Small-angle X-ray scattering and wide-angle X-ray scattering experiments combined with thermal and spectroscopic analysis techniques

Journal of

MOLECULAR STRUCTURE ELSEVIER

Journal of Molecular Structure 383 (1996) 309-314

Small-angle X-ray scattering and wide-angle X-ray scattering experiments combined with thermal and spectroscopic analysis techniques 1 W. Bras a'*, A.J. R y a n b aFOM Institute and Netherlands Organisation for Scientific Research (NWO), Kruislaan 407, 1098 SJ Amsterdam, The Netherlands bUniversity of Manchester Institute of Science and Technology, Grosvenor Street, Manchester, UK and Daresbury Laboratory, Warrrington, Cheshire WA4 4AD, UK

Received 27 September 1995;accepted 7 November 1995

Abstract

Time resolved simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) experiments can provide structural information on the changes in different levels of organisation in a system when it is subjected to a perturbation. These perturbations can be chemical, thermodynamic or physical. However, often there is a desire to gain insights into the mechanism of phase transitions beyond the purely structural information. This can be achieved by combining SAXS and WAXS experiments with thermal analysis or spectroscopic methods. In order to be able to accurately correlate the results from these techniques with the evolution in the structure it is desirable to perform these experiments simultaneously and on the same sample. The higher the time resolution the greater the importance of technique combinations. Keywords: Small angle X-ray scattering; Wide angle X-ray scattering; Differential scanning calorimetry; FT-IR spectroscopy;Raman

spectroscopy

I. Introduction

The highly collimated and very intense X-ray synchrotron radiation beamlines have made it possible to routinely perform time-resolved smallangle X-ray scattering (SAXS) experiments. The possibilities for experimentalists to gain access to these facilities have steadily increased over the last * Corresponding author. 1Paper presented at the conference on 'Horizons in Small Angle Scattering From Mesoscopic Systems', Stromboli, Italy, 27-30 September 1995.

couple of years due to the increase in the number of operational storage rings and the commissioning of new beamlines at existing facilities. This has provided us with the means not only to apply straightforward diffraction techniques but also to experiment with the possibilities of combining several experimental techniques simultaneously so that thermodynamic and chemical parameters can be obtained parallel with the structural information provided by the diffraction patterns. It is clear that such an approach introduces extra complications but in our experience these are in a great number of cases more than compensated for by the

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wealth of extra, correctly time-correlated information which becomes available. However, it is obvious that the extra effort involved in these experiments should only be taken if really required. The most obvious technique to combine with SAXS is wide-angle X-ray scattering (WAXS) so that structural changes at the atomic level can be correlated with the changes at larger scale sizes, such as in the study of semi-crystalline polymers where one wants to study the atomic arrangements inside the lamellae in combination with the lamellar configurations. This combination has successfully been implemented by several groups [1,2]. Also the use of differential scanning calorimetry (DSC) in combination with both SAXS and WAXS has been found useful for many experiments [3-5]. More complicated systems have recently been assembled which combine spectroscopic techniques such as Fourier transform infrared spectroscopy (FTIR) and Raman

spectroscopy with SAXS and WAXS. Although in their infancy these methods offer a great potential in the elucidation of phase transition mechanisms [6,7]. The diffraction techniques elucidate the structural features over a large size range; however, the spectroscopic techniques can be sensitive to parameters which are not accessible by diffraction techniques but comprise information concerning the chemical state of the system and of the molecular conformations before larger scale structure formation becomes apparent. The experiments on which we have concentrated mainly address questions encountered in the real-time studies of polymer processing.

2. Experimental techniques The combination of SAXS with WAXS, as implemented at the Synchrotron Radiation Source,

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IV. Bras, A.J. Ryan/Journal o f Molecular Structure 383 (1996) 309 314

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0.007 < q < 4.2 ~-1 with a practical time-resolution of 0.1 frame s -1 . Practical is defined here such that the data quality is still acceptable and the detectors not saturated for the majority of applications, and indicates that it is not necessary to repeat the experiment and average these values to obtain quantitative results. The latter procedure is of course possible but introduces extra experimental complications. A major limitation is the limited time resolution that can be achieved in these experiments due to the lack of detectors with a sufficiently

Daresbury, UK, on station 8.2 has been described elsewhere [2]; therefore only a brief description will be given here. This beamline has a very good collimation which, when optimised for SAXS, allows the recording of q values down to 0.006 .X.-1 (q = 27r/A). When set up in the SAXS/WAXS mode a second (curved) detector is added to collect the WAXS pattern (see Fig. 1). The time framing is achieved by a single central processing unit (CPU) which drives both detector systems. This equipment makes it possible to record the range

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Fig. 2. (a) Time-resolved Lorentz corrected SAXS pattern of a melting/re-crystallisation experiment on an oxyethylene/oxybutylene diblock copolymer. The sample was heated/cooled at 10°C min -1 . Scattering data were taken in 6 s time-frames. The data have not been smoothed. This dataset is part of a series in which the oxybutylene block length was varied [9]. In the solid state four diffraction peaks due to a lamellar ordering can be observed. (b) W A X S pattern corresponding to the SAXS data of (1). The diffraction peaks index to the crystal structure of poly(oxyethylene), a monoclinic subcell with alternating right- and left-handed 7/2 helices.

IV. Br as, ,4.J. Ryanl Journal of Molecular Structure 383 (1996) 309-314

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high count-rate capability. This is especially true for parallax-free wide-angle detectors. The best results so far have been obtained with gas filled proportional wire chambers with delay-line readout. These, however, have a physical count-rate limitation due to the read-out system and space charge effects inside the detector. A new type of curved detector, based on glass-strip technology, is at the moment under construction and should alleviate this problem. The count-rate capacity is estimated to be approximately 1 GHz with a local count-rate capacity of 50 MHz per channel [8]. This creates the possibility of, for instance, studying in more detail the crystallisation kinetics of rapidly quenched polymer samples. A typical result of the application of the SAXS/ WAXS technique is shown in Fig. 2(a, b). In this experiment the melting and re-crystallisation behaviour of a polyethyleneoxide/polybutyleneoxide block copolymer is studied [9]. In Fig. 3 data derived from the experimental spectra of Fig. 2 are co-plotted with simultaneously 5000

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obtained differential scanning calorimeter (DSC) data. With this equipment the possibility of obtaining information on the thermodynamic state of the system is achieved. This is particularly useful in the elucidation of complicated, multi-step phase transitions. The different features in the DSC trace can now accurately be correlated with structural information. An extra, important example of this type of experiment is that the possibility now exists to calibrate, in a number of important cases, the degree of crystallinity determined by DSC with that determined by SAXS/WAXS experiments [5]. The simultaneous application of FTIR and SAXS/WAXS can provide information on the chemical state of the sample together with the structure of the samples formed in conjunction with a chemical reaction. This has successfully been applied to the formation of polyurethanes in which it was unclear whether the structure formation was dominated by microphase separation or by the formation of hydrogen bonds. Independent application of these techniques had suggested that the microphase separation was the driving process in the structure formation but this was on the limit of the experimental accuracy [10]. The simultaneous application of SAXS and FTIR showed conclusively that this was indeed the case [11] (see Fig. 4). The combination of these three techniques with, for instance, a technologically important technique such as reaction injection moulding (RIM) has recently also become feasible. This requires a reasonably high time resolution of approximately 1 s per frame. The instrumentation problems that one faces with this type of experiment are that it is not feasible to perform both techniques in transmission because the optimum sample thicknesses are of different orders of magnitude. By applying the technique of attenuated total reflection (ATR) F T I R this problem has been overcome (see Fig. 5). A new combination of techniques is the addition of a Raman scattering spectrometer to SAXS/ WAXS [7]. This option would be complementary to the FTIR technique, which in synthetic macromolecules is most sensitive to the vibrations of the substituents on the carbon chain, whilst Raman spectroscopy is more sensitive to the vibrations of the carbon chain itself. This experiment has been

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tried and gives promising results (see Fig. 6). A very important advantage is that the laser beam can be focused onto a very small spot (100 mm2), comparable in size to, for instance, X-ray spots that

can be created with microfocus beamlines [12]. This allows the study of small inhomogeneities in a bulk sample. The development of growing spherulites in polymer samples is a prime example

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W. Bras, A.J. Ryan/Journal of Molecular Structure 383 (1996) 309-314 Analyser

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complicated it is, in some cases, definitely worth the extra effort involved, especially if high time resolution is essential. Furthermore, experiments in which it is difficult to accurately reproduce the experimental parameters in independent techniques will benefit from this approach.

Acknowledgements

SAXS X-Rays Sam

The work described here has been done in collaboration with a large number of colleagues. The help of the staff at Daresbury Laboratory, especially D. Bouch, P. Hindley, B.E. Komanschek, G.E. Derbyshire and D. Bogg, is gratefully acknowledged. Also the expertise and help of G.K. Bryant and H.F. Gleeson (Manchester University) and M.J. Elwell (UMIST) has been indispensable.

References Fig. 6. Set up for combined SAXS/WAXS/Raman spectroscopy. Problems arise due to the changes in scattering and absorption of the Ar-ion laser light when the samples undergo a phase transition which changes the opacity of the sample. This problem can be overcome by using a modified optical system.

of an interesting application of this technique. Initial experiments on polyethylene have shown that the combination of these techniques is feasible but that some experimental difficulties, such as the change in light scatter and absorption when the polymer goes through the melting transition and becomes transparent, have to be overcome. Equipment to perform experiments similar to those described above for anisotropic scattering systems has also been developed [13]. This will be combined with several on-line melt-spinning and extrusion machines. Due to space limitations this will not be discussed here.

3. Conclusions We have successfully developed experimental equipment which allows several parameters to be studied simultaneously in a single experiment. Although the actual experiment is more

[1] M. Bark, C. Schulze and H. Zachmann, Polym. Prep. Am. Chem., Soc. Div. Polym. Chem., 31(2) (1990) 106 107. [2] W. Bras, G.E. Derbyshire, A.J. Ryan, G.R. Mant, A. Felton, R.A. Lewis, C.J. Hall and G.N. Greaves, Nucl. Instrum. Methods Phys. Res. A, 326 (1993) 587-591. [3] T.P. Russell and J.T. Koberstein, J. Polym. Sci., 23 (1985) 1109-1115. [4] W. Bras, G.E. Derbyshire, A.J. Ryan, J. Cooke, A. Devine, B.E. Komanschek and S.M. Clark, J. Appl. Crystallogr., 28 (1995) 26-32. [5] A.J. Ryan, W. Bras, G.R. Mant and G.E. Derbyshire, Polymer, 35(21) (1994) 4537-4544. [6] W. Bras, G.E. Derbyshire, D. Bogg, J. Cooke, M.J.E. Elwell, S.N. Naylor, B.E. Komanschek and A.J. Ryan, Science, 267 (1995)996-999. [7] G.K. Bryant, H.F. Gleeson, B.E. Komanschek, A.J. Ryan and W. Bras, unpublished results. [8] F. Udo, J.E. Bateman, G.E. Derbyshire and W. Bras, unpublished results. [9] Y-W. Yang, S. Tanodekaew, C. Booth, A.J. Ryan, W. Bras and K. Viras, Macromolecules, 28 (1995) 6029 6041. [10] M.J. Elwell, Thesis, Victoria University of Manchester, 1994. [11] W. Bras, G.E. Derbyshire, D. Bogg, J, Cooke, M.J.E. Elwell, S.N. Naylor, B.E. Komanschek and A.J. Ryan, Science 267 (1995) 996-999. [12] C. Riekel et al., Nucl. Instrum. Methods Phys. Res. B, 97 (1995) 224-230. [13] W. Bras, G.R. Mant, G.E. Debyshire, A.J. Ryan, W.J. O'Kane, W.I. Helsby and C.J. Hall, J. Synchrotron Radiat., 2 (1995) 87 92.