A comparative analysis of mass losses of some aliphatic polyesters upon enzymatic degradation

Results of investigation of mass losses, geometrical surface structure changes and variations in crystallinity of poly(lactic acid) (PLA), poly(?-caprolactone) (PCL) and commercially available material (PHB) consisting of poly(3,4-hydroxybutyrate) and poly(lactic acid) are presented. These structural changes occurred due to degradation of these polymers in the presence of the following enzymes: proteinase K, protease, esterase or lipase. Independently of the enzyme type, the largest mass loss was found for PLA and the smallest for PHB. Thus, under the experimental conditions, the processes of enzymatic degradation proceeded most rapidly in PLA, more slowly in PCL, and the most slowly in PHB. It was also found that proteinase K caused the largest mass losses, protease caused smaller mass losses, and both esterase and lipase produced the least mass losses, while lipase did not bring about mass loss in PHB. Images of surfaces of individual samples, obtained by scanning electron microscopy (SEM), indirectly confirmed the results of the mass loss examination. Crystallinity of the studied polyesters increased with degradation in the presence of proteinase K and protease, while changes in the crystallinity due to esterase and lipase were not observed. The presented results illustrate well the relative susceptibilities of the individual polyesters toward degradation induced by various enzymes.     

Morphological changes of annealed poly‐ε‐caprolactone by enzymatic degradation with lipase

The effects of crystallinity and temperature on enzymatic degradation of poly‐ε‐caprolactone (PCL) films and structural changes after degradation have been studied using weight loss, differential scanning calorimetry, and optical microscopy. The weight loss during the enzymatic degradation of PCL suggested that the extent of biodegradation and the rate of degradation strongly depend on the initial crystallinity. PCL films of lower crystallinity (24%) degraded much faster than films of higher crystallinity (45%). The crystallinity of low‐crystalline PCL films increased with increasing degradation time, whereas the crystallinity of high‐crystalline PCL films decreased with time. The spherulite size increased with increasing degradation time for low‐crystalline samples but decreased with time for high‐crystalline samples. These results revealed that degradation occurs first in the amorphous region where the degradation rate is much higher, and the crystalline region of the PCL film started to degrade simultaneously for those PCL with higher crystallinity. The enzymatic degradation of PCL proceeded from the free amorphous to restricted amorphous followed by lamellar edges, where PCL chains have higher mobility irrespective of hydrolysis temperature. Caproic acid was identified as the primary product formed after degradation and confirmed by proton nuclear magnetic resonance spectroscopy, suggesting that degradation occurs through the depolymerization mechanism. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 202–211, 2010     

Degradation behaviour of PCL/PEO/PCL and PCL/PEO block copolymers under controlled hydrolytic,enzymatic and composting conditions

Short-term hydrolytic and enzymatic degradation of poly(ε-caprolactone) (PCL), one series of triblock (PCL/PEO/PCL) and the other of diblock (PCL/PEO) copolymers, with a low content of hydrophilic PEO segments is presented. The effect of the introduction of PEO as the central or lateral segment in the PCL chain on copolymer hydrolysis and biodegradation properties was investigated. FTIR results revealed higher hydrolytic degradation susceptibility of diblock copolymers due to a higher hydrophilicity compared to PCL and triblock copolymers. Enzymatic degradation was tested using cell-free extracts of Pseudomonas aeruginosa PAO1, for two weeks by following the weight loss, changes in surface roughness, and changes in carbonyl and crystallinity index. The results confirmed that all samples underwent enzymatic degradation through surface erosion which was accompanied with a decrease in molecular weights. Diblock copolymers showed significantly higher weight loss and decrease in molecular weight in comparison to PCL itself and triblock copolymers. AFM analysis confirmed significant surface erosion and increase in RMS values. In addition, biodegradation of polymer films was tested in compost model system at 37 °C, where an effective degradation of block copolymers was observed.     

Binary and ternary poly(lactic acid)/poly(ϵ-caprolactone) blends: The effects of oligo-ϵ-caprolactones upon mechanical properties

Oligo-ε-caprolactones(o-CL) have been utilized as principle secondary components within poly(lactic acid)[PLA]-based blends as well as additives within larger-sized PCL/PLA blends in an effort to fully complement the mechanical attributes of the respective polyesters. Dynamic mechanical thermal analysis(DMTA) shows that the presence of o-CL plasticizes the PLA non-crystalline phase with a more pronounced effect seen as the size of o-CL is reduced. Moreover, it appears that the size of o-CL could also affect the static mechanical properties of the ternary systems as material stiffness and strength reside between those properties measured for high molecular weight PCL/PLA binary blends and the PLA homopolyester. Future work will examine the ability of these blends to sustain these properties during hydrolytic exposure.     

Selective localization of graphene oxide in electrospun polylactic acid/poly(ε-caprolactone) blended nanofibers

Despite their immiscibility, blending polylactic acid (PLA) with poly(ε-caprolactone) (PCL) provides an efficient strategy for obtaining a biopolymer blend with tailored properties due to their complementary physical properties. In this study, graphene oxide (GO) was employed as a 2-D nanofiller and nucleating agent to improve the properties of the immiscible PLA/PCL blends at 70/30, 50/50, and 30/70 wt ratios. Nanofibers of PLA/PCL blends and PLA/PCL/GO composites were investigated. It was interesting to find that the GO selectively localized in the minor phase resulting from the phase separation. The selective localization of the GO as the nucleating agent had an influence on the degree of crystallinity and crystalline morphology in the blended composites. This study also demonstrated that the molecular chains in the PLA phase oriented along the fiber axes, while in the PCL phase, the partial crystallites changed their orientation direction to be perpendicular to the fiber axes with the addition of GO.     

Structural Effects on the Biodegradation of Aliphatic Polyesters

In advance of a discussion on structural effects on biodegradation, aliphatic polyesters as biodegradable structural materials were classified into four types regarding chemical structure, that is poly(ω-hydroxy acid), poly(β-hydroxyalkanoate), poly(ω-hydroxyalkanoate) and poly(alkylene dicarboxylate), and reviewed on synthesis route, thermal and physical properties, and biodegradability. The biodegradation mechanism of these aliphatic polyesters were discussed on the major mode of hydrolysis reaction in regard whether it was enzyme-catalyzed or not, and the substrate specificities of enzymes, such as lipases or PHA depolymerases, were discussed on the hydrolysis of the aliphatic polyesters in respect of primary structure. Moreover, the biodegradation behaviors were exceedingly influenced by solid-state morphology in addition to primary structure. The rate of enzymatic degradation of polycaprolactone fibers drawn with various draw ratios was dependent on draw ratios, suggesting that crystallinity and orientation of them affected biodegradability by lipase. In the study of enzymatic degradation of films made from butylene succinate – ethylene succinate copolymer, the dependence of degradation rate on polymeric compositions was ascribed to the degree of crystallinity rather than the primary structure. These studies revealed that the degree of crystallinity was the major rate-determining factor of biodegradation of solid polymers. © 1997 John Wiley & Sons, Ltd.     

Thermal Degradation of Poly(ε-caprolactone), Poly(L-lactic acid) and their Blends with Poly(3-hydroxy-butyrate) Studied by TGA/FT-IR Spectroscopy

Summary: The thermal degradation behavior of poly(ε-caprolactone) (PCL) and poly(L-lactic acid) (PLA) have been studied in different environment. It was found that these polymers undergo completely different degradation processes in nitrogen and oxygen atmosphere. In oxygen environment PCL and PLA mainly decompose to CO₂, CO, water and short-chain acids. In nitrogen atmosphere PCL releases 5-hexenioc acid, CO₂, CO and ε-caprolactone, whereas PLA decomposes to acetaldehyde, CO₂, CO and lactide. The polymer blends of poly(3-hydroxybutyrate) (PHB) with PCL and PLA decompose similar to the individual homopolymers with crotonic acid as the initial decomposition product of PHB.     

Morphological behaviour of poly(lactic acid) during hydrolytic degradation

The hydrolytic degradation and the morphological behaviour of a packaging grade of poly(lactic acid) (PLA) were characterized by a series of techniques. During the initial degradation process (stage 1) at a temperature near the glass transition temperature (T_g), the molecular weight of PLA decreased as degradation time increased following a bulk erosion mechanism while the crystallinity increased simultaneously, but no observable weight loss occurred at stage 1. Mainly α-form PLA crystal structure was formed for the crystalline PLA with a low content of d stereo-isomers, but the material displayed a lower regularity, smaller domain size, lower melting temperatures T_m and different motional dynamics as compared to the original PLA with a similar level of crystallinity achieved by annealing. The amorphous PLA with a higher amount of d stereo-isomers also yielded the α crystalline phase as well as stereo-complex crystals at stage 1. When the molecular weight and the crystallinity reached a stable level, PLA started erosion into the degrading aqueous medium. During this stage of degradation (stage 2), the crystalline structure in PLA residues was further modified and both pH and temperature influenced the modification. The degradation at stage 2 was likely to follow a surface erosion mechanism with lactic acid as the major product of the weight loss. Besides the crystallinity effect on the degradation, temperature also played a key role in determining the rate of PLA degradation in both stages. The process was very slow at temperatures below the T_g of PLA but the rate was greatly enhanced at temperatures above the T_g.     

Abiotic degradation of poly(dl-lactide), poly(ɛ-caprolactone) and their blends

An effective hydrolytic degradation of PDLLA, PCL and their blends in a phosphate-buffered solution of pH 4.0 at 37 °C for 18 weeks was achieved, observing a considerably faster degradation of PDLLA as compared to PCL due to the hydrophobic and semicrystalline nature of PCL matrix, able to partially prevent water diffusion into the bulk specimen.DSC and FTIR results indicated that PCL phase, in compositions rich in PCL, was very stable against hydrolysis, but the presence of PDLLA in the PDLLA/PCL blends seemed to catalyze the hydrolytic degradation of the PCL phase, probably associated to easier diffusion of water into the PCL domains by the presence of PDLLA amorphous regions. This last trend was proportional to the content of PDLLA in the blends, excepting for the composition 64%PDLLA/36%PCL. It was confirmed that PCL molecules partially delayed hydrolysis of PDLLA molecules, according to FTIR analysis, and the water diffusion prevention level was proportional to the content of PCL in the blends, except for the system 64%PDLLA/36%PCL, which presented a lower extent of degradation than neat PDLLA but higher than the blend 80%PDLLA/20%PCL. This indicated that PCL molecules did not significantly impede hydrolysis of PDLLA molecules in this blend, possibly due to the achievement of a particular structure of the PDLLA/PCL interphase in this blend. In general, hydrolysis of PDLLA/PCL blends was found to be a complex phenomenon depending not only on the content of both polymer phases, but also on the polymer phase crystallinity and morphology.     

Enzymatic degradation of poly(L-lactide) and poly(epsilon-caprolactone) electrospun fibers

Poly(L-lactide) (PLLA) and poly(epsilon-caprolactone) (PCL) ultrafine fibers were prepared by electrospinning. The influence of cationic and anionic surfactants on their enzymatic degradation behavior was investigated by measuring weight loss, molecular weight, crystallinity, and melting temperature of the fibers as a function of degradation time. Under the catalysis of proteinase K, the PLLA fibers containing the anionic surfactant sodium docecyl sulfate (SDS) exhibited a faster degradation rate than those containing cationic surfactant triethylbenzylammonium chloride (TEBAC), indicating that surface electric charge on the fibers is a critical factor for an enzymatic degradation. Similarly, TEBAC-containing PCL fibers exhibited a 47% weight loss within 8.5 h whereas SDS-containing PCL fibers showed little degradation in the presence of lipase PS. By analyzing the charge status of proteinase K and lipase PS under the experimental conditions, the importance of the surface charges of the fibers and their interactions with the charges on the enzymes were revealed. Consequently, a "two-step" degradation mechanism was proposed: (1) the enzyme approaches the fiber surface; (2) the enzyme initiates hydrolysis of the polymer. By means of differential scanning calorimetry and wide-angle X-ray diffraction, the crystallinity and orientation changes in the PLLA and PCL fibers during the enzymatic degradation were investigated, respectively.     

STUDY ON THE BLENDS OF NYLON 66 AND LIQUID CRYSTALLINE POLYESTERS

Blends of polyamide (Nylon 66) with two different kinds of liquid crystalline polyesters were studied in all the composition range. Homogeneous samples were obtained by coprccipitation from 2 wt%. solution of blends. The thermal properties, crystallinity and morphology of these blends were studied by using DSC, polarizing microscopy, and scanning electron microscopy. The phase transition and morphology of the blends are markedly-influenced by the composition of liquid crystalline polyesters. The mechanical behaviour of PHB/HNA-Nylon 66 blend was improved. although polyamidc (Nylon 66)with the liquid crystalline polyesters were incompatible, but a rather strong interaction between the polymers did exist.     

Evaluation of compatibility and properties of biodegradable polyester blends

Three different biodegradable polyesters, namely, polycaprolactone (PCL), polybutylene succinate (BIONOLLE), and a copolyester of adipic acid, terephthalic acid, and 1,4‐butanediol (EASTAR) were melt‐blended using a twin‐screw extruder. The percentage composition of each of the aforementioned polymers was varied to obtain different blends, and the mechanical properties were evaluated. Selected blends showed significant improvement in tensile strength as compared with the individual polymers used to prepare the blend. The compatibility between the polymer phases was examined via Fourier transform infrared (FTIR) and nuclear magnetic resonace (NMR) spectroscopy as well as dynamic mechanical analysis. FTIR and NMR data confirmed the occurrence of hydrogen‐bonding and ester‐interchange reactions. Thermal properties and changes in crystallinity of the blends were examined with differential scanning calorimetry and X‐ray diffraction. A considerable increase in crystallinity was shown by the blend system containing BIONOLLE/PCL. The morphology of the blends was observed and correlated to the improved mechanical properties of the blend system. Results revealed an intermediate multiphase system in which a significant degree of mixing was achieved through the chemical interaction of the functional groups present, while using the twin‐screw extruder. Significant improvement in mechanical properties of some blends was observed, and information about the miscibility of these polyesters is provided. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2003–2014, 2002     

Phase separation two polymers in a blend and selective degradation as a method for porous structure formation

A method is proposed for the formation of porous structures in polymers. The method is based on the phase separation of two polymers in a blend and selective degradation of one of them. Polymers hydrolyzable at a high rate in alkaline media (polyesters of oxalic acid) are synthesized, and their hydrolytic degradation and phase separation in mixed solutions with a polylactide are investigated. Porous polylactide samples are prepared by the selective hydrolysis of polylactide-polyoxalate blends.     

Enzymatic chain scission kinetics of poly(epsilon-caprolactone) monolayers

The hydrolytic and enzymatic degradation behavior of poly(epsilon-caprolactone) (PCL) is investigated using the Langmuir monolayer technique, and an improved data acquisition and data reduction procedure is presented. Hydrolytic and enzymatic monolayer degradation experiments of PCL with various molecular weights by Pseudomonas cepacia lipase have been carried out to analyze the influence of subphase pH, subphase temperature, enzyme concentration, and the packing density of polymer chains on the degradation kinetics. The enzymatic monolayer degradation results in an exponential increase in the number of dissolved degradation fragments with increasing degradation time, which confirms random chain scission to be the dominant scission mechanism. The increase in the enzymatic scission rate constant with decreasing initial average molecular weight of the polymers is assigned to the influence of the area density of polar terminal groups on the substrate-enzyme complex formation.     

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.     

Crystallizability of Poly(ε-caprolactone) Blends with Poly(vinylphenol) under Different Conditions

The intermolecular interaction between poly（vinylphenol） （PVPh） and polycaprolactone （PCL） and the crystallization behavior of PCL in PCL/PVPh blends with different compositions and under different conditions were investigated by Fourier transform infrared spectra （FTIR） and differential scanning calorimetry （DSC）. It has been shown that the PCL in the blends with different blend ratios all exists in crystalline state after solution casting, even though the crystallinity decreases with increasing PVPh content. For the melt crystallized samples, PCL in its 80/20 PCL/PVPh sample can still crystallize. The crystallinity is, however, lower than that of the solution cast sample. For blends containing 50% or 20% PCL, the as-cast samples are semicrystalline and can change to compatible amorphous state after heat treatment process. FTIR analysis shows the existence of hydrogen bonding between PCL and PVPh and the fraction of hydrogen bonds increases remarkably after heat treatment process.     

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.     

Moisture sorption, transport, and hydrolytic degradation in polylactide

Management of moisture penetration and hydrolytic degradation of polylactide (PLA) is extremely important during the manufacturing, shipping, storage, and end-use of PLA products. Moisture transport, crystallization, and degradation, in PLA have been measured through a variety of experimental techniques including size-exclusion chromatography, differential scanning calorimetry, and X-ray diffraction. Quartz crystal microbalance and dynamic vapor sorption experiments have also been used to measure moisture sorption isotherms in PLA films with varying crystallinity. A surprising result is that, within the accuracy of the experiments, crystalline and amorphous PLA films exhibit identical sorption isotherms.     

Moisture sorption, transport, and hydrolytic degradation in polylactide

Management of moisture penetration and hydrolytic degradation of polylactide (PLA) is extremely important during the manufacturing, shipping, storage, and end-use of PLA products. Moisture transport, crystallization, and degradation, in PLA have been measured through a variety of experimental techniques including size-exclusion chromatography, differential scanning calorimetry, and X-ray diffraction. Quartz crystal microbalance and dynamic vapor sorption experiments have also been used to measure moisture sorption isotherms in PLA films with varying crystallinity. A surprising result is that, within the accuracy of the experiments, crystalline and amorphous PLA films exhibit identical sorption isotherms.     

Stereocomplex formation in AB DI-block copolymers of poly(ϵ-caprolactone) (a) and poly(lactide)(B)

The thermal properties of two series of AB di-block copolymers of poly(ϵ-caprolactone) (A) and poly(lactide) (B) and their blends were studied. Each series contained poly(lactide) blocks of opposite chirality. The length of the poly(ϵ-caprolactone) blocks was not varied (DP = 70), whereas the poly(lactide) blocks were of varying length (DP = 5 − 80). Blends of polymers containing blocks of opposite chirality were prepared by mixing in solution. The melting temperature of the PLA phase was raised by approximately 55 °C in the blends due to stereocomplex formation. The melting temperatures of the crystalline PCL and PLA phases strongly depended on the composition of the block copolymers.