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Jul 11, 2016 - of polymer films such as the glass transition temperature (Tg) and/or coefficient of thermal expansion (CTE), using ellipsometry,12−1...
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Glass Transition and Thermal Expansion Behavior of Polystyrene Films Supported on Polystyrene-Grafted Substrates Yeseul Shin, Hoyeon Lee, Wooseop Lee, and Du Yeol Ryu* Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Korea S Supporting Information *

1. INTRODUCTION Thermal behavior of polymer films supported on the substrates has been of fundamental importance to a variety of applications in industry and science because it measures polymer chain conformation in the films confined in two-dimensional geometry that are usually applied to other layers.1−11 In film geometry, it has been believed that polymer chain behavior under interfacial interactions considerably deviates from that of the bulk, since the chain mobility or diffusivity is influenced by surface and substrate interfaces either chemically or physically.4,5 Many prior studies have investigated thermal behavior of polymer films such as the glass transition temperature (Tg) and/or coefficient of thermal expansion (CTE), using ellipsometry,12−16 positron annihilation spectroscopy,17,18 Xray,19−23 fluorescence spectroscopy,24,25 calorimetry,26−29 dielectric spectroscopy,30,31 and neutron reflectivity.32−34 Wallace and co-workers investigated thickness-dependent thermal behavior of polystyrene (PS) films supported on a passivated Si substrate that favors PS using X-ray reflectivity.35 The Tg increased and the CTE at rubbery state (αr) decreased when the film thickness decreased below ∼7Rg, where Rg denotes the radius of gyration, and above ∼7Rg, the αr of PS films approached that of the bulk. Similarly, in deuterated poly(methyl methacrylate) (dPMMA) films supported on a Si substrate that favors dPMMA, van Zanten and co-workers reported a decrease in αr with decreasing film thickness below ∼6Rg using neutron reflectivity.36 In contrast, a decrease in Tg and an increase in αr with decreasing film thickness below ∼10Rg were observed for the PS films supported on an unfavorable substrate such as the native oxide layer of Si substrate, which was investigated by Henderson and co-workers using ellipsometry.14 All the studies taken together, likewise, the thickness dependence of Tg and αr for polymer thin films appeared to be sensitive to interfacial interactions between the polymer melts and substrates. Kanaya and co-workers investigated αr of PS/deuterated PS (dPS) multilayered films supported on a Si substrate using neutron reflectivity (NR).34 Interestingly, thermal expansion of bottom layer was remarkably suppressed due to low chain mobility (or diffusivity) near the interface very close to the Si substrate, while those of the other top layers were recovered similarly to that of the bulk. Pochan and co-workers measured αr of dPS films confined in unfavorable layers of fluorinated polyimide (fPI), where the bilayer of dPS/fPI and trilayer of fPI/dPS/fPI were prepared on a Si substrate.32 They also demonstrated that the αr of dPS films decreased when the film thickness decreased. © XXXX American Chemical Society

On the other hand, it is possible to tune interfacial interactions on the substrates with soft polymers by controlling the grafting density, chemical identity, and molecular weight of polymers anchored to the substrates.37−41 A compelling system is the polymer film model supported on chemically identical polymer-grafted substrates that tend to expel the polymer melts from the surface. This behavior is referred to as autophobicity, where the dense polymer brushes are usually stretched normal to the substrate, and the intermixing interface between polymer melts and brushes is finitely limited by the entropic balance at the interface.42−47 In this study, we systematically investigated thickness dependence of Tg and CTEs at glassy and rubbery states for the PS films that were supported on chemically identical PSgrafted substrates using in situ ellipsometry. A grafting-to method with hydroxyl end-functionalized polystyrenes (PSOHs) was used to generate PS-grafted substrates. Temperature-dependent film thicknesses of PS films were evaluated to obtain Tg and CTEs (αg at glassy state and αr at rubbery state, respectively). When the film thickness decreased below ∼300 nm, both of the Tg and αr decreased, which were in contrast to the thickness dependence of Tg and αr for the polymer films supported on the hard substrates that are favorable or unfavorable with the polymer melts. When the film thickness was normalized by Rg of PS melts, the overall thermal expansion behavior of the PS films supported on PS-grafted substrates was independent of the molecular weights of PS melts and PSOHs. A limiting film thickness (hL), above which the plateau values of Tg and αr were measured to be consistent with that of the bulk, was evaluated to be ∼30Rg for all the PS films, which was larger than a range of 7−10Rg for the PS films supported on the hard substrates.

2. EXPERIMENTAL SECTION Polystyrenes (PSs) were synthesized by the living anionic polymerization of styrene in cyclohexane solution using sec-butyllithium as an initiator. Temperature condition was set at 45 °C during the synthesis under purified argon environment. Number-average molecular weights (Mn) and dispersity (Đ = Mw/Mn) were characterized by size-exclusion chromatography (SEC) with PS standards. Two hydroxyl endfunctionalized polystyrenes (PSOHs, purchased from Polymer Source) were used to prepare the PS-grafted substrates, where xx in PSOH-xx approximately indicates molecular weight of xx kg/mol. All the sample characteristics are listed in Table 1. Received: March 3, 2016 Revised: July 6, 2016

A

DOI: 10.1021/acs.macromol.6b00458 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Sample Characteristics Used in the This Study sample code

Mn (kg/mol)

Đ

Rga (nm)

PSOH-04 PSOH-20 100 kg/mol PS 40 kg/mol PS

3.7 19.5 98.6 37.7

1.08 1.05 1.03 1.04

1.67 3.82 8.60 5.32

brush thickness (d0, nm) grafting densityb (σ, chains/nm2) 4.35 12.01

0.743 0.389

distance between grafting pointsc (Dg, nm) 1.16 1.60

Radius of gyration (Rg) was evaluated by assuming each linear-type PS. bGrafting density of PS brush was calculated by σ = ρhNA/Mn,50 where ρ and NA are mass density (1.05 g/cm3) of PS and Avogadro’s number, respectively, and h is brush thickness measured by ellipsometry. cDistance between grafting points (Dg) of PS brushes at the substrate was calculated by Dg = σ−1/2.39 a

Figure 1. (a) Film thickness of entire PS layer (100 kg/mol PS + PSOH-20 brush) and PSOH-20 brush at the substrate, where the initial thickness of PS film was set to 109 nm onto 12 nm thick PS-grafted substrate. Each case of the measurements is depicted in the scheme to indicate entire PS layer (black), PSOH-20 brush (red), and the net PS film (blue color). The Tg’s of PS films were marked with the red arrows, which were determined by the intersection of the two linear regressions of glassy and rubbery temperature ranges. (b) The net thickness curves of 100 kg/mol PS films, which were normalized by the initial thickness (h0). The plots were vertically shifted by a factor of 0.015 to avoid overlapping. the film was thoroughly rinsed with toluene to remove unattached PS chains. The PS films were spin-coated onto PS-grafted substrates typically at 2000−4500 rpm for 60 s. The film thickness was controlled by varying the concentration (1.5−13 wt %) in toluene to produce 43−704 nm. Subsequently, the second thermal annealing was done at

A grafting-to method was used to generate the PS brushes onto the native oxide surface of Si substrate. PSOH solutions (1 wt %) in toluene were spin-coated on the Si substrate at 800 rpm for 60 s, and the films were annealed at 170 °C for 3 days under vacuum; this process generated a thickness of PS brushes onto the substrates when B

DOI: 10.1021/acs.macromol.6b00458 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules 120 °C for 12 h under vacuum, which was above the glass transition temperature (Tg ∼ 105 °C) of PS melts. The Tg of PS melts was measured using differential scanning calorimetry (DSC: PerkinElmer Diamond DSC) at a heating rate of 20 °C/min from 50 to 200 °C; these Tg’s of 40 and 100 kg/mol PS melts were measured to be 104.0 and 106.2 °C, respectively, during the second heating run. For the films above 300 nm, the annealing temperature was elevated to 150 °C to ensure thermal equilibrium at the interfaces. A lab-made heating stage was devised at the vacuum chamber in spectroscopic ellipsometry (SEMG-1000, Nanoview Co.), which was operated at an incidence angle of 70° with halogen light source at wavelength (λ) ranging from 350 to 850 nm (or 1.5 to 3.5 eV). For accurate measurements of the films below 100 nm, an UV−vis light source was used to cover λ ranging from 250 to 850 nm (or 1.5 to 5.0 eV). The in situ measurements of the films were conducted when the temperature increased from 40 to 180 °C at a heating rate of 2 °C/ min.

measurements was depicted in the scheme. Accordingly, the thickness curve of entire PS layer was subtracted by that of PSOH-20 brush to extract the net thickness curve of PS films, assuming an addible property of thermal expansion in film geometry. Figure 1b shows the net thickness curves of 100 kg/mol PS films as a function of temperature, as normalized by the initial thickness (h0), where the h0 of the PS films supported on the same PSOH-20 substrate was varied from 51 to 704 nm. A thickness curve of 47 nm thick PS film on PSOH-04 substrate is displayed for comparison (will be discussed later). It should be pointed out that the αg = 2.10 × 10−4/°C and αr = 6.16 × 10−4 /°C for 109 nm thick PS film were slightly larger than those for entire PS layer (Figure 1a), respectively, because the thickness variation in PSOH-20 brush with temperature was eliminated. The Tg (104.3 °C) and αr (6.59 × 10−4/°C) for 704 and 414 nm thick PS films supported on the same substrate (by PSOH20) were presumed to be consistent with those of the bulk because no further change was observed. As the film thickness decreased further, it was evident that the Tg and αr of PS films decreased, while the αg was almost invariant irrespective of film thicknesses. Figure 2 shows Tg and αr of 100 kg/mol PS films supported on PSOH-20 substrate as a function of film thickness. Here, the

3. RESULTS AND DISCUSSION PS-grafted substrates were prepared by a grafting-to method on the native oxide surface of Si substrate at high temperature. Grafting density (σ) of PS-grafted substrates was calculated using σ (chains/nm2) = ρd0NA/Mn, as listed in Table 1, where ρ and d0 are the mass density (1.05 g/cm3) of PS and brush thickness measured by ellipsometry, respectively, and NA is Avogadro’s number. The distance between grafting points (Dg) of PS brushes at the substrate was calculated by Dg = σ−1/2. When the molecular weight of PSOH increased, the brush thickness (d0) increased, but the grafting density (σ) decreased. A decrease in σ for the substrate prepared with PSOH-20 could be attributed to the deficiency in end-hydroxyl group and slower diffusional mobility of longer PS chains; this effect was in line with the elongated Dg at PSOH-20 brush. The PS films were subjected to spectroscopic ellipsometry with increasing temperature. The ellipsometric parameters such as amplitude (Ψ ) and phase difference (δ) of reflected wave were obtained and simulated to correlate with the refractive index and film thickness. Note that some deviation in film thickness might misapprehend thermal expansion behavior of polymer films due to a prefixed refractive index during heating process, although the Tg was invariable. For this reason, many iterative experiments with the same samples were done at least more than 10 times to achieve reproducibility within the reasonable refractive indices that slightly decreased during thermal expansion of PS films. Figure 1a shows temperature-dependent film thicknesses of entire PS layer (100 kg/mol PS + PSOH-20 brush) and PSOH20 brush at the substrate, where the initial thickness of PS film was set to 109 nm on 12 nm thick PS-grafted substrate. A slow heating rate of 2 °C/min was applied to the films during in situ measurements to minimize the kinetic effect on transition, even though the transition might be traced in enthalpy change at faster heating rate using differential scanning calorimetry (DSC).27−29 As the temperature increased from 40 to 180 °C, the difference in thermal expansion between glassy and rubbery states of entire amorphous PS layer was distinct, leading to a Tg = 103.1 °C as determined by the intersection of the two linear regressions of glassy and rubbery temperature ranges. The αg and αr denote CTEs of glassy and rubbery states for the films, respectively, as guided with the dashed lines. A similar behavior was observed in a thickness curve of PSOH-20 brush film, leading to a similar Tg, although the variation in thickness between glassy and rubbery states was small. Each case of the

Figure 2. Tg and αr of 100 kg/mol PS films supported on PSOH-20 substrate as a function of film thickness.

αr is directly correlated to the volumetric CTE (αv) of PS chains by αr = αv(1 + v)/(3(1 − v)), where the Poisson ratio (v) is set to 0.5 at rubbery state because the polymer chains in film geometry expand or contract normal to a large surface-area substrate.32,48 When the film thickness increased above ∼300 nm, both of the Tg and αr were invariant within experimental error range, and they were consistent with those of the bulk. For polymer thin films supported on the hard substrates that are favorable or unfavorable with the polymer melts, it is worthwhile to note that there have been the two distinct cases for thickness-dependent behavior of Tg and CTE (αr) at rubbery state below hL. The first case was defined as the polymer films supported on a favorable substrate, as found in the PS films supported on a passivated Si substrate and the PMMA films supported on a Si substrate (case I).35,36 When the film thickness decreased below hL, the Tg increased and the αr decreased since the polymer chain mobility become disturbed by the favorable interfacial interactions. An opposite case, like a decrease in Tg and an increase in αr with decreasing C

DOI: 10.1021/acs.macromol.6b00458 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules film thickness, was found in the polymer films supported on an unfavorable substrate that promotes polymer chain mobility, as the case of the PS films supported on a Si substrate (case II).14 For the two cases, a countertrend behavior between Tg and αr was distinctly observed with decreasing film thickness below hL. Intriguingly, when the film thickness decreased below ∼300 nm, both of the Tg and αr decreased to 94.9 °C and 5.40 × 10−4 /°C, respectively, at 51 nm thick PS film, which were distinguished from the above two cases. A decrease in Tg with decreasing film thickness was attributed to the autophobic behavior between the PS melts and brushes.46,49 For the polymer films supported on chemically identical polymergrafted substrates, the thinner films became influenced by the entropically unfavorable interface that possesses the positive interfacial tension (γm/b) between the PS melts and brushes. Moreover, compared with the Tg (94.9 °C) of 51 nm thick PS film supported on PSOH-20 substrate, a further decrease in Tg (91.1 °C) of 47 nm thick PS film supported on PSOH-04 substrate, as shown in Figure 1b, was also attributed to an elevation of γm/b at the short and dense PS brush substrate. This behavior was similar to the trend of Tg in the case II. However, a decrease in αr with decreasing film thickness below ∼300 nm was evident for 100 kg/mol PS films supported on PSOH-20 substrate, like the case I. A unique characteristic of soft PS-grafted substrate was that the PS brushes were stretched normal to the substrate and a finite intermixing interface with PS melts could be formed by the entropic balance at the interface with the same chemical identity.44−47 Therefore, a decrease in αr with decreasing film thickness was presumably attributed to the pinned entropic effect arising from the finite intermixing interface, which gradually suppressed thermal expansion of PS films when the film thickness decreased below ∼300 nm. Figure 3 summarizes Tg, αg, and αr of 100 kg/mol PS films supported on PSOH-04 and PSOH-20 substrates and Tg, αg,

and αr of 40 kg/mol PS films supported on PSOH-20 substrate, where the film thicknesses of 43−704 nm were normalized by Rg of PS melts to resolve the molecular weight dependence. When the normalized thickness (h0/Rg) increased above ∼30Rg, the plateau values of Tg and αr were consistent with those of the bulk. The Tg curves of PS films supported on PSOH-20 substrate were superimposable with a line curve between 100 and 40 kg/mol PS films, as also shown in Figure S1 for comparison. However, a further decrease in Tg with decreasing film thickness below ∼30Rg was found in 100 kg/mol PS films supported on PSOH-04 substrate, indicating that the chain mobility of PS melts was further enhanced at the short and dense PS brush substrate.38−41 Unlike the molecular weight dependence of PSOHs on Tg of PS films, the thermal expansion behavior of 100 kg/mol PS films fell into the two line curves of αg and αr as a function of h0/Rg, regardless of the chain length of PSOHs. Even the αg and αr of 40 kg/mol PS films supported on PSOH-20 substrate were superimposable with the two line curves when the film thicknesses were normalized by Rg = 5.32 nm, indicating that the overall thermal expansion behavior of PS films was independent of the molecular weights of PS melts and PSOHs, leading to the universality of thermal expansion behavior as a function of h0/Rg. A limiting film thickness (hL) for all the PS films was identically evaluated to be ∼30Rg, above which the plateau values of Tg ∼ 104.2 °C and αr ∼ 6.59 × 10−4 /°C were presumed to be consistent with those of the bulk because no further thickness dependence was observed within experimental error range. Surprisingly, a normalized value of hL ∼ 30Rg for PS films supported on PS-grafted substrates was larger than a range of 7−10Rg, which was observed in the PS films supported on the impenetrable, favorable or unfavorable substrates. This result suggested that the pinned entropic effect arising from the finite intermixing interface had extensive influence to suppress thermal expansion of PS melts that were supported on PS-grafted substrates.

Figure 3. Tg, αg, and αr of 100 kg/mol PS films supported on PSOH04 and PSOH-20 substrates, and those of 40 kg/mol PS films supported on PSOH-20 substrate, where the film thicknesses were normalized by Rg of PS melts. The red arrow denotes a limiting film thickness (hL) for all the PS films.

4. CONCLUSION We have investigated thickness dependence of Tg and CTEs at glassy and rubbery states for the PS films that were supported on PS-grafted substrates using in situ ellipsometry. A grafting-to method with hydroxyl end-functionalized polystyrenes (PSOHs) was used to generate PS-grafted substrates. A limiting film thickness (hL), above which the plateau values of Tg and αr were consistent with those of the bulk, was identically evaluated to be ∼30Rg for all the PS films. When the film thickness decreased below ∼30Rg, both of the Tg and αr decreased, which were in contrast to the thickness dependence of Tg and αr for the polymer films supported on the hard substrates that are favorable or unfavorable with the polymer melts. The decreases in Tg of PS films with decreasing film thickness below hL were associated with the autophobic behavior of the polymer films supported on chemically identical polymer-grafted substrates because the thinner films became influenced by the entropically unfavorable interface that possesses the positive γm/b between the PS melts and brushes. The suppressed thermal expansion at rubbery state with decreasing film thickness below hL was presumably attributed to the pinned entropic effect arising from the finite intermixing interface between PS melts and brushes. Furthermore, all the thermal expansion data were superimposable with the two line curves of αg and αr when the film thicknesses were normalized by Rg of PS melts, indicating the universality of thermal D

DOI: 10.1021/acs.macromol.6b00458 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules expansion behavior in the presence of the finite intermixing interface. Such a finding of a normalized value of hL ∼ 30Rg, which was larger than a range of 7−10Rg observed in the PS films supported on the hard substrates, represented that the pinned entropic effect arising from the finite intermixing interface had extensive influence to suppress thermal expansion of PS melts that were supported on the limitedly penetrable PSgrafted substrates.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00458. Tg curves of 100 kg/mol PS films supported on PSOH04 and PSOH-20 substrates, and Tg curve of 40 kg/mol PS films supported on PSOH-04 substrate (Figure S1) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (D.Y.R.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the APCPI ERC (2007-0056091), Nuclear R&D Program, and NRF grants (2014R1A2A2A01004364), funded by the Ministry of Science, ICT & Future Planning (MSIP), Korea.

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REFERENCES

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DOI: 10.1021/acs.macromol.6b00458 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b00458 Macromolecules XXXX, XXX, XXX−XXX