CN114258836A

CN114258836A - Nitrogen-phosphorus-containing biodegradable polymer double-crosslinked network hydrogel soilless culture substrate and preparation method thereof

Info

Publication number: CN114258836A
Application number: CN202111530998.2A
Authority: CN
Inventors: 刘亚青; 耿阳阳; 陈泰安; 向阳; 赵贵哲
Original assignee: North University of China; Shanxi Zhongbei New Material Technology Co Ltd
Current assignee: North University of China; Shanxi Zhongbei New Material Technology Co Ltd
Priority date: 2021-12-15
Filing date: 2021-12-15
Publication date: 2022-04-01
Anticipated expiration: 2041-12-15
Also published as: CN114258836B

Abstract

The invention relates to a nitrogen and phosphorus containing biodegradable polymer double-crosslinked network hydrogel soilless culture substrate and a preparation method thereof. The soilless culture substrate is composed of Ca²⁺And a crosslinked network structure formed by ionic crosslinking of sodium alginate provides mechanical properties, a physical crosslinked network structure formed by interaction of the biodegradable polymer and the methylol urea through hydrogen bonds serves as a bracket and provides nitrogen nutrients, and the two crosslinked networks form an interpenetrating network polymer composite material. According to the invention, low-temperature pore forming and chemical pore forming are organically combined, and the pore structure of the prepared biodegradable polymer double-crosslinked network hydrogel soilless culture substrate can be effectively regulated, so that the substrate not only has expected strength, but also has appropriate multistage pores to meet the requirements of plant respiration and moisture. The invention utilizes hydroxymethyl urea to release nitrogen nutrient, and the calcium dihydrogen phosphate not only can release phosphorus nutrient, but also can be mixed with alginic acidThe sodium is subjected to ion crosslinking to form a crosslinked network structure, so that the integration of the fertilizer and the matrix is realized.

Description

Nitrogen-phosphorus-containing biodegradable polymer double-crosslinked network hydrogel soilless culture substrate and preparation method thereof

Technical Field

The invention relates to the field of plant soilless culture substrates, in particular to a nitrogen and phosphorus containing biodegradable polymer double-crosslinked network hydrogel soilless culture substrate and a preparation method thereof.

Background

Soilless culture is a new vegetable culture technology developed in recent decades. In the present stage of China, the soilless culture of vegetables is mainly carried out by using common plastic greenhouses and sunlight greenhouses, matched greenhouse regulation and control equipment is not used, and the greenhouse environment regulation and control level is low; the cost is too high when equipment of advanced countries is introduced.

Most crops are aerobic organisms, depend on stable oxygen supply in the environment, and the root systems of the plants are very sensitive to oxygen and have high requirements on soilless culture substrates. Insufficient basal oxygen is the first stress factor that seriously affects each organ of the plant in the early stage, thereby affecting the growth and yield of the plant. The holes of the matrix can inhibit oxygen supply of the root system and limit the respiration of the root system, so that the energy state of cells of the root system is seriously reduced, and the important metabolic process of plants is damaged.

Hydrogel as a functional material with good hydrophilicity, biodegradability and biocompatibility has been widely studied in the fields of food, agriculture, industry, biomedicine and the like. Recently, some researchers have shown that hydrogels can serve as a medium for seed growth, providing moisture and nutrients for seed growth. More than 95% of water in the hydrogel can be gradually released to the plant root system, which shows that the hydrogel can replace soil to be used as a culture medium for plant seed germination. However, the mechanical properties of the common hydrogel are poor, and many works have been carried out by the predecessors to improve the mechanical properties of the hydrogel, and various types of hydrogels, such as double-crosslinked hydrogel, double-network hydrogel, organic-inorganic hybrid hydrogel, nanocomposite hydrogel and the like, are developed. The hydrogel soilless culture substrate prepared by utilizing biodegradable polymers such as sodium alginate and polyvinyl alcohol has the advantages of no toxicity, biodegradability, good chemical stability, strong hydrophilicity and the like, so the hydrogel soilless culture substrate has the potential to become an attractive substitute of a non-degradable material.

Disclosure of Invention

The invention aims to provide a nitrogen-phosphorus-containing biodegradable polymer double-crosslinked network hydrogel soilless culture substrate which can meet the requirements of oxygen, nutrients and water of plant roots, has good mechanical properties and can enable plants to grow vertically in the substrate, and a preparation method thereof. The preparation process is simple, easy for large-scale industrial production and low in cost.

The invention is realized by the following technical scheme: a nitrogen-phosphorus containing biodegradable polymer double-crosslinked network hydrogel soilless culture substrate is prepared from Ca²⁺And a crosslinked network structure formed by ionic crosslinking of Sodium Alginate (SA) provides mechanical properties, a physical crosslinked network structure formed by interaction of biodegradable macromolecules and Methylol Urea (MU) through hydrogen bonds serves as a bracket and provides nitrogen nutrients, and the two crosslinked networks form the interpenetrating network polymer composite material.

A preparation method of a nitrogen and phosphorus containing biodegradable polymer double-crosslinked network hydrogel soilless culture substrate comprises the following steps:

adding sodium alginate into the aqueous solution of the biodegradable high polymer material, mixing uniformly, then sequentially adding a pore-forming agent calcium carbonate, calcium dihydrogen phosphate and hydroxymethyl urea, mixing uniformly, then adding a pore-forming aid citric acid, mixing uniformly and then freezing to obtain the nitrogen-phosphorus-containing biodegradable polymer double-crosslinked network hydrogel soilless culture substrate with an adjustable pore structure.

As a further improvement of the technical scheme of the preparation method, the calcium carbonate not only can be used as a pore-forming agent and a pore-forming auxiliary agent to generate pores, but also can provide Ca for generating an ionic crosslinking reaction with sodium alginate²⁺And the mechanical property is provided for the soilless culture substrate by forming an ionic crosslinking network structure.

As a further improvement of the technical scheme of the preparation methodFurthermore, the calcium dihydrogen phosphate can not only release phosphorus nutrient, but also provide Ca for ionic crosslinking reaction with sodium alginate²⁺And the mechanical property is provided for the soilless culture substrate by forming an ionic crosslinking network structure.

As a further improvement of the technical scheme of the preparation method, the biodegradable high polymer material is a biodegradable high polymer material which contains polar water-absorbing functional groups on the main chain and can be dissolved in water.

As a further improvement of the technical scheme of the preparation method, the biodegradable high polymer material is polyvinyl alcohol (PVA).

As a further improvement of the technical scheme of the preparation method, the mass ratio of the biodegradable high polymer material to the sodium alginate is 5:1-25: 1.

As a further improvement of the technical scheme of the preparation method, the mass ratio of the sodium alginate to the calcium carbonate is 10:1-50: 1.

As a further improvement of the technical scheme of the preparation method, the freezing mode is that the mixed system is placed in liquid nitrogen for freezing.

As a further improvement of the technical scheme of the preparation method, the nitrogen-phosphorus-containing biodegradable polymer double-crosslinked network hydrogel soilless culture substrate has a multi-stage hole structure with macropores and micropores.

As a further improvement of the technical scheme of the preparation method, the pore diameter of the macropores is larger than 0.3mm, and the pore diameter of the micropores is smaller than 10 mu m.

The invention effectively combines low-temperature pore-forming and chemical pore-forming, and specifically regulates and controls the mechanism of the pore structure of the double-network hydrogel soilless culture substrate as follows: on one hand, when the hydrogel is frozen at a low temperature, ice formed by water at the low temperature occupies a certain space, and when the hydrogel is placed in a normal-temperature environment, the ice is melted, and physical holes are formed in the occupied space. On the other hand, CO generated by the chemical reaction of the pore-forming agent calcium carbonate and the pore-forming assistant citric acid in the preparation process₂Chemical pore formation was performed, and the reaction mechanism was as follows. Therefore, the nitrogen-phosphorus-containing biodegradable polymer double-crosslinked network hydrogel provided by the invention has no soilThe culture medium has a multi-stage pore structure with macropores and micropores. The number of formed holes can be regulated and controlled through the addition amount of the pore-forming agent, and the size of the holes can be regulated and controlled through the total concentration of the solution.

2C₆H₈O₇+3CaCO₃＝(C₆H₅O₇)₂Ca₃+3CO₂+3H₂O

The technical personnel in the field can prepare the double-crosslinked network hydrogel soilless culture substrate with controllable pore structures with different mechanical properties and different pore sizes by controlling the proportion of the biodegradable polymer material to the sodium alginate and the proportion of the sodium alginate to the calcium carbonate according to different requirements. According to the invention, the pore structure of the prepared biodegradable polymer double-crosslinked network hydrogel soilless culture substrate is effectively regulated and controlled in a mode of combining low-temperature pore forming and chemical pore forming, so that the substrate not only has expected strength, but also has appropriate multistage pores to meet the requirements of plant respiration and moisture.

Ca of the present invention²⁺The mechanism of the ionic crosslinking reaction with sodium alginate is shown below:

in the nitrogen-phosphorus-containing biodegradable polymer hydrogel soilless culture substrate, when the biodegradable high polymer material is PVA, hydrogen bond interaction can occur between the biodegradable high polymer material and hydroxymethyl urea, and the specific action mechanism is as follows:

compared with the prior art, the invention has the following advantages:

1) according to the invention, low-temperature pore forming and chemical pore forming are organically combined, and the pore structure of the prepared biodegradable polymer double-crosslinked network hydrogel soilless culture substrate can be effectively regulated, so that the substrate not only has expected strength, but also has appropriate multistage pores to meet the requirements of plant respiration and moisture.

2) The hydroxymethyl urea can form a physical cross-linked network structure serving as a bracket and providing nitrogen nutrients through hydrogen bond interaction with biodegradable macromolecules, and the monocalcium phosphate not only can release phosphorus nutrients, but also can be subjected to ion cross-linking with sodium alginate to form an ion cross-linked network structure, so that the integration of the fertilizer and a matrix is realized.

3) The calcium carbonate is skillfully applied, not only can be used as a pore-forming agent and a pore-forming auxiliary agent to generate pores, but also can be subjected to ion crosslinking with sodium alginate to form a crosslinked network structure to provide mechanical properties for the soilless culture substrate.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and other drawings can be obtained by those skilled in the art without creative efforts.

FIG. 1 shows Ca prepared in example 1²⁺SEM comparison of-SA/PVA-MU double-crosslinked network hydrogel soilless culture substrate and common SA/PVA hydrogel soilless culture substrate. In the figure, (a) is a schematic hole diagram of a common SA/PVA hydrogel, and (b) is Ca²⁺Schematic representation of the multi-stage pores of SA/PVA-MU hydrogels, (c) is a partial enlargement of diagram (b). In which the hydrogel has small and dense pores in the graph (a), relatively large pores in the graph (b), and pores formed by freezing and crosslinking PVA at low temperature in addition to chemical pore-forming are observed in the graph (c). SEM results show that Ca²⁺the-SA/PVA-MU hydrogel has a multi-stage pore structure of macropores and micropores.

FIG. 2 shows Ca prepared in example 1²⁺-a micropore aperture distribution diagram of the SA/PVA-MU double-crosslinked network hydrogel soilless culture substrate. As can be seen from fig. 2: the aperture obtained by chemical pore-forming assisted PVA low-temperature pore-forming is between 2 and 10 mu m, and the distribution is relatively uniformAnd the holes are denser.

FIG. 3 shows Ca prepared in example 1²⁺-a macroporous aperture distribution diagram of the SA/PVA-MU double-crosslinked network hydrogel soilless culture substrate. As can be seen from fig. 3: the aperture of the super-macroporous hydrogel prepared by the chemical method is between 0.3 and 1.3mm, which is 150 times of the aperture of the common SA/PVA hydrogel.

FIG. 4 shows Ca prepared in comparative example 1²⁺SA/PVA double-crosslinked network hydrogel with Ca prepared in example 1²⁺FTIR contrast plots for SA/PVA-MU double cross-linked network hydrogels. As can be seen from FIG. 4, the wave numbers 3481 and 3204cm are shown by comparing the MU curves^-1Bimodal belonging to Ca²⁺N-H stretching vibration of primary amide of MU component in SA/PVA-MU confirms that MU component is indeed complexed to Ca²⁺SA/PVA double-crosslinked network hydrogels.

FIG. 5 shows Ca prepared in comparative example 1²⁺SA/PVA double-crosslinked network hydrogel with Ca prepared in example 1²⁺TG comparative analysis of SA/PVA-MU double cross-linked network hydrogels.

FIG. 6 shows Ca prepared in comparative example 1²⁺SA/PVA double-crosslinked network hydrogel with Ca prepared in example 1²⁺DTG comparative analysis of SA/PVA-MU double-network hydrogels. As can be seen from the figure: when MU is added to Ca²⁺Ca in SA/PVA double-crosslinked network hydrogels²⁺The thermal stability of the-SA/PVA-MU double-network hydrogel is reduced to some extent. For Ca²⁺SA/PVA double-crosslinked network hydrogel, in which Ca is caused by strong hydrogen bonding between PVA molecular chains²⁺The network arrangement of the-SA/PVA double-crosslinked network hydrogel molecular chain is relatively regular. When MU is added to Ca²⁺After the MU in the-SA/PVA double-crosslinked network hydrogel breaks the hydrogen bonding between PVA molecular chains through the hydrogen bonding between the MU and the PVA molecular chains, so that Ca is generated²⁺Degradation temperature of SA/PVA-MU shifts to the left, and MU and Ca move²⁺The SA/PVA double-crosslinked network hydrogel forms a new hydrogen bond linkage, which enables Ca²⁺the-SA/PVA-MU double-network hydrogel network is easier to be decomposed at lower temperature.

FIG. 7 shows Ca prepared in example²⁺SA-free caused by different proportions of PVA in-SA/PVA-MU double-crosslinked network hydrogelA compressive strength stress-strain curve diagram of the soil culture substrate. The compressive strength of the hydrogel is increased along with the increase of PVA: SA-1-20: 1, because PVA-MU is used as a bracket to keep the shape of the hydrogel, and sodium alginate and Ca are added²⁺The crosslinked network of (a) can provide mechanical strength to the hydrogel. When the sodium alginate accounts for a large proportion, Ca²⁺Insufficient crosslinking with sodium alginate, and excessive sodium alginate, which results in less ion crosslinking network formed, therefore, the mechanical property of the culture medium is poor. When the ratio of PVA: SA is 25:1, the sodium alginate content is too low, so that the number of sodium alginate cross-linked networks is too small to resist external load, and the mechanical property of the culture medium is poor. As described above, the hydrogel having PVA: SA-20: 1 has the best mechanical properties.

FIG. 8 shows the presence of seedlings of Brassica napus in Ca²⁺-growth pattern on SA/PVA-MU double-crosslinked network hydrogel soilless culture substrate. As can be seen from the figure, the seedlings of Brassica napus were able to grow on Ca²⁺the-SA/PVA-MU double-crosslinked network hydrogel grows vertically in the soilless culture substrate, and the hydrogel serving as a culture medium can meet the requirements of oxygen, nutrients and water of plant roots.

FIG. 9 is a diagram showing the germination rate of rape seeds. In the figure, different SA: CaCO are recorded₃(10:1, 20:1, 30:1, 40:1, 50:1) Ca prepared²⁺-SA/PVA-MU double-crosslinked network hydrogel soilless culture substrate, SA: CaCO₃40:1 Ca²⁺-increased germination rate of SA/PVA double-crosslinked network hydrogel soilless culture substrate over time. SA CaCO₃40:1 Ca²⁺The germination rate of the-SA/PVA-MU double-crosslinked network hydrogel is highest at 3d and reaches 100% after 5 d. Ca addition to MU²⁺The germination growth rate of the-SA/PVA-MU double-crosslinked network hydrogel soilless culture substrate is integrally superior to that of Ca without MU²⁺The SA/PVA double-crosslinked network hydrogel soilless culture substrate. SA CaCO₃40:1 Ca²⁺Seeds germinated better from the-SA/PVA-MU double-crosslinked network hydrogel than from other media due to SA CaCO₃40:1 Ca²⁺the-SA/PVA-MU double-crosslinked network hydrogel has a relatively uniform multi-stage pore structure and proper MU content, and can provide a large amount of water, nutrients and oxygen.

FIG. 10 is a graph comparing the length of the roots and shoots of seedlings of canola.

FIG. 11 is a weight comparison graph of rape seedlings in fresh and dry states.

As can be seen from FIGS. 10 and 11, in SA CaCO₃40:1 Ca²⁺On the-SA/PVA-MU double-crosslinked network hydrogel soilless culture substrate, the root system of the seedling growing in germination is compared with SA: CaCO₃40:1 Ca²⁺SA/PVA and SA CaCO₃Ca 10:1, 20:1, 30:1, 50:1²⁺the-SA/PVA-MU double-crosslinked network hydrogel soilless culture substrate is long and has heavier fresh weight. The hydrogel soilless culture substrate with proper pores and MU content is favorable for seed germination and growth.

Detailed Description

The technical solutions of the present invention will be described clearly and completely with reference to the accompanying drawings, and it should be understood that the described embodiments are some, but not all embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

The measurement standards of the various properties of the invention are as follows:

measurement of compression Properties: according to GB/T1041-92, the mechanical property (compression strength) of the soilless culture substrate of the biodegradable nano polymer composite material is measured at a compression rate of 4mm/min, the sample is cylindrical, the diameter is 32 +/-1 mm, the height is 38 +/-1 mm, the surface of the sample is smooth, and the upper surface and the lower surface are parallel. The average of five measurements of the sample was taken and a statistical analysis of significant differences between treatments was determined by Duncan multi-range testing.

The specific application method of the double-crosslinked network hydrogel soilless culture substrate comprises the following steps: rape seeds are placed in a depth of 0.5cm from the surface layer of the double-crosslinked network hydrogel soilless culture substrate (with the height of 38cm and the diameter of 32cm), 9 rape seeds are uniformly planted in each culture medium, and each treatment is repeated three times. And (3) determination of the germination rate of rape seeds: the number of seeds germinated was observed and recorded on

days

1, 2, 3, 4, 5, 6, and 7, respectively.

And (3) measuring the length of the roots and buds of the rape seedlings: destructive sampling was performed in each medium at the end of the culture, the length of the roots and shoots of the seedlings of the rape were taken by a ruler, and the average results of the measurements of all the samples were taken.

Determination of fresh weight and dry weight of rape seedlings: at the end of the cultivation, destructive sampling was carried out in each medium, the weight of the seedlings was weighed, i.e. the fresh weight, and subsequently dried in an oven at 60 ℃ to constant weight, the weighed seedling weight was the dry weight, and the average of all sample measurements was taken.

The technical solution of the present invention will be described in detail by the following specific examples.

Example 1

Ca²⁺The preparation method of the-SA/PVA-MU double-network hydrogel soilless culture substrate comprises the following steps:

preparation of MU: the pH of 40g of formaldehyde is adjusted to 8, 35.6g of urea is added into the formaldehyde solution according to the molar ratio of n (urea)/n (formaldehyde) being 1.2:1, the temperature is adjusted to 40 ℃ after the urea is completely dissolved, and the MU solution is obtained after reaction for 2 hours.

Ca²⁺Preparation of a SA/PVA-MU double-crosslinked network hydrogel: adding deionized water into PVA, stirring for 1h at 96 ℃ to completely dissolve the PVA to obtain 10 wt% PVA solution, and naturally cooling to room temperature; weighing 20ml PVA solution, adding sodium alginate according to PVA: SA-20: 1, stirring for 1h, and adding calcium carbonate (SA: CaCO)₃40:1), 0.05g of monocalcium phosphate and 0.5ml of MU solution, reacting for 1 hour, adding 10ml of 5% citric acid, mixing uniformly, freezing with liquid nitrogen, and taking out to obtain Ca²⁺-SA/PVA-MU double-crosslinked network hydrogel soilless culture substrate.

Example 2

Preparation of Ca with different ratios of PVA to SA²⁺-SA/PVA-MU double cross-linked network hydrogel soilless culture substrate, wherein PVA: SA ═ 5:1, 10:1, 15:1, 20:1, 25:1, other steps are the same as in example 1.

Example 3

Preparation of SA CaCO₃Ca of different proportions²⁺-SA/PVA-MU double-crosslinked network hydrogel soilless culture substrate, wherein SA is CaCO₃The other steps were the same as in example 1, 10:1, 20:1, 30:1, and 50: 1.

Comparative example 1

Ca²⁺The preparation method of the-SA/PVA double-crosslinked network hydrogel soilless culture substrate comprises the following steps:

adding deionized water into PVA, stirring for 1h at 96 ℃ to completely dissolve the PVA to obtain 10 wt% PVA solution, and naturally cooling to room temperature; weighing 20ml PVA solution, adding sodium alginate according to PVA: SA-20: 1, stirring for 1h, and adding calcium carbonate (SA: CaCO)₃40:1) and 0.05g of monocalcium phosphate, reacting for 1h, adding 10ml of 5% citric acid, mixing uniformly, freezing in liquid nitrogen, and taking out to obtain Ca²⁺-SA/PVA double-crosslinked network hydrogel soilless culture substrate.

The Ca²⁺Specific properties and application results of the-SA/PVA double-crosslinked network hydrogel soilless culture substrate are shown in figures 1-8.

Finally, it should be noted that: the above embodiments are only used to illustrate the technical solution of the present invention, and not to limit the same; while the invention has been described in detail and with reference to the foregoing embodiments, it will be understood by those skilled in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; and the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A nitrogen-phosphorus containing biodegradable polymer double-crosslinked network hydrogel soilless culture substrate is characterized in that Ca is used²⁺And a crosslinked network structure formed by ionic crosslinking of sodium alginate provides mechanical properties, a physical crosslinked network structure formed by interaction of the biodegradable polymer and the methylol urea through hydrogen bonds serves as a bracket and provides nitrogen nutrients, and the two crosslinked networks form an interpenetrating network polymer composite material.

2. The nitrogen-phosphorus-containing biodegradable polymer double-crosslinked network hydrogel soilless culture substrate as claimed in claim 1, wherein the preparation method comprises the following steps:

3. The method for preparing nitrogen and phosphorus containing biodegradable polymer double cross-linked network hydrogel soilless culture substrate as claimed in claim 2, wherein said calcium carbonate not only can be used as pore-forming agent and pore-forming auxiliary agent to generate pores, but also can provide Ca for ionic cross-linking reaction with sodium alginate²⁺And the mechanical property is provided for the soilless culture substrate by forming an ionic crosslinking network structure.

4. The method for preparing the nitrogen and phosphorus containing biodegradable polymer double cross-linked network hydrogel soilless culture substrate as claimed in claim 2, wherein the calcium dihydrogen phosphate not only can release phosphorus nutrient, but also can provide Ca for ionic cross-linking reaction with sodium alginate²⁺And the mechanical property is provided for the soilless culture substrate by forming an ionic crosslinking network structure.

5. The preparation method of the nitrogen and phosphorus containing biodegradable polymer double-crosslinked network hydrogel soilless culture substrate according to claim 2, characterized in that the biodegradable polymer material is a biodegradable polymer material which contains polar water-absorbing functional groups on the main chain and can be dissolved in water.

6. The preparation method of the nitrogen and phosphorus containing biodegradable polymer double-crosslinked network hydrogel soilless culture substrate according to claim 2, characterized in that the biodegradable high molecular material is polyvinyl alcohol.

7. The preparation method of the nitrogen and phosphorus containing biodegradable polymer double-crosslinked network hydrogel soilless culture substrate according to claim 2, characterized in that the mass ratio of the biodegradable polymer material to sodium alginate is 5:1-25: 1.

8. The preparation method of the nitrogen and phosphorus containing biodegradable polymer double-crosslinked network hydrogel soilless culture substrate according to claim 2, characterized in that the mass ratio of sodium alginate to calcium carbonate is 10:1-50: 1.

9. The preparation method of the nitrogen and phosphorus containing biodegradable polymer double-crosslinked network hydrogel soilless culture substrate according to claim 2, characterized in that the freezing mode is that the mixed system is placed in liquid nitrogen for freezing.

10. The method for preparing the nitrogen and phosphorus containing biodegradable polymer double-crosslinked network hydrogel soilless culture substrate according to claim 2, wherein the nitrogen and phosphorus containing biodegradable polymer double-crosslinked network hydrogel soilless culture substrate has a multi-stage hole structure of macropores and micropores.