PLA Samples Research Articles

Near-infrared (NIR) spectroscopic imaging analysis of poly (lactic acid)-nanocomposite using disrelation mapping

Molecular-level interaction between poly(lactic acid) (PLA) and clay was probed by near-infrared (NIR) imaging based on disrelation mapping. Sets of NIR imaging data were measured under varying temperature. The spectra were subjected to null-space projection to remove overwhelming baseline changes arising from light scattering. Disrelation maps derived from baseline-corrected spectra revealed that the disrelation intensity between 5785 and 5825 cm−1 becomes especially significant around the boundary between the PLA and clay, suggesting the predominant development of crystalline structure in the region. Further disrelation intensities around the boundary appeared when additional heating was applied to the system. The emergence of such additional disrelation pattern reveals the growth of the PLA crystals and propagation of the crystalline region caused by the heating. Consequently, conclusive evidence on the nucleation induced by the clay is provided by the NIR imaging analysis of the PLA sample.

Surface modification of PLA nets intended for agricultural applications

Exclusion nets create a physical barrier between pests and crops, serving as a practical tool to reduce pesticide use in a wide variety of crops, and their negative impacts on the environment. While the efficiency of this method has been recently assessed, protection from pests is incomplete, as microorganisms such as fungi are not hindered by netting. In order to avoid favorable conditions for microbial development (e.g. wet leaf surfaces), the present study discusses two hydrophobic surface treatment approaches, each applied to two PLA substrates that can be used as exclusion nets to reduce water ingress on protected fruit trees. Both a commercial exclusion net and a fused deposition modelled (or 3D printed) mesh were treated via solvent-free and solvent-induced modifications. Photo-initiated chemical vapor deposition (PICVD) allowed organic thin film growth on the substrate at near-ambient conditions. Dip-dip-dry (DDD) exploited solvent-induced topology modification to yield hydrophobic PLA samples, showing various microstructures (crazes or hierarchical topology) depending on the substrate properties. Both substrates were efficiently rendered hydrophobic after surface treatments, with water contact angles climbing from 73° to 101° (PICVD) or 143° (DDD) for printed PLA samples, and to 131° (PICVD) or 135° (DDD) for commercial exclusion nets. The hierarchical features present on DDD-treated printed PLA substrates showed potential for molecular adsorption, paving the way for a method to increase the phytosanitary effect of agricultural textiles by active ingredient incorporation (limonene is used here as a test case). Surface properties were characterized with SEM imaging, contact angle measurements, tensile tests and drop penetration assessments (rain simulation).

Surface modification of PLA 3D-printed implants by electrospinning with enhanced bioactivity and cell affinity

PLA screw implants was fabricated using Fused Deposition Modeling by a bio 3D printer, and the surface modification method was also applied to enhance their properties like bioactivity and cell affinity. The crystallinity of the screws was measured using the modulated differential scanning calorimetry (MDSC). Electrospinning apparatus was used to coat the screws by poly (vinyl alcohol) (PVA)-nano Hydroxyapatite (nHA) nanofibers, with various concentrations of nHA. PVA-nHA nanofibers with a diameter ranging from 263.1 nm ± 45 nm–326.3 nm ± 40 nm was obtained. Bioactivity of the coated screws was evaluated by immersing in the simulated body fluid as well as the cytotoxicity using MTT assay. The behavior of the MC3T3-E1 cells cultured on the 3D-printed PLA sample and PVA-nHA nanofiber samples was evaluated. The results showed that the PVA-nHA nanofibers could significantly enhance the adhesion of the MC3T3-E1 cells as well as the growth of the cells.

Changes in the physical and structural properties of 3D-printed plastic samples under radiation exposure by nearly therapeutic dose

In this research, we study changes of 3D-printed samples made of PLA, ABS, and HIPS plastics under irradiation by 6 MeV electron beam in air with doses up to 1.5 kGy. We compared mechanical and structural properties of original and irradiated plastic samples. It is shown that absorption of the electron beam is not changing during irradiation process for all samples. Mechanical testing revealed no changes in the compressive stress, elastic modulus and bending stress for PLA, ABS and HIPS samples after irradiation. Testing demonstrates that thermograms of investigated polymers before and after irradiation do not have notable difference. However, polylactide glass transition peak (142–154̂C) is better distinguishable for irradiated sample of PLA plastic. The irradiated polymers demonstrate slight differences in Fourier-transform infrared (FTIR) spectra. Experimental results allow concluding that 3D-printed samples made of PLA, ABS, and HIPS plastic are capable to retain their properties under high energy electron beam irradiation with doses up to 1.5 kGy.

Kalsit Takviyeli Poli (Laktik Asit) Kompozit Malzemelerinin Hazırlanması ve Karakterizasyonu

In this work, calcite (KS) mineral was compounded with poly (lactic acid) (PLA) at the concentrations of 5, 10, 15 and 20 wt% using extrusion process. Test samples of PLA and composites were prepared by injection molding. Characterization of composites were done based on mechanical tests including tensile, hardness and impact tests, flow behavior by melt flow rate test (MFR) and morphological studies by scanning electron microscopy (SEM) method.[A1] Mechanical test results showed that the highest improvements in tensile strength and tensile modulus values were obtained for 10 wt% of KS filled composite. Further addition of KS caused remarkable decrease in tensile strength. Impact strength of PLA reduced by KS additions. The highest impact energy value was found in PLA-15 KS sample among composites. Hardness of neat PLA increased after KS inclusions. KS loaded PLA gave slightly higher MFR values compared to neat PLA. SEM micro-images of composites implied that KS particles dispersed homogeneously in PLA matrix at their lower loading ratio. Large agglomerates and poor dispersion were obtained for higher concentrations of KS since they favor particle-particle interactions. According to these results, concentrations of 10wt% and 15wt% were determined as suitable for calcite containing PLA composites. [A1]Bu cümleyi yeniden yazabilir misiniz? Biraz karışık olmuş. Konuyu bilmeyen biri okuduğunda sertlik, çekme ve darbe testlerinin mekanik olduğunu bilmeyebilir. Bunu ayrı cümlelerde yazarsanız daha iyi olur.

Electrical and Optical Properties of Silicon OxideLignin Polylactide (SiO2-L-PLA).

This paper presents a study on the electrical properties of new polylactide-based nanocomposites with the addition of silicon-dioxide–lignin nanoparticles and glycerine as a plasticizer. Four samples were prepared with nanoparticle mass fractions ranging between 0.01 to 0.15 (0.01, 0.05, 0.10, and 0.15), and three samples were prepared without nanoparticle filler—unfilled and unprocessed polylactide, unfilled and processed polylactide, and polylactide with Fusabond and glycerine. All samples were manufactured using the melt mixing extrusion technique and injection molding. Only the unfilled and unprocessed PLA sample was directly prepared by injection molding. Dielectric properties were studied with broadband spectroscopy in a frequency range from 0.1 Hz to 1 MHz in 55 steps designed on a logarithmic scale and a temperature range from 293.15 to 333.15 K with a 5 K step. Optical properties of nanocomposites were measured with UV-VIS spectroscopy at wavelengths from 190 to 1100 nm. The experimental data show that the addition of silicon-dioxide–lignin and glycerine significantly affected the electrical properties of the studied nanocomposites based on polylactide. Permittivity and electrical conductivity show a significant increase with an increasing concentration of nanoparticle filler. The optical properties are also affected by nanofiller and cause an increase in absorbance as the number of silicon-dioxide–lignin nanoparticles increase.

Biodegradable kinetics and behavior of bio-based polyblends under simulated aerobic composting conditions

The current study evaluates aerobic biodegradation of melt extruded poly(lactic acid) PLA based blends under composting conditions. Samples of neat PLA (NPLA) and bio-based polyblend composites of PLA/LLDPE (linear low-density polyethylene) having different concentration of MCC (microcrystalline cellulose crystal) were analyzed to understand the biodegradation behavior of these blends under simulated composting conditions. Biodegradation kinetics revealed that higher content of MCC and PLA accelerated the biodegradation process of the polymeric blends. Increase in the spherulite growth size and decrease in the spherulite density of the biodegraded samples confirmed the decline in amorphous portion of the test samples due to microbial assimilation, leaving behind the crystalline portion. Surface morphological analysis revealed that the samples of PLA/LLDPE/MCC blends underwent surface erosion prior to bulk biodegradation (50–80%) until the 90th day and the PLA formed fibril-like structures after degradation. This study would help in the design and preparation of biodegradable bio-based commercial blends in the future.

Surface modification of PLA scaffold using radio frequency (RF) nitrogen plasma in tissue engineering application

In this study, two models of flat film and three-dimensional porous structure made by 3D printing (scaffolding) of poly lactic acid polymer (PLA) were processed by radio frequency (RF; 13.56 MHz) low-pressure nitrogen, nitrogen/oxygen and nitrogen/hydrogen plasma to improve surface properties requested in tissue engineering. Samples were treated at different RF power (80–150 w) and time processing was 90 s. Optical emission spectroscopy was used to identify the species in plasma. A significant change in hydrophilicity and surface energy measured by contact angle was observed. Aging effect on the wettability of PLA films at two different temperatures was examined. The result showed that the samples, kept at low temperature, have not changed significantly. Morphology and surface roughness were studied by Atomic force microscopy. Chemical components at the surface were investigated by x-ray photoelectron spectroscopy (XPS). Mechanical and thermal effect on the 3D scaffold PLA were carried out by tension test and thermogravimetric analysis respectively to indicate the effects of RF plasma treatment on the samples. The structural order, interconnectivity, and scale of the scaffold holes have been recorded by an optical microscope. Surface treatment by plasma increased biocompatibility of PLA samples without any toxicity. Cell adhesion on scaffolds was approved through MTT and scanning electron microscope (SEM) analysis. MTT essay show there was significant different between N2/O2 (1:1) group than control sample. Plasma surface treatment is a convenient method to reach a perfect substrate with desired hydrophilicity for attaching cells.

재활 보조기기를 위한 FDM 3D프린팅 기반 PLA 출력물의 어닐링에 따른 최적화

3D프린터를 활용한 맞춤형 재활 기기를 제작하기 위해서 3D 프린팅으로 제작된 부품의 약한 기계적 강도와 내부 결함을 확인하기 어렵다는 점이 있다. 본 논문에서는 Poly Lactic Acid(PLA) 필라멘트를 이용하여 Fused Deposition Manufacturing(FDM) 3D프린터 출력 시 출력 온도와 출력속도, 적층 각도에 따른 기계적 강도에 미치는 영향을 인장실험을 통하여 알아보고 어닐링 방법을 통하여 기계적 강도 향상과 내부결함 판단법을 제안하고자 한다. 최적 출력 방법과 플라스틱 사출에서 주로 사용하는 어닐링 기법을 FDM 3D프린터에 적용하였고, 최적 어닐링 온도와 효과를 제안하였다. 이를 통해 맞춤형 재활 기기 제작 시 기계적 강도에 대한 이슈를 해결하고자 한다. 본 논문에서 제안하는 출력 조건은 온도 220℃ 출력 시 형상 오차가 적고 강도가 강했으며 어닐링 시 23%가 증가하였으며, 출력속도 10mm/s 시편의 강도가 가장 강했으며 어닐링 시 강도가 33% 증가하여 가장 효과가 크게 나타났다. 어닐링 온도로는 유리 전이온도 부근인 60℃에서는 모든 시편이 인장강도가 증가했으나, 결정화 온도근방인 105℃에서는 구조적 결함이 존재하는 출력물은 시편이 비틀림, 들뜸 현상에 의한 인장강도의 약화 현상이 발견되었다. 이러한 결과는 맞춤형 재활 기기 제작에서 높은 기계적 강도와 내부결함파악이 중요시 요구되는 부위에 제작되면 효과적으로 이용될 것이다.

Nonisothermal Cold Crystallization Kinetics of Poly(lactic acid)/Bacterial Poly(hydroxyoctanoate) (PHO)/Talc

AbstractThe effects of bacterial poly(hydroxyoctanoate) (PHO) and talc on the nonisothermal cold crystallization behaviours of poly(lactic acid) (PLA) were analysed with differential scanning calorimetry (DSC), and the thermal stability of the samples was observed with thermal gravimetric analysis (TGA). The modified Avrami’s model was used to describe the nonisothermal cold crystallization kinetics of neat PLA and its blends. The activation energies E for nonisothermal cold crystallization were calculated by the isoconversional method of Kissinger-Akahira-Sunose (KAS). The DSC results showed that the PLA/PHO blends were immiscible in the whole studied range, and as the PHO and talc content increased, the crystallization rate of PLA accelerated, and the crystallinity of PLA in the PLA samples increased. The values of the Avrami exponent indicated that the nonisothermal cold crystallization of the neat PLA and its blends exhibited heterogeneous, three-dimensional spherulitic growth. The E values were strongly dependent on PHO and talc. The TGA results showed that the presence of PHO and talc slightly influenced the thermal stability of PLA.

Facile fabrication of fully biodegradable and biorenewable poly (lactic acid)/poly (butylene adipate-co-terephthalate) in-situ nanofibrillar composites with high strength, good toughness and excellent heat resistance

Fully biodegradable and renewable polymer materials have shown broad application prospects with minimal environmental degradation. In the current study, eco-friendly poly(lactic acid)/poly(butylene adipate-co-terephthalate) (PLA/PBAT) in-situ nanofibrillar composites with hierarchical crystal architectures, high mechanical performance and excellent heat resistance properties were fabricated using the loop oscillating push-pull molding (LOPPM) technique and in-situ PBAT nanofibrils were used to induce highly oriented hybrid shish-kebabs. Benefiting from the flow-induced crystallization and abundant hybrid shish-kebabs, the overall crystallinity of LOPPM-processed PLA/10 wt%PBAT sample increased by 4-fold than the conventional injection-molded (CIM) neat PLA sample. The present work indicated that the tensile strength, Young's modulus, elongation at break and impact strength of LOPPM-processed PLA/10 wt%PBAT were higher than the CIM-processed PLA by 36.2%, 12.5%, 181.8% and 253.7%, respectively. The vicat softening temperature was elevated from 59.6 °C to 104.8 °C, and the thermostability also enhanced due to the construction of hybrid shish-kebabs. Strong, tough and heat-resistant PLA/PBAT in situ nanofibrillar composites exhibited outstanding properties for use under more demanding circumstances and are environmentally friendly.

A Facile and Efficient Method for Preparing Chain Extended Poly(lactic acid) Foams with High Volume Expansion Ratio

In recent years, poly(lactic acid) (PLA) foams have drawn considerable attention worldwide due to their environmental friendliness and excellent biocompatibility. However, it was a big challenge to prepare PLA foams with high volume expansion ratio owing to their poor crystallization properties and melt strength. In this work, the random copolymer of ethylene and glycidyl methacrylate as an efficient chain extender (CE) was chosen and employed to improve the crystallization behaviors and foamability of PLA. The chain extension reaction between PLA and CE was proved by torque curves, Fourier transform infrared spectra, gel fraction, and intrinsic viscosity measurements. Differential scanning calorimetry results and polarized optical microscope images showed that the crystallinity and spherocrystal number of various PLA samples increased notably as well as their spherocrystal size decreased with the CE content increasing. The viscoelasticity of various PLA samples was improved by the increment in the CE content. Various PLA samples were foamed by a batch foaming method at different foaming temperature with supercritical CO2 to research the influence of crystallization behaviors and/or rheological properties on the foaming behaviors of PLA. At the high foaming temperature, the cell density and volume expansion ratio (VER) of various PLA foams were affected by rheological properties significantly and their highest VER could reach to 42.41 ± 0.01 times. At the low foaming temperature, the cellular morphology of various PLA foams was improved notably, due to the increased melt strength induced by the cooling and the generation of crystallization region.

Amine Responsive Poly(lactic acid) (PLA) and Succinic Anhydride (SAh) Graft-Polymer: Synthesis and Characterization.

Amines are known to react with succinic anhydride (SAh), which in reactions near room temperature, undergoes a ring opening amidation reaction to form succinamic acid (succinic acid-amine). In this work, we propose to form an amine-responsive polymer by grafting SAh to a poly(lactic acid) (PLA) backbone, such that the PLA can provide chemical and mechanical stability for the functional SAh during the amidation reaction. Grafting is performed in a toluene solution at mass content from 10 wt% to 75 wt% maleic anhydride (MAh) (with respect to PLA and initiator), and films are then cast. The molecular weight and thermal properties of the various grafted polymers are measured by gel permeation chromatography and differential scanning calorimetry, and the chemical modification of these materials is examined using infrared spectroscopy. The efficiency of the grafting reaction is estimated with thermogravimetric analysis. The degree of grafting is determined to range from 5% to 42%; this high degree of grafting is desirable to engineer an amine-responsive material. The response of the graft-polymers to amines is characterized using X-ray photoelectron spectroscopy, infrared spectroscopy, and differential scanning calorimetry. Changes in the chemical and thermal properties of the graft-polymers are observed after exposure to the vapors from a 400 ppm methylamine solution. In contrast to these changes, control samples of neat PLA do not undergo comparable changes in properties upon exposure to methylamine vapor. In addition, the PLA-g-SAh do not undergo changes in structure when exposed to vapors from deionized water without amines. This work presents potential opportunities for the development of real-time amine sensors.

Effect of branching on flow-induced crystallization of poly (lactic acid)

Using rheometry in this study, we comprehensively investigated the effect of shear and extensional flow fields on the crystallization behavior of linear and branched poly(lactic-acid) (PLA). The shear-induced isothermal crystallization was explored in a stress-growth experiment during which continuous low shear rates of 0.01 and 0.1 s−1 were applied at a given temperature. To examine the effect of strong shear flow on the crystallization of PLAs, the samples were pre-sheared at high shear rates of 1 and 10 s−1 and subsequently isothermally crystallized at 140 °C. Results revealed that, at a given deformation rate, branched PLA exhibited a much faster crystallization rate than that of linear PLA. Also, altering the deformation rate at low shear rates could hardly affect the crystallization kinetics of both PLAs whereas the crystallization was strongly dependent on the shear rate value when pre-sheared at strong shear flows. The elongational flow-induced crystallization behavior of branched PLA by means of rheometry was also explored for the first time. It was shown that the elongational flow introduced a much more pronounced influence on branched PLA crystallization kinetics than that induced by the shear flows. The crystallization kinetics and structure of the sheared and stretched branched PLA samples were also investigated and confirmed using a differential scanning calorimeter (DCS), and wide- and short-angle X-ray scattering (WAXS and SAXS).

Nanocellular Foaming Behaviors of Chain-Extended Poly(lactic acid) Induced by Isothermal Crystallization.

Recently, the fabrication of semicrystalline polymer foams with a nanocellular structure by supercritical fluids has been becoming a newly developing research hotspot, owing to their peculiar properties and prospective applications. In this work, a facile and effective isothermal crystallization-induced method was proposed to prepare nanocellular semicrystalline poly(lactic acid) (PLA) foams using CO2 as a physical blowing agent. Styrene–acrylonitrile–glycidyl methacrylate (SAG) as a chain extender (CE) was introduced into PLA through a melt-mixing method to improve the crystallization behavior and melt viscoelasticity of PLA. The chain extension reaction between PLA and SAG occurred successfully as well as the branching and micro cross-linking structures were generated in chain-extended PLA (CPLA) samples, which were confirmed by Fourier transform infrared spectra, gel fraction, and intrinsic viscosity measurements. Owing to the nucleation effect of branching points and the restricted movement of PLA molecular chains by the formation of branching and/or microcross-linking structures, a large number of small spherocrystals were generated in CPLA samples, which was beneficial to produce nanocells. Nanocellular CPLA foams were prepared successfully, when the foaming temperature was 125 °C. As the SAG content increased, the cell size of various PLA foams decreased from 364 ± 198 to 249 ± 100 nm and their volume expansion ratio increased from 1.15 ± 0.05 to 2.22 ± 0.01 times, gradually. When the foaming temperature increased from 125 to 127 °C, an interesting transition from nanocells to microcells could be observed in CPLA foam with the CE content of 2 wt %. Finally, the formation mechanism of nanocells in various PLA foams was proposed and clarified using a schematic diagram.

Modification of epoxidized natural rubber as a PLA toughening agent

ABSTRACTIn the present study, the modification of epoxidized natural rubber (ENR) was investigated to improve toughness of PLA. ENR‐PLA copolymers were synthesized by 2 steps reactions. Acid hydrolysis of ENR to prepare macroinitiator (ENR‐OH) and copolymerization with lactide to obtain ENR‐PLA copolymers. The reaction time of acid hydrolysis and the lactide monomer loading was investigated. The optimum conditions were 8 h acid hydrolysis with 5wt% lactide loading to obtain ENR8‐PLA5 copolymers. A series of samples of PLA blended with ENR‐PLA copolymers was prepared using melt‐blending. The blend of PLA with ENR8‐PLA5 copolymers at 1 phr substantially improved the tensile properties. The elongation at break was increased 67%, and toughness was increased 64% compared to neat PLA. These results indicate that ENR8‐PLA5 has potential as toughening agent for PLA. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 48267.

Recycling of fiberglass wind turbine blades into reinforced filaments for use in Additive Manufacturing

The wind energy industry is one of the fastest-growing application sectors of composites, where reinforcement fibers are used in the manufacturing of light rotor blades. Considering the limited lifetime of turbine blades, a growing number of wind turbines will start to be decommissioned. Turbine blades are generally landfilled at their end-of-life, which highly impacts the environment. This paper proposes a systematic scheme combining mechanical recycling and 3D printing to recycle the valuable constituents of the scrap blades and reuse them in a Fused Filament Fabrication (FFF) process with the aim of improving the mechanical performance of 3D printed components. Mechanical grinding integrated with a double sieving mechanism is utilized to recover the reinforcement fibers. Tensile test specimens with 5 wt% fiber content are fabricated from the recycled fibers and plastic pellets and their mechanical properties as well as internal microstructure are investigated. The results demonstrate an improvement of 16% and 10% in the elastic modulus and ultimate strength of the reinforced composite filament as compared to the commercially available pure PLA filament. As well, a Young's modulus of 3.35 GPa was observed for the FFF fabricated samples, which is an 8% increase relative to pure PLA samples.

Influence of the combination of nucleating agent and plasticizer on the non-isothermal crystallization kinetics and activation energies of poly(lactic acid)

The influence of the combination of the nucleating agent (talc, N, N′-ethylene bis-stearamide (EBS) and a commercial nucleating agent NT-20) and the plasticizer (poly(ethylene glycol), PEG) on the melting and crystallization behaviors of poly(lactic acid) (PLA) was investigated by means of differential scanning calorimetry (DSC). The Jeziorny’s, Ozawa’s and Mo’s models were used to describe the non-isothermal cold and melt crystallization kinetics of the modified PLA samples. The non-isothermal cold and melt crystallization activation energies were evaluated by Kissinger’s method and Friedman’s method, respectively. The results show that the non-isothermal cold and melt crystallization kinetics of the samples are successfully analyzed by Jeziorny’s and Mo’s models, whereas Ozawa’s model is only suitable for the non-isothermal melt crystallization kinetics of PLA/talc sample. It is shown that the combination of one nucleating agent and PEG results in the synergistic effect on the cold and melting crystallization rates of PLA. Moreover, the combination of dual nucleating agents and PEG improves the cold crystallization rate but hinders the melt crystallization rate. It is indicated that dual nucleating agents act as physical hindrance to the molecular chain movement of PLA which results in the increases in the cold and melt crystallization activation energies.

Validation of an in-house-designed tensile testing machine for the mechanical characterization of 3D-printed specimens

Additive manufacturing is gaining greater and greater popularity in recent years: thanks to a dramatic reduction of costs and increasing print quality, more and more people are attracted to 3D printing, even for the production of end-use load-bearing parts. Today, there are no universally accepted norms that guide the engineer through the uncertain task of predicting the mechanical behavior of 3D-printed parts. Even though there are numerous reports and articles in literature regarding this topic, the designer will have to rely mostly on his own mechanical tests. This paper describes the design of an in-house-built machine for 3D-printed material tensile testing. The accuracy of this test-rig has been verified through comparison of results on different 3D-printed specimens: these were realized through fused deposition modelling (FDM) and were divided into four distinct sets, based on the material and their orientation on the buildplate. Some samples were printed in PLA, others in XTCF-20™: a composite material made of a matrix of Amphora™ polymer reinforced with 20% in weight carbon short fibers. PLA samples were printed flat on the buildplate, on one side and standing: this affects directly the orientation of the plastic fibers. Tensile tests were performed both on ours and on a high-end commercial testing machine, following the ISO 527 norm. Maximum stress, strain at maximum stress and Young’s modulus were calculated for each tested specimen: these measures allowed to compare the results of the two testers. Results proved coherent, thus validating the in-house machine, which was realized for a fraction of the cost of an equivalent commercial model.

NMR Analysis of Poly(Lactic Acid) via Statistical Models

The physical properties of poly(lactic acid) (PLA) are influenced by its stereoregularity and stereosequence distribution, and its polymer stereochemistry can be effectively studied by NMR spectroscopy. In previously published NMR studies of PLA tacticity, the NMR data were fitted to pair-addition Bernoullian models. In this work, we prepared several PLA samples with a tin catalyst at different L,L-lactide and D,D-lactide ratios. Upon analysis of the tetrad intensities with the pair-addition Bernoullian model, we found substantial deviations between observed and calculated intensities due to the presence of transesterification and racemization during the polymerization processes. We formulated a two-state (pair-addition Bernoullian and single-addition Bernoullian) model, and it gave a better fit to the observed data. The use of the two-state model provides a quantitative measure of the extent of transesterification and racemization, and potentially yields useful information on the polymerization mechanism.