Macromolecules 2008, 41, 4205-4215
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“Self-Foaming” Poly(phenylquinoxaline)s for the Designing of Macro and Nanoporous Materials Samuel Merlet,† Catherine Marestin,*,† Olivier Romeyer,‡ and Re´gis Mercier† Laboratoire des Mate´riaux Organiques a` Proprie´te´s Spe´cifiques (LMOPS), UMR 5041 CNRS-UniVersite´ de SaVoie, Chemin du Canal, 69 360 Solaize, France, and SerVice de Microscopie, UniVersite´ de SaVoie, SaVoie Technolac, 73 376 Le Bourget du Lac, France ReceiVed January 30, 2008; ReVised Manuscript ReceiVed April 3, 2008
ABSTRACT: This work concerns the investigation of porous poly(phenylquinoxaline)s (PPQ) films. The approach described relies on the in situ generation of foaming agents during the thermal curing of dense thin films of PPQ-containing thermolabile groups. For this purpose, a series of phenol-containing PPQ has been previously synthesized and then modified by grafting thermolabile tert-butyl carbonate groups (Boc) via a reaction involving the phenol groups. Thin dense films obtained by a solution casting method were thermally treated at different temperatures. The morphology of the resulting porous materials, characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), is discussed in relation with the chemical structure of the polymers and to the thermal treatment conditions as well.
1. Introduction Considering the growing interest given to “nanostructured materials”, many works have been devoted to the elaboration of nanoporous hybrids or organic films. One of the most widely investigated routes is based on a template method. Typically, heterogeneous materials containing a phase which can be removed either by extraction or thermal decomposition are used. In this case, the morphology of the final porosity corresponds to the removed template. On the basis of block copolymers with controlled architectures (copolymers containing thermostable and thermolabile blocks), the elaboration of nanoporous polyimides has been widely investigated at IBM in the mid 1990s.1–7 However, some difficulties concerning the control of the pore size due to porogen polydispersities or to a partial pore coalescence/collapse during the thermal treatments were reported. Another well-known method for obtaining porous structures is to use a foaming process. Carbon dioxide (in a supercritical or nonsupercritical state) is usually used as blowing agent.8,9 Two main routes are described. For the first, the polymer is saturated with CO2 at a temperature above the polymer/gas system glass transition temperature and then the porous structure is formed by depressurization. The second way involves the saturation of the polymer with CO2 at a temperature below the glass transition temperature of the polymer/gas system and then the foaming process by a rapid temperature increase (typically at a temperature above the Tg of the polymer/gas system). In both cases, the porous structure is generated from a nucleationgrowth mechanism. The morphology of the final material strongly depends on the chemical nature of the polymer as well as on the foaming experimental conditions (temperature and pressure of saturation, temperature and foaming time). Whereas microcellular foams10,11 (pore diameters ranging from 1 to 10 µm) and ultramicrocellular foams12 (diameter 223 °C. 1,4-Bis[(3,4-(Dihydroxyphenyl)glyoxlyl]benzene (3). The oxidation was performed either under microwave28 or by the conventional thermal method. A typical procedure is as follows:
Figure 1. 19F NMR spectrum of a reaction solution involving the bis(Rdiketones) 4 and 5 and the 4-fluoro-1,2-benzenediamine (after 140 min of reaction).
15 g (34.69 mmol) of 1,4-bis[(3,4-dimethoxyphenyl)glyoxylyl]benzene and 80.17 g (693 mmol) of pyridinium hydrochloride were placed in a round-bottom flask equipped with a nitrogen inlet, a magnetic stirrer, and a condenser. The reaction mixture was heated at 200 °C for 15 h. After being cooled down to room temperature, the addition of a 1.0 M aqueous hydrochloric acid solution into the reaction flask resulted in the formation of a brown precipitate. The solid was filtered, thoroughly washed with water, and dissolved in an aqueous potassium carbonate solution (5 g in 200 mL of H2O). Filtration of insoluble solids provided a clear yellow solution which was neutralized with hydrochloric acid to give a yellow powder. The powder was filtered and dried overnight at 100 °C under reduced pressure, yielding a polymergrade monomer (87%). 1H NMR (DMSO-d6, 294 K): δ (ppm) 10.15 (s, 4H), 8.08 (s, 4H), 7.35 (d, 2H), 7.24 (dd, 2H), 6.90 (d, 2H). 13C NMR (DMSO-d6, 294 K): δ (ppm) 194.9, 192.6, 153.8, 146.4, 137.0, 130.5, 124.7, 124.0, 116.2, 115.5. Anal. Calcd: C, 65.03; H, 3.47; O, 31.5. Found: C, 64.65; H, 3.8; O, 31.01. Melting point: 257 °C. 1,4-Bis(4-hydroxyphenylglyoxylyl]benzene (4). 4 was synthesized according to the procedure described earlier.26 1,4-Bis(phenylglyoxylyl]benzene (5). 5 was purified by recrystallization from acetonitrile to obtain a polymer-grade monomer. 2.4. Typical Procedure for the Synthesis of Poly(phenylquinoxaline)s. Homopolymer Synthesis. PPQ(2OH), PPQ(OH) and PPQ, respectively homopolymers based on bis(R-diketones) 3-5 and 3,3′,4,4′-Tetraaminodiphenyl sulfone were synthesized according to the general following procedure: In a 50 mL three necked round-bottom flask equipped with a nitrogen inlet and a mechanical stirrer were added 2.8962 g (7.13 mmol) of 1,4-bis[(3,4-(dihydroxyphenyl)glyoxylyl]benzene, 1.9838 g (7.13 mmol) of 3,3′,4,4′tetraaminodiphenyl sulfone, and 20 mL of m-cresol. The mixture was stirred at 60 °C for 24 h. The brown viscous solution was diluted with m-cresol and cooled down to room temperature. The polymer was precipitated in methanol. The yellow and fibrous PPQ-2OH was chopped into pieces and collected by filtration, washed with methanol and stirred overnight in methanol. The polymer was dried 48 h at 100 °C under vacuum. PPQ-2OH. 1H NMR (DMSO-d6, 360 K): δ (ppm) 8.78 (2H), 8.30 (4H), 7.58 (s, 4H), 7.15 (4H), 6.73 (4H). PPQ-OH. 1H NMR (DMSO-d6, 360 K): δ (ppm) 9.57 (2OH), 8.79 (2H), 8.30 (4H), 7.54 (s, 4H), 7.37 (4H), 6.77 (4H). PPQ. 1H NMR (NMP + (coax DMSO-d6), 360
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Scheme 3. Synthesis of Phenol Containing Poly(phenylquinoxaline)s: (a) Homopolymers; (b) “Random” Copolymers; (c) “Sequenced” Copolymers
K): δ (ppm) 8.55 (2H), 8.03-8.18 (4H), 7.25 (8H), 7.10 (6H). Typical Procedures for the Synthesis of Copolymers.
a. “Random Copolymer Synthesis” “Random” copolymers based on 1,4-bis(phenylglyoxylyl)benzene (5), 1,4-bis(4-
hydroxyphenylglyoxylyl)benzene(4),and1,4-bis[(3,4-(dihydroxyphenyl)glyoxylyl]benzene (3) were prepared according to the aforementioned procedure, mixing all comonomers at the beginning of the reaction.
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Table 1. Characterization of Poly(phenylquinoxaline)s structure of OH-containing OH content Td(∆5 wt %) Tg (°C)a PPQ (wt %) (°C)b η (dL/g) PPQ “random” PPQ/PPQ-OH “sequenced A” PPQ/ PPQ-OH PPQ-OH “random” PPQ/PPQ-2OH “sequenced A” PPQ/ PPQ-2OH “sequenced B” PPQ/ PPQ-2OH PPQ-2OH a By TGA. b By TMA.
0 2.5 2.5
470 470 470
325 348 348
0.75 0.71
5.8 5.6 5.6
450 440 440
370 357 356
0.41 0.62 0.71
5.6
440
357
0.66
11.1
430
355
0.69
Scheme 4. Postmodification of OH-Containing Poly(phenylquinoxaline)s
dried NMP, resulting in a 9 wt % polymer homogeneous solution. Then, 6.11 g (28 mmol) of Boc2O was added. The mixture was stirred at room temperature, and 34.2 mg (0.2 mmol) of DMAP dissolved in NMP was added drop by drop. After 10 h reaction at room temperature, the solution was poured into methanol. The yellow fibrous polymer was chopped into pieces, filtrated, thoroughly washed, and dried at 50 °C under vacuum for 48 h to give PPQ-Boc. 1H NMR (CDCl3, 294 K): δ (ppm) 8.96 (2H), 8.27 (4H), 7.56 (8H), 7.16 (4H), 1.52 (18H). PPQ-2Boc. 1H NMR (CDCl3, 294 K): δ (ppm) 8.96 (2H), 8.26 (4H,), 7.57-7.62 (6H), 7.34 (2H), 7.22 (2H), 1.51 (9H), 1.45 (9H). Anal. Calcd: C, 64.02; H, 5.17; N, 5.5; O, 22.10; S, 3.16. Found: C, 63.59; H, 5.12; N, 5.81; O, 21.74; S, 3.57. Random and sequenced OH-containing structures, as well as their Boc-containing analogues, were characterized by NMR. The spectra are in good accordance with the expected structures. 2.6. Dense Membrane Preparation. Thin dense membranes were prepared by casting 15 wt % polymer solutions in NMP onto clean glass plates. After a gentle solvent evaporation (24 h at 50 °C), the films were peeled off by immersion in methanol. The films were washed 48 h in methanol to remove any residual traces of NMP and finally dried at 50 °C for 24 h. TGA analyses systematically confirmed the presence of less than 0.1 wt % of remaining solvent. The dense film thickness ranged from 20 to 50 µm. 2.7. Preparation of Porous Membranes. The 1 cm2 dense films were heated for 1 min at different temperatures (Tf, foaming temperature), under nitrogen, in a Heraeus UT6060 convectional oven. The films were removed from the oven and cooled to room temperature. 2.8. Study of the Relative Reactivity of the Bis(r-diketone) Monomers by 19F NMR. General Procedure for the Synthesis of a Bisquinoxaline: (Synthesis of Q-H). A 50 mg (0.146 mmol) sample of 5 dissolved in 0.5 mL of DMSO was added into an NMR tube containing a solution of 37 mg (0.292 mmol) of 4-fluoro-1,2benzenediamine in 0.5 mL of m-cresol. 19F NMR experiments were performed using an external reference (DMSO-d6).
b. “Sequenced” Copolymer Synthesis. Method A. In a 50 mL three necked round-bottom flask equipped with a nitrogen inlet and a mechanical stirrer were suspended 2 g (5.84 mmol) of 1,4-bis(phenylglyoxylyl]benzene (5) and 1.9512 g (7.01 mmol) of 3,3′,4,4′-tetraaminodiphenyl sulfone in 16 mL of m-cresol. The reaction mixture was maintained for 1 day at room temperature. Then 2.3738 g (5.84 mmol) of 1,4-bis[(3,4(dihydroxyphenyl)glyoxylyl]benzene (3) and 1.3008 g (4.67 mmol) of 3,3′,4,4′-tetraaminodiphenyl sulfone were further added, and the polymerization was completed at 60 °C for 24 h. After precipitation in methanol, the polymer was chopped into pieces, washed with methanol, and dried under vacuum. Method B. In a 50 mL three necked round-bottom flask equipped with a nitrogen inlet and a mechanical stirrer were dissolved 2 g (5.84 mmol) of 1,4-bis(phenylglyoxylyl]benzene (5) and 1.355 g (4.87 mmol) of 3,3′,4,4′-tetraaminodiphenyl sulfone in 13 mL of m-cresol. The reaction mixture was maintained one day at room temperature. Then 2.3738 g (5.84 mmol) of 1,4-bis[(3,4-(dihydroxyphenyl)glyoxylyl]benzene (3) and 1.8970 g (6.82 mmol) of 3,3′,4,4′-tetraaminodiphenyl sulfone were further added, and the polymerization was completed at 60 °C for 24 h. After precipitation in methanol, the polymer was chopped into pieces, washed with methanol, and dried under vacuum. 2.5. General Procedure for the Postmodification of Phenol Containing Poly(phenylquinoxaline)s. In a reactor equipped with a nitrogen inlet and a magnetic stirrer was dissolved 3 g (19.6 mmol of OH groups) of a phenol containing poly(phenylquinoxaline) in
Procedure for the EValuation of the RelatiVe ReactiVity of Bis(R-diketone)s 3 and 5. Here, 45.7 mg (0.133 mmol) of 5 and 50 mg (0.133 mmol) of 3 were added to a round-bottom flask containing a solution of 33.7 mg (0.267 mmol) of 4-fluoro-1,2benzene diamine in 1 mL of m-cresol. The solution was stirred at 60 °C for 2.5 h. 19F NMR was run on samples regularly taken off from the reaction mixture.
3. Results and Discussion 3.1. Synthesis of a Bis(r-diketone) Monomer Containing Four Phenol Groups. In order to introduce an important quantity of thermolabile t-butylcarbonate groups (Boc) in polymer backbones, a new bis(R-diketone) monomer (1,4bis[(3,4-dihydroxyphenyl)glyoxylyl]benzene (3)), which possesses four phenol groups, has been designed (see Scheme 1). The synthesis of this monomer involved three steps. First, a Friedel/Crafts reaction was directly carried out from the dicarboxylic reactant, in the Eaton’s reagent. In such conditions, the desired compound was obtained in a very short time (10 min). The oxidation of this intermediate bisbenzyl structure was then performed in the presence of cupric bromide and DMSO, in ethyl acetate.29 The final phenol-containing monomer was obtained from a demethylation reaction using pyridinium chloride. While three different isomers could be formed, due to the ortho/para directing effect of the methoxy groups, only the bis(R-diketone) (3) was obtained from 1,2-dimethoxybenzene as a polymer-grade monomer. It is worth noting that the demethylation reaction in the presence of pyridinium chloride or even in HBr/AcOH of compounds prepared from 1,3dimethoxybenzene and 1,4-dimethoxybenzene led to the formation of numerous undesirable byproducts.
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Figure 2. 1H NMR Characterizations (a) PPQ-2OH (DMSO-d6, 25 °C), (b) PPQ-2Boc (CDCl3, 25 °C), and (c) PPQ-2Boc thermally treated at 200 °C (DMSO-d6, 25 °C).
Figure 3. (A) PPQ-Boc ( · - · ), PPQ/PPQ-Boc s eq 42 (s), PPQ/PPQ-Boc stat42 (--); (B) PPQ-2Boc ( · - · ), PPQ/PPQ-2Boc seq50 (s), PPQ/PPQ-2Boc stat50 (--). Table 2. Solubility of Poly(phenylquinoxaline)sa PPQ-OH, PPQ-2OH PPQ-Boc, PPQ-2Boc PPQ-Boc,PPQ-2Boc thermally treated a Key: () soluble; (×) insoluble.
CHCl3
DMSO
NMP
× ×
×
us to observe the formation of an intermediate monoquinoxaline compound as well as its disappearance in favor of the bisquinoxaline. At the end of the reaction, the presence of three to four different peaks were observed, which were characteristics of the three expected phenylquinoxaline isomers represented in Scheme 2. In order to study the relative reactivity of the different bis(Rdiketones) monomers, an equimolar amount of 4 and 5 or 3 and 5, were added to 4-fluoro-1,2-benzenediamine, in solution in m-cresol. The formation of mono and bisphenylquinoxalines at 60 °C was followed by 19F NMR. The identification of the different peaks was realized respectively from 19F analysis of each phenylquinoxaline. As illustrated in Figure 1 the bis(Rdiketone) without any phenol groups is much more reactive than the bis(R-diketone) 4. Similar results were obtained by comparing the reactivity of 5 and 3.
3.2. Relative Reactivity of the Different Bis(r-diketones). Bis(R-diketones) 5, 4, and 3 (Scheme 2) were reacted with 4-fluoro-1,2-benzene diamine, resulting, respectively, in the formation of phenylquinoxalines Q-H, Q-OH, and Q-2OH. Using a reagent containing one fluorine atom, namely 4-fluoro-1,2-benzene diamine, we were able to investigate by 19F NMR the formation of quinoxaline model compounds from the different bis(R-diketone) presented before. In each case, the analysis of different takings made during the reaction allows
Table 3. Influence of the Polymer Chemical Structure on the Limit Temperature below Which No Porosity Is Formed polymers thermally treated “random” PPQ/PPQ-Boc “sequenced A” PPQ/PPQ-Boc PPQ-Boc “random” PPQ/PPQ-Boc “sequenced A” PPQ/PPQ-Boc “sequenced B” PPQ/PPQ-Boc PPQ-2Boc a During the thermal treatment.
[gas]
b
evolved(mg/g
polym)a
150 150 345 343 343 343 654 Of the OH-containing structure.
∆m (wt %)Tha
∆m (wt %)Expa
Tg (°C)b
Tmin (°C)a
TT/O (°C)a
13 13 25.6 25.5 25.5 25.5 39.5
12.2 12.3 24.4 24.5 25.1
348 348 370 357 356 357 355
240 240 200 180 180 180 160
255 255 235 220 210 210 185
38.7
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Figure 4. Evolution of porosity formation with the foaming temperature Tf.
3.3. Synthesis of OH-Containing Poly(phenylquinoxaline)s. Homopolymers (Scheme 3a) were first prepared by condensation of 3,3′,4,4′-tetraaminodiphenyl sulfone with the different bis(Rdiketones), in m-cresol, at 60 °C. Different copolymers were synthesized by reacting 4 and 5 with 3,3′,4,4′-tetraaminodiphenyl sulfone (Scheme 3, parts b and
“Self-Foaming” Poly(phenylquinoxaline)s 4211
c). In this series of PPQs, the amount of phenol groups incorporated in the polymer chain was controlled by the relative proportion of each bis(R-diketone) involved in the polymerization reaction. In this respect, poly(phenylquinoxaline)s were obtained with an extend of OH groups ranging from 2.5 to 11.1 wt %. On the other hand, by copolymerizing monomers 3 and 5 with 3,3′,4,4′-tetraaminodiphenylsulfone, it was possible to obtain copolymers (PPQ/PPQ-2OH) having the same amount of phenol groups as those present in (PPQ-OH), which are based on bis(R-diketone) 4. When all comonomers are made to react altogether in one step (Scheme 3b), their incorporation in the polymer backbone depends of course on their relative reactivity. But, in general, they are considered as random copolymers. In our case, taking in consideration that monomer 5 is much more reactive than 3 and 4 and that both monomers are incorporated in the same proportions (50/50), one can reasonably consider that our copolymers are likely to have a sequenced character. In other words, one can suppose to have polymer chains with an alternation of PPQ/PPQ-OH or PPQ/PPQ-2OH units. The authors have then synthesized sequential PPQs in order to investigate if longer polymer sequences bearing the porogen group was directly related to the size of the final pore structure. In order to get such“sequenced” block copolymers, the polymerization was performed in a one-pot/two steps reaction. From
Figure 5. TEM and SEM characterizations of PPQ-2OH foams (in TEM analysis, bars indicate 100 nm; in SEM analysis, the image length corresponds to 34 µm).
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Figure 6. TEM and SEM characterizations of PPQ-OH foams (in TEM analysis, bars indicate 100 nm, whereas in SEM analysis, the image length corresponds to 34 µm).
a bis(R-diketone)/bisdiamine stoechiometric imbalance, a telechelic oligomer based on the most reactive bis(R-diketone) was first synthesized. Then, the less reactive bis(R-diketone) and the remainder of bis(o-diamine) monomer were added to the reaction mixture. As illustrated in Scheme 3c, two different strategies can be followed. In “method A”, a PPQ oligomer endcapped with R-diketones is formed in the first step. Even though the proportion of terminal R-diketones of oligomers is far lower than the amount of phenol containing R-diketones added in the second step, the great reactivity difference of both monomers might result in a coupling reaction between the telechelic oligomers rather than a random incorporation of the phenol containing bis(R-diketone). Such reactions might therefore lead to a fair control of the desired “sequenced” polymer architecture. In “method B” (oligomers end-capped with diamine groups), oligomer-coupling reactions are expected to be avoided, thus resulting in a more rigorous “sequenced structure”. Whereas no analytical method is able to accurately characterize the microstructure of the copolymers, the different repartition of the phenol groups within the macromolecules might be responsible for specific chain organizations and induce a particular morphology of the final materials. The authors synthesized sequential PPQs in order to investigate if the length of the polymer sequence bearing the porogen group is directly related to the size of the final pore structure. Further evidence which supports this hypothesis will be discussed later. Dense, thin, and self-standing films were elaborated from all above-mentioned structures, from the solvent casting method. 3.4. Characterization of OH Containing Poly(phenylquinoxaline)s. Even if one can reasonably suppose that the presence of phenol groups within the macromolecular chain should have a detrimental effect on the thermal stability, the thermogravimetric analysis of the different polymers (in dynamic mode)
shows that all polymers are withstanding very high temperatures, as witnessed by the 5 wt % decomposition temperatures reported in Table 1. Chain dissymmetry as well as intermolecular interactions are well-known to have a strong influence on a polymer glass transition temperature. Therefore, the formation of hydrogen bondings and the presence of increasing amounts of OH groups (from 0 to 5.8 wt %) do not surprisingly lead to increasing Tg (325, 348, and 370 °C, respectively, for PPQ, PPQ/PPQ-OH, and PPQ-OH). However, two OH on the same pendent aromatic ring (as in PPQ/PPQ-2OH and PPQ-2OH) do not induce a similar trend. Such an observation might be attributed to a loss of the macromolecular chain symmetry. 3.5. Synthesis and Characterization of Boc-Containing Poly(phenylquinoxaline)s. The tert-butoxycarbonyl (Boc) group has been used for a long time as the protective group of phenol compounds.30 Such Boc derivatives can easily be cleaved either by thermal decomposition31 or in acidic conditions.32,33 Boccontaining PPQs have already been considered as promising candidates as photoimagable dielectrics.34 The reaction of ditert-butyl dicarbonate groups with the phenol-containing PPQs was performed in NMP, according to a well-known procedure35 (Scheme 4). The postmodification of the phenol containing poly(phenylquinoxaline)s was confirmed by 1H NMR (Figure 2). It was also shown by TGA analysis (Figure 3) that the thermal degradation of the carbonate groups occurred in a narrow range of temperatures. In both cases, the postmodification reaction and the Boc thermal decomposition were quantitative and do not affect the polymer backbone integrity. As shown on TGA analyses, the polymer weight losses recorded during the thermal treatments are in very good agreement with the expected proportions of gas released
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Figure 7. TEM and SEM characterizations of random PPQ/PPQ-2OH foams (in TEM analysis, bars indicate 100 nm, whereas in SEM analysis, the image length corresponds to 34 µm).
Figure 8. Comparison of cross-section TEM image of nanoporous PPQ films (bars indicate 100 nm).
Figure 9. Comparison of Cross-Section SEM Images of Nanoporous PPQ Films (Image Lengths Correspond to 34 µm).
(Table 3). Of course dissolving an effective quantity of CO2 and isobutene mixture in such polymers using a conventional “high pressure” process should be difficult to achieve. However, it is clear that the quantity of gas evolved during the thermal treatment of our polymers is higher than those obtained from
CO2 usually dissolved in polymers such as PS,10 PC,36 PVC,27 PMMA,12 PSU,11 PESF,37 PEI,8 PEKK,37 and PI.9,38 As reported in Table 2, the introduction of Boc groups along the polymer chain strongly modifies the solubility of the polymers.
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Table 4. Dielectric Properties of Dense and Porous PPQ Thin
Films polymer structure
Τgδ porosity (% vol)a εr (30 °C, 1kHz) (30 °C, 1kHz)
“random” PPQ/PPQ-OH “sequenced” PPQ/PPQ-OH PPQ-OH “random” PPQ/PPQ-2OH “sequenced A” PPQ/ PPQ-2OH “sequenced B” PPQ/ PPQ-2OH
a
0 12.9 22.3 0 25 0 16.3 48.0 0 18.7 31.2 0
3.34 ( 0.011 3.43 ( 0.10 2.76 ( 0.07 3.45 ( 0.07 2.67 ( 0.04 3.83 ( 0.08 3.26 ( 0.08 2.44 ( 0.04 3.9 ( 0.08 3.24 ( 0.05 2.94 ( 0.04 3.82 ( 0.09
1.7 × 10-3 2.2 × 10-3 1.4 × 10-3 1.9 × 10-3 1.4 × 10-3 4.0 × 10-3 3.6 × 10-3 6.5 × 10-3 1.2 × 10-2 9.2 × 10-3 8.2 × 10-3 1.0 × 10-2
19.2 33.2 0
3.34 ( 0.007 3.00 ( 0.06 3.90 ( 0.11
9.4 × 10-3 8.8 × 10-3 1.3 × 10-2
23.0 34.0
2.98 ( 0.06 2.70 ( 0.06
8.8 × 10-3 7.8 × 10-3
By density measurements.
Figure 10. Effect of the porosity content on the dielectric constant of PPQ-OH thin films. Comparison with the Bruggeman and MaxwellGarnett predictive laws.
3.6. Preparation and Characterization of Porous Films. In a previous study,26 it has been shown that fast thermal treatments at foaming temperatures (Tf) ranging from 150 to 300 °C are sufficient to completely decompose the Boc groups. Different thin dense films elaborated from the Boc containing polymers were treated in such conditions. The porosity formed during these treatments was evaluated from density measurements. As represented in Figure 4, whatever the polymer structure, there is a temperature (denoted Tmin) below which no porosity is present, whereas at higher temperatures, the porosity increases with temperature. In our previous work,26 Tmin had been considered as the Tg of the polymer/gas system. Such a result is corroborated by the strong dependence of Tmin with the polymer chemical structure and more specifically with the quantity of gas susceptible to be released during the thermal treatment (Table 3). Comparing “Tg” with “Tmin” highlights the strong plasticization effect of the in situ formed carbon dioxide and isobutene on the polymer. Considering polymers having a given amount of thermolabile groups, but differently incorporated in the polymer chain (respectively, “random”, “sequenced A”, and “sequenced B”), no significant differences are observed. On a macroscopic scale, the film aspects depend on the thermal treatment temperature (Figure 4). Whereas the films are transparent at relatively low temperatures, they are systematically opaque at temperatures higher than TT/O (the transparent/ opaque temperature). This means that the size of the porosity strongly depends on the experimental conditions and that above
TT/O, a macroporosity (>400 nm) might be formed. Of course, it is not surprising (Table 3) that the higher the proportion of gas evolved, the lower the T(T/O). So, within a temperature range between Tmin and T(T/O), it was expected to get a small (
