Heterogeneous Chain Dynamics and Aggregate Lifetimes in Precise Acid-Containing Polyethylenes: Experiments and Simulations

Abstract

Melt state dynamics for a series of strictly linear polyethylenes with precisely spaced associating functional groups were investigated. The periodic pendant acrylic acid groups form hydrogen-bonded acid aggregates within the polyethylene (PE) matrix. The dynamics of these nanoscale heterogeneous morphologies were investigated from picosecond to nanosecond timescales by both quasi-elastic neutron scattering (QENS) measurements and fully atomistic molecular dynamics (MD) simulations. Two dynamic processes were observed. The faster dynamic processes which occur at the picosecond timescales are compositionally insensitive and indicative of spatially restricted local motions. The slower dynamic processes are highly composition dependent and indicate the structural relaxation of the polymer backbone. Higher acid contents, or shorter PE spacers between pendant acid groups, slow the structural relaxation timescale and increase the stretching parameter (β) of the structural relaxation. Additionally, the dynamics of specific hydrogen atom positions along the backbone correlate structural heterogeneity imposed by the associating acid groups with a mobility gradient along the polymer backbone. At time intervals (<2 ns), the mean-squared displacements for the four methylene groups closest to the acid groups are up to 10 times smaller than those of methylene groups further from the acid groups. At longer timescales acid aggregates rearrange and the chain dynamics of the slow, near-aggregate regions and the faster bridge regions converge, implying a characteristic timescale for the passage of chains between aggregates. The characterization of the nanoscale chain dynamics in these associating polymer systems both provides validation of simulation force fields and provides understanding of heterogeneous chain dynamics in associating polymers.

Why Enhanced Subnanosecond Relaxations Are Important for Toughness in Polymer Glasses

“…Both the time and length scales of the two methods are well matched. Because of this overlap, there are an increasing number of examples of where either QENS is used to validate MD simulations , or the nature of the motions observed in MD simulations is used to interpret experimental QENS data. − In this vein, we are encouraged that results presented here support recent assertions from Sirk and co-workers , where the ballistic impact resistance in a series of poly­(dicyclopentadiene) polymers is facilitated by enhanced nanovoid fluctuations on the picoseconds to nanoseconds time scales. These fast fluctuations dissipate energy and stave off the formation of stabilized critical cracks that lead to failure.…”

Self-assembled highly ordered acid layers in precisely sulfonated polyethylene produce efficient proton transport

“…Fully atomistic MD simulations were carried out using the LAMMPS software package 55 . Alkyl segments were modelled using the LOPLS force field 56 as in our previous work 9, 57 , sulfonate groups used the parameters developed in ref. 58 , water molecules used the TIP3P parameters 59 , and hydronium molecules used parameters developed in ref.…”

Functional Polyethylenes with Precisely Placed Thioethers and Sulfoniums through Thiol–Ene Polymerization

“…25 In addition, the required ruthenium catalyst used in ADMET can be difficult to remove, and the resulting polymer product contains double bonds in the backbone that must be hydrogenated to obtain the desired polyethylene backbone. 26 Therefore, developing an approach to include precise functional group placement that complements ADMET would represent a significant advancement toward new functional polyethylenes.…”

“…The most common method employed to synthesize precisely functionalized polyethylene utilizes acyclic diene metathesis polymerization (ADMET). ,,, By specifically designing functional diene monomers, polymers can be synthesized where the moiety is incorporated at regular, precise locations along the polymer chain, dictated by the location of the functional group in the monomer. ,,, This approach has successfully produced precise polymers containing a variety of functional groups including carboxylic acids, phosphonic acids, halogens, , alcohols, , and sulfones, demonstrating a wide range of unique properties and morphologies. , In spite of its success, ADMET has the disadvantages of requiring difficult monomer syntheses, long polymerization times, and high temperatures . In addition, the required ruthenium catalyst used in ADMET can be difficult to remove, and the resulting polymer product contains double bonds in the backbone that must be hydrogenated to obtain the desired polyethylene backbone . Therefore, developing an approach to include precise functional group placement that complements ADMET would represent a significant advancement toward new functional polyethylenes.…”