Amylose selectively includes poly(L-lactide) (PLLA) among the poly(lactide)s (PLAs) to produce an inclusion complex when the phosphorylase-catalyzed polymerization of α-D-glucose 1-phosphate is performed in the presence of PLLA, poly(D-lactide) (PDLA), or poly(DL-lactide) (PDLLA) (vine-twining polymerization). This result indicates that amylose recognizes the chirality in PLAs on the formation of an inclusion complex in vine-twining polymerization. Modeling calculations support the amylose's chiral recognition in favor of PLLA and the atomistic details of the inclusion complex which involved the preferred orientation of the constituent molecular chains with respect to their fiber axis is proposed. 相似文献
We show that resistance of densely grafted polymer layers to adsorption of chemically identical free chains, which is known to be caused by entropic expulsion of free chains from the grafted layer, can be suppressed using the grafted and free chains of opposite stereoconfiguration. Specifically, we study adsorption of poly(L-lactide) (PLLA) and its enantiomer poly(D-lactide) (PDLA) onto layers of surface-grafted PLLA in acetonitrile and chloroform by infrared spectroscopy (IR). The grafted layers with thicknesses ranging from 7 to 35 nm are produced by ring-opening polymerization of L-lactide from hydroxyl end-groups of a self-assembled monolayer on gold. The IR data indicate that adsorption on the bare gold surface is the same for the L- and D-form of the polymer. However, covering the gold with the surface-grafted PLLA produces a significant decline in the adsorption of free PLLA and, by contrast, a strong enhancement in the adsorption of free PDLA. In addition, the IR data indicate that the adsorbed PDLA chains are stereocomplexed with the grafted PLLA chains. Thus, entropic expulsion of free chains from the grafted layer, which is responsible for the resistance of surface-grafted PLLA to adsorption of free PLLA, is suppressed in the case of free PDLA by stereocomplexation between the grafted and free chains. 相似文献
The effects of chain extension and melt blending temperature on the stereocomplex formation of 50/50 (w/w) poly(L-lactide) (PLLA)/poly(D-lactide) (PDLA) blends or stereocomplex polylactides (scPLAs) were investigated. Joncryl® ADR 4368, a styrene-acrylic multifunctional oligomeric agent, was used as a chain extender. Differential scanning calorimetry and X-ray diffractometry were used to confirm the stereocomplex formation of the PLLA/PDLA blends. Melt flow indices (MFI) of the blends were also determined. The stereocomplex crystallinities gradually decreased with increasing blending temperature and Joncryl® ADR 4368 ratio. The significant decrease in the MFI of scPLAs is believed to be attributed to chain extension at the blending temperatures of 170 °C and 200 °C. The MFI values of scPLAs decreased as the Joncryl® ADR 4368 ratio and blending temperature increased. The results indicated that the chain extension has an effect on the stereocomplexation and it improved the melt strength of the scPLAs. 相似文献
Thermoset/thermoplastic blends were prepared with epoxy–aromatic diamine mixtures and poly(L-lactide) (PLLA), as semicrystalline thermoplastic, in concentrations ranging from 4 to 25 wt.%. In some cases, poly(L,D-lactide) (PDLLA), an amorphous thermoplastic, was used instead for comparative purposes. Diglycidyl ether of bisphenol-A (DGEBA) was employed as epoxy resin and 4,4′-diaminodiphenylmethane (DDM) as curing agent. Phase behavior and morphology were studied during curing at 140 °C. Initially, all blends were homogeneous; however, the curing reaction of the epoxy resin caused a liquid–liquid phase separation. A co-continuous morphology was formed at the beginning of the phase separation in all the considered blend compositions. Blends evolved to a particle/matrix structure or to a phase-inverted structure depending on the initial blend composition. At 140 °C, crystallization only occurred in blends with 16 and 25 wt.% PLLA. This crystallization originates changes in the surface of the epoxy-rich droplets developed with the phase separation. 相似文献
The linear poly(L-lactide) and poly(D-lactide) (PLLA, PDLA) with relative lower molecular weights were synthesized, and PLLA/PDLA blends at various content of PDLA were prepared by solution casting. The morphology and growth of poly(lactide) stereocomplex (PLA SC) were investigated by polarized optical microscope. Results revealed that the morphology of SC in the blends strongly depended on the content of PDLA and the annealing temperature. Dendritic and irregular SC with looser structure developed in the specimens with lower content of PDLA, and regular SC spherulites with birefringent and compact structure produced in the specimens with higher content of PDLA. The growth rate (G) of SC increased gradually as the content of PDLA enhanced in the blends. As the annealing temperature enhanced, the SC with brighter and more compact structure appeared. The G value increased at first before declining as the annealing temperature elevated from the 130 to 190°C. And the nonlinear behavior of the growth of SC in the dissimilar specimens was analyzed. 相似文献
Isotactic and optically active poly(D ‐lactic acid) (PDLA) and phenyl‐substituted poly(lactic acid)s (Ph‐PLAs), i.e., poly(D ‐phenyllactic acid) (Ph‐PDLA) and poly(L ‐phenyllactic acid) (Ph‐PLLA), were synthesized and stereospecific interactions between the synthesized polymers were investigated by their thermal properties and crystallization behavior using differential scanning calorimetry (DSC). The DSC measurements indicated that PDLA is miscible with Ph‐PLAs and that the attractive interaction between PDLA and L ‐configured Ph‐PLA is higher than that between PDLA and D ‐configured Ph‐PDLA. In other words, the latter result means that poly(lactic acid) (PLA) has a higher stereoselective attractive interaction with Ph‐PLA with the reverse configuration than with Ph‐PLA of the same configuration. These results strongly suggest that PLA‐based materials with a wide variety of physical properties and biodegradability can be fabricated by blending them with substituted PLAs with the reverse and same configurations.
Stereocomplex (SC) crystallization has been an effective way to improve the physical performances of stereoregular polymers. However, the competition between homo and SC crystallizations can lead to more complicated crystallization kinetics and polymorphic crystalline structure in stereocomplexable polymers, which influences the physical properties of obtained materials. Herein, we select the medium-molecular-weight (MMW) poly(L-lactic acid)/poly(D-lactic acid) (PLLA/PDLA) asymmetric blends with different PDLA fractions (fD=0.01–0.5) as the model system and investigate the effects of fD and crystallization temperature (Tc) on the crystallization kinetics and polymorphic crystalline structure. We observe the fractionated (i.e., multistep) crystallization kinetics and the formation of peculiar β-form homocrystals (HCs) in the asymmetric blends under quiescent conditions, which are strongly influenced by both fD and Tc. Precisely, crystallization of β-form HCs is favorable in the MMW PLLA/PDLA blends with high fD (≥0.2) at a low Tc (80–100 °C). It is proposed that the formation of metastable β-form HCs is attributed to the conformational matching between β-form HCs and SCs, and the stronger constrain effects of precedingly-formed SCs in the early stage of crystallization. Such effects can also cause the multistep crystallization kinetics of MMW PLLA/PDLA asymmetric blends in the heating process.
Novel bio-based and biodegradable block copolymers were synthesized by "click" reaction between poly(L-lactide)(PLLA) and polyamide 4(PA4). Upon tuning the molar mass of PLLA block, the properties of copolymers and electrospun ultrafine fibers were investigated and compared with those of PLLA and PA4 blends. PLLA and PA4 were found incompatible and formed individual crystalline regions, along with reciprocal inhibition in crystallization. Electrospun fibers were highly hydrophobic, even if hydrophilic PA4 was the rich component. The crystallinity of either PLLA or PA4 decreased after electrospinning and PLLA-rich as-spun fibers were almost amorphous. Immersion tests proved that fibers of block copolymers were relatively homogeneous with micro-phase separation between PLLA and PA4. The fibrous structures of copolymers were different from those of the fibers electrospun from blends, for which sheath-core structure induced by macro-phase separation between homopolymers of PLLA and PA4 was confirmed by TEM, EDS, and XPS. 相似文献
In order to study the chiral effects of polylactides on responses of osteoblastic cells, poly(l-lactide) (PLLA), poly(d-lactide) (PDLA), poly(dl-lactide) (PDLLA) and the stereocomplex of PLLA and PDLLA (SC) films with different stereoforms were prepared. The surface properties of the four polylactide films were tested and the osteoblastic ROS 17/2.8 cells were cultured on the films. The protein adsorption behaviors of fibrinogen and bovine serum albumin on films were studied. The cell proliferation, total protein amount, DNA content and alkaline phosphatase activity of osteoblastic ROS 17/2.8 cells were evaluated. The results showed that the protein adsorption was dependant on the type of proteins. The observation of cell morphologies revealed that the PDLA film provide an unfavorable surface for cell attachment. The total protein amount, DNA content and ALP activity were closely related to the stereoforms of polylactide films. All the levels of total protein amount, DNA content and ALP activity of ROS 17/2.8 cells on PDLA film were decreased. The racemic stereocomplex of PLLA and PDLA showed relatively higher positive effects on both cell growth and proliferation. 相似文献
Chinese Journal of Polymer Science - Blending of poly(levorotatory-lactic acid) (PLLA) and poly(dextrorotatory-lactic acid) (PDLA) produces the stereocomplex crystallites (PLA SC), which present... 相似文献
Stereoblock poly(lactic acid) (sb-PLA), consisting of poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) in a blocky sequence, can successfully be synthesized by solid-state polycondensation of a stereocomplexed mixture of PLLA and PDLA. First, the melt polyconden-sation of L- and D-lactic acids is conducted to obtain PLLA and PDLA with medium molecular weights. Then, both polymers are melt-blended to easily form the stereocomplex. The resulting stereocomplexed mixture (melt-blend) is subjected to solid-state polycondensation for chain extension. The molecular weight (Mw) of the resultant sb-PLA is strongly affected by the lactide/oligomer content in the melt-blend, which is determined by the melt-blending conditions, because it is directly correlated with the polymer crystallinity of the polycondensation products. 相似文献