Thermal transitions in hyperbranched polyester-polyol assemblies on carbon
E.D. Farias,[a] M.C.G. Passeggi (Jr) [a,b] and V. Brunetti [c],*
[a] Laboratorio de Física de Superficies e Interfaces, Universidad Nacional del Litoral and CONICET, Santa Fe, Argentina
[b] Departamento de Materiales, Facultad de Ingeniería Química, Universidad Nacional del Litoral,
Santa Fe, Argentina
[c] INFIQC, Universidad Nacional de Córdoba and CONICET, Departamento de Fisicoquímica,
Facultad de Ciencias Químicas, Córdoba. Argentina
* Corresponding author E-mail: verobrunetti@unc.edu.ar
Abstract
The thermal transitions of confined polymers are significant for their applications in nanoscale devices and advanced technologies. However, thermal transitions of ultra-thin polymer assemblies confined in sub-nanometer spaces are poorly understood. In this work, the self-assembly of hyperbranched polyester-polyol polymers (HPP) immobilized on carbon surfaces were investigated. The physicochemical properties and thermal transitions of the polymers under confinement were revealed by cyclic voltammetry, electrochemical impedance spectroscopy and atomic force microscopy. The adsorption of HPP with hydroxyl-terminal groups on the bare carbon surface followed typical porous solid isotherm due to hydrogen-bond forces. The dependence of the HPP nanometer-sized layers against temperature revealed a phase transition at ca. 306 K. This phase transition can be explained in terms of polymer layer reorganization due to the temperature effect on the intermolecular hydrogen-bonding instead of a glass transition. This feature is of particular relevance for further applications of hyperbranched polymers thin films in nanodevices.
Keywords: hyperbranched polymer • impedimetric determination • thermal transition • confined molecules • hydrogen-bond
1. Introduction
In recent years, highly branched dendritic polymers have gained more consideration due to their exceptional properties, which differ significantly from their linear counter parts, e.g. multiple terminal functional groups, specific rheological properties based on their globular structure, and well-defined internal cavities available to encapsulate guest molecules. In particular, the facile synthesis of hyperbranched polymers has opened a door to the design of a large variety of new dendritic molecules that promise to be strong competitors against other dendritic polymers like dendrimers in numerous applications, e.g. antifouling coatings,[1] chemical sensing,[2,3] rheological additives,[4–6] drug carriers,[7–10] theranostic devices,[11,12] enhanced oil recovery,[13] corrosion inhibition,[14] micro and nano-scale patterning,[15] cellular engineering,[16] solar cell,[17] energy storage,[18] among others. In many of these applications, hyperbranched polymers are supported as thin films on different substrates or as a part of nanocomposites, and thus, could exhibit different properties from those of the bulk. For example, Bansal et al. showed that thermomechanical responses of polymers, which provide limitations to their practical use, are favourably altered by the addition of trace amounts of a nanofiller in nanocomposites, and they proposed that glass-transition processes in confined geometries require the interaction of near-surface regions of altered mobility.[19] According to Russell et al.,[20] the film thickness introduces confinement effects on the morphology which can result on changes in the fundamental lengths scale of the morphology or even changes in the phase behaviour of polymers; as the degrees of confinement increase, totally new morphologies could emerge along with changes in the chain dynamics.
The thermal properties of polymeric materials are important for the proper function of components and as far as we know, thermal transitions of ultra-thin hyperbranched polymer layers confined in nanometer spaces are still rather unknown. Recent studies have shown that glass transition temperature of polymers (Tg) can also be modified by tuning the attractive interactions between the polymer and the substrate involved in thin films [21] and/or nanocomposites studies.[22] In this paper, a versatile family of hyperbranched polyester-polyols polymers is employed to analyze their thermal properties when they are assembled as thin films on carbon surfaces. The physicochemical properties and thermal transitions of the polymers under confinement are revealed by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and atomic force microscopy (AFM).
2. Experimental section
2.1 Materials
Boltorn®H20, Boltorn®H30, and Boltorn®H40 (see scheme 1) are produced by Polymer Factory with a theoretical molecular weight of 1749.8, 3607.6, and
7323.3 gmol-1, respectively. Hyperbranched polymers were prepared in dimethylsulfoxide (DMSO) solutions. Aqueous solutions of 0.1 M K2HPO4/KH2PO4(pH 7) and 0.002 M K3Fe(CN)6/K4Fe(CN)6 were prepared using ultra-purified water Milli-Q equipment (Millipore). All the reagents are of analytical grade and were used without extra purification.
Scheme 1
A 3.0 mm diameter glassy carbon electrode (GCE) purchased in CH Instruments, Inc. (Austin, TX) was employed as the working electrode for electrochemical measurements. Prior to the modification, GCE were polished using 1.0, 0.3, and 0.05 μm alumina (Buehler) and rinsed with copious Milli-Q water (Millipore Corp., USA). After polishing, the electrode was immersed in water into the ultrasonic cleaner, washed for 1 minute, and subsequently dried under nitrogen flow. For microscopy studies, highly oriented pyrolytic graphite (HOPG) purchased from SPI Supplies (1 cm2 of area and 1 mm of thickness) are used as substrates due to the average roughness of polished GCE is too high (about 20 nm) to evaluate the molecular adsorption. The HOPG substrates prior to the modification were exfoliated using adhesive tape, 3M®.
The carbon surfaces were modified by simple incubation of the substrate (GCE or HOPG) in DMSO solutions containing the hyperbranched polymer and subsequently washed with ethanol and water. This process was performed varying the conditions of incubation (polymer concentration or time), the surfaces were rinsed promptly and immediately employed in the measurements.
2.2 Instrumentation
The electrochemical experiments were carried out in a conventional cell of three electrodes containing the working electrode (GCE), a platinum wire for the counter electrode and Ag/AgCl/Cl- (3M) as the reference electrode. The electrolytes were deaerated under Nitrogen flux for 10 minutes previous to their use. A multifunctional electrochemical analyzer (CH Instruments Inc.) was employed to carry out the electrochemical measurements. Cyclic voltammograms were measured in the range of 0 to 0.60 V with a scan rate of 0.10 Vs-1. The impedance spectra were obtained at the open circuit potential with an amplitude of 10 mV in the frequency range from 100 KHz up to 0.1 Hz. Zview® software from Scribner Associates, Inc. was employed to analyze the impedance data.
A commercial Nanotec Electronic SPM System with a PLL/dynamic measurement board operating in tapping mode was employed to acquire AFM images at room temperature and atmosphere conditions. Rotated monolithic All-In-One-Al Budget Sensors cantilevers (Innovative Solutions Bulgaria Ltd., Bulgaria) made of silicon with a 30 nm thick aluminum reflex coating (resonant frequencies in the ranges of 80± 30 and 130± 10kHz, nominal spring constant in the ranges of 0.4-10 and 1-29 N/m, respectively, and both radius of curvature always <10 nm) were employed. The acquisition and processing of the images were done using the free software WS×M 5.0®.[23]
3. Results and discussion
3.1 Spontaneously self-assembly of Boltorn®H30 on carbon surfaces
These molecules have a roughly globular form in solution with an average diameter of about 3 nm.[24] The incubation of carbon substrates in Boltorn®H30 solutions for only a few seconds and subsequently rinsed, results in spontaneously nanometer-scale HPP films as those showed in Figs. 1(a) and 1(b). These layers in the early incubation stages grow following a sort of fractal-like structure, initially with thicknesses of approximately of 0.8 and 1.6 nm, respectively, as it is observed in the cross-section profiles of Figs. 1(e) and 1(f). When the incubation time is increased up to 15 minutes, it evolves to a densely packed worm-like bilayer structure with a thickness of about 1.6 nm, as it is observed in Fig. 1(c), and the corresponding cross-section profile (Fig. 1(g)). It is important to notice that, even at longer incubation times, there are regions where the substrate is naked. After more than four hours of incubation, although this is not so evident in Fig. 1(d), the cross-section profile (Fig. 1(h)) reveals a surface with structures more than three or four layers high, even before a complete coalescence of the first monolayer occurs, evidencing stronger adsorbate-adsorbate instead of adsorbate-substrate interactions, probably due to hydrogen-bond forces between the hydroxyl groups and ester oxygens with neighbour hyperbranched molecules.
Figure 1
The adsorption kinetics of Boltorn®H30 was followed by electrochemical techniques, taking into account or following the effect of the HPP layer on the charge transfer of a redox-active probe. Fig. 2, shows the cyclic voltammetry profiles of modified glassy carbon electrodes in Fe(CN)63+/Fe(CN)64+ solutions varying the incubation time. The presence of the HPP layer partially inhibits the charge transfer of the redox probe at lower incubation times, evidenced by a shift of the anodic and cathodic peaks to more positive and negative potential correspondingly, and a decrease of the coulometric charge up to a totally blocked behaviour observed at incubation times of about 24 hours.
Figure 2
EIS has been widely used to study the interfacial and transport properties of films and offers the opportunity to monitor the adsorption kinetics of the HPP with an enhanced sensibility regarding CV, with the aid of a redox-active label such as ferro/ferricyanide one. Fig. 3 shows typical electrochemical impedance spectra of modified electrodes varying the incubation time. The Nyquist plots show semicircles in the high frequency region and, a roughly 45o straight line in the low frequency one. The results were analyzed by applying non-linear least square fits with a Randles equivalent electrical circuit (see scheme in Fig. 3) including the solution resistance (Rs), the charge transfer resistance of the film (Rct), the double layer capacitance (Cdl) and Warburg impedance (W) associated with a semi-infinite linear diffusion. The charge transport mechanism might involve either movement of the electroactive ions by diffusion or hopping/tunnelling of electrons from one redox-site to a next one at fixed positions in the polymer chain.[25] In Fig. 3 (b), the dependence of Rct as a function of incubation time exhibits a continuous increase without reaching a plateau, in good accordance with the presence of multilayers.
Figure 3
Figure 4 shows the plot of Rct against concentration after incubation of the electrodes in Boltorn®H30/DMSO solutions for long times. The dependence of Rct on the HPP concentration, clearly exhibits an isotherm typically associated to the adsorption of porous solids,[26] in good agreement with the AFM image which exhibits an incomplete multilayers growth of HPP onto the carbon substrate after more than 4 hours of incubation.
Figure 4
3.2 Thermal properties of the HPP layers
Figure 5 shows the dependence of Nyquist plots as a function of temperature for GCE modified with Boltorn®H30. It is clearly observed that semicircles associated with the Rct values decrease rapidly upon heating above room temperature. The reorganization and higher mobility of polymer chains promotes the movement of redox probes closely to the proximity of the electrode surface and also the charge propagation across the film.
Figure 5
The transition temperature called Ttr was obtained from the intersection of two lines extrapolated from low and high temperature regions on the Rct against temperature plot, shown in Fig.6(a). For Boltorn®H30 films a Ttr value of 306 K is obtained, which is similar to the reported glass transition temperature (Tg = 308 K) of Boltorn®H30 measured by differential scanning calorimetry.[27] However, the phase transition in this case is completely different, as it will be described below. Fig. 6(b) shows the Arrhenius-type plot based on the data reported in Fig. 6(a), in which the dependence of ln(T/Rct) as a function of the inverse of the temperature (T−1) is shown. Differences between both thermal regimes, State 1 (low T regime) and State 2 (high T regime), are clearly manifested by a well-defined variation in the activation energy for the electron transfer process of the redox probe. From the slopes in Fig. 6(b), activation energies of Ea,State1 = 9.7 kJ/mol and Ea,State2 =44.4 kJ/mol were calculated for State1 and State2, respectively. These values clearly prove that phase transition from State 1 to State 2 is not a “glassy” to “rubbery” state transition, in spite of the similar values obtained for Ttr compared to the reported Tg one for Boltorn®H30 in bulk, showing that significant differences emerge when molecules are confined to distances comparable to their sizes. The phase transition from State 1 to State 2 could be explained as a reorganization of the HPP molecules from a porous multilayer to a more compact layer, probably due to a weakening of the existing H-bond network and, consequently, followed a major mobility of the polymer molecules due to heating, favouring the interaction between adsorbates and substrate (see scheme 2). Moreover, in contrast to Tg values reported for HPP in bulk, no significant differences are observed for Ttr values for different generation HPP immobilized on GCE, as it can be observed in table 1. Androulaki et al.[27] reported a significant dependence of Tg values of pure HPP depending on its generation, as a consequence of the molecular weight increase and higher degree of branching, and significant alterations in their behaviour under confinement in nanocomposites, where HPP are intercalated within the galleries of natural montmorillonite. Heating hyperbranched polyesters above Tg leads to a weakening of non-covalent interactions and, consequently, to a major mobility of molecules.
Scheme 2
Figure 6
Table 1
In order to gain insight into thermal properties of HPP layers, the reorganization of the porous layer after heating was investigated by AFM. Fig. 7 shows the topographical AFM images of modified carbon surfaces in three states: room temperature (State 1), immediately after heating up to 318 K (State 2), and after cooling down to room temperature again (State 3). Statistical parameters calculated from AFM images are depicted also in Fig. 7. The root mean square roughness (RMS) values are not too sensitive, but also denote changes around Ttr showing a slight decrease at temperatures above Ttr for HPP of generations 3 and 4, and a minor increment for generation 2. Surface skewness (Rsk) is a measure of the statistical distribution symmetry of a surface profile. A symmetric distribution should have skewness values near zero, which means that it has evenly distributed peaks and valleys in height.[28] Thus, the Rsk parameter gives an indication of the existence of deep valleys and/or sharp peaks. While surfaces with larger valleys than peaks present negative skewness values, quite the opposite, surfaces with higher peaks than valleys would be characterized by positive skewness values. From Figs. 7(a-c), it should be noticed that Rsk values are around zero at room temperature for generations 2 and 3, while for generation 4 is slightly negative. Rsk increase for generations 3 and 4, even becoming positive for generation 3 at temperatures above Ttr. On the contrary, for generation 2 it has an almost indiscernible increase. Surface kurtosis (Rku) is a measure of the “spikiness” of the surface, or the distribution of spikes above and below the mean line; it is also a measure of the randomness of surface heights and takes a value of 3 for the normal distribution. For Rku>3 the surface is dominated by sharp peaks, whereas if Rku<3 the peaks are bumpy.[29] In good agreement with EIS data, the topographical AFM images at temperatures above the Ttr exhibit an increase in the Rku values, denoting a reorganization of the HPP layer after the rupture of hydrogen-bonding between adsorbates. This change is greater for generation 4 than for generations 2 and 3, but always is reversible.
Figure 7
4. Conclusions
The self-assembly of hyperbranched polyester-polyol polymers immobilized on carbon surfaces were investigated by cyclic voltammetry, electrochemical impedance spectroscopy and atomic force microscopy, focusing on thermal transitions of the polymers under confinement. The adsorption of hyperbranched polymers with hydroxyl-terminal groups on the bare carbon surface followed a typical behaviour of an isotherm corresponding to a porous solid. The polymers form incomplete multilayers probably due to hydrogen-bond forces between adsorbates. The evolution of the nanometer-sized layers with temperature revealed a phase transition at ca. 306 K, for HPP of generations 2, 3 and 4. The phase transition can be explained as a film reorganization due to temperature effects on hydrogen-bonding. This feature is of particular relevance for further application of hyperbranched polyol-polymers as thin films or coatings.
5. Acknowledgements
We wish to express our gratitude to IFIS Litoral (SantaFe, Argentina) for the use of their SPM equipment. Financial support from CONICET (PIP 2012-2014 No577), ANPCyT (PICT 2015-2477), and SECYT-UNC are gratefully acknowledged.
E.D.F. thanks CONICET for the fellowship.
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List of Captions
Scheme 1. Schematic representation of the hyperbranched polymers, of generation 2, 3 and 4, employed in this work.
Figure 1. Topographical AFM images (1000 nm × 1000 nm) of HOPG incubated in 0.025 M Boltorn®H30/DMSO solutions for (a) 0.5, (b) 1.5, (c) 15.0, and (d) 270.0 minutes. (Accordingly, the full z scales of each image are 1.8, 4.5, 4.5 and 20.0 nm. The cross sections obtained along the lines in (a-d) are depicted in graphs (e-h), respectively).
Figure 2. Cyclic voltammograms for a GCE (solid line) and Boltorn®H30/GCE (dashed line) in 0.1 M PBS (pH 7) containing 2 mM K3Fe(CN)6 at 0.1 Vs-1 at increasing incubation times (pointed by the arrow): 1, 5, 15, 24, 60 and 1440 minutes.
Figure 3. (a) Scheme of the Randles equivalent circuit, (b) Nyquist plots for GCE (●) and Boltorn®H30/GCE ( ) in 0.1 M PBS (pH 7) containing 2 mM K3Fe(CN)6 at different incubation times: 2, 4, 17, 18, 28 and 68 hours (1, 5, 20, 30, 50, 60 minutes in insets); (c) Rct calculated from non-linear least square fits of the Nyquist plots employing the Randles equivalent circuit. Insets show enlarged graphics of the first region of each plot.
Figure 4. Rct estimated from Nyquist plots of Boltorn®H30/GCEin 0.1 M PBS (pH 7) containing 2 mM K3Fe(CN)6 at different concentrations of the Boltorn®H30/DMSO solution. Inset: enlarged graphic of the first region of the plot, and a topographical AFM image (2000 nm × 2000 nm) (full z scale 25.0 nm) acquired at the conditions pointed out by the arrow.
Figure 5. Dependence against temperature of Nyquist plots for GCE (●) and Boltorn®H30/GCE ( ) in 0.1 M PBS (pH 7) containing 2 mM K3Fe(CN)6.
Scheme 2. Idealized scheme of hyperbranched molecules at different temperatures: state 1 at room temperature and state 2 at temperature above Ttr.
Figure 6.(a) Rct as a function of temperature for Boltorn®H30 thin films on GCE.
Rct was obtained from non-linear least square fits of Nyquist plots shown in Fig. 5 employing the Randles equivalent circuit. (b) Arrhenius-type plot showing the dependence of ln(T/Rct) as a function of the inverse of temperature (T−1). At T = Ttr, a noticeable difference in the slope of the correlation line is observed, indicative of a higher activation energy, Ea.
Table 1. Thermal properties of Boltorn® polymers. *Tg values of HPP in bulk obtained by DSC and reported by Androulaki et al.[27]
Figure 7. Topographical AFM images (2000 nm × 2000 nm) of HOPG modified by
((a) Boltorn®H20 (Generation 2) (full z scale 6.5 nm), (b) Boltorn®H30 (Generation 3) (full z scale 4.5), and (c) Boltorn®H40 (Generation 4) (full z scale 4.0) at State 1 (room temperature), State 2(immediately after heating up to 318 K), and State 3 (after cooling down to room temperature), respectively. The plots on the right illustrate the statistical parameters calculated from AFM images acquired at each state.