Major Impact of Cyclic Chain Topology on the Tg-Confinement Effect

Dec 23, 2015 - High purity cyclic PS (c-PS) samples with number-average molecular weight (MW) of 3.4 and 9.1 kg/mol were synthesized via atom transfer...
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Major Impact of Cyclic Chain Topology on the Tg‑Confinement Effect of Supported Thin Films of Polystyrene Lanhe Zhang,† Ravinder Elupula,§ Scott M. Grayson,*,§ and John M. Torkelson*,†,‡ †

Department of Materials Science and Engineering and ‡Department of Chemical and Biological Engineering, Northwestern University, Evanston, Illinois 60208, United States § Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, United States S Supporting Information *

ABSTRACT: High purity cyclic PS (c-PS) samples with number-average molecular weight (MW) of 3.4 and 9.1 kg/ mol were synthesized via atom transfer radical polymerization and “click” chemistry with narrow MW distribution. Bulk glass transition temperature (Tg) measured by differential scanning calorimetry exhibited a much weaker MW dependence for c-PS relative to its linear precursor and anionically polymerized linear PS (A-PS). Using ellipsometry and fluorescence spectroscopy, major differences were observed in the Tgconfinement effect in c-PS films supported on silicon substrates compared to A-PS. Whereas a large Tg reduction with confinement is commonly observed for A-PS supported on silica, within error, no confinement effect is seen in c-PS/3.4k films on Si/SiOx substrates down to 21 nm thickness. Although the c-PS linking group contains nitrogen and oxygen atoms potentially able to undergo hydrogen bonding, Tg is invariant with confinement for c-PS/3.4k or slightly reduced for c-PS/9.1k regardless of the level of substrate-surface hydroxyl groups. Ellipsometry indicates that the near elimination of the Tgconfinement effect in c-PS originates mainly from a very weak perturbation to Tg near the free surface (in comparison to linear PS) rather than a strong perturbation at the polymer−substrate interface. We hypothesize that unlike linear polymers, the packing efficiency of cyclic PS segments, i.e., cyclic PS fragility, is not significantly perturbed by the free surface, which in turn results in at most a very weak Tg perturbation at the free surface and an invariance of average Tg across the film with confinement.

1. INTRODUCTION Extensive research has been devoted to characterizing and gaining understanding of the underlying origins associated with how the glass transition temperature (Tg) of a confined polymer film deviates from bulk Tg.1−55 The Tg value of bulk polymer (Tg,bulk) is determined by thermodynamic methods which yield different temperature (T) dependences in the liquid-state and glassy-state responses, e.g., different T-dependences of specific heat capacity (via differential scanning calorimetry) or specific volume (via dilatometry). The Tgconfinement effect in polymer films is typically determined experimentally by (pseudo)thermodynamic methods that are well designed for thin film analysis, including ellipsometry, 1 , 2 , 5 , 7 − 1 1 , 1 4 − 1 8 , 2 0 , 2 2 , 2 7 , 3 0 − 3 2 , 3 4 , 3 5 , 4 5 , 5 2 , 5 5 , 5 6 fluorescence,3,4,12,15−19,21,23,36 X-ray and neutron reflectivity,28,29,48,49 and capacitive dilatometry,26,51 among others.5,6,13,24,33,47,50,54 Competing effects originating from the free surface (polymer−air interface) and the polymer−substrate interface can readily tune the direction and magnitude of the effect of confinement on the average Tg in supported films.1−11,14−24,27−32,37−46,48−52,54,55 For example, nanoconfined films of freely deposited, linear polystyrene (PS) supported on silica substrates exhibit a decrease in Tg with decreasing thickness due to the strong free-surface effect and © XXXX American Chemical Society

the absence of significant attractive interactions at the substrate interface.1,3−5,7,8,10−12,15−18,27,48,50−52,55 On the other hand, confined films of styrene-containing copolymers supported on Si/SiOx that include 2-vinylpyridine or methyl methacrylate as comonomer can exhibit suppression of the Tg reduction at low comonomer incorporation and significant increases in Tg relative to Tg,bulk at high comonomer incorporation.20,21,28 These effects are readily explained by the increasing importance with increasing comonomer content of hydrogen-bonding interactions between hydroxyl groups present on the Si/SiOx substrate surface and nitrogen or oxygen atoms present in the comonomer repeat units. Those attractive interactions lead to a reduction of cooperative segmental mobility near the interface which propagates into film and can dominate free-surface effects at high comonomer content.20,21,28 Modification of the substrate surface can also be an effective tool for tuning the Tgconfinement behavior of polymer films.29,30,56 Recently, the important role of chain ends in tuning the Tgconfinement effect has become evident. Zhang and Torkelson16 demonstrated the significance of initiator fragments as chain Received: November 14, 2015 Revised: December 12, 2015

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DOI: 10.1021/acs.macromol.5b02474 Macromolecules XXXX, XXX, XXX−XXX

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pyridine)77 have shown that Tg increases with decreasing MW at low MW, with values exceeding that of a high-MW linear analogue. In contrast, experimental studies of cyclic PS have reported a small reduction in Tg with decreasing MW at low MW, which is strongly suppressed relative to that of linear PS.59,63,64,69,71,75−77,82 DiMarzio and co-workers63,64 developed a theoretical model based on the Gibbs−DiMarzio framework which predicted that cyclic polymers show either a small decrease or, more usually, an increase in Tg with decreasing MW at low MW. They explained this result as arising from entropic penalties imposed by decreasing conformational freedom and flex energies with decreasing ring polymer MW.63,64 In addition, the cooperative segmental relaxation dynamics of a 4.6 kg/mol cyclic PS were reported to be equivalent to those of high-MW linear PS.71 These results have been interpreted as arising from the reduction in conformational freedom due to the absence of chain ends. Only a few studies have investigated properties of ring polymers in confined films. Wang et al.89 studied surface segregation in thin films of a blend of cyclic and linear PS. They reported depleted cyclic chains at the surface, contrary to the prediction from self-consistent field theory.90 Foster and coworkers69 examined dynamics of macrocylic melts and observed slower surface fluctuations of cyclic PS films compared to linear PS analogues due to differences in Tg,bulk values. Only one study has investigated the behavior of nanoconfined cyclic polymers, which was done via Monte Carlo simulation.86 That study concluded that liquid-like to solid-like transitions occurred at lower temperature with cyclic chains than with linear chains when the chains are trapped between two parallel plates.86 We note that some studies of cyclic polymers have yielded inconsistent results due to contamination with linear chains.65−67,76,77,85 The body of research on ring polymer melts as a model system for polymers with suppressed reptation has made evident the need for high quality polymer fractions.65−68,83−85 McKenna et al.66 blended up to 15.8% linear PS chains with high purity cyclic PS fractions. They showed that the presence of linear chain contaminants decreases the plateau compliance and increases the steadystate values of the recoverable compliance. Further, the presence of small amounts of linear contaminant increases the sample viscosity dramatically. Other properties also show sensitivity to the presence of linear impurities in cyclic polymers.59,76,77,82 For example, low-MW cyclic PS synthesized by end-to-end coupling with dibromomethane showed what appeared to be increased excimer fluorescence between 300 and 320 nm.77 However, this cyclic PS sample was later revealed to contain as little as 0.1 mol % of strongly emitting linear PS contaminants.76 These results show that accurate characterization of physical properties of cyclic polymers relies on extremely effective purification, i.e., separation of cyclic polymer from linear polymer precursor. Here, we report the first study on the Tg-confinement effect in supported films of highly pure, cyclic PS and compare results to those of anionically polymerized linear PS (A-PS). Cyclic PS was synthesized via a combination of atom transfer radial polymerization (ATRP) and “click” chemistry.61,91−93 Pseudoliving radical polymerization has been widely used to form functional telechelic polymers.96,97 ATRP is attractive for making macrocyclic precursors because of its high efficiency to modify the terminal end group.57 The ring-closure method achieved through “click” chemistry links functional chain ends

ends in modifying the Tg-confinement response of low molecular weight (MW) PS supported on Si/SiOx substrates. In comparison with anionically polymerized counterparts with sec-butyl groups and hydrogen atoms at chain ends, ∼4 kg/mol linear PS with chain-end groups containing fragments from the free radical initiators azobis(isobutyronitrile) or benzoyl peroxide exhibited a 50% reduction in the magnitude of the Tg-confinement effect. This reduction is explained by hydrogen bonds formed between nitrogen or oxygen atoms in the initiator-fragment end groups and hydroxyl units on the substrate surface. The stronger effect relative to that obtained in studies with copolymers indicates that attractive interactions at chain ends can lead to greater modification of Tg behavior than interactions with interior units of chains. 16 The importance of chain ends is reinforced in a study by Lan and Torkelson,18 who showed that the effect of confinement on average Tg across the brush thickness can be eliminated within error in densely grafted PS brushes with one chain end covalently attached to the substrate. Thus, the state of chain ends can be important in the Tg-confinement behavior of polymers and provides a means by which to suppress or eliminate the effect. Recent advancements in polymer chemistry have facilitated the synthesis of a wide range of novel polymer architectures.57−61,70,75,91−99 Such advances have fueled interest in investigating the effect of chain architecture on physical properties. Studies have thus investigated star, branched (including comb, centipede, etc.), and cyclic polymer topologies.31,32,34,35,57−99 In addition to bulk properties such as Tg,57−61,63,64,70−78,82,96 fragility,71 and physical aging,32,62 the effects of confinement have been shown to be a function of molecular architecture.22,24,31,32,34,35,69,72,74 With increasing functionality and decreasing arm length, thin films of star PS (thickness < 50 nm) exhibited enhanced Tg with confinement, which was attributed to improved packing of the interfacial chains,31,32 and enhanced wetting properties compared to linear analogues.73 For thin, supported films of comb PS, the overall Tg remained unchanged from that of bulk PS down to 20 nm as measured by ellipsometry,34,35 but suppressed dynamics were measured by incoherent neutron scattering.35 Chain architectures are also known to play key roles in surface fluctuation dynamics of melt films.72,74 Confined films (∼100 nm thick) of a six-end, end-branched star PS exhibited viscosity that was 2 orders of magnitude higher (inferred from X-ray photon correlation spectroscopy) than bulk viscosity, suggesting that confinement strongly alters relaxation dynamics of branched chains.72 Films of densely branched comb PS exhibited much faster surface fluctuations than those of linear chains of the same MW.74 A full understanding of how such chain architectures affect polymer relaxation dynamics is yet to be developed. Topological constraint as a result of linking the ends of a linear chain to form macrocyclic polymers and its impact on physical properties have intrigued researchers for three decades.58−69,71,75−89,96 The influence of “end-to-end” tethering and the unusual conformational properties associated with cyclic topologies have prompted studies of bulk properties such as viscosity,65−68 Tg,59,63,64,71,75−79 and crystallization.78,80 As revealed by reduced hydrodynamic volume, ring polymers possess a more compact structure when compared to linear chains of similar MW.59,81 Experimental studies on the MW dependence of Tg of cyclic poly(dimethylsiloxane),78 cyclic poly(phenylmethylsiloxane), 79 and cyclic poly(2-vinylB

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solution of l-PS-N3 to the rapidly stirring Cu(I)Br/PMDETA solution (at room temperature) at a rate of 2 mL/h. After complete addition of the polymer solution, the reaction mixture was allowed to stir for an additional 2 h and then extracted three times from saturated aqueous NH4Cl into dichloromethane. The organic layer was dried over anhydrous MgSO4, filtered, and concentrated prior to precipitation from dichloromethane into cold methanol. The product was isolated via filtration and dried in vacuum (yield = 80%). Identical reactions were performed to produce c-PS with Mn of 9.1 kg/mol. The ATRP reaction was performed for 100 min. 2.4. Molecular Weight Characterization. IR spectra were obtained using a Perkins-Elmer Spectrum BX Fourier-transform infrared (FTIR) spectrometer. Mass spectral data were acquired using an Applied Biosystems Voyager Elite matrix-assisted laser desorption time-of-flight mass spectrometer (MALDI-TOF MS) with delayed extraction using the linear positive ion mode. 2-[(2E)-3-(4tert-Butylphenyl)-2-methylprop-2-enylidene]malononitrile (DCTB) was used as the matrix and sodium trifluoroacetate (TFA) as the cation source in a 50/5/2 matrix/Na+TFA/polymer ratio. The relaxation delay was set to 500 ns, with an acceleration voltage of 18 kV, a grid voltage of 92%, without guide wire, and low mass gate at 700 amu. Size exclusion chromatography (SEC) was carried out on a Waters model 1500 series pump (Milford, MA) with three column series from Polymer Laboratories, Inc., consisting of two PLgel 5 μm Mixed C (300 mm × 7.5 mm) and one PLgel 5 μm 500 Å (300 mm × 7.5 mm) columns. The system was fitted with a Model 2414 differential refractometer detector, and anhydrous tetrahydrofuran was used as the mobile phase (1 mL min−1 flow rate). The calculated Mn was based on calibration using linear PS standards. Data were collected and processed using Precision Acquire software. 2.5. Bulk Tg Characterization. The Tg,bulk values of linear PS precursor l-PS-N3 (3) and c-PS (4) samples were measured by differential scanning calorimetry (DSC; Mettler Toledo DSC822) under a nitrogen environment at a heating rate of 10 °C/min after annealing samples above Tg and then cooling to 25 °C at a rate of 40 °C/min. c-PS samples were annealed at Tg + 40 °C for 15 min. To avoid potential intramolecular or intermolecular reaction between the reactive azide and alkyne chain-end groups along the linear precursor, l-PS-N3 samples were annealed at Tg + 30 °C for 7 min to obtain reproducible, consecutive heat flow curves.100 All measurements were performed in duplicate. Table 1 summarizes sample MW averages and Tg values.

together on the same polymer chain, forming cyclic topology.58,61 The advantages of this method include ease of preparation, high monodispersity, nearly quantitative conversion of linear precursor, etc.61,92 Recently, the versatility of such synthetic protocols has also enabled synthesis of multicycles, molecular cages, and miktoarm stars.98,99 We prepared linear PS precursors via ATRP followed by azidation of the end group.100 Upon cyclization, cyclic PS samples are obtained that exhibit molecular weight distributions nearly identical to those of their linear precursors, as judged by MALDI-ToF MS. Using ellipsometry and fluorescence to characterize Tg, we show that the Tg-confinement effect is totally suppressed within error in supported films of 3.4 kg/mol c-PS down to 21 nm thickness. This invariance of Tg with confinement for cyclic PS is in stark contrast to the major Tgconfinement effect associated with A-PS of similar MW.4,15,16,55 Furthermore, this invariance occurs regardless of substrate hydrogen-bonding capability.

2. EXPERIMENTAL DETAILS 2.1. Materials. All reagents were from Aldrich and used without further purification, unless otherwise noted. Propargyl alcohol was distilled prior to use. Styrene monomer was passed through a plug of basic alumina to remove radical inhibiting stabilizers prior to use. All solvents were reagent grade and used as received. Propargyl 2bromoisobutyrate (ATRP initiator) was synthesized as reported in the literature109 and dried over anhydrous MgSO4 prior to use. Copper(I) bromide (Cu(I)Br) was purified by washing with boiling acetic acid. An anionically polymerized PS standard (A-PS) was from Pressure Chemical Co. with number-average MW (Mn) = 3.6 kg/mol and Tg = 72 °C (see ref 16 for detailed MW and Tg characterization). 2.2. Nomenclature. In order to simplify the identification of compounds, the following conventions are applied: As a prefix, l- is assigned to linear PS precursor, c- to cyclic PS, and A- to anionically polymerized linear PS. A suffix is added to identify the functionality at the terminus of the chain such as -N3 for the benzylic azide. 2.3. Synthesis. General Procedure for the Synthesis of l-PS-Br (2). In a representative polymerization for obtaining Mn = 3.4 kg/mol, propargyl 2-bromoisobutyrate (1) (0.089 g, 0.436 mmol), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) (0.098 g, 0.567 mmol), and styrene monomer (2.727 g, 26.183 mmol) were added to a 100 mL round-bottomed flask. After two cycles of freeze/ pump/thaw, Cu(I)Br (0.062 g, 0.436 mmol) and copper(II) bromide (Cu(II)Br2) (0.005 g, 0.022 mmol) were added to the reaction flask. After the third freeze/pump/thaw cycle, the reaction mixture was warmed to room temperature before being placed in a preheated oil bath at 70 °C and allowed to stir under nitrogen for 45 min. The reaction mixture was cooled to room temperature and purified by extraction from aqueous ammonium chloride (NH4Cl) into dichloromethane, followed by precipitation into methanol to yield the polymer as a white solid (yield = 87%). General Procedure for the Synthesis of Precursor l-PS-N3 (3). 0.500 g (0.111 mmol) l-PS-Br (2) was added to a 100 mL roundbottomed flask. The polymer was dissolved in 10 mL of dimethylformamide, and sodium azide (0.036 g, 0.555 mmol) was added as a solid in one portion. The solution was stirred overnight at room temperature before purification by precipitation into methanol to give a white solid (yield = 83%). Note: sodium azide is a potentially explosive reagent and must be handled with appropriate caution, e.g., avoiding contact with metal, ground glass joints, and chlorinated solvents. General Procedure for the Synthesis of c-PS (4) via “Click” Cyclization. A 0.06 mM solution of l-PS-N3 (3) in dichloromethane (50 mL) was degassed and backfilled with nitrogen gas for two freeze/ pump/thaw cycles. In a separate flask, PMDETA (1.049 g, 6.05 mmol) was dissolved in dichloromethane (100 mL) and degassed with one freeze/pump/thaw cycle. Immediately after a second freeze, Cu(I)Br (0.868 g, 6.05 mmol) was added, followed by a pump and thaw cycle. Upon thawing, a syringe and syringe pump were used to transfer the

Table 1. Polymer Samples with Their Molecular Weights and Tg Values sample a

l-PS/3.4k c-PS/3.4kb l-PS/9.1ka c-PS/9.1kb

Mn (g/mol)

PDI

Tg,bulkc (°C)

Tmid,bulkd (°C)

3440 3440 9120 9120

1.04 1.03 1.06 1.05

75 96 91 98

75 96 90 98

a

Naming convention: l-PS/3.4k refers to a linear PS sample with azide and alkyne end groups and number-average MW of 3440 g/mol as determined by MALDI-ToF mass spectroscopy. bNaming convention: c-PS/3.4k refers to a cyclic PS sample that was synthesized via click chemistry from its linear analogue with number-average MW of 3440 g/mol as determined by MALDI-ToF mass spectroscopy. cTg,bulk values are DSC onset values. dTmid,bulk values are numerical average of T+ and T− derived from ellipsometry analysis of a bulk film. 2.6. Substrate Surface Modification and Characterization. As-received silicon wafers (WaferNet) and green glass slides (SigmaAldrich) were cleaned with acetone followed by methanol. Silicon wafers (∼1.5 × 1.5 cm2) were hydroxylated in a piranha solution (mixture of 7:3 (v/v) 98% H2SO4 and 30% H2O2) for 30 min at room temperature. Wafers were rinsed with a copious amount of deionized water and dried under a stream of nitrogen. Hydrofluoric acid (HF) treatment was performed under clean room conditions by immerging C

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Macromolecules silicon wafers (after piranha solution treatment) in buffered oxide etch (10% HF in volume; J.T. Baker) for 5 min. Substrates were then rinsed with Milli-Q water (resistivity 18.2 MΩ cm) and dried under filtered nitrogen. Hydrophilic green glass slides were obtained by etching in 1 M hydrochloric acid for 20 min followed by soaking in a base solution of sodium hydroxide (20 g), ethanol (40 mL), and water (150 mL) for 20 min. To minimize contamination, all substrates were used for spin coating immediately after drying. Static contact angle (θs) of water was measured by a goniometer (AST Products Inc., VCA Optima XE) that takes and analyzes the image of a sessile droplet (10 μL) on a substrate surface. Three measurements were made on each substrate, and three substrates of each type were analyzed. Data are reported as averages with an error corresponding to the standard deviation between measurements. As summarized in Table 2, as-received silicon wafer and glass slides have

glassy regimes. In each regime, data points at the high and low temperature extremes served as the starting point, and more data points were added until the correlation coefficient (R2) was less than 0.990. The intersection of the two fitted lines is interpreted as the film Tg .

3. RESULTS AND DISCUSSION 3.1. Synthesis of Cyclic PS. 3.1.1. Synthesis of Linear PS Precursor. The synthesis of highly pure cyclic macromolecules is facilitated by the versatility of ATRP to produce nearly quantitatively end-functionalized polymers and the efficiency of “click” reactions such as CuAAC (copper-catalyzed alkyne− azide cycloaddition). The strategy for preparing well-defined cyclic PS involves the initial preparation of well-defined α,ωheterodifunctional linear precursors with complementary click coupling functionalities.58,61 The linear PS precursors were prepared by ATRP of styrene monomer from propargyl 2bromoisobutyrate initiator (1) followed by the postpolymerization conversion of the terminal end group (Br) to a complementary azide group. Styrene was polymerized in bulk with Cu(I)Br/PMDETA as catalyst at 70 °C (Scheme 1).

Table 2. Static Contact Angle (θs, deg) Characterization of Substrate Surface

silicon wafer green glass slide

as received after solvent cleaning

Si−OH

Si−H

62.4 ± 1.8 71.3 ± 1.5

2.0 ± 0.2 2.0 ± 0.7

82.5 ± 2.0 N/A

Scheme 1. Synthesis of Cyclic Polystyrene via a Combination of ATRP and “Click” Couplinga

static contact angles of ∼62° and ∼71°, respectively. Upon hydroxylation, both substrates exhibited superhydrophilic character with θs close to 0°, indicating a uniform coverage of surface hydroxyl groups (Si−OH). Silicon wafer after HF treatment showed an increase in θs to ∼82°, consistent with an oxide-free, hydrophobic hydrogenterminated Si surface (Si−H).104,105 A very thin layer of silicon oxide with ∼2 nm thickness was measured by a spectroscopic ellipsometer (J.A. Woollam Co., Inc. M-2000D) for as-received and piranha-treated silicon wafers. This layer was reduced to ∼0.4 nm after HF etching. 2.7. Tg Characterization of Thin Polymer Films. Films of various thicknesses were spin-coated onto Si/SiOx, Si−OH, and Si−H substrates from toluene by adjusting solution concentration and spin speed.106 Films used in fluorescence measurement were spin-coated onto green glass slides (Si−OH and as-received) from solution with less than 0.2 wt % 1,10-bis(pyrene)decane (BPD, Molecular Probes) relative to dry polymer weight. Film thickness was confirmed by measuring the thickness of a film on a silicon wafer that was spincoated under the same conditions via ellipsometry. Films characterized by spectroscopic ellipsometry and fluorescence were annealed under vacuum at ∼40 °C above Tg,bulk for at least 4 h. The Tg was determined via ellipsometry by measuring the T-dependence of film thickness upon cooling. c-PS films were annealed on the ellipsometer heating stage at bulk Tg + 40 °C for 30 min to erase thermal history and cooled at 1.0 °C/min to bulk Tg − 60 °C. Measurements were taken every 10 s at wavelengths from 400 to 1000 nm at a fixed angle of incidence of 65°. Data were fitted to a Cauchy layer model, which is composed of a silicon substrate with a 2.0 nm thick native SiO2 layer and a polymer layer on top. In the case of Si−H substrates, a 0.4 nm thick native silicon oxide layer was used in the Cauchy layer model. For film thickness below 26 nm, each film was run two or three times for reproducibility. The thermal expansion coefficient was obtained as a function of T (α(T)) by numerical differentiation:8,15−18,32

α(T ) = (h(T + ΔT /2) − h(T − ΔT /2))/(h(T0) × ΔT )

a

Reagents and conditions: (i) Cu(I)Br, N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA), styrene, 70 °C; (ii) NaN3, DMF, 25 °C; (iii) Cu(I)Br, PMDETA, CH2Cl2, high dilution, 25 °C.

Reactions were stopped before 30% conversion to optimize the end group fidelity. Conversion of the end group bromide to an azide was carried out by reacting l-PS-Br (2) with sodium azide in DMF. The α,ω-heterodifunctional approach to prepare cyclic macromolecules has significantly broadened the scope of cyclic polymer research.102,103 Unlike the ring expansion approach, the cyclization step is independent of the polymerization technique, and therefore a wide range of polymer backbones can be cyclized, as long as complementary functional groups can be placed on opposite ends. Laurent and Grayson61 demonstrated that the extremely rapid CuAAC reaction was well suited to making cyclic polymers. By adding the α,ωfunctionalized precursor slowly via a syringe pump to a solution of the copper catalyst, the intramolecular cyclization could be favored relative to intermolecular couplings. Lonsdale et al.94 systematically studied the effect of parameters such as l-PS-N3 concentration, temperature, feed rate, Cu(I)Br/PMDETA concentration, and linear polymer MW to generate highly pure cyclic polymers. Guided by

(1)

where ΔT is ∼3.5 °C and h(T0) is the thickness of the film at 25 °C. The Tg values identified from a step change in α(T) are reported to the nearest 0.5 °C. Fluorescence spectra were collected with a Photon Technology International fluorometer (front-face geometry) on films doped with trace levels (< 0.2 wt % relative to dry polymer) of BPD upon cooling. The excitation wavelength was 324 nm; emission spectra were collected from 360 to 460 nm. Films were annealed inside the sample holder at 40 °C above bulk Tg, and Tg was determined on cooling. Fluorescence was measured every 5 °C at a cooling rate of 1 °C/min. Linear regressions were fit to the data in both the rubbery and the D

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Figure 1. (a) MALDI-ToF mass spectra, (b) overlay of SEC-RI traces, and (c) IR spectra of precursor l-PS/3.4k and c-PS/3.4k samples.

Jacobson−Stockmayer theory,107 they were able to produce high purity (> 95%) cyclic polymers in less than 9 min at 25 °C. Further, Sreerama et al.92 utilized MALDI-ToF MS to confirm that the primary byproduct observed as a trace impurity during these cyclizations was cyclic dimers of the linear precursor. It is expected that these cyclic impurities would cause a less profound perturbation of the bulk physical properties than impurities of a different architecture (e.g., linear or concatenated). Two pairs of l-PS and c-PS were prepared with Mn of 3440 and 9120 g/mol as characterized by MALDI-ToF MS. All PS samples exhibited low dispersity consistent with the controlled nature of ATRP (Table 1). As seen in Figure 1a, MALDI-ToF mass spectra provide detailed end group and MW characterization. The spectra for linear PS precursor exhibited two distinct MW distributions with nearly identical Mn values and a repeat unit spacing of 104.1, consistent with PS. The two distributions were offset by ∼23 Da. Residual mass analysis108 of the weaker intensity distribution revealed that they correspond to polymer with the expected end groups [M + Na]+. The stronger intensity distribution was attributed to the metastable decay of the azide end group by the expulsion of N2 to generate a nitrene intermediate during MS analysis. This phenomenon has been observed in a range of polymers bearing azido end groups.95 As further confirmation of the end groups, the prominent IR stretching frequency of the azide functional group at 2150 cm−1 was observed for the linear precursors (see Figure 1c). 3.1.2. Cyclization of Linear PS Precursors. The continuous slow-addition “click” cyclization technique was then employed to cyclize linear precursor l-PS-N3 (3) under high dilution.

SEC, MALDI-ToF MS, and FTIR spectroscopy were utilized to ascertain the purity of c-PS. Distinct retention time shifts to longer times were observed in SEC after cyclization (Figure 1b). This observed retention time increase relative to the corresponding linear analogue provides additional evidence for the cyclization of linear precursor, as the desired cyclic product will exhibit a reduced radius of gyration.59,81 Furthermore, the disappearance of the metastable signal distribution in the MALDI-ToF mass spectra while retaining the molecular ion [M + Na]+ distribution is a clear indication of transformation of the unstable azide into the aromatic triazole linkage (Figure 1a). In addition, the complete disappearance of azide absorbance at 2150 cm−1 in the products’ FTIR spectra corroborates their cyclic nature (Figure 1c). 3.2. Comparison of Bulk Tg of Linear PS and c-PS (4) Measured by DSC. Table 1 shows Tg,bulk values as determined by DSC onset measurements. Linear PS exhibits significant MW dependence of Tg, following the Fox−Flory equation.110,111 Unconcatenated cyclic polymers have been reported to possess distinct Tg response when compared to linear counterparts.59,63,64,71,75−79 In the current study, compared to lPS that exhibited a 16 °C reduction in Tg,bulk in going from 9.1 to 3.4 kg/mol, or A-PS that exhibited an 18 °C reduction in Tg,bulk in going from 8.4 to 3.6 kg/mol,112 c-PS samples showed a much weaker Tg−MW dependence: there was a minor 2 °C reduction when MW is decreased from 9.1 kg/mol (Tg = 98 °C) to 3.4 kg/mol (Tg = 96 °C) (see Figure S4 in Supporting Information for DSC heat flow curves).113 These observations qualitatively agree with those by Santangelo et al.71 and Alberty et al.,82 who studied c-PS synthesized by anionic polymerization followed by end-to-end coupling. For example, Alberty et al. E

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Figure 2. Temperature dependence of normalized film thickness for (a) 93 nm thick (top) and 22 nm thick (bottom) A-PS/3.6k films and (b) 174 nm thick (top) and 21 nm thick (bottom) c-PS/3.4k films as measured by ellipsometry. The data have been normalized to the value at Tg and shifted vertically for clarity. Vertical dashed lines represent Tg. Temperature-dependent thermal expansivity of (c) 22 nm thick A-PS/3.6k film and (d) 21 nm thick c-PS/3.4k film. (Data smoothing is represented as the red line with the adjacent averaging method over 12 data points.117 The A-PS/3.6 kg/mol data are taken from the Supporting Information of ref 16.)

reported Tg,bulk = 99 and 89 °C for cyclic PS of Mn = 5 and 2.8 kg/mol, respectively, while their linear precursors had Tg,bulk = 80 and 68 °C, respectively.82 A weak MW dependence of Tg,bulk for cyclic PS is a result of the absence of chain ends and relatively low configurational entropy. In addition, a theoretical study has indicated that ring polymers exhibit vastly different packing entropy relative to linear chains, arising from a competition between the rings for occupying the same space.63 Fragility is a parameter that indicates the packing efficiency of glass-formers;23,115,116 research has demonstrated that in the case of linear PS fragility decreases with decreasing MW.71,114 In contrast, Santangelo et al.71 found experimentally that the fragility of a low-MW, 4.6 kg/mol c-PS was equal to the high-MW limiting value for linear PS. Although limited results from literature and the current study agree on the qualitative difference in Tg,bulk between low MW cyclic and linear PS, we need to comment further on some quantitative differences for c-PS. A key synthetic challenge in preparing c-PS is the elimination of linear impurities. Any admixture of low-MW linear chains with cyclic structures will result in a reduction of the overall Tg. Polymers synthesized in this study are of high purity with a minimal amount of linear contaminants. In addition to linear impurities, variations in Tg,bulk of ring PS can arise from differences in the linking unit structure. As will be discussed here, such changes are more prominent in low-MW cyclics. Foster and co-workers69 reported Tg = 85.4 and 84.3 °C for their c-PS with Mn =

2650 g/mol (before hydrogenation) and 2800 g/mol (after hydrogenation), respectively. Alberty et al.82 reported Tg = 89 °C for their c-PS with Mn = 2.8 kg/mol. With the current synthetic strategy, a c-PS sample with Mn = 2.5 kg/mol has a higher Tg = 95 °C (see Figure S4). The difference originates from a change in the chemical structure of the linkage upon cyclization. Metathesis ring closure of α,ω-divinylpolystyrenes introduced a linkage containing six C−C single bonds per ring;69 styrene initiation using 2,7-dimethyl-3,6-diphenyloctane dianion and cyclization using 1,4-bis(bromomethyl)benzene incorporated structural irregularities at two locations along the ring, one of which contains three C−C single bonds.82 The current synthesis initiated by propargyl 2-bromoisobutyrate and linked via alkyne−azide chemistry introduced an ester and a triazole group. The presence of such rigid coupling structures results in less flexibility when compared to C−C linkages, which could explain the higher Tg reported in the current study. In addition to entropy differences, the flex energy (energy required to flex a bond through a defined angle or one conformation to another) of the cyclic structures could be highly affected by the chemical structure of the linkages. DiMarzio and Guttman63 speculated that ring polymers require at least six units to undergo such conformational changes. With decreasing MW, the impact of the linking units on flex energies becomes significant, which in turn can result in tunable Tg. F

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Macromolecules 3.3. Tg-Confinement Effect in c-PS vs A-PS: Films Supported on Si/SiOx Substrates. Figures 2a and 2b show the T dependences of normalized thickness for bulk films and ultrathin films of A-PS/3.6k and c-PS/3.4k supported on Si/ SiOx substrates, respectively. The Tg is determined by applying linear fits to the T dependences of thickness deep in the rubbery and glassy states. This most commonly adopted method of analysis yields Tg = 72 °C in the 93 nm thick film of A-PS/3.6k and Tg = 96 °C in the 174 nm thick film of c-PS/ 3.4k, identical to Tg,bulk values obtained by DSC. Upon reducing thickness to 22 nm, A-PS/3.6k exhibits Tg = 61 °C, an 11 °C reduction relative to bulk. Interestingly, within error no effect of confinement is seen in c-PS/3.4k, which exhibits Tg = 95 °C in a 21 nm thick film (see Figure S5 for related data on c-PS/ 9.1k). Figures 2c and 2d display the T dependences of thermal expansivity (α(T)) by numerical differentiation according to eq 1 for 22 nm thick A-PS and 22 nm thick c-PS thin films. Data were smoothed using a 12-point adjacent averaging method shown as the red curve. As described first by Kawana and Jones,8 this method can be used to quantify Tg breadth by identifying the onset of the transition (T+) and the end point of the transition (T−) upon cooling. α(T) undergoes a roughly stepwise change across the glass transition, estimated by three straight lines that are fitted to the glassy, rubbery, and glass transition regimes. According to Kawana and Jones,8 the two intersections represent T+, which they interpret as the T at which a layer near the substrate falls out of equilibrium, and T−, which they interpret as the T at which a layer near the free surface falls out of equilibrium. For the 22 nm thick A-PS film, T+ = 83 °C and T− = 40 °C, resulting in a value of Tmid = 61.5 °C, where Tmid is the numerical average of T+ and T−. (The Tmid values for bulk films, Tmid,bulk, correspond well to the onset DSC Tg measurement; see Figures S6−S8 for details on data analysis for bulk films.) As a result, the value of Tmid as measured from α(T) data is commonly taken as the film Tg, i.e., Tmid = Tg. Compared to the 93 nm thick film, the 22 nm thick A-PS film shows a minor increase in T+ of ∼3 °C and a more severe reduction in T− of 22 °C. Overall, there is a 9.5 °C reduction in Tg upon confinement in A-PS/3.6k, i.e., Tg − Tg,bulk = −9.5 °C. On the other hand, for a 22 nm thick c-PS film, T+ is higher by 5.5 °C and T− lower by 8.5 °C than the bulk-like film. This leads to Tg − Tg,bulk = −1.5 °C. Compared to A-PS, a 22 nm thick c-PS film exhibits a slightly larger T+ increase and a significantly smaller T− reduction, resulting in a narrowing of the Tg breadth to 32 °C (= T+ − T−) compared to the 43 °C Tg breadth exhibited in a 22 nm thick A-PS ultrathin film. Figure 3 compares the thickness dependences of changes in T+, Tmid (= Tg), and T− from Tmid,bulk (= Tg,bulk) for c-PS/3.4k, c-PS/9.1k, and A-PS/3.6k. The effect of confinement is insignificant for c-PS/3.4k relative to A-PS of similar MW: Tg is reduced by 1−2 °C compared to an overall reduction of 9− 10 °C in the case of A-PS for the thinnest films examined here. The 1−2 °C reduction in Tg with confinement indicates that, within error, there is no Tg-confinement effect in c-PS down to 21 nm thickness. Tg breadth remains virtually independent of thickness above ∼40 nm for both low-MW c-PS and A-PS. Within error, T+ − Tg,bulk shows the same effect of confinement down to 21 nm, trending to slightly higher temperature with thicknesses below ∼40 nm. In contrast, at thickness below ∼40 nm, c-PS/3.4k exhibits a substantially suppressed T− reduction relative to A-PS/3.6k. T− is reduced by 33 °C for a 21 nm thick

Figure 3. T+ − Tmid,bulk (triangle), Tmid − Tmid,bulk (square), and T− − Tmid,bulk (circle) measured by ellipsometry for c-PS/3.4k (full symbols), c-PS/9.1k (empty symbols), and A-PS/3.6k (half-open symbols) films supported on Si/SiOx substrate. Tmid,bulk for c-PS/3.4k, c-PS/9.1k, and A-PS/3.6k are 96, 98, and 72 °C, respectively. (The A-PS/3.6k data are taken from ref 16. Symbols without error bars have errors that are smaller than the symbol size.)

film of A-PS/3.6k but only 17.5 °C for c-PS/3.4k. Based on the interpretation by Kawana and Jones,8 relative to A-PS, the effect of confinement in low-MW c-PS is significantly muted near the free surface and the same near the substrate. For linear PS, the perturbation to Tg at the free surface can propagate several tens of nanometers into the film interior, resulting in a significant Tg reduction when the film is nanoconfined.3 Based on the T− results, such perturbation at the free surface would be expected to be much weaker for cyclic PS than for linear PS. Future study of c-PS thin films by multilayer/fluorescence experiments3,15,18 could shed light on this issue. Next we consider the MW dependence of the Tg-confinement effects in A-PS and in c-PS. Past research with anionically polymerized PS has shown little or no influence of MW on the Tg-confinement effect of films supported on silicon wafers for MWs ranging from 2 to 3000 kg/mol.1,4,8,15,16,55 For an A-PS film of 21 nm thickness, Tg − Tg,bulk = −8 to −11 °C is commonly reported via ellipsometry characterization.1,15−17,55 Furthermore, this magnitude of Tg reduction upon confinement for A-PS is insensitive to the oxidation, hydrogen passivation, or piranha treatment of the silicon surface due to the lack of significant attractive interactions between the polymer and the substrate surface.1,3,8,11,21,27,28,55 As shown in Figure 3, within error, c-PS/3.4k films exhibit invariance of Tg down to 21 nm; c-PS/9.1k exhibits a maximum Tg reduction relative to Tg,bulk of 2−3 °C in a 21 nm thick film. Within error, the Tg-confinement effect is very small in c-PS/ 9.1k. Relative to A-PS, T+ shows the same effect of confinement down to 21 nm in c-PS/9.1k. The difference in Tg (= Tmid) is therefore associated with the extent of T− reduction: T− is reduced by 18 °C for a 21 nm thick film of c-PS/9.1k as opposed to more than 30 °C for A-PS. These results further indicate that the perturbation of glass transition behavior at the free surface of a cyclic PS thin film is weaker compared to A-PS. Overall, ellipsometry characterization indicates at most a weak G

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Figure 4. T+ − Tmid,bulk (triangle), Tmid − Tmid,bulk (square), and T− − Tmid,bulk (circle) measured by ellipsometry for 20−25 nm thick films measured on Si−OH substrate (half-filled symbols), Si/SiOx substrate (full symbols), and Si−H substrate (empty symbols): (a) c-PS/3.4k and (b) c-PS/9.1k films. (Symbols have errors that are smaller than the symbol size.)

MW dependence of the Tg-confinement effect for low-MW cPS films on Si/SiOx substrates. We expect that the impact of the cyclic topology in suppressing the Tg-confinement effect will eventually disappear when MW is sufficiently high, with a full recovery of the Tg-confinement effect seen in high-MW linear PS. We note that the c-PS linking group contains nitrogen and oxygen atoms potentially able to undergo hydrogen bonding with substrate-surface hydroxyl groups. To further investigate the impact of hydrogen-bonding interactions on the Tg of c-PS films, we modified the level of substratesurface hydroxyl groups. 3.4. Tg-Confinement Effect in A-PS vs c-PS: Films Supported on Si−H, Si/SiOx, and Si−OH Substrates. Figure 4a compares changes in T+, Tmid, and T− from Tg,bulk for c-PS/3.4k ultrathin films (thickness ≤ 25 nm) supported on substrates with different hydrogen-bonding capabilities. Si−OH substrates have a uniform coverage of surface hydroxyl groups with a static contact angle approaching 0° and therefore possess maximal hydrogen-bonding potential. On the other hand, Si−H substrates possess minimal hydrogen-bonding capability. Within error, T+, Tmid, and T− remain unchanged as a function of substrate. Figure 4b shows a similar plot for c-PS/9.1k. The Tgconfinement behavior of c-PS/9.1k films exhibits negligible dependence on the extent of polymer−substrate interactions. This behavior may be explained by the cyclic topology strongly restricting polymer−substrate hydrogen-bonding interactions. Compared to functional groups located at chain ends that possess greater conformational freedom, the c-PS linkage has very restricted conformational mobility, thus limiting access to hydrogen bonding. Focusing on average film Tg, these results indicate that tuning the hydroxyl group concentration on the substrate surface does not affect the invariance of Tg with confinement for c-PS/3.4k or the very small Tg-confinement effect for c-PS/9.1k films. These results may be explained by the fact that, in comparison with linear polymers, low-MW ring polymers have significantly less conformational freedom, which restricts polymer units from interacting with the substrate surface. In essence, the nearly entirely suppressed Tg-confinement effect in c-PS samples likely originates from a very weak perturbation near

the free surface (in comparison to linear PS) rather than a strong perturbation to Tg at the polymer−substrate interface. Fluorescence spectroscopy was also used to measure the overall average Tg across thin films of c-PS/3.4k. Figure 5 shows

Figure 5. Fluorescence emission spectra of BPD dopant in an 81 nm thick c-PS/3.4k film taken at 40 °C (dotted line) and 140 °C (solid line). Data have been normalized by the maximum intensity of the spectrum at 40 °C. The inset shows the structure of BPD.

emission spectra of a dopant chromophore, BPD (present at < 0.2 wt % relative to dry polymer), in an 81 nm thick c-PS/ 3.4k film in the glassy (40 °C) and rubbery (140 °C) states, normalized by the peak intensity at 40 °C. Overall intensity decreases with increasing T. Previous studies3,4,12,16−18,21 have shown that Tg values can be obtained by plotting the normalized integrated BPD emission intensity as a function of T. Figure 6 shows such a plot for c-PS/3.4k bulk and ultrathin films. Over a T range of more than 90 °C, two linear T dependences of the intensity are fitted to the rubbery and glassy states, with the intersection identified as the average film Tg. The bulk 81 nm thick film exhibits Tg,bulk = 96 °C, in agreement H

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tions.23 A-PS exhibits a strong MW dependence of bulk fragility: Santangelo et al.71,114 reported m = 97 for Mw = 2.4 kg/mol and m = 146 for Mw = 3840 kg/mol. With decreasing MW, molecular packing efficiency of short, linear chains is greatly improved by the excess configurational entropy and deficient intermolecular constraints conferred by chain ends. Importantly, Santangelo et al.71 also reported that a low MW, 4.6 kg/mol cyclic PS exhibited m = 148, within error identical to that of the high-MW linear PS. (Very high-MW linear PS and very high-MW cyclic PS are expected to have identical values of Tg,bulk and bulk fragility.) These results indicate that, in contrast to linear PS, the packing frustration experienced by cyclic PS cannot be readily reduced by going to low-MW samples. Several studies have investigated fragility and its relation to confinement effects. Evans et al.23 identified bulk fragility as a key (but not the only) parameter in determining the strength of the Tg-confinement effect for supported films of freely deposited, linear polymer chains that do not have substantial substrate interactions. Santangelo et al.71 reported that linear PS exhibits dramatic reduction in fragility with decreasing MW at low MW, in stark contrast to the nearly MW independent fragility measured for cyclic PS at similar MW. This indicates that cyclic PS experiences little or no reduction in packing frustration due to MW perturbation, i.e., major reduction in MW. In a similar fashion, we hypothesize that the fragility of cyclic PS is not significantly perturbed upon nanoscale confinement in thin films, in contrast to the major reduction in fragility of linear PS upon nanoconfinement as demonstrated by Fukao and Miyamoto53 and Glor et al.45 Marvin et al.25 further suggested that systems exhibiting an interfacial suppression of Tg under confinement should always exhibit a suppression in fragility. If the fragility-confinement effect is a fundamental precursor to the Tg-confinement effect, then it may be possible that low-MW cyclic PS exhibits little or no fragility-confinement effect. Investigation of fragility-confinement effects in cyclic PS is warranted; studies are underway.

Figure 6. Temperature dependence of the integrated fluorescence emission intensity of BPD dopant in 81 nm thick film on SiOx substrate (square), 21 nm thick film on SiOx substrate (circle), and 21 nm thick film on Si−OH substrate (triangle) for c-PS/3.4k. Data have been normalized to 1.00 at Tg and shifted vertically for clarity.

with Tg,bulk by ellipsometry and DSC. When supported on glass substrate (SiOx; θs = ∼71°), Tg = 97 °C; i.e., Tg − Tg,bulk = 1 °C, for a 21 nm thick film. When the same ultrathin film is supported on acid−base treated substrate, thereby maximizing hydroxyl units on the surface (Si−OH; θs = ∼0°), Tg = 94 °C; i.e., Tg − Tg,bulk = −2 °C. Consistent with ellipsometry results, thin films of c-PS/3.4k exhibit a negligible Tg-confinement effect that is independent of substrate-surface hydrogenbonding capability. In comparison, similar studies on A-PS films supported on glass substrate (SiOx) via fluorescence have demonstrated a much greater Tg reduction in the range of 15− 20 °C for ∼21 nm thick films.3,4,17 (The larger magnitude of the Tg-confinement effect in PS films characterized by fluorescence compared to ellipsometry has been fully addressed in ref 17.) In stark contrast to A-PS, with regard to average Tg across a film, both ellipsometry and fluorescence results support the conclusion that cyclic chain topology nearly completely suppresses the Tg-confinement effect in supported, low-MW PS films. Studies on the Tg-confinement effect of A-PS supported films have revealed little sensitivity to the substrate, including oxidation, hydrogen passivation, or piranha treatment of the surface. Via ellipsometry, Seemann et al.55 reported a Tg reduction of ∼11 °C for a ∼23 nm thick A-PS film (MW = 2 kg/mol) on piranha-treated silicon wafers (Si−OH substrates). Keddie et al.1 reported similar observations for a 120 kg/mol A-PS sample on hydrogen-passivated silica. In general, supported A-PS films possess no significant polymer− substrate interaction. Thus, the strong free-surface effect dominates the Tg response, resulting in dramatic reduction upon confinement. In contrast, for supported c-PS/3.4k films which also exhibit no apparent attractive polymer−substrate interaction, the very weak free-surface effect results in no Tg reduction within error down to 21 nm. We hypothesize that such a difference between A-PS and cyclic PS originates from the packing efficiencies of different chain topologies near the free surface.118 Dynamic fragility (m) reflects the packing efficiency which determines the susceptibility of a glass former to perturba-

4. CONCLUSIONS The Tg-confinement behavior of supported PS films is strongly affected by chain topology. Samples of l-PS with Mn = 3.4 and 9.1 kg/mol were synthesized by ATRP from propargyl 2bromoisobutyrate initiator followed by postpolymerization azidation to produce a complementary azide group at the terminal chain end. c-PS samples were obtained via a continuous addition “click” cyclization technique applied to lPS under high dilution. The high purity of cyclic polymers was confirmed by various characterization methods. Cyclic PS has a much weaker MW dependence of Tg,bulk compared to l-PS and A-PS: Tg = 96 °C for c-PS/3.4k, whereas Tg = 75 °C for l-PS/ 3.4k and 72 °C for A-PS/3.6k These results are in good qualitative agreement with previous studies of cyclic PS; variances in Tg values for c-PS can exist in the literature because of different chemical structures employed as linking units. For the MWs studied here, ellipsometry and fluorescence reveal a nearly completely suppressed confinement effect in cPS films. In contrast to A-PS/3.6k thin films that exhibit Tg − Tg,bulk = ∼−10 °C via ellipsometry at thickness = 21 nm, the Tg values of c-PS/3.4k thin films are invariant down to 21 nm thickness on Si/SiOx substrate; i.e., the Tg-confinement effect is eliminated in such films. In addition, c-PS/9.1k films exhibit a slight Tg reduction outside of error upon confinement to 21 nm. In short, low-MW c-PS films show at most a weak MW I

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(19) Rittigstein, P.; Priestley, R. D.; Broadbelt, L. J.; Torkelson, J. M. Nat. Mater. 2007, 6, 278−282. (20) Park, C. H.; Kim, J. H.; Ree, M.; Sohn, B. H.; Jung, J. C.; Zin, W. C. Polymer 2004, 45, 4507−4513. (21) Mundra, M. K.; Ellison, C. J.; Behling, R. E.; Torkelson, J. M. Polymer 2006, 47, 7747−7759. (22) Grohens, Y.; Brogly, M.; Labbe, C.; David, M. O.; Schultz, J. Langmuir 1998, 14, 2929−2932. (23) Evans, C. M.; Deng, H.; Jager, W. F.; Torkelson, J. M. Macromolecules 2013, 46, 6091−6103. (24) Ye, C. H.; Wiener, C. G.; Tyagi, M.; Uhrig, D.; Orski, S. V.; Soles, C. L.; Vogt, B. D.; Simmons, D. S. Macromolecules 2015, 48, 801−808. (25) Marvin, M. D.; Lang, R. J.; Simmons, D. S. Soft Matter 2014, 10, 3166−3170. (26) Priestley, R. D.; Cangialosi, D.; Napolitano, S. J. Non-Cryst. Solids 2015, 407, 288−295. (27) Xie, F. C.; Zhang, H. F.; Lee, F. K.; Du, B. Y.; Tsui, O. K. C.; Yokoe, Y.; Tanaka, K.; Takahara, A.; Kajiyama, T.; He, T. B. Macromolecules 2002, 35, 1491−1492. (28) Tsui, O. K. C.; Russell, T. P.; Hawker, C. J. Macromolecules 2001, 34, 5535−5539. (29) 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. Macromolecules 2001, 34, 5627−5634. (30) Zhang, C.; Fujii, Y.; Tanaka, K. ACS Macro Lett. 2012, 1, 1317− 1320. (31) Glynos, E.; Frieberg, B.; Chremos, A.; Sakellariou, G.; Gidley, D. W.; Green, P. F. Macromolecules 2015, 48, 2305−2312. (32) Glynos, E.; Frieberg, B.; Oh, H.; Liu, M.; Gidley, D. W.; Green, P. F. Phys. Rev. Lett. 2011, 106, 128301. (33) Chai, Y.; Salez, T.; McGraw, J. D.; Benzaquen, M.; DalnokiVeress, K.; Raphael, E.; Forrest, J. A. Science 2014, 343, 994−999. (34) Torres, J. M.; Wang, C. Q.; Coughlin, E. B.; Bishop, J. P.; Register, R. A.; Riggleman, R. A.; Stafford, C. M.; Vogt, B. D. Macromolecules 2011, 44, 9040−9045. (35) Torres, J. M.; Stafford, C. M.; Uhrig, D.; Vogt, B. D. J. Polym. Sci., Part B: Polym. Phys. 2012, 50, 370−377. (36) Baglay, R. R.; Roth, C. B. J. Chem. Phys. 2015, 143, 111101. (37) Long, D.; Lequeux, F. Eur. Phys. J. E: Soft Matter Biol. Phys. 2001, 4, 371−387. (38) Lipson, J. E. G.; Milner, S. T. Eur. Phys. J. B 2009, 72, 133−137. (39) Lipson, J. E. G.; Milner, S. T. Macromolecules 2010, 43, 9874− 9880. (40) White, R. P.; Price, C. C.; Lipson, J. E. G. Macromolecules 2015, 48, 4132−4141. (41) Napolitano, S.; Cangialosi, D. Macromolecules 2013, 46, 8051− 8053. (42) Lang, R. J.; Simmons, D. S. Macromolecules 2013, 46, 9818− 9825. (43) Xia, W.; Hsu, D. D.; Keten, S. Macromol. Rapid Commun. 2015, 36, 1422−1427. (44) Shavit, A.; Riggleman, R. A. Macromolecules 2013, 46, 5044− 5052. (45) Glor, E. C.; Composto, R. J.; Fakhraai, Z. Macromolecules 2015, 48, 6682−6689. (46) Ediger, M. D.; Forrest, J. A. Macromolecules 2014, 47, 471−478. (47) Arabeche, K.; Delbreilh, L.; Adhikari, R.; Michler, G. H.; Hiltner, A.; Baer, E.; Saiter, J. M. Polymer 2012, 53, 1355−1361. (48) Inoue, R.; Kawashima, K.; Matsui, K.; Kanaya, T.; Nishida, K.; Matsuba, G.; Hino, M. Phys. Rev. E 2011, 83, 021801. (49) Inoue, R.; Nakamura, M.; Matsui, K.; Kanaya, T.; Nishida, K.; Hino, M. Phys. Rev. E 2013, 88, 032601. (50) Fukao, K.; Miyamoto, Y. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 2000, 61, 1743. (51) Lupaşcu, V.; Picken, S. J.; Wübbenhorst, M. J. Non-Cryst. Solids 2006, 352, 5594−5600. (52) Sharp, J. S.; Forrest, J. A. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 2003, 67, 031805.

dependence of the Tg-confinement effect. Furthermore, the (nearly) completely suppressed Tg-confinement effect down to 21 nm thickness is observed in c-PS films regardless of the level of surface hydroxyl groups on the substrates. The cyclic topology apparently prevents hydrogen-bonding interactions between the nitrogen or oxygen atoms from the linking units and the substrate-surface hydroxyl groups. The difference in how the free surface affects c-PS and linear PS fragility is offered as a potential explanation for the very different Tg-confinement effects observed with c-PS and A-PS, both of which lack attractive interactions with silicon substrates.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02474.

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Figures S1−S9 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (S.M.G.). *E-mail [email protected] (J.M.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research by L.Z. and J.M.T. was supported by the University Partnership Initiative between Northwestern University and The Dow Chemical Company. S.M.G. and R.E. acknowledge NSF DMR 0844662 and NSF CHE 1412439 for support.

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REFERENCES

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DOI: 10.1021/acs.macromol.5b02474 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.5b02474 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules min. Onset Tg values are 90 and 72 °C for the 8.4 and 3.6 kg/mol PS, respectively. (113) Tg,1/2ΔCp values for all PS samples examined in the current study are provided in Figure S4. We note that the lower heating rate (10 °C/min) than the cooling rate (40 °C/min) used in DSC measurement of the Tg can lead to an undershoot upon heating due to physical aging/structural recovery (taking place during the slow heating scan). This may result in potential errors in the determination of the absolute Tg,onset value. Measuring fictive temperature (sometimes referred to as the limiting fictive temperature) could be more rigorous in providing quantitative comparison. However, as discussed in Badrinarayanan, P.; Zheng, W.; Li, Q.; Simon, S. L. J. Non-Cryst. Solids 2007, 353, 2603−2612. the differences between the Tg and fictive temperature are very small, ranging from 0.5 ± 0.5 to 1.4 ± 0.4 °C depending on the cooling rate, so it is reasonable to employ Tg values rather than fictive temperatures. (114) Santangelo, P. G.; Roland, C. M. Macromolecules 1998, 31, 4581−4585. (115) Dudowicz, J.; Freed, K. F.; Douglas, J. F. J. Phys. Chem. B 2005, 109, 21350. (116) Riggleman, R. A.; Yoshimoto, K.; Douglas, J. F.; de Pablo, J. Phys. Rev. Lett. 2006, 97, 045502. (117) Figure S9 shows thermal expansivity vs temperature plots with thermal expansivity recalculated using a slightly larger temperature interval (ΔT) of ∼5.3 °C for numerical differentiation to reduce data scattering. The T+ and T− values thus obtained are almost unchanged from those obtained in Figures 2c and 2d. (118) Reference 32, which investigates the Tg-confinement effect in star-shaped PS, has invoked an entropy argument. We note that a related argument could be made in the case of cyclic PS. Cyclic PS would have a lower entropy compared to linear PS, resulting in less entropy loss upon confinement.

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DOI: 10.1021/acs.macromol.5b02474 Macromolecules XXXX, XXX, XXX−XXX