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.     

Characterization of an antimicrobial poly(lactic acid) film prepared with poly(ε‐caprolactone) and thymol for active packaging

Antimicrobial active films based on poly(lactic acid) (PLA) were prepared with poly(ε‐caprolactone) (PCL) and thymol (0, 3, 6, 9, and 12 wt%) by solvent casting methods. The films were characterized by thermal, structural, mechanical, gas barrier, and antimicrobial properties. Scanning electron microscopy analysis revealed that the surface of film became rougher with certain porosity when thymol was incorporated into the PLA/PCL blends. Thymol acted as plasticizers, which reduce the intermolecular forces of polymer chains, thus improving the flexibility and extensibility of the films. The addition of PCL into the pure PLA film decreased the glass transition temperature of the films. The presence of thymol decreased the crystallinity of PLA phase, but did not affect the thermal stability of films. Water vapor barrier properties of films slightly decreased with the increase of thymol loading. The antimicrobial properties of thymol containing films showed a significant activity against Escherichia coli and Listeria monocytogenes. The results indicated the potential of PLA/PCL/thymol composites for applications in antimicrobial packaging. Copyright © 2014 John Wiley & Sons, Ltd.     

Biodegradation of poly(lactic acid) powders proposed as the reference test materials for the international standard of biodegradation evaluation methods

The methods for producing reference test materials for biodegradation evaluation tests have been studied. Mechanical crushing at low temperature of polymer pellets using dry ice was selected for the method of producing polymer powder of poly(lactic acid) (PLA). The powders were fractionated using 60 mesh (250 μm) and 120 mesh (125 μm) sieves. The size distributions were then measured. The average diameter of the PLA particles obtained by this method was 214.2 μm. The biodegradation speeds of these PLA polymer powders were evaluated by two methods based on the international standard and one in vitro method based on the enzymatic degradation. First, the degree of biodegradation for this PLA powder was 91% for 35 days in a controlled compost determined by a method based on ISO 14855-1 (JIS K6953) at 58 °C managed by the Mitsui Chemical Analysis and Consulting Service, Inc. (Japan). Second, these polymer powders were measured for biodegradation by the Microbial Oxidative Degradation Analyzer (MODA) in a controlled compost at 58 °C and 70 °C based on ISO/DIS 14855-2 under many conditions. The degree of biodegradation for this PLA powder was approximately 80% for 50 days. In addition, the polymer powders were biodegraded by Proteinase K which is a PLA degradation enzyme. This polymer powder was suitable as a reference material for the evaluation methods of biodegradation.     

The morphological,mechanical, rheological,and thermal properties of PLA/PBAT blown films with chain extender

Biodegradable poly(lactic acid) (PLA)/poly(butylene adipate‐co‐terephthalate) (PBAT) blends and films were prepared using melt blending and blowing films technique in the presence of chain extender‐Joncryl ADR 4370F. The ADR contains epoxy functional groups and used as a compatibilizer. The morphological, mechanical, rheological, thermal, and crystalline properties of the PLA/PBAT/ADR blown films were studied. Scanning electron microscopy micrographs of the films revealed more ductile deformation with increasing PBAT content. The addition of PBAT enhanced the toughness of the PLA film. Tensile tests indicated that the elongation at break increased from 20.5% to 334.6% in the machine direction and from 7.1% to 715.9% in the transverse direction. The Young modulus increased from 2690.5 to 395.6 MPa in the machine direction and from 2623.5 to 154.0 MPa in the transverse direction. The sealing strength of 40/60/0.15 PLA/PBAT/ADR film was the highest among all the samples up to 9.4 N 15 mm⁻¹. These findings gave important implications for designing and manufacturing polymer packaging materials.     

Development of biopolymer/cellulose/silica nanostructured hybrid materials and their characterization by NMR relaxometry

The objective of this study was to investigate the preparation and properties of hybrid materials composed of poly(lactic acid) (PLA) and poly(lactic acid)/poly(lactic-co-glycolic acid) (PLA/PLGA) blends employing cellulose nanocrystals (CNCs) and/or organophilic silica (R972) as nanoparticles. The CNCs were obtained by acid hydrolysis of commercially available microcrystalline cellulose (MCC). The materials were produced in film form by solution casting. Organophilic silica was incorporated at a ratio of 3 wt.%, and CNCs were added at ratios of 3 wt.% and 5 wt.% in relation to the weight of the polymer matrix. Two series of films were obtained. The first was prepared using only PLA as the matrix, and the second was obtained using blends of PLA and PLGA. The properties of the films were evaluated by X-ray diffractometry, nuclear magnetic resonance, Fourier-transform infrared spectroscopy and measurement of mechanical properties. The results revealed that each nanoparticle, whether added individually or combined with the other type of nanoparticle, induced different final material properties. Cellulose nanocrystals can act as nucleating agents for the crystallization of PLA. There was an improvement in the mechanical performance of films with the addition of CNCs. Further, the incorporation of silica combined with CNCs resulted in the generation of films with the strongest mechanical properties. The results of this study indicate that silica decreases the surface tension between PLA-cellulose and PLA/PLGA-cellulose.     

Properties of crosslinked polylactides (PLLA & PDLA) by radiation and its biodegradability

Crosslinked materials derived from poly(lactide) (PLA) have been produced by radiation modification in the presence of a suitable crosslinker (triallyl isocyanurate) (TAIC). The crosslinking structure introduced in PLA films has not only much improved the heat stability but also their mechanical properties. The properties of crosslinked samples are governed by crosslinking density and these improvement seemed to increase with radiation dose. This implied that the three dimensional networks have been introduced in material by radiation and the crosslinking density depended on the structure and length of PLA chains. Biodegradability of PLA was also determined by an enzymatic degradation test and burying in compost at 55 °C. Differing with PLLA, PDLA was insignificantly degraded by proteinase K. The degradation rate of PLA in compost was postponed with the introduction of crosslinks.     

Perfluorinated polymer film with functional group prepared by radiation-induced grafting

A perfluorinated vinyl ether monomer, 2-bromotetrafluoroethyl trifluorovinyl ether (BrTFF) was grafted into crosslinked poly(tetrafluoroethylene) (cPTFE) films by γ-rays irradiation under different conditions. The irradiation with absorbed doses more than 550 kGy gave the poly(BrTFF) grafted cPTFE membranes with grafting yields of more than 20%. FTIR spectra and SEM-EDS testified that BrTFF was successfully grafted into cPTFE films homogeneously in the film perpendicular direction. TGA analysis of the grafted films indicated that the perfluorinated poly(BrTFF) grafts were phase-separated from the PTFE region due to the crosslinked structure of the cPTFE based film, but the grafted films still had high thermal stability (above 300 °C). DSC measurements showed the decrease of melting temperature, but the increase of both heat of fusion and degree of crystallinity in cPTFE film after grafting. The grafted film, a new kind of perfluorinated polymer with bromine active groups and good mechanical properties, is expected to be modified for new applications.     

Crystallization kinetics of virgin and processed poly(lactic acid)

Poly(lactic acid) (PLA) is an emerging material mainly because it can be synthesized from renewable resources and is thus environmentally and ecologically safe. The mechanical properties, above all the thermal resistance of PLA are determined by the crystalline content: the heat deflection temperature of crystalline PLA can reach 100 °C, whereas amorphous PLA loses mechanical properties at temperatures slightly higher than 60 °C. However, PLA has a low crystallization rate, so that after processing it remains mostly amorphous. This characteristic heavily limits the use of PLA for commercial applications. Many studies have been recently published on the crystallization kinetics of PLA. The effect of processing on this feature is however often neglected. In this work, the significance of processing on the crystallization kinetics of a commercial PLA was investigated. Two processing methods were explored: extrusion and injection moulding. The obtained materials, and the starting pellets of virgin polymer, were analyzed by calorimetry in order to obtain the crystallization kinetics. Two protocols were adopted to determine the crystallization rates during cooling from the melt or heating from the solid. The parameters of a kinetic equation were determined for all the materials and protocols adopted and it was thus possible to describe the evolution of crystallinity during heating and during cooling.     

The evolution of poly(lactic acid) degradability by dielectric spectroscopy measurements

Dielectric spectroscopy is a powerful method that allows the study of the dynamics of polymers in a wide frequency range. The different regimes of the dielectric spectra can be observed and the dynamics of the primary and secondary relaxations can be found.In this work, the systems investigated included industrial and purified poly(lactic acid), PLA. This is an aliphatic polyester, one of the most important biocompatible and biodegradable material that has received increasing attention in the last 10 years.Thermal analyses indicated that the glass transition temperature of the polymer was about 320 K.The β relaxation was observed between −150 °C and −30 °C, depending on the measurement frequency (1 Hz-100 kHz), and was assigned to the secondary relaxation in the glassy state. We studied the changes that are connected with the water penetration in the polymer, and directly affect that relaxation process. Water molecules confined by polymer chains and in the polymer networks play an important role in the degradation of the matrix, and then we can observe the evolution of that degradation during some weeks, in a controlled humidity environment. It is accepted that water preferentially enters in the amorphous zones, but also affects the crystalline regions. It is observed a clear evolution of the relaxation activation energy during the degradation of the polymer.The dielectric relaxation results are complemented with molar mass measurements during the degradation process with time.     

Raman characterization of orientation in poly(lactic acid) films

Poly(lactic acid) is a new biopolymer material which is marketed by Cargill Dow Polymers under the tradename Nature Works*. One major application for this material is biaxially oriented films for food packaging because it possesses excellent barrier for flavor constituents, deadfold and heat sealability. Shrinkage must be minimized when the film is heat sealed for these applications and, therefore, characterization of the orientation of the amorphous phase of PLA films is necessary. Raman spectroscopy methodology has been developed to quantify orientation in PLA films. Bands were assigned to crystalline and amorphous phases of PLA such that orientation in both phases could be monitored. Raman depolarization ratios were used to characterize uniaxial systems but were insufficient for most biaxial draws. A new phenomenon for oriented films involving Raman band shifts was observed in these systems, and was shown to be capable of determining orientation, even for symmetrical biaxially drawn films. The origin of these shifts, as well as their use for the quantification of orientation will be discussed. Further, since the line widths of the bands could be used to quantify crystallinity, both crystallinity and orientation could be determined with one measurement.     

Difference in Glass Transition Behavior Between Semi Crystalline and Amorphous poly(lactic acid) Thin Films

Summary: Semi crystalline and amorphous poly(lactic acid) (PLA) thin films exhibit different glass transition temperature and behaviour, as revealed by ellipsometry. For semi-crystalline poly(L-lactic acid) (PLLA) thin film (with crystalline content between 40 and 60%), the glass transition temperature (T_g) is found to decrease below a film thickness of 50 nm. This depression was interpreted in term of disentenglement effect which is likely to occur upon confining the amorphous PLA phase near a non interacting surface. New results performed on non completed films, i.e. isolated objects, also reveal the lower transition temperature, thus underlying the importance of the entanglement state of the polymer chains on their mobility. For amorphous PLA thin film, obtained from the L and D copolymer, two distinct glass transitions were observed, with the highest Tg attributed to the presence of some nano-phase domains, formed by a possible cooperation of the D and L blocks to form stereocomplexes sequences, within the film. Furthermore, if these T_g remained constant as film thicknesses decrease down to 50 nm, they were also found to slightly decrease for isolated objects, thus supporting the importance of the entanglement hypothesis on the glass transition.     

Thermal degradation of poly(lactic acid) (PLA) and poly(butylene adipate-co-terephthalate) (PBAT) and their blends upon melt processing

Poly(lactic acid) (PLA) and poly(butylene adipate-co-terephthalate) (PBAT) are biodegradable aliphatic polyesters, which being semicrystalline and thermoplastic can be processed by conventional methods. Their blends give interesting materials for industrial packaging applications, due to their increased ductility as PBAT content increases. However, like many aliphatic polyesters, the PLA matrix degrades upon melt processing thus affecting the thermo-mechanical features of the blended material. In this work, we studied the effect of processing at high temperature on the molecular weight distribution, morphology, and thermo-mechanical properties of both homopolymers, as well as the PLA/PBAT 75/25 blend. Notably, different processing conditions were adopted in terms of temperature (range 150-200 °C) and other relevant processing parameters (moisture removal and nitrogen atmosphere). Analysis of PLA/PBAT blends indicated that intermolecular chain reactions took place under strong degradative conditions of PLA, yielding PLA/PBAT mixed chains (copolymers). Increasing amounts of copolymers resulted in improved phase dispersion and increased ductility, as SEM and mechanical tests indicated. Conversely, reduced PLA degradation with less copolymer formation, afforded higher modulus materials, owing to poorer dispersion of the soft phase (PBAT) into the PLA matrix.     

Degradation mechanism of poly(lactic-co-glycolic) acid block copolymer cast films in phosphate buffer solution

We have investigated the degradation of poly(lactic-co-glycolic) acid copolymer with a lactic to glycolic ratio of 50:50. Solvent-cast films were incubated at 37 °C in phosphate buffered saline solution and their degradation was followed using potentiometry, light microscopy, gravimetry, size exclusion chromatography, differential scanning calorimetry and infrared spectroscopy. The degradation process was found to have two main steps. The first step was observed from 0 to 7 days of degradation. During the first few days a soft layer formed at the surface of the film. As degradation time increased this soft surface layer was found to swell and wrinkle. The polymer molecular weight in the bulk was found to decrease as soon as the film was placed in the medium while the polymer present in the surface layer was found to degrade at a much slower rate. The second step of degradation was found to occur after 8 days. At this stage of the degradation process the molecular weight of the polymer in the bulk of the films was so low that the materials became liquid resulting in the detachment of the film from the glass slide. At this stage the mass loss and amount of acid released in the media were found to increase significantly.     

Bioplastic biodegradation activity of anaerobic sludge prepared by preincubation at 55 °C for new anaerobic biodegradation test

The new method to evaluate the anaerobic biodegradability of bioplastics, such as polycaprolactone (PCL) and poly (lactic acid) (PLA), under aquatic (slurry) conditions at 55 °C is applying. For this method, we prepared the sludge at 55 °C from the sludge at 37 °C by the method in which the sludge from the real tank operating at around 37 °C using cow manure and vegetable waste as the feed stock was preincubated at 55 °C. It was unknown at which stage the sludge during preincubation has the optimized anaerobic biodegradation activity of plastics. Four different stage sludges during preincubation (the sludge at 7 days after the start of preincubation at 55 °C, at 12 days, at 18 days, and at 40 days) were compared by the anaerobic biodegradation activity of PLA. The preincubated sludge at around 18 days (a gradual decrease in biogas evolution and a methane ratio over 60%) showed the highest biodegradation activity of PLA. In addition, the bacterial population in each sludge was analyzed by the denaturing gradient gel electrophoresis (DGGE) analysis of the amplified 16S rRNA gene fragments, however, the newly grown bacteria bands at 55 °C were not clearly detected.     

Improvement of UV stability and mechanical properties of biopolyesters through the addition of β-carotene

β-carotene, the most abundant carotenoid in nature and a natural antioxidant, has been added at a concentration of 2 × 10⁻⁴ g per mL of polymer film forming solution to three biopolyester matrices, poly(lactic acid) (PLA), polycaprolactone (PCL) and polyhydroxybutyrate-co-valerate (PHBV). The dispersion and stability of the antioxidant within the films was studied using confocal Raman imaging spectroscopy, colorimetry and UV/visible spectrophotometry. The films were characterized in terms of thermal and mechanical properties in comparison with control films without β-carotene. Addition of β-carotene to the three biopolyesters studied resulted in significant increases in the deformation at break and lower Young modulus. This antioxidant is thought to act as a plasticizer in the biopolyesters, thus increasing their free volume and causing a change in their thermal properties. Moreover, when exposed to UV light the mechanical properties of the films with β-carotene were less affected displaying, in general, higher modulus and similar ultimate tensile strength while keeping the films more ductile than the pure films. Therefore, β-carotene can be potentially used as a natural additive to increase the UV stability of the biopolyesters.     

Influence of curing temperature on the optical properties of fluorinated polyimide thin films

The influence of curing temperature (CT) on the optical properties of 6FDA/ODA poly(amic acid)-polyimide (PAA-PI) films was characterized by measuring ATR-FT-IR spectra, refractive index (RI) and birefringence of the films. The results showed that the infrared absorption intensity of characteristic peaks (IAICP) corresponding to the imide ring and the RI of PAA-PI films reached their maxima when the films had been cured at 270 °C, while the magnitude of birefringence (|Δn|) of the films reached its minimum as CT rose up to 330 °C. However, the RI decreased as CT was between 270 °C and 330 °C. Both the RI and |Δn| of the film increased obviously when CT increased after 330 °C. We think this is due to the interchain crosslink reaction (ICCR) above 330 °C and can be an evident proof of ICCR. And the evidences supporting ICCR was also discussed via IR differential spectra.     

Synthesis and characterization of novel elastomeric poly(D,L-lactide urethane) maleate composites for bone tissue engineering

Here, we report the synthesis and characterization of a novel 4-arm poly(lactic acid urethane)-maleate (4PLAUMA) elastomer and its composites with nano-hydroxyapatite (nHA) as potential weight-bearing composite. The 4PLAUMA/nHA ratios of the composites were 1:3, 2:5, 1:2 and 1:1. FTIR and NMR characterization showed urethane and maleate units integrated into the PLA matrix. Energy dispersion and Auger electron spectroscopy confirmed homogeneous distribution of nHA in the polymer matrix. Maximum moduli and strength of the composites of 4PLAUMA/nHA, respectively, are 1973.31 ± 298.53 MPa and 78.10 ± 3.82 MPa for compression, 3630.46 ± 528.32 MPa and 6.23 ± 1.44 MPa for tension, 1810.42 ± 86.10 MPa and 13.00 ± 0.72 for bending, and 282.46 ± 24.91 MPa and 5.20 ± 0.85 MPa for torsion. The maximum tensile strains of the polymer and composites are in the range of 5–93%, and their maximum torsional strains vary from 0.26 to 0.90. The composites exhibited very slow degradation rates in aqueous solution, from approximately 50% mass remaining for the pure polymer to 75% mass remaining for composites with high nHA contents, after a period of 8 weeks. Increase in ceramic content increased mechanical properties, but decreased maximum strain, degradation rate, and swelling of the composites. Human bone marrow stem cells and human endothelial cells adhered and proliferated on 4PLAUMA films and degradation products of the composites showed little-to-no toxicity. These results demonstrate that novel 4-arm poly(lactic acid urethane)-maleate (4PLAUMA) elastomer and its nHA composites may have potential applications in regenerative medicine.     

Comparative study on hydrolytic degradation and monomer recovery of poly(l-lactic acid) in the solid and in the melt

The hydrolytic degradation of poly(l-lactide) (PLLA) and the formation of its monomer in the solid and in the melt were investigated at 120-150 °C (in the solid), at 160 °C (in the solid up to 40 min and in the melt exceeding 40 min), and at 170-190 °C (in the melt). Such state difference caused the difference in the degradation behavior of PLLA and the behavior of lactic acid formation, although the degradation of PLLA proceeds via a bulk erosion mechanism, regardless of its state. The crystalline residues were formed at the degradation temperatures below 140 °C, but not at the degradation temperatures above 160 °C. The lactic acid yield exceeding 95% can be successfully attained for all the temperatures of 120-190 °C. The activation energy for hydrolytic degradation values of PLLA were 69.6 and 49.6 kJ mol⁻¹ for the temperature ranges of 120-160 °C (in the solid) and 170-250 °C (in the melt), respectively, and are compared with the reported values.     

The Influence of Peroxide on Bubble Stability and Rheological Properties of Biobased Poly(lactic acid)/Natural Rubber Blown Films

This study investigated the effects of natural rubber(NR)and an organic peroxide on the rheological properties,mechanical properties,morphology,and bubble stability during film blowing of poly(lactic acid)(PLA).The NR and peroxide contents were varied from 0 wt%to 25 wt%and 0 wt%to 0.5 wt%,respectively.The results confirmed that the presence of well-dispersed NR could significantly improve the toughness,elongation at break,and processability of PLA films,where the optimal amount of NR was 15 wt%.For the reactive blending with peroxide,a suitable peroxide content for good film toughness and clarity was 0.03 wt%,while the higher content of 0.1 wt%could provide slightly higher processability.These contents are considered much lower than that in the PLA system(without NR),which required up to 0.5 wt%peroxide.The rheological studies indicated that the melt strength,the storage modulus(G’)and complex viscosity(η*)at low frequency could be correlated with good film blowing processability of the PLA/NR films at low gel contents.These parameters failed to correlate in the films having high gel contents as the deformation rate experienced by each test was different leading to the different levels of response to the type and amount of gels.     

Photo-grafting Poly(acrylic acid) onto Poly(lactic acid) Chains in Solution

Poly(lactic acid)(PLA)is one of the most important bio-plastics,and chemical modification of the already-polymerized poly(lactic acid)chains may enable optimization of its material properties and expand its application areas.In this study,we demonstrated that poly(lactic acid)can be readily dissolved in acrylic acid at room temperature,and acrylic acid can be graft-polymerized onto poly(lactic acid)chains in solution with the help of photoinitiator benzophenone under 254 nm ultraviolet(UV)irradiation.Similar photo-grafting polymerization of acrylic acid(PAA)has only been studied before in the surface modification of polymer films.The graft ratio could be controlled by various reaction parameters,including irradiation time,benzophenone content,and monomer/polymer ratios.This photo-grafting reaction resulted in high graft ratio(graft ratio PAA/PLA up to 180%)without formation of homopolymers of acrylic acid.When the PAA/PLA graft ratio was higher than 100%,the resulting PLA-g-PAA polymer was found dispersible in water.The pros and cons of the photo-grafting reaction were also discussed.