The mixed solution of poly-L-lactic acid and ethyl cellulose in chloroform and the mixed solution of PLLA and methyl cellulose in trifluoroacetic acid were prepared, and the PLLA/cellulose ether blend was prepared by casting; The obtained blends were characterized by leaf transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). There is a hydrogen bond between PLLA and cellulose ether, and the two components are partially compatible. With the increase of cellulose ether content in the blend, the melting point, crystallinity and crystal integrity of the blend will all decrease. When the MC content is higher than 30%, almost amorphous blends can be obtained. Therefore, cellulose ether can be used to modify poly-L-lactic acid to prepare degradable polymer materials with different properties.
Keywords: poly-L-lactic acid, ethyl cellulose, methyl cellulose, blending, cellulose ether
The development and application of natural polymers and degradable synthetic polymer materials will help to solve the environmental crisis and resource crisis faced by human beings. In recent years, the research on the synthesis of biodegradable polymer materials using renewable resources as polymer raw materials has attracted widespread attention. Polylactic acid is one of the important degradable aliphatic polyesters. Lactic acid can be produced by fermentation of crops (such as corn, potatoes, sucrose, etc.), and can also be decomposed by microorganisms. It is a renewable resource. Polylactic acid is prepared from lactic acid by direct polycondensation or ring-opening polymerization. The final product of its degradation is lactic acid, which will not pollute the environment. PIA has excellent mechanical properties, processability, biodegradability and biocompatibility. Therefore, PLA not only has a wide range of applications in the field of biomedical engineering, but also has huge potential markets in the fields of coatings, plastics, and textiles.
The high cost of poly-L-lactic acid and its performance defects such as hydrophobicity and brittleness limit its application range. In order to reduce its cost and improve the performance of PLLA, the preparation, compatibility, morphology, biodegradability, mechanical properties, hydrophilic/hydrophobic balance and application fields of polylactic acid copolymers and blends have been deeply studied. Among them, PLLA forms a compatible blend with poly DL-lactic acid, polyethylene oxide, polyvinyl acetate, polyethylene glycol, etc. Cellulose is a natural polymer compound formed by the condensation of β-glucose, and is one of the most abundant renewable resources in nature. Cellulose derivatives are the earliest natural polymer materials developed by humans, the most important of which are cellulose ethers and cellulose esters. M． Nagata et al. studied the PLLA/cellulose blend system and found that the two components were incompatible, but the crystallization and degradation properties of PLLA were greatly affected by the cellulose component. N． Ogata et al studied the performance and structure of PLLA and cellulose acetate blend system. The Japanese patent also studied the biodegradability of PLLA and nitrocellulose blends. Y． Teramoto et al studied the preparation, thermal and mechanical properties of PLLA and cellulose diacetate graft copolymers. So far, there are very few studies on the blending system of polylactic acid and cellulose ether.
In recent years, our group has been engaged in the research of direct copolymerization and blending modification of polylactic acid and other polymers. In order to combine the excellent properties of polylactic acid with the low cost of cellulose and its derivatives to prepare fully biodegradable polymer materials, we choose cellulose (ether) as the modified component for blending modification. Ethyl cellulose and methyl cellulose are two important cellulose ethers. Ethyl cellulose is a water-insoluble non-ionic cellulose alkyl ether, which can be used as medical materials, plastics, adhesives and textile finishing agents. Methyl cellulose is water-soluble, has excellent wettability, cohesiveness, water retention and film-forming properties, and is widely used in the fields of building materials, coatings, cosmetics, pharmaceuticals and papermaking. Here, PLLA/EC and PLLA/MC blends were prepared by solution casting method, and the compatibility, thermal properties and crystallization properties of PLLA/cellulose ether blends were discussed.
1. Experimental part
1.1 Raw materials
Ethyl cellulose (A.R., Tianjin Huazhen Special Chemical Reagent Factory); methyl cellulose (MC450), sodium dihydrogen phosphate, disodium hydrogen phosphate, ethyl acetate, stannous isooctanoate, chloroform (the above are all products of Shanghai Chemical Reagent Co., Ltd., and the purity is A.R. grade); L-lactic acid (pharmaceutical grade, PURAC company).
1.2 Preparation of blends
1.2.1 Preparation of polylactic acid
Poly-L-lactic acid was prepared by direct polycondensation method. Weigh L-lactic acid aqueous solution with a mass fraction of 90% and add it to a three-necked flask, dehydrate at 150°C for 2 hours under normal pressure, then react for 2 hours under a vacuum pressure of 13300Pa, and finally react for 4 hours under a vacuum of 3900Pa to obtain a dehydrated prepolymer things. The total amount of lactic acid aqueous solution minus the water output is the total amount of prepolymer. Add stannous chloride (mass fraction is 0.4%) and p-toluenesulfonic acid (the ratio of stannous chloride and p-toluenesulfonic acid is 1/1 molar ratio) catalyst system in the obtained prepolymer, and in condensation Molecular sieves were installed in the tube to absorb a small amount of water, and mechanical stirring was maintained. The whole system was reacted at a vacuum of 1300 Pa and a temperature of 150° C. for 16 hours to obtain a polymer. Dissolve the obtained polymer in chloroform to prepare a 5% solution, filter and precipitate with anhydrous ether for 24 hours, filter the precipitate, and place it in a -0.1MPa vacuum oven at 60°C for 10 to 20 hours to obtain Pure dry PLLA polymer. The relative molecular weight of the obtained PLLA was determined to be 45000-58000 Daltons by high-performance liquid chromatography (GPC). Samples were kept in a desiccator containing phosphorus pentoxide.
1.2.2 Preparation of polylactic acid-ethyl cellulose blend (PLLA-EC)
Weigh the required amount of poly-L-lactic acid and ethyl cellulose to make 1% chloroform solution respectively, and then prepare PLLA-EC mixed solution. The ratio of PLLA-EC mixed solution is: 100/0, 80/20, 60/40, 40/60, 20/80, 0/l00, the first number represents the mass fraction of PLLA, and the latter number represents the mass of EC Fraction. The prepared solutions were stirred with a magnetic stirrer for 1-2 hours, and then poured into a glass dish to allow the chloroform to evaporate naturally to form a film. After the film was formed, it was placed in a vacuum oven to dry at low temperature for 10 hours to completely remove the chloroform in the film. . The blend solution is colorless and transparent, and the blend film is also colorless and transparent. The blend was dried and stored in a desiccator for later use.
1.2.3 Preparation of polylactic acid-methylcellulose blend (PLLA-MC)
Weigh the required amount of poly-L-lactic acid and methyl cellulose to make 1% trifluoroacetic acid solution respectively. The PLLA-MC blend film was prepared by the same method as the PLLA-EC blend film. The blend was dried and stored in a desiccator for later use.
1.3 Performance test
MANMNA IR-550 infrared spectrometer (Nicolet.Corp) measured the infrared spectrum of the polymer (KBr tablet). DSC2901 differential scanning calorimeter (TA company) was used to measure the DSC curve of the sample, the heating rate was 5°C/min, and the glass transition temperature, melting point and crystallinity of the polymer were measured. Use Rigaku. The D-MAX/Rb diffractometer was used to test the X-ray diffraction pattern of the polymer to study the crystallization properties of the sample.
2. Results and discussion
2.1 Infrared spectroscopy research
Fourier transform infrared spectroscopy (FT-IR) can study the interaction between the components of the blend from the perspective of molecular level. If the two homopolymers are compatible, shifts in frequency, changes in intensity, and even the appearance or disappearance of peaks characteristic of the components can be observed. If the two homopolymers are not compatible, the spectrum of the blend is simply superposition of the two homopolymers. In the PLLA spectrum, there is a stretching vibration peak of C=0 at 1755cm-1, a weak peak at 2880cm-1 caused by the C—H stretching vibration of the methine group, and a broad band at 3500 cm-1 is caused by terminal hydroxyl groups. In the EC spectrum, the characteristic peak at 3483 cm-1 is the OH stretching vibration peak, indicating that there are O—H groups remaining on the molecular chain, while 2876-2978 cm-1 is the C2H5 stretching vibration peak, and 1637 cm-1 is HOH Bending vibration peak (caused by the sample absorbing water). When PLLA is mixed with EC, in the IR spectrum of hydroxyl region of PLLA-EC blend, the O—H peak shifts to low wavenumber with the increase of EC content, and reaches the minimum when PLLA/Ec is 40/60 wavenumber, and then shifted to higher wavenumbers, indicating that the interaction between PUA and 0-H of EC is complex. In the C=O vibration region of 1758cm-1, the C=0 peak of PLLA-EC slightly shifted to a lower wave number with the increase of EC, which indicated that the interaction between C=O and O-H of EC was weak.
In the spectrogram of methylcellulose, the characteristic peak at 3480cm-1 is the O—H stretching vibration peak, that is, there are residual O—H groups on the MC molecular chain, and the HOH bending vibration peak is at 1637cm-1, and the MC ratio EC is more hygroscopic. Similar to the PLLA-EC blend system, in the infrared spectra of the hydroxyl region of the PLLA-EC blend, the O—H peak changes with the increase of the MC content, and has the minimum wave number when the PLLA/MC is 70/30. In the C=O vibration region (1758 cm-1), the C=O peak slightly shifts to lower wavenumbers with the addition of MC. As we mentioned earlier, there are many groups in PLLA that can form special interactions with other polymers, and the results of the infrared spectrum may be the combined effect of many possible special interactions. In the blend system of PLLA and cellulose ether, there may be various hydrogen bond forms between the ester group of PLLA, the terminal hydroxyl group and the ether group of cellulose ether (EC or MG), and the remaining hydroxyl groups. PLLA and EC or MCs may be partially compatible. It may be due to the existence and strength of multiple hydrogen bonds, so the changes in the O—H region are more significant. However, due to the steric hindrance of the cellulose group, the hydrogen bond between the C=O group of PLLA and the O—H group of cellulose ether is weak.
2.2 DSC research
DSC curves of PLLA, EC and PLLA-EC blends. The glass transition temperature Tg of PLLA is 56.2°C, the crystal melting temperature Tm is 174.3°C, and the crystallinity is 55.7%. EC is an amorphous polymer with a Tg of 43°C and no melting temperature. The Tg of the two components of PLLA and EC are very close, and the two transition regions overlap and cannot be distinguished, so it is difficult to use it as a criterion for system compatibility. With the increase of EC, the Tm of PLLA-EC blends decreased slightly, and the crystallinity decreased (the crystallinity of the sample with PLLA/EC 20/80 was 21.3%). The Tm of the blends decreased with the increase of MC content. When PLLA/MC is lower than 70/30, the Tm of the blend is difficult to measure, that is, almost amorphous blend can be obtained. The lowering of the melting point of blends of crystalline polymers with amorphous polymers is usually due to two reasons, one is the dilution effect of the amorphous component; the other may be structural effects such as a reduction in crystallization perfection or crystal size of the crystalline polymer. The results of DSC indicated that in the blend system of PLLA and cellulose ether, the two components were partially compatible, and the crystallization process of PLLA in the mixture was inhibited, resulting in the decrease of Tm, crystallinity and crystal size of PLLA. This shows that the two-component compatibility of PLLA-MC system may be better than that of PLLA-EC system.
2.3 X-ray diffraction
The XRD curve of PLLA has the strongest peak at 2θ of 16.64°, which corresponds to the 020 crystal plane, while the peaks at 2θ of 14.90°, 19.21° and 22.45° correspond to 101, 023, and 121 crystals, respectively. Surface, that is, PLLA is α-crystalline structure. However, there is no crystal structure peak in the diffraction curve of EC, which indicates that it is an amorphous structure. When PLLA was mixed with EC, the peak at 16.64° gradually broadened, its intensity weakened, and it moved slightly to a lower angle. When the EC content was 60%, the crystallization peak had dispersed. Narrow x-ray diffraction peaks indicate high crystallinity and large grain size. The wider the diffraction peak, the smaller the grain size. The shift of the diffraction peak to a low angle indicates that the grain spacing increases, that is, the integrity of the crystal decreases. There is a hydrogen bond between PLLA and Ec, and the grain size and crystallinity of PLLA decrease, which may be because EC is partially compatible with PLLA to form an amorphous structure, thereby reducing the integrity of the crystal structure of the blend. The X-ray diffraction results of PLLA-MC also reflect similar results. The X-ray diffraction curve reflects the effect of the ratio of PLLA/cellulose ether on the structure of the blend, and the results are completely consistent with the results of FT-IR and DSC.
3. Conclusion
The blend system of poly-L-lactic acid and cellulose ether (ethyl cellulose and methyl cellulose) was studied here. The compatibility of the two components in the blend system was studied by means of FT-IR, XRD and DSC. The results showed that hydrogen bonding existed between PLLA and cellulose ether, and the two components in the system were partially compatible. A decrease in the PLLA/cellulose ether ratio results in a decrease in the melting point, crystallinity, and crystal integrity of PLLA in the blend, resulting in the preparation of blends of different crystallinity. Therefore, cellulose ether can be used to modify poly-L-lactic acid, which will combine the excellent performance of polylactic acid and the low cost of cellulose ether, which is conducive to the preparation of fully biodegradable polymer materials.