Article pubs.acs.org/Macromolecules
Growth of Polymer Nanorods with Different Core−Shell Dynamics via Capillary Force in Nanopores Ye Sha,† Linling Li,† Xiaoliang Wang,† Yuanxin Wan,† Jie Yu,† Gi Xue,*,† and Dongshan Zhou*,†,‡ †
Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Key Laboratory of High Performance Polymer Materials and Technology (Nanjing University), Ministry of Education, State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructure, Nanjing University, Nanjing 210093, P. R. China ‡ School of Physical Science and Technology, Xinjiang Laboratory of Phase Transitions and Microstructures in Condensed Matters, Yili Normal University, Yining 835000, P. R. China S Supporting Information *
ABSTRACT: The dynamics of poly(n-butyl methacrylate) confined in porous anodic aluminum oxide (AAO) templates are investigated using differential scanning calorimetry (DSC) and fluorescence nonradiative energy transfer (NRET). Two glass transition temperatures (Tg,low and Tg,high) are obtained at higher infiltration temperatures via capillary force followed by slow cooling. Tg,low resembles the Tg of the bulk phase and represents the transition of the core layer. Tg,high represents the transition of the adsorbed layer in the confined polymer glass. The temperature threshold to form one or two glass transitions is determined by adjusting the infiltration temperatures and the pore diameters. It is shown that the adsorbed layer has increased interchain proximity relative to the bulk. In addition, the glass transition behavior is hypothesized to be mediated by the counterbalance of the size and interfacial effects in the confined space. The easily synthesized core−shell nanofibers with one glassy and one rubbery component without the need for block polymers have promising potential for use in several processing strategies.
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INTRODUCTION Polymer nanostructures have been extensively utilized in lithography, adhesives, optical sensors, electronic devices, mechanical devices, and numerous other fields.1−3 When confined to the nanoscale, many physical characteristics demonstrate anomalous behaviors compared with that of the bulk, including glass transition temperature,4,5 crystallization,6 and viscosity.7,8 An understanding of polymer properties in confined states is crucial for the design of industrial products. Among these properties, one of the most intriguing issue concerns the glass transition temperature (Tg). In past decades, researches have primarily focused on one-dimensional (1D) thin films. With decreases in freestanding thin film thickness, Tg decreases due to the increase in free surface area.9,10 When thin films are supported on substrates, interfacial interactions may dominate the glass transition dynamics, resulting in an increase in Tg with decreased film thickness.11−13 Recently, studies on the confinement of polymers with higher ordered geometries beyond thin films have been extended to structures such as nanowires14−16 and nanospheres.17−19 When the radius of gyration (Rg) of polystyrene is larger than the confined anodic aluminum oxide (AAO) nanopores, an enhanced mobility of chains with reduced interchain entanglement but unchanged glass transition temperature was observed by Shin and Russell et al., which © 2014 American Chemical Society
has opened avenues to interfacial science in tailoring the longterm stability of the nanomaterials.16 Using broadband dielectric spectroscopy, it was found that the Tg of poly(γbenzyl-L-glutamate) that was covalently attached to AAO walls reduced as much as 50 K.15 Priestley reported that polymer nanoparticles capped by silica shells demonstrated a sizeindependent Tg; however, when the shell was removed, Tg decreased with decreased particle size.18 The dynamics of polymer chains confined under higher geometries are mediated by interfacial interactions and size effects, similar to those in thin films. 2D cylindrical polymers are generally more difficult to prepare and characterize than are thin films.16,20,21 These difficulties are partially due to the complicated spectroscopic signals. Additionally, the understanding of chain dynamics of polymers under 2D confinement, including their conformation, entanglement, and interchain packing density, is limited. Fluorescence nonradiative energy transfer (NRET), also termed fluorescence resonance energy transfer (FRET), has been extensively applied to detect inter/intrachain entanglement, interchain distance and coupling, interpenetration, and Received: August 27, 2014 Revised: October 27, 2014 Published: December 2, 2014 8722
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interdiffusion.22−25 It is extremely sensitive in characterizing subtle changes in interchain proximity on short-range scales of 0.5−3 nm.26 Dramatic deviations in Tg also occur when the Rg of a polymer is much smaller than its space size.27,28 In our previous work, double glass transition temperatures of poly(methyl methacrylate) (PMMA) oligomers confined within large AAO nanopores were detected.14 We proposed that this finding was due to interfacial effects induced heterogeneities of interchain packing density that resulted in core−shell structure formation. In the present work, poly(n-butyl methacrylate) (PBMA) confined in AAO templates with pore diameters much larger than the oligomer Rg is investigated. Fast cooling of the confined melt results in a single Tg, whereas two distinct Tgs are obtained after slow cooling. After removing the AAO matrix, released nanorods present a single Tg. We aim to interpret the mechanism and required conditions for adsorbed layer generation and diffusion using NRET and differential scanning calorimetry (DSC). The critical temperature for the generation of adsorption transition is confirmed. In addition, the influences of interfacial effects, molecular weight dependence, and finite size effects on glass transition behaviors are investigated.
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Thermal Analysis. For DSC (Mettler-Toledo DSC1) measurements, the weight of the filled AAO sample was approximately 20 mg, and the heating rate was 10 K/min. For TGA (PerkinElmer TGA-Pyris system), samples containing PBMA polymer inside of AAO nanopores (approximately 10 mg) were heated from 25 to 650 °C at 10 K/min under a nitrogen atmosphere, as shown in Figure S1. NRET Measurements. After the specified thermal management, reflectance fluorescence spectra were collected with a PTI QM40 fluorometer at an excitation wavelength of 294 nm. The band-pass excitation and emission slits were both 0.25 μm. The fluorescence emission intensity was collected from 300 to 500 nm. The temperature of the sample-detecting cavity was lower than 10 °C. SEM Measurements. A Hitachi S-4800 scanning electron microscope with an accelerating voltage of 5 kV was used to investigate the nanostructure morphologies. All samples were coated with several nanometers of Au before performing SEM measurements.
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RESULTS AND DISCUSSION AAO templates of parallel, cylindrical nanopores arranged in a hexagonal cell are formed where no pore channel intersections exist.30 Such templates can be considered 2D confining spaces that are useful for the infiltration of polymer melts by capillary force.31 We utilized AAO templates to investigate the dynamics of PBMA under nanoconfinement. Solid nanofibers can be obtained if the precursor film thickness is larger than the pore size,20 and long annealing times with a bulk film on top of an AAO reservoir will promote complete filling of the nanochannels.32 Figure 1 displays the morphological investigation of self-ordered AAO membranes and the released PBMA1w nanorods after eliminating the AAO matrix. The average
EXPERIMENTAL SECTION
Materials. PBMA oligomer samples labeled with a carbazolyl chromophore (PBMA-Cz) and an anthryl chromophore (PBMA-An) were prepared using an ATRP method with random labeling of trace amounts of fluorescent monomers; the details can be found elsewhere.25 The PBMA-Cz sample had a weight-average molecular weight of 11 900 Da with a polydispersity index of 1.21. The PBMAAn sample had a weight-average molecular weight of 12 700 Da with a polydispersity index of 1.23. The oligomer sample is abbreviated as PBMA1w. Ultraviolet absorption spectra were measured using a PerkinElmer Lambda 35 UV−vis spectrophotometer. The chromophore-labeled monomer unit concentrations in mole percent were 0.414% (PBMA-Cz) and 0.351% (PBMA-An). A different PBMA was synthesized by ATRP with a weight-average molecular weight of 102 000 Da and a polydispersity index of 1.38. This sample is abbreviated as PBMA10w. The AAO membranes which are open on both ends were purchased from Hefei PUYUAN Nano Ltd. with average pore sizes of 15, 25, 50, 70, 100, and 300 nm. The thicknesses of the AAO templates ranged from 40 to 80 μm. The templates were rinsed with THF, ethanol, water, and then ethanol and dried in an oven at 120 °C for 30 min. Surface-modified hydrophobic AAO was fabricated by a vapor phase reaction using Trichloro(1H,1H,2H,2H-perfluorooctyl)silane (FOTS) according to the protocols reported elsewhere.29 The modified surfaces were uniform, as determined by contact angle measurements, EDX spectra, and element mapping, as shown in Figures S3−S5 in the Supporting Information. Preparation of PBMA-Filled AAO Samples. A PBMA1w blend powder was made with 50% (w/w) PBMA-Cz and 50% (w/w) PBMAAn. Then, the blend powder was dissolved in redistilled toluene at a concentration of 10% (w/w) and stirred vigorously overnight. A PBMA10w solution was made at a concentration of 8%. A 60 μm precursor film was casted onto a smooth glass sheet and then placed in a clean fume hood for 3 days to allow for solvent evaporation. Subsequently, it was dried under vacuum for 24 h at 80 °C. Then, the dried PBMA1w film was placed on top of the AAO membrane and heated at 110 °C under high vacuum for 10 h to obtain fully wet nanorods. Then, it was cooled slowly to lower than 10 °C. Thermal infiltration of the PBMA10w film was performed at 165 °C for 24 h. The residual sample on top of the AAO template was carefully scraped by a surgical blade. The thermal stability of PBMA content were detected by thermogravimetry, as shown in Figure S1. The PBMA nanorods were obtained by applying an aqueous sodium hydroxide solution (1.0 M) to remove the AAO matrix. The released nanorods were dried under vacuum in a freeze-dryer at −10 °C.
Figure 1. SEM images of (a) AAO template with an average nanopore diameter of 300 nm and (b) PBMA1w nanorods after removal of the AAO matrix. 8723
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diameters of the AAO template and nanowires were approximately 300 nm by statistical treatment. Conventional calorimetry was used to investigate the glass transition behavior under nanoconfinement. Figure 2 displays
Figure 3. (a) Schematic representation of the slow cooling process to yield a two-layer state. The circles represent the occupied volume of oligomer coils. The solid luminous dots represent the fluorophores, with blue dots as donors and red dots as acceptors. (b) Schematic representation of the polymer confined in AAO with two layers. The shell layer thickness ξ can be calculated by ξ = d[1 − (1 − ΔCp,hi/ ΔCp,total)1/2]/2, where d is the pore diameter of AAO.
at a cold temperature. During the release process, polymer nanorods should be kept in the glassy state. Subsequently, we detected the first heating curve of the released nanorods. Strikingly, single Tg, similar to the value of bulk, was detected. Nanorods in AAO are under hard confinement while the released nanorods are under soft confinement.15,18,40 Here the confinement geometry is much larger than the single chain dimension of PBMA1w (Rg of PBMA1w is about 2.4 nm41), so the “free-surface effect” is too weak to be ignored.42,43 So a single Tg resembles bulk phase was observed. This result demonstrates that the hard wall of AAO is the dominated reason for the observed Tg shift. Generally, we use the method termed “precursor film infiltration” to get nanowires via capillary force, and the infiltration temperature is well above Tg. But if the infiltration temperature is reduced to only slightly higher than Tg with prolonged annealing time, PMMA nanowires with still high aspect ratio can be obtained where the length diameter is approximately equal to 30.20 So we performed an experiment with tailored infiltration temperatures to investigate the influence of sample preparation on the dynamics of the confined state. First, the PBMA1w blend precursor films were placed on top of the AAO templates. Then, the infiltration temperatures were precisely controlled, as specified in Figure 4. After 10 h of infiltration, the samples were slowly cooled to below 10 °C prior to DSC testing. From the first heating curves of confined glasses shown in Figure 4, we can see that if the infiltration temperature is less than or equal to 60 °C, a single Tg is obtained. When the infiltration temperature increases to larger than or equal to 65 °C, two Tgs scenarios appear. Figure 4b provides the first derivative of heat capacity versus temperature, and the peaks clarify the generation of Tg,hi. After the infiltration at 65 °C, the adsorbed layer begins to generate (Tg,hi is at 50 °C). With increasing the infiltration temperature, the deviations between the two Tgs become larger. When the infiltration temperature is above 80 °C, these differences reach constant. Based on the concept of “adsorption transition”, when polymer chains interact with an attractive impenetrable wall, they would undergo an “adsorption transition” only if the temperature reaches the adsorption temperature, Ta.44−46 Adsorption transition is thermodynamic-like; only if the infiltration temperature is equal to or higher than Ta, the adsorption of polymer chains onto the AAO wall could happen, and the adsorption layer would be formed. So the threshold of
Figure 2. Normalized DSC heating curves for (a) PBMA1w bulk, (b) hyperquenched PBMA1w glass within 300 nm AAO, (c) slow-cooled PBMA1w glass within 300 nm AAO, and (d) slow-cooled nanorods after release from AAO template. All samples were heated at 10 K/ min. Tg is indicated on each curve by short dashed lines.
DSC heating thermographs of bulk and confined PBMA1w. Tg of bulk PBMA1w is approximately 28 °C. The glass transition behavior of confined PBMA1w strongly depends upon cooling rate. If the cooling rate is rapid, a single Tg similar to that of the bulk can be observed. Here, we used liquid nitrogen to hyperquench the confined melt, providing a cooling rate as fast as 120 K/s.33 Two separate Tgs are obtained if the cooling rate is slow (0.2 K/min). The lower Tg (denoted as Tg,low) at 30 °C is similar to that of the bulk, while the higher Tg (denoted as Tg,hi) is approximately 60 °C. Deviations between the two Tgs could be as large as 30 °C. For liquids confined in nanopores, similar glass transition behaviors have been obtained experimentally and in simulations.34−36 The enthalpy overshoot in Figure 2c confirms that the dual phase transitions are glass transitions. In our previous work, similar phenomena have been observed.14 We speculated that fast cooling of the melt results in a homogeneous state; thus, a single Tg would be obtained. If the cooling rate is slow, an adsorbed layer could be formed with decreased mobility due to the strong interfacial effects. A core− shell model was proposed to elucidate these behaviors. Figure 3a sketches the schematic representation of the confined oligomer with formation of a core−shell structure. Using this facile cooling protocol, core−shell nanowires with one glassy component and one rubbery component, providing a low-Tg core and a high-Tg shell, are obtained, as shown in Figure 3b. This type of structure is rarely found in the literature without the use of block polymers or polymer blends.37,38 We utilized the change in heat capacity to estimate the amount of polymer participating in the glass transition process; thus, the length scale of the core layer and adsorbed layer can be calculated irrespective of the gradual change of the packing densities.14,39 The glassy shell thickness of PBMA1w confined in 300 nm AAO was calculated to be approximately 25 nm using the formula in Figure 3b. After the confined sample was cooled slowly to form a glass with two layers, we removed the AAO matrix via alkaline liquor 8724
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Figure 5. Emission intensity ratio, IA/IC, for glassy PBMA1w within 300 nm AAO after infiltration at specified temperatures for 10 h.
Figure 4. (a) Normalized DSC heating curves of PBMA1w within 300 nm AAO infiltrated at different temperatures for 10 h. All samples were heated at 10 K/min. The dashed lines serve as visual guidance for the Tg evolution. (b) Temperature first derivative of the heat capacity traces displayed in (a).
larger contribution to the total NRET efficiency. The final equilibrium state is reached when the infiltration temperature is above 80 °C, which gives exact matches to the DSC traces in Figure 4. Figure 2 shows the homogeneity of the hyperquenched confined PBMA glass in comparison to the heterogeneous state of slow-cooled glass. Intuitively, the adsorbed layer is eliminated by fast cooling. As shown in Figure 6b, the fastcooled glass has lower NRET efficiency (IA/IC = 4.6) than slow-cooled glass (IA/IC = 6.4). On the basis of this result, we conclude that the slow-cooled sample has higher local interchain proximity than bulk. It is well established that a slow cooling rate produces a more stable glass than a fast cooling rate. Because the polymer confined in AAO is isochoric, a thermal annealing experiment was designed to investigate the evolution of adsorbed layer after fast cooling. The confined melt was first hyperquenched to −178 °C and then annealed at 40, 50, 60 °C, and so on, as specified in Figure 6a. After that, the sample was cooled to −20 °C and then heated. Figure 6a represents the first heating curves directly after annealing. Figure 6b depicts the NRET efficiency of the glassy samples. Tg,hi increases with increased heat capacity of the adsorbed layer as annealing temperature is increased, and Tg,low decreases with the decline in heat capacity of the core layer. This finding indicates that the adsorbed layer thickens as annealing temperature increases. The final Tg values are possibly decided by the annealing temperatures, as different annealing temperatures have different thermodynamic driving forces. Kremer detected that the dynamic exchange of two layers is dependent upon temperature for glass-forming liquids in nanoporous sol−gel glasses.48 Napolitano reported that by increasing annealing time above Tg, the glass transition behavior of a polymer in close proximity to an interface is affected, with layer thickening and densification due to chain adsorption.49 The kinetic component of the adsorption has been recently discussed by Housmans et al.; in the first stage of adsorption, the linear growth process is dominant, while a stepdown nonlinear growth takes place in the late regime when crowding suppresses the first-order reaction mechanism.50 Our results are consistent with these previous studies. The NRET efficiency increases with increased annealing temperature, which demonstrates that the interchain proximity of the adsorbed layer increases. When the annealing temperature is above 80 °C, the plateau of IA/IC value appears, which is in accordance with the results of DSC. Thus, the results
adsorption transition should be over 60 °C. Hence, no adsorption layer would be generated if the infiltration temperature did not reach 60 °C. Single Tg polystyrene nanorods were previously reported by Torkelson et al. when the infiltration temperature was only slightly higher than the bulk Tg.47 When the infiltration temperature is between 65 and 140 °C, the adsorption transition could take place, leading to the formation of adsorbed layer. It should be noted that the baseline of the DSC trace at 50 °C infiltration is not stable because at approximately 64 °C the adsorption layer may begin to form, causing the baseline to deviate toward the exothermic direction and transform through the metastable state. We also detected the NRET efficiency of the confined glassy samples prepared at different infiltration temperatures. The fluorescent donor, carbazole, was conjugated to one polymer chain, and the fluorescent acceptor, anthracene, was conjugated to the other chain. The excitation wavelength of 294 nm was only absorbed by the donor and could therefore only excite the donor to emit light. When two fluorescent moieties come close to each other (
