Nan Tian

Influence of the memory effect of a mesomorphic isotactic polypropylene melt on crystallization behavior

In this study, the memory effect of a polymer is investigated with a mesomorphic isotactic polypropylene (iPP) melt as the initial state. It is found that the nucleation density and crystallization rate at low temperatures are strongly dependent on the initial melting temperature and melting time, indicating a strong memory effect. High melting temperatures decrease the nucleation density, while a short melting time causes a faster crystallization rate than in the melt with its thermal history removed. At 180 °C, it takes about 1 hour for the crystallization rate to be restored to the normal value found in the melt with erasure of its thermal history at 220 °C. Further experiments indicate that the increase in spherulite growth rate is mainly responsible for acceleration of the overall crystallization kinetics. Through comparison between meso crystallization and self-seeding crystallization, it is suggested that some ordered structures with higher thermal stability exist in the mesomorphic iPP melt. The high thermal stability of the ordered structure may be due to the random arrangement of helices of different tacticity. We propose that the ordered structure accelerates spherulite growth, however, long melting times at 180 °C can break down the ordered structure, leading to the formation of an ideal melt and restoration of the spherulite growth rate. This study indicates that a thermodynamically unstable ordered structure can survive in a supercooled melt for a long time and is involved in the crystallization process.

Shear inhomogeneity in poly(ethylene oxide) melts

The nonlinear rheological behavior of entangled poly(ethylene oxide) (PEO) melts was studied by particle-tracking velocimetry (PTV) technique in a cone-plate geometry. In the absence of wall slip, the PEO melts exhibited internal macroscopic motions after shear cessation, when the imposed strain was larger than a critical value (between 1 and 1.5 strain units). This nonquiescent relaxation behavior of entangled melts has only recently been reported once in the literature involving a room temperature melt, and our work is the first PTV observation of nonquiescent relaxation at high temperature.

Extensional rheometer for in situ x-ray scattering study on flow-induced crystallization of polymer

We designed and constructed an extensional rheometer for in situ small and wide angle x-ray study on flow-induced crystallization of polymer. Two rotating drums with an axis distance of 20 mm are employed to impose extensional deformation on the samples. With a constant angular velocity, the two drums generate a constant Henkcy strain rate as sample length for testing keeps constant during deformation. An ionic liquid is used as heating medium to prevent polymer melt from bending downward due to gravity, which is excellent in terms of high thermal stability, low viscosity, and relative low adsorption on x-ray. Flow-induced crystallization experiments are conducted with this apparatus on x-ray scattering station in Shanghai Synchrotron Radiation Facility (SSRF), which allows us to collect rheological and structural data simultaneously and may lead to a better understanding on flow-induced crystallization of polymer.

New understanding on the memory effect of crystallized iPP

In this study, recovery processes of isotactic polypropylene (iPP) melted spherulites at 135 °C after melting at higher temperatures (170 °C–176 °C) were investigated with polarized optical microscopy and Fourier transform infrared spectroscopy. The recovery temperature was fixed to exclude the interference from heterogeneous nuclei. After melting at temperatures between 170 °C and 174 °C, the melted spherulite could recover back to the origin spherulite at low temperatures. Interestingly, a distinct infrared spectrum from iPP melt and crystal was observed in the early stage of recovery process after melting at low temperatures, where only IR bands resulting from short helices with 12 monomers or less can be seen, which indicates that the presence of crystal residues is not the necessary condition for the polymer memory effect. Avrami analysis further indicated that crystallization mainly took place in melted lamellae. After melting at higher temperatures, melted spherulite cannot recover. Based on above findings, it is proposed that the memory effect can be mainly ascribed to melted lamellae, during which crystalline order is lost but conformational order still exists. These conformational ordered segments formed aggregates, which can play as nucleation precursors at low temperatures.

Multiscale and Multistep Ordering of Flow-Induced Nucleation of Polymers

Flow-induced crystallization (FIC) is a typical nonequilibrium phase transition and a core industry subject for the largest group of commercially useful polymeric materials: semicrystalline polymers. A fundamental understanding of FIC can benefit the research of nonequilibrium ordering in matter systems and help to tailor the ultimate properties of polymeric materials. Concerning the crystallization process, flow can accelerate the kinetics by orders of magnitude and induce the formation of oriented crystallites like shish-kebab, which are associated with the major influences of flow on nucleation, that is, raised nucleation density and oriented nuclei. The topic of FIC has been studied for more than half a century. Recently, there have been many developments in experimental approaches, such as synchrotron radiation X-ray scattering, ultrafast X-ray detectors with a time resolution down to the order of milliseconds, and novel laboratory devices to mimic the severe flow field close to real processing conditions. By a combination of these advanced methods, the evolution process of FIC can be revealed more precisely (with higher time resolution and on more length scales) and quantitatively. The new findings are challenging the classical interpretations and theories that were mostly derived from quiescent or mild-flow conditions, and they are triggering the reconsideration of FIC foundations. This review mainly summarizes experimental results, advances in physical understanding, and discussions on the multiscale and multistep nature of oriented nuclei induced by strong flow. The multiscale structures include segmental conformation, packing of conformational ordering, deformation on the whole-chain scale, and macroscopic aggregation of crystallites. The multistep process involves conformation transition, isotropic-nematic transition, density fluctuation (or phase separation), formation of precursors, and shish-kebab crystallites, which are possible ordering processes during nucleation. Furthermore, some theoretical progress and modeling efforts are also included.

A novel way to monitor the sequential destruction of parent-daughter crystals in isotactic polypropylene under uniaxial tension

The cross-hatched structural evolution of isotactic polypropylene (iPP) during uniaxial tensile deformation was investigated with in situ synchrotron radiation wide angle X-ray scattering. An effective way was developed to study parent and daughter lamellae separately with in situ environment. iPP sample was preoriented to generate a bimodal orientation of lamellae for distinguishing the parent and daughter lamellae, which will orient in orthogonal directions under flow-induced crystallization. The dumbbell samples were prepared along different angles with respect to preorientation direction to achieve multisided stretching. The structural evolution of parent and daughter lamellae was followed by recording the scattering from (110) crystallographic plane. It was observed that the parent lamellae were destroyed earlier than daughter ones, no matter which the tensile direction was. Mesophase was observed at very small strain of 0.3, immediately after the damage of cross-hatched structure, which may be attributed to the destruction of parent lamellae. Deformation induced mesophase was proved to be the small crystal cluster which was transformed from parent and daughter lamellae.Graphical Abstract

A Universal Equipment for Biaxial Stretching of Polymer Films

A biaxial stretching equipment was designed and constructed to enable fundamental studies of the relationship between film processing conditions and structures of oriented film products. With programmable drive motors and scissor-like mechanism, all stretching modes, including uniaxial stretching with constant and free width, simultaneous and sequential biaxial stretching, can be applied to a square-shaped sheet. Parameters related to film stretching manufacturing, such as temperature, draw ratio and stretching speed can be set independently to meet the requirement of different polymers. The force information during stretching is recorded by two miniature tension sensors in two directions independently, which can monitor the mechanical stimulus and stress response. Using this equipment, experiments are conducted to investigate the influence of stretching parameters on the structure of polypropylene films, which provides an effective method to tailor the processing conditions to obtain the films with desired properties.

Relaxation propelled long period change in the extension induced crystallization of polyethylene oxide

The time evolution of the long period in the extension-induced crystallization of polyethylene oxide (PEO) is investigated with a combination of rheological measurement and in situ small angle X-ray scattering (SAXS), aiming to show the dynamic change in the spatial arrangement of nuclei in terms of both the chain stretching and orientation. The main findings are summarized as follows. (i) Strain hardening is observed when the strain is larger than 3.0, indicating a transition from chain orientation to chain stretching. (ii) The time evolution of the long period differs before and after strain hardening, reflecting changes in the evolution of nuclei arrangement. The full strain range is thus divided into two regions, namely the large strain region and the small strain region. In the large strain region the long period exhibits a two-step monotonic decrease. For the small strain region, an increase-plateau-decrease behavior is observed. (iii) A large gap exists between the initial values of the long period in each of the two strain regions. (iv) The crystallization kinetics shows no secondary acceleration after strain hardening. Based on these results, a qualitative model emphasizing the chain relaxation propelled rearrangement of nuclei is proposed to explain the structural evolution.

How flow affects crystallization in a heterogeneous polyethylene oxide melt

Extension-induced crystallization in a heterogeneous polyethylene oxide (PEO) melt is investigated by small angle X-ray scattering (SAXS) and extensional rheology. The crystalline complex of PEO and sodium bromide is the heterogeneous component, which simulates the roles of crystal or strongly interacted additives during flow. It is aimed to demonstrate how the altered chain relaxation by the heterogeneous particle affects the crystallization. The main findings are listed as follows: (i) strain hardening occurs in heterogeneous melts but not in pure PEO. This indicates that the crystalline complex strengthens the entanglement network of free chains, leading to slower chain relaxation. (ii) In morphology, the orientation of lamellae becomes easier with the existence of the crystalline complex. A jump in the long period is also observed with large strain, accompanied by changes in the evolutional trend at the early stage. (iii) The crystallization kinetics converge with increasing strain; even the stress response significantly differs in different samples. Based on these findings, a quasi-network consisting of the crystalline complex and the entangled free chain is anticipated. Nucleation induced by the stretch of the quasi-network is supposed to lead to the change in crystallization observed.

The influence of inertia and elastic retraction on flow-induced crystallization of isotactic polypropylene

The influence of nonquiescent relaxation of isotactic polypropylene on flow-induced crystallization (FIC) is investigated by a combination of particle tracking velocimeter (PTV) with a cone-plate shearing geometry and synchrotron radiation microbeam wide-angle X-ray diffraction (SR-μWAXD), which is aimed to correlate real flow profile and distribution of crystal orientation. With PTV technique, we observed that flow remains homogeneous during shear, while postshear movement and delayed fracture take place after a step strain when large shear rates and strains were imposed. Delayed fracture slices samples into several layers which move either forward or backward after the cessation of shear imposed externally. SR-μWAXD measurements reveal that the layers moving forward keep high crystal orientations while the layers moving backward show low orientations, which gives an inhomogeneous distribution of crystal orientation across the thickness of sheared samples. The correlation between moving direction and cry...