Biodegradation of poly(lactic acid) and its nanocomposites

PLA nanocomposites based on organically modified montmorillonites at 5% w/w loading were prepared by melt blending using an internal mixer and then degraded in a commercial compost. The addition of nanoclays was found to increase the PLA degradation rate, especially for the highest dispersed clay in the polymer matrix. Biodegradation by microorganisms isolated from the compost showed the bacterium Bacillus licheniformis as one of the responsible for PLA biodegradation in compost. It was also found that clays can influence the polymer bacterial degradation depending on their chemical structure and affinity of the bacterium towards the clay.     

Thermal properties of poly(lactic acid)/organo-montmorillonite nanocomposites

Poly(lactic acid)/organo-montmorillonite nanocomposites were prepared by melt intercalation technique. Maleic anhydride-grafted ethylene propylene rubber (EPMgMA) was added into the PLA/OMMT in order to improve the compatibility and toughness of the nanocomposites. The samples were prepared by single screw extrusion followed by compression molding. The effect of OMMT and EPMgMA on the thermal properties of PLA was studied. The thermal properties of the PLA/OMMT nanocomposites have been investigated by using differential scanning calorimeter (DSC) and thermo-gravimetry analyzer (TG). The melting temperature (T _m), glass transition temperature (T _g), crystallization temperature (T _c), degree of crystallinity (χ_c), and thermal stability of the PLA/OMMT nanocomposites have been studied. It was found that the thermal properties of PLA were greatly influenced by the addition of OMMT and EPMgMA.     

Thermal behaviors and mechanical properties of halloysite nanotube-reinforced poly(lactic acid) nanocomposites

Poly(lactic acid)/halloysite nanotubes (PLA/HNTs) nanocomposites were prepared using melt compounding followed by compression molding. N,N′-ethylenebis(stearamide) (EBS) was used to improve the dispersion of HNTs and toughen the PLA nanocomposites. The thermal properties of PLA/HNTs nanocomposites were assessed by using differential scanning calorimeter and thermogravimetric analyzer (TG). The TG measurements were performed at both nitrogen and oxygen atmosphere. The mechanical properties of PLA/HNTs were characterized through tensile and impact tests. The morphological properties of the PLA/HNTs nanocomposites were investigated by using transmission electron microscopy and field emission scanning electron microscopy. The degree of crystallinity of PLA nanocomposites was increased slightly by the addition of EBS. The decomposition process of PLA/HNTs depends on the atmosphere reaction during TG test as well as the amount of EBS. The best mechanical properties of PLA/HNTs nanocomposites expressed by the impact strength and elongation at break were achieved by the addition of 5 mass% of EBS.     

Effect of graphene nanoplatelets as nanofiller in plasticized poly(lactic acid) nanocomposites

Plasticized PLA-based nanocomposites were prepared by melt blending of the matrix with 5 mass% of epoxidized palm oils (EPO) and different amount of graphene nanoplatelets (xGnP). Plasticized PLA (p-PLA) reinforced with 0.3 mass% xGnP resulted in an increase of up to 26.5 and 60.6 % in the tensile strength and elongation at break of the nanocomposites, respectively. Thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) were performed to study the thermal behavior of the prepared nanocomposites. p-PLA reinforced with xGnP shows that increasing the xGnP content triggers a substantial increase in thermal stability. Crystallinity of the nanocomposites as well as cold crystallization and melting temperature did not show any significant changes upon addition of xGnP. However, there is a significant decrease of glass transition temperature up to 0.3 mass% of xGnP incorporation.     

Three- and four-way parallel factor (PARAFAC) analysis of photochemically induced excitation-emission kinetic fluorescence spectra

Independently emerging fluorescence profiles of unknown, photochemically induced degradation products of several naturally non-fluorescent pesticides were monitored using single exposure excitation-emission fluorescence spectroscopy. Three-way parallel factor analysis (PARAFAC) was employed to uniquely resolve the pure fluorescent spectra of the overlapping photolysis products. The quantitative utility of EEM photolysis-based determinations was demonstrated by employing four-way PARAFAC models built from EEM time cubes of multiple fenvalerate samples. The 4-way PARAFAC models were then used to predict original pesticide concentrations resulting in conservative limit of detection and root mean square errors of calibration (RMSEC) of 3 microM each.     

Effect of temperature and nanoparticle type on hydrolytic degradation of poly(lactic acid) nanocomposites

PLA and its nanocomposite films based on modified montmorillonite (CLO30B) or fluorohectorite (SOM MEE) and unmodified sepiolite (SEPS9) were processed at a clay loading of 5 wt% and hydrolytically degraded at 37 and 58 °C in a pH 7.0 phosphate-buffered solution. An effective hydrolytic degradation for neat PLA and nanocomposites was obtained at both temperatures of degradation, with higher extent at 58 °C due to more extensive micro-structural changes and molecular rearrangements, allowing a higher water absorption into the polymer matrix.The addition of CLO30B and SEPS9 delayed the degradation of PLA at 37 °C due to their inducing PLA crystallization effect and/or to their high water uptake reducing the amount of water available for polymer matrix hydrolysis. The presence of SOM MEE also induced polymer crystallization, but it was also found to catalyze hydrolysis of PLA. Concerning hydrolysis at 58 °C, the presence of any nanoparticle did not significantly affect the degradation trend of PLA, achieving similar molecular weight decreases for all the studied materials. This was related to the easy access of water molecules to the bulk material at this temperature, minimizing the effect of polymer crystallinity clay nature and aspect ratio on the polymer degradation.     

XPS and wettability characterization of modified poly(lactic acid) and poly(lactic/glycolic acid) films

Poly(lactic acid) (PLA) and poly(lactic/glycolic acid) copolymers (PLGA) are biodegradable drug carriers of great importance, although successful pharmaceutical application requires adjustment of the surface properties of the polymeric drug delivery system to be compatible with the biological environment. For that reason, reduction of the original hydrophobicity of the PLA or PLGA surfaces was performed by applying a hydrophilic polymer poly(ethylene oxide) (PEO) with the aim to improve biocompatibility of the original polymer. PEO-containing surfaces were prepared by incorporation of block copolymeric surfactants, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (Pluronic), into the hydrophobic surface. Films of polymer blends from PLA or PLGA (with lactic/glycolic acid ratios of 75/25 and 50/50) and from Pluronics (PE6800, PE6400, and PE6100) were obtained by the solvent casting method, applying the Pluronics at different concentrations between 1 and 9.1% w/w. Wettability was measured to monitor the change in surface hydrophobicity, while X-ray photoelectron spectroscopy (XPS) was applied to determine the composition and chemical structure of the polymer surface and its change with surface modification. Substantial reduction of surface hydrophobicity was achieved on both the PLA homopolymer and the PLGA copolymers by applying the Pluronics at various concentrations. In accordance with the wettability changes the accumulation of Pluronics in the surface layer was greatly affected by the initial hydrophobicity of the polymer, namely, by the lactide content of the copolymer. The extent of surface modification was also found to be dependent on the type of blended Pluronics. Surface activity of the modifying Pluronic component was interpreted by using the solubility parameters.     

Two-dimensional NMR spectra of poly(N-vinylcarbazole)

Two-dimensional NMR spectra of commercially available poly(N-vinylcarabazole) are presented and analyzed. Complete assignments for the aromatic region of the carbon spectrum are made using the edited spectrum and literature references for model compounds. The proton spectrum is assigned from its correlations with the carbon spectrum and with a J-correlated two-dimensional spectrum. One of the protons (number 1) is shifted to rather unusual high fields by the stacked structure of the carbazole rings, independent on the polymer tacticity. From the aliphatic carbon spectrum a meso probability of 0.45 is estimated for the commercial polymer, and another J-correlated two-dimensional spectrum helps assigning the triad and tetrad splittings in the aliphatic proton spectrum.     

Chemical modification of poly(lactic acid) and its use as matrix in poly(lactic acid) poly(butylene adipate-co-terephthalate) blends

Poly(lactic acid), PLA, was chemically modified with maleic anhydride (MA) by reactive extrusion. The effect of this modification on molar mass (MM) and acidity was assessed by means of size-exclusion chromatography (SEC) and titration, respectively. PLA MM decreased in the presence of MA solely and of MA and peroxide. Reduction in MM was monitored by the increase in acidity. PLA blends containing poly(butylene adipate-co-terephthalate) (PBAT) were prepared through different mixing protocols, PLA/PBAT, PLA-g-MA/PBAT and PLA/PBAT/MA/peroxide (PLA/PBAT in situ). SEC results and rheological properties revealed reduction in MM and viscosity of the modified blends. PLA/PBAT presented increase in MM and bimodal MM distribution. The calculated interfacial tension was significantly lower for the modified blends, despite the lower average particle area of PLA/PBAT. Surprisingly, the modified blends presented higher yield strength than that predicted by the rule of mixtures, which might indicate interfacial reactions.     

Conjugation of bioactive groups to poly(lactic acid) and poly[(lactic acid)-co-(glycolic acid)] films

A novel PLA-based polymer containing reactive pendent ketone or hydroxyl groups was synthesized by the copolymerization of L-lactide with epsilon-caprolactone-based monomers. The polymer was activated with NPC, resulting in an amine-reactive polymer which was then cast into thin polymeric films, either alone or as part of a blend with PLGA, before immersion into a solution of the cell adhesion peptide GRGDS in PBS buffer allowed for conjugation of GRGDS to the film surfaces. Subsequent 3T3 fibroblast cell adhesion studies demonstrated an increase in cellular adhesion and spreading over films cast from unmodified PLGA. Hence the new polymer can be used to obtain covalent linkage of amine-containing molecules to polymer surfaces.     

Biodegradable poly(lactic acid)/chitosan-modified montmorillonite nanocomposites: Preparation and characterization

In this study, the biodegradable poly(lactic acid) (PLA)/montmorillonite (MMT) nanocomposites were successfully prepared by the solution mixing process of PLA polymer with organically-modified montmorillonite (m-MMT), which was first treated by n-hexadecyl trimethyl-ammonium bromide (CTAB) cations and then modified by biocompatible/biodegradable chitosan to improve the chemical similarity between the PLA and m-MMT. Both X-ray diffraction data and transmission electron microscopy images of PLA/m-MMT nanocomposites indicate that most of the swellable silicate layers were disorderedly intercalated into the PLA matrix. Mechanical properties and thermal stability of the PLA/m-MMT nanocomposites performed by dynamic mechanical analysis and thermogravimetric analysis have significant improvements in the storage modulus and 50% loss in temperature when compared to that of neat PLA matrix. The degradation rates of PLA/m-MMT nanocomposites are also discussed in this study.     

Characterisation of multi-extruded poly(lactic acid)

The effect of multiple (up to 10 times) extrusion of polylactide on its mechanical properties (determined by a static tension test), Charpy impact strength, melt flow rate, phase transition temperatures, degradation temperature, and permeability of water vapour and oxygen is presented. It was found that, with raising the number of the extrusion cycles, the tensile strength at break slightly diminished and the impact strength clearly decreased, while the melt flow rate and water vapour and oxygen transmission rates steadily increased. Variation of the number of extrusion cycles did not affect the glass transition temperature, whereas it did cause a lowering of the cold crystallisation temperature and slight diminishing of the melting point. The presented results indicate that PLA technological waste is suitable to be reused as an additive to a neat polymer.     

Fluorine NMR spectra of poly(vinyl trifluoroacetate)

Samples of poly(vinyl trifluoroacetate) with differing stereosequences have been prepared. The NMR signals at 56.4 Mc./sec. corresponding to the three types of triadstereosequences of the trifluoroacetate groups (isotactic, heterotactic, and syndiotactic) can be resolved. The results for specific polymers are discussed.     

Thermal properties and cold crystallization kinetics of surface-treated banana fiber (BF)-reinforced poly(lactic acid) (PLA) nanocomposites

Nucleation capacity of organically modified natural montmorillonite within the surface-treated banana fiber (BF)-reinforced PLA biocomposites has been studied using DSC analysis in the present investigation. Both the surface treatments and nanoclays play vital roles in the variation in nucleation process of PLA during cold crystallization process. Biocomposite made up of silane-treated BF and its bionanocomposite prepared using cloisite 30B (C30B) were showed superior nucleation parameters, n and K values, in the Avrami plots. Enhanced equilibrium melting point and lower E _a suggests the reinforcing effect imparted by the BF surface treatments and C30B within the PLA matrix. Even though, Louritzen–Hoffmann theory was revealed that no change in crystallization regimes of PLA even after the biocomposite and bionanocomposite preparation. TG analysis revealed better heat barrier capacity for all the biocomposites and bionanocomposites in comparison with virgin PLA (V-PLA). Increased storage modulus values for biocomposites and bionanocomposites also confirm the reinforcing effects of the fillers. Heat deflection temperature and the flammability studies concluded better application window for newly developed materials than that V-PLA.     

Poly(lactic acid)‐based nanocomposites

Classic plastics accumulate in nature causing environmental pollution, yet as a counterbalance they benefit society in many ways. They are versatile, cost‐effective, and can be tailored to have desired properties. The global environment has led to the fabrication of commodity plastics from environmentally degradable polymers. Poly(lactic acid) (PLA) is the most promising among the environmentally friendly polymers available. PLA‐based plastics have mechanical, thermal, and transparency similar to traditional plastics, and they can be molded and fabricated using the same equipment and procedures. Their material properties are enhanced through nanocomposites, compatibilizers, plasticizers, and other fillers (flame retardant, ultraviolet filter, etc.). This review summarizes mass production techniques and property reinforcements (focusing on nanocomposites and plasticizers) for PLA‐based plastics for commodity use. Copyright © 2016 John Wiley & Sons, Ltd.     

Enhanced stereocomplex formation of poly(L-lactic acid) and poly(D-lactic acid) in the presence of stereoblock poly(lactic acid)

Stereoblock poly(lactic acid) (sb-PLA) is incorporated into a 1:1 polymer blend system of poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) that has a high molecular weight to study its addition effect on the stereocomplex (sc) formation of PLLA and PDLA. The ternary polymer blend films are first prepared by casting polymer solutions of sb-PLA, PLLA, and PDLA with different compositions. Upon increasing the content of sb-PLA in the blend films the sc crystallization is driven to a higher degree, while the formation of homo-chiral (hc) crystals is decreased. Lowering the molecular weight of the incorporated sb-PLA effectively increases the sc formation. Consequently, it is revealed that sb-PLA can work as a compatibilizer to improve the poor sc formation in the polymer blend of PLLA and PDLA.     

Stabilization of poly(lactic acid) by polycarbodiimide

Polycarbodiimide (CDI) was used to improve the thermal stability of poly(l-lactic acid) (PLA) during processing. The properties of PLA containing various amounts of CDI were characterized by GPC, DSC, rheology, and tensile tests. The results showed that an addition of CDI in an amount of 0.1-0.7 wt% with respect to PLA led to stabilization of PLA at even 210 °C for up to 30 min, as evidenced by much smaller changes in molecular weight, melt viscosity, and tensile strength and elongation compared to the blank PLA samples. In order to examine the possible stabilization mechanism, CDI was reacted with water, acetic acid, l-lactic acid, ethanol and low molecular weight PLA. The molecular structures of the reaction products were measured with FTIR. The results showed that CDI could react with the residual or newly formed moisture and lactic acid, or carboxyl and hydroxyl end groups in the PLA samples, and thus hamper the thermal degradation and hydrolysis of PLA.     

Chemiluminescence kinetic analysis on the oxidative degradation of poly(lactic acid)

The oxidation of polymers is characterized by the generation of free radicals, and the kinetic parameters depicting the progress of oxidation are specific for each material structure and formulation. The durability of materials depends intrinsically on the molecular structure, and degradation mechanism influences the long-term stability of products. The intimate transformations of macromolecules can be reliably characterized by chemiluminescence (CL), which depicts the evolution of oxidation state by the reaction of free radicals with molecular oxygen. The isothermal and nonisothermal CL spectra are complementary proofs for the interpretation of oxidation behavior involving hydroperoxide as the initiators of oxidation. The degradation of PLA takes place by decomposition and fragmentation to lactide. The values of five kinetic parameters (initial CL intensity, CL intensity at the first peak, temperature corresponding to the first peak, oxidation induction time and oxidation rate) obtained for oxidative degradation of PLA lead to the activation energies ranged between 49 and 99 kJ mol^?1. The evolution of thermal degradation for poly(lactic acid) is an excellent example for the explanation of the decay fate of radical intermediates.     

Gas permeation properties of poly(lactic acid)

The need for the development of polymeric materials based on renewable resources has led to the development of poly(lactic acid) (PLA) which is being produced from a feedstock of corn rather than petroleum. The present study examines the permeation of nitrogen, oxygen, carbon dioxide, and methane in amorphous films of PLA cast from solution. The properties of PLA are compared to other commodity plastics and it is shown that PLA permeation closely resembles that of polystyrene. At 30°C, N₂ permeation in PLA is 1.3 (10⁻¹⁰ cm³ (STP) cm/cm² s cmHg) and the activation energy is 11.2 kJ/mol. For oxygen the corresponding values are 3.3 (10⁻¹⁰ cm³(STP) cm/cm² s cm Hg) and 11.1 kJ/mol. The values for carbon dioxide permeation are 1.2 (10⁻¹⁰ cm³ (STP) cm/cm² s cmHg) and 6.1 kJ/mol. For methane values of 1.0 (10⁻¹⁰ cm³ (STP) cm/cm² s cmHg) and an activation energy of 13.0 kJ/mol are found. Studies with pure gases show that polymer chain branching and small changes in l:d stereochemical content have no effect on permeation properties. Crystallinity is found to dominate permeation properties in a biaxially oriented film. The separation factor for a CO₂/CH₄ mixed gas system is measured between 0 and 50°C and does not deviate significantly from the calculated ideal separation factor; at 0°C the separation factor is 16, a value that suggests continued studies of PLA as a separation medium are warranted.