Dramatic Increase in Polymer Glass Transition Temperature under

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Dramatic Increase in Polymer Glass Transition Temperature under Extreme Nanoconfinement in Weakly-Interacting Nanoparticle Films Haonan Wang, Jyo Lyn Hor, Yue Zhang, Tianyi Liu, Daeyeon Lee, and Zahra Fakhraai ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b01341 • Publication Date (Web): 24 May 2018 Downloaded from http://pubs.acs.org on May 24, 2018

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Dramatic Increase in Polymer Glass Transition Temperature under Extreme Nanoconfinement in Weakly-Interacting Nanoparticle Films Haonan Wang,† Jyo Lyn Hor,‡ Yue Zhang,† Tianyi Liu,† Daeyeon Lee,∗,‡ and Zahra Fakhraai∗,† Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States, and Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States E-mail: [email protected]; [email protected]

KEYWORDS: capillary rise infiltration, glass transition, nanoporous film, ellipsometry, nanoconfinement, polymer glass, polymer nanocomposite Abstract Properties of polymers in polymer nanocomposites and nanopores have been shown to deviate from their respective bulk properties due to chain confinement as well as polymer-particle interfacial interactions. However, separating the confinement effects from the interfacial effects under extreme nanoconfinement is experimentally challenging. Capillary Rise Infiltration (CaRI) enables polymer infiltration into nanoparticle (NP) packings, thereby confining polymers within extremely small pores and dramatically increasing the interfacial area, providing a good system to systematically distinguish the role of each effect on polymer properties. In ∗ To

whom correspondence should be addressed of Pennsylvania ‡ University of Pennsylvania † University

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this study, we investigate the effect of spatial confinement on the glass transition temperature (Tg ) of polystyrene (PS) infiltrated into SiO2 NP films. The degree of confinement is tuned by varying the molecular weight of polymers, the size of NPs (diameters between 11 nm-100 nm, producing 3 nm-30 nm average pore sizes), as well as the fill-fraction of PS in the NP films. We show that in these dense NP packings the Tg of confined PS, which interacts weakly with SiO2 NPs, significantly increases with decreasing pore size such that for the two molecular weights of PS studied, the Tg increases by up to 50 K in 11 nm NP packings, while Tg is close to the bulk Tg in 100 nm NP packings. Interestingly, as the fill-fraction of PS is decreased, resulting in the accumulation of the polymer in the contacts between nanoparticles, hence increased specific interfacial area, the Tg further increases relative to the fully-filled films by another 5-8 K, indicating the strong role of geometrical confinement as opposed to the interfacial effects on the measured Tg values.

Thin polymer films and polymer nanoparticle (NP) composites have become increasingly important in modern technology, such as microelectronics, 1 separation membranes, 2 protective coatings, 3 and photovoltaic cells. 4 At the nanometer scale, the reduction in size of the polymers in these systems results in a deviation of physical properties, such as the physical aging rate, 5–9 viscosity, 10 and the glass transition temperature (Tg ), 11–16 from their bulk values. These deviations from bulk properties could be due to either chain and segmental confinement effects 17,18 or the increased role of interfacial effects on the properties of the system. 16,19,20 In particular, Tg of supported polymer thin films have been shown to be influenced by both the substrate and the free surface. 12,21–25 The free surfaces of polymer films show enhanced mobility 14,20,26 which can result in lower average film Tg s. The effects are quite significant when the film thickness is reduced below ∼ 30 nm in both polymer and small molecule glass systems. 11,14,23,27–30 When the polymer has strong interactions with the substrate (such as hydrogen bonding), the substrate-polymer interactions can dominate and the Tg increases with decreasing film thickness. 22,23,31,32 The competing interfacial effects can even give rise to two distinct Tg s for ultra-thin films at low cooling rates. 33 Polystyrene (PS) on Silicon substrate with native oxide (Si/SiO2 ) is one of the most investigated systems with weak polymer-substrate interactions. Decreasing Tg of PS with film thickness 2


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was observed in early 1990s. 12 Since then, a large number of both experimental and theoretical studies have focused on the confinement and interfacial effects on PS glassy dynamics. 12,28,34–40 In supported PS films, Tg typically decreases with decreasing film thickness due to strongly enhanced dynamics at the free surface. After the removal of the free surface, Tg recovers to the bulk value. 17,28 As such, most studies have attributed the observation of Tg changes in supported PS thin films to the free surface effects that are the dominant factor in these films. Even the formation of highly adsorbed PS layers on native Silicon Oxide substrate has a moderate effect on Tg in this geometry. 41–43 Distinguishing interfacial effects from pure confinement effects is even more challenging when it comes to PS/SiO2 nanocomposites. Increase, 44 decrease, 45 or no change 46 in composite Tg with increasing SiO2 NP loading have all been observed. In these systems, the highest possible NP loadings are around 40% volume fraction, 44,45,47,48 limiting the degree of confinement. Furthermore, the dispersion conditions and possible aggregation of SiO2 NPs can complicate data interpretation. Recently, a new method of making polymer NP films with extremely high NP loadings has been reported via capillary rise infiltration (CaRI). 49 In this method, a NP film is placed on top of a polymer film. During annealing at high temperature, capillary forces drive the polymer to infiltrate into the interstices between the NPs without disturbing the dense (∼ 63% volume fraction) NP film packing. The viscosity and dynamics of unentangled polymers during infiltration have been studied using experiments and molecular dynamics simulations and have shown extreme slowdown of the dynamics under nano-confinement. 49–51 We have recently shown that the viscosity increase in unentangled systems is independent of the polymer/NP interactions and correlates well with the increased Tg . 51 Here, we studied the glass transition temperature of PS/SiO2 CaRI films with various NP diameters, PS molecular weights, and PS fill-fractions in under-saturated CaRI (UCaRI) 52 films. Spectroscopic ellipsometry was used to measure the thickness and the index of refraction of CaRI films as a function of temperature to determine Tg of PS confined in these films (Tg (confined)). The results show that under these extreme confinement conditions, Tg (confined) is significantly

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increased for CaRI films with NP diameters smaller than 100 nm. For UCaRI films, where there is insufficient PS to fully fill the interparticle space, PS forms capillary bridges between NPs, resides in the most confined regions, and Tg is further increased, despite having larger free surface area. The dewetting of PS on the NPs under these conditions indicates that the interfacial effects are still minimal in these systems and the Tg increase can be strongly attributed to the geometrical confinement effects due to modifications in segmental relaxation dynamics in small spaces.

Results and Discussion The sample preparation scheme for CaRI and UCaRI films in various geometries (PS-top or NPtop) are schematically shown in Figure 1. More details of sample preparation can be found in the methods section and online supporting info (SI). Tg of these films were determined via Spectroscopic Ellipsometry (SE) upon cooling the films at 10 K/min. This is the nominal cooling rate typically used in differential scanning calorimetry measurements and typically corresponds to a segmental relaxation time of ∼ 102 seconds for bulk polymers. 53–58 The thickness of the NP layer, in all three geometries shown in Figure 1 was found to be insensitive to the temperature changes and was fixed at a constant initial value to avoid over-fitting (Figure S5 of SI). This is consistent with the previous observations that, once the NP film is formed, it does not expand upon polymer infiltration. This is due to strong interactions between the NPs. 49 As such, the changes in the index of refraction (n) vs. temperature (T ) were monitored to measure Tg (confined) (Figure 2). Figure 2 shows an example of the calculated thickness and refractive index change with temperature for a PS(8K)/SiO2 (11 nm) NP-top, CaRI sample, with hPS = 242 ± 1 nm and hNP = 204 ± 1 nm. The Tg of each layer was determined as the intersection of the linear fitting of the glass and the super-cooled liquid (SCL) regions in the plot of n vs. T . The Tg s of the PS and NP films were determined to be Tg (PS) = 362 ±3 K and Tg (confined) = 403 ±4 K, respectively. We note that because of water absorption, there is a deviation from linear behavior of the refractive index of the NP layer at low temperatures. Therefore, only the linear region was fitted as shown in Figure 2a.

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Figure 1: Schematic illustration of sample preparation of (a) NP-top CaRI, (b) NP-top UCaRI, and (c) PS-top CaRI films. In Figure 2b two inflection points can be observed at 363 ± 2 K and 401 ± 7 K in the plot of hPS vs. T . These values are the same as Tg (PS) and Tg (confined) within the error, as measured by the index of refraction, respectively. This is because as the film is cooled above Tg (confined) (T > 403 K), the PS chains in both layers are in the SCL state and can further infiltrate into the NP film while hNP remains constant due to the rigidity of the nanoparticle film. This generates an apparent expansion coefficient in the PS layer that is larger than the expansion coefficient of the SCL of bulk PS. Below Tg (confined) (T < 403 K), the PS layer recovers the expansion coefficient of bulk material and goes through a glass transition upon further cooling below Tg (PS). Measurements of both the index of refraction and the film thickness as shown in Figure 2 indicate an increase of ∼ 41 K in Tg of PS(8K) upon confinement in SiO2 (11 nm) NP films. In this system the average pore diameter is estimated to be ∼ 3 nm 59 and the polymer radius of gyration is roughly 2.5 nm. 60 To investigate the changes of Tg (∆Tg ) upon further confinement, CaRI films with various NP sizes were tested, with PS of two molecular weights. Figure 3 shows Tg of PS(8K) and PS(2M), NP-top CaRI films, in NP packings with various sizes (11 nm-100 nm). In the original CaRI geometry (NP-top), Tg values are fairly similar to the corresponding bulk values in SiO2 (100 nm) NP films, increasing dramatically with decreasing NP size for both

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Figure 2: (a) Index of refraction n (at λ = 632.8 nm) vs. T for the PS (black) and NP (red) films of a PS(8K)/SiO2 (11 nm) NP-top CaRI film. The inset shows the geometry of the sample. The Tg of these layers are measured to be Tg (PS)=362±3 K and Tg (confined)=403±4 K, respectively. (b) Normalized thickness vs. temperature for PS (black) and NP (red) films. The thickness of these layers were measured to be hPS = 242 ± 1nm and hNP = 204 ± 1nm, respectively. hNP was held constant during fitting. The inset shows the mean-squared error (MSE) vs. T , which shows reliable fitting throughout this temperature range. Two inflection points are observed in hPS vs. T plot. The first inflection point occurs at the Tg (confined) upon cooling, where material is no longer sipped into the NP film at lower temperatures.

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molecular weights, up to ∆Tg = 41 ± 3 K increase for PS(8K)/SiO2 (11 nm) and ∆Tg = 23 ± 3 K increase for PS(2M)/SiO2 (11 nm). However, we note that in this geometry the polymer and NP films have mismatching expansion coefficients upon heating and the polymer infiltration results in the formation of cracks in the NP films during infiltration (See Figure S6). As such, there are some free polymers in the cracked areas and the measured Tg (confined) is a weighted average between the confined Tg and the Tg of PS in the cracked regions. In contrast, in the PS-top geometry where cracks are absent, the measured Tg (confined) is dramatically larger, up to ∆Tg = 51 ± 3 K for SiO2 (11 nm)/PS(8K) and ∆Tg = 57 ± 3 K for SiO2 (11 nm)/PS(2M).

Figure 3: Tg as a function of NP diameter for CaRI films at two different geometries (NP-top with open symbols and PS-top with solid symbols), with PS with two molecular weights PS(8K) (black) and PS(2M) (red). Black and red dashed lines show the bulk Tg values of PS(8K) (Tg = 362±2 K) and PS(2M) (Tg = 373±2 K), respectively as measured for thick PS films. Error bars represent standard error of at least 3 samples. Thicknesses of the NP and the residual PS layers are hNP ∼250 nm and hPS ∼200 nm, respectively. The observations of up to ∼ 57 K increase in Tg in CaRI films (for SiO2 (11 nm)/PS(2M) PS-top sample) is quite remarkable. SiO2 is typically considered as a substrate with weak interactions with PS. As such, in previous studies, the free surface effects were considered as the dominant factor in the observed Tg reduction of PS thin films. In these studies after the free surface was removed, bulk Tg was recovered but still no increase in Tg for films as thin as ∼ 7 nm were observed. 28 The conventional method of making polymer nanocomposites is by evaporating the solvent of polymer7


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NP mixture suspension. In these types of nanocomposite systems, low accessible NP loadings and tendency of NPs to aggregate can reduce the effect interfacial area and the spatial confinement. The force generated by extremely confined chains precludes NPs from creating regions of significant confinement, unless the polymer chain is depleted from that region. As a result, Tg of PS is either reduced or is to within a few degrees from the bulk value in conventional nanocomposites. 44–46 To put the degree of CaRI film confinement in context, the thinnest films studied in Tg measurements are typically 5-10 nm. PS in anodic aluminum oxide (AAO) templates can be confined down to a diameter of 55 nm. 61 In CaRI films, NPs are closely packed with strong interparticle interactions before the infiltration process. The space occupied by NPs is ∼ 63% of the CaRI film volume (SI table S1), which is very close to random close packing of uniform spheres (64%). As such, the PS/SiO2 interfacial area and spatial confinement is significantly larger than achievable by co-mixing of the polymer and NPs. By changing the diameter of NPs from 100 nm to 11 nm, the average pore diameter can be varied from 30 nm down to 3 nm (calculated as ∼ 0.3 of the NP diameter 59 ), with much smaller areas close to the NP junctions. Moreover, there is almost no free surface in these films and virtually all of the polymer chains are close to a SiO2 surface. There are a few possible origins for the dramatic increase of Tg in these highly confined CaRI films. Interfacial effects at the SiO2 /PS boundary, geometrical confinement of the chains, and a change in the boundary conditions from isobaric (P,N,T) to isochoric (V,N,T) can result in changes in the properties of PS in these highly confined systems. We can first rule out the effect of the boundary conditions. As shown in Figure 2b, PS can freely expand in and out of the film upon heating/cooling at temperatures above Tg (confined). This effect is observed as an apparent change in the expansion coefficient of the adjacent residual PS layer. As such, it is unlikely that the boundary conditions are isochoric (V,N,T) during these experiments. Similarly, in the NP-top geometry, PS can freely expand to the free surface, as well as the bottom PS surface, yet significantly increased Tg values are still observed. This is consistent with previous studies where it was observed that isochoric confinement for PS nanoparticles, of 100 nm diameter or larger, does not significantly affect Tg . 17

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Another possibility is the formation of highly adsorbed PS layer during the infiltration/annealing process. Previous studies have shown that during annealing at elevated temperatures, large molecular weight PS can be irreversibly adsorbed on SiO2 surface with slower dynamics. These studies have shown that the Tg of the interfacial layer is below bulk Tg before adsorption and gradually recovers to the bulk Tg with the growth of the irreversibly adsorbed layer. 43,62 The irreversibly adsorbed PS layer takes hours of annealing to grow at the infiltration temperature of 433 K used in this study. 63 Thus, by controlling the annealing time, we are able to examine the potential influence of the adsorbed layer on Tg (confined) values. Figure S9 and Figure S10 show that extended annealing of the samples (3 hours at 433 K) does not significantly affect the value of Tg and thus there is no visible evidence that the growth of a highly-adsorbed layer has an influence on the observed values of Tg (confined) for either PS(8K) and PS(2M) samples. An interfacial layer with slower dynamics is also expected to broaden the width of the glass transition 64,65 or even result in two glass transitions. 33,64–67 In the CaRI samples, the width of the glass transition can be calculated from the 1st derivative of the change of the index of refraction with temperature. Figure S11 shows T+ and T− , the high and low onsets of the glass transition, as a function of NP diameter for CaRI films. It is observed that there is no significant difference in the transition width (∆T = T+ − T− ) between confined PS and bulk PS, which indicates that the range of dynamical heterogeneity of PS inside the NP packings is similar to the bulk within our ability to resolve. To further confirm whether spatial or interfacial effects are responsible for Tg changes, series of under-saturated (UCaRI), NP-top films (Figure 1b) of PS(8K)/SiO2 (25 nm) with various PS fill-fractions were studied. 52 In this geometry, free surfaces are introduced due to underfilling. Unlike the porous glass systems used in the past, 66 underfilling of NP films forces the polymer to be confined to the narrowest regions of the pores due to capillary effects and at the same time have free surfaces. As such one can differentiate the role of interface/free surface vs. confinement on the measured Tg (confined). If interfacial effects are dominant one would expect the Tg (confined) to decrease. In contrast, because of the dewetting on SiO2 surface, PS is expected to form capil-

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lary bridges and occupy more constricting regions in the interstices of the NPs, further spatially confining the chains, resulting in increased Tg (confined). This is because, while the PS chains can eventually fill all the pores of 100 nm NPs with enough material, they fill the smaller pores (i.e., the regions of higher curvature) first and form capillary bridges between SiO2 NPs. 52 The inset of Figure 4 shows an SEM images of the capillary bridges formed for a PS(8K)/SiO2 (100 nm) NP-top UCaRI film with ∼ 50% PS fill-fraction. Here, the fill-fraction is defined as the ratio of PS content compared to the PS content of the fully-filled CaRI films. Thus 100% fill-fraction is analogous to 37% total PS volume in the NP film. Larger SEM images of both CaRI and UCaRI films can be viewed in Figure S8. Figure 4 shows Tg values of UCaRI films with various fill-fractions. As the fill-fraction is decreased, Tg (confined) further increases. This observation indicates that the stronger geometric confinement has much stronger effect than the increased free surface in changing the Tg values in CaRI films. The T+ and T− values for UCaRI samples of two UCaRI samples are also shown in Figure S11. It is observed that both T+ and T− shift uniformly with decreasing PS fill-fraction within error and the width of the transition remains constant, consistent with the negligible effect of free surfaces, which would have broadened the width of the transition. 16,68

Figure 4: Tg (confined) at a cooling rate of 10 K/min vs. fill-fraction for PS(8K)/SiO2 (25 nm), NPtop UCaRI films with ∼ 200 nm thickness. The fill-fraction is defined as the ratio of PS volume in UCaRI vs. the PS volume in fully-filled CaRI NP films. Error bars represent average ±1 standard deviation of linear fits for three separate cooling ramps. The insets show the schematic of the UCaRI sample and the SEM image of a capillary bridge observed in a PS(8K)/SiO2 (100nm) NP-top UCaRI film (∼ 50% PS fill-fraction). Scale-bar is 100 nm.

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Geometric confinement is thus the most likely explanation for the dramatically increased Tg (confined) values. Considering the configurational entropy theory of glass transition, 69,70 Tg should increase when the material is confined in small pores, as the confinement decreases the configurational entropy. 71 Previous simulations 72–77 have shown that hard walls can slow the dynamics close to interfaces thus increasing the local Tg . Geometric curvature of the hard wall was also considered as an important factor that can significantly increase the Tg 61 and even induce ordering of bonds and affect the glassy structure near the substrate. 13,78 However, it is worth noting that in previous studies of the thermal glass transition of molecules confined in controlled pore glass (CPG) or AAO templates by differential scanning calorimetry (DSC), decreasing Tg with decreasing pore size was observed. 35,55,71,79,80 Some studies also observed two Tg s, one below the bulk Tg (due to the "size effect") and the other above the bulk Tg value, 66,67,81 where the higher Tg was attributed to the adsorbed layer on the surface of the pores. In these studies, there is very weak pore sizedependence on the higher Tg value, 66 which is qualitatively different from the data reported here. We note that these DSC measurements were performed upon heating. The out of equilibrium glass can show lower Tg due to stress induced by thermal expansion mismatch 82 and potentially reduced density, which could result in reduction and broadening of the Tg transition. 65 In the current study, we measure the glass transition upon cooling from the equilibrium state and thus thermal expansion mismatch does not affect the results as the polymer can move out of the NP film as observed in Figure 2. We have also ruled out the Tg broadening as shown in Figure S11. As such, the strong increase in Tg under isobaric conditions can be attributed to the entropic confinement effects. Since the glass transition is primarily due to the arrest of segmental dynamics in polymers, the significant increase in Tg transition indicates significantly slower segmental dynamics in these systems, which is consistent with our previous measurements of increased viscosity in unentangled polymers in CaRI. 51 Future studies that directly measure the relaxation dynamics in these systems can elucidate the exact nature of the confinement effects in these systems. In summary, polymers confined in NP packings using Capillary Rise Infiltration (CaRI) are great model systems to systematically study the role of confinement effects vs. interfacial effects

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on the glass transition temperature values measured upon cooling under isobaric conditions. In this study, we demonstrated significantly increased Tg values of PS in densely-packed SiO2 NP films, with weak interfacial interactions. Tg (confined) of PS increases with decreasing pore size (increasing confinement), as well as by under-filling the polymer in UCaRI films. In this weakly interacting system, extreme geometric confinement presents a larger effect on the segmental relaxation dynamics and therefore Tg than the free surface effects that can act to reduce Tg . The dramatic increase in Tg has potential impact in applications where nanocomposites are used at elevated temperatures, but a glassy behavior is required.

Methods Preparation of PS/SiO2 CaRI films One sided-polished silicon wafers (100) purchased from Virginia Semiconductor were cut to approximately by 1 cm × 1 cm squares. Polystyrene (PS, Mn = 7.5 kg/mol, PDI = 1.06 (PS(8K)), and 1900 kg/mol, PDI = 1.18 (PS(2M)), Polymer Source, Inc.) solutions were prepared by dissolving PS in toluene. SiO2 (25 nm) NPs were purchased dispersed in water (Ludox TM-50, Sigma Aldrich). SiO2 (11 nm) and SiO2 (100 nm) NPs were purchased dispersed in isopropanol (IPA) (free samples from Nissan Chemical). The suspensions were further diluted with their respective solvents. No other treatment was applied to the NPs. To generate the nanoporous CaRI films, a PS layer was first spin-coated (Laurell, WS-400BZ6NPP/Lite spin-coater) onto the silicon wafer. Then PS film was room air plasma-treated for ∼ 2 seconds (Expanded Plasma Cleaner PDC-001, Harric Plasma) to render the film surface hydrophilic (without any effects on polymer properties as shown in SI Figure S9). For SiO2 in IPA, this latter step was not necessary. The SiO2 NP suspensions were then spin-coated onto the PS layer to form a bilayer film. The bilayer films were then annealed at 423-453 K to induce PS infiltration and homogeneous distribution throughout the voids of the SiO2 NP packings. These films are designated as NP-top CaRI films (Figure 1a). In some experiments, in order to prevent 12


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crack formation, an inverted geometry was used, where the NP film was spin-coated first, followed by the PS film. These samples are designated as PS-top CaRI films (Figure 1c). Fully-filled CaRI films are prepared such that they have a residual PS layer after full infiltration. CaRI films were made with 11 nm, 25 nm, and 100 nm diameter SiO2 NPs in both PS-top and NP-top geometries. The porosity of fully-filled NP films were calculated based on the thickness change of the PS layer after infiltration and was determined to be 37 ± 3% (See Table S1). The average pore size was calculated to be 0.3d, where d is the diameter of the NPs 59 As such, the volume occupied by the NPs is 63%. Under-saturated CaRI films (UCaRI) were prepared where the initial thickness of the PS layer was varied such that the desired fill-fraction could be achieved after all of the polymer was infiltrated in the system, leaving no residual PS layer under the NP film (Figure 1b). The PS fill-fraction, the ratio of PS volume compared to the fully-filled PS volume, was defined as (hPS )/(37% ∗ hSiO2 ), where hPS is the PS-layer thickness before infiltration and hSiO2 is the thickness of the NP layer. However, due to the formation of cracks in the NP-top geometry this method is method is subject to error. Alternatively, the fill fraction can be obtained from the index of refraction (Figure S4). Figure S4 shows a strong correlation between PS fill-fraction, calculated from the initial PS thickness, and the refractive index of the NP film after infiltration, which is consistent with previous observations. 83 Using a linear regression, we can obtain the fill-fraction from n. This method is used for the data shown in Figure 4. UCaRI samples were prepared with 25 nm SiO2 NPs and NP-top geometry only, except for SEM images where 100 nm diameter was used to highlight capillary bridges. For UCaRI films with fill-fractions > 70%, the assumption was made that the polymer is uniformly distributed in the NP films and a single-layer model with uniform index was used in ellipsometry measurements. The model is compared with an alternative model with a gradient in the refractive index and it is observed that for high filling fractions the uniform index assumption is reasonable (See Figure S7a). Lower fill-fractions where this assumption fails have not been included in this study (more details in SI and Figure S7b).

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Ellipsometry Tg measurements The thickness and refractive index values were measured using spectroscopic ellipsometry (M2000V, J.A. Woollam). The raw ellipsometry data Ψ(λ ) and ∆(λ ) were fit to a Cauchy model (n(λ ) = A + B/λ 2 , k(λ ) = 0), where n and k are the real and imaginary parts of index of refraction (Figure S2). Depending on the sample, either two-layer (for CaRI) or one-layer (UCaRI) Cauchy model on Si substrate with 1 nm native oxides were used. Optical constants of Si substrate and the native oxide layer were taken from previous publication. 84 The properties of both the PS residual layer and the NP films were modeled to fit the ellipsometry data. Different models were compared to make sure all the key parameters were included without over-fitting of the insensitive ones (Figure S1). Figure S2 shows a typical SE data and the results of the model used to fit the data. Samples were mounted onto a temperature-controlled stage (Linkam THMS 600) that was attached to the ellipsometer. The ellipsometry sampling rate was 1 s with high-accuracy zone averaging. Heating and cooling ramps under dry nitrogen flow were run for each sample (Figure S5). Cooling rate was held at 10 K/min. All Tg measurements are reported upon cooling.

Scanning electron microscopy (SEM) SEM images were taken using a JEOL 7500F HRSEM at University of Pennsylvania Materials Research Science & Engineering Center (MRSEC). Before imaging, each sample was sputtercoated with a thin gold/palladium layer using a Cressington sputter-coater 108 to prevent charging. The samples were imaged at an accelerating voltage of 5.0 kV, emission current of 20 µA, and a working distance of approximately 8 mm.

Acknowledgement This work was financially supported by NSF CAREER grant (DMR-1350044, Z.F. and H.W.) and University of Pennsylvania Materials Research Science and Engineering Center (MRSEC) grant (NSF DMR-1720530) and partially funded by NSF-1449337 (D.L. and J.L.H.) and Corning

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Research and Development Corporation (CRDC) (H.W.). We thank J. Feng and P. Mazumder from CRDC for helpful discussions. MRSEC facilities were used for SEM imaging (NSF DMR1720530).

Supporting Information Available Additional ellipsometry parameters and model selection for different samples; typical SE data and model; calculation of porosity of the NP films; characterization of NP-top UCaRI films; characterization of PS-top CaRI films; images of cracks of CaRI films; SEM images of CaRI and UCaRI films; evaluation of the effect of plasma treatment on properties. This material is available free of charge via the Internet at http://pubs.acs.org/.

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Dramatic Increase in Polymer Glass Transition Temperature under

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