Mobility Gradient of Poly(ethylene terephthalate) Chains near a

Aug 25, 2017 - We observed that hads/Rg was independent of the PET molecular weight and that the equilibrium thickness of the adsorbed layer was appro...
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Mobility Gradient of Poly(ethylene terephthalate) Chains near a Substrate Scaled by the Thickness of the Adsorbed Layer Jianquan Xu,† Zhenshan Liu,† Yang Lan,† Biao Zuo,† Xinping Wang,*,† Juping Yang,† Wei Zhang,† and Wenbing Hu*,§ †

Department of Chemistry, Key Laboratory of Advanced Textile Materials and Manufacturing Technology of the Education Ministry, Zhejiang Sci-Tech University, Hangzhou 310018, China § Department of Polymer Science and Engineering, State Key Lab of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China S Supporting Information *

ABSTRACT: The question of how to scale the mobility gradient of polymer chains near a substrate in supported ultrathin polymer films is a great challenge. In this paper, a mobility gradient of poly(ethylene terephthalate) (PET) chains near a substrate is characterized by cold crystallization. We found that either decreasing the PET film thickness or increasing the absorbed layer thickness consistently reveals three characteristic film thicknesses, which are all linearly dependent on the adsorbedlayer thickness. At the first thickness, the low-temperature peak of the top surface crystallization starts to shift toward the hightemperature peak of the bulk-like polymer crystallization; at the second thickness, it arrives there; and at the third thickness, crystallization is completely suppressed. The three kinds of film thicknesses characterize the depth profile of the local dynamics, reflecting the long-range effects of the substrate, which could be scaled by the thickness of the adsorbed layer.

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action and long/short-range interaction.6,33,34 For the majority of substrate/polymer systems, the effective interfacial interaction was found to be van der Waals interaction. Nealey31 has proposed that the interfacial free energy could be used as a parameter to evaluate interfacial interaction. When the interfacial free energy was greater than 2 mN/m, the Tg of a polymer film increased with decreasing thickness. When the interfacial free energy was less than 2 mN/m, the relationship between Tg and film thickness was the opposite. The thickness of the adsorbed layer is another intuitive parameter that can be used to evaluate interfacial interactions. Because of the numerous (∼N1/2) physical points of contact of polymer repeating units with a solid substrate,35,36 a thicker adsorbed layer results in stronger interfacial interaction. The polymer chains have been observed to be adsorbed in a flattened condition with some loops and with the absence of mobility,8,25,26 no thermal expansion,26 no interdiffusion8 and no crystallization25 even at temperatures greatly exceeding the Tg. The increase in the thickness of the adsorbed layer was attributed to the reduced space available for the formation of larger loops.37 Such a loop conformation would provide a structure to restrain the mobility of neighboring chains speculated by entanglement. Notably, the interfacial effect

INTRODUCTION Because of the unique properties that result from specific nanoscopic confinement, polymer materials in nanoscale environments have been of significant theoretical and practical interest over the past two decades. Nanoscopic confinement is generally accepted as strongly affecting polymer glass transition,1−7 viscosity,8,9 diffusion,10−12 crystallization,13,14 ion transmission,15 mechanical behaviors,16 and other pivotal properties. Such studies have shown that the physical properties of thin polymer films are thickness dependent and drastically different from the properties of the corresponding bulk polymers because of the heterogeneous polymer chain mobility in thin polymer films;1,17−20 this effect has been attributed to the free surface effect,1,3 finite-size effect,21,22 polymer− substrate interfacial effect,23−27 and the confinement-induced molecular packing and chain conformation.28−30 Focusing on the interfacial effect, Keddie and co-workers2 first revealed that the enhancement of the glass-transition temperature (Tg) of ultrathin poly(methyl methacrylate) films supported on silicon wafers and attributed this enhancement to the strong interaction between the polymer and substrate. The importance of the polymer/substrate interfacial effect on suppressing the chain mobility of polymer films was subsequently a topic of focus.5,6 The interfacial effect has been shown to be strongly dependent on the interaction between a polymer and a substrate,31,32 including physical interaction, chemical inter© XXXX American Chemical Society

Received: May 4, 2017 Revised: July 25, 2017

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Below the first film thickness (h s *), the top surface crystallization temperature begins to shift upward, reflecting the reach of the polymer/substrate interactions at the top surface; below the second film thickness (hsb*), it arrives at the bulk-like polymer crystallization temperature, reflecting the neutralization between the top surface and substrate effects; below the third film thickness (hn*), no crystallization occurs, reflecting the glassy polymers frozen by the strong polymer/ substrate interactions. These results show that these three characteristically critical film thicknesses increase linearly with various slopes when the thickness of the adsorbed layer on the substrate surface is increased; they also show that the thickness of the adsorbed layer can serve as a nanoruler to scale the distance of the long-range effect from the substrate, with the different mobility suppressed by the interfacial effect.

caused long-range perturbations in the polymer chain mobility.12 By measuring the d-polystyrene (d-PS) diffusion, Zheng et al.10 revealed that the long-range perturbations can be approximately 10 times greater than the radius of gyration (Rg). A similar result38 was reported, wherein the substrate could control the mobility of the polymer on the film surface by up to approximately 1.5REE (end-to-end distance, REE ∼ 6Rg). Koga35 reported that the diffusion dynamics at a distance 2−3Rg from the substrate was still strongly hindered even in the presence of an effective plasticizer. Thus, far, most studies on the interfacial effect aimed at the averaging interfacial effect of the whole film and how far the interfacial effect could propagate to the bulk. A few studies have been focused on the parameter employed to scale the distance of long-range perturbations from the substrate interface; however, the quantitative relation between the interfacial interaction and the distance of long-range effects remains insufficient.8,10,39 That is, the quantitative picture of the mobility gradient of local dynamics within the interfacial layer remains ambiguous. Recently, for spin-coated ultrathin poly(ethylene terephthalate) (PET) films, the three-layer model, which describes thicknesses beyond the single polymer scale that reflect longrange polymer interactions with the top and bottom surfaces, has been well established. Near the free top surface, a relatively high polymer mobility results in an additional peak of cold crystallization in the relatively low-temperature region beside the bulk-like polymer crystallization peak when the sample is heated from the glassy state.40−42 Meanwhile, near the wetting substrate surface, a relatively low polymer mobility suppresses cold crystallization at very small film thicknesses.23,25,43−47 We have observed that a decrease in the thickness of ultrathin PET film on the substrate could shift the top surface crystallization temperature toward the bulk-like polymer crystallization temperature.42 Interestingly, the top surface crystallization in thin PET films serves as a phenomenological marker to quantitatively investigate the distance of the long-range interfacial effect. In this paper, three characteristically critical film thicknesses on ultrathin PET films were investigated using ellipsometry and atomic force microscopy (AFM), as illustrated in Figure 1.

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EXPERIMENTAL SECTION

Materials and Film Preparation. PET (Mw = 30 kg/mol, Mw/Mn = 1.87; Mw = 100 kg/mol, Mw/Mn = 1.97) was purchased from Polymer Source Inc. (Montreal, QC, Canada). Prior to solution preparation, the as-received PETs were dried under vacuum at 353 K for 48 h. A mixture of trifluoroacetic acid and chloroform (w:w = 5:2) and hexafluoroisopropanol were used as solvents. Native oxide layer (∼2.2 nm) covered (SiO2−Si) and Al-coated (∼50 nm) silicon wafers were used as substrates. Si wafers (100) were precleaned using piranha solution for at least 1 h. The Al-coated layer was coated by sputtering for 120 s in a sputter Q 150T S (Quorum, U.K.) on cleaned Si wafers under an argon atmosphere. Amorphous ultrathin PET films with various thicknesses of adsorbed layers were prepared by spin-coating solutions of PET in a mixture of trifluoroacetic acid and chloroform on an SiO2−Si substrate combining a “melt-quench approach” under a controlled environment (298 K and 35% RH) according to a reported method.42,48 Amorphous ultrathin PET films were prepared on the Al substrate by spin-coating solutions of PET in hexafluoroisopropanol to avoid corroding the Al layer. After spin-coating and melt-quenching, PET thin films were annealed at 333 K (lower than the Tg) under a high vacuum for 24 h to remove the solvent and moisture and to avoid crystallization. The surface of the melt-quenched PET film was smooth and uniform without dewetting, which was confirmed by AFM. Characterization. The nonisothermal cold crystallization of ultrathin PET films was monitored in situ by an EP3SW imaging ellipsometer (Accurion GmbH Co., Germany) equipped with a HCP622-CUST (INSTEC Co., USA) heating stage with ±0.1 K temperature control accuracy at a fixed incident angle of 60° and a wavelength of 658 nm. During heating, the ellipsometric angle (delta, Δ, which is sensitive to thickness, density, and refractive index) was continuously monitored as a function of temperature (T). Both the Δ vs T curve and its temperature derivative (dΔ/dT) vs T curve were obtained, from which the crystallization temperatures of the top surface and bulk-like polymer (Tcsurface and Tcbulk) were clearly observed.42 The surface topography of the PET thin films was measured in Peak force QNM mode on a Multimode-8 atomic force microscope (Bruker Co., USA). The surface fraction of the crystalline region was estimated according to previously reported method.42 In our experiment, 8−12 pieces of supported PET films were placed on a hot stage and heated at 2 K/min. The PET films heated to a certain temperature were quickly cooled to room temperature before probing the surface morphologies by Peak force QNM mode on a Multimode8 AFM (Bruker Co., USA). In the Peak force QNM mode, the adhesion image of PET film surface was obtained, and the difference of the surface crystallization domains and amorphous domains can be observed. Utilizing Nanoscope Analysis 1.40 software, the surface fraction of the crystallization region could be estimated and the temperature evolution of the surface crystallization was obtained. The reported values were the averages of at least three samples. The experimental errors for measuring the surface fraction of the crystallization region were evaluated to be less than ±3%, thus indicating that the results were sufficiently accurate. Moreover, at

Figure 1. Schematic of the three-layer model of polymer thin films exhibiting three characteristic film thicknesses: Substrate surface effects reach hs*, the top surface effects will be counteracted by substrate effects at hsb*, and no crystallization occurs below hn*. The bottom layer represents the distance that the substrate long-range effects propagate to the bulk. B

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Macromolecules isothermal crystallization, the crystal growth rate and the incubation periods for crystal nucleation were obtained by in situ monitoring the evolution of crystallization domains. X-ray reflectivity (XRR) was used to measure the density profiles of the thin PET films at the BL14B1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF, China), using 0.124 nm X-rays.

one layer with higher density than bulk was observed in the lone adsorbed layer with 5 nm thickness shown in Figure S2. The density heterogeneity in the adsorbed layer was not observed. This result indicated that the structure of the PETadsorbed layer was not identical with that of PS interfacial sublayer, which was composed of two different chain conformations.26,51 We attributed this different structure in adsorbed layer to the difference in chemical structure of PET and PS. PET chains are less flexible, have stronger intermolecular interaction and are crystallizable. Moreover, the hydrogen bonds between PET chains and substrate can be formed in PET-supported films. These factors maybe promote PET chains to form dense packing adsorbed layer,52 resulting in forming density homogeneity in the PET adsorbed layer. Figure 3a presents the ellipsometric angle (delta Δ) and dΔ/ dT with temperature during the heating scan for a 24 nm PET film without annealing at 554 K (hads = 0.5 nm). Because the ellipsometric angle was very sensitive to change in the physical properties of the films (e.g., the Δ value can be approximated to be inversely proportional to the film thickness in the thickness range explored here), the surface and bulk-like cold crystallization temperature of thin films could be readily identified at the abrupt change of ellipsometric angle, more precisely, at the peak of dΔ/dT.53,54 Our previous work42 demonstrated that two peaks at a low temperature and relatively high temperature in Figure 3a are attributable to the top-surface crystallization temperature (Tcsurface) and the bulk-like polymer crystallization temperature (Tcbulk), respectively. With a fixed adsorbed-layer thickness of 5.6 ± 0.2 nm, the effect of the thicknesses of the thin PET films on their crystallization behavior was investigated. Figure 3b shows the temperature dependence of dΔ/dT of PET films with various film thicknesses, as denoted. Two distinct cold crystallization peaks occur when the film thicknesses (h) decrease to 54 nm. The peak at 372 K is denoted as Tcsurface, indicating the crystallization temperature of the layer near the top free surface of PET films when film thickness becomes sufficiently small to expose this free surface effect, which has been identified by in situ grazing-incidence X-ray diffraction40,41,55 as well as by AFM.42 At a film thickness of more than 54 nm, the separate peak for Tcsurface could not be observed because the change in Δ due to the crystallization at the top surface layer becomes negligible, which was attributed to a very low volume fraction of the surface layer in the whole film. Therefore, when the PET films were thicker than 54 nm, Tcsurface could be determined by investigating the evolution of the fraction of crystallization domains with temperature on the film surface using AFM (shown in Figure 3, parts c and d). The Tcsurface obtained by AFM was identical to that obtained by ellipsometry, which was confirmed for both the 61- and 43-nm-thick PET films. The other peak at 390 K is denoted as Tcbulk and represents the crystallization temperature of bulk-like polymer in the middle layer of the thin film, which is similar to that of the bulk PET (388 K). This result indicates that the crystallizations of the surface layer and bulk-like layer are well separated. With decreasing thickness of PET films, Tcsurface increased, approaching Tcbulk and merging with Tcbulk when the film thickness was less than 43 nm. To clarify what occurred on the film surface when Tcsurface and Tcbulk merged together, AFM was used to probe the surface crystallization process; the results are presented in Figure 4. Figure 4a shows the AFM topographic images of the 43 nm PET film with hads = 5.6 nm, exhibiting the growth of surface crystals at various temperatures during

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RESULTS AND DISCUSSION Effect of the Thickness of Thin PET Films with Fixed hads on their Surface Crystallization Behavior. The interactions between polymers and the substrate surface determine the wettability of ultrathin films on the substrate;31,32 thus, the thickness of the adsorbed layer can be used as a parameter to evaluate interfacial interactions. The amorphous PET films with variable thicknesses of irreversibly adsorbed layers on SiO2−Si were prepared by annealing the films at 554 K (8 K above the melting temperature) for different periods, followed by a quenching method. The thicknesses of the adsorbed layer (hads) were obtained using the previously reported solvent-leaching process26,42 and were measured with an EP3SW imaging ellipsometer. A mixture of trifluoroacetic acid and chloroform (w:w = 5:2) and hexafluoroisopropanol (which is a good solvent for PET49) were used as rinsing solvents for the SiO2−Si and Al substrates, respectively. The melt-quenched PET films were solvent leached six times in baths of fresh solvent with shaking, and each leach was for 1 h. The results in Figure 2 show that the thicknesses of the

Figure 2. Thickness of the adsorbed layer normalized by Rg (hads/Rg) as a function of the annealing time at 554 K.

adsorbed layers (hads) increase sharply with increasing annealing time up to 10 min and then reach constant values of 8.2 ± 1.0 nm for Mw = 30 kg/mol and 16.6 ± 1.0 nm for Mw = 100 kg/ mol. The radius of gyration (Rg) of PET was estimated by assuming Gaussian chain statistics50 Rg = kRMwaR, where kR = (2.8 ± 0.8) × 10−2 nm/(g/mol)aR and aR = 0.60 ± 0.03. Thus, for Mw values of 30 kg/mol and 100 kg/mol, the Rg values are approximately 13.6 and 28.0 nm, respectively. We observed that hads/Rg was independent of the PET molecular weight and that the equilibrium thickness of the adsorbed layer was approximately 0.60 Rg, which is similar to the adsorption behavior of PS on the same substrate.35,37,49 The thickness of the adsorbed layer, which was measured by the solvent-leaching process, is presented in Figure 2 and is identical to that measured by in situ XRR (Figure S1). Similar to Figure S1, only C

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Figure 3. (a) Temperature dependence of the ellipsometric angle (Δ) and its temperature derivative for a thin PET film (h = 24 nm, heating rate: 2 K/min); (b) surface and bulk-like cold crystallization temperatures of PET (Mw = 30 kg/mol, hads = 5.6 ± 0.2 nm, SiO2−Si substrate) thin films with different film thicknesses, as denoted at a heating rate of 2 K/min obtained from dΔ/dT vs temperature (T) curves; (c) AFM topographic images of a 61 nm PET film exhibiting the growth of surface crystals that were heated to various temperatures at a heating rate of 2 K/min (scale: 20 μm × 20 μm, inset image: 5 μm × 5 μm); (d) evolution of the surface fraction of crystallization as a function of temperature. The black arrow indicates the temperature (373 K) with the fastest growth rate measured by AFM.

Figure 4. Crystallization behaviors of a 43 nm-thick PET (Mw = 30 kg/mol) film with hads = 5.6 nm on an SiO2−Si substrate. Heating rate: 2 K/min. (a) AFM topographic images showing the growth of surface crystals with increasing temperature (scale: 20 μm × 20 μm, inset image: 5 μm × 5 μm). (b) Evolution of the surface fraction of crystallization as a function of temperature. The black arrow indicates the temperature (389.5 K) with the fastest growth rate for surface crystallization, as measured by AFM.

density profile of the 34 ± 3 nm-thick PET film with hads = 5.6 nm that was heated to 378 and 389 K at a 2 K/min heating rate was measured by XRR (Figure S3). The results show that the density was homogeneous. Collectively, the aforementioned results indicate that, for these films, the two-step crystallization processes became a one-step crystallization process; specifically, surface crystallization and bulk-like crystallization occurred

heating scan. The relationship between the area fraction of surface crystallization and the temperature was obtained from Figure 4a and is presented in Figure 4b. Figure 4b shows that surface crystallization exhibited the fastest growth rate when the temperature was approximately 389.5 K. This temperature is similar to the peak temperature for the 43 nm PET film in Figure 3b, indicating that Tcsurface was almost equal to Tcbulk. The D

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further decreased, the crystallization peaks gradually decrease in intensity and are eventually indiscernible when the film thickness reaches 17 nm, which implies complete suppression of cold crystallization. Three characteristic film thicknesses related to PET crystallization are observed with decreasing film thickness, separately denoted by hs*, hsb*, and hn*, below which Tcsurface begins to increase, Tcsurface arrives at Tcbulk, and no crystallization occurs, respectively. These three characteristic film thicknesses support our hypothesis that the effect of the adsorbed layer on polymer chain mobility can be divided into three different extents. Effect of the Thickness of the Adsorbed Layer on the Surface Crystallization Behavior of Thin PET Films. Because of the interactions between polymers and the substrate surface, the polymer chains are adsorbed in a flattened condition, with loss of mobility.8,25,26 An irreversibly adsorbed layer containing numerous physical contacts between the polymer chains and a solid substrate has been found.35,36 The increase in the thickness of the adsorbed layer has been attributed to the reduced space available for the formation of larger loops.37 In this section, we fixed the film thicknesses at 24.0 ± 1.0 nm with variable thicknesses of the irreversible adsorbed layers on SiO2−Si and investigated the effect of thickness of the adsorbed layer on crystallization behavior of these PET films. The effects of various adsorbed-layer thicknesses on cold crystallization in 24.0 ± 1.0 nm-thick PET films are summarized in Figure 6. Similar to Figure 5, three characteristic adsorbed-layer thicknesses are also observed. When the adsorbed-layer thicknesses reach 6.2 nm, no crystallization occurs, as confirmed by the density profile of spin-coated PET films and AFM height images (Figures S1 and S4, respectively). When 2.1 nm < hads < 6.2 nm, the Tcsurface of the films were equal to the corresponding Tcbulk. When hads < 2.1 nm, the surface crystallization and bulk-like crystallization occurred independently and the difference in Tcsurface and Tcbulk increased with decreasing hads.

simultaneously. As shown in Figure 3b, with decreasing thickness of the PET films, the peak at 390 K became small and disappeared when the thickness of the PET film was approximately 17 nm, as confirmed by AFM. The surface and bulk-like crystallization temperatures of PET films with various thicknesses at a fixed hads, as detected by ellipsometry and AFM, are summarized in Figure 5. Both

Figure 5. Cold crystallization temperature in PET films (Mw = 30 kg/ mol, hads = 5.6 ± 0.2 nm, SiO2−Si substrate) as a function of reciprocal film thickness (1/h). Solid squares and circles were data obtained by ellipsometry, whereas the hollow circles were obtained by AFM. The smooth curves and dashed lines are a visual guide.

Tcsurface and Tcbulk are independent of thickness when the thickness of the PET film (h) is greater than 54 nm. With the continuous decrease in film thicknesses, both Tcsurface and Tcbulk increase as Tcsurface shifts toward Tcbulk, and Tcsurface and Tcbulk merge together at a film thickness of 43 nm. As the thickness is

Figure 6. Cold crystallization temperatures of PET (Mw = 30 kg/mol, h = 24 ± 1 nm, SiO2−Si substrate) thin films with different thicknesses of adsorbed layers (hads), as obtained at a heating rate 2 K/min. (a) Variation of the ellipsometric angle with temperature (dΔ/dT) as a function of temperature (T) obtained from variable-temperature ellipsometry. (b) Cold crystallization temperature in 24 ± 1 nm-thick PET films as a function of the thickness of adsorbed layers (hads). The smooth curves and dashed lines are visual guides. E

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of hads. The last film thickness intercepts with the zero point. In fact, the critical film thickness required to suppress crystallization, as extracted from dielectric spectroscopy measurements of a PET film (76.4 kg/mol) on an Al substrate,25,59 also fit well with this linear relationship related to the adsorbed-layer thicknesses. A Mobility Gradient of PET Chains near Substrate Scaled by the Thickness of the Adsorbed Layers. The aforementioned results clearly indicate that the chain mobility of PET at various distances from the substrate surface can be obtained by monitoring the surface crystallization temperature of PET films with various thicknesses. The first characteristic film thickness (hs*) represents the outreach of the substrate effect, where the mobility of polymer chains on the film surface begin to be suppressed and the top-surface crystallization temperature begins to shift up. The second thickness (hsb*) reflects the beginning of complete balance between the low polymer mobility caused by the substrate effect and the high polymer mobility caused by the top free surface effect. At this distance, the enhanced mobility caused by the free surface effect is neutralized by suppressed mobility caused by the substrate effect, resulting in the temperature of the surface layer crystallization being equal to that of the bulk. The third characteristic film thickness hn* is the distance at which chain mobility begins to be completely suppressed by the substrate effect, resulting in the disappearance of crystallization behavior. Therefore, the cold crystallization of a supported PET film could behave as a phenomenological marker to characterize the depth profiles of polymer mobility influenced by the effects of the bottom substrate. The polymers adsorbed onto the substrate surface are believed to be immobile and nearly glassy.25,26 Their effects on the mobility of other polymers could be propagated into the depth of thin films via various molecular interactions, likely including chain entanglements.35 Napolitano et al.18,60 reported that prolonging annealing would expedite polymer chain adsorption onto the substrate surface, which would magnify the interfacial effect, thereby increasing the Tg of thin films. At the same time, they verified via a molding approach indicating that the shift in Tg is actually proportional to the free volume available at the interface.18 Direct experimental evidence demonstrated that the glass-transition temperatures of supported thin PS films decrease linearly with hads/Rg and that this relationship also characterizes the long-range polymer interactions with the substrate reported previously.61 The hads/ Rg is related to the conformation and the number of PS chains within an adsorbed layer, for which a high value of hads/Rg means more polymer chains are adsorbed onto the solid with a relatively stretched conformation. According to the layer model for the Tg of supported films, the thickness of the interface layer increased with hads/Rg resulting in an increase in Tg. In the case of PS films supported on grafted PS substrates, the thickness dependence of Tg has been reported to be more pronounced for films on shorter brushes with high grafting density.62 This enhanced thickness dependence was attributed to the dynamical effects in the overlap zone between the brush and melt polymers, except for the interfacial free energies at the interface.62 Koga35,37 reported that the interdiffusive chain dynamics are strongly hindered compared to those in the bulk when the distance from the substrate was less than 3Rg. They attributed this hindrance to the loop components in the adsorbed polymer chains providing a structure that can trap the neighboring polymer chains effectively, hence reducing the

Notably, dendritic crystals with dominant edge-on oriented lamellar crystals were observed on the surface of PET films with hads < 2.1 nm (Figure S5), indicating surface-induced crystallization.40,56 The crystal diameters increase linearly with time at 373 K (Figure S6), exhibiting the same kinetics as the conventional nucleation-controlled mechanism of crystal growth.57 Furthermore, with increasing thickness of the adsorbed layer, the incubation periods for crystal nucleation increase, implying that the polymer mobility is lowered by the enhanced interactions at the bottom substrate (Figure S7). Crystal orientations will switch to flat-on dominant because polymer mobility will be restricted by both sides of the thin films;39 in this case, the suppression of crystallization can be mainly attributed to the reduction in the lamellar thickness of flat-on-oriented polymer crystals, as revealed by molecular simulations.45 Other researchers have also observed a characteristic thickness of approximately 100 nm for the decrease of linear crystal growth rates in polymer thin films.58 In addition, since no crystallization was observed in lone adsorbed layers after heated at 413 K for 30 min (Figure S8), the different crystalline structures of the 24 nm PET films with various hads shown in Figure S5 were not caused by the crystallization of the adsorbed layer. Furthermore, the effect of the adsorbed layer on PET stepwise cold crystallization was explored with different molecular weights (Mw = 100 kg/mol and 76.4 kg/mol) and with a different substrate (Al substrate). The aforementioned observations of various film thicknesses, molecular weights and adsorbed-layer thicknesses are summarized in Figure 7, where

Figure 7. Relationship between three normalized characteristic film thicknesses (h*/Rg) and the normalized adsorbed-layer thicknesses (hads/Rg). The dashed lines labeled with the equations are the results of a linear regression. The critical thickness for Mw = 76.4 kg/mol PET on an Al substrate is reported in refs 25 and 59.

both the film thicknesses and the adsorbed-layer thicknesses are normalized by the radius of gyration (Rg ). All three characteristic film thicknesses exhibit good linear relationships with the adsorbed-layer thicknesses, revealing that hs*, hsb*, and hn* are approximately 13.6hads, 7.0hads, and 4.2hads, respectively. The first two film thicknesses intercept at zero adsorbed-layer thickness was 0.6Rg, which coincides with the equilibrium value F

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CONCLUSIONS Polymer materials confined in nanoscale environments are of substantial theoretical and practical interest because of the exposure of their unique properties at surfaces and interfaces. Although the distances of the long-range effects of the free surface and the polymer/substrate interface in ultrathin films have attracted extensive attention, the issue of how to quantify the depth profile of variable polymer dynamics in the thin films remains a critical question. Although constructing a schematic of the progressive growth of polymer dynamics with increasing distance from the substrate is simple, actually experimentally accessing the polymer dynamics at various depths is far more complex, mainly because of the inherent challenges of measurement. In this paper, we performed ellipsometry measurements to investigate the cold crystallization behaviors of PET confined in ultrathin films. The behaviors of cold crystallization appear to be sensitive to PET mobility at low temperatures. We observed that both a decrease in film thickness and an increase in the absorbed-layer thickness on the substrate consistently exhibit three characteristic film thicknesses (hs*, hsb*, and hn*), which are all linearly dependent upon the thicknesses of the adsorbed layers (hads). At the first thickness (hs*), the low-temperature peak of the top surface crystallization begins to shift toward the high-temperature peak of the bulk-like polymer crystallization. At the second thickness (hsb*), it arrives there. At the third thickness (hn*), crystallization is completely suppressed. These three thicknesses correspond to the distances at which chain mobility is suppressed by the substrate effect to various extentsspecifically, where they start to be suppressed, are strongly suppressed to counteract the free surface effect, and are completely suppressed, which occur at approximately 13.6, 7.0, and 4.2 times the adsorbed-layer thicknesses, respectively. To the best of our knowledge, this work represents the first time that the depth profile of the long-range effect from the substrate surface has been presented and scaled by the thickness of the adsorbed layer.

chain mobility in the close vicinity of the solid substrate. Therefore, the results shown in Figure 7 can be explained as follows. Because hads/Rg is related to the amount of adsorbed macromolecules and the conformation of the adsorbed chains, these factors determine the thickness of the overlap zone between the adsorbed layer and the bulk-like layer, which can trap the neighboring polymer chains, thus reducing their mobility. The thickness of the overlap zone between the adsorbed layer and the bulk-like layer increased with increasing hads/Rg of the adsorption layer, resulting in an increased distance of the long-range effect from the polymer/substrate interface. Therefore, the distance at which the mobility of the PET chain begins to be suppressed (hs*/Rg), the distance at which the mobility of the PET chain is suppressed moderately (hsb*/Rg) and the distance at which the chain mobility begins to be completely suppressed by the substrate effect (hn*/Rg) all increased linearly with hads/Rg. At the same time, the suppressed extent of chain mobility affected by the substrate effect faded with increasing distance far from the substrate, and these distances were approximately 13.6hads, 7.0hads, and 4hads. We report for the first time that the depth profile of local dynamics that reflect the long-range effects of the substrate can be scaled by the thickness of the absorbed layer. At the equilibrium thickness of the adsorbed layer, hads = 0.6Rg, the first distance from the substrate related to hs* reaches 8.8Rg, the second distance related to hsb* is 5.0Rg, and the third distance related to hn* is 2.6Rg. For example, for a PET molecular weight of approximately 70 kg/mol and an Rg of 22.6 nm in thin films, the first distance is 176 nm, the second distance is 100 nm, and the third distance is 52 nm. The polymer/substrate interactions have been reported to perturb the dynamics of polymer chains to a great extent, and the distance of the long-range effect for PS has been reported to persist up to 8−10Rg from the substrate.5,10,63 Therefore, 8.8Rg for PET is a reasonable estimate of the distance from the substrate when the thickness of the adsorbed layer has been saturated at 0.6Rg. The third characteristic distance shows the minimum distance at which the polymer chains are strongly suppressed by the substrate and hinder the motion required for polymer crystallization. Koga35 also observed that the interdiffusive chain dynamics of PS within 2−3Rg from the substrate were strongly hindered even in the presence of an effective plasticizer. In summary, the long-range effects of the substrate on PET chain mobility can be characterized by cold crystallization in the case of decreasing film thickness or increasing the thickness of the adsorbed layer on the substrate. The thickness of the adsorbed layer on the substrate could be used as a nanoruler to scale such long-range effects. We observed that three characteristic film thicknesses (hs*, hsb*, and hn*), corresponding to three distances for chain mobility suppression by the substrate to various extents (onset of suppression, moderate suppression to counteract the free surface effect and complete suppression), increase linearly by factors of approximately 13.6, 7.0, and 4.2 with increasing adsorbed-layer thickness. These linear relationships validate our approach that the depth profile of long-range effects from the substrate can be obtained from the thickness of the adsorbed layer. To clarify the aforementioned quantitative relationships, the mechanism of the continuous interfacial effects over almost 10Rg in a polymer matrix is worthy of further investigation.

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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.7b00922. XRR data and density profiles of 24 and 34 nm PET films with specific adsorbed layers and lone adsorbed layer with 5 nm thickness, AFM images of lone adsorbed layers and 24 nm-thick PET films with various hads, crystallization diameters of 24 nm-thick PET films as a function of crystallization time at 373 K, and interfacial effect on surface crystal growth rate and tn of supported ultrathin PET films (Figures S1−S8) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] or [email protected] (X.W.). *E-mail: [email protected] (W.H.). ORCID

Biao Zuo: 0000-0002-4921-8823 Xinping Wang: 0000-0002-9269-3275 Wenbing Hu: 0000-0002-7795-9004 G

DOI: 10.1021/acs.macromol.7b00922 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21674100, 21374104, 21504081, 21474050) and the Natural Science Foundation of Zhejiang Province (No. LQ16B040001).

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DOI: 10.1021/acs.macromol.7b00922 Macromolecules XXXX, XXX, XXX−XXX