Degrafting of Polymer Brushes by Exposure to Humid Air

Abstract

It is well-known that polymer brushes can degraft in aqueous liquids. Here we show that brushes can deteriorate in humid air too. We observe that the detachment rate of the brushes increases with increasing relative humidity and hydrophilicity of the brushes. We relate this to the increase in water absorption as these parameters are increased. Our results imply that protective measures that are at present being developed for applications of brushes in liquids will also be key in enabling the long-term storage and utilization of hydrophilic brushes in air.

Polymer brushes consist of densely end-anchored long macromolecules that stretch away from the substrate.¹ They are popular building blocks in the design of functional surface modifications.² They have been employed in sensing devices,³ in separation technologies,⁴ as smart adhesives,⁵ and as lubricants in different topologies.^6,7 It has been recognized that strong surface anchors are needed for long-term application of hydrophilic brushes in aqueous liquids⁸⁻¹⁰ because otherwise such coatings can degraft in these media.

The degradation of polymer brushes has been widely observed and attributed to tension-enhanced hydrolysis at the bonds near the anchoring surface.^11,12 When hydrophilic brushes are exposed to water, they swell and absorb the liquid. This can affect the stability of the coating, since water molecules can more easily access the hydrolysis-sensitive bonds in swollen brushes.¹³⁻¹⁵ Theoretical and experimental research has shown that the most likely candidates for hydrolysis are siloxane, amide, and ester bonds because of their low binding energies.^12,16,17 Therefore, bond rupture is expected to occur within the initiator and anchoring point interface. Moreover, swelling increases the tension at the anchor points,¹⁸ which can lower the effective activation energy for hydrolysis. In fact, recent experiments have demonstrated that chain stretching enhances hydrolytic cleavage even for hydrophobic brushes in organic solvents, provided that sufficient water is present in the brush.¹⁹ Hence, the chemistry of the anchoring points and initiators plays an important role in degrafting reactions.^20,21

Surface-initiated atom transfer radical polymerization (SI-ATRP) is a versatile polymerization technique that is commonly used to grow a wide range of polymer brushes from different substrates using surface-immobilized radical initiators.^22,23 Among the various available anchoring groups, silane-based ones are commonly used because of their facile synthesis and reproducibility.²⁴ Among the family of silane compounds, 3-aminopropyltriethoxysilane (APTES) is commonly used to attach initiators for polymer brush growth to a substrate because of its bifunctional nature and improved stability compared with monosilanes.²⁵ Previous research has shown that polymer brushes anchored with APTES molecules show hydrolysis when the substrates are submerged in aqueous solutions for a short period of time.²⁶ Nonetheless, APTES anchors are still being used when the polymer brush coatings are not immersed in aqueous environments for their specific goal.²⁷ At room temperature, air can contain a certain amount of water vapor molecules, up to 2 wt % in saturated environments (100% relative humidity (% RH)). Even at low concentrations, these water molecules induce swelling of hydrophilic polymer brushes by solvation of the polymer–air interface, with swelling ratios reaching 1.8 for polyelectrolyte and polyzwitterionic brushes.²⁷ Hence, we anticipate that degrafting of polymer brushes can also occur in air, caused by water-vapor-induced swelling.

In this Letter, we present an experimental study of the long-term stability of polymer brushes in humid air at different relative humidities. Four polymer brushes with distinct degrees of hydrophilicity were grafted from silicon substrates using earlier-reported recipes together with in-house ones.^10,28,29 The polymer brushes were synthesized according to Figure 1b. APTES layers were first grafted onto clean silicon substrates, followed by the attachment of α-bromoisobutyryl bromide (BiBB) initiators. Lastly, polymer brushes were synthesized by means of copper-catalyzed SI-ATRP. The selected polymers were poly(methyl methacrylate) (PMMA), poly(2-hydroxyethyl methacrylate) (PHEMA), poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), and poly(3-sulfopropyl methacrylate) (PSPMA), as depicted in Figure 1c). Reaction specifications can be found in Supporting Information (SI) sections 2.2 and 2.3.

Figure 1.

(a) Schematic representation of the experimental setup composed of polymer brushes inside humid chambers. (b) Synthetic route to prepare polymer brushes by substrate functionalization and SI-ATRP. (c) Chemical structures of the four polymer brushes grown by means of SI-ATRP.

The functionalized substrates with brushes were stored in sealed chambers with controlled relative humidity provided by saturated salt solutions. The salts used in these experiments were LiCl, MgCl₂, NaCl, and K₂SO₄, which provided relative humidities of 11, 33, 75 and 97%, respectively. The preparation method, specifications of the salt solutions, experimental setup, and relative humidity stability results can be found in SI section 2.5, Table S1, and Figure S2). These chambers replicated microenvironments with different relative humidities, as depicted in Figure 1a. After set periods of time, the substrates were removed from the chambers and minimally rinsed. Next, their heights were evaluated by means of spectroscopic ellipsometry (SE). The results show that deterioration of brushes grown from APTES anchors also occurs in humid air, with enhanced degrafting for hydrophilic brushes at the highest humidity levels. This is not only relevant for the selection of a robust anchor for the use of polymer brushes in air, since our results imply that the water content in humid air suffices for detaching brushes for these anchors, but also for the long-term storage of polymer brushes, since they will naturally deteriorate in air after a certain period of time.

The presence of the brushes was confirmed by means of contact angle measurements (see Table S2 and Figure S3) and Fourier transform infrared spectroscopy (FTIR) (see Figure S4), and the thickness of the brushes was measured with SE. Grafting densities were calculated by combining kinetic studies in which brushes were grown simultaneously from silicon substrates and in solution. Reaction specifications can be found in SI section 2.4. The reaction kinetics was monitored by ¹H NMR spectroscopy, from which we could estimate the molecular weights of the polymers. The thickness of the brushes was obtained by means of ellipsometry. Combining those two results and using the following equation, the grafting density (σ) was calculated as

1

where h is the thickness of the film, ρ is the bulk density of the polymer, N_A is Avogadro’s , and M_n is the number-average molecular weight found via ¹H NMR measurements (see Table S3 for all values and Figure S5 for ¹H NMR spectra). The calculated grafting densities varied in the range of σ = 0.42–0.65 chains/nm². The difference in polymerization kinetics in solution and from the surface introduces an uncertainty in our grafting density. However, the difference in M_n for polymers grown in solution and polymers grown from the surface is typically 20–50%. This was acceptable for the present purpose and confirmed that in all cases we were in the high-density brush regime, as is commonly obtained by SI-ATRP.³⁰

Next, the polymer-brush-functionalized silicon substrates were stored in closed chambers with controlled relative humidity enabled by saturated salt solutions and monitored with relative humidity sensors, as represented in Figure 1a. In order to quantify the degrafting reactions, the brushes were taken out of the chambers after intervals of 1–5 days. The brushes were minimally rinsed with water and ethanol for 2 s in order to remove the physisorbed brushes on the surface. PMMA brushes were rinsed with acetone for 2 s. Immediately after, the decrease in the brush thickness over the time was evaluated by means of SE. Degrafting kinetic plots and the grafting density decay over time for all of the polymer brushes are shown in Figures S6–S8.

Figure 2 shows a comparison of the relative height losses for all of the polymer brushes after 8 weeks of storage inside the humid chambers. The dashed line represents the initial normalized height of each brush, and the color scheme represents the humidity levels inside the chambers. The results indicate that there is an enhanced detachment for hydrophilic polymer brushes (PSPMA and PMPC) compared with the more hydrophobic brushes (PMMA and PHEMA) and that the detachment rate increases at higher relative humidities.

Figure 2.

Changes in the relative heights of the four studied polymer brushes grafted from APTES anchoring layers after 8 weeks of storage inside humid chambers. The color scheme of the bars indicates the relative humidity inside the chamber, from lighter to dark blue: 11% RH, 33% RH, 75% RH, and 97% RH, respectively. The dashed line indicates the normalized initial height of the polymer brushes before they were stored in the humid chambers. The error bars indicate the standard error with a 95% confidence interval and are mainly caused by surface roughness. The initial height of the PMMA brushes was 25 nm, with σ = 0.42 chains/nm². For PHEMA brushes, the initial height was 100 nm, with σ = 0.60 chains/nm². The initial height of PSPMA brushes was 75 nm, with σ = 0.50 chains/nm². For PMPC brushes, the initial height was 60 nm, with σ = 0.65 chains/nm².

PMMA brushes did not show any detachment after 8 weeks at any relative humidity. For PHEMA brushes, we found a maximum of 7% height loss after 8 weeks of storage in humid chambers. Next, turning our attention to hydrophilic brushes, from the stability results in Figure 2 it is clear that PSPMA and PMPC show strong detachment in humid air, with enhanced degrafting at higher humidities. After 8 weeks of storage in humid chambers, there was up to a 62 and 70% loss of the initial height, respectively, in the highest-relative-humidity environment (97% RH). Both hydrophilic brushes showed significant detachment at low humidity already. On the origin of this effect we can only speculate, and we intend to study this in more detail in future work. Interestingly, different detachment trends were observed for the different hydrophilic brushes. For PMPC brushes, there was a gradual degrafting with an increase in the relative humidity in the chamber. For PSPMA brushes, in the range from 11 to 75% RH the degrafting reactions were relatively similar (loss of 20–30% of their initial height), whereas the strongest detachment occurred only at 97% RH, with a loss of 70% of the initial height.

We attribute the minimal degrafting for PMMA to its strong hydrophobic character. Even when completely immersed in liquid, it has a water absorption of only 2 wt %.³¹ PMMA brushes provide a stable coating when fully immersed in water for long periods of time.³² Hence, it was to be expected that in an environment with a reduced water content, detachment reactions would not occur because of the minimal accessibility of water molecules to the cleavable groups. PHEMA brushes can reach swelling ratios of up to 5 when fully immersed in water.³³ Even though they show such enhanced swelling responses, after 9 days of incubation in deionized water, PHEMA brushes did not show thickness changes,²¹ so minimal variations in humid media were to be expected. Hydrophilic brushes such as PMPC and PSPMA showed strong degrafting reactions when kept fully immersed in liquid after only a couple of days.^5,34 To understand the differences in detachment for hydrophilic brushes, their swelling behavior was evaluated at different relative humidities, as depicted in Figure 3. To do so, the thicknesses of the brushes were evaluated by atomic force microscopy (AFM) measurements at different relative humidities. Experimental details on the AFM measurements can be found in SI section 3 and Figure S9.

Figure 3.

Swelling ratios of PSPMA (circles) and PMPC (diamonds) polymer brushes at different relative humidities. These results were obtained by AFM measurements over the range from 0 to 80% RH. The initial height of the PMPC brushes was 100 nm, with σ = 0.71 chains/nm². For PSPMA brushes, the initial height was 65 nm, with σ = 0.41 chains/nm².

These findings show a gradual uptake of water vapor over the relative humidity range for the PMPC brushes, which absorbed water vapor at low RH. This water vapor uptake can facilitate degrafting reactions via hydrolysis of the cleavable points located at the anchors or via swelling-induced tension already in the lower humidity range. PSPMA brushes did not show the same swelling response but instead showed enhanced swelling ratios only at higher humidities, from ∼55% RH. These findings also correlate with the detachment results showing stronger height losses only at the highest humidities (75% and 97%), which are the levels where the chains start to absorb water vapor from the environment as shown by the swelling results.

In summary, we have shown that the degrafting of polymer brushes anchored to substrates with APTES layers also occurs in humid air. The detachment rate of polymer brushes is enhanced with increasing hydrophilicity of the polymer side chain and increasing relative humidity. We relate this to the increased absorption of water molecules in humid air with increases in these parameters. Different detachment dependences on the relative humidity were observed in hydrophilic brushes. PMPC brushes showed a gradual decrease in height with increasing RH, whereas PSPMA showed enhanced degrafting at only the highest humidities. Hence, the side chain of the brush induces different detachment trends even in humid air since a polymer brush coating will lose its functionality as a result of chain loss. Our results clearly show that APTES deposited by means of simple vapor deposition should not be used as an anchoring layer for hydrophilic brushes if the purpose is to obtain a long-lasting functional coating, independent of whether the application is in full submersion or in air. Annealing of the APTES layers²⁵ or the implementation of additional protective hydrophobic blocks⁹ or alternative anchor layers such as poly(glycidyl methacrylate) (PGMA) might provide better candidates for surface functionalization for a robust and long-lasting polymer brush coatings.¹⁰

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsapm.0c00474.

• Experimental procedures, characterization data, and detachment trends for all polymer brushes (PDF)

The authors declare no competing financial interest.