Hybrid Organic-Inorganic Composites - American Chemical Society

Chapter 6

Polymer-Clay

Hybrids

Akane Okada, Arimitsu Usuki, Toshio Kurauchi, and Osami Kamigaito

Downloaded by NORTH CAROLINA STATE UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: March 21, 1995 | doi: 10.1021/bk-1995-0585.ch006

Toyota Central Research and Development Laboratories, Inc., Nagakute, Aichi 480-11, Japan

Polymer-clay hybrids, organic-inorganic molecular composites, were prepared. X-ray and T E M measurements revealed that each 10 Åtemplate of clay mineral is dispersed in the polymer matrix and that the repeat unit increased from 12 Åin unintercalated material to more than 200 Åin the intercalated material. Thus, polymer-clay hybrids are "polymer based molecular composites" or "nanometer composites". They, when molded, show excellent mechanical properties compared to unfilled polymers and/or conventional composites. In the hybrids negatively charged silicate (clay mineral) and positively charged polymer-ends are directly bonded through ionic bonds. The mechanism of reinforcement is discussed with the results of CP-MAS N M R and pulsed NMR studies.

Polymer has been successfully reinforced by glass fiber or other inorganic materials. In these reinforced composites, the polymer and additives are not homogeneously dispersed on the microscopic level. If the dispersion could be achieved on the microscopic level, the mechanical properties would be expected to be further improved and/or new unexpected features might appear(7). Clay mineral is a potential candidate for the additive since it is composed of layered silicates, 10 Â thick, and undergoes intercalation with organic molecules^. The lack of affinity between hydrophylic silicate and hydrophobic polymers makes it difficult to get homogeneously miscible with each other. Swelling of each template of silicate with organic molecules is a matter of vital importance to reach to this type of molecular composite In this paper we present two polymer-clay hybrids; nylon 6-clay hybrid and rubberclay hybrid. Swelling behavior, preparation and properties of hybrids, and interaction of the organic-inorganic surfaces are discussed. Nylon 6-Clay H y b r i d (NCH) Nylon 6 (polycaploractam) has good mechanical properties and is a commonly used engineering polymery. We tried to prepare N C H by blending commercial nylon 6 and montmorillonite, a common clay mineral, in a twin screw extruder, which gave just a phase separated, conventional nylon 6-clay composite (termed as NCC). So, we tried polymerization of ε-caprolactam in the interlayer space of montmorillonite to disperse each template of silicate into nylon 6 matrix on the molecular level. Swelling of silicate by ε-caprolactam is of key importance. We found that montmorillonite ion-exchanged with 12-aminolauric acid can be swollen by ε-caprolactam to fulfill our purpose. 0097-6156/95/0585-0055$12.00/0 © 1995 American Chemical Society

In Hybrid Organic-Inorganic Composites; Mark, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

56

HYBRID ORGANIC-INORGANIC COMPOSITES


In the following, swelling behavior, preparation and properties of N C H and the mechanism of reinforcement are presented. N C H has been used in an automotive part and has been manufactured in a large scale. Swelling Behavior of Montmorillonite with ω-Amino acid by ε-Caprolactamf 4). Natural Na-montmorillonite is hydrophylic and not compatible with most organic molecules. Sodium cation in the interlayer space of montmorillonite can be exchanged with organic cations to yield organophilic montmorillonite. For the present purpose, polymerization in the interlayer space, ammonium cations of co-amino acids were chosen as cations since they catalyze ring opening polymerization of ε-caprolactam. In a 1000 mL beaker were placed 24mmol of co-amino acid, 2.4 mL of concentrated hydrochloric acid and 200 mL of water at 80°C. The solution of the ω-amino acid was added into a dispersion composed of 10 g of montmorillonite and 1,000 mL of hot water, and this mixture was stirred vigorously for 10 min, giving a white precipitate. The product was filtered, washed with hot water, and freeze-dried. In this paper, we call the cation exchanged montmorillonites "n-montmorillonite", where η is die carbon number of co-amino acid. Mixture of 0.5 g of the n-montmorillonite powder and 2.0 g of ε-caprolactam (mp=70 °C)was heated at 100 ^ for swelling. The degree of swelling was studied by means of X-ray powder diffraction (XRD) measurement using a Rigaku RAD-B diffractometer. Figure 1 shows X R D patterns of n-montmorillonites. The basal spacings (interlayer distance) of the samples were obtained from the peak position of X R D pattern as shown in Table I. The XRDs of the mixtures of the n-montmorillonites and ε-caprolactam were measured at 25 °C and 100 °C. They suggest that swelling did occur. Basal spacings of the swollen n-montmorillonites are also shown in Table I. The spacings of the specimens were equal at 25 Χ and 100 °C for the n-montmorillonites, when η was less than 8. They corresponded with the sum of the molecular length and 10Â (template) at 25 °C. However, they exceeded the sum at 100 °C for longer n-montmorillonite. The schematic diagram is shown in Figure 2. For better swelling of co-amino acid, η should be larger than 11. We chose 12-aminolauric acid to prepare N C H since it is the most available among the longer acids . Table I. Basal Spacings of n-Montmorillonite in the Presence of ε-Caprolactam ω-Amino Acid

Spacing

Spacing in Caprolactam

NH (CH ) COOH

Â

( Â )

2

η

2

nl

Molecular length(Â)

25°C

100°C

2

6.7

12.7

14.3

14.4

3

8.1

13.1

19.3

19.7

4

9.8

13.2

19.3

19.9

5

11.0

13.2

20.3

20.4

6

12.2

13.2

23.3

23.4

8

14.7

13.4

26.2

26.4

11

18.5

17.4

30.2

35.7

12

19.7

17.2

31.5

38.7

18

27.3

28.2

43.8

71.2



6.

OKADAETAL.

57

Polymer-Clay Hybrids

1.0

5.0 20(Co-K*)

10.0

Figure 1. X R D Patterns of n-Montmorillonite



j

2 5'C

ο ο ο ο

η^8

nè11

ο

Ο

Ό Ο

W^ÊÊM

1 0 O'C

oUo ο ο

Ο Π Ο

Figure 2. Schematic Diagram of Intercalation of ε-Caprolactam

ω-amino acid

ε-caprolactam (melt)

layered clay


ο

S2 Η w

ο

η ο

ο

ο

ο

3

2

00

in


6.

OKADAETAL.

59

Polymer-Clay Hybrids

Preparation of N C H ( 5 ) . In a vessel, 113 g of ε-caprolactam and 5.97 g of 12-montmorillonite were placed. The mixture was heated at 100 °C for 30 min. Then, it was heated at 250 °C for 48 h, yielding a polymeric product. After cooling, the product was mechanically crushed. The fine particles were washed with 2 L of water at 80 °C for 1 h. Thus we obtained N C H . Nylon 6 clay composites (termed as NCC) were prepared by blending commercial nylon 6 and montmorillonite in an extruder for comparison with N C H . These materials were injection-molded for various measurements. Basal spacing, d, was directly obtained in X R D . Figure 3 shows the transmission electron micrograph (TEM) of the section of molded NCH-15 measured on a JEOL-200CX T E M applying an acceleration voltage of 200V. The suffix of N C H means the amount of 12-montmorillonite used in polymerization. The dark lines are the intersection of the sheet silicate of 10 Â thickness and the spaces between the dark lines are interlayer spaces. Table II shows the basal spacing, d, obtained by X R D and T E M . The d values agree very well. It was found to be inversely proportional to the montmorillonite content. A maximum d of 214 Â was observed. The thickness of a layer of silicates is about 10 Â. This is of the order of molecular size and this layer can be thought to be an "inorganic macromolecule", so that, in N C H , the polymer and montmorillonite are mixed on the molecular level forming a "polymer based molecular composite". On the other hand, d in the NCC was 12 Â and it is unchanged from the pristine montmorillonite and therefore N C C is not a molecular composite. Table II. Basal Spacings of NCHs Specimen

Montmorillonite (wt %)

Spacing (X-ray) (Â)

Spacing(TEM) (Â)

NCH-5

4.2

150

214

NCH-10

9.0

121

115

NCH-15

14.5

64

62

NCH-30

25.0

51

50

NCC-5

5.0

12

Properties^6). Mechanical properties of NCH-5 are shown in Table III together with nylon 6 and NCC-5 following A S T M . The tensile strength and tensile modulus of N C H were superior to others. The impact strength of N C H was identical with that of nylon 6. The most prominent effect was observed in heat distortion temperature (HDT). HDT of NCH-5 containing only 4 wt% of montmorillonite was 152 °C, which was 87 °C higher than that of nylon 6. This effect in N C H is attributed to drastic improvement in the quality of nylon 6. Resistance to water was also improved(7). The rate of water absorption in N C H was lowered by 40 % as compared to nylon 6 and N C C . Table III. Properties of NCH-5(1)

(wt %)

Tensile Strength (MPa)

Tensile Modulus (GPa)

Charpy Impact Strength (KJ/m )

NCH-5

4.2

107

2.1

6.1

NCC-5

5.0

61

1.0

5.9

nylon 6

0

69

1.1

6.2

Monmorillonite Specimen

2


60

HYBRID ORGANIC-INORGANIC COMPOSITES


The molded specimen was found to be anisotropic. The coefficient of thermal expansion of NCH-5 in the flow direction was lower than half of that in the perpendicular direction. Nylon 6 was isotropic and N C C was intermediate. T E M studies revealed that sheets of silicate were parallel to the flow direction of the mold. The nylon molecules in NCH-5 were also oriented in the same direction. It seems that anisotropy of the thermal expansion results from the orientations of silicate and polymer chains. Table III (continued). Properties of NCH-5(2) Coefficient of Thermal Rate of Water HDT Expansion Absorption at 18.5 kg/cm 23°C, 1 day Flow Pependicular Direction Direction (cm/cm

Hybrid Organic-Inorganic Composites - American Chemical Society

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