Note pubs.acs.org/Macromolecules
Enthalpic and Volumetric Changes at Phase Transitions of Polystyrene‑b‑poly(alkyl methacrylate) Copolymers and Their Pressure Dependence Dong Hyun Lee,† Hoyeon Lee,‡ Yonghoon Lee,‡ Yoonkeun Kim,‡ and Du Yeol Ryu*,‡ †
Department of Polymer Science and Engineering, Dankook University, Yongin, Gyeonggi-do 448-701, Korea Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul 120-749, Korea
‡
1. INTRODUCTION The phase behavior of block copolymers (BCPs) that are composed of two or more dissimilar polymer chains by a covalent bond has been extensively investigated both experimentally1−14 and theoretically,15−22 since the BCP selfassembly in favor of the nanoscopic morphologies offers welldefined templates and scaffolds with periodically ordered features.23−27 For diblock copolymers, the temperaturedependent phase behavior has been delineated mostly with the order−disorder transition (ODT) upon heating, where unfavorable segmental interaction between the two blocks weakens and phase mixes together (or the Flory−Huggins interaction parameter (χ) decreases).1,28,29 The weak and positive pressure dependence of the enthalpically driven ODT indicated that the miscibility between the two dissimilar blocks became more decreased by increasing pressure due to the increased unfavorable contacts.30−33 In contrast, the entropic effect arising from thermal expansion difference between the two blocks resulted in the lower disorder-to-order transition (LDOT) upon heating, particularly in weakly interacting BCPs with no specific interaction.4−6,34−39 The negative pressure dependence of the entropically driven LDOT was characterized as the compressible (or baroplastic) behavior due to the enhanced miscibility between the two blocks with increasing pressure.37−39 There have been intensive efforts to comprehend the phase behaviors of BCPs (or polymer blends). Intriguingly, a series of polystyrene-b-poly(alkyl methacrylates) (PS-b-PAMAs) that pertain to the weakly interacting BCP homologues exhibited a variety of phase behaviors by varying alkyl chain length (n) in methacrylate unit. Since a finding of a LDOT type behavior in polystyrene-b-poly(n-butyl methacrylate) (PS-b-PBMA) by Russell and co-workers,34 Mayes and co-workers shed new light on the alkyl chain length dependence of phase behaviors; these phase behaviors in BCP homologues indicated an ODT type (n = 1 and 6) and a LDOT type (n = 2, 3, and 4) as the alkyl chain length (n) in methacrylate unit increases.4 The pressure dependence of these transitions indicated the pressureenhanced miscibility (or baroplastic behavior) between the two blocks except for that of an ODT type polystyrene-bpoly(methyl methacrylate) (PS-b-PMMA) (n = 1).5,36,37 Furthermore, Kim and co-workers reported the closed-loop type phase behavior consisting of a LDOT and an UODT in polystyrene-b-poly(n-pentyl methacrylate) (PS-b-PPeMA) (n = 5), where these two transitions indicated a significant baroplasticity that has never seen before.6,36,39 Recently, the composition-dependent phase behaviors were reported in the © 2014 American Chemical Society
binary BCP blends of PS-b-PnBMA/polystyrene-b-poly(n-hexyl methacrylate) (PS-b-PnHMA) by virtue of the homogeneously mixed phase between the PnBMA and PnHMA blocks; these phase behaviors changed continuously from a LDOT to an ODT type as the blend composition varies from PS-b-PnBMA to PS-b-PnHMA. Moreover, the closed-loop phase behavior was generated in nearly symmetric composition of the miscible BCP blends.40 In present study, we provide the overview on the characteristic phase behaviors for symmetric PS-b-PAMAs (n = 1−6), and the enthalpic and volumetric changes at phase transitions, which were measured by the differential scanning calorimetry (DSC) and in-situ spectroscopic ellipsometry with increasing temperature. With the enthalpic and volumetric variables at the transitions, the pressure coefficient (dT/dP) of transition temperatures was calculated on the basis of the Clausius−Clapeyron equation and compared with the reference values.
2. EXPERIMENTAL SECTION Symmetric PS-b-PAMAs were synthesized via sequential living anionic polymerization of each monomer in tetrahydrofuran (THF) at −78 °C under a purified argon environment; these reactions were performed in the presence of LiCl (high purity, Aldrich) using sec-butyllithium (1.3 M, Aldrich). THF refluxed from CaH2 was repurified with fresh sodium−benzophenone complex until it persisted a deep purple color, indicating an oxygen- and moisture-free solvent. Degassed monomers with CaH2 (Aldrich) were vacuum-distilled over the second purifiers of dried dibutylmagnesium for styrene and trioctylaluminum for n-alkyl methacrylates, until a characteristic color persisted. The polymer samples precipitated in excess methanol were dried under vacuum and thermally annealed above the glass transition temperature (Tg) of the PS block (∼100 °C). The number- and weight-average molecular weights, Mn and Mw, and dispersitiy (Mw/Mn) of the polymer samples were evaluated using multiangle laser light scattering (MALLS) combined with size exclusion chromatography (SEC). The composition of these BCPs was determined by 1H nuclear magnetic resonance (1H NMR) spectroscopy, based on the mass densities of each block.36 The sample characteristics of PS-b-PAMAs used in this study are given in Table 1. Differential scanning calorimetry (DSC: PerkinElmer Diamond DSC) was operated at a heating rate of 10 °C/min from 80 to 280 °C, where a relatively large amount (>20 mg) of samples was loaded in order to amplify signal-to-noise in thermograms. The DSC data were obtained Received: December 11, 2013 Revised: February 23, 2014 Published: March 12, 2014 2169
dx.doi.org/10.1021/ma402535w | Macromolecules 2014, 47, 2169−2173
Macromolecules
Note
Table 1. Sample Characteristics of Block Copolymers Used in This Study sample code
Mn (g/mol)
Mw/Mn
ϕ(d)PSb
transition temp (°C)
ΔH (J/g)
PS-b-PMMA (d)PS-b-PEMAa (d)PS-b-PPMAa PS-b-PBMA PS-b-PPeMA
35 000 144 000 135 000 66 900 49 900
1.05 1.02 1.02 1.04 1.02
0.490 0.505 0.499 0.500 0.500
39 800
1.02
ODT: 237 ± 2 LDOT: 195 ± 5 LDOT: 195 ± 5 LDOT: 159 ± 2 LDOT: 148 ± 3 UODT: 206 ± 3 ODT: 227 ± 2
0.68 0.018 0.039 0.075 0.12 0.14 0.30
PS-b-PHMA a
0.508
Δh/h 3.91 6.02 1.27 2.39 1.97 −1.70 −2.94
× × × × × × ×
10−4 10−5 10−4 10−4 10−3 10−3 10−4
b
The deuterated PS block was used in block copolymers. Volume fractions of (d)PS were calculated with mass densities of the components (1.05, 1.13, 1.184, 1.125, 1.080, 1.050, 1.031, and 1.006 for PS, dPS, PMMA, PEMA, PPMA, PBMA, PPeMA, and PHMA, respectively).36 during the first heating run immediately after thermal annealing at 120 °C for 24 h under vacuum. Depolarized light scattering (DPLS) experiments, using a polarized beam from a He−Ne laser source at a wavelength of 632.8 nm, were performed to detect transition temperatures of PS-b-PAMAs. A sample thickness was set to 1.0 mm in a small brass template with a 5-nm diameter hole under an ambient condition. The intensity detected at the photodiode was recorded (through A/D converter) as a function of temperature at a heating rate of 0.7 °C/min from 120 to 260 °C under nitrogen flow. To ensure a parallel orientation of lamellar microdomains, the PS-grafted substrate was prepared with a hydroxylterminated polystyrene (PS-OH, Polymer Source Inc.) with Mn = 10 000 g/mol (Mw/Mn = 1.05) by the grafting-to surface modification on the underlying native oxide (2 nm) layer of Si substrate.41 The BCP dissolved in toluene were spin-coated onto a PS-selective substrate, where the film thicknesses were set to around 300 nm. The volumetric changes of PS-b-PAMAs were evaluated by the temperature-dependent film thickness under vacuum using in-situ spectroscopic ellipsometry (SEMG-1000, Nanoview Co.), which was operated at an incidence angle of 70° with halogen light source of wavelength (λ) ranging from 350 to 850 nm (or 1.5 to 3.5 eV). Two ellipsometric angles (Ψ and δ) related to the film thickness were continuously monitored during heating the film samples at a heating rate of 5 °C/min.
Figure 1. Depolarized light scattering (DPLS) intensity profiles for the symmetric PS-b-PAMAs with the different alkyl chain length (n = 1− 6) in methacrylate unit. A heating rate of 0.7 °C/min from 120 to 260 °C. The intensity profiles were vertically shifted in an arbitrary factor to avoid overlapping.
3. RESULTS AND DISCUSSION The phase behaviors of a series of symmetric, lamella-forming PS-b-PAMAs with the different alkyl chain length (n = 1−6) in methacrylate unit were evaluated with depolarized light scattering (DPLS) experiments that distinguish the birefringent lamellar microdomains from disordered state of BCPs. Figure 1 shows the DPLS intensity profiles for PS-b-PAMAs as a function of temperature, which were collected at a heating rate of 0.7 °C/min from 120 to 260 °C. As the temperature increases, an abrupt decrease in the DPLS intensity for polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) occurred at 237 ± 2 °C due to a typical ODT from the optical anisotropy of lamellar microdomains to the optically isotropic disordered state. In contrast to an ODT type behavior, the DPLS intensity for deuterated polystyrene-b-poly(ethyl methacrylate) (PS-b-PEMA) remained zero at lower temperatures (
