Strain and temperature dependence of deformation mechanism of lamellar stacks in HDPE and its guidance on microporous membrane preparation

Functionalization and Surface Modification of Mesoporous Hydrophobic Membranes by Oligomers and Target Additives via Environmental Crazing

This work offers an ecologically friendly and facile approach for the modification of high-tonnage commercial polymers, including polypropylene (PP), high-density polyethylene (HDPE), and poly(ethylene terephthalate) (PET), and preparation of nanocomposite polymeric membranes via incorporation of modifying oligomer hydrophilic additives, such as poly(ethylene glycol) (PEG), poly(propylene glycol) (PPG), polyvinyl alcohol (PVA), and salicylic acid (SA). Structural modification is accomplished via the deformation of polymers in PEG, PPG, and water-ethanol solutions of PVA and SA when mesoporous membranes are loaded with oligomers and target additives. The content of target additives in nanocomposite membranes is controlled by tensile strain, and the level of loading can achieve 35-62 wt.% for PEG and PPG; the content of PVA and SA is controlled by their concentration in the feed solution. This approach allows for the simultaneous incorporation of several additives which are shown to preserve their functional performance in the polymeric membranes and their functionalization. The porosity, morphology, and mechanical characteristics of the prepared membranes were studied. The proposed approach allows an efficient and facile strategy for the surface modification of hydrophobic mesoporous membranes: depending on the nature and content of target additives, their water contact angle can be reduced to 30-65°. Water vapor permeability, gas selectivity, antibacterial, and functional properties of the nanocomposite polymeric membranes were described.

In-Situ Synchrotron X-ray Study on the Structure Variation of Morphology-Identified Injection-Molded β-Nucleated iPP under Tensile Deformation

The deformation behavior of semi-crystalline polymers is strongly dependent on the morphology formed during processing. In this study, in-situ synchrotron X-ray was firstly used to identify the morphological distributions of injection-molded isotactic polypropylene (iPP) with different concentrations of β-nucleating agent. It was found that under relatively high concentration of β-nucleating agent (i.e., ≥0.03 wt.%), the outer region (skin and shear region) of the iPP was dominated by mainly highly oriented α-phase as well as certain amount γ-phase, while the core region was rich in β-crystals with little if any orientation. The addition of the β-nucleating agent was beneficial for the formation of lamellae with large lamellar stacking distance in the shear layer. Then the synchrotron X-ray was applied to study the structure variation of those morphology-identified samples under tensile deformation. It was found that voids and cavities along the stretching direction existed in the deformed iPP samples and their volume increased with increasing concentration of β-nucleating agent. The increased volume of void and cavity was associated with the β to α phase transition, which mainly occurred at the core region. In addition, upon stretching crystalline fragmentation and rearrangement took place following the formation of thinner lamellae.

Recent advances in post-stretching processing of polymer films with in situ synchrotron radiation X-ray scattering

The stretch-induced structural evolution mechanism is a long-standing scientific question in the post-stretching processing of polymer films. X-ray scattering, especially a combination of small- and wide-angle X-ray scattering (SAXS/WAXS), provides a powerful method to study the hierarchical structure of polymer films. Recent advances in synchrotron radiation (SR) light sources and detection techniques allow one to measure the structural evolution of polymer films during post-stretching processing in real time with ultrahigh time resolution, which benefits the understanding on this topic. This review summarizes some recent investigations on post-stretching processing of polymer films, which combine in situ X-ray scattering techniques with purposely designed tensile apparatus in terms of three aspects: uniaxial stretching, biaxial stretching and stretching with chemical reactions. Concerning the polymer bulk, traditional deformation mechanisms like stretch-induced crystallization (SIC), crystal slipping, phase transition and melting-recrystallization are discussed for the uniaxial and biaxial post-stretching of polymer films. New deformation models have been developed to focus on the structural evolution on the length scale of lamellar stacks, which consider the potential microphase separation of the interlamellar amorphous phase and microbuckling. For solution systems, the coupled effects of the mechanical work from external force and the chemical potential from possible chemical reactions are taken into account for the structural evolution during stretching in solution. Roadmaps of structural and morphological evolution in the processing parameter space (i.e., temperature, stress, strain and the concentration of additive in the bath solution) are eventually constructed for precursor films. The accumulation of a structural evolution database for post-stretching processing of polymer films can be expected to provide a helpful guide for industrial processing for high-performance polymers in the near future.