W
for each run. This allowed for an average value and standard deviation to be calculated for each
material and condition.
Another parameter value needed in Equation 3 so that E and W can be calculated is the radius of
curvature of the lens (R). This was found using Equation 9 and the corresponding values found from a
picture similar to that of Figure 3. In Equation 9, c is the length of a chord drawn across the lens and m
9
is the distance from the midpoint of that chord perpendicularly to the edge of the lens. A known
magnification factor was used to find the actual values of c, m, and therefore R from the measurements
taken from a print out. For each lens, multiple chords were drawn so that multiple R values could be
calculated and an average value could be used for calculations.
Equation 9
Figure 3
The Young’s modulus and work of adhesion were calculated for different materials and
conditions. Control materials were run first so that the test procedure could be practiced. As stated
before, all tests that were run used a PDMS lens. All of the substrate materials that were tested were
either solvent cast or spin coated onto a silicon wafer. The first substrate used for initial testing was a
PDMS sheet. Next practice runs were performed using the pNIPAM thermo-responsive polymer
described in the Background section earlier. Testing with this polymer substrate was run at room
temperature and at an elevated temperature above its transition temperature. The final material that
was tested was the PS-b-PMA-r-PODA-b-PS polymer that was the focus of this project. This polymer was
tested at room temperature, an elevated temperature, and at a refrigerated temperature. The
10
comparisons between these different conditions and why they were selected will be discussed further in
the Results &Discussion section.
The PDMS substrate (i.e. a thin film of PDMS) was used as a control material for the first rounds
of testing to establish the procedure. Prior to finding the measurement technique that was used
another technique was used. For all of these preliminary tests, PDMS was used as the material. PDMS
was used for these setup tests because it was a readily available material in the lab that gave consistent
results. It was also selected because there was a rough idea of the modulus and work of adhesion
expected for this condition. This allowed the method to be tested on a material that was inexpensive
and easily replaced if the method did not work as desired. Several runs on the PDMS substrate were
performed using an initial test method. This method did not yield consistent results nor results that
aligned with the expected values for the given conditions. All of the data collected during these runs
was kept and compared to the data received using the improved test method. Once the final test
method was confirmed and found to be repeatable the PDMS was used as the substrate to collect
usable data for comparison.
After the PDMS substrate was used to confirm the experimental procedure was repeatable, the
next substrate was tested was the pNIPAM thermo-responsive polymer. Using this material allowed for
experience to be gained in testing at two different temperatures and comparing results on either side of
a transition temperature. Testing of the pNIPAM polymer was done at both room temperature and at
an elevated temperature. The exact temperatures at which the tests were run were not recorded
exactly; the onlyconcern was assuring that the temperature of the polymer was maintained above the
transition temperature during testing. To ensure that the temperature of the polymer was maintained
at a high enough temperature, the polymer substrate was attached to a small piece of metal which was
heated on a hot plate prior to each run. The metal under the substrate held the temperature much
longer than the silicon wafer alone would have been able to, providing more time to run the tests.
11
Multiple runs at both room temperature and the elevated temperature were performed. The results of
each run at one temperature were averaged with the other runs at that temperature and the averages
of the two temperatures were compared against each other. An interpretation of these results gave a
chance to determine if there is a significant difference between the results at two different
temperatures.
The last set of tests run for this project was run on the PS-b-PMA-r-PODA-b-PS polymer that this
project was centered around. Measurements were taken at three temperatures for this material, room
temperature, an elevated temperature, and a refrigerated temperature. The temperature of the
polymer was maintained in the same manner as was done for the pNIPAM polymer. The same piece of
metal that was used for the pNIPAM polymer was used to maintain the temperature at both the high
and low temperatures. A hot plate was used to elevate the temperature and the metal piece was placed
in a freezer. Several runs were performed at each temperature and the results at each condition were
averaged. Comparing the averages determined whether a significant difference existed between the
works of adhesion at the three temperatures.
The masses and diameters of contact were the values actually recorded while the tests were
being run. After the runs were performed these values were entered into a spreadsheet which
calculated the slope and intercept using Equations 5 & 6 and then the Young’s modulus and work of
adhesion using Equations 7 & 8. The data was entered after each run so that the results could be
verified. Table 1in the Results & Discussion section shows an example of the values calculated from a
run. Once the results were entered and the values calculated the run was complete. The condition was
changed, either material or temperature, only after enough runs were completed to give an average and
standard deviation for that condition. After all of the results were collected, the results were analyzed
to determine if the PS-b-PMA-r-PODA-b-PS polymer could be used in an adhesive on demand
application.
12
Results & Discussion
The results of all testing are separated by the slopes of the lines generated for loading and
unloading. The work performed during loading (i.e. increasing the applied force)is known as work of
adhesion and labeled W. Work of adhesion is the thermodynamic interaction energy between to two
surfaces and minimum energy of interaction. Equation 1 is used to calculate the work of adhesion.
When unloading(i.e. decreasing applied force),the force and contact radius are recorded to obtainthe
interaction energy called adhesion strength and labeled G. To calculate the adhesion strength Equations
1 & 8 become Equations 10 & 11 respectively. In an ideal situation adhesion strength would be equal to
the work of adhesion, but because no system is able to act completely ideally, G is generally larger than
W for a given system. While W and G represent different values, both are calculated through the same
method. These values differ from one another to varying degrees depending on the properties of the
material.
Equation 10
Equation 11
Different forces can act upon the lens depending on whether lens is being applied to the
material (loading), or removed from the material (unloading). This project will be more concerned with
the adhesion strength corresponding to unloading. As described in the Introduction & Purpose section,
one possible future application of this material is a medical tape that will not damage the skin upon
removal of the tape. In this application it is more important that a significant reduction in adhesion
strength occur when removing the tape.
When taking the measurements, a microscope was used to see the contact area of the lens on
the substrate. As described earlier, the microscope output when to a computer to measure the
diameter. Figures 4 & 5 are examples of what was seen while taking measurements with marks showing
where the measurements for the diameter of the contact area were taken. Figure 4 is pNIPAM at room
13
temperature and the corresponding measurements are D=90 pixels and m=0.0580g. Figure 5 is pNIPAM
at the elevated temperature with measurements taken of D=88 pixels and m=0.0384 g.
Figure 4
Figure 5
Some of the important features outlined in Figures 4 & 5 include first the contact area in both
figures are a light circle with a dark outline near the center of the figure. The light surrounding the
contact area is the reflection of the light source off of the silicon that the polymer was spin coated onto.
In both figures arcs can be seen where the contrast changes near two of the corners. This is the outside
of the lens where it is attached to the cantilever. It is also noticeable that while these came from two
14
different conditions, there is no major difference between the two figures. In general the image seen
through the microscope was consistent regardless of the condition being measured.
As described in the Experimental Procedure section, Table 1 is an example of the spreadsheet
that was used to enter in the data from testing and generate the ordered pairs used to plot the lines
given by Equations 1 & 10. The data used to generate Table 1 came from the first run on the room
temperature PS-b-PMA-r-PODA-b-PS polymer.
Run 1 RT PS-b-PMA-r-PODA-b-PS
R (m)
0.00148 Pixels/cm
4950
unload
Load
m (g)
D (pixels) a (m)
P (N)
a^3
a^1.5
a^1.5/R
P/a^1.5
0.0120
69
6.97E-05
1.18E-04 3.39E-13 5.82E-07 3.94E-04
202.11
0.0194
0.0330
71
74
7.17E-05
7.47E-05
1.90E-04
3.23E-04
3.69E-13
4.18E-13
6.07E-07
6.46E-07
4.11E-04
4.38E-04
313.04
500.43
0.0402
0.0461
76
79
7.68E-05
7.98E-05
3.94E-04
4.52E-04
4.52E-13
5.08E-13
6.73E-07
7.13E-07
4.56E-04
4.83E-04
585.71
633.78
0.0531
0.0614
82
84
8.28E-05
8.48E-05
5.20E-04
6.02E-04
5.68E-13
6.11E-13
7.54E-07
7.82E-07
5.11E-04
5.29E-04
690.32
769.89
0.0676
0.0792
86
88
8.69E-05
8.89E-05
6.62E-04
7.76E-04
6.56E-13
7.02E-13
8.10E-07
8.38E-07
5.48E-04
5.68E-04
818.23
926.15
0.0877
0.0781
90
90
9.09E-05
9.09E-05
8.59E-04
7.65E-04
7.51E-13
7.51E-13
8.67E-07
8.67E-07
5.87E-04
5.87E-04
991.55
883.01
0.0693
0.0600
89
88
8.99E-05
8.89E-05
6.79E-04
5.88E-04
7.27E-13
7.02E-13
8.52E-07
8.38E-07
5.77E-04
5.68E-04
796.76
701.63
0.0488
88
8.89E-05
4.78E-04
7.02E-13
8.38E-07
5.68E-04
570.66
0.0395
0.0235
87
86
8.79E-05
8.69E-05
3.87E-04
2.30E-04
6.79E-13
6.56E-13
8.24E-07
8.10E-07
5.58E-04
5.48E-04
469.89
284.45
0.0085
0.0003
84
82
8.48E-05
8.28E-05
8.33E-05
2.94E-06
6.11E-13
5.68E-13
7.82E-07
7.54E-07
5.29E-04
5.11E-04
106.58
3.90
-0.0097
79
7.98E-05
-9.51E-05
5.08E-13
7.13E-07
4.83E-04
-133.36
-0.0162
76
7.68E-05
-1.59E-04
4.52E-13
6.73E-07
4.56E-04
-236.03
Table 1
In Table 1 columns 2 & 3 labeled, m and D, contain the actual data collected while running the
test. Columns 8 & 9, labeled a^1.5/R and P/a^1.5, are the values corresponding to the y and x values
respectively of the ordered pairs used to plot Equation 1. All of the columns in between were used to
15
convert the data into the ordered pairs. The value labeled R at the top of Table 1 is the radius of
curvature of the lens described earlier. Next to this value is the conversion factor used to convert the
number of pixels measured into the a value needed in meters. Looking over the data collected for this
run it is seen that some of the values of mass are negative. While physically mass cannot be negative,
these values are still valid measurements in this case. These masses are relative to the zeroing of the
balance before any load was applied and can therefore be negative if the substrate is being lifted off of
the balance slightly. Like is occurring in this case, the lens and the surface are held under tension as
opposed to compression when the force is positive. The adhesion of the polymer to the lens is strong
enough to lift the substrate slightly off of the balance. When the ordered pairs that were generated are
plotted, the resulting graphis shown in Figure 6.
Run 1 RT PS-b-PMA-r-PODA-b-PS
7.0E-04
y = 1.042E-07x + 5.026E-04
R² = 9.192E-01
6.0E-04
a^1.5/R
5.0E-04
y = 2.593E-07x + 3.258E-04
R² = 9.673E-01
4.0E-04
Load
3.0E-04
unload
2.0E-04
1.0E-04
-400
0.0E+00
-200
0
200
400
600
800
1000
1200
P/a^1.5
Figure 6
Figure 6 is the plot of the ordered pairs that were generated in Table 1. As predicted by
Equation 1, the resulting plots can be fit, with R2 values over .9, to a straight line. The line of best fit that
is created for each set of data is of the same for as Equations 1 & 2 and can therefore be used with
16
Equations 7, 8& 11 to calculate the Young’s modulus and either work of adhesion or adhesion strength.
This was done using a spreadsheet setup like that shown by Table 2.
m
Run 1 RT PS-b-PMA-r-PODA-b-PS
Load
Unload
b
m
b
2.593E-07 3.258E-04
m=9/(16E)
2.17E+06 Pa (N/m2)
E
1.042E-07 5.026E-04
m=9/(16E)
E
b=3/4(6*pi*W/E)^.5
W
21.72 mJ/m
5.40E+06 Pa (N/m2)
b=3/4(6*pi*W/E)^.5
2
G
128.65 mJ/m2
Table 2
All of the runs that were performed generated a table similar to Table 2. Each condition’s
results were grouped together and averages and standard deviations were taken for all of the
conditions. Table 3 shows the average and standard deviation values for adhesion strength and work of
adhesion for each condition that was tested. Table 4 has the average and standard deviation values of
Young’s modulus for each condition.
Average
(mJ/m2)
Stdev
38.16
5.08
38.08
6.22
Loading
Condition
PDMS
RT pNIPAM
Unloading
W
G
Hot pNIPAM
RT PS-b-PMA-r-PODA-b-PS
Hot PS-b-PMA-r-PODA-b-PS
Cold PS-b-PMA-r-PODA-b-PS
PDMS
RT pNIPAM
Hot pNIPAM
RT PS-b-PMA-r-PODA-b-PS
Hot PS-b-PMA-r-PODA-b-PS
Cold PS-b-PMA-r-PODA-b-PS
35.07
27.43
154.84
24.23
115.61
80.39
58.50
120.29
698.72
185.37
8.79
4.31
87.93
5.05
12.61
11.66
7.09
14.82
115.74
34.83
Table 3
17
Loading
Unloading
Condition
PDMS
RT pNIPAM
Hot pNIPAM
Average (Pa)
7.02E+05
2.69E+06
2.31E+06
RT PS-b-PMA-r-PODA-b-PS
Stdev
1.78E+05
5.17E+05
2.66E+05
2.43E+06 1.81E+05
Hot PS-b-PMA-r-PODA-b-PS
E Cold PS-b-PMA-r-PODA-b-PS
PDMS
RT pNIPAM
Hot pNIPAM
RT PS-b-PMA-r-PODA-b-PS
Hot PS-b-PMA-r-PODA-b-PS
1.15E+06
2.25E+06
1.37E+06
3.88E+06
2.92E+06
5.25E+06
2.51E+06
1.08E+06
2.15E+05
5.16E+05
2.36E+05
2.60E+05
6.17E+05
3.40E+06
E Cold PS-b-PMA-r-PODA-b-PS
7.48E+06 1.08E+06
Table 4
Looking over the results in Table 3 there are several important observations that can be made.
As discussed earlier, it can be seen that the adhesion strength of all of the conditions is greater than the
work of adhesion for the same condition. While comparing the W and G values from a given condition it
can also be seen that the extent to which G is larger than W varies. Looking at the work of adhesion for
PDMS, RT pNIPAM, and hot pNIPAM, the values are within 10 percent of each other. When comparing
the adhesion strength of these same conditions it is seen that the values vary by around 100 percent of
the lowest value. This shows that there are different degrees of non-ideality in the systems at different
conditions. This is what causes the G for PDMS to be around 3 times larger than the W for PDMS while
the G for the hot pNIPAM was around 1.5 times the W.
Another observation that can be made about the results in Table 3 is that the deviations for the
conditions at temperatures other than room temperature are larger than the deviations of the room
temperature conditions. During testing it was noticed that generating consistent results at the elevated
temperature was more difficult than at room temperature. This is reflected in the results. It is believed
that that cause of the larger deviation comes from the setup of the test system. The block of metal used
to maintain the temperature was elevated to the level of the cantilever by being placed on plastic petri
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dishes. It is believed that the elevated temperature of metal block caused some warping of the plastic
petri dishes which caused the reading of the scale to fluctuate more. At the refrigerated temperature it
is believed small amounts of condensation accumulated mass on the metal plate while measurements
were being taken causing the mass to fluctuate more during testing. This is believed to be the reason
why it was more difficult to receive consistent results at the temperatures other than room
temperature.
Because of the nature of why the PDMS substrate was tested, the exact results are not the most
important part of it being tested. The average modulus value from testing was compared to an
expected value for the material found from previous testing. It is important to notice the standard
deviations of the tests run on the PDMS substrate are low relative to the averages. This was the goal of
testing on the PDMS, to reduce the variation in the measurements prior to testing the PS-b-PMA-rPODA-b-PS polymer. Based on the results for the PDMS and all other substrates it is believed that the
use of PDMS as a substrate was successful.
The next substrate used was the pNIPAM polymer which was tested at both room temperature
and an elevated temperature. Comparing the works of adhesion at the two temperatures, the values
are within 10 percent of one another. This shows that thermodynamically the interaction energies at
the two temperatures does not change on either side of the transition temperature. There is a more
noticeable difference between the adhesion strengths at the two temperatures. An ANOVA test was
performed at confidence levels of 95% and 99% and the results are summarized in Table 5.
pNIPAM
Confidence F
Fcrit
Conclusion
95% 12.24 5.59 A statistically significant difference exists between the two groups
99% 12.24 12.25 No statistically significant difference exists between the two groups
Table 5
In Table 5 the two groups being compared to calculate the F statistic are the adhesion strengths
at room temperature and the results at the elevated temperature. It can be seen that the F statistic is
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unchanged by changing the confidence level only the Fcrit changes. This is as expected when recalling
the values used to calculate both values. The most important part of Table 5 is that the difference
between the two groups is significant to 95% confidence however at a 99% confidence level the F
statistic is less than Fcrit and therefore the difference is no longer statistically significant. These results of
the testing on the pNIPAM polymer were considered very successful. The results show that it is possible
to get a significant change in the adhesive properties of a thermally responsive polymer when measuring
on either side of the transition temperature. These results also show that there is a certain confidence
level above which it can no longer be said that the difference between the two groups is statistically
significant. This helped to interpret the results testing the PS-b-PMA-r-PODA-b-PS polymer.
The final substrate used for testing was the PS-b-PMA-r-PODA-b-PS polymer that was the
desired test material. This was tested at three different temperatures as opposed to the two that the
pNIPAM was tested because broader results were desired. It was originally believed that the elevated
temperature would cause the desired change in adhesive properties. Because the Tg of PMA is 10°C, the
refrigerated temperature was measured to determine the effect of the transition on the adhesive
properties.
The results for PS-b-PMA-r-PODA-b-PS showed a definite change in the adhesive properties
when at an elevated temperature. The change in properties was less definitive when tested at the
refrigerated temperature. The work of adhesion when going from room temperature to the elevated
temperature changed by over 460%, while it only changed by around 13% when going to the
refrigerated temperature from room temperature. This shows that the transition that occurred
between room temperature and the elevated temperature was thermodynamically more significant
than the transition at 15°C. The change in adhesion strength when the temperatures were changed was
even more pronounced. The change in work of adhesion from room temperature to the elevated
temperature was over 480% or over 575 mJ/m2. ANOVA tests that compared the adhesion strengths
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between room temperature and the elevated temperature and room temperature and the refrigerated
temperature show the significance of the differences. A summary of these tests are given in Table 6
below.
PS-b-PMA-r-PODA-b-PS
Temperatures Confidence F
Fcrit
95% 104.47
6.61
99% 104.47 16.26
RT & Hot
RT % Cold
99.9% 104.47 47.18
95%
11.83
5.99
99%
11.83 13.75
Conclusion
A statistically significant difference exists between the
two groups
A statistically significant difference exists between the
two groups
A statistically significant difference exists between the
two groups
A statistically significant difference exists between the
two groups
No statistically significant difference exists between the
two groups
Table 6
In Table 6 it can be seen that the jump in adhesion strength from room temperature is
statistically significant with over 99.9% confidence. This means that one can be over 99.9% sure that
this change is because of the thermal transition of the polymer and not random sampling variation.
There is less confidence that the change in adhesion strength going from room temperature to the
refrigerated temperature. There is less than 99% confidence that this change is not caused by random
sample variation.
One strange trend that can be seen in the results in Table 1 is that the adhesion strength of PSb-PMA-r-PODA-b-PS decreases going from the refrigerated temperature to room temperature and then
increases greatly from room temperature to the elevated temperature. It was expected that as the
temperature increased from one condition to the next, the trend would not change direction. The cause
of this is likely do to the nature of the transition that occurred between each of three temperatures.
The results that were obtained for PS-b-PMA-r-PODA-b-PS work of adhesion do overall show
what was expected and desired. The significant change when changing temperature means that this
material has adhesive properties that can be changed via outside stimulus. Being able to control this
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stimulus would allow the adhesive properties to be changed on demand. The results of this testing have
proven the feasibility of PS-b-PMA-r-PODA-b-PS as an adhesive on demand.
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Conclusion& Recommendations
Use of the JKR method of measuring work of adhesion and adhesion strength has shown that
PS-b-PMA-r-PODA-b-PS may have a feasible application as an adhesive on demand. The test results
show that there is a sharp increase in the adhesion strength of the polymer going from room
temperature to an elevated temperature. ANOVA testing confirms that this change is not caused by
random sample variation. Testing on another material with known thermal responsiveness showed that
there can be a significant change in adhesive properties cause by a thermal transition of a polymer. It is
believed that the reversible thermal transition of PS-b-PMA-r-PODA-b-PS when increasing temperature
caused the increase in adhesion strength. This would allow the polymer to be used in an on demand
adhesive application and be repeatedly applied and removed.
While testing performed for this project shows initial feasibility of a PS-b-PMA-r-PODA-b-PS
polymer to be used in this application additional testing still needs to be performed to confirm this
material can be used in desired applications. Further testing or research must be conducted to find the
thermal transition temperature more exactly. If the polymer is to be used as a medical tape the
transition temperature must be below the surface temperature of the human body. This must be the
case to make the adhesion strength high enough when attached to the skin.
Depending on the exact application, the desired transition temperature may wish to be
adjusted. This may be possible through different changes to the polymer structure. The current form of
the polymer has 18 carbon long side chains on the midblock. Adjusting the length of these chains could
be one way to change the crystalline properties of the polymer. This would likely shift the temperature
at which the crystallization occurs and the adhesion strength on both sides of the transition. Increasing
or decreasing the frequency of the side chains could also change the amount of crystallization that can
occur. Increasing the frequency of the side chains would likely increase the number of crosslinks up to a
certain point. This change would likely also affect the transition temperature and adhesive properties.
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A third way that these properties could be shifted would be to change the degree of polymerization of
the three blocks. Changing the lengths of the blocks in the polymer would change the amount of
primary and secondary physical linking. By changing one or a combination of these polymer properties
it could be possible to move the transition temperature into the desired range. This might also prove to
be a way to increase or decrease the adhesion strength either above or below the transition
temperature. It is recommended that there be more research into these possible changes to increase
the usability of PS-b-PMA-r-PODA-b-PS in a wider variety of applications.
In the case being tested, temperature change was the outside stimulus driving the change in
adhesive properties. While changes to the structure of the polymer would likely change the
temperature range over which the thermal transition occurs; they may also be able to change the
stimulus required to trigger the property change. There are other possible stimuli that may be more
desirable to cause the adhesive property change. One stimuli that is currently being researched further
is the application of ultraviolet (UV) light. If UV light can be used to trigger a transition in the polymer it
can also be used as the stimulus that changes adhesive properties. This would be beneficial because the
application of UV light would be less likely to do damage to sensitive skin or other surfaces to which the
adhesive is applied. Any other stimuli that could be found to generate a transition in the polymer could
be applied to causing the on demand adhesive properties of this or a similar polymer.
Additional testing that needs to be performed is testing using lenses of materials other than
PDMS. If this is to be used in medical applications a lens covered with skin or modified with proteins
found on skin. This would create test conditions more similar to the desired application and give a
better indication of whether the material is feasible to use on skin. It is recommended that other lens
materials be tested to see the affect this has on the adhesion strength.
Initial testing that has been completed shows that PS-b-PMA-r-PODA-b-PS has adhesive on
demand properties. More testing and research needs to be performed to show that this polymer can be
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used as a tape in any application especially in a medical application. All of the results received while
completing this project show that this material is feasible to use in the adhesive on demand application.
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References
1. Chaudhury, M. K. (n.d.). General Adhesion: Methods of Contact Mechanics. Retrieved from
http://polymers.nist.gov/combi/NCMC-2_Presentations/02-mChaudhury.PDF
2. Cooperstein, M. A., & Canavan, H. E. (2009). Biological Cell Detachment from Poly(N-isopropyl
acrylamide) and Its Applications. Langmuir Invited Featured Article, 7695-7707.
3. Fei, P., & Cavicchi, K. A. (2010). Synthesis and Characterization of a Poly(styrene-blockmethylacrylate-random-octadecylacrylate-block-styrene) Shape Memory ABA Triblock
Copolymer. Applied Materials & Interfaces, 2797–2803.
4. Silberzan, P., Perutz, S., & Kramer, E. J. (1994). Study of the Self-Adhesion Hysteresis of a
Siloxane Elastomer Using the JKR Method. 2466-2470.
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