Molecular Dynamics of Water Responsive Polymers and Applications.

By

Koushik .B

4th year B.Tech Chemical Engineering

SSN College of Engineering

ACKNOWLEDGEMENTThe internship opportunity I had with Applied Mechanics Department, IIT Madras was a great platform for learning and professional development. Therefore, I consider myself as a very lucky individual as I was provided with an opportunity to be a part of it. I am using this opportunity to express my deepest gratitude and special thanks to Dr. Pijush Ghosh who guided me and kept me on the correct path and allowed me to carry out my project at their esteemed organization.

I express my deepest gratitude to Ms. Amrita Rath, for taking part in useful decision & giving necessary advices and guidance. I choose this moment to acknowledge her contribution gratefully.

I perceive this opportunity as a big milestone in my career development. I will strive to use the gained skills and knowledge in the best possible way, and I will continue to work on their improvement, in order to attain the desired career objectives.

ABSTRACTChitosan is a linear polysaccharide, which is generally found in shrimp’s shells. Chitosan has significant applications in commercial and biomedical sector. It is observed that the chitosan film undergoes deformation due to water uptake, where the surface hydrophilicity undergoes changes. The report deals with the study of this self-folding property of chitosan in the presence of water. The chitosan films used in the study were prepared by the process of deacetylation of chitin where HAP or glutaraldehyde was mixed with 1:1 v/v glacial acetic acid. The parameters used in the study are, deformation of those samples at different time intervals and variation of deflection angles.

Keywords: Polysaccharides, hydrophilicity, deacetylation.

INTRODUCTIONChitosan is a polysaccharide that is obtained from chitin, which is found in the hard outer skeleton of shellfish, crab, lobster and shrimp.

It is composed of randomly distributed β-(1-4)-linked D-glucosamine (deacetylated unit) AMD N-acetyl-D-glucosamine (acetylated unit).

MANUFACTUREChitosan is a chemically processed form of chitin, which is the structural element in the exoskeleton of crustaceans (such as crabs and shrimp) and cell walls of fungi. It is done so by deacetylation of chitin. The degree of deacetylation (%DD) can be determined by NMR spectroscopy, and the %DD in commercial chitosans ranges from 60 to 100%. On average, the molecular weight of commercially produced chitosan is between 3800 and 20,000 amu (atomic mass units). A common method for the synthesis of chitosan is the deacetylation of chitin using sodium hydroxide in excess as a reagent and water as a solvent. This reaction pathway, when allowed to go to completion (complete deacetylation) yields up to 98% product. In the project done, two varied samples of chitosans, cross linked polymers and HAP polymers are used. The chitosan films used in the study were prepared by mixing HAP or glutaraldehyde with 1:1 v/v glacial acetic acid.

APPLICATIONSChitosan has a wide range of uses in pharmaceutical, agricultural and commercial sector. It can be used as a soluble dietary fiber, a

biopesticide, a seed treatment, an anti – bacterial agent and also in cosmetic and fabric industry. Though controversial, there are instances where chitosan has been used to treat obesity, high cholesterol, and Crohn’s disease. It is claimed that it might block absorption of dietary fat and cholesterol to support the former situation.

The agricultural and horticultural uses for chitosan are primarily for plant defense and yield increase. Here the glucosamine polymer influences the biochemistry and molecular biology of the plant cell. In the case of plant defense, it acts as ecologically friendly bio pesticide substance that boosts the innate ability of plants to defend themselves against fungal infections. Whereas in the other case, chitosan increases photosynthesis, promotes and enhances plant growth, stimulates nutrient uptake, increases germination and sprouting, and boosts plant vigor.

The commercial uses include those of water filtration, as a fining agent in wine making and as a polyurethane coating etc.

Whereas in the case of pharmaceutical industry, it is used as hemostatic agent in bandages. Also chitosan's properties allow it to be used in transdermal drug delivery. The popular usage includes transportation of insulin.

SELF FOLDING PROPERTY

Self-folding broadly refers to self-assembly processes wherein thin films or interconnected planar templates curve, roll-up or fold into three dimensional (3D) structures such as cylindrical tubes, spirals, corrugated sheets or polyhedral.

Self-folding methods are important for drug delivery applications since they provide a means to realize 3D, biocompatible, all-polymeric containers with well-tailored composition, size, shape, wall thickness, porosity, surface patterns and chemistry. Self-folding is also a highly parallel process, and it is

possible to encapsulate or self-load therapeutic cargo during assembly. A variety of therapeutic cargos such as small molecules, peptides, proteins, bacteria, fungi and mammalian cells have been encapsulated in self-folded polymeric containers.

Self-folding can occur spontaneously when 2D planar structures are released from a substrate, typically on dissolution of a sacrificial layer, or in response to stimuli such as electrical signals, pH, temperature, magnetic fields or chemicals.

CHITOSAN IN THE PRESENCE OF WATER

It has been observed that, chitosan samples undergo the same self – folding phenomenon in the presence of water. In this case, this effect is observed due to the differential surface hydrophilicity after water is absorbed by the polymer when exposed to it.

The deformations of different chitosan samples used are as follows:

1. CCLCSHAP200 (Successive images at a time interval of 5sec)

CLCSHAP300 (Successive images at a time interval of 5sec)

2. CLCSHAP500 (Successive images at a time interval of 5sec)

1. CLCSHAP300

2. CLCSHAP500

3. CSHAP10

4. PURE SAMPLE

The deflection angle of a chitosan sample in the folding phenomenon is the angle subtended by the tangent at the extreme point of the sample with respect to the horizontal.

The deflection of the different samples and the rate of change of those angles at different time intervals are as follows:

1. CLCSHAP200

2. CLCSHAP300

3. CLCSHAP500

4. CSHAP10

5. PURE SAMPLE

Rate of Diffusion studies

Fick's first law relates the diffusive flux to the concentration under the assumption of steady state. It postulates that the flux goes from regions of high concentration to regions of low concentration, with a magnitude that is proportional to the concentration gradient (spatial derivative), or in simplistic terms the concept that a solute will move from a region of high concentration to a region of low concentration across a concentration gradient.

Log(qT/qE)= nlog(T)+log(K)

Where,

qT= mass of water at time T.

qE= mass of water at time =infinity.

n is a constant of proportionality, which decides the type of diffusion.

Time Wt1 Wt2 Wt3 DIff1 Diff2 DIff30 3.3 3.5 3.6

10 7.14 7.3 7.22 3.84 3.8 3.4220 8.68 8.74 8.58 5.38 5.24 4.98

30 8.9 9.02 8.94 5.6 5.52 5.34

40 9.24 9.16 9.08 5.94 5.66 5.4850 10.16 10.18 9.88 6.86 6.68 6.2860 11.15 10.88 10.66 7.85 7.38 7.0670 11.27 11.15 11.2 7.97 7.65 7.680 11.33 11.32 11.39 8.03 7.82 7.7990 11.37 11.37 11.31 8.07 7.87 7.71

SUPER ABSORBING POLYMERS

Sodium polyacrylate is an example of a super-absorbing polymer. It is a cross-linked (network) polymer that contains sodium atoms. It absorbs water by a process called osmosis.

When the (sodium-containing) polymer is placed in contact with water, there is a tendency for the sodium to distribute equally between the network and the water.

That means, some of the sodium atoms want to leave the network and move to the water. When these sodium atoms leave, they are replaced with water molecules. Water swells the polymer network to try to keep the sodium concentration balanced between the polymer and the water.

The cross-links that connect the chains together prevent them from dissolving/breaking apart in the water. Sodium polyacrylate can absorb 800 times its weight in distilled water, but only 300 times its weight in tap water, since tap water contains some sodium, calcium and other mineral salts.

SWELLING MEASUREMENTS

Each sample was placed into distilled water for swelling at room temperature. Swelling process was controlled by weighting. The ability

for swelling was expressed as the swelling ratio, W, in which M1 and M2 are the weights of swollen and dry samples:

W = M1/M2

Effect of additional temperature treatment at the temperatures 120, 150, 180 °C after complete water evaporation on swelling ability was measured.

FOLDING PATTERNS ON HYDROGELS

OBSERVATIONS

The following conclusions can be drawn from these readings:

1. The deflection is seen more in Pure sample and CSHAP10, where there are chances for a deflection of 180 degrees or more.

2. In the case of cross linked polymers, as the concentration of chitosan increases, the folding property subtly decreases.

3. The folding process is slower in cross linked polymers (which in this project took around a time interval of 30sec), whereas in the case of pure sample and HAP polymer, its comparatively faster (took around 10sec to deflect completely).

4. In the case of cross linked polymers, the chances of getting a deflection angle as supplementary is comparatively less.

5. The rate of change of deflection angle decreases after a particular time. Which is obvious, as the sample gradually slows down the deformation and returns to its original state in the reverse process.

Future Scope

To study the different surface patterns on the chitosan films.

To study whether inert gases like nitrogen, helium can be absorbed in chitosan films.

To coat temperature resistant materials on the film surface to study folding characteristics at elevated temperatures.

Work on the implementation chitosan in its potential applications.

REFERENCES:

1. https://en.wikipedia.org/wiki/Chitosan 2. http://www.researchgate.net/publication/

6437739_Water_Absorption_and_Degradation_Characteristics_of_Chitosan-Based_Polyesters_and_Hydroxyapatite_Composites

3. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3462897/#R88 4. http://www.webmd.com/vitamins-supplements/ingredientmono-625-

chitosan.aspx?activeingredientid=625&activeingredientname=chitosan

http://www.drugs.com/npc/chitosan.html