Supplementary Information
Tough nanocomposite hydrogels from cellulose nanocrystals/poly(acrylamide) clusters: influence of charge density, aspect ratio and surface coating with PEG
Jun Yanga*, Jing-Jing Zhaob, Chun-Rui Hana, Jiu-Fang Duana, Feng Xua, Run-Cang Suna
aCollege of Materials Science and Technology, Beijing Forestry University, Beijing, China
bBeijing SL Pharmaceutical Co., LTD, Beijing, China
*Corresponding author:
Fourier transform infrared spectroscopy (FTIR) was performed on an infrared spectrophotometer (Nicolet iN10-MX, Thermo Scientific).
Fig.S1 Illustration of cellulose silane modification and FTIR spectra
(a)
(b)
(c)
(d)
(e)
(e)
Fig.S2 Diameter (left column) and length (right column) distribution histograms of A10S112 (a), A11S88 (b), A12S42 (c), A6S107 (d), A12S202 (d), and A22S96 (e) suspensions obtained by measuring the diameter and length on TEM micrographs of negatively stained preparations.
X-ray diffraction (XRD) tests were conducted by XRD-6000 (Shimadzu) at room temperature. The CuKα radiation source was operated at 40 kV and 30 mA. The scan speed was 2°/min with monitoring diffractions from 5° to 40°.
Fig.S3 XRD curves of cellulose nanocrystals with various surface charge and aspect ratio.
Fig.S4 TEM images and schematic illustration of CNC aggregates (green bars) dispersibility under different surface charge concentrations. It is noted that CNCs with low surface charges (B) lead to agglomerate of CNC/polymer clusters, whereas the CNCs with high surface charges (A) contributed to the well-dispersed network and reduces stress concentration. The high surface charge (red circle) concentration attributes to well dispersed CNC within the polymer matrix and extended volume fraction of constrained polymer (A), whereas low surface charge concentration leads to CNCs accumulation and local constrain zone (B).
The reinforcement of elastomers by nanoscale fillers is commonly understood to result from the formation of a fillers network, and the mobility of fillers is a key factor for the toughening of polymer materials (Pan et al 2008; Papon et al 2012). Some studies have reported that the presence of fillers slowed-down polymer chains dynamics in the vicinity of the solid surface, and a model of “glassy bridge” was proposed to interpret the polymer dynamics in filled elastomers (LeBaron et al. 1999; Jouault et al. 2009; Shah et al. 2005). Assuming connections between clusters is generated by the immobilized polymer segments, the volume fraction of constrained is related to the height of loss factor in the frequency spectra (Fig. S5). The results are analyzed in terms of loss factor and a change in the peak height. In principle, the height of loss factor (H) is related to the fraction of polymer chains that participating at immobilization on filler surface and is proportional to the number of internal degrees of freedom of molecular motion (Zhang and Loo 2009). It is noted that there is a slight shift in the H toward lower value for the CNC with high surface charge, indicating increased volume fraction of constrained chains and restricted mobility of polymer chains. Thus, this result is in consistent with the conclusion that CNC surface charges dominate the mechanical reinforcement.
Fig.S5 Representative of loss factor (tan δ) for nanocomposites at different surface charges. ((■) A12S42, (●) A11S88, (▲) A10S112)
Fig.S6 Weight loss of PEG incorporated CNC/PAM nanocomposites as a function of swelling time at 25 °C. There is almost no mass loss over 7 days, indicating strong physical adsorption of PEG on cellulose surface. The possible interaction between CNC and PEG is schemed.
Compression experiments were performed on a Zwick 005 Materials Tester at room temperature. The hydrogel sample, in the form of a cylinder (30 mm in diameter and 5 mm in initial thickness), was set on the lower plate and compressed by the upper plate. The rate of stretch was kept constant at 4 mm/min. the strain under compression is defined as the change in the thickness relative to the freestanding thickness of the specimen.
Fig.S7 Stress-strain curve for hydrogels under uniaxial compression.
X-ray Photoelectron Spectroscopy (XPS)
The silane modification through γ–methacryloxypropyl trimethoxy silane (A174) grafting on the surface of CNCs was further analyzed by XPS. The experiments were conducted using PHI-5300 ESCA (Perkin Elmer) operated at 15 kV under a current of 10 mA. Samples were set in an ultrahigh vacuum chamber (10-8 mbar) with electron collection by a hemispherical analyzer at a 90°angle. The silane agent grafting efficiency (GE%) was calculated as follows:
GE% × CA174 + (1-GE%)CCNC = CCNC-g-A174
where C is the relative silicon of the sample.
Fig.S8 XPS spectra for CNC and silane modified CNC.
There was new signal corresponding to silicon (288.5 eV) that assigned to the surface chemistry of CNC-g-A174 in comparison with pristine CNC, indicating covalent coupling from silylation reaction. The grafting efficiency (GE%) of silane agent was determined to be 6.5%.
References
Jouault N, Vallat P, Dalmas F, Said S, Jestin J, BouéF (2009) Well-dispersed fractal aggregates as filler in polymer−silica nanocomposites: long-range effects in rheology. Macromolecules 42: 2031-2040
LeBaron P C, Wang Z, Pinnavaia TJ (1999) Polymer-layered silicate nanocomposites: an overview. Appl Clay Sci 15: 11–29
Pan Y Z, Xu Y, An L, Lu H B, Yang Y L, Chen W, Nutt S (2008) Hybrid network structure and mechanical properties of rodlike silicate/cyanate ester nanocomposites. Macromolecules 41: 9245-9258
Papon A, Montes H, Lequeux F, Oberdisse J, Saalwächter K, Guy L (2012) Solid particles in an elastomer matrix: impact of colloid dispersion and polymer mobility modification on the mechanical properties. Soft Matter 8: 4090-4096
Shah D, Maiti P. Jiang D D, Batt C A, Giannelis E (2005) Effect of nanoparticle mobility on toughness of polymer nanocomposites. Adv Mater 2005 17: 525-528
Zhang X G, Loo L S (2009) Study of glass transition and reinforcement mechanism in polymer/layered silicate nanocomposites. Macromolecules 42: 5196–5207
7