2s Nuclear Instruments and Methods in Physics Research B 97 (1995) 269-272

B l!iiJ NIIMI B ELSEVIER

Beam Interactions with Materials A Atoms

Time-resolved X-ray studies of polymorphism in isotactic polybut- 1 -ene

Jennifer Cooke ‘, Anthony J. Ryan ‘,a, * , Wim Bras c-b

d Manchester Materials Science Centre, Grosrenor Street, Manchester MI 7HS. UK ’ Netherlands Organisation for Scientific Research (NWO), The Hague. The Netherlands

’ DRAL Daresbury Laboratory, UK

Abstract The morphology of melt crystallised isotactic polybut-l-ene has been studied using a combined SAXS/WAXS/DSC

technique. Time-resolved small angle and wide angle X-ray scattering patterns have been collected simultaneously during

melting, cooling and isothermal recrystallisation of several samples. An Avrami type analysis was performed on data obtained from the isothermal experiments and it was found that the Avrami exponent increased with annealing temperature

indicating a change in the crystallisation mechanism.

1. Introduction

Isotactic polybut-l-ene was first prepared by Natta in 1955 [I]. Preliminary studies of its crystalline properties using X-ray, density, infrared and thermal methods estab-

lished polybut-1-ene as being distinctly different to other polyolefins because of its unique polymorphic behaviour.

Four crystalline forms have been isolated to date [2], each modification differing in crystal and helical structure,

physical and thermal properties, and more significantly thermodynamic stability.

Polymerisation of but-1-ene produces a crystalline structure with a hexagonal unit cell (modification I) which

is not normally observed, as it dissolves in solvent present in the reactor. Evaporation of the solvent or precipitation by a non-solvent gives the orthorhombic modification III, which is also obtained when a film of polybut-1-ene is cast from solution. Forms I and II are both low melting modifi-

cations. On heating they transform to the tetragonal crys- talline form modification II, characterised by a helical

chain structure zf four-fold symmetry with an identity period of 6.8 A [2]. Form II is obtained directly by crystallisation from the melt and transforms spontaneously over a period of a few days to the “twinned” hexagonal

modification 1. Form I is characterised by a helical chain

* Corresponding author, tel. +44 61 200 3614, fax +44 61

200 3586.

structure of three-fold symmetry, identity period of 6.5 A

[2]. Once formed, modification I is stable to its melting point at 130°C [3].

The tetragonal to twinned hexagonal crystal transition is clearly the most important transformation as this step

determines the final crystalline state of thermally formed polybut-l-ene. For this reason much research has been

concentrated on following the development of form I from the melt through the tetragonal form 11.

2. Experimental

Facilities at Beamline 8.2 on the SRS, DRAL Dares- bury Laboratory were employed to perform time-resolved

simultaneous SAXS/WAXS/DSC studies on isotactic polybut-1-ene. The precise experimental setup has been described elsewhere [6].

Polybut-1-ene pellets supplied by Shell Chemicals were hot pressed to form a 1.5 mm thick sheet, which was then

aged at room temperature for several days From this sheet several small discs were cut and sealed in standard DSC

pans fitted with mica windows. Samples were put in the DSC and heated at 20”C/min to 150°C before being cooled at the same rate to room temperature. Other sam- ples were heated in the same manner but were isothermally crystallised by cooling rapidly from the melt at SO”C/min to 85, 90 or 95°C. SAXS and WAXS patterns were collected simultaneously throughout the particular thermal cycle with a time resolution of 6 s.

0168-583X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved

SSDI 0168-583X(94)00412-9 V. POLYMERS

270 .I. Cooke et al. /Nucl. Instr. and Meth. in Phys. Res. B 97 (1995) 269-272

3. Results and discussion

3.1. SAXS/ WAxS/DSC

The combination of simultaneous time-resolved

SAXS/WAXS and DSC enables detailed studies of the

polymorphic behaviour of melt crystallised polybut-1-ene to be made. On heating an aged sample, the initial small

angle pattern can be seen to show a strong peak in I(q) q2 which gradually increases in intensity before collapsing towards q = 0 as the polymer melts (Fig. 1). On cooling

and subsequent recrystallisation of the tetragonal form, only a very weak SAXS pattern is observed at low q.

It can be seen that the tetragonal form produces a small angle pattern reduced in intensity compared to that of the

hexagonal form and very similar to that of the amorphous material (Fig. 2). This is due to the small density differ-

ence between the tetragonal and amorphous polymer (0.90

and 0.85 g/cm3, respectively). The hexagonal form and amorphous polymer have a larger density difference (0.95

and 0.85 g/cm3) and therefore produce a stronger SAXS

pattern. The initial wide angle patterns are typical of a mixed

crystal sample (Fig. 31, there are strong reflections which may be indexed to either of the two crystal forms (Fig. 4).

On heating the peaks continuously change in intensity until melting at 140°C. On cooling and recrystallisation the

WAXS patterns indicate that the metastable structure has formed, only tetragonal reflections are present. By subtrac-

tion of a suitable amorphous halo, and integration of the

area underneath appropriate reflections, the amount of each crystal form present throughout the heating and cooling

cycle can be calculated, along with the degree of crys-

tallinity. The DSC curve for this heating cycle has been pub-

lished elsewhere [7]. Because the two crystal forms have melting points so close together, the melting of the tetrago- nal crystals can only be seen as a shoulder on the broader endotherm for the hexagonal crystals. At lower heating

Fig. 1. Time resolved SAXS pattern for PB-1. I(q) q2 is plotted

against q. Only a weak peak is visible on recrystallisation typical

of the tetragonal form.

0 0.05 0.1 0.15 0.2 0.25

q/Al

Fig. 2. SAXS patterns for tetragonal, hexagonal and amorphous polybut-1-ene.

rates and with less aged samples, the DSC curve may be separated as two free standing endotherms.

3.2. Isothermal crystallisation

During isothermal crystallisation two main factors in-

fluence the rate of crystallisation: (i) the rate of nucleation, and (ii) the subsequent rate of growth of these nuclei to

macroscopic dimensions [4]. The Avrami equation [8] may be used to provide some information about the kinetics of

a crystallisation process in terms of the Avrami exponent n - an integer dependent on the geometric form of the growth.

Fig. 3. Three dimensional plot of time resolved WAXS data. After

recrystallisation only tetragonal reflections are present

.I. Cooke et al. /Nucl. Instr. and Meth. in Phys. Res. B 97 (1995) 269-272 271

.!,i, recrystallised

-0.5

18 24 30

26 ldegrecs)

Fig. 4. Characteristic WAXS patterns for the hexagonal and

tetragonal forms. The tetragonal pattern is equivalent to the final

frame collected after recrystallisation, the hexagonal pattern is the

difference between tetragonal and the mixed crystal patterns.

The area beneath the tetragonal (200) peak in each

frame was calculated along with the corresponding invari- ants from the SAXS data. When plotted against time both

I.2 / ““I ” ” ” ” ” ” “““‘I”” /

I-

?

:

0.2 -

,i

_ i am Time Iminutes)

Fig. 5. Curve fit of the isothermal crystallisation at 90°C. The

Avrami exponent is represented by m2 in the table and was found

to be approximately equal to 3.

1.9 2.0 2.1 2.2 2.3 2.4 2,s

Log time

Fig. 6. Plot of Avrami exponents calculated for each of the

isothermal crystallisation temperatures.

these sets of data were found to be sigmoidal in nature.

Curve fitting the Avrami equation to the tetragonal peak area plot (Fig. 5), gave an Avrami exponent of 3.05 which

is in good agreement with literature values [5]. The ratio of (200) peak area to the adjacent amorphous

area under the WAXS plots is equivalent to the fraction

crystallised with time. For comparison, a plot of log [ln (200 area)] against log time was generated for all three annealing temperatures (Fig. 6). The gradient of each line

is equal to the Avrami exponent n, which increases as annealing temperature decreases. This has been thought to

suggest a change from homogeneous to heterogeneous nucleation [S], although it may be a result of slower than anticipated cooling of the sample. It should also be noted

that this method yielded the same values of n as the curve fitting method, and the linearity of the plots indicates that the Avrami equation is followed throughout the crystallisa- tion process.

4. Conclusions

The polymorphism of melt crystallised polybut-1-ene was studied using SAXS/WAXS/DSC small and wide angle patterns were indexed to the tetragonal or hexagonal crystals. The change in the Avrami exponent at different isothermal crystallisation temperatures was attributed to a change in nucleation mechanism. Use of this technique

V. POLYMERS

272 J. Cooke et al./Nucl. Instr. and Meth. in Phys. Res. B 97 (1995) 269-272

illustrates how other individual techniques used for study- References ing changes in morphology such as DSC are of limited

use, allowing only a restricted insight into the whole

transformation process. Simultaneous X-ray experiments of the type described allow a more comprehensive under-

standing to be obtained, as structural and thermal changes may be studied dynamically as a material is heated or

cooled.

Acknowledgements

JC is supported by a EPSRC-CASE award from ICI Paints. AJR is currently seconded half time to the EPSRC

Daresbury Laboratory.

111 121 [31

141

[51 161

171

[81

G. Natta, Atti. Accad. Naz. Lincei. 4 (1955) 61.

S. Kopp. J.C. Wittmann, B. Lotz, Polymer 35 (1994) 916.

K.W. Chau, Y.C. Yang, P.H. Geil, J. Mater. Sci. 21 (1986)

3002.

G. Polizzotti, F. Danusso, Atti. Accad. Naz. Lincei. 18 (1955)

19.

K.S. Sastry, R.D. Pate], Eur. Polym. J. 8 (1972) 63.

W. Bras, G. Derbyshire, A.J. Ryan, G.R. Mant, P. Manning,

R.E. Cameron, W. Mormann, J. Phys. (Paris) IV, C8, suppl.

J.d.PI, vol. 3 (1993).

W. Bras, G.E. Derbyshire, A.J. Ryan, J. Cooke, B. Koman-

schek, S.M. Clark, J. Appl. Crystallogr., in press (1994).

DRAL preprint DL/SCI/P909E.

C. Geancintov, R.S. Schotland and R.B. Miles, J. Polym. Sci.

B 1 (1963) 587.