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   

Synthesis, solid-state structure, and surface properties of end-capped poly(L-lactide)

End-capped poly(L-lactide) (PLLA) samples with dodecyl or 2-(2-(2-methoxyethoxy)ethoxy)ethyl (MEEE) ester were synthesized by ring-opening polymerization of L-lactide in the presence of zinc dodecanoxide or zinc 2-(2-(2-methoxyethoxy)ethoxy)ethoxide as a catalyst, respectively. On the basis of NMR analysis, it was confirmed that the carboxylic acid chain ends of PLLA molecules were selectively substituted by dodecyl or MEEE ester groups. To evaluate the wettability on the surface of end-capped PLLA films, the advancing contact angle (thetaa) with water was measured. The amorphous PLLA films showed relatively similar thetaa values regardless of the chemical structure of the polymer chain end. In contrast, the thetaa values of semicrystalline films were varied over a wide range, dependent on the chemical structure of the chain end. In addition, the thetaa values of dodecyl ester end-capped PLLA film with low molecular weight increased with an increase in the crystallization temperature. Both the crystallinity and lamellar thickness of dodecyl ester end-capped PLLA films increased with the crystallization temperature. These results suggest that the segregation of the chain ends on the PLLA film surface was strongly affected by the crystallization conditions.     

Influence of low-temperature nucleation on the crystallization process of poly(L-lactide)

The crystallization kinetics of poly(l-lactide), PLLA, is slow enough to allow a quasi-amorphous polymer to be obtained at low temperature simply by quenching from the melt. The PLLA crystallization process was followed by differential scanning calorimetry and optical microscopy after nucleation isothermal treatments at temperatures just below (53 degrees C) and just above (73 degrees C) the glass transition temperature. The crystallization exotherm shown in the heating thermograms shifts toward lower temperatures as the annealing time at 73 degrees C increases. The same effect is shown to a lesser extent when the sample nucleates at 53 degrees C, showing the ability to nucleate in the glassy state, already shown in other polymers. The shape of the DSC thermograms is modeled by using Avrami's theory and allows an estimation of the number of crystallization germs formed. The results of optical microscopy are converted to thermograms by evaluating the average gray level of the image recorded in transmission mode as a function of temperature and calculating its temperature derivative. The shape of such optical thermograms is quite similar to that of the DSC traces but shows some peculiarities after long nucleation treatments. Atomic force microscopy was used to analyze the crystal morphology and is an additional proof of the effect of nucleation in the glassy state. The crystalline morphology observed in samples crystallized after nucleation in the glassy state is qualitatively different from that of samples nucleated above the glass transition temperature, and the number of crystals seems to be much greater than what could be expected from the crystallization kinetics.     

Physical properties of poly (3,N4-ethenocytidylic acid)

Homopolymers of etheno CMP have been prepared by the action of polyribonucleotide phosphorylase upon etheno CDP. At alkaline pH the optical properties are consistent with a structure consisting of partially helical single-stranded chains whose helical regions are stabilized by base stacking. At acid pH the degree of helicity increases markedly. The degree of cooperativity displayed by the helix leads to coil transition induced by pH or temperature is less than for the case of polyribocytidylic acid. In the presence of acridine orange the alkaline form develops a strong extrinsic CD spectrum.     

Preparation and crystallization kinetics of new structurally well-defined star-shaped biodegradable poly(L-lactide)s initiated with diverse natural sugar alcohols

This study presents syntheses, structural characterization, and crystallization kinetic investigation of new structurally well-defined star-shaped poly(l-lactide)s (PLLAs). First, a series of new 3- to 6-arm star-shaped PLLAs were synthesized through SnOct(2) catalyzed ring-opening polymerization of (l)-lactide with natural sugar alcohols of glycerol, erythritol, xylitol, and sorbitol as the favorable initiators. Subsequently, their chemical structures were characterized by means of GPC, NMR, and viscometer with respect to the star-shaped structures, demonstrating the well-defined arm structures as evidenced on the g(1/2)/g' values, where g and g' denote the ratios of mean-square radius of gyration and intrinsic viscosity of a star-shaped polymer to those of a linear structural reference with similar absolute molecular weight. Furthermore, spherulite morphologies and growth rates were studied by a polarized microscopy (POM) for the synthesized star-shaped PLLAs with different molecular weights, and it was found that the more arms of a star-shaped PLLA finally resulted in a lower spherulite growth rate. With regard to the crystallization kinetics of these star-shaped PLLAs, isothermal and nonisothermal crystallization were examined by differential scanning calorimeter (DSC). It was found that Avrami exponent n values of isothermal crystallization were almost independent of the isothermal crystallization temperature T(c) for different series of star-shaped PLLAs. In contrast, the values of Avrami exponent n were observed to strongly depend on the star-shaped structures with different arms, implying their distinct nucleation mechanisms, and the more arms of a star-shaped PLLA led to a slower isothermal crystallization rate. On the basis of a modified Avrami equation, new light was shed on the nonisothermal crystallization kinetics for the star-shaped PLLAs, and the activation energies were found to vary from 146.86 kJ/mol for the linear PLLA EG-3 to 221.23 kJ/mol of the star-shaped S-3, demonstrating much decreased crystallizabilities of star-shaped PLLAs with more arms.     

Association of star-shaped poly(D,L-lactide)s containing nucleobase multiple hydrogen bonding

A new family of associating polymers based on four-arm, star-shaped poly(D,L-lactide) (PDLLA) containing peripheral complementary hydrogen-bonding sites is described. Hydroxy-terminated, four-arm, star-shaped PDLLAs of controlled molar mass were functionalized with complementary DNA base pairs, adenine (A) and thymine (T), to obtain PDLLA-A and PDLLA-T, respectively. 1H NMR spectroscopy confirmed quantitative functionalization and the subsequent formation of PDLLA-A and PDLLA-T hydrogen-bonded complexes. Job's analysis revealed a 1:1 optimal stoichiometry for the hydrogen-bonded complexes, and the association constant (Ka) that was determined using the 1H NMR-based Benesi-Hildebrand treatment was 84 M(-1) for the low molar mass complementary polymers. Furthermore, the PDLLA-based hydrogen-bonded complexes exhibited higher solution viscosities compared to the corresponding non-hydrogen-bonded precursors, which further confirmed strong complementary multiple-hydrogen-bonding associations between the star-shaped polymers with terminal adenine and thymine groups. Moreover, variable-temperature 1H NMR studies demonstrated the thermoreversibility of the hydrogen-bonded PDLLA-based complexes in solution.     

Plasticization of poly(L-lactide) with poly(propylene glycol)

A new plasticizer for poly(L-lactide) (PLA)-poly(propylene glycol) (PPG) is proposed. The advantage of using PPG is that it does not crystallize, has low glass transition temperature, and is miscible with PLA. PLA was plasticized with PPGs with nominal Mw of 425 and 1000 g/mol. Poly(ethylene glycol) (PEG), long known as a plasticizer for PLA, with nominal Mw of 600 g/mol, was also used to plasticize PLA for comparison. The thermal and tensile properties of PLA and PLA with 5-12.5 wt % of the plasticizers were studied. In blends of PLA with PPGs the glass transition temperature was lower than that of neat PLA. Both PPGs enhanced the crystallizability of PLA albeit less than PEG. All of the plasticizers increased also the ability of PLA to plastic deformation which was reflected in a decrease of yield stress and in an increase of elongation at break. The effect was enhanced by the higher PPG content and also by lower molecular weight of PPG. A phase separation occurred only in the blend containing 12.5 wt % of PPG with higher molecular weight. The evidences of crazing were found in deformed samples of PLA with low plasticizer content, whereas the samples with higher content of plasticizers crystallized due to deformation.     

Telechelic and star-shaped poly(L-lactide)s by means of bismuth(III) acetate as initiator

L-lactide was polymerized as concentrated solution in chlorobenzene with Bi(OAc)3 as initiator. When tetra(ethylene glycol) was added as co-initiator (CoI), telechelic polylactides having two CH-OH end groups were obtained. With 1,1,1-tri(hydroxy methyl)propane (THMP) as co-initiator, three-armed stars having three CH-OH end groups were formed. Analogously, tetrafunctional star-shaped poly(L-lactide)s were obtained with pentaerythritol as co-initiator. The chain lengths were varied via the monomer/CoI ratio. Time-conversion curves proved that Bi(OAc)3 is slightly less reactive as initiator than tin(II) 2-ethylhexanoate. However, bismuth acetate (or other carboxylates) have a particularly low toxicity as documented in the literature and by numerous Bi3+-containing pharmaceutical products.     

L-Phe end-capped poly(L-lactide) as macroinitiator for the synthesis of poly(L-lactide)-B-poly(L-lysine) block copolymer

A poly(L-lactide)-b-poly(Nepsilon-(Z)-L-lysine) (PLLA-b-PZLys) block copolymer was synthesized through the ring-opening polymerization of Nepsilon-(Z)-lysine-N-carboxyanhydride using L-Phe-terminated PLLA as a macroinitiator. The L-Phe-terminated PLLA was prepared through a novel three-step process. First, the hydroxyl-terminated PLLA was synthesized through the ring-opening polymerization of L-lactide initiated by n-butanol under the existence of tin(II) ethylhexanoate. Subsequently, the complete capping of the hydroxyl end group of PLLA with BOC-L-Phe was achieved by using a mixed anhydride of BOC-L-Phe under the catalysis of 4-(1-pyrrolidinyl) pyridine. Finally, the free amino end group was obtained by removal of the t-butoxycarbonyl group through trifluoroacetic acid treatment under anhydrous condition. All these treatments were conducted under mild conditions, thus avoiding the breakdown of the PLLA backbone. Poly(L-lactide)-b-poly(L-lysine) block copolymer was produced after deprotection treatment of PLLA-b-PZLys. The structure of the block copolymer was confirmed by 1H NMR, IR, and GPC. Adjustment of the ratio of the NCA monomer to the macroinitiator could control the chain length of the PLys block.     

In situ copolyesters containing poly(L-lactide) and poly(hydroxyalkanoate) units

In situ copolyesters containing polylactide (PLA) and polyhydroxyalkanoate (PHA) segments were obtained via ring-opening polymerization of L-lactide using PHA as a macroinitiator with stannous octoate as catalyst. Incorporation of PHA (20 wt %) into PLA affords a novel copolymer with Mn values ranging from 25 to 50 KDa and low polydispersities of 1.8-2.3. DSC analysis of the copolymer indicates well-defined crystallization and melting transitions different from the homopolymers and corresponding blend. The polymers were characterized by FT-IR, GPC, DSC, optical microscopy, NMR, and TGA. The results show successful reactivity of PHA as a macroinitiator for the ring-opening polymerization of lactide.     

Radiopaque organic-inorganic hybrids based on poly(D,L-lactide)

Hybrid organic-inorganic nanocomposites were prepared starting from alpha,omega-triethoxysilane-terminated poly(d,l-lactic acid) (PDLLA) to be used as potential radiopaque biocompatible coatings for medical devices. The synthesis of the organic phase precursors of given chain length was achieved via anionic polymerization of d,l-lactide using a bifunctional initiator and subsequent triethoxysilane functionalization of the end groups. PDLLA-based ceramers (ceramic polymers) were then synthesized by the sol-gel process at room temperature (rt) in the presence of different amounts of tetraethoxysilane. The rt-synthesized hybrids were then cured (at 80 or 130 degrees C), and their thermal and viscoelastic properties were investigated. All obtained hybrids were optically transparent, due to the nanometric dimension of the silica particles, and yielded clearly contrasted radiographic images.     

Crystallization, stability, and enzymatic degradation of poly(L-lactide) thin film

Poly(L-lactide) (PLLA) thin film with 100 nm thickness was crystallized at 160 degreesC for 20 min from the melt obtained at 220 degreesC. Hexagonal crystals with three types of growth (derivative growth lamellae, overgrowth multistacked lamellae, and undergrowth multistacked lamellae) were simultaneously observed by atomic force microscopy (AFM). These phenomena are due to the differences of the formative points of secondary crystal nuclei against the basal lamella. Enzymatic degradation of PLLA thin film revealed two types of amorphous regions. These regions were identified as the free amorphous region around the crystals and the restricted amorphous region between the crystal and glass substrate. In situ observation of thermal behavior of lamellar crystals was performed to understand the correlation between the chain folding and stability of the crystal by using temperature-controlled AFM. The morphology of the sectors with [100] growth plane had changed to a comblike morphology despite the fact that the [110] growth plane remained unchanged, suggesting that the stability of the chain folding and the chain-packing state affected the thermal behavior.     

Mechanical properties of laser cut poly(L-lactide) micro-specimens: implications for stent design, manufacture, and sterilization

BACKGROUND: The development of endoluminal stents from polymeric materials requires an understanding of the basic mechanical properties of the polymer and the effects of manufacturing and sterilization on those properties. METHODS: Pure poly(L-lactide) (PLLA) and PLLA containing varying amounts of triethylcitrate (TEC) as a plasticizer (5-10-15%) were studied. The specimens were solution-cast and CO2 laser-cut. Specimen dimensions were adapted to the strut size of polymeric vascular stents. The properties of the PLLA micro-specimens were assessed before and after sterilization (EtO cold gas, H2O2-plasma, beta- and gamma-irradiation). Tensile tests, and creep and recovery tests were carried out at 37 degrees C. Additionally the thermal and thermo-mechanical characteristics were investigated using dynamic-mechanical analysis (DMA) and differential scanning calorimetry (DSC). RESULTS: The results showed the dramatic influence of the plasticizer content and sterilization procedure on the mechanical properties of the material. Laser cutting had a lesser effect. Hence the effects of processing and sterilization must not be overlooked in the material selection and design phases of the development process leading to clinical use. Altogether, the results of these studies provide a clearer understanding of the complex interaction between the laser machining process and terminal sterilization on the primary mechanical properties of PLLA and PLLA plasticized with TEC.     

Amphiphilic poly(L-lactide)-b-dendritic poly(L-lysine)s synthesized with a metal-free catalyst and new dendron initiators: chemical preparation and characterization

This study presents investigations on new approaches to novel biodegradable amphiphilic poly(L-lactide)-b-dendritic poly(L-lysine)s bearing well-defined structures. First, two new Boc-protected poly(L-lysine) dendron initiators G(2)OH 4 (generation = 2) and G(3)OH 6 (generation = 3) with hydroxyl end functional groups were efficiently derived from corresponding precursors 3 and 5 via methyl ester substitution with ethanolamine. Subsequently, two series of new diblock copolymers of poly(L-lactide)-b-dendritic Boc-protected poly(L-lysine)s (S1-S2, S3-S4) were prepared in chloroform through ring-opening copolymerization of poly(L-lactide)s with a metal-free catalyst of organic 4-(dimethylamino) pyridine (DMAP) in the presence of a corresponding new poly(L-lysine) dendron initiator. Further, molecular structures of the prepared new dendron initiators as well as those of poly(L-lactide)-b-dendritic Boc-protected poly(L-lysine)s bearing different dendron blocks and PLLA lengths were examined by means of nuclear magnetic resonance spectroscopy (NMR), gel permeation chromatography (GPC), mass spectrometry (ESI-MS, MALDI-FTMS), and thermal gravimetric analysis (TGA). The results demonstrated successful formation of the synthetic precursors, functional dendron initiators, and new diblock copolymers. In addition, the very narrow molecular weight distributions (PDI = 1.10-1.14) of these poly(L-lactide)-b-dendritic Boc-protected poly(L-lysine)s further indicated their well-defined molecular structures. After the efficient Boc-deprotection for the dendron amino groups with TFA/CH(2)Cl(2), new diblock poly(L-lactide)-b-dendritic poly(L-lysine)s bearing lipophilic PLLA and hydrophilic dendritic PLL were finally prepared. It was noteworthy that the MALDI-FTMS result showed that no appreciable intermolecular chain transesterification happened during the ROP of L-lactide catalyzed by the DMAP. Moreover, self-assembly of these new biodegradable amphiphilic copolymers in diverse solvents were also preliminarily studied.     

Synthesis of poly(L-lactide) and polyglycolide by ring-opening polymerization

This protocol describes the synthesis of poly(L-lactide) by ring-opening polymerization of L-lactide using tin(II) 2-ethylhexanoate catalyst as well as the synthesis of polyglycolide by ring-opening polymerization of glycolide. Ring-opening polymerization of cyclic diesters synthesized from alpha-hydroxycarboxylic acids gives high-molecular-weight polyester in high yield. Tin(II) 2-ethylhexanoate catalyst is the most common catalyst for ring-opening polymerization of diesters owing to its high reactivity and low toxicity. Purity of monomers and the amount of water and alcohol in the reaction system are significant factors for increasing molecular weight and conversion of polyesters. The molecular weight of the polyesters is also dependent on reaction temperature and reaction time. This protocol can be completed in 3 d for the synthesis of poly(L-lactide) and 2 d for the synthesis of polyglycolide.     

Synthesis of poly(L-lactide) end-capped with lactose residue

The synthesis of poly(L-lactide) (polyLA) end-capped with lactose residue was studied from the standpoint of development of a new bioabsorbable material. After the hydroxyl group of t-butoxycarbonyl(Boc)-aminoethanol was converted to Boc-aminoethanol-OK by using potassium/naphthalene, L-lactide was polymerized in tetrahydrofuran using Boc-aminoethanol-OK as an initiator at room temperature to prepare polyLA-NHBoc. Subsequently, the removal of the Boc group in terminal Boc-aminoethanol residue was performed by treatment of formic acid to obtain the amino group end-capped polyLA (polyLA-NH(2)) as a reactive polyLA derivative. The coupling reactions of lactose with polyLA-NH(2) were investigated by two methods; the synthetic method through reductive amination of lactose with polyLA-NH(2) in the presence of sodium cyanoborohydride as a reducing agent did not give high degree of substitution of end-capped lactose residue per polyLA molecule, whereas the synthetic method through the ester interchange reaction of lactonolactone with polyLA-NH(2) gave Lac-polyLA perfectly end-capped with lactose residue.     

Influence of peripheral hydrogen bonding on the mechanical properties of photo-cross-linked star-shaped poly(D,L-lactide) networks

Four-arm, star-shaped poly(D,L-lactide) (PDLLA) oligomers of controlled molar mass and narrow molar mass distribution were successfully synthesized by use of an ethoxylated pentaerythritol initiator. Derivatization of the terminal hydroxyl groups with either methacrylic anhydride (MAAH) or 2-isocyanatoethyl methacrylate (IEM) to yield PDLLA-M (M = methacrylate end group) and PDLLA-UM (UM = urethane methacrylate end group), respectively, was monitored by in situ Fourier transform infrared (FTIR) spectroscopy. Photo-cross-linking of the functional oligomers yielded networks with high gel contents (>95%). The glass transition temperature (T(g)) of these networks was strongly dependent on prepolymer molar mass, and networks based on low molar mass precursors were more rigid than the networks obtained from higher molar mass oligomers. The tensile strength (TS) and Young's modulus of the PDLLA-M samples, approximately 7 and 17 MPa, respectively, were significantly lower than the values of 19 MPa (TS) and 113-354 MPa (Young's modulus) for the PDLLA-UM samples. The introduction of terminal hydrogen-bonding sites that were adjacent to the photo-cross-linking site resulted in higher performance poly(lactide)-based bioadhesives.     

Thermal degradation processes of end-capped poly(L-lactide)s in the presence and absence of residual zinc catalyst

Thermal degradation processes of end-capped poly(L-lactide)s (PLLAs) were investigated by means of several thermoanalytical techniques under both isothermal and nonisothermal conditions. Based on the thermogravimetric analysis, it was found that two distinct processes at temperatures below and above 330 degrees C were existed in the nonisothermal degradation for PLLA samples depending on the amounts of residual zinc compounds from synthesis process. Isothermal degradation experiments at different temperature regions showed that the sample weight of PLLA decreased linearly with time in both cases, whereas the changes in molecular weight revealed different tendency for the temperature. On the basis of characterization of the residual PLLA molecules after isothermal degradation at 330 degrees C, it was confirmed that the omega chain end of the residual molecules was an acrylic ester unit. Majority of volatile products during thermal degradation of PLLA was the lactide regardless of the degradation temperature. From these results, it is concluded that, during the thermal degradation of PLLA samples in the presence of large amounts of residual zinc compounds, the zinc compounds catalyze both the intermolecular transesterification generating PLLA with low molecular weights and the selective unzipping depolymerization of PLLA with low molecular weights at temperatures below 330 degrees C. In contrast, the primary reaction of thermal degradation for PLLA in the absence of residual zinc compounds above 330 degrees C is a competition between the random chain scission via a cis-elimination reaction and the cyclic rupture via intramolecular transesterification of PLLA molecules. In addition, it was evidenced that the racemization of lactic acid units in the main chain of PLLA molecules occurred at temperatures above 330 degrees C.