Lactic Acid Oligomers Research Articles

Processing and characterization of plasticized PLA/PHB blends for biodegradable multiphase systems

Blends of poly(lactic acid) (PLA) and poly(3-hydroxybutyrate) (PHB) plasticized with a lactic acid oligomer (OLA) added at three different concentrations (15, 20 and 30 wt% by weight), were prepared by an optimized extrusion process to improve the processability and mechanical properties of these biopolymers for flexible film manufacturing. Mor- phological, chemical, thermal, mechanical, barrier and migration properties were investigated and formulations with desired performance in eco-friendly films were selected. The efficiency of OLA as plasticizer for PLA_PHB blends was demonstrated by the significant decrease of their glass transition temperatures and a considerable improvement of their duc- tile properties. The measured improvements in the barrier properties are related to the higher crystallinity of the plasticized PLA_PHB blends, while the overall migration test underlined that all the proposed formulations maintained migration lev- els below admitted levels. The PLA_PHB blend with 30 wt% OLA was selected as the optimum formulation for food pack- aging, since it offered the best compromise between ductility and oxygen and water vapor barrier properties with practically no migration.

Synthesis and characterization of unsaturated lactic acid based thermoset bio-resins

Bio-based thermoset resins have been synthesized using lactic acid oligomers, which were functionalized with carbon–carbon double bonds, in order to allow their crosslinking by a free radical mechanism. Two different resin structures were synthesized. One resin was composed of an allyl alcohol terminated lactic acid oligomer, which was end-functionalized with methacrylic anhydride (MLA resin). The second resin was a mixture of the same allyl alcohol-lactic acid oligomer, and penthaerythritol. This mixture was end-functionalized with methacrylic anhydride, in order to get a methacrylate functionalized lactic acid oligomer, and methacrylate functionalized penthaerythritol (PMLA resin). The synthesized resins were characterized using FT-IR, 1H NMR and 13C NMR spectroscopy, differential scanning calorimetry as well as dynamic mechanical analysis to confirm the resin structure and reactivity. The flow viscosities were also measured in order to evaluate the suitability of the resins to be used as a matrix in composite applications. The results showed that the PMLA resin has better mechanical, thermal and rheological properties than the MLA resin, and both had properties which were comparable with a commercial unsaturated polyester resin. The high biobased content of 90% and the high glass transition temperature at 100°C for the PMLA resin makes it an attractive candidate for composite applications where crude oil based unsaturated polyester resins are normally used.

Synthesis and characterization of Fe3O4/poly(lactide‐co‐glycolide) composite and its coating effects on magnesium alloy

The Fe3O4/poly(lactide‐co‐glycolide) (PLGA) composites were prepared via a surface grafting technique. Initially, the poly(lactic acid) oligomer was synthesized and surface‐grafted to Fe3O4 nanoparticles. Then, the grafted Fe3O4 particles were compounded with PLGA matrix by a simple solution blending method. The grafted Fe3O4 particles presented enhanced compatibility with PLGA matrix and the composites indicated enhanced dynamic mechanical performance. Electrochemical tests showed that the Fe3O4/PLGA composite coating can help improve the impedance of magnesium samples by 100–300%, and the impedance of the metal may be tunable by altering the components ratio of LA/GA within PLGA matrix. The composites may have potential application for magnesium alloy used in degradable medical implants. POLYM. COMPOS., 37:1369–1374, 2016. © 2014 Society of Plastics Engineers

Modifying wood with bio-polyesters: analysis and performance

Oligomers of lactic and glycolic acid (OLA, OGA) and poly(butylene succinate/adipate) (OBS, OBA) were impregnated and heated into wood for dimensional stabilisation. These bio-polyesters variously provide bulking and lumen filling treatments. Solvent extraction of treated wood showed that ca. 50% of the polymer remains in wood with in situ polymerisation evidenced by GPC. OGA readily penetrates the wood cell wall, which occurred only on heating OLA, whereas OBS/OBA was lumen filling with any cell wall penetration only evident after water soaking. Stability of OLA/OGA treated wood was improved, but did not withstand extended leaching, whereas OBS/OBA treated wood presented higher stability once leached. Bending resistance was influenced by OLA/OGA treatments, but original MOE values were recovered after leaching. Overall, the results show the bio-polyester treatment can confer specific properties of the modified wood to suit selected end use applications.

Modeling the pH-dependent PLA oligomer degradation kinetics

The hydrolysis of lactic acid oligomers involves several different reactions, acid-catalyzed, base-catalyzed as well as non-catalyzed. In the present paper a pH-dependent kinetic model has been developed and its kinetic constants evaluated by fitting to experimental data of degrading lactic acid oligomers at 37 °C in aqueous solution at pH = 1, 2, 3.5, 5.7 and 7.4. The model was able to properly describe the experimental data with an average error of about 5%. The estimated values of the kinetic constants at the selected temperature are: kDα=4.44×10−3L2mol−2h−1, kDβ=9.29×10−4L2mol−2h−1 (acid-catalyzed), kRH=1.56×103L2mol−2h−1, kBB=2.88×106Lmol−1h−1 (base-catalyzed) and kW=2.49×10−5Lmol−1h−1 (non-catalyzed).

Chemical recycling of poly(lactic acid) via controlled degradation with protic (macro)molecules

Designing of a new method of poly(lactic acid) chemical recycling leading to potentially useful products is the purpose of this paper. Controlled chemical degradation processes of commercial poly(lactic acid) in the presence of small molecules, such as diols, dipentaerythriol, diamines and adipic acid or oligo(ethylene glycol) as well as polyesterodiols were studied. The processes were found to be efficient and in the presence of a catalytical amount of tin(II) octanoate lead to the formation of homo- or block diol type copolymers or oligomers of lactic acid. Diamines appear to be the most efficient, even without catalyst at quite low temperature, whereas adipic acid eventually at 200 °C reacts with moderate efficacy. It is demonstrated that Mw of the products is much lower with respect to that of poly(lactic acid). Degradation agents incorporate into the products' structures, however, in almost all cases some fraction of macro-α,ω-hydroxyacids is present in the samples, which seems to be unavoidable because of degradation mechanism and some residual moisture present in the polymers used. Products rich in ABA type triblock copolymers may be obtained by the degradation method presented utilizing macrodiols. The processes of poly(lactic acid) degradation with macrodiols can be carried out at temperatures of 120 °C or higher in solvent (e.g. xylene) or in bulk as well as an extruder. The reaction time depends on temperature and it is in the range from 30 min to 4 h. Aliphatic copolymers are homogeneous, whereas these containing aliphatic–aromatic macrodiols are of two-phases. The latter ones show different morphology based on composition. Triblock systems reveal multistage differential thermal analysis curves. Robust method of chemical recycling of poly(lactic acid) by controlled degradation with protic compounds is presented.

Polylactide stereocomplexation leads to reduced migration during microwave heating in contact with food simulants

The effect of stereocomplexation on the stability and migration resistance of polylactide during microwave and conventional heating in contact with different food simulants was evaluated. The heating effects were followed through mass loss measurements, molecular weight measurements and identification of the individual migrants by electrospray ionization-mass spectrometry (ESI-MS). Increased mass losses were observed as a function of time and temperature, but approximately 50% smaller mass losses were always measured for PLA stereocomplex as compared to the corresponding regular PLLA material. The stability of the stereocomplex material was, thus, significantly higher. Microwave heating increased the mass loss as compared to the conventional heating at the same time and temperature. This effect was especially significant when 10% ethanol was used as food simulant instead of water. The amount of water-soluble migrants was in most cases under the detection limits, but when heating temperature was increased to 95°C, ESI-MS revealed the formation of homologous series of linear lactic acid oligomers. Results indicate that PLA stereocomplex materials could have potential in single-use microwave applications.

Synthesis and characterization of bio‐based thermosetting resins from lactic acid and glycerol

ABSTRACTA bio‐based thermoset resin has been synthesized from glycerol reacted with lactic acid oligomers of three different chain lengths (n): 3, 7, and 10. Lactic acid was first reacted with glycerol by direct condensation and the resulting branched molecule was then end‐functionalized with methacrylic anhydride. The resins were characterized by Fourier‐transform infrared spectroscopy (FT‐IR), by 13C‐NMR spectroscopy to confirm the chemical structure of the resin, and by differential scanning calorimetry and dynamic mechanical thermal analysis (DMTA) to obtain the thermal properties. The resin flow viscosities were also measured using a rheometer with different stress levels for each temperature used, as this is an important characteristic of resins that are intended to be used as a matrix in composite applications. The resin with a chain length of three had better mechanical, thermal, and rheological properties than the resins with chain lengths of seven and 10. Also, its bio‐based content of 78% and glass transition temperature of 97°C makes this resin comparable to commercial unsaturated polyester resins. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40488.

Structure/Property Relationships of Poly(L-lactic Acid)/Mesoporous Silica Nanocomposites

Biodegradable poly(L-lactic acid) (PLLA)/mesoporous silica nanocomposites were prepared by grafting L-lactic acid oligomer onto silanol groups at the surface of mesoporous silica (SBA-15). The infrared results showed that the lactic acid oligomer was grafted onto the mesoporous silica. Surface characterization of mesoporous silica proved that the grafted oligomer blocked the entry of nitrogen into the mesopores. Thermal analysis measurements showed evidence that, once mixed with PLLA, SBA-15 not only nucleated the PLLA but also increased the total amount of crystallinity. Neat PLLA and its nanocomposites crystallized in the same crystal habit and, as expected, PLLA had a defined periodicity compared with the nanocomposites. This was because the grafted macromolecules on silica tended to cover the lamellar crystalline order. The g-SBA-15 nanoparticles improved the tensile moduli, increasing also the tensile strength of the resultant nanocomposites. Overall, the silica concentration tended to form a brittle material.

Synthesis and Characterization of Lactic Acid Oligomers: Evaluation of Performance as Poly(Lactic Acid) Plasticizers

The use of fully bio-based and biodegradable materials for massive applications, such as food packaging, is an emerging tendency in polymer research. But the formulations proposed in this way should preserve or even increase the functional properties of conventional polymers, such as transparency, homogeneity, mechanical properties and low migration of their components to foodstuff. This is not always trivial, in particular when brittle biopolymers, such as poly(lactic acid) (PLA), are considered. In this work the formulation of innovative materials based on PLA modified with highly compatible plasticizers, i.e. oligomers of lactic acid (OLAs) is proposed. Three different synthesis conditions for OLAs were tested and the resulting additives were further blended with commercial PLA obtaining transparent and ductile materials, able for films manufacturing. These materials were tested in their structural, thermal and tensile properties and the best formulation among the three materials was selected. OLA with molar mass (Mn) around 1,000 Da is proposed as an innovative and fully compatible and biodegradable plasticizer for PLA, able to replace conventional plasticizers (phthalates, adipates or citrates) currently used for films manufacturing in food packaging applications.

Preparation and Characterization of Poly(lactic acid) with Different Molecular Weights by Thermal Hydrolysis

A feasible and effective method of thermal hydrolysis to prepare poly (lactic acid) (PLA) oligomers with different controlled molecular weight from PLA is presented in this paper. The thermal hydrolytic reaction was carried out by immerging PLA resin pellets in boiling distilled water for a certain period of time. Ester groups in PLA chains are hydrolytically degraded in the presence of water and thermal, so PLA oligomers with different molecular weight were prepared. The structures and properties of PLA oligomers were characterized by FT-IR, GPC, DSC, etc. The results showed that thermal hydrolytic reaction could effectively reduce the molecular weight of PLA, which declines with the increase of the thermal hydrolytic reaction time. Meanwhile, the content of terminal hydroxyl group, glass transition temperature, melting point of PLA oligomers prepared from thermal hydrolytic reactions exhibit gradual changes with the extension of the thermal hydrolytic time.

Poly(ester amide)s based on (L)-lactic acid oligomers and α-amino acids: influence of the α-amino acid side chain in the poly(ester amide)s properties

Novel biodegradable and low cytotoxic poly(ester amide)s (PEAs) based on α-amino acids and (L)-lactic acid (L-LA) oligomers were successfully synthesized by interfacial polymerization. The chemical structure of the new polymers was confirmed by spectroscopic analyses. Further characterization suggests that the α-amino acid plays a critical role on the final properties of the PEA. L-phenylalanine provides PEAs with higher glass transition temperature, whereas glycine enhances the crystallinity. The hydrolytic degradation in PBS (pH = 7.4) at 37 °C also depends on the α-amino acid, being faster for glycine-based PEAs. The cytotoxic profiles using fibroblast human cells indicate that the PEAs did not elicit an acute cytotoxic effect. The strategy presented in this work opens the possibility of synthesizing biodegradable PEAs with low citotoxicity by an easy and fast method. It is worth to mention also that the properties of these materials can be fine-tuned only by changing the α-amino acid.

Synthesis and chain extension of hydroxyl‐terminated poly(lactic acid) oligomers and application in the blends

AbstractHydroxyl‐terminated poly(lactic acid) prepolymer (LA prepolymer) were prepared via L‐lactic acid as monomer, 1,4‐butanediol as blocking agent and Sn(II) octoate as catalyst by direct melt polymerization. Then the LA prepolymer was blended with starch followed by in situ chain extending reaction using different content of TDI as chain extender, producing the high molecular weight of poly(ester urethane) in the blends. The LA prepolymer/starch‐TDI blends were characterized by GPC, 1H‐NMR, SEM, DSC, tensile strength testing, and water resistance. The SEM results of cross‐section show that, compared with the simple PLA/starch blends, almost the starch granules were completely covered by ploy(ester urethane) in the LA prepolymer/starch‐TDI blends system. In comparison to the simple PLA/starch blends, the mechanical properties of LA prepolymer/starch‐TDI blends were increased, such as tensile strength increasing from 18.6 ± 3.8 to 44.2 ± 6.2 Mpa, tensile modulus increasing from 510 ± 62 to 1,850 ± 125 Mpa and elongation at break increasing from 1.8 ± 0.4 to 4.0 ± 0.5 %, respectively. This is attributed to high weight of poly (ester urethane) was formed via in situ reaction of the end of hydroxyl (LA prepolymer) and isocyanate groups and the starch granules were easily covered by ploy(ether urethane) via in situ polymerization in the blends. Moreover, covalent linkage was formed between the two phases interfaces. As a result, the interfacial adhesion was enhance and improved the mechanical property. In addition, the water resistance of LA prepolymer/starch‐TDI blends was much better that of the simple PLA/starch blends. POLYM. COMPOS., 2013 © 2013 Society of Plastics Engineers

Characterization and ageing study of poly(lactic acid) films plasticized with oligomeric lactic acid

Poly(lactic acid) (PLA) was melt-blended with a bio-based oligomeric lactic acid (OLA) plasticizer at different concentrations between 15 wt% and 25 wt% in order to enhance PLA ductility and to get a fully biodegradable material with potential application in films manufacturing. OLA was an efficient plasticizer for PLA, as it caused a significant decrease on glass transition temperature (Tg) while improving considerably ductile properties. Only one Tg value was observed in all cases and no apparent phase separation was detected. Films obtained by compression moulding were stored during 3 months under ambient controlled conditions and thermal, mechanical, structural and oxygen barrier properties were studied in order to evaluate the stability of the PLA–OLA films over time. Blends with 20 and 25 wt% OLA remained stable and compatible with PLA within the ageing period. Besides, PLA–20 wt% OLA formulation was the only one which maintained its amorphous state with adequate thermal, mechanical and oxygen barrier properties for flexible films manufacturing.

Liquefied biomass derived plasticizer for polylactide

AbstractBACKGROUND: The valorization of renewable agro‐industrial residues and their further utilization for production of polymers and polymer additives is a highly attractive alternative for replacement of oil‐based materials.RESULTS: Liquefied wood flour and rice bran derived esters were synthesized and evaluated as novel green plasticizers for polylactide (PLA). The liquefied wood flour ester (PWF) showed good miscibility with PLA and good plasticization efficiency as shown by differential scanning calorimetry (DSC) and tensile testing. Tensile strain at break increased from a few percent for pure PLA to over 100 and 300% for the materials containing 10 and 30 wt‐% of PWF. The addition of PWF accelerated the hydrolysis rate of PLA as shown by faster weight loss during aging in water and faster formation of water‐soluble lactic acid oligomers, which was shown by electrospray ionization mass spectrometry (ESI‐MS) analysis of the migrants. The liquefied rice bran based product (PRB) was not miscible with PLA and it did not improve the elongation at break of PLA. Rice bran is generally rich in arabinoxylans with only secondary less reactive alcohol groups. The larger number of un‐reacted hydroxyl‐groups in PRB was confirmed by Fourier transform infrared (FTIR) spectroscopy and could explain the immiscibility with PLA.CONCLUSIONS: The results demonstrate that the synthesized liquefied wood flour derived plasticizer could have great potential as a biobased polylactide plasticizer. © 2012 Society of Chemical Industry

Photopolymerization of biocompatible films containing poly(lactic acid)

Telechelic acrylic poly(lactic acid) (PLA) oligomers were synthesized from the transesterification of PLA with ethylene glycol diacrylate. The oligoesters were characterized by 1H NMR and GPC. The telechelic acrylic PLA oligomers were then formulated with four different reactive diluents and photopolymerized into films with photoinitiator Irgacure 2959. Films were formulated to contain either 1,4-butanediol dimethacrylate (BDDM) or triethylene glycol dimethacrylate (TEGDMA) and for each set of formulations the concentration of PLA was varied from 0 to 70wt.%. The kinetics of the photopolymerizable material was studied by photo-differential scanning calorimeter (DSC). Physical properties were evaluated using pencil hardness, reverse impact resistance, pull off adhesion, solvent resistance, and dynamic mechanical thermal analysis (DMTA) properties. The Photo-DSC results showed that the reaction rate decreased as the PLA concentration increased. Films containing lower concentrations of PLA were harder and less flexible.

Influence of ozone treatment on structure and thermal properties of bis-2-hydroxyethyl terephthalate-based copolymers

Polyesters are a particularly interesting group of polymers because of their ester bonds in the main chain, which are sensitive to degradation. It has been shown that aromatic polyesters [eg. poly(ethylene terephthalate) (PET)] can be degraded when they are copolymerized with aliphatic polyesters. In this regard, the objective of our previous study was to obtain and investigate new copolymers with some fragments of PET and its hydrolytic degradation. In this work, the impact of ozone degradation on properties of bis-2-hydroxyethyl terephthalate-based copolymers was researched. Apart from the bis-2-hydroxyethyl terephthalate, the copolymers comprised oligomers of lactic acid and glycolic acid in different combination. During ozone degradation, samples were kept for 24 h in atmosphere containing 0.51 vol.% of ozone. Structure changes were determined by means of FTIR spectroscopy. In the IR spectra of ozonised samples, new bonds characteristic for ozonised polymers were observed. Thermal properties of copolymers before and after degradation process were reviewed based on differential scanning calorimetry (DSC) and thermogravimetry. DSC results revealed that melting point increased, especially for copolymers displaying higher quantity of PET units.

Synthesis and characterization of well-defined l-lactic acid-caprolactone co-oligomers and their rhenium (I) and technetium(I) conjugates

Staring from l-lactide and ε-caprolactone, the corresponding lactic-caprolactone cooligomer with hydroxyl and carboxylic acid groups were synthesized. These oligomers were connected with chelating groups through a long chain tether, ready for transition metal binding. Coordination of organometallic rhenium(I) and technetium(I) complexes generated the conjugates in high yield and short time, satisfying the requirements for short-lived radiopharmaceuticals in clinical applications. A reasonable pharmacophore model has been established to guide the design of well-defined lactic acid oligomer for nuclear medicine.

Nanoclay effects on the degradation process and product patterns of polylactide

The addition of montmorillonite (MMT) nanoclays shifted the degradation product patterns of polylactide towards shorter lactic acid oligomers as compared to the product patterns of plain polylactide and halloysite modified polylactide hydrolysed under same conditions. The addition of the two MMT sheet type nanoclays led to substantial improvement in the barrier properties of polylactide, which was attributed to the inherent restriction offered by the inorganic filler towards the movement of gases through the film. This could be connected to the observed change in the degradation product patterns as the formed degradation products, especially the longer oligomers, could be partially trapped inside the films leading to catalytic effect, higher mass loss at later stages of hydrolysis as well as release of shorter lactic acid oligomers into the ageing water. The nanocomposites generally exhibited higher contact angles indicating increased hydrophobicity. However, hyperbranched polyesters additives were found to be preferentially present at the surface of the films which led to increased surface hydrophilicity for these materials. The hydrolytic degradation was performed at 60 °C for up to 12 weeks and the process was monitored by mass loss measurements, molar mass changes and calorimetric studies. The water-soluble degradation products were examined by electrospray ionization-mass spectrometry (ESI-MS).

Long-term properties and migration of low molecular mass compounds from modified PLLA materials during accelerated ageing

The durability of polylactide during accelerated ageing and the entire degradation process were significantly altered by different low molecular mass additives and stereocomplexation. The samples were aged in air at 60 °C and 90 °C and the degradation process was followed by monitoring mass loss, molar mass, presence and formation of low molecular mass compounds, changes in surface structure and thermal properties. Stereocomplexation increased the long-term durability of polylactide materials. Mass loss and molar mass changes also indicated that addition of TiO2 nanoparticles had a stabilizing effect at higher temperature and during longer exposure times. Interestingly addition of linear lactic acid oligomers resulted in lower mass loss compared to materials containing cyclic lactide oligomers. This is interpreted as a result of stronger interactions between the linear oligomers and PLLA chains, resulting in slower migration rate, which was also shown by ESI-MS analysis. However, the linear oligomer additives accelerated the molar mass decrease, probably due to the catalytic effect of the end groups. The stereocomplex displayed the greatest resistance towards degradation, a consequence of strong secondary interactions.