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. 2021 Apr 28;6(18):12250–12260. doi: 10.1021/acsomega.1c01133

Poly(Ethylene Glycol)/β-Cyclodextrin Pseudorotaxane Complexes as Sustainable Dispersing and Retarding Materials in a Cement-Based Mortar

Clotilde Capacchione †,‡, Paolo Della Sala †, Ilaria Quaratesi †,‡, Immacolata Bruno ‡, Antonio Pauciulo ‡, Andrea Roberta Bartiromo ‡, Patrizia Iannece †, Placido Neri †, Carmen Talotta †,*, Rocco Gliubizzi ‡,*, Carmine Gaeta †,*
PMCID: PMC8154154  PMID: 34056378

Abstract

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Pseudorotaxane complexes between β-CD and mPEG derivatives bearing a carboxylic acid function (mPEG–COOH) were synthesized and investigated for their dispersing properties in a cement-based mortar. The formation of mPEG–COOH derivatives and their pseudorotaxanes was investigated by 1D nuclear magnetic resonance, diffusion ordered spectroscopy, and thermogravimetric analysis experiments. Mortar tests clearly indicate that mPEG–COOH@β-CD-interpenetrated supramolecules show excellent dispersing abilities. In addition, the supramolecular complexes show a retarding effect, analogously to other known β-CD-based superplasticizers in which the β-CD is covalently grafted on a polymeric backbone.

Introduction

Cyclodextrins (CDs) are polyhydroxylated macrocycles obtained by enzymatic degradation of starch.1,2 CDs are constituted by d-(+)-glucose units linked together by α-1,4-glycosidic bonds and show an internal hydrophobic cavity that can host appropriate guest molecules.3 Usually, the inclusion process can modify the physical, chemical, or biological proprieties of the included molecule.1−3 On this basis, many applications of inclusion complexes of the CDs are found in different industrial sectors, such as food,4 pharmaceutical,5 chemistry,6,7 medicine,8 and environment.9 The wide expansion of CDs was made possible by their low-cost production on an industrial scale. CD-based polymers, in which the CDs are covalently grafted on a polymeric backbone, have found interesting applications as a concrete water-reducing agent [also named as superplasticizers (SPs)].10−13 SPs14−16 play a crucial role in modern concrete technology because they guarantee good fluidity of cement pastes and control their setting time, even at low water/cement ratios.14−16 Among the SPs of first generation, the sulfonated additives such as poly-β-naphthalene sulfonate (SNF)14−16 and polymelamine sulfonate (SMF)14−16 have found wide applications. Polycarboxylate ether (PCE) SPs17 show superior performance in cement-based mortars. In fact, compared with SNF, PCE can fluidize mortars with a lower water/cement ratio18 by using lower dosages and show a better slump retention ability. The action mechanism19,20 of SPs is due to the adsorption of the SP molecules on the cement particle surface.20 In particular, the dispersing abilities of SNF, SMF, and PCE additives have been explained on the basis of two effects: the electrostatic and steric repulsions among the cement particles.20 The adsorption on hydrating cement particles of anionic SPs bearing sulfonate or carboxylate groups leads to electrostatic repulsion among the particles.19,20 The electrostatic repulsive mechanism is dominant in sulfonate-based SPs such as SNF and SMF. The steric mechanism active in PCE-based SPs is based on the repulsion among the cement particles due to the steric hindrance between adsorbed polymer layers that induce short-range repulsive forces.20,21

In PCE-based SPs, the dispersing abilities are due to carboxylic groups that act as anchors to adsorb onto the surfaces of cement particles, while the poly(ethylene oxide) sidechains protrude from the cement surface into the pore solution to produce steric hindrance.22

In the past years, increasing attention has been devoted to designing ecofriendly23 SPs, and among them, β-CD-based dispersants have been playing a crucial role.10−13

The dispersing abilities of β-CD-based SPs is due to the steric hindrance between the truncated-cone macrocycles and the electrostatic repulsion between anionic24 groups of the polymer adsorbed on the surface of cement particles.11−13 Very recently, we have found out25 that water-soluble resorcin[4]arene and pyrogallol[4]arene macrocycles show an analogous behavior and are able to work as dispersing agents but show retarding effects in cement-based mortars.

Although polymeric materials constituted by CDs covalently grafted on a PEG backbone have found applications as SPs,10−13 no information has been reported to produce data about the dispersion and retarding effects of PEG@CD supramolecular complexes. Harada, pioneering in this field, reported the synthesis of many examples of PEG@CD pseudorotaxane complexes.26 Therefore, based on these considerations, we decided to study the SP properties of pseudorotaxane complexes between mPEG functionalized with carboxylic functions as the axle and β-CD as the wheel (Figure 1). In detail, we envisioned that the carboxylic functions (maleic and phthalic functions) could act as anchors24 on the cement surface (Figure 1) through coordination with metal centers, while the pseudorotaxane architecture constituted by the β-CD wheel threaded along the PEG axle would protrude from the cement surface to generate steric hindrance (Figure 1). Thus, in this work, β-CD-based pseudorotaxane complexes have been formed by mixing the components in aqueous solution, and for the first time, the abilities of these supramolecular complexes as SPs in cement-based mortars have been studied.

Figure 1.

Figure 1

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Dispersing mechanism envisioned for mPEG-COOH@β-CD pseudorotaxane complexes.

Results and Discussion

Synthesis and Characterization of Derivatives mPEG-Phth and mPEG-Mal

The synthesis of mPEG-Phth and mPEG-Mal is shown in Scheme 1. mPEG-OH was reacted with phthalic anhydride (1 equiv) at 100 °C for 12 h, under a vacuum of −0.8 bar, until H2O was distilled out. Gel permeation chromatography (GPC) analysis (Figure 2a) of the reaction mixture over time indicated that the reaction was complete after 12 h, when no hints of starting mPEG-OH material was detected. The Fourier-transform infrared (FT-IR) spectrum of the final product (Figure 2c) clearly showed the disappearance of the OH stretching at 3400 cm–1 present in the starting material mPEG-OH, while a new band appeared at 1731 cm–1 attributable to the C=O stretching of the new ester function.

Scheme 1. Synthesis of mPEG-Phth and mPEG-Mal.

Scheme 1

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Figure 2.

Figure 2

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Gel permeation chromatograms for the reaction between (a) mPEG-OH and phthalic anhydride and (b) mPEG-OH and maleic anhydride. (c) FT IR spectra of starting mPEG-OH, mPEG-Mal, and mPEG-Phth.

1H and 13C NMR experiments confirmed the esterification of mPEG-OH starting material. In particular, the 13C NMR spectrum of mPEG-Phth in D2O evidenced two signals at 171.9 and 169.8 ppm, which was attributed to the carbonyl functions, while in the 1H NMR spectrum (D2O, 600 MHz, 298 K), the −CH2OH signal of the starting mPEG-OH experienced a downfield shift of 0.56 ppm upon esterification.

Differently, when mPEG-OH and maleic anhydride (1.0 equiv) were reacted under the same conditions as described above for phthalic anhydride, after 12 h, the esterification of the starting mPEG-OH was not complete as indicated by GPC analysis. Probably, in the presence of oxygen, a thermally initiated concurrent homopolymerization of maleic anhydride occured.27 Thus, with the aim to inhibit the radical homopolymerization, we performed the reaction (Scheme 1) in the presence of phenothiazine (0.05%, w/w) and p-methoxyphenol (0.1%, w/w).27 In fact, in accordance with the literature data,27 the phenothiazine/p-methoxyphenol system can inhibit the radical polymerization of acrylate derivatives. Under these conditions, 80% of conversion of mPEG1000 to mPEG-Mal was obtained after 16 h, as confirmed by the GPC experiments (Figure 2b). In addition, the FT-IR spectrum of the mPEG-Mal (Figure 2c) revealed the absence of the OH-stretching band of the starting mPEG-OH at 3400 cm–1 and the presence of a new band at 1729 cm–1 attributable to the C=O stretching of the new ester function.

Differential scanning calorimetry (DSC) analysis of the mPEG-Phth derivative (Figure 3) evidenced a different thermal behavior with respect to the starting material PEG-OH. Indeed, during the heating ramp, the product exhibited a phase transition at 30.7 °C with an enthalpy of 115.1 J/g, whereas higher values of melting temperature and enthalpy were observed for the mPEG-OH starting material. Upon cooling, a phase change peak was observed at 16.8 °C for the mPEG-Phth product, with ΔH = 90.8 J/g, which is lower than the corresponding value measured for mPEG-OH (Figure 3b,a).The enthalpy curve of mPEG-Mal also displayed lower values of phase transition and enthalpy than the mPEG-OH starting material: the melting temperatures and the corresponding ΔH values were 32.5 °C and 122.8 J/g, respectively, during the heating process, while in the cooling ramp, the phase transition occurred at 23.0 °C with a ΔH value of 94.00 J/g (Figure 3c,a).

Figure 3.

Figure 3

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DSC heating (left, in red) and cooling (right, in blue) curves of experiments carried out with (a) mPEG1000-OH; (b) mPEG-Phth; and (c) mPEG-Mal.

This finding showed that the presence of phthalate or maleate at the end of the PEG chain influences the chemical physical properties of the PEG-based material.

At this point, we studied the thermal stability [thermogravimetric analysis (TGA)] of the products mPEG-Phth and mPEG-Mal compared to that of the starting material mPEG-OH (Figure 4). The latter starts to lose weight at 191.8 °C, while the maximum degradation rate (inflection point) is reached at 217 °C (Figure 4). The product mPEG-Phth showed an excellent thermal stability, and TGA revealed the presence of a two-step degradation, the first one at 176.4 °C due to the phthalate group (Figure 4, left), and the second one at 299.7 °C was attributable to the mPEG chain. Thus, the TGA curve of mPEG-Phth was shifted to a higher temperature when compared with the starting material mPEG-OH and phthalic anhydride, indicating an increased thermal stability of the product mPEG-Phth. Analogously, TGA indicated that the mPEG-Mal (Figure 4, right) showed an increased thermal stability when compared with the starting material mPEG-OH.

Figure 4.

Figure 4

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TGA of (left) mPEG-Phth (green) compared with mPEG-OH (black) and phthalic anhydride (dark green); (right) mPEG-Mal (blue) compared with mPEG-OH (black) and maleic anhydride (dark blue).

Synthesis and Characterization of Pseudorotaxane Complexes mPEG-Phth@βCD, mPEG-Mal@βCD, and mPEG-OH@βCD

The formation of pseudorotaxane mPEG-Phth@βCD (Scheme 2) was studied by 1H nuclear magnetic resonance (NMR) and diffusion ordered spectroscopy (DOSY) experiments and TGA analysis and was corroborated by molecular dynamics (MD) simulations.

Scheme 2. Formation of the Pseudorotaxanes mPEG-Phth@β-CD, mPEG-Mal@β-CD, and mPEG-OH@β-CD.

Scheme 2

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As known, β-CD inclusion complexes are usually obtained by mixing an aqueous solution of β-CD and a water-soluble guest molecule.26 On the basis of this information, an equimolar aqueous solution of mPEG-Phth and β-CD in D2O was stirred at room temperature for 30 min (Scheme 2) and the resulting solution was studied by 1H NMR (400 MHz, 298 K) (Figure 5). The formation of pseudorotaxane mPEG-Phth@β-CD (Scheme 2) was ascertained by a shift of the host and guest signals due to the complexation equilibrium. In Figure 5, the comparison between the 1H NMR spectra of free mPEG-Phth and β-CD with the 1H NMR spectrum of their 1:1 mixture is shown. The formation of the mPEG-Phth@β-CD complex is clearly evidenced by a complexation induced shift of the anomeric signal of β-CD (in red in Figure 5) that experienced a △δ = δcomplex– δfree = −0.1 ppm. Interestingly, a CIS of – 0.2 ppm was also experienced by −OCH2C(O)O– protons (in blue in Figure 5) of the mPEG-Phth axle. A shift was also observed for the aromatic signals of the mPEG-Phth guest that upon formation of the pseudorotaxane experienced a marked downfield shift.

Figure 5.

Figure 5

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1H NMR spectra (600 MHz, D2O, 298 K) of (a) β-CD; (b) equimolar mixture (1 × 10–3 M) of β-CD and mPEG-Phth; and (c) mPEG-Phth.

The formation of the mPEG-Phth@β-CD complex was also investigated by DOSY NMR experiments.28 In particular, DOSY spectra (400 MHz, 298 K) were acquired for the free β-CD (30 mM) for the 1:1 mixture mPEG-Phth and β-CD (15 mM each one), and for the mPEG-Phth (30 mM), under identical conditions.28c,28d The analyses of the diffusion coefficients of 2.8 × 10–10 (free β-CD), 1.95 × 10–10 (mPEG-Phth@β-CD complex), and 2.7 × 10–10 m2/s (mPEG-Phth) were in good agreement with the increasing size of the complex (Supporting Information). As previously reported,28a,28b the ratio of diffusion coefficients for two different molecular species (e.g., DmPEG-Phth@β-CD/Dβ-CD) is inversely proportional to the square root of the ratio of their molecular masses for rod-like molecules. Based on this information, the diffusion coefficient data were consistent with a 1:1 stoichiometry of the mPEG-Phth@β-CD complex.29

The structure of the pseudorotaxane mPEG-Phth@β-CD was analyzed by MD studies (Figure 6) performed by using YASARA software (version 20.8.23) and AMBER-14 force field.30,31 The simulated system consists of the mPEG-Phth@β-CD complex and 8700 water molecules in a periodic cubic box of size 6.5 nm/side. MD was simulated for 30 ns, and snapshots were captured every 10 ps. The initial coordinates of β-CD were taken by the Cambridge Crystallographic Data Centre (CCDC), while the starting structure of mPEG-Phth in a full-extended conformation was obtained by MD calculations. The minimum energy structures of the mPEG-Phth@β-CD complex obtained by MD (Figure 6b–d) showed the phthalate group and a portion of the PEG-axle included inside the cavity of β-CD to give a pseudorotaxane, while the remaining PEG chain was swimming in the aqueous medium to induce a H-bonding interaction with it.

Figure 6.

Figure 6

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(a) Ray-traced picture of the simulated system. The simulation cell boundary is set to periodic. Atoms that stick out of the simulation cell will be wrapped to the opposite side of the cell during the simulation. (b,d) Minimum energy structures of pseudorotaxane mPEG-Phth@β-CD calculated by MD simulation. (c) Representative example of a structure of mPEG-Phth@β-CD complex in which the mPEG-Phth chain is wrapped around the β-CD macrocycle (closed conformation). (e) Radius of gyration of the solute as a function of simulation time. (f) van der Waals (blue),32a molecular32b (red), and solvent accessible area32c (A2) (green) of pseudorotaxane mPEG-Phth@β-CD.

The trajectory analysis (Figure 6e) shows that over a period of 10–16 ns, when the radius of gyration of the complex assumes the maximum value corresponding to the pseudorotaxane structures in Figure 6b,d, the solvent accessible surface is the maximum. This result clearly indicates that in the minimum energy structures of mPEG-Phth@β-CD pseudorotaxane, the PEG chain adopts a fully extended conformation that shows a high solvent accessible surface (Figure 6b,d). Thus, in the fully extended conformation, mPEG-Phth@β-CD pseudorotaxane can establish H-bonding interactions with water molecules favoring the formation of the solvation water film (see Figure 11) that plays a crucial role in the dispersing abilities of SP additives (see model in Figure 1). In a similar way, the formation of the inclusion complex mPEG-Mal@β-CD was followed by 1H NMR experiment (see SI)

Figure 11.

Figure 11

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Sketch of the proposed dispersing mechanism with the pseudorotaxane complexes mPEG-COOH@β-CD.

Analogously, the formation of the inclusion complex mPEG-OH@β-CD was ascertained by 1H NMR experiments (600 MHz, D2O, 298 K, see Supporting Information) (Figure 7), which evidenced a shift of the signal of the OMe group upon addition of β-CD (1 equiv), with a △δ = (δcomplex – δfree) = 0.03 ppm.

Figure 7.

Figure 7

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1H NMR spectra (600 MHz, D2O, 298 K) of (a) mPEG-OH; (b) an equimolar mixture (1 × 10–3M) of β-CD and mPEG-OH; and (c) β-CD.

TGA of the PEG@β-CD Pseudorotaxanes

At this point, we performed a TGA of the pseudorotaxane complexes (Figure 8).33−36 As known,33−36 the TGA of free β-CD shows three zones where the weight is lost: (i) T < 100 °C, where a 10–15% of mass loss is attributed to the evaporation of superficial water associated with the macrocycle; (ii) at 110 °C ca., a 10% mass loss is attributed to the evaporation of internal water; and (iii) the third process at 307 °C ca. is related to the degradation of β-CD.33−36 TGA thermograms of mPEG-Phth@β-CD pseudorotaxane (Figure 8b) were compared with those of the starting materials, mPEG-Phth and β-CD.

Figure 8.

Figure 8

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TGA of (a) mPEG-OH (black), β-CD (red), and mPEG-OH@ β-CD (gray); (b) mPEG-Phth (green), β-CD (red), and mPEG-Phth@ β-CD (dark green); (c) mPEG-Mal (blue), β-CD (red), and mPEG-Mal@ β-CD (dark blue).

The thermogram of mPEG-Phth@β-CD pseudorotaxane in Figure 8b is quite different from those of the free components mPEG-Phth and β-CD (Figure 8b). Both the loss of superficial and internal water associated to β-CD is greatly reduced because of the inclusion of mPEG-Phth inside its cavity. As is known, the hydrophobic cavity of CDs is occupied by enthalpy-rich water molecules that upon inclusion of the guest molecule are released into the bulk. After removing moisture from the sample, it is possible to define the onset temperature that occurs at 182.2 °C, close to the value of the free guest mPEG-Phth. The thermogram of the mPEG-Phth@β-CD complex shows a three-step degradation at Td = 182.2 °C, Td = 298.0 °C, and Td = 478.2 °C. The temperatures related to the maximum degradation rate are observed with the DTG curve: 199.5, 318.4, and 496.1 °C, while free mPEG-Phth shows the maximum degradation rate at 239.5 and 324.8 °C. These results were unchanged when a 2:1 mixture of β-CD and mPEG-Phth was investigated.

Dispersing and Retarding Properties of PEG@β-CD Pseudorotaxanes

At this point, we studied the dispersing performances of PEG@β-CD pseudorotaxanes in a cement mortar by spread tests. As reported in Figure 9, different dosages of the inclusion complexes (i.e., 0.2–0.8% by weight of cement, bwoc) were used at different mPEG-COOH/β-CD molar ratios.

Figure 9.

Figure 9

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Dosage-dependent effect of the mixture mPEG-COOH/β-CD at different molar ratios on a mortar spread value.

Our aim was to obtain useful information regarding the most performing mPEG-COOH/β-CD molar ratio. As reported in Figure 9, the dispersing ability of the 1:1 mixture between mPEG-COOH and β-CD was rather limited. On the other hand, the 1:2 mPEG-COOH/β-CD mixtures showed a superior dispersing ability. For instance, a dosage of 0.50% of a 1:2 mPEG-Mal/β-CD mixture was necessary to obtain a spread value of 290 mm ca (Figure 9), while with the analogous mPEG-Mal/β-CD 1:1 ratio, at a 0.50% of dosage, a spread value of 260 mm was measured. The higher dispersing efficiency of mixtures with an excess of β-CD can be probably attributed to its ability to chelate the Ca2+ ions. Consequently, the adsorption of the complex on the cement particle surface and the formation of Ca2+ chelates lead to the formation of a thick layer, which in addition to the electrostatic repulsion between the cement particles determines the dispersion process (see Figure 1).

Considering these outcomes, we studied the dispersing performances of mPEG-COOH/β-CD mixtures in a cement mortar by spread tests (Figure 10). The slump tests of the complexes were compared to those of the free host and guest components (β-CD and mPEG-COOH derivatives) to evaluate the effect of the complexation of the mPEG-COOH chain inside the β-CD over the fluidizing properties of the system. The spread retention capability is a characteristic of SPs that confers the retention of fluidity over time to the fresh cement mortars in which they are added: therefore, a measurement of this ability allows for us to understand if the tested additive could be classified as a workability retainer and/or a water reducer. To this aim, the spread of cement mortars was evaluated by dosing the 0.5% bwoc (Figure 10) of the mPEG-COOH/β-CD mixture and by setting the water/cement ratio at 0.5; finally, the evolution of mortar fluidity was measured over time (up to 120 min). As reported in Figure 10, the dispersing ability of the free components mPEG-Phth, mPEG-Mal, mPEG-OH, and β-CD was very limited. Thus, these results clearly indicated that β-CD and the free mPEG derivatives cannot be used as dispersing materials for the concrete. Interestingly, when an equimolar mixture mPEG-Phth and β-CD was investigated, an initial spread value of 265 mm ca. (Figure 10a) was measured, which is significantly higher than that observed for the free components mPEG-Phth (170 mm) and β-CD (240 mm). The spread value significantly increased to 280 mm when a 2:1 ratio of β-CD/mPEG-Phth was investigated (Figure 10a) and the fluidity retention of the mortar declined slower than in the presence of only β-CD. Analogously, the pseudorotaxane complex mPEG-Mal@β-CD, obtained by mixing mPEG-Mal and β-CD in a 1/2 ratio, showed an excellent fluidity retention and a high initial spread of about 290 mm (Figure 10b). Thus, the inclusion complex mPEG-Mal@β-CD provides an excellent slump retention, particularly within the first 1 h, when the fluidity decreased from 280 to 270 mm, and finally, after 2 h, it reached the value of 240 mm. Interestingly, mPEG-Mal@β-CD exhibits a better slump retention because the fluidity decreased from 290 to 260 mm. Thus, the fluidity losses of the complexes mPEG-Phth@β-CD and mPEG-Mal@β-CD were, respectively, 14.2 and 10.3% within 2 h. Starting from these results, we studied the role of the terminal carboxylic function on the mPEG chain. The inclusion complex mPEG-OH@β-CD obtained by mixing mPEG-OH and β-CD in 1:1 and 1:2 ratios showed an initial spread of 240 mm very similar to that of free β-CD (Figure 10c). This result clearly indicates that the underivatized mPEG-OH does not lead to improve the fluidizing properties of β-CD. In addition, we can conclude that the carboxylic function on the PEG chain plays a crucial role in improving the adsorption capacity of the complexes on the cement surface particle.

Figure 10.

Figure 10

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Time-dependent evolution of the spread value of fresh cement mortars admixed with the pseudorotaxane complexes and their free constituents β-CD and mPEG-X with different molar ratios reported into the graphs: (a) mPEG-Phth@β-CD, mPEG-Phth, and β-CD; (b) mPEG-Mal@β-CD, mPEG-Mal, and β-CD; (c) mPEG-OH@β-CD, mPEG-OH, and β-CD. (0.5% bwoc) (w/c = 0.5).

At this point, we investigated the setting time of mPEG@β-CD pseudorotaxane complexes. As is known,11−14 the β-CD-based derivatives usually show a retarding effect that slows down the concrete’s hardening. The inclusion complexes mPEG-Phth@β-CD and mPEG-Mal@β-CD generate a serious retardation effect with an initial setting time of 8 h, which makes them unsuitable for civil engineering applications.

Analogous to other β-CD-based SPs, probably also in this case, the retarding effect is due to the steric hindrance11−14 of the macrocyclic skeleton. In addition, the formation of chelate complexes between the β-CD hydroxyl groups and Ca2+ prevents the solid-phase nucleation and growth of the hydration products and retards cement hydration (Figure 11).11−14 In detail, the formation of a thick adsorption layer constituted by macrocycle-Ca2+ chelates, and a solvation water film (Figure 11) leads to a slowdown of the growth of the hydration products.

Conclusions

In conclusion, in this work, we report an example of a sustainable supramolecular SP for applications in a cement-based mortar. Methoxy-PEG-OH was first functionalized with phthalate and maleate groups by a one-pot sustainable synthesis that does not require the use of solvents or a work-up procedure. The formation of pseudorotaxane complexes between mPEG-COOH guests and β-CD has been studied in solution by 1H NMR titration experiments and TGA measurements. The potential dispersing abilities of the mPEG-COOH/β-CD mixtures were investigated by a dosage-dependent effect and time-dependent evolution of the spread value of fresh cement mortars. Free components β-CD, mPEG-COOH derivatives, and mPEG-OH show poor dispersing properties. Differently, 2:1 mixtures of mPEG-COOH/β-CD show excellent dispersing properties with initial spread values of 280–290 mm. Finally, the mPEG-COOH/β-CD mixtures show a serious retardation effect with an initial setting time of 8 h, which makes them unsuitable for engineering applications.

Due to the current interest toward the synthesis of novel sustainable dispersing and retarding additives, it is conceivable that the information reported herein will be useful for the design of novel supramolecular-based SPs with improved fluidizing abilities and setting time performances.

Experimental Section

Materials

β-CD was purchased from Roquette (KLEPTOSE), polyglykol M 1000 was purchased from Clariant, and phthalic anhydride and maleic anhydride were purchased from Carlo Erba. All chemical reagents of analytical grade were used without further purification and were used as purchased. Reaction temperatures were measured externally.

General Procedure for the Synthesis of mPEG-Phth and mPEG-Mal

Synthesis of mPEG-Phth

Starting mPEG-OH was melted at 60 °C, and phthalic anhydride (38.1 g, 0.25 mol) was crushed into a fine powder and mixed into the reaction flask containing the polymer (250 g, 0.250 mol). The mixture was vigorously stirred at 80 °C for 12 h at P = −0.8 bar distilling off water, and finally, a white waxy product was obtained. 1H NMR (600 MHz, D2O, 298 K): δ 7.79–7.83 (overlapped, 2H, ArH), 7.66–7.71 (overlapped, ArH, 2H), 4.49 (m, −OCH2CH2OCOPh), 3.88 (m, −OCH2CH2OR) 3.62–3.70 (PEG backbone), 3.37 (s, 3H, O–CH3). 13C NMR (100 MHz, D2O, 298 K): δ 171.9, 169.8, 132.5, 132.0, 129.2, 129.0, 69.8, 68.5, 65.5, 60.6, 58.3.

Synthesis of mPEG-Mal

Starting mPEG-OH was melted at 60 °C, and the maleic acid (19.6 g, 0.21 mol) was crushed into a fine powder and added to the polymer (200 g, 0.21 mol) under stirring. After the solubilization of the starting compounds, phenothiazine (0.05%, w/w) and p-methoxyphenol (0.1%, w/w) were added to the reaction mixture. The mixture was vigorously stirred at 100 °C for 16 h at P = −0.8 bar distilling off water, until it turned into a red waxy product. 1H NMR (600 MHz, D2O, 298 K): δ 6.54 (d, J = 12.2 Hz, 1H, RCO2–CH=CH–R), 6.35 (d, J = 12.2 Hz, 1H, RCO2–CH=CH–R), 4.35 (m, −OCH2CH2OCOR), 3.59–3.69 (PEG backbone), δ 3.37 (s, 3H, O–CH3). 13C NMR (100 MHz, D2O, 298 K): δ 170.0, 167.1, 132.9, 128.8, 71.7, 68.2, 64.5, 60.3, 58.0.

Pseudorotaxane Formation

The synthesis of mPEG-COOH@β-CD pseudorotaxane complexes was conducted in a flask under atmospheric pressure. β-CD (10 g, TS = 93% wt, 8.8 × 10–3 mol) is solubilized in 250 mL of deionized water. Then, an appropriate amount of the mPEG derivative (1/0.5; 1/1; 1:2 β-CD/mPEG-COOH molar ratio) was added, and the resulting mixture was stirred at 25 °C for 30 min. Finally, the water was evaporated under vacuum, and the product was further dried and stored at room temperature.

Characterization

NMR Spectra

NMR spectra of derivatives mPEG-Mal and mPEG-Phth and their complexes with β-CD (mPEG-Mal@β-CD, mPEG-OH@ β-CD, and mPEG-Phth@β-CD) were recorded on a Bruker AVANCE-600 [600 (1H) and 150 MHz (13C)], AVANCE-400 [400 (1H), and 100 MHz (13C)] or AVANCE-300 MHz [300 (1H) and 75 MHz (13C)] spectrometers, respectively. Chemical shifts are referenced to the residual nondeuterated solvent peak (H2O: 1H NMR δ = 4.78).

DOSY Spectra

DOSY experiments of derivatives mPEG-Phth, β-CD, and mPEG-Phth@β-CD were performed on a Bruker DRX 600 spectrometer equipped with 5 mm PABBO BB|19F-1H\D Z-GRD Z114607/0109. The standard Bruker pulse program, ledbpgp2s, employing a double stimulated echo sequence and LED, bipolar gradient pulses for diffusion, and two spoil gradients was utilized. Individual rows of the quasi-2D diffusion databases were phased, and the baseline was corrected.

FT-IR Spectra

IR spectra of mPEG-Phth and mPEG-Mal were recorded with FT-IR 4100 Type A (JASCO Europe), and KBr tablets were used to prepare the samples for the FT-IR measurements.

Gel Permeation Chromatography

GPC measurements in Figure 2 were obtained with a GPC Waters Alliance e2695 instrument equipped with a refractive-index RI 2414 detector and Waters Ultrahydrogel 120, 250, 500 in series columns. H2O/NaNO3/NaOH (0.1 M) was used as an eluent at a flow rate of 0.7 mL/min at 40 °C. Data processing was performed with Empower 3 software. The molecular weight of the polymers was calculated on the basis of PEG calibration curve.

Thermogravimetric Analysis

TGA of mPEG-Phth, mPEG-Mal, and mPEG-OH and their pseudorotaxanes with β-CD (mPEG-Mal@β-CD, mPEG-OH@ β-CD, and mPEG-Phth@β-CD) was carried out in a TA Instrument Q500 thermogravimetric analyzer. The experiments were performed at a heating rate of 10 °C/min in a mixture of a nitrogen/air flow of 40/60 mL/min. The temperature of the furnace was programmed to rise from 25 to 900 °C.

Differential Scanning Calorimetry

DSC analysis was performed on mPEG-Phth and mPEG-Mal, using a DSC TA DSC25P instrument. Samples were heated from 5 to 150 °C at a rate of 10 °C/min under a dry nitrogen purge. The sample weight was approximately 5 mg.

MD Simulation

MD studies were carried out using Yasara Structure software (version 20.8.23) and AMBER 14 as the force field. Periodic boundary conditions with a cubing box (6.5 nm) as the unit cell were employed and filled with pre-equilibrated water molecules (d = 0.997 g/mL) at pH = 12 and 298 K. MD were simulated for 10 ns, and a snapshot was captured every 10 ps. The initial coordinates of β-CD were taken by The Cambridge Crystallographic Data Centre (CCDC), while the coordinates of the pseudorotaxane mPEG-Phth@β-CD were obtained by molecular mechanics calculation using a mPEG chain with a reduced number of (−CH2CH2O−) units.

Applicative Tests on Fresh Mortar

Mortar Test Materials and Laboratory-Based Equipment

Ordinary portland cement CEM IIA 42.5 RLL (Italcementi) compliant with UNI EN 197-1 and CEN Standard Sand compliant with DIN EN 196-1 (Normensand) was used for the slump and the Vicat tests in this study. The performance of the mortar was tested according to the standard regulations in force. The moisture content of all the derivatives was estimated at 105 °C for 30 min to calculate the specific dosage of additives (moisture analyzer model name Crystaltherm Gibertini).

The spreading tests were carried out at w/c = 0.5, in accordance with ASTM C143 (“standard Test method for slump hydraulic-cement concrete”) and EN196-1:2016 (methods of testing cement, part 1—Cap. 6: preparation of mortar)/EN 480–1:2011 (admixture for concrete, mortar ang grout—test methods part 1: preparation of control mortar). The equipment used was an automatic mortar mixer model E093 (Matest S.p.A.) conforming to ASTM C305M specifications (“mechanical mixing of hydraulic cement pastes and mortars of plastic consistency”) and an automatic flow table model M092 (LBG srl) conforming to ASTM C230M specifications (“standard specification for flow table for use in tests of hydraulic cement”).

A Vicat apparatus was used to determine the setting time for all the samples with a w/c = 0.5 and 0.5% bwoc of 100% active SPs in compliance with ASTM C191 specifications (“standard test method for time of setting of hydraulic cement by Vicat needle”) and EN196-1:2016/EN 480-2:2011 (admixture for concrete, mortar ang grout—test methods part 2: determination of setting time).

Study of the Dosage-Dependent Effect

Cement mortars were prepared with the following amounts of materials/conditions: 450 g of CEM II 42.5 R; 225 g of tap water (w/c = 0.5); 1350 g of CEN standard sand; and 0.5% (bwoc) of the additive (100% active). The mixing program foresees a 4-min mixing schedule (according to ASTM C305: 30 s low mixing, 30 s high mixing, 2 min rest, 1 min at high mixing). The temperature of the test room, equipment, and materials was set at 20.0 ± 2.0 °C. Cementitious pastes were assessed with 15 strokes of the automatic flow table. After mixing, paste diameters were registered three times and the average values were reported. Spread values were measured after 4′, 30′, 60′, 90′, and 120′. In order to evaluate the time dependence of the mortars’ diameter of the additive-doped mortar, pastes were doped with different percentages of active and the initial spread diameters were recorded. Setting time was carried out using the Vicat apparatus over a defined period of time to study SP trends. The initial setting time was defined at the point where the penetration of the Vicat probe into the fresh mortar was 4 ± 1 mm, in accordance with ASTM C191 specifications.

Acknowledgments

The authors thank the Programma Operativo Nazionale, Ricerca e Innovazione 2014–2020 (CCI 2014IT16M2OP005), Fondo Sociale Europeo, Azione I.1 “Dottorati Innovativi con caratterizzazione Industriale—CUP D48G18000150006, for the financial support. The work was also supported by grants from the University of Salerno and from BI-QEM SPECIALTIES S.p.A.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c01133.

  • 1H and 13C NMR spectra of mPEG derivatives and their complexes with the β-CD, FT-IR spectra, details of the MDs, and DOSY spectra (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao1c01133_si_001.pdf (696.5KB, pdf)

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Supplementary Materials

ao1c01133_si_001.pdf (696.5KB, pdf)

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