Stereocomplex formation between enantiomeric poly(lactic acid)s. 12. spherulite growth of low-molecular-weight poly(lactic acid)s from the melt

The spherulite growth of stereocomplex crystallites in the blend from low-molecular-weight poly(L-lactide) [i.e., poly(L-lactic acid) (PLLA)] and poly(D-lactide) [i.e., poly(D-lactic acid) (PDLA)] from the melt, together with that of the homocrystallites in pure PLLA and PDLA films, was investigated using polarization optical miscroscopy. The spherulite growth of stereocomplex crystallites occurred at a wider temperature range (相似文献   

In vitro hydrolysis of blends from enantiomeric poly(lactide)s. 3. Homocrystallized and amorphous blend films

Homocrystallized and amorphous enantiomeric blend films were prepared from the melt of high molecular weight poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) (1:1) by crystallization and quenching, respectively. A phosphate-buffered solution was used to investigate effects of homocrystallinity via in vitro hydrolysis as well as crystallization process during the hydrolysis, which was performed for a period of 24 months at 37 degrees C and pH 7.4. Results derived from gravimetry, gel permeation chromatography, and tensile testing showed that hydrolyzability was higher for the homocrystallized film than for the amorphous film. Thus, probable mechanisms are proposed for the enhanced hydrolysis of the homocrystallized blend film compared with that of the amorphous blend film. The hydrolysis rate constant (k) values of the homocrystallized and amorphous films estimated from the changes in number-average molecular weight (M(n)) were 5.00 x 10(-3) and 3.32 x 10(-3) day(-1), respectively. Moreover, hydrolyzability of equimolar enantiomeric poly(lactic acid) blends can be altered in the k range of 0.73 x 10(-3) and 5.00 x 10(-3) day(-1) by varying their crystalline species, crystallinity, or molecular weights.     

Electrospinning of poly(lactic acid) stereocomplex nanofibers

The electrospinning of stereocomplex nanofibers of high-molecular-weight poly(L-lactic acid) (PLLA)/poly(D-lactic acid) (PDLA) (PLLA/PDLA = 1:1) was carried out with chloroform as the spinning solvent. The stereocomplex nanofibers with diameters of 830-1400 and 400-970 nm were successfully obtained at voltages of -12 and -25 kV, respectively. Wide-angle X-ray scattering indicated that with an increasing absolute value of voltage from 0 to 25 kV the crystallinity of homo-crystallites composed of either PLLA or PDLA decreased from 5% to 1%, whereas the crystallinity of stereocomplex crystallites increased slightly from 16% to 20%. The obtained results reveal that electrospinning is an effective method to prepare stereocomplex nanofibers with a negligibly small amount of homo-crystallites, even when high-molecular-weight PLLA and PDLA are used, and that the orientation caused by high voltage (or electrically induced high shearing force) during electrospinning enhances the formation and growth of stereocomplex crystallites and suppresses the formation of homo-crystallites.     

Compatibilization effect of poly(epsilon-caprolactone)-b-poly(ethylene glycol) block copolymers and phase morphology analysis in immiscible poly(lactide)/poly(epsilon-caprolactone) blends

The miscibility and phase behavior of two stereoisomer forms of poly(lactide) (PLA: poly (L-lactide) (PLLA) and poly(DL-lactide) (PDLLA)) blends with poly(epsilon-caprolactone)-b-poly(ethylene glycol) (PCL-b-PEG) and PCL-b-monomethoxy-PEG (PCL-b-MPEG) block copolymers have been investigated by differential scanning calorimetry (DSC). The DSC thermal behavior of both the blend systems revealed that PLA is miscible with the PEG segment phase of PCL-b-(M)PEG but is still immiscible with its PCL segment phase although PCL was block-copolymerized with PEG. On the basis of these results, PCL-b-PEG was added as a compatibilizer to PLA/PCL binary blends. The improvement in mechanical properties of PLA/PCL blends was achieved as anticipated upon the addition of PCL-b-PEG. In addition, atomic force microscopy (AFM) measurements have been performed in order to study the compositional synergism to be observed in mechanical tests. AFM observations of the morphological dependency on blend composition indicate that PLA/PCL blends are immiscible but compatible to some extent and that synergism of compatibilizing may be maximized in the compositional blend ratio before apparent phase separation and coarsening.     

In-situ formation of biodegradable hydrogels by stereocomplexation of PEG-(PLLA)8 and PEG-(PDLA)8 star block copolymers

Eight-arm poly(ethylene glycol)-poly(L-lactide), PEG-(PLLA)(8), and poly(ethylene glycol)-poly(D-lactide), PEG-(PDLA)(8), star block copolymers were synthesized by ring-opening polymerization of either L-lactide or D-lactide at room temperature in the presence of a single-site ethylzinc complex and 8-arm PEG (M(n) = 21.8 x 10(3) or 43.5 x 10(3)) as a catalyst and initiator, respectively. High lactide conversions (>95%) and well-defined copolymers with PLLA or PDLA blocks of the desired molecular weights were obtained. Star block copolymers were water-soluble when the number of lactyl units per poly(lactide) (PLA) block did not exceed 14 and 17 for PEG21800-(PLA)(8) and PEG43500-(PLA)(8), respectively. PEG-(PLA)(8) stereocomplexed hydrogels were prepared by mixing aqueous solutions with equimolar amounts of PEG-(PLLA)(8) and PEG-(PDLA)(8) in a polymer concentration range of 5-25 w/v % for PEG21800-(PLA)(8) star block copolymers and of 6-8 w/v % for PEG43500-(PLA)(8) star block copolymers. The gelation is driven by stereocomplexation of the PLLA and PDLA blocks, as confirmed by wide-angle X-ray scattering experiments. The stereocomplexed hydrogels were stable in a range from 10 to 70 degrees C, depending on their aqueous concentration and the PLA block length. Stereocomplexed hydrogels at 10 w/v % polymer concentration showed larger hydrophilic and hydrophobic domains as compared to 10 w/v % single enantiomer solutions, as determined by cryo-TEM. Correspondingly, dynamic light scattering showed that 1 w/v % solutions containing both PEG-(PLLA)(8) and PEG-(PDLA)(8) have larger "micelles" as compared to 1 w/v % single enantiomer solutions. With increasing polymer concentration and PLLA and PDLA block length, the storage modulus of the stereocomplexed hydrogels increases and the gelation time decreases. Stereocomplexed hydrogels with high storage moduli (up to 14 kPa) could be obtained at 37 degrees C in PBS. These stereocomplexed hydrogels are promising for use in biomedical applications, including drug delivery and tissue engineering, because they are biodegradable and the in-situ formation allows for easy immobilization of drugs and cells.     

Phase structure and enzymatic degradation of poly(L-lactide)/atactic poly(3-hydroxybutyrate) blends: an atomic force microscopy study

Phase structures and enzymatic degradation of poly(l-lactide) (PLLA)/atactic poly(3-hydroxybutyrate) (ata-PHB) blends with different compositions were characterized by using atomic force microscopy (AFM). Differential scanning calorimetry (DSC) thermograms of PLLA/ata-PHB blends with different compositions showed two glass transition temperatures, indicating that the PLLA/ata-PHB blends are immiscible in the melt. Surface morphologies of the thin films for PLLA/ata-PHB blends were determined by AFM. Phase separated morphology was recognized from the AFM topography and phase images. The domain size of the components was dependent on the blend ratio. Enzymatic degradation of the PLLA/ata-PHB blends was performed by using both PHB depolymerase and proteinase K. Either PLLA or ata-PHB domains were eroded depending on the kinds of enzyme. Surface morphologies after enzymatic degradation have revealed the phase structure along the depth direction. Enzymatic adsorption of PHB depolymerase was examined on the surface of PLLA/ata-PHB blends. The enzyme molecules were found on both domains of the binary blends. The larger number of enzyme molecules was found on the PLLA domains relative to those on the ata-PHB domains, suggesting the higher affinity of the enzyme against PLLA domain.     

Environmental degradation of polyester blends containing atactic poly(3-hydroxybutyrate). Biodegradation in soil and ecotoxicological impact

The degradation of poly[(R,S)-3-hydroxybutyrate], a-PHB, binary blends with natural PHB (n-PHB) and poly(L-lactic acid), PLLA, respectively, has been investigated in soil. In such a natural environment, a-PHB blend component was found to biodegrade. The degradation of a-PHB-containing blends proceeded faster than that of respective plain n-PHB and PLLA. The molecular weight decrease of the n-PHB component was higher, while the same rate of bioerosion of both components was observed for the a-PHB/n-PHB binary blend. For the a-PHB blend with PLLA, the weight loss was accompanied by blend composition changes and the decrease of a-PHB content. However, the PLLA molecular weight decrease was lower in the blend in comparison with the plain PLLA sample. The increase of the number of microorganisms particularly observed for the soil where binary blends were incubated indicates that microbial degradation of a-PHB takes place. The terrestrial plant growth test (cress and barley) demonstrates no environmental toxicity of the materials studied.     

Stereocomplex formation by enantiomeric poly(lactic acid) graft-type phospholipid polymers for tissue engineering

A porous scaffold as a cell-compatible material was designed and prepared using a phospholipid copolymer composed of 2-methacryloyloxyethyl phosphorylcholine (MPC), n-butyl methacrylate, and enantiomeric macromonomers, the poly(L-lactic acid) (PLLA) macromonomer, and poly(D-lactic acid) (PDLA) macromonomer. On the basis of the wide-angle X-ray diffraction and differential scanning calorimetry measurements, the formation of a stereocomplex between the PLLA and PDLA segments of the copolymer was observed on the porous scaffold. The porous structure was prepared by a sodium chloride leaching technique, and the pore was linked to the scaffold. The pore size was confirmed by scanning electron microscopy and found to be ca. 200 microm. These observations suggest that the porous scaffold makes it possible to produce cell-compatible materials, which may involve the following advantages for tissue engineering: (i) cell compatibility using phospholipid copolymer, (ii) adequate cell adhesion by poly(lactic acid), and (iii) complete disappearance of scaffold by dissociation of stereocomplex. The cell experiment using the porous scaffold will be the next subject and reported in a forthcoming paper.     

Physical characterization of blends of poly(D-lactide) and LHRH (a leuprolide decapeptide analog)

Stereocomplexes between poly(D-lactide) (PDLA) and poly(L-lactide) (PLLA) have been extensively studied, including, in recent years, several reports on the stereocomplexation of PDLA with small molecules such as peptides. Here, the possible complexation between PDLA and luteinizing hormone releasing hormone (LHRH), a l-configured decapeptide, is considered for which several observations were made: (1) in calorimetry an additional endothermic peak appears at a lower temperature than the melting temperature of pure PDLA; (2) in Raman analyses a band splitting of the C=O stretching mode (not present in pure PDLA) shows up; (3) in X-ray diffraction, however, no change is observed after mixing the two species, indicating no crystal structure modification (and the absence of any stereocomplex crystal structure). The calorimetric double melting peak is merely explained by the presence of two distinct morphological forms of PDLA, whereas the spectroscopic band splitting can be due to simple differences of crystallinity. From these observations it is concluded that the LHRH modifies the crystallization of PDLA without, however, the formation of a stereocomplex.     

Cell adhesion and morphology in porous scaffold based on enantiomeric poly(lactic acid) graft-type phospholipid polymers

Poly(D-lactic acid) (PDLA) and poly(L-lactic acid) (PLLA) macromonomers were synthesized for preparation of a novel cytocompatible polymer. The cytocompatible polymer was composed of 2-methacryloyloxyethyl phosphorylcholine (MPC), n-butyl methacrylate (BMA), and the enantiomeric PLLA (or PDLA) macromonomer. The degree of polymerization of the lactic acid in the PLLA and PDLA segments was designed to be ca. 20. The copolymer-coated surface was analyzed with static contact angle by water. From the result, the PLLA (or PDLA) segment and MPC unit were located on the coated surface, and the monomer unit in the copolymer was reconstructed by contacting water. Fibroblast cell culture was performed to evaluate cell adhesion on the coated surface, and the cell morphology was observed. The number of cell adhesion is correlated with the PL(D)LA content, and the cell morphology is correlated with the MPC unit content. The porous scaffold was prepared by the formation of a stereocomplex between the PLLA and PDLA, and the cell adhesion and following cell intrusion was then evaluated. The fibroblast cells adhered on the surface and intruded into the scaffold through the connecting pores after 24 h. The cell morphology became round shape from spreading with the decreasing PLLA (or PDLA) content in the copolymer. It is considered that the change in the cell morphology would be induced by the MPC unit as cytocompatible unit. These findings suggest that the porous scaffold makes it possible to have cytocompatibility and to produce three-dimensional tissue regeneration.     

Biodegradable films of partly branched poly(l-lactide)-co-poly(epsilon-caprolactone) copolymer: modulation of phase morphology, plasticization properties and thermal depolymerization

We report on the modulation of phase morphology, plasticization properties, and thermal stability of films of partly branched poly(l-lactide)-co-poly(epsilon-caprolactone) copolymer (PLLA-co-PCL) with additions of low molecular weight compounds, namely, triethyl citrate ester, diethyl phthalate, diepoxy polyether (poly(propylene glycol) diglycidyl ether), and with epoxidized soybean oil (ESO). The PLLA-co-PCL/polyether films showed significant stability against thermal depolymerization, high film flexibility, and good plasticizing properties, probably due to cross-linking and chain branching formation between diepoxy groups with both the end carboxyl and hydroxyl groups of the PLLA copolymer (initially present or generated during the degradation process) to produce primary ester and ether bonds, respectively. Diethyl phthalate and triethyl citrate ester were found to be efficient plasticizers for PLLA copolymer in terms of glass transition and mechanical properties, but the more water-soluble plasticizer triethyl citrate induced a dramatic loss in the molecular weight of the copolymer. Although ESO cannot play the role of a plasticizer, it substantially stabilizes and retards thermal depolymerization of the PLLA copolymer matrix, possibly because of a reaction between epoxy groups with the end carboxyl and hydroxyl groups of the PLLA copolymer. The presence of ESO in PLLA-co-PCL/ESO/triethyl citrate blends enhanced the compatibility and miscibility of the plasticizer with the PLLA copolymer matrix, considerably improved the mechanical properties (elongation at break), and substantially stabilized the copolymer against thermal depolymerization. It seems likely that the epoxy groups interact not only with the end hydroxyl and carboxyl group of the copolymer but as well with the hydroxyl group of triethyl citrate plasticizer to produce a new ether bond (C-O-C) as the cross-linking unit. On the other hand, for PLLA-co-PCL/ESO/polyether blends, (80/10/10) epoxidized oil distorts the compactness of the blend by diminishing the proposed entanglements between carboxyl, hydroxyl, and diepoxy groups of polyether and reduces the high elongation properties otherwise observed in the PLLA-co-PCL/polyether films. The multicomponent approach toward modulating poly(l-lactide)-co-poly(epsilon-caprolactone) copolymer films using epoxy compounds and plasticizers and the insight into the nature of various PLLA matrixes presented here offer advantages to a broad engineering of PLLA copolymer films having desirable physical properties and multiphase behavior for efficient uses in future technical applications.     

Formation of and coalescence from the inclusion complex of a biodegradable block copolymer and alpha-cyclodextrin. 2: A novel way to regulate the biodegradation behavior of biodegradable block copolymers

A biodegradable block copolymer (PCL-b-PLLA, M(n) = 1.72 x 10(4), M(w)/M(n) = 1.37) of poly(epsilon-caprolactone) (PCL) and poly(L-lactide) (PLLA) with very low crystallinity was obtained by forming the inclusion complex between alpha-cyclodextrin molecules and PCL-b-PLLA followed by coalescence of the guest polymer chains. Films of the as-synthesized and coalesced copolymer samples, PCL and PLLA homopolymers of approximately the same chain lengths as the corresponding blocks of PCL-b-PLLA, and a physical blend of PCL/PLLA homopolymers with the same molar composition as PCL-b-PLLA were prepared by melt-compression molding between Teflon plates. Subsequently, the in vitro biodegradation behavior of these films was studied in phosphate buffer solution containing lipase from Rhizopus arrhizus, by means of ultraviolet spectra, attenuated total reflectance FTIR spectra, differential scanning calorimetry, wide-angle X-ray diffraction measurements, and weight loss analysis. PCL segments were found to degrade much faster than PLLA segments, both in the pure state and in copolymer or blend samples. Consistent with our expectation, suppression of the phase separation, as well as a decrease of crystallinity, in the coalesced copolymer sample led to a much faster enzymatic degradation than that of either as-synthesized copolymer or the PCL/PLLA physical blend sample, especially during the early stages of biodegradation. Thus the biodegradation behavior of biodegradable block copolymers, which is of decisive importance in drug delivery and controlled release systems, may be regulated by the novel and convenient means recently reported by us.(1)     

Interaction of poly(ethylene-glycols) with air-water interfaces and lipid monolayers: investigations on surface pressure and surface potential.

We have characterized the surface activity of different-sized poly(ethylene-glycols) (PEG; M(r) 200-100,000 Da) in the presence or absence of lipid monolayers and over a wide range of bulk PEG concentrations (10(-8)-10% w/v). Measurements of the surface potential and surface pressure demonstrate that PEGs interact with the air-water and lipid-water interfaces. Without lipid, PEG added either to the subphase or to the air-water interface forms relatively stable monolayers. Except for very low molecular weight polymers (PEGs < 1000 Da), low concentrations of PEG in the subphase (between 10(-5) and 10(-4)% w/v) increase the surface potential from zero (with respect to the potential of a pure air-water interface) to a plateau value of approximately 440 mV. At much higher polymer concentrations, > 10(-1)% (w/v), depending on the molecular weight of the PEG and corresponding to the concentration at which the polymers in solution are likely to overlap, the surface potential decreases. High concentrations of PEG in the subphase cause a similar decrease in the surface potential of densely packed lipid monolayers spread from either diphytanoyl phosphatidylcholine (DPhPC), dipalmitoyl phosphatidylcholine (DPPC), or dioleoyl phosphatidylserine (DOPS). Adding PEG as a monolayer at the air-water interface also affects the surface activity of DPhPC or DPPC monolayers. At low lipid concentration, the surface pressure and potential are determined by the polymer. For intermediate lipid concentrations, the surface pressure-area and surface potential-area isotherms show that the effects due to lipid and PEG are not always additive and that the polymer's effect is distinct for the two lipids. When PEG-lipid-mixed monolayers are compressed to surface pressures greater than the collapse pressure for a PEG monolayer, the surface pressure-area and surface potential-area isotherms approach that of the lipid alone, suggesting that for this experimental condition PEG is expelled from the interface.     

Wheat gluten-thiolated poly(vinyl alcohol) blends with improved mechanical properties

A multifunctional macromolecular thiol (TPVA) obtained by esterification of poly(vinyl alcohol) (PVA) with 3-mercaptopropionic acid was characterized by a combination of NMR, IR, transmission electron microscopy (TEM), and differential scanning calorimetry (DSC), and was used as a wheat gluten (WG) reactive modifier. The effect of TPVA molecular weight (M(w) = 2000, 9500, 50 000, and 205 000) and blend composition (5, 20, and 40% w/w TPVA/WG) on the mechanical properties of compression-molded bars indicates that TPVA/WG blends increase the fracture strength by up to 76%, the elongation by 80%, and the modulus by 25% above WG. In contrast, typical WG additives such as glycerol and sorbitol improve flexibility but decrease modulus and strength. Preliminary investigations of suspension rheology, water uptake, molecular weight distribution and electron microscopy of TPVA/WG and PVA/WG blends illustrate the different protein interactions with PVA and TPVA. Further work is underway to determine whether TPVA and WG form protein conjugates or microphase-separated morphologies.     

FT-IR imaging spectroscopy of phase separation in blends of poly(3-hydroxybutyrate) with poly(L-lactic acid) and poly(epsilon-caprolactone)

The detection of phase separation and identification of miscibility in biopolymer blends is an important aspect for the improvement of their physical properties. In this article, the phase separation in blends of poly(3-hydroxybutyrate) (PHB) with poly(L-lactic acid) (PLA) and poly(epsilon-caprolactone) (PCL), respectively, has been studied as a function of the blend composition by FT-IR imaging spectroscopy. For both polymer blend systems, a miscibility gap has been found around the 50:50% (w/w) composition of the two components. Furthermore, the separating phases have been identified as blends of the two polymer components and their compositions could be determined from calibrations based on the spectra of the blends in the compositional range of miscibility. The data derived from FT-IR spectroscopic imaging were corroborated by additional DSC analyses and mechanical stress-strain measurements of polymer blend films, which exhibited a characteristic fracture behavior as a function of PHB composition.     

Fine structure and enzymatic degradation of poly[(R)-3-hydroxybutyrate] and stereocomplexed poly(lactide) nanofibers

Fiber morphology and crystalline structure of poly[(R)-3-hydroxybutyrate] (P(3HB)) and stereocomplexed poly(lactide) (PLA) nanofibers were investigated by using scanning and transmission electron microscopies and X-ray and electron diffractions. In the P(3HB) nanofibers spun from less than 1 wt% 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) solution, planar zigzag conformation (beta-form) as well as 2(1) helix conformation (alpha-form) structure was formed. Based on the electron diffraction measurement of single P(3HB) nanofiber, it was revealed that the molecular chains of P(3HB) align parallel to the fiber direction. From the enzymatic degradation test of P(3HB) nanofiber, it was shown that beta-form molecular chains are degraded more preferentially than alpha-form chains. Stereocomplexed PLA nanofibers were electrospun from 1 wt% poly(l-lactide)/poly(d-lactide) (PLLA/PDLA) solution in HFIP, which contains equal amounts of PLLA and PDLA. While as-spun stereocomplexed PLA nanofiber was amorphous, PLA nanofiber annealed at 100 degrees C contained only racemic crystal. It was supposed that the crystallization behavior of stereocomplexed PLA in the nanofiber is affected by the electrospinning process, which forcibly exerts the strain onto the polymer chains.     

Blends of aliphatic polyesters. VI. Lipase-catalyzed hydrolysis and visualized phase structure of biodegradable blends from poly(epsilon-caprolactone) and poly(L-lactide)

Phase-separated biodegradable polymer blends were prepared from poly(epsilon-caprolactone) (PCL) and poly(L-lactide) (PLLA), and Rhizopus arrhizus lipase-catalyzed hydrolysis and phase structure of the blend films were investigated. Gravimetry revealed that the lipase-catalyzed hydrolysis of PCL in PCL- and PLLA-rich phases is disturbed by the presence of PLLA. Polarimetry confirmed the occurrence of a predominant hydrolysis of PCL and subsequent removal of the hydrolyzed water-soluble PCL oligomers in the blend films. Gravimetry and gel permeation chromatography of the non-blended PLLA film indicated that R. arrhizus lipase has no catalytic effect on the hydrolysis of PLLA. The phase structure of the blend films could be visualized by selective enzymatic removal of one component and subsequent scanning electron microscopic observation.     

Structural properties of poly C-scleroglucan complexes

Successive changes of solvent conditions can be used to dissociate and reassociate the triple-helical structure of (1,3)-beta-D-glucans. Ultramicroscopic techniques have revealed a blend of circular and other structures following renaturation. When this solvent exchange process is carried out in the presence of certain polynucleotides, the process creates a novel macromolecular complex. Here, we use size exclusion chromatography (SEC) to study such (1,3)-beta-D-glucan-polynucleotide complexes. Online multi-angle laser-light scattering (MALLS) and refractive index (RI) detectors allowed determination of molecular weight and radius of gyration of the molecules. An ultraviolet (UV) detector allowed specific detection of the polynucleotide. The poly-cytidylic acid (poly C) shifted to coelution with the linear fraction of the scleroglucan following the renaturation of poly C-scleroglucan blends, indicating that poly C is incorporated in linear, but not in circular, structures of scleroglucan. This conclusion was consistent with AFM topographs that revealed a decreased fraction of circular structures upon addition of poly C during the renaturation process. The combined information about radius of gyration (R(g)) and molecular weight (M(w)) allowed us to conclude that the poly C-scleroglucan complexes are more dense and have a higher persistence length than linear scleroglucan triple helixes. The experimentally determined mass per unit length was used as a basis for elucidating possible molecular arrangements within the poly C-scleroglucan complex.     

Morphology control in co-continuous poly(L-lactide)/polystyrene blends: a route towards highly structured and interconnected porosity in poly(L-lactide) materials

Poly(L-lactide) is a biodegradable polymer primarily used in biomedical applications. In this paper, both the microstructure and the region of dual-phase continuity are examined for binary and compatibilized poly(L-lactide)/polystyrene blends (PLLA/PS) prepared by melt mixing. The blends are shown to be completely immiscible with an interfacial tension of 6.1 mN/m. The PS-b-PLLA (24,000-b-28,000) diblock copolymer compatibilizer has an asymmetric effect on the blend. It is effective at compatibilizing 50/50 PLLA/PS blends but is only a marginal emulsifier for blends where PLLA is the dominant matrix. Percent continuity, as estimated by solvent extraction/gravimetry and also torque/composition diagrams clearly indicate an onset of the region of dual-phase continuity at 40-45%PS. It is demonstrated that highly percolated blends of the above materials exist from 40 to 75% PS and 40 to 60% PS for the binary and compatibilized blends, respectively. The scale of the microstructure of the continuous morphology is measured using BET and mercury intrusion porosimetry techniques, after extraction of the PS phase. Both the pore size and extent of continuity can be controlled through composition and interfacial modification. Static annealing of the blend after melt mixing can also be used to substantially increase the pore size of the system. Extraction of the PS phase in the blend, carried out after the above preparation protocols, is a route to generating completely interconnected porosity of highly controlled morphologies (pore size, void volume) in poly(L-lactide) materials. In this study, the pore diameter was controlled from 0.9 to 72 microm for a constant void volume of 45-47%, and the void volume was modified from 35 to 74% depending on the blend composition.     

Uniaxial drawing and mechanical properties of poly[(R)-3-hydroxybutyrate]/poly(L-lactic acid) blends

Blends of poly(L-lactic acid) (PLLA) with two kinds of poly[(R)-3-hydroxybutyrate] (PHB) having different molecular weights, commercial-grade bacterial PHB (bacterial-PHB) and ultrahigh molecular weight PHB (UHMW-PHB), were prepared by the solvent-casting method and uniaxially drawn at two drawing temperatures, around PHB's T(g) (2 degrees C) for PHB-rich blends and around PLLA's T(g) (60 degrees C) for PLLA-rich blends. Differential scanning calorimetry analysis showed that this system was immiscible over the entire composition range. Mechanical properties of all of the samples were improved in proportion to the draw ratio. Although PLLA domains in bacterial-PHB-rich blends remained almost unstretched during cold drawing, a good interfacial adhesion between two polymers and the reinforcing role of PLLA components led to enhanced mechanical properties proportionally to the PLLA content at the same draw ratio. On the contrary, in the case of UHMW-PHB-rich blends, the minor component PLLA was found to be also oriented by cold drawing in ice water due to an increase in the interfacial entanglements caused by the very long chain length of the matrix polymer. As a result, their mechanical properties were considerably improved with increasing PLLA content compared with the bacterial-PHB system. Scanning electron microscopy observations on the surface and cross-section revealed that a layered structure with uniformly oriented microporous in the interior was obtained by selectively removal of PLLA component after simple alkaline treatment.