Enhanced photodegradation of TiO2-containing poly(ε-caprolactone)/poly(lactic acid) blends

The calamitous accumulation of plastic waste in the environment, especially single-use disposables, calls for new approaches to materials design. One method to address the persistence of plastics beyond their intended use is to impart them with functionalities that will either allow for their recyclability or their degradation to basic natural components. This work focuses on the fabrication of photodegradable polyester blends and investigates the impact of compatibilization on photodegradation rates. Specifically, we blended poly(ε-caprolactone) (PCL) and poly(lactic acid) (PLA) polymers by (reactive) extrusion in the presence or absence of dicumyl peroxide (DCP), a radical generator, and titanium dioxide (TiO₂), an inorganic photocatalyst. We examined the effects of DCP and TiO₂ loadings as well as copolymer composition on the thermomechanical properties, photodegradability, and morphology. We found that the inclusion of TiO₂ dramatically increased flexural moduli and photodegradation rates in both dry and wet conditions, while reactive compatibilization had little effect of the tested properties. This simple and scalable approach is promising to fabricate materials that can readily photodegrade.     

Synthesis and evaluation of biodegradable copolyesters of octadecane-1,18-dicarboxylic acid and poly(ε-caprolactone) diol

Copolyesters of poly(ε-caprolactone) diol and 1,18-octadecane dicarboxylic acid were synthesized by polyaddition. The copolymers have a "mixed" structure of ’polyester-polyethylene’. Characterization was carried out by using ¹H and ¹³C NMR, FT-IR, viscosities, DSC, and GPC. The tensile strength varied from 11.9 to 19.9 MPa, and elongation at break varied from 370 to 660 %, respectively. Biodegradability was evaluated by enzymatic hydrolysis (lipases from Rhizopus arrhizus and Candida cylindracea and an esterase form hog liver) and by soil burial test. Both tests showed that the copolyesters were characterized by enhanced biodegradability.     

Thermal degradation of two classes of block copolymers based on poly(lactic-glycolic acid) and poly(ϵ-caprolactone) or poly(ethylene glycol)

Thermodegradative investigations of two classes of multi-block copolymers containing poly(D,L-lactic-glycolic acid) (PLGA) and either poly(ethylene glycol) (PEG) or poly(ϵ-caprolactone) diol-terminated (PCDT) segments were performed. In particular, the influence of the type and length of the segments as well as of the molar ratio between the D,L-lactic acid (LA) and glycolic acid (GA) residues was investigated at 180°C in air by viscometry, FT-IR analysis and isothermal thermogravimetry. The thermal oxidative degradation of these materials is largely affected by the LA/GA ratio, a higher LA content generally imparting higher stability. The FT-IR analysis suggests that, depending on the composition of the PLGA segments, degradative processes are triggered which can lead to a preferential degradation of the blocks.     

A convenient method to prepare random LA/CL copolymers from poly(L-lactide) and ε-caprolactone

Sequential addition of L-lactide(LA) followed by ε-caprolactone(CL), and simultaneous addition of both monomers, afforded random LA/CL copolymers in the presence of lanthanide aryloxides under mild conditions. Transesterification was proved to play a predominant role in random copolymer formation. Moreover, treatment of poly(L-lactide) with ε-CL led to random copolymer formation, which provides a new strategy not only to prepare random LA/CL copolymers, but also to directly modify PLLA.     

Purification, cloning and expression of an Aspergillus niger lipase for degradation of poly(lactic acid) and poly(ε-caprolactone)

A lipase from Aspergillus niger MTCC 2594 was purified 53.8-fold to homogeneity by hydrophobic interaction chromatography using octyl sepharose and the enzyme showed two protein bands with apparent molecular mass of 35 and 37 kDa respectively. The lipase exhibited maximum activity at pH 7.0 and 37 °C and was stable between pH 4.0 and 10.0 and temperatures up to 50 °C. The values of K_m and V_max were 3.83 mM and 32.21 μmol/min/mg respectively, using olive oil as substrate. Lipase encoding gene, lipA, coded for 297 amino acid residues with conserved pentapeptide sequence, G-H-S-L-G, was cloned and expressed in Pichia pastoris. Although lipA showed high homology with the known Aspergillus lipases, it exhibited differences in putative lid domain. Both native and recombinant lipases have potential for degradation of poly(lactic acid) and poly(ε-caprolactone), and the present study will serve as a baseline of initial studies for its exploitation in polymer degradation.     

Effect of halloysite nanotubes on the thermal degradation behaviour of poly(ε-caprolactone)/poly(lactic acid) microfibrillar composites

This paper reports on the thermal degradation behaviour and kinetics of halloysite nanotubes containing microfibrillated poly(ε-caprolactone) (PCL)/poly(lactic acid) (PLA) blends. It was found that the nanotubes probably catalyzed the PLA degradation, and that the free radicals formed during the PLA degradation initiated PCL degradation at lower temperatures, maybe in combination with halloysite nanotubes (HNT) catalysis. Drawing to form microfibrillated nanocomposites had little influence on the degradation behaviour of these materials, but pre-mixing of the HNT with PLA or PCL prior to melt-mixing and extrusion-drawing of the blends did influence the degradation behaviour, but in different ways. No evidence could be found that the presence and amount of HNT, or the mode of preparation, had an influence on the degradation mechanism, as evidenced through a Fourier-transform infrared (FTIR) analysis of the degradation products.     

Synthesis and characterization of biodegradable pH-sensitive hydrogels based on poly(?-caprolactone), methacrylic acid, and poly(ethylene glycol)

In this work, a novel biodegradable pH-sensitive hydrogel based on poly(?-caprolactone) (PCL), methoxpoly(ethylene glycol) (MPEG) and methacrylic acid (MAA) was prepared by UV-initiated free radical polymerization. The resulting macromonomers and hydrogels were characterized by FTIR and/or ¹H NMR. Swelling behaviour and pH sensitivity of the hydrogels were studied in detail. With increase in pH of aqueous medium from 1.2 to 7.2, swelling ratio of the hydrogels increased accordingly. The hydrolytic degradation behaviour was also investigated. The prepared biodegradable pH-sensitive hydrogel based on PCL, MPEG, and MAA might have great potential application in smart drug delivery system.     

Preparation and characterization of microspheres based on blend of poly(lactic acid) and poly(ɛ-caprolactone) with poly(vinyl alcohol) as emulsifier

In this paper, microspheres were prepared by oil-in-water (o/w) emulsion solvent evaporation method. Biodegradable polymer such as blend of poly (lactic acid) (PLA) and poly(?-caprolactone) (PCL) with certain compositions and characteristics was used to prepare the microspheres with poly(vinyl alcohol) (PVA) as an emulsifier. This study observed the microspheres particle’s size distribution at various concentrations of PVA (1%, 1.5%, 2%, and 2.5% PVA). The PVA volume variations effects during the process (50, 100, 150, 200, and 250 mL) were also observed. The blend of PLA and PCL is formed only by physical interaction between them. This can be seen from the FTIR spectrum which shows both PLA and PCL component. The microspheres physical size and appearance were observed by optical microscope (MO). The overall results of this study showed that the formula which used 50–150 mL of 2.5% polyvinyl alcohol produced the microspheres with the most uniform size distribution.     

Compatibilization of immiscible poly(lactic acid)/poly(ε-caprolactone) blend through electron-beam irradiation with the addition of a compatibilizing agent

The aim of this study was to compatibilize immiscible poly(lactic acid) (PLA)/poly(ε-caprolactone) (PCL) blend by using electron-beam radiation method with the addition of a compatibilizing agent. Glycidyl methacrylate (GMA) was chosen as the compatibilizing agent, in the expectation that the GMA plays a role as a monomeric compatibilizer and a reactive agent at the interface between the PLA and the PCL phases. Compatibilization process has been investigated through the melt mixing of the PLA/PCL and the GMA by using a twin-screw extruder and the exposure of the PLA/PCL/GMA mixture to electron-beam radiation at room temperature. The melt mixing process was performed to locate the GMA at the interface, thereby expecting a finer morphology due to the GMA as the monomeric plasticizer. The exposure process was carried out to induce definite interfacial adhesion at the interface through electron-beam initiated cross-copolymerization by the medium of the GMA as the reactive agent. To investigate the results of this compatibilization strategy, the morphological, mechanical, and rheological properties of the blend were analyzed. The morphological study clearly showed the reduced particle size of dispersed PCL domains and significantly improved interfacial adhesion by the electron-beam irradiation with the addition of the GMA. The stress–strain curves of the blends irradiated at less than 20 kGy showed the typical characteristics of ductile materials. The tensile properties of the blend were strongly affected by the dose of irradiation.     

Morphology and thermal degradation studies of melt-mixed poly(lactic acid) (PLA)/poly(ε-caprolactone) (PCL) biodegradable polymer blend nanocomposites with TiO2 as filler

The morphology and thermal stability of melt-mixed poly(lactic acid) (PLA)/poly(ε-caprolactone) (PCL) blend nanocomposites with small amounts of TiO₂ nanoparticles were investigated. The nanoparticles were mostly located in the PLA phase, with good dispersion of individual particles, although significant aggregation was also visible. The thermal stability and degradation behaviour of the different samples were studied using thermogravimetric analysis (TGA) and TGA-Fourier-transform infrared (FTIR) spectroscopy. Neat PCL showed better thermal stability than PLA, but the degradation kinetics revealed that PLA had a higher activation energy of degradation than PCL, indicating its degradation rate more strongly depends on temperature, probably because of a more complex degradation mechanism based on chain scission and re-formation. Blending of PLA and PCL reduced the thermal stabilities of both polymers, but the presence of TiO₂ nanoparticles improved their thermal stability. The nanoparticles also influenced the volatilization of the degradation products from the blend, acted as degradation catalyst and/or retarded the escape of volatile degradation products.     

Surface perspectives in the biomedical applications of poly(α-hydroxy acid)s and their associated copolymers

Impressive advances in biotechnology, bioengineering, and biomaterials with unique properties have led to increased interest in polymers and other novel materials in biological and biomedical research and development over the past two decades. Although biomaterials have already made an enormous impact in biomedical research and clinical practice, there is a need for better understanding of the surface and interfacial chemistry between tissue (or cells) and biomedical materials. This is because the detailed physicochemical events related to the biological response to the surface of materials still often remain obscure, even though surface properties are important determinants of biomedical material function. In this regard, data available in the literature show the complexity of the interactions (surface reorganization, non-specific/specific protein adsorption, and chemical reactions such as acid-base, ion pairing, ion exchange, hydrogen bonding, divalent-ion bridging) and the interrelationship between biological environments, interfacial properties, and surface functional groups responsible for the biological responses. Because of the multidisciplinary nature of surface and interfacial phenomena at the surface of biomedical polymers, this review focuses on several aspects of current work published on poly(alpha-hydroxy acid)s and their associated copolymers:surface structure-biomedical function relationships;physicochemical strategies for surface modification; and, finally,synthetic strategies to increase biocompatibility for specific in-vivo and/or in-vitro biomedical applications.     

Lipase-catalysed degradation of copolymers prepared from ?-caprolactone and dl-lactide

A series of copolymers were prepared by ring-opening polymerization of ?-caprolactone and dl-lactide, using zinc lactate as catalyst. The resulting PCL/PLA copolymers were characterized by various analytical techniques such as NMR, SEC, DSC and X-ray diffraction. The [CL]/[LA] ratios of the copolymers are very close to those in the feed, indicating a good conversion of monomers. The copolymers with CL contents higher than 50% appear semi-crystalline, the crystalline structure being of the PCL-type. Compression moulded polymer films were allowed to degrade in a pH = 7.6 phosphate buffer containing Pseudomonas lipase. Data show that copolymers with CL contents lower than 25% are not degradable and the degradation rate increases with CL content for CL-rich copolymers. Various soluble degradation products are detected in the degradation medium, including CL₁ to CL₃ and LA₁ to LA₄ homo-oligomers, and CL₂LA₁ co-oligomer. The presence of LA homo-oligomers and CL₂LA₁ co-oligomer suggests that Pseudomonas lipase can not only degrade PCL but also LA short blocks along PCL/PLA copolymer chains. On the other hand, little changes of composition are detected during degradation, in agreement with a surface erosion mechanism as shown by ESEM.     

Enzyme-catalyzed degradation of poly(l-lactide)/poly(?-caprolactone) diblock, triblock and four-armed copolymers

Poly(l-lactide)/poly(?-caprolactone) diblock, triblock and four-armed copolymers with the same monomer feed ratio (50/50) were synthesised by two step ring opening polymerisation of successively added ?-caprolactone and l-lactide, using isopropanol, ethylene glycol, or pentaerythritol as initiator and zinc lactate as co-initiator. The resulting copolymers were characterised by ¹H NMR, DSC, SEC, and FT-IR, which confirmed the blocky characteristic of the copolymers. Solution cast films were allowed to degrade at 37 °C in the presence of proteinase K, and the degradation was monitored by gravimetry, DSC, SEC, ¹H NMR and ESEM. The effects of chain structure, block length and crystallinity on the degradation are discussed. The four-armed block copolymer degrades the most rapidly, while the diblock copolymer exhibited the slowest degradation rate. The difference was related to the crystallinity depending on both the molecular structure and block length. Little compositional or molar mass changes were obtained during degradation, which strongly supports a surface erosion mechanism, in agreement with ESEM observations.     

Synthesis of organo-modified α-zirconium phosphate and its effect on the flame retardancy of IFR poly(lactic acid) systems

Novel intumescent flame-retardant poly(lactic acid) (PLA/IFR)/organo-modified α-zirconium phosphate(OZrP) nanocomposites were prepared via incorporation of charring agent (CA), ammonium polyphosphate (APP) and OZrP into PLA. OZrP was synthesized directly by a solvent thermal method. The morphological characterization of PLA/IFR/OZrP nanocomposites was conducted by wide angle X-ray diffraction (WXRD) and transmission electron microscopy (TEM). The effect of the OZrP on flame retardancy and the thermal stability of PLA/IFR composites were studied by thermogravimetric analysis (TGA), limiting oxygen index (LOI), vertical burning test (UL-94) and cone calorimeter test. The TGA data illustrated that the OZrP could increase the residue and significantly improve the flame retardancy of PLA/IFR/OZrP nanocomposites showing an excellent synergistic effect. The addition of OZrP to the flame-retardant PLA increases the LOI and enhances the UL-94 rating. Cone calorimeter tests gave clear evidence that the incorporation of OZrP into PLA/IFR composites resulted in the significant reduction of the heat release rate (HRR), low total heat release (THR) and high amount of char residues during combustion. The flame-retardant mechanism of PLA/IFR/OZrP nanocomposites may correspond to the intumescent flame-retardant mechanism and catalyzed carbonization mechanism caused by OZrP.     

Photodegradation of biodegradable polyesters: A comprehensive study on poly(l-lactide) and poly(?-caprolactone)

The photodegradation of melt-crystallized and amorphous-made poly(l-lactide) (PLLA-C and PLLA-A, respectively) and cast-crystallized poly(?-caprolactone) (PCL) was investigated comprehensively for the periods up to 200 h using gel permeation chromatography, differential scanning calorimetry, tensile testing, and polarization optical microscopy. The photodegradation of PLLA and PCL films proceeds via a bulk erosion mechanism, indicating that UV penetrates the specimens with no significant reduction in its intensity, irrespective of the chemical structure and the crystallinity of biodegradable polyesters. The photodegradability of PCL chains was higher than that of PLLA chains. This strongly suggests that the chemical structure of the two sequential groups adjacent to the ester oxygen rather than the density of ester group is crucial to determine the photodegradability of biodegradable polyesters. Although PLLA chains are photodegradable even in the crystalline regions, their photodegradability is lower than that in the amorphous regions. The significant increase in weight-average molecular weight (M_w)/number-average molecular weight (M_n) was observed for PLLA-A and PCL films, even when the decrease in M_n by UV irradiation was small. Most of the tensile properties of PLLA and PCL films remained unchanged during UV irradiation, while solely the elongation at break of PCL film significantly decreased. This result reflects that among the tensile properties the elongation at break was most sensitive to the change in molecular characteristics of biodegradable polyesters by UV irradiation. The contrast between bright and dark parts of Maltese crosses remained unchanged for the spherulites in PLLA-C and PCL films even after UV irradiation for 200 h. This result exhibits that the cleaved fraction of the tie chains was too low to cause the traceable disorientation of lamellae.     

The co-continuous morphology of biocompatible ethylene-vinyl acetate copolymers/poly(��-caprolactone) blend: effect of viscosity ratio and vinyl acetate content

The phase behavior and its linear viscoelastic responses of a biocompatible blend based on ethylene-vinyl acetate copolymers and poly(??-caprolactone) (EVA/PCL) were studied in this work in terms of blending ratios and annealing. The effects of viscosity ratios and vinyl acetate contents of the EVA on the co-continuous morphology were addressed. The results show that EVA/PCL is a typical immiscible blend due to the high interfacial tension between the two polymers. Thus, the blend shows a wide percolation range with a narrow fully co-continuous region. Although the phase inversion point can be well predicted by the viscous Utracki model, the dynamic viscoelastic responses of the blend cannot be well described by the emulsion model. The elasticity ratio was proposed to play an important role together with the viscosity ratio on the phase inversions. During dynamic annealing, the phase size of both the sea?Cisland and the co-continuous structures increases evidently, but the principle of time?Ctemperature superposition is only valid for the co-continuous blend while fails with that with the sea?Cisland phase structure. Beside, the phase size of the co-continuous structure is dependent strongly on the viscosity ratio between EVA and PCL. With reduced viscosity ratio, the phase size increases remarkably. However, vinyl acetate (VA) contents of the EVA have little influences on the interfacial properties and phase size of the co-continuous blends in the experimental content ranges (28?C12?wt.%).     

The behavior of poly(ε -caprolactone) and poly(ethylene oxide)-b-poly(ε -caprolactone) grafted to a poly(glycerol adipate) backbone at the air/water interface

The behavior of crystallizable poly(ε-caprolactone) (PCL) and poly(ε-caprolactone)-b-poly(ethylene oxide) (PCL-b-PEO) is studied at the air/water interface prior and after grafting to an amorphous poly(glycerol adipate) (PGA) backbone (PGA-g-PCL, PGA-g-(PCL-b-PEO)). Langmuir isotherms are measured and the structure formation in the monolayers on the water surface is followed by Brewster angle microscopy (BAM) and in Langmuir–Blodgett films after a transfer to silicon substrates by atomic force microscopy (AFM). It is observed that PGA-g-PCL forms significantly smaller crystals on the water surface and has smaller crystallization rate compared to PCL homopolymers of identical molar masses as the grafted chains. In contrast to crystals formed by linear PCL, the crystals formed by grafted PCL in PGA-g-PCL do not melt (readsorb at the water surface) in an expansion cycle on the Langmuir trough. Additionally, increasing the subphase temperature at constant surface area significantly above the melting point of linear PCL in bulk results in the formation of a mesophase, and it does lead to the disappearance of crystals. The isotherms of PGA-g-(PCL-b-PEO) show a transition at the surface pressure of ～10 mN/m. This is related to the fact that PEO chains leave the water surface and submerge into the subphase and/or the crystallization of PCL chains. The monolayer collapse appears in an extended plateau region starting at π values of ～30 mN/m. AFM images of Langmuir–Blodgett films reveal that PCL chains in PGA-g-PCL and PGA-g-(PCL-b-PEO) form lamellar crystals with a disk-shape and interconnected platelets, respectively.     

Esters ofβ -ketophosphonic acids. VII. The products of the reaction ofα-chlorocyclohexanone with the diethyl ester of ethylphosphonous acid and the sodium salt of the monoethyl ester of ethylphosphonous acid

Summary The action of diethyl ethylphosphonite on-chlorocyclohexanone gives cyclohexen-1-yl-1 -ethyl ethylphosphonate, Cyclohenen-1-yl-1-ethyl ethylphosphonate is also obtained by the action of sodium monoethyl ethylphosphonite on a-chlorocyclohexanone.     

A convenient method to prepare random LA/CL copolymers from poly(<Emphasis Type="Italic">L</Emphasis>-lactide) and ε-caprolactone

Sequential addition of L-lactide (LA) followed by ε-caprolactone (CL), and simultaneous addition of both monomers, afforded random LA/CL copolymers in the presence of lanthanide aryloxides under mild conditions. Transesterification was proved to play a predominant role in random copolymer formation. Moreover, treatment of poly(L-lactide) with ε-CL led to random copolymer formation, which provides a new strategy not only to prepare random LA/CL copolymers, but also to directly modify PLLA.     

Effect of partial crosslinking on morphology and properties of the poly(β-hydroxybutyrate)/poly(d,l-lactic acid) blends

Poly(β-hydroxybutyrate) (PHB) is a bio-based and biodegradable aliphatic polyester, however its application is limited by some disadvantages such as high price, brittleness, poor processability and low melt-strength due to serious thermal degradation. Partial crosslinking initiated by dicumyl peroxide (DCP) was applied in this work to improve the performance of poly(β-hydroxybutyrate)/poly(d,l-lactic acid) (PHB/PDLLA) blends. The partial crosslinking of the blends and its effect on the properties, morphology, rheology and thermal behavior of the blends were investigated. The tensile strength and impact toughness of the PHB were increased by incorporation of the PDLLA, which were improved further after the partial crosslinking because of an increased compatibility between the PHB and the PDLLA phases. The rheological study revealed that the storage modulus (G′) and complex viscosity (η*) of the blends were increased after addition of the DCP. On the other hand, the crystallization of PHB in the blends was restricted to a certain extent by the formation of partially crosslinked network while its crystal form was not modified.