Characterisation and Degradation of Aba Tri-Block with Disrupted Ends
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NANYANG TECHNOLOGICAL UNIVERSITY
CHARACTERISATION AND
DEGRADATION OF ABA TRI-BLOCK
WITH DISRUPTED ENDS
Foo Yong Quan
School of Materials Science & Engineering
2012
Report Spine
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FOO Y.Q. Characterisation and Degradation of ABA Tri-Block with Disrupted Ends BEng (2012)
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NANYANG TECHNOLOGICAL UNIVERSITY
CHARACTERISATION AND
DEGRADATION OF ABA TRI-BLOCK
WITH DISRUPTED ENDS
Submitted in Partial Fulfillment of the Requirements for the Degree of Bachelor of Engineering of the Nanyang Technological University
By
Foo Yong Quan
School of Materials Science and Engineering
2012
ABSTRACT
Four tri-block copolymers composed of poly(L-lactide)-poly(caprolactone-co-L-lactide)-poly(L-Lactide) (PLLA-PCLLA-PLLA), with disrupted ends were characterised according to individual’s mechanical and thermal properties. The tri-block copolymers consist of amorphous PCLLA middle block, at constant composition of CL and LLA being 60% and 40% respectively. The end blocks consist of either a semi-crystalline PLLA or various compositions of P(LLA-DLLA). Factors such as high overall crystallinity and effective crosslinking of arms led to the high modulus values of polymers.
A 16-week degradation study was done to investigate degradation behaviour. The number average molar mass, mass loss, water uptake and the changes in mechanical and thermal properties were investigated at each designated time points. Experiments revealed that the polymers degraded via bulk degradation. The 16-weeks period only showed the initial stage of degradation whereby hydrolysis occurred mainly in the amorphous region, causing a rise in crystallinity in between degradation time. There was no significant hydrolysis of the crystalline region discovered. Along with degradation, creeps recovery declined a little before an increase from the second week. Copolymers were unable to sustain high stress from the eighth week time point onwards. DSC revealed a slight hydrolysis of the crystalline phase where the trough of the DSC curve started to widen up.
ACKNOWLEDGEMENT
During the course of my Final Year Project, many have offered their valuable assistance, advice, and time such that the different component of this project was made manageable to me.
I would like to show my utmost gratitude to my Final Year Project supervisor, Prof. Subbu S. Venkatraman, for offering the opportunity to do this project under him.
A special thanks to my mentor, Mr. Kong Jen Fong, for his constant guidance and assistance throughout the course of the project. He is a very patient and helpful mentor.
Lastly, Mr. Wilson Lim of the Organic Materials Lab who had helped and showed guidance in the use and understanding of the equipment.
Table of Content
ABSTRACT I
ACKNOWLEDGEMENT II
Table of Content III
List of Tables IV
List of Figures V
CHAPTER 1: INTRODUCTION 3 1.1Background 3 1.2 Objective 5 1.3 Scope 5
CHAPTER 2: LITERATURE REVIEW 4 2.1 Biodegradable Thermoplastic Elastomers 4 2.2 Biodegradable Polymers 7 2.2.1 Poly-ε-Caprolactone 7 2.2.2 Polylactide 8 2.2.3 Homopolymer vs Copolymer 9 2.3 Possible Applications 9
CHAPTER 3: MATERIALS AND EXPERIMENTAL METHODS 11 3.1 Materials Used 11 3.2 Experimental Procedure 12 3.2.1 Polymer Films Preparation 12 3.2.2 Degradation Study (16 Weeks) 12 3.2.3 Dynamic Mechanical Analysis (DMA) 13 3.2.4 Differential Scanning Calorimetry (DSC) 13 3.2.5 Size Exclusion Chromatography (SEC) 14
CHAPTER 4: RESULTS AND DISCUSSION 15 4.1 Thermal Properties 15 4.2 Properties of Initial Sample 20 4.2.1 Elastic Modulus 20 4.2.2 Creep Recovery 22 4.2.3 Creep Recovery with Controlled force 23 4.3 Degradation Study and Mechanical Properties with Degradation 25 4.3.1 Molar Mass and Mass Loss 25 4.3.2 Creep Recovery 27
CHAPTER 5: CONCLUSION 30
CHAPTER 6: RECOMMENDATIONS 31
REFERENCES 32
List of Tables Table 1: List of Materials 11 Table 2: Melting Temperature of Samples 12 Table 3: % Crystallinity of samples 18 Table 4: % Crystallinity of samples between Initial and Week 8 19
List of Figures Figure 1: Before and after cross-linking polymer 5 Figure 2: Different copolymers forms 6 Figure 3: Star block copolymer 6 Figure 4: ε-Caprolactone to Poly-ε-Caprolactone 8 Figure 5: Lactide to PLA 8 Figure 6: DSC results of Sample 70 with Degradation 15 Figure 7: DSC results of Sample 80 with Degradation 15 Figure 8: DSC results of Sample 90 with Degradation 16 Figure 9: DSC results of Sample 100 with Degradation 16 Figure 10: Chu's Model on Degradation 17 Figure 11: Initial Elastic Modulus 21 Figure 12: Strain Recovery of Initial Samples 23 Figure 13: Creep Recovery with Controlled Force of 1.2MPa 24 Figure 14: Graph of Molar Mass with respect to Initial Molar Mass against Degradation Timeline 25 Figure 15: Graph of Mass with respect to Initial Mass against Degradation Timeline 25 Figure 16: Water Uptake of Samples 26 Figure 17: Creep Recovery with Degradation 27
CHAPTER 1: INTRODUCTION
1.1Background
The definition of biodegradable is the ability to be broken down into innocuous products by natural occurring microorganisms such as bacteria, fungi and algae [1, 2]. Since the discovery of polymethylmethacrylate (PMMA) as a biocompatible synthetic material in World War Two [3], applications of biodegradable polymers are increasing in across many industries. For example, biomedical, medicine, packaging, agricultural and automotive industry [1]. Such applications can be seen more in the biomedical sector, with controlled drug delivery, surgical fixations, like sutures, clips, bone pins and plates, and specialty packaging [4]. The term biodegradable plastic is not a term that is self-proclaimed. The material has to go through tests according to the American Society for Testing of Materials (ASTM) and the International Standards Organization (ISO) before a material can be classified as biodegradable [1].
Thermoplastic elastomer (TPE) is a kind of polymer that exhibits elastomeric behavior, yet being a thermoplastic [5]. Biodegradable TPEs, block copolymers in particular have been deemed as promising biomaterials as desired mechanical and physical properties can be achieved through adjustments of the existing block ratios or addition of new blocks. TPEs are copolymers that are in the form of A-B-A, where A is the hard segment and B, soft segment. It has also been reported that A-B-A triblock copolymers with L-lactide (LLA) and ε-Caprolactone (CL) as A and B block respectively exhibits biodegradation properties [6]. There are various biodegradable polyesters and poly(LLA) and poly(CL) are part of the main polyesters available commercially and the most widely researched [4, 7]. A comparison between PLLA and PCL shows that PLLA is semi-crystalline, a higher tensile strength and relative faster degradation as compared to a more rubbery PCL that possesses flexibility and slower degradation. Tri-block copolymers with LLA as the A and poly (CL) and trimethylene carbonate (TMC) as the soft segment has been reported, varying molar ratio of CL, TMC and LLA [8].
Therefore, biodegradable elastomers based on tri-block copolymers will be the focus. In this report there will be a study of viscoelastic characteristic, degradation behavior and mechanical properties of four synthesized elastomeric tri-block copolymers of poly (L-lactide)-poly (ε-caprolactone)poly(L-Lactide)-poly(L-Lactide) (PLLA-PCLLA-PLLA) with disrupted ends composition of L-LA and DL-lactide (DLLA).
1.2 Objective i. To characterize the tri-block copolymers with according to thermal and mechanical properties, and establish a relationship with respect to structure. ii. To analyze the degradation behavior of polyester-based tri-block copolymers. iii. To compare and study the mechanical and thermal properties of the tri-block copolymers before and after degradation.
1.3 Scope
Four synthesized tri-block copolymers were studied according to the mechanical and thermal properties and degradation behavior. These copolymers of different LLA-DLLA composition end tails, subjected to degradation studies, immersed in PBS buffer (pH 7.4), 37ºC over a period of 16 weeks seven time-point intervals. The aim is to characterize tri-block polymers before and after degradation according to the mechanical and thermal properties. This project does not include discussion on synthesis.
CHAPTER 2: LITERATURE REVIEW
In general, plastics are characterized into thermoplastics or thermosets, according to behavior with rising temperature. Thermosets are network polymers that become permanently hard after taking final shape and do not soften upon heating. As for thermoplastics, the material will soften when subjected to heating and harden upon cooling. These are reversible and repeatable [5]. Elastomers are polymers that exhibit elastic characteristics, which spontaneously return to the original state when applied force is released [9].
2.1 Biodegradable Thermoplastic Elastomers
Biodegradable TPE (BTPE) is a polymer that exhibits both thermoplastic and elastomer characteristic and are biodegradable. BTPE is considered a phase-separated material, with an elastomer component and another thermoplastic characteristic [10]. Both phases are not miscible within one another thus causing a two-phased material. This two-phased material is also known as copolymers, consisting of a soft segment (flexible) and a hard segment (crystalline). The hard segment, like the BTPE, will give the copolymer the hard and rigid thermoplastic properties and the soft segment will allow the copolymer to be more flexible and elastic. Thermoplastic elastomeric properties can be achieved through physical or chemical cross-linking. Chemical cross-linking of polymers form mechanically and thermally stable bonds but are irreversible [11]. Cross-linking in TPEs, mostly physical cross-linking can be removed by heat, thus giving TPEs the advantage like a thermoplastic to be reprocessed. The degree of cross-linking is generally related to the crystallinity of the crystalline phase [12].
Figure 1: Before and after cross-linking polymer
As shown in Figure 1, the yellow spheres denote thermoplastics, with cross-link structure. On the right of Figure 1, elastomers, depicted in pink spheres, were cross-linked with the hard phase. As thermoplastics have cross-linked structures, it is brittle and exhibit high strength. Malleability is improved with the crosslinking of elastomers. As of figure 1, the copolymer shows the distinct phase separation. Because of that, despite being combined, copolymers exhibit properties characteristics according to the components. Which means that each phase will retain individual melting temperature (Tm) and/or glass transition temperature (Tg).
Copolymer does not limit to random arrangement of segments A and B. Figure 2 below shows the different arrangement of copolymers. The various arrangements are shown using A (green block) and B (blue block). Block polymers are the simplest form of TPEs with a homopolymer of A next to homopolymer of B.
Figure 2: Different copolymers forms
Besides linear forms of copolymers, another form of copolymer is the star-block (figure 3). Star-block copolymers have been recently studied for its thermal, mechanical and degradation properties, which can be unique to linear despite similar compositions. Yuan et al. [13] had successfully synthesized hexa-armed star-shaped block copolymer poly(ε-CL)-b-poly(D,L-LA-co-glycolide) (PCL-b-D,L-LAGA). Linear PCL and star-shaped PCL were compared and investigated with regards to thermal properties. It was reported that the star-shaped copolymer showed unique thermal properties owing to the block structure in arms.
Figure 3: Star block copolymer
Despite unique properties, linear copolymers are still most common [14]. In typical ABA type tri-block copolymers, B as the soft segment and the hard segment A. Lipik et al. [8] investigated on the differences in molar ratio affecting thermal, degradation and mechanical properties. In the report, it was also noted that the concentration for TPE to exhibit both thermoplastic and elastomer properties (i.e. not too soft or not too brittle) falls between feed ratio of 0.75 to 1.5.
This report will be limited to discussion of tri-block BTPE, with B the hard segment (PCL-LLA, 60:40) and A the disrupted hard segment ends with varying compositions of LLA and DLLA.
2.2 Biodegradable Polymers
Based on the materials that were used in this report, in this section, biodegradable polymers covered are poly-ε-caprolactone and poly-lactide.
2.2.1 Poly-ε-Caprolactone
PCL is a semi-crystalline and hydrophobic polymer. Polymerization of ε-Caprolactone is prepared by ring opening polymerization as shown in figure 4. It has a low Tm of about 60°C and Tg of around -60°C, showing high elongation at break, but low tensile and storage modulus. In addition, degradation of PCL is relatively longer as compared. With properties like non-toxic, formability at low temperatures and flexibility, PCL is a preferred choice for biocompatible applications that include packaging, controlled drug delivery and medical implants, mainly in the biomedical industry [7, 15, 16].
Figure 4: ε-Caprolactone to Poly-ε-Caprolactone
2.2.2 Polylactide
Polylactide, as its name suggests, is a polymerized lactide. PLA is a semi-crystalline bioplastic with higher Tm (about 152°C) and Tg (around 58°C) as compared to PCL [7]. In comparison with PCL, PLA shows higher tensile and elastic modulus but very much lower elongation at break. PLA is given much attention, the degradation product lactic acid, is a normal intermediate of glycolysis [17]. Therefore in favor of in vivo implantations, that requires little loading or movements. Recommendation of usage of PLA is with plasticizers to enhance its flexibility [18]. PLA exists in two enantiomer forms, L-LA and DL-LA. In relation to this report, poly-DL-lactide (PDLLA) will only be in amorphous form [12].
Figure 5: Lactide to PLA
2.2.3 Homopolymer vs Copolymer
Copolymerization allows homopolymers of different degradation, mechanical and physical properties to be combined to achieve one polymer of desired properties. Certain ration and sequence of PLLA and PCL copolymer could result in better flexibility, hydrophilic/hydrophobic balance, and impact strengths than homopolymers of PLLA [19]. PLLA/PCL copolymer is less flexible and exhibits a lower elongation than PCL homopolymer, however a higher elongation than PLLA homopolymer. Huang et al. [20] studied the degradation of PLLA/PCL copolymers and found that the random copolymers degraded faster than the parent homopolymers. The reason for faster degradation rate is due to the copolymer’s lower degree of crystallinity as compared with homopolymer PLLA and PCL. Copolymerisation will cause a decrease in crystallinity and average sequence length of the homopolymers, thus possessing a much lower ultimate tensile strength, creep and storage modulus than homopolymers.
In this report, ABA tri-block copolymer will have 4 varying ratio of LLA and DLLA A block ends and B block will be a constant with 60:40 CL and LLA ratio.
2.3 Possible Applications
Biopolymers have been receiving much attention in replacing conventional plastics due to its biodegradable advantage. Applications of such polymers can be seen across many industries including but not limited to healthcare, packaging, agricultural, medicine and automotive [1].
In relation to this report, PCL based composites have been used for oral surgery, drug delivery, sutures, tissue reinforcement and torn tendon replacement patch. As for PLA, applications in similar fields have been noted and reported. The difference between PLA and PCL is the rate of degradation, with PCL having degradation period 1 time more than PLA. PCL-PLA copolymers are mostly used in nerve regenerations [21].
Recent focus has been on synthetic scaffold and tissue engineering. The reason is that requirements for tissue and scaffold engineering are demanding. In times where regeneration is needed, a synthetic replacement of similar mechanical stress and biocompatibility are the utmost requirements. Other requirements include porosity, viability of cell growth, shape and size [22].
One such application is explored, scaffolding. Possible applications of polymer in this report are deduced by means of cross- referencing with past report and characterization done based on synthetic and natural occurring polymers.
A study by Cooper et al. [23] was done and showed that current situation of synthetic scaffolds showed short-term compatibility only. The main reasons were due to the mismatch of mechanical requirements and limited integration of graft and host tissues. The report was done in relation to anterior cruciate ligament (ACL) in rabbits. The material used for was polylactic-co-glycolic acid (PLAGA) and mechanical properties in the report were also tabulated. A scaffold in biomedical terms refers to the structure that allows cells to grow onto. It is a frame for the cells to grow into the desired organs, like a bone. PLAGA single-filament yarn exhibited tensile strength of 5.3 1.8 MPa. Other variations were based on the angle of spinning into fibers.
Lipik et al. [24] characterized PLLA-b-(PCL-co-PLLA)-b-PLLA according to the %mol of LLA in PCL. Data was chosen in the range of %mol LLA in PCL of 25 to 50. Expected results should be similar of that to Lipik et al. reported as the focus of this report is on varying disrupted ends, with PCL-LLA B block constant. Results showed that the elongation at maximum load is within the range of 35.19% to 146.37%. Young’s modulus (MPa) and max tensile stress (MPa) is between 2.5 to 18.34 and 2.24 to 8.05 respectively. Based on results shown, the possible applications for polymers in this report falls between elongations <150% and tensile stress <8.05MPa. In this report, PCL(60%)-PLA(40%) is explored.
CHAPTER 3: MATERIALS AND EXPERIMENTAL METHODS
3.1 Materials Used
Briefly the synthesis procedure of the ABA triblock polymers used is described as follows. Synthesis of the polymers was performed in a one-pot ring-opening polymerization. First, the ‘B’ block is formed where CL and LLA of molar ratio 60:40 respectively were added into a reaction flask with toluene under 140oC and left to polymerize for 19 hours. Next, appropriate amount of LLA / DLLA were added to further react onto the P(CL-LLA) ‘B’ block to form the ‘A’ blocks for 5 hours under 100oC. Upon completion, the resulting polymer was precipitated with methanol. With this, ABA triblock polymers with varying ‘A’ block compositions were prepared. The table below shows the theoretical composition of the polymers used in this report.
SampleCode | Composition of LLA in Polymer(mol %) | Composition of DLLA in Polymer(mol %) | Composition of CL in Polymer(mol %) | 100 | 58.5 | 0.0 | 41.5 | 90 | 54.8 | 3.7 | 41.5 | 80 | 51.2 | 7.3 | 41.5 | 70 | 47.5 | 11.0 | 41.5 |
Table 1: List of Materials
3.2 Experimental Procedure
3.2.1 Polymer Films Preparation
Films were made from powdered polymers. Polymers are weighed at about 0.5g. With two Kapton sheets and two metal plates, the powdered sample is subjected to pre-press at 15000psi for about five seconds. After the pre-press, the polymer is then subjected to heating up to its melting temperature while stepping up to 1000psi for five minutes. The melted polymer is then subjected to a step up to 15000psi in 90 seconds and then held for another 60 seconds. Temperatures for heating samples are as follows: Sample | Temperature | 100 | 160 | 90 | 130 | 80 | 100 | 70 | 100 |
Table 2: Melting Temperature of Samples 3.2.2 Degradation Study (16 Weeks)
Dried polymer films were cut into 2 cm x 0.5 cm and immersed into 5 mL of phosphate buffer solution (PBS) of pH 7.4. Degradation was carried out over a period of 16 weeks with the polymers under incubation at 37°C. At each time point of week 1, week 2, week 4, week 6, week 8, week 12, and week 16, three films of each polymer type were removed from the PBS solution, measured the weight of wet polymer, and then dried in the oven at 37°C for one week, and then weighed again to obtain the weight of the dried polymer. The average value was considered and weight loss was then measured. 3.2.3 Dynamic Mechanical Analysis (DMA)
TA Instruments Q800 Dynamic Mechanical Analyser was used to determine the mechanical properties such as tensile strength and creep behavior, and thermal transitions such as glass transition temperature (Tg) of the polymer films. The tension deformation mode was employed and polymer films of 2 cm x 0.5 cm were used. Strain rate test was carried out at room temperature for 0 week samples (before degradation). Creep test was performed on samples before degradation and after degradation. Creep modulus and recoverable strain values were obtained from creep test by applying two different stresses to the film for 15 minutes and then allowing the sample to recover for 15 minutes. The two types of stresses are low stress which is 30% of the initial tensile strength of the sample, and high stress which is 70% of the initial tensile strength. The average stress value of the measurement of three polymer films was used for the creep test.
3.2.4 Differential Scanning Calorimetry (DSC)
Thermal transition analysis for the polymer films of week 0 to week 16 was carried out on a TA Instruments Model Q10 DSC machine. Polymer films were cut into small pieces and sample of weight 8-12mg was placed into the hermetic aluminum sample pan. The sample was first cooled down to -80°C at a rate of 20°C/min. The sample was then placed in isothermal for 5 minutes and then heated up at a rate of 10°C/min to 200°C. This was followed by cooling down the sample to -80°C at a rate of 10°C/min and then heated up again to 200°C/min at a rate of 10°C/min. Thermal transitions detected in the second heating cycle were considered so as to remove any thermal history of the polymers. 3.2.5 Size Exclusion Chromatography (SEC)
The molar masses such as the number average molar mass (Mn) and weight average molar mass (Mw) of the polymers of week 0 to week 16 of degradation with three replicates each were obtained by a Shimadzu SEC system equipped with a Shimadzu LC-20AD solvent delivery module, a Shimadzu SIL-20AC autosampler and a Shimadzu RID-10A differential refractometric detector. Dried polymer samples of approximately 1 mg were cut and dissolved in 1.0 mL of chloroform. Twelve narrow molecular weight distribution (MWD) polystyrene standards in the range of 580 - 125,000g/mol were used for calibration. All measurements were carried out with two PL gel 5μm mixed-C and one PL gel 5μm mixed-E columns connected in series with chloroform as the mobile phase maintained at a flow rate of 1.0 mL/min.
CHAPTER 4: RESULTS AND DISCUSSION
4.1 Thermal Properties
Figure 6: DSC results of Sample 70 with Degradation
Figure 7: DSC results of Sample 80 with Degradation
Figure 8: DSC results of Sample 90 with Degradation
Figure 9: DSC results of Sample 100 with Degradation
Tg of the polymers can be seen from the above figures. Taking example from figure 9, at the region of about 50°C, a slight step can be seen at the curve of 80-Film. This is the Tg of PLA. As degradation occurs along the weeks, the depressions that indicated the Tg of PLA became less obvious. The conclusion that drawn is that the amorphous loss in individual polymer was at a substantial amount along with degradation, thus making the Tg of PLA not obvious as compared. When the Tg step showed a less obvious step, the trough that signifies the crystals and Tm started to spread over a wider range. Such behavior complies with the Chu’s model.
In Chu’s model [25], during degradation, the amorphous regions break down first before crystals and are converted into shorter soluble fragments which led to the decrease in molecular weight and mass but an increase in the overall degree of crystallinity during Stage 1 as shown in Figure 10. Moreover, reorientation of the shorter fragment may promote the rise in the crystallinity. Stage 2 involves the degradation in the crystalline region. This would then lead to a decrease in the overall crystallinity of the polymer and result in a rapid loss of mass.
Figure 10: Chu's Model on Degradation
From DSC results, % crystallinity of samples can be calculated using enthalpy of Tm. with the equation:
Crystallinity=∆HmH0∙n X 100%
where ΔHm is the s the measured heat of fusion, H0 is 93J/g and n is the molar composition.
Table 3 shows the percentage crystallinity of individual samples. F1 and F2 refer to film 1 and film 2, whereas 1 and 2 on the right indicates that it is the sample in powder form. Crystallinity increases after compression moulding for sample 70 and 80, but was similar for samples 90 and 100.
From the above four figures, the trend is that the trough, which indicates melting temperature, increases to a deeper valley. This suggests that the crystallinity increases along with degradation. From the four figures, week 8 is chosen as a reference point to portray the difference in percentage crystallinity. As seen in table 4, crystallinity generally increased for all samples during degradation, with 70 having the highest increase of crystallinity. After having a deeper trough, the four samples exhibited widening of the trough. A wider range of Tm would suggest that uniform sized crystals are starting to decline, moving towards wider range of crystal-sized crystalline phase. This complies with the Chu’s model for crystallinity study with degradation.
Table 4: % Crystallinity of samples between Initial and Week 8
4.2 Properties of Initial Sample
4.2.1 Elastic Modulus
Elastic modulus is calculated using strain and stress from the Stress-Strain curves. Elastic modulus is the indication on how stiff or elastic a certain material is. It is derived from the equation: StressStrain. Since strain is in percentage or a ratio, the units for elastic modulus is in MPa or GPa, depending on the stiffness of the material. Modulus increases with increasing degree of crystallinity due to enhanced secondary interchain forces, like Van der Waal’s forces, which results from adjacent aligned chain segments as percent crystallinity increases. Crystallinity also affects elastic modulus relative to the degree of cross-linking in thermoplastic elastomers. With reference to table 3, it was expected to have a downward trend in terms of elastic modulus with increasing PDLLA.
Initial elastic modulus of the four samples in this report shows that sample 90 is the stiffest and sample 70 is the most flexible. Yield stress has no correlation with elastic modulus of the material. Young’s modulus is a more important feature of a material rather than the yield strength. The reason being that for Young’s modulus, one is able to derive the amount of strain based on stress applied. In the design process or rather the materials selection, it is important to know what are the parameters that might alter under practical usage. For yield stress, the information that can be retrieved from the figure is just at what stress would the material fracture. Specific elastic modulus is the most commonly used value for comparison between materials [26].
Figure 11: Initial Elastic Modulus
From figure 11, It is very clear sample 90 is higher in modulus than all others. As mentioned above, elastic modulus is correlated to the sample’s crystallinity. The higher the crystallinity of the polymer would be a higher elastic modulus of the material. Based on Lipik et al. [24], data has shown that longer tri-block of PLLA-b-P(CL-LLA)-b-PLLA has higher maximum elastic modulus. In this report, of such specimens are of the same chain length with only varying ends. Based on table 3, samples 80 and 70 have much lower percentage of crystallinity as compared to 90 and 100. Despite having lower percentage of crystallinity than sample 100, sample 90 showed higher elastic modulus. However, if sample 90 were to be taken out from the comparison between the other samples, it is evident that the trend mentioned and as reported in [24] is shown. Sample 70, having the lowest crystallinity, showed the lowest elastic modulus, followed by sample 80 then 100.
Comparing the DSC figures in section 4.1 and figure 11, sample 90 differs from the other three samples, as there is an existence of two different crystal sizes. Most likely it is due to a better reinforcing effect from smaller crystallites well-dispersed throughout the film. It is not known if this is the reason for its elastic modulus for not following the trend with crystallinity percentage. However this explanation is a speculation on why sample 90 has a higher tensile strength as compared to sample 100.
4.2.2 Creep Recovery
For the study on creep recovery of the four samples, two different stresses are used to investigate the impact of stress on creep recovery. The purpose of this analysis is to determine if the higher band stress versus the lower band stress would affect the recovery of the individual polymer. For elastomeric materials, it should be independent of the creep stresses. The two stresses chosen were 30% and 70% of yield stress.
Figure 14 shows that sample 90 exhibited a different trend as compared to the other three samples. When a sample shows signs of plastic deformation that is when the deformation is not recoverable. Sample 90 showed signs of little plastic deformation when the recovery showed was lower than its 30% stress recovery. With reference to figure 8 in section 4.3, samples 90’s trough has a distinct separation, which suggests that there are two different crystal size coexisting. Within the same compound, a smaller crystal has lower yield strength as compared to that of a bigger crystal. It is possible that smaller crystals are broken with the 70% yield stress applied, causing slight plastic deformation, thus giving lower recovery.
Figure 12: Strain Recovery of Initial Samples
All except sample 90 shows sign of similar or better recovery at higher stress. This means that recovery is independent of that to creep stress applied. This suggests that the materials are reasonably elastic.
4.2.3 Creep Recovery with Controlled force
Another test was done to investigate on whether difference in temperature will affect the creep recovery. The test was carried out with a constant force of 1.2MPa, at 37°C and room temperature. 37°C was chosen because the samples are suitable for biomedical purposes, the human body is about 37°C. This test would allow the understanding of the materials’ creep behaviour within the human body. For sample 100 and 80, when the deviation is taken into consideration, the difference was negligible. Sample 90 and 70 are showing more recovery at 37°C than at room temperature. At higher temperature, less work is needed to stretch a polymer to a certain elongation. With that, with the same force, samples would be stretched to a longer elongation at higher temperature. When the force is released, secondary bonds will reform with the help of thermal motion, and also helps in the "flow" of polymers.
From figure 16, all four samples showed similar recovery. Samples 90 and 70 showed similar recovery trend between the two temperatures, whereas for 100 and 80, the trend was reversed. Based from results, sample 100 showed minimal difference, whereas sample 80 showed the most difference. The difference is based on the lowest possible results after factoring standard deviation. The purpose of this comparison is that the service temperature will affect mechanical properties and that service temperature’s results hold more significance.
Figure 13: Creep Recovery with Controlled Force of 1.2MPa
4.3 Degradation Study and Mechanical Properties with Degradation
4.3.1 Molar Mass and Mass Loss
Figure 14: Graph of Molar Mass with respect to Initial Molar Mass against Degradation Timeline
Figure 15: Graph of Mass with respect to Initial Mass against Degradation Timeline
The above two figures shows the molar mass and mass loss across the 16week degradation period. Sample 100 showed the slowest degradation in both molar mass and mass loss. By visual comparison of the above two figures, it is conclusive that degradation is by bulk erosion. Mass loss of samples was not obvious until week 4. Comparing it with figure 17, at week 4, molar mass had significantly dropped. In such degradation mode, the water uptake by the polymer would be increasing before the degradation rate can match up. As water is taken in by the polymer, more linkages and bonds within the polymer is exposed and thus the polymers become susceptible to hydrolytic degradation as they contained ester linkage in the backbone.
Figure 16: Water Uptake of Samples
Figure 19 shows the water uptake comparatively. Four samples can be grouped based on the range of water uptake. In relation to the percentage crystallinity, below around 25% initial crystallinity, shows a very good water uptake rate. Comparing this with figure 17, molar mass loss can be correlated with the water uptake. The higher the water uptake of a polymer, the higher the rate is of molar mass loss.
4.3.2 Creep Recovery
Figure 17: Creep Recovery with Degradation
Creep recovery of samples is done at 30% stress. Sample 100 was the only that exhibits different trend compared to the other three samples. The general trend for these samples was that there would be an increase in creep recovery and then having lesser creep recovery.
Sample 90 showed high recovery amongst all four samples. However, as the polymer continued with degradation, its recovery was on par with sample 80 at week 4. The sample then became brittle, fracturing after week 4 when applied with 30% stress. Sample 70 showed the worst recovery decline at week 2, however, near 70%. A possible reason for this sample to be the lowest in recovery could be that the crystallinity is very low, mostly amorphous and higher elastomeric behavior as compared. Sample 100 showed a steady increase in the creep recovery. As reported by Kong et al. [27], the recoverability of materials depends mostly on the end-chains. There is an optimal crystallinity that this sample should be to achieve higher creep recovery. With sample 100 having the upward trend for creep recovery, it is expected for this sample to reach the same creep recovery as sample 90 at 3 days and follow the similar path taken. As degradation will start with the amorphous region first, overall crystallinity would increase before taking a dip.
CHAPTER 5: CONCLUSION
Four tri-block copolymer of general composition, poly (LLA)-b-poly (LLA)-co-poly (CL)-b-poly (LLA), were characterized based on thermal, mechanical and degradation properties. Copolymer of A-B-A tri-block had B block as the constant and disrupted ends of variable PLLA and PDLLA. A 16-week period with 7 individual time points were set to evaluate the effect of degradation on the mentioned properties of these tri-block copolymers.
Initial conditions showed that PDLLA’s concentration is proportionate to the decline of crystallinity in the final product. Sample 90, with PDLLA-10% and PLLA-90% at the end chains, showed the highest tensile strength and elastic modulus. Sample 100 has the highest crystallinity and the slowest degradation rate. Fastest degradation samples were samples 70 and 80, however with higher water uptake of 10%-15%. All samples showed similar results on creep recovery, with variable stress and another with variable temperature, in the range of 60% to 80%. Degradation of crystalline region can be seen from the figures in section 4.1. Tm was of sharper “V” to a wider “V”.
Based on section 2.3, it is possible that the material in this report is suitable to serve as a substitute for the PLAGA scaffold.
CHAPTER 6: RECOMMENDATIONS
Degradation timing was not substantial as the crystalline phase was just starting to degrade at the week 16. In this report, the amorphous phase affecting the mechanical and thermal properties have been explored. Further testing can be done to investigate how crystallinity affects the polymer properties, including its degradation.
It is also recommended that Scanning Electron Microscopy (SEM) can be used to study the effect of degradation on the surface of the samples. Since samples in this report are meant for bioimplants, porosity of the polymer should be taken into consideration for cells to grow on, if there is a possibility that such materials can be turned into scaffolds.
The study of crystallinity can also be furthered using X-ray Diffraction (XRD). XRD works on the basis that crystalline materials diffract X-rays, thus revealing their structure. This can be extended to the usage for effects of degradation on crystallinity.
Another method for studying of crystallinity of polymer would be using the polarized optical microscopy (POM). POM uses polarized light to illuminate birefringment samples, which will generate contrast with background as they interact strongly with polarized light. POM is capable of providing information on absorption color and optical path boundaries between materials of differing refractive indices, in a manner similar to brightfield illumination; however, it can also distinguish between isotropic and anisotropic substances. POM also reveals detailed information concerning the structure that might be too small for identification and diagnostic purposes. This would aid the study of individual sample’s crystal growth pattern from melt to film, especially to the difference in trend as mentioned in section 4.2.1.
