Swelling isPolymerVolume expansion occurs in solvent.For example, the ion exchange resin is a hydrophilic polymer compound. When the dry ion exchange resin is immersed in water, its volume often becomes larger. This phenomenon is called swelling.
Because of the high molecularrelative molecular massIt is large and has polydispersity, and the molecular shapes are linear, branched and cross-linked,Aggregate structureThere are crystalline and amorphous types, so the dissolution of macromolecules is much more complicated than that of small molecular compounds.
The dissolution of polymer is a relatively slow process, which can be divided into two stages: swelling and dissolution. Swelling refers to the phenomenon that solvent molecules diffuse into the polymer to expand its volume.Swelling is a unique phenomenon of polymer materials. The reason is that the size of solvent molecules differs greatly from that of polymers, and the molecular movement speed differs greatly. The solvent molecules diffuse rapidly, while the polymer diffuses slowly into the solvent.Therefore, when the polymer is dissolved, the solvent molecules first penetrate into the polymer material to increase its volume, that is, swelling.With the continuous infiltration of solvent molecules, the volume of swollen polymer materials increases, and the movement of macromolecular chain segments increases. Then, the movement of the entire macromolecular chain is achieved through the coordinated movement of the chain segments. Macromolecules gradually enter the solution, forming a thermodynamically stable homogeneous system, that is, the dissolution phase, as shown in Figure 1.[1]
Fig. 1 Swelling Diagram
classification
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There are two kinds of swelling:
① Infinite swelling：Linear polymers are soluble in good solvents and can absorb solvents without limitation until they are dissolved into homogeneous solutions.Therefore, the dissolution can also be regarded as the result of the infinite swelling of the polymer.
② Limited swelling：aboutCross-linked polymerAnd in bad solventlinear polymer In terms of swelling, it can only be carried out to a certain extent. No matter how long it contacts with the solvent, the amount of absorbed solvent will no longer increase, but will reach equilibrium, and the system will always maintain a two-phase state.useSwelling degreeQ (i.e. the multiple of swelling) is used to characterize this stateEquilibrium swelling methodDetermined.
Swelling process
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When the polymer is dissolved, it must first go through the process of absorbing the solvent to expand the polymer.The reason why the polymer swells first when it dissolves is that:
1. The curly shape of the polymer can provide solventMolecular diffusionThe space to enter;
2. The solvent molecule is small and the diffusion speed is fast. Before the polymer diffuses into the solvent to cause its dissolution, the solvent molecule has diffused into the polymer molecule to cause its swelling.
Because of the large molecular weight of the long chain of the polymer, when the polymer is immersed in the solvent, the polymer is not dissolved immediately, and the dissolution process is generally divided into two stages: first, the molecular weight is smallDiffusion rateFast solvent molecules penetrate into the polymer, making the polymerVolumetric expansionIs swelling.Then the polymer molecules diffuse into the solvent and dissolve.That is to say, in the dissolution process of polymers, they must go through the swelling stage before they can be dissolved.
Swelling of fiber
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Fibers are partially crystalline polymers composed of linear macromolecules, that is, fiberSupramolecular structureThere are crystalline and amorphous regions in the.The macromolecules in the crystallization area are closely arranged, and the pores are small and few;The amorphous region has loose molecular arrangement and more micro gaps.When the fiber contacts with water or water vapor, the water molecules enter the amorphous region along the micro gap of the fiber, which weakens the force between the fiber macromolecules, increasing the distance between the molecules, increasing the pores, and swelling the fiber.
Due to different structures, various fibers have different swelling degrees in water.The swelling degree of common fibers in water is shown in Figure 2.
Figure 2 Swelling degree of common fibers in water
Cellulosic fibreMolecules contain hydrophilic groups (- OH), and the degree of swelling in water is large. After swelling, the pores in the fiber increase. During dyeing, dye molecules can easily enter these pores and diffuse to the interior of the fiber.The dye uptake is related to the number and volume of pores in the fiber. When dyeing under the same conditions, the fiber with large number and volume of pores has more dye uptake.For example, the pore volume of unmixed cotton fiber is 022~0.33L/kg fiber, and 0.5L/kg fiber for mercerized cotton fiber.If dyed under the same conditions, mercerized cotton has a higher dye uptake and deeper color.[2]
Swelling performance test
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Gravimetric method is used to determine the swelling rate of hydrogel in distilled water at different temperatures (from 12 to 60 ℃).During the test, the hydrogel sample shall be soaked at a certain temperature in the water for at least 24h, and then be quickly taken out of the water. The water on the surface shall be removed with filter paper, and then be weighed to obtain the equilibrium swelling mass of the sample.The equilibrium swelling ratio (ESR) of hydrogel is calculated by the following formula.
ESR=Ws/Wd
Where, WsIs the weight of hydrogel after swelling equilibrium;Wd is the dry weight of gel.
The shrinkage behavior of the swelled hydrogel at 60 ℃ (higher than the minimum critical solution temperature) was tested by the deswelling kinetics.At a certain time interval, take out the hydrogel and immediately remove the water on the surface with filter paper before weighing.Water retention rate (WR) is defined as:
Where, WtIs the wet weight of the hydrogel at 60 ℃ for t;WsIt is the quality of hydrogel in the equilibrium state of swelling at room temperature.Other parameters are the same as those defined above.The results of each sample above are the average of three tests.[3]
