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. 2025 Sep 11;17(38):54139–54147. doi: 10.1021/acsami.5c14834

Competitive Adsorption in Polymer Nanocomposites: The Molecular Weight and End-Group Effect Revealed by SANS and MD Simulations

Tae Yeon Kong 1, WooJin Kim 2, YongJoo Kim 3,*, So Youn Kim 1,*
PMCID: PMC12464899  PMID: 40934868

Abstract

Understanding polymer adsorption at interfaces is essential for designing advanced polymer-based nanomaterials with tailored interfacial properties. Although adsorption significantly influences the macroscopic properties of polymer composites and thin films, a comprehensive understanding of molecular weight (MW)-dependent adsorption remains challenging and controversial, particularly in polydisperse polymer systems, due to the limitations of experimental approaches. Here, we investigate competitive adsorption in bidisperse poly­(ethylene glycol) (PEG) melts and demonstrate that shorter chains preferentially adsorb onto nanoparticle surfaces. Combined experiments and molecular dynamics simulations reveal that the high density of terminal hydroxyl groups in short PEG chains strengthens hydrogen bonding at the interface, thereby inducing enthalpy-driven adsorption despite identical polymer backbones. This results in a densely packed interfacial layer that alters the conformation of longer chains. These findings highlight the critical role of end-group functionality in interfacial polymer behavior and provide new insights for tailoring nanocomposite properties.

Keywords: polymer nanocomposite, polymer adsorption, competitive adsorption, interfacial chain conformation, bidisperse polymer melt, small-angle neutron scattering

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Introduction

The adsorption of polymers at interfaces is a ubiquitous phenomenon, and the unique behavior of adsorbed polymers, distinct from that of bulk polymers, has garnered significant interest over the past few decades. − This interest stems from the substantial correlation between the behavior of adsorbed polymers and the macroscopic properties of materials, such as glass transition temperature, , viscosity, thermal expansion, , and crystallization. For example, composite materials are made up of polymers with filler particles, and the physical properties of such composites are determined by the interactions between the particle surfaces and the polymer chains. −

When polymers adsorb onto surfaces, they lose conformational freedom, leading to a decrease in entropy, and the degree of entropy reduction generally depends on the chain length. − For chemically identical polymer melts without any molecular weight (MW)-dependent energetic preference for the surface, shorter and stiffer chains are theoretically predicted to favor adsorption, because the entropy loss associated with longer chains is greater due to their segmental connectivity. − Numerous theoretical studies, including density functional theory (DFT) , and self-consistent field theory (SCFT), , have reported that entropy-driven surface enrichment of shorter chains occurs in athermal bidisperse polymer melts. Hill et al. observed the surface enrichment of shorter chains using surface layer matrix-assisted laser desorption ionization time-of-flight mass spectrometry.

However, both theoretical and experimental studies have also reported that longer chains are preferentially adsorbed onto the surface, − demonstrating that confirming the MW-dependent chain preference for adsorption remains challenging. Monte Carlo (MC) simulations employing the wall polymer reference interaction site model (wall-PRISM) for binary polymer mixtures consisting of flexible and stiff chains showed that stiff longer chains preferentially partition to neutral surfaces, primarily due to their superior local packing efficiency near the interface. Experimental studies also indicated that preadsorbed short-chain polymers can be displaced by a long-chain polymer solution. For example, Fu and Santore , utilized coumarin end-labeled poly­(ethylene oxide) (PEO) and demonstrated, via total internal reflectance fluorescence, that preadsorbed short-chain PEO was replaced by long-chain PEO, attributed to the increased number of segment-surface contacts achievable by longer chains. Similarly, Dijt et al. observed through an adsorption isotherm that short PEO chains adsorbed onto silica were displaced by longer ones within minutes, attributed to the translational entropy gain. These experiments have contributed to the understanding of competitive adsorption behavior; however, they are based on chain exchange occurring under solution conditions, which may yield outcomes that differ from those in the melt state.

Competitive adsorption under solution conditions is further influenced by solvent-induced interactions and is more strongly affected by translational entropy compared to the melts, complicating the adsorption behavior. In dilute solutions, a part of the adsorbed chain can still interact with solvent molecules, thereby reducing the configurational entropy penalty for longer chains. Consequently, longer polymers may be preferentially adsorbed. Furthermore, the translational entropic gain per monomer is greater when more short chains are present in the dilute solution. , However, as the polymer concentration increases, the entropic gain from the solvent interactions becomes progressively restricted, and the impact of translational entropy is markedly reduced, causing this trend to reverse above a critical concentration. Although numerous studies have experimentally investigated MW-dependent competitive adsorption in solution, a fair comparison without solvent contribution for competitive adsorption under melt conditions has not yet been achieved and thus remains insufficiently understood, requiring further experimental investigation.

To address this gap, we designed the bidisperse polymer nanocomposites (PNCs) to investigate MW-dependent competitive adsorption of poly­(ethylene glycol) (PEG) melts on nanoparticle (NP) surfaces. Specifically, we first prepared PNCs by selectively adsorbing either long (or short) PEG chains onto nanoparticle surfaces. Subsequently, complementary short (or long) PEG chains were introduced, followed by thermal annealing under melt conditions to systematically examine competitive adsorption without solvent-induced interactions. We report the preferential adsorption of shorter chains, through direct experimental observations with small-angle neutron scattering (SANS) and Fourier transform infrared (FT-IR) spectroscopy and molecular dynamics (MD) simulations. Although this result resembles previously reported findings, our study reveal that enthalpic contributions originating from end groups play a more significant role than entropic effects. These results provide valuable insights for understanding and designing polymer interfaces in real systems, where the complete elimination of enthalpic biases is practically unattainable.

Experimental Section

Preparation of Polymer Nanocomposite (PNC)

PNCs were prepared with poly­(ethylene glycol) (PEG) and silica nanoparticles (NPs). PEG with different MWs, 0.4 and 10 kg/mol, were purchased from Sigma-Aldrich and used. Silica NPs with a diameter of 38 nm were synthesized by the Stöber method and used as dispersed in ethanol. The weight fraction of the NPs solution was about 0.1. The average size of silica NPs was determined by dynamic light scattering (DLS) and by fitting small-angle X-ray scattering (SAXS) intensity profiles to a spherical form factor.

To investigate the competitive adsorption behavior in a bidisperse polymer melt, a proper amount of the silica NP solution (ϕc = 0.1 for final PNCs) was added to either 0.4k (short) or 10k (long) PEG, ensuring that the polymer of selected MW predominantly adsorbed onto the particle surface, as illustrated in Figure S1. The mixed solutions were dried for approximately 2 h to fully remove the solvent, yielding the initial PNCs. These initial PNCs were then annealed at 70 °C for 24 h in a vacuum oven to obtain PNCs preadsorbed with either short or long polymer chains.

Subsequently, a polymer solution with a different MW from that of the preadsorbed chains (either 10k or 0.4k PEG) was added and dried to prepare the final bidisperse PNCs with selectively preadsorbed chains (Figure S1a,b).

To ensure sufficient chain mobility for chain exchange, the final PNCs were thermally annealed at 70 °C under vacuum conditions for 7 days (Figure S1c,d). After this process, both thermally annealed and nonannealed samples were characterized to compare modifications of interfacial layers.

Small-Angle Neutron Scattering (SANS)

SANS experiments were performed using the 40 m SANS instrument at the HANARO of the Korea Atomic Energy Research Institute (KAERI) in Daejeon, Republic of Korea. Two sample-to-detector distances of 1.16 and 17.5 m were employed to cover a q range of 0.004 < q < 0.4 Å–1.

For contrast-matched SANS measurements, we prepared solvent mixtures of deuterated ethanol (D-ethanol; ≥ 99.5 atom % D, Sigma-Aldrich, with all six hydrogen atoms are replaced by deuterium) and hydrogenated ethanol (H-ethanol, 99.9%). It is important to note that thermal annealing was conducted exclusively on PNC samples in the absence of solvent, and the ethanol mixtures were added only after annealing was fully completed.

The volume fractions of D-ethanol in ethanol mixtures (denoted as the d/(d + h) ratio) were systematically varied to 0.15, 0.90, 0.92, 0.94, 0.96, and 0.98 to finely tune the scattering length density (SLD) of the solvent environment and thereby achieve optimal contrast conditions for SANS analysis.

Samples were loaded into a 1 mm path-length quartz cell, and the cell temperature was maintained at 70 °C. The scattered intensity was corrected for background, empty cell scattering, and the sensitivity of individual detector pixels. The corrected data sets were placed on an absolute scale through the secondary standard method.

Fourier Transform Infrared Spectroscopy (FT-IR)

The FT–IR spectra were collected using a Tensor 27 spectrometer (Bruker) installed at the National Center for Interuniversity Research Facilities (NCIRF) at Seoul National University. Measurements were conducted over a wavenumber range of 400–4000 cm–1 with a spectral resolution of 1 cm–1, and each spectrum represents the average of 64 scans.

Simulation Method

All-atom molecular dynamics (MD) simulations were performed to investigate the behavior of polyethylene glycol (PEG) chains with different molecular weights. While experimental PEG chains had molecular weights of 0.4k and 10k, simulating such long chains was computationally prohibitive. Therefore, shorter PEG chains with molecular weights of 0.2k (N = 5) and 1.1k (N = 25) were used as proxies. The simulation system comprised a silica slab periodic in the xy-plane, PEG chains, and ethanol. Four simulations were conducted by varying the weight percentage (wt %) and mole number of PEG with two different lengths and ethanol in the system (Table S1).

The simulations were carried out using GROMACS 2020.3, − with the CHARMM36 force field (July 2022), , supplemented by the CGenFF version 4.6 − for parametrization. The NP z T ensemble was employed to equilibrate the system, as it allows for pressure coupling along the z-axis while maintaining the xy-plane dimensions fixed. Since the silica slab is periodic in the xy-plane, it represents an infinite slab in these directions, and the pressure coupling is applied only along the z-axis to accommodate density fluctuations in the bulk PEG/ethanol mixture and ensure proper equilibration at the interface. This approach is commonly used for slab-like systems to stabilize interfacial properties without distorting the periodic slab geometry. The simulation box dimensions were initially set to 8.35 × 8.51 × 12.5 nm. The density of the system, excluding the silica slab, was adjusted so that the combined PEG (0.2k and 1.1k) and ethanol molecules achieved a density of 1.0 g/cm3 at the start of the simulation.

The system was first equilibrated in the NP z T ensemble at 363 K and 1 bar for 10 ns to stabilize temperature and pressure. To achieve rapid equilibration, the velocity-rescaling thermostat was used to control the temperature, while the Berendsen barostat was employed for pressure coupling. In addition, although the experiments were performed at 70 °C, the simulations were conducted at 90 °C to promote faster equilibration and efficient sampling for the all-atom system. As the silica slab was periodic in the xy-plane, the pressure coupling was applied only along the z-axis. Following the initial equilibration, a more accurate thermodynamic ensemble was achieved by conducting an additional 10 ns NP z T simulation using the Nosé–Hoover thermostat − and Parrinello–Rahman barostat under the same conditions. Subsequently, production runs were performed in the NVT ensemble for 20 ns at 363 K, using the Nosé–Hoover thermostat. The final 2 ns of the production runs were analyzed to extract equilibrium properties. As shown in Figure S2a, the total energy fluctuation during the 20 ns NVT simulation was minimal, confirming that the system was sufficiently equilibrated within the last 2 ns. To further confirm that the system reached equilibrium, we analyzed the radial distribution functions (RDFs) between the hydrogen atoms of the silica surface and the oxygen atoms of PEG (both backbone C–O–C and terminal O–H groups) over two different time intervals: the last 5 ns and the last 2 ns of the simulation (Figure S2b). These RDFs were found to be virtually identical, indicating that the local polymer structure near the interface was stationary over time. This result supports that the system is well equilibrated not only in energy but also in spatial distribution.

All bonds involving hydrogen atoms were constrained using the LINCS algorithm, allowing a time step of 2 fs. Electrostatic interactions were treated using the Particle Mesh Ewald (PME) method with a real-space cutoff of 1.2 nm. The PME interpolation order was set to 4, and the Fourier grid spacing was set to 0.16 nm.

Density Measurement

The densities of polymers and PNCs were measured using a gas displacement pycnometer (AccuPyc II). Each sample was measured at least 9 times, and the average density was used.

Results and Discussion

To investigate MW-dependent competitive adsorption, we first prepared PNCs in which either long or short PEG chains were selectively adsorbed onto silica nanoparticles, and then introduced the complementary short or long PEG chains to produce bidisperse PNCs. Figure S1 illustrates the experimental procedure: two bidisperse PNCs were prepared where either low (0.4k) or high (10k) MW PEG was preferentially adsorbed onto NPs (Figure S1a,b). Then, two PNCs were annealed for a sufficient time (7 days) to allow possible chain exchange ((c) and (d)). We thus obtained four PNCs for comparison: “0.4k first PNC” and “10k first PNC”, both before and after annealing. Unless otherwise specified, the volume ratio of PEG 0.4k to 10k is maintained at 1:1, and particle volume fraction (ϕc) is fixed at 0.1. Then, the MW-dependent adsorption layers were compared for all four PNCs (Figure S1), where all chemical compositions are identical.

Although the chemical compositions of four PNCs are identical, the particle surface of (a) and (b) is selectively enriched with 0.4k and 10k polymers, respectively, and these selective adsorption layers can be altered in (c) and (d) after the following annealing process.

To directly observe such changes in the adsorbed polymer layers, contrast-matched small-angle neutron scattering (SANS) experiments were conducted. To minimize potential energetic biases in adsorption behavior between hydrogenous and deuterium-labeled PEG chains on silica particles, we used highly concentrated solutions and contrast-matched the SLD of the silica with deuterated ethanol (D-ethanol) polymer solutions, as previously reported , and shown in Figure S3.

Figure a,b and Figure S4 show the changes in scattered intensity, I­(q), for 10k and 0.4k first PNCs as the SLD of the medium varies. For the 10k first system, the scattered intensity progressively decreases as the SLD of the medium approaches the silica matching condition (from “b” to “f” in Figure S3). Under this condition, the characteristic oscillation peak near q∼0.03 Å–1, arising from the form factor of silica NPs (black triangle), gradually diminishes and becomes nearly undetectable at the matching point. This behavior indicates successful contrast matching with the silica particles, resulting in the polymer–polymer correlations becoming the dominant contributor to the scattering intensity.

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Contrast-matched SANS intensity profiles for (a) 10k first and (b) 0.4k first PNCs at different d/(d + h) ratios before and after thermal annealing. The curves were vertically shifted for clarity and ease of comparison. (c) Polymer shell form factor, P shell(q), calculated from the corresponding SANS intensities. Black lines represent the form factor fits. (d) Schematic illustration for bulk and adsorbed polymers in 0.4k and 10k first PNCs before and after thermal annealing. Upon the addition of the ethanol mixture, the polymer concentration becomes ∼50 wt %, and the particle volume fraction is reduced to ∼0.05.

Similarly, the 0.4k first PNC also shows a reduction in scattering intensity near the matching condition, with the absence of the silica signature peak, confirming that the measured scattering predominantly arises from the polymer–polymer correlations rather than particles themselves.

Importantly, both short (0.4k) and long (10k) PEG chains were hydrogenated, precluding direct differentiation of individual chain species by SANS; nevertheless, the observed scattering patterns remain highly informative. Specifically, we observe a distinctive scattering peak emerging at lower q∼0.02 Å–1 (purple arrow). The correlation length between bulk polymer segments is related to the high-q range (0.15 ∼ 0.76 Å–1, with radii of gyration, R g , of approximately 0.83 and 4.2 nm for 0.4k and 10k PEG, respectively). Accordingly, this low-q feature corresponds to correlations between polymer segments adsorbed onto nanoparticles (∼38 nm) at length scales significantly larger than the size of individual polymer chains, reflecting unique, nonbulk-like spatial arrangements within the adsorbed layers.

To obtain the structural information about the adsorbed layers in more detail, we extracted the polymer–polymer structure factor, S pp (q), from the series of I­(q) near the matching condition by solving the simultaneous equations, as shown in Figure S5 (see also the Supporting Information for more detailed characterization). The S pp (q) provided consistent information with the matched condition of I­(q).

We then examined changes in the adsorbed layers of PNCs after thermal annealing. During annealing, the selectively adsorbed polymer layers are expected to undergo modification, as competitive adsorption can lead to the replacement of previously adsorbed layers. The solid lines in Figure a,b show the SANS intensity profiles of the 10k first and 0.4k first PNCs after annealing. For the 0.4k first PNC (Figure b), no noticeable difference in intensity is observed before and after annealing. This indicates that the initially adsorbed short chains exhibit resistance to the challenges posed by the longer polymer.

However, intriguingly, the 10k first PNC (Figure a) exhibits significant changes in intensity profiles after annealing, suggesting that the 0.4k PEG in bulk influences the adsorbed layer. This result indicates that although PEG 10k initially adsorbs onto the particle surface, the chain conformation at the interface is altered by the penetration of PEG 0.4k from the bulk. The broad peak observed in the annealed 10k first PNC implies the formation of a thicker polymer layer.

To quantitatively analyze and solely obtain information about the adsorbed polymer shell, we extracted the polymer shell form factor, P shell(q), by dividing the S pp (q) by the S cc (q). This procedure effectively cancels contributions from particle arrangements, enabling selective characterization of the polymer shell. Subsequently, P shell(q) was fitted with a core–shell model to estimate the shell thickness, as shown in Figure c. Detailed procedures for obtaining P shell(q) are provided in the Supporting Information.

Figure d summarizes the interfacial changes and variations in the layer thickness of adsorbed polymers. The 0.4k first PNC forms an adsorbed layer of ∼1 nm thickness, corresponding to the R g of preadsorbed PEG 0.4k, which remains the same as P shell(q) shows no notable changes after annealing.

In contrast, the P shell(q) of 10k first PNC without annealing exhibits the peak at a lower q value compared to the 0.4k first PNC, indicating the formation of a thicker adsorption layer (∼2.3 nm), attributed to the longer chain length of preadsorbed PEG 10k. Following annealing, the peak further shifts to the left, implying the formation of an even thicker polymer shell (∼4.9 nm) resulting from changes in the adsorbed polymers and their conformations.

Taken together, when short chains are adsorbed first, there is no change in the polymer shell layer. However, when long chains are initially adsorbed, the layer thickness increases after annealing, presumably due to the migration of short chains from the bulk to the interface. These contrasting behaviors of adsorbed layers provide compelling evidence for the preferential adsorption of shorter chains.

Theoretically, the preferential adsorption of shorter polymers on particle surfaces is often attributed to entropy-driven mechanisms; shorter chains experience a smaller entropy penalty upon adsorption when there is no energetic preference for polymer adsorption. − However, even though short and long PEG chains are chemically identical, an energetic preference may still arise from their difference in chain length.

PEG can form hydrogen bonds with the silanol groups (Si–OH) on silica NPs through its ether (C–O–C) bonds along the backbone, as shown in Figure S8. The hydroxyl (O–H) groups at the chain end can form stronger hydrogen bonds with silanol groups. Since PEG 0.4k has significantly more end groups per unit volume than PEG 10k, it has a greater probability of forming hydrogen bonds. Thus, there can be an additional enthalpic contribution to the preferential adsorption of shorter chains.

To elucidate the driving force behind the preferential adsorption, we examined the interactions between PEG and silica using Fourier transform infrared (FT-IR) spectroscopy. Figure shows the FT-IR spectra for silica particles, neat polymers, and PNCs containing only a single MW (either 0.4k or 10k PEG), and bidisperse PNCs containing both 0.4k and 10k PEG.

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FT-IR spectra (1200 to 900 cm–1) for (a) silica NP, 10k only PNC (without 0.4k PEG), and 10k first PNCs, and (b) 0.4k only PNC (without 10k PEG) and 0.4k first PNC. (c) FT-IR spectra (4000 to 1500 cm–1) for silica NP, 10k only PNC, and 0.4k only PNC. (d) FT-IR spectra (4000 to 3000 cm–1) for 0.4k first and 10k first PNCs before and after thermal annealing.

In Figure a, the peak at ∼1095 cm–1 for neat PEG 10k is attributed to the stretching vibration (Vs) of the C–O–C bonds in the PEG backbone. Upon interacting with silica particles (10k only PNC), this peak red shifts to ∼1086 cm–1, indicating a decrease in the electron density of the C–O–C bonds due to the formation of hydrogen bonding with the silanol groups. Unfortunately, further analysis of the silanol groups was challenging due to the overlapping peaks of the Si–OH group and the C–C bond in PEG at ∼950 cm–1.

To examine changes in the adsorption behavior of bidisperse PNCs, Figure a also shows the FT-IR spectra of 10k first PNCs before and after thermal annealing. Before annealing, the particle surface is predominantly covered with 10k chains, resulting in a C–O–C peak similar to that of the 10k only PNC. However, after annealing, an intriguing blue shift of the peak is observed from 1082 to 1088 cm–1, resembling the spectrum of neat PEG 10k. Since red shifts occur when the C–O–C bonds form hydrogen bonds with silanol groups, the observed blue shift suggests the dissociation of pre-existing hydrogen bonds between PEG 10k (C–O–C) and the silanol groups.

Figure b demonstrates that PEG 0.4k also exhibits a red shift of the C–O–C peak when interacting with silica (0.4k only PNC). However, the identical peak position is found for the 0.4k only PNC and 0.4k first PNC, suggesting that the silica surface is predominantly covered with PEG 0.4k chains. Even after annealing, the peak at 1084 cm–1 remains unchanged. This invariable peak position indicates that the interaction between the 0.4k chains and the silica surface is maintained during the annealing process, and there is no additional adsorption from 10k chains.

To further investigate this disruption of hydrogen bonding observed in Figure a, we examined the free O–H peak. In Figure c, a peak at ∼3445 cm–1, attributed to the intermolecular hydrogen bonding of terminal O–H groups, is observed only when PEG 0.4k is present. This peak is not observed in PEG 10k due to the significantly fewer end groups.

In Figure d, we note that the free O–H peak is observed in all PNCs from the end O–H groups of PEG 0.4k. For quantitative analysis, the peak position was determined by fitting the data using Gaussian and linear functions.

For the 0.4k first PNC, upon nanoparticle incorporation, hydrogen bonding predominantly occurs between the free O–H groups of PEG 0.4k and the silanol groups on the particle surface, resulting in the free O–H peak at approximately 3445 cm–1. Notably, this peak position remains unchanged after thermal annealing, indicating that once PEG 0.4k chains are initially adsorbed, no further interactions between terminal O–H groups and silanol sites occur during subsequent thermal treatment.

In contrast, when PEG 10k is initially adsorbed, hydrogen bonding primarily involves interactions between the ether groups (C–O–C) and the surface silanols. Consequently, the free O–H peak initially appears at a lower wavenumber (3432 cm–1) compared to the 0.4k first case. Upon annealing, however, this peak undergoes a blue shift to 3446 cm–1, closely resembling the position observed for the 0.4k first PNC. This shift indicates that free O–H groups of PEG 0.4k migrate from the bulk polymer phase to the particle interface as a result of chain exchange during thermal annealing.

These findings indicate that initially adsorbed PEG 0.4k (short chains) maintains its interactions with the silica surface, resisting displacement by the bulk PEG 10k (long chains), even after thermal annealing. In contrast, in the 10k first PNC, the initially adsorbed PEG 10k is displaced during annealing, allowing the bulk PEG 0.4k to migrate to the interface and modify the adsorption layer.

To investigate the competitive adsorption between long and short PEG chains, MD simulations were conducted with PEG chains of varying lengths at the same weight fraction. Figure a presents MD snapshots of a system containing silica surface, PEG 5-mers, and PEG 25-mers in ethanol solvent, both at the initial state and after 40 ns. The snapshots reveal that the shorter PEG chains exhibit significantly higher adsorption onto the silica surface compared to longer chains. Figure b and Figure S9 present the radial distribution function (RDF), g(r), calculated for oppositely charged atoms between the silica surface and PEG, which exhibit attractive interactions. The RDF analysis reveals the atomic interactions that most strongly influence PEG adsorption. In particular, the interaction between the hydrogen atoms on the silica surface and the oxygen atoms in PEG exhibits a pronounced peak at approximately 2 Å, compared to those of other atomic pairs. In general, RDF peaks below 3.5 Å are attributed to electrostatic attractions, and if the interacting atoms are capable of forming hydrogen bonds, they are considered as such. This result indicates that hydrogen bonding between the silica surface’s hydrogen atoms and PEG’s oxygen atoms plays a critical role in PEG adsorption.

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(a) MD snapshots of a system with a silica surface and PEG (5-mers and 25-mers) in ethanol, shown at the initial state (left) and after 40 ns (right). (b) Radial distribution function (RDF), g(r), calculated for oppositely charged atoms between the silica and PEG molecules. (c) RDF between hydrogen atoms on the silica and oxygen atoms in the C–O–C or O–H groups of PEG. (d) Partial charges of atoms assigned by the simulation. (e) Nonbonded interaction energies, ΔE, between PEG chains and the silica surface from the MD trajectories. α indicates the molar ratio of PEG 5-mer to 25-mer.

To identify which oxygen-containing functional groups in PEG most strongly participate in hydrogen bonding with the hydrogens of silanol groups, we examined the RDF between silica surface hydrogens and oxygens of the C–O–C backbone and terminal O–H groups in PEG. Figure c shows a pronounced peak between O–H oxygens and silanol hydrogens, confirming that terminal O–H groups primarily form hydrogen bonds with silanol groups. This observation is consistent with the partial charges assigned in the simulation (Figure d), where the oxygen in the C–O–H group carries a significantly larger negative charge (−0.640) than that of the C–O–C oxygen (−0.342). Furthermore, the higher RDF peak for shorter chains compared to longer ones suggests that the greater number of terminal O–H groups in shorter PEG chains leads to relatively stronger interactions with silica particles.

To quantitatively assess the enthalpic contributions to PEG adsorption, we extracted the nonbonded interaction energies (Lennard-Jones + Coulombic interactions, denoted as ΔE) between PEG chains and the silica surface from the MD trajectories, as shown in Figure e. Interestingly, the total interaction energy for PEG 5-mer is much stronger than that of PEG 25-mer at the same weight fraction, confirming that hydrogen bonding at chain ends plays a dominant role in adsorption. Further analysis of the influence of α, which is the molar ratio of short to long PEG chains, on interaction energetics will be discussed in the subsequent section.

Considering that the difference in the number of repeat units (N) used in the simulation (5-fold) is much smaller than the actual difference between PEG 0.4k (N = 9) and PEG 10k (N = 227), it can be expected that shorter PEG chains are significantly more likely to adsorb in the experimental system. These findings suggest that the preferential adsorption of shorter PEG chains arises largely due to the strong enthalpic contributions from differences in hydrogen bonding between functional groups.

The preference for shorter chains is attributed to the substantially higher number of O–H groups per unit volume in PEG 0.4k than PEG 10k. Therefore, when the number of moles (i.e., the number of end groups) of PEG 0.4k and PEG 10k is identical, the energetic interaction preference of silanol for the O–H groups with chain lengths can be minimized.

Figure a shows the FT-IR spectra of the 10k first PNCs, prepared with varying molar ratios (α0.4k) of PEG 0.4k to PEG 10k. When the number of O–H groups in PEG 0.4k is lower than that in PEG 10k (α0.4k < 1), no shift in the C–O–C peak is observed with thermal annealing. However, when the number of O–H groups in PEG 0.4k exceeds that in PEG 10k (α0.4k ≥ 1), the C–O–C peaks undergo a blue shift. This shift indicates that preferential adsorption occurs due to the replacement of hydrogen bonds from the C–O–C groups of PEG 10k with those involving the terminal O–H groups of PEG 0.4k.

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(a) FT-IR spectra for 10k first PNCs with varying molar ratios of PEG 0.4k to and PEG 10k (α0.4k). (b) local density profiles for 10k first PNCs with varying molar ratios of PEG 5-mer to PEG 25-mer (α). (c) Schematic illustration for the preferential adsorption of shorter chains in 10k first PNC. (d) Measured densities of 0.4k first and 10k first PNCs before and after thermal annealing.

This result is further supported by MD simulation observations (Figure b), which show that when the mol ratio of short-chain PEG is low, the local density of long chains is higher. However, as the molar ratio of short-chain PEG increases, the local density of the short chains increases more significantly. Furthermore, Figure e clearly demonstrates that the total interaction energy varies significantly with the molar ratio (α). PEG chains with a higher number of terminal hydroxyl groups exhibit stronger total interaction energies with the substrate. Specifically, for all simulation systems, PEG species with higher OH density showed more negative interaction energies.

Our results confirm that the preferential adsorption observed in both experiments and simulations is enthalpy-driven nature, consistent with previous studies. , Frantz et al., particularly, showed that carboxylic acid-terminated polystyrene (PS) adsorbs more strongly onto silicon oxide surfaces than hydrogen-terminated PS, with a sticking enthalpy difference of 6.4 kT per carboxylic acid end group. This preferential adsorption becomes increasingly pronounced with larger MW differences, resulting in a greater surface excess of the carboxylic acid-terminated species.

Figure c illustrates the competitive adsorption behavior in the 10k first PNC. Initially, silanol groups predominantly form hydrogen bonds with the C–O–C bonds of PEG 10k. However, as indicated by the changes in FT-IR spectra, these hydrogen bonds are gradually replaced by new hydrogen bonds involving the free O–H groups of the added 0.4k. Due to the high energetic barrier for whole chain desorption (>100 kT), this replacement occurs through the desorption of segments of PEG 10k rather than the entire chain. In this case, a “loop” conformation is likely to form. Alternatively, the end O–H group of PEG 10k may also be replaced by the end O–H group of PEG 0.4k, leading to the detachment of PEG 10k segments near the chain ends and the formation of a “tail” conformation.

Shorter and stiffer polymer chains can pack more efficiently at interfaces. Previous studies have reported that increasing MW induces packing frustration among polymer chains in the interfacial regions, consequently reducing the mass density of PNC. , The 0.4k first and 10k first PNCs possess identical chemical compositions and volume fractions, implying that simplistic theoretical calculations might predict identical densities. However, the actual density of each PNC can differ depending on which MW of polymer initially adsorbs onto the nanoparticle surface, since this adsorption governs the initial polymer chain conformation at the interface. Specifically, the adsorption of shorter PEG 0.4k chains among longer PEG 10k chains through chain exchange can result in a denser polymer layer at the nanoparticle interface.

As shown in Figure d, before annealing, the 10k first PNC exhibited a lower density than the 0.4k first PNC. However, after annealing, unlike the 0.4k first PNC, where there was no change in density, the 10k first PNC showed a significant increase in density. Its density eventually became similar to that of the 0.4k first PNC, further supporting the preferential adsorption of PEG 0.4k. The result indicates that a densely packed interfacial layer is formed through the exchange of long PEG segments and short PEG chains, promoting the elongation of the long chains and increasing the thickness of the SIL, as confirmed by SANS results.

The stretching of the SIL induces an entropy loss due to the reduced configurational freedom of the elongated chains. However, this entropy penalty can be compensated by the enthalpy gain from replacing the hydrogen bonds between the C–O–C groups of PEG 10k and the Si–OH groups with the stronger hydrogen bonds involving the terminal O–H groups of PEG 0.4k.

Conclusions

Our comprehensive investigation, integrating both experimental observations and molecular dynamics simulations, demonstrates that shorter polymer chains preferentially adsorb onto the surface in bidisperse polymer melts. This preferential adsorption arises from enthalpic contributions due to differences in the number of functional groupsspecifically, the higher density of terminal hydroxyl groups in shorter chainswhich enhance hydrogen bonding interactions with the surface, despite the polymers having identical chemical structures. This behavior leads to a densely packed interfacial polymer layer that induces stretching of the preadsorbed longer chain conformations, significantly influencing the interfacial properties.

These results highlight the critical impact of molecular weight on polymer adsorption behavior and emphasize the importance of both chain length and end-group functionality in determining interfacial characteristics. Understanding these mechanisms provides valuable insights for tailoring interfacial interactions in polymer nanocomposites and offers a strategic pathway for designing advanced materials with optimized performance. Future work will focus on elucidating how changes in the adsorbed layer affect macroscopic properties and particle dispersion structures within the composites, thereby advancing our ability to engineer materials with targeted functionalities through precise control of interfacial phenomena.

Supplementary Material

am5c14834_si_001.pdf (1.1MB, pdf)

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIT) (NRF-2021R1A2C2007339). The Institute of Engineering Research at Seoul National University provided research facilities for this work. SANS experiments were performed using the 40 m SANS instrument at the HANARO of the Korea Atomic Energy Research Institute (KAERI) in Daejeon, Republic of Korea.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.5c14834.

  • Supporting Information: Schematic diagram for the preparation of preadsorbed bidisperse PNC samples; Scattering length density and contrast matching method for SANS; SANS intensities; Partial structure factors; Total energy for MD simulation systems; Compositions for MD simulation systems; Calculated radial distribution functions (PDF)

The authors declare no competing financial interest.

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