Thickness of the Surface Mobile Layer with Accelerated Crystallization

Apr 17, 2018 - ABSTRACT: Ultrathin polymer films exhibit a liquidlike mobile surface layer near the free surface, which plays an important role in the...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Thickness of the Surface Mobile Layer with Accelerated Crystallization Kinetics in Poly(ethylene terephthalate) Films: Direct Measurement and Analysis Jianquan Xu,† Yun Li,† Xiaoling Wu,† Biao Zuo,† Xinping Wang,*,† Wei Zhang,† and Ophelia K. C. Tsui*,‡ †

Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, China Department of Physics, Hong Kong University of Science & Technology, Clear Water Bay, Hong Kong, China

‡

S Supporting Information *

ABSTRACT: Ultrathin polymer films exhibit a liquidlike mobile surface layer near the free surface, which plays an important role in the size-dependent physical properties of polymer nanomaterials. Most research has focused on the thickness of the surface layer related to segmental relaxation, while few studies have focused on that related to largescale segmental rearrangement, such as crystallization and diffusion. In this paper, a simple measurement of the surface mobile layer thickness with accelerated crystallization kinetics in a poly(ethylene terephthalate) film was performed by examining the stepwise crystallization behavior. The thickness of the surface crystallization layer (hscry) was determined by ellipsometry or X-ray reflection and was taken as the surface mobile layer thickness. hscry was observed to increase from 4 to 13 nm when the temperature increased from 343 to 373 K without any dependence on the molecular weight or film thickness, which fits well with the theoretical prediction by the cooperative strings model. tion.28 The thickness of the surface mobile layer on a polystyrene (PS) film was reported to be in the range of dozens of nanometers to only 2−3 nm29−34 based on different analysis methods. Torkelson34 observed that the depth of the surface region with a reduced Tg was more than 36 nm, corresponding to a surface layer tens of nanometers thick. Ediger31−33 employed a photobleaching technique to probe the thickness of the surface mobile layer with fast local dynamics and showed that the thickness increased with increasing temperature, reaching 7 nm at the bulk glass transition temperature Tgbulk. Forrest27 used nanoparticle embedding to detect the surface rheology in glassy PS films and reported that the length scale of surface mobility was 5.5 nm at Tgbulk − 10 K. Tsui et al.29,30 and Zuo10 indicated that the region of the surface mobile layer that adhered to Arrhenius dynamics was approximately 2−3 nm. The surface layer thickness usually increases with increasing temperature, but some studies observed the opposite trend in the temperature dependence.23,35,36 These varying and confusing results for the surface layer thickness are due to the different definitions of the “surface mobile layer” and knowledge of the mobility gradient of the local dynamics within the surface region remains ambiguous because of the technical difficulty in extracting surface layer information from a film system for investigations.

1. INTRODUCTION The physical properties of polymer materials near their surfaces have become the subject of significant theoretical and practical interest in the past two decades. Mounting evidence1−11 indicates that the dynamics of the surface are faster than those of the bulk, while the chain mobility near the free surface is substantially higher than that in the bulk. Since the free surface effect becomes more significant with decreasing film thickness due to the increase in the volume fraction of the surface mobile layer,12−14 many physical properties of thin polymer films, such as the viscosity,15,16 modulus,17,18 and glass transition temperature (Tg),13,19,20 show a strong thickness dependence. The surface mobile layer is the transition region from the polymer film to the atmosphere and can be considered as a layer of polymer chains tethered to the glassy bulk underneath. Because of the unbalanced forces, lower entanglement density,21,22 and higher free volume at the free surface,23−26 specific conformations of the polymer chains and aggregation structures form, resulting in enhanced surface dynamics. The effect of enhancing the chain mobility at the free surface can propagate in the bulk, which leads to a mobility gradient in the chain dynamics. Understanding the physical properties of the surface mobile layer, including its thickness, correlation length for dynamics,27 and distribution of mobility, is an important approach to discover the nature of the abnormal dynamics induced by nanoscopic confinement. The thickness of the surface mobile layer is significant both theoretically for determining the local dynamics near the surface and technically for adhesion and nanoscale fabrica© XXXX American Chemical Society

Received: February 21, 2018 Revised: April 17, 2018

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DOI: 10.1021/acs.macromol.8b00396 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

speed of the substrate was fixed at 2500 rpm. After film preparation, the PET films were annealed at 333 K under high vacuum for 24 h to remove the remnant solvent, during which process the films remained amorphous, as verified by atomic force microscopy (AFM) and grazing incidence wide-angle X-ray diffraction (GIWAXD).9,39 2.2. Characterization. The crystallization processes of the amorphous ultrathin PET films were detected in situ by EP3SW ellipsometry9,39,43 (Accurion GmbH Co., Germany) at a fixed incident angle of 60° and a wavelength of 658 nm. The temperature sequence of crystallization was controlled by an HCP622-CUST (INSTEC Co., USA) heating stage with an accuracy of ±0.1 K. During the crystallization process, the variations in the ellipsometric angle (Δ, which is sensitive to the film thickness and density) were continuously monitored as a function of time (t). The curve of Δ vs t was subsequently obtained, and this plot revealed the evolution in the PET film crystallization in detail. The surface isothermal crystallization topography of the PET films was probed at various temperatures in peak force tapping mode on a Multimode-8 AFM (Bruker Co., USA). The evolution of the surface crystallization domains during isothermal crystallization was monitored in situ, and the area fraction of the crystallization domains was evaluated with Nanoscope Analysis 1.40 software according to our previous work.39,44 The three-dimensional images of the surface crystallization topographies were transformed by the Nanoscope Analysis 1.40 software to more clearly visualize the differences in the crystallization topographies. XRR was used to measure the density distribution of the surface-crystallized thin PET films and was performed at the BL14B1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF, China) using X-rays with a wavelength of 1.2398 Å. The reflectivity of the X-ray beam was detected as a function of the incident angle (θ) to delineate the reflectivity curves. The angular scans were further applied in calculations of the scattering vector (qz = 4π sin θ/λ). According to the multilayer model, the mass density profile could be represented by fitting the reflectivity curve. The XRR measurements were executed at room temperature after cooling the films. The isothermal bulk crystallization information on the PET sample was obtained by differential scanning calorimetry (DSC, Q2000, TA Co., USA) in a nitrogen atmosphere.

The surface mobile layer is difficult to define. The most common definition of this layer is as follows: at a given temperature T < Tref (Tref is a reference temperature), there exists a position (the distance from the free surface is hs), where the local relaxation time equals the bulk relaxation time at the reference temperature Tref. hs can be considered to be the thickness of the surface mobile layer. In addition, the Tgbulk is usually used as the reference temperature to reflect the segment mobility. The thickness of the surface layer determined by Ediger31−33 and Forrest27 was actually focused on the length scale of segment relaxation (hsseg) enhanced by the free surface. In this situation, the surface layer is in a rubbery state, while the bulk layer remains in a glassy state. Meanwhile, the studies of Tsui29,30 and Zuo10 approached the thickness of the surface layer through Arrhenius dynamics (hschain); that is, the viscous flow temperature (Tf) was utilized as the reference temperature to reflect the whole chain slippage. Note that whole chain motion is more difficult to achieve than segment motion,37 and the value of hschain is much lower than the value of hsseg. However, few studies have focused on the length scale of crystallization and the diffusion dynamics enhanced by the free surface, which require large-scale segmental rearrangement. In these situations, the Tref should be regulated in the range from Tgbulk to Tf. For theoretical studies, the cooperative strings model was recently proposed by Forrest38 to predict the surface mobile layer thickness as a function of temperature with diverse Tref values if necessary. In this paper, the thickness of the surface mobile layer with accelerated crystallization kinetics was investigated by examining the stepwise crystallization behavior of a poly(ethylene terephthalate) (PET) film to supplement the information on the depth profile of the local dynamics near the surface. The stepwise crystallization behavior was examined in our previous work,9,39 which revealed that crystallization occurred faster at the surface than in the bulk layer underneath and that the two crystallization processes were mutually independent. This difference in the crystallization kinetics was utilized to distinguish the molecular mobility between the surface mobile layer and the bulk layer, and the thickness of the surface mobile layer with accelerated crystallization kinetics (hscry) could then be simply obtained. To be specific, the stepwise surface and bulk crystallization processes were monitored by ellipsometry with annealing, and hscry was easily calculated by making some reasonable assumptions. hscry was also obtained directly by measuring the density profile of the surface-crystallized PET film by X-ray reflection (XRR). The thickness of the surface crystallization layer was found to be independent of the film thickness and molecular weight, while it increased from 4 to 13 nm with an increase in the temperature from 343 to 373 K. The cooperative strings model was used to theoretically predict hscry, and the result fits well with the experiment data.

3. RESULTS AND DISCUSSION 3.1. Measurement of the Surface Crystallization Layer Thickness by Examining the Stepwise Crystallization Behavior. The stepwise crystallization behavior of thin PET films under isothermal and nonisothermal conditions was observed by Shinotsuka,41 Jukes,45 and Zuo.9,39 Zuo attributed the stepwise crystallization of PET films to the layered distribution of glass transition dynamics in the thin film. Since the chain mobility near the free surface was enhanced greatly, the PET chains in the surface layer exhibited a lower crystallization temperature and a faster crystallization rate than bulk, which resulted in independent surface and bulk crystallization processes. In this situation, the thickness of the surface crystallization layer could be considered the thickness of the surface mobile layer. Temperature-controlled ellipsometry was used to expediently detect the thermal transitions of the polymer thin films by monitoring the evolution of the ellipsometric angles (ψ and Δ) with changing temperature or time.9,39,44,46−48 The ellipsometric angles are related to the physical properties of polymer films, such as the thickness, density, and refractive index. During the isothermal crystallization of PET films, the ellipsometric angles corresponding to the film properties (especially the volumetric properties) change abruptly. The angle Δ is more sensitive to the crystallization of the PET film than ψ according to our previous observations.9,39 Figure 1 presents the relationship between the normalized ellipsometric angle ((Δt − Δ0)/(Δ∞ −

2. EXPERIMENTAL SECTION 2.1. Materials and Film Preparation. PET samples (Mw = 30 kg/mol, Mw/Mn = 1.87; Mw = 100 kg/mol, Mw/Mn = 1.97, from Polymer Source Inc., Canada) were annealed under vacuum at 353 K for 48 h to remove moisture prior to solution preparation. Silicon wafers (100) with a native oxide layer (approximately 2.2 nm) were precleaned with piranha solution for at least 1 h and used as substrates. A mixture (5:2) of trifluoroacetic acid and chloroform was used as the solvent,9,39,40 and amorphous ultrathin PET films were prepared by a spin-coating method under a controlled environment (298 K and 35% RH).9,39,41,42 The thickness of the PET film was controlled by changing the concentration of the PET solution, while the rotation B

DOI: 10.1021/acs.macromol.8b00396 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 2a shows the evolution of the surface crystallization morphology of the PET film during annealing at 373 K by AFM. Dendritic crystals begin to emerge after annealing for 1 min, and the surface is completely covered by crystals at 7 min, showing the end of the surface crystallization process. Figure 2b presents the evolution in the surface fraction of dendrites as a function of the annealing time, and Figure 2c shows the process of bulk crystallization measured by DSC. Surface crystallization is clearly complete after annealing for 7 min at 373 K, but bulk crystallization is still in the incubation period of crystal nucleation at this point. These results prove that the first increase in (Δt − Δ0)/(Δ∞ − Δ0) is caused by the faster surface layer crystallization process, and the second increase is caused by the slower bulk crystallization. This layered distribution of the crystallization kinetics can also be confirmed by XRR experiments. For the measurement of the XRR, a PET film with a crystallized surface layer was obtained by quenching the PET film to room temperature after annealing at 373 K for 8 min (according to the results in Figure 1). The reflectivity curve of the surface-crystallized PET film with a thickness of 20 ± 1 nm is shown in Figure 3. A continuous multilayer model of crystallized layer/amorphous layer/SiO2/Si (substrate) was used to fit the XRR data and obtain the mass density profile. Based on the mass density profile, a 13.4 nm thick layer with a higher density (approximately 1.39 g/cm3) formed near the surface after annealing. This change could be explained by the crystallization of the surface layer. After being annealed at 373 K for 8 min, the surface layer crystallized with a higher density, but the bulk layer remained amorphous with a lower density than that of the surface layer. The layered distribution of the crystallization kinetics led to a layered distribution of the local density. Moreover, the thickness of the surface crystallization layer (hscry), which is considered the thickness of the surface

Figure 1. Normalized ellipsometric angle Δ plotted as a function of time (t) to present the stepwise crystallization behavior of a 21 nm thick PET (Mw = 30 kg/mol) film at 373 K. dΔsurf and dΔbulk represent the variations in Δ caused by the surface and bulk crystallization processes, respectively.

Δ0)) and time (t) during isothermal crystallization at 373 K. The increase in (Δt − Δ0)/(Δ∞ − Δ0) indicated that the thickness of the polymer film decreased due to the polymer crystallization process.39,47 Two distinct increases in the normalized ellipsometric angle can be clearly observed in Figure 1. The first increase in (Δt − Δ0)/(Δ∞ − Δ0) appears after annealing for 1 min at 373 K, indicating the beginning of crystallization. With increasing annealing time, an interruption occurs to form a discernible plateau from 5 to 10 min. The second increase begins at 10 min and continues up to approximately 50 min. After this continued increase with increasing annealing time, (Δt − Δ0)/(Δ∞ − Δ0) remains constant, reflecting the end of crystallization. This two-step crystallization behavior was further explored by AFM and DSC.

Figure 2. (a) Surface crystallization morphologies of a 20 ± 1 nm thick PET film annealed at 373 K for different time (scale: 20 μm × 20 μm). (b) Evolution of the surface fraction of dendrites as a function of time for a 20 ± 1 nm thick PET film (Mw = 30 kg/mol) annealed at 373 K determined by AFM. The red line was fitted by a Boltzmann function to produce a sigmoidal curve. (c) Evolution of bulk crystallization at 373 K determined by DSC. C

DOI: 10.1021/acs.macromol.8b00396 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

dΔsurf dhsurf X surf hscry = = surf cry t bulk dΔsurf + dΔbulk dhsurf + dhbulk X t hs + X t (h − hscry )

(1)

where dΔsurf and dΔbulk are the variations in Δ caused by the surface and bulk crystallization processes, respectively; dhsurf and dhbulk represent the thickness variations of the surface and bulk layers, respectively, caused by crystallization; hscry is the thickness of the surface mobile layer with accelerated crystallization kinetics; h is the original film thickness; and Xtsurf and Xtbulk are the degree of crystallinity in the surface and bulk layers, respectively, at a given temperature. According to eq 1, hscry could be obtained as follows: hscry =

Figure 3. XRR data and the associated fitting curve for a 20 ± 1 nm thick PET film after annealing at 373 K for 8 min. The inset shows the mass density profile as a function of the distance from the substrate.

X tbulk dΔsurf X tbulk dΔsurf + X tsurf dΔbulk

×h (2)

Usually, Xt is larger than Xt under isothermal conditions.45 However, the crystallization kinetics and crystallinity of the PET thin film depend on the thickness, in which both are suppressed with decreasing film thickness due to the substrate effect and the confinement effect.41,44,49−51 The surface crystallization kinetics have been reported to be more sensitive to thickness variations than the bulk crystallization kinetics.41,44 That is, the difference between Xtsurf and Xtbulk will be reduced with decreasing film thickness. As shown in Figure 3, the fitted density of the crystallized surface layer is approximately 1.39 g/ cm3. Groeninckx52 claimed that the crystallinity of PET was approximately 0.44 (calculated from the density) or 0.33 (measured by DSC) at this density. In addition, the degrees of crystallinity of the bulk PET material and the thin film (