The extensive particle-polymer surface area and local confinement effects in polymer nanocomposites can have a strong influence on the segmental mobility of the polymer chains as reflected in the characteristics of the glass-rubber relaxation and corresponding glass transition temperature, Tg. Over the last several decades, there have been a number of reports examining the glass transition characteristics of filled polymers reinforced with materials such as carbon black or silica; more recently, such systems have been studied within the context of nanoscale phenomena. In filled polymers, the inclusion of inorganic particles often leads to an increase in the glass transition temperature due to favorable interactions between the particle surface and the polymer chains that limit chain mobility. Investigators have reported positive offsets in Tg, as well as the emergence of a second (higher) Tg corresponding to the presence of a distinct, constrained population of chain segments in the vicinity of the particle surface [1]. For systems that exhibit poor wetting, Tg reductions have also been encountered. Recent studies have explored the potential agreement in Tg trends between bulk nanocomposites and polymer thin films wherein both the nature of the polymer-solid interface and the degree of geometric confinement can be carefully controlled [2-3].
In this paper, we examine the influence of nanoscale particle loading on the dynamic relaxation characteristics of two model polymer nanocomposite systems based on rubbery and glassy polymers, respectively. Both systems are of interest with respect to potential improvements in gas separation performance upon nanoparticle addition. Specifically, dynamic mechanical analysis and broadband dielectric spectroscopy techniques have been employed to elucidate the characteristics of the glass-rubber and sub-glass relaxations in these systems as a function of particle surface chemistry, particle loading and nanocomposite processing.
Rubbery nanocomposite networks were prepared by UV photopolymerization of poly(ethylene glycol) diacrylate [PEGDA] in the presence of varying loadings of MgO (~ 3 nm) or SiO2 (10 nm) nanoparticles. For the nanocomposites, the liquid pre-polymerization mixture was prepared by conventional mixing methods, as well as by planetary mixing; at higher loadings, particle dispersion was facilitated by the introduction of an appropriate diluent, and the influence of the diluent on the characteristics of the crosslinked network and resulting composites was evaluated. Glassy nanocomposites were produced via solution casting of mixtures comprised of polyetherimide [PEI] and SiO2 nanoparticles according to the method described by Takahashi and Paul [4]; nanocomposites based on the inclusion of native SiO2 as well as commercially-available surface-modified silica were examined. Characterization of the static and dynamic properties of the composites included bulk density measurement and dynamic mechanical and dielectric studies. In addition, pure gas permeability measurements were performed for the PEGDA/MgO samples.
Dynamic mechanical studies on the PEGDA/MgO and PEGDA/SiO2 networks revealed shifts in the glass transition processes of the composites that correlated with the nature and dispersion of the respective metal oxide fillers. Across the PEGDA/SiO2 series, a progressive increase in Tg on the order of 15°C was observed with increasing SiO2 loading (0 to 30 wt%). For the PEGDA/MgO nanocomposites, the bulk PEGDA glass transition temperature (Tg = -35°C at 1 Hz) was largely unchanged, but a second, distinct relaxation event emerged approximately 40°C above Tg suggesting the presence of a sub-population of chain segments subject to a higher degree of motional constraint. The characteristics of this second relaxation were found to be dependent upon filler loading and the processing conditions used for preparation of the pre-polymerization mixture. Dielectric data for the PEGDA nanocomposites were consistent with the dynamic mechanical results and provided additional insight as to the influence of the nanoparticles on the individual sub-glass relaxations. Dielectric response as a function of particle loading was analyzed according to established descriptions of inhomogeneous media [5].
In the case of the PEI nanocomposites, the introduction of native SiO2 (up to 25 wt%) appeared to have little influence on the glass transition temperature of the composite as compared to the unfilled polymer (Tg ~ 225°C at 1 Hz). However, the introduction of surface-modified (i.e., hydrophobic) fumed silica led to improved particle dispersion [4] and the appearance of a second, higher-temperature Tg in the dynamic mechanical sweeps. This second Tg was offset by approximately 60°C as compared to the bulk PEI relaxation and appears to correspond to a separate glass transition emanating from regions of reduced mobility in the vicinity of the particle-polymer interface. The properties of the dual-Tg relaxation were assessed as a function of particle loading, surface characteristics, and the resulting nanocomposite morphology.
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