Criteria of Process Optimization in Binary Polymer Blends with Both Phase Separation and Crystallization

Criteria of Process Optimization in Binary Polymer Blends with Both Phase Separation and Crystallizatio

Chain Dynamics and Crystallization Behavior of Poly(ethylene oxide) in Imidazolium-Based Ionic Liquids with Different Cationic Structures

The influence of two types of imidazolium-based ionic liquids (ILs) with different cation structures including dications (DILs) and monocations (MILs) on the dynamic and crystallization behaviors of poly­(ethylene oxide) (PEO) was investigated. Both the DILs and MILs have the same bis­[(trifluoromethyl)­sulfonyl]­imide ([TFSI]−) anion but contain variable alkyl chain lengths (C6 and C12 for DILs and C3 and C6 for MILs). It is demonstrated that even though the hydrogen-bonding interaction between PEO and DILs is slightly weaker than that between PEO and MILs, as evidenced by melting point depression and fitting results of Kwei equation, the relaxation of PEO chain in DILs is severely constricted because of the special dicationic structure of DILs, which plays a role as physical cross-links among PEO chains. However, it is more interesting that the spherulite growth rate and nucleation effects could be even enhanced in PEO/DIL mixtures. Accordingly, the activation energy for PEO diffusing across the interface between spherulites and supercooled melt, as estimated by the Lauritzen–Hoffman theory, is lower in PEO/DIL mixtures. This phenomenon could be interpreted from the formation of local ordered structure in the presence of physical networks, which might facilitate the crystallization of PEO. As the alkyl chain length of ILs becomes longer or the molecular weight of PEO increases, the difference in the crystallization kinetics of PEO in DILs and MILs reduces. In these cases, the network structure in PEO/DILs becomes looser, and the local ordered structure is difficult to be formed

Graphene-Embedded Hybrid Network Structure to Render Olefin Block Copolymer Foams with High Compression Performance

Graphene has abundant interactions with polymers by adhering to macromolecular chain segments, facilitating heterogeneous crystal nucleation and adsorbing free radicals. Hence, a hierarchical network structure using graphene as the anchor could be formed in an olefin block copolymer (OBC)/low-density polyethylene (LDPE) blend together with cross-linking points and entanglement points. Then, a nanocomposite foam was fabricated by supercritical CO2 foaming. In this work, a graphene-embedded hybrid network structure was designed to effectively control OBC/LDPE foaming. The strategy is as follows: (1) Macromolecular free radicals triggered by peroxide were adsorbed on the surface of graphene to form a dentritic-on-plate structure. (2) Both OBC and LDPE molecular chain segments were adhered to graphene to form a hybrid physical network. (3) Long-branched chains of LDPE formed entanglement points with both molecular chains whose segments adhered to graphene and macromolecular free radicals adsorbed on graphene. Finally, an optimized graphene content of 0.5 wt % in the obtained nanocomposite foam would maximize its compression stability (hysteresis loss of 53%) with a high resilience of 60%

Critical Content of Ultrahigh-Molecular-Weight Polyethylene To Induce the Highest Nucleation Rate for Isotactic Polypropylene in Blends

The influence of the addition of low amounts of ultrahigh-molecular-weight polyethylene (UHMWPE) on the crystallization kinetics of isotactic polypropylene (iPP) in iPP/UHMWPE blends has been investigated by means of differential scanning calorimetry (DSC) and polarized optical microscopy. During the nonisothermal crystallization process, the primarily formed UHMWPE crystals serve as heterogeneous nucleating agents for iPP nucleation, whereas during the isothermal crystallization process, UHMWPE is in the molten state, iPP nucleation preferentially occurs at the UHMWPE and iPP phase interfaces, and the spherulitic growth rates are not obviously affected. It is particularly interesting to find a critical UHMWPE content (2.5 wt %) in the blends to induce the highest iPP nucleation rate; however, above the critical UHMWPE content, the iPP nucleation rate slows because of aggregation of the UHMWPE component. A delicately designed DSC measurement provides insight into the nucleation mechanism of iPP at the interfaces between the UHMWPE and iPP phase domains. It is proposed that the concentration fluctuations generated from the unstable inhomogeneous phase interfaces in the iPP/UHMWPE blends promote the formation of nuclei, which eventually enhances the nucleation and overall crystallization rates of the iPP component

Three-Dimensional Polymer Nanofiber Structures for Liquid Contamination Adsorption

Green amphiphilic decontamination materials with high-performance, ultrafast, and highly efficient liquid contamination adsorption are of great significance to environmental protection. In this work, an ethylene-vinyl alcohol copolymer with both hydrophilic and hydrophobic chain fragments was selected as the matrix (amphiphilic adsorption), a topological grafting structure was designed for chain entanglement (to enhance fiber integrity), and then the aggregate structure was controlled with a soft dispersed phase (to facilitate fiber formation). Finally, environmental-friendly supercritical CO2 foaming is used to obtain the three-dimensional (3D) polymer nanofiber structures (to increase fiber-structure porosity) for liquid contamination adsorption. The strategy takes full advantage of the synergistic effect from the multi-scale structure, including the random copolymer structure (repeating unit scale in the molecular chain), topological structure (molecular chain scale), microphase separation structure (aggregate scale), and nanofiber structure (porous scale). The obtained adsorption amphiphilic material adsorbed liquid contamination with a high efficiency of 64.78 g/g and a large adsorption rate of 1.14 g/g·s–1 (kinetic constant) for carbon tetrachloride, attributing to its unique 3D polymer nanofiber structure with a large specific surface area and a large amount of porous space to adhere and to be filled by liquid contamination, respectively. This work provided a strategy for the green preparation of environment-friendly and high-performance decontamination materials

Enhanced Nucleation Rate of Polylactide in Composites Assisted by Surface Acid Oxidized Carbon Nanotubes of Different Aspect Ratios

Biodegradable polylactide (PLA) composites added with acid oxidized multiwalled carbon nanotubes (A-MWCNTs) of two different aspect ratios (length to diameter) were prepared by coagulation. The aspect ratios and surface structures of A-MWCNTs were characterized by TGA, Raman, and SEM measurements. The percolation thresholds for gelation in the PLA composites with A-MWCNTs of large and small aspect ratios are 2.5 and 4.0 wt %, respectively, which were determined by a rheological method, and in turn, the rheological result confirms the aspect ratio differences for the added two types of A-MWCNTs in the composites. Isothermal crystallization kinetics of neat PLA and its composites were further investigated by using polarized optical microscope (POM) and differential scanning calorimetry (DSC) to clarify the effects of A-MWCNTs of different aspect ratios and concentrations. The different aspect ratio A-MWCNTs with the same carboxyl group mass percent show substantial effects on PLA crystallization kinetics. Those with smaller aspect ratios enhance nucleation rate for PLA spherulites much more than those with larger aspect ratios. This phenomenon can be attributed to fewer sidewall carboxyl groups on the surfaces of A-MWCNTs with smaller aspect ratios, which provides more nucleation sites for PLA crystallization than those with larger aspect ratios at the same concentration, resulting in faster PLA nucleation rates for the former one

Influence of Chain Entanglement on Rheological and Mechanical Behaviors of Polymerized Ionic Liquids

Polymerized ionic liquids (PILs) poly[1-(4-vinylbenzyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide] (P[VBMIM][TFSI]) covering a wide range of molecular weights are synthesized using reversible addition–fragmentation chain-transfer (RAFT) polymerization. The dependence of zero-shear viscosity on weight-average molecular weight (η0 ∼ Mwα) shows two power-law regimes corresponding to disentangled and entangled regimes with α = 1.1 ± 0.1 and 3.6 ± 0.4, as is typical for neutral entangled polymers. The entanglement molecular weight (Me,calc = 1.8 × 105 g/mol) of P[VBMIM][TFSI] estimated from the packing length p = 8.6 Å and Kuhn length b = 20 Å is consistent with the experimental result (Me,rheo = 1.6 × 105 g/mol) and is much higher than the most conventional uncharged polymers due to the bulkiness of the monomers. For entangled PIL samples, reversible strain-induced disentanglement is observed under large-amplitude oscillatory shear (LAOS) and strong strain hardening in extensional flow. The chain entanglement in high-molecular-weight P[VBMIM][TFSI] brings significant improvement in the mechanical strength and robust recoverability under cyclic stretch over that of the lower-molecular-weight, unentangled PILs, making high-molecular-weight P[VBMIM][TFSI] processable for high-performance electrolytes

Step-Cycle Mechanical Processing of Gels of sPP-<i>b</i>-EPR-<i>b</i>-sPP Triblock Copolymer in Mineral Oil

Gels of syndiotactic polypropylene-b-ethylene-propylene-rubber-b-syndiotactic polypropylene (sPP-EPR-sPP) were prepared by dissolving ∼6 wt % of the triblock copolymer in mineral oil at 170 °C and then cooling to room temperature in several steps to crystallize the sPP block. The gel was subjected to step-cycle processing by first extending it to a given maximum tensile strain, followed by decreasing the load to zero. The cycle was then repeated to a higher maximum strain and so on until the sample either failed or it reached an ultimate predetermined strain. The true stress and true strain εH during each cycle were recorded, including the true strain at zero load εH,p after each cycle that resulted from the plastic deformation of the sPP crystals in the gel. The initial Young’s modulus Einit and maximum tangent modulus Emax in each cycle undergo dramatic changes as a function of εH,p, with Einit decreasing for εH,p ≤ 0.1 and then increasing slowly as εH,p increases to 1 while Emax increases rapidly over the entire range of εH,p, resulting in a ratio of Emax/Einit > 1000 at the highest maximum (nominal) strain of 20. On the basis of small-angle X-ray scattering patterns from the deformed and relaxed gels, as well as on previous results on deformation of semicrystalline random copolymers by Strobl and co-workers, we propose that the initial decrease in Einit with εH,p is due to a breakup of the network of the original sPP crystal lamellae while the increase in Emax with εH,p is caused by the conversion of the sPP lamellae into fibrils of an aspect ratio that increases with further plastic deformation. The gel elastic properties can be understood as those of a short fiber composite with a highly deformable matrix. At zero stress the random copolymer midblock chains that connect the fibrils cause these to make all angles to the tensile axis (low Einit) while at the maximum strain the stiff crystalline sPP fibrils align with the tensile axis producing a strong, relatively stiff gel