# Degradation of Cationic Polyacrylamide Flocculants upon Contact with Metal Surfaces During Rheological Measurements

N. Loukili, Ana Chitanu, L. Jossic, M. Karrouch, M. Oliveira, S. Guerin, Y.

Fayolle, P. Ginisty, A. Magnin, Yahya Rharbi

<figure><img src="image_1.png" /></figure>

► To cite this version:

N. Loukili, Ana Chitanu, L. Jossic, M. Karrouch, M. Oliveira, et al., Degradation of Cationic Polyacrylamide Flocculants upon Contact with Metal Surfaces During Rheological Measurements. Journal of Polymers and the Environment, 2024, 32 (12), pp.6636-6649. 10.1007/s10924-024-03369-4. hal-04801038

HAL Id: hal-04801038

https://hal.science/hal-04801038v1

Submitted on 24 Nov 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

Public Domain
Degradation of Cationic Polyacrylamide Flocculants upon Contact with Metal Surfaces during Rheological Measurements

This Accepted Manuscript (AM) is a PDF file of the manuscript accepted for publication after peer
review, when applicable, but does not reflect post-acceptance improvements, or any corrections. Use
of this AM is subject to the publisher's embargo period and AM terms of use. Under no circumstances
may this AM be shared or distributed under a Creative Commons or other form of open access license,
nor may it be reformatted or enhanced, whether by the Author or third parties. By using this AM (for
example, by accessing or downloading) you agree to abide by Springer Nature's terms of use for AM
versions of subscription articles:

https://www.springernature.com/gp/open-research/policies/accepted-manuscript-terms

The Version of Record (VOR) of this article, as published and maintained by the publisher, is available online at: https://doi.org/10.1007/s10924-024-03369-4. The Version of Record is the version of the article after copy-editing and typesetting, and connected to open research data, open protocols, and open code where available. Any supplementary information can be found on the journal website, connected to the Version of Record.

For research integrity purposes it is best practice to cite the published Version of Record (VOR), where
available (for example, see ICMJE's guidelines on overlapping publications). Where users do not have
access to the VOR, any citation must clearly indicate that the reference is to an Accepted Manuscript
(AM) version.
# Degradation of Cationic Polyacrylamide Flocculants upon Contact with Metal Surfaces during Rheological Measurements

N. Loukilia, Ana Chitanu, L. Jossic, M. Karroucha, M. Oliveira, S. Guerin, Y. Fayolle, P. Ginisty, A. Magnin, Y. Rharbi*

**Keywords** : Polymer degradation, flocculating polymer, Cationic polyacrylamide, polymer metal interaction, polyelectrolyte rheology.

Corresponding author : yahya.rharbi@univ-grenoble-alpes.fr
Yahya Rharbi [https://orcid.org/0000-0003-3091-2775](https://orcid.org/0000-0003-3091-2775)

## ABSTRACT

This research shows that cationic polyacrylamide (CPAM) flocculants, widely used in wastewater treatment, are susceptible to degradation when in contact with various metallic surfaces. This is evidenced by the investigation of the evolution of CPAM's rheological properties during degradation within metallic Couette tools, observing a transition from elastic to viscous behavior. The degradation is clearly evident on various metallic surfaces, while thermoplastic surfaces have significantly less effect on CPAM degradation.

Key findings indicate that chemical interactions, rather than mechanical stress, are the primary cause of degradation, and this reaction is activated by temperature. Techniques such as Fourier Transform Infrared spectroscopy, Nuclear Magnetic Resonance analysis, and polyelectrolyte titration provided

<footnote>a Univ. Grenoble Alpes, CNRS, Grenoble INP, LRP, F-38000 Grenoble, France</footnote>

<footnote>b SIAAP, Direction Innovation, Colombes, France</footnote>

<footnote>c Université Paris-Saclay, INRAE, PRocédés biOtechnologiques au Service de l'Environnement, 92761 Antony, France</footnote>

<footnote>d. IFTS, Rue Marcel Pagnol 47510 Foulayronnes, France</footnote>
some initial understanding of this mechanism. This research offers valuable insights into CPAM's interactions with metal surfaces, with important implications for environmental and industrial applications, and establishes the appropriate protocol for characterizing the intrinsic rheological properties of these materials.

# 1. INTRODUCTION

Addressing the need for efficient wastewater treatment processes has become increasingly vital due to the growing water consumption driven by industrialization and population growth [1-9]. Therefore, various approaches are being considered to enhance treatment efficiency and minimize environmental impacts. One such approach involves using flocculating agents to agglomerate sludge particles into large flocs [2,7,10]. Polymers are commonly used as flocculating agents due to their ability to form robust flocs, facilitate separation, and enhance sludge thickening and dewatering [7]. An example of such a polymer is high molecular weight Cationic polyacrylamide (CPAM), which is produced by copolymerizing acrylamide with 10 to 80% of the cationic ester acryloyloxyethyltrimethyl-ammonium chloride [11,12]. These polymers, with varying molecular weights (Mw) and architectures, are specifically designed to control the flocculation process and influence the resulting floc morphology. [4,13-15].

Flocculation is influenced by several characteristics of the polymer, such as its charge (non-ionic, anionic, cationic, or amphoteric), chemical composition, molecular weight, architecture and rheology. [4,13,16-22] The chemical composition of the polymer determines how it interacts with sludge, influencing processes such as charge neutralization, charge patching, bridging, and depletion. [4,13,16-19]. The molecular weights (Mw) and architectures (linear, branched, or crosslinked) of the polymer significantly influence the flocculation dynamics [10,13,15,23-25]. High Mw linear polymers bridge particles into large flocs, while lower Mw polymers compress the repulsive double layer, enhancing particle coagulation through van der Waals forces, resulting in smaller aggregates. [4,7,26,27]. Branched polymers produce flocs with higher mechanical strength compared to linear ones [13,28]. The rheology of the polymer emerges
as a critical factor in the flocculation process, largely due to how it influences
polymer-sludge mixing. [20-22,24,29,30]. Indeed, because flocculation is
primarily a collision-controlled process, mixing becomes critical to promote
contact between particles and polymer, particularly in orthokinetic flocculation in
the case of non-Brownian sludge particles. This ensures that particles sufficiently
contact each other to form stable flocs [20,31-35]. The importance of polymer
rheology becomes particularly evident when assessing the viscosity ratio between
the sludge and the polymer, directly affecting the mixing process [36-38].

Given this, the sensitivity of flocculation to polymer structure and rheology
highlights the importance of maintaining the polymer's stability throughout the
process to ensure effective solid-liquid separation in wastewater treatment.
Indeed, this sensitivity raises questions about how eventual polymer degradation
might impact both the flocculation mechanism and other aspects of the
wastewater treatment process [39]. This concern is particularly significant for
polyacrylamide-based (PAM-based) polymers, which are known to degrade under
various conditions[40-44]. For example, PAM-based polymers are prone to
mechanical degradation when exposed to turbulent flow in channels, leading to
chain scission [45-48]. Studies have indicated that this degradation is primarily
induced by elongational stress, particularly at the entrance of the channel [45].
Additionally, PAM can undergo chemical degradation, resulting in chain scission,
often initiated by free radicals like hydroxyl radicals. These radicals can be
generated through interactions between oxygen and dissolved transition metals,
such as Fe₂⁺, or through persulfate activation [49]. The degradation of cationic
PAM can also occur due to the hydrolysis of its ester groups, leading to a loss of
cationic charge [42,50]. Hydrolysis may further give rise to the formation of
anionic carboxylate groups and to a change in the polymer's chain conformation,
thereby altering its flocculation capabilities. This degradation is more likely to
occur under neutral or basic conditions and is influenced by the polymer's charge
content. Furthermore, the degradation of flocculating polymers, particularly in
the case of CPAM, raises environmental concerns such as contamination of local
water supplies and has led to the implementation of stringent regulations in water
treatment [49,51-55]. The degradation of PAM-based polymer can also be a
deliberate and developed process to address specific environmental needs. In the
context of wastewater treatment, several studies have explored polymer
degradation using various methods, including biological, physical, or chemical
treatments [56-60]. In other applications, such as drilling technology, there is a
necessity to degrade the polymer in a controlled and environmentally friendly manner to manage the rheological properties during specific stages of the process. For instance, post-drilling fluid removal often requires the fluid to be liquefied through polymer degradation [60].

Given the above, it becomes necessary to deepen our understanding of CPAM rheology and how it can be affected by degradation, particularly in the context of wastewater treatment. This raises two main concerns: the intrinsic rheological characteristics of CPAM and the conditions that promote its degradation.

It is essential to precisely measure the intrinsic rheological properties of high Mw CPAM while preventing degradation in rheometry tools. This information is particularly important for scaling installations in wastewater treatment plants. However, this measurement is not as straightforward as it may seem in standard rheometry procedure, as measuring high Mw CPAM in the entanglement regime necessitates prolonged periods to acquire zero shear viscosity and reptation time. Indeed, extended exposure in rheometry tools can potentially lead to CPAM degradation, posing challenges for precise measurements [49,50]. Despite the critical importance of this information for application fields, it remains poorly studied, highlighting the need for comprehensive characterization of rheological methods to acquire such data.

This study aims to report the rheological behavior of various cationic polyacrylamides (CPAM) used as flocculants in wastewater treatment when in contact with metallic surfaces and how this alters their properties. Initially, a protocol is developed to investigate several aspects of the rheology in fresh CPAM, including low shear viscosity, shear thinning behavior, and relaxation times, especially within the entangled semi_dilute regime.

The study then examines how the materials used for rheometry tools, specifically stainless steel, dural and thermoplastic, affect the evolution of CPAM's rheological properties. It investigates the degradation kinetics of CPAM during measurements with metallic tools and under static conditions. Additionally, it explores the effects of temperature, polymer concentration, polymer composition, and shear rate on rheology over time. Finally, various analytical methods such as NMR, IR, conductimetric titration, and cationic group analysis are used to understand the mechanisms responsible for the changes in CPAM properties.
## 2. MATERIAL AND METHODS

### 2.1 Material

The flocculating polymer is a high molecular slightly crosslinked cationic polyacrylamide FLOPAM™ MV 9104 from SNF Floerger (France), Superfloc XD 5500, XD 5600, C492-HMW from KEMIRA. These are polymers historically used as flocculant in the sludge treatment lines of wastewater treatment plants managed by SIAAP in Paris region (France). Sodium hydroxide (Sigma Aldrich, CAS 1310-73-2) and hydrochloric acid (Sigma Aldrich, CAS 7647-01-0) were used for conductimetric titration. Potassium polyvinyl sulfate (KPVS, Sigma Aldrich, CAS 26182-60-5), orthotoluidine blue (OTB, Sigma Aldrich, CAS 95-53-4), and cyltrimethylammonium bromide (CTAB, Sigma Aldrich, CAS 57-09-0) were used as received.

Polymer solutions with a concentration of 5 g/L were prepared in a polyethylene container by dissolving CPAM in doubly deionized water. The mixture was agitated at 250 rpm for 45 minutes using a custom-made stainless-steel mixing paddle. To counteract the upward motion of the fluid caused by the Weissenberg effect, a polyethylene blade was attached to the paddle shaft to continually scrape and push the fluid downwards. After the mixing process, the solution was left to rest for 1 hour before further use.
## 2.2 Characterisation methods

The polymer was analysed using NMR spectroscopy on an Avance III 400 Mhz Bruker (Bruker BioSpin, Germany), which allowed for the identification of the main characteristics of cationic polyacrylamide (CPAM). Fourier transform infrared (FTIR) analysis was performed using a Perkin Elmer Spectrum (Perkin Elmer, Waltham, MA, USA) and compared to the spectrum of cationic polyacrylamide. The zeta potential was measured at 25 °C using the Zetasizer Nano Series (Malvern Instruments, Malvern, UK), revealing that MV9104 has a positive charge with a surface potential ζ value of approximately +50 mV.

The polyelectrolyte titration of the CPAM was carried out following the method described in [50,61]. Initially, the charge density of the KPVS solution was determined using CTAB as a model for cationic groups. To achieve this, three drops of OTB solution were added to CTAB solution (0.36 g/L) and the resulting solution was titrated with KPVS until the color transitioned from blue to red-violet. A blank sample, containing the same amount of OTB indicator in water, underwent titration using the same procedure. The difference in the amount of KPVS used for the two titrations was employed to estimate the charges of the KPVS. Afterward, the KPVS (0.18 g/L) was utilized to titrate the CPAM (0.5 g/L), employing the previously mentioned procedure to estimate the quantity of accessible cationic groups.

## 2.3 Rheological characterization

### 2.3-1 Tools for rheological characterization.

The rheological properties of CPAM were assessed using two distinct rheometers. The DHR3 (TA Instruments) was utilized with three different Couette geometries (Concentric cylinders). The initial configuration featured a 28 mm rotor made of anodized dural and a 30 mm stator made from a dural. The second Couette setup employed a 30 mm stator and a 28 mm rotor, both fabricated from stainless steel (316L). The chemical analysis of the tool materials is provided in the supporting information (Table S1). The third Couette variant consisted of a 43mm stator lined with PVC and a 40mm rotor, available in various materials: i) dural, ii) copper, iii) stainless steel (S316L), iv) rubber, v) acrylic, and vi) PVC.
In addition, the HAAKE MARS rheometer (Thermo Fisher) was used, equipped
with a Couette assembly consisting of a CCB43/SS/SE cup and a Z38 rotor, both
constructed of stainless steel.

Roughness of the tools was 1 mm for striated tools such as CCB43/SS/SE, 500 µm for the steel Couette tools made from stainless steel (S316L), and from 1 µm to 3 µm for the other Couette tools.

2.3.2. Rheological characterization protocols for fresh polymer.

To acquire rheological properties for fresh, non-degraded polymer, a Couette
geometry was used, consisting of a PVC-lined stator with a diameter of 43 mm
and a PVC-coated rotor (40 mm). This tool was previously tested and proven to
induce minimal effects on the polymer, allowing long-term measurements at
temperatures around 23 °C for more than 12 hours. Measurements were
conducted for polymer concentrations ranging from 0.5 g/L to 5 g/L at 23 °C.

Dynamic frequency sweep measurements were carried out to assess the
viscoelastic properties of CPAM. These measurements spanned a frequency range
from 0.001 to 10 Hz, and the strain was maintained at 10%, a value confirmed to
be within the linear viscoelastic regime for all the investigated polymer
concentrations of 0.5 and 5 g/L.

The flow behavior of CPAM was examined by measuring the shear stress (σ) across shear rates (γ) from 0.001 to 100 s⁻¹, ensuring each step had adequate time to achieve a pseudo-steady state. The time required to reach this pseudo-steady state varied, spanning from 30 seconds to 20 minutes, and was dependent on the applied shear rate. The dynamic viscosity (η) was calculated by η = σ/γ.

2.3.3. Rheological characterization protocols during *in situ* polymer degradation

To acquire *in situ* time evolution of rheological properties during polymer
degradation, various Couette geometries, as described above, were utilized at
different temperatures and concentrations, along with various types of
solicitation. The rheometric protocols were selected to provide the best possible
measuring precision taking into consideration the kinetic of the degradation. The
various protocols are listed in Figure S1. In one series of experiments, the polymer
solution underwent repeated shear scans with shear rates ranging from 0.001 to
100 s⁻¹, and oscillation tests with frequencies between 0.001 and 10 Hz, over
periods extending up to 48 hours (Figure S1a). This method was suitable for
analyzing slowly evolving polymer properties. For situations where the polymer
properties changed more rapidly, the shear rate range was limited to 0.01 s⁻¹ to
10 s⁻¹, and the frequency range was set to 0.01 Hz to 10 Hz (Figure S1b). The
strain was maintained at 10%, within the linear viscoelastic regime. In another
experimental arrangement, the polymer's temporal evolution was continuously
monitored by measuring the shear stress over time at a constant shear rate of
0.01 s⁻¹, providing an estimate of the viscosity at the Newtonian plateau. In
addition, measurements at a constant frequency of 1 Hz were continuously
conducted to determine the temporal evolution of the polymer's elastic and loss
moduli at this frequency.

In another experiment, the time evolution of the polymer's rheological
properties was assessed by applying a constant shear rate of 0.01 s⁻¹ to determine
the viscosity at the Newtonian plateau every 5 minutes. Between measurements,
the solution was either left to rest without any mechanical solicitation or
subjected to constant shear at shear rates ranging from 0.1 s⁻¹ to 1000 s⁻¹ (Figure
S2)..

Degradation was investigated at temperatures ranging from 30 °C down to 5
°C, focusing on typical environmental temperatures for wastewater treatment. A
compromise was found between the precision of rheological measurements and
the kinetics of degradation, and 30 °C was set as the upper limit for this
experiment to obtain rheological measurements with the necessary precision.

2.3.4. Rheological characterization protocols during *ex situ* polymer degradation

Alternatively, other experiments were carried out to acquire information on
the *ex situ* time evolution of rheological properties during polymer degradation in
the absence of mechanical solicitation. For this purpose, a custom-built setup was
employed featuring concentric cylinders designed to replicate the Couette
geometry used in rheometry measurements. The arrangement consisted of a
cylindrical polyvinyl chloride (PVC) tube with a 43 mm internal diameter and a 40
mm cylinder insert, made of either PVC or dural.
The procedure involved introducing a 100 ml polymer solution into the tube, with the dural or PVC insert centered to create a 1.5 mm gap. Subsequently, the solution was left in the container at room temperature for a specific duration, ranging from 0 to 24 hours. At 30-minute intervals, 7 ml of the solution was extracted from the tube and subjected to rheometric analysis using the previously described protocols.

### 2.3.5 Capillary breakup rheometry.

The viscoelastic relaxation times (λ) of polymer solutions were measured using the Capillary Breakup Extensional Rheometer (CaBER). This technique involves stretching a liquid sample held between two plates (4 mm diameter, 0.5 mm gap) until capillary forces destabilize it, causing the liquid to pinch and break. A high-speed camera (Olympus i-SPEED 3) captured the process at up to 5000 frames per second. The filament's thinning, influenced by capillarity and elasticity, follows an exponential decay in radius (R), related to []:

$$R_{\min} \propto \exp\left(-\frac{t}{3\Delta}\right) \qquad (\text{Eq.1})$$

This method was used to estimate the relaxation time for solutions with concentrations between 1 and 5 g/L before and after degradation [62,63].

### 2.3.4 Quantification of different measurement uncertainties.

The uncertainty of the measurements depends on several experimental aspects: 1) Polymer preparation: Due to the extremely high molecular weight (Mw) of these polymers, which is in the range of several million kg/mol, and their sensitivity to degradation, polymer preparation is critical. The preparation procedure yields an dispersion in the measured low shear viscosity of []10%. 2) Rheometric measurement tools: The precision of the DHR3 rheometer depends on the state of the polymer. For freshly prepared polymers, the stress is high enough to require precise viscosity measurements. However, when the viscosity drops to a few times that of water, the error can increase up to []30% in the worst cases. Because low shear viscosity is used in the analysis, the uncertainty is less than []10%.
The effect of the tools on the degradation was quantified by examining the kinetics, particularly the characteristic time for degradation. At low temperatures, when degradation is slow compared to the rheological measurements, the precision of low shear viscosity measurements is sufficient. On the other hand, at higher temperatures close to 30 °C, the degradation time is of the same order of magnitude as the measurement time, and therefore the uncertainty increases. In all cases, the characteristic time is obtained with less than 20% uncertainty, leading to a reliable estimation of the activation energy as shown in Figure 10.

# 3. RESULTS AND DISCUSSION

## 3-1. Rheology of the polymer during the degradation process

### 3.1.1 Rheology of fresh polymer

Figure 1a illustrates the relationship between shear rate (γ̇) and dynamic viscosity of aqueous CPAM solution at different concentrations. The flow behavior closely fits to the Carreau model (Eq. 2) allowing the determination the zero-shear viscosity (η₀), from the Newtonian regime at low shear rates, the relaxation time (τ_shear) governing the transition from Newtonian to shear-thinning behavior, and the power law index (n) of shear-thinning regime. The Carreau fits are limited to shear rates up to γ̇= 100 s⁻¹, as flow curves beyond this range may be influenced by the effect of Taylor instabilities. The Carreau fit is performed in two ways: the η₀ estimated from the Newtonian regime is used as a fixed parameter to estimate τ_shear and n using the least squares method. Alternatively, n is independently estimated beyond the Newtonian regime through a power law fit, and the obtained value can then be used to estimate η₀ and τ_shear.

$$ \Box = \Box_0 \left( 1 + \left( \Box_{\text{shear}} \dot{\gamma} \right)^2 \right)^{\frac{n-1}{2}} \qquad (\text{Eq.2}) $$

The specific viscosity η_sp (η_sp = (η₀ - η_w)/η_w with η_w the water dynamic viscosity) of the MV9104 polymer increases with the increase in [CPAM], following a power law relationship with an exponent of approximately 1.48 within the concentration
range of 0.5 to 5 g/L (Figure 1b diamond). This exponent refers to the behavior of polyelectrolytes in the entangled semi_dilute regime, where the typical slope is predicted to be around 1.5 [64–71]. Indeed, in polyelectrolytes, η<sub>sp</sub> demonstrates different power law relationships across various regimes: C<sup>0.5</sup> in the unentangled semi-dilute regime, C<sup>3/2</sup> in the entangled semi-dilute regime, and C<sup>15/4</sup> in the concentrated regime [68]. Conversely, in neutral polymers within a good solvent, η<sub>sp</sub> varies as C<sup>1</sup> in the unentangled semi-dilute regime and C<sup>15/4</sup> in the entangled semi-dilute regime [68,72].

The concentration regime can also be roughly predicted based on the proximity to the critical overlap concentration C* or the inverse of intrinsic viscosity [η]. Indeed, η₀ was found to be primarily dependent on the product of concentration and [η] (C[η]) in various acrylamide polymers in brine solution [72]. Three concentration regimes can be identified based on the C[η] value: the dilute regime for C[η] < 1, a semi-dilute unentangled for 1 < C[η] < 10, and the semi-dilute entangled regime for C[η] > 10. The estimation of [η] and C* was based on the η₀ values obtained in the diluted regime. Initially, an empirical estimation of C* was performed by assuming η₀ to be twice the viscosity of water, resulting in C* = 0.0083 g/L and [η] = 93 L/g, satisfying the condition of C*[η] = 0.77 [73,74]. Additionally, the Wolf method was employed, yielding an intrinsic viscosity [η] of 77.7 L/g and a corresponding C* value of 0.0099 g/L [75]. Lower intrinsic viscosity values were obtained from other experiments using the Ubbelohde-type viscometer. It is important to note that the relatively high shear rate utilized in the Ubbelohde-type viscometer deviated from the condition of zero shear viscosity. Therefore, these experiments were conducted solely for the purpose of comparison. Consequently, the investigated polymer concentration is likely to fall within the semi-dilute entangled regime, with C[η] ranging between 50 and 500, which is consistent with the exponent of η<sub>sp</sub> vs. concentration [72]. Similar result is found for all the CPAM investigated in this study.

Oscillatory solicitations were performed using a rotational rheometer to investigate the viscoelastic properties of these polymer solutions (Figure 5). Within the concentration range of 1 g/L to 5 g/L, all cases exhibited a viscoelastic response above a certain crossover frequency (0.01 Hz), where the storage modulus (G') was greater than the loss modulus (G''). The crossover frequency ranged from 0.001 Hz to 0.01 Hz, corresponding to reptation times ranging from 100 to 1000 s. It is important to acknowledge that further control and quantitative analysis are required to address polymer degradation within the rheometer tool
and its impact on the sensitivity of the crossover time. However, these aspects
are beyond the scope of this paper. All examined polymers demonstrated a similar
behavior, with slight variations in the position of the crossover frequency.

<figure><img src="image_2.png" /><figcaption>**Figure 1.** a) Viscosity of fresh MV9104 as a function of shear rate for various polymer concentrations in DHR3 using a Couette geometry. Measurements were conducted at 23 °C. b) Specific viscosity, calculated from the viscosity value taken at the Newtonian plateau, plotted against concentration for fresh MV9104 solution (diamond) and degraded polymer in a Couette system (triangle).</figcaption></figure>

**3.1.2 Rheology of CPAM during degradation**

*Time evolution of the viscoelastic behavior during polymer degradation.* Each curve in Figure 2 illustrates the viscosity (η) plotted against the shear rate (γ) for a CPAM (MV9104) solution at a concentration of 5 g/L in a Couette geometry constructed from either stainless steel or dural at various time intervals. Following each measurement, the viscosity values decrease across all applied shear rates (Figure 3), and the Newtonian plateau shifts towards higher γ values. All these plots fit well to the Carreau model, indicating a reduction in the low shear viscosity (η₀) and the characteristic time (τ_shear), as well as an increase in the parameter (n) (Figure 4).

These observations strongly indicate that rheological measurements within
the Couette apparatus lead to fluidification of the aqueous CPAM solution. This
phenomenon is likely attributed to modifications in the polymer structure or
interactions between polymer chains, potentially resulting from shear-induced
degradation, confinement, aging, or chemical reactions with the environment.
<figure><img src="image_3.png" /><figcaption>**Figure 2.** a) Viscosity versus shear rate for a 5 g/L CPAM solution (MV9104) measured in a stainless-steel Couette geometry at 20 °C using a Mars rheometer (Rotor Z38S, Stator CCB43/SS/SE), with measurements taken every 3 hours. b) Viscosity measurements of the same CPAM solution conducted in a dural Couette within a DHR3 rheometer at 23 °C, with a 1 mm gap and 30-minute intervals between readings. For both graphs, the upper curve in this graph also corresponds to the fresh CPAM solution, and the lower curve to the fully degraded solution.</figcaption></figure>

Due to its sensitivity to the molecular weight of the chain and interchain interactions, the low shear viscosity undergoes noticeable changes during any structural transition of the polymer. In most experiments within this study, η₀ follows a similar trend over time: initially, it remains constant, forming a plateau for a duration dependent on temperature and the nature of the metal. Subsequently, it decreases by several orders of magnitude. For instance, in a MV9104 solution with a concentration of 5 g/L, it decreases from more than 50 Pa·s to less than 0.5 Pa·s (Figure 3a, Figure S1).

The flow index n increases during the structural evolution of the polymer, from a value of 0.2 to approximately 0.6 over time (Figure 4a). As previously observed in similar polymers, n can be described by a master curve using [η] C. This implies that the intrinsic viscosity [η] increases over time, resulting in a reduction of n [72]. On the other hand, τ_shear decreases over time, transitioning from approximately 50 seconds to a few milliseconds (Figure 4b). Moreover, this time can also be described by a master curve using [η]C and the relaxation time
in the dilute regime (τ<sub>d</sub>). This provides additional confirmation that [η] increases
during the evolution of the polymer structure, resulting in a reduction in τ<sub>shear</sub> and
eventually in τ<sub>d</sub> [72].

At the end of degradation, the polymer solution undergoes several dilutions and
is then analyzed using a rotary rheometer, which is subsequently compared to the
fresh solution. The η<sub>sp</sub> vs. [CPAM] exhibits power-law behavior with an index of
0.61 (Figure 1b), closely resembling the Fuoss index of 0.5 [76], indicating an
unentangled polymer structure. Additionally, the critical overlap concentration
(C*) for fully degraded CPAM is determined to be 0.11 g/L, representing a tenfold
reduction compared to the concentration in the fresh polymer solution (Figure
1b).

<figure><img src="image_4.png" /><figcaption>**Figure 3.** a) Zero shear viscosity (η₀) calculated from the Newtonian plateau plotted against residence time for a 5 g/L CPAM (MV9104) solution measured in a dural Couette. Measurements were conducted in DHR3 at 15 °C. b) Estimated evolution of the molecular weight of CPAM during degradation, assumed based</figcaption></figure>
on the dependence of η₀ as M<sub>w</sub><sup>3</sup> (□) or M<sub>w</sub><sup>1</sup> (□). The evolution of M<sub>w</sub> during degradation is also compared to that estimated from the evolution of the reptation time (□<sub>r</sub>) as M<sub>w</sub><sup>3</sup> (□).

<figure><img src="image_5.png" /><figcaption>**Figure 4.** a) The shear thinning index (n), b) the relaxation time (□<sub>shear</sub>) plotted against the residence time. The values of n and □<sub>shear</sub> are estimated from the fit of the viscosity *vs*. shear rate in Figure 2a to the Carreau model.</figcaption></figure>

Repeated oscillatory solicitations within dural or stainless steel Couette were used to quantify the time evolution of the viscoelastic properties of CPAM (MV9104) in the linear regime, with a 10% deformation (Figure 5). The dependence of both G' and G" on frequency continuously evolve from a situation where G'> G" for most of the investigated frequency to a situation where G">G' for all accessible frequency. This is clearly manifest in the evolution of the crossover frequency between G' and G", which shifts to higher values over time. The shift of the crossover frequency of G' and G" towards higher frequencies suggest that the CPAM solution evolves from viscoelastic fluid to viscous fluid. Over an extended duration, the solution experiences a significant alteration of its elastic properties, as indicated by G' falling below G" across all accessible frequencies. In Figure 6, it is evident that G' at 1 Hz undergoes a slight initial decrease, followed by a rapid decline to values smaller than G" at a specific time, contingent upon various experimental conditions. In contrast, G" initially increases and then decreases rapidly (further explanation required). The reptation time (τ<sub>r</sub>), derived from the inverse of the crossover frequency, follows a similar trend to G'; it initially decreases in the early stages and subsequently experiences a rapid and substantial decrease by orders of magnitude.
Additionally, the CPAM solution experiences a substantial reduction in its
elongational viscosity throughout this process. In fact, the fresh CPAM solution
forms lengthy filaments during stretching, a characteristic feature of significant
elongational viscosity (video S2a). However, under this treatment, the same
solution breaks into droplets (video S2b). In the study of freshly prepared MV9104
polymer solutions, relaxation times were observed to increase with increasing the
concentration of the polymer. For a concentration of 1 g/L, the relaxation time
exceeded 150 milliseconds. At a concentration of 5 g/L, the relaxation time
extended to more than one second, leading to challenges in achieving stable
measurements. In contrast, for fully degraded polymer within Dural Couette, the
relaxation times decreased drastically, falling below the detection capability of
our instruments, indicative of significantly lower elongational viscosity.

<figure><img src="image_6.png" /><figcaption>**Figure 5.** Storage modulus G' and loss modulus G" plotted versus frequency for a CPAM solution (MV 9104) at 5g/L under oscillatory solicitation with a 10% amplitude in a dural Couette with a 1 mm gap at 23 °C in DHR3. The solution was measured at different times, indicated from top to bottom: 0 h, 1 h, 2.5 h, and 3.5 h, with a resting period between measurements.</figcaption></figure>
<figure><img src="image_7.png" /></figure>

**Figure 6.** Storage modulus G' (□) and loss modulus G" (■) at 1 Hz and the reptation time τ<sub>r</sub> (■) plotted against time for MV9104 at 5 g/L. The measurements were carried out under oscillatory solicitation with a 10% amplitude by cyclically varying the oscillation frequency at different time intervals using a dural Couette in DHR3. τ<sub>r</sub> is taken as the inverse of the crossover frequency for G' and G".

**3.1.3 Parameters affecting the degradation**

The behavior of various flocculating CPAM polymers was characterized
using a low shear rate to quantify the evolution of zero shear viscosity over time
within a dural Couette. As depicted in Figure 7, it is apparent that the viscosity of
all tested CPAMs decreases in a manner closely resembling that of MV9104 over
time. This observation implies a consistent trend in the degradation of these
polymers, suggesting a general degradation process for all these chemicals,
regardless of their individual characteristics.

<figure><img src="image_8.png" /></figure>

**Figure 7.** Evolution of the normalized low shear viscosity *vs.* time for different CPAM samples (indicated in the legend) at 5 g/L. η₀ is measured in the Newtonian
plateau at 23 °C in a dural Couette in DHR3. $\eta_0(t)$ is normalized by $\eta_0(0)$ before degradation.

To investigate the impact of mechanical stress on polymer degradation, various stress procedures were employed (Figure 8). In the initial experiment, the solution was placed at rest in a Couette geometry with a 1-2 mm gap, and the $\eta_0$ was measured at the Newtonian plateau by applying a low shear rate of 0.01 s⁻¹ for 100 seconds every 5 minutes (Figure S2). In the second experiment, the same procedure was repeated, but the rest phases were substituted with a constant shear rate ranging from 0.1 s⁻¹ to 1000 s⁻¹ (Figure S2). The results suggest that the decline in $\eta_0$ primarily occurred during the rest phase, indicating that shear plays a limited role in polymer degradation.

<figure><img src="image_9.png" /><figcaption>**Figure 8.** Low shear viscosity ($\eta_0$) vs. time for MV9104 at 5 g/L in a dural Couette at 23 °C measured at a shear rate of 0.01 s⁻¹ every 5 minutes. The sample is either left at rest between measurements or sheared at 1000 s⁻¹.</figcaption></figure>

To investigate the cause of this degradation process, a series of experiments were conducted using different Couette materials. The results indicate that the nature of the Couette surface significantly influences degradation, with dural surfaces causing faster degradation than stainless steel surfaces (Figure 9). When using thermoplastics like PVC or acrylate as the Couette surface, the polymer remains unchanged over an extended period. For example, the polymer remains almost unchanged over a period of three weeks within a PVC Couette at 5 °C.

This study was extended to other Couette geometries made from different materials (e.g., DHR3 made of stainless steel), DHR3 (made of dural), and other home-fabricated Couette geometries made from iron steel, copper, and
galvanized steel. A 1 mm gap was used for all these experiments. The results consistently show that polymer degradation occurs within metallic Couette geometries, with some dependence of the degradation kinetics on the nature of the metal.

<figure><img src="image_10.png" /></figure>

**Figure 9.** Time evolution of the low shear viscosity of an MV9104 solution (concentration: 5 g/L) measured in different Couette geometries utilizing various stator/rotor materials: dural/dural, stainless steel/stainless steel, PVC/dural, acrylic/rubber, and PVC/PVC. The low shear viscosity is measured at a shear rate of 0.01 s⁻¹. Included in the plot are data from *ex situ* degradation experiments (labeled as *ex situ*) employing a custom-developed setup that mimics the Couette geometry, composed of a concentric PVC tube and dural cylinder (refer to the experimental procedure for details). These measurements are juxtaposed with those from the solution stored with a large polyethylene container (large beaker). In the last two specific cases, the solution was transferred to a PVC/PVC Couette configuration to determine the low shear viscosity at various time intervals.

Temperature effectively activates polymer degradation. As depicted in Figure 10, the reduction in η₀ of MV9104 within a dural/dural Couette at different temperatures exhibits a similar trend, accelerating with increasing temperature.

Temperatures were tested from 30 °C down to 5 °C, focusing on typical environmental temperatures for wastewater treatment. Higher temperatures yield rapid degradation within the rheometer, even faster than the time necessary to perform a precise rheological measurement within the Newtonian plateau. A temperature of 30 °C was set as the upper limit for this experiment to obtain rheological measurements with the necessary precision. The degradation kinetics were quantified by determining the time (t_e) for which the viscosity reaches η₀/e
for temperatures between 30 °C and 5 °C. The time $t_e$ is found to decrease with increasing temperature, following an Arrhenius behavior with an activation energy of E = 47.1 kJ/mol. These observations underscore once again that the degradation mechanism is likely a chemical reaction rather than a mechanical process.

The above results show that the polymer undergoes degradation, which is affected by surface materials as highlighted in Figure 9 and by temperature as shown in Figure 10. Polymer concentration does not have a significant effect on the process, at least within the tested concentration range. The same behavior is observed for all CPAM investigated in the present study

<figure><img src="image_11.png" /><figcaption>**Figure 10.** a) Low shear viscosity ($\eta_0$) plotted against time for MV9104 CPAM solution (5 g/L) measured in a Couette geometry made of dural at 5 °C, 15 °C, and 23 °C. b) Plot of the degradation rate ($1/t_e$) vs. inverse temperature. The characteristic time for degradation ($t_e$) is taken as the time for which viscosity equals $\eta_0(0)/e$. Measurements are carried out for a range of temperature from 30 °C down to 5 °C.</figcaption></figure>

## 3-2. Possible modes of CPAM degradation within metallic rheometry tools.

The decrease in both the low shear viscosity and the shift in the reptation time strongly suggests that the polymer undergoes degradation over time within the dural and stainless-steel Couette. The degradation of CPAM can manifest in several forms: a) chain breaking leading to a reduction in molecular weight, b) hydrolysis resulting in the evolution of cationic charges, and c) an increase in ionic strength due to ions released from the metal surface.
### 3.2.1 Reduction of the molecular weight.

Polyacrylamide-based polymers is reported to exhibit susceptibility to degradation through mechanical solicitation and chemical reactions. Previous studies have noted the potential for polyacrylamide (PAM) chains to undergo scission in channel flow conditions, attributed to elongational stress at the channel entrance [45,47,49]. However, our current experimentation indicates that shear within the Couette apparatus does not impact the degradation of cationic polyacrylamide (CPAM), at least within the range of applied shear in this study (1000 s⁻¹). Consequently, mechanical degradation can be ruled out as a contributing factor to the observed viscosity reduction in these CPAMs.

Chemical degradation pathways for polyacrylamide-based polymers involving chain scission are reported to be initiated by free radicals, such as hydroxyl radicals, stemming from interactions between oxygen and dissolved transition metals like Fe₂⁺ or through persulfate activation [60]. This raises questions about whether such species can be present within the solution when in contact with metallic surfaces. First, it is important to recall that most of these types of CPAM are brought to a low pH via the addition of citric acid, particularly to avoid hydrolysis. In contact with various surfaces such as duralumin, the citric acid reacts with the metal surface, leading to an increase in pH. Metallic ions, when present in the metal composition (see Table S1), are likely to be released. However, this is difficult to imagine, especially for stainless-steel because of its stability compared to other materials used. The stability of stainless steel could also explain the slower degradation kinetics compared to duralumin. Therefore, there is a need to further deepen the understanding of the mechanisms involved in the reaction between stainless steel and CPAM.

It is instructive to estimate the evolution of the Mw if degradation occurs via chain cission. Assuming that the majority of the reduction in η<sub>sp</sub> and τ<sub>r</sub> is a consequence of a decrease in M<sub>w</sub>, one can anticipate the evolution of M<sub>w</sub> during degradation. Considering that the CPAM solution remains in a semi-diluted, entangled polyelectrolyte state throughout the degradation process, with η<sub>sp</sub> exhibiting a dependence on Mw³, and using an initial Mw of 10,000 kg/mol for the fresh polymer, one can predict the reduction of M<sub>w</sub> to be between 10,000 kg/mol to 2,000 kg/mol at the end of degradation. On the other hand, in the case of an unentangled semi-dilute solution with η<sub>sp</sub> varying as Mw¹, Mw would decrease from about 10,000 kg/mol to approximately 100 kg/mol (Figure 3b).
Conversely, it is probable that during degradation, the solution transitions
from being entangled and semi-dilute to unentangled and semi-dilute, resulting
in a change in the dependence of η<sub>sp</sub> on Mw from M<sub>w</sub><sup>3</sup> behavior to M<sub>w</sub><sup>1</sup> behavior.
Therefore, Figure 3b provides a range of M<sub>w</sub> in case of degradation resulting from
chain breaking.

When utilizing the evolution of the relaxation time to estimate changes in
Mw, one can arrive at the same conclusion as with specific viscosity, albeit with
some variation in the time dependence of the recovered Mw.

**3.2.2. Evolution of the molecular structure of the CPAM**

To gain a deeper understanding of the degradation of CPAM, several techniques
were employed to acquire further information on the molecular changes in the
polymer: FTIR, RMN, and GPC analyses, as well as pH and conductimetry
titration, polyelectrolyte titration of the cationic groups. The figures are shown in
the Supplementary data section.

First, molecular structure of the as received CPAMs powder were analyzed by
mean of FTIR (Figures 11 and S4) and NMR (Figure S5). The FTIR spectra of the
CPAMs investigated in this study exhibit all the characteristic absorption peaks
typically observed in copolymers of acrylamides (AM) and acryloxyethyltrimethyl
ammonium chloride. (AAC) (Guan et al., 2010). For example, the 1659 cm⁻¹ and
3344 cm⁻¹ bands could be attributed to the stretching vibrations of C=O and NH₂,
respectively of AM. The 1164 cm⁻¹ and around 1730 cm⁻¹ peaks are attributed to
the C=O and C-O stretching vibrations from the ester groups of AAC. The peak
at 1470 cm⁻¹ is assigned to -CH₂- flexural vibrations in -CH₂-N⁺. The 2928 cm⁻¹
peak could be due to stretching vibration of CH in N-CH₃. The ¹H-NMR spectra
also show most of the characteristic peaks of the copolymer's AM and AAC (Guan
et al. 2016). The peaks at 3.30 ppm and 3.82 ppm are attributed, respectively, to
the protons in -N⁺(CH₃)₃ and in -CH₂-N⁺ of the cationic component's groups. The
peak at 4.60 ppm is attributed to the protons in -O-CH₂- of the AAC group. In
addition, two other groups are observed: the asymmetric peaks at 1.63 ppm and
2.54 ppm are attributed to protons located in -CH₂- and -CH- in the PAM backbone
[77]. Two other peaks appearing at 3.55 ppm and 4.08 ppm are characteristic of
the copolymer PAM and AAC. Their intensities compared to that of the 3.82 ppm
peak have been found to be sensitive to the copolymer composition.
<figure><img src="image_12.png" /><figcaption>**Figure 11.** IR spectra of degraded MV9104 (20 g/L) dried at room temperature and then at 120 °C to remove residual water, compared to a fresh sample from a 20 g/L solution dried under identical conditions.</figcaption></figure>

FTIR and RMN analyses on degraded polymer were conducted on MV9104 CPAM prepared at a concentration of 20 g/L. Preparation of such a high concentration required mixing the polymer using a thermoplastic propeller for several hours to achieve a relatively homogeneous solution. The solution was degraded within the dural Couette for more than 6 hours and then dried at room temperature for several days. Control experiment shows that the applied degradation procedure effectively reduced the polymer viscosity in a similar manner to the reductions observed in the previously mentioned experiments. A reference sample of MV9104 was prepared by drying the non-degraded solution at room temperature. The FTIR and NMR samples of MV9104 exhibited the same characteristics, and no clear differences were observed between these spectra. This suggests that, at least within the resolution of our experiment, no changes occur in the molecular structure of these CPAMs (Figure S4, S5).

The degradation of CPAM may also occur through the hydrolysis of the cationic group, resulting in anionic carboxylic groups, as demonstrated by some authors [50]. Previous studies indicate that CPAM hydrolysis follows a time scale similar to that observed in the present experiment under conditions of pH greater than 7 and temperatures exceeding 30 °C. Conversely, under low pH and low-
temperature conditions, this form of degradation may take a few days. For instance, at 22 °C and pH = 6, the estimated half-life of the cationic group ranges from 10 to 14 days.

The initial pH of the prepared CPAM solution was approximately 2.3. To adjust the pH, citric acid was introduced into the CPAM composition. Within the context of the experiment, the pH of the CPAM within the reaction vessel gradually rises over the first 4 hours, reaching up to pH=4, contingent on the specific metal involved. Within the pH range of less than 4 and the temperature range of 5 °C to 30 °C employed in this study, hydrolysis is anticipated to be minimal, at least in the initial hours. It is noteworthy that, concurrently, the conductivity of the CPAM solution experiences a 50% reduction in the first 4 hours, even as the pH increases.

The hydrolysis of cationic groups within the metallic Couette was assessed through polyelectrolyte titration using KPVS/OTB, following the methodology outlined in some papers [50,61]. The estimated quantity of cationic groups using KPVS/OTB titration of the fresh polymer is noticeably lower than the value provided by the supplier, which might be attributed to the limited accessibility of cationic groups resulting from the high molecular weight (Mw) of CPAM. Thus, this experiment will be used as a qualitative assessment of the relevance of the degradation by means of hydrolysis. Following the degradation of MV9104 in the dural Couette, the CPAM was titrated using KPVS/OTB, and the quantity of cationic groups was compared to the fresh polymer. Surprisingly, the estimated amount of cationic groups within the degraded CPAM exhibited a significant increase of 24% compared to the fresh one. This observation could be interpreted as the molecular weight of CPAM decreasing, leading to more accessible titration sites.

The CPAMs in this study contain citric acid, raising the possibility of the acid interacting with the metal surface and removing ions, thereby increasing ionic strength and reducing CPAM viscosity. To address this potential effect, a citric acid solution with a pH similar to that of the CPAM solution was added to the dural Couette. The solution's pH and conductivity were monitored for over 6 hours. The pH of the citric acid solution increased similarly to that of the CPAM, reaching about 4. This indicates that the increase in pH within the metal Couette is mainly due to the reaction of citric acid with the metal surface.
The resulting citric acid solution was used to dissolve fresh MV9104 powder, preparing a 5 g/L CPAM solution. Subsequently, the viscosity of this solution was compared to that of MV9104 prepared using DDI water. The viscosity of this CPAM solution was found to be only 0.3 times that of the fresh CPAM solution, significantly higher than the measured viscosity for the degraded polymer in the dural Couette. This suggests that a change in ionic strength during the reaction of citric acid with the metal surface is not likely the main explanation for the observed degradation of CPAM polymer within the metallic Couette.

Similarly, to the discussion on scission-induced degradation in section 3.2.1, it is instructive to propose a mechanism by which hydrolysis reduces low shear viscosity and inhibits all viscoelastic properties. As seen in section 3.11, the researched CPAM likely behaves as a charged polyelectrolyte in the semi-dilute entangled regime, leading to low shear viscosity scaling as $M_w^3$ and $C^{1.48}$.[68,72] Hydrolysis, leading to a loss of charged cationic groups, can cause the charge density to evolve, leading either to a partial collapse of the chain or at least a change in the chain conformation. This results in a lower scaling exponent for $M_w$ and concentration, which qualitatively accounts for the observed reduction in low shear viscosity and the reduction of reptation time.

## 4. CONCLUSION

This study examines the degradation of cationic polyacrylamide (CPAM) when it contacts metal surfaces. The findings reveal significant changes in CPAM's rheological properties during measurements with metallic rheometry Couette cells. Specifically, low shear viscosity decreases over time, and viscoelastic properties shift towards viscous behavior. The main cause of these changes is not shear but chemical interactions between the metal surface and CPAM.

Different metals impact CPAM differently, with duralumin causing faster degradation than stainless steel. In contrast, when in PVC and other thermoplastics, this degradation is significantly less prevalent. These effects are consistent across various CPAM concentrations, and higher temperatures accelerate the degradation process. Analytical methods such as NMR, FTIR, and cationic group titration provide insights into the mechanisms of CPAM degradation by metal surfaces.
The study concludes that the intrinsic rheological properties of high Mw CPAM
can only be accurately measured using thermoplastic tools, due to the extended
time needed for characterization. This information is crucial for scaling up
installations in wastewater treatment plants. The findings suggest that the
wastewater treatment industry should carefully select tools to avoid CPAM
degradation. Understanding CPAM degradation on metal surfaces can enhance
the effectiveness of flocculating agents, improving treatment processes and
maintaining environmental safety. Further research is needed to explore the
chemical and physical mechanisms behind this degradation and its impact on the
flocculation process.

ACKNOWLEDGEMENTS

The Rhéologie et Procédés Laboratory is affiliated with the LabEx Tec 21
(Investissements d'Avenir - Grant agreement no ANR-11-LABX-0030).
Additionally, the laboratory is associated with the PolyNat Carnot Institute
(Investissements d'Avenir - Grant agreement no ANR-11-CARN-030-01). The
authors express their gratitude to F. Hugenell, Didier Blésès, and Vincent Verdoot
for their invaluable assistance in conducting these experiments. The authors
would like to thank the MOCOPEE program for its financial support.

REFERENCES

[1] A. Abbasi Moud, Polymer based flocculants: Review of water purification applications, J. Water Process Eng. 48 (2022) 102938.
https://doi.org/10.1016/j.jwpe.2022.102938.

[2] J. Boráň, L. Houdková, T. Elsäßer, Processing of sewage sludge: Dependence of sludge dewatering efficiency on amount of flocculant, Resour. Conserv. Recycl. 54 (2010) 278-282. https://doi.org/10.1016/j.resconrec.2009.08.010.

[3] B. Cao, T. Zhang, W. Zhang, D. Wang, Enhanced technology based for sewage sludge deep dewatering: A critical review, Water Res. 189 (2021) 116650. https://doi.org/10.1016/j.watres.2020.116650.

[4] M. Hyrycz, M. Ochowiak, A. Krupińska, S. Włodarczak, M. Matuszak, A review of flocculants as an efficient method for increasing the efficiency of municipal sludge dewatering: Mechanisms, performances, influencing factors and perspectives, Sci. Total Environ. 820 (2022) 153328.
https://doi.org/10.1016/j.scitotenv.2022.153328.

[5] P. Jarvis, B. Jefferson, J. Gregory, S.A. Parsons, A review of floc strength and breakage, Water Res. 39 (2005) 3121-3137.
https://doi.org/10.1016/j.watres.2005.05.022.
[6] A. Suresh, E. Grygolowicz-Pawlak, S. Pathak, L.S. Poh, M. bin Abdul Majid, D. Dominiak, T.V. Bugge, X. Gao, W.J. Ng, Understanding and optimization of the flocculation process in biological wastewater treatment processes: A review, Chemosphere 210 (2018) 401-416.
https://doi.org/10.1016/j.chemosphere.2018.07.021.

[7] V. Vajihinejad, S.P. Gumfekar, B. Bazoubandi, Z. Rostami Najafabadi, J.B.P. Soares, Water Soluble Polymer Flocculants: Synthesis, Characterization, and Performance Assessment, Macromol. Mater. Eng. 304 (2019).
https://doi.org/10.1002/mame.201800526.

[8] H. Wei, B. Gao, J. Ren, A. Li, H. Yang, Coagulation/flocculation in dewatering of sludge: A review, Water Res. 143 (2018) 608-631.
https://doi.org/10.1016/j.watres.2018.07.029.

[9] B. Wu, X. Dai, X. Chai, Critical review on dewatering of sewage sludge: Influential mechanism, conditioning technologies and implications to sludge re-utilizations, Water Res. 180 (2020) 115912.
https://doi.org/10.1016/j.watres.2020.115912.

[10] P. Mpofu, J. Addai-Mensah, J. Ralston, Investigation of the effect of polymer structure type on flocculation, rheology and dewatering behaviour of kaolinite dispersions, Int. J. Miner. Process. 71 (2003) 247-268.
https://doi.org/10.1016/S0301-7516(03)00062-0.

[11] W. Baade, D. Hunkeler, A.E. Hamielec, Copolymerization of acrylamide with cationic monomers in solution and inverse-microsuspension, J. Appl. Polym. Sci. 38 (1989) 185-201. https://doi.org/10.1002/app.1989.070380117.

[12] Y. Zhou, G.V. Franks, Flocculation Mechanism Induced by Cationic Polymers Investigated by Light Scattering, Langmuir 22 (2006) 6775-6786.
https://doi.org/10.1021/la060281+.

[13] Q.N. Ho, M. Fettweis, K.L. Spencer, B.J. Lee, Flocculation with heterogeneous composition in water environments: A review, Water Res. 213 (2022) 118147. https://doi.org/10.1016/j.watres.2022.118147.

[14] B. Lee, M. Schlautman, Effects of Polymer Molecular Weight on Adsorption and Flocculation in Aqueous Kaolinite Suspensions Dosed with Nonionic Polyacrylamides, Water 7 (2015) 5896-5909.
https://doi.org/10.3390/w7115896.

[15] V.H. Lim, Y. Yamashita, K. Ogawa, Y. Adachi, Comparison of cationic flocculants with different branching structure for the flocculation of negatively charged particles coexisting with humic substances, J. Environ. Chem. Eng. 10 (2022) 108478. https://doi.org/10.1016/j.jece.2022.108478.

[16] Y. Adachi, J. Xiao, Initial stage of bridging flocculation of PSL particles induced by an addition of polyelectrolyte under high ionic strength, Colloids Surf. Physicochem. Eng. Asp. 435 (2013) 127-131.
https://doi.org/10.1016/j.colsurfa.2012.12.042.

[17] B. Bolto, J. Gregory, Organic polyelectrolytes in water treatment, Water Res. 41 (2007) 2301-2324. https://doi.org/10.1016/j.watres.2007.03.012.

[18] M. Lapointe, B. Barbeau, Understanding the roles and characterizing the intrinsic properties of synthetic vs. natural polymers to improve clarification through interparticle Bridging: A review, Sep. Purif. Technol. 231 (2020) 115893. https://doi.org/10.1016/j.seppur.2019.115893.

[19] C.S. Lee, J. Robinson, M.F. Chong, A review on application of flocculants in wastewater treatment, Process Saf. Environ. Prot. 92 (2014) 489-508.
https://doi.org/10.1016/j.psep.2014.04.010.

[20] J. Gregory, L. Guibai, EFFECTS OF DOSING AND MIXING CONDITIONS ON POLYMER FLOCCULATION OF CONCENTRATED SUSPENSIONS, Chem. Eng. Commun. 108 (1991) 3-21. https://doi.org/10.1080/00986449108910948.
[21] W. Yu, J. Gregory, L. Campos, G. Li, The role of mixing conditions on floc growth, breakage and re-growth, Chem. Eng. J. 171 (2011) 425-430.
https://doi.org/10.1016/j.cej.2011.03.098.

[22] M.A. Yukselen, J. Gregory, The reversibility of floc breakage, Int. J. Miner. Process. 73 (2004) 251-259. https://doi.org/10.1016/S0301-7516(03)00077-2.

[23] Y.-H. Bae, S.-S. Lee, A SINGLE FLOCCULANT/DUAL FLOCCULATION SYSTEM FOR DEWATERING USING A BRANCHED SELF INVERSING EMULSION FLOCCULANT, Environ. Eng. Res. 11 (2006) 208-216.
https://doi.org/10.4491/eer.2006.11.4.208.

[24] T.O. Chimamkpam, M.G. Rasteiro, F.A.P. Garcia, E. Antunes, P. Ferreira, D. Hunkeler, C. Wandrey, Solution viscosity and flocculation characteristics of linear polymeric flocculants in various media, Chem. Eng. Res. Des. 89 (2011) 1037-1044. https://doi.org/10.1016/j.cherd.2010.10.022.

[25] M.L. Taylor, G.E. Morris, P.G. Self, R.St.C. Smart, Kinetics of Adsorption of High Molecular Weight Anionic Polyacrylamide onto Kaolinite: The Flocculation Process, J. Colloid Interface Sci. 250 (2002) 28-36.
https://doi.org/10.1006/jcis.2002.8341.

[26] P. Fawell, Solid-Liquid Separation of Clay Tailings, in: M. Gräfe, C. Klauber, A.J. McFarlane, D.J. Robinson (Eds.), Clays Miner. Process. Value Chain, 1st ed., Cambridge University Press, 2017: pp. 327-380.
https://doi.org/10.1017/9781316661888.010.

[27] W. Sung Ng, L.A. Connal, E. Forbes, K. Mohanarangam, G.V. Franks, *In situ* study of aggregate sizes formed in chalcopyrite-quartz mixture using temperature-responsive polymers, Adv. Powder Technol. 29 (2018) 1940-1949. https://doi.org/10.1016/j.apt.2018.04.027.

[28] D. Palomino, D. Hunkeler, S. Stoll, Comparison of two cationic polymeric flocculant architectures on the destabilization of negatively charged latex suspensions, Polymer 52 (2011) 1019-1026.
https://doi.org/10.1016/j.polymer.2010.12.033.

[29] K. Mohanarangam, J. Hopkins, K. Simic, D. Stephens, P. Fawell, Turbulent mixing of concentrated viscoelastic polymer solution: Influence of submerged sparge shape and orientation, Miner. Eng. 128 (2018) 1-16.

[30] J. Henderson, A. Wheatley, Factors effecting a loss of flocculation activity of polyacrylamide solutions: Shear degradation, cation complexation, and solution aging, J. Appl. Polym. Sci. 33 (1987) 669-684.

[31] P.-H. Chavanis, The Generalized Stochastic Smoluchowski Equation, Entropy 21 (2019) 1006. https://doi.org/10.3390/e21101006.

[32] J. Gregory, Polymer adsorption and flocculation in sheared suspensions, Colloids Surf. 31 (1988) 231-253. https://doi.org/10.1016/0166-6622(88)80196-3.

[33] R. Hogg, The role of polymer adsorption kinetics in flocculation, Colloids Surf. Physicochem. Eng. Asp. 146 (1999) 253-263. https://doi.org/10.1016/S0927-7757(98)00723-7.

[34] M.V. Smoluchowski, Drei vortrage über diffusion, brownsche molekularbewegung und koagulation von kolloidteilchen, Z Phys. 17 (1916) 585-599.

[35] M. v. Smoluchowski, Versuch einer mathematischen Theorie der Koagulationskinetik kolloider Lösungen, Z. Für Phys. Chem. 92U (1918) 129-168. https://doi.org/10.1515/zpch-1918-9209.

[36] S. Burmester, C. Rielly, M. Edwards, The mixing of miscible liquids with large differences in density and viscosity, Fluid Mech. Mix. Model. Oper. Exp. Tech. (1992) 83-90.
[37] B. Jha, L. Cueto-Felgueroso, R. Juanes, Fluid mixing from viscous fingering, Phys. Rev. Lett. 106 (2011) 194502.

[38] P. Yecko, S. Zaleski, J.-M. Fullana, Viscous modes in two-phase mixing layers, Phys. Fluids 14 (2002) 4115-4122.

[39] A.T. Owen, P.D. Fawell, J.D. Swift, The preparation and ageing of acrylamide/acrylate copolymer flocculant solutions, Int. J. Miner. Process. 84 (2007) 3-14. https://doi.org/10.1016/j.minpro.2007.05.003.

[40] V. Kurenkov, H.-G. Hartan, F. Lobanov, Degradation of polyacrylamide and its derivatives in aqueous solutions, Russ. J. Appl. Chem. 75 (2002) 1039-1050.

[41] D.B. Levitt, G.A. Pope, S. Jouenne, Chemical degradation of polyacrylamide polymers under alkaline conditions, SPE Reserv. Eval. Eng. 14 (2011) 281-286.

[42] P.F. Oliveira, J.A. Costa, L.F.S. Oliveira, L.S. Mota, L.A. de Oliveira, C.R. Mansur, Hydrolysis and thermal stability of partially hydrolyzed polyacrylamide in high-salinity environments, J. Appl. Polym. Sci. 136 (2019) 47793.

[43] Y. Pei, L. Zhao, G. Du, N. Li, K. Xu, H. Yang, Investigation of the degradation and stability of acrylamide-based polymers in acid solution: Functional monomer modified polyacrylamide, Petroleum 2 (2016) 399-407.

[44] D. Ramsden, K. McKay, Degradation of polyacrylamide in aqueous solution induced by chemically generated hydroxyl radicals: Part I—Fenton's reagent, Polym. Degrad. Stab. 14 (1986) 217-229.

[45] A.R. Al Hashmi, R.S. Al Maamari, I.S. Al Shabibi, A.M. Mansoor, A. Zaitoun, H.H. Al Sharji, Rheology and mechanical degradation of high-molecular-weight partially hydrolyzed polyacrylamide during flow through capillaries, J. Pet. Sci. Eng. 105 (2013) 100-106. https://doi.org/10.1016/j.petrol.2013.03.021.

[46] S. Garrepally, S. Jouenne, P.D. Olmsted, F. Lequeux, Scission of flexible polymers in contraction flow: Predicting the effects of multiple passages, J. Rheol. 64 (2020) 601-614.

[47] A.F. Horn, E.W. Merrill, Midpoint scission of macromolecules in dilute solution in turbulent flow, Nature 312 (1984) 140-141. https://doi.org/10.1038/312140a0.

[48] K. Zhang, G.H. Lim, H.J. Choi, Mechanical degradation of water-soluble acrylamide copolymer under a turbulent flow: Effect of molecular weight and temperature, J. Ind. Eng. Chem. 33 (2016) 156-161.

[49] B. Xiong, R.D. Loss, D. Shields, T. Pawlik, R. Hochreiter, A.L. Zydney, M. Kumar, Polyacrylamide degradation and its implications in environmental systems, Npj Clean Water 1 (2018). https://doi.org/10.1038/s41545-018-0016-8.

[50] R. Aksberg, L. Wågberg, Hydrolysis of cationic polyacrylamides, J. Appl. Polym. Sci. 38 (1989) 297-304. https://doi.org/10.1002/app.1989.070380211.

[51] M.J. Caulfield, X. Hao, G.G. Qiao, D.H. Solomon, Degradation on polyacrylamides. Part I. Linear polyacrylamide, Polymer 44 (2003) 1331-1337.

[52] M.J. Caulfield, G.G. Qiao, D.H. Solomon, Some Aspects of the Properties and Degradation of Polyacrylamides, Chem. Rev. 102 (2002) 3067-3084. https://doi.org/10.1021/cr010439p.

[53] Q. Fu, X. Liu, Y. Wu, D. Wang, Q. Xu, J. Yang, The fate and impact of coagulants/flocculants in sludge treatment systems, Environ. Sci. Water Res. Technol. 7 (2021) 1387-1401.

[54] A.-G. Guezennec, C. Michel, K. Bru, S. Touze, N. Desroche, I. Mnif, M. Motelica-Heino, Transfer and degradation of polyacrylamide-based
flocculants in hydrosystems: a review, Environ. Sci. Pollut. Res. 22 (2015) 6390-6406.

[55] R.D. Letterman, R.W. Pero, Contaminants in Polyelectrolytes Used in Water Treatment, J. AWWA 82 (1990) 87-97. https://doi.org/10.1002/j.1551-8833.1990.tb07056.x.

[56] R. Barati, J. Liang, A review of fracturing fluid systems used for hydraulic fracturing of oil and gas wells, J. Appl. Polym. Sci. 131 (2014). https://doi.org/10.1002/app.40735.

[57] W. Liang, Y. Luo, S. Song, X. Dong, X. Yu, High photocatalytic degradation activity of polyethylene containing polyacrylamide grafted TiO2, Polym. Degrad. Stab. 98 (2013) 1754-1761.

[58] S. Vijayalakshmi, D. Senapati, G. Madras, Pulsed laser degradation of polyethylene oxide and polyacrylamide in aqueous solution, Polym. Degrad. Stab. 87 (2005) 521-526.

[59] R. Vinu, G. Madras, Photocatalytic degradation of poly (acrylamide-co-acrylic acid), J. Phys. Chem. B 112 (2008) 8928-8935.

[60] B. Xiong, Z. Miller, S. Roman-White, T. Tasker, B. Farina, B. Piechowicz, W.D. Burgos, P. Joshi, L. Zhu, C.A. Gorski, Chemical degradation of polyacrylamide during hydraulic fracturing, Environ. Sci. Technol. 52 (2018) 327-336.

[61] S. Kam, J. Gregory, Charge determination of synthetic cationic polyelectrolytes by colloid titration, Colloids Surf. Physicochem. Eng. Asp. 159 (1999) 165-179. https://doi.org/10.1016/S0927-7757(99)00172-7.

[62] N.J. Kim, C.J. Pipe, K.H. Ahn, S.J. Lee, G.H. McKinley, Capillary breakup extensional rheometry of a wormlike micellar solution, Korea-Aust. Rheol. J. 22 (2010) 31-41.

[63] A. Naillon, C. de Loubens, W. Chèvremont, S. Rouze, M. Leonetti, H. Bodiguel, Dynamics of particle migration in confined viscoelastic Poiseuille flows, Phys. Rev. Fluids 4 (2019) 053301.

[64] D.C. Boris, R.H. Colby, Rheology of sulfonated polystyrene solutions, Macromolecules 31 (1998) 5746-5755.

[65] A.V. Dobrynin, R.H. Colby, M. Rubinstein, Scaling theory of polyelectrolyte solutions, Macromolecules 28 (1995) 1859-1871.

[66] S. Dou, R.H. Colby, Charge density effects in salt-free polyelectrolyte solution rheology, J. Polym. Sci. Part B Polym. Phys. 44 (2006) 2001-2013.

[67] A. Gulati, M. Jacobs, C.G. Lopez, A.V. Dobrynin, Salt Effect on the Viscosity of Semi-dilute Polyelectrolyte Solutions: Sodium Polystyrenesulfonate, Macromolecules 56 (2023) 2183-2193.

[68] A. Han, R.H. Colby, Rheology of entangled polyelectrolyte solutions, Macromolecules 54 (2021) 1375-1387.

[69] Y. Heo, R.G. Larson, The scaling of zero-shear viscosities of semi-dilute polymer solutions with concentration, J. Rheol. 49 (2005) 1117-1128.

[70] A. Matsumoto, R. Yoshizawa, O. Urakawa, T. Inoue, A.Q. Shen, Rheological Scaling of Ionic Liquid-Based Polyelectrolytes in the Semi-dilute Unentangled Regime from Low to High Salt Concentrations, Macromolecules 54 (2021) 5648-5661.

[71] A. Matsumoto, C. Zhang, F. Scheffold, A.Q. Shen, Microrheological approach for probing the entanglement properties of polyelectrolyte solutions, ACS Macro Lett. 11 (2021) 84-90.

[72] S. Jouenne, B. Levache, Universal viscosifying behavior of acrylamide-based polymers used in enhanced oil recovery, J. Rheol. 64 (2020) 1295-1313.

[73] C. Castelain, J. Doublier, J. Lefebvre, A study of the viscosity of cellulose derivatives in aqueous solutions, Carbohydr. Polym. 7 (1987) 1-16.
[74] W.W. Graessley, Polymer chain dimensions and the dependence of viscoelastic properties on concentration, molecular weight and solvent power, Polymer 21 (1980) 258-262.

[75] B.A. Wolf, Polyelectrolytes revisited: Reliable determination of intrinsic viscosities, Macromol. Rapid Commun. 28 (2007) 164-170.

[76] R.M. Fuoss, U.P. Strauss, Polyelectrolytes. II. Poly-4-vinylpyridonium chloride and poly-4-vinyl-N-n-butylpyridonium bromide, J. Polym. Sci. 3 (1948) 246-263.

[77] Q. Guan, H. Zheng, J. Xub, F. Tian, X. Sun, Effect of charge density on structural characteristics of cationic polyacrylamide: models based on reactivity ratio and characterization, J. Polym. Mater. 33 (2016) 365.
SUPPLEMENTARY DATA

<figure><img src="image_13.png" /><figcaption>**Figure S1.** Rheological characterization protocols for *in situ* polymer degradation. For analyzing slowly evolving polymer degradation $\dot{\gamma}_1$= 0.001s⁻¹, $\dot{\gamma}_1$= 100 s⁻¹, $\dot{\gamma}_2$= 0.001Hz, $\dot{\gamma}_2$= 10 Hz. For analyzing rapidly evolving polymer degradation $\dot{\gamma}_1$= 0.01s⁻¹, $\dot{\gamma}_1$= 10 s⁻¹, $\dot{\gamma}_2$= 0.01Hz, $\dot{\gamma}_2$= 10 Hz</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Figure S2.** Rheological characterization protocol for *in situ* CPAM degradation. Step (a) involves measuring low shear viscosity at a constant shear rate of 0.01 s⁻¹. Step (b) involves solicitation of the CPAM, where it was either left to rest at a shear rate of 0 s⁻¹ or subjected to a constant shear rate of either 0.01 s⁻¹ or 1000 s⁻¹.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>**Figure S3.** Viscosity vs. time for a 5 g/L MV9104 polymer solution, measured using a stainless steel CCB43/SS/SE cup and a Z38 rotor within a MARS rheometer. The measurement was taken at a constant shear rate of 0.1 s⁻¹ at 20 °C.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Figure S4.** IR spectra of MV9104 comparing the as-received sample (granulat) to samples prepared from an aqueous 20 g/L solution and dried at room temperature and at 60 °C (HD).</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_18.png" /></figure>

**Figure S5.** Proton NMR spectra of MV9104 degraded at 20 g/l and dried (b) compared to that of the fresh sample (a).
<figure><img src="image_19.png" /></figure>

a

b

**Video S2** Filament break up (a) before degradation and (b) after degradation for a 5 g/L MV9104 solution, observed using an elongation rheometer (CABER) at room temperature. Measurements were carried out using a tool of 4 mm diameter and the solution was within a 0.5 mm gap.

<table><caption>**Table S1.** Chemical composition of the materials used in Couette tools.</caption><thead><tr><th>Atoms</th><th>Fe</th><th>Cu</th><th>Al</th><th>Zn</th><th>Mn</th><th>Mg</th><th>Cr</th><th>Ni</th><th>Mb</th><th>C</th></tr><tr><th></th><th>%</th><th>%</th><th>%</th><th>%</th><th>%</th><th>%</th><th>%</th><th>%</th><th>%</th><th>%</th></tr></thead><tbody><tr><th>Copper</th><td></td><td>100</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><th>Dural</th><td>0.7 max</td><td>3.5 to 4.5</td><td>97.65 to 94.35</td><td>0.25</td><td>0.4 to 1</td><td>0.4 to 1</td><td>0.1</td><td></td><td></td><td></td></tr><tr><th>Stainless-steel</th><td>68.95 to 71.95</td><td></td><td></td><td></td><td></td><td></td><td>16</td><td>10 to 12</td><td>2 to 3</td><td>0.05</td></tr></tbody></table>
GRAPHICAL ABSTRACT

<figure><img src="image_20.png" /><figcaption>Impact of rheometry tools on reducing low shear viscosity in cationic polyacrylamide</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>Figure 1a</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>Figure 1a</figcaption></figure>
<figure><img src="image_23.png" /><figcaption>Figure 2a</figcaption></figure>
<figure><img src="image_24.png" /><figcaption>Figure 2b</figcaption></figure>
<figure><img src="image_25.png" /><figcaption>Figure 4b</figcaption></figure>
<figure><img src="image_26.png" /><figcaption>Figure 4a</figcaption></figure>
Oh

<figure><img src="image_27.png" /><figcaption>Figure 5a</figcaption></figure>
<figure><img src="image_28.png" /><figcaption>Figure 3b</figcaption></figure>
<figure><img src="image_29.png" /><figcaption>Figure 3a</figcaption></figure>
<figure><img src="image_30.png" /><figcaption>Figure 8</figcaption></figure>
<figure><img src="image_31.png" /><figcaption>Figure 5b</figcaption></figure>
<figure><img src="image_32.png" /><figcaption>Figure 7</figcaption></figure>
<figure><img src="image_33.png" /><figcaption>Figure 5c</figcaption></figure>
<figure><img src="image_34.png" /><figcaption>Figure 6</figcaption></figure>
3.5h

<figure><img src="image_35.png" /><figcaption>Figure 5d</figcaption></figure>
<figure><img src="image_36.png" /><figcaption>Figure 11</figcaption></figure>
<figure><img src="image_37.png" /><figcaption>Figure 10a</figcaption></figure>
<figure><img src="image_38.png" /><figcaption>Figure 10b</figcaption></figure>
<figure><img src="image_39.png" /></figure>