The effects of varying concentrations of incorporated PDLA on the acceleration of PLLA homo‐crystallization due to stereocomplex (SC) crystallite formation are investigated in PLLA films doped with PDLA over the wide concentration range of 1–10 wt%. PLLA homo‐crystallization is accelerated for all the PDLA concentrations when the processing temperature Tp is just above the endset melting temperature of the SC crystallites (Tp = 226–238 °C), although the appropriate Tp range becomes narrow at low concentrations of PDLA. The accelerating effects of SC crystallites depend on the SC crystalline thickness and the interaction between the SC crystalline regions and PLLA amorphous regions for Tps below and above the melting peak temperature of the SC crystallites, respectively.
In the general processing temperature range of poly(L ‐lactic acid) (PLLA) articles (210–240 °C), PLLA/poly(D ‐lactic acid) (PDLA) stereocomplex (SC) crystallites melted just above the endset temperature of SC melting (228–238 °C) and recrystallized during cooling were found to act as the most effective nucleating agents for enhancing the crystallization of PLLA compared to partially melted SC crystallites (211–227 °C) or those melted far above the endset temperature of SC melting (240 and 250 °C) and recrystallized during cooling. The high nucleating effect of the SC crystallites melted in the temperature range of 228–238 °C was found to be caused by their smaller sizes or the larger number of SC crystallites per unit mass. The incorporation of such SC crystallites facilitates the processing of PLLA articles having high crystallinity and, therefore, high heat‐resistance in a shorter period to reduce the production cost.
PLLA/PDLA blend films with only stereocomplex crystallites as a crystalline species together with pure PLLA and PDLA films with only homo‐crystallites as a crystalline species were prepared, and the effects of enantiomeric polymer blending, crystalline species, and crystallinity on the water vapor permeability were investigated. The WVT coefficient P of PLLA/PDLA blend films was 14–23% lower than that of pure PLLA and PDLA films in the crystallinity Xc range of 0–30%. Amorphous PLLA/PDLA blend films have a much lower P than pure PLLA and PDLA films. The dependence of P on Xc for blend films was stronger for Xc = 0–30% than for Xc = 30–100%. This dependence is discussed using the Nielsen model and the concept of “restricted” (or “restrained”) and “free” amorphous regions.
Summary: The effects of various additives: poly(D ‐lactic acid) (PDLA), talc, fullerene C60, montmorillonite, and various polysaccharides, on the non‐isothermal crystallization behavior of poly(L ‐lactic acid) (PLLA), during both the heating of melt‐quenched films from room temperature, and the cooling of as‐cast films from the melt, was investigated. When the melt‐quenched PLLA films were heated from room temperature, the overall PLLA crystallization was accelerated upon addition of PDLA or the stereocomplex crystallites formed between PDLA and PLLA, the mixtures containing PDLA, and the mixture of talc and montmorillonite. No significant effects on the overall PLLA crystallization were observed for talc, C60, montmorillonite, and the mixtures containing C60. Such rapid completion of the overall PLLA crystallization upon addition of the aforementioned additives can be ascribed to the increased density (number per unit volume or area) of PLLA spherulites. When the as‐cast PLLA films were cooled from the melt, the overall PLLA crystallization completed rapidly, upon addition of PDLA, talc, C60, montmorillonite, and their mixtures. Such rapid overall PLLA crystallization is attributable to the increased density of the PLLA spherulites and the higher nucleation temperature for PLLA crystallization. In contrast, the addition of various polysaccharides has no significant effect, or only a very small effect, on the overall PLLA crystallization during heating from room temperature or during cooling from the melt. This finding means that the polysaccharides can be utilized as low‐cost fillers for PLLA‐based materials, without disturbing the crystallization of the PLLA. The effect of additives in accelerating the overall PLLA crystallization during cooling from the melt, decreased in the following order: PDLA > talc > C60 > montmorillonite > polysaccharides.
Polarization optical photomicrographs of pure PLLA, and the PLLA‐F film, with the fullerene additive, during cooling from the melt (Process IIB). Both of the photomicrographs were taken at 120 °C. 相似文献
Elastomeric EPDM fibers with diameters of 200–400 nm are prepared by coaxial electrospinning of PVP/EPDM fibers, subsequent vulcanization of the polymers and finally removal of the outer PVP layer using ethanol. The initially applied PVP layer restricts the elastic recovery of the EPDM fibers. The crosslinking density of the EPDM fibers reaches 8.44 × 10?5 mol · cm?3. The original morphology of EPDM is preserved after removing the PVP layer. The ultrafine EPDM fibers are expected to be useful in many fields, such as brittle plastics toughening, as well as applications in extremely high or low temperatures.
Bio‐stereo nanocomposite polylactides are prepared by polymerization followed by stereocomplexation in scCO2/dichloromethane through in situ polymerization and master batch processes. The bio‐stereo nanocomposite polylactides show intercalated‐exfoliated and fully exfoliated nanoscale clay distribution in a perfect stereocomplex polylactide matrix. In situ polymerization proves better strategy to produce well‐exfoliated silicate layers in the stereocomplex matrix compared to the MB route that increases the melting temperature by up to ≈64 °C. The thermal properties of these materials maintain both stereocomplex and nanocomposite features simultaneously. The results open a new and versatile way to develop polylactide‐based materials.
TPU was infiltrated into vertically aligned, 3.5 mm‐long MWNT forests to produce continuously reinforced anisotropic nanocomposites, and thermomechanical and electrical testing has revealed multifunctionality which shows promise for numerous applications. A 1000% increase in the storage modulus at 70 °C was observed as compared to the neat TPU, and these continuously aligned composites showed electrical conductivity two orders‐of‐magnitude greater (≈1.5 S · cm?1) than randomly aligned composites prepared using CNTs from these forests. The heightened improvement for the continuously reinforced composite appears to be owed to the extremely high aspect ratio of these CNTs and the interconnected network which remains after infiltration.
DSC indicated that the nucleation of PLLA is enhanced in the presence of PGA even at a PGA content as low as 0.1 wt.‐%. However, the enhancing behavior of PGA was different to that of other nucleating agents for PLLA. Polarized optical microscopy revealed that the presence of PGA increased the number of PLLA spherulites per unit area. WAXD showed that in the PLLA/PGA films, PLLA and PGA crystallize separately to form their respective crystallites and PGA crystallites were formed at a PGA content at above 3 wt.‐% (at least). FTIR spectroscopy indicated that that there are significant interactions between PLLA and PGA chains in amorphous regions. Such interactions should have enhanced the growth of PLLA crystallites from the surface of PGA crystallites.
Summary: Novel light‐sensitive hollow capsules were fabricated from the small molecule 3‐sulfopropylacrylate potassium (SPA) and poly(allylamine hydrochloride) (PAH). With UV irradiation, SPA could be photopolymerized in the wall of hollow capsules. After photopolymerization the capsule size and surfaces showed pronounced differences. The capsules became much more rigid as indicated by an increase in the modulus of more than a factor of 5.
CLSM image of SPA/PAH hollow capsule emission at 554 nm, from rhodamine B after photopolymerization. 相似文献
Spherical silica nanoparticles were mixed with a PP matrix and the thermal behavior of the nanocomposites was studied. The nanocomposites presented drastic improvements in the degradation behavior under thermo‐oxidative conditions, showing complex multistep processes. Under inert conditions the improvements were lower. Our results indicate that mechanisms based on the labyrinth effect, nanoconfinement or trapping model, are not able to explain the whole enhanced thermal stability in these nanocomposites. Moreover, the high specific area of the nanoparticles (≈70 m2 · g?1) indicates that processes based on the adsorption of volatile polar products coming from the oxidative degradation mechanism are plausible.
A new carboxylated styrene‐butadiene rubber (CSBR) in ultrafine powder form was used to modify the properties of nylon 6. The nylon 6/CSBR blends possessed higher toughness than nylon 6/maleic anhydride‐grafted polyethylene‐octene elastomer (POE‐g‐MAH) system. TEM micrographs revealed the fine dispersion of CSBR particles with a diameter of 150 nm. The effective toughening of nylon 6 with CSBR was attributed to the good interface, fine dispersion, and shear yielding.
TEM photograph of undeformed Nylon 6/CSBR (80/20) blend (×40 000). 相似文献
In the present work, we report on the synthesis and characterization of poly(vinylidene fluoride) (PVDF) with N‐isopropylacrylamide (NIPAAM) polymer side chains from molecular graft copolymerization in solution. The copolymer can be readily cast into temperature‐sensitive microfiltration (MF) membranes by the phase inversion technique. The copolymer approach to membrane fabrication allows a much better control of the physicochemical nature of the membrane pores through the variation in graft concentration, membrane casting temperature and concentration of the membrane casting solution.
Thermo‐responsive PNIPAAm/PLLA nanofibrous films with tunable surface morphologies and better biocompatibility were prepared by electrospinning technique. The electrospun composite films possessed a “bead‐on‐string” structure. The wettability of nanofibrous films was observed by water CA measurements. The results showed that the electrospinning process and addition of PLLA did not change the thermo‐sensitivity of PNIPAAm. The wettability of electrospun PNIPAAm/PLLA composite films could switch from superhydrophilic to superhydrophobic when the temperature increased from 20 to 50 °C. Electrospinning is a promising way to create stimuli‐responsive surfaces with potential application in the design and tactics of controllable drug delivery system.
New results on the method of preparation of phenol‐formaldehyde resins from phenol and multihydroxymethyl derivatives of some ketones are presented. The latter, known as the reactive solvents of melamine, were prepared by reacting acetone and methyl ethyl ketone with excess of formaldehyde. A novel group of resins of resol type has been obtained. The structure of products is discussed and compared to that of classical resols. The fragments of ketones have been found incorporated into the structure of resins.
Temperature dependence of viscosity of a classical resol and of the resols modified with reactive solvents. 相似文献
PLLA and stereocomplexed polylactide (sc‐PLA) nanofibers were formed by electrospinning solutions of the polymers in HFIP. A highly semi‐crystalline sc‐PLA nanofiber having only sc crystallites was confirmed by WAXD analysis. The diameters of the nanofibers of both polymers decreased slightly when they were annealed at 60 °C, which was near Tg. Enzyme degradation of both as‐spun PLLA and sc‐PLA nanofibers by proteinase K from Tritirachium album was carried out. The rate of degradation of the nanofibers can be controlled by varying annealing conditions, hence the extent of crystallinity.
Poly(methyl‐co‐trifluoropropyl)silsesquioxanes (P(M‐co‐TFP)SSQs) were prepared using methyltrimethoxysilane (MTMS) and trifluoropropyltrimethoxysilane (TFPTMS). The molecular weight, microstructure of the copolymers and properties of their thin films have been changed by adjusting reaction parameters such as the molar ratio of water to silane, the molar ratio of catalyst to silane, reaction time, solvent content, and temperature. The refractive index of the copolymer thin film decreased from 1.404 to ca. 1.348 as curing temperature was increased to 420 °C. The dielectric constant of the film decreased with an increase of the molecular weight of the copolymer, and the lowest dielectric constant obtained was ca. 2.2. Hardness and elastic modulus of the thin films were 0.7 and 5 GPa, respectively. Crack velocity was measured to be 10?11 m/s at the film thickness of around 0.9 μm under aqueous environment.
Low‐MW urethanes were investigated to control domain formation as well as inhibiting cleavage reactions during vapor phase polymerization of PEDOT. A diurethanediol (DUDO) was identified as a highly efficient mediator for the process, resulting in PEDOT films exceeding conductivities of 1 000 S · cm?1. All the urethanes investigated had the desired inhibiting effect on the polymerization, but all apart from DUDO also introduced unwanted domain formation on the micrometer scale. The addition of PEG generally improved conductivity by suppressing the domain formation and, with an optimized combination of DUDO and PEG, conductivities over 1 200 S · cm?1 were achieved.
There is a dramatic loss in the mechanical performances at the end of the first life application of bumpers made by a poly(propylene) matrix (PP) and an ethylene‐propylene rubber copolymer as dispersed phase. The use of specific additives during the recycling strongly enhances the mechanical properties of these items. The on purpose designed additive contained a regenerative agent and an antioxidant system. The mixing action of the recycling process re‐establishes the phase compatibility; the antioxidants inhibit oxidation, which speeds up the degradation reactions in the recycling process; eventually the regenerative agent joins short chains possessing suitable reactive groups. These three effects play a different role in restoring original material performance in dependence of the recycling process: for instance the regenerative agent may be nearly useless if chain scission is not the dominant process in the recycling operation and the interfacial adhesion between rubber and the matrix does not assure automatically a ductile mechanical behaviour under impact conditions.
SEM micrograph of the fractured surface of the used bumper recycled with additives. 相似文献
The crystallization behavior of poly(L ‐lactide) (PLLA) was investigated in the presence of benzenetricarboxylamide (BTA) derivatives as crystal nucleators. BTA‐cyclohexyl (BTA‐cHe) was the most effective nucleating agent, but induced a complete loss of transparency of the processed material. On the other hand, BTA‐n‐hexyl (BTA‐nHe) enhanced crystallization with little increase in haze. PLLA containing BTA‐cHe enhanced PLLA crystallization in α‐form crystal whereas BTA‐nHe enhanced α′‐form (incomplete α‐form) with forming smaller spherulites. TEM revealed BTA‐nHe had completely dissolved in the PLLA matrix in melt and recrystallized during the thermal annealing process. It was also found that the size of the recrystallized BTA‐nHe was in the nanometer range to effectively nucleate the PLLA crystals.
The serviceability of non‐return valves has a major influence on the productivity of the injection molding process. During a meeting of experts held at our Institute, it was seen that closing behavior and wear are the key problems encountered in practice. The conducted investigations to tackle these questions have shown that both an improved closing behavior and a lower level of wear can be achieved by reducing the inside radius of the locking ring.
Pressure profile over the length of a non‐return valve (n = 0.4; = 25 000 mm3/s). 相似文献
