Feifei Xue

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.

Deformation and structure evolution of glassy poly(lactic acid) below the glass transition temperature

Poly(lactic acid) (PLA) is a bio-based and compostable thermoplastic polyester that has rapidly evolved into a competitive commodity material over the last decade. One key bottleneck in expanding the field of application of PLA is the control of its structure and properties. Therefore, in situ investigations under cooling are necessary for understanding the relationship between them. The most intriguing feature of a supercooled liquid is its dramatic rise in viscosity as it is cooled toward the glass transition temperature (Tg) though accompanied by very little change in the structural features observable by typical X-ray experiments. The deformation behaviors and structure evolution of glassy PLA during uniaxially stretching below Tg were investigated in situ by synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) techniques. The stretched samples were measured by differential scanning calorimetry (DSC). The obtained results showed that the deformation and yield stress of glassy PLA are strongly dependent on the stretching temperatures together with the transition from mesophase to mesocrystal and the formation of cavities. With the increase in drawing temperature, the onset of the mesocrystal formation is delayed to a higher strain value, whereas corresponding to the same critical orientation degree of amorphous chains (fam ≈ 0.45). The DSC results indicated that the post-Tg endothermic peak corresponding to the melting of mesocrystal appears and shifts to a higher temperature with increasing stretching temperature, followed by the down-shifts (to a lower temperature) of the exothermic peak of cold crystallization of PLA. The appearance of a small exothermic peak just before the melting peak related to the transition of the α′ to α crystal implies the formation of an α′ crystal during cold crystallization in the drawn PLA samples. The structure evolution of glassy PLA stretched below Tg was discussed in details.

Crystallization induced layer-to-layer transitions in symmetric PEO-b-PLLA block copolymer with synchrotron simultaneous SAXS/WAXS investigations

The time dependent hierarchical structures and crystallization behaviors of synthetic crystalline–crystalline symmetric diblock copolymer poly(ethylene oxide)-b-polylactide (PEO-b-PLLA) were investigated by DSC and synchrotron simultaneous small-angle/wide-angle X-ray scattering (SAXS/WAXS)measurements with various isothermal crystallization conditions. DSC measurements indicated that both blocks can crystallize and melt independently. Synchrotron SAXS/WAXS results showed that the structures and crystallization behaviors of PLLA and PEO blocks influence each other and the crystallization of both blocks could induce microphase separation into layer-layer structures, however, the crystalline structures of both blocks are not affected. It is assumed that the final structure of the crystalline–crystalline block copolymer is determined by the crystallization behaviors of the blocks rather than the glass transition temperature of the PLLA block and the theoretical microphase structure of the block copolymers.

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.

Constructional details of polystyrene-block-poly(4-vinylpyridine) ordered thin film morphology

One of the most important issues for spin-coated diblock copolymer thin films is to distinguish the blocks by common techniques, especially for the diblock copolymers containing one block could form active domain. Aiming different estimations on the composition of surface microdomains in polystyrene-block-poly (4-vinylpyridine) (PS-b-P4VP) atomic force microscope (AFM) phase images, AFM, transmission electron microscopy, and droplet shape analyzer were used to identify the constructional details of the polymer thin film morphology. It was confirmed that PS block is harder than P4VP block in symmetric PS-b-P4VP films and corresponds to brighter regions in AFM phase image. It is helpful to distinguish the nanodomains that originate from the different blocks in PS-b-P4VP thin films. The structure evolutions of PS-b-P4VP film annealed with different selective solvents were studied and discussed. The results indicated that the core-corona inversion process is due to the cores of micelles swell and coalesce together rather than local reorganization of the chains.

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.