Why Does Sodium Polyacrylate Absorb Water?

What is a Super absorbent polymer

Super absorbent polymer (sodium polyacrylate, also called SAP) is a special functional polymer material that contains strong hydrophilic groups such as carboxyl and hydroxyl groups and has a certain cross-network structure.

Through hydration, it can

  • quickly absorb hundreds of times to more than a thousand times its own mass in water,
  • and it can also absorb tens to one hundred times of saltwater, blood, urine, and other liquids.

Compared to traditional absorbent materials

The water absorption of traditional water-absorbing materials such as cotton, sponge, paper, etc. is carried out by the principle of capillary, which belongs to physical adsorption.

The super absorbent resin has a certain degree of cross-linking due to its molecular structure, and the internal water cannot be easily extruded by simple mechanical methods, so it has strong water retention.

Comparison of water absorption capacity of several traditional water-absorbing materials and superabsorbent polymers:

Absorbent material Water absorbing capacity (Weight fraction) / %
Waterman No. 3 filter paper 180
Tissue paper 400
Polyurethane sponge 1050
Wood pulp pile 1200
Cotton ball 1890
Sodium polyacrylate 20200

Why Does It Absorb Water?

One of the intrinsic reasons for the high water absorption rate of SAP is that there are a large number of hydrophilic groups such as carboxyl, amido, and hydroxyl groups on its macromolecular chain.

Crosslinked acrylate polymers are an important part of synthetic resin-based water-absorbing materials, and are considered to be the most promising water-absorbing polymers.

Water absorption mechanism

Based on the different compositions of SAP, the water absorption mechanism is also different.

  • For polyacrylate type water-absorbent polymer, mainly relies on osmotic pressure to complete the water absorption process.
  • For non-ionic SAP, it relies on the hydrophilic effect of hydrophilic groups to complete water absorption.

The swelling properties of SAP directly affect its product quality and application.

At present, there are many research reports on the swelling properties of SAP, among which the research theories on the water absorption mechanism of superabsorbent resins can be summarized into three aspects:

  1. thermodynamic mechanism of water absorption;
  2. water absorption mechanism of flexible molecular chains;
  3. swelling kinetic mechanism.

thermodynamic mechanism of water absorption

The adsorption of SAP on the water can be divided into physical adsorption and chemical adsorption.

Physical adsorption

Physical adsorption refers to the adsorption of water through the capillary, so the water absorption capacity is limited, and it will quickly overflow under a certain pressure.

The SAP molecule contains strong hydrophilic polar groups and has a three-dimensional cross-linked structure.
Unlike traditional water-absorbing materials, SAP first absorbs water through capillary adsorption and dispersion, and then the hydrophilic groups of the resin interact with water molecules through hydrogen bonds. The ionic hydrophilic groups begin to dissociate when they meet water, and the anions are fixed at high On the molecular chain, cations are mobile ions.

With the dissociation of the hydrophilic group, the number of anions increases, the electrostatic repulsion increases, and the three-dimensional cross-linked network of the polymer expands. At the same time, in order to maintain electrical neutrality, the cations cannot diffuse to the external solvent, so the concentration increases, resulting in an increase in the osmotic pressure inside and outside the resin cross-linked network, and further infiltration of water molecules.

With the increase in water absorption, the osmotic pressure difference inside and outside the network tends to zero. As the network expands, its elastic contraction force also increases, gradually offsetting the electrostatic repulsion of anions, and finally reaching a water absorption balance. Water molecules penetrate and diffuse into the resin under the capillary action caused by the osmotic pressure difference and the expansion of the three-dimensional cross-linked structure of the resin, so as to achieve the purpose of water absorption.

Chemical adsorption

Chemical adsorption means that the hydrophilic groups in the resin firmly adsorb water molecules through chemical bonds, the adsorption capacity is very strong, and it is difficult to overflow under high pressure.

Due to the cross-network structure of SAP itself and the combination with hydrogen bonds, the adsorption of the resin is limited.
The molecular network cannot expand indefinitely when water is present, which ensures that the resin will not dissolve in water after absorbing water.

In this way, there are two forces inside the SAP, one is the osmotic pressure generated by the repulsion between the internal ions, which makes the water enter the resin and causes the expansion of the space network; the other is the elastic force generated by the cross-linking effect, which makes the polymer after water absorption has a certain strength. These two forces restrict each other, and finally reach a balance, the resin is saturated with water, and the water absorption at this time is the water absorption rate.

water absorption mechanism of flexible molecular chains

The thermodynamic mechanism of water absorption of SAP can well explain the water absorption mechanism of ionic SAP, but it is difficult to explain the water absorption mechanism of non-ionic SAP. Therefore, it is necessary to explain the water absorption mechanism of SAP from the aspect of the molecular chain.

According to the second law of thermodynamics, the system always spontaneously balances in the direction of increasing entropy. In the absence of external energy, the SAP in a completely dry state moves the macromolecular chain randomly, and the conformation of each carbon-carbon σ bond tends to be inconsistent. At this time, the macromolecular chain of SAP always tends to the coiled molecular conformation spontaneously.

For an ideal flexible macromolecular chain, its C-C bond can rotate freely, and its rotation is only limited by
Pendant groups and hydrogen bonding effects, with ideal flexibility. However, for SAP, the rotation of the macromolecular chain near the cross-linking point is hindered. In the case of uniform cross-linking density, each cross-linked lattice has the same size. It can be considered that the macromolecular chains that constitute the cross-linked lattice have ideal flexibility. , that is, each water-absorbing network is ideal, and the number of carbon atoms in the molecular chain between each cross-linking point is the same.

The changes of the SAP cross grid before and after water absorption

Therefore, the lower the cross-linking density of SAP, the stronger the flexibility of the macromolecular chain, the longer the effective chain length, the easier its conformational change, the stronger the water absorption capacity, and the smaller the external energy required to overcome the conformational change of the macromolecular chain. That is, the gel strength of SAP is lower.

From the perspective of macromolecular chain conformational change, the monomer with the greater affinity between the side groups of the main carbon chain and water molecules helps to enhance the flexibility of the SAP macromolecular chain.

swelling kinetic mechanism

The swelling kinetics of the resin was explained using the Berens-Hopfenberg diffusion relaxation model equation.

The diffusion-relaxation model equation proposes that the diffusion of water molecules and the relaxation of resin macromolecular segments satisfy a linear relationship.