Crystallization of Polyethylenes Containing Chlorines: Precise vs Random Placement

Crystallization of Poly(l-lactic acid) on Water Surfaces via Controlled Solvent Evaporation and Langmuir-Blodgett Films

Solvent evaporation is one of the most fundamental processes in soft matter. Structures formed via solvent evaporation are often complex yet tunable via the competition between solute diffusion and solvent evaporation time scales. This work concerns the polymer evaporative crystallization on the water surface (ECWS). The dynamic and two-dimensional (2D) nature of the water surface offers a unique way to control the crystallization pathway of polymeric materials. Using poly(l-lactic acid) (PLLA) as the model polymer, we demonstrate that both one-dimensional (1D) crystalline filaments and two-dimensional (2D) lamellae are formed via ECWS, in stark contrast to the 2D Langmuir-Blodgett monolayer systems as well as polymer solution crystallization. Results show that this filament-lamella biphasic structure is tunable via chemical structures such as molecular weight and processing conditions such as temperature and evaporation rate.

Tuning layered superstructures in precision polymers

An approach to influence and control layered superstructures by varying the methylene sequence length between two consecutive functional groups in linear precision polymers containing 2,6-diaminopyridine (DAP) groups is presented. Layered superstructures with repeating units involving three monomeric units along the chain direction with very high coherence lengths upto 110 nm are observed in case of shorter alkyl segments, (16 and 18 [Formula: see text] units), while more conventional layer superstructures incorporating only one monomer are found for related polymers with 20 [Formula: see text] units per methylene sequence. A building block model explaining the unusually large periodicity of three monomeric units is proposed wherein layers containing crystalline or amorphous methylene sequences occur in different combinations. Occurrence of different layered structures depending on crystallization conditions, methylene sequence length as well as functional group type is explained by a competition of H-interactions between the DAP groups and the van der Waal forces between the hydrophobic methylene groups.

Experimental Study on Thermal and UV-enhanced Gas-Solid Chlorination of High-Density Polyethylene

The dynamic process of gas-solid chlorination of high-density polyethylene (HDPE) was revealed using a vibrated-bed reactor, where a UV-Vis measurement system was used to record the kinetics of chlorination online. Thermal and UV-enhanced chlorination at fixed temperature below the melting point (Tm) of HDPE were firstly investigated. It was observed that UV irradiation could accelerate the reaction rate. However, all of the prepared chlorinated polyethylene (CPE) products had unchlorinated crystal structures and a limitation of chlorine content. Multi-stage chlorination was hence designed for more homogeneous chlorination and decreasing the residual crystallinity. The differential scanning calorimetry (DSC) results showed effective decrease of the melting enthalpy (ΔHm) while the final reaction temperature was beyond Tm. Meanwhile, the presence of -CH = CH- bonds in the polymer chain revealed by solid-state nuclear magnetic resonance (S-NMR) indicated that the final reaction temperature should not be higher than 150 °C.

Phosphate-Functionalized Polyethylene with High Adsorption of Uranium(VI)

For uranium extraction from seawater, development of new stable and reusable sorbents with high affinity and good selectivity for U(VI) is required. Herein, a new phosphate-functionalized polyethylene (denoted PO₄/PE) was synthesized via a simple Ar-jet plasma treatment of PE in concentrated H₃PO₄ and was employed in U(VI) extraction from seawater. The prepared PO₄/PE shows superior performance in the extraction of trace U(VI) from seawater. The adsorption process followed the second-order kinetics model and the Langmuir model. The maximum adsorption capacity of PO₄/PE for U(VI) reaches 173.8 mg/g at pH 8.2 and 298 K. PO₄/PE can be effectively regenerated by 0.1 mol/L Na₂CO₃ and reused well even after eight cycles. Experimental results offer a new approach for U(VI) uptake from seawater.

Nanoscale Aggregation in Acid- and Ion-Containing Polymers

In this review we summarize recent efforts in understanding nano-aggregation in acid- and ion-containing polymer systems. The acid and ionic groups have specific interactions that drive aggregation and alter polymer behavior at the nano-, micro-, and bulk length scales. Advancements in synthetic methods, characterization techniques, and computer simulations have enabled researchers to better understand the morphologies and dynamics, particularly at the nanoscale. This overview of recent advancements in nano-aggregated polymer systems highlights the current understanding of the field and presents promising directions for future investigations and new applications.

Epitaxial crystallization of precisely bromine-substituted polyethylene induced by carbon nanotubes and graphene

Epitaxial crystallization of precisely bromine-substituted polyethylene induced by carbon nanotubes and graphene.

Precision polymers containing main-chain-amino acids: ADMET polymerization and crystallization

New PE-type precision oligomers displaying different amino acids (chiral/achiral, polar/non-polar) placed at every 19th carbon atom are presented.

Heterogeneous Coordination Environments in Lithium-Neutralized Ionomers Identified Using 1H and 7Li MAS NMR

The carboxylic acid proton and the lithium coordination environments for precise and random Li-neutralized polyethylene acrylic acid P(E-AA) ionomers were explored using high speed solid-state ¹H and ⁷Li MAS NMR. While the ⁷Li NMR revealed only a single Li coordination environment, the chemical shift temperature variation was dependent on the precise or random nature of the P(E-AA) ionomer. The ¹H MAS NMR revealed two different carboxylic acid proton environments in these materials. By utilizing ¹H-⁷Li rotational echo double resonance (REDOR) MAS NMR experiments, it was demonstrated that the proton environments correspond to different average ¹H-⁷Li distances, with the majority of the protonated carboxylic acids having a close through space contact with the Li. Molecular dynamics simulations suggest that the shortest ¹H-⁷Li distance corresponds to un-neutralized carboxylic acids directly involved in the coordination environment of Li clusters. These solid-state NMR results show that heterogeneous structural motifs need to be included when developing descriptions of these ionomer materials.

Nanoscale morphology in precisely sequenced poly(ethylene-co-acrylic acid) zinc ionomers

The morphology of a series of linear poly(ethylene-co-acrylic acid) zinc-neutralized ionomers with either precisely or randomly spaced acid groups was investigated using X-ray scattering, differential scanning calorimetry (DSC), and scanning transmission electron microscopy (STEM). Scattering from semicrystalline, precise ionomers has contributions from acid layers associated with the crystallites and ionic aggregates dispersed in the amorphous phase. The precisely controlled acid spacing in these ionomers reduces the polydispersity in the aggregate correlation length and yields more intense, well-defined scattering peaks. Remarkably, the ionic aggregates in an amorphous, precise ionomer with 22 mol % acid and 66% neutralization adopt a cubic lattice; this is the first report of ionic aggregate self-assembly onto a lattice in an ionomer with an all-carbon backbone. Aggregate size is insensitive to acid content or neutralization level. As the acid content increases from 9.5 to 22 mol % at approximately 75% neutralization, the number density of aggregates increases by approximately 5 times, suggesting that the ionic aggregates become less ionic with increasing acid content.