Jingqing Li

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.

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.

Morphological Aspects of In-Situ Compatiblized Binary Polymer Blends With Variable Viscosity Ratios of Components

The in-situ compatiblized binary polymer blend polypropylene(PP)/polystyrene(PS)/ anhydrous aluminum chloride(AlCl3) was selected as a model system of a reactive polymer blend to investigate the effect of viscosity ratio of components at a constant shear rate on the phase morphological behavior in in-situ compatibilized systems. The results showed that the well-known interfacial compatibilization effect was related to variations of viscosity ratios of components in the reactive PP/PS blends with different contents of AlCl3 catalyst. The phase morphology evolution of the in-situ compatiblized reactive blend was determined by both the interfacial compatibilization and the variation of the viscosity ratio of components under the fixed mixing conditions, which showed characteristics obviously different from and much more complex than those in binary polymer blends generally compatiblized by added compatiblizers. The results implied that the variation of the viscosity ratio of components should be checked carefully and taken into account if necessary, when the phase morphology of binary polymer blends is investigated, especially in complex in-situ compatiblized reactive polymer blends.

Rheological Characterization of in situ Compatibilized Binary Polymer Blends With Variable Steady Shear Viscosities of Components

Zero shear viscosities of binary polymer blends, η0,bc, of polypropylene (PP)/polystyrene (PS), in situ compatibilized by anhydrous aluminum chloride (AlCl3) catalyst, were obtained by fitting their shear rate sweep curves according to the modified Carreau model. The results showed that the dependence of η0,bc on AlCl3 content was complicated and obviously influenced by viscosity variations of the components as well as the interfacial compatibilization effect of the in situ formed PP-g-PS copolymer. For further investigation, η0,bc was divided into three parts: contribution of the viscosity of components, contribution of phase geometry, and contribution of the interfacial compatibilization effect. The results showed that when the apparent value of the third part was experimentally determined, the significant influence of viscosity variations of the components had to be considered, while the influence of phase morphology geometry resulting from viscosity variations of the components could be ignored experimentally and reasonably within the whole experimental range of AlCl3 content. The contribution of the interfacial compatibilization effect to η0,bc could be used as the rheological parameter to characterize the interfacial character and could be used to interpret the variations of η0,bc of the in situ compatibilized polymer blend successfully. In addition, η0,bc is more sensitive to the shear viscosity variations of the components than the phase structure geometry evolution of the reactive blends.

Confined Crystallization in Polymer Blends: DSC Studies of the Behavior and Kinetics of Isothermal Crystallization of Polypropylene in Poly(cis-butadiene) Rubber Blends

Thermal properties of polypropylene with poly(cis-butadiene) rubber (iPP/PcBR) blends have been measured by differential scanning calorimetry (DSC), and the melting point Tm, crystallization temperature Tc, enthalpy Δ H (melting enthalpies and crystalline enthalpies), and equilibrium melting point T0 m have been measured and calculated. The variation of Tm, Tc, Δ H and T0 m with composition in the blends was discussed, showing that an interaction between phases is present in iPP/PcBR blends. The degree of supercooling characterizing the interaction between two phases in the blends and the crystallizability of the blends which bears a relationship to the composition of the blends was discussed. The kinetics of isothermal crystallization of the crystalline phase in iPP/PcBR blends was studied in terms of the Avrami equation, and the Avrami exponent n and velocity constant K were obtained. The Avrami exponent n is between 3 and 2, meaning that iPP has a thermal nucleation with two dimensional growths. The variation of the Avrami exponent n, velocity constant K, and crystallization rate G bear a relation to the composition of the blends, n increases with increasing content ofPcBR. K also increased with increasing content of PcBR. All of the K for the blends are greater than for pure iPP. The crystallization rate G (t1/2) depends on the compositions in the blends; all G of the blends are greater than for iPP.

Compositional Dependence of Static Shear Viscosity of Immiscible PP/PS Blends

Phase structures of immiscible polypropylene (PP)/polystyrene (PS) blends with different volume proportions, PP90/PS10, PP80/PS20, PP70/PS30, PP60/PS40, PP50/PS50, PP40/PS60, PP30/PS70, PP20/PS80, PP10/PS90, were observed by means of scanning electronic microscopy (SEM). The zero shear viscosities of the blends were determined according to a modified Carreau model by fitting the curves of static shear rate sweeps of blends tested at 190°C in a Stress Tech Fluids Rheometer. The results showed that the compositional dependence of zero shear viscosity of PP/PS deviated greatly from linear or log‐linear additivity. When PS was dispersed in a PP continuous phase, the blends showed negative deviation, while for blends with PP dispersed in a PS matrix, positive deviation was generated. When different theoretical equations of Nielsen, Utracki, Taylor, Frankel‐Acrivos (FA), Choi‐Schowalter (CS), and Han‐King (HK) were used to fit the experimental data of zero shear viscosities of blends, none of them was suitable for PP/PS blends. These experimental phenomena may result from the complex phase structures of the blends and their response to shear conditions, which are discussed in detail and compared with the experimental analysis.

Mechanistic insights into the shear effects on isotactic polypropylene crystallization containing β-nucleating agent

The crystalline structures and crystallization behaviors of iPP containing β nucleation agent TMB-5 (iPP/TMB-5) were investigated by synchrotron radiation wide angel X-ray diffraction (SR-WAXD), differential scanning calorimeter (DSC) and polarized light microscope (PLM). It was found that α-crystallization lagged behind β-crystallization at normal temperatures, but the discrepancy reduced with increasing temperature. TMB-5 could not induce β-iPP when the nucleation agent is wrapped up with α-crystal that crystallized at high temperatures. The polymorphic composition of iPP/TMB-5 was susceptible to the introductory moment of shear. New crystallization process of β-nucleated iPP was proposed to understand the experimental phenomena which could not be explained by those reported in the literature. It was supposed that polymer crystallization initiated from mesophase, and the formations of iPP crystals involved the organization of helical conformation ordering within mesophase. It was proposed that the iPP melt contained mesophases with stereocomplex-type ordering of right-handed and left-handed helical chains which could be disturbed by shear or TMB-5, leading to different polymorphic structures.

Temperature dependence of poly(lactic acid) mechanical properties

The mechanical properties of polymers are not only determined by their structures, but also related to the temperature field in which they are located. The yield behaviors, Youngs modulus and structures of injection-molded poly(lactic acid) (PLA) samples after annealing at different temperatures were measured during stretching within 30–150 °C. The in situ photographs were recorded with a camera to observe deformation behavior during stretching. The less ordered α′ form crystal of PLA can be formed in the un-annealed PLA samples and those that were annealed at 70 °C with low crystallinity. The crystallinity increases with increasing annealing temperature and α form crystal is formed when the annealing temperature is higher than 100 °C. The stretched samples with low crystallinity show the first yield at draw temperatures below the glass transition temperature (Tg) and the second yield above Tg. For the samples annealed between 80 and 120 °C, a peculiar double yield appears when stretched within 50–60 °C and only the first or the second yield can be found at the lower and higher draw temperatures. The yield strain and yield stress together with Youngs modulus were obtained and discussed in terms of the effects of the draw temperature and crystalline structure of PLA samples.