2 - Macromolecular solutions and hydrogels

Exercise 12.

Determination of polymer molecular mass from viscosity measurements

Means: Ostwald viscometer, stop-watch, 50-ml volumetric flask, 25-ml volumetric pipette,

10-ml volumetric pipette, 5 pieces of 50-ml beaker, balance (accuracy 0.01 g)

Materials: polymer: PEG (polyethylene glycol), samples of different molar masses, solvent:

distilled water

Instruction:

Prepare PEG stock solution by measuring a given amount between 2.5-6.5 g PEG and

dissolve it into a 50-ml volumetric flask.

IMPORTANT: Polymers can dissolve slowly in good solvent only. Their dissolution starts

with swelling. Observe the swelling of solid PEG pieces. Remove the air bubbles adhered on

solid phase by gentle rotation. Do not shake the flask before filling up to the meniscus! PEG

solution foams strongly.

Make dilution series by repeated double dilution of stock solution in such a way that

remove 25 ml of stock solution from 50-ml volumetric flask into a 50-ml beaker, then dilute

the remaining 25 ml by distilled water, homogenize it and take out 25 ml of dilute solution

and pour into another 50-ml beaker. Repeat this dilution process twice more.

Measure the viscosity of aqueous solutions with capillary viscometer. Pipette 10 ml of water

first and measure the flow time of water three times, then that of the solutions with increasing

concentration, each three times. The dilute aqueous solutions of PEG (polyethylene glycol)

are Newtonian liquids, their viscosity does not depend on the flow rate, it is constant at a

given concentration and constant temperature.

Summarize the result in a Table

Plot the ηspec/c as a function of c to determine the intrinsic viscosity [η] (see Fig. 26), finally

calculate the molecular weight by using Mark and Houwink equation and constants K = 4.28

10-2

cm3/g and a = 0.64.

Hydrogels

Hydrogels are crosslinked polymeric networks, which have the ability to hold water

within the spaces available among the polymeric chains. The hydrogels have been used

extensively in various biomedical applications, viz. drug delivery, cell carriers and/or

entrapment, wound management and tissue engineering. The water holding capacity of the

hydrogels arise mainly due to the presence of hydrophilic groups, viz. amino, carboxyl and

hydroxyl groups, in the polymer chains, it is dependent on the number of the hydrophilic groups

and crosslinking density.

Hydrogels can be classified into two groups depending on the nature of the

crosslinking reaction. If the crosslinking reaction involves formation of covalent bonds, then

PEG solutions Flow time, s ηrel =tsol/tw ηrel-1 = ηspec ηspec/c

Dilution c, g/cm3 1. 2. 3. Average

∞ (water) 0 1 0 - 8x 4x 2x no

the hydrogels are termed as permanent or chemical hydrogel. If the hydrogels are formed

due to the physical interactions, viz. molecular entanglement, ionic interaction and hydrogen

bonding, among the polymeric chains then the hydrogels are termed as physical hydrogels.

The examples of physical hydrogels include polyvinyl alcohol-glycine hydrogels, gelatin gels

and agar-agar gels. There are so-called stimuli responsive hydrogels, which change their

equilibrium swelling with the change of the surrounding environment. E.g.., the pH sensitive

hydrogels have been used since long in the pharmaceutical industry. The swelling of hydrogels is

characterized by the percentage swelling of the hydrogel, which is directly proportional to the

amount of water imbibed within the hydrogel. Rheological analysis The characterization of hydrogels using rheological properties

has been done since long. The hydrogels have been well classified by rheological techniques.

Taking a lesson from the food industries, scientists are trying to use this powerful technique

for the characterization of the polymers and hydrogels.

Rheology is the study of the deformation of matter including flow. The flow is

primarily assigned to the liquid state, but also as 'soft solids' or solids under conditions in

which they respond with plastic flow rather than deforming elastically in response to an

applied force. Newtonian fluids can be characterized by a single coefficient of viscosity for

a specific temperature. Although this viscosity will change with temperature, it does not

change with the flow rate or strain rate. But for a large class of fluids, the viscosity change

with the strain rate (or relative velocity of flow) and are called non-Newtonian fluids.

Basic deviations from Newtonian behaviour of liquid flow are summarized and compared to

the Newtonian fluids in a Table below; their characteristic flow curves are showed in Fig. 27.

Character Types Characterization Examples

Non-Newtonian fluids

Shear thickening

(dilatant)

Apparent viscosity increases

with increased stress.

Suspensions of corn

starch or sand in water

Non-Newtonian fluids

Shear thinning

(pseudoplastic)

Apparent viscosity decreases

with increased stress.

Paper pulp in water,

latex paint, ice, blood,

syrup, molasses

Time-

independent

viscosity

Newtonian fluids

Viscosity is constant

Stress depends on normal and

shear strain rates and also the

pressure applied on it

Blood plasma, water

Time-

dependent

viscosity

Thixotropic Apparent viscosity decreases

with duration of stress.

Some clays, some

drilling mud, many

paints, synovial fluid.

Thixotropy is the property of certain gels or fluids that are thick (viscous) under

normal conditions, but flow (become thin, less viscous) over time when shaken, agitated, or

otherwise stressed. They then take a fixed time to return to a more viscous state. In more

technical language: some non-Newtonian pseudoplastic fluids show a time-dependent change

in viscosity; the longer the fluid undergoes shear stress, the lower its viscosity. A thixotropic

fluid is a fluid which takes a finite time to attain equilibrium viscosity when introduced to a

step change in shear rate. Some thixotropic fluids return to a gel state almost instantly, such

as ketchup, and are called pseudoplastic fluids. Others such as yogurt take much longer and

can become nearly solid. Many gels and colloids are thixotropic materials, exhibiting a stable

form at rest but becoming fluid when agitated.

Non-Newtonian and Newtonian liquids

Thixotropic system

Pseudoplastic system

Flow curve Viscosity curve

Fig. 27. Flow curves characteristic of different flow types.

Exercise 13.

Rheological characterization of CMA hydrogels

The CMA (carboximetil (-CH2-COO-Na+) amylopectin (Fig. 28)) is a branched chain

polysaccharide, it is a gelatinizing material made of starch. Starch occurs in different plants,

it is built up from different sugar molecules with chemical formula (C6H1005)n. The number

of monomers is between 10 and 500 thousands.

Fig. 28. A part molecule structure of a branched chain polysaccharide (left side) and a

carboximetil amylopectin (right side)

Shear thinning

Shear thickening Newtonian

Shear rate gradient

Sh

ear

stre

ss

D, 1/s

ττττ, Pa

ττττ = ηηηη D

slope

ηηηη, Pa s

D, 1/s

ηηηη0

ηηηη∞∞∞∞

Shea

r st

ress

Thixotropic loop

Shear rate gradient

The aim of exercise is to show the effect of CMA concentration and solution pH on the

structure formation in CMA gels and their rheological behaviour.

Means: RHEOTEST-II rotational viscometer, balance (accuracy 0,01 g), 3 pieces of 100-ml

beaker, 3 glass rod, 50-ml measuring cylinder, 10-ml measuring pipette, spoon

Materials: CMA samples, distilled water, 0.1 M NaOH solution, universal pH paper

Instruction:

Study either the concentration (1) or the pH (2) dependence!

1) CMA concentration dependence at constant pH: Weigh three different amounts (x)

of CMA between 1 to 1.8 g in three pieces of 100-ml beaker and add (50-x) ml of

distilled water by a measuring cylinder to each, and mix them with a glass rod

thoroughly. Pay attention that each beaker contains different mass of CMA and water, but

the total mass of each CMA gel is the same. CMA swells well and transparent hydrogel

forms during an hour.

2) pH dependence at constant CMA concentration: Weigh a given amount (x) of

CMA between 1 to 1.8 g three times in three pieces of 100-ml beaker and add (50-x-y) ml

of water by a measuring cylinder to each, and mix them with a glass rod thoroughly. After

~10-minute-standstill, keep the original pH in one of the beakers, and add 2 different

volumes of 0.1 M NaOH (y) between 1– 5 ml (e.g., 1 and 3, 2 and 4 or 2,5 and 5 ml) by

means of measuring pipette into the other two beakers. Pay attention that each beaker

contains the same mass of CMA and the sum of water and base solution is also the same,

therefore the CMA concentration is constant, only the pH is different. CMA swells well

and transparent hydrogel forms during an hour.

After about 1 hour standing, spoon a necessary amount of gel into the given

measuring cylinder of viscometer (Fig. 29). Choose an appropriate cylinder in the order of

S1, S2 or S3 as the viscosity of gels increased. Measure the torsion moment (α) first in the

direction of increasing shear rate (up or forward curve) by switching gear gradually from 1a,

2a, … up to 12a, then of decreasing shear rate (down or downward curve) from 11a, 10a,…..

1a. Attention: if α value reaches 100 scale, switch the measuring-limit from I to II.

To plot flow curves, first copy the shear rate gradient, D (1/s) values belonging to the

given cylinder and the gears 1a, 2a, etc. from Table 8, and calculate the shear stress, τ (Pa)

values by means of Z constant in Tables 9 belonging to the given cylinder of Rheotest II.

Summarize the values in a Table.

Shear rate Scale, α measured Shear stress

Class 1a ... D, 1/s up down up τ, Pa down τ, Pa

Plot the flow curves, i.e., the measured values of shear stress, τ (Pa) as a function shear rate

gradient, D (1/s). Evaluate the flow character of CMA gels.

Sign of inner cylinder S1 S2 S3

Volume required, ml 25 30 35

Fig. 29. The schematic picture of Rheotest II rotational viscometer

1. measuring-limit switch 3. inner cylinder

5. outer cylinder 7. display of gear

8. switch of gear 9. α-meter (torsion)

12. button for motor 13. button for α-meter

Table 8. Values of shear rate gradient (D, 1/s) at the different gear for use of Rheotest II

rotational viscometer

Gear Inner

cylinder class 1a 2a 3a 4a 5a 6a 7a 8a 9a 10a 11a 12a

S1 3.0 5.4 9.0 16.2 27.0 48.6 81.0 145.8 243 437.4 729.0 1312.0

S2 1.0 1.8 3.0 5.4 9.0 16.2 27.0 48.6 81.0 145.8 243.0 437.4

S3

D

1/s 0.33 0.6 1.0 1.8 3.0 5.4 9.0 16.2 27.0 48.6 81.0 145.8

Table 9. The Z constants of cylinders to calculate the shear stress (τ = Z·α, Pa) values from

torsion (α) for use of Rheotest II rotational viscometer

Measuring-limit

I II

Cylinder Needed volume Z (Pa/scale)

S/S1 25 mL 0.584 5.82

S/S2 30 mL 0.608 5.93

S/S3 35 mL 0.792 7.88

Questions:

1. Write the most important properties of macromolecular solutions!

2. What kind of solvent is appropriate for dissolution of linear polymers?

3. How can you characterize the conformation of macromolecular coils? How does it change

in good solvent?

4. What is theta-state? Does it influence by temperature?

5. Which properties of macromolecular solutions are suitable for determination of molecular

mass?

6. What kind of viscosity is characteristic of macromolecules dissolved in a solvent? Is it

related to the molecular weight?

7. How can you measure the relative viscosity of a solution?

8. How can you determine the intrinsic viscosity of macromolecular solution from the

measured viscosity data?

9. What are hydrogels? Classify them into two groups depending on the nature of the

crosslinking reaction!

10. What is rheology? What is flow curve?

11. List the basic deviations from Newtonian behaviour of liquids!

12. Explain the shear thinning flow behaviour!

13. What is the thixotropy?

14. Draw the flow and viscosity curves of a pseudoplastic system!

15. What is CMA?