The hydrolytic degradation of a segmented glycolide-trimethylene carbonate copolymer (Maxon™)

The hydrolytic degradation of a segmented copolymer constituted by glycolide and trimethylene carbonate units (Maxon™) has been investigated by means of ¹H NMR spectroscopy and calorimetry. Degradation experiments have been performed either under a pH 7.4 buffered solution or in distilled water at temperatures of 37 and 50 °C. At both temperatures, the degradation proceeds through two main stages where an increase in the degree of crystallinity occurs in the first place. NMR peaks associated with glycolyl end units and appearing progressively can be well distinguished. Their chemical shifts coincide with those observed in the hydrolytic degradation of polyglycolide threads, which was also assayed for comparative purposes. Glycolide rings are also produced in a significant amount during the bulk degradation of Maxon. They are partially retained in the polymer, which may be a consequence of an intramolecular cyclization of the terminal hydroxyl groups.Thermal degradation in the presence of a transesterification catalyst has also been investigated and the changes in the polymer microstructure evaluated through the NMR analyses of signals were sensitive to the chemical sequence.     

Influence of degradation on the crystallization behaviour of a biodegradable segmented copolymer constituted by glycolide and trimethylene carbonate units

The influence of degradation on non-isothermal crystallization from the melt of a segmented copolymer constituted of glycolide and trimethylene carbonate units and used as a bioabsorbable surgical suture was studied by optical microscopy, differential scanning calorimetry and time-resolved X-ray diffraction. Fibrillar positive spherulites were obtained with slightly degraded samples but new axialitic morphologies were detected when samples had a molecular weight, M_w, lower than 29,000 g/mol and the crystallization started at a high temperature.Crystal growth kinetics of samples degraded under different conditions was evaluated over a wide temperature range by a non-isothermal method. Two crystallization regimes (I and II) were determined for the more degraded samples (i.e., those able to crystallize according to axialitic and spherulitic morphologies), whereas only regime II was found for samples of higher molecular weights. Primary nucleation density decreased with the extent of degradation provided no morphological changes occurred, and so did the regularity of lamellar stacking, as shown by synchrotron measurements, although the morphological parameters remained practically constant.     

Studies on hydrolytic degradation of poly(ester-anhydride)s based on oligosuccinate and aliphatic diacids

This paper describes synthesis, characteristics and hydrolytic degradation of functional poly(ester-anhydride)s based on oligo(3-allyloxy-1,2-propylene succinate) (OSAGE) and aliphatic diacids (DA). The polymers were obtained by polycondensation of OSAGE with adipic (ADP), sebacic (SBA) or dodecanedicarboxylic acid (DDC). The carboxyl groups in OSAGE and in diacids were converted to mixed anhydride groups by acetylation with acetic anhydride. After that, prepolymers thus obtained were condensed in vacuum to yield poly(ester-anhydride)s. The structure of copolymers was confirmed by NMR spectroscopy. Influence of the kind of diacid and the OSAGE to diacid ratio on selected properties of poly(ester-anhydride)s were examined. Poly(ester-anhydride)s were subjected to hydrolytic degradation at 37 °C, in aqueous phosphate buffer solution of pH 7.41 (PBS). The course of degradation was monitored by determination of weight loss of samples, ¹H NMR and DSC. Fracture surfaces of samples during degradation were examined by scanning electron microscopy.     

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.     

Preparation, characterization and hydrolytic degradation of poly[p-dioxanone-(butylene succinate)] multiblockcopolymer

A series of poly[p-dioxanone-(butylene succinate)] (PPDOBS) copolymers were prepared from p-dioxanone (PDO), 1,4-butanediol and succinate acids through a two-step process including the initial prepolymer preparation of poly(p-dioxanone)diol (PPDO-OH) and poly(butylene succinate)diol (PBS-OH) and the following copolymerization of the two kinds of prepolymers by coupling with hexamethylene diisocyanate (HDI). The molecular structures of the prepared PPDO-OH, PBS-OH and PPDOBS were characterized by hydrogen nuclear magnetic resonance spectroscopy (¹H NMR). The crystallization of the copolymers was investigated by using differential scanning calorimetry (DSC), polarized optical microscopy (POM) and wide angle X-ray diffraction (WAXD). It has been shown that the crystallization rate and the degree of crystallization increases with the increase of the weight fraction of poly(butylene succinate) (PBS) blocks in the copolymers. In phosphate buffer solution with pH 7.4 at 37 °C for 18 weeks, the hydrolytic degradation behaviors of the copolymers were studied. The changes of retention weight, water absorption, pH value, and surface morphologies with the degradation time showed that the hydrolytic degradation rate of PPDOBS could be controlled by adjusting the weight fraction of poly(p-dioxanone) (PPDO) and PBS blocks in the copolymers. The changes of the thermal properties of PPDOBS during the degradation were also investigated by DSC.     

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.     

Synthesis and hydrolytic degradation of poly(ethylene succinate) and poly(ethylene terephthalate) copolymers

The conditions of synthesis of statistical poly(ethylene succinate-co-terephthalate) copolymers (2GTS) and high molecular weight poly(ethylene succinate) (PES) with good hydrolytic and optical parameters, designed for the production of biodegradable products and resins, are presented in this article. Copolymers were prepared by melt polycondensation of bis-(β-hydroxyethylene terephthalate) (BHET) and succinic acid (SA) with excess of ethylene glycol (2G) in the presence of a novel titanium/silicate catalyst (C-94) and catalytic grade of germanium dioxide (GeO₂) as cocatalyst. The chemical structure and physical properties of those materials were characterized by ¹H NMR, FT-IR, dynamical-mechanical thermal analyses (DMTA), differential scanning calorimetry (DSC), solution viscosity and spectroscopic methods. The hydrolytic degradation was performed in a water solution with variable pH, also in garden soil and in compost. The highest hydrolytic degradation rate was observed for pH 4 and for compost. Better hydrolytic degradation values in compost medium were observed for copolyester prepared in the presence of GeO₂ as polycondensation cocatalyst. The copolyester with 40 mol% of aliphatic units was chosen for industrial syntheses which were performed in ELANA and subsequently the processing parameters and compatibility with potato starch of this polyester were checked by BIOP Biopolymer Technologies AG.     

In vitro and in vivo degradation behaviors of thermosensitive poly(organophosphazene) hydrogels

Biodegradable and thermosensitive poly(organophosphazenes) with various substituents were synthesized and their hydrolytic degradation properties were investigated in vitro and in vivo. The aqueous solutions of all polymers showed a sol-gel phase transition behavior depending on temperature changes. The side groups of polymers significantly affected the polymer degradation and accelerated hydrolysis of polymers in the order of carboxylic acid > depsipeptide > without carboxylic acid and depsipeptide. The increased gel strength led to the decreased hydrolysis rate. The polymer hydrogels with 750 Da of α-amino-ω-methoxy poly(ethylene glycol) were rapidly decreased by dissolution. The polymer degradation was also influenced by pH and temperature. The in vivo behaviors of mass decrease of the polymer hydrogels were similar with the in vitro results. These results suggest that the biodegradable and thermosensitive poly(organophosphazenes) hold great potentials as an injectable and biodegradable hydrogel for biomedical applications with controllable degradation rate.     

Thermal degradation of poly(p-phenylene-graft-?-caprolactone) copolymer

The thermal degradation of poly(p-phenylene-graft-?-caprolactone) (PPP), synthesized by Suzuki polycondensation of poly(?-caprolactone) (PCL) with a central 2,5-dibromo-1,4-benzene on the chain with 1,4-phenylene-diboronic acid, has been studied via direct pyrolysis mass spectrometry. The thermal degradation occurred mainly in two steps. In the first step, decomposition of PCL chains occurred. A slight increase in thermal stability of PCL chains was noted. In the second stage of pyrolysis, the decomposition of the polyphenylene backbone takes place. The evolution of CL monomer or small CL segments left on the phenyl ring continued also in the temperature region where degradation of PPP backbone started.     

The influence of soft segment length on the properties of poly(butylene terephthalate-co-succinate)-b-poly(ethylene glycol) segmented random copolymers

Three series of poly(butylene terephthalate-co-succinate)-b-poly(ethylene glycol) segmented random copolymers with starting PEG number-average molecular weight (M_n(PEG)) at 600, 1000 and 2000, respectively, as well as hard segment poly(butylene succinate) (PBS) molar fraction (M_PBS) increasing from 10% to 30% were synthesized through a transesterification/polycondensation process and characterized by means of GPC, NMR, DSC, WAXD and mechanical testing etc. The investigations were mainly focused on the influence of M_n(PEG) on the properties of resulting copolymers bearing two sorts of hard segments. It is revealed that all the samples show a relatively symmetrical GPC curves with the number-average molecular weight more than 4 × 10⁴, while the polydispersity decreases from 1.9 to 1.4 as the increasing M_n(PEG) because of the prolonged time for polycondensation and the faster exclusion of small molecules by-product with the decreased molten viscosity. The sequence distribution analysis shows that the average sequence length of hard segment PBT decreases while that of PBS increases with the increasing M_PBS and are independent of the soft segment length. The approximate unit degree of randomness as well as the soft segment length turns out that the segments take a statistically random distribution along the backbone. Micro-phase separation structure is verified for the appearance of two glass transition temperatures and two melting points, respectively, in DSC thermograms of most samples. The depression of melting points and the reduction of crystallinity of hard segments with increasing M_PBS are related to the crystal lattice transition from α-PBT to PBS and discussed in the viewpoint of cohensive energy. Mechanical testing results demonstrate that the increase of amorphous domains the increase of M_PBS as well as M_n(PEG) will provide high elongation and good flexibility of copolymer chain. The in vitro degradation experiments show that the partial substitution of aromatic segment PBT with aliphatic PBS will substantially accelerate the degradation rate with enhanced safety of degradation by-products and while changing M_n(PEG) broaden the spectrum to tailor the properties.     

Preparation of block copolymer of poly(trimethylene carbonate) with oligo(ethylene glycol) and the surface properties of the dip coated film

To persuade the stent coating materials for the better sustainable development, a block copolymer composed of hydrophobicity of poly(trimethylene carbonate) (PTMC) and hydrophilicity of PTMC bearing ethylene glycol chains was synthesized as a potential candidate. The result of thermal stability of the polymer was analyzed which reached up to 206 °C (T₁₀) and it is considered that sufficient for sterilization during the treatment. Moreover, the dip coated films of polymer were coated on polyethylene (PE) and stainless steel (SS) substrates in order to stimulate the stability upon the physiological environment. In addition, the preliminary in vitro test of the films were evaluated by protein adsorption and blood platelet adhesion tests. Hence, this study tends to convince that the synthetic block copolymer based on PTMC derivatives were approached for stent coating materials.     

Influence of chain microstructure on the hydrolytic degradation of copolymers from 1,3‐trimethylene carbonate and L‐lactide

Ring‐opening copolymerization of L ‐lactide (LLA) and 1,3‐trimethylene carbonate (TMC) blends with LLA/TMC feed ratios from 90/10 to 50/50 was realized at 110 or at 180 °C for various time periods, using low toxic zirconium (IV) acetylacetonate (Zr(Acac)₄) as initiator. The resulting copolymers exhibit different chain microstructures. Copolymers obtained at 110 °C exhibit a gradient chain structure with the presence of lactidyl sequences next to very short ones, and are semicrystalline. In contrast, copolymers obtained at 180 °C are amorphous because of a more random chain microstructure with the presence of larger amounts of medium sequences. Degradation of the copolymers was carried out in pH 7.4 phosphate buffer at 37 °C. Analytical techniques such as ¹H NMR, DSC, GPC, and XRD were used to monitor the degradation. Initially amorphous copolymers can remain amorphous during degradation because of the highly random unit's distribution, and equivalent LLA and TMC contents. However, initially amorphous copolymers containing larger amounts of lactidyl units are able to crystallize during degradation because of the presence of relatively long LLA blocks. Insofar, as initially semicrystalline copolymers are concerned, degradation occurs preferentially in the amorphous zones. Therefore, various degradation behaviors and degradation rates can be obtained by varying the chemical composition, chain microstructure, and morphology of PLLA‐PTMC copolymers. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3869–3879, 2009     

Influence of cellulose nanowhiskers on the hydrolytic degradation behavior of poly(d,l-lactide)

This paper reports the preparation of bionanocomposites based on poly(d,l-lactide) and cellulose nanowhiskers (PDLLA/CNWs) and studies the influence of the CNWs on the hydrolytic degradation behavior of the polylactide. The hydrolytic degradation process was studied in a phosphate buffer medium through the sample weight loss and also by FTIR, DSC and TGA measurements. The presence of CNWs induced a strong delay in the hydrolytic degradation of the PDLLA, even when the concentration of the nanofillers was only 1%. This effect was related to the physical barrier created by the highly crystalline CNWs that inhibited water absorption and hence retarded the hydrolytic degradation of the bionanocomposites. In addition, the incorporation of cellulose nanocrystals in the PDLLA also made the biopolymer more thermally stable, increasing the initial temperature of mass loss even after the degradation in phosphate medium. The results presented here show the possibility of controlling the biodegradability and prolonging the service life of a polylactide through the incorporation of a small quantity of nanofillers obtained from renewable materials.     

Thermal degradation behaviour of aromatic poly(ester-amide) with pendant phosphorus groups investigated by pyrolysis-GC/MS

The thermal stability of a novel phosphorus-containing aromatic poly(ester-amide) ODOP-PEA was investigated by thermogravimetric analysis (TGA). The weight of ODOP-PEA fell slightly at the temperature range of 300-400 °C in the TGA analysis, and the major weight loss occurred at 500 °C. The structural identification of the volatile products resulted from the ODOP-PEA pyrolysis at different temperatures was performed by pyrolysis-gas chromatography/mass spectrometry (pyrolysis-GC/MS). The P-C bond linked between the pendant DOPO group and the polymer chain disconnected first at approximately 275 °C, indicating that it is the weakest bond in the ODOP-PEA. The P-O bond in the pendant DOPO group was stable up to 300 °C. The cleavage of the ester linkage within the polymer main chain initiated at 400 °C, and the amide bond scission occurred at greater than 400 °C. The structures of the decomposition products were used to propose the degradation processes happening during the pyrolysis of the polymer.     

Effects of molecular weight and small amounts of d-lactide units on hydrolytic degradation of poly(l-lactic acid)s

Films of poly(l-lactic acid) (PLLA) with different number-average molecular weights (M_n) and d-lactide unit contents (X_d) were made amorphous and the effects of molecular weight and small amounts of d-lactide units on the hydrolytic degradation behavior in phosphate-buffered solution at 37 °C of PLLA were investigated. The degraded films were investigated using gravimetry, gel permeation chromatography, polarimetry, differential scanning calorimetry, X-ray diffractometry, and tensile testing. To exclude the effects of crystallinity on the hydrolytic degradation, the films were made amorphous by melt-quenching. The incorporation of small amounts of d-lactide units drastically enhanced the hydrolytic degradation of PLLA. In the period of 0-32 weeks, the hydrolytic degradation rate constant (k) of PLLA films increased with increasing X_d, while the k values did not depend on M_n. This means that the effects of X_d on the hydrolytic degradation rate of the films are higher than those of M_n. In contrast, in the period of 32-60 weeks neither X_d nor M_n was a crucial parameter to determine k values, probably because in addition to these parameters the differences in the amount of catalytic oligomers accumulated in films and crystallinity affect the hydrolytic degradation behavior of the films. The initially amorphous PLLA films remained amorphous even after the hydrolytic degradation for 60 weeks.     

In situ prepared PBSu/SiO2 nanocomposites. Study of thermal degradation mechanism

A series of nanocomposites consisted of poly(butylene succinate) (PBSu) and fumed silica nanoparticles (SiO₂) were prepared using the in situ polymerization technique. The amount of SiO₂ used directly affected the final molecular weight of the prepared polyesters. At a low SiO₂ content (0.5 wt.%) the molecular weight obtained was higher compared to neat PBSu, however at higher concentrations this was gradually reduced. The melting point of the matrix remained unaffected by the addition of the nanoparticles, in contrast to the crystallinity, which was dramatically reduced at higher SiO₂ contents. This was mainly due to the extended branching and cross-linking reactions that took place between the carboxylic end groups of PBSu and the surface silanols of the nanoparticles. Thermal degradation of the PBSu/SiO₂ nanocomposites was studied by determining theirs mass loss during heating. From the variations of the activation energies, calculated from the thermogravimetric curves, it was clear that nanocomposites containing 1 wt.% SiO₂ content had a higher activation energy compared to pure PBSu, indicating that the addition of the nanoparticles could slightly increase the thermal stability of the matrix. However, in PBSu/SiO₂ nanocomposite containing 5 wt.% SiO₂ the activation energy was smaller. This phenomenon should be attributed to the existence of extended branched and cross-linked macromolecules, which reduce the thermal stability of PBSu, rather than to the addition of fumed silica nanoparticles.     

Racemization on thermal degradation of poly( -lactide) with calcium salt end structure

Poly( -lactide) with calcium salt end structure (PLLA-Ca) is a promising material for PLLA recycling because of the ease of lactide recovery through the unzipping depolymerization process. However, the pyrolysis of PLLA-Ca also causes meso-lactide to form. In this article, the racemization in PLLA-Ca pyrolysis was analyzed in detail with Py-MS, Py-GC/MS, and a glass tube oven. The results suggested that at a temperature lower than 250 °C, nucleophilic attack by a carboxylate anion end on an asymmetrical methyne carbon in a penultimate lactate unit occurred, resulting in the predominant formation of meso-lactide. On the other hand, also at temperatures over 320 °C, by-reactions, such as enolization reactions, caused the meso-lactide to form, but not dominantly. In the temperature range of 250–320 °C, -lactide was produced exclusively, because unzipping depolymerization proceeded as the main reaction. This is a very significant result for PLLA recycling, because PLLA-Ca is an easily recyclable material, which depolymerizes based on the 1st-order weight loss process.     

Hydrolytic aging of crystallizable shape memory poly(ester urethane): Effects on the thermo-mechanical properties and visco-elastic modeling

The shape memory functionality of a segmented poly(ester urethane) and its hydrolytically aged specimens has been studied by cyclic thermo-mechanical measurements with an imposed strain of 100%. The shape memory effect was triggered by a melting transition in the soft segment phase. Aging was enforced by immersion in hot de-ionized water. In the course of the immersion the tensile properties (secant moduli, stress and strain at yield and break) were impaired by hydrolysis. Advanced specimen embrittlement finally led to rupture during the first thermo-mechanical cycle. This happened after 68 days of aging at 55 °C and correspondingly after 8 days at 80 °C. The residual strain after the first cycle, which was about 25%, increased significantly with aging time. Therefore, the total strain recoverability became ever smaller: aged specimens needed conditioning by at least two cycles for a full development of shape recoverability. Likewise the recovery force decreased continuously. Despite these degradation effects, it was observed that the shape fixity and the cycle-related shape recovery of appropriately conditioned specimens (number of cycles N > 2) remained on a constant high level (at round 100% and between 90% and 100%, respectively) throughout the whole aging period. These observations are discussed within the framework of a simplified model of the behavior of crystallizable shape memory polymers. The amorphous state of the polymer is described by the equation of the linear visco-elastic solid. As for the semi-crystalline state the material is assumed to react elastically with respect to deviations from the configuration, which was frozen up under constraint conditions. The curves of the dependence of the material behavior on aging time at 55 °C match perfectly those at 80 °C when the time axis is adjusted by a factor of 8.5, from which the apparent activation energy for hydrolytic aging in the amorphous state of 82 kJ mol⁻¹ could be deduced.     

Thermal degradation during melt processing of poly(ethylene oxide), poly(vinylidenefluoride-co-hexafluoropropylene) and their blends in the presence of additives, for conducting applications

In this paper, the thermal degradation during compounding of poly(ethylene oxide) and poly(vinylidenefluoride-co-hexafluoropropylene) and their immiscible blends was investigated. Infrared spectroscopy, size exclusion chromatography, dynamic rheology, thermogravimetric analysis, and coupled gas chromatography-mass spectroscopy were used. Most of the identified degradation products or substances are glycols, formates, lactones, ethers or even acids for long processing times. The mechanisms for the production of these chemical compounds were already reported in previous publications. Additionally, the influence of carbon black and LiV₃O₈ additives, currently used for enhancing the conductive properties, was studied. Addition of carbon black causes an increase in the PEO stability, whereas addition of LiV₃O₈ has an opposite and dramatic effect. PVDF-HFP is found to be less susceptible to degradation because of the inherent stability of fluorine-containing polymers. However, melt processing provides additional contributions to degradation because of the high viscosities.     

The utilization of TG/GC/MS in the establishment of the mechanism of poly(styrene) degradation

General purpose poly(styrene) is a large volume commodity polymer used in a variety of applications. It is widely used in food packaging, particularly for baked goods. In this application, the presence of styrene monomer, which has a distinctive taste and aroma, cannot be tolerated. Processing of the polymer and forming of the food container at an unacceptably high temperature leads to the formation of styrene monomer and finished articles with unacceptable aroma characteristics. An examination of the thermal degradation of poly(styrene) has revealed the origin of monomer formation. The thermal decomposition of poly(styrene) has been widely studied. However, most studies have been carried out at high temperature (>300°C) where many processes are occurring simultaneously. Degradation at lower temperature, 280°C, occurs in two well-defined steps. The first is thermolysis of a head-to-head bond present in the mainchain as a consequence of polymerization termination by radical coupling. This generates macroradicals which smoothly depolymerize to expel styrene monomer. The nature of the degradation is readily apparent from kinetic analysis of the isothermal thermogravimetry (TG) data and the identity of the single volatile product may be readily established by gas chromatography/mass spectrometry (GC/MS) analysis of the effluent from the TG analysis.