Elsevier

Materials Research Bulletin

Volume 36, Issue 9, July–August 2001, Pages 1665-1673
Materials Research Bulletin

Cold-crystallization of Poly(ether-ether-ketone) amorphized by ball milling

https://doi.org/10.1016/S0025-5408(01)00648-1Get rights and content

Abstract

A partial amorphization of semicrystalline poly(ether-ether-ketone) has been induced mechanically by means of the ball-milling technique. The thermal behavior of the polymer has been studied using temperature-modulated differential scanning calorimetry. The cold crystallization of the amorphized material, taking place close to the glass transition temperature, can be detected on the nonreversing heat flow signal clearly separated from the glass transition effect.

Introduction

Poly(ether-ether-ketone), PEEK, is a semicrystalline polymer of great interest because of its engineering properties, such as high chemical resistance, good thermal stability, and mechanical behavior. PEEK can be partially crystallized by cooling from the melt or heating the amorphous glass. The melting behavior of semicrystalline PEEK depends on the thermal treatment applied for the crystallization of the polymer. This dependence shows a special complexity due to the metastable character of semicrystalline structures [1], [2], [3].

Even though the mechanical milling technique has been more extensively applied to metallic systems, recent studies carried out in polymeric materials have shown that this technique is suitable to introduce structural changes affecting the physical properties of the polymer. Mechanical milling has been also used for blending two polymers in the solid state, overcoming the problems associated with the most common methods such as solution or melting. Pan and Shaw [4] applied mechanical milling to polyamide, obtaining a material with high hardness and compressive strength. The effect of milling on the molecular weight of the polymer was studied by Shaw et al. [5] on acrylonitrile–butadiene–styrene, and by Castricum et al. [6] on polyethylene. Investigations performed by Ishida [7] on polyethylene, poly(tetrafluoroethylene), and blends of both polymers showed the amorphization induced by the mechanical treatment. More recently, Bai et al. [8] have analyzed the structural changes of poly(ethylene terephthalate) obtained after milling at different temperatures.

In our previous works, we studied the influence of milling on the thermal behavior of poly(ethylene terephthalate) [9] and bisphenol A polycarbonate [10]. We also applied this technique to obtain mixtures of two semicrystalline polymers, namely poly(butylene terephthalate) and poly(ethylene terephthalate) [11]. Also, mixtures of these polymers with a noncrystalline one such as poly(ether imide) were obtained [12]. Our preliminary milling results on PEEK were reported in [12]. In the present work a more detailed study of the effect of mechanical milling on PEEK is presented. Particularly, the effect of milling on the crystallization behavior has been analyzed. The crystallization taking place above the glass transition temperature, on heating the polymer, (cold crystallization) has been monitored by temperature-modulated differential scanning calorimetry (TMDSC). In TMDSC measurements a sample is subjected to a linear heating with a superimposed low-frequency temperature oscillation, resulting in a modulation in the heating proflle. The results obtained are reported in terms of reversing and nonreversing signals. The reversing component is obtained from the amplitude of the harmonic heat flow using a Fourier transform of the data, and the nonreversing heat flow is the difference between the total and the reversing signals. Changes in the heat capacity are attributed to the reversing signal, while nonreversing stands for all kinetic effects. TMDSC is widely applied to separate to endothermic step change observed at the glass transition temperature, appearing in the reversing signal, from the annealing peaks detected as nonreversing effects [13].

Section snippets

Experimental

PEEK was supplied by Aldrich in the form of fibers of 0.3 mm in diameter. The material was ground in an IKA A10 grinder to obtain a powder with a grain size ranging between 100 and 200 μm mesh. To homogenize the degree of crystallinity, PEEK samples were annealed for 10 min at 473 K.

The milling device was a Frisch (Pulverisette 6) centrifugal ball mill working with a rotation speed of about 500 rpm. We used a cylindrical stainless steel grinding jar of 80 cm3 (maintained at room temperature

Results and discussion

After the annealing process (10 min at 473 K) the polymer shows a crystallinity of 30%, determined from the enthalpy change for melting (assuming it has a value of 130 J/g for fully crystalline material [2]). Glass transition appears at 425 K and melting at 620 K. DSC curves obtained with heating rates of 5 and 2 K/min are presented in Fig. 1 (curves a and c).

When PEEK was milled an exothermic effect, attributed to a recrystallization process, appeared at approximately 430 K. No further

Conclusions

The studies presented in this work aim at clarifying the effect of ball milling on the thermal behavior of a polymeric material. In the case of semicrystalline PEEK, we obtain a polymer with a higher degree of amorphization, as a result of this mechanical treatment. The material amorphized by milling recrystallizes on heating near the glass transition temperature.

TMDSC is a relatively new technique of special interest to study materials having a complex thermal behavior. In the case of

Acknowledgments

The authors thank Dr. Joan Cifre (SCT-UIB) for his assistance in X-ray and modulated DSC measurements.

References (13)

  • C. Bas et al.

    Eur. Polym. J.

    (1995)
  • C. Bai et al.

    Polymer

    (2000)
  • J. Font et al.

    Thermochim. Acta

    (1999)
  • J. Font et al.

    Mater. Res. Bull.

    (2000)
  • J. Font et al.

    Mater. Res. Bull.

    (1999)
  • J. Font et al.

    Mater. Res. Bull.

    (1999)
There are more references available in the full text version of this article.

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