Amorphization of organic compounds by ball milling

Materials Research Bulletin, Vol. 32, No. 12, pp. 1691-1696, 1997 Copyright 0 1997 Elsevier Science Ltd Printed in the USA. All tights reserved 0025.5408/97 $17.00 + .oo

Pergamon

PI1 SO0255408(97)00162-l

AMORPHIZATION

OF ORGANIC

COMPOUNDS

BY BALL MILLING

Joan Font’*, Joaquim Muntasell’, and Eduard Cesari* de Fisica i Eng. Nuclear, Univ. Politecnica de Catalunya Avda. Diagonal, 647, E-08028 Barcelona, Spain 2Departament de Fisica, Univ. Illes Balears, Crtra. de Valldemossa km 7.5 E-07071 Palma de Mallorca, Spain

‘Departament

(Received

(Refereed) May 10, 1997; Accepted June 26, 1997)

ABSTRACT We have applied the ball-milling technique to amorphize the organic compounds sucrose and poly(ethylene terephthalate). Differential scanning colorimetry and X-ray diffraction measurements show that the samples have been amorphized by milling. However, differential scanning calorimetry heating runs reveal some differences between the crystallization process of samples mechanically amorphized and those obtained by quenching. o 1997 ~lre~r Science Lrd

KEYWORDS: A. amorphous material, A. organic compounds, C. differential scanning calorimetry (DSC), C. X-ray diffraction, D. specific heat

INTRODUCTION Since the mid 1980s ball milling has been widely applied to transition metals and inorganic compounds in order to obtain materials with good mechanical properties. This technique has been used basically for mechanical alloying of a mixture of elemental powders and mechanical milling of intermetallic compounds [l-3]. Solid solutions, amorphous phases, and nanocrystalline structures can be obtained. In contrast to the many studies done concerning transition metals and inorganic compounds, ball-milling experiments on organic materials are scarce [4,5]. Our first studies [6,7] using this technique were performed on plastic crystals derived from neopentane. We showed

*To whom correspondence

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in those works that mechanical alloying is an adequate method to obtain solid-state solutions of these compounds. Amorphous solids are generally prepared by rapid solidification, quenching the liquid state or vapor deposition on a cold substrate. Earlier studies [2,X] performed on metallic systems showed that mechanical milling is an adequate procedure for the amorphization of metals. The aim of the present work was to determine the ability of the ball-milling technique to amorphize organic compounds. The materials analyzed in this study were sucrose and poly(ethylene terephthalate) havin g a glass transition above room temperature. A comparative analysis between mechanical milling effects and the conventional amorphization by quenching has been performed. Although extensive information about these compounds can be found in the literature. few studies applying ball milling as an amorphization method have been done up to the present 191.

EXPERIMENTAL Sucrose and poly(ethylene terephthalate} (PET) were mechanically ground in a Fritsch (Pulverisette 6) centrifugal ball mill. The rotation speed of the mill was S/IO of the potentiometer arbitrary scale. We used a cylindrical stainless steel grinding jar of 80 cm3 with two balls 2 cm in diameter made of the same material. The sample mass into the vial was 5 g, giving rise to a product-to-ball weight ratio of I : 12. The vial temperature was maintained at about 280 K. The compounds were characterized by means of a differential scanning calorimeter (Setaram DSC92). We employed open aluminum crucibles with sample masses of about 50 mg. The heating rate used in our measurements was 5 K mm ‘. X-ray diffraction measurements were performed at room temperature with a Siemens D-5000 diffractometer. using Cu Ka, radiation A = 0. IS4 nm. The goniometer speed was 0.005” (20) ss’. The sucrose used was purchased from Aldrich with a purity level of 99+%. The melting point determined from our differential scanning calorimetry (DSC) measurements was 453 K. The value obtained for the melting enthalpy change. 140 kJ kg- ‘. indicates that the sucrose was fully crystalline. The standard PET tilm used in this study melts at 533 K with the enthalpy change being 35 kJ kg- ‘. This enthalpy value points out that the degree of crystallinity of PET film is about 30’1 ( I I8 kJ kg ’ characterizes the melting enthalpy of fully crystalline PET I IO]).

RESULTS

AND DISCUSSION

Sucrose. Amorphous sucrose can be obtained by quenching the liquid. The glass transition temperature ranges between 325 and 350 K. depending on the thermal history [9,1 I]. In Figure I we show the specitic heat evolution determined from DSC runs at 5 K min-’ for two samples: a crystalline and a quenched from the liquid state with a rapid drop in liquid nitrogen. This figure shows a specitic heat jump of 0.85 kJ kgg’ K-’ corresponding to the glass transition at 345 K of the amorphous sample. We submitted the crystalline sucrose to a grinding treatment in the ball mill. In Figure 2. we observed that the amorphization degree increases with increasing milling time. Curve b (I h of milling) indicates an exothermic effect around 340 K attributed to the crystallization

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Specific heat evolution (filled circles).

of crystalline

FIG. 1 sucrose (open circles) and amorphized

by quenching

of the supercooled liquid. For a milling time of 60 h, a great exothermic peak appears at 365 K, and the glass transition is clearly visible about 328 K. The enthalpy change of this crystallization process is 75 kJ kg-‘. We have not detected further evolution for longer milling periods. Figure 3 shows the DSC curves of a crystalline sample and two amorphous samples. A comparison of curves b (sample quenched from liquid) and c (milled 60 h) reflect that the glass transition appears at lower temperatures in the sample milled, but the specific heat jump is of the same order in both samples. Curve b shows a slight, smooth endothermic effect at 450 K and, taking into account that no exothermic crystallization can be observed, we attribute this effect to the melting of a small crystalline fraction of sucrose that coexists with the amorphous state at room temperature. Figure 4 shows the diffraction patterns of crystalline sucrose (curve a) as well as that of quenched and milled samples (curves b and c, respectively). We can observe in this figure a

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Sucrose DSC curves: a = crystalline, e = milled 60 h.

FIG. 2 b = milled 1 h, c = milled 2 h, d = milled 10 h, and

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FIG. 3 Sucrose DSC curves: a = crystalline, b = amorphous amorphous obtained by ball milling for 60 h.

obtained

by quenching,

and c =

clear similitude between the patterns of both amorphous samples as well as the disappearance of the intense peaks corresponding to the monoclinic phase of crystalline sucrose. PET. PET film has been melted and slowly cooled to room temperature, to increase the degree of crystallinity. The DSC run of the sample submitted previously to this thermal treatment is shown in Figure 5, curve a. The enthalpy variation of melting is 55 kJ kg-‘, which corresponds to a crystallinity of approximately 45%. Curve b in the same figure shows the behavior, on heating, of a sample previously quenched from the melt dropping in liquid nitrogen. The glass transition at 360 K (specific heat jump AC, = 0.35 kJ kg-’ K-‘) and the exothermic crystallization at 405 K (enthalpy variation AH = 37 kJ kg-‘) can be observed in this DSC curve. From the value of the enthalpy crystallization at the melting temperature determined from AC, and AH results [91, we assume that the sample, after quenching treatment, is fully amorphous.

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FIG. 4 Sucrose XRD patterns: a = crystalline, b = amorphous amorphous obtained by ball milling for 60 h.

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obtained

by quenching,

and c =

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FIG. 5 PET DSC curves:

amorphous

a = obtained by slow cooling from the melt (45% of crystallinity), b = obtained by quenching, and c = amorphous obtained by ball milling for 20 h.

We have analyzed the effect of ball milling on a sample obtained by slow cooling from the liquid state. As an example, in Figure 5, curve c, we show the DSC curve for a sample milled for 20 h. The glass transition appears at 335 K and the crystallization takes place in two steps: a smooth process just after the glass transition and the other, very strong, in the beginning of the melting. For shorter milling times, both effects are smaller. In Figure 6, we show the XRD patterns of a sample cooled slowly from the melt (curve a) and a sample obtained after 20 h of milling (curve b). The XRD peaks in curve a, corresponding to a triclinic cell, are broadened and show a slight intensity due to the particle grain size, although this sample had been previously crushed in a mortar. Curve b shows only the halo-like pattern typical of amorphous materials. Comparison between sucrose and PET results points out some differences in the ball milling effects. The amorphous sucrose obtained by milling is more unstable than that prepared by quenching. The appearance of crystallization confirms this fact. However, the

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FIG. 6 PET XRD patterns:

b = amorphous

a = obtained by slow cooling from the melt (45% of crystallinity) obtained by ball milling for 20 h.

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behavior of PET is clearly different: for milled samples. although a slight exothermic effect appears near to the glass transition. the greater crystallization takes place at a higher temperature than that of quenching.

CONCLUSIONS We have analyzed the effect that milling has on the organic compounds sucrose and PET standard film. In both cases, the amorphous materials obtained mechanically and by means of rapid soliditication from the melt are similar with respect to the heat capacity jump as well as the XRD patterns. However. a different behavior is clearly observed when the amorphized compounds are heated. In PET that is mechanically amorphized, crystallization takes place at a higher temperature than in PET quenched, just before melting; quenched sucrose does not crystallize, whereas, amorphous sucrose that is prepared by milling does, at 365 K.

ACKNOWLEDGMENTS Partial financial support from the CICYT (project PTR94-0049)

is gratefully acknowledged.

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