enhancement of the mechanical properties of polylactides by solid-state extrusion. ii....

Enhancement of the mechanical properties of polylactides by solid-state extru&on: 11. Poly(L-lactide), poly(L/D-lactide), and poly(L/DL-lactide)

Stephen Ferguson, Dieter Wahl, and Sylwester Gogolewski, Polymer Research, AOlASIF Research Institute, Clavadelerstrasse, CH-7270 Davos, Switzerland

Polylactides with various degrees of chain regularity- crystalline poly(L-lactide); amorphous poly(L/D-lactide), and amorphous poly(r./DL-lactide)-were processed into high- strength, high-modulus resorbable rods using solid-state extrusion through a conical die. The rods with a highly fibril- lated structure had flexural strengths at a yield of up to 215 MPa and flexural modulus of up to 13.7 GPa. The initially amorphous poly(L/DL-lactide) developed approximately 25% of crystallinity after solid-state extrusion. POly(L/D-laC-

tide) remained amorphous independently of the treatment procedure applied. The shear strengths, flexural strengths, and flexural moduli of the solid-state extruded rods increased with increasing draw ratio. For the semicrystalline materials, solid-state extrusion enhanced the crystallinity. The achiev- able draw ratios during solid-state extrusion, and in con- sequence the final mechanical properties, were lower for poly(L/D-lactide) and poly(L/DL-lactide) than for poly- (L-lactide). 0 1996 John Wiley & Sons, Inc.

INTRODUCTION

There is increasing interest in the use of resorbable polymers for the preparation of internal fixation devices. Many such devices produced from various polyhydroxyacids are available at present, mainly for use in maxillofacial/craniofacial surgery and the treatment of small fragments and osteochondral defects. One lim- itation on the wider use of resorbable devices in bone fracture treatment relates to their mechanical properties.' While these seem to be sufficient when the implants are used in areas of restricted load, they are inadequate, however, to allow for the use of implants in high-load situations.

Various techniques are applied to produce resorbable internal fixation devices with enhanced mechanical properties. Compression moulding of fibers embed- ded in a matrix of the same polymer or different p0lymers,2.~ and "orientr~sion"~ are examples of such techniques. In these devices, strength-enhancing elements are formed as a result of the spherulite-to-fibril transformation leading to a high degree of chain orien- t a t i ~ n . ~ Chain orientation in polymeric objects is obtained by cold and/or hot-drawing, at temperatures above the glass transition temperature and below the melting temperature, calandering, blowing, extrusion

*To whom correspondence should be addressed.

at high shear rates, flow-controlled crystallization, etc.6 The routine hot-drawing process is used for polymeric objects with small cross-sections, e.g., fibers. A ram (piston-cylinder) extrusion, hydrostatic extrusion, or die drawing (solid-state extrusion) can be applied for orienting the chains in objects with large cross-sections and thus producing high-strength, high-modulus implants which might be considered for the internal fixation of loaded bone fractures. These techniques were successfully applied to nonresorbable

On the other hand, the increase of the chain orientation and crystallinity in implants produced from semicrystalline polyhydroxyacids decreases the rate of implant re~orption.'~ In addition, it has been observed that highly crystalline debris may remain at the im- plantation site for a longer time than required and, when located directly under the skin, may cause mechanical tissue irritation. Hence, it seems very attrac- tive to produce high-strength, high-modulus resorbable implants from resorbable polymers which develop a low degree of crystallinity and/or are fully amorphous. It might be expected that such low-crystallinity or amorphous implantable devices, while fulfilling their functions, will not upon in vivo degradation produce crystalline debris.

It has been reported in the previous study that solid- state extrusion of poly(D-lactide) produces high- strength, high-modulus bars with flexural strengths at a yield of up to 200 MPa and flexural modulus of up to 9 GPa.14

Journal of Biomedical Materials Research, Vol. 30, 543-551 (1996) 0 1996 John Wiley & Sons, Inc. CCC 0021-9304/96/040543-09

544 FERGUSON, WAHL, AND GOGOLEWSKI

This article reports the use of the solid-state extrusion technique for the processing of polylactides with various degrees of chain regularity (crystallinity) such as poly(L-lactide), poly(L/D-lactide), and PO~Y(L/DL- lactide) into high-strength, high-modulus rods. These can be processed further into internal fixation devices with more complex shapes and enhanced mechanical properties.

EXPERIMENTAL

Materials

Poly(L-lactide), poly(L/D-lactide) 30/70%, and poly (L/DL-lactide) 80/20% were purchased from CCA Purac Biochem (Gorinchem, Holland). The raw material was dried to constant weight under vacuum and stored in a desiccator prior to use. The molecular and thermal characteristics of the materials are given in Table I.

Sample preparation

Cylindrical bars (billets) with a diameter of 4 mm were melt-extruded using a Brabender (Brabender OHG, Duisburg, Germany) single-screw extruder Model PL331 at temperatures in the range of 120- 230°C, depending on the polymer used. The bars were evacuated at 4 X lo-' mbar and stored in a desiccator prior to use. Some of the bars were extracted with

alcohol to reduce the low-molecular-weight component content.

Solid-state extrusion

Solid-state extrusion of the polymeric billet was car- ried out at temperatures in the range of 50-165°C using exchangeable conical dies (ID 1, 1.5, 2, 3, and 4 mm) mounted in a heated metallic block. The temperature of the block was maintained within kO.3"C. An Instron model 4302 tensile testing machine was used to pull the billet through the die. The cross-head speed was in the range of 20-240 mm/min. The draw ratio (DR) of the solid-state extruded elements was defined as the initial cross-sectional area divided by the final cross- sectional area, also called the actual deformation ratio.' To introduce the bar into the die, it was necessary to predraw one end of the bar into a "nose" to reduce its diameter.

Thermal analysis

A Perkin-Elmer differential scanning calorimeter (DSC7) calibrated with indium was used for evaluation of the samples' thermal characteristics. The samples had an average weight of 3-7 mg and were scanned at the heating rate of 10"C/min under dry, oxygen- free nitrogen flowing at the rate of 30-40 ml/min. Three scans were taken from each sample. The relative degree of crystallinity of the material was calculated using the value of the heat of melting (H,) of the crystalline regions of poly(t-lactide), AH, = 93.1 J/g.15

TABLE I Molecular and Thermal Properties of Poly(L-lactide), Poly(L/DL-lactide), and

Poly(L/D-lactide) Samples before and after Processing

Sample M u (Da) r, r c ) AH, (J/g) x [%I -

Poly (L-lactide) Raw material Melt extruded Solid-state extruded (DR =

165"C/20 mm per min 10)

Poly(L/D-lactide) Raw material Melt extruded Solid-state extruded (DR = 7)

65"C/20 mm per min

Poly(L/DL-lactide) Raw material Melt extruded Solid-state extruded (DR = 8)

90"C/20 mm per min

415,000 187 ? 0.5 65.0 t 1.4 70 200,000 179 t 0.6 44.0 ? 0.7 47 175,000 184 t 0.1 59.4 2 1.9 64

160,000 85,000 80,000

205,000 80,000 70,000 134 ? 0.8 23.9 t 0.2 26

~

M,,, viscosity-average molecular weight; T,, melting temperature; A Hm, heat of melting; x, crystallinity. Melting temperature and heat of melting values are means of three measurements i: relative standard deviation (RSD) (%).

MECHANICAL PROPERTIES OF POLYLACTIDES. I1 545

-

~

-

~

Viscosity measurement

160 n r

140 c e

E 120 5

100 ; Y

Intrinsic viscosities of polymer solutions in chloro- form were measured at 25°C using an Ubbelohde Oc viscometer. The following constants were used in the Mark-Houwink equation for calculation of molecular weight: K = 5.45 X ml/g, a = 0.73 for poly- (L-lactide) and K = 2.21 X ml/g, a = 0.77 for both poly(L/D-lactide) and poly(L/DL-lactide).*6

Mechanical tests

Both the flexural modulus and flexural yield strength (0.2% strain offset) of the 50-mm-long samples were measured at room temperature in a four-point-bending mode according to ASTM D 79017 (Instron testing machine, cross-head speed of 1 mm/min, span of the two lower supports 30 mm, 3 : 1 span ratio, diameter of the supports 5 mm). The measured cross-head displace- ment was corrected to give the actual midspan deflec- tion. All of the values given are the means of five measurements. The relative standard deviation (RSD) was calculated (RDS = SD/Mean X 100%).

Scanning electron microscopy (SEMI

A Hitachi model S-4100 field emission SEM was used to characterize the morphology of the samples before and after solid-state extrusion. The instrument was operated at 2.0 kV.

RESULTS AND DISCUSSION

Melt-extrusion

Table I presents the molecular and thermal characteristics of the polylactides which were subjected to

I , 220 T T

16.0 - - 0" cr - 9 12.0 - a

I T)

!? 8.0

ii 4.0 1

200 - m n 180 r

5 m

160 f! i5 - E

c 140

120

0 1 I , l o 0 5 10 15 20

Draw Ratio

Figure 1. The effect of the draw ratio on the flexural strength and modulus of poly(L-lactide). 0, flexural modulus; 0, flexural strength; mean ? SD.

300 1 ---1 I

250 1

P tj ?! 150

f " 100

0 0 5 10 15 20

Draw Ratio

Figure 2. The effect of the draw ratio on the shear strength of polyk-lactide).

various treatment procedures. The melt-extrusion re- sulted in a significant decrease of molecular weight resulting from a thermo-oxidative and possibly ther- momechanical degradation. The extent of degradation varied from 50 to 60% depending on the material used, and was most extensive for the poly(L/DL-lactide). Even more extensive degradation was reported pre- viously for injection-moulded polylactides.'s

Solid-state extrusion

The mechanical properties of the solid-state extruded polylactides were strongly affected by the draw ratio and draw temperature (Figs. 1-6).

As shown in Figure 1, the flexural strength of poly- (L-lactide) increased with increasing draw ratio, reaching a maximum of 215 MPa at DR = 10 and temperature of 158"C, and subsequently decreased with the draw ratio to DR = 18. The draw ratio of 18 could only be obtained if the bars were extracted prior to solid-

r 12.0 -

m n 9 f 10.0 - - a I 8.0 - !?

T)

- X d 6.0 -

1 180

yo 0

O O 5 10 15 Draw Ratio

Figure 3. The effect of the draw ratio on the flexural strength and modulus of poly(L/DL-lactide). ., flexural modulus; 0, flexural strength.

546

-

-

-

FERGUSON, WAHL, AND GOGOLEWSKI

E 120 P

5

? 3i E -

100 0 U -

80

180

160

m 140

v

s p 120 2 3i k 100

' 80

0

60

0 . L-

4 0 5 10 15

Draw Ratio

Figure 4. of poly( L / DL-lactide).

The effect of the draw ratio on the shear strength

state extrusion and the draw temperature was increased to 165°C. Hence, it can be assumed that at this high temperature the flow drawing process is predomi- nant. As during the flow drawing the material flows with minimal chain orientation, the observed decrease of the flexural strength with the draw ratio can easily be understood. It cannot be excluded, however, that the liquid residue in the extracted samples might enhance chain mobility and hence chain refolding, resulting in the decrease of the flexural strength. The flexural modulus and crystallinity increased with the draw ratio up to DR = 10, reaching maximum values of 13.7 GPa and 73%, respectively, and above this draw ratio remained practically constant. The shear strength increased with the draw ratio to a maximum of 257 MPa at DR = 18 (Fig. 2).

For poly(L/DL-lactide), both the flexural modulus and strength increased with increasing draw ratios, reaching maxima of 9.5 and 177 MPa, respectively, at DR = 10 (Fig. 3). Above this draw ratio, where higher temperatures were used for drawing there was a drop

1 160 I

0 ' , ' 0

0 5 10 Draw Ratio

Figure 5. The effect of the draw ratio on the flexural strength and modulus of poly(L/D-lactide). A, flexural modulus; A, flexural strength.

140

120 - 100 5

5 5 80 3i k 2 60 cn

W

40

0 0 5

Draw Ratio

10

Figure 6. The effect of the draw ratio on the shear strength of poly(r. / D-lactide).

in flexural modulus and strength, which was probably due to the flow drawing and/or partial melting. The shear strength of the solid-state extruded poly- (L/DL-lactide) increased from 60 to 140 MPa for DR = 3, and then gradually decreased to 120 MPa for DR = 13 (Fig. 4). This may indicate that increasing the draw ratio above 3 enhances mechanical chain scission. It cannot be excluded, however, that upon drawing, the chain network formed in this low-molecular-weight material partially disentangles, which does not allow for achieving a high degree of chain orientation.

The flexural modulus and flexural strength of the poly(L/D-lactide) linearly increased with the draw ratio reaching at DR = 8, maxima of 7.7 and 147 MPa, respectively (Fig. 5). Shear strength increased with increasing draw ratio from 50 to 128 MPa at DR = 6 and subsequently decreased with the draw ratio up to

TABLE I1 Mechanical Properties of Poly(L-lactide), PO~Y(L/DL-

lactide), and Poly(L/D-lactide) Rods before and after Solid-state Extrusion*

Sample E (GPa) uu2 (MPa) us (MPa)

Melt extruded 4.5 f 5.4 129 t 2.6 67 2 4.0 Solid-state extruded 13.7 I: 2.8 211 i 1.6 248 i 4.0

Poly(L-lactide)

(DR = 16)

Poly(L / D-lactide) Melt extruded 3.9 f 2.2 88 +- 2.1 54 % 2.6 Solid-state extruded 7.0 5 4.9 145 t 0.8 121 2 5.1

(DR = 7.5)

Poly(L/DL-lactide) Melt extruded 4.1 5 2.5 104 ? 3.0 61 ? 3.7 Solid-state extruded 9.0 5 1.7 175 i 1.2 129 t 3.4

(DR = 10)

E, flexural modulus; C T ~ . ~ , flexural strength at yield; us, shear strength. All values are means of five measurements 2 relative standard deviation (RSD) (%).

*Maximum values.

MECHANICAL PROPERTIES OF POLYLACTIDES. I1

100 , I 189

547

200 7 -

40

0 i- 0 5 10 15

Draw Ratio

m

4 178 2

1 1 7 6

c -

20

Figure 7. The effect of the draw ratio on the thermal properties of poly(L-lactide). 0, crystallinity; 0, melting temperature.

DR = 8 (Fig. 6). It was not possible to draw poly- (L/D-lactide) above this draw ratio with the presently used equipment. This may be because the molecular weight of the billets was too low and the design of the nozzles for solid-state extrusion was not optimal. In the latter case, the high shear forces acting on the billet surface may lead to additional chain scission. The mechanical properties of the polylactides used in this study are summarized in Table 11.

The values of flexural moduli and flexural strength of experimental and commercial polylactide internal fixation devices given in the literature vary from 80 to 300 MPa and 3 to 13 GPa, respectively, depending on the technique used for the implant prepara t i~n . '~-~~ Even higher values of tensile strength and modulus were reported for gel-spun fibers of poly(~-lactide)."

As shown in Table 11, the values of flexural strength and modulus obtained for the solid-state extruded polylactides with various degrees of chain regularity

190 7

165

0 L.0 0 105 120 135 150 165 180

Solid-state Extrusion Temperature ("C)

Figure 8. The effect of the solid extrusion temperature on the thermal properties of poly(L-lactide) at an extrusion rate of 80 mm/min. 0, crystallinity; 0, melting temperature.

1 0 I , I

0 20 40 60 80 DSC Heating Rate ('Chin)

Figure 9. The effect of the DSC heating rate on the measured melting temperature of poly(L-lactide). The superheating effect of solid-extruded rods is indicative of high chain or- dering.

compare favorably with those given in the literature for the composite systems. They exceed those of polylactides processed using standard techniques, such as melt-extrusion or injection-moulding, and are much higher than the values reported for machined, as-polymerized blocks. The mechanical properties of polymers increase with increasing molecular weight. While for high-molecular-weight materials this effect is less pronounced, it can be critical for low-molecular-weight polymers. The polylactides used in the present study had a low molecular weight as a result of the degradation caused by melt-extrusion. Hence, it can be assumed that the mechanical properties of solid-state extruded polylactides can further be enhanced if the extrusion conditions applied in the experiments are optimized, and the billets used for solid-state extrusion will have a higher molecular weight. Such billets with a high molecular weight can, for instance, be produced by gel-extr~sion.~~

24.0

23.5

23.0

2 - 22.5 I 22.0

5 215 I

21 .o

20.5

- U

0 0 50 75 100 125 150 175

Temperature ("C)

Figure 10. DSC thermogram of poly(L/DL-lactide) (A) before and (B) after solid-state extrusion to DR = 11.


30 r- 25 1

c 20 - 1 X .d .-

6 1 5 1 l o

5 t

I 140

134

i i 132 i

0 0 5 10 15

Draw Ratio

Figure 11. The effect of the draw ratio on the thermal properties of poly(L/DL-lactide). ., crystallinity; 0, melting temperature.

The values of flexural strengths obtained here for solid-state extruded polylactides were measured at yield using four-point bending. This is more appro- priate for the implant design and its intended function. The values given in the literature were measured at

maximum load (ultimate flexural strength) using three- point bending.20,21 The values of flexural strength measured in three-point bending are usually higher than those measured in four-point bendingz4 Hence, the comparison of the values of flexural strength of polylactides obtained in the present study with the literature data may be misleading.

Thermal characteristics

The effects of the polymer treatment procedure on the melting temperature and heat of melting (degree of crystallinity) of polylactides are illustrated in Table I and Figures 7-11.

Melt-extruded poly(L-lactide) showed a substantial reduction in the melting temperature and crystallinity. This can be easily understood, as the raw material was obtained by chopping of as-polymerized blocks. These polymer chips usually develop high crystallinity, which is further enhanced upon storage, especially in the presence of trace amounts of moisture. Melt-extrusion is accompanied by quenching of the extrudate. This leads to the formation of a material which consists

Figure 12. Scanning electron micrographs of the longitudinal split surface of poly(L-lactide) rods solid-state extruded to various draw ratios at 165°C. (A) DR = 2.7; (B) DR = 5.2; (C) DR = 13; (D) DR = 17. Melting of low-molecular-weight com- ponents and/or flow drawing at the higher extrusion temperature obscures fibril boundaries. The scale bars represent 6 pm.


of a limited amount of poorly organized crystalline phase and high amount of the amorphous phase.

For the solid-state extruded poly(L-lactide), the melting temperature and crystallinity increased with increasing draw ratio and increasing extrusion temperature (Figs. 7 and 8). Figure 9 shows the superheating effect of the solid-state extruded material, which is indicative of the high degree of chain orientation/ex- tension.

Poly(L/D-lactide) 30/70% showed a glass transition at 54°C and no melt transition. It is known that poly(L / DL-lactide) 70/30%, racemic polyf DL-lactide) 50/50%, and poly(L/D-lactides) containing more than 15% of the D-unit in the chain are amorphous. It was not reported, however, that 30/70% poly(L/D-lactide) is also amorphous, or what the "eutectic point" for this polylactide system is.

Raw and melt-extruded poly(L/DL-lactide) 80/20% were amorphous. Solid-state extrusion developed crystallinity in the material. The measured degree of crystallinity and melting peak temperature increased with

the draw ratio, reaching maxima of 25% and 134"C, respectively, for DR = 9 (Fig. 10). The maximum melting temperature of 138°C was recorded for the samples drawn at DR = 5. At draw ratios higher than 5, which required higher draw temperatures, the melting peak temperature dropped to 134°C as a result of the flow drawing (Fig. 11).

Sample morphology

Solid-state extrusion of the polylactides under inves- tigation led to the formation of a highly oriented fibrillar structure. Figure 12(A-D) shows the longitudinally split surface of poly(L-lactide) rods, solid-state extruded at 165°C to the draw ratios of 2.7,5.2,13, and 17, respectively. It can be seen that the extent of fibrillation increases with the increasing draw ratio. Figure 13(A, B) presents the longitudinally split surface of a polyk- lactide) rod, solid-state extruded at 145°C to a draw

Figure 14. Scanning electron micrographs of poly(L-lactide) rods fractured perpendicularly to the longitudinal axis. (A) Fracture surface of melt-extruded rods shows typical ductile fracture characteristic of folded-chain spherulitic morphology. (B) Fracture surface of solid-state extruded rod (DR = 5.2) shows numerous fibrils ends. The scale bars represent 40 pm.

Figure 13. Scanning electron micrographs of the split surface of poly(L-lactide) solid-state extruded at 145°C to DR = 6. (A) The surface split longitudinally shows numerous fibrils. (B) Aggregates of fibrils run transversely connecting the longitudinal elements of the rod. The scale bars represent (A) 6 pm and (B) 40 pm.


ratio of 6. The aggregates of fibrils run transversally connecting longitudinal bundles of fibrils. The separa- tion between the fibrils is more pronounced than for the material shown in Figure 12. The differences in the surface texture may result from the fact that at higher draw temperatures, there is a partial melting of a low- molecular-weight component which upon solidifica- tion obscures the fibril boundaries, and/or that the solid-state extrusion which leads to chain orientation may be accompanied by flow drawing. Figure 14(A) shows the fracture surface of a melt-extruded poly(L- lactide) rod, and Figure 14(B) shows the fracture surface of the same rod after solid-state extrusion at 145°C to a draw ratio of 5.2. The rods were fractured perpendicularly to the longitudinal axis. While the surface of the melt-extruded bar exhibits the typical ductile fracture characteristic of the folded-chain spherulitic morphology, the surface of the solid-state extruded rod has a brushlike texture with numerous fibrils fractured at various planes.

Figure 15(A, B) shows the longitudinally split surface of a poly(L/DL-lactide) rod, solid-state extruded at 80°C

to DR = 7. The fibrillar morphology is very similar to that shown for polyb-lactide), with numerous fibrils pulled out from the bar surface.

The surface morphology of a fully amorphous poly- (L/D-lactide) rod, solid-state extruded at 65°C to DR = 7.5 is shown in Figure 16(A, B). The texture of the

Figure 16. Scanning electron micrographs of the split surface of amorphous polyfL/D-lactide) rods solid-state extruded at 65°C to DR = 7.5 showing the highly oriented structure. Unlike poly(L-lactide) and poly(L/DL-lactide), the fibril aggregates have a ribbon-like structure. The scale bars represent (A) 300 pm; (B) 100 pm; and (C) 10 pm.

Figure 15. Scanning electron micrographs of the split surface of poly(L/DL-lactide) rods solid-state extruded at 80°C to DR = 7 showing the highly oriented, fibrillar structure. The scale bars represent (A) 100 p m and (B) 6 pm.


fracture surface of this material is somewhat different from those of other two polylactides, in the sense that the fibrils aggregate in a ribbon-like structural elements.

CONCLUSIONS

Solid-state extrusion of poly(L-lactide), PO~Y(L/D- lactide) and poly(L/DL-lactide) at temperatures above the glass transition temperature leads to highly fibril- lated, high-strength, high-modulus materials.

The flexural modulus, flexural strength, and shear strength of polylactide rods produced by solid-state extrusion were strongly dependent on the degree of polymer chain regularity. These were highest for the highly crystalline poly(~-lactide) ( E = 13.7 GPa, uo.z = 215 MPa, us = 257 MPa) and lowest for the amorphous polyb/D-lactide) ( E = 7.6 GPa, ( T ~ . ~ = 147 MPa, us = 128 MPa).

The mechanical properties can further be enhanced if the billets used for solid-state extrusion have higher molecular weight (i.e., are produced using a technique other than melt-extrusion, which does cause polymer degradation). Such a technique of choice can be gel- extrusion.

The solid-state extruded rods can be processed into orthopedic implants with more complex structures by compression-moulding .

8.

9.

10.

11.

12.

13.

14.

15.

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17.

18.

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Received April 7, 1995 Accepted August 16, 1995

enhancement of the mechanical properties of polylactides by solid-state extrusion. ii....

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