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Chemical and Dynamical Processes in Solution; Polymers, Glasses, and Soft Matter

First Clear cut Experimental Evidence for a Glass Transition in a Polymer with Intrinsic Microporosity – PIM-1 Huajie Yin, Yeong Zen Chua, Bin Yang, Christoph Schick, Wayne J. Harrison, Peter Martin Budd, Martin Böhning, and Andreas Schönhals J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00422 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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The Journal of Physical Chemistry Letters

First Clear-cut Experimental Evidence for a Glass Transition in a Polymer with Intrinsic Microporosity – PIM-1 Huajie Yin1, Yeong Zen Chua2, Bin Yang2, Christoph Schick2,3, Wayne J. Harrison4, Peter M. Budd4, Martin Böhning1, Andreas Schönhals1,* 1

Bundesanstalt für Materialforschung und –prüfung (BAM), Unter den Eichen 87, 12205

Berlin, Germany 2

University of Rostock, Institute of Physics and Competence Center CALOR, Albert-Einstein-

Str. 23–24, 18059 Rostock, Germany 3

Kazan Federal University, 18 Kremlyovskaya Street, Kazan 420008, Russian Federation

4

School of Chemistry, University of Manchester, Manchester M13 9PL, U.K.

*

CORRESPONDING AUTHOR: A. Schönhals, BAM Bundesanstalt für Materialforschung

und -prüfung (Fachbereich 6.6), Unter den Eichen 87, 12205 Berlin, Germany; Tel. +49 30 / 8104-3384; Fax: +49 30 / 8104-1617; Email: [email protected]

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ABSTRACT: Polymers with intrinsic microporosity (PIMs) represent a novel, innovative class of materials with great potential in various applications from high-performance gas separation membranes to electronic devices. Here for the first time, for PIM-1, as the archetypal PIM, fast scanning calorimetry provides definitive evidence for a glass transition (Tg=715 K, heating rate 3·104 K/s) by decoupling the time-scales responsible for glass transition and decomposition. As the rigid molecular structure of PIM-1 prevents any conformational changes, small-scale bend and flex fluctuations must be considered the origin of its glass transition. This result has strong implications for the fundamental understanding of the glass transition and for the physical aging of PIMs and other complex polymers, both topical problems of materials science. TABLE OF CONTENTS GRAPHIC:


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Since the advent of polymers with intrinsic microporosity, with the synthesis of PIM-11 (the polycondensation product of 5,5',6,6'-Tetrahydroxy-3,3,3',3'-tetramethyl-1,1'-spirobisindane and 1,4-dicyanotetrafluorobenzene, see Supporting Information) about 15 years ago, this new class of polymers initiated a substantial amount of research effort, besides various others especially in the field of gas separation membranes.2,3 The outstanding gas transport properties of PIMs in general are due to a distinct intrinsic microporosity, with corresponding pore sizes in the range of 5.2 to 10.7 Å.4,5 These pore sizes are consistent with a possible continuous free volume phase deduced from atomistic molecular dynamic simulations.5,6 The microporosity of polymers of intrinsic microporosity1 (BET surface areas > 700 m2/g) is due to their rigid, ladder-like chain architecture preventing segmental rotations. So far it was impossible to determine a glass transition temperature Tg for PIMs unambiguously, being a key characteristic of polymers determining their possible applications. Scientifically the question is raised whether such rigid polymers can undergo a glass transition at all. This aspect is essential for the fundamental understanding of the glass transition and also for the physical aging of PIMs and other complex polymers, both current issues in materials science. 79It should be noted that a deeper general understanding of the glass transition is of interest not only for polymeric or silicate glasses but also for metallic glasses, new types of electrolytes (e.g. ionic liquids) and also for biological matter like proteins. Despite the intensive research during the last decades this phenomenon is still a long-term unsolved issue in physical chemistry and materials sciences. In contrast to high-free volume polymers developed earlier, such as polyacetylenes like poly(trimethylsilyl propyne) (PTMSP)10, PIMs exhibit a favorable combination of high gas permeabilities with good permselectivities, being the basis for high-performance membrane materials for gas separation. Such membrane processes are regarded as a key technology for energy efficient separations in processing of natural gas and biogas as well as in the chemical industry. They constitute a green alternative compared to conventional separation techniques, which are usually much more energy consuming. For air and hydrogen separations, PIMs are already established as state-of-the-art membrane materials.11 Recently also the interesting optical and electronic properties of PIMs have been addressed.12-14The aromatic moieties, providing a certain electron mobility, together with the intrinsic microporosity, allowing for a partially free movement of charge carriers, both contribute to the electrical conductivity of PIMs. This was also proven by broadband dielectric spectroscopy (BDS) revealing a considerable conductivity deep in the glassy state.15 These properties could make PIMs an innovative choice for the next-generation of electronic devices, for instance OLEDs12 or parts


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of battery electrodes.13,16 Due to their intrinsic microporosity PIMs are also promising materials for supercapacitors.17 For any application the material properties have to be stable over time. Physical aging – which leads to time dependent properties – is a critical issue with regard to long-term performance or stability18 which PIMs share with other rigid superglassy or microporous polymers, such as polyimides19 or polynorbornenes.20 For PIM-1 it was shown that physical aging reduces dramatically its gas permeability2 which is ascribed to a partial collapse of the microporous structure.21 Physical aging is closely related to the molecular mobility and to the glass transition phenomenon. Utilizing conventional DSC measurements and dynamic-mechanical thermal analysis (DMTA) no glass transition temperature Tg could be definitively observed for PIMs before the onset of thermal decomposition.22 Data of a glass transition temperature of 709 K (436 °C) at a heating rate of 10 K/min reported in ref.23 for PIM-1 are not convincing enough and probably overlaid by chemical degradation found to set-in at temperatures below 673 K (400 °C) by thermogravimetric analysis in nitrogen atmosphere. 15 Also the temperature dependence of the heat flow does not show the typical step-like change for a glass transition and the reported values of the heat capacity step at Tg (ca. 18 J/g K) are much too high for a glass transition and rather in the order of magnitude expected for a degradation endotherm. However, DMTA results reported in ref.22 show slight upturns in the tensile loss modulus and the loss tangent above 573 K (300 °C) up to 623 K (350 °C) which might indicate the beginning of a glass transition process located at temperatures above the decomposition temperature. In conventional polymers, the glass transition is related to cooperative conformational dynamics of segments. As PIM-1 has a ladder-like stiff chain structure (see Figure 1a) with a spiro-center as site of contortion, which decreases the number of conformational degrees of freedom dramatically and prohibits segmental rotations, PIM-1 might be expected not to exhibit a glass transition. Recently, it was shown by BDS that the structure of PIM-1 allows for some molecular mobility15. A so-called β*-relaxation process with an Arrhenius-like temperature dependence of the relaxation times with a high activation energy was observed. This relaxation process of PIM-1 is non-cooperative in nature. An a-relaxation process related to the glass transition is not found up to 523 K (250 °C). Moreover, computer simulations have revealed that spirobisindane (SBI) and dioxane linkages (see Figure 1a) in PIM-1 can bend and flex to a considerable extent 24. These processes might provide enough mobility to manifest a glass


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transition in PIM-1. Furthermore, a glass transition taking place in the temperature range around 701 K (428 °C) was also predicted by computer simulations 25. Here, for the first time, fast scanning calorimetry (FSC) reaching heating/cooling rates of several thousand K/s 26 is employed to investigate the thermal behavior of PIM-1. At these high heating rates, the sample is subjected to high temperatures only for a few milliseconds, avoiding thermal decomposition related to longer time scales. In a series of preliminary experiments in which heating runs were performed with stepwise increase of the upper temperature limit, the temperature range of the glass transition could be located between 600 and 850 K (for details see Supporting Information). As shown in Figure 1b, no indication of a glass transition is observed until the 4th heating (up to 700 K). In the 5th heating cycle, the onset of glass transition is noticed as an endothermic step-like change of the heat flow, and it develops fully in the subsequent runs. Such an endothermic step-like change of the heat flow is the characteristic signature of the glass transition observed during a differential scanning calorimetry experiment. Microscopic pictures of the PIM-1 sample on the sensor chip taken after each heating supports the conclusions drawn from the calorimetric data (see Figure S5 in the Supporting Information).

a)

b)

Figure 1. (a) Chemical structure of PIM-1 indicating the SBI and dioxane linkages; (b) first series of FSC heating runs with stepwise increase of the upper indicated temperature limit (heating rate 3·104 K/s). The detected Tg is indicated by an arrow. The curves are shifted along the y-scale for clarity. The peak-like signal in the heat flow at temperatures around 525 K is caused by processes reaching the stationary state at the beginning of the scanning experiment.


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Then a series of three complete heating/cooling cycles up to 873 K was performed on a fresh sample (Figure 2a). Both series were performed at 3·104 K/s, i.e. a complete heating run from room temperature up to 873 K takes only about 0.03 s. Deviations observed during the first heating may be mainly ascribed to changes of the thermal contact between the sensor chip and the sample, but residual traces of solvent or internal stresses may also have had an influence. The following two cycles overlap completely, confirming the reproducibility of the Tg measurement and the absence of decomposition. This confirms a glass transition temperature of 715 K (442 °C), which is also near to the temperature range of the prediction in ref. 25 In ref.25 the calculations were carried out by molecular dynamics and grand canonical Monte Carlo simulations. Although in the reported calorimetric experiments rather high heating rates are employed, a large gap in the time scales of the experiment and the simulation still remains. Even after nine subsequent heating/cooling cycles no signs of thermal decomposition were observed (see Supporting Information). After these nine measurements, the sample can be completely dissolved from the sensor, indicating that no crosslinking took place. From FTIR spectra taken directly from the sample on the sensor, before and after heating, it was further confirmed that no degradation occurred during the repeated heating cycles (see Supporting Information). Figure 2c and d display microscopic pictures taken of the PIM-1 sample on the sensor chip before and after the measurements, respectively. After heating to temperatures higher than Tg, the visible size of the material became smaller and its edges become smoother. An animation prepared from microscopic images taken before and after heating cycles shows the shrinkage of the thin film on the sensor (see Supporting Information, Animation S1). Such shrinkage can only be explained by a softening of the sample induced by the glass transition.


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a)

c)

b)

d)

Figure 2. (a) Heat flow data for 3 consecutive heating / cooling cycles up to 873 K at a heating rate of 3·104 K/s; (b) Heat flow data for 2nd and 3rd heating run (2H, 3H) revealing a glass transition (curves were shifted along the heat flow axis for clarity); Microscopic images of the PIM-1 sample on the sensor chip obtained at room temperature (magnification 20´) before (c) and after (d) three heating runs to 873 K. The blue shape indicates the contour of the material before the measurement. The black spots in the images are optical effects and have nothing to do with the heating process as they appear in a similar way before and after heating. The observed Tg value is considerably high, which is consistent with the rigid structure of PIM-1 resulting in very limited molecular fluctuations. Furthermore, the temperature range of the glass transition for PIM-1 is extraordinarily broad (dT~150 K) compared to conventional polymers, where dT is found to be of the order of 10 to 20 K for comparable heating rates.27 This quite broad glass transition range cannot be explained by a thermal lag, due to the sample thickness (see Supporting Information). Also, the increment in the heat flow corresponding to the change of the specific heat capacity at the glass transition is small. In the cooperativity


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approach the glass transition is related to a cooperativity length scale x. 28 In the fluctuation theory to the glass transition 29 x can be roughly estimated by 2

𝜉≈#

+ ) ,/ (1&)(

$% &'( ∆(

.

(1)

kB is the Boltzmann constant and r the density, D(1/cp) is the step of the reciprocal specific heat capacity at Tg, where cV»cp was assumed. dT is the width of the glass transition. From Equ. 1 it is inferred that the value of x is mainly determined by dT. Because dT is large, it has to be expected that x is quite small (estimated< 1 nm, see Supporting Information). As conformational changes are to be excluded as the origin of the glass transition in PIM-1, it must be concluded that the small-scale bend and flex fluctuations evidenced by molecular dynamics simulations are responsible for the glass transition of PIM-1 on a molecular level. This line of argument is in agreement with the small expected cooperativity length scale. These small-scale bend and flex fluctuations can be considered as a completely new mechanism for glassy dynamics, alongside with molecular rotational fluctuations, segmental transitions, or orientational degrees of freedom.29 Finally, experiments with different heating rates (from 1·104 to 4·104 K/s) were carried out to prove whether Tg shifts to higher values with increasing heating rates, as expected for a glass transition. This is confirmed by Figure 3a showing the expected shift of Tg. This shift also proofs that the observed response originates from the sample and is not an artefact of the measuring device. In Figure 3b the heating rate dependence of the glass transition temperature is presented in a so-called activation diagram, i.e. log(heating rate) vs. 1000/Tg. In general, for a glass transition the heating rate dependence of the glass transition temperature should be curved when plotted in the activation plot and should follow the Vogel/Fulcher/Tammann (VFT-) dependence. 29 Here a linear dependence is obtained. This linear dependence can be understood taking into consideration the narrow range of heating rates which can be covered experimentally. By fitting the Arrhenius equation to the data, an apparent activation energy of ca. 55 kJ/mol is deduced. At first glance, this seems to be a low value for a glass transition. First, one has to consider that the rather large error in the estimated glass transition temperature of ± 20 K (see Supporting Information) leads also to large errors of the activation energy for which a lowest value of 30 kJ/mol and a highest value of 288 kJ/mol may be estimated. This means that the deduced value is only a crude estimation. Second, to rationalize this result further, one has to consider that for the employed fast heating rate the curvature of the VFT-dependence is expected to be low. Moreover, a low value of the apparent


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activation energy is also in agreement with a quite strong behavior in the sense of the fragility classification8 of glass formation as anticipated for a small value of the cooperativity length scale. Strong glass formers are materials for which the glassy dynamics is closer to an Arrhenius-dependence, whereas for fragile ones a pronounced VFT-dependence is found. The majority of polymers are classified as fragile glass formers. This is assumed to be related to the underlying cooperative conformational dynamics of segments30, a mechanism which is structurally not possible for PIM-1. The strong behavior observed for PIM-1 further supports the conclusion that the glass transition of PIM-1 is due to localized fluctuations rather than segmental motions. In further, more time-consuming experiments, the influence of physical aging on the thermal response of PIM-1 will be investigated in more detail (see also references31-34).

a)

b)

Figure 3. (a) FSC curves for consecutive heating runs at indicated heating rates; (b) log(heating rate) vs. inverse Tg. Error bars were estimated based on DTg=±20 K, partly due to the broad and weak glass transition. A deeper consideration of the errors is given in the Supporting Information. In conclusion, clear-cut experimental evidence of the glass transition of PIM-1 has been provided by fast scanning calorimetry, which avoids thermal decomposition through ultrafast heating rates. A glass transition temperature is determined to be ca. (715 ± 20) K (442 °C, heating rate of 3·104 K/s). Due to the rigid structure of PIM-1 and the resulting strong limitation of conformational degrees of freedom, the observed glass transition must be ascribed to a local bend and flex fluctuation rather than a cooperative segmental motion as in conventional polymers. In order to establish a general understanding of the underlying ACS Paragon Plus Environment

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structure property relationships, this has to be proven in the future for other rigid polymers, e.g. other PIMs35, polynorbornenes36 or microporous polyimides37 - preferentially by a combination of calorimetric methods with methods directly addressing the dynamic behavior, such as broadband dielectric spectroscopy15 or neutron scattering techniques38 complemented with theoretical and modelling approaches.


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Experimental Methods Synthesis of PIM-1 The synthesis of PIM-1 was carried out according to the procedure reported in ref.15. More details are given in the Supporting Information. Fast Scanning calorimetry (FSC) The FSC measurements were performed based on a thin film chip sensor XI-415 of Xensor Integration (Netherlands) by using a recently developed differential power compensated calorimeter. Detailed information about the measurement principle and instrumental setup can be found in the Supporting Information. Ultrahigh heating and cooling rates of approximately 1·106 K/s can be reached in the case of low sample mass and large changes of the heat capacity at the glass transition under optimized conditions. For the samples investigated here, the heat flow can be measured at heating rates up to 4·104 K/s. The heating rate is controlled precisely whereas the cooling is ballistic. This means that the cooling rates can be higher than the heating rates, which is in agreement with recent literature.34 The temperature calibration of the fast scanning calorimetry was conducted according to ref. A, with standard metals of indium, tin, lead, and zinc, as well as with potassium sulphate for the high temperature region.39 A small piece of PIM-1 film was placed on the heating area of the sensor. Good thermal contact between sensor surface and sample was established during the measurements. Test experiments were carried out with Argon and air as surrounding atmospheres. Because no difference in the results was found for both atmospheres, all measurements reported here were carried out in air atmosphere. Fourier transform infrared spectroscopy (FTIR) The thin film sample mounted on the chip sensor was characterized before and after heating up to 873 K by FTIR using a spectrometer (VERTEX 70, Bruker, Germany) coupled with a FTIR-microscope (HYPERION 3000, Bruker, Germany). The position of the sample was adjusted microscopically. The spot area for incident infrared irradiation was set to 50*50 µm2. Spectra were recorded in reflection mode in a spectral range between 600 and 4000 cm−1 with a spectral resolution of 2 cm−1 at a scan rate of 180 scans. Temperature Modulated Differential Scanning Calorimetry (TMDSC) To estimate the specific heat capacity of PIM-1 in the glassy sate TMDSC measurements were carried out using a DSC 8500 (Perkin Elmer, USA) utilizing the step scan mode. More details are given in the Supporting Information. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Details of synthesis, characterization and sample preparation of PIM-1; further details of FSC and TMDSC measurements, estimation of the cooperativity length scale, and microscopic images of the sample during heating including a short animation; FT-IR spectra of the sample before and after FSC measurements. (PDF) AUTHOR INFORMATIION Corresponding Author: *E-mail: [email protected] ORCID: Huajie Yin:

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Yeong Zen Chua: Bin Yang: Christoph Schick Peter M. Budd Martin Böhning Andreas Schönhals

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0000-0003-1375-3629 0000-0001-5534-9187 0000-0001-6736-5491 0000-0003-3606-1158 0000-0001-9753-345X 0000-0003-4330-9107

Notes: The authors declare no competing financial interests. ACKNOWLEDGMENTS We thank A. Hertwig and G. Hidde for their help with the FTIR measurements. P. Szymoniak is acknowledged for the TMDSC measurements. W. J. Harrison is supported by EPSRC grant EP/K016946/1 “Graphene-based membranes”. C. Schick acknowledges financial support from the Ministry of Education and Science of the Russian Federation (Grant 14.Y26.31.0019)


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(17) Ravi, S.; Gopi, C. V. V.; Kim, H. Enhanced electrochemical capacitance of polyimidazole coated covellite CuS dispersed CNT composite materials for application in supercapacitors. Dalton Trans. 2016, 45, 12362-12371. (18) Hodge, I. M. Physical Aging in Polymer Glasses. Science 1995, 267, 1945-1947. (19) Swaidan, R.; Ghanem, B.; Litwiller, E; Pinnau, I. Physical aging, plasticization and their effects on gas permeation in “rigid” polymers of intrinsic microporosity. Macromolecules 2015, 48, 6553-6561. (20) Yin, H.; Chapala, P.; Bermeshev, M.; Schönhals, A.; Böhning, M. Molecular mobility and physical aging of a highly permeable glassy polynorbornene as revealed by dielectric spectroscopy. ACS Macro Lett. 2017, 6, 813-818. (21) McDermott, A. G.; Budd, P. M.; McKeown, N. B.; Colina, C. M.; Runt, J. Physical aging of polymers of intrinsic microporosity: a SAXS/WAXS study. J. Mater. Chem. A 2014, 2, 11742-11752. (22) Budd, P. M.; Elabas, E. S.; Ghanem, B. S.; Makhseed, S.; McKeown, N. B.; Msayib, K. J.; Tattershall, C. E. Solution-processed, organophilic membrane derived from a polymer of intrinsic microporosity. Adv. Mater. 2004, 16, 456-459. (23) Staiger, C. L.; Pas, S. J.; Hill, A. J.; Cornelius, C. J. Gas separation, free volume distribution, and physical aging of a highly microporous spirobisindane polymer. Chem. Mater. 2008, 20, 2606-2608. (24) Heuchel, M.; Fritsch, D.; Budd, P. M.; McKeown, N. B.; Hofmann, D. Atomistic packing model and free volume distribution of a polymer with intrinsic microporosity (PIM1). J. Membr. Sci. 2008, 318, 84-99. (25) Golzar, K.; Modarress, H.; Amjad-Iranagh, S. Effect of pristine and functionalized single- and multi-walled carbon nanotubes on CO2 separation of mixed matrix membranes based on polymers of intrinsic microporosity (PIM-1): a molecular dynamics simulation study. J. Mol. Model. 2017, 23, 266. (26) Schick, C.; Mathot, V. Fast scanning calorimetry, Springer International Publishing: New York, USA; 2016. (27) Schawe, J. E. K. Vitrification in a wide cooling rate range: The relations between cooling rate, relaxation time, transition width, and fragility. J. Chem. Phys. 2014, 141, 184905. (28) Adam, G.; Gibbs, J. H. On the temperature dependence of cooperative relaxation properties in glass-forming liquids. J. Chem. Phys. 1965, 43, 139-146. (29) Donth, E. The glass transition: relaxation dynamics in liquids and disordered materials, Springer: Berlin, Germany; 2001. (30) Dalle-Ferrier, C.; Kisliuk, A.; Hong, L.; Carini Jr., G.; Garini, G.; Angelo, G. D´; AlbaSimionesco, C.; Novikov, V. N.; Sokolov, A. P. Why many polymers are so fragile: a new perspective. J. Chem. Phys. 2016, 145, 154901. (31) Boucher, V. M.; Cangialosi, D.; Alegría, A.; Colmenero, J. Enthalpy recovery in nanometer to micrometer thick polystyrene films. Macomolecules 2012, 45, 5296–5306. (32) Cangialosi, D.; Alegria, A.; Colmenero, J. Cooling rate dependent glass transition in thin films and in bulk. In: Schick, C.; Mathot, V. (Eds.) Fast Scanning Calorimetry; Springer International Publishing; New York; 2016; pp. 403-431.


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First Clear-cut Experimental Evidence for a Glass ... - ACS Publications

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