An investigation of physico-chemical properties of a new polyimide-silica composites

Article abstract of DOI:10.1039/c4ra06025c

In the present study, a novel diamine 1,4-bis[4-(hydrazonomethyl)phenoxy]butane (4-BHPB) has been successfully synthesized by a facile method. 4-BHPB was reacted with pyromellitic dianhydride (PMDA) to derive a novel polyimide (PI). Highly compatibilized

Full text of DOI:10.1039/c4ra06025c

RSC Advances
View Article Online
View Journal | View Issue
PAPER
An investigation of physico-chemical properties of
a new polyimide–silica composites
Cite this: RSC Adv., 2014, 4, 46587
a
a
b
cd
d
T. Akhter, O. Ok Park, H. M. Siddiqi,* S. Saeed and Khaled Mohammad Saoud
In the present study, a novel diamine 1,4-bis[4-(hydrazonomethyl)phenoxy]butane (4-BHPB) has been
successfully synthesized by a facile method. 4-BHPB was reacted with pyromellitic dianhydride (PMDA)
to derive a novel polyimide (PI). Highly compatibilized PI–SiO
synthesized PI matrix and modified silica nanoparticles. The compatibility between organic–inorganic
O–I) components was remarkably improved by charge transfer complex (CTC) formed between PI
2
nanocomposites were tailored using
(
chains and modified silica nanoparticles. PI chains have electron donor and acceptor groups (diamine
and dianhydride portion, respectively). These groups were generated in silica nanoparticles by the
organic modification through 2,6-bis(3-(triethoxysilyl)propyl)pyrrolo[3,4-f]isoindole-1,3,5,7(2H,6H)-
tetraone (M-SiO
gel method was used for in situ synthesis of silica nanoparticles from a mixture of M-SiO
enhanced compatibility between O–I phases through CTC formation furnished PI–SiO nanocomposites
designated as OI-M) with improved thermal stability, hydrophobicity, and surface smoothness. For the
2
), which in turn, was prepared reacting PMDA with 3-aminopropyltriethoxysilane. Sol–
2
and TEOS. The
2
(
2
comparison of properties, another series of PI–SiO composites (OI-UM) were prepared dispersing
unmodified silica micro-particles into PI matrix. OI-UM hybrid system does not contain CTC between PI
matrix and silica particles. The structure of the monomer, PI, and organically modified silica network was
analyzed by FTIR, and NMR spectroscopy. Thermogravimetric analysis, FE-SEM, contact angle
measurement, and AFM were used to study thermal and morphological properties of the synthesized PI–
Received 20th June 2014
Accepted 1st September 2014
DOI: 10.1039/c4ra06025c
www.rsc.org/advances
2
SiO hybrids.
Among these techniques, sol–gel reaction is the most widely
explored method for the preparation of O–I hybrids because of
Introduction
Polyimides (PIs) have attracted substantial importance as its versatility and provision of better control over surface
12,14–17
coating materials in microelectronics, aircra, and electrical morphology.
However, inorganic silicates and organic
industries because they exhibit high mechanical and thermal polymers behave differently during the condensation process of
stability, good optical properties, excellent resistance against sol–gel reaction, as a result O–I phases are segregated from each
1–4
4,18–20
organic solvents. However, certain applications like wave- other.
Properties of O–I composites are believed to be the
guide materials, spacecra, electronics, and aircra industries result of interfacial interaction between matrix and ller pha-
2
1–24
demand more improvements in hydrophobicity, thermal prop- ses.
A number of research efforts are reported to avoid or at
5
erties, and surface smoothness. The properties and perfor- least minimize the phase separation between O–I components
mance of PIs may be improved further by the addition of nano- by adding different coupling agents.
12,25–30
Various researchers
scale silica into PI matrix giving organic–inorganic (O–I) hybrid studied the surface modication of silica particles to make
6–9
materials. O–I hybrids merge together the synergistic prop- them more compatible with organic matrix. Their studies
erties of PI matrix (ductility, and low temperature processing) demonstrated the improvement in the properties of O–I hybrid
9,15
10
with low coefficient of thermal expansion and high thermal materials. For instance, Musto et al. explored the effect of
1
0–12
stability of silica nanoparticles.
A number of techniques controlled particle size and improved morphology on the
1,2,13
have been attempted for the preparation of O–I hybrid.
properties of PI–SiO composites prepared via sol–gel method
2
by introducing g-glycidoxypropyltrimethoxysilane as interphase
coupling agent. Similarly, enhanced thermal and mechanical
properties of O–I hybrids were achieved by Khalil et al. using
a
Department of Chemical and Biomolecular Engineering (BK21+ Graduate Program),
KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea
31
b
aminophenyltrimethoxysilane as a coupling agent owning to
Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan
c
Department of Metallurgy and Materials Engineering, Pakistan Institute of controlled particles size and uniform dispersion of nano-scale
Engineering and Applied Sciences (PIEAS), Islamabad 45650, Pakistan
silica phase into PI matrix.
d
Department of Liberal Arts and Sciences, Virginia Commonwealth University in Qatar,
PO Box 8095, Doha, Qatar
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 46587–46594 | 46587

View Article Online
RSC Advances
Paper
Many researchers studied the tendency of PI chains to form
charge transfer complex (CTC) with other polymer chains
through their electron donor (diamine portion) and electron
32–34
acceptor groups (dianhydride).
The present study reports the improvement of compatibility
between polyimide matrix and silica nanoparticles through
CTC. For this purpose a new diamine 1,4-bis[4-(hydrazono-
methyl) phenoxy]butane (4-BHPB) was synthesized. 4-BHPB was
subsequently reacted with pyromellitic dianhydride (PMDA) to
prepare a novel PI. The PI chain inherently possesses electron-
donor groups (diamine portion) and electron-acceptor groups
Fig.
1
Synthesis of 1,4-bis[4-(hydrazonomethyl)phenoxy]butane
(4-BHPB).
(
-
dianhydride portion). Existence of electron-donor and
acceptor groups in the reinforcement and thereby formation of
CTC with PI chains was conrmed by the organic modication
of SiO nanoparticles. Silica nanoparticles were synthesized
using a mixture of TEOS and 2,6-bis(3-(triethoxysilyl)propyl)
pyrrolo[3,4-f]isoindole-1,3,5,7(2H,6H)-tetraone (M-SiO ). M-SiO

ask. The reaction mixture was stirred at the same temperature
for 8 h. Aer the complete consumption of the reactants, which
was observed by TLC (chloroform–methanol, 9 : 1), the reaction
mixture was cooled to room temperature. Allowing the reaction
mixture to stand for 1 h resulted in the precipitation of crude
product, which was retrieved by ltration. The dried product
was re-dissolved in THF. The nal product was precipitated by
2
2
2
was prepared by reacting APrTEOS with PMDA. Subsequently,
synthesized PI–SiO2 nanocomposites (designated as OI-M)
exhibited signicantly improved thermal, morphological, and
hydrophobic properties owing to enhanced phase connectivity
between PI matrix and silica nanoparticles, which is a direct
consequence of CTC formed between O–I components. More-
over, PI matrix and silica nanoparticles both have alkyl groups,
which may enhance the interfacial interaction between two
phases through nonpolar interactions. For the evaluation of
property enhancement and comparative study, a series of PI–SiO2
composites (OI-UM) was also prepared where the unmodied
silica network was dispersed into PI matrix and resulting
composites lack in CTC between O–I components. The
percentage of silica phase in both hybrid system (OI-M and OI-
UM) was varied from 0–30 wt% and their properties were
studied using appropriate analytical techniques.
ꢀ
cooling the solution to 0 C and was separated by ltration.
ꢀ
ꢁ
ꢁ1
Yield: 78%. m.p 262–263 C; FTIR: n (cm ) 3387, 3347
1
2
(NH2), 2942, 2863 (–CH ), 1623 (–CH]N). H NMR (300 MHz,
1
DMSO-d
6
): d ppm 7.65 ( H, s, methine proton), 6.52 (4H, s,
0
0
NH2), 4.02 (4H, s, 5, 5 ), 1.85 (4H, s, 6, 6 ), 6.91–6.88 (4H, d, J ¼
0
0
13
8
.7 Hz, 2, 2 ), 7.41–7.38 (4H, d, J ¼ 8.7 Hz, 3, 3 ); C NMR (75
0
0
MHz, DMSO-d ): d ppm 129.45 (1), 114.96 (2, 2 ), 126.96 (3, 3 ),
6
0
0
1
58.70 (4), 67.56 (5, 5 ), 25.87 (6, 6 ) 139.17 (C]N). Anal. calcd
for C18 : C, 66.24; H, 6.79; N, 17.17. Found: C, 66.04; H,
.45; N, 16.99.
Synthesis of polyimide. High molecular weight PI was
successfully synthesized by reacting diamine monomers
22 4 2
H N O
6
(
(
4-BHPB and ODA) with PMDA in a stoichiometric amount
Fig. 2). 4-BHPB (1.55 g, 4.55 mmol) and ODA (8.37 g, 41.80
Experimental
Materials
4-Hydroxybenzaldehyde (Merck, Germany), 1,4-dibromobutane
(Merck, Germany), potassium carbonate (Merck, Germany),
ethanol (Merck, Germany), hydrazine monohydrate (64–65%,
Sigma-Aldrich, USA), dimethylformamide (DMF, Merck, Ger-
0
many), 4,4 -oxydianiline (ODA, 97%, Sigma-Aldrich, USA),
pyromellitic dianhydride (PMDA, 99.2%, Sigma-Aldrich, USA),
dimethylacetamide (DMAc, 99.8%, water content <0.005%,
Sigma-Aldrich, USA), tetraethoxysilane (TEOS, 97.5%, Acros
Organics, Fair Lawn, N J), and 3-aminopropyltriehoxysilane
(APrTEOS, 95%, ABCR GmbH & Co, Germany).
Synthesis
Synthesis of 1,4-bis[4-(hydrazonomethyl)phenoxy]butane
4-BHPB). A novel and facile route was adopted for the synthesis
(
of 4-BHPB as given in Fig. 1. Hydrazine hydrate (2.5 g, 50 mmol),
˚
molecular sieves (5 g, 4 A), and absolute methanol (15 mL) were
ꢀ
stirred for 30 min under inert atmosphere at about 0–5 C. Next,
0
35,36
a solution of 4,4 -(butane-1,4-diylbis(oxy))dibenzaldehyde
1 mmol) in methanol (10 mL) was slowly added to the reaction Fig. 2 Synthesis of PI.
(
46588 | RSC Adv., 2014, 4, 46587–46594
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
a
mmol) were dissolved in anhydrous DMAc (100 mL) by Table 1 Compositions of PI–SiO
mechanically stirring the mixture for 1 h in a sealed glove box
2
hybrid films
Film composition
under inert and moisture free environment. Aer complete
PI–SiO
2
2
M-SiO :
dissolution of both the diamines in DMAc, PMDA (9.95 g, 45.61 composites
mmol) was added in one portion with continuous stirring. With
the advent of polymerization reaction, the viscosity of the
Codes
0-PI
TEOS (%)
PI (wt%)
100
SiO (wt%)
Film
F, T
2
Pristine PI
00 : 00
0
solution was rapidly increased. In order to facilitate stirring,
OI-UM
more DMAc was added to the solution. The polymerization
reaction was carried out under the complete anhydrous condi-
1
0UM
0 : 1
0 : 1
0 : 1
90
80
70
10
20
30
F, O
F, O
F, O
20UM
30UM
ꢀ
tions at about 5 C. In order to ensure anhydrides as chain-end-
groups instead of amino groups and compensate the loss of
anhydride functional group, dianhydride was added further
OI-M
1
2
30M
0M
0M
0.05 : 0.95
0.05 : 095
0.05 : 0.95
90
80
70
10
20
30
F, T
F, T
F, T
(
1% of initial weight) aer stirring the polymer solution for 6 h.
The reaction mixture was stirred under the same conditions for
8 h in order to complete the polymerization reaction. Next, the
1
a
F ¼ exible, T ¼ transparent, O ¼ opaque.
solvent elution technique was used to cast PAA lms, which
ꢀ
were thermally imidized into PI by successive heating at 100 C,
ꢀ
ꢀ
2
00 C, and 300 C each for 1 h.
Modication of silica nanoparticles. SiO2 network was
organically modied for increasing the interphase compatibility
and uniform dispersion of nanoparticles into PI matrix (Fig. 3).
compositions and codes of hybrid lms of OI-M and OI-UM
composites.
This goal was achieved by preparing 2,6-bis(3-(triethoxysilyl)
7
Characterization
propyl)pyrrolo[3,4-f]isoindole-1,3,5,7(2H,6H)-tetraone (M-SiO
2
)
by stirring APrTEOS and PMDA (2 : 1) in DMAc for 4 h. Subse-
FTIR spectroscopy. FTIR spectroscopy was employed to
quently, an appropriate amount of TEOS (40% solution in evaluate the structure of diamine monomer, nature of silica
DMAc) was added and the oligomeric solution was stirred network, and conversion of PAA into PI. The FTIR spectra of all
ꢁ1
further for 1 h.
the species were recorded in the range of 4000–400 cm on a
Synthesis of PI–SiO composites. Two types of O–I hybrids Perkin-Elmer FTIR 2000 spectrometer.
2
were prepared as discussed below.
Synthesis of polyimide-unmodied SiO
OI-UM hybrid system, unmodied silica micro-particles were synthesized monomer were elucidated by H and C NMR
dispersed in situ in PI matrix. An appropriate amount of TEOS spectroscopy. Bruker AM-300 spectrophotometer was used to
NMR spectroscopy
H and C NMR spectroscopy. Structures and purity of the
1
13
2
composite (OI-UM). In
1
13
1
13
(40 wt% solution in DMAc) was added to the PAA solution as record H and C NMR spectra at 300 MHz and 75 MHz,
given in Table 1. The reaction mixture was stirred for 1 h fol- respectively.
2
9
29
lowed by the addition of acidied water. The temperature of the
Solid state Si NMR spectroscopy. The Si NMR spectra,
ꢀ
reaction mixture was raised to 60 C followed by stirring for recorded on Bruker Advance 400WB NMR Spectrometer, were
another 6 h. Subsequently, the reaction mixture was transferred used to investigate the structure of organically modied silica
ꢀ
to Teon petri dishes and heated at 70 C for 12 h to cast OI-UM network. The hybrid lms were ground to powder form in freeze
hybrid lms. The resultant exible lms were thermally imi- drier. Various silicate nuclei were differentiated from each other
ꢀ
ꢀ
ꢀ
dized at 100 C, 200 C, and 300 C each for 1 h.
using very facile nomenclature. The silicate species having one
Synthesis of polyimide-modied SiO composite (OI-M). The organic side group were denoted by letter “T”. “Qi” were used to
2
OI-M hybrid system was prepared by in situ generation of denote the species, which do not have any organic side group,
modied silica nanoparticles. A mixture of M-SiO , PAA solu- whereas “i” stands for the number of –O–Si groups directly
2
tion, and acidied water was subjected to sol–gel reaction. Next, bonded to Si atom.
the solvent elution technique and thermal imidization process
Field emission scanning electron microscopy (FE-SEM). The
were exercised to cast and imidized hybrid lms, respectively, as surface morphology of both OI-M and OI-UM hybrid lms was
discussed earlier.
–30 wt% SiO
studied using eld emission scanning electron microscope
was incorporated into the PI matrix (both in S-4800. Flexible composite lms were dipped in liquid nitrogen,
0
2
OI-UM and OI-M) via sol–gel reaction. Table 1 presents the and then were cryo-fractured. Before examining, the samples
were sputter-coated with platinum and vaulted on aluminium
holders with the help of carbon tape. Secondary electron images
were collected with a beam voltage of 10 keV using an in-lens
detector.
Atomic force microscopy (AFM). Surface roughness analysis of
the hybrid lms was carried out with the help of multimode
digital instruments DI 3100 Nanoman AFM, Veeco Metrology,
Fig. 3 Synthesis of M-SiO
2
.
LLC, USA. Scanning probe microscopy (triangular cantilever
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 46587–46594 | 46589

View Article Online
RSC Advances
Paper
containing a pyramidal tip made of silicon nitride) soware successful formation of the product (Fig. 4). Four protons of
WSxM 3.0 Beta v. 8.1 was utilized to capture AFM micrographs amino group resonated at 6.52 ppm as singlet in the spectrum.
in taping-mode at ambient conditions.
Contact angle measurements. The contact angle of pristine PI, (5,5 ) bonded to the oxygen atom resonated at 4.02 ppm, while
OI-M, and OI-UM composites was analyzed on Phoenix 300 protons of the central methylene groups (6,6 ) appeared at 1.85
In the aliphatic region, the four protons of methylene groups
0
0
Plus, SEO Co, Ltd Contact Angle Analyzer using deionized water ppm.
as providing liquid.
Further conrmation about the structure of 4-BHPB was
1
3
Thermogravimetric analysis (TGA). TGA analysis at TG 209 F3, derived from the C NMR spectrum (Fig. 5) where the peak for
NETZSCH Germany, was used to access the thermal stability of carbon atom of azomethine group was observed at 139.17 ppm.
neat PI and its silica reinforced composites. The vacuum dried Thus, the NMR data conrmed that the structure of 4-BHPB
ꢀ
ꢁ1
samples were analyzed at a heating rate of 10 C min from 30 agreed with the proposed molecular structure.
ꢀ
to 800 C in an inert nitrogen atmosphere.
Characterization of polyimide–SiO composites
2
Results and discussion
Spectroscopic analysis
Monomer synthesis
FTIR spectroscopy. FTIR spectroscopy was used to analyze the
conversion of monomers into PAA, and PAA into PI.
Fig. 1 presents the synthesis of 1,4-bis[4-(hydrazonomethyl)
FTIR spectrum of PI (Fig. 6) shows that an absorption band at
phenoxy]butane (4-BHPB), which comprises of two steps. The
ꢁ1
0
around 1650 cm (characteristic of amide carbonyl group in PAA)
was not observed. Thus, the presence of silica network imposed no
detrimental effect on PI curing, which is in good agreement with

rst step includes synthesis of 4,4 -(ethane-1,2-diylbis(oxy))
dibenzaldehyde by the nucleophilic substitution reaction of 4-
hydroxybenzaldehyde with dibromoethane. Whereas, in the
21,37
0
the previously published reports.
Conversion of PAA into PI was
second step condensation reaction between 4,4 -(ethane-1,2-
further supported by the presence of characteristic absorption
band for asymmetric and symmetric stretching vibration of
diylbis(oxy))dibenzaldehyde and hydrazine monohydrate
resulted in the preparation of 4-BHPB. The purity and structure
of the synthesized product were comprehensively conrmed by
ꢁ
1
ꢁ1
carbonyl groups (1759 cm , 1720 cm , respectively), C–N
ꢁ
1
1
13
stretching vibration (1360 cm ), and imide ring deformation
FTIR, elemental analysis, H and C NMR spectroscopic
techniques.
ꢁ
1
(
722 cm ).
In the FTIR spectra of the OI-M and OI-UM hybrid system, a
In the FTIR spectra, the diagnostic absorption band, attrib-
ꢁ1
ꢁ1
broad absorption band around 1100–1020 cm is present, the
intensity of this broad absorption band was observed to increase as
the silica contents increased from 10–30 wt%. This absorption
band appeared because of the stretching vibrations of Si–O–Si
uted to –C]N, was observed at 1623 cm . The absorption
ꢁ1
ꢁ1
bands observed at 3387 cm and 3347 cm were assigned to
asymmetric and symmetric stretching vibrations of the amino
group, respectively. Similarly, the para-substitution pattern of
the benzene ring in the product was conrmed by an absorption
37
linkages in the silica network. Moreover, it is also evident from
Fig. 6 that the height of this band was smaller and located at lower
wavenumbers in case of the OI-M system as compared to the OI-
UM system. Thus, it may be inferred that in the OI-UM hybrid
system the structure of silica network is dense and cyclic as
compared to the OI-M hybrid system where more open silica
ꢁ1
band at 820 cm
.
1
The structure of 4-BHPB was further evaluated by the
H
NMR spectroscopy where a singlet of two protons at 7.65 ppm,
assigned to the protons of azomethine groups, conrmed the
37
networks prevailed due to preceding chemical modication.
1
13
Fig. 4 H HMR spectrum of 4-BHPB.
Fig. 5
C NMR spectrum of 4-BHPB.
46590 | RSC Adv., 2014, 4, 46587–46594
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
29
Fig. 7 Solid-state Si NMR spectrum of 10M.
Microstructural analysis
FE-SEM. The FE-SEM micrographs in Fig. 8a–d for both the
OI-M and the OI-UM hybrid systems (reinforced with 10 and 20
wt% silica) illustrate the inuence of organic modication of
the silica precursor. In the OI-UM hybrid system (Fig. 8a and c)
silica micro-beads (1–2 mm) have clear sharp boundaries and
smooth surfaces. Wide gaps between PI matrix and silica micro-
particles expose the weak interfacial interaction between O–I
components. Such phase separation and thermodynamic
immiscibility between O–I components are liable to act as stress
concentration defects rather than competent reinforcement
31
materials. On the contrary, silica nanoparticles (30–40 nm)
appear as co-continuous and nely mixed with PI matrix in OI-
M nanocomposites (Fig. 8b and d). An improved interfacial
interaction brought about a morphological transformation
2
Fig. 6 FTIR spectra of PI and PI–SiO hybrids: (a) PI, PAA (b) OI-M
hybrids (c) OI-UM hybrids.
29
29
Solid-state Si NMR spectroscopy. Si NMR spectroscopy was
used to evaluate the structure of silica network and to investigate
29
the bonding environment of silicate nuclei. Si NMR spectrum
was recorded using cross polarization magic angle spinning
(
CP-MAS) technique with a relaxation delay of 3 s. Fig. 7 presents
29
the Si NMR spectrum of 10M from the OI-M hybrid system. Silica
network in the OI-M hybrid system comprised of four different
types of silicate nuclei i.e., Si(OSi)
4
(Q4) at ꢁ110 ppm, Si(OSi)
3
(OH)
Si–C
(
Q3) at ꢁ100 ppm, Si(OSi)
2
OH
2
(Q2) at ꢁ91 ppm, and (SiO)
3
(T) at ꢁ65 ppm. These chemical shi values are in accordance with
38–40
the published literature.
The resonance peak corresponding to
(
SiO)
of silica phase in the OI-M hybrid system.
cation of silica nanoparticles resulted in uniform dispersion of
3
Si–C nuclei (T) exhibits the successful organic modication
9,38,39
The organic modi-

particles in the PI matrix and controlling the phase segregation
between O–I component imparting the advanced properties to
OI-M hybrid lms.
Fig. 8 FE-SEM micrographs (a) 10UM, (b) 10M, (c) 20UM, (d) 20M, (e)
20UM at 500 nm, (f) 20M at 5 mm.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 46587–46594 | 46591

View Article Online
RSC Advances
Paper
from a dispersed particle microstructure to nely inter-
connected and co-continuous phase morphology. An enhanced
compatibility between O–I phases through the CTC and
nonpolar interactions (Fig. 9) are credited for the uniform
dispersion of silica nanoparticles into PI matrix.
Atomic force microscopy (AFM). AFM was used to study the
surface roughness and morphology of the OI-UM and OI-M
composite systems. Fig. 10a–d illustrates the two- and three-
dimensional AFM micrographs of both the types of PI–SiO
2
hybrids. It is obvious from micrographs (Fig. 10a and b) that the
surface of the OI-UM hybrid system is occupied with contours of
different sizes. The values of root mean square roughness (Rq)
a
and average roughness (R ) were found to be 3.82 nm and 4.37
nm, respectively. In comparison, OI-M hybrid lms (Fig. 10c
and d) because of the efficient dispersion of silica nano-
particles, exhibited considerably uniform and smooth surface
Fig. 10 AFM micrographs (3 ꢂ 3 mm) of (a and b) 3D and 2D images of
OI-UM hybrid system (c and d) 3D and 2D images of OI-M hybrid
q a
with R and R values as low as 0.42 nm and 0.33 nm, respec- system.
tively. It can be concluded that the uniformity and planarity of
the OI-M hybrid lms are signicantly enhanced as compared
to hybrid lms of the OI-UM system.
Table 2 Values of contact angles of PI–SiO hybrids
2
Codes
-PI
Contact angle (q)
Contact angle measurement
0
65.5
Contact angle analysis furnishes useful information about the
hydrophobicity or hydrophilicity of the surface of the polymer OI-UM
1
2
3
0UM
0UM
0UM
74.9
79.2
83.2
materials. Hydrophobic surfaces have contact angles in the
ꢀ
ꢀ
range of about 70 to 90 , while hydrophilic surfaces have low
ꢀ
41
values (0–30 ). The values of contact angle (q) for OI-M and OI-
UM systems are given in Table 2. The variations in the contact OI-M
angle as a function of silica contents are shown in Fig. 11. In the 10M
case of the OI-M, integration of silica nanoparticles into the PI
79.6
87.3
93.3
2
3
0M
0M
ꢀ
matrix lead to an increase in the contact angle values from 65.5
ꢀ
for the pristine PI to 93.3 at 30 wt% silica loading. On the other
hand, this value increased to 83.2 in the case of the OI-UM
ꢀ
hybrid at 30 wt% silica content.
Thus, OI-M hybrid lms were found more hydrophobic with
ꢀ
an increase of 27.8 in contact angle values (0 to 30 wt%) as
compared to OI-UM hybrid lms, which exhibit an increment of
ꢀ
1
7.7 . Higher values of contact angle and enhanced hydropho-
bicity of the OI-M system as compared to OI-UM system may be
Fig. 11 Comparison of the contact angle of the OI-M and OI-UM
hybrid systems as a function of silica content.
attributed to nonpolar interactions of alkyl groups and CTC as a
result of electron donor and electron acceptor groups present
together in the O–I phases. Enhanced hydrophobicity of PI–SiO
coatings prevent the infusion of water to the substrate
2
(
e.g. metal and/or ceramics), thereby minimizing the chances of
Fig. 9 Charge transfer complex and nonpolar interactions between PI
chains and silica nanoparticles.
42
deterioration of the coating.
46592 | RSC Adv., 2014, 4, 46587–46594
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
consequently reduced the degradation at elevated tempera-
31
ture. In contrast to the unmodied silica particles, the modi-
ed silica phase was well adhered and well mixed to the matrix.

The existence of CTC because of the electron donor–acceptor
groups and nonpolar interactions due to alkyl groups simulta-
neously present in the O–I phases makes organically modied
silica nanoparticles effective heat and mass transport barriers;
hence, conserved the matrix more efficiently.
Conclusions
The charge transfer complex (through novel diamine 4-BHPB in
PI matrix and M-SiO
2
in silica nanoparticles), as well as
nonpolar interaction between the alkyl groups of O–I compo-
nents signicantly raised the compatibility between PI matrix
and nano-scale silica phase. The presence of different types of
silicate nuclei and organic modication of silica domain was
Fig. 12 Comparison of thermal stability of OI-M and OI-UM hybrid
systems.
2
9
investigated by solid state Si NMR spectroscopy. FE-SEM
micrographs revealed that M-SiO led to controlled particle size
below 50 nm), considerably reduced agglomeration, and
Thermal properties
2
The effect of improved miscibility of PI matrix with silica
nanoparticles through CTC on the thermal properties of
resulting hybrid lms was evaluated using the thermogravi-
metric analysis. The hybrid lms were analyzed at a heating rate
(
uniform dispersion of silica nanoparticles into the PI matrix in
case of OI-M hybrids. In comparison, OI-UM hybrids displayed
silica micro-beads completely detached from the PI matrix with
particle size ranging between 1–2 mm. Similarly, OI-M hybrid
ꢀ
ꢁ1
of 10 C min in the inert nitrogen atmosphere. Fig. 12 shows
the TGA thermograms of both hybrid systems reinforced with

lms were found more hydrophobic as compared to OI-UM
10–30 wt% silica phase, whereas Table 3 gives some derived
hybrid lms. Thermal stability exhibited the similar trend, i.e.
data. The thermal stability of the hybrids was assessed in terms
of temperatures at 10% weight loss (T10), and residual mass at
ꢀ
values of T10 and residual mass at 800 C remained always
higher for the OI-M hybrid system as compared to the OI-UM
hybrid system. Thus, it can be inferred that CTC between O–I
phases presented a remarkable enhancement to the eventual
properties of the OI-M hybrid system by ensuring efficient
dispersion of silica nanoparticles, strengthening interfacial
interactions, and inhibiting phase segregation.
ꢀ
8
00 C. Table 3 shows that the value of thermal decomposition
ꢀ
ꢀ
temperature T10 is 558 C for pristine PI, while it was 20 C
higher for OI-UM and 34 C higher for the OI-M hybrid system at
ꢀ
30 wt% silica content as compared to pristine PI. A comparison
of the thermal stability among both the hybrid systems
exhibited that modied silica nanoparticles raised the values of
T
10 to a greater extent as compared to the unmodied silica
ꢀ
networks. Similarly, the residual mass at 800 C has higher
values for the OI-M hybrid system as compared to the OI-UM
system. The delayed decomposition process of the PI matrix
revealed that incorporation of modied silica nanoparticles
resisted the diffusion of oxygen into the PI matrix and
Notes and references
1 M. Miki, T. Suzuki and Y. Yamada, J. Appl. Polym. Sci., 2013,
130, 54–62.
2 T. Suzuki and Y. Yamada, J. Appl. Polym. Sci., 2013, 127, 316–
322.
3
T. Akhter, S. Saeed, H. Siddiqi, O. O. Park and G. Ali, J. Polym.
Res., 2013, 21, 1–11.
Table 3 Thermal properties of pristine PI and PI–SiO
2
hybrids
4 A. Romero, M. Parentis, A. Habert and E. Gonzo, J. Mater.
Sci., 2011, 46, 4701–4709.
ꢀ
Codes
-PI
OI-M
T
10 ( C)
Residual mass (wt%)
25
5
6
7
8
9
D.-J. Liaw, K.-L. Wang, Y.-C. Huang, K.-R. Lee, J.-Y. Lai and
C.-S. Ha, Prog. Polym. Sci., 2012, 37, 907–974.
T. Akhter, S. Saeed, H. M. Siddiqi and O. Ok Park, Polym. Adv.
Technol., 2013, 24, 407–414.
T. Akhter, H. Siddiqi, S. Saeed, O. O. Park and S. Mun, J.
Polym. Res., 2014, 21, 1–10.
X. Wang, L. Wang, Q. Su and J. Zheng, Compos. Sci. Technol.,
2013, 89, 52–60.
L. Zou, S. Roddecha and M. Anthamatten, Polymer, 2009, 50,
0
558
1
2
3
0M
0M
0M
569
580
592
33
41
49
OI-UM
1
2
3
0UM
0UM
0UM
563
570
578
30
36
43
3
136–3144.
0 P. Musto, G. Ragosta, G. Scarinzi and L. Mascia, Polymer,
004, 45, 1697–1706.
1
2
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 46587–46594 | 46593

View Article Online
RSC Advances
Paper
1
1
1
1 M. Nandi, J. A. Conklin, L. Salvati and A. Sen, Chem. Mater., 29 C. Joly, M. Smaihi, L. Porcar and R. D. Noble, Chem. Mater.,
991, 3, 201–206. 1999, 11, 2331–2338.
2 P. Musto, M. Abbate, M. Lavorgna, G. Ragosta and 30 J. W. Gilman, C. L. Jackson, A. B. Morgan, R. Harris,
1
G. Scarinzi, Polymer, 2006, 47, 6172–6186.
E. Manias, E. P. Giannelis, M. Wuthenow, D. Hilton and
S. H. Phillips, Chem. Mater., 2000, 12, 1866–1873.
31 M. Khalil, S. Saeed and Z. Ahmad, J. Appl. Polym. Sci., 2008,
107, 1257–1268.
3 Y. Li, G. He, S. Wang, S. Yu, F. Pan, H. Wu and Z. Jiang, J.
Mater. Chem. A, 2013, 1, 10058–10077.
1
1
4 G. Kickelbick, Prog. Polym. Sci., 2003, 28, 83–114.
5 G. Ragosta, P. Musto, M. Abbate and G. Scarinzi, J. Appl. 32 B. V. Kotov, T. A. Gordina, V. S. Voishchev, O. V. Kolninov
Polym. Sci., 2011, 121, 2168–2186.
6 A. Morikawa, H. Yamaguchi, M. Kakimoto and Y. Imai,
and A. N. Pravednikov, Polym. Sci. U.S.S.R., 1977, 19, 711–
716.
1
Chem. Mater., 1994, 6, 913–917.
7 Z. Ahmad and J. E. Mark, Chem. Mater., 2001, 13, 3320–3330.
33 M. Nishihara, L. Christiani, A. Staykov and K. Sasaki, J.
Polym. Sci., Part B: Polym. Phys., 2014, 52, 293–298.
1
1
1
8 H. Zou, S. Wu and J. Shen, Chem. Rev., 2008, 108, 3893–3957. 34 J. Ferraris, D. O. Cowan, V. Walatka and J. H. Perlstein, J. Am.
9 Y. Deng, M. Wu, X. Liu and Y. Gu, Eur. Polym. J., 2010, 46,
255–2260.
0 C. Kizilkaya, S. Karata ¸s , N.-K. Apohan and A. G u¨ ng ¨o r, J. Appl.
Polym. Sci., 2010, 115, 3256–3264.
1 S. Saeed, M. Khalil and Z. Ahmad, J. Macromol. Sci., Part A:
Pure Appl.Chem., 2009, 46, 152–162.
Chem. Soc., 1973, 95, 948–949.
35 K. Balaji and S. C. Murugavel, J. Polym. Sci., Part A: Polym.
Chem., 2011, 49, 4809–4819.
36 A. Tabbusum, Design, synthesis & characterization of some
novel multifunctional molecules, Quaid-i-Azam University,
Islamabad 45320, Pakistan, 2012.
2
2
2
2
2
2 S. Sinha Ray and M. Okamoto, Prog. Polym. Sci., 2003, 28, 37 W. Stober, A. Fink and E. Bohn, J. Colloid Interface Sci., 1968,
539–1641. 26, 62–69.
3 C. J. Cornelius and E. Marand, J. Membr. Sci., 2002, 202, 97– 38 Y. Gao, N. R. Choudhury, N. Dutta, J. Matisons, M. Reading
18. and L. Delmotte, Chem. Mater., 2001, 13, 3644–3652.
4 C. J. Cornelius and E. Marand, Polymer, 2002, 43, 2385–2400. 39 V. Moravetski, J. R. Hill, U. Eichler, A. K. Cheetham and
1
1
2
2
5 S. H. Wang, Z. Ahmad and J. E. Mark, Chem. Mater., 1994, 6,
43–946.
6 L. Mascia and A. Kioul, Polymer, 1995, 36, 3649–3659.
J. Sauer, J. Am. Chem. Soc., 1996, 118, 13015–13020.
40 D. H. Gray, S. Hu, E. Juang and D. L. Gin, Adv. Mater., 1997, 9,
731–736.
9
2
2
7 S. Al-Kandary, A. A. M. Ali and Z. Ahmad, J. Mater. Sci., 2006, 41 C. J. Chen, H. T. Lu, W. Y. Tseng, I. H. Tseng, S. L. Huang and
1, 2907–2914. M. H. Tsai, J. Appl. Polym. Sci., 2011, 122, 648–656.
8 S. Al-Kandary, A. A. M. Ali and Z. Ahmad, J. Appl. Polym. Sci., 42 Y. Chen and J. O. Iroh, Chem. Mater., 1999, 11, 1218–1222.
005, 98, 2521–2531.
4
2
2
46594 | RSC Adv., 2014, 4, 46587–46594
This journal is © The Royal Society of Chemistry 2014