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
PEG solutions Flow time, s ηrel =t sol /t w ηrel 1 = ηspec ηspec /c Dilution c, g/cm 3 1. 2. 3. Average ∞ (water) 0 1 0 8x 4x 2x no
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 cm 3/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 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 fluid s 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 Apparent viscosity increases Suspensions of corn Shear thickening with increased stress. starch or sand in water (dilatant) Non Newtonian fluids Paper pulp in water, Time Apparent viscosity decreases Shear thinning latex paint, ice, blood, independent with increased stress. (pseudoplastic) syrup, molasses viscosity Viscosity is constant Stress depends on normal and Newtonian fluids Blood plasma, water shear strain rates and also the pressure applied on it Time Some clays, some Apparent viscosity decreases dependent Thixotropic drilling mud, many with duration of stress. viscosity 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
Newtonian Shear thickening
stress
stress hear hear Shear thinning S hear hear S Thixotropic loop
Shear rate gradient
Shear rate gradient
Pseudoplastic system Flow curve Viscosity curve
τττ, Pa ηηη, Pa s ηηη 0
τττ = ηηη D slope
ηηη∞ ∞∞
D, 1/s D, 1/s
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 (C 6H10 05)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) 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 Inner Gear cylinder class 1a 2a 3a 4a 5a 6a 7a 8a 9a 10a 11a 12a S1 D 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/s 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 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?