Structural Changes in Drawn Poly(L-lactide)/Clay Hybrid Films

The films composed of poly(L-lactide)(PLLA)/organoclay hybrids (PLACHs) have been prepared via a melt-compounding process, in which the organoclay paticles are uniformly dispersed in the PLLA matrix. The changes in PLLA crystal orientation for the uniaxially drawn films were studied by differential scanning calorimetry, X-ray diffractometry and transmission electron microscopy. Additionally, temperature dependence of the mechanical properties for these drawn films were examined by a dynamic viscoelastometer. After drawing the quenched PLACHs film at 90 °C, the orientation and crystallinity of PLLA crystal increased rapidly with increasing drawing ratio (λ) in the range more than 3 times. At the higher λ, the organoclay platelet particles in the PLACHs became parallel to the draw direction. The mechanical properties of drawn PLACHs were strongly dependent on both clay concentration and λ.     

Novel Nanostructures from Self‐Assembly of Chiral Block Copolymers

A diblock copolymer system constituting both achiral and chiral blocks, polystyrene‐block‐poly(L ‐lactide) (PS‐PLLA), was designed for the examination of chiral effects on the self‐assembly of block copolymers (BCPs). A unique phase with three‐dimensional hexagonally packed PLLA helices in PS matrix, a helical phase (H*), can be obtained from the self‐assembly of PS‐rich PS‐PLLA with volume fraction of PLLA f = 0.34, whereas no such phase was found in racemic polystyrene‐block‐poly(D .L ‐lactide) (PS‐PLA) BCPs. Moreover, various interesting crystalline PS‐PLLA nanostructures can be obtained by controlling the crystallization temperature of PLLA (T_c,PLLA), leading to the formation of crystalline helices (PLLA crystallization directed by helical confined microdomain) and crystalline cylinders (phase transformation of helical nanostructure dictated by crystallization) when T_c,PLLA < T_g,PS (the glass transition temperature of PS) and T_c,PLLA ≧ T_g,PS, respectively. As a result, a spring‐like behavior of the helical nanostructure can be driven by crystallization so as to dictate the transformation (i.e., stretching) of helices and to result in crystalline cylinders. For PS‐PLLA with PLLA‐rich fraction (f = 0.65), another unique phase, a hexagonally packed core‐shell cylinder phase with helical sense (CS*), in which the PS microdomains appear as shells and PLLA microdomains appear as matrix and cores, can be found in the self‐assembly of PLLA‐rich PS‐PLLA BCPs. The formation of those novel phases: helix and core‐shell cylinder is attributed to the chiral effect on the self‐assembly of BCPs, so we named this PS‐PLLA BCP as chiral BCP (BCP*). For potential applications of those materials, the spring‐like behavior with thermal reversibility might provide a method for the design of switchable nanodevices, such as nanoscale actuators. In addition, the PLLA blocks can be hydrolyzed. After hydrolysis, helical nanoporous PS bulk and PS tubular texture can be obtained and used as templates for the formation of nanocomposites.

     

Oriented crystallization of poly(L‐lactic acid) in uniaxially oriented blends with poly(vinylidene fluoride)

This article describes the oriented crystallization of poly(L ‐lactic acid) (PLLA) in uniaxially oriented blends with poly(vinylidene fluoride) (PVDF). Uniaxially drawn films of PLLA/PVDF blend with fixed ends were heat‐treated in two ways to crystallize PLLA in oriented blend films. The crystal orientation of PLLA depended upon the heat‐treatment process. The crystal c‐axis of the α form crystal of PLLA was highly oriented in the drawing direction in a sample cold‐crystallized at T_c = 120 °C, whereas the tilt‐orientation of the [200]/ [110] axes of PLLA was induced in the sample crystallized at T_c = 120 °C after preheating at T_p = 164.5–168.5 °C. Detailed analysis of the wide‐angle X‐ray diffraction (WAXD) indicated that the [020]/ [310] crystal axes were oriented parallel to the drawing direction, which causes the tilt‐orientation of the [200]/ [110] axes and other crystal axes. Scanning electron microscopy (SEM) suggested that oriented crystallization occurs in the stretched domains of PLLA with diameters of 0.5–2.0 μm in the uniaxially drawn films of PVDF/PLLA = 90/10 blend. Although the mechanism for the oriented crystallization of PLLA was not clear, a possibility was heteroepitaxy of the [200]/[110] axes of the α form crystal of PLLA along the [201]/[111] axes of the β form crystal of PVDF that is induced by lattice matching of d₁₀₀(PLLA) ≈ 5d₂₀₁(PVDF). © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1376–1389, 2008     

Visualization of spontaneous aggregates by diblock poly(styrene)‐b‐poly(L‐lactide)/poly(D‐lactide) pairs in solution with new fluorescent CdSe quantum dot labels

Spontaneous stereocomplex aggregation of diblock poly(styrene)‐b‐poly(L ‐lactide) PS‐b‐PLLA/poly(D ‐lactide) PDLA pairs has been investigated under ambient temperature in tetrahydrofuran solution. First, diblock PS₂₆₀‐b‐PLLA₁₆₅ and PS₂₆₀‐b‐PDLA₁₆₂ bearing similar lengths of respective PLLA and PDLA blocks were synthesized through controlled atom‐transfer radical polymerization of styrene, and a subsequent living ring‐opening polymerization of optically pure lactides, and their structures were further characterized by nuclear magnetic resonance spectroscopy (NMR) and gel‐permeation chromatography (GPC). Subsequently, new enantiomeric poly(D ‐lactide) stabilized core‐shell fluorescent CdSe quantum dots (CdSe/PDLA QD) were designed and prepared as sensitive fluorescence labels to shed new lights on the spontaneous stereocomplex aggregation in THF, which was mediated by stereocomplexation of the PLLA and PDLA chains. Upon simply mixing two individual THF solution of diblock PS₂₆₀‐b‐PLLA₁₆₅ and HO‐PDLA₃₀‐SH, spontaneous stereocomplex aggregation was studied, and the aggregated uniform spherical particles were observed by scanning electronic microscopy (SEM) to exhibit average particle diameters of 2.0 μm. Finally, utilizing the prepared CdSe/PDLA QDs as new fluorescent labels, morphologies of the spontaneous aggregates by new diblock PS₂₆₀‐b‐PLLA₁₆₅/HO‐PDLA₃₀‐SH pair were for the first time directly visualized by a confocal laser scanning fluorescence microscopy (CLSFM). These results might suggest alternative ways to simply prepare functional fluorescent particles with tunable diameter sizes and would be helpful to understand the mechanism of stereocomplex particle aggregation. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1393–1405, 2009     

Crystallization and morphology of poly(ethylene oxide‐b‐lactide) crystalline–crystalline diblock copolymers

The crystallization behaviors and morphology of asymmetric crystalline–crystalline diblock copolymers poly(ethylene oxide‐lactide) (PEO‐b‐PLLA) were investigated using differential scanning calorimetry (DSC), wide angle X‐ray diffraction (WAXD), and microscopic techniques (polarized optical microscopy (POM) and atomic force microscopy (AFM)). Both blocks of PEO₅‐b‐PLLA₁₆ can be crystallized, which was confirmed by WAXD, while PEO block in PEO₅‐b‐PLLA₃₀ is difficult to crystallize because of the confinement induced by the high glass transition temperature and crystallization of PLLA block with the microphase separation of the block copolymer. Comparing with the crystallization and morphology of PLLA homopolymer and differences between the two copolymers, we studied the influence of PEO block and microphase separation on the crystallization and morphology of PLLA block. The boundary temperature (T_b) was observed, which distinguishes the crystallization into high‐ and low‐temperature ranges, the growth rate and morphology were quite different between the ranges. Crystalline morphologies including banded spherulite, dendritic crystal, and dense branching in PEO₅‐b‐PLLA₁₆ copolymer were formed. The typical morphology of dendritic crystals including two different sectors were observed in PEO₅‐b‐PLLA₃₀ copolymer, which can be explained by secondary nucleation, chain growth direction, and phase separation between the two blocks during the crystallization process. Lozenge‐shaped crystals of PLLA with screw dislocation were also observed employing AFM, but the crystalline morphology of PEO block was not observed using microscopy techniques because of its small size. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1400–1411, 2008     

Zone‐Drawn Poly(p‐phenylene) Films. Effects of Molecular Orientation on Electrical and Mechanical Properties

The electrochemically synthesized poly(p‐phenylene) film could be zone drawn by a factor of 1.57, where the orientation function (f) increased with the draw ratio (λ), regardless of the heater temperature (T_h) or applied tension (σ), and reached 0.428 for the resulting film. The electrical conductivity in the drawing direction rose with f but decreased as T_h became higher due to dedoping that occurred simultaneously with drawing. Young's modulus and tensile strength significantly increased to 4.5 GPa and 155 MPa by zone drawing from 1.1 GPa and 79 MPa of the as‐synthesized film.     

Synthesis of amphiphilic block copolymer consisting of glycopolymer and poly(l‐lactide) and preparation of sugar‐coated polymer aggregates

The block glycopolymer, poly(2‐(α‐d ‐mannopyranosyloxy)ethyl methacrylate)‐b‐poly(l ‐lactide) (PManEMA‐b‐PLLA), was synthesized via a coupling approach. PLLA having an ethynyl group was successfully synthesized via ring‐opening polymerization using 2‐propyn‐1‐ol as an initiator. The ethynyl functionality of the resulting polymer was confirmed by MALDI‐TOF mass spectroscopy. In contrast, PManEMA having an azide group was prepared via AGET ATRP using 2‐azidopropyl 2‐bromo‐2‐methylpropanoate as an initiator. The azide functionality of the resulting polymer was confirmed by IR spectroscopy. The Cu(I)‐catalyzed 1,3‐dipolar cycloaddition between PLLA and PManEMA was performed to afford PManEMA‐b‐PLLA. The block structure was confirmed by ¹H NMR spectroscopy and size exclusion chromatography. The aggregating properties of the block glycopolymer, PManEMA_16k‐b‐PLLA_6.4k (M _n,PManEMA = 16,000, M _n,PLLA = 6400) was examined by ¹H NMR spectroscopy, fluorometry using pyrene, and dynamic light scattering. The block glycopolymer formed complicated aggregates at concentrations above 21 mg·L^?1 in water. The d ‐mannose presenting property of the aggregates was also characterized by turbidimetric assay using concanavalin A. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 395–403     

Effects of top confinement and diluents on morphology in crystallization of poly(l‐lactic acid) interacting with poly(ethylene oxide)

Effects of top confinement and diluent poly(ethylene oxide) (PEO) on poly(l ‐lactic acid) (PLLA) crystal morphology have been investigated. When crystallized at 120 °C, uncovered neat PLLA sample exhibits higher growth rate ringless spherulites; while the covered sample exhibits lower growth rate ring‐banded spherulites. As PEO is introduced into PLLA, the morphology also undergoes significant changes. For the same T_c,PLLA = 120 °C, the PEO/PLLA blend with PEO composition greater than 25% exhibits ring‐banded patterns even in uncovered sample. However, in much greater PEO composition (>80 wt %), uncovered samples exhibit ring bands diverging into dendritic patterns, while top covered samples tend to maintain the spiral ring‐band patterns. Both PEO inclusion in PLLA and top cover on films impose growth kinetic alterations. Additionally, the top glass cover tends to prevent the lower surface tension PLLA to be accumulated on the surface, resulting in the formation of ring‐band pattern. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1160–1170     

Thermoelasticity and stress relaxation behavior of polychloroprene/organoclay nanocomposites

Polychloroprene/organoclay nanocomposites (ENC) were characterized by X-ray scattering, DSC and stretching calorimetry techniques.The infinite cluster of highly anisometric organoclay particles was likely to set on at filler volume fraction as low as ? ? 0.04. Quantitative analysis of thermoelastic behavior over the entire range of relative elongations provided additional support for the concept of strain-dependent strain amplification factor as the major parameter controlling the thermomechanical properties of the ENC. Low values of the fitting parameter C in the modified equation for the total heats of stretching were regarded as evidence for the reduced exothermal effects of external friction between organoclay nanoparticles coated with the boundary interphase. Stress relaxation behavior of the ENC was qualitatively consistent with the original assumption that after initial stretching to the highest elongation (λ_lim) the pre-existing infinite clusters of filler particles were broken into isolated clusters which remained structurally similar, whatever the subsequent stretching to λ_f < λ_lim.     

Synthesis,characterization, effect of architecture on crystallization,and spherulitic growth of poly(L‐lactide)‐b‐poly(ethylene oxide) copolymers with different branch arms

Well‐defined poly(L ‐lactide)‐b‐poly(ethylene oxide) (PLLA‐b‐PEO) copolymers with different branch arms were synthesized via the controlled ring‐opening polymerization of L ‐lactide followed by a coupling reaction with carboxyl‐terminated poly(ethylene oxide) (PEO); these copolymers included both star‐shaped copolymers having four arms (4sPLLA‐b‐PEO) and six arms (6sPLLA‐b‐PEO) and linear analogues having one arm (LPLLA‐b‐PEO) and two arms (2LPLLA‐b‐PEO). The maximal melting point, cold‐crystallization temperature, and degree of crystallinity (X_c) of the poly(L ‐lactide) (PLLA) block within PLLA‐b‐PEO decreased as the branch arm number increased, whereas X_c of the PEO block within the copolymers inversely increased. This was mainly attributed to the relatively decreasing arm length ratio of PLLA to PEO, which resulted in various PLLA crystallization effects restricting the PEO block. These results indicated that both the PLLA and PEO blocks within the block copolymers mutually influenced each other, and the crystallization of both the PLLA and PEO blocks within the PLLA‐b‐PEO copolymers could be adjusted through both the branch arm number and the arm length of each block. Moreover, the spherulitic growth rate (G) decreased as the branch arm number increased: G_{6sPLLA‐b‐PEO} < G_{4sPLLA‐b‐PEO} < G_{2LPLLA‐b‐PEO} < G_{LPLLA‐b‐PEO}. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2034–2044, 2006     

Nanocomposites of poly(ethylene terephthalate‐co‐ethylene naphthalate) with organoclay

Poly(ethylene terephthalate‐co‐ethylene naphthalate) (PETN)/organoclay was synthesized with the solution intercalation method. Hexadecylamine was used as an organophilic alkylamine in organoclay. Our aim was to clarify the intercalation of PETN chains to hexadecylamine–montmorillonite (C₁₆–MMT) and to improve both the thermal stability and tensile property. We found that the addition of only a small amount of organoclay was enough to improve the thermal stabilities and mechanical properties of PETN/C₁₆–MMT hybrid films. Maximum enhancement in both the ultimate tensile strength and initial modulus for the hybrids was observed in blends containing 4 wt % C₁₆–MMT. Below a 4 wt % clay loading, the clay particles could be highly dispersed in the polymer matrix without a large agglomeration of particles. However, an agglomerated structure did form in the polymer matrix at a 6 wt % clay content. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2581–2588, 2001     

Porous Biodegradable Polyesters, 3. Preparation of Porous Poly(ε‐caprolactone) Films from Blends by Selective Enzymatic Removal of Poly(L‐lactide)

Porous poly(ε‐caprolactone) (PCL) films were prepared by the removal of poly(L ‐lactide) (PLLA) from phase‐separated PLLA/PCL blend films using the selective Proteinase K™‐catalyzed hydrolysis of PLLA and subsequent elution of its water‐soluble oligomers and monomer into the surrounding hydrolysis media. Polarimetry, gravimetry, and differential scanning calorimetry (DSC) confirmed the complete removal of PLLA molecules from the blend films within 5 d of the Proteinase K‐catalyzed hydrolysis and therefore the formation of porous PCL films when the initial PLLA content [X_PLLA(0)(w/w) = PLLA/(PCL + PLLA)] of the blend films was in the range 0.3–0.5. The fragmentation of the blend film with X_PLLA(0) = 0.7 occurred when the Proteinase K‐catalyzed hydrolysis was continued for longer than 5 d. These findings exhibited that both the PLLA‐rich and PCL‐rich phases were continuous in the blend films for X_PLLA(0) ranges of 0.3–0.7 and of 0.3–0.5, respectively, and that the PCL‐rich phase became dispersed when X_PLLA(0) was increased to 0.7. The dependence of enzymatic hydrolysis rate on X_PLLA(0) strongly suggests that the Proteinase K‐catalyzed hydrolysis of the blend films occurs at the interfaces of PLLA‐rich and PCL‐rich phases as well as at the film surfaces.     

Functionally integrated liquid crystalline photochromic triple block copolymer with locally light‐ and thermal‐controllable sub‐blocks

Photoorientation and reorientation processes induced by illumination of the samples with oppositely directed polarized light and by the thermal treatment were studied for the films of triblock copolymer pAzo₁₀‐b‐pPhM₈₀‐b‐pAzo₁₀ consisting of a nematic phenyl benzoate сentral sub‐block (PhM, DP = 80) with two terminal smectic azobenzene sub‐blocks (Azo, DP = 10). For amorphized films of triblock copolymer, illumination with polarized light (λ = 546 nm) is shown to be by orientation of only Azo‐containing groups, but upon following annealing of the film, PhM groups are adjusted to the orientation of Azo fragments. It was found, that the subsequent illumination of the block copolymer sample with oppositely directed polarized light changes the orientation of azobenzene groups, while the orientation of phenyl benzoate groups is remained unchanged. Thus, the cyclic illumination of the triblock copolymer samples by the linear polarized light and subsequent thermal treatment make it possible to control and fix orientation of azobenzene and phenyl benzoate groups located in different sub‐blocks in the desired and independent manner. The comparison of these results with the data on random p(Azo₇‐ran‐PhM₃₀) copolymer of the similar composition revealed, that in the random copolymer, both Azo and PhM mesogenic groups are involved in the orientational cooperative process regardless of films process treatment. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1602–1611     

Photopyroelectric spectroscopy of polyaniline films

Photopyroelectric spectroscopy (PPES), in the 400 < λ < 900 nm wavelength range, was used to study thermal properties of differently doped polyaniline (PAN) films. The photopyroelectric intensity signal V_n(λ) and its phase F_n(λ) were independently measured, as well as the intensity V_n(f) and the phase F_n(f) (f being the chopping frequency) for a given λ of the saturation part of the PPES spectrum. Equations of both the intensity and the phase of the PPES signal, taking into account the thermal and the optical characteristics of the PAN films and the pyroelectric detector, were used to fit the experimental results. From the fittings we obtained, with great accuracy, the values of thermal conductivity k and thermal diffusivity coefficient α of PAN films of different doping degrees. It was observed that, in contrast with the strong doping‐dependence of the electrical conductivity, the thermal parameters of PAN films remained practically unchanged under doping. This apparent discrepancy is explained by the granular metal model of doped PAN. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1294–1300, 2000     

Preparation of oriented β‐form poly(L‐lactic acid) by solid‐state extrusion

Melt‐crystallized, low molecular weight poly(L ‐lactic acid) (PLLA) consisting of α crystals was uniaxially drawn by solid‐state extrusion at an extrusion temperature (T_ext) of 130–170 °C. A series of extrusion‐drawn samples were prepared at an optimum T_ext value of 170 °C, slightly below the melting temperature (T_m) of α crystals (～180 °C). The drawn products were characterized by deformation flow profiles, differential scanning calorimetry (DSC) melting thermograms, wide‐angle X‐ray scattering (WAXD), and small‐angle X‐ray scattering as a function of the extrusion draw ratio (EDR). The deformation mode in the solid‐state extrusion of semicrystalline PLLA was more variable and complex than that in the extensional deformation expected in tensile drawing, which generally gave a mixture of α and β crystals. The deformation profile was extensional at a low EDR and transformed to a parabolic shear pattern at a higher EDR. At a given EDR, the central portion of an extrudate showed extensional deformation and the shear component became progressively more significant, moving from the center to the surface region. The WAXD intensities of the (0010)_α and (003)_β reflections on the meridian as well as the DSC melting thermograms showed that the crystal transformation from the initial α form to the oriented β form proceeded rapidly with increasing EDR at an EDR greater than 4. Furthermore, WAXD showed that the crystal transformation proceeded slightly more rapidly at the sheath region than at the core region. This fact, combined with the deformation profiles (shear at the sheath and extensional at the core), indicated that the crystal transformation was promoted by shear deformation under a high pressure rather than by extensional deformation. Thus, a highly oriented rod consisting of only β crystals was obtained by solid‐state extrusion of melt‐crystallized, low molecular weight PLLA slightly below T_m. The structure and properties of the α‐ and β‐form crystals were also studied. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 95–104, 2002     

Alkaline and enzymatic degradation of L‐lactide copolymers. II. Crystallized films of poly(L‐lactide‐co‐D‐lactide) and poly(L‐lactide) with similar crystallinities

Films of poly(L ‐lactide‐co‐D ‐lactide) [P(LLA‐DLA); 95/5] and poly(L ‐lactide) [i.e., poly(L ‐lactide acid) (PLLA)] were prepared by crystallization from the melt, and a comparative study of the crystallization effects on the alkaline and proteinase K catalyzed hydrolysis of the films was carried out. The hydrolyzed films were investigated with gravimetry, differential scanning calorimetry, polarimetry, and gel permeation chromatography, and the results were compared with those reported for amorphous‐made specimens. The alkaline hydrolyzability of the P(LLA‐DLA) (95/5) and PLLA films was determined solely by the initial crystallinity (X_c) and was not affected by the content of the incorporated D ‐lactide (DLA) unit in the polymer chain; this was in marked contrast to the fact that the enzymatic hydrolyzability depended on not only the initial X_c value but also the DLA unit content. The alkaline hydrolysis rate of the P(LLA‐DLA) (95/5) and PLLA films and the enzymatic hydrolysis rate (R_EH) of the P(LLA‐DLA) (95/5) films decreased linearly as the initial X_c value increased. This meant that the hydrolyzability of the restricted amorphous regions was very similar to that of the free amorphous regions. In contrast, R_EH of the PLLA films decreased nonlinearly with the initial X_c value, and this nonlinear dependence was caused by the fact that in the PLLA films the restricted amorphous regions were much more hydrolysis‐resistant than the free amorphous regions. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1064‐1075, 2005     

Polyurethane‐nanocomposite materials via in situ polymerization into organoclay interlayers

New nanocomposite materials based on polyurethane intercalated into organoclay layers have been synthesized via in situ polymerization. The syntheses of polyurethane–organoclay hybrid films were carried out by swelling the organoclay [12‐aminododecanoic acid montmorillonite] into different kinds of diols followed by addition of diisocyanate then casting in a film. The homogeneous dispersion of MMT in the polymer matrix is evidenced by scanning electron microscope and x‐ray diffraction, which showed the disappearance of the peak characteristic to d₀₀₁ spacing. It was found that the presence of organoclay has improved the thermal, solvent resistance and mechanical properties. Also, the tensile strength is increased with increasing the organoclay contents to 20% by the ratio 182% related to the PU with 0% organoclay. On the contrary, the elongation has decreased with increasing the organoclay contents. Copyright © 2007 John Wiley & Sons, Ltd.     

Effects of take‐up speed on the structure and properties of melt‐spun poly(L‐lactic acid) fibers

Poly(L ‐lactic acid) (PLLA) filament fibers were prepared by one‐step melt spinning process and the effects of variations in take‐up speed on their thermal properties, mechanical properties, and crystalline structures were investigated. Differential scanning calorimetry (DSC) results revealed that the PLLA fibers showed multiple melting peaks and that the melting peak appearing at a lower temperature moved lower while that at a higher temperature moved higher with increasing take‐up speed. The glass transition temperature (T_g) obtained from dynamic mechanical analysis (DMA) increased with increasing take‐up speed. The tenacity increased and the boiling water shrinkage (BWS) decreased with increasing take‐up speed. However, these mechanical and thermal properties were stabilized at take‐up speeds over 3500 m/min. The melt‐spun PLLA fibers of this study showed an α‐form crystal structure which was not affected by the take‐up speed. The change in the tendency of the thermal and mechanical properties at around 3500 m/min did not appear to result from the change in crystal form but rather from the change in crystallite size and crystallite orientation. Copyright © 2008 John Wiley & Sons, Ltd.     

Toughening modification of PLLA with PCL in the presence of PCL‐b‐PLLA diblock copolymers as compatibilizer

To obtain an effective compatibilizer for the blends of poly(L‐lactide) (PLLA) and poly(ε‐caprolactone) (PCL), the diblock copolymers PCL‐b‐PLLA with different ratios of PCL/PLLA (CL/LA) and different molecular weights (M_n) were synthesized by ring‐opening polymerization (ROP) of L‐lactide with monohydric poly(ε‐caprolactone) (PCL‐OH) as a macro‐initiator. These copolymers were melt blended with PLLA/PCL (80/20) blend at contents between 3.0 and 20 phr (parts per hundred resin), and the effects of added PCL‐b‐PLLA on the mechanical, morphological, rheological, and thermodynamic properties of the PLLA/PCL/PCL‐b‐PLLA blends were investigated. The compatibility between PLLA matrix and PCL phase was enhanced with decreasing in CL/LA ratios or increasing in M_n for the added PCL‐b‐PLLA. Moreover, the crystallinity of PLLA matrix increased because of the added compatibilizers. The PCL‐b‐PLLA with the ratio of CL/LA (50/50) and M_n ≥ 39.0 kg/mol were effective compatibilizers for PLLA/PCL blends. When the content of PCL‐b‐PLLA is greater than or equal to 5 phr, the elongations at break of the PLLA/PCL/PCL‐b‐PLLA blends all reached approximately 180%, about 25 times more than the pristine PLLA/PCL(80/20) blend.     

Macromolecular Organization of Poly(L‐lactide)‐block‐Polyoxyethylene into Bio‐Inspired Nano‐Architectures

Block copolymers create various types of nano‐structures, e. g., spheres, rods, cubes, and lamellae. This review discloses the dynamic macromolecular organization of block copolymers comprising poly(L ‐lactide) (PLLA) and poly(oxyethylene) (PEG) that allows to simulate elaborate biological systems. The block copolymers, AB‐ (PLLA‐PEG) and ABA‐type (PLLA‐PEG‐PLLA), are synthesized by ordinary lactide polymerization to have a controlled block length. They are dispersed into an aqueous medium to prepare nano‐scale particles, consisting of hydrophobic PLLA and hydrophilic PEG in the core and shell, respectively. Then, the particles are placed on a flat substrate by the casting method. The particles are detected as discoids by AFM, having shrunk with loss of water. Heat‐treatment of these particles at 60°C (above T_g of PLLA) gives rise to a collapse into small fragments, which then aggregate into bands with nano‐size width and thickness. The PLLA‐PEG bands align parallel to each other, while the PLLA‐PEG‐PLLA bands form a characteristic network resembling the neuron system created in animal tissue. As analyzed by TEM diffraction, each is composed of α‐crystal of PLLA whose c‐axis (molecular axis) is perpendicular to the substrate surface. Based on this fact, a doubly twisted chain structure of PLLA is proposed in addition to a plausible mechanism for the self‐organization of the block copolymers. Derivatives of the PLLA‐PEG block copolymers can form far more interesting nano‐architectures. An equimolar mixture of enantiomeric copolymers, PLLA‐PEG‐PLLA and PDLA‐PEG‐PDLA, forms a hydrogel that is thermo‐responsive. The terminal‐modified poly(L ‐lactide)‐block‐polyoxyethylene monocinnamate (PLLA‐PEG‐C) forms a highly stabilized nanofiber by the photo‐reaction of the cinnamates placed in the outer layer of the nanobands.