The nanocompsites of star‐shaped poly(D‐lactide)‐co‐poly(L‐lactide) stereoblock copolymers (s‐PDLA‐PLLA) with two‐dimensional graphene nanosheets (GNSs) were prepared by solution mixing method. Crystallization behaviors were investigated using differential scanning calorimetry, polarized optical microscopy, and wide angle X‐ray diffraction. The results of isothermal crystallization behaviors of the nanocompsites clearly indicated that the GNS could remarkably accelerate the overall crystallization rate of s‐PDLA‐PLLA copolymer. Unique stereocomplex crystallites with melting temperature about 207.0°C formed in isothermal crystallization for all samples. The crystallization temperatures of s‐PDLA‐PLLAs shifted to higher temperatures, and the crystallization peak shapes became sharper with increasing GNS contents. The maximum crystallization temperature of the sample with 3 wt% GNS was about 128.2°C, ie, 15°C higher than pure s‐PDLA‐PLLA. At isothermal crystallization processes, the halftime of crystallization (t0.5) of the sample with 3 wt% GNS decreased to 6.4 minutes from 12.9 minutes of pure s‐PDLA‐PLLA at 160°C.The Avrami exponent n values for the nanocomposites samples were 2.6 to 3.0 indicating the crystallization mechanism with three‐dimensional heterogeneous nucleation and spherulites growth. The morphology and average diameter of spherulites of s‐PDLA‐PLLA with various GNS contents were observed in isothermal crystallization processes by polarized optical microscopy. Spherulite growth rates of samples were evaluated by using combined isothermal and nonisothermal procedures and analyzed by the secondary nucleation theory. The results evidenced that the GNS has acceleration effects on the crystallization of s‐PDLA‐PLLA with good nucleation ability in the s‐PDLA‐PLLA material. 相似文献
Summary: A series of new polyisoprene‐block‐polylactide and polystyrene‐block‐polylactide diblock copolymers was prepared by combining the living anionic polymerization of isoprene or styrene, and the stereoselective ring‐opening polymerization of rac‐lactide. Aluminum and yttrium‐based polystyrene or polyisoprene macroinitiators yielded isotactic‐stereoblock and heterotactic‐enriched polylactide segments, respectively. A strong influence of the microstructure of the polylactide block on the aggregation properties in solution and morphological behavior of the solid materials in thin films has been observed.
General strategy used for the preparation of the diblock copolymers, illustrated here for poly(isoprene‐block‐lactide). Poly(styrene‐block‐lactide) copolymers were prepared similarly. 相似文献
As a potential replacement for petroleum-based plastics, biodegradable bio-based polymers such as poly(lactic acid) (PLA) have received much attention in recent years. PLA is a biodegradable polymer with major applications in packaging and medicine. Unfortunately, PLA is less flexible and has less impact resistance than petroleum-based plastics. To improve the mechanical properties of PLA, PLA-based blends are very often used, but the outcome does not meet expectations because of the non-compatibility of the polymer blends. From a chemical point of view, the use of graft copolymers as a compatibilizer with a PLA backbone bearing side chains is an interesting option for improving the compatibility of these blends, which remains challenging. This review article reports on the various graft copolymers based on a PLA backbone and their syntheses following two chemical strategies: the synthesis and polymerization of modified lactide or direct chemical post-polymerization modification of PLA. The main applications of these PLA graft copolymers in the environmental and biomedical fields are presented. 相似文献
The thermogravimetric analysis (TG) of two series
of tri-block copolymers based on poly(L,L-lactide) (PLLA) and poly(ethyleneglycol) (PEG)
segments, having molar mass of 4000 or 600 g mol–1,
respectively, is reported. The prepared block copolymers presented wide range
of molecular masses (800 to 47500 g mol–1)
and compositions (16 to 80 mass% PEG). The thermal stability increased with
the PLLA and/or PEG segment size and the tri-block copolymers prepared from
PEG 4000 started to decompose at higher temperatures compared to those copolymers
from PEG 600. The copolymers compositions were determined by thermogravimetric
analysis and the results were compared to other traditional quantitative spectroscopic
methods, hydrogen nuclear magnetic resonance spectrometry (1HNMR)
and Fourier transform infrared spectrometry (FTIR). The PEG 4000 copolymer
compositions calculated by TG and by 1HNMR, presented
differences of 1%, demonstrating feasibility of using thermogravimetric analysis
for quantitative purposes. 相似文献
The direct polycondensation of D ,L ‐lactic acid in the absence and presence of different catalysts at various temperatures has been studied experimentally. Two types of reactions were carried out, one under closed conditions to estimate the equilibrium constant and the other under flow of nitrogen to estimate the polymerization rate constant. A mathematical model was developed based on a suitable kinetic scheme for polycondensation reaction accounting for the rate of water removal. The effects of different operating conditions (temperature and pressure) on the average molecular weight of the polymer have been explored through experiments and model simulations.
In this paper, we combine hydrophilic oligoagarose (DPn = 10–15), issued from enzymatic degradation of the natural biopolymer agarose, with polycaprolactone, a synthetic biodegradable and hydrophobic polyester. To synthesize these amphiphilic graft copolymers, we use partially acetylated oligoagarose as macroinitiator in combination with tin (II) octanoate for the bulk polymerization of ε-caprolactone. The grafting was confirmed by NMR and SEC which showed a monomodal distribution. After removal of the acetyl protecting groups, copolymers with 30–60% free hydroxyl groups were soluble in water and insoluble in chloroform, thus indicating that they probably adopt micelle-like structures in aqueous solution with a PCL hydrophobic core and spherical oligoagarose side chains. 相似文献
The blends of poly(1,3‐trimethylene carbonate‐b‐(l ‐lactide‐ran‐glycolide)) (PTLG) with poly(d ‐lactide) (PDLA) were prepared via solution‐casting method using CH2Cl2 as solvent. The poly(l ‐lactide) (PLLA) segments of PTLG with PDLA chain constructed as stereocomplex structures and growth stereocomplex crystals of PLA (sc‐PLA). The effects of sc‐PLA crystals on thermal behavior, mechanical properties, thermal decomposition of the PTLG/PDLA blends were investigated, respectively. The differential scanning calorimetry (DSC) and wide‐angle X‐ray diffraction (WAXD) results showed that the total crystallinity of the PTLG/PDLA blends was increased with the PDLA content increasing. Heterogeneous nucleation of sc‐PLA crystals induced crystallization of the PLLA segments in PTLG. The crystallization temperature of samples shifted to 107.5°C for the PTLG/PDLA‐20 blends compared with that of the PTLG matrix, and decreased the half‐time of crystallization. The mechanical measurement results indicated that the tensile strength of the PTLG/PDLA blends was improved from 21.1 MPa of the PTLG matrix to 39.5 MPa of PTLG/PDLA‐20 blends. The results of kinetics of thermal decomposition of the PTLG/PDLA blends by TGA showed that the apparent activation energy of the PTLG/PDLA blends was increased from 59.1 to 72.1 kJ/mol with the increasing of the PDLA content from 3 wt% to 20 wt%, which indicated the enhancement of thermal stability of the PTLG/PDLA blends by addition of PDLA. Furthermore, the biocompatibility of the PTLG/PDLA blends cultured with human adipose‐derived stem cells was evaluated by CCK‐8 and live/dead staining. The experiment results proved the PTLG/PDLA blends were a kind of biomaterial with excellent physical performances with very low cytotoxicity. 相似文献
A magnesium complex of the type {ONNN}Mg‐HMDS wherein {ONNN} is a sequential tetradentate monoanionic ligand is introduced. In the presence of an alcohol initiator this complex catalyzes the living and immortal homopolymerization of the lactide enantiomers and ?‐caprolactone at room‐temperature with exceptionally high activities, as well as the precise block copolymerization of these monomers in a one‐pot synthesis by sequential monomer addition. Copolymers of unprecedented microstructures such as the PCL‐b‐PLLA‐b‐PDLA and PDLA‐b‐PLLA‐b‐PCL‐b‐PLLA‐b‐PDLA block–stereoblock microstructures that feature unique thermal properties are readily accessed. 相似文献
