The influence of different confinements active during crystallization within polybutadiene‐block‐polyisoprene‐block‐poly(ethylene oxide) (PB‐b‐PI‐b‐PEO) and the corresponding hydrogenated polyethylene‐block‐poly(ethylene‐alt‐propylene)‐block‐poly(ethylene oxide) (PE‐b‐PEP‐b‐PEO) triblock copolymers on the self‐nucleation behavior of the crystallizable PEO and PE blocks is investigated by means of differential scanning calorimetry (DSC). In triblock copolymers with PEO contents ≤ 20 wt.‐% crystallization of PEO is confined within small isolated microdomains (spheres or cylinders), and PEO crystallization takes place exclusively at high supercoolings. Self‐nucleation experiments reveal an anomalous behavior in comparison to the classical self‐nucleation behavior found in semicrystalline homopolymers. In these systems, domainII (exclusive self‐nucleation domain) vanishes, and self‐nucleation can only take place at lower temperatures in domainIIISA, when annealing is already active. The self‐nucleation behavior of the PE blocks is significantly different compared with that of the PEO blocks. Regardless of the low PE content (10–25 wt.‐%) in the investigated PE‐b‐PEP‐b‐PEO triblock copolymers a classical self‐nucleation behavior is observed, i.e., all three self‐nucleation domains, usually present in crystallizable homopolymers, can be located. This is a direct result of the small segmental interaction parameter of the PEP and PE segments in the melt. As a consequence, crystallization of PE occurs without confinement from a homogeneous mixture of PE and PEP segments.
Self‐nucleation regimes of a block copolymer showing confined crystallization by means of DSC. 相似文献
Isothermal crystallization of the poly(ferrocenyl dimethylsilane) (PFDMS) segments in a poly[styrene‐block‐(ferrocenyl dimethylsilane)] (PS‐b‐PFDMS) diblock copolymer of lamellar micro‐morphology has been investigated. The PFDMS is shown to crystallize in a confined and grain‐by‐grain fashion. Here a ‘grain’ is defined as an ensemble of stacked lamellae wherein the PFDMS crystallization spreads quickly but stops at its surroundings. Such crystallization propagates not only along the PFDMS lamellae but across the amorphous PS layers as well. We suggest that conformational changes in the PS as induced by the PFDMS crystallization (‘squeezing transfer’) are responsible for the latter pathway of the crystallization's spread.
The reorganization kinetics of the “original” lamellar diblock copolymer poly(ε‐caprolactone)‐block‐poly(4‐vinylpyridine) crystals formed at 260 K is studied in the melting region from 270 K (10 K below the onset of the melting peak of original crystals) to 310 K (the melting peak temperature) on the time scale starting from 10?4 to 102 s by ultrafast differential scanning calorimetry. Different reorganization pathways are observed in this temperature range. Annealing at temperatures below 295 K leads to further stabilization of original crystals by secondary crystallization. At annealing temperatures higher than 295 K, crystals partially melt and the reorganization occurs via the melting–recrystallization. For even higher temperature, such as 310 K, the melting is completed within a few milliseconds and recrystallization starts from the nuclei formation. The sigmoidal recrystallization kinetics is analyzed by the Avrami equation. It is found that the copolymer experiences about one order of magnitude slower recrystallization rate and has higher melting peak temperatures of crystals formed after recrystallization than the homopolymer. The slower recrystallization kinetics in the copolymer is discussed from the viewpoint of the nanoscale spatial constraint and the intermediate state prior to the recrystallization.
A series of highly branched star‐comb poly(ε‐caprolactone)‐block‐poly(l ‐lactide) (scPCL‐b‐PLLA) are successfully achieved using star‐shaped hydroxylated polybutadiene as the macroinitiator by a simple “grafting from” strategy. The ration of each segment can be controlled by the feed ratio of comonomers. These star‐comb double crystalline copolymers are well‐defined and expected to illustrate the influences of the polymer chain topology by comparing with their counterparts in linear‐shaped, star‐shaped, and linear‐comb shape. The crystallization behaviors of PCL‐b‐PLLA copolymers with different architectures are investigated systematically by means of wide‐angle X‐ray diffraction, differential scanning calorimetry, and polarized optical microscopy analysis. It is shown that the comb branched architectures promote the crystallization behavior of each constituent significantly. Both crystallinity and melting temperature greatly raise from linear to comb‐shaped copolymers. Compared to linear‐comb topology, the star‐comb shape presents some steric hindrance of the graft points, which decrease the crystallinity of scPCL‐b‐PLLA. Effects of copolymer composition and chain topology on the crystallization are studied and discussed.
The isothermal crystallization of star‐shaped four‐armed equimolar stereo diblock poly(lactide) (4‐LD) polymers with different molecular weights is investigated. Solely stereocomplex (SC) crystallites are formed in all equimolar 4‐LD polymers, irrespective of molecular weight and crystallization temperature (Tc). The wide‐angle X‐ray diffractometry, differential scanning calorimetry, and polarized optical microscopy results for crystalline species, crystallinity, and maximum radial growth rate of spherulites values indicate that both branching and diblock architectures disturb the SC crystallization and spherulites growth of equimolar 4‐LD polymers, and the disturbance effect is larger for branching architecture than for diblock architecture. The equilibrium melting temperature (Tm0) values are 181.9–266.0 °C, which are comparable with or lower than the value reported earlier (279 °C). The crystallite growth geometries of equimolar of 4‐LD polymers are independent of molecular weight and Tc.
A comprehensive investigation of in situ aggregation of structurally well‐defined enantiomeric poly(styrene)‐block‐poly(lactide) (PS‐b‐PLLA and PS‐b‐PDLA) in a non‐selective solvent, tetrahydrofuran (THF), is presented. The isolated aggregates are found to form poly(L ‐lactide) (PLLA)/poly(D ‐lactide) (PDLA) racemic crystals by differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD), and Fourier transform infrared (FTIR) spectroscopy. The kinetic study reveals that the growth rate of the aggregates depends on the molecular weight of the enantiomeric PLA blocks, as well as the preparation conditions. The proposed mechanism demonstrates a new PS (shell)–PLA (core) structural hierarchy solely driven by stereocomplexation between enantiomeric PLLA and PDLA blocks.
Summary: The degradation behavior of the diblock copolymers PPDX‐b‐PCL was studied under in vitro conditions, in samples with high PPDX content. Molded films were immersed in phosphate buffer solution at pH = 7.4 and 37 °C for 9 months. The samples were periodically extracted, dried and evaluated by weighing, SEC, 1H NMR, DSC, and POM. The results point out that an increase in PCL content reduced the weight loss in the diblock copolymers. 1H NMR and DSC analysis showed that degradation occurred almost exclusively in the PPDX block during the first 9 months of hydrolysis. POM results for the diblock copolymer with high PPDX content (77%) indicated the presence of some typical homo‐PPDX spherulites in the 0.8 months degraded sample when no weight loss was detected. This result demonstrated that random chain scission during the early stages of degradation can produce homo‐PPDX chains that cannot be dissolved in the hydrolysis medium because their molecular weight is still too high. It was found that a small increase in PCL content in the diblock copolymers produced a synergistic increase in the PPDX block degradation stability. This is a direct result of the inter‐digitized lamellar morphology present in the copolymers where PCL and PPDX lamellae are alternated within mixed spherulites. In view of its much higher resistance to hydrolysis, the PCL lamellae offer a barrier‐type protection to the PPDX within the copolymer. A schematic morphological model is proposed to explain the observed changes during the different degradation stages encountered by the diblock copolymers.
Proposed scheme for the hydrolytic degradation process of PPDX‐b‐PCL diblock copolymers. 相似文献
Well‐defined amphiphilic PLA‐b‐PMPC diblock copolymers were synthesized. Bimimetic micelles were prepared and applied for release of anti‐cancer drugs (DOX). TEM and DLS analysis revealed a regular spherical shape with small diameter (less than 50 nm) of the micelle. The biocompatibility of PLA‐b‐PMPC micelles was studied, and it was found that the micelles possessed excellent cytocompatibility due to the zwitterionic phosphorylcholine group. DOX could be efficiently loaded into the micelles with a loading efficiency of 44–67%. The DOX‐loaded micelles showed lower cytotoxicity than free drugs and efficiently delivered and released the drug into cancer cells. With these properties, the PLA‐b‐PMPC polymer micelles are attractive as drug carriers for pharmaceutical application.
This paper describes an unusual contrast inversion phenomenon in TEM imaging of PFS‐b‐PDMS block copolymer bulk samples. It is clearly observed especially in samples that show a lamellar morphology that the contrast inversion is accompanied by a contraction of the PDMS domains and an expansion of the Fe‐rich domains. The location of the iron‐ and silicon‐rich domains was monitored by EDX analysis. We infer that the contrast inversion was caused by electron beam radiation‐induced damage to, and possible cross‐linking of, PDMS chains. A simple way to selectively deposit metal on electron beam patterned polymer film was demonstrated.
Summary: Well‐defined star‐shaped poly(ε‐caprolactone)‐b‐poly(L ‐lactide) copolymers (PCL‐b‐PLLA) were synthesized via sequential block copolymerization, and their molecular weights and arm length ratio could be accurately controlled. Both differential scanning calorimetry and wide angle X‐ray diffraction analysis indicated that the crystallization of both the PLLA and PCL blocks within the star‐shaped PCL‐b‐PLLA copolymer could be adjusted from the arm length of each block, and both blocks mutually influenced each other. The sequential isothermal crystallization process of both the PLLA and PCL blocks within the PCL‐b‐PLLA copolymers was directly observed with a polarized optical microscope, and the isothermal crystallization of the PCL segments was mainly templated by the existing spherulites of PLLA. Moreover, the PLLA blocks within the star‐shaped PCL‐b‐PLLA copolymer progressively changed from ordinary spherulites to banded spherulites when the arm length ratio of PCL to PLLA was increased while concentric spherulites were observed for the linear analog. Significantly, these novel spherulites with concentric or banded textures and the morphological evolution of the spherulites have been observed for the first time in the PCL‐b‐PLLA block copolymers.
In isothermal crystallization from the melt, only stereocomplex crystallites as a crystalline species were formed in all the blends at crystallization temperature above 130 °C. The spherulite growth rate and crystallinity values decreased monotonically with deviation of the PDLA content from 50%. Surprisingly, regime analysis revealed that the crystallization mechanism of the blends was independent of PDLA content. In non‐isothermal crystallization of melt‐quenched specimens during heating, the cold crystallization of blends takes place rapidly at a lower temperature compared to that of pure PLLA and PDLA. This is attributable to the rapid stereocomplex crystallization or the nucleating effect of stereocomplex crystallites formed during quenching from the melt or second heating.
Summary: The relationship between the architecture of block copolymers and their micellar properties was investigated. Diblock, 3‐arm star‐shaped and 4‐arm star‐shaped block copolymers based on poly(ethylene glycol) and poly(ε‐caprolactone) were synthesized. Micelles of star‐shaped block copolymer in an aqueous solution were then prepared by a solvent evaporation method. The critical micelle concentration and the size of the micelles were measured by the steady‐state pyrene fluorescence method and dynamic light scattering, respectively. The CMC decreased in the order di‐, 3‐arm star‐shaped and 4‐arm star‐shaped block copolymer. The size of the micelles increased in the same order as the CMC. Theory also predicts that the formation of micelles becomes easier for 4‐arm star‐shaped block copolymers than for di‐ and 3‐arm star‐shaped block copolymers, which qualitatively agrees with the experiments.
Graft copolyesters with a PCL backbone and PLLA side chains were successfully prepared in three steps avoiding transesterification. First ε‐caprolactone was polymerised with 1,6‐hexane diol as initiator to obtain hydroxytelechelic oligo(ε‐caprolactone)s. These diols were then subjected—in the second step—to polycondensation with L ‐malic acid yielding in linear poly[oligo(ε‐caprolactone)L ‐malate] having secondary hydroxyl functions in the side chain. For both reactions scandium triflate Sc(OTf)3 was used as a catalyst. In the third step various amounts of L ‐lactide were grafted from the polymer backbone using Zn(oct)2 as catalyst. The successful reaction was confirmed by NMR and SEC (size exclusion chromatography) analysis. Further the thermal properties of the graft copolymers with different graft lengths were determined via differential scanning calorimetry.
Two different synthetic pathways give access to the amphiphilic block copolymer poly(ethylene oxide)‐block‐poly(tert‐butoxycarbonylaminomethylacrylate). In the first approach, two end‐functionalized segments are linked via click chemistry; and in the second approach, a poly(ethylene oxide) (PEO) based macroinitiator is chain extended via atom transfer radical polymerization (ATRP). In both cases the linking unit consists of an amide group, which is necessary to effectively deprotect the corresponding polymer precursor without cleavage of both segments. For this, amide‐containing ATRP initiators are employed and successful synthesis by nuclear magnetic resonance (NMR) and size exclusion chromatography (SEC) analyses before comparing both pathways is demonstrated. After deprotection, a novel double hydrophilic block copolymer, poly(ethylene oxide)‐block‐poly(dehydroalanine), is obtained, which is investigated using SEC (aqueous and DMSO) and 1H‐NMR spectroscopy. Containing a potentially zwitterionic PDha segment and a high density of both amino and carboxylic groups, pH‐dependent aggregation of the block copolymer is expected and is studied using dynamic light scattering, revealing interesting solution properties. The corresponding polymers are applied in various areas including drug delivery systems or in biomineralization. 相似文献
Summary: A binary blend of poly (L ‐lactide) (PLLA) and poly(ε‐caprolactone) (PCL) of composition 70:30 by weight was prepared using a twin screw miniextruder and investigated by differential scanning calorimetry (DSC), optical microscopy and scanning electron microscopy (SEM). Ternary 70:30:2 blends were also obtained by adding either a diblock copolymer of PLLA and poly(oxyethylene) (PEO) or a triblock PLLA‐PCL‐PLLA copolymer as a third component. Optical microscopy revealed that the domain size of dispersed PCL domains is reduced by one order of magnitude in the presence of both copolymers. SEM confirmed the strong reduction in particle size upon the addition of the copolymers, with an indication of an enhanced emulsifying effect in the case of the PLLA‐PEO copolymer. These results are analyzed on the basis of solubility parameters of the blend components.
Optical micrograph of M3EG2 blend melt quenched at 125 °C. 相似文献
The amphiphilic triblock copolymer PLA‐b‐PLL‐b‐MPEG is prepared in three steps through acylation coupling between the terminal amino groups of PLA‐b‐PZLL‐NH2 and carboxyl‐terminal MPEG, followed by the deprotection of amines. The block copolymers are characterized via FT‐IR, 1H NMR, DSC, GPC, and TEM. TEM analysis shows that the triblock polymers can form polymeric micelles in aqueous solution with a homogeneous spherical morphology. The cytotoxicity assay indicates that the final triblock polymer micelles after deprotection show low cytotoxicity against Bel7402 human hepatoma cells. MPEG and PLL were introduced into the main chain of PLA affording a kind of ideal bioabsorbable polymer materials, which is expected to be useful in drug and gene delivery.
Summary: Atomic force microscopy (AFM) has been applied to get molecular images of diblock poly(propylene)‐block‐poly(ethylene‐co‐propylene) copolymers deposited on mica from diluted solutions at elevated temperatures. Both isolated molecules and their small aggregates have been visualized as compact particles of various sizes with outspreading poly(ethylene‐co‐propylene) chains. On the base of the height, volume and morphological analysis AFM images were divided into three groups. In the first group the compact particles are suggested to be small regular‐shaped crystallites formed by a few poly(propylene) blocks. Some isolated particles of this group were connected with single copolymer chains. In the second group the compact particles have larger dimensions and irregular or round shapes implying unordered packing of constituents. The third group were represented by isolated poly(ethylene‐co‐propylene) coils. The two‐dimensional expansion of coils on mica both isolated and included in aggregates exceeds several times their dimensions in a solution. The probable mechanism of such an expansion is proposed relying on the existence of van‐der‐Waals surface force field of a sufficient strength in the vicinity of the crystal surface.
Enlarged AFM height images of block‐copolymer aggregates of group A with small compact particles of regular shapes (frames a, b, c) and group B (frame d) with a large globular compact particle. 相似文献
The microphase separation and morphology of PDEG‐b‐PCH‐b‐PDEG and PCH‐b‐PDEG‐b‐PCH amphiphilic triblock copolymers have been studied by DSC, SAXS and AFM. A clear first‐order scattering peak was observed for most of the triblock copolymers, independent of the macroinitiator used. This diffraction has been ascribed to the development of a lamellar structure, which was confirmed by AFM. On the other hand, the existence of an ODT upon heating was observed for most of the triblock copolymers. The ODT location was dependent on the outer segment molecular weights, shifting at higher temperatures as the polymerisation degree increased. WAXS profiles were checked to determine the glass transition and ODT temperatures.
Summary: Poly(ester amide)s derived from glycolic acid and ω‐amino acid units, such as aminohexanoic or aminoundecanoic acids, are synthesized by a thermal polycondensation reaction that involves the formation of metal halide salts. Polymerization kinetics of different metal salts are studied by isothermal and nonisothermal methods and the corresponding parameters compared. The condensation reaction begins in the solid state for the aminohexanoic derivatives, although a rapid liquefaction is observed. On the other hand, the melting temperatures of the sodium and the potassium chloroacetylaminoundecanoate salts are lower than the reaction temperatures, and consequently polycondensation proceeds fully in the liquefied state. These polymers are characterized by an alternate disposition of ester and amide groups and can be obtained with high molecular weights and short polymerization times. Thermal properties (glass transition and melting temperatures) of the two new polymers are determined and compared. Thermal stability is also investigated; the results indicated that decomposition temperatures were always far from both reaction and polymer fusion temperatures.
DSC heating scans performed at different rates for potassium chloroacetylaminoundecanote. 相似文献
The structure and molecular motions of the triblock copolymer PCL‐PEO‐PCL and its inclusion complexes with α‐ and γ‐cyclodextrins (α‐ and γ‐CDs) have been studied by solid‐state NMR. Different cross‐polarization dynamics have been observed for the guest polymer and host CDs. Guest–host magnetization exchange has been observed by proton spin lattice relaxation T1, proton spin lattice frame relaxation T1ρ and 2D heteronuclear correlation experiments. A homogeneous phase has been observed for these complexes. Conventional relaxation experiments and 2D wide‐line separation NMR with windowless isotropic mixing have been used to measure the chain dynamics. The results show that for localized molecular motion in the megahertz regime, the included PCL block chains are much more mobile than the crystalline PCL blocks in the bulk triblock copolymer. However, the mobility of the included PEO block chains is not very different from the amorphous PEO blocks of the bulk sample. The cooperative, long chain motions in the mid‐kilohertz regime for pairs of PCL‐PEO‐PCL chains in their γ‐CD channels seem more restricted than for the single PCL‐PEO‐PCL chains in the α‐CD channels, however, they are not influencing the more localized, higher frequency megahertz motions. 相似文献
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