Effect of Interfacial Adsorption on the Stability of Thin Polymer Films in a Solvent-induced Process

The stability of ultrathin polymer films plays a crucial role in their technological applications. Here, we systematically investigated the influence of interfacial adsorption in physical aging and the stability of thin polymer films in the solvent-induced process. We further identify the stability mechanism from the theory of thin film stability. Our results show that the aging temperature and film thickness can strongly influence the stability of thin PS films in acetone vapor. Physical aging can greatly improve the stability of thin polymer films when the aging temperature Taging1 > Tg. A thinner PS film more quickly reaches a stable state via physical aging. At short aging time, the formation of the adsorbed layer can reduce the polar interaction; however, it slightly influences the stability of thin polymer films in the solvent-induced process. At later aging stage, the conformational rearrangement of the polymer chains induced by the interfacial effect at the aging temperature Taging1 plays an important role in stabilizing the thin polymer films. However, at Taging2 < Tg, the process of physical aging slightly influences the stability of the thin polymer films. The formation of the adsorbed layer at Taging2 can reduce the short-range polar interaction of the thin film system and cannot suppress the instability of thin polymer films in the solvent-induced process. These results provide further insight into the stable mechanism of thin polymer films in the solvent-induced process.


INTRODUCTION
Ultrathin (< 100 nm) polymer films are of great interest due to an increased demand for nanoscale devices, including nanoelectronics, nano-coatings and high precision sensors. [1−4] The stability of ultrathin polymer films plays a crucial role in their technological applications. [5,6] With an increase in the environmental adaptability requirements of thin polymer films (e.g., gas sensing, wearable nanoelectronic devices, and underwater vehicles [4−7] ), thin polymer films need to be stable in air, vapors, and solvents. The unstable process is called the dewetting process. Dewetting of an ultrathin polymer films initiates with the formation of randomly placed holes; and then the holes grow with the polygonal networks of liquid rims appearing, which further break up into a collection of droplets. [8−10] Various approaches have been utilized to stabilize thin polymer films. [11−14] In principle, the molecular mechanisms against dewetting include a thermodynamic mechanism and a kinetic mechanism. Most inhibiting approaches need the introduction of specialized compounds into thin polymer films to change the shape of the curve of the total excess free energy (ΔG) and the thickness (h) of thin polymer films (the thermodynamic mechanism) and thus increase the local viscosity of the films (the kinetic mechanism). However, when the solvent molecules are introduced, they can greatly change the instability interaction forces and reduce the local viscosity of the films. This induces dewetting and phase separation of thin polymer films. For example, previous studies have demonstrated that the dewetting of thin polystyrene films can be triggered via long-range van der Waals interaction forces during thermal annealing; [15,16] however, the shortrange polar interactions induced via the polar solvent lead to the instability of the films during the solvent-induced process. [17−27] Many methods of stabilizing thin polymer films in air have failed in solvent-induced processes. The stabilization of thin polymer films against dewetting in the solventinduced process therefore remains a major challenge.
Some recent studies have shown that the stability of thin polymer films can be considerably improved only via physical aging [28−34] and without the introduction of any specialized compounds into the thin polymer films. Ultrathin polymer films are usually fabricated by spin-coating. During the rapid solvent evaporation process, polymer coils are subjected to a marked state change from isolated individual coils in solution to a dry polymer film. This leads to a frozen non-equilibrated chain conformation. The properties of the thin polymer films are very sensitive to the annealing conditions due to the nonequilibrated chain conformation.
During physical aging, the polymer chains change their conformations from the non-equilibrated state to equilibrated state to minimize their free energy. Recently, Chandran et al. [35] proposed a new concept of "molecular process design" to design polymeric structures with desired and tunable properties by processing-induced non-equilibrium conformations. The interfacial effect plays an important role in the relaxation of the chain conformation.
Several studies have reported that the adsorbed layerwhich strongly influences the glass transition temperature, viscosity, crystallization, and dewetting-can form on the solid substrate during the process of conformational relaxation via the interfacial effect. [36−46] Several research groups have shown that the formation of the adsorbed layer can increase the effective viscosity of the PS chains in thin film systems, which is closely related to the stability of the thin polymer films. [38−40] Napolitano et al. [47] reported that the equilibrium thickness of the adsorbed layer forming in the thermal annealing keeps a linear relation with the effective Hamaker constant of thin polymer films, which indicates the strength of the long-range interactions at the polymer/substrate interface.
While several studies have discussed the interfacial adsorption of thin polymer films, few studies have disclosed the effect of the interface adsorption on the stability of thin polymer films in the solvent-induced process. The molecular mechanism underlying interfacial adsorption on the instability interactions in the solvent-induced process still remains largely unexplored. Here, we evaluate the effect of interfacial adsorption on the long-range van der Waals and short-range polar interactions of thin polymer films in the solvent-induced process and identify the stability mechanism of thin polystyrene (PS) films in solvent vapor.
Our results show that the formation of the adsorbed layer can greatly reduce the short-range polar interaction of the thin film system but cannot completely suppress the instability of thin polymer films in the solvent-induced process; the conformational rearrangement of the polymer chains at the aging temperature T>T g plays an important role in stabilizing the thin polymer films.

EXPERIMENTAL Preparation of Thin Polymer Films
The system under investigation was a thin polystyrene (PS) film with acetone as a solvent to induce dewetting of the thin PS film. Thin PS films (M w =97 kg/mol; M w /M n <1.1, supplied from Aldrich) were spin coated from a toluene solution onto the Si wafer with a 1.9 nm oxide layer. Before spin-coating, the Si wafers were cleaned in a bath of 2/1 (V/V) solution of 98% H 2 SO 4 /30% H 2 O 2 for 30 min at 100 °C, rinsed with deionized water, and then dried with compressed nitrogen. The thin PS films were first stored in the vacuum oven for 12 h at room temperature (0 h of aging time).

Physical Aging of Thin Polymer Films
Thin polymer films were annealed for different aging time at 60 and 135 °C in air. To induce dewetting of thin PS films at room temperature, thin PS films were put in the apparatus (see Fig. 1). First, a thin PS film was placed in the experimental apparatus, and the thickness of the films was measured by ellipsometry; then acetone was injected into the experimental apparatus to induce swelling and dewetting.

Characterization
Spectroscopic ellipsometry (MM-16, France, HORIBA Jobin Yvon) was used to measure the thickness of thin PS films. The incidence angle of the ellipsometry was 70°, and the wavelength range for fitting was 430 nm to 825 nm. The morphologies of thin polymer film were observed by an optical microscope (OM, VHX-1000, Japan, Keyence) in reflection mode with a charge coupled device camera attachment. The morphologies of the rim and the holes floor were observed by atomic force microscopy (SPA-300, Japan, Seiko Instruments Inc.) operating in tapping mode.

RESULTS AND DISCUSSION
The apolar PS films were first annealed in vacuum. After thermal annealing, the thin PS films were treated by a saturated acetone vapor at room temperature. The dewetting process of the thin polymer film was triggered via the saturated acetone vapor when the aging time, t aging , was 0 ( Fig. 2a). After the holes formed during the dewetting process, the liquid polymer solution pulled away from the solid substrate and formed a rim surrounding the holes because the removed liquid polymer solution accumulated around the holes. "Finger-like" patterns formed, and these fingers generated droplets during the dewetting process.
The selected temperature of physical aging, T aging1 , was 135 °C, which was far higher than the glass transition temperature of PS (T g =103 °C). The thin PS films (M w =97 kg/mol, 60 nm) were stored in vacuum for different aging time at 135 °C. Figs. 2(b)−2(f) show a series of optical microscopic images of thin PS films with different t aging at 135 °C. These PS films with different t aging were treated by the saturated acetone vapor at room temperature. The shape of the rim changes from a "finger-like" pattern to a "nose-like" pattern with increasing aging time before finally becoming stable (Figs. 2b−2f). When t aging reached 48 h, the dewetting process could not be observed, and the thin polymer film became stable. Next, we selected 60 °C (T aging2 ) as the aging temperature, which was far lower than the T g of PS (T g =103 °C). Thin PS films (M w =97 kg/mol, 60 nm) were stored in vacuum for different aging time at 60 °C. of the thin PS films, which were treated by the saturated acetone vapor at room temperature. All PS films were unstable under the acetone vapor, and the shape of their rim was "finger-like." The low T aging slightly influenced the stability of the thin PS films and the shape of the rim during the dewetting process. The aging temperature was an important factor to influence the stability of the thin polymer films. When T aging >T g , the aging process could improve the stability of the thin polymer films; however, the aging process slightly influenced the stability of the thin polymer films when T aging <T g . In order to further understand the results, the morphologies of the hole floor are shown in Fig. 3. A few PS droplets remained on the solid substrate when t aging =0 (Fig. 3a). In the case of T aging1 , as shown in Figs. 3    droplets could be observed on the solid substrate with increasing t aging . This indicates that the PS chains can adsorb on the solid substrate in physical aging at T aging1 . For T aging2 (Figs. 3b′−3f′), nanodroplets were also observed on the solid substrate with increasing t aging . This implies that the PS chains can also adsorb on the solid substrate in the aging process at T aging2 . The nanodroplets are remnants of a very thin wetting layer. According to the previous literature, [48] the 3D AFM image of nanodroplets was analyzed with the IMAGEJ software to calculate the total volume per unit area of the nanodroplets. The residual film thickness was calculated by the value of the total volume per unit area of the nanodroplets.
Our results show that the residual film thickness increased with increasing t aging . Fig. 3(g) shows that the thickness of the adsorbed layer changed with increasing t aging . Fig. 3(g) also shows that the equilibrium thickness of the adsorbing layer at T aging1 (~1.8 nm) is far higher than that at T aging2 (~1.0 nm). The film thickness was also changed to study the impact of physical aging on the stability of thin polymer films in solvent vapor. Here, thin PS films with different thicknesses were annealed at 135 °C. Fig. 4 shows a series of optical microscopy images of thin PS films, which were treated via saturated acetone vapor at room temperature. The black dash line in Fig. 4 indicates the boundary between the dewetted regions and the stable regions. The dewetting-wetting transition boundary shows that the aging time of the dewettingwetting transition decreases with decreasing film thickness from 48 h at 60 nm to 7 h at 15 nm (see Figs. 2 and 4). Fig. 5(a) shows a series of AFM images of the hole floor of the thin polymer films. From Fig. 5(a), it is seen that the adsorbed layer can form during the aging process. Fig. 5(b) shows the thickness of the adsorbed layer changes with t aging , which suggests that the film thickness slightly influences the change in the thickness of the adsorbed layer with t aging during the process of the physical aging.
The results above reveal that the aging temperature and film thickness can strongly influence the stability of thin PS films in acetone vapor. At T aging1 , the interfacial effect can be transmitted from the solid substrate/polymer interface to the polymer film via the mobility of polymer chains because T aging1 is far higher than T g-PS . The transmission of the interfacial effect greatly improves the adsorption of polymer chains at the solid substrate/polymer interface as well as the stability of the thin polymer film. A thinner PS film causes the thin PS film to reach the stable state more quickly during physical aging. However, the polymer segments cannot move freely during the process of physical aging at T aging2 . The interfacial adsorption only occurs at the solid substrate/polymer interface. The process of physical aging at T aging2 slightly influences the stability of thin polymer films. The mechanism for this increased stability of thin polymer films during a solventinduced process will be discussed in detail later.
The stability of the thin polymer film was next analyzed via thermodynamic aspects. The stability of the thin polymer films is determined by the interplay between the film/substrate and film/medium interface energies. The spreading coefficient can be calculated from the literature [17,18] as follows: where γ f is the surface energy of polymer film; and the contact angle, θ e , is the equilibrium contact angle of PS droplets on the solid substrate. The film is unstable when S is negative. Fig. 6 shows the value of the equilibrium contact angles (θ e ) of PS droplets on the solid substrate after dewetting in acetone vapor. The θ e slowly decreases with t aging at temperatures lower than T aging2 (−0.02 (°)/h in Fig. 6a); they then reach equilibrium. However, at temperatures higher than T aging1 , the θ e rapidly decreases with t aging in the first hour (−5.08 (°)/h in Fig. 6a) and then the rate of decrease becomes slower (−0.13 (°)/h in Fig. 6a). tained from Figs. 6(a) and 6(b). Fig. 6(c) shows that at a lower temperature than T aging2 , the value of the S of the film system slowly increases with t aging and then reaches equilibrium; however, at a higher temperature than T aging1 , the θ e rapidly increases with t aging in the first hour and then the rate of increase becomes slower. Fig. 6(d) shows that S reaches zero faster for a thinner PS film; this result is consistent with Fig. 4.
Previous studies have shown that both the apolar van der Waals (ΔG LW ) and the polar acid-base (ΔG P ) interaction energies can determine the stability of a thin polymer film. [17,18] For a thin apolar polymer film like polystyrene, the effective Hamaker constant of the film is the main parameter influencing the stability of the thin PS film. However, the polar interactions must be considered after the polar solvent molecules (acetone molecules) enter the PS film.
We used the total excess free energy per unit area (ΔG) of the thin film system to disclose the contribution of the ΔG LW and the ΔG P . The ΔG can be expressed as the sum of the apolar van der Waals (ΔG LW ) and the polar acid-base (ΔG P ) interaction energies to analyze the stability of thin polymer films. In the physical aging process, the polymer film system can be thought of as a five-layered system like Si/SiO 2 /the adsorbed PS layer/solvated polymer film/air due to the formation of the adsorbed layer (see the inset of Fig. 7a). Previous papers [17] have shown that the solvated polymer film is approximated as a solvent layer. The adsorbed PS layer is approximated as a PS layer because acetone is difficult to enter into the adsorbed PS layer. According to the theory of thin film stability, the total excess free energy per unit area (ΔG) for the system can be expressed as: [16−18] ΔG (h) = ΔG LW + ΔG P where A PS , , and A Si are the effective Hamaker constants, h is the thickness of the solvated polymer layer, h ads is the thickness of the adsorbed PS layer, and is the thickness of the SiO 2 layer. S P is the polar component of the spreading coefficient, d min is the cutoff distance, and l is the correlation length.
The effective Hamaker constants are calculated using the relation: [17,18] In our case, Eq. (5) can be written as: where A PS-PS is the Hamaker constants of the PS; A ace-ace is the Hamaker constants of the acetone; A air-air is the Hamaker constants of the air; is the Hamaker constants of the SiO 2 ; and A Si-Si is the Hamaker constants of the Si.
To calculate the three effective Hamaker constants of the system, Hamaker constants for each item were identified from the literature [18] (Table 1). The polar component of the spreading coefficient can be calculated from: [17,18] S = S LW + S P (9) where the dispersion component is S LW = ΔG LW (d min ). Table 1 Hamaker constants reported in the literature. [18] The values of Hamaker constants reported in literature [18] (J)     Fig. 7(a) is the schematic diagram of a five-layered system. To calculate the dispersion component S LW , we consider the film thickness at d min , which is the equilibrium film thickness of our system at which the free energy is minimized. Thus, by considering the effective contribution of the apolar van der Waals ΔG LW and the polar acid-base interaction ΔG P , the total excess free energy per unit area ΔG and its second derivative ΔG″ as a function of film thickness, h, for the polymer systems could be obtained (Fig. 7). Figs. 7(a) and 7(d) indicate that the apolar van der Waals forces play a stabilizing role. From the total excess free energy per unit area in Figs. 7(b) and 7(e), the polar interactions introduced by acetone lead to instability of the film, and the polar interactions decrease with increasing t aging . This implies that the formation of the adsorbed layer can decrease polar interactions in the system. The second derivative of the effective free energy in Figs. 7(c) and 7(f) show that the transition thickness at which the film undergoes spinodal dewetting, h trans , decreased with increasing aging time at 135 °C. This implies that the physical aging at 135 °C can inhabit instability of thin PS films in acetone vapor. However, h trans barely changes with increasing aging time at 60 °C. Although the formation of the adsorbing layer reduces the polar interactions, it slightly affects the h trans . Thus, physical aging at 60 °C slightly influences the stability of thin PS films in acetone vapor. Fig. 8 shows the influence of film thickness on the total excess free energy per unit area ΔG and its second derivative ΔG″. The polar interactions decrease with increasing t aging (Figs. 8a−8c). Figs. 8(d)−8(f) show that h trans rapidly decreased in the first hour. The rate of decrease then becomes slower with time; a thinner PS film leads to a faster h trans decline.
The results above show that the polar interactions of the film system introduced by acetone destabilize the film during a solvent-induced process. Physical aging at 135 °C improves the stability of the thin PS films, and we evaluate this finding in detail to understand the origin of the stability of thin polymer films.
In this work, thin polymer films were fabricated via a spincoating process. During the rapid solvent evaporation, the polymer coils begin to overlap, and polymer conformations are subjected to a marked state change from isolated individual coils in solution to a dry polymer film; this leads to materials frozen in a non-equilibrated chain conformation. The non-equilibrated chain conformation has a reduced interpenetration with other chains at the interface. Before physical aging, the local free volume exists at the polymer/substrate interface.
Napolitano et al. reported that the non-equilibrated conformations in freshly spin-coated PS films can induce a higher local free volume at the polymer/substrate interface. [49] The thin polymer film without physical aging is treated in acetone vapor, and acetone molecules occupy the interfacial free volume during swelling; this interaction is because the attractive nature of the silicon oxide layer for acetone induces an increase in the chemical potential near the solid substrate. Our previous study indicated that acetone molecules can form an ultrathin enriched layer between the PS film and the solid substrate during the swelling process. [26] The contribution of the polar interactions on instability is the largest for films without physical aging (Fig. 7).
During physical aging at a temperature T>T g , the polymer conformations change from a non-equilibrated state to an equilibrated state, and the interface adsorption and conformational rearrangement of polymer chains take place. Previous studies have shown that a broad distribution of relaxation time is observed during the annealing process of the spincoated polymer films at T>T g . [50,51] During physical aging at 135 °C, equilibrium contact angles (θ e ) of PS droplets on the solid substrate rapidly decrease irrespectively of the film thicknesses at short time (the first hour). This implies a lack of nanoconfinement in stage I (see Fig. 5b). In stage I, the shortrange interfacial potential drives the process of the interfacial adsorption and the formation of the adsorbing layer via the adsorption of the local polymer chains near the solid substrate. The polymer segment at 60 °C cannot move freely, however, and the short-range interface potential can also lead the polymer chains to occupy the local free volume at the polymer/substrate interface. Fig. 2 shows that the value of the h ads-1h at higher aging temperatures (135 °C) is similar to that of equilibrium h ads at lower aging temperatures (60 °C). This result is consistent with the time-temperature equivalence principle. The interfacial free volume is occupied by the polymer segments, and this prevents the acetone molecules from contacting the solid substrate. The formation of the first adsorbing layer can considerably reduce the polar interaction (see Figs. 7b and 7e). However, the first adsorbing layer slightly influences the stability of the thin polymer films in the solvent-induced process at this stage.
At later time at 135 °C, further chain adsorption is mainly due to the conformational rearrangements of previously adsorbed chains. We see that the growth rate of the h ads slows (see Figs. 2 and 4); however, this stage is important to the stability of thin polymer films in the solvent-induced process. The conformational rearrangement can induce interpenetration of the polymer chain with other chains. Previous works [51−54] have shown that the interfacial effect can be transmitted via the conformational rearrangement of the polymer chains, which increases the number of the interpenetrating chains to improve the stability of thin polymer films. The conformational rearrangement of the polymer chains is a classic kinetic process. A thinner polymer film implies a shorter aging time to achieve stability (see the dewetting-wetting transition boundary in Fig. 4).

CONCLUSIONS
Thin polymer films that are stable in the thermal annealing process can be destabilized at room temperature during the solvent-induced process. This work investigated the influence of interfacial adsorption in physical aging on the stability of thin polymer films during a solvent-induced process. At an aging temperature of T aging1 =135 °C>T g , the formation of the adsorbed layer in stage I can reduce the polar interactions; however, this slightly influences the stability of the thin polymer films during the solvent-induced process. The conformational rearrangement of the polymer chains in stage II may be an important factor influencing the stability of thin polymer films during the solvent-induced process.
The h trans also decreased with aging time at 135 °C. The results show that physical aging at 135 °C can greatly inhibit the instability of the thin PS films in acetone vapor. At an aging temperature of T aging2 =60 °C<T g , the formation of the adsorbed layer can reduce the polar interactions, and physical aging at this temperature slightly influences the stability of thin PS films in acetone vapor. These results explain the stabilizing mechanism of thin polymer films during the solvent-induced process.