Low-Temperature Synthesis of EPEG-Based Superplasticizers: Kinetic Optimization and Structure–Property Relationships

Abstract

Conventional synthesis of polycarboxylate superplasticizers (PCEs) typically relies on high-temperature processes, posing challenges for sustainable production. Ethylene glycol monovinyl polyethylene glycol ether (EPEG), characterized by the high reactivity of its vinyloxy double bond, offers a promising sustainable alternative for low-temperature synthesis. This study systematically investigates the aqueous free radical copolymerization of EPEG and acrylic acid, identifying a reaction temperature of 20 °C as the kinetic optimum that achieves a macromonomer conversion rate exceeding 95% under ambient conditions. Through the variation in five key process parameters, a clear “synthesis–structure–property” relationship was established, revealing that the weight-average molecular weight (M_w) acts as the pivotal regulator of performance. High-M_w PCEs exhibited superior initial dispersion driven by strong electrostatic repulsion and high adsorption but suffered from poor slump retention due to the rapid depletion of free polymers. Conversely, low-M_w variants, regulated by chain transfer agent dosage, significantly reduced the pore solution surface tension, thereby enhancing wetting ability and workability retention. The optimal synthesis conditions (20 °C, 4:1 acid-to-ether ratio, 2.5% initiator, 1.5% chain transfer agent) yielded PCEs with an ideal balance between initial dispersion and retention. Furthermore, the synthesis demonstrated excellent process robustness with a broad dosing window (>60 min). These findings provide a vital theoretical basis for the robust and low-temperature industrial production of EPEG-based PCEs for sustainable infrastructure materials.

1. Introduction

As the third generation of high-performance concrete water reducers, Polycarboxylate superplasticizers (PCEs) have become indispensable core admixtures in modern High-Performance Concrete [1,2,3,4]. This is attributed to their low dosage requirements, high water-reducing rates, substantial strengthening effects, and excellent molecular designability [5,6,7]. PCEs typically exhibit a comb-like molecular architecture, consisting of a charged polymeric backbone and multiple hydrophilic polyether side chains [8,9]. The dispersion mechanism of PCEs is widely regarded as the synergistic result of electrostatic repulsion and steric hindrance: anionic groups on the backbone (e.g., carboxyl groups) adsorb onto the positively charged surfaces of cement particles, providing electrostatic repulsion, while the grafted hydrophilic polyether side chains (e.g., polyethylene glycol, PEG) form a steric adsorption layer on the particle surface, preventing agglomeration via steric hindrance effects [10,11,12,13].

The synthesis, performance, and cost of PCEs are largely contingent upon the type of macromonomer selected [3,14]. Historically, the industry has predominantly utilized vinyl alcohol-based macromonomers, such as methallyl polyoxyethylene ether (HPEG) and isoprenyl polyoxyethylene ether (IPEG). However, due to the relatively low reactivity of the allyl or methallyl double bonds in these monomers, their free radical copolymerization in aqueous solution typically requires elevated temperatures (e.g., 60–90 °C) to proceed effectively [15,16]. This high-temperature synthesis process not only significantly increases energy consumption but also poses risks under acidic conditions typically involving acrylic acid (AA) as a comonomer. Such conditions can induce side reactions, including the hydrolytic cleavage of polyether side chains, thereby compromising the molecular uniformity and performance stability of the resultant PCEs. Beyond these process-related issues, recent reviews have highlighted intrinsic limitations in current materials. For instance, traditional free radical polymerization methods often suffer from uncontrollable chain growth and termination tendencies, making it difficult to precisely tailor the molecular structure [3]. Furthermore, in terms of application, the anionic charge density and specific architectures of conventional PCEs can inevitably retard cement hydration, leading to delayed setting times and compromised early-age strength [17]. Therefore, developing a novel synthesis strategy that is both energy-efficient and capable of precise structural control to balance dispersion and hydration kinetics is urgently needed.

In recent years, vinyloxy-based macromonomers have garnered significant attention due to their superior reactivity. This study focuses on ethylene glycol monovinyl polyethylene glycol ether (EPEG), characterized by a chemical structure where the C=C double bond (H₂C=CH-O-) is directly connected to an ether oxygen atom. Unlike vinyl alcohol-based macromonomers (H₂C=C(CH₃)-CH₂-O-), the lone pair electrons on the ether oxygen of EPEG form a p-π conjugation with the C=C double bond. This interaction significantly enhances the electron cloud density and reactivity of the double bond [16]. Such high reactivity facilitates the copolymerization of EPEG with acrylic acid (AA), enabling efficient polymerization at low temperatures (e.g., 0–30 °C). Developing a low-temperature synthesis protocol offers substantial advantages in energy conservation and cost reduction [18]. Furthermore, it effectively mitigates high-temperature side reactions, showing great promise for producing PCEs with more regular structures and superior performance.

However, the reaction kinetics and process control mechanisms of EPEG under low-temperature conditions remain not fully understood. To bridge the gap between synthesis protocols and performance regulation, this study systematically investigates the aqueous free radical copolymerization of EPEG and AA. The primary limitations of existing research and the main contributions of this work are summarized as follows:

• Conventional synthesis typically relies on vinyl alcohol-based macromonomers which exhibit low reactivity, necessitating energy-intensive high-temperature processes (60–90 °C). Such conditions often induce side reactions that compromise molecular uniformity. Furthermore, there is a lack of systematic research elucidating how key process parameters regulate the microscopic structure and macroscopic performance of EPEG-based PCEs synthesized at low temperatures.
• Leveraging the high reactivity of the vinyloxy double bond in EPEG, this study establishes a robust low-temperature (20 °C) synthesis protocol that achieves high conversion (>95%) with significantly reduced energy consumption. By systematically varying five key process parameters, a clear “synthesis–structure–property” relationship is constructed. Crucially, the weight-average molecular weight (M_w) is identified as the central regulator that balances initial dispersion and slump retention, mediated by mechanisms involving adsorption behavior and pore solution surface tension.

The findings of this study provide a vital theoretical basis for the robust, low-temperature industrial production of sustainable EPEG-based superplasticizers. It is noted that the performance evaluations in this study are based on cement paste to elucidate fundamental mechanisms; verification in concrete systems will be the subject of future work.

2. Experimental

2.1. Materials

The raw materials utilized in this study include industrial-grade ethylene glycol monovinyl polyethylene glycol ether (EPEG, weight-average molecular weight M_w ≈ 3000 Da, Satellite Chemical Co., Ltd., Jiaxing, China) as the macromonomer and analytical reagent (AR) grade acrylic acid (AA, Sinopharm Chemical Reagent Co., Ltd. Shanghai, China) as the comonomer. The redox initiation system comprised an 8 wt% aqueous solution of hydrogen peroxide (H₂O₂) and AR-grade dioctyl sodium sulfosuccinate (E51, Sinopharm Chemical Reagent Co., Ltd.). Sodium hypophosphite (SHPP, AR grade, Sinopharm Chemical Reagent Co., Ltd.) was utilized as the chain transfer agent, and ferrous sulfate heptahydrate (FeSO₄·7H₂O, AR grade) served as the catalyst. A 32 wt% sodium hydroxide (NaOH) solution was used as the neutralizing agent. Performance evaluations were conducted using P·I 42.5 Portland cement supplied by Fushun Cement Co., Ltd. (Fushun, China), which has a specific gravity of 3.15 g/cm³ and a Blaine fineness of 360 m²/kg. The chemical oxide composition of the cement is listed in

Table 1. Chemical composition of the cement.

Chemical Composition wt.%	CaO	SiO2	Al2O3	Fe2O3	MgO	K2O	SO3	LOI
Cement	61.34	20.82	6.34	3.07	1.03	0.85	2.3	4.26

. Deionized water was used throughout all experiments.

2.2. Low-Temperature Synthesis of EPEG-PCEs

The synthesis was conducted via aqueous free radical copolymerization. A predetermined amount of EPEG macromonomer and deionized water were charged into a four-neck round-bottom flask equipped with a mechanical stirrer, a thermometer, and peristaltic pump inlets. The mixture was stirred until the macromonomer was completely dissolved. Separately, Solution A was prepared by mixing AA with a specific amount of deionized water, and Solution B was prepared by dissolving E51 in deionized water.

The reaction was carried out at a controlled water bath temperature (baseline temperature: 20 °C). First, the chain transfer agent (SHPP) and a 1 wt% FeSO₄·7H₂O solution were added sequentially to the flask. Subsequently, the oxidant H₂O₂ was added all at once. Solution A (baseline duration: 60 min) and Solution B (baseline duration: 70 min) were then added dropwise simultaneously using peristaltic pumps at constant rates. Upon completion of the additions, the reaction mixture was maintained at the set temperature for an additional 30 min to ensure conversion. Finally, the synthesized PCEs were neutralized to a pH of approximately 7 using the NaOH solution.

This protocol employs a H₂O₂-E51 redox initiation system in a semi-batch process. The oxidant (H₂O₂) is pre-charged into the reactor, while the reducing agent (E51) and the primary comonomer (AA) are introduced gradually. Given the high reactivity of EPEG, an excessively rapid generation of free radicals could lead to uncontrolled polymerization. By adding the reducing agent (E51) dropwise at a constant rate, the instantaneous generation rate of free radicals can be precisely controlled to match the consumption rate of the highly reactive monomers, thereby ensuring a smooth and controllable polymerization process at low temperatures (20 °C).

In this study, a single-factor experimental design was adopted. Based on a reference synthesis condition yielding excellent dispersion performance (Sample ID: A3/B3/C3/D3/E2; Parameters: acid-to-ether molar ratio 4:1, temperature 20 °C, initiator dosage 2.5%, chain transfer agent dosage 1.5%, dropping times A: 60 min/B: 70 min), five key process parameters were systematically varied to investigate their effects on the structure and performance of the PCEs: acid-to-ether molar ratio, reaction temperature, initiator dosage, chain transfer agent dosage, and dropping time. The specific synthesis parameters for each sample series are detailed in

Table 2. Synthesis parameters of EPEG-PCEs.

Sample ID	Acid-to-Ether Ratio (AA:EPEG)	Temperature (°C)	Initiator Dosage (%)	Chain Transfer Agent (%)	Dropping Time (Sol. A/Sol. B) (min)
A1	2:1	20	2.5	1.5	60/70
A2	3:1	20	2.5	1.5	60/70
A3 (B3/C3/D3/E2, Ref)	4:1	20	2.5	1.5	60/70
A4	5:1	20	2.5	1.5	60/70
A5	6:1	20	2.5	1.5	60/70
B1	4:1	0	2.5	1.5	60/70
B2	4:1	10	2.5	1.5	60/70
B4	4:1	30	2.5	1.5	60/70
B5	4:1	40	2.5	1.5	60/70
C1	4:1	20	1.5	1.5	60/70
C2	4:1	20	2.0	1.5	60/70
C4	4:1	20	3.0	1.5	60/70
C5	4:1	20	3.5	1.5	60/70
D1	4:1	20	2.5	0.5	60/70
D2	4:1	20	2.5	1.0	60/70
D4	4:1	20	2.5	2.0	60/70
D5	4:1	20	2.5	2.5	60/70
E1	4:1	20	2.5	1.5	40/50
E3	4:1	20	2.5	1.5	80/90
E4	4:1	20	2.5	1.5	100/110
E5	4:1	20	2.5	1.5	120/130

Note 1: The dosages of initiator and chain transfer agent are calculated as mass percentages relative to the total mass of monomers. Note 2: Sample A3 is simultaneously used as the reference sample (labeled as B3, C3, D3, and E2) in the corresponding series. No additional synthesis was conducted for these IDs; they represent the same experimental batch.

.

2.3. Molecular Structure Characterization

Fourier Transform Infrared Spectroscopy (FTIR): The synthesized PCE samples were purified by precipitation in isopropyl alcohol, followed by washing and vacuum drying. The qualitative analysis of functional groups was conducted using a Nicolet iS20 spectrometer (Thermo Fisher Scientific, WI, USA) via the KBr pellet method.

Gel Permeation Chromatography (GPC): The molecular weight and distribution were determined using an Agilent 1260 Infinity GPC system (Agilent Technologies, CA, USA) equipped with a refractive index detector (RID). The mobile phase was a 0.1 mol/L NaNO₃ aqueous solution, eluted at a flow rate of 1.0 mL/min with a column temperature of 40 °C. The separation was performed using Agilent PL aquagel–OH mixed columns. Polyethylene glycol (PEG) standards with molecular weights ranging from 1000 to 20,000 Da were used for calibration. Key parameters, including M_w, M_n, polydispersity index (PDI = M_w/M_n), and the macromonomer conversion rate, were calculated from the GPC data. Given the broad screening nature of this study involving a large matrix of synthesis parameters, the data reported represents single-batch measurements validated by the continuity of the trends across the gradient series.

2.4. Performance and Mechanism Characterization

Cement Paste Fluidity: The fluidity of cement pastes was measured in accordance with the Chinese National Standard GB/T 8076-2012 [19]. A mixture was prepared using 300 g of reference cement and 87 g of water (water-to-cement ratio, w/c/C = 0.29) in a planetary cement mixer. The mixing procedure followed the GB/T 8076 standard: slow mixing for 120 s, a 15 s pause, followed by fast mixing for 120 s. The paste was then poured into a truncated cone placed on a glass plate. The dosage of PCEs was fixed at 0.1% (solid content) by mass of cement. The fluidity was recorded immediately after mixing (T0) and at 40 min and 80 min post-mixing to evaluate slump retention.

Surface Tension: The surface tension of the PCE aqueous solutions was characterized at 25 °C using a BZY-2 automatic surface tensiometer equipped with a platinum plate (Wilhelmy plate method). The measurements were performed on solutions with varying mass concentrations prepared in deionized water. The platinum plate was thoroughly cleaned and flamed between measurements to ensure accuracy.

Adsorption (TOC Method): The adsorption behavior of PCEs on cement particles was quantified using a Shimadzu TOC-L Total Organic Carbon analyzer via the depletion method. PCE solutions were mixed with cement, stirred for 10 min, and centrifuged at 8000 rpm for 10 min to obtain the supernatant. The supernatant was then diluted significantly to fall within the linear calibration range of the instrument. The TOC value of the supernatant (C) was measured and compared with the initial TOC value of the PCE solution (C₀). The amount of PCE adsorbed (Q, mg/g cement) was calculated using the equation Q = V(C₀ − C)/m, where V is the solution volume and m is the mass of cement.

Zeta Potential: The Zeta potential of the cement suspensions was determined using a Malvern ZS90 Zetasizer (Malvern Instruments, Malvern, UK) based on electrophoretic light scattering. To systematically investigate the influence of PCE dosage on particle surface charge, an automatic titration procedure was employed. The PCE aqueous solution was incrementally added to a fresh cement suspension (w/c = 0.5) under continuous stirring. The Zeta potential values were recorded as a function of the cumulative PCE dosage to evaluate the adsorption behavior and charge evolution of the cement particles.

Heat of Hydration: The hydration kinetics of the cement pastes were characterized using a custom-designed adiabatic temperature rise testing system. Freshly mixed cement pastes, prepared at a water-to-cement ratio (w/c) of 0.5, were immediately cast into a thermally insulated mold integrated with a high-precision temperature sensor. The temperature evolution profile was continuously logged for a minimum duration of 70 h to capture the complete hydration process, including the induction and acceleration periods.

3. Results and Discussion

3.1. Structural Confirmation of PCE Copolymers via FTIR

To confirm the successful copolymerization of EPEG and AA, the reference sample (A3) was characterized by FTIR. As illustrated in Figure 1, the broad absorption peak centered at 3436 cm⁻¹ is attributed to the stretching vibrations of the hydroxyl groups (-OH) at the terminals of the EPEG side chains and adsorbed water molecules. The absorption peak at 2887 cm⁻¹ corresponds to the symmetric and asymmetric stretching vibrations of the abundant methylene groups (-CH₂-) within the polyether side chains of EPEG.

The strong absorption peak observed at 1731 cm⁻¹ is assigned to the carbonyl stretching vibration (-C=O) of the carboxylic groups introduced by AA. Furthermore, the most intense peak appearing at 1111 cm⁻¹ represents the characteristic stretching vibration of aliphatic ether bonds (-C-O-C-), originating from both the main chain and the side chains of the EPEG macromonomer.

Crucially, no distinct absorption peak was observed in the vicinity of 1680 cm⁻¹, which is characteristic of the C=C double bond stretching vibration. This absence indicates that the double bonds in both the EPEG macromonomer and the AA comonomer were substantially consumed during the reaction. The FTIR spectrum thus confirms the formation of the expected molecular architecture, in which AA constitutes the polymer backbone while EPEG is successfully grafted as side chains, resulting in an EPEG-AA comb-like copolymer.

3.2. Regulation of Molecular Structure and Dispersion Performance via Process Parameters

To systematically elucidate the “synthesis–structure–property” relationships, five series of PCE samples (Series A–E) were synthesized. Their molecular architectures, macroscopic performances, and underlying working mechanisms were comprehensively analyzed.

3.2.1. Effect of Acid-to-Ether Molar Ratio (Series A)

The acid-to-ether molar ratio is the decisive parameter governing the charge density of the PCE backbone [20,21]. As indicated in the GPC data (Figure 2a), increasing the acid-to-ether ratio from 2:1 (A1) to 6:1 (A5) resulted in a steady increase in the M_w from 3.01 × 10⁴ Da to 5.52 × 10⁴ Da. Concurrently, the polydispersity index (PDI) broadened from 1.40 to 1.70.

This trend can be attributed to polymerization kinetics. Under a constant initiator concentration (implying a constant rate of radical generation), increasing the acid-to-ether ratio primarily elevates the total monomer concentration [M]. Since the rate of chain propagation (R_p) is proportional to [M], the ratio of propagation to chain transfer and termination reactions increases, leading to a higher degree of polymerization and thus a higher M_w. Additionally, the increased system viscosity at high monomer concentrations hinders the diffusion of growing macroradicals. This phenomenon is consistent with the classical auto-acceleration effect reported in concentrated radical polymerizations [22]. It leads to diffusion-controlled termination, where the reduced termination rate allows polymer chains to grow longer and more unevenly, resulting in a broader PDI.

Figure 2b demonstrates that increasing the acid-to-ether ratio significantly enhances the initial fluidity of the cement paste (from 204 mm for A1 to 255 mm for A5). Mechanism analysis via TOC adsorption and Zeta potential measurements (Figure 3a,b) reveals the cause: a higher acid-to-ether ratio leads to a marked increase in the adsorbed amount of PCEs on cement particles (from 1.50 mg/g to 2.40 mg/g) and a more negative Zeta potential. Interestingly, varying the acid-to-ether ratio resulted in negligible variations in the surface tension of the pore solution. This is consistent with the fact that Sample A5, despite its increased M_w, remains within the medium-to-high molecular weight range and lacks the distinct surface tension-reducing capability characteristic of low-molecular-weight PCEs.

Increasing the acid-to-ether ratio induces two simultaneous structural changes: (1) an increase in M_w, and (2) an increase in backbone charge density (-COOH groups). These factors synergistically enhance the adsorption affinity (TOC data) and electrostatic repulsion (Zeta potential), while the longer polymer chains provide stronger steric hindrance, thereby improving initial dispersion. This positive correlation between the acid-to-ether ratio and adsorption capacity aligns well with the anionic charge density mechanism reported in previous studies [6,20], confirming that a higher abundance of carboxyl groups facilitates the anchoring of polymer chains onto the cement surface. However, when the ratio is excessive (e.g., 6:1 in A5), although the initial fluidity is maximized, the slump retention deteriorates significantly (the 80-min loss rate reached 47.1%, compared to 28.6% for A3). This is attributed to the excessively high charge density and M_w, which cause rapid and excessive adsorption of PCEs during the initial mixing stage, quickly depleting the free PCEs in the interstitial solution. As hydration proceeds, the continuous formation of hydration products creates new solid surfaces, and the lack of reserve free PCEs to adsorb onto these new surfaces leads to significant fluidity loss. Therefore, an acid-to-ether ratio of 4:1 (A3) represents the optimal balance point between initial dispersion (high adsorption) and slump retention (preventing premature depletion).

3.2.2. Effect of Reaction Temperature (Series B)

Reaction temperature is the core parameter for regulating free radical polymerization kinetics [23]. As shown in Figure 4a, temperature serves as a potent tool for controlling M_w. Increasing the temperature from 0 °C (B1) to 40 °C (B5) caused a drastic decrease in M_w and a significant narrowing of the PDI. Kinetically, elevated temperatures accelerate chain initiation, propagation, and transfer reactions simultaneously. However, the activation energies for initiation and transfer are typically higher than that for propagation. Consequently, higher temperatures favor the reactions that “shorten” the chains (initiation and transfer) over the reaction that extends the chains (propagation), resulting in a reduced average chain length and a more uniform distribution.

Crucially, the macromonomer conversion rate exhibited a non-monotonic trend with temperature (Figure 4b). The conversion peaked (>95%) at 10–20 °C (B2, B3). At 0 °C (B1), the initiation rate was too slow, leading to incomplete reaction (~92.1% conversion). Conversely, at higher temperatures (30–40 °C, B4, B5), the rapid generation of free radicals led to the swift consumption of the highly reactive and fast-diffusing AA comonomer. This caused a rapid drop in AA concentration, leaving the bulkier, slower-diffusing EPEG macromonomers unreacted. This suggests that 20 °C (B3) is not only the balance point for performance but also the kinetic optimum for achieving high EPEG conversion.

The relationship between M_w and performance follows a classic structure–property relationship (Figure 4c). High-M_w PCEs synthesized at a low temperature (0 °C, B1) exhibited superior initial fluidity, whereas low-M_w PCEs synthesized at high temperature (40 °C, B5) showed poor initial fluidity. Mechanism analysis confirms that high-M_w PCEs possess higher initial adsorption capacity (Figure 5a) and more negative Zeta potential (Figure 5b), providing strong initial dispersion forces. Conversely, surface tension measurements revealed an opposite trend: a decrease in M_w significantly enhances the surfactant capability of PCEs. The low-M_w sample (B5) reduced the solution surface tension to a minimum (~60.3 mN/m), facilitating the wetting of cement particles, whereas the high-Mw sample (B1) showed a much weaker surface tension reducing capability.

Specifically, for high M_w PCEs synthesized at low temperatures (e.g., B1), the longer backbone and high anionic content result in increased adsorption, stronger electrostatic repulsion, and a larger steric radius. These factors collectively contribute to excellent initial dispersion. However, this comes with a trade-off: the excessive initial adsorption rapidly depletes free PCE molecules in the solution, leading to poor slump retention, as evidenced by a 48.1% loss at 80 min. Conversely, low M_w PCEs synthesized at high temperatures (e.g., B5) possess shorter backbones and fewer adsorption sites, resulting in lower adsorption and weaker repulsion, which translates to poor initial dispersion. Nevertheless, these polymers offer distinct advantages. Firstly, the lower initial consumption leaves a higher concentration of free PCEs available for sustained dispersion, thereby ensuring good retention with only an 11.6% loss. Secondly, their stronger surfactant effect significantly lowers surface tension, which aids in the wetting of cement particles.

The PCE synthesized at 20 °C achieved the best equilibrium among high conversion, excellent initial fluidity, and acceptable retention. Thus, M_w is identified as the central parameter for tuning EPEG-PCE performance.

3.2.3. Effect of Initiator and Chain Transfer Agent Dosage (Series C & D)

The dosages of initiator and chain transfer agent (CTA) serve as two additional key factors for regulating M_w. Increasing the initiator dosage or CTA dosage significantly reduced the M_w and narrowed the PDI of the PCEs (Figure 6a and Figure 7a). The mechanisms are distinct: increasing the initiator increases the number of starting radical chains, thereby reducing the average length per chain; increasing the CTA frequency terminates chain growth prematurely, also reducing M_w.

Notably, when the CTA dosage was as low as 0.5% (D1), the macromonomer conversion plummeted to 65.8%. The GPC chromatograms provided in the Supplementary Materials (Figure S1) reveal that sample D1 displays a severe bimodal distribution with a fraction of ultra-high molecular weight species. This indicates that insufficient CTA fails to control the high reactivity of EPEG, leading to localized uncontrolled polymerization (incipient gelation). This confirms that a sufficient amount of CTA is mandatory in EPEG low-temperature systems to ensure controllability and high conversion.

The structure–property relationship observed here aligns perfectly with that of the temperature series. Reducing M_w—whether via increased initiator (Figure 6c) or CTA (Figure 7c)—resulted in decreased initial fluidity but improved slump retention.

This relationship is further corroborated by the mechanistic data. For the initiator series, although the effect was less pronounced than for CTA, the trends remained consistent with the M_w-driven mechanism (Figure 8). Specifically, variations in initiator dosage induced only minor fluctuations in Zeta potential (Figure 8b) and surface tension (Figure 8c), reflecting the relatively smaller impact of initiator-regulated M_w changes on interfacial properties compared to the other parameters.

In contrast, for the CTA series (Series D), the mechanistic correlations were stark (Figure 9). As CTA dosage increased and M_w decreased, the initial adsorption amount on cement particles declined (Figure 9a), consistent with the positive correlation between polymer chain length and adsorption affinity. Consequently, the Zeta potential became less negative (lower absolute value) as M_w decreased (Figure 9b). Surface tension data (Figure 9c) further underscored the critical role of M_w: any parameter that reduced M_w (especially CTA) significantly lowered the solution surface tension.

Among the three parameters available for M_w regulation (Temperature, Initiator, CTA), Temperature (20 °C) and Initiator (2.5%) should be fixed to guarantee maximum macromonomer conversion (>95%) for kinetic and economic efficiency. Therefore, within the effective range (≥1.0%), CTA dosage is identified as the ideal, independent parameter for “fine-tuning” the M_w of PCEs. This allows for the customization of PCE products to meet specific engineering requirements (prioritizing either initial flow or retention) without compromising conversion efficiency.

3.2.4. Effect of Dropping Time (Series E)

The dropping time controls the instantaneous concentration of monomers and the reductant. As indicated in Figure 10, extending the dropping time from 40 to 120 min had a negligible impact on M_w, PDI, initial fluidity, and slump retention. The only deviation occurred at 40 min (E1), which showed slightly lower M_w and inferior fluidity. This is likely due to the excessively fast addition rate, which caused a high instantaneous concentration of AA, leading to AA homopolymerization or localized defects in copolymerization.

Similarly, dropping time had minimal influence on the microscopic mechanisms, as shown in Figure 11. Adsorption amount (Figure 11a), Zeta potential (Figure 11b), and surface tension (Figure 11c) all remained relatively stable across the variations. These results demonstrate the excellent process robustness of this low-temperature EPEG synthesis system, provided the dropping time exceeds 60 min. This is a significant advantage for industrial scale-up, as slight fluctuations in process parameters (e.g., pump rates) will not significantly alter the final product performance.

3.3. Effect of EPEG-PCEs on Cement Hydration and Gel Formation

To elucidate the influence of PCE molecular structure on the cement hydration process, the hydration kinetics of cement pastes incorporating Series B samples (synthesized at different temperatures and exhibiting significant differences in molecular weight) were evaluated via adiabatic temperature rise measurement.

As illustrated in Figure 12a, the main hydration exothermic peak of the reference sample appears at approximately 8 h. Upon the addition of any Series B PCEs, the main exothermic peak was delayed to beyond 9 h, indicating a distinct retarding effect of the EPEG-PCEs. This retardation is generally attributed to two mechanisms, the complexation of Ca²⁺ by the carboxyl groups on the PCE backbone [24,25], and the adsorption of PCE molecules onto cement particles (particularly C₃S and C₃A), forming a semi-permeable barrier adsorption layer that hinders water transport and ion exchange necessary for the nucleation of calcium silicate hydrate (C-S-H) gels [13,26,27], effectively prolonging the induction period before the onset of the acceleration stage.

Notably, the retardation effect exhibited a non-monotonic dependence on molecular weight. The most pronounced retardation was observed neither in the sample with the highest M_w (B1) nor the lowest M_w (B5), but in sample B3 (synthesized at 20 °C), which possesses an intermediate M_w. Sample B3 significantly delayed the heat flow peak to approximately 12 h and suppressed the peak intensity below that of the reference sample. This kinetic signature implies that the specific adsorption behavior of B3 significantly suppresses the growth rate of C-S-H gels after nucleation. This anomaly suggests that retardation is governed by the specific adsorption conformation and layer compactness rather than total adsorption amount alone. It is hypothesized that high-M_w PCEs (B1), despite their high adsorption capacity, adopt a “loop-and-tail” conformation to maximize steric hindrance; while this guarantees superior physical dispersion, it creates a relatively porous adsorption layer that allows some water and ions to diffuse to the cement surface, permitting a relatively faster precipitation of C-S-H gels. In contrast, intermediate-M_w PCEs (B3), characterized by optimized chain lengths and a narrow PDI, are hypothesized to adopt a flatter “train-rich” conformation or pack more densely on the surface of early hydration products [28]. This results in a more compact and impermeable passivation layer that effectively blocks the dissolution of C₃S and poisons the nucleation sites for C-S-H formation. Meanwhile, low-M_w PCEs (B5) exhibit the weakest retardation due to insufficient surface coverage resulting from their lower adsorption affinity.

Figure 12b presents the cumulative heat evolution over 70 h. The samples with high M_w (B1, B2) exhibited total heat evolution slightly higher than that of the reference sample. From the perspective of microstructure development, this enhanced heat evolution indicates that the superior steric dispersion prevents the formation of localized dense gel clusters often found in agglomerated pastes. Instead, it exposes more effective surface area, thereby promoting a more extensive and continuous formation of the C-S-H gel network in the later stages. Conversely, the samples with medium-to-low M_w (B3, B4, B5), which showed strong initial retardation, exhibited lower total heat evolution compared to the reference. This indicates that their intense adsorption and complexation effects not only delay the onset of hydration but also suppress the ultimate degree of hydration to a certain extent within the tested period.

4. Conclusions

This study systematically investigated the low-temperature synthesis of EPEG-based PCEs, establishing a clear “synthesis–structure–property” relationship. The principal conclusions are drawn as follows:

Utilizing the high reactivity of EPEG, a robust aqueous free radical copolymerization protocol was established. A reaction temperature of 20 °C was identified as the kinetic optimum, achieving a macromonomer conversion rate exceeding 95% under ambient conditions, thereby avoiding the side reactions often associated with high-temperature processes.

The M_w acts as the central regulator of performance. High-M_w PCEs provide superior initial dispersion driven by strong electrostatic repulsion and steric hindrance but suffer from rapid slump loss. Conversely, low-M_w variants, regulated efficiently by the CTA, significantly enhance slump retention by reducing the pore solution surface tension and maintaining a sustained polymer concentration.

The optimal balance between initial dispersion and retention was achieved at 20 °C with an acid-to-ether molar ratio of 4:1, 2.5% initiator, and 1.5% CTA. The synthesis process demonstrates excellent robustness with a broad dosing window (>60 min), providing a feasible route for consistent industrial scale-up.

Finally, it is noted that this study employed a single-factor experimental design. Consequently, potential interactions between synthesis parameters were not quantitatively evaluated and warrant further investigation in future studies. Furthermore, while the current results on cement paste provide a solid theoretical foundation, further validation in concrete mix designs is required to assess the practical applicability of these EPEG-PCEs in complex aggregate systems.