nanomaterials

Article Preparation and Characterization of Polyvinylpyrrolidone/Cellulose Nanocrystals Composites

Marina Voronova 1, Natalia Rubleva 1, Nataliya Kochkina 1 , Andrei Afineevskii 2, Anatoly Zakharov 1 and Oleg Surov 1,* 1 G.A. Krestov Institute of Solution Chemistry of the Russian Academy of Sciences, 1 Akademicheskaya St., Ivanovo 153045, Russia; [email protected] (M.V.); [email protected] (N.R.); [email protected] (N.K.); [email protected] (A.Z.) 2 Department of Physical and Colloid Chemistry, Ivanovo State University of Chemistry and Technology, 7 Sheremetevsky Prospect, Ivanovo 153000, Russia; afi[email protected] * Correspondence: [email protected]; Tel.: +7-4932-351545



Received: 7 November 2018; Accepted: 4 December 2018; Published: 5 December 2018


Abstract: Composite films and aerogels of polyvinylpyrrolidone/cellulose nanocrystals (PVP/CNC) were prepared by solution casting and freeze-drying, respectively. Investigations into the PVP/CNC composite films and aerogels over a wide composition range were conducted. Thermal stability, morphology, and the resulting reinforcing effect on the PVP matrix were explored. FTIR, TGA, DSC, X-ray diffraction, SEM, and tensile testing were used to examine the properties of the composites. It was revealed PVP-assisted CNC self-assembly that produces uniform CNC aggregates with a high aspect ratio (length/width). A possible model of the PVP-assisted CNC self-assembly has been considered. Dispersibility of the composite aerogels in water and some organic solvents was studied. It was shown that dispersing the composite aerogels in water resulted in stable colloidal suspensions. CNC particles size in the redispersed aqueous suspensions was near similar to the CNC particles size in never-dried CNC aqueous suspensions.

Keywords: cellulose nanocrystals; nanocomposites; self-assembly; dispersibility

1. Introduction Cellulose nanocrystals (CNC) are crystalline nano-sized rod-like particles, that can be easily produced from natural renewable cellulosic materials by controlled acid hydrolysis. When sulfuric acid is used, CNC form stable aqueous suspensions due to negatively charged sulfate ester groups at the CNC particle surface which cause electrostatic repulsion between the rod-like colloidal particles. The diameter of the anisotropic CNC particles is from 10 to 20 nm, the length is from 100 to 300 nm, depending on processing conditions and the raw material [1]. The main features that promote the use of CNC are an anisotropic particle shape, a rather large surface area, a low density, a very high modulus of elasticity and a significant reinforcing effect at a low content in a composite. As one of the most plentiful natural polymers with superb mechanical properties, excellent biodegradability and biocompatibility, nanocrystalline cellulose has been considered as a perfect reinforcing element for low-cost nanocomposites [2,3]. The properties of CNC (capacity to chemical modification of the surface hydroxyl groups, high mechanical strength, formation of a chiral nematic liquid-crystalline phase) attract considerable attention of researchers designing new functional materials [4–9]. These days the most studies on CNC composites relate to materials with low CNC content because of the challenge to prevent CNC agglomeration at high CNC content. Another issue is a choice of suitable polymer matrix providing high CNC content, nano-structural control, and interface tailoring

Nanomaterials 2018, 8, 1011; doi:10.3390/nano8121011 www.mdpi.com/journal/nanomaterials Nanomaterials 2018, 8, 1011 2 of 21 at molecular scale level. In this regard, model systems consisting of crystalline (CNC) and amorphous (polymer matrix) components can provide a mimetic approach for research of natural phenomena [10]. In recent years, intensive efforts have been provided to design new eco-friendly polymeric materials from renewable sources as well as hybrid structures comprising nanoscale natural biodegradable and biocompatible polymers and inorganic materials [11,12]. Polyvinylpyrrolidone (PVP), a water-soluble, non-toxic, non-ionic amorphous polymer with high solubility in polar solvents, has been widely used in nanoparticles synthesis [13]. Due to the amphiphilic nature, PVP can affect morphology and nanoparticle growth by ensuring solubility in various solvents, discriminatory surface stabilization, controlled crystal growth, playing the role of a shape-control agent and facilitating the growth of specific crystal faces while preventing others [14–16]. PVP, with its versatile features such as a lack of toxicity, film formation, water solubility, and adhesive power, is one of the most promising polymers for nanogels preparation [17–21]. PVP can serve as a nanoparticle dispersant, growth modifier, surface stabilizer, hampering the agglomeration of nanoparticles through the repulsive forces that arise from its hydrophobic carbon chains interacting with each other in a solvent. PVP has been thought as a perspective material with a very good film-forming ability for potential application in production of cosmetics, printing inks, detergents, paints, coatings, shielding devices, adhesives, plastics, medicine, and pharmaceuticals [22–25]. Wu et al. [26] developed a method of polypyrrole (PPy) polymerization on CNC coated with PVP. They achieved high conductivity for the PPy/PVP/CNC composite with an excellent specific capacitance and improved cycling stability. The authors emphasize that physical adsorption of PVP onto the CNC surface plays a critical role in ensuring uniform PPy coating. PVP makes the CNC surface hydrophobic and, thus, much more appropriate for the growth of the hydrophobic PPy cover that acts as a steric stabilizer stopping further aggregation of the nanoparticles. Going et al. [27] reported a method for achieving dispersion of the freeze-dried CNC/PVP composite in a water–methanol system using magnetic stirring and sonication. The effects of CNC loading and preparation method on dispersion, solution viscosity, and mechanical properties of electrospun PVP/CNC nanocomposites were inspected. Huang et al. [28] produced PVP/CNC/Ag nanocomposite fibers via electrospinning and studied their properties. The authors showed that improved strength and antimicrobial characteristics of PVP/CNC/Ag electrospun composite fibers made the material a perspective candidate for a biomedical application. Chitosan/PVP/CNC composite films were prepared by Hasan et al. [29] for the development of an efficient sustained drug release in wound dressing application. It was revealed that incorporation of CNC in films enhanced swelling, thermal, and mechanical properties and improved resistance to enzymatic degradation. Recently, Gao and Jin [30] manufactured iridescent chiral nematic CNC/PVP nanocomposite films for fast and easy distinguishing similar organic solvents, such as halogenated hydrocarbons, skeletal isomers, and homologues. They showed that the CNC/PVP nanocomposite film could work as a sensor for the solvents recognition due to an apparent structural color change. Here, we hypothesize that PVP adsorption onto CNC can block lateral interactions between the CNC and prevents their agglomeration in the lateral direction, i.e., hinders growth of the CNC particles width upon the concentration increase or drying of the composites. On the other hand, we suppose that freezing of CNC suspensions can align rod-like CNC particles in direction of ice crystal growth allowing formation of the CNC aggregates with a high aspect ratio. We assume also that these aggregates can be broken down easily in water and some organic solvents, providing good dispersibility of the composites. The objective of this study is to produce and characterize PVP/CNC composite films and aerogels over a wide composition range, investigate the composites morphology and PVP-assisted CNC self-assembly as well as explore dispersibility of the PVP/CNC aerogels in water and some organic solvents. Nanomaterials 2018, 8, 1011 3 of 21

2.Nanomaterials Materials 2018 and, 8,Methods x FOR PEER REVIEW 3 of 21

2.1.2.1. Materials Microcrystalline cellulose (MCC)(MCC) (~20 micron, powder) and polyvinylpyrrolidone (powder, averageaverage MwMw 40,000) 40,000) (Figure (Figure1) were 1) were purchased purchased from Sigma-Aldrichfrom Sigma-Aldrich (Saint Louis, (Saint MO, Louis, USA). MO, Propanol, USA). dioxane,Propanol, and dioxane, chloroform and chloroform of analytical of gradeanalytical were grade purchased were purchased from Sigma-Aldrich from Sigma-Aldrich Rus and were Rus used and withoutwere used further without purification further (Moscow,purification Russia). (Moscow, Deionized Russia). water Deionized (18 m Ωwater, Millipore (18 m systems)Ω, Millipore was usedsystems) throughout was used the throughout experiment. the Sulfuricexperiment. acid Su (chemicallylfuric acid pure,(chemically GOST pure, 4204-77) GOST was 4204-77) purchased was frompurchased Chimmed from (Moscow, Chimmed Russia). (Moscow, Potassium Russia). bromide Potassium (FT-IR bromide grade, >99%)(FT-IR was grade, purchased >99%) fromwas Sigma-Aldrichpurchased from Rus Sigma-Aldrich (Moscow, Russia). Rus (Moscow, Russia).

( CH2 CH )n N O

Figure 1. Structural formula of polyvinylpyrrolidone. 2.2. Preparation of CNC 2.2. Preparation of CNC CNC were produced from MCC using 62 wt% H2SO4 hydrolysis as described earlier [31]. CNC were produced from MCC using 62 wt% H2SO4 hydrolysis as described earlier [31]. The The prepared CNC had a rod-like morphology with an average width of 15–20 nm and length of prepared CNC had a rod-like morphology with an average width of 15–20 nm and length of 100–150 100–150 nm (aspect ratio of about 7) from transmission electron microscopy (TEM) analysis (Figure S1, nm (aspect ratio of about 7) from transmission electron microscopy (TEM) analysis (Figure S1, Table Table S1). S1). 2.3. Preparation of PVP/CNC Composites 2.3. Preparation of PVP/CNC Composites PVP/CNC composites were prepared by solution casting method similar as described earlier [32]. In brief,PVP/CNC the PVP composites solution was were prepared prepared by by dissolving solution 0.5casting g of PVPmethod in 10 similar mL of as deionized described water earlier at room[32]. In temperature brief, the PVP for solution 2 h under was agitation. prepared Theby dissolving PVP/CNC 0.5 ratio g of wasPVP controlled in 10 mL of by deionized stoichiometric water additionat room temperature of a CNC suspension for 2 h under to a fixedagitation. quantity The ofPVP/CNC the PVP solutionratio was followed controlled by by vigorous stoichiometric stirring foraddition 1 h. The of a solutions CNC suspension were cast to into a fixed glass quantity Petri dishes of the and PVP dried solution at ambient followed conditions by vigorous for 24–48stirring h. Thefor 1 resulting h. The solutions PVP/CNC were composite cast into films glass were Petri about dishes 100 andµm dried thick. at ambient conditions for 24–48 h. The resultingThe aerogels PVP/CNC based oncomposite CNC and films PVP were were about prepared 100 byμm freeze-drying thick. of aqueous mixtures of the two components.The aerogels The based aqueous on CNC mixtures and PVP of CNC were and prepared PVP were by preparedfreeze-drying by ultrasonication of aqueous mixtures before the of samplesthe two werecomponents. frozen at The−40 aqueous◦C for 48 mixtures h and subsequently of CNC and lyophilized PVP were under prepared reduced by pressureultrasonication of 6 Pa atbefore−54 ◦theC for samples 36–48 h.were frozen at −40 °C for 48 h and subsequently lyophilized under reduced pressureThe compositeof 6 Pa at − films54 °C or for aerogels 36–48 h. with different CNC contents (for example, 2 or 4 wt.%, etc.) were denotedThe ascomposite PVP/CNC-2, films PVP/CNC-4, or aerogels with etc., ordifferent PVP/CNC-2 CNC contents aero, PVP/CNC-4 (for example, aero, 2 etc., or 4 respectively. wt.%, etc.) were denoted as PVP/CNC-2, PVP/CNC-4, etc., or PVP/CNC-2 aero, PVP/CNC-4 aero, etc., 2.4.respectively. Characterization

2.4.1.2.4. Characterization Determination of Size and Surface Charge of CNC Particles The size of CNC particles was determined using an EMV-100L (Sumy, Ukraine) transmission 2.4.1. Determination of Size and Surface Charge of CNC Particles electron microscope (TEM) (accelerating voltage of 50 kV). The particlesize of CNC size distributionparticles was in determined aqueous and using organic an EMV-100L CNC suspensions (Sumy, Ukraine) was determined transmission by dynamicelectron microscope light scattering (TEM) method (accelerating (DLS) (wavelength voltage of 50 of kV). 633 nm) using a Zetasizer Nano ZS (Malvern InstrumentsThe particle Ltd, Malvern,size distribution UK) device. in aqueous This equipment and organic provides CNC suspensions measurement was of determined particles sizes by rangingdynamic fromlight 0.3scattering nm to method 6 µm. The (DLS) measurements (wavelength wereof 633 carried nm) using out a at Zetasizer temperature Nano of ZS 20 (Malvern◦C and concentrationInstruments Ltd, of 0.1Malvern, mg/mL UK) in disposable device. This polystyrene equipment cuvettes. provides The measurement obtained particle of particles size values sizes areranging the results from 0.3 of averagingnm to 6 μ overm. The five measurements consecutive measurement were carried cycles. out at The temperature value obtained of 20 in°C eachand cycleconcentration is, in turn, of the0.1 mg/mL result of in automatic disposable processing polystyren ofe cuvettes. 10–15 measurements. The obtained particle Measured size by values the DLS are the results of averaging over five consecutive measurement cycles. The value obtained in each cycle is, in turn, the result of automatic processing of 10–15 measurements. Measured by the DLS method, CNC particle sizes are the mean hydrodynamic diameters of equivalent spheres. They do not represent actual physical dimensions of the rod-like CNC particles but are valid for comparison

Nanomaterials 2018, 8, 1011 4 of 21 method, CNC particle sizes are the mean hydrodynamic diameters of equivalent spheres. They do not represent actual physical dimensions of the rod-like CNC particles but are valid for comparison purposes. However, the hydrodynamic diameter strongly correlates with the length of a rod-shaped particle evaluated by electron microscopy or atomic force microscopy [33,34]. The CNC particle surface charge in aqueous suspension was evaluated by measuring the ζ-potential (Zetasizer Nano ZS, Malvern Instruments Ltd, Malvern, UK).

2.4.2. Scanning Electron Microscopy (SEM) The investigations of the surface morphology of the composites were carried out using a VEGA3 TESCAN instrument (Brno, Czech Republic) (accelerating voltage of 5 kV, a chamber pressure of about 9 × 10−3 Pa, contrast in a secondary electron mode).

2.4.3. Fourier Transform Infrared Spectroscopy (FTIR) The FTIR measurements of the samples (pressed tablets with KBr) were carried out at room temperature using a VERTEX 80v FTIR spectrometer (Bruker, Ettlingen, Germany) in the range of 4000 to 400 cm−1.

2.4.4. X-ray Diffraction Analysis The X-ray diffraction analysis was performed using a Bruker D8 Advance powder diffractometer (Bruker BioSpin GmbH, Rheinstetten, Germany) according to the Bragg–Brentano scheme with Cu-Kα radiation (λ = 0.1542 nm). CNC crystallite sizes were estimated using the Scherrer equation [35]:

L = Kλ/ß1/2Cosθ, (1) where, L is the crystal dimension perpendicular to the diffracting planes with hkl Miller indices, λ is the wavelength of X-ray radiation, ß1/2 is the full width of the diffraction peak measured at half maximum height (FWHM), θ is the diffraction angle, and K = 0.94.

2.4.5. Thermogravimetric Analysis (TGA) Thermogravimetric analysis was performed on a TG 209 F1 (Netzsch Gerätebau GmbH, Selb, Germany) at a heating rate of 10 K min−1 in an atmosphere of dry argon at a flow rate of 30 mL min−1.

2.4.6. Differential Scanning Calorimetry (DSC) The DSC measurements were performed on a DSC 204 F1 (Netzsch Gerätebau GmbH, Selb, Germany) in an atmosphere of ultra pure grade dry argon at a flow rate of 15 mL min−1 and a heating rate of 10 K min−1 using standard aluminum crucibles.

2.4.7. Mechanical Properties The tensile strength and elongation at break were measured using an IR 5046-5 tensile-testing machine (Ivanovo, Russia) at a loading rate of 1 mm/min and at room temperature with a solid clamp in the tension mode [36]. Four specimens with the dimensions of 15 mm (length) × 5 mm (width) × 0.1 mm (thickness) were used for each sample group. The stress and strain values were calculated from the machine-recorded force and displacement based on the initial cross-section area and the original gauge length (10 mm) of each sample, respectively. The Young’s modulus for each sample was calculated from the initial linear portion of the stress-strain curves through a linear regression analysis. The obtained values of the Young’s modulus were within ±10%, while the stress and the elongation at break fluctuated in the range of ±15%. Glycerol was added (2% v/v) to the CNC solution to enhance the elasticity of the films, and this solution was further used to dissolve PVP. Nanomaterials 2018, 8, 1011 5 of 21

2.4.8. Water Uptake Nanomaterials 2018, 8, x FOR PEER REVIEW 5 of 21 Isotherms of water adsorption and desorption upon films and aerogels of the PVP/CNC composites25 °C under werecontrolled obtained. humidity The sorption (aqueous experiment solutions of was sulfuric conducted acid of in a a certain desiccator concentration). at temperature The ◦ ofwater 25 uptakeC under was controlled determined humidity gravimetrically. (aqueous solutions of sulfuric acid of a certain concentration). The water uptake was determined gravimetrically. 3. Results and Discussion 3. Results and Discussion 3.1. FTIR Analysis 3.1. FTIR Analysis Figure 2 shows FTIR spectra of PVP/CNC composite films. The peaks located at about 1291, Figure2 shows FTIR spectra of PVP/CNC composite films. The peaks located at about 1291, 1426, 1426, 1666, and 2953 cm−1 were assigned to the stretching vibrations of C–N, C=C, C=O, and C–H of 1666, and 2953 cm−1 were assigned to the stretching vibrations of C–N, C=C, C=O, and C–H of PVP, PVP, respectively [13]. The appearance of a broad band at about 3440 cm−1 suggests that PVP respectively [13]. The appearance of a broad band at about 3440 cm−1 suggests that PVP contains contains adsorbed water. The large bands near 2900 cm−1 and at 3300–3500 cm−1 assigned adsorbed water. The large bands near 2900 cm−1 and at 3300–3500 cm−1 assigned respectively to respectively to the C–H and O–H stretching vibrations of cellulose, were found for all the the C–H and O–H stretching vibrations of cellulose, were found for all the characterized materials. characterized materials. The characteristic bands at 1060 and 1030 cm−1 and at 1165, corresponded to The characteristic bands at 1060 and 1030 cm−1 and at 1165, corresponded to the C–OH stretching the C–OH stretching vibrations of the cellulose secondary and primary alcohols and to the vibrations of the cellulose secondary and primary alcohols and to the asymmetric ring breathing mode asymmetric ring breathing mode of cellulose, respectively [37]. The stretching vibration of the of cellulose, respectively [37]. The stretching vibration of the cellulose C–O–C bond was identified at cellulose C–O–C bond was identified at 1110 cm−1 and 1050 cm−1. The FTIR spectra of all the 1110 cm−1 and 1050 cm−1. The FTIR spectra of all the PVP/CNC composites demonstrated an intense PVP/CNC composites demonstrated an intense wide band at about 3440 cm−1 related to adsorbed wide band at about 3440 cm−1 related to adsorbed water. As the CNC content in the composites water. As the CNC content in the composites increased, almost no changes in the positions of all increased, almost no changes in the positions of all these peaks were noted. Thus, the FTIR spectra these peaks were noted. Thus, the FTIR spectra indicated that the molecular interactions between indicated that the molecular interactions between CNC and PVP are weak (see Section 3.9). However, CNC and PVP are weak (see Section 3.9). However, the data obtained [38] suggest that in the data obtained [38] suggest that in complexation reactions with substances of a different chemical complexation reactions with substances of a different chemical nature, PVP participates in hydrated nature, PVP participates in hydrated state through bound water. In this case, water can stabilize state through bound water. In this case, water can stabilize the resulting intermolecular complexes. It the resulting intermolecular complexes. It is obvious that the PVP interaction with water occurs on is obvious that the PVP interaction with water occurs on account of hydrogen bonding. It is believed account of hydrogen bonding. It is believed that a PVP unit forms hydrogen bonds with two water that a PVP unit forms hydrogen bonds with two water molecules through lone-electron pairs of the molecules through lone-electron pairs of the oxygen atom of the carbonyl group of the pyrrolidone oxygen atom of the carbonyl group of the pyrrolidone heterocycle [38]. heterocycle [38].

1 0 1 0 2 2 3 4 50 3 50 5 4 6 7 100 5 6 100 8 150 7 8 Transmittance, % 200Transmittance, % 150

250

200 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber, cm-1 Wavenumber, cm-1

(a) (b)

Figure 2. FTIRFTIR spectra spectra of ofthe the PVP/CNC PVP/CNC composite composite films: films: 1 - neat 1—neat PVP; PVP; 2 - PVP/CNC-4.6; 2—PVP/CNC-4.6; 3 - 3—PVP/CNC-10.9;PVP/CNC-10.9; 4 - PVP/CNC-16.3; 4—PVP/CNC-16.3; 5 - PVP/CNC-19.6; 5—PVP/CNC-19.6; 6 - PVP/CNC-28.9; 6—PVP/CNC-28.9; 7 - PVP/CNC-37.9; 7—PVP/CNC-37.9; 8 - neat 8—neatCNC: (a CNC:) in the (a) inrange the rangeof wavenumbers of wavenumbers of 4000–2500 of 4000–2500 cm cm−1; −(b1);( inb) inthe the range range of of wavenumbers wavenumbers of 2500–500 cm−11. .PVP, PVP, polyvinylpyrrolidone; polyvinylpyrrolidone; CNC, CNC, cellulose cellulose nanocrystals. nanocrystals.

3.2. Water Adsorption uponupon the PVP/CNC Composites Water sorptionsorption is one of the majormajor problems limiting applications for CNC based materials. Water uptakeuptake affectsaffects mechanicalmechanical properties,properties, chemicalchemical and dimensional stabilities. On the other hand, the water absorptionabsorption capacity together with the biodegradabilitybiodegradability are among the mostmost importantimportant properties for thethe practicalpractical applicationsapplications andand post-usepost-use ofof thethe biodegradablebiodegradable materials.materials. There is a plethora of studies that have investigated the moisture sorption and retention by CNC reinforced polymer composites [39–44]. Absorbed water molecules can be accumulated in three regions within a CNC-based nanocomposite: on the CNC, in a polymer matrix, and at the polymer/CNC interface [45]. The accessible hydroxyl groups on the CNC particles surface are commonly considered as the

Nanomaterials 2018, 8, 1011 6 of 21 plethora of studies that have investigated the moisture sorption and retention by CNC reinforced polymer composites [39–44]. Absorbed water molecules can be accumulated in three regions within a CNC-basedNanomaterials 2018 nanocomposite:, 8, x FOR PEER REVIEW on the CNC, in a polymer matrix, and at the polymer/CNC interface 6 of [45 21]. The accessible hydroxyl groups on the CNC particles surface are commonly considered as the initial initial site of water sorption. However, for polymer composites, the CNC networks can decrease site of water sorption. However, for polymer composites, the CNC networks can decrease water water sorption as compared to the neat polymer matrices that readily adsorb water [46]. sorption as compared to the neat polymer matrices that readily adsorb water [46]. Isotherms of water adsorption and desorption upon films and aerogels of the PVP/CNC Isotherms of water adsorption and desorption upon films and aerogels of the PVP/CNC composites were obtained. The adsorption experiment was carried out as described earlier [32]. A composites were obtained. The adsorption experiment was carried out as described earlier [32]. detailed analysis of the isotherms is depicted in Figure 3. Obviously, water uptake increases with a A detailed analysis of the isotherms is depicted in Figure3. Obviously, water uptake increases with a relative pressure increase. Meanwhile, the water uptake on the composites decreases gradually with relative pressure increase. Meanwhile, the water uptake on the composites decreases gradually with the CNC content increase. Water uptake by the aerogels is very similar to the uptake by the films in the CNC content increase. Water uptake by the aerogels is very similar to the uptake by the films in spite of their different densities (see Section 3.3). spite of their different densities (see Section 3.3).

100 90 90 80 P/P =1 80 70 0 P/P =1 0 70 60 P/P =0.959 0 P/P =0.959 60 0 50 P/P =0.883 P/P =0.883 50 0 40 0 40 30 P/P =0.754 P/P =0.754 0 30 0 Wateruptake, % 20 Water uptake, % P/P =0.579 20 P/P =0.579 0 P/P =0/358 0 10 0 P/P =0.358 P/P =0.166 10 P/P0=0.166 0 0 P/P =0.037 P/P =0.037 0 0 0 0 0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40 CNC content, % CNC content, %

(a) (b)

100 100

90 90

80 80

70 70 P/P =1 0 60 60 P/P =1 0 50 50 P/P =0.959 40 0 40 P/P =0.959 0 30 P/P =0.883 30 % uptake, Water 0

Water uptake, % uptake, Water P/P =0.883 0 P/P =0.754 20 P/P =0.754 20 0 0 P/P =0.579 P/P =0.579 0 10 P/P0=0.358 10 P/P =0.358 0 P/P 0=0.166 0 P/P =0.166 0 P/P =0.037 0 P/P0 =0.037 0 0 10203040506070800 0 10203040506070 CNC content, % CNC content, %

(c) (d)

Figure 3.3.Equilibrium Equilibrium water water content content in thein compositethe composite films films (a,b) and(a, aerogelsb) and aerogels (c,d) at water (c, d adsorption) at water (adsorptiona,c) and desorption (a, c) and ( bdesorption,d) depending (b, d on) depending the CNC content on the (P/PCNC0 contentis a relative (P/P pressure0 is a relative of water pressure vapor). of water vapor). As PVP is a hydrophilic polymer, it has a great tendency to adsorb water. Evidently, the water uptakeAs byPVP the is composites a hydrophilic does polymer, not depend it has on a great either tendency morphology to adsorb (film orwater. aerogel) Evidently, or density the ofwater the sample,uptake by but the is composites determined does only not by depend the PVP on content either morphology in the composite. (film or The aerogel) molecular or density interactions of the betweensample, but CNC is determined and PVP are only weak, by butthe PVP they content may interact in the with composite. each other The throughmolecular bound interactions water. Apparently,between CNC the and reduced PVP waterare weak, binding but capacitythey may of interact the PVP/CNC with each composites other through as compared bound water. to the neatApparently, PVP can the be relatedreduced to water the increase binding in capacity the intramolecular of the PVP/CNC CNC–CNC composites and PVP–PVP as compared interactions to the (seeneat SectionsPVP can 3.5 be andrelated 3.9). to the increase in the intramolecular CNC–CNC and PVP–PVP interactions (see Sections 3.5, 3.9). 3.3. Density of the PVP/CNC Composite Films 3.3. DensityFigure4 of shows the PVP/CNC the composites Composite density Films versus the CNC content. Figure 4 shows the composites density versus the CNC content.

NanomaterialsNanomaterials2018 2018, 8,, 8 1011, x FOR PEER REVIEW 7 7 of of 21 21

1,8 0,06 1,7

1,6 0,05 -3 1,5 -3 0,04 1,4

1,3 0,03

1,2 Density, g cm Density, g cm g Density, 0,02 1,1

1,0 0,01

0,9 0,00 0 20406080100 0 1020304050607080 CNC content, % CNC content, %

(a) (b)

FigureFigure 4. 4.Density Density of of PVP/CNC PVP/CNC composites composites as as a a function function of of the the CNC CNC content: content: ( a()a) films; films; ( b()b aerogels.) aerogels. TheThe error error bars bars are are the the standard standard deviations deviations in in repeated repeated experiments. experiments.

TheThe density density was was determined determined by by measuring measuring the the samples samples geometry geometry and and dividing dividing the weight the weight by the by volume.the volume. The reported The reported results results are the are average the average values obtainedvalues obtained from at leastfrom three at least different three samples. different Itsamples. can be seen It can that be the seen dependence that the ofdependence the film density of the on film the CNCdensity content on the is ofCNC complex content nature: is of thecomplex film densitynature: is the maximal film density at the CNCis maximal content at ofthe about CNC 5–10 content wt% of and about minimal 5–10 wt% at the and CNC minimal content at of the about CNC 55–60content wt%. of However,about 55–60 the wt%. aerogel However, density graduallythe aerogel decreases density gradually with the increase decreases in thewith CNC the contentincrease in in thethe composite. CNC content in the composite. TheThe strange strange density density behavior behavior of of the the composite composite films films deserves deserves a a special special attention attention and and some some speculation.speculation. If If we we assume assume that that in in the the composite composite films films the the PVP PVP molecules molecules are are in in a a coil coil (or (or globule) globule) conformation,conformation, then then their their shape shape can becan considered be considered to be sphericalto be spherical (see Section (see 3.9Section). The phase3.9). The diagram phase fordiagram packing for of mixturespacking of of spheresmixtures and of rodsspheres is very and complexrods is very because complex it depends because on theit depends stoichiometry, on the thestoichiometry, rod aspect ratio, the rod and aspect size ratio ratio, of and the rodsize andratio the of spherethe rod (andRsphere the/ sphereRrod). Simulations (Rsphere/Rrod). predictSimulations the densestpredict packing the densest for binarypacking structure for bina ofry spheres structure and of rodsspheres (of 2:1and stoichiometry) rods (of 2:1 stoichiometry) at an appropriate at an Rsphereappropriate/Rrod range, Rsphere that/Rrod exceeds range, thethat densities exceeds ofthe the densities demixed of phases the demixed (rods and phases spheres) (rods [47 and]. However, spheres) at[47]. intermediate However, volume at intermediate fractions, simulationsvolume fractions, predict simulations phase separation predict into phase rods separation and spheres. into Moreover, rods and dependingspheres. onMoreover, volume fractions,depending size on ratio volume or aspect fraction ratio, packings, size ofratio phase-separated or aspect ratio, rods andpacking spheres of mayphase-separated be more effective rods and than spheres packing may of binarybe more structure effective of than spheres packing and of rods. binary Thus, structure we observe of spheres a complexand rods. density Thus, behavior we observe of the a compositecomplex density films as behavi a resultor ofof the the packing composite efficiency films ofas rod-likea result CNCof the particlespacking and efficiency spherical of rod-like PVP molecules. CNC particles and spherical PVP molecules.

3.4.3.4. TG TG Analysis Analysis TheThe TG TG and and derived derived differential differential TG TG (DTG) (DTG) curves curves of of the the PVP/CNC PVP/CNC composite composite films films are are shown shown inin Figure Figure S2a,b, S2a,b, respectively. respectively. The The thermal thermal parameters, parameters, including including the the maximum maximum thermal thermal degradation degradation temperature (Tmax) and the onset thermal degradation temperature (Ton) are summarized in Table1. temperature (Tmax) and the onset thermal degradation temperature (Ton) are summarized in Table 1.

Table 1. Results of TG analysis of the PVP/CNC composite films. Table 1. Results of TG analysis of the PVP/CNC composite films. Initial Mass Loss First Degradation Stage Second Degradation Stage Initial mass loss First degradation stage Second degradation stage Total Sample MassTotal mass Sample Ton, Tmax, MassMass Ton, Tmax, MassMass Ton, Tmax, MassMass Ton◦, °C T◦max, °C ◦ Ton, °C ◦ Tmax, °C ◦ Ton, °C ◦ Tmax, °C Loss,loss, % % C C Loss,loss, % % C C Loss,loss, % % C C Loss, %loss, % PVPPVP - 76.476.4 14.614.6 ------405.4405.4 432.3432.3 80.980.9 95.5 95.5 PVP/PVP/CNC-2.3СNС-2.3 - 77.9 77.9 15.1 15.1 ------403.3 403.3 432.6 432.6 78.4 78.4 93.5 93.5 PVP/PVP/CNC-6.7СNС-6.7 - 80.6 80.6 13.7 13.7 240.1240.1 264.5264.5 3.93.9 402.9402.9 433.8 433.8 76.6 76.6 94.2 94.2 PVP/PVP/CNC-10.9СNС-10.9 - 69.0 69.0 12.8 12.8 239.5239.5 259.8259.8 6.2 6.2 402.9402.9 434.9 434.9 73.7 73.7 92.7 92.7 PVP/PVP/CNC-19.6СNС-19.6 - 72.872.8 12.212.2 232.6232.6 252.1252.1 11.711.7 403.3403.3 436.6436.6 64.064.0 87.9 87.9 PVP/CNC-28.9 - 66.6 11.0 229.8 246.3 16.0 401.9 434.1 59.9 86.9 PVP/PVP/CNC-37.9СNС-28.9 - 64.866.6 10.611.0 226.4229.8 242.4246.3 19.616.0 398.2401.9 430.4434.1 53.859.9 84.0 86.9 PVP/СCNCNС-37.9 63.0- 82.264.8 3.010.6 154.5226.4 169.8242.4 37.819.6 334.1398.2 376.4430.4 30.153.8 70.9 84.0 CNC 63.0 82.2 3.0 154.5 169.8 37.8 334.1 376.4 30.1 70.9

Nanomaterials 2018, 8, 1011 8 of 21

The PVP-based composites with a small CNC content have one main characteristic mass loss region (excluding the initial region of water desorption). On the thermograms of the composites with the CNC content of about 7% or more, two main mass loss regions are observed. The water content in the samples gradually decreases with the growing CNC content, and for the neat CNC it equals 3.0%. The first main degradation range is located between ~230 and 400 ◦C and is attributed to pyrolysis of the CNC catalyzed by acid sulfate groups [48]. The second stage mass loss occurs at about 400 ◦C and involves decomposition of PVP and CNC carbonaceous matter [31].

3.5. DSC Analysis Before the DSC scanning, all the samples were first equilibrated at 20 ◦C and heated to 200 ◦C at a heating rate of 10 K min−1, and then cooled down to 20 ◦C at the same rate (Figure S3). The glass transition temperatures (Tg) for all the samples determined under heating and cooling are reported in Table2. The results of DSC analysis of the PVP/CNC films show that Tg tends to shift to higher values with increasing the CNC content in the composite. This higher Tg value can point out that the association of PVP molecules is enhanced by the CNC presence. A notable increase in Tg may be ascribed to the macromolecular confinement provided by the CNC surfaces [49]. On the other hand, the nanoconfinement effects may facilitate significant enhancement of mechanical properties.

Table 2. Results of DSC analysis of the PVP/CNC composite films.

◦ Tg, C Sample Heating Cooling PVP 180.2 166.7 PVP/CNC-4.6 176.5 173.9 PVP/CNC-10.6 178.3 174.7 PVP/CNC-19.9 184.2 177.7 PVP/CNC-28.9 198.5 186.4 PVP/CNC-37.9 198.6 184.1

However, analyzing the data on density, as well as the results of TG and DSC analysis, one can observe an interesting feature. Composite films with a CNC content of about 5–10% are characterized by the maximum density, the maximum temperature of water desorption and the minimum Tg under heating. Apparently, this is due to the firmly bound water, which is not completely removed from PVP even under heating up to 250 ◦C[38]. Indeed, the strongly bound water can act as a plasticizer and increase mobility of the polymer chains (the Tg decreases).

3.6. Mechanical Properties Figure S4 shows stress-strain curves of the PVP/CNC composite films. The values of ultimate tensile strength (σmax), Young’s modulus (E), and elongation at break (εb) are summarized in Table3.

Table 3. Tensile properties of the PVP/CNC composite films.

Ultimate Tensile Elongation at Young’s Modulus Sample Strength (σmax), MPa Break (εb), % (E), MPa PVP 6.6 5.5 120 PVP/CNC-4.4 9.7 7.4 132 PVP/CNC-8.4 17.2 4.2 413 PVP/CNC-17.7 31.8 6.9 457 PVP/CNC-26.8 38.4 3.0 1284 PVP/CNC-29.2 43.8 2.5 1755 PVP/CNC-39.9 25.3 2.1 1216 Nanomaterials 2018, 8, 1011 9 of 21

Upon the addition of CNC, σmax of the composites increases, indicating a reinforcing effect of the CNC. However, when CNC is added, εb decreases gradually, which suggests that the composites become brittle compared with the neat PVP. As the CNC content grows, σmax and E increase and reach maxima at the CNC content of 29.2 wt.%. However, upon further increasing of the CNC content, σmax and E decrease. Therefore, a CNC content of about 30 wt.% is optimal for the composite films based on the σmax and E values. Various modeling approaches have investigated the effect of CNC fillers within polymer matrices on effective nanocomposite properties. Typically, Halpin–Tsai, Halpin–Kardos, and percolation approaches have been used to understand the reinforcing effect of low concentration CNC in low-modulus polymers [2]. Experimentally obtained tensile modulus could be compared with theoretical predictions by using a modified Halpin–Tsai model as [50]:

Ec/Em = (3/8)·(1 + 2ρηLVCNC)/(1 − ηLVCNC) + (5/8)·(1 + 2ηTVCNC)/(1 − ηTVCNC), ηL = (Er − 1)/(Er + 2ρ), (2) ηT = (Er − 1)/(Er + 2), where Ec and Em are the Young’s modulus of the composite and matrix, respectively; ρ is the CNC aspect ratio; Er is the ratio between the Young’s modulus of CNC and the matrix; VCNC is CNC volume fraction in the composite. Based on the weight percentage (WCNC), one can estimate the corresponding volume fraction VCNC by knowing the density of CNC and PVP as:

VCNC = (WCNC/dCNC)/(WCNC/dCNC + (1 − WCNC)/dPVP) (3)

The Young’s modulus of CNC nanoparticles has been set at 105 GPa; the CNC aspect ratio has been assumed as 7; the densities for CNC and PVP have been assumed as 1.46 and 1.2 g/cm3, respectively. Figure5 compares the experimentally obtained data and predicted Young’s modulus values according to random Halpin–Tsai equation. The Halpin–Tsai model tends to lower predictions for high modulus ratio systems Er, that is common in nanocomposites based on rubbery matrices [2]. These predicted values are not accurate for composites with aligned stiff short fillers. Besides inaccuracy at high modulus ratio, the Halpin–Tsai equation has no information about filler anisotropy or about the quality of the interface. However, a stiffening of the interphase region might be very important and useful for stress transfer from the matrix to the CNC. Thus, an observation that experiments exceed the Halpin–Tsai model is the expected result in case of the CNC filler and a low-modulus polymer.

Figure 5. Experimental data (1) and fitting results according to Halpin–Tsai model (2). The error bars are the standard deviations in repeated experiments. NanomaterialsNanomaterials 20182018, 8,, x8 ,FOR 1011 PEER REVIEW 10 of10 21of 21

The observed experimental results exceeding Halpin–Tsai model can be explained in terms of percolationThe observedconcept in experimental CNC based results composites. exceeding The Halpin–Tsaiconcept of modelpercolation can beis explainedrelated with in termsthe developmentof percolation of a concept connected in CNC network based in composites.a multiphase The system. concept The of percolating percolation phase is related reaches with the the percolationdevelopment threshold, of a connected i.e., a networkconnected in networ a multiphasek is formed, system. when The percolating the concentration phase reaches of the the percolatingpercolation phase threshold, increases. i.e., a connectedSome effective network properties is formed, increase when the rapidly concentration and dramatically of the percolating for concentrationsphase increases. above Some the effective percolation properties threshold. increase In nanocomposites, rapidly and dramatically percolatio forn is concentrations most dramatic above for highthe percolationmodulus ratio threshold. systems, In nanocomposites, and the percolat percolationion threshold is most can dramatic be shifted for high to modulus very low ratio concentrationssystems, and by the using percolation high aspect threshold ratio can nanofillers be shifted [51]. to very low concentrations by using high aspect ratioThe nanofillers Young’s modulus [51]. and tensile strength of CNC reinforced polymer matrix composites can be comparedThe Young’s to other modulus material and tensilesystems strength via ‘Ash ofby CNC plots’. reinforced According polymer to matrixGibson–Ashby composites theory, can be Young'scompared modulus to other and material strength systems of a solid via ‘Ashby are proportional plots’. According to the torelative Gibson–Ashby density theory,[52]. Possible Young’s reasonsmodulus for andpoor strength enough ofmechanical a solid are properties proportional in CNC to the based relative composites density [ 52are]. CNC Possible aggregation, reasons for lowpoor interfacial enough properties mechanical of properties CNC/matrix, in CNC low particle based composites aspect ratio, are and CNC low aggregation, CNC alignment low interfacial[2]. The experimentalproperties of data CNC/matrix, obtained are low in particle good agreemen aspect ratio,t with and the low density CNC alignmentdata, according [2]. The to experimental which the densitydata obtained of composite are in films good decreases agreement sharply with the with density a CNC data, content according of more to which than the 30% density (Figure of 4a). composite films decreases sharply with a CNC content of more than 30% (Figure4a). 3.7. X-ray Diffraction Analysis 3.7. X-ray Diffraction Analysis TheThe X-ray X-ray diffraction diffraction patterns patterns of ofthe the PVP/CNC PVP/CNC films films are are shown shown in inFigure Figure 6. 6.

PVP 2000 (200) PVP/CNC-4.6 PVP/CNC-10.9 PVP/CNC-19.6 PVP/CNC-28.9 PVP/CNC-37.9 PVP/CNC-54.5 (1-10) PVP/CNC-70.6 (110) 1000 Intensity

0 5 101520253035 2θ, deg.

Figure 6. X-ray diffraction patterns of the PVP/CNC composite films. Figure 6. X-ray diffraction patterns of the PVP/CNC composite films. As PVP is an amorphous polymer, the diffraction peak arising at about 2θ = 22.9 is attributed to the As PVP is an amorphous polymer, the diffraction peak arising at about 2θ = 22.9 is attributed◦ to ◦ (200) plane of cellulose Iß, whereas the two overlapped weaker diffractions at 2θ close to 16.6 and 14.8 the (200) plane of cellulose Iß, whereas the two overlapped weaker diffractions at 2θ close to 16.6° are assigned to the (110) and (1–10) lattice planes of cellulose Iß [53]. The intensity of the diffraction and 14.8° are assigned to the (110) and (1–10) lattice planes of cellulose Iß [53]. The intensity of the peaks for CNC becomes more pronounced when the CNC content in the composite is increased. diffraction peaks for CNC becomes more pronounced when the CNC content in the composite is The powder diffraction peaks were deconvoluated with the Pseudo-Voigt function, assuming a increased. linear background [54]. The average dimensions of the elementary crystallites in the diffracting planes The powder diffraction peaks were deconvoluated with the Pseudo-Voigt function, assuming a were evaluated by applying the Scherrer equation (1). From these values, we suppose the crystallites linear background [54]. The average dimensions of the elementary crystallites in the diffracting have a square cross-sectional geometry with widths of about 2.6 nm and a diagonal of 3.9 nm (Table4, planes were evaluated by applying the Scherrer equation (1). From these values, we suppose the Figure7). crystallites have a square cross-sectional geometry with widths of about 2.6 nm and a diagonal of 3.9 nm (Table 4, Figure 7).Table 4. CNC crystallite sizes (nm) in the PVP/CNC composite films.

Crystallographic Plane Table 4. CNC crystalliteSample sizes (nm) in the PVP/CNC composite films. (1–10) (110) (200) Crystallographic plane Sample PVP/CNC-28.9(1–10) 2.96(110) 2.62 3.74 (200) PVP/CNC-37.9 2.15 2.53 4.19 PVP/CNC-28.9 PVP/CNC-54.52.96 2.67 2.62 2.37 3.81 3.74 PVP/CNC-37.9 PVP/CNC-70.62.15 2.48 2.53 2.42 3.74 4.19 PVP/CNC-54.5 2.67 2.37 3.81 PVP/CNC-70.6 2.48 2.42 3.74

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Figure 7. Cross-section ofof elementaryelementarycrystallites crystallites in in CNC CNC particles. particles. Projection Projection of of cellulose cellulose molecules molecules is Figure 7. Cross-section of elementary crystallites in CNC particles. Projection of cellulose molecules clearlyis clearly visible. visible. The The indexation indexation of corresponding of correspondi latticeng planeslattice isplanes described is described according according to the monoclinic to the is clearly visible. The indexation of corresponding lattice planes is described according to the unitmonoclinic cell of allomorphunit cell of Iallomorphß [55]. Iß [55]. monoclinic unit cell of allomorph Iß [55]. 3.8. Morphologies of the PVP/CNC Composites 3.8. Morphologies of the PVP/CNC Composites SEM images were recorded toto analyzeanalyze thethe microstructuremicrostructure ofof thethe PVP/CNCPVP/CNC composite films films and SEM images were recorded to analyze the microstructure of the PVP/CNC composite films and aerogels (Figure(Figures8 ,8, Figure 9 and9 S5).and Figure S5). aerogels (Figures 8, 9 and S5).

(a) (b) (a) (b)

(c) (d) (c) (d)

(e) (f) (e) (f) Figure 8.8. Cross-sectionalCross-sectional SEM SEM images images of of the the neat neat PVP PVP film film (a) and(a) and the PVP/CNCthe PVP/CNC composite composite films films with Figure 8. Cross-sectional SEM images of the neat PVP film (a) and the PVP/CNC composite films CNCwith CNC content content (wt.%) (wt.%) of: 4.6 of: (b 4.6); 16.3 (b); ( c16.3); 28.9 (c); ( d28.9); 54.5 (d); ( e54.5); 85.7 (e); ( f85.7), respectively. (f), respectively. The scale The bar scale is 2barµm. is 2 with CNC content (wt.%) of: 4.6 (b); 16.3 (c); 28.9 (d); 54.5 (e); 85.7 (f), respectively. The scale bar is 2 μm. μm.

Nanomaterials 2018, 8, 1011 12 of 21 Nanomaterials 2018, 8, x FOR PEER REVIEW 12 of 21

(a) (b)

(c) (d)

(e) (f)

Figure 9. SEM images of the neatneat PVPPVP aerogelaerogel ((aa)) andand thethe PVP/CNCPVP/CNC compos compositeite aerogels with CNC content (wt.%) (wt.%) of: of: 4.6 4.6 (b (b);); 10.9 10.9 (c ();c );19.6 19.6 (d); (d 28.9); 28.9 (e); ( e70.6); 70.6 (f), (respectively.f), respectively. Scales: Scales: 2 μm 2 (µa,m c, (da,c e,d, ,fe);,f 1); 1μmµm (b ().b ).

The cross-sectional SEM image image of of the the neat neat PVP PVP film film showed a a smooth morphology morphology (Figure (Figure 88a).a). With the increase in the CNC content in thethe compositecomposite films,films, the fractured surface became rougher due to the CNCCNC particlesparticles aggregationaggregation (Figure(Figure8 b–f).8b–f). SeparateSeparate CNCCNC particlesparticles inin thethe filmsfilms werewere notnot observed. AnalyzingAnalyzing SEMSEM images images of of the the composite composite aerogels, aerogels, we unexpectedlywe unexpectedly found found that in that a certain in a certaincomposition composition range (the range CNC (the content CNC content from about from 5 about to 30 wt.%),5 to 30 the wt.%), individual the individual CNC particles CNC particles became becamevisible (Figure visible9 (Figureb–e). 9b–e). Using the SEM images, we estimated the CNC particle size in the composite aerogels. For this purpose, we measured the widths and lengths of of a a set of 100 particles manually (Figures (Figures S6–S9). S6–S9). The analysis results are shown as histograms inin FigureFigure 1010..

Nanomaterials 2018, 8, 1011 13 of 21 Nanomaterials 2018, 8, x FOR PEER REVIEW 13 of 21

60 PVP/CNC-4.6 aero 30 PVP/CNC-4.6 aero PVP/CNC-10.5 aero PVP/CNC-10.5 aero PVP/CNC-19.6 aero PVP/CNC-19.6 aero 25 PVP/CNC-28.9 aero 40 PVP/CNC-28.9 aero 20

15 Frequency, % Frequency,

Frequency, % Frequency, 20 10

5

0 0 20 30 40 50 60 70 80 90 100 110 120 0 500 1000 1500 Particle width, nm Particle length, nm (a) (b)

FigureFigure 10. 10. TheThe CNC CNC particle sizesize distributiondistribution in in the the composite composite aerogels aerogels across across the widthsthe widths (a) and (a) alongand alongthe lengths the lengths (b). (b).

TheThe analysis analysis of of the the images images shows shows that that the the widths widths of of the the CNC CNC particles particles in in the the composite composite aerogels aerogels areare about about 55–65 55–65 nm nm and and do do not not depend depend on on the the co composition.mposition. Meanwhile, Meanwhile, the the lengths lengths of of the the CNC CNC particlesparticles in thethe aerogelsaerogels are are much much larger larger than than the lengths the lengths of the CNCof the particles CNC particles in the initial in suspension,the initial suspension,these composites these composites were produced were produced from. In addition,from. In addition, as the CNC as the content CNC content in the compositein the composite grows, grows,the particles the particles lengths lengths increase increase gradually gradually and exceed and exceed 1 micron 1 micron for the for composition the composition with a CNCwith a content CNC contentof 28.9 wt.%.of 28.9 The wt.%. fraction The fraction of particles of particles with the wi lengthsth the lengths exceeding exceeding 600 nm 600 also nm increases. also increases. ItIt is is reasonablereasonable to to assume assume that that such such an increase an increase in the in CNC the particles CNC particles size is due size to theiris due aggregation. to their aggregation.It is well known It is thatwell CNC known strongly that agglomerateCNC strongly to stackagglomerate up in parallel to stack giving up anin apparentparallel giving increase an in apparentparticle width increase [54 ].in However, particle width it should [54]. be However, noted that init should this case be their noted lengths that in increase, this case while their the lengths widths increase,remain unchanged.while the widths Evidently, remain PVP unchanged. assists CNC Evidently, self-assembly PVP which assists produces CNC self-assembly reasonably uniform which producesCNC aggregates reasonably with uniform a high CNC aspect aggregates ratio (length/width). with a high Theaspect aspect ratio ratio (length/width). of the CNC The aggregates aspect ratioincrease of the as CNC the CNC aggregates content increase grows from as the about CNC 5 to content 30 wt.% grows because from the about lengths 5 to increase 30 wt.% up because to fourfold, the lengthswhilethe increase widths up remain to fourfold, constant. while Apparently, the widths hereremain we constant. observe theApparently, result of ahere synergistic we observe effect the of resultPVP adsorptionof a synergistic onto CNC effect particles of PVP and adsorption ice-templating onto CNC under particles freeze-drying and ice-templating [56,57]. As inter-chain under freeze-dryinghydrogen bonding [56,57]. isprevailing As inter-chain within hydrogen the (200) plane bonding for cellulose is prevailing Iß (the so-calledwithin the ”hydrogen-bonded” (200) plane for celluloseplane) [2 I,ß58 (the,59], so-called one can suppose”hydrogen-bonded” that adsorbed plane) PVP [2,58,59], blocks the one possibility can suppose of forming that adsorbed lateral bonds PVP blocksbetween the thepossibility CNC particles of forming and, lateral thus, prevents bonds be theirtween aggregation. the CNC particles Freezing and, of CNC thus, suspensions prevents their can aggregation.align rod-like Freezing particles of becauseCNC suspensions the particles can can align be subjectedrod-like particles to an elongation because the action particles as the can speed be subjectedof ice crystals to angrowth elongation along action the length as the differs speed fromof ice their crystals growth growth across along the widththe length [60,61 differs]. Moreover, from theirsome growth tribology across findings the width highlight [60,61]. that Moreover, polymer adsorption some tribology on CNC findings surface highlight together that with polymer the CNC adsorptionsurface hydration on CNC can surface act as together a lubrication with and the reduceCNC surface a friction hydration in the CNC can contacts act as a considerablylubrication and [46 ]. reduce a friction in the CNC contacts considerably [46]. 3.9. PVP Adsorption onto CNC Particles 3.9. PVPPVP Adsorption as a linear onto amorphous CNC Particles polymer with a large heterocyclic unit in the polymer chain is characterizedPVP as a bylinear a coil-globule amorphous conformational polymer with transition a large inheterocyclic solution, which unit isin caused the polymer by the interaction chain is characterizedof distant units by in a the coil-globule chain. If the conformational process is driven tr byansition units repulsion,in solution, the coilwhich swells. is caused If the monomer by the interactionunits are attracted of distant to eachunits other, in the the chain. polymer If the chain process condenses is driven onto by itself units into repulsion, a dense conformation—thethe coil swells. If theso-called monomer polymer units are globule. attracted It is to thought each other, that the at higher polymer temperatures chain condenses or in aonto good itself solvent, into a mutualdense conformation—therepulsion of the polymer so-called units polymer prevails, globule. and the It coilis thought swells duethat toat thehigher excluded temperatures volume effect.or in a Atgood low solvent,temperatures mutual or repulsion in a poor solvent,of the polymer the chains units are mostlyprevails, attracted and the to coil each swells other, due and to the the coil excluded collapses volumeinto a globule. effect. At A verylow temperatures small attraction or betweenin a poor the solvent, polymer the chain chains units are ismo usuallystly attracted enough to causeeach other,the polymer and the tocoil condense collapses into into a a globule. globule. In A polymers very small with attraction flexible between chains (as the PVP), polymer the coil-globulechain units istransition usually enough is a continuous to cause process.the polymer to condense into a globule. In polymers with flexible chains (as PVP),Figure the 11 coil-globule shows the transition size distribution is a continuous of PVP particles process. depending on the concentration in solution andFigure their surface 11 shows charge the determined size distribution by the ζ-potential of PVP particles value. With depending an increase on in the the PVPconcentration concentration in solutionfrom 0.05 and to their 7%, thesurface fraction charge of particlesdetermined with by the the size ζ-potential of 60–70 value. nm disappears, With an increase while thein the fraction PVP concentration from 0.05 to 7%, the fraction of particles with the size of 60–70 nm disappears, while

NanomaterialsNanomaterials 20182018, 8, ,8 x, 1011FOR PEER REVIEW 1414 of of 21 21 Nanomaterials 2018, 8, x FOR PEER REVIEW 14 of 21 the fraction of particles with sizes less than 10 nm grows. At increased PVP concentrations, the averagetheofparticles fraction size withofof theseparticles sizes particles less with than issizes 10reducednm less grows. thanapproxim 10 At nm increasedately grows. to 3–6 PVP At nm. increased concentrations, The experimental PVP concentrations, the average values of size thethe of PVPaveragethese particle particles size sizeof isthese reducedobtained particles approximately by dynamicis reduced light toapproxim 3–6 sca nm.tteringately The are experimentalto in3–6 good nm. Theagreement values experimental of thewith PVP the values particle estimated of sizethe ones.PVPobtained Thus,particle by the dynamicsize length obtained lightof the scatteringby expanded dynamic are polymer inlight good sca ch agreementtteringain of arePVP with in with good the the estimated agreement molecular ones. with mass Thus, the of estimated the40,000 length is aboutones.of the 50Thus, expanded nm, the and length polymerthe radius of the chain of expanded gyration of PVP polymerof with the thecoil, ch molecular Raing, variesof PVPmass from with of1 theto 40,000 7molecular nm is that about can mass 50 be nm, ofcalculated 40,000 and the is fromaboutradius the 50 of known nm, gyration and relations the of the radiuscoil, Rofg ,gyration varies from of the 1 to coil, 7 nm Rg that, varies can befrom calculated 1 to 7 nm from that the can known be calculated relations from the known relations Rg = 0.0195·M 0.550.55, (4) Rg = 0.0195·M , (4) Rg = 0.0195·M 0.55, (4) where M is the molecular weight of PVP [62]; where M is the molecular weight of PVP [62]; [62]; Rg = (L·N/6) 0.5, (5) R R=g = (L (·LN·N/6)/6)0.5 0.5,, (5) (5) where L is the bond length in the polymer chain,g nm; N is the number of the bonds [63]. where L is the bond length in the polymer chain, nm; N is the number of the bonds [63]. where L is the bond length in the polymer chain, nm; N is the number of the bonds [63]. 50 0 45 50 0 40 45 35 40 7.0 30 35 7.0 -5 25 5.0 30 20 -5 25 3.05.0 Intensity, %

15 mV -potential, 20 3.0 ζ

Intensity, % 1.0

10 mV -potential, 15 ζ -10 5 1.0 10 0.1 0 0.05 -10 5 0.1 012345678 0 1 10 100 1000 0.05 Hydrodynamic diameter, nm PVP concentration, % 1 10 100 1000 012345678 Hydrodynamic diameter, nm PVP concentration, % (a) (b) (a) (b) Figure 11. PVP particles size distribution (the curves are marked by PVP concentration in wt.%) (a) andFigureFigure the PVP11. PVP particles particles surface sizesize distributionchdistributionarge in aqueous (the (the curves curves solutions are are marked (markedb). by by PVP PVP concentration concentration inwt.%) in wt.%) (a) and(a) andthe PVPthe PVP particles particles surface surface charge charge in aqueous in aqueous solutions solutions (b). (b). It is seen that a concentration increase reduces the fraction of expanded polymer chains, and the degreeItIt isof is seen seencoil swellingthat that a a concentration concentration becomes lower, increase increase which reduces reduces leads the the to fraction fractionthe coil of ofcontraction expanded expanded andpolymer polymer decrease chains, chains, in itsand and size the the (Figuredegreedegree 11a).of of coil coil The swelling swelling PVP coil becomes becomes contraction lower, lower, is which whichaccompanied leads leads to to by the the an coil increase contraction in the and andabsolute decrease decrease values in in its itsof thesize ζ(Figure(Figure-potential 11a).11 a).(Figure The The PVP PVP11b), coil coilwhich contraction contraction agrees well is is accompanied accompanied with the results by by an anof increase increasequantum-chemical in in the the absolute absolute calculations values values of of[38], the the ζ accordingζ-potential-potential to (Figure (Figurewhich 11b),the11b), negative which which agrees agreescharge well wellis caused with with the theby resultsthe results formation of of quantum-chemical quantum-chemical of H-bonded complexes calculations calculations of [38],the [38], pyrrolidoneaccordingaccording to to ring which which with the the water negative negative molecules charge charge and is is caused should by become the formation lower (more of H-bonded positive) complexes complexesat dehydration. of of the the Thepyrrolidonepyrrolidone PVP adsorption ring ring with with onto water water CNC molecules moleculesparticles is and and accompanied should should become become by growth lower lower in (more (morethe ζ -potentialpositive) positive) absoluteat at dehydration. dehydration. value, ζ butTheThe does PVP PVP not adsorption adsorption significantly onto onto affect CNC CNC theparticles particles CNC isparticle is accompanied accompanied size (Figure by by growth growth 12a,b). in in the the ζ-potential-potential absolute absolute value, value, butbut does does not not significantly significantly affect the CNC particle size (Figure 1212a,b).a,b).

0 30

0 30 -10

-10 25:1 -20 20 10:125:1 -20 20 -30 10:15:1 -30 -40 10 1.5:15:1 Intensity, % Intensity,

-potential, mV -40 10 0.7:11.5:1 ζ -50 Intensity, % Intensity, -potential, mV 0.7:1 ζ -50 -60 0 neat CNC

-60 0 neat CNC 0 20406080100 1 10 100 1000 0 20406080100CNC/PVP ratio, % 1Hydrodynamic 10 diameter, 100 nm 1000 CNC/PVP ratio, % Hydrodynamic diameter, nm (a) (b) (a) (b) FigureFigure 12. 12. CNCCNC particles particles size size distribution distribution in in the the pres presenceence of of PVP PVP (the (the curves curves are are marked marked by by the the PVP/CNCFigurePVP/CNC 12. concentration CNC concentration particles ratio) size ratio) ( adistribution) (anda) and the theCNC in CNC theparticles pres particlesence surface surfaceof PVPcharge charge(the in curvesaqueous in aqueous are suspensions marked suspensions by (the the CNCPVP/CNC(the CNCconcentration concentrationconcentration of 0.07 ofratio) wt.%) 0.07 ( wt.%)a in) and the in thepresence the CNC presence particlesof PVP of ( PVPb surface). (b). charge in aqueous suspensions (the CNC concentration of 0.07 wt.%) in the presence of PVP (b).

Nanomaterials 2018, 8, 1011 15 of 21

It is usually believed that PVP molecules tend to be adsorbed onto nanoparticles in a linear open-chain-like pattern as opposed to a compact coiled structure, i.e., the molecular coils are flattened at the surface [64]. Nevertheless, the surface features of the solid adsorbents can affect the properties of the adsorbed layer. Hydrogen bonding, electrostatic attraction, or hydrophobic interactions can highly influence the adsorption behavior [63]. One can hypothesize that strong adsorption due to the high affinity of PVP for the adsorbent surface causes the PVP molecules to be adsorbed in an open-chain-like pattern. On the other hand, if the PVP does not interact well with the adsorbent surface, the PVP molecules tend to be adsorbed in more compact coiled shaped structures. For example, atomic force microscopy was used by Verma et al. for research the interaction of PVP with ibuprofen [65]. The obtained AFM images showed that PVP tended to be adsorbed in a compact coiled structures, demonstrating a lack of affinity for the ibuprofen surface. In addition, the authors observed patchy preferential adsorption of PVP on one face of the ibuprofen crystal as compared to the other. Kotel’nikova et al. [66] studied the effects of PVP molecular weight and aqueous solution concentration of PVP on adsorption upon microcrystalline cellulose. The authors observed a noticeable increase in the amount of adsorbed PVP with an increase in its molecular weight, and at higher solution concentration and temperature. The authors classified the obtained two-step S-shaped isotherm as an isotherm of type IV and drew a conclusion about the polymolecular character of the PVP adsorption upon the porous surface of microcrystalline cellulose. Besides, the authors noted that PVP with a larger molecular weight is both more easily adsorbed and desorbed. Apparently, this indicates that the polymer is adsorbed in compact globular structures rather than in a flattened conformation as longer linear chains with a larger molecular mass should bond to the adsorbent surface more firmly. Hirai and Yakura [62] studied PVP adsorption on palladium (Pd) nanoparticles in methanol. They determined that the thickness of PVP adsorbed layer on Pd nanoparticles as well as the PVP radius of gyration increased proportionally to Mw0.55, where Mw was an averaged molecular weight of PVP. According to the authors, the adsorbed layer thickness is almost completely determined by the particular segmental sequence of the adsorbed polymer chain which takes the conformation similar to that of the free polymer in solution. Consequently, a polymer coiled conformation in solution should lead to the polymer adsorption in a globule-like structure. Thus, analyzing the data obtained, one can suppose that PVP is adsorbed onto CNC in a globule-like structure rather than in a flattened conformation and due to electrostatic attraction rather than via hydrogen bonding. Nevertheless, the situation can be more complicated because of PVP strong binding via hydrogen bonding to the (200) facet of the CNC particle which may result in a flattened conformation of the adsorbed PVP molecules. Computer simulation could reveal and prove the appropriate mechanism of PVP adsorption onto different facets of CNC particles. However, this will be the aim of our further research.

3.10. Dispersibility of Freeze-Dried PVP/CNC Composites in Water and Some Organic Solvents As a PVP molecule contains a strongly hydrophilic pyrrolidone moiety and a considerable hydrophobic alkyl group, adsorbed PVP can prevent aggregation of nanoparticles through repulsive forces which arise from the hydrophobic carbon chains. PVP can provide a steric hindrance that will allow the dried PVP/CNC to be readily redispersed in aqueous systems [27,67,68]. Thus, PVP adsorption onto the CNC surface can lead to a steric hindrance preventing hydrogen bonding between the CNC particles, which can impart good redispersibility of dried PVP/CNC in water. To test whether irreversible CNC particle aggregation occurred after drying, we redispersed freeze-dried PVP/CNC composites in water and some organic solvents. The redispersibility of the dried PVP/CNC aerogels in water was evaluated in terms of particle-size distribution (Figures 13 and 14) and colloidal stability of the suspensions (Figure S10). Nanomaterials 2018, 8, x FOR PEER REVIEW 16 of 21

0

30 -10

CNC aero -20

20 7 6 -30 5 4 -40

Intensity, % 10 3 -potential, mV ζ 2 -50 1 -60 0 PVP 0 20406080100 1 10 100 1000 NanomaterialsNanomaterials 20182018, ,88, ,x 1011 FOR PEER REVIEW CNC content in the composite aerogel, % 1616 of of 21 21 Hydrodynamic diameter, nm

(a) 0 (b)

30 Figure 13. (a) Particle size distribution of the PVP/CNC aerogels-10 in water (for comparison, the curves for the neat PVP and CNC are shown): 1 - PVP/CNC-4.6 aero; 2 - PVP/CNC-10.9 aero; 3 - CNC aero -20 7 PVP/CNC-16.320 aero; 4 - PVP/CNC-19.6 aero; 5 - PVP/CNC-28.9 aero; 6 - PVP/CNC-37.9 aero; 7 - 6 -30 PVP/CNC-54.5 aero. (b) Surface charge of 5PVP coated CNC particles after redispersion of the PVP/CNC aerogels in water. 4 -40

Intensity, % 10 3 -potential, mV ζ 2 -50 Figure S10 displays the photographs showing1 the suspension stability for all the PVP/CNC PVP -60 samples. The0 photographs of the suspensions in the testing vials are taken some time after 0 20406080100 1 10 100 1000 ultrasonication. All the CNC suspensions are turbid and no sedimentationCNC content in the can composite be observed aerogel, % at the Hydrodynamic diameter, nm bottom of the vials at the initial stage. As time passes, lots of sedimentation is flocculated in propanol, chloroform, and (dioxane,a) whereas aqueous suspensions retain their(b) good stability even a month later. FigureFigure 13. 13. (a) (Particlea) Particle size distribution size distribution of the ofPVP/CNC the PVP/CNC aerogels aerogelsin water (for in watercomparison, (for comparison, the curves Figures 13a and 14 show particle size distribution of the PVP/CNC aerogels in water and forthe the curves neat for PVP the neatand PVPCNC and are CNC shown): are shown): 1 - PV 1—PVP/CNC-4.6P/CNC-4.6 aero; 2 aero; - PVP/CNC-10.9 2—PVP/CNC-10.9 aero; aero;3 - propanol, respectively. Figure 13b demonstrates surface charge of PVP coated CNC particles after PVP/CNC-16.33—PVP/CNC-16.3 aero; aero; 4 - PVP/CNC-19.6 4—PVP/CNC-19.6 aero; aero; 5 - PVP/CNC-28.9 5—PVP/CNC-28.9 aero; aero; 6 - PVP/CNC-37.9 6—PVP/CNC-37.9 aero; aero; 7 - redispersion of the PVP/CNC aerogels in water. PVP/CNC-54.57—PVP/CNC-54.5 aero. aero. (b) Surface (b) Surface charge charge of PVP of PVP coated coated CNC CNC particles particles after after redispersion redispersion of of the the PVP/CNCPVP/CNC aerogels aerogels in in water. water.

60 Figure S10 displays the photographs showing the suspension stability for all the PVP/CNC 50 CNC aero samples. The photographs of the suspensions in the testing vials7 are taken some time after 40 ultrasonication. All the CNC suspensions are turbid and no sedimentation6 can be observed at the bottom of the vials at the initial30 stage. As time passes, lots of 5 sedimentation is flocculated in 4 propanol, chloroform, and dioxane,20 whereas aqueous suspensions retain their good stability even a Intensity, % month later. 3 10 2 1 Figures 13a and 14 show particle size distribution of the PVP/CNCPVP aerogels in water and 0 propanol, respectively. Figure 13b demonstrates surface charge of PVP coated CNC particles after 1 10 100 1000 redispersion of the PVP/CNC aerogels inHydrodynamic water. diameter, nm

Figure 14. Particle size distribution of the PVP/CNC aerogels in propanol (for comparison, Figure 14. Particle size distribution of the PVP/CNC aerogels in propanol (for comparison, the curves the curves for the neat PVP and60 CNC are shown): 1—PVP/CNC-4.6 aero; 2—PVP/CNC-10.9 aero; for the neat PVP and CNC are shown): 1 - PVP/CNC-4.6 aero; 2 - PVP/CNC-10.9 aero; 3 - 3—PVP/CNC-16.3 aero; 4—PVP/CNC-19.6 aero; 5—PVP/CNC-28.9CNC aero; aero 6—PVP/CNC-37.9 aero; PVP/CNC-16.3 aero; 4 - PVP/CNC-19.650 aero; 5 - PVP/CNC-28.9 aero; 6 - PVP/CNC-37.9 aero; 7 - 7—PVP/CNC-54.5 aero. 7 PVP/CNC-54.5 aero. 40 6 Figure S10 displays the photographs30 showing the suspension5 stability for all the PVP/CNC Figures 12 and 13 show that the particle size and surface 4charge after redispersing the samples. The photographs of20 the suspensions in the testing vials are taken some time after Intensity, % PVP/CNCultrasonication. aerogels All in thewater CNC remain suspensions practically are turbidunchanged and nocompared sedimentation3 to the caninitial be values observed for atthe the 10 2 aqueous never-dried CNC in the presence of PVP. The redispersed1 aqueous suspensions bottom of the vials at the initial stage. As time passes, lots of sedimentationPVP is flocculated in propanol, 0 demonstratechloroform, high and colloidal dioxane, whereasstability after aqueous 1 month suspensions of holding retain (Figure their S10). good stability even a month later. The redispersibility of PVP/CNC aerogels1 in 10 propan 100ol is 1000not as effective. If the PVP content in Figures 13 and 14 show particle size distributionHydrodynamic diameter, of the nm PVP/CNC aerogels in water and propanol, therespectively. composite Figureis less 13thanb demonstrates 60%, aggregates surface of 1 charge μm in of size PVP are coated formed CNC (Figure particles 14). after However, redispersion the redispersed suspensions with a high PVP content are stable in propanol for 10 days (Figure S10). of theFigure PVP/CNC 14. Particle aerogels size distribution in water. of the PVP/CNC aerogels in propanol (for comparison, the curves forFigures the neat 12 and PVP 13 andshow CNC that are the shown): particle size1 - PV andP/CNC-4.6 surface charge aero; 2 after - PVP/CNC-10.9 redispersing theaero; PVP/CNC 3 - aerogelsPVP/CNC-16.3 in water aero; remain 4 - PVP/CNC-19.6 practically unchanged aero; 5 - PVP/CNC-28.9 compared to aero; the initial6 - PVP/CNC-37.9 values for aero; the aqueous7 - never-driedPVP/CNC-54.5 CNC aero. in the presence of PVP. The redispersed aqueous suspensions demonstrate high colloidal stability after 1 month of holding (Figure S10). FiguresThe redispersibility 12 and 13 ofshow PVP/CNC that the aerogels particle in propanolsize and is surface not as effective. charge Ifafter the PVPredispersing content in the the PVP/CNCcomposite isaerogels less than in 60%,water aggregates remain practically of 1 µm in sizeunchanged are formed compared (Figure 14to). the However, initial thevalues redispersed for the aqueoussuspensions never-dried with a high CNC PVP contentin the arepresence stable inof propanol PVP. The for 10redispersed days (Figure aqueous S10). suspensions demonstrateIn chloroform high colloidal and dioxane, stability the after redispersed 1 month of particles holding of(Figure PVP/CNC S10). aerogels form aggregates largerThe than redispersibility 3 µm. However, of PVP/CNC these suspensions aerogels in are propan stableol for is somenot as time. effective. It is worthIf the PVP noting content that thein theredispersed composite suspensions is less than behave 60%, aggregates differently of in 1 chloroform μm in size and are dioxane. formed If(Figure the former 14). However, suspension the is redispersed suspensions with a high PVP content are stable in propanol for 10 days (Figure S10).

Nanomaterials 2018, 8, 1011 17 of 21 most stable at minimum and maximum PVP dosages, the latter, on the contrary, is most unstable under the same conditions (Figure S10). These results testify that the enhanced redispersibility of dried CNC/PVP is not due to the electrostatic mechanism only, rather is specified by both the electrostatic repulsion and the steric hindrance from the PVP that are adsorbed onto the surface of CNC particles [69]. Figure S11 illustrates PVP particles size distribution in propanol and chloroform (0.2 wt.% concentration). It is evident that compared to aqueous solutions, the hydrodynamic diameters of PVP particles in propanol and especially in chloroform tend to increase, probably due to the polymer coils extension into linear chains. It would be interesting to reveal the relationship of PVP conformations in various solvents (linear chain, coil, condensed globula) with the possibility to stabilize CNC suspensions in these solvents. However, this aim is well beyond the scope of the article and will be achieved in the future work. The specific application of such a system could involve the development of an effective drug delivery system. The aim of the further research is to enhance physiochemical properties as well as bioavailability of poorly water-soluble drugs, through preparation of freeze-dried optimized formulations based on PVP and CNC as hydrophilic carriers.

4. Conclusions The work describes the preparation of PVP/CNC composite films and aerogels with extensive characterization. The composites morphology and PVP-assisted CNC self-assembly are explored. The redispersibility of the aerogels prepared by freeze-drying is investigated in detail. In this work, we have discovered PVP-assisted CNC self-assembly which produces CNC aggregates with a high aspect ratio. It was revealed that the particle size of the CNC aggregates after redispersing in water returned to the initial values for the aqueous never-dried CNC. The redispersed aqueous suspensions demonstrated high colloidal stability after 1 month of holding. It was shown that the redispersibility of CNC in an organic solvent (propanol, dioxane, chloroform) could be significantly improved using a PVP content of 50 wt.%.

Supplementary Materials: The following are available online at http://www.mdpi.com/2079-4991/8/12/1011/ s1, Table S1: CNC characteristics, Figure S1: TEM image of CNC, Figure S2: TG and DTG curves of the PVP/CNC composite films, Figure S3: DSC traces for heating and cooling of the PVP/CNC composite films, Figure S4: Typical stress-strain curves of the PVP/CNC composite films, Figure S5: SEM images of the neat PVP aerogel and the PVP/CNC composite aerogels, Figures S6–S9: SEM images of PVP/CNC composite aerogels for the CNC particle size estimation, Figure S10: Photos of redispersed suspensions during storage, Figure S11: PVP particles size distribution in propanol and chloroform. Author Contributions: Conceptualization, M.V. and O.S.; Methodology, M.V. and O.S.; Validation, M.V., O.S., and A.Z.; Formal analysis, M.V., O.S., and A.Z.; Investigation, M.V., N.R., N.K., and A.A.; Data curation, M.V. and O.S.; Writing—original draft preparation, O.S.; Writing—review and editing, M.V., O.S., and A.Z.; Supervision, A.Z..; Project administration, A.Z.; Funding acquisition, A.Z. All authors approved the manuscript. Funding: This research was supported by the Russian Science Foundation, grant number 17-13-01240. Acknowledgments: The authors would like to thank The Upper Volga Region Centre of Physicochemical Research (Ivanovo, Russia) and Centre for collective use of scientific equipment of Ivanovo State University of Chemistry and Technology (Russia) for some measurements carried out using the centers’ equipment. Conflicts of Interest: The authors declare no conflict of interest.

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