Heterogeneous chain dynamics and aggregate lifetimes in precise acid-containing polyethylenes: Experiments and simulations

Journal Article · Thu Nov 10 00:00:00 EST 2016 · Macromolecules

DOI:https://doi.org/10.1021/acs.macromol.6b01918· OSTI ID:1332950

Middleton, L. Robert ^[1]; Tarver, Jacob D. ^[2]; Cordaro, Joseph ^[3]; Tyagi, Madhusudan ^[4]; Soles, Christopher L. ^[5]; Frischknecht, Amalie L. ^[6]; Winey, Karen I. ^[1]

---

1. Univ. of Pennsylvania, Philadelphia, PA (United States)
2. National Institute of Standards and Technology, Gaithersburg, MD (United States)
3. Sandia National Lab. (SNL-CA), Livermore, CA (United States)
4. NIST Center for Neutron Research, Gaithersburg, MD (United States); Univ. of Maryland, College Park, MD (United States)
5. National Institute of Standards and Technology, Gaithersburg, MD (United States); NIST Center for Neutron Research, Gaithersburg, MD (United States)
6. Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)

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. As a result, 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.

• Research Organization: Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: AC04-94AL85000
• OSTI ID: 1332950
• Report Number(s): SAND-2016-11233J; 648999
• Journal Information: Macromolecules, Journal Name: Macromolecules; ISSN 0024-9297
• Publisher: American Chemical Society
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
36 MATERIALS SCIENCE