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


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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)


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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)


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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


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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.


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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.


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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


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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.


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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.


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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.


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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.


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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.


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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.


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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 compression of the hydrogel.

pH test-

The pH of the hydrogel at different temperature was noted.


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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


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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!


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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.


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Result:

1% Solution-

For 1% concentrated solution the absorbency for the peak wavelength is found

to be 0.142


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2% solution

For 2% concentrated solution the absorbency at peak wavelength is

0.197.


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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”.


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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.


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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.


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thermochromic hydrogel

Science

thermochromic stimulus

external stimulus

piezochromic stimulus

biochromic stimulus

ionochromic stimulus

electrochromic stimulus

photochromic stimulus

polymer engineering