Mater. Proc. 2020 , 2 , 29
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J/(g°C)), the lowest for the talc fillers (to a final value of NJ c p = 0.4510 J/(g°C), and intermediate for the kaolinite fillers (to a final value of NJ c p = 0.4142 J/(g°C). This would indicate that the fillers assist in creating cross-links between the molecular chains of the natural rubber, preventing molecular mobility during the glass transition. The nanoparticles are indeed most efficient in creating cross- links, likely due to the surface chemistry of the nanoparticles with residual free amic acid groups and imidized moieties, as detailed before [5], in combination with the nanoscale surface area effect. The effects of fillers on the structure of natural rubbers are further illustrated from the ATR-FTIR spectra shown in Figure 3. The spectra both confirm the presence of the fillers in different concentrations in parallel with some changes in the natural latex molecular structure. The FTIR spectra of natural rubbers are characterized by the presence of characteristic bands for cis-1,4- polyisoprene, including 2960 cm ƺ 1 (CH 3 symmetric stretching), 2913, 2852 cm ƺ 1 (CH 2 asymmetric and symmetric stretching), 1655 (-C=C-), 1445 cm ƺ 1 (CH 3 and CH 2 bending), 1376 cm ƺ 1 (CH 3 bending), and 842 cm ƺ 1 (=CH wagging). Apart from that, the spectra of different fillers are characterized by separate absorption bands characteristic for SMI nanoparticles: 1713 cm ƺ 1 (C=O, imide), 701 cm ƺ 1 (aromatic, styrene); kaolinite: 500 to 700 cm ƺ 1 (Si-O), 900 cm ƺ 1 (OH deformation), 1000 cm ƺ 1 (Si-O stretching) 3600 cm ƺ 1 (OH stretching, Al-OH stretching); talc: 672 cm ƺ 1 (Si-O-Si symmetric stretching), 1017 cm ƺ 1 (Si- O-Si asymmetric stretching), and sharp band at 3670 cm ƺ 1 (OH).
Figure 3. FTIR spectra of natural rubber composites with different concentrations of fillers, ( a ) SMI nanoparticles, ( b ) Kaolinite, ( c ) Talc (same color legends for overview spectra and details on the right).
The related spectral bands of the fillers are independent of the natural rubber matrix and progressively increase at the higher filler concentrations. A single interaction between the SMI nanoparticles and the matrix can be seen at the shoulder peak around 1360 cm ƺ 1 : the band is present in pure natural rubbers and gradually intensifies with the higher SMI concentrations, while the band
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