Nanoporous Poly(methyl silsesquioxane) By The Templating of Amphiphilic Block Polymers, PS-b-P2VP and PS-b-PAA
C. C. Yang,1 P. T. Wu,1 Y. Chang,1 and C. Y. Chen,1 and W. C. Chen1,2,*
1: Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan 106
2: Institute of Polymer Science and Engineering, National Taiwan University, Taipei, Taiwan 106 Fax:886-2-23623040 E-mail:
Abstract
In this study, nanoporous films were prepared from poly(methyl silsesquioxane) (MSSQ) by the templating of amphiphilic block copolymers, poly(styrene-b-2-vinylpyridine) (PS-b-P2VP) and poly(styrene-b-acrylic acid) (PS-b-PAA). The different characteristic of hydrogen bonding on the prepared block copolymers resulted in a significant variation on the morphology and properties of the obtained nanoporous MSSQ films. The intermolecular hydrogen bonding interaction between the MSSQ and PS-b-P2VP prevents the aggregation of the porogens before curing, which is supported from the results of FTIR, MDSC and molecular simulation. The pore size of the nanoporous MSSQ thin films through the templating of the PS-b-P2VP was less than 10 nm from the TEM characterization, which was close to the porogen size. However, the
experimental results suggested that the intramolecular hydrogen bonding existed in the system of the
MSSQ/PS-b-PAA hybrid. Thus, a meso-phase separation was occurred and resulted in non-uniform nanopores after pyrolysis in the case of the MSSQ/PS-b-PAA hybrid. The refractive index and dielectric constant of the prepared nanoporous films could be tuned in the range of 1.361 ~ 1.111 and 2.67 ~ 1.30, respectively, by varying the porogen ratio. Besides the produced nanopores, the retardation of the structural transformation of the MSSQ by the porogens and the thermal decomposition characteristic of the porogens both contributed to the obtained dielectric constant.
Keywords: PS-b-P2VP, PS-b-PAA, amphiphilic block copolymer, porous MSSQ film, hydrogen bonding Introduction
Nanoporous materials have represented as a new class of low dielectric (k) constant materials for integrated circuit (IC) applications.1-3 One of the major issues in the nanoporous materials is controlling the pore size and distribution through the molecular structure of the templating agent.1 For the IC devices, the low k films with closed nanopores and medium porosity are required to avoid the poor mechanical strength and dielectric properties. Hence, the introducing of the porosity into the currently existed low k materials could achieve the goal of k smaller than 2.0 at a medium porosity, such as methyl silsesquioxane (MSSQ).2-5 The nanoporous MSSQ thin films were prepared by various thermal decomposable pore generators (porogens), such as dendrimers,2,6 star-shaped,7 and hyperbranced linear polymers,2,8 norbornene-derivatives,4,9,10 and amphiphilic block copolymers (ABCs).5,11-22
We are particularly interested in employing the ABCs as the porogens since they offer the microphase morphologies
for controlling the pore size and shape. Yang et al. obtained the nanoporous films through the hybrids of the MSSQ with the ABC of poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) (PEO-b-PPO-b-PEO).14,15 The hydrophobic/hydrophilic interaction between the MSSQ and the PEO-b-PPO-b-PEO created the nanophase separation and resulted in small pores of a few nm. Frank, Miller, and their coworkers used another porogen, poly(methyl methacrylate-co-dimethylaminoethyl methacrylate) P(MMA-co-DMAEMA) for nanoporous MSSQ films.18 The pore size of the resulted films was estimated to be around 10 nm for the 50% polymer loading ratio. For the above studies, the chemical interaction between the MSSQ and the ABC was regarded as an important factor for controlling the resulted pore size distribution. For example, the DMAEMA of the P(MMA-co-DMAEMA) should be larger than 15 mole % for preventing the macro-phase separation. However, the experimental and theoretical investigations on the chemical interaction between the MSSQ and the ABC have not been fully explored in the previous studies.14,15,18
In this study, two kinds of ABCs, poly(styrene-b-2-vinyl pyridine) (PS-b-P2VP) and poly(styrene-b-acrylic acid) (PS-b-PAA), were used as the porogens for obtaining the nanoporous MSSQ. The variation on the ABC chemical structure was used to address the difference on the chemical interaction and resulted morphology of the prepared films.
Figure 1 shows the scheme for preparing the nanoporous MSSQ films by the templating of the ABC using the PS-b-P2VP as an example. Miscible hybrid materials of the MSSQ and PS-b-P2VP were obtained through the proposed hydrogen bonding interaction between the 2VP moiety and the Si-OH end group. The amphiphilic characteristic of the MSSQ precursor shifted to hydrophobic since the polycondensation of the Si-OH groups makes the Si-O-Si linkages during the thermal curing. Hence, microphase separation is expected through the hydrophobic- amphiphilic interaction of the MSSQ and the ABC. The ABC in the microphase domain could be removed by thermal curing at high temperature and leave a pore structure in the films. The shifting on the FTIR spectra of the prepared MSSQ/ABC hybrids was used to verify the hydrogen bonding interaction. The miscibility between the MSSQ and the porogen was further analyzed by the modulated DSC. The morphology of the prepared nanoporous MSSQ films was investigated by TEM. The correlation of the refractive index and dielectric constant of the prepared nanoporous films with the loading ratio of the ABCs was analyzed and discussed.
Experimental
The detailed synthesis of copolymer PS-b-P2VP, PS-b-PAA and MSSQ precursor and the preparation of the nanoporous MSSQ thin films are reported recently.23
The Mn and PDI of the PS-b-P2VP were 15,400 (volume ratio of the PS block (fPS) = 0.46) and 1.14, respectively. The Mn and PDI of the PS-b-PAA were 14,800 (volume ratio of the PS block (fPS) = 0.54) and 1.15, respectively. The OH content of the prepared MSSQ precursor estimated from the FTIR analysis was 6.72% from the comparison with a reference MSSQ sample from Gelest with a reported OH content of 5%.
The obtained nanoporous MSSQ thin films through templating of the PS-b-P2VP and the PS-b-PAA were named as the P1 and P2, respectively.
Results and Discussion
Figure 2 shows the FTIR absorption spectra of the as-spun PS-b-P2VP/MSSQ hybrid thin films. The absorption band shown in Figure 2 is assigned to the Si-O stretching on the Si-OH bond of the MSSQ.24,25 As observed from Figure 2,
the position of the Si-O band gradually shifts from 904 to 938 cm-1 as increasing the loading of the PS-b-P2VP. It indicates the formation of the hydrogen bonding between the N of the 2VP segment and H of the Si-OH of the MSSQ, as suggested by Figure 1(A). The hydrogen bonding existing between silanol group and pyridine was also observed by the blend of poly (styrene-b-vinylphenyldimethylsilanol) and
poly(vinylpyridine).26 The suggested hydrogen bonding decreases the bond strength between the H and Si-O of the Si-OH bond and thus results in the shifting of the Si-O stretching band to a higher wavenumber. However, for the case of the MSSQ/PS-b-PAA hybrid films, the position of the Si-O only shows an insignificant variation in the range of 904~908 cm-1 as varying the PS-b-PAA loading. This result suggests that intermolecular hydrogen bonding for the case of the
MSSQ/PS-b-PAA is much weaker than that of the MSSQ/PS-b-P2VP.
Figure 3 shows the FTIR spectra of the
MSSQ/PS-b-P2VP hybrid thin films in the wavenumber range of 650 to 1650 cm-1 at the curing temperature of 25 (as-spun), 120 (after baked overnight), and 400 0C (after complete pyrolysis of the ABC), respectively. The absorption peak at 938 cm-1 assigned to the Si-O stretching of the Si-OH bond decreases as increasing the temperature and completely disappears at 400 0C. This suggests the condensation reaction of the Si-OH bond of the MSSQ precursor to the Si-O-Si bond.
The peak at 1100 cm-1 is splitting into two peaks as increasing the curing temperature, which is assigned to the Si-O-Si stretching. The two peaks at 1130 and 1030 cm-1 assigned to the symmetric (e.g., cage-like) and non-symmetric structures (e.g., network), respectively. Such structural transformation is similar to that reported in the literature.18,24,25 The characteristic peaks of the PS-b-P2VP at 700, and 1434 to 1590 cm-1 are completed disappeared after pyrolysis at 4000C.
Figure 4 shows the TGA curves of the prepared PS-b-P2VP and PS-b-PAA, respectively. The PS-b-P2VP has only one Td about 387 0C. However, the PS-b-PAA, gradually decomposes from 150 0C to a higher temperature. The degree on the cross-linking of the MSSQ determined by IR analysis was 50 % at 1000C and 95 % at 390 0C, respectively.25 The collapse of the symmetry structure of the MSSQ is around 4500C,25 which is larger than the Td of the ABC. Hence, the initial pyrolysis of the PS-b-PAA prior to the complete cross-linking reaction of the MSSQ precursor by thermal curing probably did the damage to the formation of the pore structure in the films. Therefore, the PS-b-P2VP would be a better candidate as a porogen than the PS-b-PAA for preparing nanoporous materials from the above thermal analysis.
Figure 5 show the modulated DSC (MDSC) curves of the MSSQ/PS-b-P2VP and MSSQ/PS-b-PAA hybrid materials.
The glassy transition temperature (Tg) of the pure PS-b-P2VP and PS-b-PAA determined from MDSC are 100 and 950C, respectively. Hence, they are mobile under the curing temperature of 1200C for the preparing the hybrid materials.
For the case of MSSQ/PS-b-P2VP (30%), a broad featureless curve is shown. It might be due to the domain is too small or the porogen loading is too low. A recent report on the MDSC study of the MSSQ/P(MMA-co-DMAEMA) also shows a similar result.27 As increasing the loading of the PS-b-P2VP, a single Tg of 110oC is shown in the case of MSSQ/PS-b-P2VP (50%). It suggests the miscibility at the length scale of 10 nm at a high porogen loading of the PS-b-P2VP. The higher Tg of the P1(50%) than that of the pure PS-b-P2VP might be due to the inorganic matrix of the MSSQ or intermolecular hydrogen bonding. The pure PS-b-PAA shows a Tg of 950C but its hybrids with MSSQ show two Tg for both cases of 30 and 50%.
One Tg of the hybrid system at 96oC is similar to that of the
pure PS-b-PAA, which suggests an immiscible domain of the PS-b-PAA in the hybrid. The higher Tg (123 oC for P2(30%) and 121 oC for the P2(50%)) than that of the pure PS-b-PAA might be due to the intermolecular hydrogen bonding between the MSSQ and PS-b-PAA, which is the miscible domain of the hybrid system. Hence, the MDSC result suggests that the hybrid system of the MSSQ/PS-b-PAA is a partially miscible system.
Figures 6(a) and (b) show the TEM diagrams of the prepared nanoporous P1 and P2 thin films, respectively. The pore sizes estimated from Figure 6(a) are 8.5 nm for the loading of 30 wt % PS-b-P2VP. The pores in the MSSQ thin films could be correlated from the size of the PS-b-P2VP micelle. Here, the dimension size of the PS-b-P2VP is estimated as below. The reduced radius of the gyration,
2
of the atactic polystyrene in the glassy state according to small angle neutron scattering is a constant of 0.0278.28 Since the size of the 2-vinylpyridine is similar to that of the styrene, the radius of gyration of the PS-b-P2VP with Mn = 15,400 was estimated to be 3.4 nm according to the above relationship.
Hence, the diameter of the prepared PS-b-P2VP is around 6.8 nm, which is close to the pore size determined from Figure 6(a).
This suggests that the insignificant aggregation of the 30%
PS-b-P2VP loading in the MSSQ matrix and results in a small and uniform pore size after pyrolysis. For the case of the PS-b-PAA porogen, non-uniform pore aggregation could be observed from Figure 6(b) from the cross-section TEM. This morphology with interconnected pores suggests the aggregation of the PS-b-PAA in the MSSQ/PS-b-PAA hybrid thin films due to the strong intramolecular hydrogen bonding. Form the comparison of the PS-b-P2VP and PS-b-PAA as the porogen for the nanoporous MSSQ films, it is suggested that the intermolecular hydrogen bonding should be much stronger than the intramolecular interaction for obtaining uniform
morphology.
Table 1 shows the film thickness, refractive index, dielectric constant of the prepared nanoporous P1 and P2. The variation of the refractive index and dielectric constant of the prepared P1 and P2 with the ABC porogen loading is shown in Figures 7 and 8, respectively. The refractive index and dielectric constant of the P1 decrease from 1.361 to 1.111 and 2.67 to 1.30, respectively, as increasing the PS-b-P2VP loading from 0 to 70 wt%. The dash line shown in the Figure 7 is the refractive index predicted by Maxwell/ Garnett model.3 The significant decreasing of the refractive index and dielectric constant suggests the increased porosity in the prepared films.
The relative porosity of the prepared films estimated by the Maxwell/ Garnett model is 0 to 67.4% as increasing the PS-b-P2VP loading. The relative low dielectric constant of the prepared P1 suggests its potential application for deep submicron IC devices. The trend on the variation of refractive index of the P2 is similar to the P1 at the range below 50% of the PS-b-PAA loading but arising with further loading. It suggests that the PS-b-PAA residue might remain in the prepared P2, as suggested from the TGA analysis of Figure 4.
Conclusions
In this study, the effects of the chemical interaction on the morphology and properties of the nanoporous MSSQ films were characterized by two kinds of amphiphilic block copolymers. The experimental results supported the strong intermolecular hydrogen bonding existed between the PS-b-P2VP and the MSSQ. It created uniform nanpore distribution and relatively low dielectric constant in the prepared films. However, both intermolecular and
intramolecular hydrogen bonding existed in the
MSSQ/PS-b-PAA hybrid and resulted in a partially miscible system. Thus, the aggregation of the PS-b-PAA occurred in the prepared hybrid films and resulted in non-uniform pores. The narrow thermal decomposition of the PS-b-P2VP than that of the PS-b-PAA also resulted in a significant difference on the morphology and properties of the prepared films. Hence, the consideration of both chemical interaction and thermal characteristics are crucial on preparing nanoporous films by the templating of amphiphilic block copolymers.
Acknowledgements
We thank the National Science Council, Department of Education, and Ministry of the Economic Affairs of Taiwan for financial support of this work.
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Table 1. The film thickness, refractive index, dielectric constant of the prepared nanoporous P1 and P2.
P1 P2 60 4119 1.154 1.60 1893 1.274 1.83 70 4182 1.111 1.30 1496 1.320 2.14
Hydrogen Bonding between H atom of SiOH and N atom of 2VP block PS block
P2VP block MSSQ Precursor MSSQ/PS-b-P2VP film, 120 Co
After 400 C curingo
Figure 1 The preparation scheme for the nanoporous MSSQ film by the templating with ABC, PS-b-P2VP copolymer.
1000 980 960 940 920 900 880 860 840
70 %
IR Peak Position (cm-1)
PS-b-P2VP Loading (wt%)
increasing PS-b-P2VP loading from 0 to 70 wt%
938 cm-1 904 cm-1
Absorbance (Arbitrary unit)
Wavenumber (cm-1)
Figure 2 The FTIR absorption spectra of as-spun PS-b-P2VP/MSSQ hybrid thin films in the wavenumber
ranging from 860 to 990 cm-1. Inset shows the variation of the Si-O IR peak assigned with the PS-b-P2VP loading.
1600 1400 1200 1000 800 600
MSSQ
PS-b-P2VP
{
400 oC 120 oC 25 oC
Absorbance (Arbitrary unit)
Wavenumber (cm-1)
Figure 3 The FTIR spectra of the prepared MSSQ/PS-b-P2VP hybrid films at 25 (as-spun), 120 (after baked overnight), and 400 0C (after complete pyrolysis), respectively.
100 200 300 400 500 600 700 800 900
0 10 20 30 40 50 60 70 80 90
100 PS-b-P2VP
PS-b-PAA
Weight Loss (wt%)
Tempearature (oC)
Figure 4 The TGA curves of the PS-b-P2VP and PS-b-PAA copolymers.
70 80 90 100 110 120 130 140 150
(e) (f)
(d)
(c) (b) (a)
Heat Flow (Arbitrary Unit)
Temperature (oC)
Figures 5 The modulated DSC curves of the prepared nanocomposite thin films of MSSQ/PS-b-P2VP and MSSQ/PS-b-PAA, respectively. (a) pure PS-b-P2VP; (b) MSSQ/PS-b-P2VP, 50%; (c) MSSQ/PS-b-P2VP, 30%; (d) pure PS-b-PAA; (e) MSSQ/PS-b-PAA, 50%; (f)
MSSQ/PS-b-PAA, 30%. The percentage represents the ABC porogen loading level.
Figures 6 The TEM diagrams of the prepared nanoporous thin films. (a) P1 (plane view). (b) P2 (cross-section view).
0 10 20 30 40 50 60 70
1.05 1.10 1.15 1.20 1.25 1.30 1.35 1.40
P1 P2
Maxwell/Garnett Model
Refractive Index
ABC Porogen Loading (wt%)
Figure 7 The variation on the refractive index of the P1 and the P2 with the porogen loading. The dash line represents the Maxwell/ Garnett model.
0 10 20 30 40 50 60 70
1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6
P1 Dielectric Constant P2
ABC Porogen Loading (wt%)
Figure 8 The variation on the dielectric constant of the P1 and the P2 with the porogen loading.