Thermal Properties and Segmental Dynamics of Polymer Melt Chains

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Thermal properties and segmental dynamics of polymer melt chains adsorbed on solid surfaces Naisheng Jiang, Mani Sen, Maya K Endoh, Tadanori Koga, Elin Maria Larsson Langhammer, Patrik Bjöörn, and Mesfin Tsige Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00122 • Publication Date (Web): 12 Mar 2018 Downloaded from http://pubs.acs.org on March 12, 2018

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Thermal properties and segmental dynamics of polymer melt chains adsorbed on solid surfaces

Naisheng Jiang1*, Mani Sen1, Maya K. Endoh1, Tadanori Koga1,2*, Elin Langhammer3, Patrik Bjöörn3, Mesfin Tsige4*

1

Department of Materials Science and Chemical Engineering, Stony Brook University, Stony Brook, NY 11794-2275, USA

2

Department of Chemistry, Stony Brook University, Stony Brook, New York 11794-3400, USA

3

4

Insplorion AB, Medicinaregatan 8A Gothenburg, 413 90, Sweden

Department of Polymer Science, The University of Akron, Akron, Ohio 44325-3909, USA

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ABSTRACT The glass transition of supported polystyrene (PS) and poly(2-vinylpyridine) (P2VP) thin films in the vicinity of the substrate interface was studied by using a nanoplasmonic sensing (NPS) method. This “nanocalorimetric” approach utilizes localized surface plasmon resonance from two-dimensional arrangements of sensor-nanoparticles deposited on SiO2 coated glass substrates. The NPS results demonstrated the existence of a high glass transition temperature (Tg, high) along with the bulk glass transition temperature (Tg, bulk ≈ 100 ºC for PS and P2VP) within the thin films: Tg, high ≈ 160 ºC for PS and Tg, high ≈ 200 ºC for P2VP. To understand the origin of the Tg, high, we also studied the thermal transitions of lone polymer chains strongly adsorbed onto the substrate surface using solvent rinsing. Interestingly, the NPS data indicated that the Tg, high is attributed to the adsorbed polymer chains. To provide a better understanding of the mechanism of the Tg, high, molecular dynamics simulations were performed on a PS film on hydrophobic and hydrophilic substrates. The simulation results illuminated the presence of a higher density region closest to the substrate surface regardless of the magnitude of the polymer-solid interactions. We postulate that the highly packed chain conformation reduces the free volume at the substrate interface, resulting in the Tg,

high.

Moreover, the simulation results revealed that the deviation of the Tg, high from the bulk Tg, bulk becomes larger as the polymer-substrate interaction increases, which is in line with the experimental findings.


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I. Introduction The glass transition temperature (Tg) is one of the key parameters determining the mechanical properties of polymers for various applications. The Tgs of supported polymer thin films have been extensively investigated over the last two decades1 because of strong demand of polymer-based nanotechnologies such as organic photovoltaics, semiconductor chips, and biosensors. Early studies in the mid-1990s2-7 have shown that the polymer dynamics at various length scales in polymer thin films is greatly influenced by the presence of two interfaces: a polymer-air interface (i.e., a free surface) and a polymer-substrate interface. To explain the observed thickness dependences of Tg of polymer thin films, the so-called “layer model”, in which the dynamics is considered to be spatially heterogeneous in the direction perpendicular to the substrate surface, was proposed; a polymer film can be divided into multiple layers with different Tg values (e.g., a free surface layer, a bulk layer, and a polymer-substrate interfacial layer)6, 8-15. It has been long believed that the molecular motion of the polymer-substrate interfacial layer can be either enhanced or suppressed, depending on the interactions between polymers and substrates3-5, 16-22, while the free surface is considered as the origin of the Tg reduction of supported polymer thin films2, 4, 8, 10, 14, 20. For example, for weakly interacting polymer-solid systems, such as polystyrene (PS) on a glass or silicon substrate covered with a native oxide layer (SiOx/Si), the enhanced mobility layer associated with the Tg reduction at the free surface can extend into the film interior and overwhelm the relatively weak substrate effect2,

4, 8, 10, 14, 20

. In contrast, for strongly

interacting systems, such as poly(2-vinylpyridine) (P2VP) on the SiOx/Si substrate, the substrate effect overcomes the free surface effect, leading to an increase in the overall film Tg compared to the bulk5, 20, 23-24. Recently, several groups further looked into the substrate effect and illuminated the role of a physically adsorbed (i.e., via physisorption) polymer layer of several nanometers thick (“adsorbed nanolayer”) at the polymer-substrate interface in the deviation of Tg in polymer thin films from the bulk11,

13, 25-30

. Napolitano and Wübbenhorst13 showed that different

thermal annealing procedures yielded a range of different Tg values for PS thin films prepared on aluminum substrates (i.e., a weakly interacting system). They indicated that the deviation 3 ACS Paragon Plus Environment

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in Tg from the bulk was linked to the growth and relaxation of the adsorbed nanolayer. Priestley and co-workers29 reported that the free surface effect influences the Tg of the PS adsorbed nanolayer largely at short annealing time, while the effect is diminished with the increased degree of polymer adsorption via prolonged annealing times. This finding was further established by Napolitano and co-workers, who used poly(4-tert-butylstyrene) thin films on Si substrates that showed the largest depression (50 °C) in Tg associated with the free surface effect31, demonstrating that the free surface effect is completely erased upon the growth of the adsorbed nanolayer via prolonged thermal annealing30. Priestley and co-workers29 also indicated a considerable Tg increase of the adsorbed nanolayer. Additionally, using nanosized dielectric relaxation and thermal spectroscopy, Madkour and co-workers reported the presence of an approximately 3.5 nm thick “immobile” (i.e., high Tg) layer (with regard to molecular fluctuations) at the poly(vinyl methyl ether)/substrate interface even when temperatures were raised to 80 °C higher than bulk Tg.32 On the other hand, on the basis of molecular fluctuations, Kremer and co-workers studied the glassy dynamics of “P2VP islands” on a Si substrate prepared from dilute solution, where the islands contained just a few or only one polymer chains, by means of broadband dielectric spectroscopy (BDS)33. They presented that, based on the BDS dipolar fluctuations that reflect segmental motions, the confinement of the polymer chain to small-surface geometries has virtually no influence on the α-relaxation dynamics (i.e., the dynamic glass transition) of the chain segments. However, a clear understanding of equivalence between the thermodynamic and dynamic measurements of the Tg in confined polymers at the substrate interface is still lacking34. This is mainly due to the lack of in-situ local interfacial measurements of the thermodynamic and dynamic properties. In order to overcome the difficulty, we employ a versatile plasmonic nanospectroscopy technique, the so-called “NanoPlasmonic Sensing (NPS)” method, which has been proved as a noninvasive, “nanocalorimetric” approach for probing thermal transitions (i.e., glass transition and melting) at various buried interfaces35-37 in conjunction with molecular dynamics (MD) simulations. This NPS technique utilizes localized surface plasmon resonance

(LSPR)

from

two-dimensional

arrangements


of

nanoplasmonic

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sensor-nanoparticles (gold in this case) on glass substrates (Figure S1)35. The Au sensor on a conventional glass slide is then covered with a thin (a few tens of nanometers) spacer layer of a dielectric material (SiO2 used in this study). This field typically extends a few tens of nanometers into film interior from the plasmonic nanoparticle surface35-37. Through coupling of this enhanced near field with the local environment, the LSPR frequency is sensitive to dielectric (refractive index) changes of a surrounding medium38. Since phase changes are accompanied with a change in thermal expansion coefficient = ∆ × ∆ , where V and T are the volume of the polymer and temperature, respectively, thermal transitions (such as glass transition and melting) can be identified as changes in the rate of ∆ / , where ∆ is a spectral shift of the characteristic peak associated with the LSPR in an optical extinction or scattering spectrum (Figure S1)35, 39. The NPS results for PS and P2VP polymer thin films prepared on the sensors intriguingly revealed the emergence of a high Tg (Tg,high) value, which is 80-100 °C higher than the bulk Tg (Tg, bulk), within the thin films. In addition, in conjunction with the established solvent leaching40, we could successfully unveil the polymer chains adsorbed on the sensor surfaces and measure its Tg value. Interestingly, it was found that the origin of the Tg,high is the “dead layer”41 or the “flattened” layer 42-45 which lies flat on a solid surface and forms a compact, higher density region (2~3 nm in thickness) relative to the bulk42-43,46-47. Furthermore, molecular dynamics (MD) simulations were performed to provide a molecular level understanding of the chain structure and segmental dynamics. The results demonstrated the formation of an about 3 nm-thick PS high-density region (i.e., the flattened layer) with an extremely high Tg value on graphite (hydrophobic) and a smaller Tg value compared to graphite but still much higher than the bulk Tg value on α-quartz (hydrophilic) substrates. We also found that the Tg,high - Tg,

bulk

values tend to

increase with increasing the polymer-substrate interaction and molecular weights, which are in good agreement with the NPS results.

II. Experimental Section II-1. Sample Preparation Monodisperse PS (Mw = 290 kDa, Mw/Mn = 1.06, Pressure Chemical Co. hereafter 5 ACS Paragon Plus Environment

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referred to as PS290k) and P2VP (Mw = 219 kDa, Mw/Mn = 1.11, Scientific Polymer Products Inc., hereafter referred to as P2VP219k) were used. The bulk glass transition temperatures (Tg, bulk)

of PS and P2VP were determined to be 100 and 98 °C, respectively, by differential

scanning calorimetry (DSC). The NPS sensor chip (1 cm × 1 cm in a surface area, obtained from Insplorion AB (Gothenburg, Sweden)) was composed of a glass substrate covered with a 10 nm-thick SiO2 spacer layer. Au sensor particles (170 ± 10 nm in average diameter based on AFM analysis) were deposited on the surface of the SiO2 layer (Figure S2). The details of the sensor geometry and fabrication method have been reported elsewhere.35 Note that all the NPS sensor chips were thermally annealed at 350 ºC for 3 h during a post-fabrication process and were further cleaned through ultrasonication in acetone before preparation of spin-cast films. The PS or P2VP thin films were deposited onto the NPS sensor chips by spin coating from PS/toluene or P2VP/dimethylformamide (DMF) solutions with a fixed rotation speed of 2500 rpm and a fixed acceleration speed of about 2000 rpm/sec. The thicknesses of the spin cast films were then controlled by the polymer concentrations of the solutions. A 1.5 wt % PS290k or P2VP219k solution resulted in a film thickness of approximately 60 nm. The thicknesses of the spin cast films were determined by AFM after the NPS measurements via cross sectional analysis for a scratched region using plastic tweezers (Figure S3). In this paper, we also aim to probe the Tg of the lone PS and P2VP flattened layers on the NPS sensor chips. Polymer adsorption is an unavoidable event even when weakly attractive surfaces come into contact with a polymer solution/melt48. This is due to the fact that, like any thermodynamic process, polymer adsorption on a surface is governed by entropic and enthalpic contributions to the free energy: by giving up their bulk translational entropy, which costs free energy of only a few kBT, chains are able to achieve an energy advantage proportional to the number of monomers per chain49. Despite experimental difficulties, several research groups have proved that solvent leaching proposed by Guiselin40 is practical for deriving adsorbed nanolayers composed of homopolymers11, 13, 25, 29, 42-45, 50-56, polymer nanocomposites57, and block copolymers58 grown from the polymer melts. Taking advantage of the Guiselin approach40, we previously revealed that an adsorbed homopolymer layer via physisorption consists of two different chain conformations regardless of the magnitude of 6 ACS Paragon Plus Environment

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attractive solid-segment interactions: early arriving chains lie flat on solids (“flattened chains”), while late arriving chains form bridges jointing up nearby empty sites, resulting in “loosely adsorbed polymer chains”42-43. The driving force for the formation of the flattened chains is to overcome a loss in the conformational entropy of the polymer chains at the interface by increasing the surface-segment contacts49, 59. We have also demonstrated how to extract the lone flattened chains from the adsorbed layer by optimizing the solvent-leaching process42-43. Such selective extraction of the two different adsorbed chains was possible due to the large difference in the desorption energy between the outer loosely adsorbed chains and the inner flattened chains, which is proportional to the number of segment-surface contacts60. In this paper, we aim to reveal the Tg of the flattened chains next to the substrate surface. Hereafter we assign the flattened layer as the residual layer composed of the flattened chains alone unless otherwise stated. The PS and P2VP flattened layers were prepared by the protocol reported previously43, 45, 61

: firstly, about 60 nm-thick PS or P2VP films were pre-annealed at 190 °C for given times

(up to 150 h) in an oil-free vacuum oven (below 10-3 Torr). The annealed PS and P2VP thin films were then solvent leached in baths of fresh chloroform for PS or dimethylformamide (DMF, Sigma-Aldrich, ACS reagent, > 99.8%) for P2VP at room temperature until the resultant film thickness remained constant (typically a total of 3 cycles with 10 min per cycle) 43, 45, 61

. The resultant flattened layers were post-annealed at 190 °C under vacuum overnight

to remove any excess solvent molecules trapped.

II-2. Nanoplasmonic Sensing (NPS) Measurements NPS measurements were performed in an Insplorion X1 Measurement reactor (Insplorion AB). The details of the experimental set-up and data analysis have been described elsewhere37. The principle of the NPS technique has been described previously (Figure S1)35. As mentioned above, a spectral shift of the characteristic peak associated with the LSPR ∆ follows a change in the thermal expansion coefficient of a polymer thin film. Therefore, glass transition can be identified as a change in the rate of ∆ / . The X1 measurement chamber is a quartz tube flow reactor with a controlled temperature and gas 7 ACS Paragon Plus Environment

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atmosphere. The samples prepared on NPS sensors were placed in a sensor holder of the instrument and subjected to temperature ramps under a constant flow of N2 during all measurements. It should be noted that the Au nanodisks are embedded in the SiO2 layer and hence not physically interacted with the studied material but indirectly interacted via a LSPR dipole field35 (Figure S1). The dipole field from the LSPR penetrates through the space layer and has considerable strength on and in proximity to its surface62. Before each NPS measurement, the spin-cast films and flattened layers were first quickly ramped from room temperature to 230 °C to remove any water or other contaminations adsorbed onto the sample surface and reactor walls. The sample was then cooled to 50 °C before heating it back to 230 °C at a rate of 1 °C/min or 5 °C/min. We confirmed no significant differences in Tg between the two heating rates. The data was acquired by monitoring the spectral position for the NPS sensors during the ramping/cooling procedure, and the ramping/cooling procedure was repeated up to 5 times to ensure stability and reproducibility. In fact, we confirmed that the NPS data for the spin cast films and flattened layers was reproducible, implying no degradation of the polymers during the ramping/cooling procedures in between 50 °C and 230 °C. It should be addressed that since the intrinsic temperature dependence of the blank sensor is perfectly linear and when focusing only on the intersection points as in Tg measurement, this calibration does not affect the determination of Tg.37

II-3. Atomic Force Microscopy Measurements Atomic force microscopy (AFM) (Agilent AFM 5500 and Digital Nanoscope III) was used to study the surface morphologies of the flattened layers, spin-cast films, and the blank sensor. A standard tapping mode was conducted in air using a cantilever with a frequency of 300 kHz and a spring constant of 40 N/m. The scan rate was 0.5 – 1.0 line/sec with scanning density of 512 lines per frame.

II-4. Differential scanning calorimetry (DSC) Measurements The glass transition temperatures of the bulk PS and P2VP were determined by DSC (Perkin Elmer DSC 7) in a nitrogen atmosphere. Two heating ramps of 1 ºC /min and 5 ºC 8 ACS Paragon Plus Environment

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/min were used for the experiments. We confirmed that the Tg values determined via DSC curves with the different heating rates remained constant.

II-5. Molecular Dynamics Simulation All-atom molecular dynamics simulations were used to study the glass transition of an atactic PS thin film supported by graphite and fully hydroxylated α-quartz. In the present study, the PS film consisted of 87 chains of each 20 monomers long. Both substrates were of comparable lateral dimension of about 5.4 nm × 5.1 nm and the thicknesses of the graphite substrate and the α-quartz substrate were 1.31 nm (5 layers) and 1.8 nm, respectively. The OPLS-AA force field63, which has all the required force field parameters for both substrates and PS, was used to model the intra- and inter-molecular interactions between the different types of atoms in the system. During the simulation, the entire graphite substrate and the α-quartz substrate except the hydroxyl groups at the surface were frozen. All the simulations were carried out using the LAMMPS MD package64 with the Lennard-Jones cutoff radius of 1.2 nm and an integration timestep of 1 fs. The particle-particle/particle-mesh Ewald (PPPM) algorithm65 was employed for the calculation of the Coulomb interactions. The PS film of about 12 nm in thickness was generated following the same procedure discussed in our previous work66. The film was then placed 0.5 nm above the substrates, within the range of van der Waals interactions between the film and substrate, so that the polymer chains have the widest possible latitude for selecting adsorption sites. Then, the PS/substrate systems were cooled down from 600 K to 300 K at a rate of 20 K/5 ns and then heated up to 560 K at a similar rate. After this initial cycle of cooling and heating equilibration, the systems were cooled down from 560 K at variable cooling rates in steps of 20 K. The cooling rate at the higher temperatures was determined by the relaxation dynamics of the phenyl rings of the PS chains at that given temperature and was varied from 20 K/40 ns at high temperatures to 20 K/300 ns at low temperatures. During this cooling cycle, the position of each atom was recorded every 2 ps for determining the structural and thermodynamic properties of the film summarized in the Results and Discussion section. III. Results and Discussion 9 ACS Paragon Plus Environment

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III-1. NPS Results We begin with the thin films composed of P2VP, which has strong affinity with the SiO2 surface of the NPS sensor due to the presence of polar forces11, 43-44. Figure 1 (a) shows a representative AFM height image of the 60 nm-thick P2VP219k spin cast film prepared on the NPS sensor chip. Note that the films were annealed at 190 °C for 72 h before any further characterization. From the AFM image, we can see that the spin cast film fully covers the NPS sensor. It should be noted that the observable protrusions (~ 1 nm in average height from the surface) correspond to the buried Au nanodisks (Figure S2 (a)). The thickness of the film was determined by the AFM scratch test, as shown in Figure S3. Figure 1 (b) shows the NPS result for the 60 nm-thick P2VP219k thin film. From the figure we can see three linear regions with different ∆ / values within the temperature range between 55 °C and 230 °C, yielding the two intersections that correspond to the glass transition temperatures (since the PS and P2VP used are atactic polymers such that melting does not occur): the

(a)

0.0 ∆λpeak (nm)

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P2VP 60nm

-0.5 -1.0 -1.5 -2.0 -2.5

(b) 100 150 200 Temperature (ºC)

250

Figure 1. (a) AFM height image of the 60 nm-thick P2VP219k spin cast film prepared on the NPS sensor chip. The image size and height scale are 3 µm × 3 µm and 0-8 nm, respectively. (b) Determination of Tg from the NPS measuements for the 60 nm-thick P2VP219k thin film. Two transition temperatures were indicated by the black arrows.

lower one (Tg,low) is 98 ± 2 °C, which corresponds to the bulk Tg, and the other (Tg,high) is 203 ± 2 °C. Hence, the NPS result show the presence of the much higher glass transition temperature far above the bulk Tg within the P2VP thin film. Below, we show that the presence of this Tg,high value is rather general regardless of the polymer-solid interactions and


originates from the polymer

Figure 3 Tg vs. film thickness for the (a) 0.2 P2VP219k and (b) PS290k thin films.

segments that are in direct contact with the sensor surface.

0.0 -0.2

Figure 2 shows the NPS

-0.4

result for the 60 nm-thick

-0.6

PS290k thin film. Note that PS

80

100

120

140

160

180

Temperature (ºC)

has a much weaker interaction with SiO2 compared to P2VP5,

Figure 2. Determination of Tg from the NPS measuements for the 60 nm-thick PS290k thin film.

23, 67

. We also confirmed that

the PS films were homogenous on the sensors (data not shown

here).

From the figure, the NPS result

also

shows the two glass transition temperatures: the lower one

250

(a) 219kP2VP

Tg (ºC)

(Tg,low) is 98 ± 2 °C, which corresponds to the bulk Tg, and the other (Tg,high) is 155 ± 2 °C.

200 Tg, low Tg, high

150

Figure 3 summarizes the thickness dependence of the Tg values of the PS290k

100 200

and P2VP219k thin films obtained from the

180

NPS measurements. No clear film thickness

160 Tg (ºC)

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∆λpeak (nm)

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dependence was seen in both the Tg,low and

(b) 290kPS

140 Tg, low Tg, high

120

Tg,high values of the PS290k and P2VP219k

100

thin films. The Tg,low values of the PS and

80

P2VP thin films are in good agreement with

2

10

technique and DSC are considered as

4

5 6 7 89

2

100 Film Thickness (nm)

the bulk Tg values obtained from DSC. This is consistent with the fact that the NPS

3

Figure 3 Tg vs. film thickness for the (a) P2VP219k and (b) PS290k thin films.

“thermodynamic” Tg measurements34 so 11 ACS Paragon Plus Environment

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that the indedendent experimental techqniues provide the identical bulk Tg value. On the other hand, it is interesting to point out that the Tg,high values of the P2VP thin films are more than 40 °C higher than those of the PS thin films prepared on the same NPS substrates, while the bulk Tg values remained nearly identical. Since this NPS technique is sensitive to the region with a few tens of nanometers into film interior from the polymer-solid interface35-37, the results suggest that the observed Tg,high values are associated with the glass transition of

Figure 4. AFM height images of (a) the P2VP219k and (b) the PS290k flattened layers prepared on the NPS sensors. The image sizes are 2.5 µm × 2.5 µm. The corresponding height (from the SiO2 surface) profiles along the red lines in (a) and (b) are plotted in (c) and (d).

the polymers in proximity to the polymer-solid interface. To answer the origin of the interfacial Tg,high values, we utilized the established protocol to unveil the flattened layer alone. Here, the original PS and P2VP thin films were firstly thermally annealed at 190 °C for 72 h, then subjected to solvent leaching to remove the unadsorbed chains and loosely adsorbed chains, as discussed in the Experimental section. Prior to NPS measurements, we first characterized the surface topographies of the flattened layers using AFM. Figure 4 shows representative height images of the P2VP and the PS flattened layers on the NPS sensors. The Au nanodisks (~ 170 nm in diameter and ~ 30 nm in height) were identified based on individual AFM results of the bare NPS sensor (Figure S2). We can see that, unlike the thin polymer films, the P2VP flattened layer did not cover the 12 ACS Paragon Plus Environment

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substrate surface entirely (Figure. 4a), resulting in the surface coverage of about 68 % (excluding the surface areas occupied by the Au nanodisks) based on detailed image analysis (Figure S4 (a)). In the case of the PS flattened layer, it showed even a lower surface coverage (Figure 4b): The PS flattened chains form nanodroplets with the average diameter of approximately 60 nm on the surface of the NPS sensor with the surface coverage of about 23% (excluding the surface areas occupied by the Au nanodisks) (Figure S4 (b)). Hence, it is clear that P2VP adsorbs more onto the NPS

0.0 sensor than PS due to the stronger 43-44

. We

anticipate that the formation of the PS

(nm)

interaction11,

λpeak

polymer-solid

(a)

-0.1 -0.2

nanodroplets is induced by spinodal

-0.3

dewetting of the PS flattened layer on the

-0.4

weakly interactive silicon oxide surface

80

during a post thermal annealing process,

AFM

cross

sectional

. From the

analysis,

the

average heights of the P2VP219k and

160

0.4

61

(nm)

as reported previously55,

120

200

Temperature (°C)

∆λpeak

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(b)

0.3 0.2 0.1

PS290k flattened layers were determined

0.0 to be 11 nm (~ 0.87 Rg, P2VP, where Rg,

-0.1 P2VP

= 12.7 nm for P2VP219k) and 10

100

nm (~ 0.69 Rg,PS, where Rg, PS = 14.4 nm for PS290k), respectively. The results indicate that the PS and P2VP flattened

150

200

250

Temperature (ºC)

Figure 5. Determination of Tg from the NPS measuements for (a) the PS adsorbed layer and (b) P2VP adsorbed layer.

layers are not the “equilibrated” flattened layers (about 2 nm in thickness for PS and about 3 nm in thickness for P2VP, independent of Mw42-44) that form a compact and more flattened chain conformation to maximize the surface-segment contacts42-43, 45. Rather, it is likely that a large portion of the flattened chains are more loosely attached to the NPS sensor surface with much less solid-segmental contacts, forming the expanding chain conformation in the direction normal to the solid surface (see, 13 ACS Paragon Plus Environment

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the inset of Figure 5a). This reminds us of a previous finding about the flattened chain conformation in polymer nanocomposite thin films: Due to the presence of gold nanoparticles (the average diameter of 5.8 nm and the interparticle spacing of 100-200 nm) dispersed in the PS (Mw = 30kDa and 50kDa) flattened layers, the conformations of the PS flattened chains were extended in the direction normal to the film surface57. Further experiments are needed to study the flattened chain conformations between the Au nanodisk sensors. It should be noted that the surface morphologies of the PS and P2VP adsorbed layers were found to be identical before and after the NPS measurements. Figure 5 shows the NPS results for the lone PS290k and P2VP219k flattened layers on the NPS sensor chips. Hence, we clearly indicate the two Tg values of 94 ± 2 °C and 183 ± 2 °C for the PS290k flattened layer and 98 ± 2 °C and 205 ± 2 °C for the P2VP219k flattened layer. It is therefore reasonable to suppose that the Tg,high values observed in the PS and P2VP thin films are attributed to the high Tg value of the lone flattened layers grown on the sensor surface. It should be pointed out that the higher Tg value of the lone PS flattened layer is about 30 °C higher than that the Tg,high values observed in the spin-cast PS films. This may be due to the fact that the lone PS flattened layer is quite heterogeneous (Figure 4b). According to a previous study,35 the indirect NPS approach is capable of probing thermal properties of polymeric materials with different shapes from small nanoparticles to ultrathin films. The LSPR spectral shift ∆ in response to a change in refractive index is approximated as38: ∆ ≈ − , ! 1 − # $/% where m is the sensitivity factor (in nm per refractive index unit), nadsorbate and nmedium are the refractive indices of the adsorbate and medium surrounding the nanoparticle, respectively, d is the thickness of the adsorbate layer (in nm), and l is the penetration depth of the electric field (in nm). This equation is valid only for a homogenous adsorbed layer onto a support such that the PS290k flattened layer composed of nanoscopic dewetting droplets on the NPS sensor would make the accuracy of Tg determination somewhat ambiguous. Nevertheless, the existence of the high Tg at the polymer-solid interface is confirmed. Priestley and co-workers reported the Tg value of the PS adsorbed layer (we believe that 14 ACS Paragon Plus Environment

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their adsorbed layer is composed of the flattened chains and loosely adsorbed chains, while they never mentioned the detailed chain conformation) obtained by washing a 200 nm thick PS thin film annealed at 190 °C29. Their fluorescence spectroscopy experiments gave the Tg value of 90 °C for the PS adsorbed layer (after annealing beyond 6 h). They also explained that the deviation of Tg of the adsorbed layer from Tg,bulk is due to the presence of a free surface of the adsorbed layer. Hence, it is reasonable to suppose that the observed lower Tg value of 94 ± 2 °C in this study corresponds to the Tg of the “tails” of the PS flattened chains preferentially segregated at the free surface7, 68-72 (see, the inset of Figure 5 (a)), while the higher Tg value of 183 ± 2 °C corresponds to the Tg of the polymer segments that are in strong contact with the solid surface (i.e., “trains” or “loops” of the flattened chain73). This will be further confirmed by the following MD simulations. On the other hand, in the case of the P2VP flattened layer, the lower Tg value (98 ± 2 °C) is in good agreement with the bulk Tg value, suggesting that the free-surface effect74 is eliminated. This difference in the lower Tg between the PS and P2VP flattened layers is probably due to their interactions with the substrate; The strong substrate effects, which have been observed in thin films of P2VP on SiOx/Si substrates5, 20, 23-24, may easily overwhelm the free-surface effect74 when the film thickness is less than 10 nm67, 75. It is also interesting to point out that the surface Tg value (94 ± 2 °C) observed in the exposed flattened PS chains (Figure 2) recovers the bulk Tg in the 60 nm-thick film. This can be explained by the erase of the surface effect due to interdiffusion of chains between the extended tails of the flattened chains and free chains in the bulk of the film, as previously indicated by Priestley and co-workers29.

III-2. MD Results The experimental results point out another important aspect: Although PS and P2VP have very similar bulk Tg, the Tg, high value of the P2VP flattened chains is about 20 °C higher than that in the PS flattened chains on the same sensor. This indicates the effect of the polymer-substrate interaction on Tg,

high

of the strongly adsorbed polymer segments. To

further provide better understanding of the role of the polymer-substrate interaction in Tg, high, MD simulations were performed on an atactic PS film adsorbed on planar graphite 15 ACS Paragon Plus Environment

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(hydrophobic) and hydroxylated α-quartz (hydrophilic) substrates. Note that we here ignored the effect of the Au nanodisks on the sensor which may affect the chain conformation. To understand the effect of the substrates on the properties of the PS film, we analyzed the local segmental reorientation dynamics of the chains as a function of the distance from

On graphite substrate

0.8

On α-quartz substrate

0.6 P2(t)

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0-0.5 nm 0.4 9.5-10 nm 0.2

0

-2

10

-1

10

0

10 t (ns)

1

10

2

10

10

-2

-1

10

0

10 t (ns)

10

1

2

10

Figure 6. P2(t) time-autocorrelation functions of the phenyl ring vector on graphite (left) and α-quartz (right) substrates at T=500 K. The simulation box in the z-direction, i.e., the direction perpendicular to the substrate surface, was divided into slabs of 0.5 nm thickness and the z-dependent time-autocorrelations are shown by different colors ranging from green, for phenyl rings within 0-0.5 nm from the substrate surface, to black, for phenyl rings within 9.5-10.0 nm from the substrate surface and represents the top surface of the PS film. The black arrow shows an increase in the location of a slab from the substrate in steps of 0.5 nm.

the substrate. This was accomplished through calculations of the autocorrelation function of the phenyl ring vector joining the backbone carbon atom in contact with the phenyl ring to the carbon atom in the para position of the ring. The quantities of interest here are & ' = 〈cos ,' 〉 and &$ ' = 32 〈cos2 ,' 〉 − 12, which are the first order and second order Legendre polynomial, respectively. θ(t) in both cases is the angle of rotation of the phenyl ring vector in time t. The benefit of calculating P1(t) and P2(t) is that the quantities can in principle be compared to those obtained through experiments such as dielectric spectroscopy 16 ACS Paragon Plus Environment

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(DS) for P1(t) and nuclear magnetic resonance (NMR) for P2(t). Figure 6 shows the semi-logarithmic plot of P2(t) time-autocorrelation functions of the phenyl ring vector on the graphite and α-quartz substrates at 500 K. In calculating the autocorrleation function, the PS film in the z-direction (i.e., the direction perpendicular to the substrate surface) was divided into slabs of 0.5 nm thickness and the time-autocorrelation of the phenyl ring vector in each of the slabs is represented by different colors in the figure. Based on the z-dependence of P2(t) shown in the figure, the PS film can be divided into three different regions: (1) the strongly adsorbed region where the P2(t) functions are represented by broken lines, (2) the bulk region where the P2(t) functions are represented by solid lines,

2

10

Region 1

10

1

Region 2

Region 3

Film

〈τ〉 (ns)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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PS on Graphite PS on α-quartz 0

10

bulk

0

20

40 60 Distance from the substrates (Å)

80

100

Figure 7. The relaxation time 〈0〉 computed from the time integral of the P2(t) curves shown in Figure 6 as a function of distance from the substrates. Based on the data, three different regions are identified where the effect of the substrates is observed only in the region 1.

and (3) the surface region where the P2(t) functions are represented by dotted lines. We observe a large difference in P2(t) between the strongly adsorbed regions on the graphite substrate and α-quartz substrate: P2(t) in the strongly adsorbed region on the α-quartz decays very fast. This region on both substrates is about 3 nm in thickness, which is similar in size to 17 ACS Paragon Plus Environment

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the equilibrated flattened layer that form a compact and more flattened chain conformation to maximize the surface-segment contacts42-44. P2(t) in the bulk and surface regions is independent of the substrate type. To quantify this observation, we calculated the mean relaxation time (〈0〉) of the phenyl ring vector from the time integral of the P2(t) curve76 and is shown in Figure 7 as a function of the distance from the substrate. The relaxation times were

determined

using

a

stretched

exponential

fit

of

the

form

of

the

Kohlrausch-Williams-Watts (KWW) function77. In Figure 7, we clearly see the three different regions and the free-surface region is about 2 nm in thickness. The same behavior is also observed at different temperatures investigated as well as in the P1(t) curves (data not shown here). Extracting 〈0〉 from P1(t) and P2(t) at low temperatures (below T = 460 K (or about 187 °C) for the region 1 and below T = 380 K (or about 107 °C) for the regions 2 and 3) was challenging since the autocorrelation curves did not decay to below 0.37. Hence, the extracted 〈0〉 values should be considered as rough estimates. To determine the glass transition temperatures in the three different regions of the supported PS film, similar calculations described above were performed at different temperatures, and the relaxation time τ in each region was calculated by taking the average of 〈0〉 of the P1(t) curves in each region. This part of the discussion focuses on P1(t), since the Tg determined from simulation results of P1(t) is reasonable to compare with experimental Tg values78. Tg for the different regions is estimated from fit of 〈0〉 vs. T using two popular fitting functional forms known as Vogel-Fulcher-Tamman equation (VFT)79-81 and the recently proposed COOP model82. The VFT is the most commonly used functional form but it is found to usually over-predict Tg while the COOP model is now becoming popular since extrapolated Tg values are very reliable and are usually within a few percent from experimental Tg values. We believe that the two fitting functional forms will provide us upper and lower bound Tg values for our systems.


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(a)

(b)

Figure 8. Temperature dependence of the phenyl ring vector relaxation time obtained through the analysis of P1(t) curves in the three different regions of the PS film. The red plus symbols represent the region 1 of the PS film on graphite, the purple square symbols represent the region 1 of PS film on α-quartz, the green x symbols and blue star symbols represent the region 2 and region 3, respectively. The solid lines in (a) are best fits to the data using the VFT relationship and (b) are best fits to the data using the COOP model. Note that the data for the region 2 and region 3 are the averages of the data obtained for the PS films on graphite and α-quartz.

The temperature dependence of the relaxation time for the three different regions on both 20 ACS Paragon Plus Environment

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substrates is shown in Figure 8. The solid lines are best fits to the data using the Vogel-Fulcher-Tamman (VFT)79-81 relationship (Fig. 8a) and COOP (Fig. 8b). The Tg is usually identified as the temperature at which the fit extrapolates to the usual 100 sec. Based on the extrapolation using VFT relationship, the Tg of the region 1 on the graphite, the region 1 on the α-quartz, the region 2, and the region 3 were found to be around 95 °C, 62 °C, 49 °C, and 32 °C, respectively. Similarly, the extrapolated Tg values using the COOP model are 89 °C, 44 °C, 21 °C, and 5 °C, respectively. On the graphite substrate, there is a large difference (about 46 °C from VFT and 78 °C from COOP) between the Tg of the bulk region and the Tg of the strongly adsorbed region, while this difference decreases to about 13 °C (VFT) and 23 °C (COOP) for the α-quartz substrate. To understand this large difference in the relative Tg values on the two substrates, we computed the total interaction energy between the PS film and the substrates. The result pointed out that the PS film/graphite substrate interaction is about twice more favorable than

2

PS on α-quartz PS on Graphite

1.5

3

ρ (g/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1

0.5

0

0

20

40

60 Z (Å)

80

100

120

Figure 9. Density profiles of PS as a function of the distance Z from the substrates at T=223 °C. The density profiles are identical beyond 2 nm from the substrate.


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the PS film/α-quartz substrate interaction at all temperatures. Additionally, as shown in Figure 9, we found that the more favorable interaction between the PS and graphite substrate leads to a higher density of the polymer segments of the adsorbed chains closest to the substrates, resulting in less free volume and consequently a reduction in the mobility of the chain segments, as confirmed through mean-squared displacement calculations (data not shown). This is in agreement with several simulation results in the literature that investigated the effect of polymer/substrate interaction on the dynamics of polymer chains near the substrate83-87. It should also be emphasized that the density profile shown in Fig. 9 is in good agreement with a previous X-ray reflectivity result for a PS290k spin cast film (about 10 nm-thick) prepared on a Si substrate after annealing at 150 °C for 24 h43. Hence, this simulation data supports the experimental results: As the polymer-substrate interaction is stronger, the higher the density of the flattened layer is, resulting in the larger deviation of Tg, high

value of the polymer adsorbed segments from the bulk Tg. Furthermore, based on our preliminary investigation on the effect of PS chain lengths of

up to 100 monomers on the phenyl ring relaxation time, we have observed: (i) the relaxation time of the adsorbed region increases much faster than the increases observed in the bulk and surface regions as a function of the chain length and (ii) the size of the adsorbed layer is about 3 nm in thickness independent of molecular weight. The former implies that the difference between the Tg of the bulk region and the Tg of the adsorbed region will increase even more with increasing chain lengths, verifying the present experimental results with the use of high Mw polymers. The latter suggests that the effect of the substrate on the dynamics of the chain segments of the adsorbed polymer chains may be of a short range. It should be emphasized that the second observation is in good agreement with previous experimental results on the quasiequilibrium flattened layer42-44. Further simulation studies on the above two observations are currently in progress and will be summarized as a forthcoming publication.

IV. Conclusion In summary, we have studied the glass transition temperatures of the PS and P2VP thin 22 ACS Paragon Plus Environment

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films prepared onto the SiO2 coated glass substrate surfaces using the nanoplasmonic sensing (NPS) method. The results were intriguing to highlight the presence of the very high Tg (Tg, high):

Tg, high ≈155 ºC for PS and Tg, high ≈ 200 ºC for P2VP, both of which are far above their

bulk Tg values (Tg,bulk ~ 100 ºC). In conjunction with the established solvent leaching protocol for unveiling the adsorbed chains, we found that the polymer segments that are in direct contact with the solid surface is responsible for Tg,

high.

MD simulations were further

performed to investigate the Tg behavior of the PS thin films on the hydrophilic and hydrophobic surfaces. The simulation results (i) revealed the presence of a very thin (~ 3 nm in thickness), highly packed layer with high Tg next to the substrate surface regardless of hydrophobicity, and (ii) demonstrated that the high Tg value increases with increasing the magnitude of the polymer-solid attractive interactions. Since a 10 °C change in Tg in the bulk would be accompanied by a 1000 times change in molecular mobility88, one would expect an extremely slowing down of molecular dynamics of such strongly bound polymer segments. Physisorption of polymer chains with many surface-segment contacts would have a similar effect to that of cross-linking on a bulk polymer where it inhibits the chains from attaining the entropy necessary to reach the melt state.3,89 For example, bulk PS with the presence of a high degree of cross linking can elevate the Tg by 70 °C.90 It should be noted that the slopes of ∆ / , which is related to the thermal expansion coefficient, at the temperature ranges above Tg,high are always gradual compared to those at the temperature ranges below Tg,high for the polymer thin films and flattened layers used in this study. A similar trend was previously reported in the NPS experiments for PS nanoparticles35. The authors explained that this is related to different occupied fractions of the sensing volume around the nanoplasmonic sensor-nanoparticles, which gives rise to different sensitivities of the sensor for different polymer thicknesses. In addition, as discussed above, the density of the flattened layer is higher than the bulk such that the dielectric property (i.e., index of refraction) of the flattened layer would be different from the bulk, resulting in different slopes in NPS measurements as well. An interesting observation was the formation of the nanodroplets induced by spinodal dewetting of the PS flattened layer during a post thermal annealing process, as previously 23 ACS Paragon Plus Environment

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observed55, 61. This dewetting of the flattened layer is entropically driven and was observed when the “effective” grafting density of the flattened layer (i.e., the number of solid–segment contacts per area) was not high enough43. This is in line with a previous report by Green and coworkers91 who showed that multi-arm, star-shaped adsorbed PS chains on a native oxide Si substrate form a thermally stable wetting layer on the substrate surface due to much lower entropic penalty upon adsorption, while the linear PS adsorbed chains dewet on the same substrate into nanodroplets. Further thermal Tg measurements by NPS for polymer thin films with different chain architectures (including stars, rings, and so on) deserve future work. Finally, it is also interesting to comment on a gradient in the segment dynamics within a supported polymer thin film in the direction normal to the film surface. Roth and co-workers showed that the strong polymer-solid interaction between P2VP and silica dominates the Tg of P2VP thin films through chain connectivity and overwhelms the impact of the free surface effect23. Recently, Fakhraai and co-workers demonstrated the existence of two glass transition temperatures of P2VP due to the completing interfacial effects between the substrate and free-surface75. Further NPS experiments with varying thickness of PS and P2VP ultrathin films and annealing times are currently in progress to identify (i) the glass transition temperatures of the strongly adsorbed region, the bulk region, and the surface region, as suggested by the present simulation results, and (ii) the effect of the adsorbed chains, whose impact on the vitrification of an adjacent layer would extend far beyond the volume occupied by monomers belonging to chains directly pinned onto the substrate47, on the emergence of the gradient in the glassy dynamics.


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Acknowledgements We thank Jenny Andersson for her assistance to the NPS experiments and Selemon Bekele for his assistance in fitting the simulated relaxation time data with VFT and COOP models. T. K. acknowledges partial financial support from the NSF Grant (CMMI-1332499) and M.T. acknowledges support from the NSF Grant (DMR-1410290). Use of the National Synchrotron Light Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contracts No. DE-AC02-98CH10886.

Author Information Corresponding Authors E-mails:

[email protected]

(N.J.);

[email protected]

(T.K.);

[email protected] (M.T.) Notes: The authors declare no competing financial interest.

Supporting Information The Supporting Information contains the details of the with the LSPR measurements, additional AFM results on the NPS sensor chip, the scratch test, and the surface coverage analysis. This material is available free of charge via the Internet at xxxx.


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References 1. Kremer, F.; Tress, M.; Mapesa, E. U. Glassy dynamics and glass transition in nanometric layers and films: A silver lining on the horizon. J. Non-Cryst. Solids 2015, 407, 277-283. 2. Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Size-Dependent Depression of the Glass Transition Temperature in Polymer Films. Europhys. Lett. 1994, 27, 59-64. 3. Wallace, W. E.; Vanzanten, J. H.; Wu, W. L. Influence of an Impenetrable Interface on a Polymer Glass-Transition Temperature. Phys. Rev. E 1995, 52, R3329-R3332. 4. Forrest, J. A.; Dalnoki-Veress, K.; Stevens, J. R.; Dutcher, J. R. Effect of Free Surfaces on the Glass Transition Temperature of Thin Polymer Films. Phys. Rev. Lett. 1996, 77, 2002-2005. 5. van Zanten, J. H.; Wallace, W. E.; Wu, W.-l. Effect of strongly favorable substrate interactions on the thermal properties of ultrathin polymer films. Phys. Rev. E 1996, 53, R2053-R2056. 6. DeMaggio, G. B.; Frieze, W. E.; Gidley, D. W.; Zhu, M.; Hristov, H. A.; Yee, A. F. Interface and surface effects on the glass transition in thin polystyrene films. Phys. Rev. Lett. 1997, 78, 1524-1527. 7. Kajiyama, T.; Tanaka, K.; Takahara, A. Surface molecular motion of the monodisperse polystyrene films. Macromolecules 1997, 30, 280-285. 8. Ellison, C. J.; Torkelson, J. M. The distribution of glass-transition temperatures in nanoscopically confined glass formers. Nat. Mater. 2003, 2, 695-700. 9. Priestley, R. D.; Ellison, C. J.; Broadbelt, L. J.; Torkelson, J. M. Structural Relaxation of Polymer Glasses at Surfaces, Interfaces, and In Between. Science 2005, 309, 456-459. 10. Tsui, O. K. Anomalous dynamics of polymer films. In Polymer Thin Films, Tsui, O. K. C.; Russell, T. P., Eds.; World Scientific: Singapore; Hackensack, NJ, 2008; Vol. 1, pp 267-294. 11. Fujii, Y.; Yang, Z. H.; Leach, J.; Atarashi, H.; Tanaka, K.; Tsui, O. K. C. Affinity of Polystyrene Films to Hydrogen-Passivated Silicon and Its Relevance to the T-g of the Films. Macromolecules 2009, 42, 7418-7422. 12. Yang, Z. H.; Fujii, Y.; Lee, F. K.; Lam, C. H.; Tsui, O. K. C. Glass Transition Dynamics and Surface Layer Mobility in Unentangled Polystyrene Films. Science 2010, 328, 1676-1679. 13. Napolitano, S.; Wubbenhorst, M. The lifetime of the deviations from bulk behaviour in polymers confined at the nanoscale. Nat. Commun. 2011, 2, 260-267. 14. Ediger, M. D.; Forrest, J. A. Dynamics near Free Surfaces and the Glass Transition in Thin Polymer Films: A View to the Future. Macromolecules 2014, 47, 471-478. 15. Geng, K.; Katsumata, R.; Yu, X.; Ha, H.; Dulaney, A. R.; Ellison, C. J.; Tsui, O. K. C. Conflicting Confinement Effects on the Tg, Diffusivity, and Effective Viscosity of Polymer Films: A Case Study with Poly(isobutyl methacrylate) on Silica and Possible Resolution. Macromolecules 2017, 50, 609-617. 16. Frank, C. W.; Rao, V.; Despotopoulou, M. M.; Pease, R. F. W.; Hinsberg, W. D.; Miller, R. D.; Rabolt, J. F. Structure in thin and ultrathin spin-cast polymer films. Science 1996, 273, 912-915. 17. Fryer, D. S.; Peters, R. D.; Kim, E. J.; Tomaszewski, J. E.; de Pablo, J. J.; Nealey, P. F.; White, C. C.; Wu, W. L. Dependence of the glass transition temperature of polymer films on 26 ACS Paragon Plus Environment

Page 26 of 32

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interfacial energy and thickness. Macromolecules 2001, 34, 5627-5634. 18. Tsui, O. K. C.; Russell, T. P.; Hawker, C. J. Effect of interfacial interactions on the glass transition of polymer thin films. Macromolecules 2001, 34, 5535-5539. 19. Sharp, J. S.; Forrest, J. A. Dielectric and ellipsometric studies of the dynamics in thin films of isotactic poly(methylmethacrylate) with one free surface. Phys. Rev. E 2003, 67, 0318051. 20. Park, C.-H.; Kim, J.-H.; Ree, M.; Sohn, B.-H.; Jung, J.-C.; Zin, W.-C. Thickness and composition dependence of the glass transition temperature in thin random copolymer films. Polymer 2004, 45, 4507-4513. 21. Mundra, M. K.; Ellison, C. J.; Behling, R. E.; Torkelson, J. M. Confinement, composition, and spin-coating effects on the glass transition and stress relaxation of thin films of polystyrene and styrene-containing random copolymers: Sensing by intrinsic fluorescence. Polymer 2006, 47, 7747-7759. 22. Rittigstein, P.; Priestley, R. D.; Broadbelt, L. J.; Torkelson, J. M. Model polymer nanocomposites provide an understanding of confinement effects in real nanocomposites. Nat. Mater. 2007, 6, 278-282. 23. Roth, C. B.; McNerny, K. L.; Jager, W. F.; Torkelson, J. M. Eliminating the enhanced mobility at the free surface of polystyrene: Fluorescence studies of the glass transition temperature in thin bilayer films of immiscible polymers. Macromolecules 2007, 40, 2568-2574. 24. Ellison, C. J.; Kim, S. D.; Hall, D. B.; Torkelson, J. M. Confinement and processing effects on glass transition temperature and physical aging in ultrathin polymer films: Novel fluorescence measurements. Eur. Phys. J. E 2002, 8, 155-166. 25. Napolitano, S.; Rotella, C.; Wübbenhorst, M. Can Thickness and Interfacial Interactions Univocally Determine the Behavior of Polymers Confined at the Nanoscale? ACS Macro Lett. 2012, 1, 1189-1193. 26. Rotella, C.; Napolitano, S.; Vandendriessche, S.; Valev, V. K.; Verbiest, T.; Larkowska, M.; Kucharski, S.; Wubbenhorst, M. Adsorption Kinetics of Ultrathin Polymer Films in the Melt Probed by Dielectric Spectroscopy and Second-Harmonic Generation. Langmuir 2011, 27, 13533-13538. 27. Yin, H. J.; Napolitano, S.; Schonhals, A. Molecular Mobility and Glass Transition of Thin Films of Poly(bisphenol A carbonate). Macromolecules 2012, 45, 1652-1662. 28. Napolitano, S.; Capponi, S.; Vanroy, B. Glassy dynamics of soft matter under 1D confinement: How irreversible adsorption affects molecular packing, mobility gradients and orientational polarization in thin films. Eur. Phys. J. E 2013, 36, 1-37. 29. Burroughs, M. J.; Napolitano, S.; Cangialosi, D.; Priestley, R. D. Direct Measurement of Glass Transition Temperature in Exposed and Buried Adsorbed Polymer Nanolayers. Macromolecules 2016, 49, 4647-4655. 30. Perez-de-Eulate, N. G.; Sferrazza, M.; Cangialosi, D.; Napolitano, S. Irreversible Adsorption Erases the Free Surface Effect on the Tg of Supported Films of Poly(4-tert-butylstyrene). ACS Macro Lett. 2017, 6, 354-358. 31. Ellison, C. J.; Mundra, M. K.; Torkelson, J. M. Impacts of polystyrene molecular weight and modification to the repeat unit structure on the glass transition-nanoconfinement effect and the cooperativity length scale. Macromolecules 2005, 38, 1767-1778. 27 ACS Paragon Plus Environment

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32. Madkour, S.; Szymoniak, P.; Heidari, M.; von Klitzing, R.; Schönhals, A. Unveiling the Dynamics of Self-Assembled Layers of Thin Films of Poly(vinyl methyl ether) (PVME) by Nanosized Relaxation Spectroscopy. ACS Appl. Mater. Interfaces 2017, 9, 7535-7546. 33. Tress, M.; Mapesa, E. U.; Kossack, W.; Kipnusu, W. K.; Reiche, M.; Kremer, F. Glassy Dynamics in Condensed Isolated Polymer Chains. Science 2013, 341, 1371-1374. 34. Priestley, R. D.; Cangialosi, D.; Napolitano, S. On the equivalence between the thermodynamic and dynamic measurements of the glass transition in confined polymers. J. Non-Cryst. Solids 2015, 407, 288-295. 35. Langhammer, C.; Larsson, E. M.; Kasemo, B.; Zorić, I. Indirect Nanoplasmonic Sensing: Ultrasensitive Experimental Platform for Nanomaterials Science and Optical Nanocalorimetry. Nano Lett. 2010, 10, 3529-3538. 36. Diaz de Zerio Mendaza, A.; Melianas, A.; Nugroho, F. A. A.; Backe, O.; Olsson, E.; Langhammer, C.; Inganas, O.; Muller, C. A fullerene alloy based photovoltaic blend with a glass transition temperature above 200 [degree]C. J. Mater. Chem. A 2017, 5, 4156-4162. 37. Nugroho, F. A. A.; Diaz de Zerio Mendaza, A.; Lindqvist, C.; Antosiewicz, T. J.; Müller, C.; Langhammer, C. Plasmonic Nanospectroscopy for Thermal Analysis of Organic Semiconductor Thin Films. Anal. Chem. 2017, 89, 2575-2582. 38. Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P. Biosensing with plasmonic nanosensors. Nat. Mater. 2008, 7, 442-453. 39. Fredriksson, H.; Alaverdyan, Y.; Dmitriev, A.; Langhammer, C.; Sutherland, D. S.; Zäch, M.; Kasemo, B. Hole–Mask Colloidal Lithography. Adv. Mater. 2007, 19, 4297-4302. 40. Guiselin, O. Irreversible Adsorption of a Concentrated Polymer Solution. Europhys. Lett. 1992, 17, 225-230. 41. Rotella, C.; Wubbenhorst, M.; Napolitano, S. Probing interfacial mobility profiles via the impact of nanoscopic confinement on the strength of the dynamic glass transition. Soft Matter 2011, 7, 5260-5266. 42. Gin, P.; Jiang, N.; Liang, C.; Taniguchi, T.; Akgun, B.; Satija, S. K.; Endoh, M. K.; Koga, T. Revealed Architectures of Adsorbed Polymer Chains at Solid-Polymer Melt Interfaces. Phys. Rev. Lett. 2012, 109, 265501. 43. Jiang, N.; Shang, J.; Di, X.; Endoh, M. K.; Koga, T. Formation Mechanism of High-Density, Flattened Polymer Nanolayers Adsorbed on Planar Solids. Macromolecules 2014, 47, 2682-2689. 44. Jiang, N.; Wang, J.; Di, X.; Cheung, J.; Zeng, W.; Endoh, M. K.; Koga, T.; Satija, S. K. Nanoscale adsorbed structures as a robust approach for tailoring polymer film stability. Soft Matter 2016, 12, 1801-1809. 45. Sen, M.; Jiang, N.; Cheung, J.; Endoh, M. K.; Koga, T.; Kawaguchi, D.; Tanaka, K. Flattening Process of Polymer Chains Irreversibly Adsorbed on a Solid. ACS Macro Lett. 2016, 5, 504-508. 46. Linse, P.; Källrot, N. Polymer Adsorption from Bulk Solution onto Planar Surfaces: Effect of Polymer Flexibility and Surface Attraction in Good Solvent. Macromolecules 2010, 43, 2054-2068. 47. Carrillo, J.-M. Y.; Cheng, S.; Kumar, R.; Goswami, M.; Sokolov, A. P.; Sumpter, B. G. Untangling the Effects of Chain Rigidity on the Structure and Dynamics of Strongly Adsorbed Polymer Melts. Macromolecules 2015, 48, 4207-4219. 28 ACS Paragon Plus Environment

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Langmuir

48. de Gennes, P. G. Polymers at an interface; a simplified view. Adv. Colloid Interface Sci. 1987, 27, 189-209. 49. Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman and Hall: London, 1993. 50. Koga, T.; Jiang, N.; Gin, P.; Endoh, M. K.; Narayanan, S.; Lurio, L. B.; Sinha, S. K. Impact of an Irreversibly Adsorbed Layer on Local Viscosity of Nanoconfined Polymer Melts. Phys. Rev. Lett. 2011, 107, 225901. 51. Jiang, N.; Endoh, M. K.; Koga, T. Structures and Dynamics of Adsorbed Polymer Nanolayers on Planar Solids. In Non-equilibrium Phenomena in Confined Soft Matter, Napolitano, S., Ed.; Springer International Publishing, 2015, pp 129-160. 52. Asada, M.; Jiang, N.; Sendogdular, L.; Gin, P.; Wang, Y.; Endoh, M. K.; Koga, T.; Fukuto, M.; Schultz, D.; Lee, M.; Li, X.; Wang, J.; Kikuchi, M.; Takahara, A. Heterogeneous Lamellar Structures Near the Polymer/Substrate Interface. Macromolecules 2012, 45, 7098-7106. 53. Asada, M.; Jiang, N.; Sendogdular, L.; Sokolov, J.; Endoh, M. K.; Koga, T.; Fukuto, M.; Yang, L.; Akgun, B.; Dimitriou, M.; Satija, S. Melt crystallization/dewetting of ultrathin PEO films via carbon dioxide annealing: the effects of polymer adsorbed layers. Soft Matter 2014, 10, 6392-6403. 54. Housmans, C.; Sferrazza, M.; Napolitano, S. Kinetics of Irreversible Chain Adsorption. Macromolecules 2014, 47, 3390-3393. 55. Beena Unni, A.; Vignaud, G.; Bal, J. K.; Delorme, N.; Beuvier, T.; Thomas, S.; Grohens, Y.; Gibaud, A. Solvent Assisted Rinsing: Stability/Instability of Ultrathin Polymer Residual Layer. Macromolecules 2016, 49, 1807-1815. 56. Bal, J. K.; Beuvier, T.; Unni, A. B.; Chavez Panduro, E. A.; Vignaud, G.; Delorme, N.; Chebil, M. S.; Grohens, Y.; Gibaud, A. Stability of Polymer Ultrathin Films (

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