Polymer adsorption – reversible or irreversible?

Sanat K. Kumar

Andrew M. Jimenez

The concept of a strongly adsorbed macromolecular layer at a surface has been appreciated since the early 1900's. A recent revival of the topic has focused on understanding the kinetics of bound layer formation and its exchange with surrounding chains, notably by Granick’s pioneering experiments.^1–3 As suggested by de Gennes, this layer can be temporally persistent due to the number of contacts that a random walk chain with a degree of polymerization N makes with a flat surface.⁴ Thus, a chain of N = 2500, with an adsorption energy of ∼k_BT per segment, has a net adsorption energy of ∼50k_BT per chain. Evidently, while each chain segment can adsorb and desorb freely, the time scale of the simultaneous desorption of a whole chain is likely to be long and strongly temperature dependent. That such localization occurs, and its effects on the apparent thermodynamic^5,6 and dynamic⁷ properties of thin polymer films, is now well-documented. The corresponding phenomenon of chain adsorption on nanoparticles (NPs) has been appreciated for over 50 years; this bound polymer layer has been shown to play a critical role in sterically stabilizing NPs, avoiding agglomeration,⁸ and leading to mechanical reinforcement⁹ beyond that of macroscopically-inspired two-phase models. The relevance of these adsorbed layers to the macroscopic properties of thin films and nanocomposites makes these issues of great current interest.

Two recent articles in Soft Matter, a review by Napolitano (“Irreversible adsorption of polymer melts and nanoconfinement effects”, DOI: 10.1039/D0SM00361A) and a paper by Roth and coworkers (“Review and reproducibility of forming adsorbed layers from solvent washing of melt annealed films”, DOI: 10.1039/D0SM00565G), take different approaches to this problem. While there are many similarities in these papers, the nature of the adsorbed layer – whether it is dynamic or glassy (which has also been argued about in the case of NP-based systems^10,11) – is an essential distinction. Napolitano argues that these layers are effectively irreversibly adsorbed such that, upon reaching steady-state, any dynamic measurements of these systems have essentially no time dependence. That is, there is postulated to be a strong decoupling between the relaxational time scales for the adsorbed layer and the free (unadsorbed) chains – the adsorbed chains relax (and exchange) so slowly, if at all, that they basically do not participate in most dynamic measurements, which focus on the more mobile bulk chains. This non-equilibrium viewpoint is reiterated by Roth: “We find h_ads(t) curves to be far less reproducible and reliable than implied in the literature, strongly dependent on solvent washing and substrate cleaning conditions… the glassy film harder to wash off…” Such sensitivity to preparation techniques is a classic signature of a system out of equilibrium.

An important difference in their perspectives, however, is that, as Roth states, “polymer chains in solution are highly mobile, diffusing and exchanging on the surface even in the limit of strong adsorption, contradicting Guiselin's assumption” and “However, even in these strongly bonded systems, the surface bound chains are not irreversibly adsorbed, but can exchange with matrix chains.”

While the overall perspectives of Napolitano and Roth, especially on the reversibility of the adsorption process, appear, initially, to be at odds with each other, they are nevertheless consistent with the strongly temperature dependent time scale of the desorption process, and how it relates to the observation time scale of a particular experiment. To date, most melt experiments, e.g. P2VP melts mixed with silica NPs,¹² operate at temperatures (typically less than 1.2T_g) where the desorption times of the bound layer are on the order of days or much longer. Indeed, according to Koga et al.,¹³ one has to go well above 200 °C before the bound P2VP chains will desorb on the time scale of a calorimetry experiment. Performing experiments at these high temperatures is restricted by the degradation of the polymer. Thus, we believe that, while desorbed chains must exchange freely with melt chains of the same chemistry, whether this process occurs on time scales where they affect other dynamic measurements is strongly temperature dependent; under typical conditions studied to date the time scales of these phenomena are strongly decoupled so that the bound layer appears temporally frozen. This appears to be consistent with almost all macroscopic property measurements made on such systems.

So where do these studies leave us and how do we reconcile the apparently different pictures embodied in these two approaches? The first is the realization that the consequences of having equilibrium vs. non-equilibrium adsorbed layers can only be ascertained by carefully designed experiments that can controllably vary the adsorption energies across a spectrum of values. This should make the relevant adsorption/desorption processes occur on time/temperature scales relevant to (or, in the opposite case, completely decoupled from) other dynamic relaxations in the system. Further studies, potentially extending to nanocomposites, where the surface to volume ratio is large even in macroscopically large samples, can help to alleviate signal-to-noise issues encountered in typical thin film experiments. These studies can also focus on decoupling confinement effects from those of bound layer interactions in highly loaded polymer nanocomposites where every chain participates in adsorption. Thin film and composite studies are both important for understanding the effective “irreversibility” of these layers; understanding this underpinning issue should lead to greater insight in our understanding of their thermomechanical properties.

References

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12. A. M. Jimenez, D. Zhao, K. Misquitta, J. Jestin and S. K. Kumar, Exchange Lifetimes of the Bound Polymer Layer on Silica Nanoparticles, ACS Macro Lett., 2019, 8, 166–171.
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