Thermochromic hydrogel

Thermochromic Coatings
1
A Project Report
On
Surfactant Free
PVA/Borax Thermochromic Hydrogel
containing pH indicator Dyes
Submitted by
Chetan Chatale(T4708)
Anshul Gautampurkar(T4715)
Nitin Golait(T4719)
in partial fulfilment for the award of the degree
of
Degree of Bachelor of Technology
in
Plastics and Polymer Engineering
At
MAHARASHTRA INSTITUTE OF TECHNOLOGY
AURANGABAD
2013-14

2
CERTIFICATE
This is to certify that the project report
Submitted by
Anshul Gautampurkar (T4715)
Nitin Golait(T4719)
Is completed as per the requirement of the Dr. Babasaheb Ambedkar
Marathwada University, Aurangabad in partial fulfilment of Degree of
Bachelor of Technology, Plastics and Polymer Engineering
For the academic year 2013- 2014.
Mrs. A.S. Dutta Mrs. S. Mandal Dr. S.P. Bhosale
(Guide) (Head of Department) (Principal)

3
ACKNOWLEDGEMENT
We take this opportunity to express our profound gratitude and deep regards to
our guide, Mrs. Astha Dutta for her exemplary guidance, monitoring and
constant encouragement throughout the course of this report. The blessing, help
and guidance given by her time to time shall carry us a long way in the journey
of life on which we are about to embark.
We also take this opportunity to express a deep sense of gratitude to Mrs.
Suranjana Mandal, Head of Plastics and Polymer Engineering Department,
MIT, for her cordial support, valuable information and guidance, which helped
us in completing this task through various stages.
We are obliged to the staff members of Plastics and Polymer Engineering
Department, for the valuable information provided by them in their respective
fields. We are grateful for their cooperation during the period of our assignment.
Lastly, we thank almighty, our families and friends for their constant
encouragement without which this assignment would not be possible.
Anshul Gautampurkar(T4715)
Nitin Golait(T4719)

4
INDEX
Sr.
No.
Content Page No.
1. Introduction 5
2. Hydrogel 6
3. Nature- Phenolphthalein 7
4. Nature- Bromothymol Blue 8
5. Borax 9
6. Process- Experimental 10-13
7. Testing
a) Durability and Weatherability 14
b) pH Testing 14
c) UV-Vis Spectroscopy Basics 15-17
d) UV- Vis Spectroscopy Testing 18-19
8. Discussion 20
9. Conclusion 21
10. References 22-23

5
INTRODUCTION
In recent years, functional polymers changing their visible optical properties in
response to an external stimulus have met with growing interest. According to
the external stimulus which affects the optical properties, these so-called
chromogenic polymers are classified as thermochromic (stimulus: temperature),
photochromic (stimulus: light), electrochromic (stimulus: electric field),
piezochromic( stimulus: pressure), ionochromic ( stimulus: ion concentration)
and biochromic (stimulus: biochemical reaction). Because of their advanced
properties the demand on chromogenic polymers for future applications will
become enormous. Smart windows, tunable light filters, large area displays as
well as sensors, which can visualize, eg., temperature or pressure profiles, are
the most important potential innovations based on chromogenic polymers. For
all these applications laboratory prototypes demonstrating the effect have been
presented. A few of them have already reached the readiness for marketing and
certainly others will follow in the near future.
BASIS
The well-known pH-indicators, phenolphthalein and bromothymol blue,
exhibit an outstanding thermochromism, when they are embedded in aqueous
polyvinyl alcohol/borax gel networks. The color of phenolphthalein in
hydrogels changes gradually from colorless at 20°C to dark red at 70°C. The pH
change in a hydrogel system was from 8.0 to 8.9 with increasing temperature. In
the case of bromothymol blue, the color was optically clear green at room
temperature and changed to clear blue with increasing temperature.

6
HYDROGELS
The advantages of using hydrogels are that they are:-
 Biologically degradable,
 Innocuous,
 Free of an organic solvent,
 Inexpensive,
 Available in large quantities,
 Non-flammable,
 High transparency, and
 Have a reasonable colour-transition temperature.
However, it is not easy to select an appropriate surfactant that enables
organic dyes to be miscible with aqueous hydrogel networks. Therefore, we
present a surfactant-free system that may simplify the manufacturing process
and improve the maintenance of transparency over time.

7
NATURE- PHENOLPHTHALEIN
pH <0 0-8.2 8.2-12 >12.0
Conditions Strongly
Acidic
Acidic or
Near
Neutral
Basic Strongly
Basic
Colour Orange Colourless Pink to
Fuchsia
Colourless
Image

8
BROTHYMOL BLUE- NATURE
BTB indicator in acidic, neutral, and alkaline solutions (left to right)
Bromothymol blue acts as a weak acid in solution. It can thus be in protonated
or deprotonated form, appearing yellow or blue respectively. It is bluish green
in neutral solution. The deprotonation of the neutral form results in a
highly conjugated structure, accounting for the difference in colour.

9
BORAX
• Borax, also known as sodium borate, sodium tetraborate, or disodium
tetraborate, is an important boron compound, a mineral, and
a salt of boric acid. Powdered borax is white, consisting of soft colorless
crystals that dissolve easily in water.
• Borax has a wide variety of uses. It is a component of
many detergents, cosmetics, and enamel glazes. It is also used to
make buffer solutions in biochemistry, as a fire retardant, as an anti-
fungal compound for fiberglass, as a flux in metallurgy, neutron-capture
shields for radioactive sources, a texturing agent in cooking, and as a
precursor for other boron compounds
• Buffer
• Sodium borate is used in biochemical and chemical laboratories to
make buffers, e.g. for gel electrophoresis of DNA, such as TBE or the
newer SB buffer or BBS (borate buffered saline) in coating procedures.
Borate buffers (usually at pH 8) are also used as preferential equilibration
solution in DMP-based crosslinking reactions.
• Co-complexing
• Borax as a source of borate has been used to take advantage of the co-
complexing ability of borate with other agents in water to form complex
ions with various substances. Borate and a suitable polymer bed are used
to chromatograph non-glycosylated hemoglobin differentially from
glycosylated hemoglobin (chiefly HbA1c), which is an indicator of long
term hyperglycemia in diabetes mellitus.

10
PROCESS
EXPERIMENTAL
Preparation:
Various attempts were made for the purpose of preparing a surfactant free
thermochromic hydrogel which showed excellent colour change properties.
Three of the indicators were tried as pH indicator with the PVA/Borax
hydrogel, namely,
 Phenolphthalein
 Bromophenol Blue
 Bromothymol Blue
Experiment No. 1
Steps-
 A 15% aqueous solution of Poly-Vinyl Alcohol was prepared by adding
5.4 grams PVA granules to 36 grams of distilled water. After constant
heating and stirring, a viscous liquid is produced which is the desired
solution.
 A 3% borax solution was prepared by adding 0.06 grams of borax powder
into 2 ml of distilled water.
 Butanolic solution of phenolphthalein indicator was prepared by adding
0.07 grams of phenolphthalein indicator into 10 ml of butanol. As a
surfactant free system was to be prepared, butanol was used in place of
the commonly used ethanol as butanol itself acts like a surfactant.
 A 2ml of 1M NaOH solution was prepared for maintaining the
environment of the reaction.
 Now, the aq. PVA solution was added to the aq. Borax solution along
with the butanolic solution of phenolphthalein indicator and a few drops
of NaOH solution in a 3 necked flask and the whole reaction mixture was
subjected to refluxing.

11
 The temperature of the reaction was maintained to 40°C, but after 10
minutes the stirring stopped due to the formation of a highly crosslinked
dark pink coloured structure, without any signs of colour change.
The possible reasons for the failure of prepared sample were:
 A very high concentration of aq. PVA was used which itself had a high
molecular weight (1,40,000 Mw), thereby resulting in a highly
crosslinked structure.
 The inclusion of NaOH which accelerated the crosslinking process.
Experiment No. 2
Steps-
 Raw material 1:
5% aqueous PVA solution (1.6 gm PVA in 40 gm distilled water).
 Raw material 2:
3% borax solution (0.3gm of borax in 10 ml of distilled water).
 Raw material 3:
Butanolic solution of phenolphthalein indicator (0.07gm phenolphthalein
in 7 ml butanol).
 The stated raw materials were added to a three necked flask
simultaneously.
 The reaction mixture was then subjected to continuous stirring and
temperature of about 40⁰C to 50⁰C was supplied to it for about 50
minutes within a refluxing assembly.
 Result:
A jelly mass which at elevated temperatures showed red colour, while at
low temperatures appeared translucent was achieved.

12
The exclusion of NaOH from the recipe worked as only physical
crosslinking between PVA and Borax occurred resulting in a hydrogel
formation.
Experiment No. 3
Steps-
 Raw material 1:
 Raw material 2:
3% borax solution (0.3gm of borax in 10 ml of distilled water).
 Raw material 3:
Butanolic solution of Bromophenol Blue indicator (0.07gm
phenolphthalein in 7 ml butanol).
 Similar process was imparted for preparing the third batch, with an only
exception of the change in the pH indicator, that was changed from
phenolphthalein to bromophenol blue.
 Result-
A viscous, dark purple coloured mass which did not exhibit thermo-
chromic behaviour was achieved.
 Reasons for the result-
The amount of borax used was more for the particular pH indicator, now
as borax is basic in nature the bromophenol blue indicator showed dark
purple colour in basic environment.
This system cannot show thermochromic behaviour as, with other
indicators as the temperature rises the pH of the solution increases which
triggers the pH indictor to change its colour, but in the case of
bromophenol blue indicator the colour change only occurs when the pH
of the environment is lowered.

13
Experiment No. 4
Steps-
 Raw material 1:
 Raw material 2:
2.5ml of 3% borax solution (0.3gm of borax in 10 ml of distilled water).
 Raw material 3:
Butanolic solution of Bromothymol Blue indicator (0.07gm bromothymol
blue in 7 ml butanol).
 The stated raw materials were added to a three necked flask
simultaneously.
 The reaction mixture was then subjected to continuous stirring and
temperature of about 40⁰C to 50⁰C was supplied to it for about 50
minutes within a refluxing assembly, with an only exception of the
change in the pH indicator, that was changed from phenolphthalein to
bromothymol blue.
 Result-
A jelly like substance showing thermochromism with the colour change
from clear green to clear blue at elevated temperatures.

14
Testing
 Durability and Weatherability-
 Depending upon the application, the durability of the gel varies.
 As, if it is exposed to outer atmosphere the gel solidifies and then exhibits
very slight colour change properties.
 But if it is stored in a packed container it has good durability.
 Has a good shelf life.
 The hydrogel cannot bear any physical stress as water exudes out of it
possibly because of the compressionof the hydrogel.
 pH test-
The pH of the hydrogel at different temperature was noted.

15
UV-Vis Spectroscopy:
Basic Concepts
Radiation is a form of energy and we are constantly reminded of its presence via
our sense of sight and ability to feel radiant
heat. It may be considered in terms of a wave motion where the wavelength, λ,
is the distance between two successive peaks.
The frequency, ν, is the number of peaks passing a given point per second.
These terms are related so that:
c =νλ
where c is the velocity of light in a vacuum.
The wavelength λ of electromagnetic radiation
The full electromagnetic radiation spectrum is continuous and each region
merges slowly into the next. For spectroscopy purposes, we choose to
characterize light in the ultraviolet and visible regions in terms of wavelength
expressed in nanometers.
Other units which may be encountered, but whose use is now discouraged, are
the Angstrom (Å) and the millimicron (mμ).
1nm = 1mμ = 10Å = 10-9 meters

16
The human eye is only sensitive to a tiny proportion of the total electromagnetic
spectrum between approximately 380 and 780
nm and within this area we perceive the colors of the rainbow from violet
through to red. If the full electromagnetic spectrum
shown in Figure 2 was redrawn on a linear scale and the visible region was
represented by the length of one centimeter, then
the boundary between radio and microwaves would have to be drawn
approximately 25 kilometers away!

17
The electromagnetic spectrum
Hydrogel was fully transparent and colorless at room temperature. On heating,
the color changed gradually from colorless at 20°C to dark red at 70°C. The
temperature dependent visible absorption spectrum of gel in the temperature
range from 20 to 70°C. The absorption intensity of the gel at λmax 552 nm
increases with increasing temperature, indicating that the phenol form of
phenolphthalein is converted to the phenolate form. The pH value in the
hydrogel increased with increasing temperature and with decreasing viscosity of
the hydrogel. It is known that the crosslink density in the hydrogel decreases
when the temperature is increased. The reactions between borate ions and
hydroxyl groups of PVA led to the monodiol–borate complex formation and
didiol–borate complexes, so-called crosslinks, governed by change of
temperature.

18
Result:
1% Solution-
For 1% concentrated solution the absorbency for the peak wavelength is found
to be 0.142

19
2% solution
For 2% concentrated solution the absorbency at peak wavelength is
0.197.

20
Application I
The main application of the prepared hydrogel lies in “Smart Windows”.
In this the hydrogel is sandwiched between two layers of glass.
This assembly is itself used as a window.
At lower temperatures, which mainly occurs in winters, the passage of sunlight
in the form of heat energy should be more, the hydrogel remains as it is, i.e.,
transparent, which serves for the purpose.
While in summers, the hydrogel is supposed to change its color from
transparent to opaque, which apparently inhibits the passage of light as well as
heat through the glasses.
2,6-diphenyl-4-(2,4,6-triphenylpyridinio)phenolate (DTPP), an indicator die
which can be incorporated into the hydrogel thereby obtaining an excellent
thermochromism.
This assembly can be effectively used in the case of the so called “Smart or
Intelligent Windows”.
Application II
Phenolphthalein-PVA/Borax hydrogel is the cheapest assembly amongst all the
tried combinations.
This assembly can be used to test the nature of a reaction mixture/solution.
If the gel dissolves into the solution then the solution can be said to be acidic.
If the gel turns red inside the solution, then the solution can be said to be basic.
The test can be named as “Slime test”.

21
Discussion
Instead of the using a surfactant to prevent turbidity, normal butanol was used.
Butanol exhibited the same striking thermochromic behavior of phenolphthalein
and bromothymol blue in the hydrogel networks without turbidity. Unlike these
surfactant-free hydrogel networks, thermochromic hydrogels that contain
surfactant can become turbid when an unsuitable surfactant is used. A suitable
surfactant can be selected only experimentally. In contrast to the neutral or basic
PVA–gel networks where the pH value increased with increasing temperature,
we observed a significant decrease in pH of the acidic hydrogel networks with
increasing temperature. Generally, the borax aqueous solution acts as a buffer
(pH =9.5), and there is no change in pH with increasing temperature. Because of
this buffering, we assumed that appropriate indicators, which change their color
under acidic conditions, could show the thermochromic behavior in the acidic
hydrogel networks. Bromophenol blue, which has a color transition from purple
at pH 4.6 to yellow at pH 3.0, was applied to the acidic PVA–gel network, but
the bromophenol blue in our investigation exhibited no significant color change
with increasing temperature in the hydrogel networks. A temperature-induced
decrease in pH might not be sufficient to change their color.

22
Conclusion
The new surfactant-free thermochromic hydrogel system was made from
PVA/borax gel networks containing phenolphthalein or bromothymol
blue, which are cheap and common pH indicators. It is likely that other
pH-sensitive dyes will show a similar behaviour in hydrogel networks.
The PVA/Borax Hydrogel network thus prepared shows excellent elastic-
jelly like properties which can itself be applied into many applications,
and the inclusion of pH sensitive dies into the hydrogel only multiplies its
applications, with the most important property of the gel being its
thermochromism. Various pH indicators have the needed compatibility
with the gel, thus show the colour changing property. Additional studies
are currently being conducted on the thermochromic gels having
improved properties and more desirable colors. In the future, these
networks with high transparency will be economical substitutes in
electrochromic or photochromic applications, such as car rearview
mirrors, large-scale traffic direction boards, sunprotecting smart
windows, and optical memory cells. The two main applications of the gel
have been indicated in this report, while there are numerous other
applications, which still not found, and are waiting to be discovered.

23
REFERENCES: -
• P. Banfield, Chromic Phenomena – Technological applications of colour
chemistry, The Royal Society of Cemistry, Cambridge, (2001).
• T. Tsutsui and T. Tanaka, Polymer 21, 1351 (1980).
• T.-A. Yamagishi and P. Sixou, Polymer 36, 2315 (1995).
• N. Tamaoki, Adv. Mater. 13, 1135 (2001).
• H. Finkelmann, J. Koldehoff, and H. Ringsdorf, Angew. Chem., 17, 935
(1978).
• D.N. Batchelder, Contemp. Phys. 29, 3 (1988).
• M. Leclerc, Adv. Mater. 11, 1491 (1999).
• P.T. Hammond and M.F. Rubner, Macromolecules 30, 5773 (1997).
• T. Skotheim, J.R. Reynolds and R.L. Elsenbaumer, eds, Handbook of
conducting polymers, Second Edition, Marcel Dekker, New York,
(1997).
• H.R. Wilson, SPIE 2255, 214 (1994).
• H.R. Wilson, SPIE 2255, 473 (1994).
• R.S. Werbowji and D.G. Gray, Macromolecules 13, 69, (1980).
• B. Yildiz, B. Isik, and M. Kis, Eur. Polymers.J.38, 1343, (2002).
• A. Seeboth, D. Lotzsch, and E. Potechius, Colloid Polymer Science 279,
696, (2001).
• J. Kriwanek, R.Vetter, D. Lotzch, and A. Seeboth, Polym., Advc.
Technol. 14, 79 (2003)
• Lee, K. Y.; Mooney, D. Chem Rev 2002, 101, 1869 . Watanabe, M.;
Akahoshi, T.; Tabata, Y.; Nakayama, D. J Am Chem Soc 1998, 120,
5577

Thermochromic hydrogel

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Thermochromic hydrogel

Similar to Thermochromic hydrogel (20)

Recently uploaded

Recently uploaded (20)

Thermochromic hydrogel