Perfluorooctanoic acid-mediated self-assembly behaviour of linear and star block copolymers: the impact of intermolecular interactions on polymer micelles

Abstract

In this study, we present results from high-sensitivity differential scanning calorimetry (HSDSC), dynamic light scattering (DLS), small-angle neutron scattering (SANS), and computational simulation performed for understanding the micellization and micelle morphology of two EO–PO block copolymeric surfactants, namely, the linear triblock copolymer Pluronic® F127 and the star block copolymer Tetronic® T1107 (both with 70% PEO content), in the presence of perfluorooctanoic acid (PFOA). Increasing the concentration of PFOA in aqueous solutions of both copolymers resulted in a significant reduction in the critical micellization temperature (CMT), as inferred from the calorimetric findings. Scattering experiments revealed intriguing results, indicating that the micelle radius (R_c, R_hs) and aggregation number (N_agg) of polymeric micelles increased with increasing PFOA concentration. The addition of PFOA did not alter the morphology of the micelles; however, comparing the correlation peaks in SANS suggested that the distance between the spherical micelles decreased as the PFOA concentration increased. Thus, the outcome of this study helps to fine-tune the performance-based properties of these widely used polymeric surfactants with a fluorinated hydrophobic additive.

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1. Introduction

Self-assembly of amphiphilic block copolymers with chemically incompatible blocks in selected solvents provides nanostructures with well-defined forms and sizes that have various potential uses, especially in biomedicine and nanomaterials.^1–4 Over the past two decades, the synthesis of newly developed block copolymers and the enhancement of manipulation technologies have led to the creation of unique morphologies and hierarchical nanostructures, some of which exhibit self-assembly and morphologies similar to those of biomacromolecules.^5–7 Uncovering the physicochemical principles behind microstructural changes in micellar solutions helps us understand the evolution of life and provides ideas for building useful functional nanomaterials.

Amphiphilic EO–PO block copolymers have been studied for their micellization behaviour and have sol-to-gel transition properties. Pluronics® and Tetronics® have covalently bonded polyethylene oxide (PEO) and polypropylene oxide (PPO). Linear PEO, PPO, and PEO blocks make Pluronics®. Tetronics®, which are star block copolymers with a central ethylene diamine core, are pH sensitive and have a slight ionic conductivity. Tetronics’ X-shaped arms have different PPO–PEO ratios. These block copolymers form micelles in aqueous solutions above a critical micelle temperature (CMT) and concentration (critical micelle concentration, CMC). Scientists prefer amphiphilic block copolymers for future research due to their micellization property.^8–10 Depending on the structure of the amphiphilic copolymer used and the solution parameters (polymer concentration, solvent type, pH, ionic strength, temperature, solvent/cosolvent ratio, and others), different types of polymeric micelles can be formed.^11,12 It is possible to obtain diverse micelle-like structures with varying morphologies in this manner, which is an essential requirement for a variety of advanced nanotechnology applications.¹³ In Pluronics® and Tetronics®, the length of the PEO and PPO blocks, in addition to the previously mentioned parameters, will affect the micelle shape.¹⁴ The simplest method for achieving spherical micelles is to use a polymer with a hydrophilic block that is longer than the hydrophobic block.^15,16 The micellization and solubilization properties of the Pluronics® and Tetronics® make them promising candidates for various applications, including drug delivery, nanotechnology, and the removal of hydrocarbons. Luo et al.¹⁷ studied the influence of small molecules, including octanol, octylamine, and octanoic acid, on the micellization of P123. They found that the difference in the functional group of the compounds has a significant effect on the micelles’ morphology. Senthil Kumar et al.¹⁸ report the interaction of methyl paraben and propyl paraben with P123, F127, and the mixed micelles of P123 and F127. They found that the paraben molecules are incorporated into the surface cavities of both pure and mixed micelles, changing their morphology. Chakrabarti et al.¹⁹ used P104 and T1107 polymers to improve polyaromatic hydrocarbon (PAH) solubility and discovered that the micellar morphology changes in the presence of PAHs. As a result, they concluded that this could be a new method for removing PAHs. Luo et al.²⁰ investigated the effect of hydrogen bonding on the self-assembly behaviour of P123 micelles by introducing three small molecules into the aqueous solution: propyl benzoate, propyl paraben, and propyl gallate. They discovered that hydrogen bonding is essential in understanding the mechanism of micellar morphology in terms of hydrophobic interactions.

The unique structure of certain block copolymers, such as Pluronic® F127 and Tetronic® T1107, and their potential applications, including drug delivery and pollutant remediation, have attracted the attention of researchers. Several hydrocarbon derivatives, including polycyclic aromatic hydrocarbons (PAHs),¹⁹ alcohols,²¹ phenol,^22,23 organic acid,^24,25 and parabens,²⁶ can be incorporated into these polymers to modify their performance-based properties. These derivatives cause the polymers to adopt different shapes, such as spheres or rods. There is still a lack of understanding regarding the effects of fluorinated additives, particularly PFOA. The purpose of this study is to clarify the impact of PFOA on micelle properties, specifically their size and molecular composition. In this manuscript, the impact of PFOA on micellar properties, such as CMT, hydrodynamic size, and aggregation number, of linear and nonlinear EO–PO block copolymers is provided and compared with the literature reported on nonfluorinated hydrocarbon additives. The impact of PFOA is also assessed and compared in terms of micellar transitions. To elucidate the underlying mechanisms, various experiments, including HSDSC, DLS, SANS, and computer simulations, were performed. The present study is the first to elucidate the concentration-dependent, step-wise interaction mechanism of PFOA with block copolymer micelles. It also offers both application-relevant and ultimate insights that differentiate it from the previously reported studies on hydrocarbon additives. This study also provides a platform to compare the study of PFOA in modulating the micellar properties of linear and non-linear block copolymers, which enhances the depth and applicability of the research findings.

2. Materials and methods

2.1. Materials

Pluronics® F127 and Tetronic® T1107 copolymer (structural constitution presented in Fig. 1) were used as received from BASF Corp., Parsippany, NJ, USA. Perfluorooctanoic acid (PFOA) was purchased from Sigma Aldrich, India. The 5% F127 and T1107 solutions were prepared by weighing the calculated amount of F127 and water and kept in the refrigerator overnight in tightly sealed glass vials. Millipore water from the Milli-Q system was used to prepare samples for DLS, while D₂O (99%) was used for sample preparation for the SANS experiment.

Fig. 1 Molecular structures of (a) F127, (b) perfluorooctanoic acid (PFOA), and (c) T1107.

2.2. Methods

2.2.1. Cloud point (CP). The cloud points of 5% F127 and 5% T1107 solutions in the presence of PFOA were measured. The 3–4 mL copolymer solutions were heated gently in 20 mL glass-stoppered borosilicate test tubes, which were immersed in a water bath. The temperature of the water bath was increased using an electric heater at a heating rate of 1 °C min⁻¹, and the solution was constantly stirred. A magnetic stirrer was placed in the water bath for uniform distribution of heat. The glass caps were kept on the test tube to prevent any possibility of evaporation. During the measurement of CP, the first appearance of cloudiness, as assessed visually, was taken as CP, as per the procedure reported in the literature.^27,28 The CP measurements were repeated three times for each solution, and the results were found to be constant within ±0.2 °C.

2.2.2. Viscosity. Calibrated Cannon Ubbelohde viscometers were employed to measure the solution viscosity of 5% F127 and 5% T1107 solutions as a function of PFOA concentration. Viscometers were immersed in the water bath, ensuring a temperature stability of ±0.1 °C. The flow times of the solutions were measured by a viscometer of size 50, having a viscometer constant of 0.0040903 cSt per s. The kinematic viscosities of the solution (cSt) were obtained by multiplying the flow times of the solutions by the viscometer constant. The relative viscosities of the solutions were obtained by multiplying the kinematic viscosities by the density of water to give viscosities in centipoise, followed by dividing them by the viscosity of water.²⁹

2.2.3. High-sensitivity differential scanning calorimetry (HSDSC). HSDSC (VP-DSC, Micro Cal Inc., Amherst, MA) studies were conducted in Taiwan to determine the critical micellization temperature (CMT) for 5% F127 and T1107 aqueous solutions in the presence of perfluorooctanoic acid. The measurements were conducted over the temperature range of 5 to 120 °C at a scanning rate of 20 °C h⁻¹, and the data were evaluated using Origin 8.5 software (OriginLab, US). The progressive baseline for the thermograms was evaluated by extrapolating the pre- and post-transitional portions of the peak into the transition region. The area of the peak, based on the progressive baseline, was integrated to calculate the heat of micellization (ΔH). Other parameters, namely free energy change of micellization (ΔG) and entropy change of micellization (ΔS), were determined by closely following the mass the action model as described in an earlier study.³⁰

2.2.4. Dynamic light scattering. The effect of PFOA on the hydrodynamic diameter of F127 and T1107 micelles was examined using dynamic light scattering, which was carried out at a fixed scattering angle of 173° at different concentrations using Zetasizer Nano-ZS 4800 (Malvern Instruments, UK) furnished with a He–Ne laser operating at a wavelength of 633 nm using the CONTIN method of analysis. To verify the reproducibility of the work, each measurement was repeated twice and was found to be reproducible within ±0.2 nm.

2.2.5. Small-angle neutron scattering (SANS). SANS measurements were carried out for F127 and T1107 solutions, both in the presence and the absence of PFOA, at the SANS-I facility at Dhruva Reactor, Bhabha Atomic Research Centre, Mumbai. The solutions were placed in Hellma quartz cells having a 2-mm thickness with tight-fitting Teflon stoppers. A neutron beam of 5.2 Å wavelength (λ) was used for the measurements, and a one-dimensional position-sensitive detector was used to measure the scattering of neutrons from the sample. The distance between the sample and detector was 2 m to measure wave vector transfer (Q = 4πsin θ/λ, where θ is the scattering angle), having a 0.015–0.25 Å⁻¹ range. The measured data were corrected for background, empty-cell contribution, and sample transmission, and normalised to an absolute scale using standard procedures. The differential scattering cross-section, dΣ/dΩ per unit volume of solution for micelles is expressed byhere, n signifies the number density of the micelles of volume V. ρ_P and ρ_S are scattering length densities of the micelle and solvent, respectively. P(Q) and S(Q) are single-particle (intraparticle) structure factor and the interparticle structure factor, respectively. B is a constant term that represents the incoherent scattering background, which is generally owing to hydrogen in the sample.

2.2.6. Computational simulation. The GaussView 5.0.9 software package was used to determine the energies of the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), and the energy gap (ΔE = E_LUMO − E_HOMO) of molecules using density functional theory (DFT) calculations. The B3LYP functional is the DFT approach most frequently utilized because of its ability to predict molecular structures and other properties precisely. The lower basis set, 3-21G, was used to conserve the computer power. Also, reported studies used the DFT/B3LYP/3-21G basis set for large molecules.^31–33 Additionally, the 3D molecular electrostatic potential (3D-MEP) surface data were estimated for the examined system. The optimized molecular structures illustrating HOMO and LUMO lobes for F127, T1107 and PFOA are provided in Fig. 2.

Fig. 2 Optimized molecular structures illustrating HOMO and LUMO lobes of the pure components: (a) F127, (b) T1107, and (c) PFOA.

3. Results and discussion

3.1. Cloud point

Pluronic® F127 and Tetronic® T1107 are block copolymers that form core–shell micelles in aqueous medium. Their aqueous solubility is due to the formation of H-bonds with water molecules. With an increase in temperature, H-bonds are progressively broken down. At a specific temperature, all the H-bonds are broken down, resulting in the separation of two phases, the aqueous phase and the surfactant phase, thereby giving a cloudy appearance to the solution. The temperature at which a solution becomes cloudy is referred to as the cloud point. The cloud point of block copolymers changes strongly depending on their hydrophobicity. The aqueous solution of hydrophilic block copolymers becomes cloudy at much higher temperatures. Both Pluronic® F127 and Tetronic® T1107 contain a relatively higher amount of PEO (70%), which is likely the cause of their cloud points being higher than the boiling point of water. When hydrophobic or lipophilic additives are added, they replace water molecules from the interior of micelles, thereby dehydrating the micelles, resulting in a lowering of the cloud point. The addition of PFOA lowers the cloud point of both copolymeric solutions; however, it is the highly hydrophilic nature of both copolymers that predominates over the dehydrating effect of PFOA. Accordingly, solubilization of PFOA up to its maximum concentration (30 mM) is insufficient to lower the cloud point of both 5% F127 and 5% T1107 solutions below 100 °C. Comparatively, hydrocarbons are more effective in lowering the cloud point than fluorinated PFOA, as they are incorporated into the micellar core, increase the hydrophobicity and disrupt the hydration shell.^19,34

3.2. Viscosity

Solubilization of hydrophobic additives into the different interiors of copolymeric micelles replaces the water molecules and dehydrates the micelles, resulting in the aggregation of copolymers.^19,21 Under this perspective, we have measured the solution viscosity of 5% F127 and 5% T1107 as a function of PFOA concentration at 30 °C, and the findings are presented in Fig. 3. A first outlook of Fig. 3 implies that there is an increase in solution viscosity of both copolymers because of solubilization of PFOA with a dominating effect on the 5% F127 solution. It has been attributed to the dehydration of block copolymeric micelles.^21,22 Ganguly et al.²² have demonstrated that the addition of phenol dehydrates Pluronic® P123 micelles, resulting in a spherical-to-rod-like micellar transition as a function of time. Similarly, dehydration-mediated micellar transition of Pluronic® P123 with time has been observed in the presence of hexan-1-ol.²¹ These additives dehydrate the hydrated shell region of Pluronic® micelles that facilitate the morphological changes. At a lower level of solubilization, due to H-bonding and hydrophobic interaction, added PFOA removes the water molecules from the outer shell (corona) of copolymeric micelles, resulting in swelling of micelles. It stimulates the solution viscosity to increase. A close look at Fig. 3 clearly demonstrates that at a low level of solubilization of PFOA up to 10 mM, which mainly contributes to the increase in solution viscosity. The highest solution viscosity is achieved at 10 mM PFOA concentration. At a higher level of solubilization, addition of PFOA does not have a significant effect on the solution viscosity of 5% F127 and 5% T1107 solutions. Accordingly, solution viscosity remains almost unaltered at the concentrations higher than 10 mM PFOA. Generally, solubilization of highly hydrophobic additives into the core of micelles does not have a significant effect on the viscosity of the solution containing copolymeric micelles.^21,25 Due to the hydrophobic nature, PFOA is expected to interact with the core of copolymeric micelles. Accordingly, the hydrophobicity of the micellar core is increased due to replacement of a water molecule by the long fluorinated chain of PFOA. Consequently, tighter packing of polymeric chains results in an increase in solution viscosity. The interaction of PFOA with the core of F127 and T1107 micelles is confirmed in the SANS part of the manuscript. The addition of hexane-1-ol increases the viscosity of the 5% P123 solution due to its solubilization in the palisade layer of micelles. However, solubilization of the much more hydrophobic decan-1-ol directly into the core of micelles does not significantly increase the solution viscosity.²¹ Direct penetration of hydrocarbon additive is possible for hydrocarbon additives that do not result in micellar transition. Similarly, the addition of polyaromatic hydrocarbons (PAHs), viz. naphthalene, phenanthrene, anthracene, and pyrene, does not significantly alter the solution viscosity due to their solubilization into the core of T1304 micelles.¹⁹ Conclusively, it can be stated that at lower PFOA concentration, mainly the dehydration of the corona region of 5% F127 and 5% T1107 micelles, which contain hydrophilic PEO groups, increases the solution viscosity. In contrast, at higher PFOA concentration, a significant increase in viscosity is not observed due to the solubilization of PFOA into the core of copolymeric micelles. It can be concluded that fluorinated PFOA displays stepwise interaction with different interior of polymeric micelles. Comparatively, hydrophobic hydrocarbon additives may directly penetrate into the core of micelles. We have also observed the effect of temperature on solution viscosity. It was perceived that the temperature has a negligible effect on the viscosity of both polymers.

Fig. 3 Effect of PFOA on the solution viscosity of 5% F127 and 5% T1107 solutions at 30 °C.

3.3. High-sensitivity differential scanning calorimetry (HSDSC)

HSDSC is used to determine the effect of PFOA concentration on the micellization of the linear block copolymer (F127) and the star block copolymer (T1107) and the results are presented in Fig. 4. Here, the CMT values of 5% F127 and 5% T1107 in the absence and presence of PFOA are presented in Table 1. In our case, T_m is considered as the critical micellization temperature (CMT) of the polymer. In the presence of PFOA, the CMTs of both block copolymers are noted to decrease, regardless of structural variation or micellization behaviour. The interaction between PEO groups and PFOA is stronger compared to that of PPO groups. This may be due to the involvement of ethereal oxygen atom of PEO groups in hydrogen bonding and ion–dipole interactions with the carboxylate headgroup of PFOA. In contrast, the fluorinated tail of PFOA is lipophilic in nature and displays limited compatibility with conventional hydrocarbon-based PPO groups. It resulted in the weaker PPO–PFOA interactions.^35,36 Similar results were found for tetrafluoroethanol,³⁷ haloethane, and isoflurane³⁸ in aqueous block copolymer solutions, where the effect of additives on the micellization behaviour was explained. Notably, the decrease in CMT of 5% F127 in the presence of PFOA is more pronounced than in T1107, which is attributed to the differences in micellar size, molecular weight, hydrophobicity, and water content within the micelles. It can be safely concluded that doping with PFOA facilitated the micellization of both block copolymers.

Fig. 4 HSDSC thermogram of (a) 5% F127 and (b) 5% T1107 as a function of PFOA concentration.

Table 1 CMT of 5% F127 and T1107 in the presence of PFOA determined from HSDSC thermogram

5% Block copolymer + [PFOA], mM	Tonset, °C	Tinf, °C	Tm, °C
F127
H2O	19.6	21.2	23.4
1 mM PFOA	16.0	17.6	21.5
5 mM PFOA	12.2	14.4	20.6
10 mM PFOA	—	—	19.4
20 mM PFOA	—	—	17.5
30 mM PFOA	—	—	13.6
T1107
H2O	25.7	27.1	30.4
1 mM PFOA	23.7	25.3	31.4
5 mM PFOA	18.8	22.0	29.5
10 mM PFOA	14.2	18.0	26.5
20 mM PFOA	—	—	22.7
30 mM PFOA	—	—	22.1

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3.4. Dynamic light scattering

The effect of PFOA on the aggregation behaviour of 5% F127 and 5% T1107 can be well understood by measuring the hydrodynamic size of the micelles. Fig. 5 shows the apparent hydrodynamic diameter (D_h) of 5% F127 and 5% T1107 at varying concentrations of PFOA at 30 °C.^21,39 The polydispersity of micelles is found to be low. The evaluated D_h of PFOA-free 5% F127 and 5% T1107 matches well with the reported work.^18,40,41

Fig. 5 Size distribution plots of (a) 5% F127 and (b) 5% T1107 in the presence of PFOA at 30 °C.

The D_h decreases as the concentration of PFOA increases at 30 °C. The change in D_h of block copolymer micelles depends on the location or interaction of the additive within the different micelle interiors.⁴² The diameter of 5% F127 was found to be 23.7 nm in the absence of PFOA. Doping with 5 mM PFOA reduces D_h to 15.1 nm, further decreasing to 10.7 nm with 10 mM PFOA. Increasing PFOA concentration beyond this level does not significantly reduce the D_h of the 5% F127 micelles. Specifically, raising PFOA from 10 mM to 30 mM results in only a slight decrease in D_h from 10.7 nm to 9.7 nm. Similarly, the addition of PFOA decreases the D_h of 5% T1107. Several factors may explain this phenomenon, including (i) its unique amphiphilic character, featuring a hydrophilic carboxylic acid head and a hydrophobic perfluorinated tail, which enables interactions with micelles and disrupts their structures, leading to smaller micelles; (ii) the incorporation of PFOA into the micelle core, altering its composition and size; (iii) its influence on the solvent environment surrounding the Tetronic® micelles. Changes in solvent quality can affect micelle swelling, leading to a reduction in size.^43,44 At lower concentrations, PFOA is thought to partition into both the water phase and the micellar phase and interact with the corona region containing PEO groups. Therefore, at this level, it binds water molecules to the PEO chain via hydrogen bonds, tightening the chain and decreasing D_h.⁴² The further addition of PFOA to Pluronic® and Tetronic® micelles makes them more compact, as their amphiphilic nature favours residing at the micelle–water interface, with their hydrophobic fluorocarbon tails inserting into the PPO core. This results in a tighter core packing and less hydration in the corona, mainly by pushing water out of the PEO chains. As a result, the micelle becomes smaller and more compact, with reduced hydrodynamic size. Such changes are in good agreement with the dehydration-induced decrease in size of micelles in the presence of PAHs. At higher PFOA concentrations, it penetrates deeper into micelles and dehydrates them. The distribution of D_h remains relatively stable despite increasing [PFOA], indicating these aggregates are stable and do not undergo morphological transitions with higher PFOA levels.²¹ Conversely, an increase in the hydrodynamic radius of micelles formed by the EO–PO block copolymer was noted in the presence of hydrocarbons. The increase in diameter resulted from hydrophobic interactions between PPO and hydrocarbons, leading to swelling of the hydrocarbon core and changes in micellar geometry.^19,45,46

3.5. Small-angle neutron scattering (SANS)

It should be noted that the D_h of the aggregate obtained from DLS corresponds to the diameter of micelles with water molecules surrounding them. Therefore, it is challenging to determine whether the decrease in the D_h of micelles is due to the micelles becoming smaller or the number of water molecules surrounding them decreasing. To understand what this indicates, SANS measurements are used to determine the actual size of the micelles. Fig. 6 depicts the scattering profile of 5% F127 and 5% T1107 in the presence and absence of PFOA at 30 °C. The 5% F127 and 5% T1107 form spherical micelles with a core radius of 50.3 Å and 31.7 Å, respectively. When PFOA is added to the aqueous solution of such block copolymer micelles, the scattering intensity increases, thereby indicating that the size of micelles in the solution increases. Still, there is no evidence of the evolution of large micelles. Initially, the addition of PFOA has little effect on the core of the 5% T1107 and 5% F127 spherical micelles but has a significant impact on the R_hs of the micelles. In the presence of 5 mM PFOA, the R_c of the 5% T1107 and 5% F127 micelles is slightly increased. This indicates that at low PFOA concentrations, the majority of PFOA remains more in the water continuous phase (near the shell region of the micelles) rather than the micellar phase, which allows the removal of water molecules near the PEO shell. Due to the presence of an eight-carbon chain and the fluorine element in the compound, it can reside between the two polymer chains by removing water molecules, acting as a non-ionic surfactant rather than a simple additive. As a result, the aggregation of PFOA in the polymeric micelles increases with an increase in the concentration of PFOA. The data obtained after fitting the SANS data are presented in Table 2. It shows that the addition of PFOA up to a 10 mM concentration results in a nominal increase in R_c of 5% F127 as well as 5% T1107 micelles, which increase to 54.9 Å from 50.3 Å and 35.5 Å from 31.7 Å, respectively. It can be stated that up to 10 mM concentration, PFOA preferably interacts with the hydrated shell of both the copolymeric micelles and dehydrates them such that very few PFOA molecules penetrate inside the micelles. However, it is reported that interaction and dehydration of the corona region of Pluronic® P123 micelles by hexan-1-ol results in an increase in size of micelles presenting the growth of micelles that resulted in an increase in the solution viscosity.²¹ This can be interlinked with the increase in solution viscosity of both block copolymer solutions up to 10 mM PFOA. The tightening of the PPO chain due to dehydration of micelles by PFOA in the core also contributes to the swelling of the core of micelles. Such increase in the size of micelles is also associated with an increase in aggregation number and a decrease in number density of micelles.²⁸ Because of the negative Gibbs transfer energy, the PFOA molecules have a strong thermodynamic tendency to transfer from the aqueous phase to the micellar phase.^45,47 The weak adhesive interactions between the transferred PFOA molecules and the water molecules are not nearly strong enough to offset the strong water–water cohesive interactions as these PFOA molecules are transferred to the interior of the aggregates. A close outlook of Table 2 also depicts that further increase in concentration of PFOA significantly increases the R_c of both 5% F127 and 5% T1107 micelles. At the higher level of solubilization, a large number of PFOA penetrates the block copolymeric micelles and squeeze water molecules from the core of the swollen micelles, resulting in dehydration of the polymer's PPO core to a larger extent. Accordingly, the R_c jumps to 65.9 Å and 50.0 Å upon addition of 30 mM PFOA. The removal of water molecules from micelles allows the incorporation of more copolymer monomers inside the micelles that facilitates micellization and results in an increase in the aggregation number (N_agg) of copolymers and PFOA. This causes the diffusion coefficient to decrease and the R_c of micelles to increase. There are two possibilities for improving the PFOA aggregation number in mixed micelles: (i) hydrogen bonding between the PFOA molecules and the PEO and PPO chains and (ii) hydrophobic interaction as well as the van der Waals force between the PFOA molecules and the PPO chain.

Fig. 6 Normalised scattering cross-section profile (d∑/dΩ) vs. the scattering vector (Q) for (a) 5% F127 and (b) 5% T1107 as a function of [PFOA] at 30 °C. The solid lines represent the fitted data.

Table 2 Micellar parameters for 5% F127 and 5% T1107 in the presence of PFOA at 30 °C obtained after fitting the SANS data

5% Block copolymer + [PFOA], mM	Rc, (Å)	σ	Rhs, (Å)	ϕ	Aggregation number, Nagg		Nd
					F127	PFOA
Rc = core radius, σ = polydispersity, Rhs = hard sphere radius, ϕ = micelle volume fraction, Nd = number density of micelles (×10^16 cm^−3)
F127
0	50.3	0.48	100.2	0.17	85	0	4.04
5	51.2	0.44	102.4	0.18	84	96	4.00
10	54.9	0.40	105.6	0.19	97	222	3.85
20	61.4	0.35	110.7	0.20	121	554	3.52
30	65.9	0.30	116.6	0.17	133	956	2.56
T1107
0	31.7	0.36	57.0	0.07	17	0	9.03
5	33.0	0.33	59.6	0.12	18	26	13.53
10	35.5	0.30	64.6	0.16	21	61	14.18
20	43.8	0.25	77.8	0.19	36	203	9.64
30	50.0	0.23	88.5	0.19	48	408	6.55

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Also, a decrease in polydispersity of micelles is observed in the case of both copolymers which indicates that micelles become more uniform in size and shape and are thermodynamically more stable. It is important to note that the morphology of the micelles remains changed with the addition of PFOA; however, we can assume that the PFOA causes the polymer to dehydrate, which may reduce the distance between the micelles. These findings are supported by the polymer's correlation peak in the presence of PFOA. The correlation peak position provides information about the interaction between neighbouring micelles.^48,49 The absence of a correlation peak in the absence of PFOA revealed that micelles are sparsely distributed, while its emergence suggests that micelles are within interacting distance. Gjerde et al.⁴⁹ suggested that the correlation peak emerges as a consequence of local ordering, tight micellar packing, and significant interactions between adjacent micelles resulting from an increase of hydrophobicity and dehydration. It can be correlated with the spatial correlation between neighbouring micelles. Mortensen et al.⁴⁸ reported that correlation peaks in SANS profiles become more prominent due to the growth of F127 micelles with an increase in concentration and temperature; however, they retained their spherical shape. For both polymers, the hard sphere radius is approximately 100.2 Å for 5% F127 and 57 Å for 5% T1107 at 30 °C in the absence of PFOA. This implies that the average distance between the micelles must be much greater than the diameter of the micelles. With the addition of PFOA, the emergence of a correlation peak in SANS profiles indicates a much tighter ordering caused by PFOA-induced dehydration and aggregation, resulting in tightly packed micelles without significant morphological change. It is a straight manifestation of a reduction in intermicellar distance, presenting denser packing. This finding suggests that strong ordering occurs, which could be attributed to significant local dehydration as N_agg increases. The emergence as well as sharpening of the correlation peak in SANS profiles offer solid experimental evidence for the growth of micelles and a decrease in the average distance between micelles.^48,49 However, hydrocarbons such as PAHs, cresols, and Surfynols® solubilize into the hydrophobic core of the polymer or surfactant micelles, leading to micellar growth due to hydrophobic restructuring to minimise free energy and increase the core volume.^19,34,46 On the other hand, fluorocarbons do not significantly alter the micellar size due to their lipophobicity and reduced interaction with the hydrocarbon chains of the polymer, resulting from the van der Waals forces. Such behaviour has been elaborately validated using a computational simulation approach.

3.6. Computational simulation

The optimised structures for the F127 + PFOA and T1107 + PFOA systems are depicted in Fig. 7, along with the orbital energy difference and frontier molecular orbitals (HOMO and LUMO). Table 3 displays the optimised quantum descriptions, where the band energy (ΔE) between the HOMO and LUMO energy gaps of the frontier molecular orbitals is used to characterise the charge-transfer interaction and stability of the system. In this case, the ΔE values for the F127 + PFOA and T1107 + PFOA systems are lower than those of their respective individual components, indicating the likelihood of undergoing favourable interactions. The F127/T1107 + PFOA system has a larger negative total energy (TE), which denotes the favourable stability of the system.

Fig. 7 Optimized structures of (a) F127 + PFOA and (b) T1107 + PFOA showing HOMO and LUMO.

Table 3 Computational descriptors for block copolymer + PFOA systems

System	T.E. (a.u.)	ELUMO (eV)	EHOMO (eV)	ΔE (eV)
F127	−574.04	−0.0592	−0.3621	0.3029
T1107	−1868.88	0.0435	−0.2064	0.2499
PFOA	−1942.84	−0.0504	−0.3077	0.2573
F127 + PFOA	−2516.93	−0.0566	−0.2212	0.1646
T1107 + PFOA	−3811.76	−0.0505	−0.1975	0.1470

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The counter plots of the 3D-molecular electrostatic potential (MEP) surface and Mulliken charge distribution of corresponding atoms evidently predicted the coordinating electrophilic centres of PFOA and the nucleophilic sites of the F127 and T1107 head groups. Here, the colour spectrum of the MEP is mapped to all other values by linear interpolation, where the most negative potential is assigned to red (nucleophilic centres). In contrast, the most positive potential is assigned to blue (electrophilic sites).

Additionally, the quantum chemical descriptors (QCDs) are also calculated by employing the following expressions:

IP = − E_HOMO (2)

EA = − E_LUMO (3)

where IP = ionisation potential, EA = electron affinity, GH = global hardness, GS = global softness.^50–52

The highest occupied molecular orbital (HOMO) relates to a molecule's ability to donate electrons to empty orbitals, while the lowest unoccupied molecular orbital (LUMO) represents the molecule's ability to accept electrons from filled orbitals. The energy band gap (ΔE) between E_HOMO and E_LUMO is used to evaluate the stability and reactivity of the examined system.⁵³ Table 3 suggests that ΔE for F127-PFOA and T1107-PFOA are smaller compared to their individual components, suggesting a stronger interaction between PFOA and F127/T1107. It can also be considered a thermodynamically convenient environment for dehydration. Here, we have found that the value of ΔE for the T1107-PFOA system is 0.1470 eV, which appears to be lower than its individual ingredients, T1107 (0.2499 eV) and PFOA (0.2573 eV), and also lower than F127-PFOA (0.1646). This demonstrates that T1107 and PFOA have the ability to interact well. The HOMO and LUMO energies are considered equal to the IP and EA, respectively.^54,55 The lower the IP, the easier it is for molecules to donate electrons. As can be seen from Fig. 8(a), T1107 has a lesser IP than PFOA; hence, T1107 can transfer charge easily. Similarly, EA is the measure of electron-accepting tendency. This can be easily visualised by frontier molecular orbitals, viz., HOMO and LUMO. As a result, the T1107 and PFOA molecules will attempt to approach one another, which will cause aggregates to form. GH measures the resistance of an atom to a charge transfer. It quantifies how difficult it is to either donate or accept electrons. In our case, T1107 and PFOA have GH values of 0.125 eV and 0.129 eV, respectively. Since the two values differ from each other, there is a strong tendency for the two molecules to interact. From these values, it is clear that T1107 has a lower GH value; hence, it donates charge readily to the PFOA molecule (Fig. 8).

Fig. 8 Plots of showing the (a) ionisation potential (IP), (b) electron affinity (EA), (c) global hardness (GH) and (d) global softness (GS) of F127, T1107, PFOA, F127 + PFOA and T1107 + PFOA.

4. Conclusion

In this study, we investigated the influence of PFOA on the aggregation behaviour of hydrophilic Pluronic® F127 and Tetronic® T1107 using an array of techniques. A very high hydrophilic nature (high PEO content) of F127 and T1107 is responsible for their cloud points being above the boiling point of water. The dehydration of copolymer micelles by doping with PFOA is not capable of lowering the CP below 100 °C. Dehydration of micelles results in an increase in solution viscosity, but the D_h obtained from DLS measurements suggests a lowering of the size of F127 and T1107 micelles. It was concluded that, at lower concentrations, the depletion of the hydration shell (corona) of copolymer micelles due to removal of water molecules (dehydration effect) caused by PFOA is responsible for tightening the PEO chain, and a decrease in the volume of aggregates is a probable cause for the decrease in D_h. HSDSC results clearly demonstrate that the addition of PFOA lowers the CMT of F127 and T1107, thereby confirming the PFOA-facilitated micellization of F127 and T1107. SANS results suggest that, irrespective of an increase in viscosity, the micelles remain spherical in shape in the presence of PFOA. PFOA, at lower concentrations, interacts with PEO groups in the corona region, while it penetrates into the core of polymeric micelles, where it dehydrates the core and increases the size of the core. It is a probable reason for retention of the morphology of block copolymer micelles. These findings are validated by the evaluated lower ΔE (eV) and larger negative T.E. (a.u.) values compared to their individual counterparts. A larger negative total energy (TE) clearly suggests that solubilization of PFOA leads to the stabilization of the F127 and T1107 copolymer micelles. It is evident from the GH values that T1107, having a lower hardness value gives off the charge easily to harder PFOA molecules, resulting in the attractive interactions causing the aggregation of molecules. Similarly, in the case of F127-PFOA, PFOA has a lower GH than F127, which indicates that PFOA gives off charge easily to F127. All these findings collectively confirm that PFOA displays preferably a stronger interaction with the linear block copolymer F127 than with the nonlinear block copolymer T1107. Also, doping of PFOA results in an additional thermodynamical stability to F127 and T1107 micelles. Generally, hydrocarbon-based additives can typically be solubilized in the core without dehydrating the corona, while fluorinated additives display fundamentally different behaviour, where stepwise penetration inside the micelle is observed: dehydration of the corona at lower concentration, followed by core penetration at higher concentrations. Accordingly, PFOA displays a dual interaction mechanism. These outcomes can be employed in designing the polymer-based PFOA remediation systems. The ability to stabilize and dehydrate micelles without modulating their geometry is valuable for designing soft materials. The findings offer valuable insights into the design of functional nanostructured materials using fluorinated additives.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgements

Ankur Patel greatly acknowledges the SHODH Fellowship (Fellowship No. KCG/SHODH/2022-23/2021017477), Education Department, Government of Gujarat, for the financial assistance. Paresh Parekh thanks the UGC-DAE Consortium for Scientific Research, Mumbai Centre, India, for the Collaboration Research Scheme Project CRS/2021-22/03/579, for the financial assistance. Vijay Patel thanks the Research Project (No. UGC/3402/2022) sponsored by the IPR-UGC section, Veer Narmad South Gujarat University, India, for the financial assistance. Vijay Patel also thanks Sardar Vallabhbhai National Institute of Technology (SVNIT), Gujarat, India, for providing the computational simulation and DLS facilities and the honourable Principal of Navyug Science College, Surat, Dr Ashwin S. Patel, for providing a good laboratory facility.

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