Investigation of Silicone Rubber Blends and Their Shape

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INVESTIGATION OF SILICONE RUBBER BLENDS AND THEIR SHAPE

MEMORY PROPERTIES

A Thesis

Presented to

The Graduate Faculty of The University of Akron

In Partial Fulfillment

Of the Requirement for the Degree

Master of Science

Yuelei Guo

May, 2018

INVESTIGATION OF SILICONE RUBBER

BLENDS AND THEIR SHAPE

MEMORY PROPERTIES

Yuelei Guo

Thesis

Approved: Accepted:

Advisor Department Chair Dr. Kevin Cavicchi Dr. Sadhan C. Jana

Committee Member Interim-Dean of the College Dr. Erol Sancaktar Dr. Ali Dhinojwala

Committee Member Dean of the Graduate School Dr. Nicole Zacharia Dr. Chand Midha

Date

ABSTRACT

Silicone rubber is widely used as an elastomer. This thesis investigates the

fabrication and material properties of silicone rubber blended with some inexpensive

fillers.

The section of this thesis focuses on cornstarch-silicone rubber blends. In

acetoxy cured silicone rubber, cornstarch has good compatibility and is able to blend

well with the silicone resin. Samples were prepared by simple physical mixing. The

crosslinking degree and other mechanical abilities can be investigated as a function of

the weight percent of cornstarch and water.

The second section of this thesis focuses on shape memory polymers. Shape

memory polymers were fabricated by mixing acetoxy silicone with crystalline small

molecules. The solid-liquid transition of the small molecule is able to generate reversible solid networks, which enable the shape memory effect. The shape memory effect properties tested as a function of the weight percent of the small molecule in

samples. With increasing weight percent of the small molecule, the fixity is increased

to ca. 99% and the recovery decreased to ca. 87%. Based on these tests, an optimum

blend formulation is found, which shows the best shape memory performance can be

found and repeatability through several test cycles. Due to the differences in the

interaction degree between the small molecule and the silicone rubber, the choice of

the small molecule is also an important factor to influence the shape memory effect.

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ACKNOWLEDGEMENTS

First, I would like to show my deepest gratitude to my supervisor, Prof. Kevin A

Cavicchi, who has provided me with valuable guidance in the last two years. Without

his academic and constructive advice, the completion of this thesis would be

impossible. I have learned a lot by joining his group. It is a valuable experience to

have him as my supervisor.

Second, I would like to express my heartfelt gratitude to all members in our

group: Guodong Deng, Junyoung Seo, Marcos Pantoja, Pei-zhen Jian, Tzu-yu Lai,

Juan Marin Angel, Bangan Peng, Luis Ruiz-Santiago, Xindi Li, Cheng Li, Lei Zhou and Yi Ting Lo. I’m so glad that I have the chance to work together with you. I appreciate your help and I’ll always remember the time we spent together.

Last, my thanks would go to my beloved family for their loving considerations

and great confidence in me all through these years. My family always gives me the

strongest support and I’ll continue to be a better person.

TABLE OF CONTENTS

LIST OF FIGURES ...... VII

LIST OF SCHEMES ...... X

LIST OF TABLES ...... XI

CHAPTER I ...... 1

INTRODUCTION ...... 1

1.1 Silicone rubber ...... 1

1.1.1 Introduction of Silicone rubber ...... 1

1.1.2 Vulcanization methods of silicone rubber ...... 3

1.1.3 Silicone Rubber Blend ...... 6

1.1.4 Applications of silicone rubber ...... 8

1.2 Shape memory polymer ...... 11

1.2.1 Introduction of shape memory polymers ...... 11

1.2.2 Application of shape memory polymers ...... 14

CHAPTER II ...... 16

EXPERIMENTAL ...... 16

2.1 Materials and tools ...... 16

2.2 Sample Preparation...... 17

2.3 Characterization ...... 18

2.3.1 Crosslinking Degree ...... 19

2.3.2 Strain-Controlled Shape Memory Experiments ...... 20

2.3.3. DMA (Dynamic Mechanical Analysis) ...... 21

2.3.4 TGA (Thermogravimetric Analysis) ...... 22

2.3.5 DSC (Differential Scanning Calorimetry) ...... 22

2.3.6 Microscope ...... 22

CHAPTER III ...... 24

RESULTS AND DISCUSSION ...... 24

3.1 Cornstarch-PDMS Blend ...... 24

3.2 Shape Memory Silicone Rubber Blend ...... 33

CHAPTER IV ...... 47

CONCLUSIONS ...... 47

REFERENCES ...... 49

LIST OF FIGURES

Figure Page 1.1 Structure and the basic unit of the silicone rubber...... 3

1.2 Structure of fillers (a) Cornstarch (b)1,10-Decanediol (c)12-Hroxystearicacid (d)

Stearic acid ...... 7

1.3 Structure of shape memory polymer ...... 13

1.4 Mechanism of thermally induced SMP (schematic) ...... 14

2.1 (a) Mixer (b) Micro milling machine (c) Mold (5mmx5mmx1mm) (d) Modified

crescent wrench ...... 17

2.2 Procedure to prepare a sample ...... 18

3.1 Solubility test of non-crosslinked silicone rubber in different solvents (a)

silicone rubber in Toluene (b) silicone rubber in Chloroform ...... 24

3.2 Swelling capacity vs Different amount of cornstarch ...... 25

3.3 Swelling capacity vs Amount of water ...... 26

3.4 The amount of cornstarch vs Swelling capacity ...... 27

3.5 DMA strain sweep curves (storage modulus vs strain) for different weight

percent sample, the ratio means water:PDMS:cornstarch (a) strain sweep curves for

different weight percent cornstarch sample (b) strain sweep curves for different

weight percent water ...... 28

3.6 DMA stress-strain test for different weight percent cornstarch samples (the

vii

amount of silicone:cornstarch = 30:30, 30:35, 30:40, 30:45, 30:50, 30:55 and pure

PDMS) ...... 30

3.7 TGA tests for (a) PDMS, cornstarch and 1:30:30 sample (b) different weight percent samples (the amount of water: silicone: cornstarch=1:30:30, 1:30:35,

1:30:40, 1:30:45, 1:30:50, 1:30:55) ...... 32

3.8 A series of shape memory effect (a) the initial shape (b) use tweezer to keep a shape (c) put the sample into hot water for 5 seconds (d) the temporary shape (e) put the sample back into the hot water for 5 seconds (f) the sample recovers to the initial shape ...... 33

3.9 TGA curves for PDMS, Decanediol and 50% Decanediol-Silicone blend.

Experiments were conducted using a nitrogen atmosphere with a heating rate of

10oC min-1 from 25°C to 800°C ...... 34

3.10 (a) DSC curves for Decanediol Silicone Blend with different weight percent

Decanediol (10%, 20%, 30%, 40%, 50%, 100%). Experiment used a heating rate of

10oC min-1 from 25°C to 100°C. (b) Melting temperature vs weight percent

Decanediol ...... 35

3.11 DMA Temperature ramp test (storage modulus vs temperature) for different weight percent of Decanediol (10%, 20%, 30%, 40%, 50%, 100%)...... 36

3.12 (a) DMA strain sweep cures (storage modulus vs strain) for different weight percent Decanediol sample (10%, 20%, 30%, 40%, 50%). (b) Storage modulus vs

viii

weight percent Decanediol ...... 37

3.13 Fixity and Recovery test by the wrench for different weight percent

Decanediol sample (10%, 20%, 30%, 40%, 50%, 60%) at 95°C oven...... 39

3.14 DMA controlled stress shape memory cycle test for 50% weight percent

Decanediol silicone rubber blend sample at (a) 0.11MPa (b) 0.13MPa (c) 0.2MPa (d)

0.215MPa ...... 40

3.15 Fixity and recovery results of 50% Decanediol blend of shape memory cycles under different stress...... 41

3.16 Microscope picture for (a) Decanediol powder (b) Crystallization in 50%

Decanediol-Silicone Blend sample (c) Holes in the 50% Decanediol-Silicone Blend sample after swelling in THF to remove the Decanediol ...... 44

3.17 Fixity and recovery test by the wrench for samples with different fillers (50%

Decanediol-Silicone rubber blend, 50% 12-Hydroxystearic Acid-Silicone rubber blend, 50% Stearic Acid-Silicone rubber blend) at 95°C oven...... 45

LIST OF SCHEMES

Scheme Page 1.1 Room temperature vulcanization (RTV) crosslinking reaction ...... 6

LIST OF TABLES

Table Page 3.1 Mechanical characteristics of different weight percent cornstarch samples ...... 31

3.2 Solubility test results of Decanediol and swelling test results of PDMS ...... 43

CHAPTER I

INTRODUCTION

1.1 Silicone rubber

Silicone rubber is a kind of elastomer with an inorganic -Si-O-repeat structure in the main chain and organic side groups. It is classified as a semi-inorganic, saturated, hetero chain and non-polar elastomer.[1-3] It is one of the important varieties

of specialty synthetic rubber. Compared with common rubber, silicone rubber has excellent heat resistance, cold resistance, and electrical properties.[1-4] Silicone rubber is widely used in the fields of aerospace,[1] electrical,[3] biomedicine[5] and so on. At

present, rubber technology is developing in the direction of high performance, multi-

functionality and composites.

1.1.1 Introduction of Silicone rubber

Rubbers or elastomers are polymers with high elasticity and low glass transition

[1-4] temperature (Tg). At room temperature, they exhibit high-elasticity, so it is flexible

and has reversible deformation. Rubber can produce large deformation under small

external force, and when the external force is removed, it can revert to the initial

shape.

12 Silicone rubber is a low glass transition temperature, crosslinked polymer,

which is also called an elastomer. As a high elasticity elastomer, silicone rubber has

good rebound resilience, sealing properties, heat resistance, oil resistance, and

chemical resistance.[2-5] The heat resistance is an especially notable property as,

silicone rubbers can resist the temperature range of -55~300°C. Compared with other

types of rubber, the advantage of silicone rubbers is the high stability under high

temperature, even at 200°C.[3-5]Therefore, because of these good properties, silicone rubbers have been widely used in many applications, such as electronics, toys, the

construction industry, and the biomedical industry.

As shown in Figure 1.1, the structure of silicone rubbers is conceptually

simple. The main chain of silicone rubbers consists of alternating silicon and oxygen

atoms, and the silicon atoms are usually connected with two organic substituent

groups. The two groups are most always methyl and vinyl in normal silicone

rubbers.[4-5] By using different substituent groups, the silicone rubbers can be

modified to have different properties, such as using phenyl group to increase the

glass transition temperature.

Figure 1.1 Structure and the basic unit of the silicone rubber

1.1.2 Vulcanization methods of silicone rubber

Silicone rubbers can always be divided into room temperature vulcanized silicone rubbers (RTV)[6] and high temperature vulcanized silicone rubbers (HTV)[10] by their vulcanization characteristics.

Room temperature vulcanized silicone rubbers (RTV) have one or two functional groups at the two ends (sometimes in the middle) of the polymer.

Generally, RTV silicone rubber always starts as a liquid and is converted into a solid elastomer after curing. RTV silicone rubber can also be divided into one-part silicone rubber (RTV-1) and two-part silicone rubber (RTV-2).[6] For one-part condensation curing silicone rubber, the vulcanization reaction can be initiated by the moisture in the air, and does not need other initiators. For two-part silicone rubber, it cannot be initiated by the moisture in the air but needs other catalysts to drive crosslinking.[6-9]

RTV silicone rubbers have a good ability to withstand thermal stress and mechanical stress, and they can be used in many relevant applications.[9]

14 High temperature vulcanized silicone rubbers (HTV) are produced under high-temperature vulcanization (110-170°C). They use a linear high molecular weight polysiloxane as the base silicone, and mix with reinforcing fillers and curing agents.[10] The notable feature of HTV silicone rubbers is their high-temperature stability. Although under room temperature, the strength of HTV silicone rubbers is just half of the strength of some other synthetic rubbers, when under high temperature such as more than 200°C, HTV silicone rubbers can keep the flexibility,

[10-11] elastic resilience and surface hardness.

Silicone curing is one of the important synthesis steps for generally silicone rubber. There are generally three routes that are used in silicone curing[12-13]:

a. Peroxide curing

The peroxide curing is a peroxide-initiated free radical reaction. Under heating, the peroxides decompose and produce highly reactive radicals. These radicals will react with the liquid silicone and crosslink the polymer to form a three- dimensional network.[9-13] This method can be applied to solid silicone rubber.

b. Platinum-catalyzed addition curing

Platinum-catalyzed addition curing is a basic addition reaction. Under the catalysis of platinum, the Si-H groups of the silicone polymers will react with polymers which have vinyl groups. After a series of addition reactions, a three- dimensional network is formed. Some advantages of this curing chemistry are that no byproducts are released, rapid curing, and wide application. This route can be applied to solid silicone rubber, liquid silicone rubber (LSR), and 2-part silicone

[12-14] rubber (RTV-2). 15 c. Condensation curing

Curing agents always be used in this way. The curing agents participate in condensation reactions with the hydroxyl groups of the silicone polymers and then form a network. At the same time, some small compounds can be released, such as acetic acid, or alcohol. The curing process is insensitive to the inhibition by some substances. This way can be applied to 1-part silicone rubber (RTV-1) and 2-part

[12-15] silicone rubber (RTV-2).

In this research, the silicone rubber is a 1-RTV that crosslinks by condensation curing. For 1-RTV vulcanization, the crosslinking reaction is initiated by the moisture in the air. As the Scheme 1.1 shows, the basic crosslinking reaction is a hydrolysis condensation reaction. The first step is a hydrolysis reaction, the silicone will react with water where acetoxy groups are converted to hydroxyl groups producing acetic acid. The second step is a condensation reaction. The hydroxyl groups which are produced by the first reaction step will condense to form a three-dimensional network.[14-16] As this reaction proceeds the release of acetic acid and the formation of water in the second step provides the reactant and catalyst for

[16-17] the further reaction driving crosslinking.

Scheme 1.1 Room temperature vulcanization (RTV) crosslinking reaction

1.1.3 Silicone Rubber Blend

With the improvement of industrialization level, the demand for silicone

rubber become much higher. The traditional silicone rubber products do not always

meet people’s requirements.[17-18] Silicone rubber needs to be filled with filler to

regulate various properties, such as the tensile strength, elongation, hardness, and

heat resistance, so that it can meet the need of use. Fillers are the solid substance that

could improve the properties of materials, or increase capacity and reduce the cost of

materials.[18-20] Many kinds of filling agents are used in the plastics processing,

[21-24] rubber processing, coating, medicine and other products.

17 Different fillers correspond to different properties. In this research the aim was to introduce new properties, such as shape memory effect, through the inclusion of a filler. For a good mixing of the filler and silicone rubber, filler was chosen which has hydroxy bonds, so that they can have hydrogen bonding with each other. As shown in Figure 1.4, four fillers were investigated in this research: cornstarch, 1,10- decanediol, 12-hydroxystearic acid, stearic acid.

(a)

(b)

(c)

(d) Figure 1.2 Structure of fillers (a) Cornstarch[25] (b) 1,10-Decanediol (c) 12- Hydroxystearic acid (d) Stearic acid

Cornstarch was the first filler investigated. Figure 1.2(a) shows the structure of

cornstarch. Cornstarch is a kind of hygroscopic polymers. Even for the cornstarch

under the normal environment, the moisture content (12%-14%, influenced by

18 temperature and humidity) is very high.[25-26] By adding the cornstarch, the sticky

silicone is somewhat stiffened and starts to set up from the inside out quickly

because of its moisture content.

The other three fillers all can produce the shape memory effect when blended

with silicone rubber. They are all small molecule substances. As shown in Figure

1.2(b), 1,10-Decanediol has two hydroxy bonds at the end of the chain. These two bonds could lead to interactions with silicone rubber network. It forms white, needle-

shaped crystals and the melting temperature is around 73°C. These characteristics

show that it may be a good filler to make a shape memory sample. From Figure

1.2(c), we can also see the structure of 12-hydroxystearic acid. Compared with

decanediol, it has a carboxylic acid group that may also interact with the polar

functionality in the silicone. The melting temperature is around 75°C, similar to

decanediol. The last filler is stearic acid. As shown in Figure 1.2(d), stearic acid only

has one carboxylic acid, so it would have a weaker interaction compared with the

other filler.

1.1.4 Applications of silicone rubber

Among many synthetic rubbers, silicone rubber has excellent properties. It

has many characteristics such as low odor, low toxicity and high temperature

resistance. From 300°C or -90°C, silicone rubber can maintain its original strength

and elasticity.[27-28] Silicone rubber also has good electrical insulation, oxygen

resistance, light resistance and chemical stability.[27-30] Because of these excellent

properties, silicone rubber has played an important role in modern society.

19 Silicone rubber products can be found in our daily life such as automotive,

food storage products, clothing, electronics[32] and hardware.[30-32] Silicone rubber

also has a wide application in modern medicine. A variety of silicone medical

products have been successfully developed such as silicone rubber noise prevention

earplug; silicone rubber artificial blood vessel; artificial trachea; and artificial

[31] lung.

Silicone rubber has an important position in the application of advanced

technology. With appropriate technology, silicone rubber can produce innovative

products, especially in simplifying automatic production process.[32-34] Silicone

rubber is reliable and cost-effective. It could help industry simplify the production

process because it has many vulcanization, molding and manufacturing

formulations.[33-34] Silicone rubber products also do not produce harmful odors or byproducts, and have low allergenic property. Silicone rubber and its technology have been applied in many industries. The following is a part of the application.

Silicone rubber is almost used in all aspects of the automotive industry.

Excellent insulation, heat and chemical resistance, adhesion and teat strength are only a few of the important properties of silicone rubber.[27-28] As a result, silicone

[30] rubber is especially suitable for automobile manufacturing and component supply.

The sealed self-lubricating silicone rubber can provide a strong and durable

seal for many devices, such as the headlamp and the oil filter. The gasket silicone

rubber seal is leakproof and durable. It will not break or crack under extreme

temperature and pressure. Silicone rubber low rolling resistance tire can reduce fuel

20 [32-35] consumption and has excellent grip.

Oil resistant and heat resistant silicone rubber can be used in cylinder cover

gaskets, engine covers or pumps, which can help heat dissipation, withstand extreme

temperatures and maintain stability and durability under extreme environmental and

[30] chemical pressure.

The baking pan made of silicone rubber is more sturdy, convenient, easy to use, and durable. The elastic non-stick surface is easy to clean and does not bring any odor. The material removed from the refrigerator can be directly placed in the stove, microwave oven or dishwasher without affecting the quality of products or foods.[29-30]

Silicone rubber has many characteristics such as good biocompatibility,

stable performance, low blood coagulation and can withstand high temperature and

high pressure. And it can be processed into various shapes such as thin film, sponge,

airbag and so on.[32-35] Nowadays, it is one of the most widely used materials in

[31] medicine.

Another important application is silicone rubber sealant. This is also the type of silicone rubber used in this research. One of the typical applications of silicone sealant is glass curtain wall. The glass and aluminum alloy frames are bonded with silicone structural adhesive as external wall materials, and the expansion joints are made of silicone weatherproof adhesive for waterproofing and sealing.[36] Besides

sealant, RTV also includes sealing materials used in aerospace, nuclear power plants,

electronics, machinery, automotive and other industries.

21 1.2 Shape memory polymer

As a new unique member of shape memory materials, the shape memory polymers attract more and more attention because of their scientific significance.

SMP can change some of their functions with the change of environment according

[37-38] to a certain purpose.

1.2.1 Introduction of shape memory polymers

Shape memory polymer (SMP) refers to a kind of responsive materials that can have physical transformation between initial shape and temporary shape under certain external stimulus (such as heat, electricity, light chemical induction, etc.)[37-40]

SMP have several advantages such as: many kinds of stimulation can induce the shape memory effect; they can be designed in a broad range of ways, and have a

[40] flexible application.

Like the structure showed in Figure 1.3, shape memory polymer (SMP) has two structure elements: permanent network and reversible network. The permanent network is an elastic network which is the part to keep the permanent shape and drives recovery upon the application of a stimulus. The reversible network is able to counterbalance the load stored in the permanent network to fix a temporary shape and is sensitive to the stimulus-allowing recovery when the network is reversed.[48]

In this research the permanent network is the crosslinked silicone rubber and the reversible network is the solid network formed by the crystalline small molecule.

Figure 1.3 Structure of shape memory polymer

According to its recovery principle, SMP could be divided into thermally

induced SMP, light-induced, electro-active SMP, etc. Thermal-induced SMP is the

most common type. The thermally induced SMP is mainly derived from the

reversible network that undergoes a thermal transition, such as a melting or glass

transition.[44-45] By switching the temperature higher than the transition temperature

and lower than it, the shape memory effect can be achieved. Based on the structural

characteristics of the permanent network, SMP can be divided into two categories:

[45] thermosetting SMP and thermoplastic SMP.

Light-active SMP is realized by introducing a specific photochromic group

(PCG) into the main chain or side chain of the polymer. When light is irradiated,

PCG has photoisomerization which makes the state of the molecular chain change

significantly, that is, the material has their temporary shape. When the light stops, the

PCG has reversible photoisomerization, the state of the molecular chain restores, and

the material reverts to the initial shape. This kind of material is used as the printing

[42] material, optical recording material and drug release agent, etc.

Electro-active SMP is a composite of thermal induced shape memory polymer

materials and conductive material, such as graphitized carbon black, metallic powder,

23 etc. The mechanism is similar with that of the thermal induced shape memory polymer. The conductive material increases the temperature of the system through the heat generated by the current, resulting in the shape recovery, so it has both

[41-42] conductive and shape memory effects.

The shape memory effect (SME) represents the ability of the polymer to remember the shape through the shape memory cycle from the initial shape to the temporary shape and then back to the recovered shape. Fixity and recovery are two important features to judge the effect. Fixity indicates the ability to keep the temporary shape in the absence of the stress, and recovery refers to the ability to recover to the initial shape from the temporary shape when the polymer has been

[46-47] exposed to a stimulus.

Figure 1.4 shows the mechanism of thermally induced SMP. In this research the sample is physically mixed. The polymer chain is silicone rubber and the “square” indicates the small molecule crystal. For the small molecule, the transition temperature is the melting temperature Tm. When the temperature rises to higher than

Tm, the small molecule part melts and softens. Under the external force, the sample can be made into any shape and keep it. By maintaining the external force and cooling down the temperature, the small molecule crystallizes forming a solid network that allows the deformed shape to be fixed. The stress from the deformation of the polymer is stored in the hard segment. After the temperature has risen again to higher than Tm, the small molecular chain is melted again. Under the restoring stress of the elastic phase, the sample gradually reaches a thermodynamic equilibrium

24 state.[48-50] The macroscopic performance is that the sample recovers to the original

shape.

Figure 1.3 Mechanism of thermally induced SMP (schematic).

1.2.2 Application of shape memory polymers

In recent years, SMPs have developed rapidly because of some of its obvious

advantages such as low cost, large recovery deformation, structural versatility, and

low recovery temperature.[51-52] SMPs have applications in many areas. As a main branch in the application of shape memory polymers, the application of thermal induced SMP will be introduced.

Compared with other functional materials, the thermal induced SMP material

has some obvious advantages such as abundant raw materials, large deformation,

easy transportation. And the cost is only one percent of the shape memory metal

alloy.[52-54] The production process is simple. The recovery temperature range is wide.

And it is easy to make complex products with low energy consumption.

25 Polyene thermal induced SMP materials can be divided into two categories:

the general type and the flame retardant type.[54] The general type has many

advantages such as inexpensive and can be widely used in the packaging industry.

Most of the flame retardant type products are used in advanced technology such as

[54-55] missiles, rockets, and aircraft.

Polyester thermal induced SMP materials have good electrical properties and

[53-54] mechanical properties, so it is widely applied in the electrical industry.

For fluoroplastics, due to the high-temperature resistance, aging resistance,

chemical corrosion resistance and excellent electrical properties, they are mainly

used in the field of military and advanced industry. And they can be used in the

nozzles and rivets of different caliber polymer pipes, medical fixers, and temperature

[54-56] sensing devices of fire alarms.

Some portable containers and toys made from shape memory polymer materials are very convenient for climbing and traveling. The container can be

heated by hot water to make it return to the original shape when it is needed. The

high-strength shape memory polymer material can also be used as a car’s baffle and bumper. After a collision, the deformation part can be regenerated by hot air

[56] heating.

As a new functional polymer material, shape memory polymer can get good

economic and social benefits. And with the further research, the properties of shape

memory materials will be continuously improved.

CHAPTER II

EXPERIMENTAL

2.1 Materials and tools

Triactoxysilane (clear waterproof silicone, 100%, GE), cornstarch (100%,

ACH), 1,10-decanediol (98%, Sigma-Aldrich), 12-hydroxystearic acid (>80%, TCI), stearic acid (97%, ACROS), chloroform (>99%, Sigma-Aldrich), tetrahydrofuran

(THF, 99%, ACS, Sigma-Aldrich), toluene (>99%, Fisher Scientific), methanol

(>98.5%, Sigma-Aldrich), acetone (>98.5%, Sigma-Aldrich) were used as received.

Stand mixer (Figure 2.1(a), Sunbeam MixMaster); micro milling machine

(Figure 2.1(b), Bel-Art Products); mold (Figure 2.1(c), made by stainless steel; square, length: 5mm, thickness: 1mm. The size can be changed by requirements); modified crescent wrench (Figure 2.1 (d), used to test the shape memory effect).

(a) (b)

2.2 Sample Preparation

Figure 2.4 shows the main procedure to prepare a sample. For the first blend sample which the cornstarch was added as the filler, first add the filler into a cup

(plastic, the diameter of its bottom is 2cm larger than the stir). Then add the silicone to the cup and mix them together by the mixer (shown in Figure 2.1 (a)) at speed 1 until almost no cornstarch left as residue. If still much cornstarch left in the cup, hands can be used to press it for several times to make it mix better, Next, put the blend sample in a mold (shown in Figure 2.1(c)) and move it in a vacuum compression press machine at a pressure of 15MPa (the pressure can be changed

28 according to the elasticity of samples) under more than 28’’ Hg vacuum to have a

square shape. The sample was held in the compression press machine for 3-5 minutes after initial compression and then removed. Finally, the material still need 12 hours to have a complete curing before being tested. All these steps are operated at room temperature under air.

For the shape memory blend sample which different small molecules were added as fillers. Most of the steps to make a sample are similar with the cornstarch-

PDMS sample. The only difference is that small molecules need to be milled into powder before mixing by the micro milling machine (shown in Figure (b)).

Figure 2.2 Procedure to prepare a sample

2.3 Characterization

29 2.3.1 Crosslinking Degree

The degree of crosslinking was measured by the swelling capacity and the gel

fraction. These two can be measured in one continuous test.

First, the sample was cut into a 3cm x 3cm square, and then put in a vacuum

oven to dry it for 24h. After 24h it was taken out and weighed to obtain the weight W1.

The sample was then soaked in chloroform for 24h. After 24h, the sample was weighed to obtain weight W2. The sample was then put in a vacuum oven to dry for

24h. After 24h, the sample was weighed for a third time and get the weight W0. Then

the swelling capacity and the gel fraction are calculated by the following equations:

2 1 Swelling capacity = x 100% (1)

𝑊𝑊 −𝑊𝑊1 1 0 Gel fraction = 𝑊𝑊 x 100% (2) −𝑊𝑊0 𝑊𝑊 The swelling of the polymer leads to𝑊𝑊 the expansion of the three-dimensional

molecular network, which reduces the entropy. Then the network would try to

contract by the elastic contraction force. When these two forces balance each other,

the swelling equilibrium is reached. The swelling capacity shows the ability of the

sample to swell in solvent. The gel part of the material cannot be dissolved in the

solvent, the solvent can only penetrate in the polymer network. The swelling capacity

is the measure of polymer weight changes before and after swelling in the solvent.

Generally, the degree of crosslinking varies inversely with the swelling capacity. The increasing crosslink density of the network would reduce the swelling capacity.

The gel fraction determines the fraction of the crosslinked polymer. Before swelling, the dried mass of the sample is determined. But during swelling, the

30 non-crosslinked polymers are dissolved in the swelling solvent, and removed from the

sample. Therefore, the mass of polymer remaining when the swollen sample is dried

is the gel fraction.

2.3.2 Strain-Controlled Shape Memory Experiments

The fixity and recovery are two important factors for a shape memory polymer.

A crescent wrench with a sample holding bars was used to control the strain as 50%,

100%, 150%, 200%. While a water bath could be used to vary the temperature of the

sample, the solubility of the small molecule crystal in water prevented its use. Instead,

an oven was used to regulate the sample temperature. Under strain, the sample was

put into an oven at elevated temperature (>Tm) for 10 mins. After removing the

sample from the oven for 10 mins, the sample was cooled to room temperature and

removed from the wrench. After measuring the fixed strain, the sample was placed

back to the oven and recovered to its initial shape. Each sample was tested several

times and to obtain average values of the fixity and recovery.

By marking an initial length on the sample, this length was measured after each step of the shape memory cycle. The measured lengths were the initial length (Li), the

length after stretching (La), the length after being removed from the wrench (Lt), and

the length after recovery (Lp). By the following equation, the strain at each step of the

shape memory cycle was calculated:

= (3) 𝑥𝑥 𝑖𝑖 𝐿𝐿 −𝐿𝐿 𝑥𝑥 (x𝜀𝜀 = i, a, t,𝐿𝐿 𝑖𝑖p)

Initial strain ( ), applied strain ( ), temporary strain ( ), permanent strain ( )

𝑖𝑖 𝑎𝑎 𝑡𝑡 𝑝𝑝 𝜀𝜀 𝜀𝜀 𝜀𝜀 𝜀𝜀 31 can be given. And then the fixity (Rf) and recovery (Rr) are given by:

= x 100% (4) 𝜀𝜀𝑡𝑡 𝑓𝑓 𝑎𝑎 𝑅𝑅 𝜀𝜀

R = x 100% (5) 𝜀𝜀𝑎𝑎−𝜀𝜀𝑝𝑝 𝑟𝑟 𝑎𝑎 𝑖𝑖 𝜀𝜀 −𝜀𝜀 2.3.3. DMA (Dynamic Mechanical Analysis)

Dynamic properties were tested by Dynamic Mechanical Analysis (DMA).

Strain sweeps were used to measure the storage modulus and the loss modulus.

In a strain sweep, samples were tested with different strains, but constant temperature

and frequency. By testing under this mode, we can get a graph of storage modulus and

loss modulus vs strain.

Strain rate mode is used to measure the tensile strength and the fracture strength

and the corresponding strain. Under strain rate mode, samples are stretched by

constant strain rate. Then we can get the tensile curve and compare the stress and

[4] strain at the broken point, and consider the variation among the different samples.

Temperature ramp is used to measure the relationship between the storage and

loss modulus and temperature. The curve can show transition temperatures, such as

the glass transition temperature Tg, the melting temperature Tm, and the variation of the viscoelasticity at different temperatures.

Controlled force mode is used to test the shape memory cycle. By controlling a certain stress and varying temperature, the change of the strain is measured and used to calculate the recovery, fixity, and the repeatability of the shape memory effect.

Through this test, we can have an quantitative judgment of the shape memory effect

32 of the material.

2.3.4 TGA (Thermogravimetric Analysis)

TGA is a technique that used to measure the thermal stability and compositions of the samples. Through a controlled temperature ramp the weight of sample is monitored to observe thermal changes, such as decomposition. The normal graph shows a step profile, which means that the weight of sample decreases by parts. A decreasing weight in the graph means substances in the sample have been removed from the sample, so by analyzing the graph, the volatilization or decomposition temperature corresponding to each type of substance is determined.

2.3.5 DSC (Differential Scanning Calorimetry)

DSC is a common thermal analysis technique. By testing the relationship between sample temperature and the difference of the required heat flow of the reference and sample under controlled temperature, a variety of thermodynamic and kinetic parameters such like melting temperature, specific heat capacity, reaction heat, transformation heat and crystallization rate can be measured. In this research DSC was used to measure the melting temperature of the small molecule which controls the temperature range over which the shape memory effect can occur.

2.3.6 Microscope

Microscope is mainly used to amplify tiny objects to make people can observe them with their eyes. In this research, the optical microscope is used to get much

33 information about the morphology of samples such like sample’s surface and internal structure, the porous size, and the particle size of the small molecule powder.

CHAPTER III

RESULTS AND DISCUSSION

3.1 Cornstarch-PDMS Blend

Choosing a suitable solvent is a very important step for the swelling capacity

test. The non-crosslinked part should dissolve in the solvent, to allow for extraction of

the sol fraction.

(a) (b) Figure 3.1 (a) Solubility test of non-crosslinked silicone rubber in toluene (b) Solubility test of non-crosslinked silicone rubber in chloroform

Based on the solubility parameters and the polarity of different solvents,

chloroform shows a large degree of swelling compared with other solvents. Figure

3.1(a) and (b) shows chloroform could dissolve the silicone monomer completely, to

be compared, toluene cannot dissolve all the monomer. Chloroform should be a good

solvent for the swelling test.

Figure 3.2 Swelling capacity vs Different amount of cornstarch

Figure 3.2 shows the variation of swelling capacity with the increasing amount

of cornstarch in the sample. These samples were made of the same amount of water

and silicone, but different amount of cornstarch. For the polymer, generally, with

increasing degree of crosslinking, the crosslink density of polymer chains will

increase, so the swelling capacity will go down. From the graph, with increasing

amount of cornstarch, the swelling capacity decreases linearly with the weight of cornstarch. As previously mentioned, the cornstarch contains~20% water itself, and the water can promote the crosslinking reaction. The cornstarch is also a polymeric material and may form hydrogen bonds with the hydroxy groups with the silicone rubber, this physical crosslinking could also restrict swelling to some degree.

Figure 3.3 Swelling capacity vs amount of water

Figure 3.3 shows the variation of swelling capacity vs the amount of water in the formulation. For this test, samples are made using the same amount of silicone and cornstarch, and different amount of water. By adding the water, this varies the moisture in the sample. But the variation of swelling capacity in Figure 3.3 is much smaller than it in Figure 3.2, and almost invariant with an increasing amount of water.

Compared with the moisture in cornstarch, the water added has a smaller influence.

Changing the amount of water does not have much effect on samples.

Figure 3.4 The amount of cornstarch vs swelling capacity

Figure 3.4 shows the relationship between the amount of water and the swelling

capacity. These three lines show samples with different amount of water but the same

amount of cornstarch and silicone. For this test, the effect of the initial moisture in the

cornstarch was considered, so dried cornstarch group was used to compare with the

samples prepared with as relieved cornstarch. From the graph, that differences between curves are small, therefore, the influence of water on the crosslinked elastomer is much small. The moisture in cornstarch does not have a large influence.

For this research, the amount of water is not critical to the sample preparation. But we

still can see the basic trend is the swelling capacity will decrease with increasing

amount of water, which means that the degree of crosslinking increases with

increasing amount of water.

(a)

(b) Figure 3.5 DMA strain sweep curves (storage modulus vs strain) for different weight percent sample, the ratio means water:PDMS:cornstarch (a) strain sweep curves for different weight percent cornstarch sample (b) strain sweep curves for different weight percent water

Figure 3.5 shows the strain sweeps by DMA, which means samples are tested under a constant frequency and temperature, but advancing strain. The ratio in the legend refers to the material proportion of samples; the amount of water; the amount of silicone; the amount of cornstarch. By this test, the storage modulus and loss

39 modulus can be obtained. Here, the storage modulus would be mainly discussed.

Storage modulus shows the stiffness of the material which means the ability to resist deformation with applied force. From the Figure 3.5 (a), with increasing amount of cornstarch, the stiffness increases, which means that the materials are more difficult to deform. From Figure 3.5 (b), it shows that increasing the amount of in the formulation increases the storage modulus. Compared to Figure 3.5 (a), differences of the storage modulus between different amounts of water are less than them between different amounts of cornstarch. The influence of water on dynamic properties is smaller than cornstarch.

Behavior similar to the Payne effect can be obviously seen in these two graphs.[39-40] The filler-polymer interaction causes a gradual drop of the E’ with the increasing deformation. The filler-polymer interaction becomes stronger with the increasing amount of cornstarch, which leads to the differences of slopes of the curves in Figure 3.5 (a).[60-61] In Figure 3.5 (b), the curves are more similar because the samples are less likely to be influenced by the amount of water.

Figure 3.6 DMA stress-strain test for different weight percent cornstarch samples (the amount of silicone:cornstarch = 30:30, 30:35, 30:40, 30:45, 30:50, 30:55 and pure PDMS)

The results of the stress-strain tests are shown in Figure 3.6. Compared with pure PDMS, the storage modulus of the silicone rubber blend samples increases compared to the neat silicone. By adding cornstarch, the silicone rubber is converted from a soft gel to a stiff elastomer. Both PDMS and rubber blends have nonlinear stress-strain curves. PDMS has a typical J-shaped stress-strain curve and the characteristics of J-shaped curve gradual disappeared by adding cornstarch. For these nonlinear stress-strain curves, they can show a brief linear part at the beginning part

(below 30% elongation), and then the Young’s modulus can be calculated from this part.[58-59] Beyond this part, these curves show an increasing nonlinearity which is

caused by the alignment of the polymer chains.

SAMPLE E’/MPA ELONGATION AT BREAK (%) UTS/MPA

PDMS 0.55 449 0.66 30:30 1.32 309 1.34 30:35 1.58 314 1.22 30:40 1.54 219 1.07 30:45 1.58 242 0.99 30:50 1.87 229 1.00 30:55 1.95 235 1.06 Table 3.1 Mechanical characteristics of different weight percent cornstarch samples

As shown in Table 3.1, with increasing amount of cornstarch, the stiffness of the sample increases. Compared with PDMS, blend samples have a larger storage modulus, lower strain at break and larger ultimate tensile strength(UTS). This formula can also be applied in samples with different compositions. The storage modulus increases, the strain at break decreases and the ultimate tensile stress increases as a function of increasing weight percent of cornstarch. Because the difference of composition ratio is small among these blend samples, this trend is more obvious when compared with PDMS.

(a)

(b) Figure 3.7 TGA tests for (a) PDMS, cornstarch and 1:30:30 sample (b) different weight percent samples (the amount of water: silicone: cornstarch=1:30:30, 1:30:35, 1:30:40, 1:30:45, 1:30:50, 1:30:55)

Figure 3.7(a) shows the TGA test results of PDMS, cornstarch and the 1:30:30 sample. From this graph, the weight loss of each part could be observed. The decomposition temperature of the blend sample is around 110°C. As for the sample, weight loss can be divided into three parts. Weight loss from 100% to about 85% is mainly due to the loss of absorbed and bounded water and the thermal decomposition of the non-crosslinking part. Weight loss from about 85% to 55% is caused by the

43 thermal decomposition of cornstarch. Weight loss from about 55% to 5% is due to the

thermal decomposition of both cornstarch and silicone. Because of the nitrogen

atmosphere, the carbon and silica can remain as the residue. As shown in Figure

3.7(b), different composition results in the different weight loss percent of curves.

3.2 Shape Memory Silicone Rubber Blend

For the shape memory polymer, the most important thing is the shape memory

effect. Some pictures which demonstrate the shape memory effect of the blend sample

are shown in the Figure 3.8.

(a) (b) (c)

(d) (e) (f) Figure 3.8 A series of shape memory effect (a) the initial shape (b) use tweezer to keep a shape (c) put the sample into hot water for 5 seconds (d) the temporary shape (e) put the sample back into the hot water for 5 seconds (f) the sample recovers to the initial shape

44 Figure 3.8 shows a whole cycle of the shape memory test. These pictures

indicate that the blend sample has a good shape memory effect. The sample could fix

the temporary shape in only 5 seconds, and it can also recover to the initial shape in 5

seconds. It means that the sample have a quick response under the external stimuli.

After the first view of the shape memory effect, some characterization tests have been

done about the sample in the following research.

Figure 3.9 TGA curves for PDMS, decanediol and 50% decanediol-silicone blend. Experiments were conducted using a nitrogen atmosphere with a heating rate of 10°C min-1 from 25°C to 800°C

Figure 3.9 shows TGA curves of decanediol, PDMS and the blend sample.

Decomposition temperatures of them are all at about 130°C. Weight loss from 100%

to 50% of the sample is mainly due to the decomposition of decanediol and the non-

crosslinked part of silicone rubber. The weight loss from 50% to 10% is mainly caused by the decomposition of PDMS. The residual part is remaining carbon and silica because of the nitrogen atmosphere.

45 Based on the TGA test for the sample, a temperature 50˚C below the

decomposition temperature was chosen for the DSC test of the samples. The

decomposition temperature is around 180°C and the melting temperature of

decanediol is around 70°C-80°C, so 25°C-100°C is an appropriate temperature range for the DSC test.

(a)

(b) Figure 3.10 (a) DSC curves for decanediol-silicone blend with different weight percent decanediol (10%, 20%, 30%, 40%, 50%, 100%). Experiment used a heating rate of 10°C min-1 from 25°C to 100°C. (b) Melting temperature vs weight percent decanediol

46 Figure 3.10 shows the DSC curves for different weight percent decanediol

samples. From the graph, both the melting temperature and the melting temperature

range increase with increasing weight percent of decanediol. It is unclear what the

underlying thermodynamic explanation is for this behavior, but it deserves further

study.

Figure 3.11 DMA Temperature ramp tests (storage modulus vs temperature) for different weight percent of decanediol (10%, 20%, 30%, 40%, 50%, 100%)

Figure 3.11 shows temperature ramp tests of the storage modulus vs temperature. In the graph, curves all show a decrease part of the storage modulus which means the internal structure changed. In this research, this is due to the melting temperature of the crystals. Melting temperature is an important parameter in this

research so the DMA test helps to make sure the melting temperature again. By

adding more decanediol, the storage modulus of blend samples increases because of

the physical network of decanediol. That also explains why the sample with lower

47 weight percent of decanediol has a larger E’ when the temperature is higher than the melting temperature. After the temperature is higher than the melting temperature, only PDMS network left in samples to resist the applied force. The sample with higher weight percent of PDMS has a higher modulus at a high temperature (>Tm).

(a)

(b) Figure 3.12 (a) DMA strain sweep cures (storage modulus vs strain) for different weight percent decanediol samples (10%, 20%, 30%, 40%, 50%). (b) Storage modulus vs weight percent decanediol

48 Figure 3.12 (a) shows the storage modulus vs strain for different weight percent

decanediol. With increasing weight percent of decanediol, the storage modulus

increases. The modulus decreases with the increase of strain. This is attributed to

Payne like effect where the crystal network is disrupted at high strain. Payne like

effect can be seen. The effect is much more significant for a higher weight percent

decanediol sample. If higher deformation is applied, the filler interaction can be

broken then the polymer could be released. The storage modulus has a larger decrease

for higher weight percent decanediol sample. Figure 3.12 (b) shows the E’ (at about

0.02% strain) as a function of weight percent of decanediol. The curves between E’ and weight percent decanediol have a plateau at low strain, so the E’ at 0.02% should be representative of the network for disruption. The graph shows that with increasing amount of decanediol, the blend sample has a larger E’. The relationship is non-linear

[60-61] and the trend of growth becomes larger with increasing amount of decanediol.

After some thermal tests and mechanical tests, the next step is to test the fixity

and recovery of samples. Fixity and recovery are two important parameters to judge

the shape memory effect. These tests are tested by the crescent wrench.

(a)

(b) Figure 3.13 Fixity and recovery tests by the crescent wrench for different weight percent decanediol sample (10%, 20%, 30%, 40%, 50%, 60%) at 95°C oven.

Figure 3.13 shows fixity and recovery for different weight percent decanediol silicone rubber blends. As shown in the graph, for the fixity, there is a large difference between different samples. With increasing amount of weight percent decanediol, the fixity shows an increase trend. For 40%, 50%, 60% weight percent decanediol blends, the fixity is more than 90% at 200% strain or larger, and 50% sample can even reach ca. 98%. As to the recovery, 10%-50% weight percent decanediol blends do not show

50 a large difference and are all higher than 90%. But the recovery of 60% blend sample

has an obviously decrease compared with other weight percent blend samples.

Considering both the fixity and recovery, the 50% weight percent decanediol blend

sample has the best performance and is used for further tests.

The above data just shows the fixity and recovery of the first cycle for samples.

But for the shape memory polymer, a good repeatability is required. More shape memory cycle tests need to be done.

Figure 3.14 DMA controlled stress shape memory cycle test for 50% weight percent decanediol silicone rubber blends under different stress (0.11MPa, 0.13MPa, 0.2MPa, 0.215MPa)

As shown in Figure 3.14, when the stress is unloaded, the strain has a little

decrease and then a plateau. Besides the first cycle, the permenent strain for each

51 cycle does not show a large difference in the graph. The recovery magnitude of the

first cycle is low, this is attributed to the none-zero load applied by the DMA clamps.

Compared with the blend sample under 0.11 MPa, the sample under 0.215 MPa does not show a large decrease in fixity and recovery. These curves directly indicate that blends have a good shape memory repeatability and can still keep good shape memory ability under different loaded stress.

(a)

(b) Figure 3.15 Fixity and recovery results of 50% decanediol blend of shape memory cycles under different stress

Based on the curves shown in Figure 3.14, the fixity and recovery can be

calculated. In Figure 3.15 (a), the fixity data at different stress are close to each other

and all exceed 95%. As mentioned above, the recovery data of the first cycle has been

influenced by instrument effects. Although the recovery of the first cycle is reduced,

the recovery already reached 90%. For the following cycles, the recovery data could

even approach 100% and do not have a great difference with different stress applied.

Based on these data points, it is seen that the 50% weight percent decanediol blend not only has a great performance in the first cycle but also can have a repeatable

performance for the next few cycles. It indicates that the silicone rubber network did

not have a large reduction in elasticity during several stretching tests.

The next question is the morphology of silicone rubber blend samples.

According to the mechanism of the shape memory polymer, decanediol provides the

reversible network, so the distribution of the decanediol particle is also a factor related

with the shape memory effect.

53 Solubility of Swelling capacity of Decanediol PDMS

H2O x x

CHCl3 x o

THF o o

Methanol o x

Toluene x o

Acetone o x Table 3.1 Solubility test results of decanediol and swelling test results of PDMS

First, choosing a good solvent to remove the decanediol particle in the sample to have a directly view of the porosity. As the results shown in Table 3.2, THF could be the best solvent for the test.

(a) (b)

(c)

54 Figure 3.16 Microscope picture for (a) Decanediol powder (b) Crystallization in 50% decanediol-silicone blend sample (c) Holes in the 50% decanediol-silicone blend sample after swelling in THF to remove the decanediol

Figure 3.16 (a) shows the particle size of the decanediol powder. Because of the

physical processing method, the decanediol powder particle cannot reach a highly

even size. From the picture, the particle size is from 78μm to 289μm. Figure 3.16 (b)

shows the crystallization situation in the sample. From this view, many crystallized

particles can be seen. Figure 3.16 (c) shows the internal structure of the sample after

swelling in THF to remove the decanediol. After removing decanediol particles, the

distribution of them can be directly observed. The holes are not all the same size

because the decanediol particle size is varies. By a rough calculation of the area

fraction of the holes in the whole picture, the area of the holes in Figure 3.16 (c) takes

nearly 50% of the entire area which correspond with the weight percent of decanediol,

And the holes are not all together, but dispersed in the whole picture.

After a series of tests, decanediol-silicone blend samples reveal a good shape

memory effect. Then some other fillers which also have hydroxy groups have been

tested to see differences in the shape memory effect between them and decanediol.

(a)

(b) Figure 3.17 Fixity and recovery test by the wrench for samples with different fillers (50% decanediol-silicone rubber blend, 50% 12-hydroxystearic acid-silicone rubber blend, 50% stearic acid-silicone rubber blend) at 95°C oven.

Figure 3.17 (a) shows the fixity of these three samples. From the graph, these three samples all has a large value of fixity and do not show a large difference. All the fixity of these samples is above 90%, and only the fixity of the 12-hydroxystearic acid blend sample is a little bit larger than the others. But from the Figure 3.17 (b), compared with other samples, the 12-hydroxystearic acid blend sample do not

56 perform well, its recovery is only around 80%. The decanediol silicon rubber blend has the best performance, the next is stearic acid, the 12-hydroxystearic acid is the last.

For shape memory polymer, fixity and recovery are all important. Among the comprehensive performance of these three samples, the decanediol blend sample has the best shape memory effect.

CHAPTER IV

CONCLUSIONS

Silicone rubber blend is a kind of widely used materials. In this research, silicone rubber was blended with different fillers and then been tested for some abilities.

The first section is focused on cornstarch-silicone rubber blend. Homogeneous samples can be fabricated by mixing different concentration of cornstarch with PDMS in a simple physical way. The blended rubber could convert from soft state to rigid gradually by adding cornstarch. The mechanical properties and thermal abilities were tested with different weight proportions. The results of several tests show that cornstarch is an effective reinforcing filler for silicone rubber.

The second section is about shape memory polymer. By mixing silicone rubber with decanediol, the shape memory effect can be realized. The crystalline small molecule (decanediol) could provide the reversible network. The silicone rubber part results in a elasticity. Samples can be stretched to 200% elongation and display good shape memory properties. According to the shape memory cycle test, 50% decanediol

silicone rubber blend shows the best performance in terms of high fixity and recovery, and it displays good repeatability after several shape memory cycles. Last, by comparing decanediol with other crystalline small molecules, decanediol silicone rubber blend still displays the best performance.

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