Opposite Effects of SiO
2 Nanoparticles on the Local
α and Larger-Scale
α' Segmental Relaxation Dynamics of PMMA Nanocomposites.
Polymers (Basel) 2019;
11:polym11060979. [PMID:
31163669 PMCID:
PMC6630292 DOI:
10.3390/polym11060979]
[Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/23/2019] [Revised: 05/24/2019] [Accepted: 05/24/2019] [Indexed: 12/27/2022] Open
Abstract
The segmental relaxation dynamics of poly(methyl methacrylate)/silica (PMMA/SiO2) nanocomposites with different compositions (ϕSiO2) near and above the glass transition temperature were investigated by mechanical spectroscopy. At ϕSiO2 ≤ 0.5%, the α peak temperature hardly changes with ϕSiO2, but that of α’ relaxation composed of Rouse and sub-Rouse modes decreases by 15 °C due to the increase of free volume. At ϕSiO2 ≥ 0.7%, both α and α’ relaxations shift to high temperatures because of the steric hindrance introduced by nanoparticle agglomeration. On the other hand, with increasing ϕSiO2, the peak height for α relaxation increases at ϕSiO2 ≤ 0.5% and then decreases at ϕSiO2 ≥ 0.7%, but that for α’ relaxation shows an opposite behavior. This is because at low ϕSiO2, the short-chain segments related to α relaxation can easily bypass the particles, but the longer-chain segments related to α’ relaxation cannot. At high ϕSiO2, the polymer chains were bound to the nanoparticles due to the physical adsorption effect, leading to the decrease of relaxation unit concentration involved in α relaxation. However, the dissociation of those bonds with heating and the concentration heterogeneity of polymer chains result in the increase of peak height for α’ relaxation.
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