PLA Homopolymer Research Articles

Widening the Application Range of PLA‐Based Thermoplastic Materials through the Synthesis of PLA‐Polyether Block Copolymers: Thermal, Tensile, and Rheological Properties

AbstractPolyethylene glycol (PEG), polypropylene glycol (PPG), and poly(ethylene glycol‐co‐propylene glycol) (PEPG) of number average molar masses (Mn) of 6–20 kDa are used as macroinitiators in the ring‐opening polymerization (ROP) of l‐lactide to synthesize high molar mass (50 kDa < MnCopo < 120 kDa) PLLA‐b‐polyether‐b‐PLLA linear triblock and starblock copolymers. At the studied block lengths, PEG and PEPG blocks exhibit miscibility with the PLLA blocks in the amorphous domains leading to a plasticization effect. As the PEG or PEPG block content is increased to 18 %wt, the Tg (Tg ≈ 24 °C) and the elastic modulus (E ≈ 500 MPa) are reduced, while the elongation at break (εb ≈ 280%) and crystallization rate are increased. At the same time, small angle oscillatory shear (SAOS) rheometric measurements show that the plasticized copolymers have a reduced melt viscosity. In contrast, SAOS and DSC measurements of the PPG‐containing block copolymers reveal phase separation of the PPG and PLA blocks leading to microstructures in the melt. Tensile tests show that the phase‐separated PPG‐containing block copolymers are more ductile than PLA homopolymers, but more brittle than the PEG‐ or PEPG‐plasticized block copolymers.

Effects of molecular weight on stereocomplex and crystallization of PLLA/PDLA blends

The chain dynamics affects the chain motions and the crystallization behaviors of polymers, and the crystallization ability of poly(lactic acid) (PLA) homopolymers can be substantially enhanced by incorporating stereocomplex-PLA (s-PLA). The effects of s-PLA molecular weight on the enhanced crystallization of the homopolymer were characterized and analyzed in this study. The crystallization behaviors, morphologies and structures of the homopolymer in PLA/s-PLA blends were investigated using DSC, POM, WAXD and FTIR during or after isothermal and non-isothermal crystallization. The blends compose homopolymer and the same content of the s-PLAs with three different molecular weights. The DSC results showed that the homopolymer in the blends containing the higher molecular weight of the s-PLA crystallizes at higher crystallization temperature than that containing the lower molecular weight of the s-PLA during the non-isothermal crystallization. The crystallization half-time of the homopolymer blending with the higher molecular weight s-PLA is shorter than that blending with the lower molecular weight of the s-PLA during the isothermal crystallization. POM images showed that the spherulite density of the homopolymer blending with the higher molecular weight of the s-PLA is significantly higher than that with the lower molecular weight of the s-PLA. The Mpemba effect was first experimentally observed in the melting crystallization of polymer blends containing PDLA and s-PLA. A possible explanation is proposed and discussed to elucidate the impact of s-PLA molecular weight on the crystallization behaviors of PLA homopolymer in the blends.

Evaluating the in vivo stability of water-soluble PEG-PLA copolymers using FRET imaging

Biodegradable and biocompatible polymer materials with tunable physical properties present a great interest for controlled drug delivery applications. A good example is BEPO®, a clinical-stage in situ-forming depot technology based on the utilization of a blend of poly(ethylene glycol)-b-poly(D,L-lactic acid) (PEG-PLA) diblock and triblock amphiphilic copolymers dissolved in an organic solvent. Once injected, this technology will form a bioresorbable solid polymer depot that will allow the release of a drug from weeks to months. The safety of the final degradation products from this technology, i.e., PEG and lactic acid, is well-documented. However, little information exists about the fate of intermediate degradants, specifically of water-soluble PEG-PLA chains where the molecular weights of the PLA block are short. Herein, we designed a Förster Resonance Energy Transfer (FRET) system for short copolymers, suitable for longitudinal in vivo imaging in the subcutaneous space, allowing to follow the stability of these products. Our results confirm that these species, that might be leaked from BEPO® depots during degradation, are rapidly hydrolyzed in the subcutaneous space of mice, forming approved products by Health Authorities, i.e., PEG and PLA homopolymers and/or lactic acid.

Recent advances in enhancing stereocomplexation between poly(lactide) enantiomeric chains.

Over the past three decades, its excellent biodegradability and biocompatibility have enabled poly(lactide) (PLA) to be extensively explored as a replacement for oil-based thermoplastics in biomedical and industrial applications. However, PLA homopolymers have "facilitative" limitations such as low mechanical properties, low processing temperatures, slow recrystallization, and insufficient crystallinity, which have usually hindered commercial PLA in industrial and biomedical applications. The formation of stereo-complexation between enantiomeric poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) chains offers an effective approach to PLA-based engineering materials with improved properties. In this review, we have understandably summarized recent progress in improving the SC crystallization of PLA-based plastics into two aspects, i.e., enantiomeric PLA homopolymers, and enantiomeric PLA-based copolymers. One important point to be noted is that much emphasis is focused on improving SC crystallization by enhancing interactions in the enantiomeric PLA-based copolymers. There is an insightful discussion about the effect of enhanced SC crystallization as well as intermolecular interactions between PLLA and PDLA chains in various stereocomplexable systems. Most importantly, this review starts with the basic understanding of SC crystallization and further elaborates on the rational mechanism of enhanced SC crystallization to provide a broad idea for broadening the road toward PLA-based materials.

Raman Study of Block Copolymers of Methyl Ethylene Phosphate with Caprolactone and L-lactide.

We present an in-depth analysis of Raman spectra of novel block copolymers of methyl ethylene phosphate (MeOEP) with caprolactone (CL) and L-lactide (LA), recorded with the excitation wavelengths of 532 and 785 nm. The experimental peak positions, relative intensities and profiles of the poly(methyl ethylene phosphate) (PMeOEP), polycaprolactone (PCL) and poly(L-lactide) (PLA) bands in the spectra of the copolymers and in the spectra of the PMeOEP, PCL and PLA homopolymers turn out to be very similar. This clearly indicates the similarity between the conformational and phase compositions of PMeOEP, PCL and PLA parts in molecules of the copolymers and in the PMeOEP, PCL and PLA homopolymers. Experimental ratios of the peak intensities of PMeOEP bands at 737 and 2963 cm-1 and the PCL bands at 1109, 1724 and 2918 cm-1 can be used for the estimation of the PCL-b-PMeOEP copolymers chemical composition. Even though only one sample of the PMeOEP-b-PLA copolymers was experimentally studied in this work, we assume that the ratios of the peak intensities of PLA bands at 402, 874 and 1768 cm-1 and the PMeOEP band at 737 cm-1 can be used to characterize the copolymer chemical composition.

A Dense Layer of Polyethyleneglycol and Zwitterionic Bone Targeting Peptide on the Surface of Stereocomplex Polylactide-Polyethyleneglycol Nanoparticles Improves Shelf-storage Stability and the Serum Compatibility

The surface properties of nanoparticles (NPs) affect their stability and formation of the protein corona, which influence their targeting abilities. We evaluated these properties using bone (hydroxyapatite; HAP) targeting peptide on tamoxifen (TAM)-loaded stereocomplexformed polylactide-polyethyleneglycol (SC-PLA-PEG) NPs. Octaaspartic acid-octaglycine-cysteine (D8G8C) anionic derivative (Ani. pep.) and octa-aspartic acid-octa lysine-cysteine (D8K8C), a zwitterionic derivative (Zwi. pep.) were conjugated with SC-PLA-PEG NPs as HAP-targeting peptides. The addition of hydrophobic PLA homopolymers increased the surface PEG density on the NPs. Denser PEG chains on NPs decreased their specific surface area, reducing protein adsorption on the NPs and TAM release from NPs. NPs with dense PEG chains and Zwi. pep. showed superior shelf stability and lower protein adsorption than NPs with dense PEG chains and Ani. pep. in murine serum. Furthermore, the HAP-binding ability of NPs with Zwi. pep. was significantly higher than that of NPs with Ani. pep. These results indicate that decreasing the specific surface area and zwitterionization of HAP-targeting peptides on NPs are promising approaches to improve the serum compatibility and stability of NPs.

Concurrent ring-opening and atom transfer radical polymerization for synthesis of block copolymers, and their comprehensive chromatographic characterization

In this study, we report concurrent ring-opening and atom transfer radical polymerization for synthesis of the block copolymers of L-lactide and styrene. Simultaneous polymerization of two different polymerization mechanisms requires careful selection of reaction conditions and reagents for targeted comparable propagation of both types. Several different di-block copolymers are synthesized having different total molar mass, chemical composition, and individual block lengths. Such simultaneous polymerization mechanisms require different reagents, and the regents required for one type of polymerization may hamper the other parallel mechanism. Hence, synthesized block copolymers are analysed comprehensively using different polymer chromatographic techniques and NMR spectroscopy. Total molar mass and chemical composition of BCPs are determined by size exclusion chromatography and NMR, respectively. PLA block length in PS-b-PLA and quantification of PS homopolymers is revealed by liquid chromatography at critical conditions of PS. For determination of PS block length in PS-b-PLA and quantification of PLA homopolymers, a gradient polymer elution chromatography (GPEC) method is applied. The comprehensive analysis of synthesized PS-b-PLA by parallel polymerization mechanisms revealed uninterrupted simultaneous polymerization of both monomers under applied conditions.

Labeling PLA-PEG nanocarriers with IR780: physical entrapment versus covalent attachment to polylactide.

Near-infrared fluorescent dyes, such as IR780, are promising theranostics, acting as photosensitizers for photodynamic therapy and in vivo tracers in image-guided diagnosis. This work compared the uptake by macrophage-like cells of IR780 either physically associated or covalently attached to poly(D,L-lactide) (PLA) formulated as polymeric nanocapsules (NC) from a blend of PLA homopolymer and PLA-PEG block copolymer. The physicochemical characterization of both NC was conducted using asymmetric flow field-flow fractionation (AF4) analysis with static and dynamic light scattering and atomic force microscopy. The interaction of IR780 with serum proteins was evidenced by AF4 with fluorescence detection and flow cytometry in cell uptake studies. The average diameters of NC were around 120 nm and zeta potentials close to -40 mV for all NC. NC uptake by cells in different media and experimental conditions shows significantly lower fluorescence intensities for IR780 covalently linked to PLA and correspondingly low quantitative uptake. Different mechanisms of internalization were evidenced depending on the IR780 type of association to NC. Serum proteins mediate IR780 interaction with cells in a dose-dependent manner. Our results show that non-covalently linked IR780 was released from NC and accumulated in macrophage cells. Oppositely, IR780 conjugated to PLA provides stable association with NC, and its fluorescence is representative of cell uptake of the nanocarrier itself. This work strongly reinforces the importance of covalent attachment of a fluorescence dye such as IR780 to the nanocarrier to study their interaction with cells in vitro and to obtain reliable tracking in image-guided therapy. Graphical abstract.

Lactic acid containing polymers produced in engineered Sinorhizobium meliloti and Pseudomonas putida.

This study demonstrates that novel polymer production can be achieved by introducing pTAM, a broad-host-range plasmid expressing codon-optimized genes encoding Clostridium propionicum propionate CoA transferase (PctCp, Pct532) and a modified Pseudomonas sp. MBEL 6-19 polyhydroxyalkanoate (PHA) synthase 1 (PhaC1Ps6-19, PhaC1400), into phaC mutant strains of the native polymer producers Sinorhizobium meliloti and Pseudomonas putida. Both phenotypic analysis and gas chromatography analysis indicated the synthesis and accumulation of biopolymers in S. meliloti and P. putida strains. Expression in S. meliloti resulted in the production of PLA homopolymer up to 3.2% dried cell weight (DCW). The quaterpolymer P (3HB-co-LA-co-3HHx-co-3HO) was produced by expression in P. putida. The P. putida phaC mutant strain produced this type of polymer the most efficiently with polymer content of 42% DCW when cultured in defined media with the addition of sodium octanoate. This is the first report, to our knowledge, of the production of a range of different biopolymers using the same plasmid-based system in different backgrounds. In addition, it is the first time that the novel polymer (P(3HB-co-LA-co-3HHx-co-3HO)), has been reported being produced in bacteria.

Mechanical, rheological and anaerobic biodegradation behavior of a Poly(lactic acid) blend containing a Poly(lactic acid)-co-poly(glycolic acid) copolymer

The goal of this work is to examine the thermal, rheological, mechanical and thermophilic anaerobic biodegradation performance of PLA blends containing a highly degradable polymer. Here random copolymers consisting of Poly(l-lactic acid) (PLLA) and Poly(glycolic acid) (PGA) structural units (75:25 M ratio) were synthesized at three different molecular weights (Mw = 16, 58 and 113 kg mol−1) and melt blended with a high molecular weight PLA homopolymer. The glass transition temperature (Tg) of poly(lactic acid-co-glycolic acid) (PLGA) was lower than that of PLA and increased with copolymer molecular weight. A single Tg intermediate to that of the two blend constituents was observed suggesting miscibility. Polymer blends showed enhanced methane production at early stages of anaerobic degradation with the rate increasing with increasing PLGA content and decreasing PLGA molecular weight. Blends exhibited a decrease in modulus and tensile strength as compared to pure PLA. Likewise, a decrease in ductility for all but the lowest molecular weight copolymer containing blend was observed. The zero-shear viscosity of polymer blends scaled predictably with PLGA content and exhibited reduced sensitivity to shear-rate. It is envisioned that this strategy could be applied for those applications where recycling is prohibitive such as at universities, hospitals and stadiums where mixed waste streams containing plastics and other waste types, such as food and paper, can be anaerobically co-digested and the resulting biogas used as fuel.

Furfuryl Alcohol and Lactic Acid Blends: Homo- or Co-Polymerization?

Furfuryl alcohol (FA) and lactic acid (LA) are two of the most interesting biomolecules, easily obtainable from sugars and hence extremely attractive for green chemistry solutions. These substances undergo homopolymerization and they have been rarely considered for copolymerization. Typically, FA homopolymerizes exothermically in an acid environment producing inhomogeneous porous materials, but recent studies have shown that this reaction can be controlled and therefore we have implemented this process to trigger the copolymerization with LA. The mechanical tests have shown that the blend containing small amount of FA were rigid and the fracture showed patterns more similar to the one of neat polyfurfuryl alcohol (PFA). This LA-rich blend exhibited higher chloroform and water resistances, while thermal analyses (TG and DSC) also indicated a higher furanic character than expected. These observations suggested an intimate interconnection between precursors which was highlighted by the presence of a small band in the ester region of the solid state 13C–NMR, even if the FT-IR did not evidence any new signal. These studies show that these bioplastics are basically constituted of PLA and PFA homopolymers with some small portion of covalent bonds between the two moieties.

Controlled random copolymerization of rac‐lactide and ɛ‐caprolactone by well‐designed phenoxyimine Al complexes

ABSTRACTThe random copolymers poly(LA‐ran‐CL) have improved properties of degradability, mechanical strength, elasticity, and permeability over the PLA and PCL homopolymers. However, the synthesis of such copolymers is still a great challenge in polymer chemistry. In this contribution, we develop a simple but well‐designed phenoxyimine Al complex (4) with bulky Ph2CH groups, which achieves controlled random copolymerization of rac‐LA and ɛ‐CL in a living manner (Ð = 1.06–1.09). The reactivity ratios of rac‐LA and ɛ‐CL (rLA and rCL) are 1.09 and 1.05 and the average sequence lengths of the lactidyl unit (LLA) and the caproyl unit (LCL) are in the range of 1.9–2.0 at different stages of conversion. In marked contrast, Al complexes (1–3) having less bulky substituents on the ligands only produce gradient copolymers. Furthermore, this strategy of catalyst design would be readily extended to other catalytic systems including β‐ketiminato Al complex (5). © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 611–617

PS-b-PMMA/PLA blends for nanoporous templates with hierarchical and tunable pore size

Blends of poly(styrene)-block-poly(methyl methacrylate) (PS-b-PMMA) and poly(lactide) (PLA) were deposited in the form of thin films on the surface of modified silicon wafers and exposed to tetrahydrofuran (THF) vapor annealing. It was shown that in specific experimental conditions, a core-shell morphology consisting in cylinders with a PMMA shell and a PLA core, within a continuous matrix of PS, was formed. In this case, PLA naturally segregated in the core of the PMMA cylinders, minimizing the PS/PLA interaction, which constitutes the most incompatible pair (the interaction strength between the various components was confirmed in thin films of the corresponding polymer blends). Compared to other block copolymer/homopolymer blends described in the literature, this system exhibits unexpected high increase of the characteristic lengths of the system (center-to-center distance and diameter). This was attributed to a partial solubilization of the PLA in the PMMA corona (the two polymers are highly compatible), inducing an enhanced level of PS and PLA stretching caused by the strong repulsion between these two polymers. The selective extraction of the PLA yielded to porous domains with small dimensions (6±2.5nm), reaching the performances that are currently attained in highly incompatible block polymers with low molecular weight. Further PMMA removal revealed a second porosity level, with higher pores diameter and center-to-center distance compared to the neat PS-b-PMMA system. This work highlights how PS-b-PMMA, that currently represents one of the industrial standards nanoporous template precursors, can be modified in an easy and costless approach using PLA homopolymer addition.

Morphology control in thin films of PS:PLA homopolymer blends by dip-coating deposition

In this work, smooth polymer films of PS, PLA and their blends, with thicknesses ranging from 20nm up to 400nm and very few defects on the surface were obtained by dip-coating. In contrast to the process of spin-coating which is conventionally used to prepare thin films of polymer blends, we showed that depending on the deposition parameters (withdrawal speed and geometry of the reservoir), various morphologies such as layered films and laterally phase-separated domains could be formed for a given blend/solvent pair, offering much more opportunities compared to the spin-coating process. This diversity of morphologies was explained by considering the superposition of different phenomena such as phase separation process, dewetting and vitrification in which parameters such as the drying time, the compatibility of the polymer/solvent pairs and the affinity of the polymer towards the interfaces were suspected to play a significant role. For that purpose, the process of dip-coating was examined within the capillary and the draining regimes (for low and high withdrawal speed respectively) in order to get a full description of the thickness variation and evaporation rate as a function of the deposition parameters.

聚乳酸/聚氧乙烯薄膜的性能研究

本文采用DSC测试了聚氧乙烯(PEO)和聚乳酸(PLA)的结晶温度(TC)和熔融温度(Tm)，用溶液共混方法制备了不同组成的PLA/PEO共混物薄膜，并对其进行了差示扫描量热(DSC)、广角X射线衍射(WAXD)及偏光显微镜(POM)分析。结果表明：共混物中薄膜PLA的结晶温度和熔融温度随PEO在共混物中含量的增加而减小；而熔融温度降低的幅度较小，但是都比PLA均聚物的熔融温度低；PLA在共混物薄膜中的结晶形态与PLA均聚物一致，但结晶尺寸相差很大。 The crystalline temperature and melt temperature of PEO were measured by DSC. PLA/PEO blend thin films of different composition were synthesized by solution blending, and the thin films were tested by Differential Scanning Calorimetry (DSC), Wide-Angle X-ray Diffraction (WAXD) and Po-larized Optical Microscope (POM). The results demonstrate that the crystalline temperature and the melting temperature of PLA decrease when the content of PEO in blend materials increases. The range of the decrease of the melting temperature is smaller. Its melting temperature is lower than the melting temperature of PLA homopolymer. Crystalline morphology in blends is consistent with PLA homopolymer, but the crystal size is difference.

Well-Ordered Nanoporous ABA Copolymer Thin Films via Solvent Vapor Annealing, Homopolymer Blending, and Selective Etching of ABAC Tetrablock Terpolymers.

Solvent vapor annealing treatments are used to control the orientation of nanostructures produced in thin films of a poly(styrene)-block-poly(isoprene)-block-poly(styrene)-block-poly((±)-lactide) (PS-PI-PS-PLA) and its blends with PLA homopolymer. The PS-PI-PS-PLA tetrablock terpolymer, previously determined to adopt a core(PLA)-shell(PS) cylindrical morphology in the bulk, gave perpendicular alignment of PLA cylinders over a limited range of thicknesses using a mixed solvent environment of tetrahydrofuran and acetone. On the other hand, perpendicular alignment was achieved regardless of film thickness by inclusion of 5 wt % homopolymer PLA in the PS-PI-PS-PLA tetrablock. Tapping mode atomic force microscopy (AFM) was used to visualize film surface morphologies. Subsequent reactive ion etching (RIE) and basic hydrolysis of PLA produced 15 nm pores in a PS-PI-PS triblock thin film matrix. Nanoporosity was confirmed by scanning electron microscopy (SEM) images and the vertical continuity of pores was confirmed by cross-sectional SEM analysis.

Fully biobased triblock copolyesters from l-lactide and sulfur-containing castor oil derivatives: Preparation, oxidation and characterization

The synthesis and characterization of fully biobased ABA triblock copolyesters from PLA and castor oil derivatives are described. The synthetic strategy uses a dihydroxy telechelic polyester as a macroinitiator for the ring opening polymerization of l-lactide. This macroinitiator was obtained from a sulfur-containing hydroxyacid derivative of 10-undecenoic acid, which undergoes self-polycondensation with low amounts of biobased 1,4-butanediol, enables the obtention of two prepolymers of different weight. Two series of triblock copolyesters with different percentage of PLA contents were prepared and the resulting thioether-bearing copolyester oxidized to polysulfones. Both series of copolyesters showed improved thermal stability, decrease of crystallinity and a similar hydrophilicity with respect to the PLA homopolymer.

Synthesis of poly(vinyl alcohol-graft-lactic acid) copolymer and its application as medical anti-tissue adhesion thin film

Graft copolymers of poly(vinyl alcohol-g-lactic acid) (PVA-g-PLA) were synthesized through a direct polycondensation of d,l-lactic acid grafting from polyvinyl alcohol (PVA) using 1-ethyl-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC·HCl) as the catalyst. The molecular weights of the graft copolymers varied from 98,000 to 106,000, which were determined by gel permeation chromatography. It was observed that the glass transition temperatures of the copolymers decreased with the increase of lactic acid units in the copolymers. Thin films with the thickness of 0.04–0.06 mm were prepared using the copolymers. The films showed improved hydrophilicity and flexibility compared with PLA homopolymer. Small pieces of the films were implanted under the skin of mice for anti-tissue adhesion purpose. The films demonstrated good anti-tissue adhesion behavior and suitable degradability in the mouse body. The graft copolymers were non-toxic, flexible, blood compatible, non-cytotoxic and had potential applications as a medical anti-tissue adhesion agent.

Selective etching of polylactic acid in poly(styrene)‐block‐poly(d,l)lactide diblock copolymer for nanoscale patterning

ABSTRACTSelf‐assembled thin films of a lamellar forming polystyrene‐block‐poly(d,l)lactide (PS‐b‐PLA) block copolymer (BCP) contain a “reactive” block that can be readily removed to provide a template for substrate pattern formation. Various methods of PLA removal were studied here with a view to develop the system as an on‐chip etch mask for substrate patterning. Solvo‐microwave annealing was used to induce microphase separation in PS‐b‐PLA BCP with a periodicity of 34 nm (Lo) on silicon and silicon on insulator (SOI) substrates. Wet etches based on alkaline and enzymatic solutions were studied in depth. Fourier transform‐infrared (FT‐IR) analysis showed that basic hydrolysis using sodium hydroxide (NaOH) or ammonium hydroxide (NH4OH) solutions resulted in greater PLA removal in comparison to an enzymatic approach using Proteinase K in a Tris‐HCl buffer solution. However, in the enzymatic approach, the characteristic self‐assembled fingerprint patterns were retained with less damage. Comparison to a dry etch procedure using a reactive ion etch (RIE) technique was made. A detailed study of the etch rate of PS and PLA homopolymer and PS‐b‐PLA shows depending on DC bias, the etch selectivity of PLA and PS can be almost doubled from 1.7 at DC bias 145 V to 3 at DC bias 270 V. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40798.Together with Krebs et al., J. Appl. Polym. Sci. (2014) 131, 40795, doi:10.1002/app.40795, this article is part of a Special Issue on Polymers for Microelectronics. The remaining articles appear in J. Appl. Polym. Sci. (2014) volume 131, issue 24. This note was added on 1st July 2014.

Magnetic Microrheology of Block Copolymer Solutions

The viscosity of poly(styrene)-b-poly(lactide) [PS-b-PLA] solutions in a neutral solvent was characterized by magnetic microrheology. The effect of polymer concentration on the viscosity of the block polymer solutions was compared with that of the PS and PLA homopolymers in the same solvent. The viscosity of PS-b-PLA solution, unlike the homopolymer solutions, showed a steep increase over a narrow concentration range. The steep rise was concomitant with microphase separation into an ordered cylindrical microstructure as determined by small-angle X-ray scattering. Hence microrheology proved effective as a means of characterizing the order-disorder transition concentration. During an in situ drying experiment, changes in local viscosity through the depth of a block copolymer solution were characterized as a function of drying time. Early in the drying process, the viscosity rose steadily and was uniform through the depth, a result consistent with steadily increasing and uniform polymer concentration. However, later in the drying process as the overall polymer concentration approached that required for microphase separation, the viscosity of the polymer solution near the free surface became an order of magnitude higher than that near the bottom of the container. The zone of high viscosity moved downward as drying proceeded, consistent with a microphase separation front.