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Fabricating and Characterizing Biodegradable Polymeric Fibers for Medical Applications: Experiments with Poly(L-Lactic Acid) New Jersey Governor’s School of Engineering and Technology 2014 Jenna Barton [email protected] Hyeri Cho [email protected] Sara Lee [email protected] Ryan Oakley [email protected] Abstract Poly-L-lactic acid (PLLA) is accepted as a suitable material for bone pins and sutures in the medical field due to its biodegradable nature and ease of manipulation. Through this study, many of PLLA’s notable attributes were tested in order to further characterize the polymer and encourage expansion in its biomedical applications. Using X-ray diffraction, the material’s molecular structure was determined to be semi-crystalline after being annealed and amorphous prior to the heating process. Through differential scanning calorimetry it was found that PLLA has a glass transition temperature of 46.5˚C, demonstrating that the material will not lose any of its characteristics when used for biomedical purposes, as the body’s natural temperature is 37˚C. Additionally, both the annealed and non-annealed fibers were characterized using the stress/strain curve through the tensile strength test. The study established that the more crystalline annealed fibers were stiffer and more resistant to strain. Based on the results, it was concluded that the annealing process and subsequent more crystalline structure improved the fibers for their intended purpose of biomedical aids for bone and muscle injuries. 1 Introduction With perpetual advancements in the research and applications of macromolecules, polymers continue to impact all aspects of the modern population. These materials are prevalent in a wide range of industries, including large-scale commercial production and the biomedical field. This study specifically revolves around testing poly-L-lactic acid, PLLA, in order to analyze its characteristics in respect to the focus of further expanding its biomedical applications in surgery and patient treatment. PLLA is a stereoisomer of poly(lactic acid), which is a biodegradable polymer produced from lactic acid found in renewable resources. Focusing on the biomedical field, this biodegradable characteristic is highly valuable. Many operations require bone pins, sutures, screws, wires, and plates, all of which are often made from non-biodegradable materials. Subsequently, the use of these materials requires a second surgery for removal after the patient heals. Due to PLLA’s biodegradable nature, a second surgery is not necessary, making it ideal for such applications. Additionally, if PLLA can be made with
Transcript
Page 1: Fabricating and Characterizing Biodegradable Polymeric ... · Fabricating and Characterizing Biodegradable Polymeric Fibers for Medical Applications: Experiments with Poly(L-Lactic

Fabricating and Characterizing Biodegradable Polymeric Fibers

for Medical Applications: Experiments with Poly(L-Lactic Acid)

New Jersey Governor’s School of Engineering and Technology 2014

Jenna Barton

[email protected]

Hyeri Cho

[email protected]

Sara Lee

[email protected]

Ryan Oakley

[email protected]

Abstract Poly-L-lactic acid (PLLA) is accepted as a

suitable material for bone pins and sutures in the

medical field due to its biodegradable nature and ease

of manipulation. Through this study, many of

PLLA’s notable attributes were tested in order to

further characterize the polymer and encourage

expansion in its biomedical applications. Using X-ray

diffraction, the material’s molecular structure was

determined to be semi-crystalline after being

annealed and amorphous prior to the heating process.

Through differential scanning calorimetry it was

found that PLLA has a glass transition temperature of

46.5˚C, demonstrating that the material will not lose

any of its characteristics when used for biomedical

purposes, as the body’s natural temperature is 37˚C.

Additionally, both the annealed and non-annealed

fibers were characterized using the stress/strain curve

through the tensile strength test. The study

established that the more crystalline annealed fibers

were stiffer and more resistant to strain. Based on the

results, it was concluded that the annealing process

and subsequent more crystalline structure improved

the fibers for their intended purpose of biomedical

aids for bone and muscle injuries.

1 Introduction

With perpetual advancements in the research

and applications of macromolecules, polymers

continue to impact all aspects of the modern

population. These materials are prevalent in a wide

range of industries, including large-scale commercial

production and the biomedical field. This study

specifically revolves around testing poly-L-lactic

acid, PLLA, in order to analyze its characteristics in

respect to the focus of further expanding its

biomedical applications in surgery and patient

treatment.

PLLA is a stereoisomer of poly(lactic acid),

which is a biodegradable polymer produced from

lactic acid found in renewable resources. Focusing on

the biomedical field, this biodegradable characteristic

is highly valuable. Many operations require bone

pins, sutures, screws, wires, and plates, all of which

are often made from non-biodegradable materials.

Subsequently, the use of these materials requires a

second surgery for removal after the patient heals.

Due to PLLA’s biodegradable nature, a second

surgery is not necessary, making it ideal for such

applications. Additionally, if PLLA can be made with

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a similar strength to the material it is replacing,

patient recuperation time and comfort could be

improved. During the study, tensile strength, ease of

extrusion, and crystallinity of PLLA fibers were

tested in order to characterize the polymer for future

biomedical applications.

2 Background

2.1 Poly-L-Lactic Acid (PLLA)

A polymer is a chain of repeating units called

monomers that are representative of the original

molecule. The polymer chains can consist of

hundreds of monomer units, often having a molecular

weight ranging in the thousands of atomic mass units

(amu).1

The monomer used in the polymer PLLA is

either lactic acid or lactide, each of which undergoes

a different process to reach PLLA. Lactide is

polymerized through a ‘ring-opening’ process during

which the ring structure of lactide is broken and the

ends are stitched together. Lactic acid is polymerized

through a condensation reaction where the ends of

the lactic acid molecules react with each other to

combine and give off water (see Figure 1).

Polymers have a large scope of interesting

properties, ranging from shape-memory to pH-

sensitivity. Poly-L-lactic acid is studied for its

potential applications in the medical field due to its

biodegradability and tensile strength.2

Additionally, PLLA has a similar strength to

bone and is compatible with human bone cells when

copolymerized, facilitating the healing process.3

2.2 Mechanical Properties of Polymers

The mechanical properties of a polymer are

mainly concerned with how the polymer reacts to

stress and strain. Stress, represented by the symbol σ,

is the pressure applied to the polymer in units of

force per area. Strain, represented by the symbol ε, is

the elongation that the polymer experiences as a

percentile difference from the original length. To test

the tensile strength of a polymer, a machine pulls a

fiber from two ends while taking stress and strain

data. The data is then plotted on a stress-strain curve

which contains multiple vital points of data. The first

point is the tangent modulus (a.k.a. Young’s

Modulus), which is the linear region at the beginning

of the graph that represents the overall stiffness of the

polymer.4 The yield point appears after the tangent

modulus and is the first vertex that the graph shows.

This point represents the transition from elastic to

plastic: to the left of the yield point the polymer will

revert back to its original shape if the applied stress is

removed, where to the right of the yield point the

polymer will be permanently deformed and not revert

back to its original shape when the applied stress is

removed.5 Another important point of data within the

stress-strain curve is the point of rupture, which

shows how long the polymer can be stretched before

it snaps. The study of the mechanical properties of

polymers in relation to biomedical applications is

motivated by the desire to replace biological parts

with the studied polymers such that the replacements

are able to withstand the same stress and strain of the

originals and therefore maintain similar mechanical

properties. The mechanical properties that should be

studied include the polymer’s intrinsic, structural,

and viscoelastic properties. A polymer’s viscoelastic

property is a time-dependent property that relates to

the deformation of the polymer, meaning that it is

dependent on how quickly or slowly the polymer

deforms under a force.6 If the polymer deforms with

a constant stress and the strain increases with time, it

is experiencing creep. If the polymer deforms with a

constant strain and the stress decreases with time, it is

experiencing stress-relaxation. Both creep and stress-

relaxation are viscoelastic properties. All of these

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properties must be taken into account when designing

and choosing polymers for biomedical use.

2.3 Calorimetry

Calorimetry is defined as the measurement of

the flow of heat into or out of a sample. A

calorimeter is a piece of equipment that identifies the

thermochemical properties of a sample, such as its

melting point or specific heat. A differential

calorimeter measures the gain or loss in a sample

relative to a fixed reference point and heats the

sample at a constant rate, plots the data, and scans a

whole temperature range.7 This type of calorimeter

clearly communicates the thermochemical properties

of a sample such as glass transition temperature,

crystallization temperature, and melting point by

putting the sample through a heating cycle. This

cycling makes the sample more amorphous and is

necessary if the sample is originally crystalline due to

the fact that it cannot go through a crystallization

process if it is already in a crystal state.8

2.4 Extrusion

Extrusion is the process of melting the

polymer and pushing it through an opening called a

die in order to form a continuous profile in the shape

of the die’s cross-section. This process successfully

produces uniform fibers, pipes, or sheets.9 Extrusion

is a suitable method for polymers that are robust

against heat and results in an amorphous structure

due to rapid cooling. Specifically used in this

experiment, plunger extrusion pushes the polymer

through a die using the pressure produced by the

apparatus and is considered to produce less waste

material than the more common extrusion process,

twin-screw extrusion.10

2.5 X-Ray Diffraction

X-ray diffraction (XRD) is a crucial analytical

technique in determining crystalline structure and

chemical composition of a substance. The

crystallinity of a substance can be determined by

emitting high energy X-ray light on the sample and

analyzing its diffraction pattern (see Figure 2).11

The

degree of crystallinity can be determined by

measuring the width of the peaks on the integrated

graph of diffraction intensity.12

This technique not

only reveals the degree of crystallinity, but it also

identifies the angle of diffraction which can then be

used to determine the crystalline structure through

Bragg’s Law.13

Since many polymer composites used

in the laboratory and industry are semi-crystalline,

XRD is an important tool when analyzing and

characterizing polymers.

Through the integration of collected data, one

can determine the percent crystallinity with the ratios

of integrated intensities calculated from the

diffraction pattern.14

From the XRD data one can also

determine the preferred orientation of the sample.

Using the accompanying software, the integrations

can be manipulated to eliminate background noise

from the air as well as amorphous data in order to

better identify the crystalline structure. Through the

process of X-ray diffraction and the analysis of the

interference patterns, many characteristics of the

molecular structure can be identified and studied.

2.6 Bragg’s Law

Bragg’s law is an optical law used in X-ray

diffraction that is mathematically defined as

, where n is an integer that represents the

number of times the wave oscillates beneath the first

layer, λ is the wavelength of the X-rays, d is the

distance between two layers of the crystal, and θ is

the angle of incidence (see Figure 3).15

In X-ray

diffraction, Bragg’s law is treated as a function of

theta that outputs the d-spacing. Substituting values

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into the equation allows for the production of a graph

of intensity vs. incident angle which produces peaks

that can be used as a fingerprint for the material.16

The graph can be used to identify an unknown

sample by comparing it to known materials in the

database. A large intensity is indicative of

constructive interference after the refraction of the X-

rays, and the angle at which it occurs is specific to

certain crystalline compounds.17

Data-wise, this

produces a Bragg peak which can be used to

characterize the sample.

2.7 Annealing

Mechanical characteristics of a polymer can

be altered via the process of annealing. Annealing

polymer involves heating it below its melting point,

resulting in changes of physical properties and

crystalline structure.18

The polymer can undergo

recrystallization depending on the annealing

temperature and cooling temperature. Slow

crystallization after the heating yields a more

crystalline structure while rapid cooling or

inadequate heating will not result in much

recrystallization. In order to achieve greater

crystallinity, the sample has to be cooled down

slowly at a constant rate. Annealing also affects

optical properties of the polymer, such as

transparency, by changing the crystalline structure as

well as increasing hardness and brittleness.19

3 Experimental Design

3.1 Fabricating the Fibers

In order to begin the research, an anhydrous

sample of PLLA was extruded into fibers using

Rosand’s Bench-top RH2000 Capillary Rheometer

(see Figure 4). Although the machine is

conventionally used to measure the viscosity of

materials, the means by which the system runs

worked well to extrude the PLLA into filaments.

Large granules of PLLA were loaded into a

single barrel of the machine through the repetitive

process of pouring a small amount into the barrel and

packing it down with a plunger. This packing

removed any air bubbles that would become

imperfections in the filaments during extrusion. Once

the store of PLLA was heated to over 200˚C and the

pressure had surpassed 2.4 MPa, the sample was

ready for extrusion.

The extrusion process involved the usage of a

system of wheels and a rotating dowel in order to

systemize collection. The PLLA was forced through

a die with a 0.5 mm diameter at a rate of 2 mm/min

by the machine’s plunger. The sample then exited the

machine as a cylindrical strand and hung above the

table. Initially the extruded fiber was thicker because

the polymer was not flowing as fast through the die.

Once the filament was long and thin enough, it was

wound through a series of wheels that kept it on track

towards the mechanized spool. The uneven end of the

fiber was removed so that the entirety of the sample

was ideally free of any irregularities that were caused

by the start of the extrusion process. The fresh end

was then taped to the cardstock spool which was

constructed to fit perfectly around the rotating dowel.

Once the clean end of the filament was secure, the

spooling machine (the CS-194T) was turned on and

the dowel began rotating, effectively collecting the

sample as it was being extruded (see Figure 5).

During this process, the samples were wound

according to differing radial velocities of the spool.

Each sample was extruded at a rate of 2 mm/min by

the RH2000. The fibers had different diameters due

to the different speeds of extrusion. Once an ample

amount of the fiber was collected on the spool, the

CS-194T and the plunger were stopped, the fiber was

cut, and the sample was labeled and placed to the

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side. This process produced the five samples used

throughout the experiment.

3.2 Annealing the Fibers

In order to create semi crystalline fibers,

samples of the extruded filaments underwent

recrystallization through the annealing process. The

length and diameter of five different samples were

analyzed using rulers and an Olympus optical

microscope. The samples were then labeled with

tape, placed in a covered glass dish, and heated for

110 minutes in an oven at 145˚C (see Figure 6). After

the heating, the fibers were slowly cooled and then

reanalyzed.

3.3 X-Ray Diffraction (XRD)

The samples of the fibers were tested using

X-ray diffractometers in order to analyze the

molecular structure of PLLA. Sample cards were first

constructed out of cardstock to support the fiber

samples and keep them straight in front of the X-ray

beams. Next, the newly extruded fibers were attached

to the sample cards and labeled. Using XRD and the

General Area Detector Diffraction System (GADDS)

software V4.1.42, the different samples were

analyzed for characteristics indicative of molecular

structure and preference (see Figure 7). Once data

collection for the amorphous fibers was complete, the

annealed fibers were attached to sample cards and

analyzed using X-ray diffraction. With data

collection complete, the different samples, non-

annealed and annealed, were easily compared to one

another in order to determine the effect of the

annealing process on the fibers.

3.4 Testing Tensile Strength

The Sintech 5D Universal Testing Machine

was used to test the stiffness, stress, and strain of the

fibers. Each fiber specimen’s diameter was measured

and strands of the polymer were attached to card

stock tabs in order to ensure consistent sample

lengths and to facilitate setting up the experiment

(see Figure 8). In order for the test to be performed,

the following data was entered into the computer

program: diameter of the sample, grip separation,

initial speed, and length. Due to the common factor

of the card stock frame, the length and grip

separation remained constant at one inch, or 25.4

mm. Likewise, in order to maintain consistency, the

speed, which is the rate of elongation of the sample,

remained constant at 5 mm/min. With this

experiment, the fiber spun at a speed level of 12

(arbitrary) with diameter 90 microns was tested as

well as its annealed counterpart with diameter 95

microns.

In the experiment, the specimen card with the

fiber was placed between the grips of the machine,

and the program was set to the corresponding

parameters. Then the card stock frame was cut to

ensure that the material being tested was truly the

polymer sample. With verification that the specimen

was set, the program was started and the data was

collected on the computer. Once all of the specimens

were recorded, the best results were used in order to

form the specific sample that was used for analysis.

3.5 Differential Scanning Calorimetry

Heat flow was tested using the Mettler Toledo

TSO801RO, a differential scanning calorimeter that

measured heat gained and lost during a heating cycle

with a linear temperature ramp. A small, 9.2 mg

sample of crystalline PLLA went through a heat,

cool, heat, cool cycle over a period of 90 minutes

through a temperature range of 25 to 300˚C. The

machine was set to change at a rate of 10˚C per

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minute during both heating cycles, -0˚C during the

first cooling cycle, and -25˚C during the second

cooling cycle. Between each endothermic and

exothermic process the sample was held for sixty

seconds in isothermal conditions before beginning

the next step in the test. Through this experiment the

glass transition temperature and melting temperatures

were identified for the sample in order to determine

possible conditions in which the material can be

utilized.

For data interpretations, only the second

heating cycle was considered due to the nature of the

PLLA. Since the material being tested was already in

crystalline form, the first heating cycle served as a

means to get the PLLA sample into an amorphous

state in order to measure the crystallization in the

second heating cycle.

3.6 Manipulating the Data: X-ray

Diffraction

In order to interpret the XRD data, the raw

calculations collected during the experiments were

mapped into comprehensible graphs and plots. The

following steps were taken to acquire such visuals

using the Jade 7 software. First, the .xy file from the

integrated XRD detector image was read into the

program. Then, the range was clipped such that the 2-

theta value (the x-axis) was between 3 and 32

degrees. Data from another integrated XRD detector

image measuring air diffraction was read into the

program to approximate a background profile and

subtracted from the collective data, subsequently

leaving measurements describing only the sample.

Next, using a nine degree peak, the approximate

amorphous portion was subtracted, leaving behind

the derived pattern. In order to characterize the

crystalline data, ten peaks were found automatically

using the software and two additional peaks were

manually added. These twelve peaks were then

refined into profiles starting with 0.5 degree widths.

From here, the X-ray diffraction data was prepared

for analysis.

4 Results and Discussion

4.1 Dimensions of PLLA Fibers

When analyzing the fiber samples after the

extrusion process, certain data measurements,

including diameter of the fiber and speed of the

spool, were used in order to characterize the fibers.

For this portion of the experiment, the volumetric

flow rate of the extruder can be calculated by

multiplying the cross sectional area of the barrel by

the rate of compression. The diameter of the barrel

was 14.7 mm, and the rate at which the plunger

lowered was 2 mm/min. Based on the information,

the volumetric flow rate constant was calculated to be

5.66×10-9

m3/s.

Referring to Table II in the appendix, the

number of the sample represents its relative speed

that appeared on the dial of the spooling apparatus.

The average diameter of the fiber as well as the

standard deviation of the diameters decreased as the

speed of the spool increased.

The calculated volumetric flow rates from the

sample data showed significant variability. The

calculated volumetric flow rate of sample number 3

showed an extreme positive deviation from the

constant, suggesting unevenness in the thickness of

the fiber. Other calculated volumetric flow rates

exhibited overall negative deviation from the

constant. This negative deviation is most likely a

result of imperfect compaction of PLLA granules in

the barrel of the extruder.

The analysis of the relationship between fiber

diameter and speed of the spool suggests that there is

an inverse square correlation between the diameter

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and speed (see Figure 9). The diameter of the fiber

decreased dramatically and appeared to reach an

asymptote as the speed increased. The graph of

diameter plotted against the inverse square of the

speed shows that there is a linear relationship

between the two, indicating an inverse square

relationship between the diameter and speed. The line

of best fit is heavily influenced by the outlier,

calculated based on data obtained on the number 3

fiber. This point is providing significant leverage,

increasing the slope of the overall graph (see Figure

10).

4.2 Annealing and Physical Changes

The annealing process affected the structural

characteristics of the polymer. Although heating the

polymer would not have altered its intrinsic material

properties, the change in its structure due to the

annealing process made significant changes in terms

of its strength, hardness, and even some optical

characteristics. The fibers became milky in color

compared to their original clear appearance and their

dimensions were altered: their diameters increased

and lengths decreased based on measured values

accurate up to ±1 cm (for length) and ±15 μm for

diameter (see Figure 6 for optical change and Table 1

for physical changes).

4.3 Tensile Strength

Using the stress and strain data collected

during the tensile strength test, the Young’s Modulus

was calculated through Hooke’s Law and

successfully used to characterize the polymer. For the

non-annealed samples, there were a total of five

successful specimens that produced a mean modulus

of 2.63×103 MPa with a standard deviation of

1.88×102 MPa (see Table III). When testing the

annealed sample, there were only two specimens that

produced a mean modulus of 3.08×103 MPa with a

standard deviation of 3.19×102 MPa (see Table IV).

Such results are indicative of increased stiffness due

to the crystallization of the polymer during the

annealing process. Additionally it suggests that

depending on the intended application, the polymer

can be modified in order to have varying degrees of

stiffness.

The tensile strength test also revealed the

peak stress and percent strain at the breaking point.

Remaining consistent with the conclusion drawn

based on the modulus of elasticity, the non-annealed

fibers had a lower mean peak stress of 44.0 MPa

(with a standard deviation of 6.3 MPa) when

compared to the annealed fibers’ mean peak stress of

51.3 MPa (with a standard deviation of 11.5 MPa).

Additionally, the mean percent strain at break for the

non-annealed fibers was lower than that of the

annealed, having been measured to be 4.60% (with

standard deviation of 4.18%) in comparison to the

13.76% (with a standard deviation of 9.38%). For a

visual representation of this data, see Figures 11 and

12.

The yield point was also identified on each

graph, further verifying the superior strength of the

annealed fiber. Visually estimated at point Y, the

yield point was determined to fall before 3% strain

for the annealed fiber and 2% strain for the non-

annealed fibers. For a visual representation of this

data, see Figures 11 and 12.

Considering this data, one must take into

account the discrepancy in sample set. When

analyzing the mean values for the non-annealed

fibers, the calculations are taking into account five

specimens, creating a more thorough, though still

small, sample set. In comparison, the annealed fiber

set contained only two fibers, limiting the accuracy

of the measurements. Regardless, the results

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remained consistent in showing the improved

strength and stiffness of the annealed fibers.

4.4 Differential Scanning Calorimetry

Figure 13 is the visual representation of the

results of the DSC test. In this graph, the heat flow of

the initial heating is represented by the red curve and

subsequent cooling of the polymer corresponds to the

blue curve. Initial heating and cooling was done to

obtain amorphous structure in the polymer to analyze

its crystal transition. Glass transition temperature of

poly(L-lactic acid) was experimentally determined to

be 46.49 °C, which is approximately 10 degrees

higher than the normal human body temperature.

The literature on the subject puts the glass

transition temperature of PLLA at 58°C, which is

much higher than the experimentally obtained

value.20

This negative deviation of glass transition

temperature might be a result of using a hydrous

sample with too much exposure to atmosphere. High

glass transition temperature suggests that the material

will not undergo structural changes in the body. Such

robustness against body heat makes PLLA a

desirable material for biomedical applications.

The crystallization of the polymer is

represented by the peak at 98.17 °C. The process is

exothermic because the polymer chains reorganize to

obtain minimum energy level. Rapid cooling of the

polymer within this temperature rate will result in

amorphous structure. The melting point of the

polymer is represented by the downward peak at

161.3 °C, supporting its use in the human body.

4.5 X-Ray Diffraction

X-ray diffraction (XRD) was used to

determine the degree of crystallinity of the polymer

and the orientation of the crystals in the sample

PLLA fibers. The diffraction patterns seen as a result

of this test are evidence of crystalline structures in

the polymer complex.

In X-ray diffraction analyses, background

noises may cause some confounding effects. The two

most prevalent background noises are the scattering

of X-rays by air and peaks caused by microcrystalline

cellulose in the cardboard used to hold the specimen.

The most important and influential background

pattern for diffraction analysis is air scattering, due to

the fact that it is used as a control. Air scattering can

be characterized with a broad peak at low 2-theta

values. The X-ray intensity reaches its maximum at

approximately 3 degrees, and then slowly decreases

as 2-theta value increases (see Figure 14). Cardboard

can also cause interference that can be compounded

with the diffraction from the crystal. The crystalline

structure of microcrystalline cellulose shows few

peaks in its diffraction pattern, making it hard to

distinguish the actual diffraction pattern of the

specimen from the cardboard noise. However, unlike

air scattering, the diffraction from the cardboard

holder can be avoided by adjusting the location and

the direction of the specimen before the start of the

test.

Since the diffraction patterns are additive,

subtracting such background noise from the actual

data helps enhance the visibility of various peaks and

determine whether the diffraction was correctly done

on the specimen.

4.6 XRD: Amorphous Fibers

The amorphous fibers had smooth curves in

their diffraction patterns rather than sharp peaks,

indicating that there is no specific structure inside

(Figure 15). Although the graph did not show

patterns at lower 2-theta angles, the shape of the

pattern is very similar to that of air scattering pattern.

Notably, peaks are not present in this graph.

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The thicker fibers seem to have more

deviation from the air scatter, but clear peaks are not

present in their graphs either (see Figure 16). Overall,

the graph follows the general air scattering pattern.

4.7 XRD: Annealed Fibers

The annealed fibers showed more crystallinity

compared to their amorphous counterparts, but air

scattering was still an issue when conducting X-ray

diffraction on thinner fibers.

Initially, the diffraction pattern showed little

deviation from the air scatter (see Figure 17). There

are two visible peaks between 2-theta values of 16

degrees and 18 degrees, though the relative intensity

of the peak is much smaller than the peak caused by

the air scattering on the background.

Similarly, the pattern created by the fiber with

a diameter of 75 microns follows the air scatter and

then shows two small peaks around 16 and 18

degrees (see Figure 18). The shape of the diffraction

pattern is almost identical to the diffraction pattern of

fiber with diameter 63 microns, however the intensity

at the first peak was slightly higher (over 400

counts), compared to that of the pattern of the thinner

65 micron fiber (400 counts).

The diffraction pattern of the 100 micron

PLLA fiber shows greater intensity level at the peaks

(see Figure 19). The highest peaks are seen around 18

and 19 degrees. These peaks show much greater

intensity than those of the other two fibers. In

addition, a third peak is visible around 27 degrees.

The thinner fibers were hard to locate in the

X-ray scanner and their diffraction patterns were

heavily influenced by background noise. Such fibers

also did not show extreme peaks in their X-ray

diffraction pattern; however, a comparison of the

three patterns revealed that better X-ray scattering

patterns can be obtained by using a much thicker

fiber.

Although the air scattering is still visible at

low 2-theta values for the thicker fiber, the intensity

of the diffracted X-rays is much higher at the peak

(see Figure 20). Two sharp peaks occur at 16.5

degrees and 18.5 degrees. There are also many

smaller peaks that were not observed in the thinner

fibers. The peak intensity is as high as 3300,

indicating a clear chain-chain structure at the given

theta value.

4.8 Orientation

The orientation of the crystals in the fiber can

also affect the diffraction pattern. When polymer

molecules crystallize, they may prefer certain

orientations over others, resulting in an uneven

diffraction pattern. Any unevenness can be analyzed

using chi-measurements with the X-ray diffraction

data. Chi-measurements calculate the intensity of

diffracted X-ray beams as the specimen rotates.

On the meridian plane the intensity versus chi

plot shows only minor fluctuations, signifying that

there was no preferred orientation (see Figure 21). In

contrast, there are visible peaks on the chi plot on the

axial plane suggesting that the crystals are aligned on

the axial plane and stretch across the length of the

fiber (see Figure 22). The alignment of polymer

chain on the axial plane gives it structural advantage

in enduring vertical strain.

4.9 Peaks and Miller Indices

The plot seen in Figure 23 and described in

Table V shows the 2-theta values and intensity of X-

ray diffraction at the peaks. Using the Jade program,

the amorphous background was subtracted from the

plot in order to isolate the 11 remaining peaks.

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The peak with the greatest intensity occurs at

a 2-theta value of 16.36 degrees and has a d-spacing

of 5.308 Å. The angle measure of this peak

corresponds to the index (110). The second most

intense peak corresponds to the index (111) and

occurs at a 2-theta value of 18.6 degrees and has a d-

spacing of 4.664 Å. The third notable peak is at 14.5

degrees and has a d-spacing of 5.984 Å. Although it

is not very intense, this specific peak is significant

because it corresponds to the index (010), which is on

the same plane as the index (110) (See Figure 24).

Using peak (111) is not favorable because it is not

normal to the meridian plane like the other two

planes. Therefore, peak (010) was used instead of

peak (111).

The analysis of peaks (110) and (010)

revealed the structure of the PLLA unit cell. Using

the two d-spacing values as cell parameters, the

planes and their miller indices can be determined.

The cell parameter would be

and , but the angle between the two

axes, , is unknown. Assuming that ,

ideally should equal √ (See Figure 24). According

to this assumption,

√ ,

which does not concur with the empirical result. The

difference suggests that either or there was a

significant error in selecting peak centroids.

5 Conclusions PLLA proposes numerous advantages to the

biomedical field due to its biodegradability,

biocompatibility, and strength.21

During the

experiment, PLLA was fabricated and characterized

in order to expand its usage in the medical industry.

First, PLLA was extruded and analyzed using

X-ray diffraction to determine the degree of

crystallinity and orientation of polymer crystals. The

peaks on the collected data displayed that degree of

crystallinity was higher in the annealed sample fibers.

Thus the annealing process and subsequent increase

of crystalline structure could improve the fibers for

their intended purpose of biomedical devices.

In order to verify the biocompatibility of

PLLA with the temperature of the human body, a

sample of non-extruded PLLA was subjected to

differential scanning calorimetry. The study proved

that PLLA has a glass transition temperature of

46.5 , which is significantly higher than the human

body temperature of 37 , verifying that PLLA will

not undergo characteristic changes inside the body.

Finally, a tensile strength test was performed on the

PLLA fibers in order to determine peak stress and

percent strain at the breaking point of various

diameter fibers. It was concluded that the annealing

process increased the tensile strength of PLLA,

which is a necessary aspect in customizing

biomedical devices. The study suggests that PLLA

can be modified to varying degrees of stiffness and

brittleness through annealing to accommodate the

biomedical device needed.

The study of PLLA can be expanded upon

through further research, and it can also potentially

lead to numerous groundbreaking biomedical

advances.22

The copolymerization of PLLA with

other polymers, human tissue, and even minerals in

the human bone is promising in the medical industry.

Additionally, further experimentation of crystallinity

and tensile strength is essential to the improvement of

biomedical devices such as bone pins, sutures,

screws, and wires.23

Future work should also look to

expand the usage of PLLA throughout more

biomedical devices, including stronger devices such

as plates and bone fixtures. Because PLLA has a

variety of favorable characteristics, further testing of

PLLA will considerably broaden its usage in the

biomedical field.

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6 Acknowledgements

The authors would like to recognize and

thank everyone who invested time in the creation of

this paper. Everyone involved was a priceless asset,

and the authors want to express their deepest

gratitude. Thank you to mentor Dr. Thomas Emge,

the Chief Crystallography Engineer of Rutgers

University, who provided the smiles and knowledge

that led to this paper. Thank you to mentor Dr.

Sanjeeva Murthy, an Associate Research Professor at

the Rutgers Center for Biomaterials, who shared his

passion and expertise with everyone involved in this

project. Thank you to Edmund Han, the RTA who

gave amazing help and never failed to keep his team

on task.

The authors would also like to thank the

people who made their Governor’s School experience

possible. Thank you Dean Jean Patrick Antoine and

Director Ilene Rosen for providing such an amazing

opportunity for the students attending Governor’s

School. We would also like to thank Sarah Sprawka

and Purac Biomaterials (a Corbion company) for

donating the PLLA used in our experiments. Finally,

thank you to the sponsors of the 2014 New Jersey

Governor’s School of Engineering and Technology:

Rutgers University, The State of New Jersey,

Lockheed Martin, Morgan Stanley, Novo Nordisk,

The Provident Bank Foundation, Silver Line

Windows and Doors, and South Jersey Industries Inc.

References 1. Murthy, Sanjeeva. “Polymers in Biomedical

Devices for use in Regenerative Medicine.” Lecture,

NJCBM from Rutgers University, Piscataway, NJ,

July 2, 2014. 2. Ibid. 3. Ibid.

4. Murthy, Sanjeeva. “Polymer Properties and Future

Directions.” Lecture, NJCBM from Rutgers

University, Piscataway, NJ, July 17, 2014. 5. Ibid. 6. Ibid. 7. Ibid. 8. Ibid. 9. Murthy, Sanjeeva. “Polymers in Biomedical

Devices for use in Regenerative Medicine.” Lecture,

NJCBM from Rutgers University, Piscataway, NJ,

July 2, 2014. 10. Ibid. 11. Emge, Thomas. “X-Ray Diffraction (XRD).”

Lecture, Rutgers University, Piscataway, NJ, July 1,

2014. 12. Ibid. 13. Ibid. 14. Ibid. 15. Ibid. 16. Ibid. 17. Ibid. 18. Kopeliovich, Dmitri. “Annealing of Plastics.”

Substech: Substances and Technology, Knowledge

Source on Material Engineering.

http://www.substech.com/dokuwiki/doku.php?id=ann

ealing_of_plastics (retrieved July 10, 2014). 19. Ibid. 20. Chitoshi N, Shin-ya T. “Glass Transition and

Mechanical Properties of PLLA and PDLLA-PGA

Copolymer Blends.” Journal of Applied Polymer

Science June (2004) 21. Avérous, Luc. “Bioplastics: Biodegradable

Polyesters.” http://www.biodeg.net/bioplastic.html

(retrieved July 19, 2014) 22. Tatsumi A, Kanemitsu N, Nakamura T, Shimizu

Y. “Bioabsorbable poly-L-lactide costal coaptation

pins and their clinical application in thoracotomy.”

The Annals of Thoracic Surgery March (1999). 23. Saito T, Iguchi A, Sakurai M, Tabayashi K.

“Biomechanical study of a poly-L-lactide (PLLA)

Page 12: Fabricating and Characterizing Biodegradable Polymeric ... · Fabricating and Characterizing Biodegradable Polymeric Fibers for Medical Applications: Experiments with Poly(L-Lactic

sternal pin in sternal closure after cardiothoracic

surgery.” The Annals of Thoracic Surgery February

(2004)

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Appendix

Figures

FIGURE 1: THE FORMATION OF PLA

The above image demonstrates the two main routes of synthesis of PLA through a hydration

reaction with lactic acid (top left) or a ring-opening reaction with lactide (top right).

FIGURE 2: THE BASIC LAYOUT OF AN X-RAY DIFFRACTION MACHINE.

This image is the basic layout of an X-ray Diffraction machine. The X-rays travel from the X-ray

source, hit the sample, and then travel to the X-ray detector. The X-ray detector measures the

intensity of the reflected X-rays, and plots the data based on the θ value.

Image courtesy of University of California, Davis.

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FIGURE 3: THE PATH OF X-RAYS BASED ON BRAGG’S LAW

The path of x-rays based on Bragg’s Law. The d-spacing is represented by the letter d, and is the

calculated value, since θ is controlled by the machine, λ is constant, and n is constant.

Image courtesy of University of California, Davis

FIGURE 4: IMAGE OF THE EXTRUSION PROCESS

This is an image of the extrusion process. The die is not pictured, but it would be at the top of the

picture from where the fiber seems to originate.

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FIGURE 5: IMAGE OF THE SPOOLING PROCESS

This is an image of the spooling process. The fiber travels through the arm in front of the yellow

spool, and the arm moves back and forth to create a uniform spool of fiber.

FIGURE 6: BEFORE AND AFTER IMAGES OF THE FIBERS AFTER THE ANNEALING

PROCESS

These images are of the fibers before and

after the annealing process. The amorphous

fibers before the heating are transparent

whereas the annealed fibers are opaque.

Additionally, the annealed fibers curled up

in the heat, suggesting a change in

crystalline structure.

Amorphous Annealed

Amorphous Annealed

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FIGURE 7: IMAGE OF SAMPLE 20A IN CARD STOCK FRAME DURING X-RAY

DIFFRACTION

This is an image of the PLLA fiber 20A in the card stock during the x-ray diffraction process.

The cardstock was used to support the fiber sample and keep it straight in front of the X-ray

beams. The XRD system analyzed characteristics of the annealed fiber with molecular level

detail.

FIGURE 8: SAMPLES FOR TENSILE STRENGTH TEST

The above image is an example of the card stock frames and the fibers before the Tensile

Strength Test. In this case, the sample is of the amorphous fiber 27, our thinnest sample.

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FIGURE 9: DIAMETER OF THE FIBER VS. SPEED

The relationship between the speed of fiber in m/s and the diameter in microns is displayed.

Although as speed increases, diameter decreases, the graph does not show a linear relationship

between speed and diameter.

FIGURE 10: DIAMETER OF THE FIBER VS. INVERSE SQUARE OF SPEED

The relationship between the inverse square speed of fiber in m/s and the diameter in microns is

displayed to be linear. Thus, the graph supports the equation.

0.00

100.00

200.00

300.00

400.00

500.00

600.00

0.00 0.20 0.40 0.60 0.80 1.00 1.20

DIA

MET

ER (μ

m)

SPEED (m/s)

DIAMETER vs. SPEED

y = 11.971x + 59.68 R² = 0.9981

0.00

100.00

200.00

300.00

400.00

500.00

600.00

0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00

DIA

MET

ER (μ

m)

1/SPEED2 (s2/m2)

DIAMETER vs. 1/SPEED2

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FIGURE 11: STRESS-STRAIN GRAPH FOR AMORPHOUS FIBERS

The above graph is a combination of all the specimens for Amorphous Fiber 12. Point B marks

the beginning of the linear section, point M marks the end of the linear portion, point Y indicates

the yield point when the fiber transitions from elastic to plastic, and point F indicates the point of

failure.

FIGURE 12: STRESS-STRAIN GRAPH FOR ANNEALED FIBERS

The above graph is a combination of all the specimens for Annealed Fiber 12. Point B marks the

beginning of the linear section, point M marks the end of the linear portion, point Y indicates the

yield point when the fiber transitions from elastic to plastic, and point F indicates the point of

failure.

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FIGURE 13: DIFFERENTIAL SCANNING CALORIMETRY OF PLLA

The above image is of the results of the Differential Scanning Calorimetry of Poly-L-Lactic Acid.

The red line represents the first heating cycle, the one that began with the semi-crystalline

sample. The blue line is the first cooling cycle, which occurs at a rate specifically determined to

not re-crystallize the polymer. The green line is the final heating cycle and since it started with

an amorphous sample, the green line gives much better data than the red line.

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FIGURE 14: X-RAY SCATTERING DUE TO AIR

This data is the result of an X-ray diffraction test with no sample in the machine. The scattering

is just due to the air, and can be subtracted from other results to eliminate the noise that the air

creates.

FIGURE 15: X-RAY DIFFRACTION OF AMORPHOUS PLLA FIBER –

DIAMETER 100 μm

The above image is of X-Ray Diffraction of the Amorphous PLLA Fiber of diameter 100 μm.

The amorphous PLLA shows no Bragg peaks and looks very similar to the air scattering. Even

though the sample was tested multiple times in multiple locations, the data stayed the same: there

was almost nothing that differed from the air scattering.

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0.8 5.8 10.8 15.8 20.8 25.8 30.8

Inte

nsi

ty

2-theta (Degrees)

Air

0

20

40

60

80

100

120

140

160

0 10 20 30 40 50 60 70

Inte

nsi

ty

2-Theta (Degrees)

PLLA Fiber - Amorphous, d = 100 µm

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FIGURE 16: X-RAY DIFFRACTION OF AMORPHOUS PLLA FIBER –

DIAMETER 600 μm

The amorphous PLLA shows no obvious Bragg peaks, but differs from the air scattering. At

around 33 degrees, there is a tiny deformation that may be a weak peak.

FIGURE 17: X-RAY DIFFRACTION OF ANNEALED PLLA FIBER –

DIAMETER 63 μm

The figure shows the X-ray diffraction pattern of annealed 63 micron diameter fiber. The sample

shows two small peaks, but the peak caused by the air scattering is far more intense than the little

peaks.

0

50

100

150

200

250

13 23 33 43 53 63

Inte

nsi

ty

2-Theta (Degrees)

PLLA Fiber - Amorphous, d = 600μm

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0.8 5.8 10.8 15.8 20.8

Inte

nsi

ty

2-Theta (Degrees)

PLLA Fiber - Annealed, d = 63μm

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FIGURE 18: X-RAY DIFFRACTION OF ANNEALED PLLA FIBER –

DIAMETER 75 μm

The figure shows the X-ray diffraction pattern of annealed 75 micron diameter fiber. The sample

shows two small peaks, yet air scattering dominates the diffraction pattern.

FIGURE 19: X-RAY DIFFRACTION OF ANNEALED PLLA FIBER –

DIAMETER 100 μm

The figure shows the X-ray diffraction pattern of annealed 100 micron diameter fiber. The

sample shows two small Bragg peaks.

0

200

400

600

800

1000

1200

1400

1600

1800

2000

1.5 6.5 11.5 16.5 21.5

Inte

nsi

ty

2-Theta (Degrees)

PLLA Fiber - Annealed, d = 75μm

0

100

200

300

400

500

600

700

800

10 15 20 25 30

Inte

nsi

ty

2-theta (Degrees)

PLLA Fiber - Annealed, d = 100 μm

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FIGURE 20: X-RAY DIFFRACTION OF ANNEALED PLLA FIBER –

DIAMETER 600 μm

The figure shows the X-ray diffraction pattern of annealed 600 micron diameter fiber. The

sample shows very clear Bragg peaks and some amorphous background.

FIGURE 21: X-RAY DIFFRACTION OF ANNEALED FIBER, CHI –

DIAMETER 100 μm, MERIDIAN

The figure shows the intensity plotted against the chi values when the fiber was rotated on the

meridian plane. There is no distinct or periodic pattern, suggesting that there is no specific

orientation that the polymers prefer on the meridian plane.

0

500

1000

1500

2000

2500

3000

3500

1.5 6.5 11.5 16.5 21.5 26.5 31.5

Inte

nsi

ty

2-Theta (Degrees)

PLLA Fiber - Annealed, d = 600μm

0

20

40

60

80

100

120

140

160

-350 -300 -250 -200 -150 -100 -50 0 50 100

Inte

nsi

ty

Chi (Degree)

PLLA Fiber - Annealed, 100 µm - Meridian

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FIGURE 22: X-RAY DIFFRACTION OF ANNEALED FIBER, CHI –

DIAMETER 100 μm, AXIAL

The figure shows the intensity plotted against the chi values when the fiber was rotated on the

axial plane. There are two periodic peaks on the plot, indicating that the polymer chains have

some kind of orientation on the axial plane.

FIGURE 23: INTENSITY VERSUS 2-THETA WITHOUT AMORPHOUS BACKGROUND

The graph shows the peaks from the X-ray diffraction pattern of annealed 600 micron diameter

fiber. The peaks were isolated from the background by subtracting the amorphous background

from the diffraction pattern. There are 11 distinct peaks that were isolated.

0

100

200

300

400

500

600

700

800

-350 -300 -250 -200 -150 -100 -50 0 50 100

Inte

nsi

ty

Chi (Degrees)

PLLA Fiber - Annealed, d = 100 µm - Axial

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FIGURE 24: INDICES OF PLANES AND CELL PARAMETERS

The diagram represents the cross section of the fiber. Two planes, (110) and (010), correspond to

the peaks at 16.4° and 14.5°. The triangle on the bottom right of the diagram represents the

relative length of a and b, assuming that they form a right angle.

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Tables

TABLE I: MEASUREMENTS OF THE FIBERS BEFORE AND AFTER THE ANNEALING

PROCESS (110 MINUTES)

The table compares the length and diameter of amorphous and annealed fiber in order after a 110

minute annealing process . The number of the fiber corresponds to the spool speed

displayed on the dial of the machine.

TABLE II: DIMENSIONS AND VOLUMETRIC FLOW RATE

Number Diameter

(μm)

STDEV

(μm)

Speed

(m/s)

1/Speed2

(s2/m

2)

Radius

(μm)

Cross-Sectional

Area

(m2)

VF Ratec

(m3/s)

%

Error

3 500.00 81.65 0.17 36.73 250.00 1.96E-07 3.24E-08*

472

8 129.14 23.41 0.40 6.22 64.57 1.31E-08 5.25E-09 7.24

12 94.86 16.77 0.56 3.20 47.43 7.06E-09 3.95E-09 25.3

16 88.43 10.08 0.74 1.84 44.21 6.14E-09 4.53E-09 20.0

20 85.43 20.11 0.88 1.30 42.71 5.73E-09 5.02E-09 11.3

27 59.86 8.23 1.12 0.80 29.93 2.81E-09 3.15E-09 44.3

*Outlier

The table shows the diameter, standard deviation of the diameter, spooling speed, and cross

sectional area. It also shows volumetric flow rate calculated based on these values and their

percent error from the constant.

Speed Level

Amorphous Annealed

Diameter

(μm)

Length

(cm)

Diameter

(μm)

Length

(cm)

12 100.00 22.75 100.00 21.50

16 75.00 30.00 82.50 29.25

20 67.50 23.50 75.00 22.50

27 57.50 24.75 62.50 23.50

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TABLE III: SAMPLE DATA FOR TENSILE STRENGTH TEST OF AMORPHOUS

FIBER

Specimen

#

Initial

Speed

mm/min

Diameter

mm

Length

mm

Peak Load

N

Peak Stress

MPa

Strain At

Break

%

Modulus

MPa

1 5.0 0.090 0.090 0.254 40.0 1.730 2.702e+003

2 5.0 0.090 0.090 0.288 45.2 5.214 2.662e+003

3 5.0 0.090 0.090 0.316 49.6 2.188 2.527e+003

4 5.0 0.090 0.090 0.224 35.3 11.661 2.391e+003

5 5.0 0.090 0.090 0.316 49.7 2.219 2.889e+003

Mean 5.0 0.090 0.090 0.280 44.0 4.602 2.634e+003

Std. Dev. 0.0 0.000 0.000 0.040 6.3 4.182 1.878e+002

The table shows the data obtained from the tensile strength test of amorphous fiber. It shows the

dimension of the fibers, speed at which the fibers were pulled, the peak load and stress, strain at

break, and Young’s modulus of the fiber determined by the values.

TABLE IV: SAMPLE DATA FOR TENSILE STRENGTH TEST OF ANNEALED FIBER

Specime

n #

Initial

Speed

mm/min

Diameter

mm

Length

mm

Peak Load

N

Peak

Stress

MPa

Strain At

Break

%

Modulus

MPa

1 5.0 0.095 25.400 0.306 43.2 7.128 2.857e+003

2 5.0 0.095 25.400 0.421 59.4 20.389 3.307e+003

Mean 5.0 0.095 25.400 0.363 51.3 13.759 3.082e+003

Std. Dev. 0.0 0.000 0.000 0.081 11.5 9.376 3.188e+002

The table shows the data obtained from the tensile strength test of annealed fiber.

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TABLE V: D-SPACING AND RELATIVE INTENSITIES

d(Å) 2 θ (°) Intensity Relative

Intensity

7.093 12.22 72 0.027

5.984 14.50 91 0.034

5.308 16.36 2645 1.000

4.664 18.63 500 0.189

3.986 21.84 123 0.047

3.753 23.20 67 0.025

3.576 24.38 53 0.020

3.269 26.71 47 0.018

3.089 28.30 78 0.029

2.879 30.40 41 0.016

2.810 31.18 117 0.044

The table analyzes the d-spacing values, 2-theta, and intensity of the 11 prominent peaks.

Relative intensity of the peak is the intensity of the peak divided by the intensity of the strongest

peak. Intensity of the peak has no set unit.


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