Opposite Effects of SiO2 Nanoparticles on the Local α and Larger-Scale α’ Segmental Relaxation Dynamics of PMMA Nanocomposites

Abstract

The segmental relaxation dynamics of poly(methyl methacrylate)/silica (PMMA/SiO2) nanocomposites with different compositions ( ϕ SiO 2 ) near and above the glass transition temperature were investigated by mechanical spectroscopy. At ϕ SiO 2 ≤ 0.5%, the α peak temperature hardly changes with ϕ SiO 2 , but that of α’ relaxation composed of Rouse and sub-Rouse modes decreases by 15 °C due to the increase of free volume. At ϕ SiO 2 ≥ 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 ϕ SiO 2 , the peak height for α relaxation increases at ϕ SiO 2 ≤ 0.5% and then decreases at ϕ SiO 2 ≥ 0.7%, but that for α’ relaxation shows an opposite behavior. This is because at low ϕ SiO 2 , 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 ϕ SiO 2 , 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.

Decoupled Polymer Dynamics in Weakly Attractive Poly(methyl methacrylate)/Silica Nanocomposites

“…Although both the glass-transition temperature and the Vogel temperature represent the retarded dynamics due to the NP–polymer interaction, the change in T ∞ is much more significant than that in T g . The maximum variation in T g is less than 5 K, while that in T ∞ is over 15 K. A similar trend of T ∞ has also been reported by Wang et al, in which T ∞ increased by 9 K for PMMA/silica PNCs with 2.0 wt % NP loading . In contrast, Baeza and co-workers reported a significant decrease of T ∞ in P2VP/silica systems .…”

“…The maximum variation in T g is less than 5 K, while that in T ∞ is over 15 K. A similar trend of T ∞ has also been reported by Wang et al, in which T ∞ increased by 9 K for PMMA/silica PNCs with 2.0 wt % NP loading. 71 In contrast, Baeza and co-workers reported a significant decrease of T ∞ in P2VP/silica systems. 66 The difference might be attributed to different NP−polymer interactions due to the difference in chain stiffness.…”

Mechanical and thermal properties of carbon fiber reinforced composite with silanized graphene as nano-inclusions

“…On the contrary, when 1 wt% pristine graphene is added as the nano-inclusion, the agglomeration becomes high enough to introduce steric hindrance to polymer segmental motion and hence the T g increases compared to the 0.5 wt% pristine graphene sample. 39 In case of 0.25 wt% and 0.5 wt% silanized graphene samples the agglomeration is far lower compared to their corresponding counterpart for pristine graphene. The agglomeration in 0.25 wt% and 0.5 wt% silanized graphene samples is small enough such that with increased nano-inclusion the attractive interaction between silanized graphene and polymer enhances, restricting the segmental polymer chain motion.…”

“…The agglomeration in 0.25 wt% and 0.5 wt% silanized graphene samples is small enough such that with increased nano-inclusion the attractive interaction between silanized graphene and polymer enhances, restricting the segmental polymer chain motion. 39 This caused an increased T g for 0.5 wt% silanized graphene sample compared to 0.25 wt% silanized graphene nanocomposite. In case of 1 wt% silanized graphene nanocomposite, the agglomeration could become high enough that graphene-rich and graphene-poor regions could form as in the case of 0.5 wt% pristine graphene, and hence the T g decreases compared to 0.25 wt% and 0.5 wt% silanized graphene nanocomposites.…”

Mediating the slow dynamics of polyacrylates by small molecule-bridged hydrogen bonds

“…The addition of a rigid (or high-T g ) component to miscible polymer blends generally increases the energy barrier for chain rotation/ reptation of flexible ones, which retards the slow dynamics of a flexible matrix (with higher peak temperatures). 25,26 However, the addition of flexible components typically accelerates the slow mode of a rigid matrix (with lower peak temperatures). 21,24 It must be noted that in those van der Waals systems, the peak temperature of two relaxation modes usually moves in the same direction, since both modes are controlled by the same monomeric friction coefficient of the polymer.…”