Gel Preparation Experiment Discussion Examples | Relationship Between Water Absorption, Crosslink Density, and Swelling Ratio

What Is a Gel Preparation Experiment?

A gel preparation experiment is an experiment in which polymer chains are crosslinked to form a three-dimensional network structure capable of retaining water or solvent inside.
Although gels contain a large amount of liquid, they have the property of maintaining an overall solid-like shape.
This property arises because the polymer chains are connected to one another by crosslinking points.

The properties of a gel are affected by the type of polymer, crosslink density, amount of hydrophilic functional groups, interactions with the solvent, ionic strength, pH, temperature, and other factors.
A gel with high water absorption or a high swelling ratio can take up a large amount of water, but it does not necessarily have high mechanical strength.
In discussing gels, it is important to consider the balance between water absorption and structure.

Main Items to Include in the Results

In the results of a gel preparation experiment, organize the polymer or monomer used, crosslinking agent, reaction conditions, appearance of the prepared gel, dry mass, mass after water absorption, swelling ratio, water content, hardness, transparency, shape retention, and other information.
Tabulating the mass changes before and after water absorption makes it easier to compare swelling ratios and water absorption.

Main Items to Include in the Results

• Polymer or monomer used
• Type of crosslinking agent
• Crosslinking-agent concentration
• Presence or absence of an initiator or catalyst
• Reaction temperature
• Reaction time
• Appearance of the gel
• Color and transparency of the gel
• Hardness of the gel
• Elasticity of the gel
• Mass before drying
• Mass after drying
• Mass after water absorption
• Amount of water absorbed
• Swelling ratio
• Water content
• Water-absorption time
• Shape retention
• Sources of error and points for improvement

Reference Experimental Values and Calculation Examples for the Swelling Ratio of Gels

Here, dry mass, mass after water absorption, swelling ratio, water content, and the effects of crosslinking-agent concentration obtained in gel preparation experiments are organized as reference experimental values that are easy to discuss in reports.

Gels have three-dimensionally crosslinked polymer-network structures and can retain water or solvents inside.
When the crosslink density is low, the network can expand easily and the gel swells greatly.
On the other hand, when the crosslink density is high, movement of the network is restricted, and the amount of water absorbed and the swelling ratio become smaller.

Reference Experimental Conditions

Equations for Swelling Ratio and Water Content

The water absorption of a gel is evaluated from the dry-gel mass and the mass after water absorption.

Swelling ratio Q = Mass after water absorption Ws ÷ Dry mass Wd

Amount of water absorbed = Mass after water absorption Ws − Dry mass Wd

Water content (%) = (Ws − Wd) ÷ Ws × 100

For example, if the dry-gel mass is 0.100 g and the mass after water absorption is 12.0 g,

Swelling ratio Q = 12.0 ÷ 0.100 = 120 times

Water content = (12.0 − 0.100) ÷ 12.0 × 100 = 99.2%

This gel is considered to absorb water up to 120 times its dry mass, with most of its mass after water absorption consisting of water.

Changes in Swelling Ratio With Crosslinking-Agent Concentration

The following is a reference example in which polyacrylic acid-based gels were prepared using different crosslinking-agent concentrations and swollen in distilled water for 24 hours.

The swelling ratio decreases as the crosslinking-agent concentration increases.
This is considered to be because the number of crosslinking points connecting the polymer chains increases, making the network structure more difficult to expand.

Changes in Swelling With Water-Absorption Time

When a gel is immersed in water, water gradually diffuses into the interior and the swelling ratio increases over time.
After a certain period, water absorption and the elastic force of the network become balanced, and the gel approaches equilibrium swelling.

Water absorption is rapid at first, but the amount of increase becomes smaller over time.
This is considered to be because the amount of water inside the gel increases and the elastic force of the network suppresses further swelling.

Relationship Between Crosslink Density and Swelling

The higher the crosslink density, the more strongly the free movement of the polymer chains is restricted and the smaller the network size becomes.
Therefore, even when water is absorbed, the network cannot expand greatly and the swelling ratio becomes smaller.

Changes in Swelling Ratio With pH

In gels containing ionic groups, such as polyacrylic acid-based gels, the ionization state changes with pH, causing the swelling ratio to change.

Under neutral to weakly basic conditions, carboxyl groups become ionized and negative charges inside the gel repel one another, making the network easier to expand and increasing the swelling ratio.

Changes in Swelling Ratio With Salt Concentration

In ionic gels, increasing the salt concentration of the external solution may shield the charge repulsion inside the gel and reduce the swelling ratio.

The swelling ratio decreases as the salt concentration increases.
This is considered to be because the osmotic-pressure difference and charge repulsion caused by fixed charges inside the gel were weakened by ions in the external solution.

Example of Temperature-Dependent Swelling Behavior

In temperature-responsive gels, the balance between hydrophilicity and hydrophobicity changes with temperature, and the swelling ratio may change greatly.
A reference example of a temperature-responsive gel is shown here.

If the swelling ratio decreases as the temperature increases, the polymer chains in the gel may have aggregated hydrophobically, released water, and contracted.

Relationship Between Gel Strength and Swelling Ratio

Gels with large swelling ratios contain large amounts of water but may become mechanically weak.
The following is a reference example in which shape retention was simply evaluated by pressing the gel with a finger.

To increase water absorption, the crosslink density must be reduced, but if it becomes too low, the gel becomes more likely to break apart.
It is necessary to balance water absorption and mechanical strength according to the intended application.

Differences in Reswelling Depending on Drying Conditions

The conditions used to dry a gel may change its ability to swell again when placed back in water.

During high-temperature drying, the network inside the gel may contract and adhere together, making reswelling more difficult.
Freeze-drying tends to preserve the porous structure and may allow faster reswelling.

Repeated Water-Absorption and Drying Tests

To evaluate the reusability of a gel, the swelling ratio may be examined after repeated cycles of water absorption and drying.

If the swelling ratio decreases with repeated use, drying shrinkage, damage to the network structure, or leaching of components may have occurred.

Differences in Measurement Caused by the Method Used to Remove Surface Water

When measuring the mass after water absorption, the swelling ratio may change depending on how the water adhering to the surface of the gel is handled.

When comparing swelling ratios, the method used to remove surface water and the time before measurement must be standardized.

Example of How to Write the Results

When a sample with a dry-gel mass of 0.100 g was swollen in distilled water for 24 hours, the mass after water absorption was 12.0 g.
The swelling ratio was therefore 12.0 ÷ 0.100 = 120 times, and the water content was (12.0 − 0.100) ÷ 12.0 × 100 = 99.2%.
This result shows that the prepared gel could retain a very large amount of water relative to its dry mass.

When the crosslinking-agent concentration was increased from 0.1 mol% to 2.0 mol%, the swelling ratio decreased from 185 times to 26 times.
This is considered to be because increasing the crosslinking-agent concentration increased the number of crosslinking points connecting the polymer chains, making the network structure more difficult to expand.
On the other hand, samples with excessively low crosslinking-agent concentrations had large swelling ratios but were very soft and partly easy to break apart.

Regarding the effect of pH, the swelling ratio was low under acidic conditions and increased greatly under neutral to weakly basic conditions.
In polyacrylic acid-based gels, increasing the pH ionizes the carboxyl groups and increases the number of negative charges inside the gel.
As a result, the network is considered to have expanded because of repulsion among negative charges and the osmotic-pressure difference, allowing the gel to take up more water.

Points for Connecting the Results to the Discussion

In a discussion of gel preparation, it is important to explain not only the magnitude of the swelling ratio but also its relationship with crosslink density, charge, external-solution conditions, and mechanical strength.

• Has the swelling ratio been calculated correctly from the dry mass and the mass after water absorption?
• Has the water content been calculated, and can it be explained that most of the swollen gel consists of water?
• Can the reason the swelling ratio decreases as the crosslinking-agent concentration increases be explained in relation to the network structure?
• Can it be discussed that if the crosslink density is too low, water absorption is high but gel strength decreases?
• Can changes in swelling with pH be explained in relation to the ionization state of functional groups and charge repulsion?
• Can the reason swelling is suppressed at high salt concentrations be explained in relation to charge shielding by ions?
• For temperature-responsive gels, can changes in hydrophilicity and hydrophobicity with temperature be explained?
• Can it be discussed that the method used to remove surface water and the measurement timing affect the swelling ratio?
• Can decreases in reswelling ability caused by drying conditions and repeated use be explained in relation to changes in the network structure?

Example Discussion

In this experiment, gels were prepared using different crosslinking-agent concentrations, and their swelling ratios in distilled water were compared.
When a gel with a dry mass of 0.100 g was allowed to absorb water for 24 hours, the sample containing 0.3 mol% crosslinking agent had a mass after water absorption of 12.0 g, a swelling ratio of 120 times, and a water content of 99.2%.
This shows that the prepared gel could retain a large amount of water inside the polymer network.

As the crosslinking-agent concentration increased, the swelling ratio decreased greatly.
At 0.1 mol%, the gel swelled to 185 times its dry mass, whereas at 2.0 mol%, the swelling ratio remained at 26 times.
This is considered to be because increasing the number of crosslinking points restricted the movement of the polymer chains and made it difficult for the network structure to expand in water.
On the other hand, samples with low crosslinking-agent concentrations were extremely soft and partly easy to break apart, indicating a trade-off between water absorption and mechanical strength.

Regarding changes in swelling ratio with pH, swelling was small under acidic conditions and large under neutral to basic conditions.
In polyacrylic acid-based gels, the carboxyl groups are nonionized under acidic conditions, so charge repulsion is small.
On the other hand, as the pH increases, the carboxyl groups ionize and acquire negative charges, increasing charge repulsion inside the gel and making the network easier to expand.
As a result, more water was taken up and the swelling ratio is considered to have increased.

When the salt concentration was increased, the swelling ratio decreased from 120 times to 18 times.
This is considered to be because Na⁺ and Cl⁻ in the external solution shielded the repulsion caused by fixed charges inside the gel and reduced the osmotic-pressure difference between the inside and outside of the gel.
Therefore, the swelling of ionic gels is strongly affected not only by pH but also by the ionic strength of the external solution.

Possible sources of error include insufficient drying, the method used to remove surface water, water-absorption time, chipping of the gel, and the time before mass measurement.
In particular, water droplets readily adhere to the surface of a swollen gel, and measuring the gel without removing them may overestimate the swelling ratio.
Conversely, pressing the gel too strongly with filter paper may remove water from inside the gel and underestimate the swelling ratio.
Therefore, it is important to standardize the method used to remove surface water and the measurement procedure.

Summary

The water absorption of a gel can be evaluated as a swelling ratio or water content from the dry mass and the mass after water absorption.
The lower the crosslinking-agent concentration, the larger the swelling ratio tends to become, but shape retention and mechanical strength also tend to decrease.

This reference example addressed the effects of crosslinking-agent concentration, water-absorption time, pH, salt concentration, temperature, drying conditions, repeated water absorption, and the method used to remove surface water.
In a report, it is useful to discuss the calculated swelling ratio in relation to network structure, crosslink density, charge repulsion, osmotic pressure, and measurement procedures.

Why Gels Absorb Water

Gels absorb water because hydrophilic functional groups are present in the polymer chains and interact with water molecules.
For example, carboxyl groups, hydroxyl groups, amide groups, and ionic groups attract water through hydrogen bonding and electrostatic interactions.
When water enters the gel, the polymer network expands and the gel swells.

However, a gel does not absorb water without limit.
As the network structure expands, an elastic force acts through the crosslinking points to pull the molecular chains back.
The final swelling state is determined by the balance between the force that promotes water uptake and the force that tends to restore the network.

What Is Crosslinking?

Crosslinking is the connection of polymer chains to one another through chemical bonds or strong interactions.
Through crosslinking, polymers form three-dimensional network structures rather than dissolving freely as individual chains.
Because this network structure exists, a gel can retain its shape even when it contains water and does not dissolve completely.

Crosslinking includes chemical crosslinking through covalent bonds and physical crosslinking through hydrogen bonds, ionic bonds, hydrophobic interactions, and similar interactions.
Chemical crosslinking is relatively stable, while physical crosslinking may change readily with temperature, pH, ionic concentration, and other conditions.

What Is Crosslink Density?

Crosslink density is a concept representing how many crosslinking points are present in a gel.
The higher the crosslink density, the more finely the polymer chains are connected and the smaller the network size becomes.
The lower the crosslink density, the larger the network becomes and the more freely the polymer chains can move.

Crosslink density greatly affects the water absorption, swelling ratio, hardness, elasticity, and solubility of a gel.
In general, gels with high crosslink densities are hard and retain their shape easily, but they are more difficult to swell.
Gels with low crosslink densities are soft and readily absorb large amounts of water, but they may lose their shape more easily.

What Is Swelling Ratio?

Swelling ratio is a value indicating how much a gel has expanded after absorbing water or solvent.
It is generally determined by comparing the mass after water absorption with the dry mass.
A larger swelling ratio means that the gel absorbed more water.

Swelling ratio = Mass of gel after water absorption ÷ Dry-gel mass

In some cases, the amount of water absorbed is divided by the dry-gel mass instead.
The equation to use must follow the instructions in the laboratory manual.
Stating the equation used in the report makes the meaning of the results easier to understand.

What Is Water Content?

Water content is a value indicating how much of the entire swollen gel consists of water.
Whereas swelling ratio is an index of “how much water was absorbed relative to the dry gel,” water content is an index of “what proportion of the swollen gel as a whole is water.”

Water content (%) = Mass of water in the swollen gel ÷ Mass of the swollen gel × 100

A gel with a high water content tends to be a soft material containing a large amount of water.
However, if the water content is too high, shape retention and mechanical strength may decrease.

Relationship Between Water Absorption and Crosslink Density

Water absorption indicates how much water can be taken into the gel.
In a gel with a low crosslink density, the network expands easily and can take up a large amount of water, so the water absorption and swelling ratio tend to be high.
On the other hand, in a gel with a high crosslink density, the network is small and movement of the molecular chains is restricted, so water absorption tends to be low.

However, if the crosslink density is too low, the gel may break apart or dissolve in water.
It cannot simply be said that reducing the amount of crosslinking always improves water absorption; the balance between water absorption and shape retention is important.

Relationship Between Swelling Ratio and Gel Hardness

A gel with a high swelling ratio tends to become soft because it contains a large amount of water.
In contrast, a gel with a low swelling ratio tends to become hard because the proportion of water is lower and the polymer network is denser.
However, hardness is affected not only by crosslink density but also by the type of polymer, crystallinity, ionic bonding, drying state, and other factors.

Gel hardness and water absorption often have a trade-off relationship.
Increasing water absorption may make the gel softer and easier to break apart, while increasing hardness may reduce the amount of water absorbed.
In gel materials, this balance is adjusted according to the intended application.

Effect of Crosslinking-Agent Concentration

In gel preparation, the crosslinking-agent concentration greatly affects the properties of the gel.
When the crosslinking-agent concentration is high, the number of crosslinking points increases and the network structure becomes denser.
As a result, the gel tends to become harder and the swelling ratio tends to decrease.

Conversely, when the crosslinking-agent concentration is low, the network structure becomes coarser and the gel can readily absorb more water.
However, if the amount of crosslinking is too small, gelation may be insufficient and the gel may lose its shape or dissolve.

Effect of Hydrophilic Functional Groups

Hydrophilic functional groups in polymer chains play a major role in the water absorption of gels.
Carboxyl groups, hydroxyl groups, amide groups, sulfonic acid groups, and similar groups readily interact with water molecules and increase the water absorption of the gel.
In particular, gels containing ionic groups may swell greatly because of electrostatic repulsion and osmotic-pressure effects.

However, even if hydrophilicity is high, insufficient crosslinking may cause the polymer to dissolve into the water.
Both hydrophilic functional groups and a crosslinked structure are necessary to achieve high water absorption.

Effect of Ionic Strength

In ionic gels, the ionic strength of the external solution greatly affects the swelling ratio.
In pure water, the gel may swell greatly because of osmotic pressure caused by the difference in ion concentration between the inside and outside of the gel and repulsion between functional groups carrying the same charge.
In contrast, in electrolyte solutions such as saline, the charges may be shielded by ions and swelling may be suppressed.

This is why water-absorbing polymers swell greatly in pure water but do not swell as much in salt water.
When water and salt water are compared experimentally, ionic strength and electrostatic interactions can be related in the discussion.

Effect of pH

In gels containing acidic or basic groups, the ionization state of the functional groups changes with pH.
For example, in gels containing carboxyl groups, the carboxyl groups are more readily ionized under high-pH conditions, and repulsion among negative charges may make the gel swell more easily.
Conversely, at low pH, ionization is suppressed and the swelling ratio may decrease.

In pH-responsive gels, swelling and contraction occur in response to changes in pH.
This property may be used in drug-release materials, sensor materials, and similar applications.

Effect of Temperature

The swelling ratio of a gel may change with temperature.
As the temperature increases, the mobility of the polymer chains and their interactions with water change, potentially altering the swelling ratio.
In temperature-responsive gels, the swelling state may change greatly at a certain temperature.

In general student experiments, measurement errors caused by temperature changes must also be considered.
If the temperature during water absorption differs among samples, the diffusion rate of water and the softness of the gel may change, affecting comparisons of the amount of water absorbed.

Effect of Water-Absorption Time

Water absorption by a gel is not completed instantaneously.
Water gradually diffuses from the surface of the gel into the interior, and the amount of water absorbed increases over time.
When the amount of water absorbed becomes almost constant after a certain period, the gel can be considered to be approaching swelling equilibrium.

If the water-absorption time is too short, water has not sufficiently penetrated into the interior of the gel and the swelling ratio may be underestimated.
When comparing conditions, the water-absorption time must be kept the same for all samples.

Effect of Drying Conditions

When determining the swelling ratio, the mass of the dry gel is used as the reference.
If drying is insufficient, water or solvent remains in the dry gel and the dry mass is measured as larger than it actually is.
As a result, the calculated swelling ratio may be lower than the actual value.

On the other hand, excessive drying or high-temperature drying may change the gel structure and affect its water absorption.
Drying conditions are important not only for mass measurement but also because they affect the reswelling ability of the gel.

Error Caused by Wiping Water From the Surface

When measuring the mass of a gel after water absorption, the extent to which water adhering to the gel surface is removed affects the result.
If surface water remains, water that was not actually absorbed inside the gel is also included in the mass, causing the swelling ratio to be overestimated.
Conversely, if the gel is wiped too strongly, water inside the gel may also be squeezed out, potentially causing the swelling ratio to be underestimated.

Therefore, when measuring mass after water absorption, it is important to use the same method for treating surface water for every sample.
Follow the method specified in the laboratory manual, such as lightly pressing with filter paper or allowing the gel to drain for a fixed period.

Discussion When the Gel Breaks Apart

If a gel breaks apart after water absorption, possible causes include excessively low crosslink density, insufficient polymerization or crosslinking, short polymer chains, excessive expansion of the network caused by water absorption, and excessively rough handling during measurement.
If the gel breaks apart, it becomes difficult to measure the mass after water absorption accurately.

A gel that breaks apart easily may indicate that although its water absorption is high, its mechanical strength is insufficient.
It is important to discuss the balance between water absorption and shape retention.

Discussion When the Gel Is Too Hard

If a gel is too hard, possible causes include excessively high crosslink density, high polymer concentration, excessive drying, crystallinity, or strong interactions.
A hard gel retains its shape easily, but it may have less room for water uptake and therefore a lower swelling ratio.

For gels in which water absorption is important, excessive hardness is not necessarily a good result.
Depending on the application, an appropriate degree of softness and water absorption may be required.

Discussion When the Gel Is Too Soft

If a gel is too soft, possible causes include low crosslink density, low polymer concentration, insufficient reaction, or excessive water absorption.
A soft gel may contain a large amount of water, but its mechanical strength and shape retention may be low.

A gel that is too soft may tear when picked up with tweezers or release water during mass measurement.
Therefore, measurement errors also tend to become larger.

Discussion of Transparency and Cloudiness

The transparency or cloudiness of a gel also reflects its structure and preparation conditions.
A uniform gel may appear transparent or translucent.
On the other hand, if cloudiness is observed, possible causes include phase separation, bubbles, crystallization, nonuniform crosslinking, and residual unreacted components.

In particular, insufficient mixing or rapid gelation may create nonuniform structures inside the gel, causing light scattering and a white appearance.
Cloudiness is not merely an appearance issue but provides a clue for considering the uniformity of the internal gel structure.

Discussion When Bubbles Are Present

Bubbles introduced during gel preparation affect the appearance and physical properties of the gel.
Because no polymer network exists in the bubble regions, the strength decreases locally.
In addition, bubbles may cause variation in water absorption and mass measurements.

Bubbles may be generated by stirring during mixing, entrainment of air during casting, or gas generation during the reaction.
Methods for reducing bubbles include mixing slowly, degassing, and avoiding entrainment of air during casting.

Discussion When Gelation Does Not Occur

If gelation does not occur after the reaction, possible causes include insufficient crosslinking agent, failure of the crosslinking reaction to proceed, failure of the initiator or catalyst to function, insufficient reaction temperature or time, low monomer concentration, and inappropriate pH or ionic conditions.
Gelation requires sufficient formation of polymer chains and crosslinking points.

If crosslinking does not occur, the polymer may dissolve in water or remain as a viscous liquid.
Failure to gel is an important observation indicating insufficient formation of a crosslinked structure.

Discussion When Water Absorption Is Low

If water absorption is low, possible causes include excessively high crosslink density, a small amount of hydrophilic functional groups, changes in the network structure caused by drying or reaction, a short water-absorption time, and high ionic strength of the external solution.
If the gel is hard and the network is dense, water has difficulty entering the interior.

The apparent amount of water absorbed also changes depending on the drying state before water absorption and the treatment of surface water after absorption.
When water absorption is low, structural causes and causes related to measurement procedures should be considered separately.

Discussion When Water Absorption Is Too High

If water absorption is extremely high, possible explanations include high hydrophilicity of the gel, low crosslink density, a large number of ionic groups, and a large osmotic-pressure difference in pure water.
However, if the amount of water absorbed is excessively large, surface water may have been included in the measurement, or the gel may have broken apart, making the distinction between internal and external water unclear.

A high swelling ratio indicates high water absorption, but it may be overestimated if the measurement method is inappropriate.
The treatment of surface water, water-absorption time, and whether the gel broke apart must be checked.

Causes of Variation in Measured Values

Causes of variation in swelling ratio and water absorption include differences in gel size, differences in drying state, differences in water-absorption time, differences in surface-water treatment, bubbles inside the gel, nonuniform crosslinking, evaporation of water during measurement, and errors in reading the balance.
Because gels are soft and contain water, their measured values readily change depending on handling.

When comparing multiple samples, it is important to standardize sample size, drying time, water-absorption time, the method used to remove surface water, and the timing of mass measurement.

When the Results Can Be Considered Good

Results in a gel preparation experiment can be considered good when the gel retains its shape, shows a clear increase in mass after water absorption, and the swelling ratio and hardness change reasonably according to differences in conditions.
For example, if the gel is harder and has a lower swelling ratio at high crosslinking-agent concentrations and is softer with a higher swelling ratio at low crosslinking-agent concentrations, the results can be considered consistent with theory.

In addition, if variation in measured values is small and the gel size and drying and water-absorption conditions are standardized, the reliability of the results increases.
It is important to observe both water absorption and shape retention.

Example Discussion When the Experiment Did Not Go Well

If gel preparation does not go well, the cause is considered from results such as failure to gel, breakdown of the gel, low water absorption, excessively high water absorption, large variation in swelling ratio, cloudiness of the gel, or the presence of many bubbles.
Organizing the causes separately into reaction conditions, crosslink density, drying, water-absorption time, surface-water treatment, and measurement procedures makes the discussion easier.

How to Write Points for Improvement

In a discussion of gel preparation experiments, including not only sources of error but also points for improvement makes the report easier to organize.
Improvements can be organized by dividing them into gel preparation, drying, water absorption, mass measurement, and comparison of conditions.

Improvements to Gel Preparation

• Prepare the monomer or polymer concentration accurately
• Keep the crosslinking-agent concentration accurate and consistent
• Keep the reaction time and reaction temperature constant
• Mix uniformly
• Avoid introducing bubbles
• Prepare samples of the same size

Improvements to Drying and Water Absorption

• Keep the drying time and drying temperature constant
• Confirm that the mass after drying has stabilized
• Keep the water-absorption time constant
• Keep the water-absorption temperature constant
• Clearly distinguish pure-water, salt-water, and pH conditions

Improvements to Mass Measurement

• Remove surface water after water absorption using the same method
• Do not wipe too strongly
• Keep the time before mass measurement constant
• Handle the gel carefully so that it does not break apart
• Perform multiple measurements and calculate the average value

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of gel preparation experiments, simply writing that “the gel absorbed water,” “the gel swelled,” or “the gel was hard” results in a superficial discussion.
Relating water absorption, hydrophilic functional groups, crosslink density, swelling ratio, mechanical strength, and measurement errors produces a more persuasive discussion.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of a gel preparation experiment.
Adjust the necessary parts according to your own experimental results.

• A gel has a three-dimensional network structure formed by crosslinked polymer chains.
• Water is incorporated into the gel because hydrophilic functional groups interact with water molecules.
• The higher the crosslink density, the more difficult the network structure is to expand, and the swelling ratio tends to decrease.
• A gel with a low crosslink density readily absorbs a large amount of water, but its shape retention may decrease.
• The swelling ratio can be determined from the relationship between the mass after water absorption and the dry mass.
• Water content indicates the proportion of water contained in the swollen gel.
• Water absorption and mechanical strength may have a trade-off relationship.
• In salt water, swelling of ionic gels may be suppressed because ions shield the charges.
• If the water-absorption time is short, the gel may not reach swelling equilibrium and the swelling ratio may be underestimated.
• If the method used to remove surface water is not consistent, errors occur in the mass after water absorption.

Points to Check When Discussing a Gel Preparation Experiment

Checking the following points before writing the report makes the discussion easier to write.

• Is the three-dimensional network structure of the gel explained?
• Is water absorption related to hydrophilic functional groups?
• Is the relationship between crosslink density and swelling ratio described?
• Is the relationship between crosslink density and gel hardness described?
• Is the equation for the swelling ratio clearly stated?
• Are water content and swelling ratio distinguished?
• Are water-absorption time and swelling equilibrium considered?
• Are the effects of salt concentration and pH discussed?
• Are the effects of drying conditions considered?
• Are errors caused by surface-water treatment described?
• Are causes of gels breaking apart, being too hard, or being too soft considered?
• Do the points for improvement correspond to the sources of error?

Summary

In gel preparation experiments, polymer chains are crosslinked to form a three-dimensional network structure, and the gel swells by retaining water inside this network.
Water absorption changes depending on hydrophilic functional groups in the polymer chains, interactions with water, crosslink density, and the conditions of the external solution.
A gel with a high crosslink density is hard and retains its shape easily, but the network is difficult to expand and the swelling ratio tends to be low.

A gel with a low crosslink density readily absorbs a large amount of water, but its shape retention and mechanical strength may decrease.
In other words, there is a balance between water absorption and strength.
In addition, salt concentration, pH, temperature, water-absorption time, drying conditions, and the method used to treat surface water greatly affect the measured swelling ratio.

In a report, rather than simply writing that “the gel swelled,” discuss the three-dimensional network structure, hydrophilic functional groups, crosslink density, swelling ratio, water content, gel hardness, and measurement errors in relation to one another.
In gel preparation experiments, not only the magnitude of water absorption but also shape retention and standardization of measurement conditions are important points of evaluation.