Synthesis of Polyimides in Molecular-Scale Confinement for Low

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Synthesis of Polyimides in Molecular-Scale Confinement for Low Density Hybrid Nanocomposites Scott G. Isaacson, Jade I. Fostvedt, Hilmar Koerner, Jeffery W Baur, Krystelle Lionti, Willi Volksen, Geraud Jean-michel Dubois, and Reinhold H. Dauskardt Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b03725 • Publication Date (Web): 09 Oct 2017 Downloaded from http://pubs.acs.org on October 13, 2017

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Synthesis of Polyimides in Molecular-Scale Confinement for Low Density Hybrid Nanocomposites Scott G. Isaacson1, Jade I. Fostvedt1, Hilmar Koerner2, Jeffery W. Baur2, Krystelle Lionti3, Willi Volksen3, Geraud Dubois1,3, and Reinhold H. Dauskardt1* S.G. Isaacson, J.I. Fostvedt, Prof. R.H. Dauskardt Department of Materials Science and Engineering, Stanford University, 496 Lomita Mall, Stanford, California 94305, USA. Dr. H. Koerner, Dr. J.W. Baur Air Force Research Laboratory, Materials and Manufacturing Directorate (AFRL/RXCC), Wright-Patterson Air Force Base, Ohio 45429, USA. Dr. K. Lionti, Dr. W. Volksen, Dr. G. Dubois Hybrid Polymeric Materials, IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120-6099, USA. Keywords: hybrid materials, polyimides, molecular confinement, nanocomposites, low density materials

1 ACS Paragon Plus Environment

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Abstract:

In this work we exploit a confinement-induced molecular synthesis and a resulting bridging mechanism to create confined polyimide thermoset nanocomposites that couple molecular confinement-enhanced toughening with an unprecedented combination of high-temperature properties at low density. We describe a synthesis strategy which involves the infiltration of individual polymer chains through a nanoscale porous network while simultaneous imidization reactions increase the molecular backbone stiffness. In the extreme limit where the confinement length scale is much smaller than the polymer’s molecular size, confinement-induced molecular mechanisms give rise to exceptional mechanical properties. We find that polyimide oligomers can undergo crosslinking reactions even in such molecular-scale confinement, increasing the molecular weight of the organic phase and toughening the nanocomposite through a confinement-induced energy dissipation mechanism. This work demonstrates that the confinement-induced molecular bridging mechanism can be extended to thermoset polymers with multifunctional properties, such as excellent thermo-oxidative stability and high service temperatures (> 350 °C).


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Main Text: Polyimide nanocomposites generally use inorganic fillers dispersed within a polyimide matrix to generate improved materials properties at increased densities. 1-5 In contrast, this work demonstrates the synthesis of confined polyimide nanocomposites in which polyimide oligomers infiltrate into nanoscale pores and subsequent cross-linking reactions in nanoscale confinement form the final nanocomposite. This strategy exploits a newly-discovered molecular bridging mechanism6 to fabricate toughened, low-density nanocomposite materials containing the polyimide thermoset in molecular-scale confinement. Confined polyimide nanocomposite hybrids are desirable for lightweight components and thermal barrier coatings in high temperature applications due to the low-density, thermally stable, and chemically-resistant properties of their individual phases as well as the synergistic toughening effects of molecularscale confinement. Their overall densities are also much lower than that of both bulk polyimides and traditional polyimide nanocomposites.1 We recently reported that when un-crosslinked polystyrene molecules are confined to molecular length scales in a nanoporous matrix, energy dissipation through a confinementinduced molecular bridging mechanism leads to significant toughening. 6 The work involved infiltration of the matrix with nearly monodisperse polystyrene molecules that interacted only through a reduced density of confinement-suppressed entanglements. The synthesis method developed in the current work begins with the infiltration of polyimide precursors and involves several significant challenges, including cyclization reactions that occur during filling, poor polyimide solubility in organic solvents, the relatively large persistence length of polyimides 7-8 in comparison to polystyrene9 (contributing to reduced conformational freedom and low molecular mobilities in confinement), and the unknown effects of molecular confinement on the 3 ACS Paragon Plus Environment

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imidization and crosslinking reactions. We report and describe the successful synthesis of confined polyimide hybrid nanocomposites and show that cyclization and crosslinking reactions within nanoscale pores leads to increased nanocomposite toughness through a molecular confinement-induced toughening mechanism. The confined polyimide nanocomposites described in this study were composed of a polyimide thermoset phase within a nanoporous organosilicate matrix that served as the stiff confining phase for the polyimide molecules (Figure 1a).6 The matrix was obtained by spincasting and thermally curing a porogen-containing ethylene oxycarbosilane (Et-OCS) sol-gel formulation atop a silicon wafer coated with a dense Et-OCS adhesion layer.10-11 This process created matrix films approximately 600 nm in thickness with a porosity of 47%. The matrix porosity was composed of a random interconnected network of approximately cylindrical pores with an average diameter of 7 nm (Figure S1).


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a

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Figure 1. The structure and filling of confined polyimide nanocomposite hybrids. a) A polyimide thermoset is confined within a nanoporous organosilicate matrix. b) The synthesis proceeds in three steps: spin coating of the poly(amic ester) precursor, simultaneous infiltration and imidization, and confined crosslinking of the polyimide phase. c) XPS depth profiles of polyimide hybrid nanocomposites for selected filling temperatures. The plateaus in the carbon concentration provide evidence for uniform dispersion of polyimide throughout the full thickness of the organosilicate matrix. Note that the profile with the lowest carbon concentration was filled by heating to 170 °C for four hours and then crosslinked by raising the temperature to 370 °C for one hour. For all profiles the sputtering time is approximately proportional to the sputtering depth into the nanocomposite film.


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The infiltration of the polyimide phase within the nanoporous matrix and the subsequent curing reaction proceeded in three steps (Figure 1b). First, a layer of a poly(amic ester) – an ethanol-soluble polyimide precursor – with a degree of polymerization less than 10 was spin cast from an ethanol solution onto the matrix. Second, this bilayer structure was heated to a temperature Tfill (200-300°C, 3°C min-1 ramp rate) in a nitrogen atmosphere, allowing capillary forces to draw the poly(amic ester) into the pores and infiltrate the nanoporous matrix. During this elevated-temperature filling step, the poly(amic ester) underwent imidization reactions that lead to an increase in glass transition temperature and a reduction in solubility. 12 Finally, the nanocomposite films were heated to a temperature of 370°C (3°C min-1 ramp rate), which induced the thermally- and radical-activated crosslinking of the polyimide within the pores to form a toughened nanocomposite material with low density (1.36 g cm -2). The simultaneous infiltration and imidization of the polymer molecules during the filling step was monitored and characterized using X-ray photoelectron spectroscopy (XPS) depth profiling and Fourier transform infrared spectroscopy (FTIR). The effect of the filling temperature (Tfill) on the penetration of the polyimide chains into the nanoporous matrix is shown in the XPS depth profiles in Figure 1c. Each profile shows an initial region of high carbon content corresponding to the residual polyimide overburden (ca. 0-1 min sputtering) followed by a plateau corresponding to polyimide uniformly distributed throughout the Et-OCS matrix (ca. 12.5 min). The terminal region of declining carbon content indicates a transition to the silicon substrate. Based on the known carbon content of the polyimide and the Et-OCS matrix, we estimate that a fully filled film would exhibit a plateau carbon concentration of ~46%. The plateau carbon concentrations shown in Figure 1C range from 43% (Tfill = 300°C) to 47% (Tfill = 200°C) for 6 ACS Paragon Plus Environment

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uncrosslinked samples, corresponding to fill levels of 91% and greater. This demonstrates that polyimides can be uniformly infiltrated into nanoscale pores over a wide range of filling temperatures in spite of simultaneous imidization reactions that decrease their solubility, decrease their conformational freedom, and raise their glass transition temperature.12 Increasing T fill led to decreases in the polyimide overburden thickness and the plateau carbon concentration, suggesting volatilization of low-molecular-weight species during the filling step that increases in rate with T fill. This volatilization is especially apparent in the crosslinked polyimide nanocomposites (370°C, Figure 1c), which exhibit a plateau carbon concentration of 36%, corresponding to a fill level of 76%. X-ray reflectivity density measurements provided supporting evidence for this volatilization, showing a decrease in overall density from 1.50 g cm-2 (Tfill = 280°C) to 1.36 g cm-2 after crosslinking (Figure S2). This final density is substantially lower than traditional polyimide composite materials. 1 The imidization reactions that occur during filling were studied as a function of Tfill and the duration of the filling step through Fourier transform infrared spectroscopy. FTIR absorption spectra of confined polyimide hybrids taken after filling at various temperatures (Figure 2a) show that the filling process induces the thermal imidization and cyclization of the amic ester precursor, producing the desired imide (Figure 2b) as well as a dianhydride side product. 13-14 We observed that the absorption peak at 1858 cm -1 corresponding to the dianhydride13-14 appears mostly strongly at low Tfill and decreases with higher filling temperatures, indicating that higher filling temperatures increase the selectivity for the thermodynamically-favored imide.15-16


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Figure 2. FTIR characterization showing evidence of the imidization and crosslinking reactions that occur during infiltration and in molecular-scale confinement. a) Characterization of the imidization reaction at different filling temperatures, T fill. The imidization reaction and the infiltration of the polymer chains into the nanopores occur simultaneously. b) The imidization reaction converts the poly(amic ester) precursor into the desired polyimide product. Here the “Ar” groups with superscripts denote different aromatic moieties. c) The radical addition reaction between phenylethynyl groups produces a polyene backbone with pendant oligoimide groups. Each oligoimide possesses two phenylethynyl groups, allowing for the crosslinking of multiple polyene chains into a molecular network. Here “Im” (left) and bolded lines (right) 8 ACS Paragon Plus Environment

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denote the polyimide backbone shown in panel B. d) Representative phenylethynyl triple bond absorption peaks used to monitor the confined crosslinking reaction. The spectrum corresponding to the unconfined (bulk film) reaction shows nearly complete elimination of the phenylethynyl groups, while the confined spectrum displays a small but repeatable residual peak after thermal treatments. e) The average degree of cure was reduced when the crosslinking reaction took place in confinement.

The confined imidization reaction discussed above was compared to imidization of the amic ester oligomer in an unconfined neat film on a silicon substrate (Figure S3). No substantial differences between confined and unconfined imidization were observed. Due to the intramolecular nature of the imidization reaction, no significant effects of confinement were expected. The similarity of the FTIR absorption spectra for confined and unconfined imidization also suggests that the oligoimide did not substantially react with the Et-OCS pore surfaces. The dependence of these reactions on the duration of the filling step was also examined by comparing the FTIR spectra of nanocomposites with filling durations of one hour and four hours (Figure S4). No substantial difference in the imidization reaction or formation of side products was observed. This indicates that the extent and selectivity of the imidization reaction are not kinetically limited within the examined processing window. After synthesizing a nanocomposite film containing polyimide molecules uniformly dispersed within the nanoporous matrix, we characterized the effects of molecular confinement on the polyimide crosslinking reaction. This reaction was induced by heating the polyimide nanocomposite to 370°C for one hour. At this temperature, the phenylethynyl endcaps of the polyimide chains are known to undergo radical addition reactions 17-20 and aromatic formation21-22 9 ACS Paragon Plus Environment

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that crosslink the polyimide molecules into a polymer network (Figure 2c). The presence of the phenylethynyl groups was monitored through FTIR analysis of the absorption peak at 2215 cm-1 corresponding to the phenylethynyl triple bond (Figure 2d). The extent of the crosslinking reaction  was calculated through the equation:  A2215cm 1    A 1  1516cm  370C

  1  

 A2215cm 1  A  1516cm 1

    200C

where A2215cm 1 is the phenylethynyl triple bond peak area and A1516cm1 is the aromatic peak area, which is used as an internal standard. Subscripts refer to the temperature of the one-hour thermal treatments. Samples filled for one hour with a T fill of 200°C were chosen as a reference because at this temperature imidization had completed, all residual solvent had been eliminated, and no significant reaction of the phenylethynyl groups is expected. 17-19,23 We observed that polyimide crosslinking can occur even under confinement within 7 nm pores despite the relatively stiff backbone and 5 nm end-to-end distance of the polyimide molecule. However, the extent of the crosslinking reaction was suppressed compared to a bulk, unconfined polyimide film about 300 nm in thickness. Figure 2e shows that ~74% of the phenylethynyl groups reacted in the confined case, while 96% reacted in the unconfined film. Previous studies with similar polymers have shown that the phenylethynyl crosslinking reaction is expected to proceed nearly to completion for treatments of 370°C for 1 hour,17,19 consistent with our own characterization of the crosslinking of unconfined polyimide films. The observed suppression of polyimide crosslinking in nanoscale confinement is likely due to the reduced number of intermolecular interactions (due to competing interactions with the pore surfaces) and hindered ability of the polyimide molecules to undergo the conformational changes and rotations necessary to bring the phenylethynyl groups into contact for the reaction. It is further anticipated


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that confinement affects the crosslink topology of the resulting thermoset by increasing the formation of extended polyene chains and suppressing the formation of aromatics due to the enhanced influence of steric effects in nanoscale confinement. Additional studies are required to confirm this hypothesis. Having demonstrated the ability of polyimide molecules to undergo crosslinking in molecular-scale confinement, we then examined the effects of polyimide infiltration and crosslinking on the fracture toughness of the overall nanocomposite material using double cantilever beam fracture mechanics specimens (Figure S5). The unfilled organosilicate matrix exhibited a fracture energy of 2.3 J m-2 that is characteristic of nanoporous glasses (Figure 3a). 24 When the matrix was filled with uncrosslinked polyimide, the fracture energy remained unchanged. This is consistent with previous studies of nanocomposites filled with low molecular weight (1.5 kDa) polystyrene, which also show no toughening effect. 6 Both the uncrosslinked polyimide and the low molecular weight polystyrene impart no toughening to the nanocomposite because of their small end-to-end distances (~5 nm and 3 nm, respectively). When the end-to-end distance is smaller than the confining pore size (7 nm in this case), molecules are in a bulk-like state and are unable to effectively participate in the confinement-induced molecular bridging toughening mechanism.6


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Figure 3. Fracture properties of polyimide hybrids. a) The cohesive fracture energy of the confined polyimide nanocomposite with an uncrosslinked polyimide phase exhibits no toughening relative to the unfilled nanoporous matrix. When the polyimide phase is crosslinked, however, the toughness increases by nearly a factor of two. b) An idealized representation of the polyimide network undergoing confined molecular bridging. c) The cohesive fracture energy of hybrid nanocomposites heated to 350 °C in air for one hour. The data are normalized to the cohesive fracture energy before high-temperature exposure, Gc,0.

When the polyimide was crosslinked into higher molecular weight species, however, the nanocomposite fracture energy increased by nearly a factor of two, from 2.3 J m-2 to 4.3 J m-2. This increase in toughness comes despite a concomitant decrease in density from approximately 1.50 g cm-2 to 1.36 g cm-2. One might initially attribute this increase in fracture energy upon crosslinking to the standard toughening mechanisms associated with crosslinked polymers such as shear banding.25-26 However, these energy dissipation processes that are active in bulk thermoset polymers involve extensive plastic deformation near the crack tip 26-27 and would be limited in pore-confined polymers for several reasons. First, the polymer chains are confined within a brittle organosilicate matrix that will fracture before reaching the strains necessary for large-scale deformation of the polymer phase. Second, the confined polymers have reduced intermolecular interactions and connectivity due to the presence of the organosilicate matrix, inhibiting their ability for polymer chains to transfer the stresses away from the crack tip. We therefore propose that the observed toughening for crosslinked polyimide thermoset nanocomposites does not result from the traditional shear banding mechanism, but instead is due to the molecular bridging of polyimide chains that have formed a molecular network as a result


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of the crosslinking reaction. In this mechanism, chain segments that are intercepted by the crack front bridge the fracture surfaces and pull out of the pore structure as the crack advances (Figure 3b). The pullout and associated elongation and deformation of each bridging molecular segment dissipates energy and contributes to the toughening of the nanocomposite material. 6 The toughening through molecular bridging is activated only for confined molecules and provides an alternate method of toughening nanocomposite materials where bulk toughening mechanisms may be absent. We further characterized the high temperature resistance of the polyimide thermoset nanocomposites by heating them to 350°C in air for one hour and then measuring their cohesive fracture energy at room temperature (Figure 3c). Nanocomposites filled with crosslinked polyimide experienced no degradation in fracture energy after high temperature exposure. In contrast, nanocomposites filled with polystyrene retained only 80% of their original toughness. As expected,6 the polystyrene degrades at high temperature, reducing the fracture energy of the hybrid nanocomposite. High-temperature exposure also led to increased roughness of the polystyrene nanocomposite fracture surface, likely from the formation of volatile polystyrene degradation products. These data show that confined polyimide hybrids have the potential to perform as thermal barrier coatings for high temperature polymer substrates and as lightweight materials for aerospace application with high service temperatures of at least 350°C and densities much lower than that of traditional polyimide nanocomposites. This study provides a proof-of-concept that nanocomposite materials may be synthesized with confined, crosslinked molecules and that such materials may be toughened through a confinement-induced molecular bridging mechanism. Optimization of the toughening mechanism through methods such as changing the pore-polymer interactions and tuning the


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degree of crosslinking may produce multifunctional nanocomposites that more effectively leverage the molecular bridging mechanism for even larger increases in fracture toughness.


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ASSOCIATED CONTENT Supporting Information. The following files are available free of charge: Additional characterization data and experimental methods (Figures S1-S2, Supplementary Methods) (PDF)

AUTHOR INFORMATION Corresponding Author * [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was supported by the Air Force Office of Scientific Research Grant No. FA9550-121-0120. Part of this work was performed at the Stanford Nano Shared Facilities (SNSF) and the Stanford Nanofabrication Facility (SNF).

ACKNOWLEDGMENT This work was supported by the Air Force Office of Scientific Research Grant No. FA9550-121-0120. Part of this work was performed at the Stanford Nano Shared Facilities (SNSF) and the Stanford Nanofabrication Facility (SNF).


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Table of Contents graphic: Low Density Hybrid Nanocomposites with Polyimides in Molecular-Scale Confinement Scott G. Isaacson, Jade I. Fostvedt, Hilmar Koerner, Jeffery W. Baur, Krystelle Lionti, Willi Volksen, Geraud Dubois, and Reinhold H. Dauskardt*

ToC figure: confined molecular bridging

nanoporous matrix

confined polyimide thermoset


Synthesis of Polyimides in Molecular-Scale Confinement for Low

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