Electron-beam irradiation on poly(propylene carbonate) in the presence of polyfunctional monomers

Poly(propylene carbonate) (PPC) showed predominantly degradation under electron-beam irradiation, accompanied by deterioration of its mechanical performance due to sharp decrease of the molecular weight. Crosslinked PPC was prepared by addition of polyfunctional monomer (PFM) to enhance the mechanical performance of PPC. When 8 wt% of PFM like triallyl isocyanurate (TAIL) was added, crosslinked PPC with a gel fraction of 60.7% was prepared at 50 kGy irradiation dose, which showed a tensile strength at 20 degrees C of 45.5 MPa, whereas it was only 38.5 MPa for pure PPC. The onset degradation temperature (T-i) and glass transition temperature (T-g) of this crosslinked PPC was 246 degrees C and 45 degrees C, respectively, a significant increase related to pure PPC of 211 degrees C and 36 C. Therefore, thermal and mechanical performances of PPC could be improved via electron-beam irradiation in the presence of suitable PFM.

Self-assembly morphology effects on the crystallization of semicrystalline block copolymer thin film

Self-assembly morphology effects on the crystalline behavior of asymmetric semicrystalline block copolymer polystyrene-block-poly(L-lactic acid) thin film were investigated. Firstly, a series of distinctive self-assembly aggregates, from spherical to ellipsoid and rhombic lamellar micelles (two different kinds of rhombic micelles, defined as rhomb 1 and rhomb 2) was prepared by means of promoting the solvent selectivity. Then, the effects of these self-assembly aggregates on crystallization at the early stage of film evolution were investigated by in situ hot stage atomic force microscopy. Heterogeneous nucleation initiated from the spherical micelles and dendrites with flat on crystals appeared with increasing temperature. At high temperature, protruding structures were observed due to the thickening of the flat-on crystals and finally more thermodynamically stable crystallization formed. Annealing the rhombic lamellar micelles resulted in different phenomena. Turtle-shell-like crystalline structure initiated from the periphery of the rhombic micelle 1 and spread over the whole film surface in the presence of mostly noncrystalline domain interior. Erosion and small hole appeared at the surface of the rhombic lamellar micelle 2; no crystallization like that in rhomb 1 occurred. It indicated that the chain-folding degree was different in these two micelles, which resulted in different annealing behaviors.

Study of hydrogen-bonded blend of polylactide with biodegradable hyperbranched poly(ester amide)

Polylactide (PLA) was melt blended with a biodegradable hyperbranched poly(ester amide) (HBP) to enhance its flexibility and toughness without sacrificing comprehensive performance. The advantage of using HBP was due to its unique spherical shape, low melt viscosity, and abundant functional end groups together with its easy access. Rheological measurement showed that blending PLA with as little as 2.5% HBP resulted in a 40% reduction of melt viscosity. The glass transition temperature (T-g) of PLA in the blends decreased slightly with the increase of HBP content, indicating partial miscibility which resulted from intermolecular interactions via H-bonding. The H-bonding involving CO of PLA with OH and NH of HBP was evidenced by FTIR analysis for the first time. The HBP component, as a heterogeneous nucleating agent, accelerated the crystallization rate of PLA. Remarkably, with the increase of HBP content, the elongation at break of PLA blends dramatically increased without severe loss in tensile strength, even the tensile strength increased within 10% content of HBP. The stress-strain curves and the SEM photos of impact-fractured surface showed the material changed from brittle to ductile failure with the addition of HBP. Reasonable interfacial adhesion via H-bonding and finely dispersed particulate structure of HBP in PLA were proposed to be responsible for the improved mechanical properties.

Self-assembly of a polymer pair through poly(lactide) stereocomplexation

A polymer pair composed of poly( N-isopropylacrylamide-co-2-hydroxyethyl methacrylate terminated oligo( L-lactide)) ( poly( NIPAAm-co-HEMAOLLA)) graft random copolymer and poly( D-lactide) ( PDLA) homopolymer was self-assembled into micelles with a diameter around 100 nm through the stereocomplexation between the OLLA branches of the graft copolymer and the PDLA homopolymer. The specific intermolecular stereocomplexation was considered as the powerful ordered aggregation force in the micelle cores. The shell's component of poly( NIPAAm-co-HEMA) and its thermosensitivity were proved by H-1 nuclear magnetic resonance ( NMR) and dynamic light scattering ( DLS), respectively. The incorporation of PDLA homopolymer into the graft copolymer affected the micelle size and the critical micelle concentration ( CMC). The incorporation of even a small quantity ( 11 wt%) of PDLA into the graft copolymer micelles resulted in a great decrease of the micelle size. For the graft copolymer with low per cent grafting of 18%, the size of the corresponding micelles decreased slightly even if the PDLA content increased up to 33 wt%. For the graft copolymer with high per cent grafting of 58%, with the further increase of PDLA content, the size of the corresponding micelles at first decreased further and then began to increase. The molecular weight of the PDLA did not significantly affect the micelle size.

Polymerization of rac-lactide using schiff base aluminum catalysts: Structure, activity, and stereoselectivity

A series of aluminum ethyls and isopropoxides based upon N,N,O,O-tetradentate Schiff base ligand framework have been prepared. X-ray diffraction analysis and H-1 NMR confirmed that these Schiff base aluminum ethyls and isopropoxides were all monomeric species with a five-coordinated central aluminum in their solid structures. Compared to the aluminum ethyls which all retain their monomeric structure in the solution, the dinucleating phenomenons of aluminum isopropoxides with less steric hindered substituents in the solution have also been observed. The activities and stereoselectivities of these complexes toward the ring-opening polymerization of rac-lactide have been investigated. Polymerization experiments indicated that (SB-2d)(AlOPr)-Pr-i [(SB-2d) = 2,2-dimethyl-1,3-propylenebis(3,5-di-tert-butylsalicylideneiminato)] exhibited the highest stereoselectivity and (SB-3b)(AlOPr)-Pr-i [(SB-3b) = 2,2-dimethyl-1,3-propylenebis(3,5-dichlorinesalicylideneiminato)] possessed the highest activity among these aluminum isopropoxides. The substituents and the mode of the bridging part between the two nitrogen atoms both exerted significant influences upon the progress of the polymerizations, influencing either the tacticity of isolated polymers or the rate of polymerization.

Biodegradable poly(L-lactide)/poly(epsilon-caprolactone)-modified montmorillonite nanocomposites: Preparation and characterization

New nanocomposites were prepared by melt blending poly(L-lactide) (PLLA), poly(epsilon-caprolactone) (PCL), and organically modified montmorillonite (OMMT). The obtained nanocomposites showed enhanced tensile strength, modulus and elongation at break than that of PLLA/PCL blends. The dynamic mechanical analysis showed the increasing mechanical properties with temperature dependence of nanocomposites. Wide-angle X-ray diffraction analysis and transmission electron microscopy indicated that the material formed the nanostructure. Adding OMMT improved the thermal stability and crystalline abilities of nanocomposites. The morphology was investigated by environmental scanning electron microscopy, which showed that increasing content of OMMT reduces the domain size of phase-separated particles. The specific interaction between each polymer and OMMT was characterized by the Flory-Huggins interaction parameter, B, which was determined by the equilibrium melting point depression of nanocomposites. The final values of B showed that PLLA was more compatible with OMMT than PCL.

Formation of nonextinct ring-banded textures and multistacked lamella of tetra-aniline-block-poly(L-lactide) rod-coil diblock oligomer films induced by solvent vapor treatment

The self-assembly processes of the rod-coil diblock oligomer thin film of tetra-aniline (TANI)-block-poly(L-lactide) (PLLA) with different film thicknesses induced in the coil-selective solvent of acetone vapor at room temperature were studied. The morphologies of the oligomer films were determined by the film thickness. For the thicker film (232 nm), the nonextinct concentric ring-banded textures could form. While for the thinner and appropriate film (about 6 nm), multistacked diamond-shaped appearances with the periodic thickness being about 8.5 nm(6-nm-thick extended PLLA chain and 2.5-nm-thick p-pi conjugating TANI bimolecular layer) formed. The possible formation models of those two regular morphologies were presented in detail.

Self-assembly of crystalline-coil diblock copolymer in solvents with varying selectivity: From spinodal-like aggregates to spheres, cylinders, and lamellae

We have investigated systematically the morphology of thin films spin-coated from solutions of a semicrystalline diblock copolymer, poly(L-lactic acid)-block-polystyrene (PLLA-b-PS), in solvents with varying selectivity. In neutral solvents (chloroform and tetrahydrofuran (THF)), a spinodal-like pattern was obtained and the pattern boundary was sharpened by diluting the solution. Meanwhile, loose spherical associates, together with larger aggregates composed of these associates by unimer bridges, formed partly due to crystallization of the PLLA blocks in relatively concentrated solutions. In slightly PS-selective solvent (e.g., benzene), both loose and compact spherical micelles were obtained, depending on the polymer concentration, coexisting with unimers. When enhancing the selectivity with mixed solvents, for example, mixing the neutral solvent and the slightly selective solvent with a highly PS-selective solvent, CS2, loose assemblies (nanorods in CS2/THF mixtures and polydisperse aggregates in CS2/benzene mixtures) and well-developed lamellar micelles were obtained.

Stereoselective polymerization of rac-lactide with a bulky aluminum/Schiff base complex

An aluminum/Schiff base complex {[2,2-dimethyl-1,3-propylenebis(3,5-di-tert-butylsalicylideneiminato)](isopropanolato)aluminum(III) (2)} based on a bulky ligand and aluminum isopropoxide was prepared and employed for the stereoselective ring-opening polymerization (ROP) of rac-lactide (rac-LA). The initiator was characterized with nuclear magnetic resonance (NMR), crystal structure measurements, and elemental analysis. It contained a five-coordinate aluminum atom that was trigonal bipyramidal in the solid state according to the crystal structure measurements. The two conformational stereoisomers of 2 exchanged quickly on the NMR scale. Compound 2 polymerized rac-LA into a crystalline polymer that was characterized with H-1 NMR, wide-angle X-ray diffraction, electrospray ionization mass spectrometry, and gel permeation chromatography. The kinetics of the polymerization were first-order in both the monomer and initiator, and there was a linear relationship between the rac-LA conversion and the number-average molecular weight of poly(rac-LA) with a narrow molecular distribution (1.04-1.08). These features showed that the polymerization was well controlled. The high melting temperature (196-201 degreesC) and isotacticity of poly(rac-LA) indicated that complex 2 was a highly stereoselective initiator for the ROP of rac-LA.

Stereoselective polymerization of rac-lactide using a monoethylaluminum Schiff base complex

A monoethylaluminum Schiff base complex (2) with formula LA1Et (L = N,N'-(2,2-dimethylpropylene)bis(3,5-di-tei-t-butylsalicylideneimine) was synthesized and employed for the stercoselective ring-opening polymerization of rac-lactide (rac-LA). The complex 2 was characterized by nuclear magnetic resonance, crystal structure, and elemental analysis. It contains a five-coordinate aluminum atom with distorted trigonal bipyramidal geornetry in the solid state. In the presence of 2-propanol, 2 showed high stereoselectivity for the polymerization of rac-LA. The polymerization yielded crystalline poly(rac-LA) with a high melting temperature (193-201 degreesC). NMR, differential scanning calorimetry, and wide-angle X-ray diffraction indicated that the poly(rac-LA) was highly isotactic, and a stereocomplex was formed between poly-L- and poly-D-lactide block sequences. By the analysis of electrospray-ionization mass spectrometry and H-1 NMR, the polymer was demonstrated to be endcapped in both terminals with an isopropyl ester and a hydroxy group, respectively. The polymerization was of first order in rac-LA concentration. The relationship between the rac-LA conversion and molecular weights of the polymer was linear so that the polymerization could be well controlled.

Enzymatic degradation of poly(L-lactide) and poly (epsilon-caprolactone) electrospun fibers

Poly(L-lactide) (PLLA) and poly(epsilon-caprolactone) (PCL) ultrafine fibers were prepared by electrospinning. The influence of cationic and anionic surfactants on their enzymatic degradation behavior was investigated by measuring weight loss, molecular weight, crystallinity, and melting temperature of the fibers as a function of degradation time. Under the catalysis of proteinase K, the PLLA fibers containing the anionic surfactant sodium docecyl sulfate (SDS) exhibited a faster degradation rate than those containing cationic surfactant triethylbenzylammonium chloride (TEBAC), indicating that surface electric charge on the fibers is a critical factor for an enzymatic degradation. Similarly, TEBAC-containing PCL fibers exhibited a 47% weight loss within 8.5 h whereas SDS-containing PCL fibers showed little degradation in the presence of lipase PS. By analyzing the charge status of proteinase K and lipase PS under the experimental conditions, the importance of the surface charges of the fibers and their interactions with the charges on the enzymes were revealed. Consequently, a "two-step" degradation mechanism was proposed: (1) the enzyme approaches the fiber surface; (2) the enzyme initiates hydrolysis of the polymer.

Effect of crystallization on the lamellar orientation in thin films of symmetric poly(styrene)-b-poly(L-lactide) diblock copolymer

The effect of crystallization on the lamellar orientation of poly( styrene)-b-poly(L-lactide) (PS-PLLA) semicrystalline diblock copolymer in thin films has been investigated by atomic force microscopy (AFM), transmission electronic microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). In the melt state, microphase separation leads to a symmetric wetting structure with PLLA blocks located at both polymer/substrate and polymer/air interfaces. The lamellar period is equal to the long period L in bulk determined by small-angle X-ray scattering (SAXS). Symmetric wetting structure formed in the melt state provides a model structure to study the crystallization of PLLA monolayer tethered on glassy (T-c < T-g,T-PS) or rubber (T-c > T-g,T-PS) PS substrate. In both cases, it is found that the crystallization of PLLA results in a "sandwich" structure with amorphous PS layer located at both folding surfaces. For T-c <= T-g,T- PS, the crystallization induces a transition of the lamellar orientation from parallel to perpendicular to substrate in between and front of the crystals. In addition, the depletion of materials around the crystals leads to the formation of holes of 1/2 L, leaving the adsorbed monolayer exposure at the bottom of the holes.

BCNU-loaded PEG-PLLA ultrafine fibers and their in vitro antitumor activity against glioma C6 cells

The purpose of the present study was to develop implantable BCNU-toaded poly(ethylene glycol)poly(L-lactic acid) (PEG-PLLA) diblock copolymer fibers for the controlled release of 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). BCNU was well incorporated and dispersed uniformly in biodegradable PEG-PLLA fibers by using electrospinning method. Environmental Scanning Electron Microscope (ESEM) images indicated that the BCNU-loaded PEG-PLLA fibers looked uniform and their surfaces were reasonably smooth. Their average diameters were below 1500 nm. The release rate of BCNU from the fiber mats increased with the increase of BCNU loading amount. In vitro cytotoxicity assay showed that the PEG-PLLA fibers themselves did not affect the growth of rat Glioma C6 cells. Antitumor activity of the BCNU-loaded fibers against the cells was kept over the whole experiment process, while that of pristine BCNU disappeared within 48 h. These results strongly suggest that the BCNU/PEG-PLLA fibers have an effect of controlled release of BCNU and are suitable for postoperative chemotherapy of cancers.

Biodegradable amphiphilic triblock copolymer bearing pendant glucose residues: Preparation and specific interaction with Concanavalin A molecules

A novel biodegradable amphiphilic block copolymer PLGG-PEG-PLGG bearing pendant glucose residues is successfully prepared by the coupling reaction of 3-(2-aminoethylthio) propyl-R-D-glucopyranoside with the pendant carboxyl groups of PLGG-PEG-PLGG in the presence of N,N'-carbonyldiimidazole. The polymer PLGG-PEG-PLGG, i.e., poly {(lactic acid)-co-[(glycolic acid)-alt-(L-glutamic acid)]}-block-poly(ethylene glycol)-block-poly{( lactic acid)-co-[( glycolic acid)-alt-(L-glutamic acid)]}, is prepared by ring-opening copolymerization of L-lactide (LLA) with (3s)-benzoxylcarbonylethylmorpholine-2,5-dione (BEMD) in the presence of dihydroxyl PEG with molecular weight of 2000 as macroinitiator and Sn(Oct)(2) as catalyst, and then by catalytic hydrogenation. The glucose-grafted copolymer shows a lower degree of cytotoxicity to ECV-304 cells and improved specific recognition and binding with Concanavalin A (Con A). Therefore, this kind of glucose-grafted copolymer may find biomedical applications.

Preparation of core-sheath composite nanofibers by emulsion electrospinning

Uniform core-sheath nanofibers are prepared by electrospinning a water-in-oil emulsion in which the aqueous phase consists of a poly(ethylene oxide) (PEO) solution in water and the oily phase is a chloroform solution of an amphiphilic poly(ethylene glycol)-poly(L-lactic acid) (PEGPLA) diblock copolymer. The obtained fibers are composed of a PEO core and a PEG-PLA sheath with a sharp boundary in between. By adjusting the emulsion composition and the emulsification parameters, the overall fiber size and the relative diameters of the core and the sheath can be changed. A mechanism is proposed to explain the process of transformation from the emulsion to the core-sheath fibers, i.e., the stretching and evaporation induced de-emulsification. In principle, this process can be applied to other systems to prepare core-sheath fibers in place of concentric electrospinning and it is especially suitable for fabricating composite nanofibers that contain water-soluble drugs.