Deformation-Induced Phase Transitions in iPP Polymorphs

Structure and Mechanical Properties of iPP-Based Nanocomposites Crystallized under High Pressure

The unique nonparallel chain arrangement in the orthorhombic γ-form lamellae of isotactic polypropylene (iPP) results in the enhancement of the mechanical properties of γ-iPP. Our study aimed at the investigation of the mechanical properties of γ-iPP nanocomposites with 1-5 wt.% multiwall carbon nanotubes (MWCNT) and 5 wt.% organo-modified montmorillonite prepared by melt-mixing and high-pressure crystallization. Neat iPP and the nanocomposites were crystallized under high pressures of 200 MPa and 300 MPa, and for comparison under 1.4 MPa, in a custom-built high-pressure cell. The structure of the materials was studied using WAXS, SAXS, DSC, and SEM, whereas their mechanical properties were tested in plane-strain compression. Under a small pressure of 1.4 MPa, polymer matrix in all materials crystallized predominantly in the α-form, the most common monoclinic form of iPP, whereas under high pressure it crystallized in the γ-form. This caused a significant increase in the elastic modulus, yield stress, and stress at break. Moreover, due to the presence of MWCNT, these parameters of the nanocomposites exceeded those of the neat polymer. As a result, a 60-70% increase in the elastic modulus, yield stress, and stress at break was achieved by filling of iPP with MWCNT and high-pressure crystallization.

Additive Manufacturing of Polyolefins

Polyolefins are semi-crystalline thermoplastic polymers known for their good mechanical properties, low production cost, and chemical resistance. They are amongst the most commonly used plastics, and many polyolefin grades are regarded as engineering polymers. The two main additive manufacturing techniques that can be used to fabricate 3D-printed parts are fused filament fabrication and selective laser sintering. Polyolefins, like polypropylene and polyethylene, can, in principle, be processed with both these techniques. However, the semi-crystalline nature of polyolefins adds complexity to the use of additive manufacturing methods compared to amorphous polymers. First, the crystallization process results in severe shrinkage upon cooling, while the processing temperature and cooling rate affect the mechanical properties and mesoscopic structure of the fabricated parts. In addition, for ultra-high-molecular weight polyolefins, limited chain diffusion is a major obstacle to achieving proper adhesion between adjunct layers. Finally, polyolefins are typically apolar polymers, which reduces the adhesion of the 3D-printed part to the substrate. Notwithstanding these difficulties, it is clear that the successful processing of polyolefins via additive manufacturing techniques would enable the fabrication of high-end engineering products with enormous design flexibility. In addition, additive manufacturing could be utilized for the increased recycling of plastics. This manuscript reviews the work that has been conducted in developing experimental protocols for the additive manufacturing of polyolefins, presenting a comparison between the different approaches with a focus on the use of polyethylene and polypropylene grades. This review is concluded with an outlook for future research to overcome the current challenges that impede the addition of polyolefins to the standard palette of materials processed through additive manufacturing.

Smectic Liquid Crystalline Polymer Membranes with Aligned Nanopores in an Anisotropic Scaffold

Bottom-up methods for the fabrication of nanoporous polymer membranes have numerous advantages. However, it remains challenging to fabricate nanoporous membranes that are mechanically robust and have aligned pores, that is, with a low tortuosity. Here, a mechanically robust thin-film composite membrane was fabricated consisting of a two-dimensional (2D) porous smectic liquid crystalline polymer network inside an anisotropic, microporous polymer scaffold. The polymer scaffold allows for relatively straightforward planar alignment of the smectic liquid crystalline mixture, which consisted of a diacrylate cross-linker and a dimer forming benzoic acid-based monoacrylate. Polymerized samples displayed a smectic A (SmA) phase, which formed the eventual 2D porous channels after base treatment. The aligned 2D nanoporous membranes showed a high rejection of anionic solutes bigger than 322 g/mol. Cleaning and reusability of the system were demonstrated by intentionally fouling the porous channels with a cationic dye and subsequently cleaning the membrane with an acidic solution. After cleaning, the membrane properties were unaffected; this, combined with numerous pressurizing cycles, demonstrated reusability of the system.

Molecular Weight-Dependent, Flexible Phase Behaviors of Amphiphilic Block Copolymer/Additive Complexes in Aqueous Solution

Pluronic amphiphilic block copolymers, well known to have a phase behavior can be controlled by external conditions, have a wide range of potential for applications such as nanotemplates or nanobuilding blocks. However, the phase behaviors of Pluronic block copolymer/additive complexes with highly ordered phases have not been fully investigated. Here, we report the unusual molecular weight-dependent self-assembly of Pluronic block copolymer/additive complexes. Depending on the temperature and additive, Pluronic P65 block copolymer with a lower molecular weight showed the closed loop-like (CLL) phase behavior with the disorder-order-disorder-order phase transition in aqueous solution, whereas Pluronic P105 and P85 block copolymers with higher molecular weights underwent highly ordered continuous phase transitions with face centered cubic (FCC), hexagonal, and lamellar phases. It is expected that the specific phase behavior of the block copolymer/additive complex can be applied in optical devices such as nanotemplates or optical sensors for a highly ordered superlattice. Furthermore, this study provides a new route to control the phase behavior of the block copolymers without a complicated process.

A filament stretching rheometer for in situ X-ray experiments: Combining rheology and crystalline morphology characterization

We present a rheometer that combines the possibility to perform in situ X-ray experiments with a precise and locally controlled uniaxial extensional flow. It thus allows us to study the crystallization kinetics and morphology evolution combined with the rheological response to the applied flow field. A constant uniaxial deformation rate is ensured, thanks to a fast control scheme that drives the simultaneous movement of the top and bottom plates during a pulling experiment. A laser micrometer measures the time evolution of the smallest diameter, where the highest stress is concentrated. The rheometer has a copper temperature-controlled oven with the ability to reach 250 °C and a N₂ connection to create an inert atmosphere during the experiments. The innovation of our rheometer is the fixed location of the midfilament position, which is possible because of the simultaneous controlled movement of the two end plates. The copper oven has been constructed with four ad hoc windows: two glass windows for laser access and two Kapton windows for X-ray access. The key feature is the ability to perfectly align the midfilament of the sample to the laser micrometer and to the incoming X-ray beam in a synchrotron radiation facility, making it possible to investigate the structure and morphologies developed during extensional flow. The rheological response measured with our rheometer for low-density polyethylene (LDPE) is in agreement with the linear viscoelastic envelope and with the results obtained from the existing extensional rheometers. To demonstrate the capability of the instrument, we have performed in situ-resolved X-ray experiments on LDPE samples exhibiting extensional flow-induced crystallization.

In Situ WAXD and SAXS during Tensile Deformation Of Moulded and Sintered Polyamide 12

To provide knowledge to improve the mechanical performance of Polyamide 12 (PA12) sintered products, we have studied experimentally the mechanical response and structure development under constant strain rate of compression moulded and laser sintered PA12 by means of in situ small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) experiments. It is found that at low temperatures, i.e., below the glass transition temperature, the brittle failure of laser sintered samples is determined by the fast formation of voids that originate at the beginning of the macroscopic plastic deformation. This effect appears to be faster at temperatures below room temperature and it is less effective at higher temperatures. When tested at 120 ∘ C, sintered PA12 shows a better mechanical response in terms of yield stress and a comparable strain at break with respect to moulded PA12. This can be explained by considering that sintered samples have slightly thicker crystals that can sustain higher stress at high temperature. However, this also leads to the formation of a larger number of voids at low testing temperatures. This work does not attempt to quantify the micromechanics behind crystals deformation and disruption, but it provides a deeper insight in the difference between the mechanical response of moulded and sintered PA12.

Time-Resolving Study of Stress-Induced Transformations of Isotactic Polypropylene through Wide Angle X-ray Scattering Measurements

The development of a highly oriented fiber morphology by effect of tensile deformation of stereodefective isotactic polypropylene (iPP) samples, starting from the unoriented γ form, is studied by following the transformation in real time during stretching through wide angle X-ray scattering (WAXS) measurements. In the stretching process, after yielding, the initial γ form transforms into the mesomorphic form of iPP through mechanical melting and re-crystallization. The analysis of the scattering invariant measured in the WAXS region highlights that the size of the mesomorphic domains included in the well oriented fiber morphology obtained at high deformations increases through a process which involves the coalescence of the small fragments formed by effect of tensile stress during lamellar destruction with the domain of higher dimensions.