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CHAPTER 7

PREPARATION AND PROPERTIES OF PLLA/PLCL

FILAMENTS FOR POTENTIAL USE AS A

MONOFILAMENT SUTURE

7.1 INTRODUCTION

In recent years, biodegradable polymers have attracted increasing

attention as a candidate for use in biomedical applications (Furuhashi et al

2006). Among medical devices made out of biodegradable polymers, fibrous

materials are playing an increasing important role. This is attributed to the

development of sophisticated surgical procedures, as well as the

advancements made in synthetic fibres. The use of fibres as absorbable

sutures represents a major biomedical application of this form of polymer. If

fibres are to be used as absorbable sutures in medicine and surgery it is

necessary to meet a number of specific criteria pertinent to biological

properties, tensile properties, handling characteristics and surface properties

(Penning et al 1993).

Today the selection of a proper suture is not a simple task. In

earlier days it was much less difficult for the surgeon to select the sutures due

to the only availability of natural suture materials. With the introduction of

synthetic polymer materials, particularly synthetic degradable materials, a

revolution have occurred in this field. A wide variety of new suture materials

were developed in order to satisfy the requirements of modern surgery.

However, there is no single suture material which can fulfill all the crucial

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requirements of sutures. The surgeon should choose the right suture for the

type of surgery that he is performing because different tissues have differing

requirements for suture support. The present surgeon has several choices of

suture material available and he may choose them based on availability and

his familiarity (Pillai et al 2010).

Poly (L-lactic acid) (PLLA) represents a class of materials of

growing importance, especially in the field of biomedical engineering due to

its favorable chemical, biological, and mechanical characteristics. These

characteristics include structural simplicity, comparatively easy synthesis, and

compatibility with biological tissues, applicability of mechanical

characteristics, and a simple degradation mechanism with no complex

products (Jamshidi et al 1988).

PLLA can be transformed by spinning into filaments for

subsequent fabrication of desirable textile structures. Several researchers have

prepared PLLA fibres by melt spinning and solution spinning. Cicero et al

(2001) employed two step melt-spinning to produce PLLA fibres. Higher-

order structures and mechanical properties of stereocomplex-type poly (lactic

acid) melt spun fibres were studied by Furuhashi et al (2006). Eling et al

(1982) reported that the PLLA fibres prepared by dry spinning have higher

mechanical properties than melt spinning. Wet spinning of PLLA was carried

out by Nelson et al (2003). It was reported that solvent systems, polymer

blends, and winding rates alter mechanical and morphological properties of

the fibres. Gupta et al (2006) used a dry-jet-wet spinning and hot drawing

process to obtain PLLA fibres.

In a recent study PLLA hompolymer fibres has been prepared using

dry-jet-wet spinning technique and observed that the glass transition

temperature (Tg) and crystallinity are 76°C and 48% respectively, which is

higher for suture applications (Gupta et al 2006). Furthermore,

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Hiljanen-Vainio (1996) also reported that for monofilament suture purposes

PLLA is too stiff and inflexible due to its greater glass transition temperature

and crystallinity, therefore it is difficult to make knots at room temperature.

The knottability of the fibres may be improved by modifying the PLLA fibre

properties through reducing the glass transition temperature and crystallinity.

Blending of PLLA with other polymers has been recognized as a valuable

tool for modifying the properties of PLLA (Sodergard and Stolt 2002).

However, manifestation of superior properties depends upon the miscibility of

polymers at the molecular level (Hiljanen-Vainio 1996).

Thus, several polymer blend systems have been investigated, such

as PLLA / Poly (ethylene glycol) (PEG) (Sheth et al 1997), PLLA/Poly (D, L-

lactic acid) (PDLLA) (Tsuji et al 1991), PLLA/PCL (Yang et al 1997),

PDLLA/PCL (Tsuji and Ikada 1996). However, most of the blends are found

to be immiscible or partly miscible depending on their composition. Hiljanen-

Vainio (1996) produced different combinations of PLLA/PLCL using an

injection molding apparatus and stated that blending of PLLA with

Poly (L-Lactide-co- -caprolactone) (PLCL) provides a unique means to

achieve intimately compatible blends.

Previous studies in the literature showed that, no extensive study is

carried out for PLLA/PLCL fibres using dry–jet–wet spinning technique

(Gupta et al 2006). In this study attempts have been made to prepare

PLLA/PLCL fibres by dry-jet-wet spinning. The main advantages of

producing the fibres by this technique are that the minimum polymer

degradation occurs during the spinning process and leads to better molecular

chain orientation, which ultimately results in the development of the fibres

with superior properties (Gupta et al 2006). The effect of draw ratio on the

PLLA/PLCL fibres is also studied.

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7.2 MATERIALS AND METHODS

7.2.1 Materials

Two kinds of polymers, poly L-lactic acid (PLLA) and poly

(lactide-co- -caprolactone) (PLCL) (both obtained from Polymer laboratory,

University of Uppsala, Sweden) were used in this study. Both the polymers

were stored under vacuum at ambient conditions. Chloroform and methanol

were supplied by Merck India. Chloroform was dried over P2O5 prior to the

polymer dissolution. The intrinsic viscosity of the dried PLLA and PLCL

were 3.45 dL/g and 3.27 dL/g respectively.

7.2.2 Methods

The process of manufacture of suture is presented in the Figure 7.1.

The following section discusses the each step in detail.

7.2.2.1 Spinning

PLLA/PLCL (90%/10% w/w) fibres were prepared by dry-jet-wet

spinning of the polymer from chloroform solution and with methanol as the

precipitating medium. Polymer was dried at 100°C under vacuum for 24

hours prior to the spinning. The polymer was dissolved in chloroform using a

moisture-free glass assembly, under constant stirring at ambient conditions

for 24 hours. The polymer solution was subsequently spun on a dry–jet–wet

spinning machine as shown in Figure 7.2.

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Figure 7.1 Process of Manufacture of PLLA/PLCL Monofilament Suture

Sourcing of PLLA/ PLCLpolymers and other chemicals

Spinning by Dry-jet-wet spinningmachine

Drawing and heat settingof as-spun fibres

Characterization of PLLA/PLCLmono filaments

Tenacity measurement

Knot strength

measurement

Glass transition

temperature

Crystallinity

Surface structure analysis

Comparison of properties ofPLLA homopolymer with

PLLA/PLCL fibres

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Spinneret Coagulation bath

Spinningzone

PLLA/PLCLMonofilamentfibres

Take up rollerRam

Cylinder

Figure 7.2 Schematic Diagram of Dry-Jet-Wet Spinning

The polymer solution was extruded through the spinneret of 0.5

mm into the coagulation bath containing methanol. Both the coagulation bath

had methanol as the coagulant and fibre was collected using two sets of take

up rollers, both operating at the speed of 6m/min. The air gap between the

spinneret and coagulation bath was kept as 25 mm. The fibre was collected on

bobbins and was subjected to drawing process in the second step.

7.2.2.2 Drawing

The as spun fibre produced at a take-up speed 6 m/min was

subjected to drawing and heat setting operations. The fibre was subjected to

two stages drawing with varying draw ratios in the range of 3–15 at the

drawing temperature of 75°C, followed by the heat setting at 100°C under

taut conditions.

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7.2.2.3 Tenacity and knot strength tests

Instron tensile tester was used to measure the tenacity and knot

strength of PLLA/PLCL sutures. The PLLA/PLCL sutures were tested for

tenacity and knot strength at a gauge length of 150 mm and extension rate of

90 mm/min. In the knot strength measurement, knot was formed with square

knot method as described previously. The details of the knot tying procedure

have been explained in chapter 3, section 3.3.4. For each test method at least

20 readings are taken.

7.2.2.4 Differential Scanning Calorimetry (DSC)

Differential scanning calorimetry (DSC) was performed using a

Perkin Elmer DSC-7 system. Vacuum-dried samples were loaded and the

thermograms were run in the temperature range of 40–200°C under nitrogen

atmosphere at a heating rate of 10°C/min. The heat of fusion ( Hf) values

were obtained from the area under the melting thermograms. The crystallinity

was obtained by the following expression:

Crystallinity (%) = Hf / Hf (cry) X 100

Where, Hf is the heat of fusion of the sample and Hf (crys) is the heat of

fusion of 100% crystalline PLLA and is taken as 93.7 J/g.

7.2.2.5 X-Ray Diffraction

PLLA/PLCL monofilaments were analyzed using a Phillips wide-

angle X-ray diffraction apparatus equipped with a scintillation counter. X-ray

scans were made over a 2 range of 10°-35.

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7.2.2.6 Scanning Electron Microscope

The surface characteristics of fibres were studied using a

STEREOSCAN 360 (Cambridge Scientific Industries Ltd.), scanning electron

microscope after coating them with silver.

7.2.2.7 Statistics

The datas are expressed as mean ± standard deviation. Statistical

analysis was carried out using the unpaired student’s t test. A value of P <0.05

has been considered to be statistically significant.

7.3 RESULTS AND DISCUSSIONS

7.3.1 Draw Ratio and Its Effect on Tenacity and Knot Strength

The PLLA/PLCL fibres are prepared by dry-jet-wet spinning

method. The as spun fibres obtained using the dry-jet-wet spinning is

subjected to drawing and heat-setting operations. The drawing of the fibre is

carried out at a temperature range of 75°C, followed by the heat setting at

100°C. It is important to mention that in the spinning conditions, a maximum

draw ratio of 15 could be achieved. This draw ratio is comparatively higher

than that reported so for in melt spinning of PLLA homopolymer fibres. The

achieved higher draw ratio may be due to the fact that the chain entanglement

is largely suppressed in the solution state as compared with the polymer melt.

Since the entanglements are the key factors in limiting the maximum

drawability of a polymer, the low entanglements are effectively transferred to

the fibre structure (Fu et al 2002).

Further, it is observed that the draw ratio obtained for PLLA/PLCL

fibre is also higher than that of PLLA homopolymer fibres obtained by dry-

jet-wet spinning. The higher draw ratio in PLLA/PLCL fibres may be

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interpreted in terms of a reduction of molecular interactions associated with

the introduction of the flexible polymer PLCL. Smook et al (1990)

demonstrated that the maximum draw ratio of flexible polymers is governed

by molecular attraction forces expressed as the cohesive energy for a wide

range of polymers. Based on this relation one expects a higher drawability for

PLLA/PLCL fibres.

The variation of tenacity of the PLLA/PLCL fibre with the draw

ratio is presented in Figure 7.3. It is clear from the figure that tenacity

increases with increase in draw ratio and reaches a maximum value of

30cN/tex for the PLLA/PLCL fibre at draw ratio of 9 and then decreases with

further increase in draw ratio. The increase in tenacity with increase in draw

ratio is due to the high orientation of molecular chains and crystallinity.

Beyond draw ratio 9, tenacity decreases, the decrease in strength can be

associated with void generation due to overdrawing.

Figure 7.3 Tenacity as a Function of Draw Ratio for PLLA/PLCL Fibre

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An effect of draw ratio on knot strength is similar to that of the

tenacity as shown in Figure 7.4. The maximum knot strength of 22cN/tex for

blended fibre is achieved for a draw ratio of 9. Further it is observed that the

maximum tenacity and knot strength value achieved for the PLLA/PLCL

fibre is substantially lower than that reported for PLLA fibre. Further it is

observed that the maximum tenacity of 30 cN/tex and knot strength value of

22cN/tex achieved for the PLLA/PLCL fibre is substantially lower than the

tenacity value of 36 cN/tex and knot strength value 27 cN/tex reported for

PLLA fibres by Gupta et al (2006). The decrease of crystallinity of

PLLA/PLCL fibres is due to the presence of PLCL, which interrupt the chain

regularity and inhibit the formation of highly ordered structures.

Figure 7.4 Knot Strength as a Function of Draw Ratio for PLLA/PLCL

Fibres

7.3.2 Differential Scanning Calorimetry Analysis

The results of DSC heating scans of the PLLA/PLCL as spun and

the fibres drawn fibres at draw ratio (DR) 9 are shown in Figure 7.5. The

glass transition temperature of the asspun and the drawn fibres are 50°C and

58°C respectively. As the glass transition is associated with the mobility of

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the fibre in the amorphous region, orientation is introduced in these regions

during the drawing process. Further, the crystalline regions in a semi

crystalline polymer are known to act as the anchors between the amorphous

regions and therefore tend to restrict the molecular mobility (Jamshidi et al

1988).

The observed behavior of the Tg enhancement with drawing and

heat setting may, therefore, be attributed to the cumulative effect of the

enhanced orientation in the amorphous region and the high crystallinity.

Similar results are observed by Cicero et al (2002). Furthermore, it is

important to mention that the maximum Tg value of 58°C for the PLLA/PLCL

fibres is considerably lower than the Tg value 76°C stated for PLLA

hompolymer fibres by Gupta et al (2006). This reduction in Tg value for the

PLLA/PLCL fibre may be due to the increase in the chain flexibility and

mobility associated with the introduction of PLCL.

Endo

Temperature (0C)

160 200 220180140120605040 80

Asspun

DR 9

Exo

Figure 7.5 DSC Thermograms of the PLLA /PLCL Fibres

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7.3.3 X-Ray Diffraction Analysis

The X-ray diffraction pattern of the PLLA/PLCL asspun fibre and

the fibres drawn fibres at draw ratio (DR) 9 are presented in the Figure 7.6.

Sharp peaks with higher intensity are observed in case of drawn fibre as

compared to asspun fibre. This shows that X-ray crystallinity is higher in

drawn fibre as compared to asspun fibres. This could be due to orientation-

induced crystallization. All diffractions lie at identical angles, and no

additional diffraction peaks are visible in the drawn fibres, indicating that the

crystalline phase does not change during drawing process. More over it is

observed that the maximum value of crystallinity of 40 % for the

PLLA/PLCL fibre is also lower than the crystallinity 48% stated for PLLA

homopolymer fibres by Gupta et al (2006). The decrease of crystallinity of

PLLA/PLCL fibres is due to the presence of PLCL, which interrupt the chain

regularity and inhibit the formation of highly ordered structures.

Intensity

10 20 30

DR= 9

Asspun

Angle 2

Figure 7.6 X-Ray Diffraction Patterns of PLLA/PLCL Fibres

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7.3.4 Surface Topography from SEM

The Scanning electron micrograph of the asspun and drawn

PLLA/PLCL fibres is presented in Figure 7.7. The asspun fibre shows a

porous structure throughout the surface, whereas considerable change in the

surface morphology is observed in the drawn fibres, this may be due to the

collapse of the porous structure.

(a) (b)

(c)

Figure 7.7 SEM Micrographs of PLLA /PLCL Filaments (a) Asspun

(b) Drawn at Draw Ratio 3 (c) Drawn at Draw Ratio 9

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7.3.5 Comparison of Properties of PLLA Homopolymer Fibres with

PLLA/PLCL Fibres

The properties of PLLA homopolymer fibres and PLLA/PLCL

fibres are presented in the Table 7.1. From the Table 7.1, it is noted that the

maximum tenacity of 30 cN/tex and knot strength value of 22cN/tex achieved

for the PLLA/PLCL fibre is substantially lower than the tenacity value of 36

cN/tex and knot strength value 27 cN/tex reported for PLLA fibres by Gupta

et al (2006). The decrease of crystallinity of PLLA/PLCL fibres is due to the

presence of PLCL, which interrupt the chain regularity and inhibit the

formation of highly ordered structures. Furthermore, it is noted that the

maximum Tg value for the PLLA/PLCL fibres is 58°C that is considerably

lower than the Tg value 76°C stated for PLLA hompolymer fibres by Gupta et

al (2006). This reduction in Tg value for the PLLA/PLCL fibre may be due to

the increase in the chain flexibility and mobility associated with the

introduction of PLCL. More over it is observed that the maximum value of

crystallinity of 40 % for the PLLA/PLCL fibre is also lower than the

crystallinity 48% stated for PLLA homopolymer fibres by Gupta et al (2006).

The decrease of crystallinity of PLLA/PLCL fibres is due to the presence of

PLCL, which interrupt the chain regularity and inhibit the formation of highly

ordered structures.

Table 7.1 Comparison of Properties of PLLA homopolymer fibres with

PLLA/PLCL fibres

Property PLLA homopolymer fibres PLLA/PLCL fibres

Tenacity (cN/tex) 36 30

Knot strength (cN/tex) 27 22

Tg (°C) 76 58

Crystallinity (%) 48 40

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7.4 CONCLUSIONS

The dry–jet–wet spinning process is employed to spin poly

(lactic acid) (PLLA)/ poly (lactide-co- -caprolactone) (PLCL)

(90/10 %) fibres. The as spun fibres are subsequently subjected

to two-step process of drawing and subsequent heat setting. It is

important to mention that a maximum draw ratio of 15 could be

achieved for PLLA/PLCL fibres.

The effect of draw ratio on tenacity and knot strength are

studied. The tenacity value increases with increase in draw ratio

and a maximum value of 30cN/tex is achieved at draw ratio 9.

Beyond draw ratio 9 tenacity decreases. The effect of draw ratio

on knot strength is similar to that of the tenacity. The maximum

knot strength of 22cN/tex is achieved at draw ratio 9 for

PLLA/PLCL fibres.

The addition of PLCL in PLLA, in order to reduce the glass

transition temperature and crystallinity has shown a strong

effect on the fibre properties.

The fibre with the crystallinity of 40% and glass transition

temperature of 58°C are achieved for a draw ratio 9, which is

considerably lower than the PLLA homo polymer fibres.

The asspun fibre shows a porous structure throughout the

surface, whereas considerable change in the surface

morphology is observed in the drawn fibres.

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In conclusion, manufacture of PLLA/PLCL fibres using dry-jet-

wet spinning technique may be an effective way to modify the

property of PLLA and more suitable to use in suture

applications.