The overall crystallization kinetics and spherulite morphologies of miscible poly(ethylene oxide) (PEO)/1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) mixtures were studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM) and rheological measurements. The finer crystal structures were further detected by wide angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS). Crystallization of PEO is largely suppressed by [BMIM][PF6] addition especially at higher ionic liquid (IL) concentrations above 20 wt%. Both the overall crystallization rate and the spherulite growth decrease with the increase of IL content and crystallization temperature; however, the crystallization mechanism keeps unchanged as evidenced by the similar Avrami exponent n and WAXD results. The addition of [BMIM][PF6] could induce more nuclei to some extent, but the induction time of crystallization is evidently prolonged, and a linear to non-linear transition of the spherulite growth (R ∝ t to R ∝ t1/2) can be observed. At higher IL concentration, the spherulite texture changes apparently from particular serrated to branch surface due to the diffusion-controlled growth and the dilution effect, which also as a main factor contributes to the increasing trend of the long period of crystals.

•Phase separation kinetics strongly relies on PEO concentration.•Distinct phase morphologies are observed: network, co-continuous and droplets.•Rheological results are discussed combining with morphological evolution.•The interfacial tension is estimated by using G-M model and Palierne model.Phase separation kinetics of dynamically asymmetric mixtures poly(ethylene oxide)/ionic liquid (PEO/[EMIM][BF4]) was investigated through optical microscopy and rheological measurements. The distinct phase-separating morphologies including network structure at lower PEO concentrations of 10∼30 wt % (P10, P30) driven by viscoelastic phase separation (VPS), co-continuous structure at the moderate concentrations (P40, P50) governed by spinodal decomposition (SD) and droplets-matrix at higher concentrations (P60, P70) driven by either SD or nucleation and growth (NG) are observed under different temperatures. Evident deviation of the storage modulus G′ and gel-like behavior on loss tangent tanδ at low frequencies can be seen upon entering into the two-phase region dependent on different morphologies. During time sweeps, the initial increase of G′ can be ascribed to the growth of concentration fluctuation, and the subsequent decrease possibly due to the phase domain coarsening, breaking up or coalescence. The interfacial tension σ at the late stage of phase separation with droplet-matrix morphologies is estimated by using the Gramespacher-Meissner (G-M) model and Palierne model.Download high-res image (506KB)Download full-size image

Crosslinking reactions of high density polyethylene with low peroxide concentrations ranging from 0.1 wt% to 1.0 wt% at temperatures of 170, 180 and 190 °C were monitored by rheological measurements. A critical gel forms at the peroxide concentration of 0.2 wt%, where the transition from long chain branching generation to crosslinking network formation could occur. Rheokinetics of crosslinking can be fitted well by Ding-Leonov’s model. The curing rate k2 at the earlier stage exhibits about 3 times acceleration per 10 °C with increasing temperature, while the equilibrium modulus G′ at the fully cured stage is almost independent of temperature. Influences of crosslinking on the subsequent crystallization behaviors were detected by DSC measurements. Above the critical gel concentration, crystallization is largely retarded as evidenced by the lower crystallization temperature Tc and crystallinity Xc due to the network formation. The secondary crystallization valley located at the temperature near 80 °C can be observed above the critical concentration, which becomes more evident with the increasing peroxide concentration and curing temperature. This phenomenon provides another evidence of crystallization retardation by the crosslinking network.

High performance polypropylene/glass fiber (PP/GF) composites were prepared by treating GF with a film former (GFf) containing a certain ratio of maleic anhydride grafted polypropylene (MPP) and a kind of β-nucleating agent (TMB-5) via impregnation method, and meanwhile introducing different amount of MPP in the matrix. The coupling effects of GF treatment and the matrix modification on the mechanical properties and interfacial microstructures were investigated in detail. It is found that the film former can efficiently facilitate both the interfacial adhesion and β-transcrystallinity with the aid of appropriate MPP in the matrix. Addition of small amount of MPP (2 wt%) to the matrix in GFf system could significantly improve the mechanical properties with ca. 100% and 178% enhancements in tensile and impact strength, respectively, compared to the commercial GF reinforced PP (PP/GFc). With further increase of MPP content, the tensile strength of the composite increases slightly, accompanied by deterioration of the impact strength. The dramatically improved mechanical properties can be attributed to the simultaneously enhanced interfacial adhesion and β-form transcrystallinity through the coupling effects of nucleating agent locked by MPP on the surface of GFf and those MPP in both the film former and matrix. A schematic mechanism of interfacial microstructure formation is depicted, in which the diffusivity of MPP and PP, the nucleating agent and the molecular interactions are considered as key factors influencing the formation of β-transcrystallinity and interfacial adhesion.