In this study, the poly(L-lactide)/poly(D-lactide)(PLLA/PDLA) blends with different optical purities of PLLA and various molecular weights of PDLA are prepared by solution mixing, and the stereocomplex formation and phase separation behaviors of these blends are investigated. Results reveal that optical purity and molecular weight do not vary the crystal structure of PLA stereocomplex(sc) and homochiral crystallites(hc). As the optical purity increasing in the blends, the melting temperature of sc(Tsc) and the content of sc(ΔHsc) increased, while the melting temperature of hc(Thm) hardly changes, although the content of hc(ΔHhm) decreased gradually. The Tsc and ΔHsc are also enhanced as the molecular weight of PDLA reduces, and the ΔHhm reduces rapidly even though the Thm does not vary apparently. With lower optical purities of PLLA and higher molecular weights of PDLA, three types of crystals form in the blends, i.e., PLA sc, PLLA hc and PDLA hc. As molecular weight decreases and optical purity enhances, the crystal phase decreases to two(sc and PDLA hc), and one(sc) finally. This investigation indicates that the phase separation behavior between PLLA and PDLA in the PLLA/PDLA blends not only depends on molecular weights, but also relies on the optical purities of polymers. 相似文献
Poly[oxy[[2-(perfluorooctyl)ethyl]thiomethyl]ethylene]s (H2F8TP-Xs, where X is mole% of perfluorooctyl groups in the side chain) with different levels of conversion were synthesized using polymer analogous reactions from poly[oxy(chloromethyl)ethylene] and 2-perfluorooctyl ethane thioacetate. H2F8TP-20, 41, 64, and 85 were obtained by changing the poly[oxy(chloromethyl)ethylene] to 2-perfluorooctyl ethane thioacetate mole ratio in the reaction from 0.35 to 1.50. H2F8TP-85 (85% conversion) was found to have an extremely low surface energy of 6.2 mN/m at room temperature, which was attributed to the highly ordered perfluorinated alkyl groups on the surface as a result of phase separation between the perfluorinated side chain part and the hydrogenated flexible backbone. The films of the polymers were characterized by electron spectroscopy for chemical analysis (ESCA) and near edge X-ray absorption fine structure (NEXAFS). 相似文献
In the current work we present results on the controlled/living radical copolymerization of acrylonitrile (AN) and 1,3‐butadiene (BD) via reversible addition fragmentation chain transfer (RAFT) polymerization techniques. For the first time, a solution polymerization process for the synthesis of nitrile butadiene rubber (NBR) via the use of dithioacetate and trithiocarbonate RAFT agents is described. It is demonstrated that the number average molar mass, , of the NBR can be varied between a few thousand and 60 000 g · mol−1 with polydispersities between 1.2 and 2.0 (depending on the monomer to polymer conversion). Excellent agreement between the experimentally observed and the theoretically expected molar masses is found. Detailed information on the structure of the synthesized polymers is obtained by variable analytical techniques such as infrared spectroscopy (IR), nuclear magnetic resonance (NMR) spectroscopy, differential scanning calorimetry, and electrospray ionization‐mass spectrometry (ESI‐MS).
The phase equilibrium boundaries of the membrane forming system, water/triethyl phosphate (TEP)/PVDF, at 25 °C were determined experimentally using cloud-point and equilibrium absorption methods. Based on the phase diagram, appropriate dope and bath compositions were selected to prepare microporous membranes by means of the isothermal immersion-precipitation technique. As a metastable casting dope with respect to crystallization was adopted, the formed membranes exhibited a uniform cross-section composed of interlocked crystal elements coexisting with the network of continuous pores, as was revealed by high resolution FESEM imaging. Morphologies of the membranes’ top surfaces were found to depend heavily on the bath strength, which was controlled by the TEP content. By changing the bath gradually from pure water to 70% TEP, the top surface evolved from a dense skin (asymmetric membrane) to a totally porous morphology (symmetric membrane). Wide angle X-ray diffraction analysis indicated that PVDF crystallized into α-type structure for all of the synthesized membranes. The crystallinity as determined from diffraction peak deconvolution was ≈65%, which value was confirmed by Differential Scanning Calorimetry (DSC). The obtained thermograms also showed a similar melting peak temperature (Tm ≈ 169 °C) for all membranes. Furthermore, water fluxes and tensile strengths of the membranes were measured. The results were found to correlate with the morphologies of the membranes. 相似文献
Copolyimide membranes with different poly(ethylene oxide) (PEO) content (from 28 to 68 wt percent, wt.%) have been thermally treated at different temperatures (from 200 to 300 °C) to evaluate the effect of the thermal protocol on the gas transport properties to O2, N2, CO2 and CH4. The permeability coefficients (P) for all gases increased after the thermal treatment of the membranes and were related to the PEO content, being this enhancement higher for membranes with lower PEO content. Thermal treatment at 300 °C of the membranes with 28 and 43 wt.% of PEO, yielded more productive materials for CO2/N2 separation since the permeability coefficients for CO2 (PCO2) increased 9.8 and 3.2 times, respectively, while the selectivity just suffered a small drop (less than 1.3 times in both cases). Overall, the membrane with 43 wt.% of PEO exhibited the best performance, with a PCO2 of 78 Barrer and a CO2/N2 selectivity of 52. For CO2/CH4 separation, an increase on selectivity of 1.8 times was obtained in the copolyimide with 43 wt.% of PEO, reaching the selectivity a value of 18. This enhancement of productivity has been associated to an improvement of phase segregation. 相似文献
