Yingrui Shang

In-situ synchrotron SAXS and WAXS investigations on deformation and α–β transformation of uniaxial stretched poly(vinylidene fluoride)

The crystalline structure evolution of poly(vinylidene fluoride) (PVDF) during tensile deformation at 60 °C, 140 °C and 160 °C, i.e. between the glass transition temperature (Tg) and the melting temperature (Tm), was investigated by in-situ synchrotron SAXS and WAXS techniques. The analysis of the obtained scattering results indicated either yielding or α–β transformation in PVDF occurred and initiated at almost the same strain level with different stretching temperatures. A deformation mechanism was proposed for PVDF to illustrate the structure evolution during uniaxial stretching at high temperature indicating that the initial crystallite and crystalline lamellae structures of stretched PVDF are destroyed and orientated not simultaneously, which is intimately related to the yield point and the initial of α–β transformation on a certain degree of orientation. The long period along tensile direction increases to a maximum and then drops into a lower but stable value during this stage of deformation.

Crystalline structures and crystallization behaviors of poly(L-lactide) in poly(L-lactide)/graphene nanosheet composites

Poly(L-lactide) (PLLA)/graphene nanosheet (GNS) composites and pure PLLA were prepared by the solution blending method. Crystalline structures and crystallization behaviors of PLLA in the composite were investigated by XRD, POM, SAXS, and DSC. It was found that α′ form PLLA formation seemed to be more preferred than α form PLLA formation in PLLA/GNS composites at crystallization temperatures Tcs within the α′–α crystal formation transition region due to the existence of GNSs, resulting in an obvious shift of the α′–α crystal formation transition of PLLA in PLLA/GNSs towards high Tcs compared with that of pure PLLA. At Tcs below α′–α crystal formation transition, the formed α′ crystal turned to be more imperfect due to GNS addition, while at Tcs above α′–α crystal formation transition, the crystal structure of α form PLLA was not affected by GNSs. Further POM observations at high Tcs with only α crystal formed showed that PLLA spherulites were well formed in both PLLA/GNSs and pure PLLA, however with very different crystallization kinetics while isothermally crystallizing at different Tcs. The PLLA crystallization process of PLLA in PLLA/GNSs was accelerated by GNSs with both the nucleation rate and spherulite growth rate increased mainly because of the increasing segmental mobility of PLLA chains due to GNS addition; whereas, GNSs showed no observable influence on the determined zero growth temperature Tzg of α form PLLA and the Tzg was estimated lower than the equilibrium melting point of PLLA, indicating that the crystal growth of PLLA is mediated by a transient mesophase with the transition temperature of Tzg between the mesophase and melt not influenced by GNSs in PLLA. Synchrotron on-line SAXS results revealed that the long periods of PLLA in PLLA/GNS composites isothermally crystallized at different Tcs are much smaller than those in pure PLLA. The GNSs are helpful in forming more perfect recrystallized α form PLLA after the α′ form PLLA is melted with increasing Tcs. The presence of GNSs resulted in imperfect α form PLLA from melt directly when it is isothermally crystallized at different Tcs within the temperature range of α′–α crystal formation transition.

Shear effects on crystalline structures of poly(L-lactide)

The shearing effects of sheared polymer melts on their finally formed crystalline structures of poly(L-lactide) (PLLA) were investigated by means of small angle X-ray scattering (SAXS) and wide angle X-ray diffraction (WAXD). The results of WAXD prove that shear has no effects on the crystal structure of PLLA. The SAXS results demonstrate that both of the long period and the thickness of crystalline lamellae increase with rising shear rates when vertical to the shear direction, but remains constant when being parallel to the shear direction. The structural changes for samples prepared with different shear temperature or under the same shear strain but different shear rate were investigated. The mesophase of polymer melts and shearing effects on their pre-ordered phase turned out to be the key factor affecting the crystal structure of PLLA under different shearing conditions.

Chloroform micro-evaporation induced ordered structures of poly(L-lactide) thin films

Self-assembly of poly(L-lactide) (PLLA) in thin films induced by chloroform micro-evaporation was investigated by microscopic techniques and X-ray diffraction studies. A film-thickness dependent on highly ordered structures has been derived from disordered films. Ring-banded spherulitic and dendritic morphologies with radial periodic variation of thicknesses were formed in dilute solution driven by micro-evaporation of the solvent. Bunched morphologies stacked with a flat-on lozenge-shaped lamellae were created in thinner films. The formation of the concentric ring banded structures was attributed to the periodic rhythmic growth associated with radial periodic changes in the concentration gradient of PLLA. A diffusion-induced rhythmic growth mechanism was proposed to explain the formation of the ring banded morphologies with periodic variation of thicknesses.

A qualitative analysis of particle-induced viscosity reduction in polymeric composites

This work proposes a qualitative analysis of the particle-induced viscosity reduction phenomenon observed in polymeric composites like polylactic acid (PLA)/partially methylated silica particles, but not in composites like PLA/hydroxyl-terminated silica particles. The analysis is mainly based on considerations of the friction coefficients of spherical particles in the composites. The analysis reveals that sufficiently small particles, relatively weak polymer–particle interfacial interactions, particle rotation, and the existence of interfacial slipping in a polymeric composite are necessary conditions for the occurrence of particle-induced viscosity reduction, while long-distance particle–particle interactions based on polymer–particle interactions, and possibly existing additional interfacial elastic behaviors conversely lead to an increase in viscosity. The ratio of the radius of the particles to the gyration radius of the coils of polymer chains in polymer melts (R/Rg) is found to be not crucial.

Crystallization behavior of poly(ε-caprolactone) and poly (ε-caprolactone)/LiClO4 complexes from the melt

The crystalline structures and isothermal crystallization behavior of poly(e-caprolactone) (PCL) and PCL/LiClO4 complexes from the melt were investigated via time-resolved simultaneous synchrotron small-angle and wide-angle X-ray scattering (SAXS/WAXS) measurements in this paper. The influences of Li salts and crystallization temperature on the isothermal crystallization behavior and structural evolution dynamics of PCL were discussed. The results indicated that Li salts affected the crystallization behavior of PCL without changing the structure of the crystalline. It was also shown that the formation of pre-ordered phase (according to SAXS) in the induction stage prior to the appearance of crystalline peaks (according to WAXS) during the isothermal crystallization of PCL and PCL/LiClO4 complexes. The collected SAXS data were analyzed according to Guinier plot and correlation function. By comparing the average radius size of isolated domains formed in the melt obtained from the Guinier plot in the early stage of crystallization with the crystalline lamellar thickness calculated according to the correlation function in the later stage of crystallization, it is concluded that the final crystalline structure may evolve from pre-ordered phase in the initial stage of the melt. Through accurate analysis the PCL and PCL/LiClO4 complexes depicted invariant SAXS intensities, it is found that the LiClO4 coordinated PCL chains mainly locate at the interphase between the crystalline lamellae and amorphous layer. The structural evolution during crystallization in the induction period and subsequent stages of PCL and PCL/LiClO4 were analyzed according to a proposed model.

Shear effects on crystallization behaviors and structure transitions of isotactic poly-1-butene

Different melt pre-shear conditions were applied to isotactic poly-1-butene (iP-1-B) and the effect on the crystallization behaviors and the crystalline structure transitions of iP-1-B were investigated. The polarized optical microscope observations during isothermal crystallization process revealed that the applied melt pre-shear within the experimental range could enhance the nucleation of crystal II and accelerate the diameter growth of the formed spherulites. If the applied melt pre-shear rate was large enough, Shish-Kebabs structure could be formed. After the isothermal crystallization process, the following crystal transition from form II to form I of iP-1-B at room temperature was characterized by X-ray diffraction. It was found that the applied melt pre-shear within the experimental range seemed to have no influence on the crystal transition, implying no strong enough internal stress was formed in the melt pre-sheared iP-1-B samples. Further investigations were applied with synchrotron radiation instruments. Wide angle X-ray scattering (WAXS) and small angle X-ray scattering (SAXS) after the crystal transition showed that the applied melt pre-shear could result in orientated fine crystalline structures. With the melt pre-shear rate increasing, the lattice spaces of the crystallites decreased and the long period, L, and the amorphous layer thickness, La, along the equator direction increased slightly, but L and La along the meridian direction was not affected by melt pre-shear flow. Though the orientated crystalline structures existed in the iP-1-B samples, no accelerating effect on crystal transition from II to I was found. Importantly, the final crystalline structures of iP-1-B in form I was found tunable under different melt pre-shear conditions, even though there was an additional crystal transition process after the isothermal crystallization process of iP-1-B.

Deformation-induced structure evolution of poly(butylene terephthalate)/poly(carbonate) blends during uniaxial stretching

The crystalline structure evolution of poly(butylene terephthalate)/poly(carbonate) (PBT/PC) during stretching was investigated by in situ synchrotron wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS) at 70 °C. The WAXS results demonstrated that the crystallinity and crystallite size (D100) decrease with deformation. The α–β transition was observed during the deformation of PBT/PC blends. The orientation degree of the PBT (100) diffraction increases with strain in the PBT/PC blends. The SAXS data showed that PC is located in the interlamellar region of PBT for the undeformed samples. A structure model was proposed to describe the stretching-induced structure evolution of the PBT/PC blends.

Influence of Crystallization on Molecular Dynamics of the Amorphous Phase in Poly(ε-caprolactone) and Poly(ε-caprolactone)/LiClO4 Complexes Investigated by Dielectric Relaxation Spectroscopy

The dielectric relaxation spectroscopy results indicate that the relaxation peak of PCL for pure PCL and PCL/LiClO4 complexes shifts to a lower frequency with increasing crystallization temperature, while the amorphous, condensed structure of PCL for pure PCL and PCL/LiClO4 complexes is slightly affected by crystallization temperature. These findings were obtained from small-angle X-ray scattering (SAXS) measurements. To further understand the effect of crystallization temperature on the relaxation behavior of PCL and PCL/LiClO4 complexes, the isothermal crystallization process of PCL and PCL/LiClO4 was probed in situ by dielectric spectra measurement. The analysis indicated that the relaxation dynamics for pure PCL and PCL/LiClO4 complexes decrease continuously during the isothermal crystallization process, even at the crystallization induction period. This paper presents a detailed discussion of the crystallization mechanisms of the PCL and PCL/LiClO4 complexes, and the probable explanation as to the effect of crystallization temperature on the relaxation dynamics of PCL and PCL/LiClO4 .