We study the effect of in situ synthesized 10 nm silica nanoparticles on the glass transition and dynamics of natural rubber networks using differential scanning calorimetry, broadband dielectric ...relaxation spectroscopy and thermally stimulated depolarization currents. Even in the absence of specific polymer-filler interactions, polymer segments within a few nanometers of the filler particles exhibit relaxation times up to 2–3 orders of magnitude slower and reduced heat capacity increment at the glass transition compared to bulk natural rubber. These effects are only observed when the nanoparticles are uniformly distributed in the polymer matrix.
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We report new results on segmental dynamics and glass transition in a series of poly(dimethylsiloxane) networks filled with silica nanoparticles prepared by sol-gel techniques, obtained by ...differential scanning calorimetry (DSC), thermally stimulated depolarization currents (TSDC), broadband dielectric relaxation spectroscopy (DRS) and dynamic mechanical analysis (DMA). The nanocomposites are characterized by a fine dispersion of 10nm silica particles and hydrogen bonding polymer/filler interactions. The first three techniques indicate, in agreement with each other, that a fraction of polymer in an interfacial layer around the silica particles with a thickness of 2–3nm shows modified dynamics. The DSC data, in particular measurements of heat capacity jump at Tg, are analyzed in terms of immobilized polymer in the interfacial layer. The dielectric TSDC and DRS data are analyzed in terms of slower dynamics in the interfacial layer as compared to bulk dynamics. We employ a special version of TSDC, the so-called thermal sampling (TS) technique, and provide experimental evidence for a continuous distribution of glass transition temperatures (Tg) and molecular mobility of the polymer in the interfacial layer, which is consistent with the DRS data. Finally, DMA results show a moderate slowing down of segmental dynamics of the whole polymer matrix (increase of glass transition temperature by about 10K as compared to the pure matrix).
The molecular dynamics of a series of poly(dimethylsiloxane) networks filled with silica nanoparticles synthesized in situ was investigated using thermally stimulated depolarization currents, ...broadband dielectric relaxation spectroscopy and differential scanning calorimetry. The techniques used cover together a broad frequency range (10
−3–10
9
Hz), thus allowing to gain a more complete understanding of the effects of the nanoparticles on the chain dynamics. In addition to the
α relaxation associated with the glass transition of the polymer matrix, we observe in dielectric measurements a slower
α relaxation which is assigned to polymer chains close to the polymer/filler interface whose mobility is restricted due to interactions with the filler surface. The thickness of the interfacial layer is estimated to be about 2.1–2.4
nm. Differential scanning calorimetry shows a change in the shape of the glass transition step, as well as a decrease in both the degree of crystallinity and the crystallization rate by the addition of silica.
Effects of hydration on the interfacial dynamics of polydimethylsiloxane (PDMS) adsorbed on silica and titania nanoparticles in the form of aggregates of different specific surface area (S BET, ...25–342 m2/g) were studied employing equilibrium water sorption/desorption isotherms (ESI/EDI) and broad-band dielectric spectroscopy (BDS). ESI revealed a systematic increase of the water content, h, of initial oxides (varying up to 60 wt %) with increasing S BET, whereas changes in h in nanocomposites (PNCs) are very weak (0.1–0.3 wt %) and practically independent from S BET. BDS allowed for the detection of an increase in the interfacial polymer fraction with hydration, accompanied by a significant enhancement of dynamics and cooperativity of the corresponding segmental relaxation in the interfacial layer (αint relaxation), changes being reversible as recorded by hydration and subsequent dehydration of the samples. At the same time, bulk PDMS dynamics in PNCs (α relaxation) was not significantly affected. Effects of hydration on the characteristics of αint in PNCs were found to be qualitatively similar here with changes imposed by thermal annealing (resulting in increasing crystallinity) and surface modifications (resulting in changes in S BET) recorded recently in similar PNCs. Hydration effects were additionally followed via monitoring the dielectric relaxation process attributed to hydrated hydroxyls at the surfaces of the oxides (S relaxation) in the PNCs. The results may suggest that with increasing h the concentration of contact points between the solid surface (oxides) and the polymer chains increases, while the polymer chain conformations at the interface change toward an increase in looplike interfacial polymer segments.
Results by differential scanning calorimetry (DSC) and broadband dielectric spectroscopy (BDS) are presented in this work, monitoring the interplay between confined, interfacial, and bulk polymer ...dynamics in silica–gel/polydimethylsiloxane (PDMS) systems. Confined and bulk mobility could be distinguished by recording two well separated glass transition steps in DSC. For low content of adsorbed PDMS (40 wt%), two contributions to segmental dynamics were detected by BDS, arising from the polymer in the interfacial layer on the surfaces of nanoparticles (αint relaxation) and from polymer chains confined in the pores (∼6–20 nm in diameter) of silica–gel (αp), while no bulk segmental relaxation was detected. Nanozirconia grafted onto inner and external surfaces of silica–gel resulted, on the one hand, in reduction of the number of polymer–filler contact points and, on the other hand, in more severe spatial confinement of the polymer in pores. Two additional processes contribute to the dielectric response for the higher polymer loading (80 wt%), arising from the bulk–unaffected amorphous polymer chains (α relaxation) and the constrained polymer mobility between condensed crystal regions (αc). Thermal annealing suppressed the interfacial and bulk (via crystallization) polymer mobility, whereas the confined polymer dynamics remained completely unaffected. The interfacial characteristics could be described well in terms of bimodal conformations of polymer chains, similarly to polymers adsorbed on flat solid surfaces, and in terms of changes in apparent interfacial layer thickness.
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•Glass transition of bulk and confined PDMS could be recorded separately in DSC.•Modification of silica–gel pores with nanozirconia enhanced spatial 2D polymer confinement.•Interfacial dynamics and fraction were suppressed by surface modification and annealing.•Confined polymer remained unaffected after thermal (crystallization) annealing.
This work deals with effects of polymer molecular weight, W m, below the entanglement threshold, W m,e, on molecular dynamics of polydimethylsiloxane (PDMS) adsorbed onto silica particles, employing ...differential scanning calorimetry (DSC) and two dielectric techniques: broadband dielectric spectroscopy (BDS) and thermally stimulated depolarization currents (TSDC). The rigid amorphous polymer fraction at interfaces, RAFint, was found suppressed for larger W m by all techniques in qualitative agreement with each other. Results on RAFint were supported by evaluating, for the first time, the coverage of hydroxyls at the surfaces of nanoparticles by polymer chains (S relaxation). The mobility of interfacial polymer (αint relaxation) was followed by BDS and TSDC, showing suppression of dynamics and cooperativity with decreasing W m. We suggest that interfacial polymer fraction and dynamics are dominated by the concentration of polymer–particle contact points, the latter increasing for smaller W m due to more free chain ends, as expected below W m,e. Furthermore, adopting models that describe multiple conformations for polymers adsorbed on solid surfaces, we explain our results in terms of promotion of tail/loop-like conformations in the particle–polymer interfacial layer for shorter/longer polymer chains, respectively. The model was further checked by employing surface modification of initial silica, which resulted in smoothening of nanoparticle surface and led to further suppression of RAFint and interfacial polymer dynamics.
The glass transition of partially crystallized gelatin–water mixtures was investigated using broadband dielectric spectroscopy (BDS) over a wide range of frequencies (10 mHz to 10 MHz), temperatures ...(113–298 K), and concentrations (10–45 wt %). Three dielectric relaxation processes (processes I, II, and III) were clearly observed. Processes I, II, and III originate from uncrystallized water (UCW) in the hydration shells of gelatin, ice, and hydrated gelatin, respectively. A dynamic crossover, called the Arrhenius to non-Arrhenius transition of UCW, was observed at the glass transition temperature of the relaxation process of hydrated gelatin for all mixtures. The amount of UCW increases with increasing gelatin content. However, above 35 wt % gelatin, the amount of UCW became more dependent on the gelatin concentration. This increase in UCW causes a decrease in the glass transition temperature of the cooperative motion of gelatin and UCW, which appears to result from a change in the aggregation structure of gelatin in the mixture at a gelatin concentration of approximately 35 wt %. The temperature dependence of the relaxation time of process II has nearly the same activation energy as pure ice made by slow crystallization of ice Ih. This implies that process II originates from the dynamics of slowly crystallized ice Ih.
We report for the first time on a polyurethane system with polyhedral oligomeric silsesquioxanne (POSS) particles chemically bonded along its main chain as nanobuilding blocks. Morphology and ...molecular dynamics of this novel system are studied with imaging (SEM), thermal (DSC, MDSC) dielectric (TSDC, DRS), and mechanical (DMA) techniques. Two distinct phases occur: one chain extended with POSS and one with chemistry and morphology similar to the matrix. Although T g remains practically unaffected upon incorporation of particles, a relaxation α′ slower than the main dynamic glass transition α and present also in the matrix is enhanced in the presence of POSS. We attribute it to decelerated dynamics in the presence of heavy moieties like particles and rigid hard microdomains. No effect on the time scale of these relaxations is imposed by the presence of POSS, but rubbery modulus is significantly reduced due to the soft POSS-extended phase.
We employ a partly new experimental approach to polymer nanocomposites (PNCs) based on a semicrystalline matrix, in order to distinguish between phenomena affiliated, on the one hand, to interactions ...between polymer and nanoparticles, and, on the other hand, to polymer crystals. Thus, effects of silica nanoparticles and of crystalline fraction (CF) on glass transition and segmental dynamics in poly (l–lactic acid) were investigated by means of differential scanning calorimetry (DSC) and broadband dielectric relaxation spectroscopy (DRS). Analysis of results involves combination of measurements on initially amorphous and on semicrystalline (annealed) samples. No change in the glass transition temperature by the filler is observed by DSC, whereas the heat capacity step decreases in the PNCs. The segmental α relaxation (dynamic glass transition) in DRS becomes, however, faster and weaker in the PNCs. Results are rationalized in terms of the rigid amorphous fraction (RAF) due to filler (interfacial polymer, RAFfiller) and due to polymer crystals (RAFcrystal). RAFfiller and RAFcrystal were disentangled from the total RAF via two assumptions: (a) assuming the same RAFcrystal to CF ratio in the neat matrix and the PNCs, and (b) assuming the same RAFfiller in amorphous and semicrystalline samples. Changes of the various polymer fractions with composition show similar trends in DSC and DRS. RAFfiller was found to increase with filler fraction, with a saturation for the largest loading (20 wt%), assigned to filler aggregation confirmed by morphological characterization. Both RAFcrystal and mobile amorphous fraction, MAF, were found within experimental error almost constant with filler loading. The overall results suggest that interfacial interactions (RAFfiller) in combination with changes in semicrystalline morphology dominate polymer dynamics in semicrystalline PNCs.
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•Heat capacity step and dielectric strength of α relaxation decrease in PLA PNCs.•Separation of RAF due to filler, RAFfiller, and to crystals, RAFcrystal, under assumptions.•Similar trends of RAF and MAF by DSC and DRS.•Interfacial layer thickness for PLLA PNCs estimated in the range from 2 to 5 nm.•Semicrystalline morphology explains effects on segmental dynamics and Tg.
