Article pubs.acs.org/Macromolecules
Rheological Consequences of Hydrogen Bonding: Linear Viscoelastic Response of Linear Polyglycerol and Its Permethylated Analogues as a General Model for Hydroxyl-Functional Polymers Carina Osterwinter,† Christian Schubert,‡ Christoph Tonhauser,‡ Daniel Wilms,‡ Holger Frey,*,‡ and Christian Friedrich*,† †
Freiburg Materials Research Center (FMF), University of Freiburg, Stefan-Meier-Str. 21, D-79104 Freiburg, Germany Institute of Organic Chemistry, Organic and Macromolecular Chemistry, University of Mainz, Duesbergweg 10-14, D-55128 Mainz, Germany
‡
S Supporting Information *
ABSTRACT: Viscoelastic properties of linear, hydroxyl-functional polymers are only little understood with respect to the effect of functional group interactions. Melt rheology and thermal phase transitions of linear polyethers (polyglycerol, linPG-OH) and their methylated analogues (linPG-OMe) in a broad molecular weight range (Mn = 1−100 kg/mol) with low polydispersities (PDI) have been investigated as a general model for hydroxyl-functional polymers with respect to their functionality and hydrogen bond interactions. We provide detailed insight into the rheodynamics of nonentangled and well-entangled polyethers bearing one functional group per monomer unit. Booij−Palmen plots (BBP) revealed failure of the time−temperature superposition principle (TTS) for both types of polymers in the segmental relaxation region, while TTS holds in the terminal relaxation region. The characteristic modulus of linPG-OMe derived from the BBP clearly reflects the transition from the nonentangled to the fully entangled state with increasing molecular weight. Quantitative analysis of these data allows for different estimates of the entanglement molecular weight, which is approximately 14 kg/mol. In case of linPG-OH a lower apparent entanglement molecular weight (8 kg/mol) leads to estimated 36 entanglement interactions in a cube of 10 nm edge length together with 47 association sites in the same volume. This can be determined from the molecular-weightindependent plateau modulus only, which is significantly lower than for linPG-OMe. This is explained as a consequence of the overlay of an entanglement network and an association network created by hydrogen bonding of the OH groups with themselves and with the ether linkages.
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g/mol were reported by Brooks et al.4 Recently, a systematic thermorheological investigation of a series of hbPGs5 with a broad range of molecular weights and similar degrees of branching was published by Tonhauser et al.6 A transition to an entangled regime at high molecular weights was observed for molecular weights exceeding 80 000 g/mol, and an empirical equation was derived to describe the η0−M relation. Because of the lack of information on the entanglement molecular weight of this polymer, a quantitative discussion of the results is still missing. Thus, for a detailed understanding of these hyperbranched structures in melt as well as in solution,7 it is essential to investigate their linear analogues. Surprisingly, only little research has targeted a detailed understanding of the rheological and thermal behavior of linear polyglycerol to date. This is certainly due to the fact that high
INTRODUCTION It is well-known that polymer architecturefor instance linear versus branched topologytogether with polarity, type, and number of functional groups determines the rheological properties of polymers.1 A special class of such polymers that permit to study these effects in a systematic and fundamental manner are linear (lin) and hyperbranched (hb) polyglycerols (PG).2 These materials combine low glass transition temperatures and the typical high backbone flexibility of polyethers, such as poly(ethylene glycol) (PEG), with a high number of functional groups, as also present in established polyols, such as poly(vinyl alcohol) (PVA). In view of their promising properties in biomedical applications, linear polyglycerols attract increasing interest.3 Since linPG is a good alternative to poly(ethylene glycol) with respect to several biomedical and pharmaceutical applications,3 it is essential to understand its thermorheological properties. First rheological studies of its hyperbranched analogue, hbPG, with molecular weights exceeding 700 000 © 2014 American Chemical Society
Received: August 15, 2014 Revised: November 14, 2014 Published: December 18, 2014 119
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molecular weight linPG has been made accessible only recently by monomer-activated anionic polymerization.8,9 In an even more general context, to the best of our knowledge no systematic thermorheological studies of linear, polyhydroxyl functional polymers in melt have been reported to date as well. The structurally related poly(vinyl alcohol), a commercial polymer that has been known for a long time, is not suitable for such studies due to its high melting point (523 K10) close to its decomposition temperature (513 K11). To the best of our knowledge, rheological research on other established hydroxylfunctional polymers, such as poly(hydroxystyrene) or poly(hydroxyethyl methacrylate) (PHEMA), has not been reported. Existing reports focus on application-related aspects only (contact lenses in the case of PHEMA and photoresists in the case of PHS). Moreover, the latter polymers show high glass transition temperatures (100 °C in the case of PHEMA12 and up to 180 °C in the case of PHS13), and thus in the relevant temperature ranges hydrogen bonds are expected to be weak and short-termed. The atactic chain structure of linear polyglycerol results in an amorphous material with low glass transition temperature. This allows for facile rheological handling and renders polyglycerol an ideal representative of functional polymers with interacting functional monomer units in general. Clearly to be distinguished from the current work, a number of studies have focused on model polymers with a low number of hydrogen-bonding units, introduced deliberately. Following a pioneering study on the influence of hydrogen bonding on the melt viscosity of styrene/methacrylic acid copolymers,14 Tobolsky and Shen were able to show at the example of a series of functional poly(methacrylate)s (poly(2-hydroxyethyl methacrylate), poly(n-propyl methacrylate), and a 50/50 random copolymer) that the rubbery plateau modulus was significantly higher than “might have reasonably been predicted”.15 This “discrepancy” was attributed to hydrogen bonds. H-bonds in the mentioned polymers belong to the class of weak bonds with association energies in the range of 20 kJ/mol and weak directionality.16 To study the influence of Hbonding on the viscoelasticity of linear poly(butadienes) more rigorously, Stadler and co-workers introduced a low fraction of 4-phenyl-1,2,4-triazoline-3,5-dione (urazole) units.17 The authors observed broadening of the rubbery plateau zone, an increase in zero shear viscosity, steady state recoverable compliance, and activation energy of flow with increasing urazole content.18,19 Moreover, the thermorheological properties of these melts could be described using a horizontal shift factor following the WLF equation and a negligible vertical shift factor.19 The influence of the latter was on the order of T/T0, with T0 being the reference temperature. Subsequently, several research groups extended this approach by using different polymer backbones and more complex interaction motifs.20−23 An overview of the structural diversity of the H-bond motifs employed is given in an excellent review article by Binder and colleagues.16 In the most recent works on this topic it was demonstrated that for nonentangled H-bond forming polymers the strength of the H-bonds changes the dynamics of these materials from “nonentangled” to an “apparently entangled” state, going from weakly (25 kJ/mol for acrylamidopyridine group) to strongly (70 kJ/mol for ureidopyrimidinone acrylate) interacting groups.23 In all cases, the temperature dependence of the material parameters was such that time−temperature superposition (TTS) holds fairly well. In particular, the temperature dependence of the characteristic moduli, especially
the plateau modulus, seems to be no strong function of temperature. For many polymers, bT (= G(T)/G(T0), the vertical shift factor) is chosen identical to one17,22,23 or proportional to T/T0.18,19 In contrast to these studies, in the current work we elucidate the entanglement behavior of a typical functional polymer, bearing an interacting, weakly H-bonding moiety at every monomer unit with respect to its associating interactions. For this purpose, linear polyglycerol (linPG-OH, Figure 1, left) was
Figure 1. Structural illustration of linear polyglycerol (left) and its permethylated analogue (right).
chosen. To be able to account for the specific interactions, we also analyze the fully methylated, perfect analogues of these linear polymers, thereby “switching off” the hydrogen bonds (linPG-OMe, Figure 1, right). The comparative characterization of rheological properties will help to assess a key issue, namely the influence of associations on the entanglement behavior of these polymers.
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EXPERIMENTAL PART
Reagents. All solvents and reagents were purchased from Acros Organics or Sigma-Aldrich and used as received, unless otherwise stated. Ethoxyethyl glycidyl ether (EEGE) was prepared as described by Fitton et al.24 Glycidol (96%) and EEGE were purified by vacuum distillation over CaH2 directly prior to use. Toluene was distilled over sodium prior to use. DMSO-d6 was purchased from Deutero GmbH. Synthesis of Linear Polyglycerols. linPG-OH was obtained by two different synthetic protocols, depending on the molecular weight targeted. linPG-OH with low molecular weight (Mn ≤ 3000 g/mol) was prepared according to the procedure established by Dworak et al.24 linPG-OH with elevated molecular weights up to 100 000 g/mol was prepared according to a procedure published recently by Deffieux et al.9 In the latter case, full removal of the catalyst and initiator salt was achieved by consecutive washing of the reaction mixture with saturated NaHCO3 solution, NaCl solution (10%), and water. After acidic cleavage of the acetal protecting group, the polymers were precipitated twice in tetrahydrofuran (THF). All samples were dried accurately in vacuo at 80 °C for 2 days and stored in a drier in vacuo at room temperature. Methylation of Linear Polyglycerol. The permethylation of linPG-OH giving linPG-OMe was carried out according to a procedure described previously.26,27 To a solution of polyglycerol in water (0.32 g/mL), NaOH (5.8 equiv per OH group) was slowly added. To the turbid solution tetrabutylammonium bromide (TBAB) (10 mol %) and tetrahydrofuran (THF) (3.14 mL per 1 g polymer) were added prior to the addition of methyl iodide (MeI) (1.5 equiv per OH group) under vigorous stirring. After 3 h further MeI (0.2 equiv per OH group) was added, and the mixture was stirred overnight. The reaction mixture was extracted with CHCl3 (3 × 5 mL) and dried over CaCl2. Concentration and centrifugation of remaining NaI gave the permethylated linear polyglycerol as a light yellow oil in yields between 70% and 90%. All samples were dried accurately in vacuo at 80 °C for 2 days and stored in a drier in vacuo at room temperature. NMR. 1H NMR spectra were recorded on a Bruker AC at 300 MHz. The degree of modification of linPG-OH was determined via 1H NMR by comparing the decreasing hydroxyl signal of the linPG-OH starting material with the modified samples. SEC. SEC measurements were performed in DMF at 50 °C for all PG samples, using PS standards. SEC measurements are only comparable to a very limited extent, as the polymer standard differs 120
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from PG in their structure. Therefore, the molecular weights of linPGOMe were calculated on the basis of the degree of modification and the molecular weight of the linPG-OH starting materials. IR. IR spectra were recorded on a Bruker Vektor 22 FT-IR in a wavelength range between 600 and 4000 cm−1. All spectra were measured with a resolution of 4 cm−1. Room temperature behavior as well as temperature ranges up to 393 K were investigated in the measurements. DSC. DSC measurements were carried out with a PerkinElmer DSC7 and confirmed using a PerkinElmer Pyris1 instrument. The temperature range was chosen from 153 K (DSC7) and 183 K (Pyris1) up to 323 K, employing a heating rate of 10 K/min. The glass transition temperature (Tg) was determined from the maximum of the first derivative of the second heating rate. Rheology. Rheological properties were measured on a Paar Physica MCR-301 under nitrogen with an 8 mm plate−plate geometry. Temperature-dependent oscillatory measurements were carried out in a frequency range of 0.1−100 rad/s with deformations of 2% for the highest temperatures down to 0.01% at temperatures close to Tg. The temperature range was individually chosen for every sample, starting at 263 K for the highest molecular weight sample as low as 203 K, depending on the individual glass transition temperature Tg of the samples. Temperature steps of 10 K were used at high temperatures, and steps of 2 K at temperatures close to Tg were chosen for the oscillatory measurements. Master curves were constructed by shifting of isotherms relative to a reference temperature (here Tg + 50 K) using the commercially available software IRIS Rheo-Hub software.28 Data Analysis. Usually, the Ueberreiter−Kanig equation
Tg = Tg∞ −
a M
EaT = Ea∞
is the activation energy at infinite temperature: In this equation, 0 0 E∞ a = 2.303Rc1c2. The determination of characteristic moduli from the master curves is based on the applicability of the Booij−Palmen approach.32 According to this method, a characteristic modulus Gc is determined as that storage modulus, for which a minimum in the δ(|G*|) curve is observed. The resulting characteristic moduli, which can be correlated with the entanglement modulus Ge, or the plateau modulus, G0N, are related to the entanglement molecular weight, Me, and density ρ by the following equation:33 Ge =
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c10(T − T0) c01
(2)
c02
In this equation and are the two WLF parameters determined at the reference temperature T0. A parameter characterizing the sensitivity of molecular dynamics to changes in temperature is the fragility m.31 Using zero shear viscosity as the probe of molecular dynamicsthis corresponds to the so-called “fragility of chain relaxation” mn used in the mentioned reference together with WLF equation, the following expressions for m can be found:
m=
Tgc1g c 2g
= c10c 20
Tg 2
(Tg − TV )
=
Tg 1 ln(10)fg Tg − TV
(3)
In these equations, TV is the Vogel temperature, a reference temperature invariant of the WLF equation, which is related to WLF parameters in the following way: TV = T0 − c02. fg is the fractional free volume at glass transition temperature and related to cg1 by the following equation: fg =
1 ln(10)c1g
ρRT 5 0 GN = 4 Me
(6)
RESULTS AND DISCUSSION A. Synthesis and Molecular Characterization. linPG is commonly synthesized by anionic ring-opening polymerization (AROP) of ethoxyethyl glycidyl ether (EEGE), as first described by Taton et al.34 EGGE is the most popular monomer for the synthesis of linPG due to the mild acidic cleavage of the acetal protecting group. Dworak et al. demonstrated the use of potassium or cesium alkoxides as initiators for the polymerization of EEGE.25,35 In 2008 Deffieux, Carlotti and co-workers published an elegant method to obtain higher molecular weights of P(EEGE), using tetraoctylammonium bromide (TOAB) as an initiator and triisobutylaluminum (i-Bu3Al) as monomer activator.9 For this study, a systematic series of linear polyglycerols with molecular weights from 1000 up to 100 000 g/mol with narrow and consistent polydispersities (PDI) have been prepared, relying on established literature procedures (Table 1).9,25 For rheological characterization full removal of catalyst and initiator salt was essential. This was achieved by consecutive washing with several aqueous solutions and precipitation in THF after acidic cleavage of the protection group. Full catalyst and initiator removal was confirmed by 1H NMR spectra (see SIFigure 1). Permethylation of all linPG-OH samples was carried out under optimized phase transfer conditions, following a published method.26,27 To this end, an aqueous sodium hydroxide solution and THF with TBAB as phase transfer catalyst was used. For all samples molecular weights were obtained from SEC measurements using polystyrene standards (Table 1). 1 H NMR spectroscopy in DMSO-d6 was used to ensure quantitative conversion of all hydroxyl groups in the methylation step (Figure 2, right). The OH protons, which are completely absent in the case of all linPG-OMe samples, are located in the region around δ = 4.5 ppm, while the signals of the polymer backbone appear between δ = 3.2 and 3.7 ppm. The temperature dependence of the OH signals for the linPGOH with a molecular weight of 7000 g/mol was qualitatively investigated by 1H NMR spectroscopy as well. Figure 2 (left) shows a significant upfield shift of the OH protons with a simultaneous decrease of signal intensity with increasing temperature. This is a result of decreasing Hbonding with increasing temperature and the resulting shielding of OH protons.36 For temperatures exceeding 400 K there is no temperature dependence of H-bonds observable in solution. B. Solid State Properties. Since interaction of the hydroxyl groups plays a key role for the viscoelastic behavior of PG, as
(1)
c 20 + T − T0
(5)
E∞ a
is used to account for the influence of molar mass M on glass transition temperature Tg.29 In this equation, T∞ g is the glass transition temperature at infinite molecular weight and a is a constant that fixes a characteristic molar mass, for which the plateau value is reached. To construct master curves, the single isothermal frequency sweeps are shifted horizontally by the shift factor aT. The temperature dependence of that shift factor is described with the empirical Williams−Landel−Ferry (WLF) equation:30
log aT = −
T2 (T − TV )2
(4)
Another material function closely related to temperature dependence of molecular dynamics is the activation energy of viscous flow, Ea.30 If this quantity is calculated by the help of the WLF equation, a temperature-dependent activation energy results: 121
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decreasing bond strength of the OH bonds and attenuation of hydrogen bonds. Consequently, higher energies are required to excite OH-bond vibrations. Interestingly, no complete loss of H-bonds was observed in this temperature range. Both in solution by NMR and in the solid by IR spectroscopy qualitative evidence on the strength of hydrogen bonding interactions was obtained. While the NMR spectra show a quantitative proton exchange in solution at elevated temperature, the FTIR spectra suggest a continuous decrease of Hbonding efficiency. The glass transition temperatures of all polymers measured by DSC are presented in dependence of the number-average molecular weight Mn in Figure 4. As expected, due to the polymer’s atactic chain architecture, the glass transition temperature is the only characteristic temperature of these polymers. No crystallization occurs for all PG samples. As expected and clearly visible in Figure 4, with increasing molecular weight Tg approaches a plateau. Using eq 1, the glass transition temperature for infinite molecular weight, T∞ g , is established: 262 K for linPG-OH and 213 K for the methylated linPG-OH analogues. As will be detailed below, the determined characteristic temperatures depend on the heating rate of the DSC experiment (in our case: 10 K/min). Glass transition temperatures determined for low molar mass PGs by rheological means (temperature, at which the zero shear viscosity exceeds 1012 Pa·s) are lower by 10 K and correspond to significantly lower heating rates (
