SOLID STATE NMR EVALUTION OF NATURAL RESIN/CLAY

NANOCOMPOSITES

Emerson O. da Silva1, Maria Inês B. Tavares1*, José S. Nogueira2

1* Instituto de Macromoléculas Professora Eloisa Mano/ UFRJ, Centro de

Tecnologia, Bloco J, Ilha do Fundão. PO Box: 68525, 21945-970, Rio de

Janeiro, RJ, Brazil,

mibt@ima.ufrj.br, eos@ima.ufrj.br.

2DF/ICET/UFMT, Cuiabá, Mato Grosso, Brazil, nogueira@cpd.ufmt.br

Abstract

NMR nuclear relaxation times have been used by Tavares et al as a

methodology to characterize the nano materials, especially nanocomposites,

because NMR offers a great variety of relaxation parameters. The spin-lattice

relaxation time, with a time constant T1, have been explored to get as much

information as possible from the measurements of the spin-lattice proton

relaxation (T1H), which can measure the fraction of available polymer/clay

interface as well as the dispersion homogeneity of those interfaces actually

formed. The spin-lattice has been evaluated since this relaxation time confirms

the T1 and can give additional information to nanocomposite clay dispersion.

The NMR relaxation times are sensitive to the chemical environmental, changes

in the polymer matrix; chemical structure and interaction process, because they

depend on the domain distribution and sample homogeneity, since they are

measured in the solid state via intermolecular chains interaction and/or spin

diffusion. The T1 relaxation time of the nanocomposite decreased very much in

relation to the natural resin, according to the increase in the exfoliation clay

process, forming a nanocomposite with polymer matrix around the clay lamella.

Keywords: natural resin, NMR, Nanocomposite

Introduction

Nanotechnology permits obtaining very good and specific materials for

different areas. Generally, nanocomposites have their mechanical and thermal

properties improved comparing to virgin polymers. Modified clay produce

hierarchically ordered hybrid organic-inorganic nanocomposites. The

nanocomposites can be prepared by bulk polymerization; solution

polymerization; solution mixing and melt process. To prepare nanocomposites

some questions have to be answered: 1) to understand how the structure of the

clay relates to the structure of the nanocomposite; 2) to determine if there is a

relationship between the preparative method and the final structure of the

material, i.e., intercalated or exfoliated; and 3) to understand the pathway by

which nanocomposites enhance the thermal and fire stability of polymers, and

other properties. An enormous quantity of synthetic polymer nanocomposites

has been investigated, focusing different visions and necessity to obtain a good

nanocomposite material [1-4], because they often exhibit remarkable

improvement in materials properties when compared with virgin polymer or

conventional micro and macro composite. However, there are few works about

the nanocomposites based on natural polymeric matrix. As the synthetic

polymers are from non-renewable source, it is of strategic importance to study

natural polymer material as one of the possible substitute for some synthetic

polymers. Natural polymers can be extracted from trees in the form of

exsudates and they are called natural resins. Thus, the natural resins have very

good adhesion properties and they are normally used in composition of

varnishes, however, it can be used in different materials, for instance,

nanocomposites. The natural resin obtained from Amescla tree, which is the

subject of this study, which is a natural resin that is mainly constituted by

polyterpenoid compounds, and because of its particularity it is a good matrix to

prepare nanocomposite with organoclay.

Nanocomposites are basically characterized by X-ray diffraction; thermal

properties such as glass transition and thermal stability; electronic transmission

microscopy (TEM) [1-4] and mechanical properties. Therefore, other techniques

must be used to support and understand the nanostructured material. In this

case nuclear magnetic resonance (NMR) via nuclear relaxation times has been

used by Tavares et al [5-9], as a methodology to characterize the

nanomaterials, especially nanocomposites, because NMR offers a great variety

of relaxation parameters. The spin-lattice relaxation time have been explored to

get as much information as possible from the measurements of the spin-lattice

proton relaxation, which can measure the fraction of available polymer/clay

interface as well as the dispersion homogeneity of the those interfaces actually

formed [5-14].

According to the scope, this work evaluated an innovative methodology,

using solid state NMR to characterize the dispersion and the nanostructure of

the hybrid polymer nanocomposite. The NMR relaxation times are sensitive to

the chemical environmental; changes in the polymer matrix structure and

interaction process, because they depend on the domain distribution and

sample homogeneity, since they are measured in solid state via intermolecular

chains interaction and/or spin diffusion. Thus, the purpose of this work is the

preparation of a nanocomposite of NaRA and montmorillonite with and without

organic treatment and the characterization of formed nanocomposites, using X-

ray diffraction and nuclear magnetic resonance [15-17]. The NMR responses

were obtained through the determination of proton spin-lattice relaxation time.

T1H relaxation time was measured for each sample and they were interpreted in

terms of clay layer dispersion; homogeneity and interaction between clay layer

and resin polymer matrix.

Materials and Methods

Samples

Natural resin from Amescla tree was received from Universidade Federal

de Mato Grosso, collected in Sinop city, MT, Brazil. The pondered molar mass

was 56.000. The NaRA was purified by extraction by ethanol and precipitation

in water.

The nanoparticle used were nanoclay viscogel B7 (MMT), which was

treated with alquil ammonium salt with 12 atoms of carbon – C-12, and

homoinioc clay (MT), which were kindly supplied from Bentec.

Nanocomposite Preparation

Separated dispersions of NaRA and MMT or MT in chloroform were

prepared and stirred for 6 hours. The amounts of NaRA and MMT were

calculated to obtain a nanocomposite 5% (w/w). Then, the MMT suspension

was slowly added to NaRA solution and stirred for 3 days. Thus, this

nanocomposite was obtained by solution induced intercalation method.

Materials Characterization

The characterization methods were performed on XRD diffraction and

low field NMR, through the measurements of spin-lattice relaxation time.

X-Ray Diffraction

The extent of clay intercalation and / or exfoliation was determinate by X-

Ray Diffraction (XRD) analysis. The films were characterized using an X-ray

diffractometer, XRD Rigaku Miniflex, with CuK ( = 1,5418 Å) radiation

operated at 30 KV and 15 mA. The data were recorded at 2 rates of 2° per

minute. The basal spacing of nanocomposite was calculated by through the

Bragg’s relation: = 2 d sin.

Low Field NMR

Low field NMR MARAN ultra 23 spectrometer, operating at 23 MHz (for

protons), and equipped with an 18 mm variable temperature probe, was used

for the determination of relaxation measurements. Proton spin-lattice relaxation

times (T1H) were determined directly by the traditional inversion recovery pulse

sequence (180°- - 90°) the 90° pulse of 4.6µs was calibrated automatically by

the instrument software. The amplitude of the FID was sampled for twenty

data points, ranging from 0.1 to 5000 ms, with 4 scans for each point and 5s of

recycle delay. The relaxation values and relative intensities were obtained by

fitting the exponential data with the aid of the program WINFIT.

Results and Discussions

Figure 1 shows the X-ray pattern of the organofilic clay; natural resin and

its nanocomposite.

Figure 1 - X-ray diffraction of organofilic clay; natural resin and nano2

The nano2 exhibited no diffraction peaks in the 2 α range of 2-8 degree.

The absence of a Bragg scattering peak seems to indicate that the clay tactoids

were intercalated and/or exfoliated by the solution intercalated process.

However, the MMT can only delaminate into much smaller stacks, containing far

fewer silicate layers. The decrease and polydispersion of the clay layer number

NaRA

MMT

NANO2

result in the absence of XRD peaks. The decrease in the intensity and an

increase in the width of the X-ray nanocomposite peak, comparing to the natural

resin, can be an indication that a less ordained sample was formed.

According to the X-ray results another technique can be used to confirm

the formation of nanocomposite. One of the characterization techniques that

have been used is the Transmission Electron Microscopy, but its disadvantage

is that only a specific region of the sample is detected and it does not represent

an average behavior of the material.

According to what it was exposed, NMR is a non destructive method and

the response reflects the average behavior of the nanomaterial. We have

chosen to first measure the proton spin-lattice relaxation time in the rotating

frame (T1H), which was determined indirectly through the carbon-13 decay

during the variable contact time, with the increase in the contact-time. The

relaxation parameter data are listed in Table 1 and the resolved carbon decay

for the resin, nano1 and nano2 are shown in Figures 2, 3 and 4, respectively.

Table 1 – Proton Spin-Lattice Relaxation time in the rotating frame for ResNA-p, Nano1 and Nano2

Sample

T1H (ms)

78

ppm

48 ppm 40 ppm 28 ppm 24 ppm 17 ppm

ResNa-p 9 9 13 15 17 18

Nano1 nd nd 13 12 12 11

Nano2 nd nd 7 5 12 9

nd= non determined

From the data listed in the Table 1 nano1 presents better clay dispersion

comparing to nano2, because of the homogeneity in the relaxation data, which

is a first indication that this nanocomposite presents a good dispersion of the

clay layers in the polymeric matrix.

Figure 2 – Carbon-13 decay during the variable contact time experiment for the

amescla resin

Figure 3 – Carbon-13 decay curve for the determination of Nano1 T1H

Figure 4 - Carbon-13 decay curve for the determination of Nano2 T1H

The patterns of signal decays of Nano1 present great parallelism in

relation to Nano2, which corroborates with the data obtained either by relaxation

values or by 13C NMR spectra.

To continue the evaluation of the molecular dynamic of the amescla resin

and its nanocomposite, we have chosen to determine of proton spin-lattice

relaxation time directly in low field NMR spectrometer, because those

measurements are rapid; precise and reproductive. Thus, it turns an interesting

method to evaluate the intercalated and/or exfoliated nanocomposite formation

and clay homogeneity dispersion, because the relaxation parameter analyses

the complete sample without any previous treatment.

According to the purpose of this work, the measurements of proton spin-

lattice relaxation time of natural resin; homoionic clay; organophylic clay and the

nanocomposites, nano1 (NaRA/MT) and nano2 (NaRA/MMT) are listed in Table

2.

Table 2 – Proton spin-lattice relaxation time of the resin (NaRA), clay (MMT)

and nanocomposite (Nano1 and Nano2)

The natural resin showed three distinct relaxation values. One refers to

free water (9 ms), an intermediate one (398 ms), which was attributed to the

amorphous domain and a high one (775 ms) due to the ordained domain and

constricted amorphous chains.

The homoionic and organophylic clay presented only one domain due to

the free water (0.6 ms). Therefore, the nanocomposites presented different

relaxation values comparing both nano1 and nano2 with resin matrix.

In the case of nano1, the first value was derived from the free water (4

ms); the intermediate value 111 ms is very much lower than the value found for

the resin, this difference shows that this relaxation value comes from the

interface region between natural resin and organoclay. The deep decrease in

T1H, is derived from the metals present in the clay layer that promotes a in

interference in the value of this parameter to lower values, because the metals

behave as relaxation agent, which confirms that the polymer chains are very

close to the organoclay layers. Thus, this relaxation data was attributed to the

exfoliated/intercalated regions, with a higher percentage of exfoliation. The

higher T1H value (772 ms) is from the resin matrix that has a contribution from

ordinate region as well as the intercalated polymer chains among the clay

layers forming an intercalated nanocomposite, which has lower molecular

mobility due to be confined between layers.

Sample T1H (ms)

MMT-org 0.6 ______ ______

MMT-Na+ 0.5 ______ _____

ResNa-p 9 398 775

Nano1 4 111 772

Nano2 20 192 227

For the nano2, three values of relaxation parameter were also detected

and the low T1H value and the intermediate T1H value are higher than the

values compared for nano1. Moreover, the higher T1H value (227) for nano2 is

lower comparing to the higher T1H value determined for both natural resin and

nano1, which is due to the Na+ present in the clay structure that promotes the

decrease in the relaxation value, acting as a relaxation agent. But, some

intercalated and exfoliated nanocomposites can be found in nano2.

This affirmation can be supported by the fact that, if the nanocomposite

was only intercalated the relaxation value would increase due to the formation

of constricted polymer domains inside the clay layers. The exfoliation process

was facilitated because the natural resin presents chains with different sizes,

which can promote a plasticization effect promoting an exfoliation process. The

higher relaxation value does not change, because it belongs to the resin matrix

not affected by the organoclay presence.

Both nanocomposites showed to have good clay layer dispersion in the

resin matrix, but according to the relaxation values nano1 seems to be

homogeneous than nano2.

The clay structure was evaluated by the silicon-29 NMR spectrum, which

permits to evaluate changes in the clay structure due to proximity of polymer

chains. The silicon-29 spectra of clay, viscogel B7 and both nano1 and nano2

are showed in Figures 5, 6, 7 and 8, respectively. Comparing the Si-29 MAS

NMR spectra, the nanocomposites showed changes in the NMR signal form

and also a displacement in the chemical shift values. The nano1 and nano2 Si-

29 NMR spectra presented a pattern different than viscogel B7 and clay,

respectively, which can be due to the random clay layer dispersion in the

polymeric matrix, because of the break in the clay layer ordination. Polymer

chains could promote a clay layer intercalation and/or exfoliation. Confirming

the results obtained by the other techniques.

Figure 5 – Si-29 MAS NMR spectrum of clay

Figure 6 – Si-29 MAS NMR spectrum of viscogel B7

ssb ssb ssb ssb

ssb ssb

Figure 7 – Si-29 MAS NMR spectrum of nano1

Figure 8 – Si-29 MAS NMR spectrum of nano2

Conclusions

The synthesis of NaRA/MMT and NaRA/MT materials were carried out

successfully. XRD curves indicated that the nanocomposite can be formed. The

measurements of NMR through T1H, determined by Low Field NMR, and High

Field NMR analyses from the polymeric matrix (C-13 and H-1) and from the

nanoparticle (Si-29) confirm and explain the x-ray results. Thus, from this work it

was shown that the NMR spectroscopy can be one important technique to

evaluate the nanocomposite structure and molecular behavior.

Acknowledgements

To CNPq, for the financial support

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