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1.
Polymer blends consisting of linear poly(l-lactide) (PLLA) and different proportions of dendritic PLLA-based copolyesters (hb-PLLA) characterized by different degrees of branching (DB) were obtained in melt. The solid-state properties of poly(l-lactide)s and their blends were investigated by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), atomic force microscopy (AFM) and stress-strain measurements. DSC and DMA methods proved miscibility of PLLA/hb-PLLA blends for the studied composition range. AFM indicated that no phase separation occurs in PLLA/hb-PLLA blends and that PLLA and hb-PLLA cocrystallize in one single lamellae type. The mechanical characteristics of PLLA/hb-PLLA blends deteriorated with an increase of the DB and with changing blend composition. Susceptibility of the blends to biodegradation was studied by measuring the weight loss in two different biodegradation media. PLLA/hb-PLLA blends showed more pronounced hydrophilic character and higher susceptibility to biodegradation with an increase in the degree of branching.  相似文献   

2.
Yong He  Ying Xu  Zhongyong Fan 《Polymer》2008,49(26):5670-5675
A unique crystallization behavior of poly(l-lactide) (PLLA)/poly(d-lactide) (PDLA) stereocomplex was observed when a PLLA/PDLA blend (50/50) was subjected to specific melting conditions. PLLA and PDLA were synthesized by ring opening polymerization of l- or d-lactide using zinc lactate as catalyst. PLLA/PDLA blend was prepared through solution mixing followed by vacuum drying. The blend was melted under various melting conditions and subsequent crystallization behaviors were analyzed by using DSC, XRD, NMR and ESEM. Stereocomplex was exclusively formed from the 50/50 blend of PLLA and PDLA with relatively low molecular weights. Surprisingly, stereocomplex crystallization was distinctly depressed when higher melting temperature and longer melting period were applied, in contrast to homopolymer crystallization. Considering predominant interactions between PLLA and PDLA chains, a novel model of melting process is proposed to illustrate this behavior. It is assumed that PLLA and PDLA chain couples would preserve their interactions (melt memory) when the stereocomplex crystal melts smoothly, thus resulting in a heterogeneous melt which can easily crystallize. The melt could gradually become homogeneous at higher temperature or longer melting time. The strong interactions between PLLA and PDLA chain segments are randomly distributed in a homogeneous melt, thus preventing subsequent stereocomplex crystallization. However, the homogeneous melt can recover its ability to crystallize via dissolution in a solvent.  相似文献   

3.
J Blomqvist  B Mannfors  L.-O Pietilä 《Polymer》2002,43(17):4571-4583
In this paper static amorphous state properties (solubility parameter, free volume (using the Voorintholt method and the Voronoi tessellations) and pair correlation functions, the last ones also by including water molecules in the cells), which can be related to the probability for water uptake, have been studied for polyglycolic (PGA), poly(l-lactic) (PLLA), poly(l,d-lactic) (PLLA/PDLA) and poly(glycolic/l-lactic) (PGA/PLLA) acids, known to be biodegradable polymers. The polymer consistent force field, as modified by the authors, has been used in the calculations. The main purpose of this paper is to investigate, which of the amorphous state properties would be relevant for water uptake. We also discuss the validity of th6e methods used for these kinds of studies, and the related reliability of the computed results. Chain flexibilities of the studied polyesters in the amorphous phase have been analyzed, and the intermolecular interactions are found to cause the most significant variations in the distributions of the adjacent chain dihedral angle pairs and in the related populations of the low-energy regions of the comonomers. The solubility parameters, as calculated from the cohesion energy densities of the constructed models, suggest PGA being most compatible with water, in agreement with experiments. On the other hand, the quantitative structure-property relationships method ‘Synthia’ suggests a very similar solubility in water for all particular polyesters. In the PLAs and PGA/PLLA, however, a larger number of hydrogen bonds is formed between the water molecules and the carbonyl oxygen atoms of the chains showing a better possibility of PLLA and its copolymers to break into shorter chains. As an explanation, the hydrophobic methyl groups of the lactide units are suggested to push the water molecules closer to the carbonyl groups than in homo-PGA.  相似文献   

4.
Among the various inorganic nucleators examined, Talc and an aluminum complex of a phosphoric ester combined with hydrotalcite (NA) were found to be effective for the melt-crystallization of poly(l-lactide) (PLLA) and PLLA/poly(d-lactide) (PDLA) stereo mixture, respectively. NA (1.0 phr (per one hundred resin)) can exclusively nucleate the stereocomplex crystals, while Talc cannot suppress the homo crystallization of PLLA and PDLA in the stereo mixture. Double use of Talc and NA (in 1.0 phr each) is highly effective for enhancing the crystallization temperature of the stereo complex without forming the homo crystals. The stereocomplex crystals nucleated by NA show a significantly lower melting temperature (207 °C) than the single crystal of the stereocomplex (230 °C) in spite of recording a large heat of crystallization ΔHc (54 J/g). Photomicrographic study suggests that the spherulites with a symmetric morphology are formed in the stereo mixture added with NA while the spherulites do not grow in size in the mixture added with Talc. The exclusive growth of the stereocomplex crystals by the melt-crystallization process will open a processing window for the PLLA/PDLA.  相似文献   

5.
The crystallization behavior of poly(l-lactic acid) was studied in the range of 80-160 °C. The peak crystallization time (τp) was defined and obtained from the crystallization isotherm measured with a differential scanning calorimeter (DSC). Isothermal crystallization temperature (Tc) dependence of log(τp) discretely changed at 113 °C (= Tb). The linear growth rate of spherulite, G, was measured with a polarizing microscope. The Tc dependence of G and the size of the spherulite also discretely changed at Tb. Crystal structures for samples isothermally crystallized at temperatures which were higher and lower than Tb were orthorhombic (α-form) and trigonal (β-form), respectively. The discrete change of the crystallization behavior was explained by the formation of different crystal.  相似文献   

6.
Plasticized poly(l-lactide) (PLA) montmorillonite layered silicate (MLS) nanocomposites were compounded and blown-film processed using a co-rotating twin screw extruder. PLA was mixed with 10 wt% acetyltriethyl citrate ester plasticizer and 5 wt% of an organically modified montmorillonite at various screw speeds. Wide-angle X-ray diffraction (WAXD) and transmission electron microscopy (TEM) determined that the compounded pellets and the blown film PLA/MLS nanocomposites were intercalated. The effect of processing screw speeds on the barrier, thermal, mechanical, and biodegradation properties of the nanocomposites were analyzed and compared to the neat polymer. Nanocomposite films show a 48% improvement in oxygen barrier and a 50% improvement in water vapor barrier in comparison to the neat PLA. The thermogravimetric analysis (TGA) showed an overall 9 °C increase in the decomposition temperature for all of the nanocomposites. Differential scanning calorimetry (DSC) has determined that the glass transition, cold crystallization and melting point temperatures were not significantly influenced by the presence of MLS. Mechanical properties of the nanocomposites showed that the Young's modulus increased by 20% and the ultimate elongation of the nanocomposites were not sacrificed in comparison to the neat samples. Biodegradation rates in soil were slightly greater for the PLA/MLS nanocomposite than the pure PLA. However, none of the PLA pure and nanocomposites achieved significant biodegradation levels after 180 days.  相似文献   

7.
Naoya Ninomiya  Toru Masuko 《Polymer》2007,48(16):4874-4882
Poly(l-lactide) (PLLA) film containing transcrystalline (TC) structures can easily be obtained by placing PLLA films melted between two poly(tetrafluoroethylene) (PTFE) sheets, followed by isothermal crystallization at 122 °C. The fine structures of the PLLA-TC film were studied by various structural techniques such as X-ray diffractometry, optical microscopy and transmission electron microscopy. We also examined the purification effect upon the morphology of PLLA-TC film. The formation of the TC structures revealed that one-dimensional spherulitic growth occurred from the assembling impurities as nucleation agent near the PTFE substrate in the heterogeneous nucleation system. We found that the b-axis of PLLA crystal was parallel to the lamellae growth direction confirmed using X-ray diffraction. The precipitated PLLA film crystallized in a similar process exhibited scanty TC textures, suggesting that the existence of impurity in the PLLA sample was an important factor for the formation of those structures.  相似文献   

8.
Eamor M. Woo  Ling Chang 《Polymer》2011,52(26):6080-6089
Crystallization of nonequimolar compositions of poly(d-lactic acid) with low-molecular-weight poly(l-lactic acid) (PDLA/LMw-PLLA) blends leads to formation of various fractions of stereocomplexed PLA (sc-crystallites) and homocrystallites (PDLA or PLLA). For the PDLA/LMw-PLLA blends within the composition window of LMw-PLLA content between 30 and 50 wt%, only sc-crystal exists and no homocrystal is present. On the other hand, for PDLA/LMw-PLLA blends with excess PDLA, e.g. PDLA/LMw-PLLA = 90/10, atomic-force microscopy (AFM) characterization on various stages of crystallization of sc-PLA crystal with PDLA homocrystal shows a repetitive stacking of excess PDLA on pre-formed sc-PLA crystal serving as crystallizing templates. The crystallization initially begins with string-like (fibril-like) PDLA lamellae, followed with PDLA aggregating on sc-PLA crystal into a bead-on-string crystal, then growing to thicker irregularly-shaped dough-like lamellae. Repetitive growth cycle from strings to bead-on-string lamellae continues on top of the dough-like lamellae as new substrates, until ending impingement of the PDLA spherulites.  相似文献   

9.
Jian-Bing Zeng 《Polymer》2009,50(5):1178-2282
A novel biodegradable multiblock poly(ester urethane) (PEU), consisting of poly(l-lactic acid) (PLLA) and poly(butylene succinate) (PBS) blocks, has been successfully synthesized via chain-extension reaction of dihydroxyl terminated PLLA (PLLA-OH) and PBS prepolymers (PBS-OH) using toluene-2,4-diisocyanate (TDI) as a chain extender. The chemical structures and molecular weights of PEUs, containing different block lengths and weight fractions of PLLA and PBS, were characterized by 1H NMR and GPC. The effects of the structures on the physical properties of PEUs were systematically studied by means of DSC, TGA, WAXD and tensile testing. The DSC results indicated that PLLA segment was compatible well with PBS segment in amorphous phase and the crystallization of PEU was predominantly caused by PBS segment, which was also confirmed by WAXD. The results of tensile testing showed that the extensibility of PLLA was largely improved by incorporating PBS segment. The PEU can be used as a potential substitute for some petroleum-based thermoplastics.  相似文献   

10.
In this study, a series of poly(l-lactic acid) (PLLA)/poly(ethylene oxide) (PEO) blends with different PLLA concentrations was prepared. Films of these blends crystallized with and without a coverslip were characterized by the presence and absence of banded structures, respectively. This difference in morphology was observed because the PEO component of the blends was oxidized at a high temperature (125 °C) in air without the protection of a coverslip. X-ray photoelectron spectroscopy (XPS) results showed that the surface of the blends crystallized in nitrogen without a coverslip contained mostly PLLA while the surfaces of the same blends crystallized under a coverslip contained only a moderately higher concentration of PLLA than their bulks. The migration of PLLA to the surface of the blends during crystallization in nitrogen when no coverslip was used was due to its low surface tension. Phase images obtained using atomic force microscopy (AFM) indicated that the banded structures consisted of valleys and ridges, which were in fact flat-on and edge-on lamellae, respectively. A detailed time-of-flight secondary ion mass spectrometry (ToF-SIMS) examination suggested that PLLA and PEO were located mainly on the surfaces of the ridges and valleys, respectively.  相似文献   

11.
H. Yamane  K. Sasai 《Polymer》2003,44(8):2569-2575
Thermal property and crystallization behavior of PLLA blended with a small amount of PDLA (1-5 wt%) were studied. PDLA molecules added in PLLA formed stereocomplex crystallites in the PLLA matrix. When the blend was cooled to a temperature below Tm of PLLA, stereocomplex crystallites acted as nucleation sites of PLLA and enhanced the crystallization of PLLA significantly (heterogeneous nucleation). Such crystallization enhancement was not observed when the blend with lower PDLA content was cooled from 240 °C at which both PLLA crystal and the stereocomplex disappeared. Low molecular weight PDLA isolated in the matrix of PLLA did not form a stereocomplex crystallite with a large surface area enough to act as a nucleation site. On the other hand, high molecular weight PDLA chains formed a large stereocomplex crystallite. With increasing PDLA content, stereocomplex crystallites were more easily formed and they acted as nucleation sites. PLLA crystal near the stereocomplex crystallites has an incomplete structure and showed a melting peak at a lower temperature than pure PLLA crystal.  相似文献   

12.
To achieve the feed stock recycling of poly(l-lactide) (PLLA) to l,l-lactide, PLLA composites including alkali earth metal oxides, such as calcium oxide (CaO) and magnesium oxide (MgO), were prepared and the effect of such metal oxides on the thermal degradation was investigated from the viewpoint of selective l,l-lactide formation. Metal oxides both lowered the degradation temperature range of PLLA and completely suppressed the production of oligomers other than lactides. CaO markedly lowered the degradation temperature, but caused some racemization of lactide, especially in a temperature range lower than 250 °C. Interestingly, with MgO racemization was avoided even in the lower temperature range. It is considered that the effect of MgO on the racemization is due to the lower basicity of Mg compared to Ca. At temperatures lower than 270 °C, the pyrolysis of PLLA/MgO (5 wt%) composite occurred smoothly causing unzipping depolymerization, resulting in selective l,l-lactide production. A degradation mechanism was discussed based on the results of kinetic analysis. A practical approach for the selective production of l,l-lactide from PLLA is proposed by using the PLLA/MgO composite.  相似文献   

13.
A novel method of grafting ring-opening polymerization of l-lactide (LLA) onto the surface of hydroxyapatite nano-particles (n-HAP) was developed. PLLA was directly connected onto the HAP surface through a chemical linkage. The PLLA-g-HAP particles could be stably dispersed in organic solvent such as chloroform for several weeks. The n-HAP particles still retained the original dimension and shape after the grafting of PLLA. Compared with the 31P MAS-NMR spectrum of pure HAP powders, there appeared a downfield displacement of 1.2 ppm in the spectrum of PLLA-g-HAP. Fourier transformation infrared (FT-IR) spectra further confirmed the existence of PLLA on the surface of PLLA-g-HAP. The amount of grafted polymer determined by thermal gravimetric analysis (TGA) was about 6% in weight. The tensile strength and elongation at break of the PLLA/PLLA-g-HAP composite containing 8 wt% of PLLA-g-HAP were 55 MPa and about 10-13%, respectively, while those of the PLLA/n-HAP composites were 40 MPa and 3-5%, respectively.  相似文献   

14.
Plasticized poly(l-lactide) (PLA) based nanocomposites were prepared by melt blending of the matrix with 20 wt% of poly(ethyleneglycol) 1000 (PEG 1000) and different amounts of montmorillonite, organo-modified or not. The intercalation of the polymer chains between the aluminosilicates layers and morphological structure of the filled PLAs were analysed by wide-angle X-ray scattering (WAXS). Thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC) were performed to study the thermal behaviour of the prepared composites. At constant filler level, it appears that from all the clays studied, the montmorillonite organo-modified by bis-(2-hydroxyethyl)methyl (hydrogenated tallowalkyl) ammonium cations brings the greater effect in terms of thermal stability. Increasing the amount of clay allows to delay the onset of thermal degradation of the plasticized polymer matrix. It was also pointed out, by WAXS and DSC analyses, that it exists a real competition between PEG 1000 and PLA for the intercalation into the interlayer spacing of the clay.  相似文献   

15.
Poly(l-lactide-co-succinic anhydride) networks were synthesised via the carbodiimide-mediated coupling of poly(l-lactide) (PLLA) star polymers. When 4-(dimethylamino)pyridine (DMAP) alone was used as the catalyst, gelation did not occur. However, when 4-(dimethylamino)pyridinium p-toluenesulfonate (DPTS), the salt of DMAP and p-toluenesulfonic acid (PTSA), was the catalyst, the networks obtained had gel fractions comparable to those which were reported for networks synthesised by conventional methods. Greater gel fractions and conversion of the prepolymer terminal hydroxyl groups were observed when the hydroxyl-terminated star prepolymers reacted with succinic anhydride in a one-pot procedure than when the hydroxyl-terminated star prepolymers reacted with presynthesised succinic-terminated star prepolymers. The thermal properties of the networks, glass transition temperature (Tg), melting temperature (Tm) and crystallinity (Xc) were all strongly influenced by the average molecular weights between the crosslinks (). The network with the smallest (1400 g/mol) was amorphous and had a Tg of 59 °C while the network with the largest (7800 g/mol) was 15% crystalline and had a Tg of 56 °C.  相似文献   

16.
E. Piorkowska  R. Masirek 《Polymer》2006,47(20):7178-7188
Plasticization of semicrystalline poly(l-lactide) (PLA) with a new plasticizer - poly(propylene glycol) (PPG) is described. PLA was plasticized with PPG with nominal Mw of 425 g/mol (PPG4) and 1000 g/mol (PPG1) and crystallized. The plasticization decreased Tg, which was reflected in a lower yield stress and improved elongation at break. The crystallization in the blends was accompanied by a phase separation facilitated by an increase of plasticizer concentration in the amorphous phase and by annealing of blends at crystallization temperature. The ultimate properties of the blends with high plasticizer contents correlated with the acceleration of spherulite growth rate that reflected accumulation of plasticizer in front of growing spherulites causing weakness of interspherulitic boundaries. In PLA/PPG1 blends the phase separation was the most intense leading to the formation of PPG1 droplets, which facilitated plastic deformation of the blends that enabled to achieve the elongation at break of about 90-100% for 10 and 12.5 wt% PPG1 content in spite of relatively high Tg of PLA rich phase of the respective blends, 46.1-47.6 °C. 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.  相似文献   

17.
The non-isothermally and isothermally crystallized stereodiblock copolymers of poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) with equimolar l-lactyl and d-lactyl units and different number-average molecular weights (Mn) of 3.9 × 103, 9.3 × 103, and 1.1 × 104 g mol−1, which are abbreviated as PLLA-b-PDLA copolymers, contained only stereocomplex crystallites as crystalline species, causing higher melting temperatures of the PLLA-b-PDLA copolymers compared to those of PLLA homopolymers. In the case of non-isothermal crystallization, the cold crystallization temperatures of the PLLA-b-PDLA copolymers during heating and cooling were respectively lower and higher than those of PLLA homopolymers, indicating accelerated crystallization of PLLA-b-PDLA copolymers. In the case of isothermal crystallization, in the crystallizable temperature range, the crystallinity (Xc) values of the PLLA-b-PDLA copolymers were lower than those of the PLLA homopolymers, and were susceptible to the effect of crystallization temperature in contrast to that of homopolymers. The radial growth rate of the spherulites (G) of the PLLA-b-PDLA copolymers was the highest at the middle Mn of 9.3 × 103 g mol−1. This trend is different from that of the PLLA homopolymers where the G values increased monotonically with a decrease in Mn, but seems to be caused by the upper critical Mn values of PLLA and PDLA chains as in the case of PLLA/PDLA blends (in other papers), above which homo-crystallites are formed in addition to stereocomplex crystallites. The disturbed crystallization of PLLA-b-PDLA copolymers compared to that of the PLLA/PDLA blend is attributable to the segmental connection between the PLLA and PDLA chains, which interrupted the free movement of those chains of the PLLA-b-PDLA copolymers during crystallization. The crystallite growth mechanism of the PLLA-b-PDLA copolymers was different from that of the PLLA/PDLA blend.  相似文献   

18.
A new method of the surface modified starch (SM-St) by grafting reaction of the hydroxyl groups on the starch with l-lactic acid was developed. A novel biodegradable starch grafted copolymer, starch-g-poly(l-lactide) (St-g-PLLA), was synthesized in situ by the ring-opening graft polymerization of a l-lactide (LLA) monomer onto the surface of the SM-St granules in the presence of Sn(Oct)2 as a catalyst. The PLLA grafting efficiency could reach as high as 64 wt%. The structure of St-g-PLLA was characterized by IR, DSC and WAXD. The good adhesion between the two components had been evidenced by SEM observations. The medium-resistance of St-g-PLLA and the mechanical properties of the PLLA composite blending with St-g-PLLA were much better than that of the PLLA/starch blending composite.  相似文献   

19.
Stereoregular high polymers of poly(l-lactic acid) (PLLA) (Mw 1.2 × 105, isotacticity 96.0%) and poly(d-lactic acid) (PDLA) (Mw 1.0 × 105, isotacticity 98.6%) were successfully synthesized via melt/solid polycondensation (MP/SSP) using a biogenic catalyst creatinine (CR). The follow-up monitor of the polycondensation products with 13C NMR technique revealed that the polymerization of MP/SSP proceeded in a stereochemical controlled way throughout the whole process as evidenced by the constant high values of isotacticity (97.8–99.4%) of produced polymers. Thermogravimetric analysis demonstrated that the decomposition temperatures (Td,init 324.3 °C, Td, 5% 347.0 °C, Td, max 400.2 °C) of PLLA synthesized with catalyst CR are over 100 °C above those of PLLA synthesized with catalyst SnCl2·2H2O.  相似文献   

20.
A novel cytocompatible graft copolymer of chitosan and l-lactic acid (CL) was prepared by grafting l-lactic acid onto the amino groups in chitosan without a catalyst. The structures of the CL graft copolymers were characterized by FTIR, 13C-NMR and X-ray measurements. Degree of substitution and side-chain length were evaluated from salicylaldehyde and elemental analysis. The tensile strength and water uptake of the CL copolymers films were investigated as a function of feed ratio of LA/CS. The influence of pH on the swelling behavior of the copolymer films was determined and interpreted. Fibroblast culture was performed to evaluate cell proliferation on the copolymers films. The results showed that the cell growth rate on the copolymers films is faster than chitosan obviously.  相似文献   

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