Nanosize and microsize clay effects on the kinetics of the thermal degradation of polylactides

Polylactide (PLA)-montmorillonite (MMT) micro- and nanocomposites based on semicrystalline and amorphous polymers and unmodified or organomodified clays at 5 wt% content were produced by melt mixing. Based on the three different test methods that were used to follow thermal degradation, different conclusions were obtained. During melt processing, thermomechanical degradation was more pronounced in the presence of all fillers, which apparently acted catalytically, but to different degrees. During isothermal degradation in air from 180 °C to 200 °C, degradation rate constants were calculated from novel equations incorporating changes in intrinsic viscosity (IV). Results show that the thermal degradation rate constants of the amorphous PLA and its composites are lower than those of the semicrystalline PLA and its composites. Due to better filler dispersion in the polymer matrix, the thermal degradation rate constants of the nanocomposites are significantly lower than those of the unfilled polymers and their microcomposites under air. As per dynamic TGA data and thermal kinetic analysis from weight losses and activation energy calculations, organomodified nanofillers have a complex effect on the polymer thermal stability; the unmodified fillers, however, reduce polymer thermal stability. These TGA data and kinetic analysis results also support the findings that the thermal stability of the amorphous PLA and its composites is higher than that of the semicrystalline polymer and its composites and the thermal stability of the nanocomposites is higher than that of the microcomposites. In general, mathematical modeling based on random thermal scission equations was satisfactory for fitting the TGA experimental data.     

A study on the degradation of polylactic acid in the presence of phosphonium ionic liquids

In this overview study, two ionic liquids (IL) with different anions (decanoate, tetrafluoroborate) but with the same phosphonium-based cation that showed promising plasticizing/lubricating behavior in polylactic acid (PLA) were screened for their effects on the polymer degradation under thermomechanical, thermo-oxidative (at 160 °C), hydrolytic (100% humidity, 60 °C), conditions, and during soil immersion. Depending on the particular medium and conditions used, degradation was followed by changes in molecular weight, melt viscosity, sample weight and appearance, morphology, crystallinity, acid number, and pH. The effects of the IL containing a decanoate anion were more pronounced on lubrication and also on degradation as evidenced by reduced melt viscosities and accelerated thermomechanical, isothermal, hydrolytic, and soil degradation. The IL containing the tetrafluoroborate anion showed higher thermal stability compared with the IL containing decanoate anion as also confirmed from thermal degradation rate constants which were calculated from random chain scission statistics. Accelerated hydrolytic degradation was observed in PLA containing the tetrafluoroborate based IL but to a lesser extent than the decanoate based IL. The catalytic role of the decanoate anion in hydrolytic degradation was confirmed through experiments with model compounds. X-ray diffraction (XRD) data on the materials exposed to soil degradation provided evidence that the initially amorphous polymer attained a certain degree of crystallinity as a result of the significant MW reduction.     

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

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

TGA/FTIR studies of segmented aliphatic polyurethanes and their nanocomposites prepared with commercial montmorillonites

Nanocomposites prepared with segmented polyurethane (SPU) and commercially available nanoclays (Cloisite™ Na⁺, Cloisite™ 15A, Cloisite™ 30B) were studied using thermogravimetric analysis coupled with Fourier Transform Infrared Spectroscopy (TGA/FTIR). The results showed that the thermal degradation of unfilled SPU and the 4, 6 and 10 wt% hand mixed nanocomposites occurred in two stages being the first due to degradation of hard segments and the second due to the degradation of soft segments. It was also found that the thermal stability of these nanocomposites was not improved by increasing nanoclay concentration except for SPU/Cloisite™ 15A nanocomposites were a 40 °C increase was observed. In a similar manner, FTIR spectra of the evolved gases obtained after the thermal degradation of these nanocomposites were qualitatively similar to the unfilled polymer except in those containing Cloisite™ 30B where isocyanate absorptions were detected. In contrast, SPU/Cloisite™ 30B nanocomposites prepared by in-situ polymerization, exhibited higher thermal stability than the corresponding hand mixed nanocomposites. In addition, these nanocomposites exhibited the presence of carbon dioxide in the evolved gases during its second degradation stage which was not observed in the hand mixed nanocomposites. In this case, it can be said that the presence of clays in the nanocomposites has a significant effect on the thermal degradation pathways.     

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.     

The amount of immobilized polymer in PMMA SiO2 nanocomposites determined from calorimetric data

For semicrystalline polymers there is an ongoing debate at what temperature the immobilized or rigid amorphous fraction (RAF) devitrifies (relaxes). The question if the polymer crystals are melting first and simultaneously the RAF devitrifies or the RAF devitrifies first and later on the crystals melt cannot be answered easily on the example of semicrystalline polymers. This is because the crystals, which are the reason for the immobilization of the polymer, often disappear (melt) in the same temperature range as the RAF. For polymer nanocomposites the situation is simpler. Silica nanoparticles do not melt or undergo other phase transitions altering the polymer-nanoparticle interaction in the temperature range where the polymer is thermally stable (does not degrade). The existence of an immobilized fraction in PMMA SiO₂ nanocomposites was shown on the basis of heat capacity measurements at the glass transition of the polymer. The results were verified by enthalpy relaxation experiments below the glass transition. The immobilized layer is about 2 nm thick at low filler content if agglomeration is not dominant. The thickness of the layer is similar to that found in semicrystalline polymers and independent from the shape of the nanoparticles. Nanocomposites therefore offer a unique opportunity to study the devitrification of the immobilized fraction (RAF) without interference of melting of crystals as in semicrystalline polymers. It was found that the interaction between the SiO₂ nanoparticles and the PMMA is so strong that no devitrification occurs before degradation of the polymer. No gradual increase of heat capacity or a broadening of the glass transition was found. The cooperatively rearranging regions (CRR) are either immobilized or mobile. No intermediate states are found. The results obtained for the polymer nanocomposites support the view that the reason for the restricted mobility must disappear before the RAF can devitrify. For semicrystalline polymers this means that rigid crystals must melt before the RAF can relax.     

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.     

Preparation, characterization and biodegradation of biopolymer nanocomposites based on fumed silica

PLA and PCL based nanocomposites prepared by adding three different types of fumed silica were obtained by melt blending. Materials were characterized by means of Scanning Electron Microscopy (SEM), Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA) and Dynamic–Mechanical Thermal Analysis (DMTA).A good distribution of the fumed silica into both polymer matrices was observed. The highest thermo-mechanical improvements were reached by addition of the fumed silica with higher surface area. PLA and its nanocomposites were degraded in compost at 58 °C; at this temperature all samples presented a significant level of polymer degradation, but a certain protection action of silica towards PLA degradation was observed, whereas the addition of fumed silica did not show considerable influence on the degradation trend of PCL. These dissimilarities were attributed to the different degradation mechanism of the two polymers.     

Nanocomposite of montmorillonite with telechelic sulfonated poly(butylene terephthalate): Effect of ionic groups on clay dispersion, mechanical and thermal properties

Telechelic ionomeric poly(butylene terephthalate) nanocomposites with organically modified clays have been prepared by the melt intercalation technique both in Brabender mixer and in twin screw-extruder. The presence of ionic groups tethered at the end of the polymer chains permits electrostatic interaction between the polymer and the surface of an organically modified clays providing a thermodynamic driving force for the dispersion of the clay platelets in the polymer matrix. The improved dispersion has been verified by TEM and XRD analyses. Nanocomposites with telechelic polymers present therefore consistently higher thermo-mechanical properties and improved thermal and hydrolytic stability respect to nanocomposites with standard PBT. Nanocomposite obtained using PBT with 3% telechelic ionic groups and with 5% of clay present a heat deflection temperature that is 48 °C higher compared to that of the commercial material. The presence of the clay also slightly increases the thermal and hydrolytic stability respect to standard PBT.     

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.     

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, characterization and in vitro degradation study of a novel and rapidly degradable elastomer

Biodegradable elastomers represent a useful class of biomaterials. In this paper, a novel biodegradable elastomer, poly(PEG-co-CA) (PEC), was synthesized by condensation of poly(ethylene glycol) (PEG) and citric acid (CA) under atmospheric pressure without any catalyst. We first synthesized a pre-polymer by carrying out a controlled condensation reaction between PEG and citric acid, and then post-polymerised and simultaneously cross-linked the pre-polymer in the mould at 120 °C. The pre-polymer was characterized by FT-IR, ¹H NMR, ¹³C NMR, GPC and DSC. A series of polymers were prepared at different post-polymerisation time and different monomer ratios. Measurements on the mechanical properties of PEC testified that the new polymers are elastomers with low hardness and big elongation, and hydrolytic degradation of the polymer films in a buffer of pH 7.4 at 37 °C showed that PEC had excellent degradability (all the films show the weight losses more than 60% after 96 h incubation). The different post-polymerisation time and monomer ratio had strong influence on the degradation rates and mechanical performances. The material is expected to be useful for controlled drug delivery and other biomedical applications.     

Thermo-oxidative stability of polypropylene/layered silicate nanocomposites

Several series of experiments were carried out to check the effect of components on the stability of PP/layered silicate nanocomposites. The amount of organophilic montmorillonite (OMMT) changed between 0 and 6, while that of maleated polypropylene (MAPP) between 0 and 50 vol%. The composites were prepared in an internal mixer at 190 °C. Mixing speed and time were changed to study the effect of processing conditions on stability. The structure of the samples was characterised by various methods, while stability by the induction time of oxidation (OIT), the onset temperature of degradation (OOT) and by colour. Contrary to numerous claims published in the literature, which indicate the positive effect of layered silicates on the stability of polymer nanocomposites, our results clearly proved that both OMMT and MAPP accelerate degradation during processing and deteriorate the properties of PP composites. Residual stability decreases drastically with increasing amounts of both components, chain scission leads to the decrease of viscosity and to inferior strength and deformability. In spite of expectations, the effect of the components is independent of each other. Discoloration is caused mainly by the inherent colour of the filler and it decreases with increasing exfoliation. The most probable reason for decreased stability is the reaction of the components with the stabilisers, but this explanation needs further verification. Processing conditions influence degradation considerably, increasing shear rate and longer residence times lead to more pronounced degradation. The basic stabilization of commercial grade polypropylenes is insufficient to protect the polymer against degradation and without additional stabilization processing under normal conditions results in products with inferior quality.     

TGA/FTIR study on thermal degradation of polymethacrylates containing carboxylic groups

In this work, the thermal degradation of polymethacrylates containing carboxylic groups namely poly(methacryloyloxy butanoic acid), PMBA; poly(methacryloyloxy hexanoic acid), PMHA; and poly(p-methacryloyloxy benzoic acid), PMBeA was investigated by TGA/FTIR. Moreover, in order to shed more light on the reaction pathways during the thermal decomposition of these polymers, an FTIR spectroscopic study of structural changes in the degrading material was performed. By TGA it was observed that PMBA exhibited two well-defined degradation stages at 327 and 450 °C; PMHA presents only one main weight loss at ca. 402 °C although from DTG curve it was noted that the single step degradation was composed by two overlapped peaks located at 414 and 449 °C and a small shoulder at 317 °C; finally PMBeA showed three weight loss regions at 265, 353 and 468 °C. From FTIR analysis of the partially degraded samples it was found that the thermal degradation of these polymers resembled that of polymethacrylic acid, i.e. anhydrides were initially formed and then the modified structure is broken to yield an aromatic structure with phenolic groups. In contrast, the analysis by FTIR of the volatile products from the studied polymers differs notably than those obtained for polymethacrylic acid: β-lactones and γ-lactones were released from PMBA and PMHA, respectively, during its thermal degradation, whereas an ester derivative from benzoic acid evolves from PMBeA probably through depolymerization.     

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.     

Hydrolytic degradation of carbohydrate-based polyamides of the AABB type derived from l-arabinose and d-xylose

The hydrolytic degradation of a series of aregic carbohydrate-based polyamides derived from l-arabinose and d-xylose is described. These polyamides are those that are fully sugar-based (PA-SuSu), those derived from aldaric acids and polyalkylene diamines (PA-mSu), and those derived from diamine sugars and polyalkylene dicarboxylic acids (PA-Sun). Their physical properties and crystal structures depend on their constitution and the configuration of the carbohydrate-based moiety. The feasibility of the hydrolysis of these polyamides was, in general, related with such structural properties. Thus, the fully sugar-based PA-SuSu were amorphous, water-soluble materials, and were hydrolysed in water at 70 °C. PA-mSu were crystalline and more resistant to hydrolysis — they were degraded at pH 2 and 70 °C [T_g(s) 60-90 °C]. PA-Sun were amorphous and highly hygroscopic materials — they were hydrolysed in water at 37 °C [T_g(s) 25-40 °C].     

Hydrolytic degradation of carbohydrate-based aromatic homo- and co-polyesters analogous to PET and PEI

The hydrolytic degradation of a series of homo- and co-polyesters analogous to poly(ethylene terephthalate) (PET) and poly(ethylene isophthalate) (PEI), prepared from carbohydrate-based monomers, was studied. The degradation process was carried out at temperatures of approximately 10 °C above the T_g of the polymers. All the studied polyesters were found to degrade at significant rates, and degradability showed a clear dependence on the configuration of the sugar units present in the polymer chain. No weight loss was detected upon degradation, apparently due to the non-solubility of the degraded products in the aqueous incubation medium. Hydrolysis of co-polyesters took place preferentially by cleavage of the ester groups of the sugar units.     

Effects of synthetic and natural zeolites on morphology and thermal degradation of poly(lactic acid) composites

Poly(lactic acid) (PLA) composites containing 5 wt% synthetic (type 4A) and natural (chabazite) zeolites were prepared using extrusion/injection molding. Morphological, structural, and thermal properties of composites were investigated by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC). DSC results revealed that the glass transition and melting temperatures were not significantly changed; however, the incorporation of both type 4A and chabazite zeolites enhanced the nucleation of PLA crystallites as well as increased the percent crystallinity. Thermal degradation properties of PLA and PLA/zeolite composites were studied by non-isothermal thermogravimetric analysis (TGA) in nitrogen atmosphere. TGA results showed that at temperatures above 300 °C, PLA/type 4A synthetic zeolite composites were thermally decomposed more easily than the PLA and PLA/chabazite natural zeolite composites. The apparent activation energies of thermal degradation of PLA and PLA/zeolites composites estimated using both the Flynn-Wall-Ozawa and Kissinger methods followed the same order: PLA/type 4A < PLA/chabazite < PLA.     

Understanding the role of nanosilica particle surfaces in the thermal degradation of nanosilica-poly(methyl methacrylate) solution-blended nanocomposites: From low to high silica concentration

Nanocomposites of poly(methyl methacrylate) and 12 nm silica particles have been prepared by a casting procedure which allows the homogeneous dispersion of up to 35 wt% of silica. Twelve nanocomposites with compositions ranging from 1 to 35 wt% have been prepared and studied by Scanning Electron Microscopy, Fourier Transform Infrared Spectroscopy, Differential Scanning Calorimetry, Thermogravimetric Analysis and Isothermal Chemiluminescence. Thermal stability increases outstandingly as soon as 1 wt% of silica is added to PMMA. This effect is well-known but had not been explained up to now. Of the two main processes which initiate degradation in PMMA, radical formation at labile chain ends and random chain scission, it is the former which disappears in composites with 3 wt% of silica. The origin of the thermal stabilization in these polymer composites is thus the blocking of the PMMA chain end by the silica particle. At the same time, viscous flow decreases progressively, and composites with 20 wt% of silica or over are dimensionally stable even if heated at 300 °C for several hours. The concomitant decrease of viscous flow and increase of low temperature thermal stability shift the temperature range of application of these composites strongly, and the understanding of the silica surface role allows envisaging the control of the behaviour.     

Preparation of novel cross-linked fluoroalkyl end-capped adamantane cooligomer/copper nanocomposites

Copper(II) oxide reacted with hydrazine in the presence of fluoroalkyl end-capped oxime-blocked isocyanato cooligomer containing adamantyl segments at 90 °C for 30 min in ethylene glycol, and the deprotecting reaction of the cooligomer in the resulting product at 120 °C for 15 min was found to afford the colloidal red-brown colored cross-linked fluorinated cooligomer/Cu nanocomposites, which were stabilized by fluoroalkyl end-capped cooligomer containing benzotriazole segments. On the other hand, the corresponding cross-linked fluorinated cooligomer possessing no adamantyl segments could not afford the copper nanocomposites under similar conditions. Dynamic light scattering measurements and TEM photography of these cross-linked fluorinated cooligomer/Cu composites showed that these composites are nanometer size-controlled very fine nanoparticles. These fluorinated Cu nanocomposites have a good dispersibility and stability in mixed solvents of ethylene glycol and methanol for two weeks under air atmosphere conditions. In addition, these fluorinated Cu nanocomposite powders have been found to be stable for more than three months.