CN113429553A

CN113429553A - Low-viscosity reversible crosslinked polycaprolactone and preparation method and application thereof

Info

Publication number: CN113429553A
Application number: CN202110509202.9A
Authority: CN
Inventors: 章明秋; 王泽浦; 阮文红; 容敏智
Original assignee: Sun Yat Sen University
Current assignee: Sun Yat Sen University
Priority date: 2021-05-11
Filing date: 2021-05-11
Publication date: 2021-09-24

Abstract

The invention provides low-viscosity reversible crosslinked polycaprolactone and a preparation method and application thereof. The low-viscosity reversible crosslinked polycaprolactone is obtained by introducing a reversible Diels-Alder (DA) bond to polycaprolactone; wherein the number average molecular weight of the polycaprolactone is 4000-12000. The invention can obviously reduce the melt viscosity of the polycaprolactone polymer, improve the processing performance of the polycaprolactone polymer and enable the polycaprolactone polymer to be processed and reused for many times on the premise of ensuring the mechanical property of the material by using reversible Diels-Alder bonds to reversibly crosslink the polycaprolactone with specific molecular weight.

Description

Low-viscosity reversible crosslinked polycaprolactone and preparation method and application thereof

Technical Field

The invention relates to the field of biodegradable polymer materials, and in particular relates to low-viscosity reversible crosslinked polycaprolactone and a preparation method and application thereof.

Background

Polycaprolactone (PCL) has certain rigidity and strength, good compatibility with high polymer materials, good biodegradability and shape memory function, and thus has wide application prospect. However, the mechanical strength of PCL is not sufficient to limit its application and development. In order to overcome this disadvantage of PCL, it has been modified accordingly:

1) inorganic particle filling modification (as in chinese patent CN 110643028A): the inorganic particles need a certain content for filling, but under the filling of the filler with ultrahigh content, the molecular chain movement is greatly limited due to the interaction between the fillers and the action between the fillers and the polymer molecular chain, so that the system viscosity is sharply increased, the fluidity is greatly reduced, the problems of difficult plastic processing (such as extrusion and injection) and incapability of uniformly mixing occur, and the high-efficiency production and the product quality are influenced. The conventional solution is to reduce the processing viscosity of the polymer composite by adding compatibilizers, plasticizers, lubricants, etc., and to improve the processability by graft-modifying the filler and matrix. However, these methods cannot fundamentally solve the problem of high viscosity of the polymer system due to high filling and also have the problem of additive precipitation.

2) Crosslinking modification: among all structural factors influencing the rheological behavior of the polymer, the molecular weight is the most important, and the polymer with small molecular weight has low melt viscosity and good fluidity, but as the molecular weight becomes smaller, the mechanical property of the polymer is gradually weakened, even no mechanical strength exists, so that a method for bonding molecular chains for reinforcement by crosslinking is adopted (such as Chinese patent CN107254186A), but the crosslinked material cannot be dissolved and melted any more, and the material cannot be reprocessed, thereby bringing about the problem of recycling.

Therefore, it is required to develop polycaprolactone having good reversible processability and excellent mechanical properties.

Disclosure of Invention

The invention aims to overcome the defects of the prior art and provide the reversible crosslinked polycaprolactone which has ultralow melt viscosity and excellent mechanical property and can be processed for multiple times. The low-viscosity reversible crosslinked polycaprolactone is obtained by introducing reversible Diels-Alder (DA) bonds to specific polycaprolactone with lower molecular weight, and reversible crosslinking is carried out on the polycaprolactone with specific molecular weight by using the reversible DA bonds, so that the melt viscosity of the polycaprolactone polymer can be remarkably reduced, the processability of the polycaprolactone polymer can be improved on the premise of ensuring the mechanical property of the material, and the material can be processed and reused for multiple times.

Another object of the present invention is to provide a method for preparing the low viscosity reversibly crosslinked polycaprolactone.

The invention also aims to provide application of the low-viscosity reversible crosslinked polycaprolactone in preparation of a high-filling composite material.

In order to achieve the purpose, the invention adopts the following technical scheme:

a low-viscosity reversible cross-linked polycaprolactone is obtained by introducing a reversible DA bond to polycaprolactone; wherein the number average molecular weight of the polycaprolactone is 4000-12000.

Since molecular weight plays an important role in polymer viscosity, the molecular weight is too low, and although the viscosity is small, the processing is facilitated, but the mechanical properties of the polymer are poor, so that the existing modification is carried out on the polymer with higher molecular weight.

Polycaprolactone is an excellent biodegradable material, and the mechanical property is gradually improved along with the increase of molecular weight, but the viscosity is also increased. The invention selects reversible DA bond to reversibly crosslink the polycaprolactone with specific small molecular weight, so that the obtained crosslinked polycaprolactone has low processing viscosity and good mechanical property at normal temperature. The reversible DA covalent bond can generate reversible 'breaking' and 'combination' under the action of a certain temperature condition, a dynamic cross-linked polycaprolactone network is constructed by using a thermally reversible covalent bond, when the temperature of the reverse DA reaction is matched with the processing temperature of polycaprolactone, the reversible bond can be broken by utilizing temperature rise in the processing process of polycaprolactone, the cross-linked network is depolymerized, the molecular weight is reduced, and when the molecular chains are broken and are short enough to be not entangled with each other, the viscosity of the system is obviously reduced, so that the flowability of polycaprolactone is greatly improved; and after the material is cooled, reversible DA bonds can be recombined, a cross-linked network with chemical bond links is formed among the micromolecule polycaprolactone, and the mechanical property of the polycaprolactone is recovered. This reversible crosslinking allows the material to be processed and recycled many times.

As an embodiment (see examples 1, 4, 5), preferably, the low molecular weight polycaprolactone has a number average molecular weight of 4300 to 8000.

Preferably, the low molecular weight polycaprolactone is multi-arm polycaprolactone, specifically, one or more of two-arm polycaprolactone, three-arm polycaprolactone or four-arm polycaprolactone (see example 1). The multi-arm polycaprolactone can obtain higher crosslinking degree under the condition of the same molecular weight, and has wider regulation range.

Common reversible DA bonds, which are obtained by reacting a dienophile with a dienophile, can be used in the present invention.

Preferably, the diene contains conjugated diene, specifically one or a combination of furan and its derivatives (see example 1), pyrrole and its derivatives, or thiophene and its derivatives.

Further preferably, the diene is furan and its derivatives, and further preferably one or a combination of two of furfuryl amine (see example 1) and furfuryl alcohol.

Preferably, the dienophile is an unsaturated compound containing a double or triple bond.

Further preferably, the unsaturated compound containing a double bond or a triple bond is a maleimide group-containing compound, and more preferably one or a combination of diphenylmethane bismaleimide, 2-methylpentanediamine bismaleimide (see example 1), tris (2-maleimidoethyl) amine, 1, 6-bis (maleimide) hexane, 1, 12-bis (maleimide) dodecane or 1, 8-bismaleimide-3, 6-dioxaoctane.

In one embodiment (e.g., examples 1 to 3), the molar ratio of the conjugated diene in the diene to the double bond or triple bond in the dienophile is preferably 1:0.6 to 1.4.

By controlling the molar ratio of the diene to the dienophile, the crosslinking degree of the low-viscosity reversible crosslinked polycaprolactone can be controlled, and the mechanical property and the processing temperature range of the prepared low-viscosity reversible crosslinked polycaprolactone can be regulated, wherein too high or too low a molar ratio can cause too low crosslinking degree of the material and unsatisfactory mechanical property. Therefore, it is further preferable that the molar ratio of the conjugated diene in the diene to the double bond or triple bond in the dienophile is 1:0.8 to 1.2, and it is further preferable that the molar ratio is 1: 1.

Preferably, the melt viscosity of the low-viscosity reversible crosslinked polycaprolactone at 135 ℃ is 1-10 Pa.s.

The preparation method of the low-viscosity reversible crosslinked polycaprolactone comprises the following steps:

s1, functional modification of polycaprolactone

Reacting polycaprolactone with anhydride in the presence of a catalyst to obtain carboxyl-terminated polycaprolactone, further carrying out a grafting reaction with a compound containing a functional group A, and introducing the functional group A into the polycaprolactone to obtain functionalized polycaprolactone;

s2, melting and blending the functionalized polycaprolactone obtained in the step S1 and a compound containing a functional group B, and keeping the mixture at the temperature of 20-80 ℃ for 24-48 hours to obtain the low-viscosity reversible crosslinked polycaprolactone;

wherein, reversible DA bonds are formed between the functional group A and the functional group B.

Preferably, the compound containing the functional group A is a diene, and the compound containing the functional group B is a dienophile; or the compound containing the functional group A is dienophile, and the compound containing the functional group B is dienophile.

Preferably, the anhydride is succinic anhydride in step S1 (see example 1).

Preferably, the catalyst in step S1 is one or a combination of two of triethylamine and 4-dimethylaminopyridine.

Preferably, the temperature of the melt blending in the step S2 is 100-130 ℃ (see example 1).

The application of the low-viscosity reversible crosslinked polycaprolactone in the preparation of the high-filling composite material is also within the protection scope of the invention.

Compared with the prior art, the invention has the following beneficial effects:

the invention uses reversible DA bond to reversibly crosslink polycaprolactone with specific molecular weight, which can obviously reduce the melt viscosity of polycaprolactone polymer, improve the processing property and enable the material to be processed and reused for many times on the premise of ensuring the mechanical property of the material. Wherein the melt viscosity of the obtained low-viscosity reversible crosslinked polycaprolactone at 135 ℃ can be as low as 1-10 Pa.s; the tensile strength at normal temperature can reach 20-25 MPa, the tensile modulus can reach 227-260 MPa, and the elongation at break can reach more than 600%; after the three-time recovery, the retention rate of the tensile strength can reach 95-100%, the retention rate of the tensile modulus can reach 95-100%, and the retention rate of the elongation at break can reach 97-100%, which shows that the recycling capacity is excellent.

Drawings

FIG. 1 is a graph of the melt viscosity as a function of temperature for the low viscosity reversibly crosslinked polycaprolactone prepared in example 1;

FIG. 2 is a graph showing the melt viscosity of polycaprolactone obtained in comparative example 3 as a function of temperature;

FIG. 3 is a plot of melt viscosity as a function of temperature for the polycaprolactone of comparative example 4;

FIG. 4 is a graph of the melt viscosity as a function of temperature of the low viscosity reversibly crosslinked polycaprolactone prepared in example 1 after 3 recoveries.

Detailed Description

The present invention will be further described with reference to the following specific examples and drawings, which are not intended to limit the invention in any manner. Reagents, methods and apparatus used in the present invention are conventional in the art unless otherwise indicated. Unless otherwise indicated, reagents and materials used in the present invention are commercially available.

Example 1

The embodiment provides a low-viscosity reversible crosslinked polycaprolactone, and the preparation method comprises the following steps:

s1, polycaprolactone grafted furan group

S11.1mol of caprolactone, 0.02mol of pentaerythritol and 0.2g of stannous octoate are added into a single-mouth bottle protected by nitrogen, the temperature is rapidly raised to 120 ℃, and the mixture is stirred for 12 hours to obtain polycaprolactone;

s12, adding 400mL of N, N-dimethylformamide into the polycaprolactone obtained in S11, and stirring under the conditions of dry nitrogen and room temperature until the N, N-dimethylformamide is completely dissolved; adding 0.096mol of succinic anhydride and 0.096mol of triethylamine; stirring for 12h at 50 ℃ after the addition; distilling under reduced pressure for 30min, adding ether to precipitate product, centrifuging, and drying to obtain carboxyl-functionalized polycaprolactone;

the number average molecular weight Mn of the carboxyl functionalized polycaprolactone is 6300 through a nuclear magnetic resonance test;

s13.0.01mol of polycaprolactone obtained from S12 is dissolved in 400mL of dichloromethane to obtain a dissolved solution, then 0.011mol of furfuryl amine, 0.001mol of 4-Dimethylaminopyridine (DMAP) and 0.011mol of dicyclohexyl carbodiimide (DCC) are added into the dissolved solution, stirred for 24 hours at 25 ℃, filtered, the filtrate is poured into ether, the volume ratio of the ether to the filtrate is 10:1, and the polycaprolactone with the terminal grafted with furan groups is obtained, filtered and dried in vacuum for later use;

s2, preparing low-viscosity reversible cross-linked polycaprolactone

S21. preparation of compound containing maleimide group

Dissolving 0.37mol of maleic anhydride in 80mL of N, N-dimethylformamide, dropwise adding 2-methylpentamethylenediamine 0.17mol, reacting at 30 ℃ for 2h, adding 64mL of acetic anhydride, 8mL of triethylamine and 0.4g of nickel acetate, keeping the temperature at 80 ℃ for 2h, adding 200mL of water, and then distilling under reduced pressure for 30 min; dissolving the mixture obtained by distillation in ethanol at 50 ℃, cooling to room temperature, precipitating white powder which is a compound 2-methyl pentanediamine bismaleimide containing maleimide groups, filtering and drying for later use;

s22, preparation of low-viscosity reversible crosslinked polycaprolactone

And (2) melting and mixing polycaprolactone with the terminal grafted furan group prepared in the step (S1) and 2-methylpentanediamine bismaleimide obtained in the step (S21) according to the molar ratio of the furan group to the maleimide functional group of 1:1 at 120 ℃ for 10min, then pressing and forming, reacting the formed sample in an oven at 60 ℃ for 24h, cooling to normal temperature, and standing for 24h to obtain the low-viscosity reversible crosslinked polycaprolactone.

Example 2

This example provides a low viscosity reversibly crosslinked polycaprolactone, which was prepared by a method different from that of example 1 in that the molar ratio of furan groups to maleimide functional groups in step S22 was 1: 0.8.

Example 3

This example provides a low viscosity reversibly crosslinked polycaprolactone, which was prepared by a method different from that of example 1 in that the molar ratio of furan groups to maleimide functional groups in step S22 was 1: 0.6.

Example 4

This example provides a low viscosity reversible crosslinked polycaprolactone, which is prepared by a method different from that of example 1, wherein the starting materials in step S11 are 1mol caprolactone, 0.025mol pentaerythritol and 0.2g stannous octoate, and the number average molecular weight of polycaprolactone measured in step S12 is 4700.

Example 5

This example provides a low viscosity reversibly crosslinked polycaprolactone, which is prepared by the following method, except that: in the step S11, the raw materials are 1mol of caprolactone, 0.016mol of pentaerythritol and 0.2g of stannous octoate, and the number average molecular weight of polycaprolactone measured in the step S12 is 7200.

Comparative example 1

The comparative example provides a reversible crosslinked polycaprolactone, and the preparation method is different from that of example 1 in that: in step S11, the raw materials include 1mol caprolactone, 0.033mol pentaerythritol and 0.2g stannous octoate, and the number average molecular weight of polycaprolactone determined in step S12 is 3500.

Comparative example 2

The comparative example provides a reversible crosslinked polycaprolactone, and the preparation method is different from that of example 1 in that: in step S11, the materials are 1mol caprolactone, 0.009mol pentaerythritol and 0.2g stannous octoate, and the number average molecular weight of polycaprolactone determined in step S12 is 12800.

Comparative example 3

The comparative example provides polycaprolactone, and the preparation method is different from that of example 1 in that: step S21 is eliminated, and only the polycaprolactone with the furan group grafted at the end obtained in step S1 is selected in step S22 for compression molding (i.e., DA reversible crosslinking is not performed).

Comparative example 4

In the comparative example, the polycaprolactone with the molecular weight of 80000, which is commercially available, is selected, is molded at 120 ℃ and then is cooled to the normal temperature and placed for 24 hours.

And (3) performance testing:

the processing performance and the mechanical property of the polycaprolactone prepared by the method are tested, and specific test items and test steps are as follows:

1. melt viscosity: measuring according to the standard of ISO 20965-2005, wherein the test result is shown in FIGS. 1-4;

2. tensile property: the measurement was carried out according to ASTM-D63, wherein the sample shape was dumbbell-shaped, the test temperature was 25 ℃ and the tensile rate was 50mm/min, five samples were prepared from the material obtained in each example/comparative example, the results were averaged, and the tensile properties were represented by tensile strength and modulus and elongation at break, and the specific test results are shown in Table 1;

3. the recycling performance is as follows: shearing a sample, and then pressing and forming again; the mechanical properties of the processed polycaprolactone after 3 times of recovery were tested, and the test results are detailed in table 2.

FIG. 1 is a graph showing the viscosity of the low viscosity reversibly crosslinked polycaprolactone prepared in example 1 of the present invention as a function of temperature, from which it can be seen that: the viscosity has two steep decline slopes, the first one is between 40 and 50 ℃, which is the melting temperature of the polycaprolactone crystal; as the crystals melt, the polymer viscosity decreases significantly; the second descending steep slope is 110-140 ℃, which is the temperature range of inverse DA reaction, and the crosslinked polycaprolactone is gradually cracked into low-molecular-weight polycaprolactone prepolymer and micromolecular bismaleimide along with the occurrence of inverse DA, so that the viscosity is obviously reduced (the viscosity of the material is between 1-10 Pa & s at 135 ℃); when the temperature rises to 140 ℃, the crosslinked polycaprolactone is completely cracked into polycaprolactone with lower molecular weight and micromolecule bismaleimide, and the viscosity tends to be horizontal (the viscosity tends to 1Pa s). The viscosity change curve graph of the low-viscosity reversible crosslinked polycaprolactone prepared in the embodiments 2-5 along with the temperature is similar to that of the embodiment 1, the viscosity is reduced in the temperature ranges of 40-50 ℃ and 110-140 ℃, and the viscosity of the material above 135 ℃ is reduced to 1-10 Pa.s.

FIG. 2 is a graph showing the viscosity change with temperature of the uncrosslinked modified polycaprolactone obtained in comparative example 3, from which it can be seen that: when the temperature is increased from 40 ℃ to 50 ℃, the viscosity is sharply reduced to about 10 Pa.s, and the reduction range reaches 10 DEG C⁴This is because the polycaprolactone prepolymer has a very low molecular weight, a low degree of entanglement, and a rapid drop in viscosity after the crystals have melted; when the temperature exceeds 50 ℃, the viscosity slowly decreases with temperature, because the molecular mobility increases with increasing temperature, which is a common feature of most thermoplastic polymers.

FIG. 3 is a graph of the viscosity of the high molecular weight polycaprolactone of comparative example 4 as a function of temperature, from which it can be seen that: the viscosity change rule is consistent with that of the comparative example 3, but the viscosity (the viscosity at 135 ℃ is 1000-10000 Pa.s) is far higher than that of the comparative example 3, so that the processing is not facilitated.

The low-viscosity reversible crosslinked polycaprolactone prepared by the method has obviously reduced melt viscosity at the temperature of over 135 ℃, and compared with the conventional high-molecular-weight polycaprolactone, the viscosity is reduced by over 1000 times, so that the processing is facilitated.

Table 1 tensile properties test results

PCL	Tensile Strength (MPa)	Tensile modulus (MPa)	Elongation at Break (%)
				Example 1	21.6±1.6	260.5±21.4	>600.0
Example 2	22.8±2.4	227.6±17.7	>600.0
				Example 3	19.3±2.1	232.7±18.3	530.0±38.0
Example 4	20.1±1.5	240.3±15.1	>600
				Example 5	21.5±1.8	230±12.1	>600
Comparative example 1	15.2±1.2	180±10.6	330.0±28.0
				Comparative example 2	8.5±2.1	240.3±15.1	15.8±1.2
Comparative example 3	—	—	—
				Comparative example 4	40.2±1.6	271.4±18.4	>600.0

。

As can be seen from table 1: the low-viscosity reversible crosslinked polycaprolactone obtained by the embodiments of the invention has a tensile strength of 20-23 MPa at 25 ℃, a tensile modulus of 230-260 MPa and an elongation at break of more than 500%; the polycaprolactone with lower or higher molecular weight is selected in comparative examples 1 and 2 respectively, and the prepared low-viscosity reversible crosslinked polycaprolactone has poor mechanical property at 25 ℃; comparative example 3, in which no crosslinking modification was performed, tensile data could not be obtained due to the brittle and weak properties of the polymer itself; comparative example 4 is a high molecular weight polycaprolactone (molecular weight 80000) with good mechanical properties, but not good for processing due to high viscosity.

From examples 1 to 3, it can be seen that, with the increase of the molar ratio of the furan group to the maleimide functional group, the tensile strength is increased and then decreased, which indicates that by controlling the molar ratio of the furan group to the maleimide functional group, the crosslinking degree of the low-viscosity reversible crosslinked polycaprolactone can be controlled, and the mechanical properties of the prepared material can be regulated.

TABLE 2 mechanical Properties after 3 recyclings

As can be seen from table 2: even after three times of recovery, the tensile property of the low-viscosity reversible crosslinked resin obtained in each embodiment of the invention is hardly changed obviously (the retention rate is 95-100%), which shows that the polycaprolactone containing reversible DA bond crosslinking has very excellent recovery performance.

Wherein, the viscosity of the low viscosity reversible crosslinked polycaprolactone of the example 1 after being recovered for three times is changed along with the temperature as shown in fig. 4, the change trend is basically consistent with that before being recovered (fig. 1), and after the temperature is over 140 ℃, the viscosity is reduced to be less than 10Pa s, which shows that even after being recovered for three times, the side reaction in the material is little, no obvious irreversible crosslinking is formed, and the processing performance is excellent. Once again, the crosslinked polycaprolactone of the present invention was demonstrated to have excellent recyclability.

The above-mentioned embodiments are intended to illustrate the objects, technical solutions and advantages of the present invention in further detail, and it should be understood that the above-mentioned embodiments are merely exemplary embodiments of the present invention, and are not intended to limit the scope of the present invention, and any modifications, equivalent substitutions, improvements and the like made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims

1. A low-viscosity reversible crosslinked polycaprolactone is characterized in that the low-viscosity reversible crosslinked polycaprolactone is obtained by introducing a reversible DA bond into polycaprolactone; wherein the number average molecular weight of the polycaprolactone is 4000-12000.

2. The low viscosity reversibly crosslinked polycaprolactone according to claim 1, wherein the polycaprolactone has a number average molecular weight of 4300 to 8000.

3. The low viscosity reversibly crosslinked polycaprolactone of claim 1, wherein the polycaprolactone is one or a combination of two-arm polycaprolactone, four-arm polycaprolactone or eight-arm polycaprolactone.

4. The low viscosity reversibly crosslinked polycaprolactone of claim 1, characterized in that the reversible DA bond is obtained by reaction of a diene and a dienophile; the diene is one or a combination of more of furan and derivatives thereof, pyrrole and derivatives thereof or thiophene and derivatives thereof; the dienophile is an unsaturated compound containing double bonds or triple bonds.

5. The low-viscosity reversible crosslinked polycaprolactone according to claim 4, wherein the molar ratio of the conjugated diene in the diene to the double bond or triple bond in the dienophile is 1: 0.6-1.4.

6. The low-viscosity reversible crosslinked polycaprolactone according to claim 5, wherein the molar ratio of the conjugated diene in the diene to the double bond or triple bond in the dienophile is 1: 0.8-1.2.

7. The low viscosity reversible crosslinked polycaprolactone according to claim 1, wherein the melt viscosity of the low viscosity reversible crosslinked polycaprolactone at 135 ℃ is 1-10 Pa-s.

8. A method for preparing the low viscosity reversible crosslinked polycaprolactone described in any one of claims 1-7, comprising the following steps:

s1, functional modification of polycaprolactone

9. The method for preparing low viscosity reversible crosslinked polycaprolactone according to claim 8, characterized in that the compound containing functional group A is a diene, and the compound containing functional group B is a dienophile; or the compound containing the functional group A is dienophile, and the compound containing the functional group B is dienophile.

10. Use of the low viscosity reversibly crosslinked polycaprolactone of any one of claims 1 to 7 for the preparation of highly filled composites.