Chengbo Zhou

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.

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.

Thermal strain-induced cold crystallization of amorphous poly(lactic acid)

The cold crystallization behaviors and structure evolutions of amorphous PLA stretched within a high-temperature range (100–150 °C) were investigated via in situ synchrotron WAXS and SAXS measurements. The PLA stretched at 120 °C exhibits excellent mechanical properties with the highest modulus, stretching strength and moderate elongation, which is attributed to the coordination effect of thermally-induced initial crystallinity and moderate chain segment mobility. The WAXS results showed that the diffraction peaks of (200/110) and (203) planes shift to higher 2θ values, which indicates the formation of more ordered α′ and α forms during PLA stretching within 100–150 °C. More α forms can be formed from the stretched α′ form and/or from the amorphous phase at 100 °C. The phase diagram of α and α′ forms in stretched PLA with stretching temperature was proposed according to the obtained data of lattice spacing. The crystallinity of stretched PLA increases faster than that which underwent quiescent cold crystallization at the same temperature. The SAXS results indicated that the long period and lamellar thickness along and vertical to the stretching direction of PLA stretched within 100–150 °C exhibit distinct variations, which is attributed to the difference of strain-induced crystallization and lamellar orientation.

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.

Local conformation controlled crystallization of isotactic poly(butene-1)

The slow spontaneous solid-solid transition from metastable form II to stable form I in typical isotactic polybutene-1 (iPB-1) homopolymer after melt crystallization may be unavoidable and it is challenging to obtain form I directly from bulk iPB-1 melt. In this work, by DSC and WAXS, form I was observed to directly crystallize from bulk iPB-1 while cooling to T far below the crystallization temperature Tc of form II. It is further proposed that the local formation of 3/1 helix conformation is the determining process of direct formation of form I and there exists a temperature window for 3/1 helix conformation to form at low T due to the competition between the entropy effect and the synergistic effect of enthalpy and the energy barrier. The key of polymorph selection of iPB-1 lies in the T-dependent formation of the crystal-favored helix conformations. The results could shed lights on developing charming industrial approaches to obtain the stable form I directly in bulk iPB-1 as well as on understandings to the physics behind polymer crystallization and polymorph selection processes.