some properties ofpoly(3-hydroxybutyrate)-poly(3-hydroxyvalerate) blends

Polymer International 39 (1996) 215-219

Some Properties of Poly(3- hydroxybutyrate)-

Poly(3-hydroxyvalerate) Blends

F. Gassner & A. J. Owen*

Institut Physik 111, Universitaet Regensburg, Universitaetsstrasse 3 1,93053 Regensburg, Germany

(Received 8 March 1995; revised version received 29 September 1995; accepted 21 October 1995)

Abstract: The melting, crystallization and dynamic mechanical behaviour of blends of bacterially produced poly[~(-)-3-hydroxybutyrate] (PHB) and poly[~(-)-3-hydroxyvalerate] (PHV) have been investigated. Results showed that melt-pressed PHB-PHV blends contained phase-separated domains in the melt which subsequently crystallized as PHB and PHV type spherulites respectively. The two melting regions detected by DTA related to separate melting of PHB and PHV crystallites, which were almost unaffected by the blend composition. The mechanical behaviour of a random copolymer of PHB/HV was compared with that of a blend of almost the same composition, and found to be markedly different.

Key words: poly-3-hydroxybutyrate (PHB), poly-3-hydroxyvalerate (PHV), biodegradable polymers, polymer blends.

1 INTRODUCTION

Bacterially produced poly[~(-)-3-hydroxybutyrate] (PHB) has been made available in large quantities by Zeneca Bio Products, Billingham, UK. It is of considerable interest, since it is a biodegradable thermoplastic with many potential applications (see for example, Refs 1-7). Zeneca also produce random copolymers of PHB with up to about 27% hydroxyvalerate in the PHB polymer chains. These PHB/HV copolymers, which are known under the tradename Biopol, have properties which differ markedly from those of the pure homopolymer PHB.*-’O

Recently, Steinbuechl and coworkers’ have suc- ceeded in obtaining useful quantities of the pure homopolymer poly-3-hydroxyvalerate (PHV) from the bacterium Chromobacterium violaceum using valeric acid as carbon source. This breakthrough now enables inves- tigation of the physical properties of pure PHV as well

* To whom correspondence should be addressed at: Polymer & Colloids Group, University of Cambridge, Cavendish Laboratory, Madingley Road, Cambridge CB3 OHE, UK.

as those of PHB, together with blends of these two homopolymers.” The present work describes some results on the melting behaviour and dynamic mechanical properties of PHB-PHV blends, together with a comparison of the properties of a PHB-PHV blend with a copolymer of the same overall composition. Apart from being bacterially produced and biodegradable, these components are useful test substances for understanding the physical properties of polymer mixtures.

2 EXPERIMENTAL

High purity grade, bacterially produced PHB powder was provided by Zeneca Bio Products (formerly ICI Biological Products), Billingham, UK (molar mass Mn = 222 000; M , = 794 000; particle size approximately 3-5 f 2.5,um). The PHV used was kindly sup- plied by A. D. Jendrossek of A. Steinbuechl’s research group, University of Goettingen, Germany. The molar mass of the powder was given as M, = 102500. The

215 Polymer Znternational0959-8103/96/$09.00 0 1996 SCI. Printed in Great Britain

216 F . Gassner, A . J . Owen

PHV particles were approximately 70 f 10pm in diam- eter. Some results were also obtained for a PHB/HV random copolymer from Zeneca containing 18 mol% HV.

Attempts to obtain satisfactory films for mechanical measurements by solvent evaporation using chloroform were unsuccessful. Consequently, we adopted a powder blending procedure, in which the powders were thor- oughly mixed in various proportions prior to melting in a press at 190°C for 2min, and moulding into films 0.2 mm thick. The films were cooled rapidly from the melt and subsequently stored at room temperature (20°C) to allow crystallization to proceed.

Samples cut from the melt-pressed films were investigated by differential thermal analysis (DTA) using Mettler equipment, at a heating rate of 5"C/min.

The morphology of the blends was investigated using a Zeiss Axioplan optical microscope in polarizing mode, with the sample mounted in a Linkam heating stage.

A laboratory-built dynamic mechanical testing rig was used to obtain the dynamic modulus and loss factor of sample strips at a frequency of 5Hz over the temperature range from approximately - 80 to + 160°C.

3 RESULTS AND DISCUSSION

3.1 DTA of PHV homopolymer

Firstly, DTA measurements were obtained in order to compare the melting behaviour of the original PHV powder with that of the melt-pressed films (Fig. 1). The powder showed a relatively narrow melting endotherm beginning at about 90°C and ending at 120°C. From the peak area the melting enthalpy was calculated to be 99 J/g. Melt-pressing of the powder into films at 190°C and subsequent re-crystallization at 20°C led to a some- what different melting endotherm with a shoulder on

film

powder i - 5 0 0 50 100 1SO

temperature ["Cl Fig. 1. DTA scan of PHV powder and melt-pressed film

(heating rate = S"C/min).

the lower side of the peak and a melting enthalpy of 86 J/g. The slight shift of the shape of the melting peak to lower temperatures suggests that thinner crystallites with reduced thermal stability had been formed. The maximum in the endotherm lay at 116°C in both cases.

3.2 DTA of PHB-PHV blends

DTA scans for the various blend compositions (Fig. 2) showed that there were no further significant shifts of the melting peak maximum for PHV. The PHB melting peak near 178°C showed a slight shift to lower temperatures with increasing PHV content in the blend, suggesting that the presence of PHV disturbs the PHB crystal growth in some way (see later). Within the limits of experimental error, the melting enthalpy values for both PHB and PHV were found to be constant for all blend compositions (including the pure samples), when normalized with respect to the mass of each polymer present in the blend. These results suggest that the two polymers did not mix to any great extent.

By means of Flory-Huggins' mean field theory and using estimated Hildebrand solubility parameters (for example Refs 13, 14), we calculated that in thermal equi- librium we would indeed expect the melt to be phase- separated into a PHB-rich phase containing at most 3% PHV and a PHV-rich phase containing at most 7% PHB. (The upper critical solution temperature would lie at approximately 2000 K, which is of course completely inaccessible, as the system would degrade at much lower temperatures.)

3.3 Optical microscopy-Simulation' of the melt-pressing process

In order to obtain information about the morphology of the PHB-PHV blend samples used for mechanical measurements, we 'simulated the melt-pressing procedure in a hot-stage under a light microscope. On

01 100

0 20180

5

*E a) 4 0 1 60

0 2! 6 0 1 4 0 a)

e o 1 2 0

- 50 0 50 100 1 so temperature ["C]

Fig. 2. DTA scan of PHB-PHV blends (heating rate = 5"C/ min).

POLYMER INTERNATIONAL VOL. 39, NO. 3, 1996

Poly(3-hydroxybutyrate)-poly(3-hydroxyvalerate) blends 217

heating a mixed powder with approximately equal proportions of PHB and PHV, we observed firstly that the relatively large PHV particles melted near 110°C. The PHB powder was then clearly identifiable as solid islands floating in PHV liquid. On approaching 190°C the PHB subsequently melted. The original positions of the PHB particles remained visible at first. The sample was then pressed via the cover slip, and cooled as rapidly as possible to just above room temperature (at a rate of approximately 75"C/min). Immediately after being pressed, the melt appeared to be homogeneous, even though we believe two separate phases existed. (This is presumably due to the refractive indices of the two components being very similar.)

After an induction time of less than 2 min, spherulites nucleated and grew in the melt in domains of varying size (usually much larger than the original particle sizes). These spherulites clearly resembled those observed in pure PHB samples, and will be referred to as PHB-type spherulites. According to the above con- siderations, these PHB-type spherulites are likely to contain a small fraction of PHV, some of which may enter the PHB crystal lattice, resulting in a slight effect on the subsequent melting temperature (Fig. 2). After a further induction time of about lSmin, a different type of spherulite nucleated and grew slowly in the rest of the melt, preferentially nucleating at the boundaries of PHB domains but not exclusively there (see Fig. 3). These spherulites were characteristic of those observed in pure PHV samples. The PHV-type spherulites were easily identifiable, since they were generally larger and more highly birefringent (showing much brighter inter-

ference colours) than the PHB-type spherulites under these conditions. (Both types of spherulite were opti- cally similar in that they were both banded, biaxial and had the same sign of birefringence.)

We believe that the morphology described above approximates that occurring in the melt-pressed films used for the mechanical measurements. However, it should be mentioned that under other conditions of heating and crystallization (not discussed here), different types and sizes of spherulites have been observed. Under some conditions it is possible that co- crystallization of PHB and PHV could take place. Work by Organ & Barham on blends of PHB with PHB/HV copolymer has shown that interesting and complicated mixing behaviour and resultant morphol- ogies can occur.15 It is, therefore, necessary to perform further systematic work, in order to understand the crystallization behaviour of these blends more thor- oughly.

3.4 Dynamic mechanical measurements of PHB-PHV blends

Figures 4 and 5 show modulus and loss factor results for the various blend compositions. The large fall in modulus and corresponding peak in the loss factor tan 6 at approx. 10°C for PHV are related to the glass transition of the amorphous regions of PHV. The blends soften at higher temperatures as a consequence of the increased quantity of PHB present, which is mechanically stiffer. The glass transition in PHB is not resolved in these samples, presumably due to the quite

Fig. 3. Optical micrograph of large PHV-type spherulites still growing in PHB-PHV melt. O n the right of the figure is a large domain of much smaller PHB-type spherulites which crystallized first. Some PHB spherulites (on left) have been engulfed by the

growing PHV spherulites. (Magnification on print: 8.5 mm corresponds to 100 pm).


218 F . Gassner, A. J . Owen

0.5 1 I I I 0.0 I I

-50 0 50 100 150 temperature [“CJ

Fig. 4. Dynamic mechanical modulus for PHB-PHV blends at 5 Hz.

high degree of crystallinity of the PHB component. (Such a transition would show up as a loss factor peak at approximately 20°C at 5 Hz for PHB.) The large rise in loss factor above about 100°C is due to PHV melting.

3.5 Comparison of PHB-PHVblend with PHBfHV copolymer

In a random copolymer the two components are forced to be neighbours within a single molecule, whereas in a blend, separation of the two types of molecule may occur. It is therefore of considerable interest to compare the properties of a copolymer and a blend of the same overall monomer concentration. In this work we had

. 04 -

.02- 0-

. 2 - 0-

c .02- lu - 0-

.02 - 0 -

02-

I 1 I 100 \ s o

temperature [“C] Fig. 5. Dynamic loss factor for PHB-PHV blends at 5 Hz. 0,

100/0; +, 80/20; [7,60/40; x , 40/60, A, 20/80; *, 0/100.

available a random copolymer of HB and HV with 18% HV in the chains and a blend of PHB and PHV with 20% PHV, for comparison.

Figures 6 and 7 show the corresponding dynamic mechanical results. Above 5°C the copolymer had a lower dynamic elastic modulus than the equivalent blend, and the copolymer showed a much more pro- nounced dynamic glass transition loss peak near room temperature. Thus, demixing of the two components of the blend does not by necessity lead to poorer mechanical properties. The reason for this behaviour is as follows : PHB/HV copolymers crystallize only in the PHB crystallographic structural form, as PHB type spherulites, below an HV content of c. 40%.16 The HV units act as defects in the crystals, and form part of the amorphous component within these PHB spherulites. On the other hand, in blends of PHB and PHV the two components can crystallize separately as PHB and PHV spherulites, with a correspondingly higher overall

c. a [L

2 Y, 3 7 0 0 E

5 , 1 I I 1 I 1 ; - 2 -

1 -

0 -50 0 50 100 150

temperature rC] Fig. 6. Dynamic modulus of PHB/HV (18%) copolymer and

P H E P H V (80/20) blend.

I I I I I I I

! + PHWPHV(80hO) blend t .121.08

co .081.04

- 50 0 s o 100 150

temperature [“C] Fig. 7. Loss factor of PHB/HV (18%) copolymer and

P H E P H V (80/20) blend.


Poly(3-hydroxybutyrate)-poly(3-hydroxyvalerate) blends 219

degree of crystallinity in the sample. This leads to higher modulus and lower mechanical losses due to amorphous softening at the glass transitions.

4 CONCLUSION

A PHB-PHV powder-blended system formed phase- separated domains in the melt. The domains could be identified neatly via their different melting and crystallization behaviour. Two types of spherulite were observed to nucleate and grow, which were characteristic of pure PHB and PHV spherulites respectively. DTA measurements showed separate melting endo- therms for the two blend components, which were characteristic of the melting behaviour of the individual homopolymers.

Dynamic mechanical measurements gave widely dif- fering viscoelastic properties depending on blend composition. The dynamic modulus and loss factor obtained were essentially a superposition of the behaviour for the two separate polymers; for high PHB contents the behaviour was clearly dominated by PHB, whereas for low PHB contents the PHV dominated the behaviour. Comparison of a copolymer and a blend of almost the same overall monomer composition showed that the blend had a higher modulus and lower loss over a large temperature range, this being attributable

to different degrees and types of crystallization in the two samples. These properties are now being investigated in more detail.

REFERENCES

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5 6

7

8 9

10

11

12

13

14 15 16

1 Holmes, P. A., in Developments in Crystalline Polymers-2, ed. D.

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Sci., 19 (1984) 2781.

Nicholson. T. M.. Unwin. A. P. & Ward, I. M., Proc. POLYMkT’94 Cona, Imperial College, London, September 1994. Koning, G. J. M. & Lemstra, P. J., Polymer, 33 (1992) 3295. Pearce, R., Jesudason, J., Orts, W., Marchessault, R. H. & Bloem- bergen, S., Polymer, 33 (1992) 4647. Revol, J-F., Chazy, H. D., Deslandes, Y. & Marchessault, R. H., Polymer, 30 (1989) 1973. Bauer, H. & Owen, A. J., Colloid Polymer Sci., 266 (1988) 241. Owen, A. J., Heinzel, J., Divjakovic, V. & Skrbic, Z., Polymer Commun., 33 (1992) 1563. Scandola, M., Ceccorulli, G. & Doi, Y., Int. J . Biol. Macromol., 12 (1990) 112. Steinbuechl, A., Debzi, E.-M., Marchessault, R. H. & Timm, A., Appl. Microbiol. Biotechnol., 39 (1993) 443; Plast., Rubb. Compos. Proc. Appl., 19 (1993) 15. Pearce, R. P. & Marchessault, R. H., Macromolecules, 27 (1994) 3869. Eisele, U., Introduction to Polymer Physics. Springer Verlag, Heidelberg, 1990, Chapter 13. Bluemm, E. & Owen, A. J., Polymer, 36 (1995) 4077. Organ, S. J. & Barham, P. J., Polymer, 34 (1993) 459. Scandola, M., Ceccorulli, G., Pizzoli, M. & Gazzano, M., Macro- molecules, 25 (1992) 1405.


some properties ofpoly(3-hydroxybutyrate)-poly(3-hydroxyvalerate) blends

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