Direct evidence from filled elastomers

Early work to identify and characterise modified regions of polymer in proximity to filler surfaces was undertaken in the 1960s and 1970s and utilised NMR, thermal expansion and mechanical analysis techniques, the results of which were reviewed by Dannenburg in 1975 [50] and later by Kraus in 1978 [146]. The results of these early studies were often contradictory with some authors claiming no effect of filler particles on the molecular mobility of the elastomer phase and others claiming to have observed a layer (or layers, or a gradient) of restricted mobility polymer of defined thickness. To further complicate this, the interpretation of NMR data in these early papers has now been called into question by Robertson and Roland in their review of this topic [29].

More recent experimental work is very extensive, therefore only a selection of key papers on this topic are summarised in Table 2.2. Papers are highlighted light grey to indicate that glassy or immobilised polymer was reported. Dark grey highlighting indicates that the findings or conclusions of the paper have subsequently been called into question. As can be seen, a wide range of experimental techniques have now been used to characterise the dynamics of polymers in filled rubbers though consensus of opinion has yet to be reached. As has been highlighted in the literature, this may be due to the wide range of filler-polymer combinations studied by different groups.

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Table 2.2: Review of papers addressing the immobilisation of polymer by filler particles

Author Year Technique(s) Materials Conclusion

Mason [147] 1960 Dilatometry NR/CB No change in Tg

Smit [148] 1966 Mechanical SBR/CB Absorbed, modified polymer layer. Thickness >

expansion SBR/CBs Little effect of carbon black on the Tg

Gruver [152] 1971 Mechanical SBR/PS Modifications of dynamic properties are not

[156] 1990 Mechanical Various/AP,

alumina Interphase relaxation

Berriot et al. [28] 2002 H-NMR PEA/Silica Identified gradient of Tg

around filler particles Fragiadakis et al.

[160] 2005 DSC, TSDC &

BDS PDMS/Silica Interfacial layer of polymer of 2.1-2.4 nm thickness with restricted mobility Fragiadakis et al.

[161] 2006 BDS PDMS/Silica Restricted interfacial layer

of polymer of ~3 nm

AFM SBR/Silica No effect of filler on dynamic loss modulus

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PVAC/silica No change in temperature dependence of segmental

BDS NR/Silica Restricted interfacial layer detected for

[92] 2011 DMA & BDS S-SBR/CB Immobilisation of polymer between filler aggregates subject to temperature dependence

Robertson et al.

[166] 2011 Rheometry PB/CB Concludes that 'second

glass transition' of

& AFM SBR/Silica No effect of commercial fillers on the polymer samples. Tg of interfacial layer is up to 65K higher

oxide (GO) Segmental dynamics depends on interfacial bonding and GO volume fraction

Mujtaba et al.

[172] 2014 ¹H-NMR SBR/Silica Glassy layer is detected and related to viscoelastic properties

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As an example of such disagreement, the papers of Berriot et al. (2002) [28] and Robertson et al. (2011) [26] are compared. Berriot et al. reported a gradient of glass transition in the polymer in proximity to silica particles in model, silica-filled PEA elastomers. In their samples the filler particles were synthesised using the Stöber method and yield spherical silica filler particles of ~50 nm diameter with little to no particle flocculation or aggregation. Their primary evidence for a Tg shift is derived from fitting of proton NMR T2 relaxation times obtained from Hahn spin echo experiments. They deconvoluted the filled material relaxation response in terms of rubbery and glassy responses (both measured in separate experiments on unfilled material above and below the Tg). Figure 2.61 shows the normalised time domain free induction decay (M(t)/M0) of their silica-filled PEA (solid dark line). They fit their data, at long time scales, using an exponential function which describes the relaxation of the mobile elastomer above Tg. By subtracting this fitted function from the total T1 relaxation, a rapid relaxation component is revealed which is found to correlate with the rapid relaxation measured for unfilled PEA below the Tg (inset in Figure 2.61).

Figure 2.61: NMR T2 relaxation of silica-filled PEA. The solid line is the decay of the filled material above Tg. The dotted line is the decay of the unfilled material above Tg. Open circles correspond to an exponential fit (performed at long timescales only) describing the unfilled matrix relaxation above Tg. Inset is the residual data compared with the relaxation of unfilled material measured below the Tg. (From Berriot et al. [28])

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Thus they claim that a separate glassy layer exists around the filler particles which is subject to a temperature dependence (temperature dependence data not shown). They propose a gradient in Tg with respect to distance from the filler surface, 𝑧𝑧, which they describe as

𝑇𝑇g𝜔𝜔(𝑧𝑧) = 𝑇𝑇g𝜔𝜔�1 ± �𝜁𝜁

𝑧𝑧�^{1 𝑣𝑣�} � 2.58

where 𝑇𝑇g𝜔𝜔(𝑧𝑧) is the local Tg, 𝑇𝑇g𝜔𝜔 is the bulk Tg, 𝜁𝜁 is a constant with units of dimension which relates to the nature of the attachment at the interface and 𝑣𝑣 is an exponent related to 3D percolation (see the PFVD model of the glass transition). This equation in similar in form to that originally proposed by Keddie et al. [27] to account for the size dependence of the glass transition in polymer thin films.

This contrasts with the work of Robertson et al. [26] who studied commercial precipitated silica-filled SBR. In this case, the silica particles exist as particle aggregates. They found no evidence from dilatometry, calorimetric and mechanical testing of any modification of polymer dynamics due to the incorporation of filler. Figure 2.62 shows their calorimetric data showing no change in Tg and heat capacity step change from the glass to the rubber. It is important to note that these DSC experiments were performed on samples of filled rubber extracted with solvent prior to crosslinking. What remains after this process is the filler phase and the portion of polymer which is tightly bound to the filler particles (the so-called bound rubber). Even for measurement of just the polymer in intimate contact with the filler, no change in Tg

calorimetric properties was detected. Figure 2.62 shows the dynamic loss modulus as a function of temperature. As can be seen, there is no shift or broadening of the mechanical dissipation resulting from the Tg.

In these investigations, diametrically opposed conclusions about the nature of the interfacial polymer were reached. It is worthwhile noting that a direct comparison between these studies may be difficult, as different polymer phases were used and, although both groups studied silica filler, the particle sizes and morphologies were very different.

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Figure 2.62: A) Calorimetric data of Robertson et al. [26] showing no deviation in Tg or heat capacity step as a function of bound rubber content. B) Dynamic mechanical dissipation from Robertson et al. [26]. As can be seen the shape of the peak in loss modulus is unaffected by the presence of filler

A highly relevant paper has also been published by Fragiadakis et al. (2011) [165] in which they investigated the role of silica particle morphology and dispersion on the formation of a glassy or immobilised layer of polymer. They studied natural rubber filled with silica prepared via the sol-gel method. In one series of samples the silica was precipitated into swollen NR networks and, in the second series, into NR melts in a toluene solution. In this way they prepared a series of well dispersed spherical silica-filled samples (with diameter = 10 nm) and a series of samples with aggregated silica particle structure (with the lower viscosity of the NR/toluene solution allowing for greater particle aggregation). Using a combination of DSC, BDS and Thermally Stimulated Depolarisation Current (TSDC) techniques they demonstrated that a distinct, immobilised phase is found in NR filled with near perfect dispersions of spherical filler particles which is absent from NR filled with silica aggregates (Figure 2.63). The state of particle dispersion and therefore polymer-filler contact appears to play a key role in the formation of an immobilised layer of polymer.

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Figure 2.63: Calorimetric (left) and BDS (right) data for dispersed and aggregated silica-filed NR systems. For dispersed systems the calorimetric step change at the Tg is reduced in magnitude and broadened indicating a gradient of Tg. BDS data for the dispersed system shows the appearance of a secondary, slower α' relaxation associated with interphase polymer.

Aggregated systems essentially behave like the unfilled material (From Fragiadakis [165])