Ternary composites by an: In situ hydrolytic polymerization process

Article abstract of DOI:10.1039/c8ra02402b

Polyamide 6/modified silica composite materials have been prepared by a coupled polymerization procedure. For this purpose, the three-component-system we presented in a previous publication, consisting of ?-aminocaproic acid (?-ACA), ?-caprolactam (?-CL), and 1,1′,1′′,1′′′-silanetetrayltetrakis-(azepan-2-one) (Si(?-CL)₄), has been combined with other silicon monomers with one or two methyl groups (MeSi(?-CL)₃ and Me₂Si(?-CL)₂). The simultaneous polymerization of ?-CL and silicon monomers leads to the in situ formation of silica/polysiloxane particles and the surrounding polyamide 6 matrix in one step. Moreover, 3-aminopropyltriethoxysilane has been added to the three-component-system to achieve covalent bonding between organic and inorganic phases and to inhibit agglomeration of the silica particles. Chemical structures and morphologies of the composites have been investigated by solid-state NMR and FTIR spectroscopy as well as electron microscopy and SEC measurements. Structural effects on thermal properties have been studied by DSC and TGA measurements.

Full text of DOI:10.1039/c8ra02402b

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Ternary composites by an in situ hydrolytic
polymerization process†
Cite this: RSC Adv., 2018, 8, 14713
a
a
a
b
b
a
K. Nagel,
L. Kaßner, A. Seifert, R.-E. Grutzner, G. Cox and S. Spange*
¨
Polyamide 6/modified silica composite materials have been prepared by a coupled polymerization
procedure. For this purpose, the three-component-system we presented in a previous publication,
0
00 000
consisting of 3-aminocaproic acid (3-ACA), 3-caprolactam (3-CL), and 1,1 ,1 ,1 -silanetetrayltetrakis-
azepan-2-one) (Si(3-CL) ), has been combined with other silicon monomers with one or two methyl
groups (MeSi(3-CL) and Me Si(3-CL) ). The simultaneous polymerization of 3-CL and silicon monomers
(
4
3
2
2
leads to the in situ formation of silica/polysiloxane particles and the surrounding polyamide 6 matrix in
one step. Moreover, 3-aminopropyltriethoxysilane has been added to the three-component-system to
achieve covalent bonding between organic and inorganic phases and to inhibit agglomeration of the
silica particles. Chemical structures and morphologies of the composites have been investigated by
solid-state NMR and FTIR spectroscopy as well as electron microscopy and SEC measurements.
Structural effects on thermal properties have been studied by DSC and TGA measurements.
Received 19th March 2018
Accepted 2nd April 2018
DOI: 10.1039/c8ra02402b
rsc.li/rsc-advances
interfacial attraction between inorganic particles and polymer
matrices is the surface modication of the ller. Especially for
Introduction
Polyamides are widely used in automotive industry, electronics, silica nanoparticles, there are manifold possibilities to improve
and packaging due to their mechanical performance, stiffness particle dispersion in organic polymers by surface modication.
and strength, chemical resistance, and good processability. One way is the increase of hydrophobicity of the silica particles
However, especially in load-bearing appliances, such as under- by alkylation reactions of silanol groups with alkoxy- or chlor-
the-hood applications, the low heat deection temperature, oalkylsilanes. The surface modication leads to a reduced
1
4
high water absorption, and dimension instability limit the use interfacial energy between nanoparticles and organic polymer.
1
of polyamides. A possibility to overcome some of these draw- But oen reproducibility is problematic due to exact setting of
backs is the incorporation of inorganic llers into the poly- reaction parameters like temperature, molar ratios, humidity,
15–18
amide matrix. Especially the use of ller particles with domain or mixing conditions.
Another route is the attachment of
size in the nanometer scale (<100 nm) has caused considerable functional groups on the silica surface, which can interact with
attention in material research for over twenty years owing to the the polymer matrix by electrostatic forces, hydrogen bonds, or
20,21
19
strong improvement of the mechanical strength and thermal covalent bonding. In literature, 3-amino-, 3-methacryloxy-,
2
22
resistance even at low ller content. Various inorganic llers or 3-glycidoxypropyltrialkoxysilanes are used to obtain phase-
2–5
6,7
such as clay minerals, carbon materials, and silica nano- interconnected nanocomposites. Preformed functionalized
8–12
particles
have been used for this purpose. The strong silica particles are used in most approaches for nanocomposite
improvement of rigidity and stability observed in polymer synthesis. These particles are mixed with monomer or dissolved
nanocomposites, in comparison with conventional macro- or molten polymers. In the rst case, monomer is polymerized
20,23
scopic composites, is caused by the large interface boundary in situ around the ller particles.
Beside the use of pre-
between both components relative to the volume. Therefore, it formed functionalized silica particles, the in situ formation of
is important to achieve a homogeneous distribution of the ller by the sol–gel process during polymerization of the organic
2
4,25
inorganic ller, which is still a major challenge because of the matrix
or by mixing with polymer are possible routes for the
13
pronounced agglomeration tendency of nanoparticles. One synthesis of nanocomposites. In a previous work we introduced
possible solution to retard agglomeration and to increase the a method for the one-pot synthesis of polyamide 6 (PA6)/silica
composite materials by a coupled polymerization reaction of
3
-caprolactam (3-CL), 3-aminocaproic acid (3-ACA), and
0
00 000
26
a
1,1 ,1 ,1 -silane-tetrayltetrakis-(azepan-2-one) (Si(3-CL)
4
, 1).
Polymer Chemistry, Technische Universit a¨ t Chemnitz, 09107 Chemnitz, Germany.
E-mail: stefan.spange@chemie.tu-chemnitz.de
The silicon monomer serves as precursor for PA6 as well as SiO .
The water, which is necessary for the hydrolysis of 1 and the
formation of the organic and inorganic polymers, is delivered by
2
b
BASF SE, 67056 Ludwigshafen am Rhein, Germany
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ra02402b
1
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Synthesis of monomer 2 and 3
In a typical procedure, 3-CL (20.0 g, 0.177 mol) was dissolved
under stirring in dry toluene (300 mL). Triethylamine (20.2 g,
0
.200 mol) was added in slight excess in a single portion. This
solution was cooled by water bath and either MeSiCl (8.82 g,
.059 mol) or Me SiCl (11.49 g, 0.089 mol), dissolved in 100 mL
3
0
2
2
of dry toluene, was added slowly through a dropping funnel
under vigorous stirring. Immediately, a white precipitate from
triethylammonium chloride was observed. Subsequently, the
solution was stirred at room temperature for 16 h.
Triethylammonium chloride was separated by ltration and the
solvent was removed under vacuum. 2 was recrystallized in n-
hexane to remove residual 3-CL. Compound 3 can be puried
ꢀ
by Kugelrohr distillation at 170 C and 0.5 mbar.
0
00
1
,1 ,1 -Methylsilanetriyltris(azepan-2-one) (2). Colorless
solid. Yield: 90%. d (250 MHz; CDCl ): 0.68 (3H, s, SiCH ), 1.69
H
3
3
(
18H, m, CH ), 2.48–2.52 (6H, m, C(O)CH ), 3.21 (6H, m, NCH ).
2 2 2
d
(63 MHz; CDCl ): ꢁ0.3 (SiCH ), 23.6, 29.8, 30.4 (CH ), 38.4
C
3
3
2
(C(O)CH
2
2 3
), 45.2 (NCH ), 183.5 (C(O)). dSi (50 MHz; CDCl ):
ꢁ
1
ꢁ
19.2. nmax/cm : 2923, 2855 (CH), 1638 (CO), 920 (SiN), 729
(
SiC). Found: C, 59.7; H, 9.0; N, 10.9. Calc. for C19 Si: C,
33 3 3
H N O
Scheme
1 Strategy for fabrication of PA6/silica/polysiloxane
6
0.1; H, 8.8; N 11.1.
composites by coupling the reaction of silicon monomers 1 and 2 or 3
with the polycondensation of 3-ACA among the hydrolytic ring
opening polymerization of 3-CL.
0
1,1 -Dimethylsilandiylbis(azepan-2-one) (3). Colorless oil.
Yield: 96%. dH (250 MHz; CDCl ): 0.42 (6H, s, SiCH ), 1.58–1.68
3
3
(
12H, m, CH ), 2.48–2.52 (4H, m, C(O)CH ), 3.27–3.30 (4H, m,
2
2
NCH ). d (63 MHz; CDCl ): ꢁ0.3 (SiCH ), 23.7, 30.0, 30.2, (CH ),
2
C
3
3
2
3
8.2 (C(O)CH
2
), 45.0 (NCH
2
), 183.6 (C(O)). dSi (50 MHz; CDCl
3
):
the condensation of 3-ACA. This amino acid also catalyzes the
hydrolytic polymerization of 3-CL. The latter has two different
functions during composite synthesis as well. First, it is
necessary for the adjustment of the nal ller content.
Furthermore, it facilitates homogenization of the three reaction
components. Therefore, it is necessary to adjust the stoichi-
ometry of all reaction components precisely.
ꢁ
1
ꢁ
0.7. nmax/cm : 2925, 2855 (CH), 1644 (CO), 918 (SiN), 718
Si: C,
(
SiC). Found: C, 58.4; H, 9.3; N, 9.6. Calc. for C14
H
26
N
2
O
2
59.5; H, 9.3; N, 9.9.
Synthesis of composites
In this strategy, the coupling of the polymerization reactions
leads to the formation of nanometer sized silica particles
embedded in a PA6 matrix.
Composites were synthesized in a high-pressure lab autoclave
of Berghof company. A Teon beaker was used as insert. Prior to
heating to the dened reaction temperature (Table 1), the
reactants 3-ACA, 3-CL, 1, and the fourth reaction component 2–4
were lled in the autoclave in the specied molar ratios (Table
26
In the here presented work, the established three-
component-system is extended by an additional, methyl-
substituted silicon monomer to a four-component-system (see
Scheme 1). The polymerizations of the heterobifunctional
silicon monomer 1 as well as 2 and 3, respectively, in combi-
nation with the hydrolytic 3-CL polymerization, allows the
synthesis of homogeneous PA6/silica/polysiloxane composites
in one single step. By simultaneous polymerization of different
silicon monomers, inorganic particles with covalently linked
polar silica and nonpolar polysiloxane units could be obtained.
1) at room temperature. Then it was purged thrice with argon up
to 8 bar with relaxation to atmospheric pressure in between. The
reaction took place under an initial argon pressure of 8 bar for
at least 210 min, including ca. 60 min heating phase. During
reaction, pressure increased to approximately 14 bar. 15 min
before the reaction termination, the pressure was released
slowly to start the post-condensation phase. Aer cooling down
to ambient temperature, the white to beige-colored monolithic
samples were crushed into smaller pieces. To remove residual
monomers and oligomers the composites were puried by
soxhlet extraction for 48 h with methanol and dried in a vacuum
Furthermore,
commercially
available
3-amino-
propyltriethoxysilane (APTES, 4) has been proven suitably as
fourth reaction component. In this case, the primary amino
functionality allows a covalent bonding between the organic
PA6 phase and the inorganic silica phase.
ꢀ
oven at 40 C to constant mass.
For sample nomenclature an explanation should be given:
letter P means polymerization product. In reference sample
P_0, which was synthesized according to the same polymeriza-
tion procedure without the addition of silicon monomers, no
Experimental
Materials and methods are described in ESI.†
2
ller is present. In second reference P_SiO only bare silica
arises during reaction.
26
Synthesis of monomer 1 is known from the literature.
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Table 1 Polymerization experiments for PA6 composites including molar ratios of reactants, reaction temperatures and amounts of extractables
Molar ratios of reactants
4
Reactant
Reaction
Amount of
No.
Filler amount
0
3-ACA
Si(3-CL)
4
1
3-CL
(monomer)
temperature
extractables
ꢀ
b
P_0
P_SiO
P_2_2.2
P_2_2.5
1.0
4.0
0
4.4
—
—
1.0 (2)
1.0 (2)
1.0 (2)
1.0 (2)
1.0 (2)
1.0 (2)
0.25 (3)
0.54 (3)
0.78 (3)
1 (3)
0.002 (0.2 mol% 4)
0.005 (0.5 mol% 4)
0.25 (20 mol% 4)
0.25 (20 mol% 4)
230 C
10 wt%
ꢀ
b
2
2.0 wt% SiO
2.2 wt% SiO
2.5 wt% SiO
5.1 wt% SiO
3.3 wt% SiO
3.3 wt% SiO
3.3 wt% SiO
2.1 wt% SiO
2.4 wt% SiO
2.7 wt% SiO
2
2
2
2
2
2
2
2
2
2
1.0
4.0
2.0
4.0
3.6
3.6
3.6
0.75
0.46
0.22
0
18.0
83.9
42.0
11.3
36.6
36.6
36.6
18.8
20.1
21.4
22.1
18.1
18.1
23.0
2.0
230 C
11 wt%
ꢀ
a
/MeSiO1.5
/MeSiO1.5
/MeSiO1.5
/MeSiO1.5
/MeSiO1.5
/MeSiO1.5
/Me
/Me
/Me
19.0
11.0
19.1
17.5
17.5
17.5
3.5
2.8
2.2
1.7
4.0
230 C
13 wt%
ꢀ
a
230 C
7 wt%
ꢀ
a
P_2_5.1
230 C
4 wt%
ꢀ
b
P_2_3.3-210
P_2_3.3-220
P_2_3.3-230
P_3_2.1
P_3_2.4
P_3_2.7
210 C
15 wt%
ꢀ
b
220 C
14 wt%
ꢀ
b
230 C
12 wt%
ꢀ
b
2
2
2
SiO
SiO
SiO
230 C
24 wt%
ꢀ
b
230 C
18 wt%
ꢀ
b
230 C
26 wt%
ꢀ
b
P_3_2.9
2.9 wt% Me
2.0 wt% SiO
2.0 wt% SiO
2.0 wt% SiO
6.0 wt% SiO
2
SiO
230 C
28 wt%
ꢀ
b
P_4_2.0-1
P_4_2.0-2
P_4_2.0-3
P_4_6.0
2
2
2
2
/APTES
/APTES
/APTES
/APTES
1.0
1.0
1.0
1.0
230 C
12 wt%
ꢀ
b
4.0
5.0
4.0
230 C
11 wt%
ꢀ
b
230 C
7 wt%
ꢀ
b
230 C
10 wt%
a
b
2
4 h H
2
O, 24 h EtOH. 48 h MeOH.
For all other samples, the rst number P_X stands for the groups at the surface of the silica particles would occur,
second silicon monomer used as fourth reaction component. covering or blocking free silanol groups. Thus, the condensa-
The following number P_X_X stands for the ller content. If tion reaction leading to agglomeration and the formation of
existent, the last number P_X_X-X stands for variations for the silica particles in the mm-scale should be inhibited. The effect of
given ller content, e.g., in reaction temperature or concentra- introduced methyl units on materials' homogeneity is shown
tion of fourth reaction component. Samples P_3_X contain later with the help of electron microscopic images as well as the
monomer 1 and 3 in different ratios; the overall amount of analysis of thermal behavior of composite materials.
silicon monomers is equal to the molar amount of monomer 1
in P_SiO . Because of the different molecular weights of SiO
and Me SiO-units, the ller content rises with increasing
The chemical structure of composite materials was investi-
gated by means of ATR-FTIR spectroscopy as well as solid-state
2
2
-
1
3
1
29
1
2
C{ H} NMR and Si{ H} NMR spectroscopy. FTIR spectra
ꢁ
1
amount of 3.
indicate typical bands for PA6 like amide I at 1636 cm and
ꢁ
1
ꢁ1
amide II at 1536 cm as well as N–H vibration at 3295 cm
.
Results and discussion
The four reaction components 3-aminocaproic acid, 3-capro-
lactam, monomer 1, and one of the monomers 2, 3, or 4 were
used for composite material synthesis (see Scheme 1 and Table
1). For these four-component-systems, homogeneity, thermal
properties, and molar mass distribution in dependency of ller
content, molar ratios of the reactants, and reaction conditions
were investigated. The inuence of added monomer 4 was
studied additionally in perspective to an enhanced interaction
between organic and inorganic phases due to covalent bonding.
Owing to the different types of interactions between organic and
inorganic phase, results for composites with poly-
methylsiloxane units (monomers 2 and 3) and composites with
chemical bonding introduced by monomer 4 are presented
separately.
Polyamide 6/silica/polymethylsiloxane composites
Through combination of silicon monomers 1 and 2 or 3 during
Fig. 1 ATR-FTIR spectra of samples P_2_2.2 and P_3_2.4 synthesized
with different fourth components in comparison to pure PA6 (P_0) and
polymerization,
a silica/polysiloxane network, containing
2 2
methyl groups, is formed. Favorably, an assembly of methyl PA6/SiO composite (P_SiO ).
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Table
2
Results of size exclusion chromatography of extracted
2
composites P_2 and P_3 compared to P_0 and P_SiO , including
number average and weight average molecular weight as well as
polydispersity (PDI). The given filler content is theoretically calculated
from the reactant quantity
Polymer
Filler
content
[wt%]
M
n
M
w
w
PDI M /
ꢁ
1
ꢁ1
Sample
P_0
Filler
[g mol ] [g mol ] M
n
0
—
17 500
17 400
/MeSiO1.5 17 100
16 400
48 000
49 700
45 700
43 600
37 400
53 500
47 500
44 500
32 600
41 700
36 800
28 900
2.7
2.8
2.7
2.7
2.4
3.1
2.8
2.7
2.3
2.5
2.4
2.4
P_SiO
2
2.0
2.2
2.5
5.1
SiO
SiO
2
P_2_2.2
P_2_2.5
P_2_5.1
2
15 500
17 500
17 000
16 500
13 900
16 500
15 500
12 200
P_2_3.3-210 3.3
P_2_3.3-220 3.3
P_2_3.3-230 3.3
P_3_2.1
P_3_2.4
P_3_2.7
P_3_2.9
2.1
2.4
2.7
2.9
SiO
2
/Me
2
SiO
2
Me SiO
Solid-state NMR spectra of selected samples are shown in
1
3
Fig. 2. All PA6-typical signals can be assigned in these state
{
C
1
H} CP/MAS NMR spectra. Additionally, position of carbon
signals 2–6 between 20–45 ppm indicate crystallization mainly
in a-modication, conrming the results from FTIR measure-
30
ments. Methyl groups 7 and 8 are detectable between 0 ppm
2
9
1
and ꢁ5 ppm. The Si{ H} CP/MAS NMR spectrum of P_SiO2
shows Q and Q
signals at ꢁ100 ppm and ꢁ110 ppm, respec-
tively, resulting from a strongly cross-linked silica network.
Composites synthesized under addition of methyl derivatives 2
3
4
31
31,32
and 3 further show T
a broad D
signal at ꢁ16 ppm, respectively.
Pure PDMS with its long exible chains leads to a narrow
2
and T
3
signals at ca. ꢁ64 ppm
and
3
3,34
2
2
9
34
signal at ꢁ22 ppm in Si NMR spectroscopy. The observed
1
3
1
29
1
Fig. 2 Solid-state C{ H} CP/MAS (top) and Si{ H} CP/MAS NMR
broadening of D signal and shi to lower eld is caused by the
2
spectra (bottom) of the extracted samples P_SiO
2
, P_2_5.1, and
P3_2.1.
2
bonding of short chain segments of Me SiO-units on the silica
surface and the resulting change in bond angles and
35
distances.
To further investigate the formation of
ꢁ1
polydimethylsiloxane/silica particles, sample P_3_2.7 was
extracted with dichloromethane due to the better solubility of
PDMS. The extracted composite material still show signals of
siloxane units (Fig. S5 in ESI†). Additionally, the sample P_3_2.4
was dissolved in formic acid and the insoluble inorganic phase
was separated by centrifugation and washed with deionized
water three times. The obtained white powder was dried and
studied by FTIR spectroscopy (Fig. S6 in ESI†). Absorption
Furthermore, the typical Si–O stretching vibration at 1073 cm
for SiO is observed (see Fig. 1 and S3†). Due to sub-
2
stoichiometric contents of monomer 2 or 3, the polymers
contain little polysiloxane and therefore no signicant IR bands
for methyl groups bonded to silicon atoms are observed in the
ꢁ
1
range of 865–750 cm . However, for the weak IR band due to
the methyl group's symmetric deformation vibration between
ꢁ
1
1
250 and 1260 cm an increase in intensity compared to P_0
ꢁ
1
ꢁ1
ꢁ1
bands at 1262 cm (CH
3
deformation), 846 cm and 800 cm
and P_SiO could be detected.
2
(
Si–C stretching vibration) next to the band of the asymmetric
ATR-FTIR spectra give additional information about the
crystallization behavior of PA6. The positions of the amide V
ꢁ
1
Si–O–Si stretching vibration at 1000–1100 cm
prove the
ꢁ
1
ꢁ1
presence of polydimethylsiloxane fragments. Therefore, the
formation of polysiloxane/silica particles could be expected.
Organic polymers were investigated by size exclusion chro-
matography in 1,1,1,3,3,3-hexauoro-2-propanol as shown on
selected examples in Fig. S7–S9† and Table 2. For all samples,
a monomodal molecular mass distribution was observed. The
band at 690 cm and amide VI band at 580 cm indicate
2
7–29
crystallization mainly in a-modication.
Variation of reac-
ꢀ
ꢀ
tion temperature between 210 C and 230 C seems to have no
inuence on the chemical structure of the obtained composites,
indicated by the non-changing PA6-typical IR bands in Fig. S3.†
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Table
3
Thermogravimetric analysis of P_0, P_SiO
2
,
P_2_2.2, with rising reaction temperature, since equilibrium conditions
ꢀ
P_2_3.3, and P_2_5.1, temperature program 30–700 C, heating rate
are not attained due to the relatively short reaction times.
ꢁ
ꢀ
1
20 K min
under He, then gas change for temperature range
ꢁ1
ꢀ
Samples containing monomer 3 have lower molecular
7
00ꢁ900 C, heating rate 40 K min under air; 900 C for 15 min; filler
a
weights than P_0 or P_SiO (Fig. S9 in ESI†), whereas molecular
content is theoretically calculated
2
weight decreases and amount of extractables rises with
Filler content [wt%] increasing amount of monomer 3.
ꢀ
Sample
P_0
T
D
( C)
Residue [wt%]
Methanol extracts of samples P_0, P_2_3.3_210, and
P_2_3.3_230 were investigated by SEC and H NMR spectros-
copy (Fig. S10 in ESI†). All extracts show comparable composi-
tions, containing a major proportion of unreacted 3-
458.1
454.7
458.4
470.9
464.9
463.5
468.3
0.1
1.8
2.0
2.7
2.6
1.3
3.9
0
1
P_SiO
2
2.0
2.2
3.3
3.3
3.3
5.1
P_2_2.2
P_2_3.3-210
P_2_3.3-220
P_2_3.3-230
P_2_5.1
caprolactam and smaller fractions of dimers, trimers, and
higher oligomers as described in literature for related synthetic
36–40
procedures.
a
T
D
– maximum of rst derivation; wt% residue referred to mass at
Thermal properties of PA6 composites were investigated by
thermogravimetric analysis as well as differential scanning
calorimetry (see Table 3, Fig. 3, S11, S12† and Table S1). TGA
measurements show an increased temperature stability with
higher ller contents.
ꢀ
50 C.
2
For sample P_0 without ller a decomposition temperature
ꢀ
T of 458 C is observed, while P_2_5.1 with 5.1 wt% ller shows
D
ꢀ
an increased T
D
of 468 C.
2
Most likely, the containing SiO protects the organic polymer
against degradation to a certain degree. The use of 1 as only
silicon monomer leads to composites; which show a slight
increase in thermal stability at low ller content of 1 wt% but
26
the opposite effect for higher ller contents. Hence, addition
of 2 has a positive effect on thermal stability at higher silica
content. Beside the amount of ller, also reaction temperature
has an inuence on decomposition behavior. Samples with
constant ller content of 3.3 wt% but different reaction
temperatures (P_2_3.3) show a decreasing T
D
with increasing
ꢀ
ꢀ
polymerization temperature from 210 C up to 230 C. Differ-
ences between mass of residue and theoretical ller content can
be ascribed to inhomogeneities within the composites.
In contrast to this, but accordingly to ATR-FTIR and solid-
Fig. 3 TGA measurements of extracted samples P_2_2.2, P_2_3.3-
2
30, and P_2_5.1 with increasing filler content compared to P_0 with
1
3
1
ꢀ
gas change from helium to air and change of heating rate at 700 C. state C{ H} NMR measurements, both reaction temperature
ꢀ
Mass loss is referred to mass at 250 C.
as well as ller content have no signicant inuence on crys-
tallization behavior as observed in DSC measurements (see ESI,
Table S1 and Fig. S11†). The composites show a melting peak at
results of samples obtained from monomer 2 as fourth reactant
(P_2_2.2, P_2_2.5, and P_2_5.1 in comparison to P_0 (Fig. S8†))
show a minor decrease in molar mass with increasing ller
content, whereas polydispersity hardly changes at all. Further-
more, average molecular weight as well as polydispersity for
samples P_2_3.3_210, P_2_3.3_220, and P_2_3.3_230 decrease
with elevated reaction temperature as shown in Table 2. The
broadening of the molecular weight distribution may be
explained by an increase in viscosity with decreasing tempera-
ture as well as formation of crystallites because reaction
temperatures are in range of the melting temperature of the
resulting PA6. Therefore, trapping of reactive chain ends,
diffusion effects, and non-equilibrium conditions can lead to an
additional broadening of the molecular weight distribution as
28
has been published for the solid-state polymerization of PA6.
The decreasing extractable contents of the samples P_2_3.3-
10, P_2_3.3-220, and P_2_3.3-230 indicate the faster reaction
Fig. 4 Scanning electron microscopic images of samples P_2_2.2,
P_2_2.5, and P_2_5.1 as well as P_3_2.1, P_3_2.4, and P_3_2.7.
Distribution of silicon is marked cyan.
2
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Fig.
5 Bright-field TEM images of sample P_2_2.2 in different
magnifications (freezing ultra-thin cut).
ꢀ
ꢀ
ca. 220 C with a shoulder at 210 C during the second heating
in a cyclic measurement.
The doublet melting indicates the presence of g- and a-
ꢀ
ꢀ
crystallites, which melt around 212–215 C and 220–225 C,
respectively.
41,42
However, crystal size and perfection as well as

llers can inuence the melting temperature. Li et al. observed
ꢀ
different melting peaks above 200 C for isothermal crystallized
PA6, which could be assigned to a-crystals of different size and
perfection by XRD. For this reason, melting peaks of the
43
I
m
II
m
second heating are just labeled as T and T without assign-
ment to crystal modication. The melting peak in the rst
Scheme
2
Illustration of synthesis of PA6/silica/polysiloxane
heating curve could be assigned to the a-modication, veried composites with covalent bonding between the organic and inorganic
1
3
phase introduced by 3-aminopropyltriethoxysilane 4.
by C solid-state NMR and FTIR spectroscopy.
Electron microscopic images show homogeneity of
composites (Fig. 4, and S13ꢁS17 in ESI†) synthesized as four-
component-system with monomer 2.
Additionally, ionic interactions between ammonium and
Generally, an increasing tendency for agglomeration can be carboxylate units as well as the formation of hydrogen bonds are
observed with increasing ller content as well as for increasing possible. Two different total ller amounts of 2 wt% and 6 wt%
polymerization temperature. For sample P_2_2.2 with 2.2 wt% were chosen for investigations.

ller the most homogeneous particle distribution could be
Furthermore, overall ller content of 2 wt% was examined in
realized compared to higher ller contents up to 5.1 wt%. For detail with regard to different contents of 4 as fourth reaction
P_2_2.2 relatively loose agglomerates as well as very homoge- component. The molar fraction of 0.2 mol% to 20 mol% given
neous areas were obtained. TEM images validate this observa- in Table 1 is related to the total amount of silicon species used
tion (Fig. 5). The obtained particles are a few micrometers in as reagent.
size consisting of primary particles of around 5 nm. For ller
content of 5.1 wt% agglomerates of several hundred mm were investigated but also thermal properties have been of interest.
received. For composites P_3 with varying portion of monomer
Chemical structure of composite materials was investigated
a slightly better homogeneity can be observed, but there are by ATR-FTIR spectroscopy (ESI, Fig. S22†). Stretching vibrations
Especially, distribution and size of silica particles were
3
some larger agglomerates with sizes of a few mm (see Fig. 4, and of N–H, C–H, and Si–O bonds were observed as well as amide I
S18–S21 in ESI†). Probably, capping of the surface silanol and II bands. Probably because of the small fraction of 4, no
groups lead to an inhibition of hydrogen bonding between silica signicant differences to P_SiO
2
can be detected.
1
3
1
particles and PA6 chains. Hence, the methyl groups of mono-
The solid-state C{ H} NMR spectrum of P_4_6.0 (ESI,
mer 2 and 3 did not result in the purposed improvement of Fig. S23†) shows signals for PA6 which indicate crystallization
homogeneity compared to systems with the exclusive use of mainly in a-modication. No signal for the propyl group could
2
9
1
monomer 1.
be detected due to the low content of 4. The Si{ H} NMR
spectrum (ESI, Fig. S23†) shows Q and Q as well as T signals,
4
3
3
proving the formation of a highly cross-linked silica network
with siloxane units.
Covalent linkage between organic and inorganic phase
Extraction experiments with 2,2,2-triuoroethanol (TFE) are
an useful tool for the investigation of interactions between
organic and inorganic phase because PA6 covalently bonded to
silica is not soluble in TFE. As a result, the amount of residue
was signicantly higher than the theoretical calculated ller
content (Tables 1 and 4). Moreover, signals of PA6 could be
detected beside typical bands for silica and polysiloxane units
Beside investigations of four-component mixtures with methyl-
substituted monomers 2 or 3 also 3-aminopropyltriethoxysilane
4
was added to the reaction mixture as fourth component with
additional functionality. The amino function leads to formation
of amide bonds between carbonic acid functionalities of the
organic PA6 matrix and the inorganic silica phase due to the
high reaction temperature (Scheme 2).
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Table 4 Results of soxhlet extraction with 2,2,2-trifluoroethanol (TFE), behavior is discussed as result of a decrease in lamellar thick-
a
and TGA measurements (n. a. not analyzed)
44
ness of the polymer crystals. The effect is clearly stronger than
for P_2_5.1 with similar ller content, and just non-covalent
interactions between organic and inorganic matrix. This
difference could originate from a ner dispersion of the silica
particles, which was observed by electron microscopy. Another
possible reason is the formation of branching points due to the
covalent linkage between inorganic ller particles and the
polymer inhibiting the crystallization. Lower degrees of crys-
tallization have been observed for all samples P_4 in compar-
ison to P_2 as well as reference experiments P_0, and P_SiO2.
Beside crystallization behavior, the ller further inuences
decomposition behavior of the organic phase (Table 4 and
Residue
TFE-extraction
(48 h)
Total ller
content
Residue
TGA
ꢀ
Sample
P_SiO
P_4_2.0-1
P_4_2.0-2
P_4_2.0-3
T
D
in C
2
2 wt%
2 wt%
2 wt%
2 wt%
454.7
457.2
461.7
479.8
1.8 wt%
4.1 wt%
0.6 wt%
1.9 wt%
3.3 wt%
2.2 wt%
9.4 wt%
n. a.
5.4 wt%
7.9 wt%
n. a.
4
4
80.7
86.7
P_4_6.0
6 wt%
460.6
63.7 wt%
(
black)
a
T – maximum of rst derivation; wt% residue referred to mass at Fig. S24†). TGA measurements show that covalent bonding of
D
ꢀ
50 C.
2
PA6 on silica slightly increases decomposition temperature of
composites P_4 compared to samples without covalent bonding
2
(P_SiO ). The strongest effects can be observed for sample
P_4_2.0-3 with the highest amount of 4 (20 mol%) regarding the
total amount of silicon-containing monomers.
Table 5 Scanning electron microscopic images of four-component
4
materials synthesized using 3-ACA, 3-CL, Si(3-CL) (1), and 3-amino-
propyltriethoxysilane (4). Distribution of silicon is marked cyan
Sample
Filler content Content 4 Electron microscopic images
P_4_2.0-1 2 wt%
0.2 mol%
Fig. 6 Cyclic DSC measurements of extracted materials P_4 with
different filler content (see Table 4) as well as different portions of 4
compared to P_SiO
and g-crystallites are marked gray.
2
. Range of literature values for melting points of a-
41,42
P_4_2.0-2 2 wt%
0.5 mol%
in the FTIR spectra of the residues. Unfortunately, it was
impossible to investigate composite P_4_2.0-3 by this method.
Despite several trials, either clogging of the extraction tube or
nearly complete extraction was observed. In the latter case,
about 1 wt% residue could be detected by TGA measurements at
P_4_2.0-3 2 wt%
20 mol%
ꢀ
900 C in air atmosphere, indicating extraction of silica.
Thermal properties of composites were investigated by
differential scanning calorimetry (Fig. 6 and Table S1† in ESI),
and thermogravimetric analysis (Table 4 and Fig. S24†).
DSC measurements of P_4_2.0 show, corresponding to three-
component-systems with 2 wt% SiO (P_SiO ), a signal
2
2
ꢀ
II
maximum at 220 C (T
m
), which indicates the formation of a-
1
3
1
crystallites in accordance with
observed shoulder at 210 C (T
C{ H} NMR results. An
m
) may be interpreted as melting
ꢀ
I
P_4_6.0
6 wt%
20 mol%
of g-crystalline regions, with the uncertainty discussed above.
For sample P_4_6.0 a shi to lower temperatures as well as
a broadening of the melting peak could be observed. This
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silicon-containing monomers 1 (Si(3-CL)
4 3
), and 2 (MeSi(3-CL) )
or 3 (Me Si(3-CL) ) was carried out for synthesis of polyamide 6
2
2
composite materials with embedded methyl-functionalized
silica. Composites showed a distinct dependency of homoge-
neity on ller content. It was found that agglomeration
tendency increased with rising ller content independent of the
used silicon monomer 1, 2 or 3. Melting points were not inu-
enced by ller content or fourth reactant in contrast to
decomposition behavior. Thermal stability could be improved
with increasing ller content.
An additional four-component-system, namely a mixture of
3-ACA, 3-CL, 1, and 3-aminopropyltriethoxysilane 4 was exam-
ined. The latter facilitates a covalent bonding of PA6 on silica
surface. The covalently bonded ller inuenced crystallization
as well as decomposition behavior of PA6 due to strong inter-
actions between silica surface and PA6 chains. Higher amounts
of 4 led to an improvement in homogeneity and thermal
stability. Agglomeration tendency of silica particles increased
with higher ller content comparable to other three- or four-
component systems with the same ller content.
Fig. 7 Energy filtered transmission electron microscopic (EFTEM)
images of sample P_4_2.0-2 with 2 wt% filler (0.5 mol% 4) (freezing
ultra-thin cut). Silica particles are identifiable as darker areas.
Three independently prepared samples of P_4_2.0-3 have
been investigated concerning their decomposition behavior to
study reproducibility. All samples show an increase of T
D
of at
least 25 K compared to P_SiO without modication of silica
particles. An increase of the ller content to 6 wt% leads to
2
Conflicts of interest
a decreased T . This is probably the result of distinct agglom-
D
There are no conicts to declare.
eration with higher silica content. The variable amount of
residue of TGA measurements hint at partly inhomogeneous
distribution of ller in the organic matrix. It should be
mentioned, that residue of sample P_4_6.0 aer TGA
measurement was black. This indicates protection of organic
residues against burning due to encapsulation in the silica
network.
Table 5 and Fig. S25–S28 in ESI† show scanning electron
microscopic images of the samples P_4. Noticeable is the
tendency for agglomeration in case of the highest ller content
of 6 wt% (P_4_6.0), which is comparable to samples with 5 wt%
Acknowledgements
We thank Fonds der Chemischen Industrie and Deutsche For-
schungsgemeinscha (DFG Sp 392/39-1) for nancial support.
We acknowledge BASF SE for nancial support as well as
composite characterization by SEC and TEM. Furthermore, we
thank Prof. Hietschold and T. Jagemann (TU Chemnitz) for the
opportunity of measuring SEM images. We acknowledge T.
Windberg and R. John for their practical work as well as S.
Schmidt for proof-reading.
2
SiO in the three-component-system and the four-component-
systems of monomers 2 and 3 (e.g., P_2_5.1). Furthermore,
proportion of 4 seems to have an inuence on particle distri-
bution for 2 wt% total ller content (P_4_2.0). Although, large
agglomerates of several tens of micrometers could be obtained
for 0.2 mol% 4 (P_4_2.0-1), a homogeneous dispersion could be
observed by SEM measurements for samples with 0.5 mol% 4
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