CN111423704A - Polylactic acid stereo complex and preparation method thereof - Google Patents
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Abstract
The invention provides a polylactic acid stereo complex and a preparation method thereof, wherein the preparation method comprises the following steps: adding graphite oxide into a mixture of the levorotatory polylactic acid and the dextrorotatory polylactic acid, and mixing to prepare; the addition amount of the graphite oxide is 1.5-10 wt% of the mixture, and the hydroxyl content of the graphite oxide is not lower than 0.01%; the mass ratio of the levorotatory polylactic acid to the dextrorotatory polylactic acid is 30: 70-70: 30. The method can obtain the polylactic acid stereo compound consisting of more than 99 percent of stereo compound crystals, and the melting temperature of the polylactic acid stereo compound is increased to about 220 ℃, thereby improving the use temperature and the strength of the polylactic acid material. The preparation method is simple and short in time consumption, and can quickly obtain the polylactic acid stereo complex. In addition, the polylactic acid stereocomplex of the present invention is excellent in biodegradability and moldability, and is suitable for industrial production.
Description
Technical Field
The invention relates to the field of biodegradable high polymer materials, and more particularly relates to a polylactic acid stereo complex and a preparation method thereof.
Background
With the increasing importance of people on environmental problems, biodegradable plastics are gradually favored, and among numerous biodegradable materials, polylactic acid (P L A) is the most attractive material.
In addition, P L A is derived from renewable resources (such as corn), so that the consumption of petrochemical products can be reduced, and the human resource crisis is relieved.
In order to improve the high temperature resistance of polylactic acid, researchers have made many studies in this regard, and one of them is a method of preparing a stereocomplex crystal having a higher melting point than P L a by mixing optical isomers of poly-L-lactic acid and poly-D-lactic acid, however, this method forms a homogeneous P L a crystal in addition to the stereocomplex crystal, and it is difficult to obtain a stereocomplex consisting of only the stereocomplex crystal, or the method is prepared for a long time, and the production cost is increased invisibly.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides a polylactic acid stereocomplex and a preparation method thereof.
The invention provides a preparation method of a polylactic acid stereo complex, which comprises the following steps: adding graphite oxide into a mixture of the levorotatory polylactic acid and the dextrorotatory polylactic acid, and mixing to prepare;
the addition amount of the graphite oxide is 1.5-10 wt% of the mixture, and the hydroxyl content of the graphite oxide is not lower than 0.01%;
the mass ratio of the levorotatory polylactic acid to the dextrorotatory polylactic acid is 30: 70-70: 30.
Through a large number of experimental researches, the invention discovers that the graphite oxide is added into the mixture of the levorotatory polylactic acid and the dextrorotatory polylactic acid, so that the preferential formation of a stereo compound crystal can be promoted, the generation of a single polylactic acid crystal is inhibited, and the content of the stereo compound of the polylactic acid is greatly improved. When the addition amount of the graphite oxide is 1.5-10 wt%, the hydroxyl content of the graphite oxide is not less than 0.01%, and the mass ratio of the levorotatory polylactic acid to the dextrorotatory polylactic acid is 30: 70-70: 30, a polylactic acid stereo compound consisting of more than 99% of stereo compound crystals can be obtained, the melting temperature of the polylactic acid stereo compound is increased to about 220 ℃, and the use temperature of the polylactic acid material is increased. The preparation method of the invention has short time consumption, can quickly obtain the polylactic acid stereo complex, and is suitable for industrial production.
The three conditions of the addition amount of the graphite oxide, the hydroxyl content of the graphite oxide and the mass ratio of the levorotatory polylactic acid to the dextrorotatory polylactic acid are required to be simultaneously satisfied, and if one of the conditions is not satisfied, a polylactic acid three-dimensional compound crystal and a single crystal are simultaneously formed.
Further preferably, the content of hydroxyl groups in the graphite oxide is 20-30%, and the mass ratio of the levorotatory polylactic acid to the dextrorotatory polylactic acid is 40: 60-60: 40, at present, the existence of the hydroxyl groups is supposed to be favorable for adsorbing a P L A molecular chain, and the P LL A molecular chain and the PD L A molecular chain are alternately arranged to form a stereo complex crystal.
Furthermore, the molecular weight of the levorotatory polylactic acid is 100-200 kg/mol, and the molecular weight distribution is 1.20-2.50.
Furthermore, the molecular weight of the poly-D-lactic acid is 100-200 kg/mol, and the molecular weight distribution is 1.20-2.50.
Further, the particle size of the graphite oxide is 200 to 1000 mesh, and more preferably 500 to 800 mesh.
As an alternative, the preparation method specifically includes: and mixing the levorotatory polylactic acid, the dextrorotatory polylactic acid and the graphite oxide, and banburying for 1-10 min at the temperature of 200-250 ℃.
Further preferably, the banburying temperature is 230-240 ℃ and the banburying time is 3-6 min.
Or, as another alternative, the preparation method specifically includes: respectively dissolving the levorotatory polylactic acid and the dextrorotatory polylactic acid by using a solvent, mixing, then adding the graphite oxide, uniformly mixing, and removing the solvent.
Further, the solvent is one or more of dichloromethane, trichloromethane, N-dimethylformamide and tetrahydrofuran.
Further preferably, the solvent is dichloromethane and/or tetrahydrofuran.
Further, the levorotatory polylactic acid, the dextrorotatory polylactic acid and the graphite oxide are all preferably industrial-grade products. The industrial-grade product generally has good product uniformity, can be produced in batches and provides a premise for industrial application.
The invention also provides the polylactic acid stereocomplex prepared by the preparation method. The melting temperature of the polylactic acid stereo compound is about 220 ℃, and the polylactic acid stereo compound is more resistant to high temperature than a common polylactic acid material.
The method can obtain the polylactic acid stereo compound consisting of more than 99 percent of stereo compound crystals, and the melting temperature of the polylactic acid stereo compound is increased to about 220 ℃, thereby increasing the use temperature of the polylactic acid material. The preparation method is simple and short in time consumption, and can quickly obtain the polylactic acid stereo complex. In addition, the polylactic acid stereocomplex of the present invention is excellent in biodegradability and moldability, and is suitable for industrial production.
Drawings
FIG. 1 is a differential scanning calorimetry curve of a polylactic acid mixture according to examples 1-5 of the present invention;
FIG. 2 is an infrared spectrum of a polylactic acid mixture according to examples 1 to 5 of the present invention;
FIG. 3 is a thermogravimetric plot of the polylactic acid blends provided in examples 1, 3 and 5 of the present invention;
FIG. 4 is a photograph of injection molded bars provided in example 25 of the present invention.
Detailed Description
The following examples are intended to illustrate the invention but are not intended to limit the scope of the invention. The examples do not show the specific techniques or conditions, according to the technical or conditions described in the literature in the field, or according to the product specifications. The reagents or instruments used are conventional products available from regular distributors, not indicated by the manufacturer.
Examples 1 to 5
30g of L-polylactic acid (P LL A, M)n150k), 30g d-polylactic acid (PD L a, M)n130k) and a certain amount of graphite oxide (with 20 percent of hydroxyl content and 500 meshes) are internally mixed in an internal mixer at 230 ℃ for 6 minutes to obtain the polylactic acid mixture. The mass ratio of the levorotatory polylactic acid to the dextrorotatory polylactic acid is 50: 50.
A series of polylactic acid mixtures can be obtained by changing the adding amount of the graphite oxide, and the specific parameters are shown in Table 1.
TABLE 1 examples 1-5 composition of polylactic acid mixtures
| Sample (I) | PLLA(g) | PDLA(g) | Graphite oxide (g) | Addition amount of graphite oxide |
| Example 1 | 30 | 30 | 0 | 0％ |
| Example 2 | 30 | 30 | 0.3 | 0.5％ |
| Example 3 | 30 | 30 | 0.61 | 1.0％ |
| Example 4 | 30 | 30 | 0.91 | 1.5％ |
| Example 5 | 30 | 30 | 1.2 | 2.0％ |
The polylactic acid mixtures obtained in examples 1 to 5 were analyzed by Differential Scanning Calorimetry (DSC), and as shown in FIG. 1, the first melting temperature was about 170 ℃ and the second melting temperature was about 220 ℃. The first melting temperature corresponds to the crystalline melting temperature of the polylactic acid alone, and the second melting temperature corresponds to the crystalline melting temperature of the stereocomplex. From FIG. 1, it can be seen that the first melting temperature and the second melting temperature are simultaneously present in examples 1-3, indicating that both the polylactic acid crystals alone and the stereocomplex crystals are simultaneously present in the polylactic acid mixture; however, as the amount of graphite oxide added was increased, the peak value of the first melting temperature was decreased, and the peak value of the second melting temperature was gradually increased, indicating that the content of stereocomplex crystals was gradually increased and the content of polylactic acid crystals alone was gradually decreased. The first melting temperature was almost disappeared in example 4, and when the content of graphite oxide was further increased to 2%, the first melting temperature was completely disappeared (corresponding to example 5), which confirmed that no polylactic acid crystal alone and only a stereocomplex crystal existed in the polylactic acid mixture.
The present inventors also tried to prolong the banburying time in example 1 (graphite oxide addition amount is 0%), but as a result, the stereocomplex crystal content was not increased.
The polylactic acid mixtures obtained in examples 1 to 5 were characterized by an infrared spectrometer, and the test results are shown in FIG. 2, in which the spectrum of the infrared spectrum is 909cm-1The point is a characteristic absorption peak of the stereo composite crystal. From the analysis of FIG. 2, it was found that the polylactic acid mixture obtained was 909cm in length as the amount of graphite oxide added was increased-1The absorption peak becomes stronger, indicating that the content of the stereocomplex crystal of the polylactic acid mixture increases with the increase of the addition amount of the graphite oxide.
As a result of thermogravimetric analysis of the polylactic acid mixtures obtained in examples 1, 3 and 5, as shown in fig. 3, the temperature at which the polylactic acid mixture without graphite oxide (example 1) started to decompose was about 320 ℃; as the amount of graphite oxide added increases, the temperature at which the polylactic acid mixture starts to decompose decreases to about 300 ℃ (example 5). The result shows that the addition of graphite oxide has little influence on the thermal stability of the polylactic acid blend, and the thermal decomposition temperature is above 300 ℃, which indicates that the product can be molded and processed at 250 ℃ without causing obvious degradation.
Examples 6 to 10
18g of L-polylactic acid (P LL A, M)n180k), 42g d-polylactic acid (PD L a, M)n150k) and a certain amount of graphite oxide (with 30 percent of hydroxyl group content and 800 meshes) are internally mixed in an internal mixer at the temperature of 200 ℃ for 10 minutes to obtain the polylactic acid mixture. The mass ratio of the levorotatory polylactic acid to the dextrorotatory polylactic acid is 30: 70.
A series of polylactic acid mixtures can be obtained by changing the adding amount of the graphite oxide, and the specific parameters are shown in Table 2.
TABLE 2 examples 6-10 compositions of polylactic acid blends
The polylactic acid mixtures obtained in examples 6 to 10 were analyzed by DSC to obtain a first melting temperature of about 170 ℃ and a second melting temperature of about 220 ℃. The first melting temperature corresponds to the crystalline melting temperature of the polylactic acid alone, and the second melting temperature corresponds to the crystalline melting temperature of the stereocomplex. The simultaneous presence of the first melting temperature and the second melting temperature in examples 6-8 illustrates the simultaneous presence of separate polylactic acid crystals and stereocomplex crystals in the polylactic acid mixture; however, as the amount of graphite oxide added was increased, the peak value of the first melting temperature was decreased, and the peak value of the second melting temperature was gradually increased, indicating that the content of stereocomplex crystals was gradually increased and the content of polylactic acid crystals alone was gradually decreased. The first melting temperature in example 9 almost disappeared, and when the content of graphite oxide was further increased to 2%, the first melting temperature disappeared completely (corresponding to example 10), demonstrating that no polylactic acid crystal alone and only the stereocomplex crystal existed in the polylactic acid mixture at this time.
The polylactic acid mixtures obtained in examples 6 to 10 were characterized by an infrared spectrometer, and it was found by analysis that the polylactic acid mixtures obtained were at 909cm with increasing addition of graphite oxide-1The absorption peak becomes stronger, indicating that the content of the stereocomplex crystal of the polylactic acid mixture increases with the increase of the addition amount of the graphite oxide.
The thermogravimetric analyses of the polylactic acid mixtures obtained in examples 6 to 10 revealed that the temperature at which the polylactic acid mixture started to decompose was decreased from 320 ℃ to about 300 ℃ as the amount of graphite oxide added was increased (example 10). The result shows that the addition of graphite oxide has little influence on the thermal stability of the polylactic acid blend, and the thermal decomposition temperature is above 300 ℃, which indicates that the product can be molded and processed at 250 ℃ without causing obvious degradation.
Examples 11 to 15
42g of L-polylactic acid (P LL A, M)n140k), 18g d-polylactic acid (PD L a, M)n120k) and a certain amount of graphite oxide (hydroxyl content 0.01%, 1000 mesh) are banburying in a banbury mixer at 250 ℃ for 3 minutes to obtain a polylactic acid mixture. The mass ratio of the levorotatory polylactic acid to the dextrorotatory polylactic acid is 70: 30.
A series of polylactic acid mixtures can be obtained by changing the adding amount of the graphite oxide, and the specific parameters are shown in Table 3.
TABLE 3 examples 11-15 composition of polylactic acid mixtures
| Sample (I) | PLLA(g) | PDLA(g) | Graphite oxide (g) | Addition amount of graphite oxide |
| Example 11 | 42 | 18 | 1.2 | 2.0％ |
| Example 12 | 42 | 18 | 2.5 | 4.0％ |
| Example 13 | 42 | 18 | 3.8 | 6.0％ |
| Example 14 | 42 | 18 | 5.2 | 8.0％ |
| Example 15 | 42 | 18 | 6.7 | 10.0％ |
The polylactic acid mixtures obtained in examples 11 to 15 were analyzed by DSC, and only a single melting temperature corresponding to the crystal melting temperature of the stereocomplex was found at around 220 ℃ which confirmed that no single polylactic acid crystal existed in the polylactic acid mixture and only the stereocomplex crystal existed.
The polylactic acid mixtures obtained in examples 11 to 15 were characterized by an infrared spectrometer and analyzed to find that the polylactic acid mixtures obtained were at 909cm with increasing addition of graphite oxide-1There was almost no difference in the absorption peak intensity.
The thermogravimetric analyses of the polylactic acid mixtures obtained in examples 11 to 15 revealed that the initial decomposition temperature of the polylactic acid mixture hardly changed with the increase in the amount of graphite oxide added, i.e., the initial decomposition temperature of the polylactic acid mixture was maintained at about 300 ℃ and did not further decrease with the amount of graphite oxide added in the range of 2 to 10%. The result shows that the addition of graphite oxide has little influence on the thermal stability of the polylactic acid blend, and the thermal decomposition temperature is above 300 ℃, which indicates that the product can be molded and processed at 250 ℃ without causing obvious degradation.
Examples 16 to 20
Mixing 36g L-polylactic acid (P LL A, M)n130k), 24g d-polylactic acid (PD L a, M)n150k) and a certain amount of graphite oxide (with hydroxyl content of 50 percent and 200 meshes) are internally mixed in an internal mixer at the temperature of 240 ℃ for 1 minute to obtain the polylactic acid mixture. The mass ratio of the levorotatory polylactic acid to the dextrorotatory polylactic acid is 60: 40.
A series of polylactic acid mixtures can be obtained by changing the addition amount of the graphite oxide, and the specific parameters are shown in Table 4.
TABLE 4 examples 16-20 composition of polylactic acid mixtures
| Sample (I) | PLLA(g) | PDLA(g) | Graphite oxide (g) | Addition amount of graphite oxide |
| Example 16 | 36 | 24 | 0.006 | 0.01％ |
| Example 17 | 36 | 24 | 0.3 | 0.5％ |
| Example 18 | 36 | 24 | 0.61 | 1.0％ |
| Example 19 | 36 | 24 | 0.91 | 1.5％ |
| Example 20 | 36 | 24 | 1.2 | 2.0％ |
The polylactic acid mixtures obtained in examples 16 to 20 were analyzed by DSC to obtain a first melting temperature of about 170 ℃ and a second melting temperature of about 220 ℃. The first melting temperature corresponds to the crystalline melting temperature of the polylactic acid alone, and the second melting temperature corresponds to the crystalline melting temperature of the stereocomplex. The simultaneous presence of the first melting temperature and the second melting temperature in examples 16-20 illustrates the simultaneous presence of separate polylactic acid crystals and stereocomplex crystals in the polylactic acid mixture; however, as the amount of graphite oxide added was increased, the peak value of the first melting temperature was decreased, and the peak value of the second melting temperature was gradually increased, indicating that the content of stereocomplex crystals was gradually increased and the content of polylactic acid crystals alone was gradually decreased.
The polylactic acid mixtures obtained in examples 16 to 20 were characterized by an infrared spectrometer, and it was found by analysis that the polylactic acid mixtures obtained were at 909cm with increasing addition of graphite oxide-1Absorption peak ofAnd also gradually becomes stronger, indicating that the content of the stereocomplex crystal of the polylactic acid mixture increases with the increase of the content of the graphite oxide.
The present inventors conducted thermogravimetric analysis of the polylactic acid mixtures obtained in examples 16 to 20 and found that the temperature at which the polylactic acid mixture starts to decompose decreases from 320 c to about 300 c (example 20) as the content of graphite oxide increases. The result shows that the addition of graphite oxide has little influence on the thermal stability of the polylactic acid blend, and the thermal decomposition temperature is above 300 ℃, which indicates that the product can be molded and processed at 250 ℃ without causing obvious degradation.
Example 21
Mixing L-polylactic acid (P LL A, 5g, M)n150k) and d-polylactic acid (PD L a, 5g, M)n130k) were dissolved in 40m L dichloromethane, respectively, and then mixed, stirred for 3 hours, poured into an ultra-flat petri dish, evaporated to form a film at room temperature, and dried to constant weight at 40 ℃ under vacuum to obtain a polylactic acid mixture.
Through a series of characterization, it is found that the polylactic acid mixture obtained in example 21 is basically not different from the polylactic acid mixture obtained in example 1.
Example 22
Mixing L-polylactic acid (P LL A, 3g, M)n150k) and d-polylactic acid (PD L a, 3g, M)n130k) were dissolved in 40m of L m of tetrahydrofuran, and then mixed, 0.03g of graphite oxide (hydroxyl group content 20%, 500 mesh) was added, stirred for 3 hours, poured into an ultra-flat petri dish, evaporated to form a film at room temperature, and dried to constant weight at 40 ℃ in vacuo to obtain a polylactic acid mixture.
Through a series of characterization, it is found that the polylactic acid mixture obtained in example 22 is basically not different from the polylactic acid mixture obtained in example 2.
Example 23
Mixing L-polylactic acid (P LL A, 3g, M)n150k) and d-polylactic acid (PD L a, 3g, M)n130k) were dissolved in 40ml of N, N-dimethylformamide, respectively, and then mixed, 0.06g of graphite oxide (hydroxyl group content 20%, 500 mesh) was added, stirred for 3 hours, poured into an ultra-flat petri dish, evaporated at 100 ℃ to form a film, and dried at 40 ℃ under vacuum to constant weight to obtain a polylactic acid mixture.
Through a series of characterization, it is found that the polylactic acid mixture obtained in example 23 is not substantially different from the polylactic acid mixture obtained in example 3.
Example 24
Mixing L-polylactic acid (P LL A, 3g, M)n150k) and d-polylactic acid (PD L a, 3g, M)n130k) were dissolved in 40m L of a solvent (chloroform to tetrahydrofuran volume ratio 1: 1), mixed, added with 0.09g of graphite oxide (hydroxyl group content 20%, 500 mesh), stirred for 3 hours, poured into an ultra-flat petri dish, evaporated to a film at room temperature, and dried to constant weight at 40 ℃ in vacuo to obtain a polylactic acid mixture.
Through a series of characterization, it is found that the polylactic acid mixture obtained in example 24 is not substantially different from the polylactic acid mixture obtained in example 4.
Example 25
The polylactic acid mixtures obtained in examples 1, 2 and 4 were injection molded under the following conditions: the cylinder temperature is 230 ℃, the mold temperature is 120 ℃, the injection pressure is 150bar for 5 seconds, and the pressure is maintained at 70bar for 7 seconds. The resulting splines are shown in FIG. 4 (examples 1, 2, 4, in order from left to right). It can be seen that the polylactic acid three-dimensional composite obtained in the examples of the present invention is excellent in moldability.
Although the invention has been described in detail hereinabove with respect to a general description and specific embodiments thereof, it will be apparent to those skilled in the art that modifications or improvements may be made thereto based on the invention. Accordingly, such modifications and improvements are intended to be within the scope of the invention as claimed.
Claims (9)
1. A method for preparing a polylactic acid stereocomplex, comprising: adding graphite oxide into a mixture of the levorotatory polylactic acid and the dextrorotatory polylactic acid, and mixing to prepare;
the addition amount of the graphite oxide is 1.5-10 wt% of the mixture, and the hydroxyl content of the graphite oxide is not lower than 0.01%;
the mass ratio of the levorotatory polylactic acid to the dextrorotatory polylactic acid is 30: 70-70: 30.
2. The preparation method according to claim 1, wherein the graphite oxide has a hydroxyl group content of 20 to 30%, and the mass ratio of the L-polylactic acid to the D-polylactic acid is 40:60 to 60: 40.
3. The method according to claim 1, wherein the molecular weight of the L-polylactic acid is 100 to 200kg/mol, and the molecular weight distribution is 1.20 to 2.50;
and/or the molecular weight of the poly-D-lactic acid is 100-200 kg/mol, and the molecular weight distribution is 1.20-2.50.
4. The method according to claim 1, wherein the graphite oxide has a particle size of 200 to 1000 mesh.
5. The preparation method according to any one of claims 1 to 4, characterized by specifically comprising: and mixing the levorotatory polylactic acid, the dextrorotatory polylactic acid and the graphite oxide, and banburying for 1-10 min at the temperature of 200-250 ℃.
6. The preparation method according to claim 5, wherein the L-polylactic acid, the D-polylactic acid and the graphite oxide are mixed and internally mixed at 230-240 ℃ for 3-6 min.
7. The preparation method according to any one of claims 1 to 4, characterized by specifically comprising: respectively dissolving the levorotatory polylactic acid and the dextrorotatory polylactic acid by using a solvent, mixing, then adding the graphite oxide, uniformly mixing, and removing the solvent.
8. The method according to claim 7, wherein the solvent is one or more of dichloromethane, chloroform, N-dimethylformamide, and tetrahydrofuran.
9. A polylactic acid stereocomplex, which is produced by the production method according to any one of claims 1 to 8.
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Citations (4)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN102167894A (en) * | 2011-01-24 | 2011-08-31 | 中国科学院长春应用化学研究所 | Graphene/polylactic acid composite material and preparation method thereof |
| CN108752885A (en) * | 2018-06-06 | 2018-11-06 | 四川大学 | Stereocomplex PLA/carbon nano-particles product and preparation method thereof |
| WO2019180292A1 (en) * | 2018-03-23 | 2019-09-26 | Avanzare Innovacion Tecnologica S.L. | Use of high-aspect-ratio graphene materials as additives for thermoplastic materials |
| US20190362867A1 (en) * | 2017-02-07 | 2019-11-28 | Colorado State University Research Foundation | Thermoplastic carbon composite electrodes |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102167894A (en) * | 2011-01-24 | 2011-08-31 | 中国科学院长春应用化学研究所 | Graphene/polylactic acid composite material and preparation method thereof |
| US20190362867A1 (en) * | 2017-02-07 | 2019-11-28 | Colorado State University Research Foundation | Thermoplastic carbon composite electrodes |
| WO2019180292A1 (en) * | 2018-03-23 | 2019-09-26 | Avanzare Innovacion Tecnologica S.L. | Use of high-aspect-ratio graphene materials as additives for thermoplastic materials |
| CN108752885A (en) * | 2018-06-06 | 2018-11-06 | 四川大学 | Stereocomplex PLA/carbon nano-particles product and preparation method thereof |
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